1 Introduction

1.1 Importance and background of zirconia as a dental material

The evolution of dental materials has been a journey of innovation, aimed at finding the optimal balance of durability, esthetics, and biocompatibility [1]. Starting from the early utilization of gold and amalgam to the advanced ceramics and polymers of today, the quest for more esthetic materials led to the development of dental ceramics. Among these, zirconia (zirconium dioxide, ZrO2), with its esthetic resemblance to natural teeth and high mechanical strength, has become an essential ceramic material in modern dentistry [2, 3]. It excels particularly in dental restorations such as crowns, bridges, and veneers, maintaining high performance even after long-term use due to its outstanding compressive strength and wear resistance [4]. The further enhancement of the precision and customization of dental restorations has been achieved by leading to the development of CAD/CAM (computer-aided design and computer-aided manufacturing) technologies [5]. Additionally, ongoing research into the optimization of zirconia’s properties, such as improving its translucency and minimizing the risk of low-temperature degradation (LTD), continues to extend its applications and performance in dentistry.

1.2 Purpose and scope of the research trend review

Several in-depth reviews of different aspects of zirconia ceramic denture preparation have been carried out over the past few decades [6,7,8,9]. The novelty of this paper lies in critically analyzing the progression of zirconia dental ceramics, focusing on the microstructure, mechanical properties such as esthetic outcomes, fracture toughness, and wear resistance. Especially, it scrutinizes the role of stabilizers, particularly yttria, play in defining zirconia’s phases, as well as their impact on both esthetic properties and mechanical strength. Addressing specific challenges such as low-temperature degradation and discoloration, this paper aims to sheds light on recent breakthroughs that mitigate inherent shortcomings of zirconia. Covering the depth of current research on zirconia-based materials, it underscores the strides made towards optimizing zirconia for restorative dentistry, thereby enhancing patient care and driving future material innovations.

2 Dental ceramics overview

Dental ceramics are inorganic, non-metallic materials used for the restoration or replacement of damaged tooth structures. They are predominantly used in various dental treatments, such as esthetic restoratives, crowns, bridges, veneers, inlays, onlays, and orthodontic devices.Dental ceramics can be categorized based on the different base materials, each with its unique properties and applications [10,11,12].

The most commonly used dental ceramic systems include silica-based, leucite-based, lithium disilicate-based, alumina-based, and zirconia-based ceramics, shown in Fig. 1 [11, 12]. Silica-based ceramics, with silica (silicon dioxide, SiO2) as their main component, provide excellent esthetics and are primarily used for veneers and anterior crowns. Leucite-based ceramics include materials with leucite (potassium aluminum silicate, KAl2[AlSi3O10](OH)2) crystals, offering enhanced strength and durability for minor restorations. Lithium disilicate-based ceramics, composed of lithium disilicate (Li2O5Si2) crystals, offer high strength and superior esthetics, making them suitable for both anterior and posterior tooth restorations. Alumina-based ceramics, made primarily of aluminum oxide (Al2O3), provide high strength and wear resistance, though their esthetics may be less superior compared to silica-based ceramics. Zirconia-based ceramics, mainly consisting of zirconium oxide (ZrO2), possess very high strength and toughness and are widely employed for crowns, bridges, and implants [2]. Each dental ceramic system is designed to satisfy specific clinical requirements.

Fig. 1
figure 1

Classification of dental ceramics with fracture strength [11, 12]

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3 Zirconia in dentistry: from microstructure to clinical applications

3.1 Crystal structure and phase transition of zirconia (ZrO2)

Discovered in 1789 by Martin Klaproth, zirconium (Zr) is not found in its pure form in nature but as part of minerals like zircon (ZrO2·SiO2) or as free oxide (zirconia, ZrO2), known as Baddeleyite [13]. Through rigorous processing, zirconium oxide powder is produced, which can then be formed into zirconia blocks for various applications, including dental restorations (Fig. 2).

