Recent Development in Advance Ceramic Materials and Understanding the Mechanisms of Thermal Barrier Coatings Degradation

Abstract

Metallic alloys' behavior at high temperatures, especially their response to corrosion and formation of protective surface layers, has long been a focus of scientific inquiry. Although certain alloy compositions require an initiation period before hot corrosion advances to the propagation stage, no combination of alloys can be considered impervious to hot corrosion indefinitely. The capacity of nickel-based materials to tolerate extreme circumstances such high temperatures, acidity, corrosion, and scratching is highly valued. However, they are unable to satisfy the strict demands of today's high-temperature applications. The durability of thermal barrier coatings (TBCs), which are prone to oxidation, rust, and degradation from sulphates and foreign object damage, has been the subject of recent study. For sophisticated ceramic materials exposed to high temperatures, hot rust degradation poses a considerable challenge. The main objective of this study is to investigate the effects of severe degradation on several advanced ceramic material types and their level of advancement. The purpose of the inquiry is to comprehend the deteriorating processes at the long term working condition, including the function of oxidation and liquid salts. Additionally, we investigate the effects of temperature, environment, and contact duration on the heated weathering behavior of earthenware. Finally, we discuss strategies for mitigating hot corrosion degradation in ceramics, such as protective coatings like new design of TBCs, doping, and composition optimization. This paper aims to offer a thorough understanding of the hot corrosion behavior of ceramics, which is crucial for developing durable materials suitable for high-temperature applications. Additionally, it explores the fabrication of protective coatings and addresses the challenges faced in this regard. The insights gained from this research can contribute to the advancement of resilient ceramic fabrics and the development of effective protective coatings.

1 Introduction

The choice of materials in contemporary manufacturing is significantly influenced by high temperature corrosion. Several industrial operations, including those in electric power plants, aircraft, gas turbines, heat-treating, mineral and metallurgical processing, run at temperatures higher than 500 °C. According to Liu et al., 2021 in many industrial processes, the highest-temperature rusting process that oxidation frequently dominates. In actuality, most high temperature metals are designed to engage with the oxidizing atmosphere in a way that causes a protective oxide layer to develop [1]. A high temperature alloy's ability to resist deterioration is dependent on its ability to maintain the creation of this protective scale. The degree of protection that may be offered depends on the scale's characteristics. The scale should ideally have a modest growth rate, excellent adhesion to the metal substrate, high stability, be continuous, and be devoid of flaws like microcracks or significant voids.

For next-generation aircraft and/or engines using gas turbines (with working temps surpassing 1500 °C), the standard yttrium oxide (Y₂O₃) stabilized by zirconia (ZrO₂) YSZ is inappropriate. First, YSZ will lose its phase stability and damage endurance when sintering at temperatures above 1300 °C. Lower thermal transmission is necessary to increase the operating temperature of gas turbine and/or jet motors even more. Determining novel TBC materials with enhanced high temperature stability, decreased thermal conductivity, and no phase shift during high temperature cycles is therefore crucial. There is now a global search for new and promising TBC materials that will replace 7–8 mol percentage of Y₂O₃ Stabilized by ZrO₂ and have better high-temperature resistance properties [2]. Advanced materials such as Lanthanum aluminate (LaAlO₃), Magnesium zirconate (MgZrO₃), Gadolinium Zirconate (GZO), Thermal barrier coatings (TBCs), which shield metals and ceramics from heated degradation, can be made from these and other cutting-edge materials. A lot of interest has been paid to GZO in particular because of its high melting point and poor heat transmission. The rare earth compounds yttrium oxide, yttrium aluminum garnet (YAG), and scandium oxide are among the other sophisticated TBC materials. Lanthanum zirconate, ceria-stabilized zirconia, and zirconium oxide are also available (Sc₂O₃), lanthanum hex aluminate (LaAl₁₁O₁₈), calcium zirconate (CaZrO₃), ceria-stabilized zirconia (CSZ), and cerium oxide. TBCs may become more resilient to wear and heated rust in high-temperature settings thanks to these cutting-edge materials. These materials have limited heat transmission, chemical resilience, and great temperature stability. New substances being investigated by researchers as possible TBC options include tungsten carbide and silicon carbide. The purpose and working circumstances determine the best TBC substance to use, and a number of variables, including thermal resistance, thermal expansion, and longevity, must be taken into account. Although a thermal barrier coating’s (TBC) main purpose is to act as a thermal barrier, choosing the right TBC material can be difficult because the coating must meet a number of strict performance requirements while working in a very harsh thermo-mechanical environment [3]. The performance requirements for a TBC include a high melting point, resistance to thermal cycling, corrosion and damage, good damage tolerance, and a low sintering rate. However, accurately evaluating the performance of TBC materials in service poses a significant challenge. Laboratory simulations struggle to replicate the harsh operating conditions that TBCs experience, strong heated gas pressures, fast temperature changes, sharp temperature slopes, high temps, and extra mechanical and corrosion stresses, among others.

Molten salts, typically composed of potassium and sodium sulfates, attack the turbine hot section as contaminants from the air and fuel and may result in rapid material loss. This is what causes hot corrosion. Literature suggest that there are two types of hot corrosion: high-temperature and low-temperature hot corrosion. While the fundamental mechanics of hot corrosion are very well known, there are still areas of contention. High-temperature hot corrosion occurs at temperatures above 850 °C and involves the formation of molten salt deposits on the surface of the material, which can penetrate and react with the substrate material, leading to degradation. The most common type of salt deposit in high-temperature hot corrosion is sodium sulfate, which can cause oxide scale spallation and ultimately lead to material failure. In contrast, low-temperature hot corrosion occurs at temperatures between 400 and 850 °C and involves the formation of non-molten salt deposits, such as sulfates and chlorides, on the material's surface. These deposits can induce stress corrosion cracking, causing the material to fail over time. Both types of hot corrosion can be particularly problematic in gas turbine engines, which operate at high temperatures and are exposed to various types of contaminants that can lead to hot corrosion. Therefore, understanding the mechanisms and prevention methods for both types of hot corrosion is crucial for designing durable materials for high-temperature applications [4]. According to Rapp second-tier components, (such as environmental pollutants like chlorine as well, calcium, and iron) have a less apparent influences on hot corrosion environment [5]. Maintaining the protective scale and the hot-corrosion process in its development stage are essential for ongoing defense against hot corrosion. The covering and base metal core will be quickly lost once the scale is lost or broken. The ideal safe scale would lessen the solubility of combined salt, which is determined by the burning chemistry and working conditions. Both chromia and alumina can frequently meet this requirement, but alumina shields at higher operating temps than chromia. According to reports, liquid surface layers in use can cause plasma and electron-beam physical vapor deposition (EB-PVD) coatings to spall more frequently. It is possible for salt species in fumes from substances like dirty fuel or saltwater to settle on the surface beneath them. The salt may leak through the coating's interconnected cracks and pores if the coating temperature is greater than the freezing point of the salt. When it cools, the wicked-in salt solidifies, disabling the coating's tension tolerance mechanism and accelerating spallation [6]. As the oxidation process continues, the oxidation products interact with the salt crystals to produce a complicated mixture of salts that typically has a lower freezing point than the initial salt and is known as the eutectic phase. At this time, the rust reaction sharply rises. Using a coating of sodium sulphate (Na₂SO₄), for instance, Fig. 1 shows the weathering of nickel at 700 °C. In the beginning, Na₂SO₄ is solid, and during the rusting process, nickel is oxidized to create NiO, which when coupled with SO₂ forms NiSO₄. Around 671 °C, the Na₂SO₄ and NiSO₄ combination melts, and at this point, the reaction rate accelerates.

Fig. 1

Corrosion of nickel at 700 °C, covered with Na₂SO₄ deposit (2.5 gm/cm²) in 1 atm. O₂ + 4% SO₂ at different total gas pressures [7]

Hot corrosion is the hearth corrosion that occurs in coal-fired power plants. The main salt deposits in the ash are Na₂SO₄ or K₂SO₄, which are produced when SO₂ reacts with NaCl or KCl found in the coal [7]. The highest exterior wall temperature of steam producing tubes is between 550 and 650 °C, so the heated weathering does not start until the sulphate crystals combine with SO₃ to create pyrosulfates.

1.1 High Temperature Induced Corrosion

In the material selection process, high temperature rust is becoming more important, especially in the power producing industry. Materials are being forced past their initial design limits due to the need for engines to operate at higher temps in order to improve engine yields and continuous exposure to hostile environments and high pressures. Due to their thermal instability, most metals in atmospheric vapors usually combine to form oxides, sulfides, carbides, or a mix of these based on their makeup and working conditions.surface reactions may be minor or unnoticeable at low temperatures and humidity, but as temperatures rise, reaction rates rise as well, implying that an alloy's corrosion resistance becomes critical [8].

It is interesting to note that while some research suggests that salt deposits dissolve on materials at high temperatures, other studies have reached different conclusions. For instance, research conducted by Shinata et al. found that between 640 and 650 °C, or 913 and 923 K, the same processes that operate at low temperatures also occur at higher temperatures. However, in this case, chlorine does not vaporize, but instead condenses into the melt. This discrepancy highlights the complexity of hot corrosion and the need for further research to fully understand the mechanisms and behaviors of materials under high-temperature conditions. It is also important to note that different materials and operating conditions may result in different outcomes, emphasizing the need for tailored solutions when designing materials for high-temperature applications [9]. The process of heated rusting seems to operate differently at greater temperatures than it does at lower temps. A reduction in chlorine evaporation is brought on by the addition of oxygen, which interacts with the transition metal chlorides in the chloride liquid. As opposed to being at the oxide surface as it would be at lower temps, this indicates that chlorine is still present in the dissolve. As a result, rusting may continue for an extended period of time. Cr and Cl combine to form CrCl₂, which is thought to seep from the matrix and form networked holes beneath the material's surface. Then, capillary forces fill these spaces with combined ions. Instead of atmospheric chlorine and chlorides, the rate-limiting species at higher temps seems to be liquid oxygen in the salt solution [10].

It is crucial to address also the impact of CMAS (Calcium-Magnesium-Alumino-Silicate) on coating degradation at high temperatures. CMAS is known to be a significant factor in the deterioration of coatings, particularly in thermal barrier coatings (TBCs). The infiltration of molten CMAS into the coating matrix can lead to various detrimental effects such as phase transformations, spallation, and increased porosity. Understanding the behavior of CMAS and its interaction with coatings is vital for developing effective strategies to mitigate its corrosive effects and enhance the durability of coatings in high-temperature environments.

1.2 Temperature Effect on Conventional YSZ Coatings and Failure

By looking at the thermal and ambient phase change of 7-8YSZ, it was possible to calculate the average temperature of the TBC surface throughout operation. The temperature zones of the combustor plates in turbine engines with different histories were sampled using four-point deformation. With a 50 K rise in surface temperature, the bond coat (BC)/(TBC) interfacial strain that causes TBC spallation at atmospheric temperature reduces by 10% (relatively). The working cycles and lengths of the gas turbines (GTs) under evaluation have little impact on the behavior. In any of the liners analyzed, there was no sign of oxidation-related life limitation. The deterioration of the TBC is shown in Fig. 2 by the strain-to-strong spallation's temperature relationship. Temperature and working time cause the TBC surface to become marginally firmer, but thermocycling causes micro- and macrocracks and lessens the TBC's rigidity, as shown in Fig. 2 [11].

Fig. 2

Strain-to-failure of 4-point-bend samples, taken from ex-service combustor liner of various GTs with different operation schemes (operation hours [OH]/cycles [Starts]) [11]

Four-point bending with acoustic emission was used to detect the critical strain of the start of TBC delamination, macro-cracking, or spallation. TBC was tested under compressive loading to simulate cooling loading.

By analyzing the thermal and ambient phase change of 7YSZ, it was possible to calculate the average temperature of the surface of the TBC while it was in operation. Additionally, the study observed that with a 50 K rise in surface temperature, the BC/TBC interface tension that caused TBC spallation (the process of the TBC separating from the underlying material) at ambient temperature decreased by 10% relative to the original value. The study also examined various GT combustor liners with different histories and found that the operating cycles and durations of the GTs had a minimal impact on the TBC spallation behavior. The TBC degradation was found to be related to the strain-to-strong spallation's temperature dependency, which showed that the stiffness of the TBC decreased with thermocycling, leading to micro- and macrocracks.

In general, The maximum temperature limit for YSZ depends on various factors such as composition, microstructure, and specific application requirements. Generally, YSZ can withstand temperatures up to approximately 1500–1600 °C (2732–2912 °F) in oxidizing environments. Beyond this temperature range, YSZ starts to experience degradation and may not provide optimal performance. At elevated temperatures, YSZ undergoes phase transformations and can develop thermal stresses due to differences in thermal expansion coefficients between the coating and substrate materials. These stresses can lead to cracking, spallation, and reduced coating integrity. Additionally, YSZ may experience chemical reactions with the environment, such as reaction with water vapor or certain corrosive species, further impacting its stability.

1.3 Aim and Scope of the Study

This review article aims to provide a comprehensive understanding of advanced ceramic materials, their various chemical, mechanical and thermal properties, their advanced coating designs, and the challenges associated with their practical implementation. The primary objective is to explore the behavior of ceramic materials under extreme conditions, including issues such as cracking, stress corrosion cracking, and hot corrosion. The article will examine the latest advancements in coating design strategies, including thermal barrier coatings, and protective surface treatments. By addressing the challenges faced by advanced ceramic materials, the article aims to facilitate the development of more resilient materials for high-temperature applications. Additionally, the scope of this review encompasses the fabrication and characterization of protective coatings, along with the identification of potential solutions to overcome the encountered challenges.

2 Advanced Ceramic Materials used for Thermal Coatings

There are many Ceramic materials other than conventional YSZ used for the thermal coatings but the some of the advanced ceramic materials are comprehensively discussed in the literature will interest for this studies [12].

Ceramic materials such as gadolinium zirconate (GZO), hafnium oxide (HfO₂), lanthanum zirconate (LZO), Yttrium aluminum garnet (YAG), Yttrium oxide (Y₂O₃), Scandium oxide (Sc₂O₃), Lanthanum hex aluminate (LaAl₁₁O₁₈), Calcium zirconate (CaZrO₃), Ceria-stabilized zirconia (CSZ), Lanthanum aluminate (LaAlO₃), Magnesium zirconate (MgZrO₃) and Strontium zirconate (SrZrO₃) are advanced ceramic materials that have high resistance against high-temperature hot corrosion. Thermal barrier coatings (TBCs) are added to steel components in gas turbine engines to provide thermal shielding and rust protection for maritime, power production, and aviation transportation. These TBCs have a two-layered construction with an oxidation-resistant binding coat and a thermally shielding top coat made of yttria-stabilized zirconia (YSZ). The bond coat’s oxidation behavior, which incorporates both the MCrAlY and Pt aluminide layer, has a major effect on the TBCs’ life [13, 14].

Numerous studies have been done on the oxidation behavior as well as failure causes of TBC devices. In addition, the deterioration processes that TBC systems are exposed to deposits have been investigated. These deposits can arise from the fuel or environment and can lead to a reduction in TBC performance. Therefore, the study of these processes is crucial to understanding TBC performance and improving TBC design [15].

2.1 Gadolinium Zirconate (GZO)

Rare-Earth Zirconate (A₂B₂O₇) Gadolinium zirconate (GZO), as opposed to Yttria-stabilized zirconia, has reportedly attracted more interest and demand in recent years. This is because of pyrochlore-structured oxide compounds GZO is more effective at providing thermal shielding than YSZ because it has a greater melting point, superior heated rust resistance, high thermal stability, and reduced thermal transmission. GZO has a cubic crystal structure and a lesser heat conductivity than YSZ at about 1.5 W/mK. Additionally, GZO has excellent mechanical qualities, including high toughness, good adherence, and exceptional thermal shock protection, making it a strong contender for TBC uses. Additionally, GZO is more robust and dependable than YSZ in terms of spallation resilience, making it an excellent choice for TBC uses. Nevertheless, both materials are still being researched and used in a variety of sectors, based on particular working circumstances and performance criteria.

The temperature and climatic durability of GZO-based compounds has been examined in a number of investigations. For instance, after being exposed to high temperatures in air according to Zuo et al. examined the morphology and phase stability of GZO coatings and discovered that the coating had good phase stability up to 1400 °C [16]. Additionally, it has been demonstrated that GZO compounds are immune to heated rusting, which is a critical issue in gas turbine applications. It has been reported that GZO coatings exhibit excellent resistance to Na₂SO₄–26%NaCl at 1050 °C [17].

A thermal barrier coating (TBC) is known to be more stable and less porous in hazy turbine circumstances because of the low thermal conductivity, high thermal stability, and advantageous fast reaction with CaO–MgO–Al₂O₃–SiO₂ (CMAS) layers of gadolinium zirconate (GZO). The possible next-generation thermal barrier covering substance gadolinium zirconate must further increase its (CMAS) resilience. Several techniques, including doping, makeup tuning, surface alteration, and stacked coatings, can be used to increase the CMAS resilience of gadolinium zirconate (GZO), a possible next-generation thermal barrier covering material [18].

Due to its low thermal transmission, high thermal stability, and favorable reaction to layers of CaO, MgO, Al₂O₃, and SiO₂, gadolinium zirconate (GZO) is used as a thermal barrier coating (TBC) substance. (CMAS). For the next iteration of TBCs, its CMAS resilience must be improved. In a recent research, atmospheric plasma discharge and amorphous material were used to create three GZO layers with various Gd/Zr ratios. The coverings' mechanical and rust protection qualities were assessed. The coatings had similar Young's moduli and hardness to those reported in earlier research. The results showed that an increase in Gd content led to faster CMAS corrosion and the formation of Gd-apatite. The Gd_2.3Zr_1.7O_6.85 coating exhibited up to a 35% reduction in penetration depth after a 24-h annealing process, indicating improved long-term corrosion resistance compared to Gd_2.0Zr_2.0O_7.0 and Gd1.8Zr2.2O_7.1 coatings. This study suggests that developing practical fabrication techniques for non-stoichiometric GZO coatings with adaptable CMAS corrosion resistance could lead to the development of more durable TBCs in the future [19, 20].

In Fig. 3, a double layer coating that has undergone a burner rig test is seen in a scanning electron microscope (SEM) picture. The picture shows that the coating failed extremely late following the -phase depletion of the bond layer. In addition to performing well in burner rig testing, optimized multilayered systems also show improved resistance against calcium, magnesium, aluminum, and silicon oxide (CMAS) assault, which is a major worry, especially for aeroengines. The future development of long-lasting thermal barrier coatings is anticipated to be advanced by the development of a feasible technique for producing non-stoichiometric gadolinium zirconate coatings with adaptive CMAS corrosion resistance. It is anticipated that the operating temperature of the next-generation aeroengine will be higher than 1500 °C, which is much over the threshold at which partly stable Y₂O₃ stabilized by ZrO₂ can exist. Due to their extraordinary high-temperature stability and advantageous thermal characteristics, rare-earth (RE) zirconates in the pyrochlore structure are being explored as promising materials for thermal barrier coatings (TBCs) in the future. The adherence of gadolinium zirconate to the substrate can be increased using a variety of ways.

Fig. 3

SEM micrograph of a GZO/YSZ double layer after test in burner rig with rather complete β-phase depletion of the bond coat [21]

For instance, the substrate’s surface may be roughened to improve the mechanical interlocking between the coating and the substrate. connection coatings and other intermediary layers may also aid in strengthening the connection between GZ and the substrate. The thermal expansion coefficient of the coating must be properly matched to that of the substrate in order to increase the thermal cycling resistance of GZ. During thermal cycling, mismatched thermal expansion coefficients may cause the coating to peel off and shatter. Using graded composition coatings, which progressively modify the coating's composition from the substrate to the top layer to produce a better match of thermal expansion coefficients, is one technique to deal with this problem. To improve the resistance of GZ to environmental degradation, it is important to minimize its exposure to corrosive environments, such as those containing sulfur or alkali metal salts. Additionally, the use of dopants or compositional modifications can be employed to enhance the resistance of GZ to such environments [21].

Aero-engines are vulnerable to damage by molten aviation ash (CMAS) when subjected to elevated operating temperatures; as a result, one of the most crucial factors to evaluate the in-service efficiency of aero-engines is the corrosion resistance of TBCs. In the presence of molten CMAS, GZ exhibits high corrosion resistance. During the corrosion process, Gd₂O₃ will leak out of GZ and react with Ca and Si in molten CMAS to create a solid solution that resembles an appetite. The resistance to corrosion of GZ, when exposed to molten CMAS, is thought to be further extended by the practical approach of increasing the Gd content in GZ. The accelerated Gd-apatite production caused by the elevated Gd concentration aids in preventing molten CMAS from entering the porous nature of the coatings. The produced Gd-apatite fills in the coatings' porous structure in the interim, obstructing the penetration of molten CMAS. It is clear that the Gd element, through participating in the formation of apatite, considerably adds to GZ’s CMAS corrosion resistance [22].

The swiftly formed Gd-apatite is metastable and will ultimately dissolve again in the Gd³⁺ undersaturated CMAS melt, hence it cannot offer long-term CMAS penetration mitigation. Therefore, further study is required to create plans to increase the CMAS resistance of GZ coatings over the long term. In comparison to yttria-stabilized zirconia, gadolinium zirconate has a melting point that is higher than 2000 °C and phase transition temperatures that are higher. When temperatures above 1600 °C, gadolinium zirconate degrades; it remains stable between 1500 and 1550 °C. In comparison to yttria-stabilized zirconia, gadolinium zirconate has a melting point that is higher than 2000 °C and phase transition temperatures that are higher. When temperatures above 1600 °C, gadolinium zirconate degrades; it remains stable between 1500 and 1550 °C.

The employment of GZO with YSZ in the form two phase thermal barrier coatings, has been an interesting designed of TBCs system. It was observed that Two-phase GZO-YSZ coatings are stable for high temperature oxidation, but there is mutual interaction between the two ceramic materials at high temperature which leads to dissolution of one phase pyrochlore to another defective fluorite structure. The further detail discussion are available at the reference [23].

2.2 Hafnium Oxide (HfO₂)

New ceramic materials for thermal barrier coatings (TBC) must be developed in order to construct next-generation gas engines that are more effective and potent. In the literature, two new dopants—Ta₂O₅ and Y₂O₃—are examined in relation to how they affect the thermophysical, mechanical, and phase stability of ceramic materials based on HfO₂ (Ta₂O₅–HfO₂ and Y₂O₃–HfO₂). While the Y₂O₃–HfO₂ system creates stabilized HfO₂, which may include up to 18 mol% YO_1.5 (YSH), converting monoclinic HfO₂ to cubic HfO₂, the Ta₂O₅–HfO₂ system creates a special substance known as Hf6Ta₂O₁₇. At temperatures up to 1300 °C, cubic YSH ceramics have superior phase stability than Hf₆Ta₂O₁₇. Additionally, cubic YSH (YO_1.5 = 18–24 mol%)’s mechanical characteristics, thermal expansion, and fracture hardness indicate that it may survive greater temperatures. Consequently, creating YSH ceramics has the potential to enhance TBC materials for gas engines in the future [18].

In an effort to address this issue, new TBC materials for higher-temperature applications have attracted a lot of interest. Co-doped ZrO₂ (YTaO₄–ZrO₂), rare-earth zirconates (La₂Zr₂O₇), fluorite-type Ln₂Ce₂O₇ (Ln = La and Nd), hexaluminate (SrAl₁₂O₁₉, LaMgAl₁₁O₁₉), and perovskite-type SrZrO₃ are among the ceramic materials that have lately been evaluated. But most of them have a poor thermal cycle lifespan due to either a low thermal expansion coefficient (TEC) or a weak fracture toughness. Hafnia (HfO₂) is recognized for having a stronger sintering resistance and improved temperature stability while having a composition that is similar to ZrO₂ [24].

Hafnia and hafnia-based ceramic devices have been proposed as an alternate TBC solution for higher temperature operations. Similar to ZrO₂, HfO₂ porcelain changes into three distinct crystalline structures as the temperature rises. At normal temperature, pure HfO₂ exists in the monoclinic phase (m-HfO₂); at high temperature (T 1700 °C), it transforms into the tetragonal phase (t-HfO₂); and at higher temperatures (beyond 2600 °C), it eventually takes on the cubic phase (c-HfO₂). Purified t-HfO₂ and c-HfO₂ are regarded to be impossible to manufacture at room temperature. Pure HfO₂ also seems to be undesirable for TBC usage in high-temperature environments, as shown by the low but extremely asymmetrical thermal expansion coefficients of monoclinic HfO₂ (a = 9.34 106 K1, b = 2.98 106 K1, c = 13.1 106 K1) [25].

Additionally, pure HfO₂ transmits heat quite effectively. These elements need the modification of HfO₂ and may make it suitable for use in high-temperature applications. The HfO₂ lattice has a rather open structure due to its high ability to receive aliovalent stabilizers. Through the addition of cations like Ca²⁺ or Y³⁺ , which may greatly minimize the asymmetry of thermal expansion in the HfO₂ ceramics, tetragonal or cubic crystal structures may be maintained at normal temperature. In studies to date, rare earth metals with larger cationic radii and atomic weights have been employed to dope HfO₂. Examples include Er³⁺ , Gd³⁺ , or Y³⁺ . Roy et al. explain the effects of Gd₂O₃ doping on the crystal structure, surface morphology, and molecular make-up in the Gd₂O₃–HfO₂ system. The examined research indicate the mechanical and thermal conductivities of ceramics composed of totally stable Er₂O₃ and HfO₂ [26, 27].

According to recent studies, introducing dopants with higher oxidation states seems to improve tetragonality and phase stability. As a result, zirconia stabilization employing high valency oxides like Ta₂O₅ and Nb₂O₅ has received a lot of interest. The two lighter and smaller doping ions would also function as "rattles" in the cation sublattice, extensively scattering phonons and lowering thermal conductivity dramatically. Contrarily, we hypothesize that Ta⁵⁺ or Nb⁵⁺ could be able to preserve HfO₂ and improve its thermophysical and mechanical properties. As far as we know, little research has been done on aliovalent cations stabilizing HfO₂, particularly those with smaller cation radii and greater atomic weights. Here, trivalent rare earth Y₂O₃ and pentavalent Ta₂O₅ are used as dopant cations to partially replace hafnia. We conduct a comparative study to examine the effects of different dopants on HfO₂. Additionally, these dopants’ effects on the thermophysical properties, mechanical properties, and long-term phase stability of HfO₂-based ceramic materials (such as Ta₂O₅–HfO₂ and Y₂O₃–HfO₂) [28].

A promising material for heat barrier coatings is hafnium oxide (HfO₂). (TBCs). It is a good contender for TBCs in high-temperature applications due to its high melting point (2800 °C), moderate thermal conductivity (about 2.5 W/(mK)), and great chemical stability. It also has high mechanical characteristics and is thermal shock resistant.

The thermal stability and thermal shock resistance of HfO₂-based TBCs have been shown via research. Additionally, they exhibit high corrosion resistance to CMAS deposits, which are a significant contributor to TBC failure in aviation engines. Several methods, including atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD), have been used to create HfO₂-based TBCs [29].

However, the application of HfO₂ as a TBC material is still in its infancy compared to other TBC materials like yttria-stabilized zirconia (YSZ). To improve the coating procedure and comprehend the long-term performance of HfO₂-based TBCs in high-temperature settings, further study is required.

According to a recent research by Liu et al. [8], in both short-term and long-term testing, Hf₆Ta₂O₁₇ ceramic outperforms certain classic and innovative CMAS-resistant ceramic materials in TBCs. Temperature, which is controlled by CMAS viscosity, is the most important factor influencing the CMAS behavior of Hf₆Ta₂O₁₇ ceramic. The self-crystallization products of CMAS at 1250 °C consist of anorthite CaAl₂Si₂O₈ and wollastonite CaSiO₃. The reaction layer is made of HfSiO₄, while the thick layer is made of CaXHf_6−xTa₂O_17−x. At temperatures of 1300 and 1400 °C, however, the reaction layer and dense layer are made of CaTa₂O₆ and m-HfO₂.

Figure 4 illustrates how the CMAS resistance mechanism of Hf₆Ta₂O₁₇ ceramic changes with temperature. CMAS infiltration is successfully prevented at 1250 °C by the creation of CMAS self-crystallization products composed of anorthite CaAl₂Si₂O₈ and wollastonite CaSiO₃, as well as the formation of HfSiO₄ in a reaction layer and CaXHf_6xTa₂O_17x in a dense layer. The development and thickening of CaTa₂O₆ and m-HfO₂ in the dense layer at higher temperatures of 1300 and 1400 °C improves the CMAS corrosion resistance of Hf₆Ta₂O₁₇ ceramic.

Fig. 4

a Micros Structure cross-sectional Hf₆Ta₂O₁₇ at 1400 °C for 16 h b Circle from figure (a) was magnified. c, d Rectangle from figure (a) was magnified; e cross-sectional EDS mapping results of Hf₆Ta₂O₁₇ ceramic [8]

2.3 Lanthanum Zirconate (LZO)

Rare-Earth Zirconate (A₂Zr₂O₇)/pyrochlore-structured oxide compounds another substance that has been investigated for use in thermal barrier coatings is lanthanum zirconate (La₂Zr₂O₇). (TBCs). LZO is a viable choice for TBCs since it has a pyrochlore crystal structure and a low thermal conductivity of around 1.5–2 W/(mK). According to studies, LZO has strong resistance to CMAS assault, and when exposed to CMAS, a thick and persistent La-apatite layer forms. In addition, LZO has been found to have good high-temperature stability, with no significant phase changes observed up to 1600 °C [30].

However, LZO also has some drawbacks as a TBC material. It has been found to have poor adherence to metallic substrates and is prone to spallation under thermal cycling conditions. In addition, LZO coatings have been found to be more brittle than other TBC materials, such as YSZ. Despite these challenges, research is ongoing to further improve the performance of LZO as a TBC material, particularly with regards to improving its adhesion to metallic substrates and enhancing its durability under thermal cycling conditions [31].

Lanthanum zirconate is a typical pyrochlore-structured ceramic material. (La₂Zr₂O₇, LZ). The common chemical formula for pyrochlore is A₂B₂O₇. A rare earth or element with an inactive solitary pair of electrons makes up element A in A₂B₂O₇, which often comprises a transitional metal or post-transition metal with a changing oxidation state as element B. LZ is chosen over 8YSZ in different TBC applications due to multiple benefits. Unlike 8YSZ, LZ does not go through a phase transition from room temperature to its melting point, has better sintering resistance, low heat conductivity (1.5–1.8 W/m/K at 1000 °C for a fully dense substance), and lower oxygen ion diffusivity, all of which contribute to the bond coat and substrate not oxidizing. However, one significant drawback of LZ is its low CTE value in compared to the base and bond layers' high CTE values. The numerous research available where, researcher compare the production procedures of LZ granules and LZ-based thermal barrier coatings with 8YSZ, the most advanced thermal barrier coating material currently available. At temperatures exceeding 1300 °C, there are major mechanisms for LZO disintegration. By heating a mixture of lanthanum oxide (La₂O₃, 99.9%) and zirconium oxide (99%) particles to high temperatures (1773 K) for 10 h while maintaining an argon atmosphere, LZ powder for thermal spray purposes may be produced. To create pure La₂O₃, La₂(CO₃)38H₂O is often dissolved in nitric acid. During the chilling process, the t phase will transition into the monoclinic (m) phase., with a volume increase of 4% or less. The hardening of the covering is the other process, which will densify the substructure and raise heat conductivity. High temperature generated tension and a shorter covering lifespan are the results of phase, microstructure, and characteristic changes as well [32]. However, it was found that the Young's modulus of the pyrochlore La₂Zr₂O₇ was smaller than that of YSZ. The YSZ coatings that were sprayed using plasma had a breaking hardness that was comparable to that of this substance. La₂Zr₂O₇ has a more advantageous thermal conductivity at high temps than YSZ, which is about 20% less. The La₂Zr₂O₇ layer didn't break during the initial thermal cycle tests at temps above 1200 °C, and it showed thermal resilience. So La₂Zr₂O₇ is a very hopeful substance for modern TBCs.

Satpathy et al. [33] investigated the potential of two kinds of thermal barrier coatings (TBCs), APS lanthanum zirconate (LZ) and APS 8 wt% yttria stabilized zirconia (YSZ), for usage in advanced gas turbine components. The coatings were tested for resistance to corrosive calcium-magnesium-aluminosilicate (CMAS) deposits. The researchers discovered that LZ was more resistant to CMAS than YSZ. One explanation for this is because LZ has lesser wettability, making it less prone to be penetrated by CMAS. Furthermore, when LZ comes into touch with CMAS, it develops a thick apatite phase on its surface, thereby preventing molten CMAS from penetrating interior. The development of this apatite phase rises with temperature and reaction time with CMAS, and it stays stable for over 100 h at 1450 °C. YSZ, on the other hand, does not produce the apatite phase and experiences unfavorable phase change after just 10 h of interaction with CMAS at 1250 °C and 1 h at 1450 °C. Because of its better CMAS resistance, our findings imply that LZ may be a suitable choice for application in advanced gas turbine components.