Fig. 2
figure 2

The Evolution of zirconia-based materials from left to right: natural zircon mineral, processed zirconium oxide powder, and final zirconia dental restorations

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Zirconia naturally exists in three major crystal phases: monoclinic, tetragonal, and cubic (as shown in Fig. 3a). The crystal structures of zirconia are as follows: From room temperature up to 1170 °C, zirconia is in the monoclinic phase (Monoclinic phase; ICSD #41763; Space group P21/c); When the temperature increases beyond approximately 1170 °C and up to 2370 °C, zirconia transforms into the tetragonal structure (Tetragonal phase; ICSD #89428; Space group P42/nmc); Above 2370 °C, zirconia exists in the cubic structure (Cubic phase ICSD #89429; Space group Fm-3 m), shown in Fig. 3b [14]. In dental applications, the tetragonal phase of zirconia is particularly valued for its toughness and aesthetic appeal, suitable for implants and restorations [15]. The ISO 13356 standards specify conducting an artificial aging test at 134 °C for 5 h under 2.2 bars of pressure to evaluate the stability of tetragonal zirconia in dental applications [16]. This test is critical for assessing the long-term performance of dental implants and restorations made from zirconia. A transformation rate of less than 25% to the monoclinic phase during this test indicates that the tetragonal form of zirconia retains its structural integrity and remains effective as a material for dental implants. This requirement highlights the necessity for tetragonal zirconia to withstand the demanding conditions of the oral environment over extended periods, ensuring its viability for durable and reliable dental restorations.

Fig. 3
figure 3

a Various crystal structures of zirconia with temperature, b phase diagram zirconia with yttria stabilization, c crystal structure of yttria doped zirconia [27]

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3.2 Stabilization of zirconia (ZrO2) for dental applications

The stabilization of zirconia is crucial to maintaining the tetragonal phase at room temperature, which offers superior mechanical properties [15, 17, 18]. Stabilizers like yttria (Y2O3), alumina (Al2O3), calcia (CaO), ceria (CeO2), or magnesia (MgO) are incorporated into the zirconia matrix [19,20,21,22,23,24]. This addition not only prevents the undesirable transition to the monoclinic phase but also enhances mechanical properties [25]. Among these, yttria (Y2O3), the most established and fully recorded dopant for stabilizing the tetragonal phase, results in the replacement of Zr4+ ions with Y3+ ions in the lattice, creating oxygen vacancies essential for preserving electrical neutrality (Fig. 3c) [26]. This substitution disrupts the crystal lattice, which helps to stabilize the tetragonal and cubic phases, thus preventing the spontaneous transformation to the monoclinic phase. The type and amount of stabilizer define zirconia’s categorization into: (1) Fully stabilized zirconia (FSZ), with the highest stabilizer content (above 8 mol%), yielding a cubic structure without phase transitions up to 2500 °C, (2) Partially stabilized zirconia (PSZ), which incorporates a modest stabilizer amount for a mix of tetragonal and cubic phases at room temperature, and (3) Tetragonal zirconia polycrystals (TZP) with low yttria content (2 ~ 3 mol%) that secures the tetragonal phase at room temperature [27]. Y-TZP, with varying levels of yttria, displays distinct properties, making each specification uniquely suitable for particular applications, as summarized in Table 1.

Table 1 Comparative analysis of yttria content in tetragonal zirconia polycrystals: 3Y-TZP vs. 4Y-TZP vs. 5Y-TZP [28]
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Other stabilizers, such as magnesium oxide (MgO), cerium oxide (CeO2), and alumina (Al2O3) are utilized to enhance specific properties of zirconia. Magnesia-stabilized zirconia (MSZ), with MgO as a stabilizer, offers high-temperature stability and has applications in industrial ceramics [20, 29]. Its potential in dental implants is being recognized due to its ability to withstand temperatures up to 800 °C before the crystalline phase begins to degrade at around 1200 °C. Cerium-stabilized zirconia (CSZ), which incorporates CeO2, is valued for its exceptional resistance to thermal shock, making it suitable for applications exposed to high temperature changes [22, 23]. CSZ also demonstrates considerable fracture toughness and good resistance to low-temperature hydrothermal aging, although its hardness and strength are somewhat lower compared to 3Y-TZP. Additionally, zirconia can be reinforced with alumina (Al2O3) to create alumina-toughened zirconia (ATZ), offering superior hardness and wear resistance for applications where these properties are critical [19, 20]. It is important to consider the specific needs of the application and the esthetic expectations, balancing the mechanical requirements with the translucency and color that match the patient’s teeth.