While LZ has a higher thermal diffusivity at high temperatures than YSZ in the presence of CMAS, LZ as the top coat can preserve the YSZ layer below and extend coating life due to its quick interaction with CMAS in producing the stable apatite phase (Figs. 5, 6).

Fig. 5

After CMAS Surface Analysis of the LZ TBC: a 1250 °C—1 h, d 1450 °C—1 h, and e 1450 °C—50 h. The typical EDS plots indicated needle and nodular phases shown in a–c. f in the paper displays a close-up image of the cross-section of the reaction zone between CMAS and LZ after being exposed to a temperature of 1250 °C for a duration of 1 h [33]

Fig. 6

After CMAS Surface Analysis of the 8YSZ TBC: a 1250 °C—1 h, b 1250 °C—10 h, d 1450 °C—1 h, and e 1450 °C—50 h. The typical EDS plots b and c. f in the paper displays a close-up image of the cross-section of the reaction zone between CMAS and LZ after being exposed to a temperature of 1250 °C for a duration of 1 h [33]

2.4 Neodymium Zirconates (NZO)

The high melting point, poor thermal conductivity, and high temperature chemical stability of neodymium zirconate (Nd₂Zr₂O₇) rare-Earth Zirconate (A₂Zr₂O₇)/pyrochlore-structured oxide compounds have made it a promising contender for thermal barrier coatings (TBCs). In comparison to more conventional TBC materials like yttria-stabilized zirconia, recent research have showed encouraging findings for the use of Nd₂Zr₂O₇ as a TBC material. This material has superior thermal insulation capabilities and enhanced resistance to high-temperature oxidation. (YSZ). For TBCs used in gas turbine engines exposed to salt deposits and other corrosive elements at high temperatures, Nd₂Zr₂O₇ has also been discovered to have excellent hot corrosion resistance. According to one research, Nd₂Zr₂O₇ coatings performed better than YSZ coatings in a hot corrosion environment in terms of weight increase and oxide spallation area [34]. Nd₂Zr₂O₇ has the potential to be a useful TBC material, however additional study is required to fully understand its mechanical and long-term durability.

In the recent research, different Nd₂Zr₂O₇-yttria-stabilized zirconia (YSZ) thermal barrier films were examined for their heated weathering behavior. Using an atmospheric plasma spraying technique, functionally graded systems (FGS) constructed of Nd₂Zr₂O₇–YSZ and double-phase upper layers with 25, 50, or 75% by weight Nd₂Zr₂O₇ were used on a base. We looked examined how the coatings responded to pure Na₂SO₄ contact at 920, 970, and 1000 °C. The results demonstrate that only chromite and neodymium sulfates are generated on the coated surface during FGS. In contrast, zirconate phases with lower neodymium levels and fluorite structure were formed in dual-phase films made of Nd₂Zr₂O₇ + YSZ by the diffusive breakdown of the pyrochlore Nd₂Zr₂O₇ (NZO) phase. The basic and acidic processes involving the byproducts of the Na₂SO₄ breakdown control the mechanism of pyrochlore phase decomposition [35].

Cross-sectional analysis was conducted on NZO–YSZ coatings that underwent hot corrosion tests, and the internal microstructures were revealed in Fig. 7. The SEM micrographs of the coatings' internal morphologies after the hot corrosion tests were displayed in the figure, with (a–c) corresponding to double-phase coatings and (d) providing an overall view of the cross-sectional microstructure of the FGS coating. The microstructural features of the double-phase topcoats were found to be similar to those observed in the FGS. Therefore, different zones of the FGS TBC were further analyzed to characterize the topcoat and hot corrosion products. Fortunately, no significant destruction caused by aggressive S- and Na-containing salts was observed, and the morphology of the FGS TBC remained consistent with that of coatings after hot corrosion. A TGO zone was generated by high-temperature exposure. To examine degradation related to chemical interactions between the ceramic layer and the corrosive medium, the ceramic topcoat (e), the interface between the topcoat and the bond coat (f), and the zone between the bond coat and the substrate (g) were analyzed. The findings indicate that there was no significant damage caused by S- and Na-containing salts, and the morphology of the coatings remained consistent with that of coatings after hot corrosion. The analysis also revealed the generation of a TGO zone by high-temperature exposure, and further characterization of different zones of the FGS TBC provided insights into the degradation related to chemical interactions between the ceramic layer and the corrosive medium. These observations are valuable in understanding the performance and potential applications of NZO-YSZ coatings in advanced gas turbine components.

Fig. 7

Microstructural cross-sectional SEM images of TBC systems after hot corrosion: a 25NZO-75YSZ; b 50NZO-50YSZ; c 75NZO-25YSZ; d FGS NZO-YSZ; e upper zone of FGS TBC; f middle zone of FGS TBC; g lower zone of FGS TBC SEM cross section and surface image was taken After 239 h of hot corrosion at 920 °C and 216 h of an analogous experiment at 970 °C [35]

2.5 Samarium Zirconates (SZO)

Rare-Earth Zirconate (A₂Zr₂O₇)/pyrochlore-structured oxide compounds due to its durability at high temperatures, low thermal conductivity, and favorable chemical compatibility with the underlying substrate, samarium zirconate Sm₂Zr₂O₇ (SZO) is a viable candidate material for thermal barrier coatings (TBCs). Due to its pyrochlore structure, Sm₂Zr₂O₇ exhibits remarkable thermal stability and sintering resistance even at high temperatures. Improved resistance to deterioration in extreme conditions, such as high-temperature oxidation and hot corrosion, has been the focus of recent research on Sm₂Zr₂O₇-based TBCs. Modifying the coating's microstructure is one way to lessen its susceptibility to deterioration [36]. For instance, studies into the use of porous Sm₂Zr₂O₇ coatings, which provide a superior substrate for the development of a protective oxide layer on the surface, are ongoing.

Another approach is to modify the composition of the coating to enhance its resistance to degradation. For example, researchers have investigated the use of Sm₂Zr₂O₇ coatings doped with other rare earth elements, such as Yb or La, to lessen their sensitivity to heat corrosion and increase their high-temperature stability. Additionally, researchers have explored the use of Sm₂Zr₂O₇ coatings with a graded composition, where the concentration of Sm gradually decreases towards the surface, to increase their oxidation resistance and high-temperature stability.

Thermal barrier coatings (TBCs) made of rare earth zirconates are applied to mechanical surfaces that are treated at high temperatures in order to offer heat management and prolong heat loads. Yttria-stabilized zirconia (YSZ) and samarium zirconate (SZO), whose pyrochore designs are stable up to 2200 °C but whose sensitivity to thermal cycling is poorly known, are the least thermally conductive materials among rare earth zirconates. Here, a nickel-based super alloy (Inconel) is coated with two rare earth metals, yttria-stabilized zirconia (YSZ) and samarium zirconate using a plasma spray technique. (SZO). The porcelain has a fluorite design when it is cast, and when heat cycling between 100 and 1600 °C is applied, it changes into the pyrochlore phase. Here, multiple combos of the layering techniques are used to produce varying thermal cycling outcomes. Different thermal load properties for the super metal are predicted to be provided by the chemical changes of YSZ and SZO. In this study, the theory has been closely examined as surface shape, roughness, thermal cycle fatigue, porosity, rust behavior, surface profilometry, and electrochemical characteristics have been investigated.

The research undertaken by Vorozhtcov et al. [37] is noteworthy in that it provides useful information for high-temperature Sm₂Zr₂O₇ applications. The glycol-citrate process for producing the Sm₂Zr₂O₇ chemical and subsequent characterization using various techniques are critical in evaluating its characteristics and appropriateness for various applications. The study's X-ray phase analysis, infrared spectroscopy, scanning electron microscopy, and high-temperature mass spectrometry give vital insights into the compound's crystal structure, thermal stability, and partial vapor pressures. The rough estimation of the Sm₂O₃ partial mole enthalpy of mixing in Sm₂Zr₂O₇, as well as the recalculation of the Gibbs energy value of Sm₂Zr₂O₇ production from 2460 to 298.15 K, contributes to a better understanding of the compound's characteristics. This study's findings can help in the development of innovative materials for high-temperature applications, such as refractory ceramic materials and TBCs (Fig. 8).

Fig. 8

Microstructure of the Sm2Zr2O7 powders obtained after annealing at the temperatures 1073 K and 1673 K for 2 h [37]

2.6 Cerates Rare-Earth Cerium Oxides (A₂Ce₂O₇)

Given its high melting point (2627 °C), low heat conductivity (6–7 W/mK), and remarkable chemical stability, cerium oxide (CeO₂), commonly known as ceria, has been researched as a viable material for thermal barrier coatings (TBCs). Ceria may also be used in oxygen sensors and solid oxide fuel cells due to its strong oxygen ion conductivity. Ceria-based TBCs may provide strong thermal stability, durability against erosion, and thermal shock resistance, according to studies. Ceria-based TBCs may function better with the inclusion of additional elements like zirconium, yttrium, or gadolinium. For instance, Ce_0.8Zr_0.2O₂ and Ce_0.9Gd_0.1O₂ have been found to exhibit outstanding thermal shock resistance, great thermal stability, and good erosion resistance.

General, while ceria-based TBCs are still in the early stages of research, they seem to have promise as a material for high-temperature applications, particularly in harsh environments where erosion and hot corrosion are major concerns [38].

The temperature impacts IN738's thermal efficiency, thermal expansion, and specific heat, among other properties. This study compares the results of tests on two different surfaces for hot corrosion, microhardness, microstructural analysis, and degrading behavior. Two specimens have Ce_0.8Zr_0.2O₂ thermal barrier coatings (TBCs). have the thermal stability as compared to the Ce_0.9Gd_0.1O₂ is more and the corrosion and degradation occur on high temperature of 1960 degree centigrade. With thermal coating and a finish made of glass–ceramic bond with 25% CeO₂ etched in it are being produced. In terms of the rust test, the tensile test, and the microstructural analysis, the example with the finish displays superior findings to the other [39].

Ren et al. [40] used a solid-state reaction sintering process to create multicomponent cerate materials for use as TBCs. The materials, (Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂Ce₂O₇ (5RC) and zirconocerate (Sm_0.2Eu_0.2Tb_0.2Dy_0.2Lu_0.2)₂ZrCeO₇ (5RZC), exhibited a pristine fluorite crystal structure and were equally dispersed with rare-earth elements. The materials were studied and found to have a greater CTE than YSZ and single-component zirconates and cerates, making them attractive as TBC materials. The lattice shrinkage previously seen in some single-component cerates at intermediate temperatures was discovered to be absent in the multicomponent 5RC. Furthermore, the multicomponent materials had low conductivity at high temperatures (as low as 1.2 W/(mK) at 1000 °C), which might be attributable to increased phonon scattering. Due of greater phonon scattering, the multicomponent materials also exhibited poor conductivity, and Zr₄₊ doping improved their stability. The researchers found that multicomponent ceramics 5RC and 5RZC show promise as TBC materials.

The aforementioned [41] research, Fig. 9 suggests that coatings made from A₂B₂O₇ type compounds have demonstrated favorable properties, including lower thermal conductivity, higher thermal stability, and higher thermal expansion coefficient when compared to conventional ceramic materials. The introduction of various dopants, whether through single or multi-element doping, has led to improvements in the thermodynamic properties of these materials to varying degrees. However, there are still some limitations, such as a thermal expansion coefficient that does not fully match that of the bond coat (although it is larger than 8YSZ) and relatively low fracture toughness.

Fig. 9

Thermal expansion coefficient trend chart of La₂Ce₂O₇ (LC) coatings, Ta₂O₅ doped La₂Ce₂O₇ (LCT) coatings and DZ125 substrate [41]

2.7 Yttrium Aluminum Garnet (YAG)

It has been investigated if the ceramic substance yttrium aluminum garnet (YAG) might be used for thermal barrier coatings. (TBCs). YAG is a strong contender for TBC applications because to its great thermal stability and outstanding mechanical qualities at high temperatures. In addition, YAG has a strong sintering resistance and low heat conductivity, making it highly resistant to thermal shock and erosion caused by hot gas flow and particulate matter.

Yttrium aluminum garnet (YAG) is a very stable material with a thermal expansion coefficient of 7.5–9.1 106/K, Vickers hardness of 16.5–17 GPa, and fracture toughness of 1.8 MPam1/2 at 298 K. It is also creep resistant and has low oxygen permeability, making bond coatings difficult to oxidize and react with CMAS. However, at high temperatures, its thermal conductivity is somewhat greater (2.4–3.1 W/(mK)), which may be lowered by structural design (0.91 W/(mK) at 1273 K). Gd³⁺ can be used in place of Y³⁺ to lower thermal conductivity to 1.51 W/(mK) at 1473 K while significantly increasing thermal expansion coefficient. Pd doping can also significantly improve its high-temperature oxidation resistance and thermal stability. Nonetheless, its poor thermal expansion coefficient and exceedingly low fracture toughness remain significant constraints to its use [42,43,44].

Research has shown that YAG coatings deposited by plasma spray exhibit excellent thermal insulation and erosion resistance, making them suitable for TBC applications. However, YAG coatings are susceptible to degradation due to the presence of moisture, which can cause hydrothermal degradation of the coating, reducing its thermal insulation properties. To overcome this limitation, various modifications have been proposed, such as doping YAG with rare earth oxides or adding a protective layer to prevent moisture penetration. Overall, YAG has shown promise as a TBC material, and continued research is being conducted to optimize its properties and durability for use in aero-engines and other high-temperature applications.

According to XUE [45] synthesis of Y₃Al₅O₁₂ (YAG) nano-powders using a chemical co-precipitation method, and sintered them at 1700 °C for 10 h, resulting in a single-phase YAG ceramic. The tested the material's resistance to infiltration by molten CMAS, which is a common issue in TBC systems. It was found that the molten CMAS did not penetrate into the YAG ceramic and was completely stopped on its surface. Additionally, it was not observe any chemical reaction between YAG and the molten CMAS glass at 1250 °C for 24 h, indicating that YAG has excellent chemical stability in molten CMAS. Therefore, the authors concluded that YAG is a promising material for use in TBC systems to prevent molten CMAS infiltration.

3 Yttrium Oxide (Y₂O₃) Co-doping

Doping with other oxide and stabilized ZrO₂ is also new way to synthesis advance ceramic materials for TBCs. Scientist had observer, much improved properties, and stable phase with these dual doped based oxide stabilized by zirconia. Newly, Chen et al. experimented on the Ta₂O₅–Y₂O₃ co-doped ZrO₂ (TYSZ) based ceramic coatings. The key findings produced that TYSZ spherical powder particles have a compact and smooth surface, good flowability, and loose density, meeting the requirements for air plasma spraying (APS). TYSZ exhibits good phase stability at high temperature with a low coefficient of thermal expansion (CTE) due to the specially designed substitutional defects. However, its thermal conductivity is much lower than that of the YSZ coating. The TYSZ coating has a typical layered structure and high adhesion strength. It undergoes phase transformation during plasma spraying and heat treatment at high temperatures, which affects its structural stability. Thermal shock test results show that the TYSZ coating experiences cracking and spalling near the bond coat interface due to the mismatch of thermal expansion coefficient and phase transformation.

Figure 10 from the XRD patterns of the YSZ and TYSZ coatings after thermal shock failure at 1400 °C showed that the YSZ coating exhibited a significant decomposition of the t’ phase into c and t phases, while the TYSZ coating underwent a clear phase transformation to the stable t phase with good phase stability. This indicates that the TYSZ coating, with its designed defects and composition, has better phase stability than the YSZ coating after thermal shock failure at high temperatures. The surface temperature of the coating reached 1400 °C during thermal shock, causing oxygen diffusion into the coating and forming the stable t phase with theoretical oxygen content and arrangement distribution [46, 47].

Fig. 10

XRD patterns of the YSZ and TYSZ coatings after thermal shock [46]

In Fig. 11a, the monoclinic (m) phase peaks of YSZ were detected, along with peaks of c and t phases at diffraction angles of 72°–75°. However, after heat treatment at 1400 °C, YSZ showed the c and m phases, with no t phase observed, suggesting a transformation of t phase into m and c phases. Excessive phase transformation can reduce mechanical properties. In Fig. 11b, TYSZ, Y₂O₃ and Ta₂O₅ were completely dissolved in ZrO2 crystal, stabilizing the lattice through large local distortions produced by the substituted cations. The TYSZ tetragonal phase was formed due to lattice distortion, rather than oxygen vacancies. After heat treatment at 1400 °C, TYSZ maintained its original t phase, indicating good high-temperature phase stability. The appearance of the YTO4 phase was due to the excessive local areas of Y³⁺ and Ta⁵⁺, which gathered at grain boundaries and other defects to form the YTO₄ phase.

Fig. 11

XRD patterns of high-temperature solid-synthesized powders and after treatment at 1400 °C for different periods: a YSZ and b TYSZ [46]

In another study [48] they successfully utilized vacuum sintering to produce Y₂O₃ ceramics doped with sintering additives. The addition of La₂O₃ and ZrO₂ as additives during sintering at 1700 °C for 4 h resulted in a pore-free and homogeneous microstructure, with an average grain size ranging from 15.9 to 16.7 μm. UV–Vis spectra analysis revealed that the maximum band gap energy of the pure Y₂O₃ ceramic was 5.12 eV. The addition of La₂O₃ and ZrO₂ improved the structural and morphological properties of Y₂O₃, indicating that the sintering additives can be used to produce high-quality Y₂O₃ ceramics for solid-state laser applications.

Zhang et al.’s [49] investigation of the multi-rare earth oxide co-doped modification Gd₂O₃–Yb₂O₃–Y₂O₃–ZrO₂ GYYZO powders and coatings and comparison to conventional 8YSZ coatings evaluated high-temperature phase stability and sintering resistance. The solid solution state of the multi-rare earth oxides Gd₂O₃–Yb₂O₃–Y₂O₃ was discovered to be uniform, with no noticeable element segregation and enrichment in the composition of the GYYZO coating created by APS. GYYZO powders were created via the solid sintering process. While 8YSZ coating clearly displayed Y element segregation and enrichment as well as phase change after high temperature heat treatment, GYYZO coating did not exhibit any overt element enrichment phenomena. The rare earth elements Yb and Gd can maintain the phase structure of GYYZO and prevent the segregation and enrichment of Y. In comparison to standard 8YSZ coatings, the GYYZO coating has demonstrated exceptional resistance to sintering at high temperatures. The GYYZO coating showed a lesser reduction in porosity (20.6%) and greater sintering healing of cracks and pores after high temperature heat treatment. The GYYZO coating's grains were refined and consistently dispersed, but the 8YSZ grains increased dramatically with diverse diameters. Furthermore, after high temperature heat treatment, a regular organization and distribution of nano-pores were detected on the grain boundary of GYYZO coating, which hindered grain expansion via a considerable pinning effect on grain boundary movement. The substitution of Gd³⁺, Yb³⁺, and Y³⁺ ions for Zr⁴⁺ resulted in the formation of oxygen vacancies, which promoted the second phase of grain boundary segregation, whereas the inclusion of Gd₂O₃–Yb₂O₃ inhibited the diffusion behavior of Y₂O₃ at the grain boundary, thereby impeding the coating's phase transition.

Based on the recent research Y₂O₃ high melting point, strong thermal stability, low thermal conductivity, and good chemical stability, yttrium oxide (Y₂O₃) is a popular material for thermal barrier coatings (TBC). Physical vapor deposition (PVD) techniques including electron-beam physical vapor deposition (EB-PVD) and sputtering are often used to produce Y₂O₃-based TBCs.

The vulnerability of Y₂O₃-based TBCs to sintering and phase transition at high temperatures is one of its difficulties. To solve this problem, scientists have looked at Y₂O₃-based TBCs’ capacity to withstand high temperatures by including additional oxides like ZrO₂ and HfO₂. Additionally, the creation of fresh deposition methods, including solution precursor plasma spraying (SPPS), has showed promise in enhancing the microstructure and functionality of Y₂O₃-based TBCs.

3.1 Scandium Oxide (Sc₂O₃)

Scandium oxide (Sc₂O₃) is a potential candidate for TBCs due to its high melting point (2410 °C), low thermal conductivity (about 6 W/(mK)), and good thermomechanical properties. Sc₂O₃ also exhibits excellent stability against high-temperature oxidation, making it attractive for use in harsh environments. However, Sc₂O₃ is a rare and expensive material, which limits its practical application for TBCs. Recent research has focused on developing composite TBCs, where Sc₂O₃ is combined with other materials to improve performance and reduce costs [50]. For example, Sc₂O₃–ZrO₂ composites have shown promising results, demonstrating improved thermal cycling performance and reduced thermal conductivity compared to traditional YSZ coatings.

At a temperature of 1320 °C, the study examined how CMAS interacts with YSZ and ScYSZ coatings. The results revealed that the solubility of Sc³⁺ in CMAS was lower than that of Y³⁺ , making it challenging to form the m-phase. This suggests that ScYSZ has a superior t-phase stability compared to traditional YSZ coatings when exposed to CMAS. Furthermore, increasing the density of ScYSZ can effectively prevent molten CMAS infiltration, which is not observed with YSZ coatings [51]. In a related study, Wang et al. [52] investigated four different SCZ compositions (4S8CZ, 4S10CZ, 4S12CZ, and 4S16CZ) synthesized using the solid-state synthesis method. These compositions had a single tetragonal phase structure. After being subjected to high-temperature treatment at 1500 °C, the 4S8CZ and 4S10CZ ceramics transformed from the tetragonal phase to the monoclinic phase, while the 4S12CZ and 4S16CZ ceramics maintained the tetragonal phase structure. The researchers found that the addition of CeO₂ to SCZ ceramics increased the coefficient of thermal expansion (TEC) but decreased the thermal conductivity. They also observed that the 4S16CZ composition exhibited the highest fracture toughness and best thermophysical and mechanical properties. Due to its excellent sintering resistance ability and high fracture toughness, the 4S16CZ ceramic has the potential to be a promising material for thermal barrier coatings with a long thermal cycling lifetime.

The corrosion behaviors for Sc_0.04Ce_0.16Zr_0.80O_1.98 (SCZ) as well as YSZ (4 mol% Y₂O₃-stabilized ZrO₂) in CMAS (Ca–Mg–Al–Si) at 1250 °C were thoroughly investigated in this study using a combination of field emission scanning electron microscopy (FE-SEM), a combination of X-ray diffraction (XRD), Raman spectroscopy, and field emission transmission electron microscopy. (FE-TEM). The results showed that Y³⁺ is highly soluble in the CMAS corrosion environment and that it may diffuse into the liquid salt and easily dissociate from YSZ, changing the phase from tetragonal (t) to monoclinic (m). The reverse migration of Mg and Ca into the YSZ crystal structure resulted in a little amount of cubic (c) phase, which increased the stabilizer content during the YSZ powder's breakdown. In the CMAS environment, the buffer CeO₂ was chemically inert for SCZ compounds. Sc³⁺ only readily precipitated from the top layer of the ceramic bulk at high CMAS concentrations and showed a limited affinity for Ca²⁺, leading to the development of the m phase. In terms of t phase stability and CMAS corrosion protection, the SCZ material fared better than the YSZ material [53].

3.2 Lanthanum Hexaaluminate (LaAl₁₁O₁₈)

Due to its outstanding thermal and chemical stability, lanthanum hex aluminate (LaAl₁₁O₁₈) is a potential material for applications involving high temperatures such thermal barrier coatings (TBCs). Its unique crystal structure, composed of a three-dimensional network of aluminum and oxygen ions with lanthanum ions occupying the octahedral voids, provides high resistance to thermal shock and chemical attack. Recent studies have shown that LaAl₁₁O₁₈ can maintain its structural integrity and mechanical properties even at temperatures exceeding 1700 °C. Furthermore, LaAl₁₁O₁₈ has shown excellent compatibility with metallic bond coats commonly used in TBC systems, resulting in good adhesion and reduced interdiffusion between the layers. Despite its promising properties, further research is needed to optimize the processing methods and understand the long-term stability of LaAl₁₁O₁₈-based TBCs under real-world operating conditions [54].

Since most zirconia coatings age considerably and undergo undesirable densification at temps above 1100 °C, as a thermal barrier coating, yttria partially stabilized zirconia (Y-PSZ) may face competition from lanthanum hexa aluminate (LHA), which has a magnetoplumbite structure. (TBC). The morphology of chemically applied films and calcined lanthanum hexaaluminate particles exhibits platelets. The magnetoplumbite structure is characterized by the highly charged La³⁺ cation, which is located in an oxygen spot in the hexagonally close-packed arrangement of oxygen ions. Ion transport perpendicular to the crystalline c-axis has been significantly suppressed, which hinders sintering densification. In contrast to oxygen ion conducting zirconia, lanthanum hexaaluminate’s thermal stability and electrical shielding properties enable operating temperatures beyond 1300 °C. In order to produce homogeneous crystalline layers with controlled micro-porosity and residual stresses, this study describes the design of variables for atmospheric plasma spraying (APS) as well as the improvement of particle preparation for thermal spraying by spray drying. The steps were described using X-ray diffraction (XRD) [55].

Oxides that have a magnetoplumbite structure and a general composition of LnMAl₁₁O₁₉ (Ln = La to Gd; M = Mg, Mn to Zn) possess desirable properties such as high melting points, low thermal conductivities, and high thermal expansion, which make them ideal for high-temperature thermal barrier coatings (TBCs). The magnetoplumbite structure inhibits sintering due to low ionic diffusion. LaMgAl₁₁O₁₉, SmMgAl₁₁O₁₉, and GdMgAl11O19 have similar thermal expansion behaviors, which are determined by the basic magnetoplumbite crystal structure rather than individual rare earth elements. These oxides exhibit thermal conductivities between 2 and 3 Wm⁻¹ K⁻¹ from room temperature to 1150 °C, with the SmMgAl₁₁O₁₉ and GdMgAl₁₁O₁₉ compositions having lower thermal conductivities than LaMgAl₁₁O₁₉. Doping with Yb in GdMgAl₁₁O₁₉ reduces its thermal conductivity effectively. Plasma-sprayed coatings of LaMgAl₁₁O₁₉ and LaMnAl₁₁O₁₉ exhibit excellent sintering resistance and maintain their thermal conductivity even after exposure to temperatures of up to 1600 °C for 20 h. The microhardness of these coatings is higher than that of hardened steel but lower than that of ZrO₂, and their elastic behavior is superior to YSZ due to their lower indentation modulus. However, the La-containing magnetoplumbites are not stable in water-containing atmospheres at elevated temperatures, such as those found in turbine engine combustion environments, and exhibit continuous weight loss [56, 57].

Lanthanum hexa aluminate (LHA) covering shows significantly lower sintering rates and long-term structural and thermochemical durability up to 1673 K than zirconia-based TBCs. Due to the particles' random organization and resulting microporous coating, LHA has a weak heat transmission. The shielding properties of the substance are related to its crystalline property. To meet the needs of modern turbine engines, LaTi₂Al₉O₁₉ (LTA) was proposed and studied as a novel TBC substance for use at 1300 °C. LTA showed extraordinary phase stability up to 1600 °C. The thermal conductivities with LTA covering varied from 300 to 1500 °C, or 1.0 to 1.3 W/mK. The thermal expansion factors increased until they reached YSZ-equivalent values at 200, 400 °C. The microhardness values of the LTA and YSZ compounds were both around 7 GPa, but YSZ had a significantly reduced breaking toughness value. The double-ceramic LTA/YSZ layer construction, however, offset the decreased fracture durability. The LTA/YSZ TBC had an approximate 700-h heat cycle life at 1300 °C. Lanthanum phosphate (LaPO₄) is considered a potential TBC material for Ni-based superalloys due to its high-temperature stability, high thermal expansion, and poor thermal conductivity. In addition, it's expected that lanthanum phosphate will have better weathering protection in environments that contain sulfur and vanadium ions. But very simply, this kind of coverage cannot be produced by plasma blasting. The suitability of LaPO₄ as TBC needs to be thoroughly investigated [58].

3.3 Ceria-Stabilized Zirconia (CSZ)

Ceria-stabilized zirconia (CSZ) is a composite material that combines the high-temperature stability of zirconia with the oxygen ion conductivity of ceria. Zirconia is stabilized at high temperatures by the inclusion of ceria by suppressing grain expansion and the transition of the zirconia crystal structure from tetragonal to monoclinic. Additionally, ceria’s strong oxygen ion conductivity enables the material to transport oxygen more effectively, which may be advantageous in certain applications like solid oxide fuel cells. Due to its exceptional mechanical and thermal durability, CSZ has been researched as a viable material for thermal barrier coatings (TBCs). The material's microstructure, composition, and coating preparation technique all have a significant impact on the material's performance as a TBC material. The microstructure and content of CSZ coatings are now being optimized in order to enhance their functionality as TBCs, especially in high-temperature and challenging conditions [59].

The recent research evaluated the hot corrosion (HC) resistance of thick Ceria-Yttria Stabilized Zirconia (CYSZ) and Yttria-Stabilized Zirconia (YSZ) multilayer thermal barrier coatings (TBC). Thermal cycling (adding additional salt between cycles) at a constant test temperature (900 °C) and salt concentration were used in two distinct HC studies. (Na₂SO₄-V₂O₅). The results showed that the testing procedure changes how the HC works and that the external, thick CYSZ layer acts as a buffer to protect the inside layers from HC degradation. However, the TBC systems' resistance to rust was diminished as a result of the formation of vertical cracks during the covering application process. The study also compared the hot corrosion properties of YSZ, CSZ, and TiSZ thermal barrier coatings under a sodium sulfate + vanadium oxide salt environment at 1050 °C. YSZ TBCs failed after 20 h of testing, while CSZ coating degraded after 28 h. TiSZ coating showed better hot corrosion resistance, with no monoclinic zirconia formation after 40 h of testing, and started to degrade after 40 h of hot corrosion testing. TiSZ coating was found to have better hot corrosion resistance at 1050 °C than both YSZ and CSZ coatings [60].

3.4 Lanthanum Aluminate (LaAlO₃)

The magnetoplumbite structure oxides, with a general composition of LnMAl₁₁O₁₉ (Ln = La to Gd; M = Mg, Mn to Zn), exhibit desirable properties for high-temperature thermal barrier coatings (TBCs) including high melting points, low thermal conductivities, and high thermal expansion coefficients. Additionally, their sintering behavior is hindered due to the low ionic diffusion in magnetoplumbites. A prospective contender for a variety of uses, such as thermal barrier coatings, is lanthanum aluminate (LaAlO₃), a perovskite-type oxide with a special combination of chemical and physical properties. (TBCs). LaAlO₃ is outstanding thermally stable, has a high melting point, and has strong resistance to oxidation at high temperatures, which are desirable characteristics for TBC materials. In addition, LaAlO₃ has a low thermal conductivity, which makes it an effective thermal insulator. Its unique crystal structure also allows for the formation of epitaxial thin films, which can further enhance its performance as a TBC material. The use of LaAlO₃ in TBCs has shown promising results, with improved thermal barrier performance and increased durability under high-temperature and thermal cycling conditions [61].

Bespalko et al. [62] synthesized the La–Zr–O/La–Al–O nanocomposite by first combining separate precipitates and then aging them. It has a disordered structure with weak connections among its constituents, which include nanoparticles of La–Zr–O and La–Al–O mixed oxide precursors. At 700 °C, pronounced anion desorption results in considerable disordering of the composite due to the formation of an uneven structure from mixed layered hydroxycarbonate/carbonate. After 900 °C treatment, the La–Zr-containing phase crystallizes as PSC ZrO₂ stabilized by La (Al) cations, with the appearance of the LaAlO₃ structural phase. Because of an increase in cation mobility at sintering temperatures of 1100–1300 °C, the particle size of perovskite grows and the pyrochlore phase La₂Zr₂O₇ forms from PSC ZrO₂. The presence of both phases and their interfaces hinders sintering and favors ordering for the first structure and disordering for the second structure in the La₂Zr₂O₇/LaAlO₃ composite. This is owing to the unique influence of domain borders on defects in these oxides’ oxygen sublattices, which are distinguished by distinct coordination of oxygen anions by cations and packing type. The unique features of domain boundaries present in the nanostructured La–Zr–O/La–Al–O composite prevent the growth of phase particles in large clusters, leading to the stabilization of porosity and the durability of coating layers. This composite material is also cost-effective to manufacture, making it a promising material for use in TBCs. However, CePO₄ and LaPO₄ ceramics can tolerate high temperatures, but their coatings are susceptible to cracking or decomposition when exposed to acidic or alkaline aqueous solutions at high temperatures, limiting their potential use in TBCs. Scientists are currently working to enhance the properties and performance of LaAlO₃-based TBCs to meet the increasing demands of advanced applications in the aerospace and power generation industries.

3.5 Magnesium Zirconate (MZO)

Perovskite oxides have a crystal structure of ABO₃ that can incorporate various ions, even those with a large atomic mass, making them a preferred material for researchers. Perovskite-type oxides, particularly ABO₃ perovskites with A as calcium, strontium, or barium and B as zirconium, titanium, or cerium, have exceptional structural characteristics and properties. Using perovskite oxides as thermal barrier coatings offers a significant advantage, with a 20% lower thermal conductivity compared to YSZ, resulting in excellent thermal stability at elevated temperatures.

Due to its low heat conductivity, high melting point, and strong chemical stability, the ceramic material magnesium zirconate (MgZrO₃) has been suggested as a suitable option for thermal barrier coatings (TBCs). Thermal conductivity of MgZrO₃ is around 3.5 W/(mK), which is less than that of yttria-stabilized zirconia. It also has a tetragonal crystal structure (YSZ). Additionally, MgZrO₃ is chemically stable, especially when it comes to molten calcium-magnesium-alumino-silicates (CMAS), which are found in the hot zones of gas turbines. But since MgZrO₃ is challenging to make and deposit as a coating, further study is required to create workable manufacturing methods for MgZrO₃-based TBCs. MgZrO₃, however, is a promising substance for future TBC uses [63].

Several perovskite structure materials, such as SrZrO₃, BaZrO₃, Ba(Mg_1/3Ta_2/3)O₃ (BMT), and La(Al_1/4Mg_1/2Ta_1/4)O₃ (LAMT), are being investigated as potential TBCs because of their high-temperature capabilities. The coefficient of thermal expansion (CTE) of these materials ranges from 7.9 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹, which is comparable to YSZ, except for BaZrO₃. Their thermal conductivities are below 2.8 Wm⁻¹ K⁻¹ at 1000 °C, with BMT > YSZ > SrZrO₃ > LAMT. However, their fracture toughness values are lower than YSZ, with the lowest being four times lower than YSZ. Preliminary thermal cycling results indicate that single-layer coatings of these materials have shorter lifetimes than YSZ. However, double-ceramic-layer coatings of SrZrO₃/YSZ and LAMT/YSZ have similar lifetimes. Doping SrZrO₃ with Yb₂O₃ or Gd₂O₃ can decrease thermal conductivity, and a double-ceramic-layer coating of Yb₂O₃-doped SrZrO₃ combined with YSZ has a 25% longer cycling lifetime than the optimized YSZ coating at a surface temperature of ∼ 1350 °C. These findings suggest that perovskite structure materials have potential as TBCs, but further research is needed to optimize their properties and performance [64,65,66].

BaZrO₃ was initially considered as a potential candidate for TBC applications due to its high melting temperature of 2600 °C. However, it was found to have a relatively poor thermal expansion coefficient and interior chemical stability, which resulted in the coating failing during thermal cycle tests. This significantly reduced the TBC’s service lifetime [67].

3.6 Strontium Zirconate (SrZO)

Given its high-temperature stability, low coefficient of thermal conductivity, and favorable thermal expansion compatibility with metallic substrates, strontium zirconate (SrZrO₃) is a potential material for thermal barrier coatings (TBCs). Solid-state reactions, sol–gel processes, and combustion synthesis are just a few of the ways that SrZrO₃ may be made. It has a cubic perovskite crystal structure. Y₂O₃-stabilized ZrO₂ (YSZ) may be replaced with SrZrO₃ in TBCs because SrZrO₃ may enhance YSZ's thermal stability and phase stability. Additionally, SrZrO₃ may improve TBCs’ resistance to CMAS corrosion by producing an appetite-like layer with Sr at the coating/CMAS interface. This layer inhibits CMAS penetration and extends the useful life of TBCs. Overall, SrZrO₃ shows great potential for the development of advanced TBCs with improved thermal and corrosion resistance properties [68].

SrZrO₃ has shown improved performance in cyclic tests at surface temperatures above 1250 °C due to its high melting temperature (2800 °C), low thermal conductivity, and high thermal expansion coefficient (approximately 11 × 10⁻⁶ K⁻¹ between 30 and 1000 °C). Additionally, SrZrO₃ has a lower sintering rate and Young's modulus compared to YSZ, which leads to more favorable mechanical responses. However, there have been reports of temperature-induced phase transformation in SrZrO₃, which negatively impacts its TBC performance. Some studies suggest that this issue can be addressed by doping with gadolinium oxide (Gd₂O₃) or ytterbium (III) oxide (Yb₂O₃), which not only helps suppress the phase transformation but also enhances the thermophysical properties of the coatings [10, 67].

The hot corrosion behaviors of Sr(Y_0.05Yb_0.05Zr_0.9)O_2.95 (SYYZ) ceramic in Na₂SO₄, V₂O₅, and Na₂SO₄ + V₂O₅ salt solutions were explored, respectively. At 900, 950, or 1000 °C, SYYZ porcelain did not react with Na₂SO₄. At 800 and 900 °C, m-ZrO₂, YVO₄, and YbVO₄ were the predominant corrosion products on the SYYZ ceramic surface in V₂O₅, whereas Sr₃V₂O₈ and t-ZrO₂ emerged at 1000 °C. The corrosion products on the Na₂SO₄ + V₂O₅ salt mixture-coated SYYZ ceramic surface at 800 and 900 °C were Sr₃V₂O₈ and t-ZrO₂, but at 1000 °C a new phase of SrZrO₃ emerged. The two main weathering processes for SYYZ ceramic deterioration are phase change and molecular contact [69]. The Fig. 12 [70] show the thermal properties of various ceramic, and it can be visualized that why advanced ceramic materials are not used for commercial application yet due not comply with thermal requirement of TBCs coatings.

Fig. 12

Advance materials thermal properties [70]

3.7 Calcium Zirconate (CZO)

Due to its good high-temperature stability with relatively low thermal conductivity, the ceramic material calcium zirconate (CaZrO₃) has been investigated for application in thermal barrier coatings (TBCs). CaZrO₃ is very stable at temperatures over 1500 °C and has a high melting point of 2530 °C. CaZrO₃ is a desirable option for use in TBCs due to its low thermal conductivity of 2.5 W/(mK). To assess the long-term reliability and corrosion resistance of CaZrO₃-based TBCs in high temperatures settings, especially in the presence of molten airborne ash, further study is required. (CMAS) [71].

CaZrO₃ is a novel material under consideration for TBC applications in this category. Despite having a lower melting point than YSZ, it has a promising thermal conductivity of around 2 W/mK and outstanding mechanical characteristics [72]. Garcia et al. [72], for example, conducted a research comparing CaZrO₃ coatings produced by air plasma and flame spray techniques. According to the findings, the two approaches created coatings with differing microstructures and characteristics. All of the coatings were porous, however the flame-sprayed coatings showed interplast fractures, and the atmospheric plasma-sprayed coatings had bigger circular pores. Despite this, all CaZrO₃ coatings had extremely poor heat conductivity.

To withstand heated rusting conditions, a new thermal barrier covering technology was developed. In this coating system, conventional yttria-stabilized zirconia received an addition of 5% CaZrO₃. (YSZ). In the presence of a combination of 50% Na₂SO₄ and 50% V₂O₅, the aforementioned hybrid coating system was contrasted with the traditional YSZ system at 950 C. The outcomes showed that the CaZrO₃-added composite system had a longer lifetime in this extremely hostile environment than the baseline YSZ system. This is because CaV₂O₄ was formed instead of YVO₄ as a result of the preferential reaction of NaVO₃ switching from yttria to calcia. The preferential reactions are explained using the acid–base theory of molten salts with ceramic oxides and changes in free energy. The hybrid system with CaZrO₃ added also showed less deformation, according to studies done on lattice distortion, extending the lifespan of the covering system altogether [73]. Studies also shows that SrZrO₃ cannot be used as a top coat in TBC systems when the coating showed a phase change with a volume increase at 730 °C, causing the samples to fail. BaZrO₃ showed comparatively weak thermal and chemical durability in thermal cycling experiments, which resulted in early disintegration.

3.8 Rare Earth Niobates (RENbO₄), and Tantalates (RETaO₄)

Rare-earth niobates are a family of materials that are composed of a rare-earth element, such as cerium or lanthanum, and niobium oxide. These materials exhibit interesting and useful properties that make them attractive for a variety of applications. In addition to their potential as TBC and structural materials, rare-earth niobates also have potential as dielectric and piezoelectric materials. Dielectric materials are used in capacitors, while piezoelectric materials are used in sensors, actuators, and other electromechanical devices. Rare-earth niobates can also be doped with other elements to tune their properties. For example, doping with strontium can increase the conductivity of RENbO₄ ceramics, making them useful for solid oxide fuel cell applications. It has two basic crystal structures: pyrochlore with a defective fluorite structure (RE₃NbO₇) and perovskite structure (RENbO₄). The perovskite rare-earth niobate RENbO₄ ceramics have the potential to be used as a new thermal barrier coating (TBC) material.

At temperatures over 200 C, RENbO₄ ceramics have a comparable crystal structure as RETaO₄ ceramics and have poorer thermal conductivity than YSZ. RENbO₄ has a thermal conductivity range of 1.80–2.26 W/mK at 1000 C. The poor thermal conductivity of RENbO₄ is caused by chemical inhomogeneity dis terms of cation charge differences and a changing bonding length. The ferroelastic structure of rare-earth niobate provides for improved fracture toughness. This feature makes it a possible contender for use in structural applications requiring high strength and toughness [74]. A material’s crystal structure refers to the arrangement of atoms in a three-dimensional repeating pattern. RENbO₄ and RETaO₄ ceramics have a comparable crystal structure, which implies they have a similar atom arrangement. Because of their similar crystal structure, they exhibit equivalent thermal characteristics such as thermal conductivity. Chemical inhomogeneity in RENbO₄ refers to cation charge variations and bonding lengths that are not homogeneous across the material. These discrepancies can have an influence on thermal characteristics, but they also give unique qualities, such as increased fracture toughness due to the material’s ferroelastic structure.

The resistance of a substance to crack propagation is referred to as fracture toughness. A high fracture toughness material is less likely to crack under stress than a low fracture toughness material. Because of its enhanced fracture toughness, RENbO₄ is a viable contender for use in structural applications that need high strength and toughness.

Thermal conductivity is the capacity of a substance to transport heat. A low thermal conductivity substance will transmit heat more slowly than a high thermal conductivity material. The thermal conductivity of RENbO₄ is lower than that of YSZ, making it a potential material for use in TBCs. Chemical inhomogeneity in RENbO₄ refers to the fact that the cation charge is not uniform.

Rare-earth tantalates are another family of ceramic materials that share some similarities with rare-earth niobates. Like rare-earth niobates, rare-earth tantalates are composed of a rare-earth element, such as cerium or yttrium, and tantalum oxide. Rare-earth tantalates have interesting properties that make them attractive for a variety of applications. For example, they have a high dielectric constant, which makes them useful in capacitors and other electronic devices. They also have high thermal stability and low thermal expansion, which makes them attractive for use in high-temperature applications such as thermal barrier coatings.

One specific example of a rare-earth tantalate is yttrium tantalate (YTaO₄), which has a perovskite crystal structure and is used in some optical applications. Yttrium tantalate has a high refractive index and is transparent in the visible and near-infrared regions of the electromagnetic spectrum, making it useful for lenses and other optical components. Like rare-earth niobates, rare-earth tantalates can also be doped with other elements to tune their properties. For example, doping with calcium can increase the conductivity of yttrium tantalate, making it useful for solid oxide fuel cell applications.

Clarke and Levi [75, 76] conducted a study on the crystal structure of yttrium tantalate (YTaO₄) and found that it is a ferroelastic material. Ferroelastic materials exhibit a reversible strain when an external stress is applied, which makes them useful in applications such as actuators and sensors.

Clarke and Levi discovered that the stable phases of YTaO₄ at high and room temperatures are tetragonal (t) and monoclinic (m), respectively. This means that the crystal structure of YTaO₄ changes as it is cooled from high temperatures to room temperature. The phase transition temperature between these two phases is approximately 1430 °C. The t-m ferroelastic transformation that occurs during the cooling process of YTaO₄ is a result of a distortion in the crystal lattice. As the crystal structure changes from tetragonal to monoclinic, the lattice undergoes a shear deformation, which causes the material to exhibit a reversible strain. This reversible strain can be harnessed for use in a variety of applications.

Rare earth oxide coatings (La₂O₃, CeO₂, Pr₂O₃, and Nb₂O₅ as main phases) have a higher thermal expansion coefficient and a lower thermal diffusivity than ZrO₂ coatings, rendering them ideal for use as TBCs. Phase instability of the rare earth oxides, the majority of which are polymorphic at high temperatures, has some effect on these compounds' ability to withstand thermal stress. When zircon is used as a TBC substance, it dissociates during plasma blasting, producing layers made of a mix of solid ZrO₂ and porous SiO₂. The deteriorated SiO₂ in the layer may present a challenge for diesel engines due to the loss of SiO and Si(OH)₂. It is believed that the coating's ZrO₂ phase produces the thermal shield effect. Only a few other silicates, such as garnet almandine (Fe₃Al₂(SiO₄)₃), garnet pyrope (Mg₃Al₂(SiO₄)₃), garnet andradite-grossular (Ca₃Al₂(SiO₄)₃), and others, have the potential to be used as TBC materials. 10 to 30% by weight of CaO and 2CaO⋅SiO₂ make up the compound oxide layer. ZrO₂ exhibits excellent resilience to thermal stress and heated rusting [58, 81].

4 TBCs Materials Selection Principles

4.1 Requirements for the Optimal TBCs Properties

When selecting ceramic materials for Thermal Barrier Coatings (TBCs), several key requirements should be considered. (1) High-temperature stability: Ceramic materials used in TBCs must possess excellent stability at elevated temperatures to withstand the harsh thermal conditions experienced in turbine engines or other high-temperature applications. (2) Thermal insulation: The chosen ceramics should have low thermal conductivity to provide effective thermal insulation, reducing heat transfer to the underlying substrate and enhancing overall thermal efficiency. (3) Chemical compatibility: It is crucial to ensure that the ceramic material is chemically compatible with the environment in which the TBC operates. This includes resistance to corrosive gases, molten salts, and other potential chemical reactions that may occur at high temperatures. (4) Mechanical strength: The selected ceramic should exhibit sufficient mechanical strength to withstand thermal cycling, mechanical stresses, and potential impacts during operation. It should be able to maintain its structural integrity and resist cracking or spallation. (5) Thermal expansion compatibility: Ceramic materials in TBCs should have a similar thermal expansion coefficient to that of the underlying substrate. This helps to minimize thermal stress and prevent delamination or failure due to the mismatch in thermal expansion between the layers. (6) Environmental durability: The ceramics should demonstrate good resistance to environmental factors such as moisture, oxidation, erosion, and thermal shocks to ensure long-term durability and performance stability. (7) Manufacturing feasibility: Consideration should be given to the feasibility of manufacturing the selected ceramic material into the desired form, whether it be a coating or a bulk component. Factors such as availability, cost, and processability should be evaluated. By taking these requirements into account when selecting ceramic materials for TBCs, it becomes possible to ensure the desired performance, longevity, and reliability of the coating system in high-temperature applications.

When creating ceramic materials for advanced thermal barrier coatings, it might be difficult to strike a compromise between a number of different parameters, as shown in Fig. 13 [77] (TBCs). Table 1 shows the optimize properties of advance ceramic materials. New ceramic materials that can balance a variety of roles, performance traits, process constraints, and economic demands Thermal barrier coatings (TBCs) can be made from different ceramic materials with different phases. Some common phases of TBCs include:

• 1. Cubic: Cubic phase TBCs are made from a mixture of zirconia and yttria. They have good phase stability and low thermal conductivity, but lower toughness compared to other phases. They were initially developed in the mid-1970s.

• 2. Tetragonal: Tetragonal phase TBCs are made from partially stabilized zirconia with 7–8 wt% yttria. They have high toughness and relatively high thermal conductivity, making them the ceramic of choice for TBCs. However, they can experience phase transformation under thermal cycling, which can lead to coating failure.

• 3. Monoclinic: Monoclinic phase TBCs are made from zirconia with a higher content of yttria (10–12 wt%). They have better thermal shock resistance compared to tetragonal phase TBCs, but lower thermal conductivity.

• 4. Pyrochlore: Pyrochlore phase TBCs are made from a mixture of rare earth oxides and other ceramic oxides. They have excellent thermal stability and low thermal conductivity, making them suitable for high-temperature applications.

Fig. 13

The development of new ceramic materials for advanced TBCs [77]

Table 1 Protentional properties of advanced ceramic TBCs materials

The development of TBCs has focused on improving the properties of these phases through modifications in the composition, processing, and microstructure. For example, the addition of dopants like ceria and titania to zirconia can improve the coating's thermal stability and resistance to phase transformation. The use of novel processing techniques like plasma spraying and electron beam physical vapor deposition (EBPVD) can also improve the coating's microstructure and properties are being developed to meet this problem.

When zirconia-yttria TBC was first created in the middle of the 1970s, it had a single cubic phase (c), up to 20 weight percent of yttria, and provided high phase stability. It also had a low conductivity. The toughness and durability of the ceramic might be improved by reducing the yttria content to 6–8 weight percent, but at the expense of phase stability and thermal conductivity. The result is that the preferred ceramic material for TBCs is partially stabilized zirconia with 7–8 wt% yttria (8YSZ), excellent toughness, and reasonably high thermal conductivity of 2.0–2.3 W/m K for fully dense samples and 0.9–1.2 W/m K for 10–15% porosity at about 1000 °C. According to a review of the development of zirconia-based TBCs, the phase structure and its distinct characteristics, such as thermal stability and toughness, are essential for the performance and longevity of TBCs, especially in harsh settings.

5 Preserving Metallic Materials Against Oxidation and Hot Corrosion: A Scientific Approach

When metals are exposed to high temperatures, they can undergo oxidation reactions, which involve the loss of electrons and the formation of metal oxides, sulfides, or carbides. These reactions occurs when metals are exposed to air or other oxidizing agents at high temperatures. The resulting oxide layers can be protective, in some cases, preventing further corrosion or degradation of the metal. Hot corrosion is a more specific type of oxidation process that occurs in high-temperature environments where the metal is exposed to contaminants such as sulfur-containing gases or other corrosive agents. As a result, complicated molecules that are not just metal oxides may develop, but also sulfides or other compounds. Hot corrosion can be particularly problematic in industrial settings such as petrochemical plants where high temperatures and corrosive environments are common [100].

According to Guo et al. [101] Hot-salt corrosion is a phrase used to describe situations in which exposure to molten salts occurs either directly or via condensation processes. Turbine blades constructed of Ni-base superalloys' degradation in actual end-use settings The properties of the substance degrade when Ni-base superalloys are applied as rotor blades in a practical setting. Corrosion may cause this degradation. Corrosion happens when hard particles strike a turbine blade and scrape away surface material. The particles may come from loosened scale deposits in the combustor or may enter via the turbine intake. Hot corrosion and sulfidation processes are two terms used to characterize corrosion.

Hot corrosion can be accelerated by the deposition of compounds such as Na₂SO₄, which can react with metals at high temperatures to form sulfides and other corrosion products. The sulfur released during fuel combustion can also contribute to hot corrosion, as can other contaminants present in the engine or industrial environment.

Sulfides are formed on the surface of metal components during sulfidation corrosion, a particular kind of heated corrosion. During sulfidation corrosion, alkaline sulfates may be found in the sediment, which can hasten the corrosion process even more. The degradation of blade materials and other metal components due to hot corrosion and sulfidation corrosion can both reduce their usable lives and raise the possibility of component failure. Therefore, in order to ensure the dependability and safety of industrial machinery and other systems subjected to high-temperature, acidic conditions, it is crucial to comprehend and minimize the impacts of hot corrosion and sulfidation corrosion [78].

5.1 Thermally Sprayed Coatings

The application of thermal barrier coatings is frequently done using thermally sprayed coatings (TBCs). Using a high-velocity flame or plasma jet, a coating material is melted and sprayed onto the substrate in this process, forming a solid layer after cooling. Because of its adaptability, affordability, and capacity to handle various forms and sizes, this approach is popular. Thermally sprayed coatings come in a variety of varieties that are utilized for TBCs, such as: The procedure for TBCs that is most frequently employed is called air plasma spray (APS). The coating material is heated up by a high-temperature plasma arc and then propelled onto the substrate. High-Velocity Oxygen Fuel (HVOF) technique melts the coating material with a supersonic jet of fuel and oxygen, resulting in a dense and uniform coating. Detonation Gun (D-Gun) technique, the coating material is accelerated and melted using a detonation wave, resulting in a dense and solidly bonded layer. Cold spray technique creates a dense, incredibly adherent covering without melting by impacting and bonding solid particles at high velocity onto the substrate. Nanostructured materials, like nanostructured ceramics, are now being used in thermally sprayed coatings for TBCs, which can enhance the coatings' thermal and mechanical properties. Gradient coatings are another innovation that can help to reduce the danger of cracking and delamination by enabling a smoother transition between the coating and the substrate [102].

The deposition of thermal barrier coatings can be done using a variety of specialized methods (TBCs) which includes Electron beam physical vapor deposition (EBPVD), Air plasma spray (APS), High velocity oxygen fuel (HVOF), Suspension plasma spray (SPS), and Solution precursor plasma spray (SPPS). TBC material is vaporized using an electron beam in the process of electron beam physical vapor deposition (EB-PVD), and the vaporized material is then deposited onto a substrate using a nozzle. This procedure enables the careful control of the coating microstructure while depositing highly homogeneous and dense coatings [103, 104].

Using a plasma torch, a liquid precursor solution is sprayed onto the substrate in the solution precursor plasma spray (SPPS) method. The precursor solution then decomposes into a solid TBC coating. This process can create TBCs with distinct microstructures and qualities that are not possible using conventional thermal spray methods. In the case of suspension plasma spray (SPS), Using a plasma torch, this approach sprays a liquid suspension of tiny TBC particles onto the substrate. This process can result in coatings with a high density, little porosity, and great resistance to thermal shock. High-velocity oxy-fuel (HVOF) spraying approach uses a combustion flame to spray TBC particles onto the substrate at extremely high speeds. With this technique, coatings with great substrate adherence, high density, and low porosity can be created [105,106,107].

As shown in Fig. 14 [108], typical TBCs comprise of a topcoat (TC), a bond coat (BC), a thermally grown oxide (TGO), and a substrate. Thermal insulation and resistance to calcium-magnesium-alumina-silicate (CMAS) corrosion are functions of the TC, whereas bond strengthening, oxidation resistance, and mechanical property transition are functions of the BC. To withstand thermal and mechanical impacts in harsh settings, the TC material used must exceed tight performance limitations. Plasma spraying (PS) and electron beam physical vapor deposition (EB-PVD) are the primary methods for producing conventional TBCs. The EB-PVD TBC had a columnar structure with multi-scale pores, whereas the PS-TBC coating had approximately 60% poorer thermal conductivity than the bulk due to the lamella structure with numerous inter-splats. Many procedures for the production of TBCs are now being investigated; possible and exemplary approaches include suspension plasma spraying (SPS) and plasma spraying physical vapor deposition (PS-PVD). TBCs made using these two processes have a columnar structure comparable to that of EB-PVD, which can increase the coatings’ strain tolerance. Furthermore, because they have larger intracolumnar porosity, which can lower the thermal conductivity of the coating, they have been extensively explored for the manufacture of next-generation improved TBCs.

Fig. 14

Schematic diagram of the deposited coatings: a sprayed machines, b coatings structures, c surface SEM images [108]

The development of TBCs has been driven by the need to protect critical components of high-temperature machinery from the harsh operating environments they encounter. The typical TBC structure, consisting of a topcoat, bond coat, thermally grown oxide, and substrate, provides critical functionalities, including thermal insulation, corrosion resistance, and mechanical strengthening. The choice of TC material is of utmost importance, as it must meet strict performance criteria to resist thermal and mechanical stresses in extreme conditions.

Traditional TBC preparation techniques, such as plasma spraying and electron beam physical vapor deposition, have been widely used. However, with the advent of newer techniques such as suspension plasma spraying and plasma spraying physical vapor deposition, there is potential for the development of more advanced TBCs with improved strain tolerance and reduced thermal conductivity. The study of TBCs is crucial to improve the efficiency and durability of gas turbines and other high-temperature machinery in various industries. As such, ongoing research is focused on the development of novel TBC materials and preparation techniques to meet the increasing demands of these applications.

5.2 Designed of Fabricated Coatings

Ceramic coatings are often used to improve the performance of materials by providing them with enhanced properties such as wear resistance, corrosion resistance, and thermal protection. There are various types of ceramic coatings available in the TBCs systems, and they can be classified based on their composition, structure, and thickness. Some common types of ceramic coatings include: The schematic diagram Fig. 15 is presented below.

• 1. Monolayer ceramic coatings: These coatings consist of a single layer of ceramic material deposited onto a substrate. They are relatively simple to produce and can offer good protection against wear and corrosion.

• 2. Double ceramic coatings: These coatings consist of two layers of ceramic material, with each layer having a different composition or structure. This design can provide improved performance compared to monolayer coatings, as it can offer a combination of different properties.

• 3. Triple ceramic coatings: These coatings consist of three layers of ceramic material, with each layer having a different composition or structure. This design can offer even better performance than double coatings, as it can provide a wider range of properties.

• 4. Multi-layer ceramic coatings (functionally graded coatings): These coatings consist of multiple layers of ceramic material, with each layer having a different composition, structure, and/or thickness. This design can provide a graded transition between the properties of the substrate and the coating, which can help to reduce thermal stresses and improve adhesion.

Fig. 15

The architecture of a Fabricated System and level of advancements

In addition to these types of composite coatings, there are also single-layer and double-layer ceramic coatings Fig. 15e and f that can provide different combinations of properties depending on their composition and structure. Figure 16 shows the cross sectional analysis of mono, double ceramic, dense layer of multi ceramic layers [109].

Fig. 16

Showing as-sprayed microstructures of a YSZ, b GZ/YSZ, and c DGZ/GZ/YSZ [109]

Composite ceramic coatings are a particular kind of covering that consists of several layers of various materials, such as ceramics, metals, and plastics. High toughness, abrasion resistance, heat resistance, and rust resistance are just a few of the desirable qualities that these compounds are made to offer. Protecting metal components from heated rust is one essential use for composite ceramic coverings. By creating a layer of protection between the metal component and the acidic atmosphere, composite ceramic coverings can help to avoid heated rusting. The coating's clay layers are frequently very resistant to chemical assault, while the metal layers can offer extra defense against corrosion and mechanical wear [1].

Composite ceramic coatings, based on the particular elements and form of the covering, can offer a variety of other advantages in addition to preventing heated rusting. Some hybrid ceramic coatings, for instance, are self-healing, which means they can fix fractures or other harm to the covering without the help of a person. For a variety of high-tech uses, other coverings may be created to have particular electrical or temperature characteristics [110].

Metal components in hostile settings are frequently covered with ceramic composite coverings to prevent rusting. These coverings are usually made of ceramic materials, which are put to the metal component's surface to form a shield that prevents rusting. The makeup of the covering substance, its thickness, and the characteristics of the metal base are just a few of the variables that can affect the rate of erosion of ceramic composite coatings at the molecular level. For example, if the ceramic coating is too thin or porous, it may not provide adequate protection against corrosion, and the rate of corrosion may increase.

However, ceramic composite coatings can also provide some unique advantages at the nanoscale level. By expanding the surface area of the coating and enhancing its capacity to cling to the metal base, for instance, the use of nanoparticles as well as nanofibers in the ceramic substance can improve the coating's defensive qualities. Additionally, the use of nanotechnology compounds, which include corrosion inhibitors as well as self-healing agents, can lower rust rates even more and lengthen the usable life of the covering.

6 Hot Corrosion: Mechanism of Hot Corrosion: TBCs Failures

Hot corrosion is a complex corrosion process that can occur in high-temperature environments when metal components are exposed to corrosive substances such as molten salts or sulfurous gases. The mechanism of hot corrosion involves several steps:

• Formation of an Oxide Scale: When a metal component is exposed to high temperatures, it can form an oxide scale on its surface. This scale is typically composed of metal oxides and can provide some protection against corrosion.

• Penetration of Corrosive Substances: Corrosive substances such as molten salts or sulfurous gases can penetrate the oxide scale and react with the underlying metal. These substances can react with the metal to form metal sulfides or other corrosive compounds.

• Disruption of the Oxide Scale: The formation of metal sulfides and other corrosive compounds can cause the oxide scale to become disrupted, allowing more corrosive substances to penetrate the surface of the metal.

• Accelerated Corrosion: Once the oxide scale is disrupted, the rate of corrosion can increase significantly. The metal can become pitted, cracked, or otherwise degraded, leading to component failure.

The exact mechanism of hot corrosion can vary depending on the specific environment and materials involved. For example, in some cases, hot corrosion can involve the formation of complex sulfide compounds on the surface of the metal, while in other cases, it may involve the formation of oxide compounds or other types of corrosion products. General, the mechanism of hot corrosion is complex and can be influenced by a variety of factors, including temperature, composition of the metal and surrounding environment, and the presence of corrosive substances [111].

When Sulphur in the fuel reacts with alkali metal impurities like sodium and potassium to generate molten sulphates, hot corrosion, a quick kind of attack, occurs. Just a few parts per million (ppm) of the aforementioned contaminants in the gasoline or their counterpart in the air may be necessary for this degradation to start. One way sodium can infiltrate a system is through liquid fuel that contains salt, tainted water/steam infusions, turbine air inlets near saline water, and other sources. According to Habibi et al. Zirconia-titania composite (ZrO₂–TiO₂), Lanthanum aluminate (LaAlO₃), Yttria-stabilized scandia (Y₂O₃–Sc₂O₃) are used in TBCs to control the several chemical components, outside the alkali metals like sodium and potassium, might affect or contribute to corrosion on bucketing [112]. Vanadium, which is typically found in crude and leftover oils, is noteworthy in this context. Even though the outcome is the same, the industry now distinguishes between high-temperature as well as low-temperature hot corrosion, corrosion at low temperatures occurs at temperatures below 200 °C and typically involves the reaction of metal with a corrosive environment, such as acidic gases or liquids. Common forms of low-temperature corrosion include acid corrosion, which occurs in environments containing acid gases such as hydrogen sulfide or carbon dioxide, and oxygen corrosion, which occurs in the presence of dissolved oxygen in water or other liquids. Low-temperature corrosion can lead to localized pitting, cracking, and other forms of material degradation, which can ultimately lead to equipment failure.

High-temperature corrosion occurs at temperatures above 400 °C and typically involves the reaction of metal with molten salts, oxides, or sulfides. This type of corrosion can occur in a range of industrial environments, including gas turbines, boilers, and furnaces. As discussed earlier, hot corrosion is a type of high-temperature corrosion that specifically involves the reaction of metal with molten salts or high-temperature gases containing oxides of sulfur [113].

The tetragonal (t-phase) to monoclinic (m-phase) phase transition of ZrO₂, which results in a 3–5% volume increase, is prevented by adding yttrium. Thermal conductivity is decreased when yttrium and oxygen vacancies are present together. However, YSZ cannot be used in turbine engines with temperatures over 1200 °C.

Figures 16a–c show the cross sections of single-layer YSZ, double-layer GZ/YSZ, and three-layer density gadolinium zirconate (DGZ)/GZ/YSZ, respectively, as they were sprayed [109]. None of the three covering techniques had any flaws or fractures at the intersection of the different top finish layers. Columns and columnar limits that formed during the layering process are visible in the layers. The third layer of the coating has a structure that is rather compact and dense rather than columnar. In contrast to a columnar design, a compressed structure may delay the entry of the liquid caustic ions, according to Song et al. (2020). the presence of sodium sulphate causes an extremely rapid type of oxidation to take place between 1500 °F (816 °C) and 1700 °F (927 °C) (Na₂SO₄) [108]. Sodium sulphate is created as a result of the reaction between sodium, sulphur, and oxygen during combustion. Sulfur is a natural impurity in the fuel. Low-temperature heated corrosion was recognized as a unique process of corrosion assault in the middle of the 1970s. If the conditions are right, this attack could be quite severe. It requires a significant partial pressure of SO₂ and takes place between 1100 and 1400 °F (593 and 760 °C) [114]. It is caused by low-melting eutectic substances that result from the admixture of sodium sulfate with particular metal constituents, such as nickel and cobalt. In truth, it resembles fireplace rust that happens in coal-fired furnaces in some ways. The two types of heated rust cause various types of assaults. High-temperature corrosion exhibits intergranular assault, sulphur particulates, and a base metal denuded zone. Metal oxidation occurs when oxygen atoms combine with metal atoms to form oxide scales. The synthesis of these oxides proceeds more quickly at higher temperatures, which raises the possibility of component failure if too much of the substrate material is used up in the process.

The Figs. 17 and 18 below shows the results of a hot corrosion test conducted on a laser remelted thermal barrier coating (TBC). The test involved exposing the TBC to a corrosive environment for a period of time and observing the changes in its surface microstructure over time. After 10 h of hot corrosion testing, the TBC surface remained flat and smooth. However, as the test continued, network cracks began to appear on the surface of the coating. This cracking is attributed to the volume expansion that occurs during the hot corrosion process, which is caused by the phase transition of t'-ZrO₂ to m-ZrO₂. After 100 h of hot corrosion testing, the delamination of the coating surface became more obvious, and the surface roughness of the coating [115].

Fig. 17

Microstructure as-sprayed TBC after hot corrosion test a 10 h, b 40 h, c 100 h and d cross-section [115]

Fig. 18

Microstructure of the laser remelted TBC after hot corrosion test a 10 h, b 40 h, c 100 h and d cross-section [115]

The SEM images showed that in the as-sprayed TBC, a large number of holes and horizontal and vertical cracks appeared after hot corrosion testing. This was attributed to the diffusion of molten salts into the TBC, which reacted with Y element to form Y₂(SO₄)₃, leading to a decrease in Y content and resulting in volume expansion due to the phase transition of t'-ZrO₂ to m-ZrO₂. This, in turn, caused crack propagation and damage to the protective coating. In contrast, the laser remelted TBC showed a significant reduction in the number of cracks and holes compared to the as-sprayed TBC. The results indicated that the corrosion reaction primarily occurred on the coating surface, and only a few molten salts penetrated the coating and reacted with Y element. The laser remelting treatment was therefore shown to be beneficial in reducing the susceptibility of the TBC to hot corrosion. As shown in the figures the degradation occurs in the both the as-sprayed and laser remelted TBCs showed the formation of continuous thermally grown oxide (TGO) at the interface between the ceramic layer and bonding layer after the hot corrosion process. Microstructure as-sprayed TBC after hot corrosion test (a) 10 h (b) 40 h (c) 100 h and (d) cross-section. However, the thickness of the TGO in the laser remelted TBC was significantly thinner than that in the as-sprayed TBC, further indicating the superior hot corrosion resistance of the laser remelted TBC.

The two kinds of hot corrosion that occur are high temperature and low temperature hot corrosion in the metal's hot corrosion process. The electrochemical reaction occurs between liquid ions and a target, typically sodium and potassium sulfates. Generally speaking in terms of ceramic coatings, there are two types of hot corrosion: Type I (high temperature), which usually takes place between 820 or 920 °C (1500°F along with 1700°F), with a maximum temperature of about 870 °C (1600°F), and the second type (low temperature), which typically occurs between 590 or 820 °C (1100 or 1500°F), with a maximum temperature of about 700 °C (1300°F), is frequently exhibited. [116].

6.1 Role of Molten Salts

Melting salt combinations made of Na₂SO₄ and V₂O₅ are often what cause corrosion in TBCs at high temperatures. At high temperatures, the combination of V₂O₅ and Na₂SO₄ salt combines with the top coating to create the following equations, especially in TBCs with top coating YSZ.when metals are elevated to temperatures between 700 and 900 °C in the presence of sulphate deposits created by the interaction between sodium chloride and sulphur molecules in the vapor phase around the metals, hot corrosion, an intensified type of oxidation, occurs. If the sulphate content is higher than the equilibrium vapor pressure at the metal temperature needed to run the components (700–1100 °C), Na₂SO₄ will settle on the surface of the turbine blades and vanes. If masses of liquid Na₂SO₄ are present (m.p. = 884 °C), Ni- and Co-base superalloys may be attacked more rapidly at greater temperatures. This sort of attack is referred to as hot rust. Rapid weathering may also be caused by the existence of solid or liquid compounds like chlorides, vanadates, and sulfate-vanadate mixtures. Recently The study examined [117] the impact of molten phase on superheater corrosion using two model salt systems and two stainless steel qualities. The results revealed that there is a threshold content of melt above which the corrosion rate increases due to increased steel/salt-melt contact. The study suggests that the first melting temperature T0 is not the only critical limit for severe superheater corrosion, and the amount of liquid phase at T₁ is also crucial. The critical amount of liquid phase at T0 for severe corrosion is steel-specific. The findings emphasize the importance of understanding the effect of molten phase on superheater corrosion and developing material-specific guidelines for designing and operating high-temperature systems. The study's results are relevant for industries such as power generation, chemical processing, and metallurgy, where high-temperature corrosion is a significant concern. The dissociation reaction of Na₂SO₄ at an elevated temperature is given by Eq. 1 and generation of corrosive species by Eq. (2), and degradation of ceramic top coat by Eq. 3 and 4 [78].

$${\text{Na}}_{{2}} {\text{SO}}_{{4}} \to {\text{ Na}}_{{2}} {\text{O }}\left( {{\text{Basic}}} \right) \, + {\text{ SO}}_{{3}} \left( {{\text{Acidic}}} \right)$$

(1)

$${\text{Na}}_{{2}} {\text{O }} + {\text{V}}_{{2}} {\text{O}}_{{5}} \to {\text{2NaVO}}_{{3}}$$

(2)

$${\text{t}}-\text{Zr}{{\text{O}}_{2}}\left( {{\text{Y}}_{2}}{{\text{O}}_{3}} \right)\left( \text{s} \right)+\text{2NaV}{{\text{O}}_{3}}\left( \text{S} \right)\to \text{m}-\text{Zr}{{\text{O}}_{2}}\left( \text{s} \right)+\text{2YV}{{\text{O}}_{4}}\left( \text{S} \right)+\text{N}{{\text{a}}_{2}} {\text{O}}$$

(3)

$${\text{ZrO}}_{{2}} ({\text{Y}}_{{2}} {\text{O}}_{{3}} ) \, + {\text{ V}}_{{2}} {\text{O}}_{{5}} \to {\text{ m - ZrO}}_{{2}} + {\text{ 2YVO}}_{{4}}$$

(4)

The most promising TBC materials are made of rare-earth pyrochlore oxide layer (RE₂T₂O₇, where RE is short for rare-earth and T is short for tetravalent metal), which has the advantages of low thermal conductivity, high temperature expansion coefficient (TEC), highest melting point, ability to eliminate defects, and phase stability at high temperatures. By swapping out 12 T atoms for RE atoms, TO₂ is transformed into RE₂T₂O₇ (pyrochlore structure), which leads to the creation of 1/8 oxygen gaps and allows for charge neutrality. (fluorite structure) The structure's internal oxygen gaps and altered chemical bonds cause a reduction in heat conductivity. These advance ceramic also faces the hot corrosion during their operation. Xu et al. [118] explain the degradation of pyrochlore type lanthanum zirconate, and cerate. LZ coating to a molten mixture of Na₂SO₄ + V₂O₅ at 1173 K marks an extra XRD peaks which are attributed to LaVO₄, m-ZrO₂, La₂O₂SO₄ and La₂(SO₄)₃ together with pyrochlore peak. The possible reactions that would have produced these phases are mentioned below. For every corrosion product the XRD peak was detected.

$${\text{La}}_{{2}} {\text{Ce}}_{{2}} {\text{O}}_{{7}} \left( {\text{s}} \right) + {\text{ 2NaVO}}_{{3}} \left( {\text{l}} \right) \, \to {\text{ 2LaVO}}_{{4}} \left( {\text{s}} \right) \, + {\text{2Ce}}_{{2}} {\text{O }}\left( {\text{s}} \right) \, + {\text{ Na}}_{{2}} {\text{O }}\left( {\text{l}} \right)$$

(5)

$${\text{La}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) \, + {\text{ 2NaVO}}_{{3}} \left( {\text{l}} \right) \, \to {\text{ 2LaVO}}_{{4}} \left( {\text{s}} \right) \, + {\text{ Na}}_{{2}} {\text{O}}\left( {\text{l}} \right)$$

(6)

$${\text{La}}_{{2}} {\text{Zr}}_{{2}} {\text{O}}_{{7}} \left( {\text{s}} \right) \, + {\text{ 2NaVO}}_{{3}} \left( {\text{l}} \right) \, \to {\text{ 2LaVO}}_{{4}} \left( {\text{s}} \right) + {\text{ 2m}} - {\text{ZrO}}_{{2}} \left( {\text{s}} \right) \, + {\text{ Na}}_{{2}} {\text{O}}\left( {\text{l}} \right)$$

(7)

$${\text{2La}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) + {\text{ 5Na}}_{{2}} {\text{SO}}_{{4}} \left( {\text{l}} \right)\, \to {\text{ La}}_{{2}} {\text{O}}_{{2}} {\text{SO}}_{{4}} \left( {\text{s}} \right) + {\text{ La}}_{{2}} \left( {{\text{SO}}_{{4}} } \right)_{{3}} \left( {\text{s}} \right) + {\text{ SO}}_{{3}} \left( {\text{g}} \right) + {\text{ 5Na}}_{{2}} {\text{O }}\left( {\text{l}} \right)$$

(8)

$${\text{2La}}_{{2}} {\text{Zr}}_{{2}} {\text{O}}_{{7}} \left( {\text{s}} \right) + {\text{ 6Na}}_{{2}} {\text{SO}}_{{4}} \left( {\text{l}} \right) \, \to {\text{ La}}_{{2}} {\text{O}}_{{2}} {\text{SO}}_{{4}} \left( {\text{s}} \right) + {\text{ La}}_{{2}} \left( {{\text{SO}}_{{4}} } \right)_{{3}} \left( {\text{s}} \right) + {\text{ 4m}} - {\text{ZrO}}_{{2}} \left( {\text{s}} \right) + {\text{ 2SO}}_{{3}} \left( {\text{g}} \right) + {\text{ 6Na}}_{{2}} {\text{O }}\left( {\text{l}} \right)$$

(9)

Failure of thermal barrier coatings (TBC) systems under thermomechanical stress is very difficult because it depends on a number of factors that must all be taken into account, including temperature disparity, oxidation, surface roughness, creep, and sintering.

At a first stage, determining the induced residual stress fields provides information about the key places for crack nucleation and may aid in interpreting the experimental findings of cracking behavior. The predicted residual stresses are very localized and may be adequate for fracture nucleation but not for crack progression and failure. As a result, a fracture mechanics approach was developed that uses the modified crack closure integral procedure to simulate and evaluate cracks in the important parts of the TBC system in order to determine the mode-dependent crack loading. A freshly developed mixed mode failure criterion and pertinent fracture hardness data are then used to project the crack propagation capacity. This method is used to study various empirically discovered failure mechanisms for TBC systems. A TBC under Stress may spall off after a considerable number of rounds. This is brought on by both an increase in overall wear damage and sizable interior stresses brought on by oxidation at the bond coat/topcoat junction. Additionally, there are stress discontinuities near the extremities of the covering, which is where early fatigue cracks commonly develop. Large-scale bowing and spallation, which primarily starts at the bond coat/TGO contact, frequently result in the catastrophic collapse of thermal barrier coatings (TBCs). The struggle between bending and interface delamination, which is brought on by the waviness of the interface, occurs before spallation in TBCs. To predict the lifetime of TBCs during in-service operation, a thorough study of the covering failing process is the first requirement. TBCs are subjected to heated rust, fast heating and cooling, nonuniform heating in both the through-thickness and surface orientations, high temps, and demanding mechanical loadings in the operating environment. Due to the lack of adequate service condition modeling, the majority of study is concentrated on the failure process of TBCs in a homogeneous temperature field or when subjected to mechanical stresses [119].

6.2 Residual Stress in Coatings; TBCs Failures

Because of its close relationship to thermally grown oxide (TGO) splitting during cycle oxidation, the tension growth in the covering system is important. As a result of "rumpling," "ratcheting," or "scalloping," which are all terms for splitting and eventual failing of TBCs, the contact between TGO and bond layer may become rougher. A correlation between rumpling and coating thickness was discovered in the majority of pertinent studies, which focused on widely available (Ni,Pt)Al films with a relatively narrow coating thickness of 40–50 m. Understanding tension development in coating still aids in describing TGO spallation, even though the use of MCrAlY coatings frequently requires a considerably higher layer in industrial uses where rumpling is less noticeable. The volume shrinkage of the phase transformation is what causes stress to build up in coatings, and grain sliding in response to high temperature stress in coatings is the dominant factor for surface rumpling [120].

According to mechanical considerations, one of the major forces behind the formation of fractures in the oxide layer is the accumulated tension energy in the TGO that has not been harmed. The analysis of TGO stress growth may reveal details about the ways in which TBCs falter. Two factors contribute to the formation of residual stress in TGO: (i) thermal mismatch strains, which are brought on by the thermal-expansion mismatch between the metal and the oxide scales during cooling; and (ii) growth or intrinsic strains, which are primarily brought on by the lateral growth of TGO constrained by the underlying metal during oxidation. Utilizing piezospectroscopy to measure the luminescence from the minute Cr³⁺ content, it is possible to identify the leftover stress in cemented Al₂O₃.

As depicted in Fig. 19 while the initial inherent growing stress is instantly decreased by the phase change in the underlying layer, the remaining stress in sintered Al2O3 is primarily brought on by thermal-mismatch stresses during cooling. The retention of a high tensile stress level in TGO is a sign of the stability of the TGO because it depends on the local state of the oxide layer and could be quickly diminished due to splitting or the formation of interfacial gaps [121].

Fig. 19

The residual stresses (S22) at room temperature in different FE models after 799 h oxidation: a TBC in the absence of internal oxides, b TBC with ten [121]

The presence of internal oxides in Thermal Barrier Coatings (TBCs) affects their interfacial morphology and stress distribution. Swelling of internal oxides increases interfacial roughness and plumps up the middle area of the interface. The presence of internal oxides generates a complicated stress state in TBCs, altering stress component patterns and magnitudes, including a significant decrease at the peak of the BC layer. Internal oxidation refers to the process of oxidation occurring within a material, typically in the form of metallic alloys or ceramics. In the context of TBCs, internal oxidation can occur in the bond coat layer due to the high-temperature exposure during service, resulting in the formation of oxides within the layer. These oxides can have a significant impact on the stress distribution within the TBC, affecting its mechanical and thermal properties. The number, location, and distance to the interface of the internal oxides can all impact the stress distribution, and this can have implications for crack initiation and propagation. Therefore, understanding the role of internal oxidation in TBC failure is important for developing more durable and reliable TBCs. The stress distribution is affected by the number, position, and distance to the interface of the internal oxides, with the highest tensile stress changing from the peak area to the off-peak region in the BC layer. While this may prevent fracture formation at the peak, significant normal tensile stresses in the intermediate and off-valley areas may result in further crack initiation. Internal oxidation has a significant impact on TBC failure because it alters the probability and position of micro-cracks.

6.3 Stress Corrosion Crack in Coatings; TBCs Failures

A variety of rusting known as stress corrosion cracking (SCC) takes place under compressive tension and allows for the formation and spread of fractures through a substance, even when a protective covering is present. In building structures, such as coverings like heat barrier coatings, this can be a significant issue. (TBCs). To prevent steel surfaces from rusting and heated gas damage, TBCs are frequently used in high-temperature uses, such as gas engines. TBCs, however, are also vulnerable to SCC because of the interaction of chemical assault and mechanical tension [122].

Moisture or other acidic substances, which can interact with the covering and compromise its mechanical qualities, are a frequent cause of SCC in TBCs. This may cause cracks to develop and spread, eventually leading to covering failure. Carefully choosing covering materials and production techniques that can endure the unique working circumstances of the application is crucial to reducing SCC in TBCs. Utilizing coatings with better mechanical and chemical characteristics, such as stronger and more flexible coatings, or adding surface processes to increase the coating's rust protection are two examples of how to do this. Additionally, SCC early warning indicators and disastrous breakdowns can be avoided by using surveillance and examination methods. This could involve non-destructive testing techniques like X-ray diffraction or ultrasound examination, which can spot covering flaws like fractures before they become serious issues [123].

Stress corrosion cracking (SCC) is a subject of continuing study, with a focus on knowing the processes of fracture onset and spread as well as creating novel materials and treatments to reduce SCC. Here are some recent scientific references related to SCC research:

Kth is the critical stress intensity factor range for the beginning of Stage 2 growth under cycle loading, whereas KISCC is the critical stress intensity factor for the start of Stage 2 growth under continuous loading. The stress corrosion cracking (SCC) of materials is conceptually represented by the bathtub model. It is predicated on the notion that the SCC process can be broken down into separate phases with varying rates of fracture development and failure.

Figure 20 illustrates the five consecutive phases of the SCC process for high-pH circumstances as described by the bathtub model. The planning stage, the first step, is where the prerequisites for SCC start are created. Stage 1a follows, during which time small fractures start to appear but stop spreading. Early-stage development of fractures as they connect to the primary crack point is a feature of stage 1b. Stage 2 is characterized by the rapid growth of fractures under mechanical pushing forces above the barrier, while Stage 3 is characterized by the rapid disintegration of the substance.

Fig. 20

Schematic illustration of the dependence of crack growth rate on time or mechanical driving force of material [124]

The fracture development rate moves from Stage 1b to Stage 2 once the applied mechanical moving force, such as the critical stress intensity factor range for cycle loading or the critical stress intensity factor for static loading, surpasses a predetermined level. With a fracture development rate in Stage 2 that ranges from 1 10^–7 to 3 10^–7 mm/s, the pipeline's lifespan may be shortened. According to reports, the crucial stress intensity factor (KISCC value), which determines when Stage 2 development begins, is around 21 MPa m0.5.

The bathtub model can be used to forecast the development of SCC in materials by calculating the stress intensity factor for a specific fracture shape. Different formulae and techniques for determining stress intensity factors have been created by researchers. Hydrostatic testing is frequently employed to ensure the safety of coverings by removing fractures larger than a crucial dimension. If the coverings can resist the hydrotest pressure, a crack's maximal size can be presumed, and the amount of time that the system can operate safely can be calculated. However, during typical hydraulic testing, already-existing fractures can still develop or reignite, negating the test's advantages. Understanding how tiny fractures (Stage-1b) develop is essential for managing coatings integrity because the impact of hydraulic testing on them has been ignored. Hydrostatic testing has been shown to briefly halt the expansion of big fractures, but its impact on the development of tiny cracks is distinct and must be taken into account [125].

The development of fractures is significantly influenced by the tension rate at the split point. Even if the mechanical pushing power is insufficient for the fracture to develop, high strain rates can lead to high crack growth rates. When the local strain rate is significant, microcrack start and coalescence are more likely to happen in the plastic zone before the crack point. The stress–strain reaction and the stress rate applied at the fracture apex have an impact on the strain rate. Prior to pipeline operation, hydrostatic testing may raise the local strain rate upstream of the fracture point and help with early-stage crack onset and development. Cracks in coatings are a common issue and can result from a variety of factors, such as improper surface preparation, poor adhesion, or substrate movement. Cracks can compromise the performance of the coating by allowing moisture or other substances to penetrate the substrate, leading to corrosion or other forms of damage. To prevent cracks in coatings, proper surface preparation, selection of suitable coating materials, and application techniques must be employed [126]. Stress corrosion cracking (SCC) is a subject of continuing study, with a focus on knowing the processes of fracture onset and spread as well as creating novel materials and treatments to reduce SCC.

Brooking et al. [127] provide the findings of a study of the static stress-hot corrosion cracking process in various specimen geometries. When compared to the three-point bend and plain specimens, C-rings generated the most aggressive stress state and exhibited evident cracking in the shortest exposure durations. The cracking process was amplified by the stress gradient created by bending. As cracks propagate into the C-ring section, their propagation rates drop, most likely due to a reduction in stress gradient at the fracture tip and stress relaxation caused by the existence of the crack. Cracks establish a wide fronted propagation in C-rings, culminating in conjoined thumbnail cracks across the specimen breadth. Beach type patterns are visible on the fracture surfaces of statically stressed specimens. Beach type patterns on the fracture surfaces of statically loaded specimens are connected with the "stop-start" time-dependent character of hot corrosion stress cracking, but further research is needed to properly understand the variations in growth rate. The SEM and optical pictures for the following figure are shown below Fig. 21. Hot corrosion and stress corrosion cracking are complex and interrelated phenomena that can cause significant damage to TBCs in high-temperature environments. Hot corrosion refers to the degradation of a material due to the presence of molten salts or other corrosive species at high temperatures, while stress corrosion cracking refers to the initiation and propagation of cracks under the simultaneous influence of mechanical stress and a corrosive environment. The interaction between these two phenomena can accelerate the degradation of TBCs and ultimately lead to their failure. In addition to the experimental studies mentioned, numerical simulations can also be used to gain insight into the mechanisms of hot corrosion and stress corrosion cracking in TBCs. Finite element analysis, for example, can be used to model the stress distribution and crack propagation in TBCs under different loading and environmental conditions. Multi-scale modeling, which combines atomistic and continuum-level simulations, can also provide insights into the chemical and mechanical processes occurring at the nanoscale and their effect on the macroscopic behavior of TBCs. Furthermore, advances in material characterization techniques, such as electron microscopy and X-ray diffraction, have enabled researchers to study the microstructure and chemistry of TBCs at high resolution. This has led to a better understanding of the mechanisms of internal oxidation and other degradation processes, as well as the role of microstructure and composition in determining the performance of TBCs.

Fig. 21

SEM (a) secondary electron SEM image at 550 °C, 200 h, statically loaded plain specimen (b) optical image of 550 °C, 100 h, statically loaded C-ring specimen [127]

During service, numerous dynamic events take place simultaneously. Between the bond layer and the superalloy, there is interdiffusion. Al diffuses from the binding layer as a consequence, creating the TGO. All components, including the ceramic top layer, undergo microstructural, chemical, and phase changes. Temperature is projected to boost the speeds of these thermally triggered activities enormously. These processes usually result in coating deterioration and failure of the both bond coat and top coat.

The role and mechanics of the bond coat layer in TBC systems are being studied, and several ways are being investigated to increase its performance. To improve oxidation resistance and minimize TGO spallation, one method is to change the composition and structure of the bond coat layer. Another strategy is to develop innovative diagnostic tools, such as the PLPS method stated above, to quantify residual stresses in TGO and other layers of TBCs non-destructively and better understand their failure causes. Furthermore, developments in computer modeling and simulation can help in the development and optimization of TBC systems by anticipating the behavior and performance of various coating designs under varied situations. Overall, ongoing TBC system research targets. The binding layer increases the life of the TBC system even further. However, the bond layer's role is extremely intricate and little understood. The deterioration of the bond coat layer is the principal covering failure mechanism in most real-world situations. When exposed to high temperatures, the NiCrAlY bond binder oxidizes and forms a TGO layer. As the TGO thickens, it is more prone to spallate, causing the TBC system to fail. It is extremely challenging to pinpoint the precise mechanisms causing bond coat-induced TBC failure for different types of coating. However, all research acknowledge the significance of TGO spallation or fracture in the failure of the air plasma-sprayed TBC system. A thorough investigation is being carried out to discover a solution to the bond coat layer degrading problem in the TBC system. Heat shield coverings may have a longer life if they have a NiCrAlY bond layer with nanostructures. Using this equipment and the relevant crack spread characteristics, TBC failure models may be constructed and validated. Jiang et al. [128] developed a unique PLPS (Photoluminescence Piezospectroscopy) technique to detect residual stress in TGO of APS TBCs after high-temperature oxidation.

For the first time, the residual stress distribution in the TGO of APS TBCs was measured non-destructively, and the stress evolution process was discovered to be separated into two stages: the development stage and the relaxation stage. The stress accumulated to a high value of 3.45 GPa during the development stage due to the expansion of TGO. During the relaxation period, stress reduced immediately after the commencement of peak stress but then decreased at a consistent rate. Jiang et al. [127] in their study on measuring residual stress in the TGO of APS TBCs. While the results of the study are interesting and informative, a critical analysis of the methodology, results, and conclusions is necessary to fully evaluate the significance and implications of the findings. One limitation of the study is the use of a novel PLPS (Photoluminescence Piezospectroscopy) method for non-destructively measuring residual stress. While this method has advantages over traditional destructive testing methods, such as X-ray diffraction or Raman spectroscopy, its accuracy and precision have not been fully evaluated or compared to other methods. Further validation and standardization of the PLPS method are needed to establish its reliability and reproducibility.

Another issue is the limited scope of the study, which only investigated residual stress evolution in the TGO of APS TBCs subjected to high-temperature oxidation. The findings may not be directly applicable to other types of TBCs or failure mechanisms. Moreover, the study did not address the effect of other factors, such as coating thickness, processing parameters, and environmental conditions, on residual stress in the TBC system. Finally, the study did not provide a detailed explanation or mechanism for the observed stress evolution process, particularly the rapid stress drop after the onset of peak stress. More research is needed to elucidate the underlying mechanisms of residual stress formation and relaxation in TBCs and to develop predictive models for TBC performance and failure.

With a final thickness of 9.6 m, the TGO thickness followed a subparabolic growth law, and significant microcracks were formed after 80 h of oxidation and were mostly localized just above the TGO up to roughly 10–20 m in height. The residual stress, TGO thickness, and crucial microcracks are all components that contribute to the eventual failure, and their synchronized development as a function of oxidation time has been discovered. The residual stress rose with TGO development until microcrack nucleation, and the transition point at which the stress in TGO began to diminish was shown to be closely related to the crucial moment of microcrack coalescence. This observation demonstrates directly that TBC delamination may be successfully identified by monitoring the residue stress in TGO of APS TBCs. The subparabolic growth law of TGO thickness and the localization of microcracks just above the TGO up to roughly 10–20 m in height indicate that the TGO thickness and microcracks are correlated. Additionally, the residual stress in TGO increases with TGO development until microcrack nucleation. The transition point at which the stress in TGO begins to diminish is closely related to the crucial moment of microcrack coalescence, which demonstrates that TBC delamination can be identified by monitoring the residue stress in TGO of APS TBCs.

The findings highlight the importance of understanding the growth and development of TGO and microcracks in TBCs to prevent their failure. Monitoring the residual stress in TGO of APS TBCs could provide a valuable method for identifying TBC delamination before it occurs. However, it is worth noting that this study focused on APS TBCs, and it is not clear how generalizable these findings are to other types of TBCs. Therefore, more research is needed to investigate the role of TGO and microcracks in different types of TBCs and to develop strategies for preventing TBC failure.

In the future, the authors propose to develop a portable TGO detection system based on the proposed method, which will be applicable to the preparatory test for topcoat delamination inspection and will allow flexibility in matching actual turbine blade and coatings with curved surfaces.

The intact zone at which TGO suffers residual stress is likely the region where the TGO layer is still attached to the substrate, and therefore experiences compressive residual stresses due to thermal expansion mismatch between the TGO and the substrate. The cracked interface at which TGO stress is relieved is likely the region where the TGO layer has delaminated or cracked away from the substrate, resulting in a decrease in the compressive residual stresses that were previously present. The schematic in Fig. 22 may illustrate the changes in TGO thickness and delamination over time, and how these changes affect the residual stresses experienced by the TGO layer. The authors of the text you provided suggest that measuring TGO stress can provide valuable information for assessing the structural integrity of materials, and that non-destructive measurement of TGO stress is easier than non-destructive detection of delamination.

Fig. 22

Graphics of the intact zone at which TGO suffers and relieved [128]

6.4 Prevention and Mitigation

It is challenging to develop straightforward, all-encompassing protection measures since the corrosion mechanism is complex and includes a number of crucial processes that are not fully understood at the atomic level. The selection of an alloy composition, the use of coatings, the promotion of the production of a protective layer of corrosion products, and the use of inhibitors will all be briefly covered in the sections that follow. The preferred material for technical infrastructure, particularly those for oil and gas pipelines, is carbon steel, which normally contains more than 98% iron. Iron is alloyed with metals that passivate surfaces, such as Cr, Ni, and Mo, to produce stainless steels with excellent corrosion resistance. In addition to the materials previously mentioned, here are some other advanced ceramic materials used for thermal barrier coatings: Gadolinium zirconate (GZO), Hafnium oxide (HfO₂), Lanthanum zirconate (LZO), Yttrium aluminum garnet (YAG), Lanthanum hexaaluminate (LaAl₁₁O₁₈), Calcium zirconate (CaZrO₃), Lanthanum aluminate (LaAlO₃) and Magnesium zirconate (MgZrO₃) are the new ceramics material that use in TBCs to prevent or increase the life time of the coated metal [93].

7 Conclusions

The current study used a step-by-step technique to explore the advanced ceramic materials for TBCs, architecture of fabricated coatings, degradation due to corrosion and stresses. This research showed that there are various types of ceramic materials available, but they are facing the issues, hot corrosion, oxidation, Stress Corrosion Cracking, Calcium-Magnesium–Aluminum-Silicate corrosion, and Foreign Object Damage. These all cause a variety of degradations when they interact with ceramic materials. The original tetragonal YSZ, cubic GZO and pyrochlore are destabilized by thermochemical interactions via a dissolution/reprecipitation pathway, leading to significant microstructural deterioration. The character of the corroded atmosphere, the coating substance, the coating's substructure, and the manufacturing methods all play a role in the complicated occurrence of thermal barrier coatings degrading due to hot corrosion. Investigations into the hot corrosion behavior of various TBC varieties, including YSZ, Gd₂Zr₂O₇, La₂Zr₂O₇, and other doped-ceramic, revealed that TBCs with optimized coatings and advanced ceramics has the greatest corrosion protection. Bond layers top coat layer, cooling layer with TBCs and different surface treatments like shot peening and laser surface shaping can also help TBCs withstand heated rusting. But the deterioration of TBCs brought on by heated rust happens gradually, and it is challenging to predict the service life accurately. Therefore, it is crucial to continue research efforts to improve our understanding of the degradation mechanisms and develop more effective strategies to mitigate hot corrosion-induced degradation of TBCs. Overall, the use of TBCs remains critical for the efficient and reliable operation of high-temperature components in various industrial applications, and continued research and development are necessary to ensure their long-term performance and reliability.

8 Future Perspective

Future studies in this area should concentrate on creating new materials with increased resilience to heated rust as well as streamlining the procedures used to create coatings. Advanced measurement methods like X-ray diffraction, Raman spectroscopy, and electron microscopy can offer important insights into the processes of heated rust and the deterioration of thermal barrier coatings. Additionally, since thermal barrier coatings are frequently exposed to both kinds of stress in real-world uses, additional research is required to examine the combined impacts of heated rust and mechanical pressure on the efficacy of thermal barrier coatings. The creation of more accurate and efficient techniques for evaluating the performance and longevity of thermal barrier coverings under heated rust circumstances should also be the focus of research efforts.

Gildersleeve and Vaßen [129] reveals their finding about the future outlook of advance ceramic materials and thermal spray techniques. A significant portion of current ceramic materials, thermal spray applications and research efforts are focused on industries with established markets and consistent demand, such as aerospace and wear and corrosion control. This indicates that the direction of research in thermal spray technology and advanced ceramic materials is heavily influenced by the needs of the industrial market. For example, the shift from thermal barrier coatings to environmental barrier coatings and now to thermal-environmental barrier coatings for next-generation turbines demonstrates this market-driven research direction. Similarly, the COVID-19 pandemic has spurred research in bactericidal coatings for public surface protection.

While this market-driven research approach can be advantageous by exploring cutting-edge technologies with practical applications, it also poses risks to the overall growth of the technology. The thermal spray market is vulnerable to changes in the industry, such as the emergence of a new materials processing technology that outperforms thermal spray in a key market segment like aerospace. Such a shift could severely impact the financial growth and future expansion of the entire thermal spray industry.

In the cases examined in this overview, the implementation and feasibility of thermal spray technology vary from moderately to highly advanced stages. For instance, in the biomedical field, commercial companies continue to sell plasma-sprayed hydroxyapatite-coated implants for hip replacement surgeries. In the sensor and electronics field, there are companies that have successfully integrated and commercialized thermal spray direct-write technology, providing services for thermally sprayed sensor devices and antennae. However, in other areas like thermally sprayed thermoelectric devices and thermally sprayed gas sensors, the technological maturity level is significantly lower, indicating further development is required.

Bierwisch et al. [130] presented the roadmaps for various applications and knowledge fields highlight the need for significant development goals in the future to ensure sustainable growth in the advanced ceramics markets. These goals can be categorized into four major research topics:

• Novel Ceramics with Enhanced and New Properties: The development of advanced ceramics with improved properties and novel functionalities is crucial for meeting the evolving needs of various industries.

• High-Performance Key Components for System Application: The focus should be on designing and producing high-performance ceramic components that play essential roles in system applications, ensuring reliability and efficiency.

• Highly Efficient Processing Technology: The advancement of processing techniques for advanced ceramics is vital to achieve cost-effectiveness, scalability, and quality control in manufacturing processes.

• Holistic Modeling and Simulation Techniques: The use of comprehensive modeling and simulation tools can facilitate the understanding of complex phenomena, optimize material design, and predict performance in diverse applications.

To achieve progress in these research areas, interdisciplinary approaches and collaborations between academia and industry will be essential. Both sides need to contribute their expertise, share leadership roles, and foster research and development cooperations. It is important to recognize that breakthrough developments, both small and large, will be required at various levels, from ceramic materials to practical applications. The entire value-added chain must be considered to drive effective improvements and innovations.

To unlock the full innovation potential of advanced ceramics, significant efforts and investments are necessary. This includes research funding, which should not be limited to large framework programs but also allocated to individual contributors and unconventional ideas that may lead to fundamental breakthroughs. Despite the high risks involved, such endeavors offer opportunities for transformative advancements in the field. Collaboration, innovation, and targeted funding are crucial elements for realizing the ambitious but essential goals of sustainable development and growth in the advanced ceramics industry.