4 Microstructure-esthetics-mechanical properties relationship in dental zirconia

4.1 Optical properties of zirconia

When selecting dental materials, both esthetics and mechanical strength play a crucial role in patient satisfaction and the long-term success of restorations. Zirconia has been a material of choice for its notable strength but has historically faced challenges in aesthetics due to its opacity. This opacity stems from the intrinsic light scattering within its polycrystalline structure. A key goal in dental material development has been to enhance the translucency of zirconia, bringing it closer to the natural appearance of teeth, and thereby broadening its application for esthetically critical dental procedures.

The optical behavior of dental ceramics, including zirconia, is influenced by light interaction—transmittance, absorption, and reflection [30]. These properties are quantitatively evaluated using parameters such as translucency (TP), light transmittance (T%), and contrast ratio (CR), which provide objective measures of a material’s ability to replicate the appearance of natural teeth. Typically, higher TP values indicate greater translucency, while lower CR values suggest less opacity. Additionally, color is systematically quantified using the Commission Internationale de l’Eclairage CIEL*a*b* values for color space or the CIEDE 2000 color difference formula [31]. In commercial yttria partially stabilized zirconia (Y-PSZ) materials, with a standard thickness of 1 mm, translucency and contrast varied between types. Commercial yttria partially stabilized zirconia materials demonstrate that increased yttria content can improve translucency. For instance, 3Y-TZP shows a TP of 24.0, while 5Y-PSZ registers a TP of 29.7, and a corresponding decrease in CR from 0.48 to 0.37, indicating a clear trend toward greater transparency with higher yttria levels [32].

Despite the theoretical potential for transparency indicated by zirconia’s band gap, practical applications reveal a more complex scenario where translucency is affected by numerous factors such as grain distribution, phase composition, porosity, and material thickness [33, 34]. These factors are intricately linked to the manufacturing process, which includes the sintering protocol and the incorporation of additives like yttria and alumina [35, 36]. While increased yttria traditionally meant a compromise in mechanical strength for the sake of translucency (as shown in Fig. 4a), new research suggests a less straightforward relationship, with certain compositions of yttria-stabilized zirconia displaying similar strength across different yttria percentages. Recent studies, including those by Fei Zhang et al., have investigated this relationship, showing that higher yttria contents do not uniformly lead to diminished strength, contradicting previous assumptions [37]. They demonstrated that increasing yttria content does not always result in lower strength, as observed with 3 mol% and 4 mol% yttria-stabilized zirconia having similar strength values. These findings indicate a nuanced interplay between yttria content, mechanical properties, and translucency, calling for a more sophisticated approach in optimizing zirconia for dental use, as shown in Fig. 4b–d. This complex relationship highlights the need for continued research and innovation, which will be further explored in upcoming discussions on the latest trends in zirconia research and development.

Fig. 4
figure 4

Comparative analysis of yttria-stabilized zirconia ceramics: a Flexural strength versus translucency parameter [35], b mechanical strength, c translucency, d crack growth resistance [37]

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Li Wang and colleagues’ research further explores this by comparing the mechanical properties, translucency, and aging stability of yttria partially stabilized zirconia (PSZ) ceramics, specifically those created using stereolithography-based additive manufacturing [38]. Their work contrasts 5 mol% yttria-stabilized zirconia (5Y-PSZ) with a mixed variant (3Y + 8Y-PSZ), revealing that a balance of tetragonal phase content and yttria concentration, as seen in the mixed variant, results in improved strength and toughness, albeit with some trade-off in translucency and aging resistance(as shown in Fig. 5). This also underscores the intricate balance between yttria and zirconia phases in determining the final properties of the material.

Fig. 5
figure 5

a CR values of two PSZ ceramics aged different time scale, b SEM images of the cross section of sample after aging for 20 h: (left) 5Y-PSZ and (right) 3Y + 8Y-PSZ, the darker area indicates the transformed phase change layer [38]

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Zhang et al. provides an in-depth analysis of how the yttria distribution and the balance between cubic and tetragonal phases in PSZ influence its translucency, strength, and aging stability [39]. They compared two samples with 5 mol% yttria-stabilized PSZ ceramics prepared by different processing routes: co-precipitated zirconia (T5Y) and bimodal grain sized powder mixture (B5Y). While both T5Y and B5Y contain the same amount of yttria stabilizer, the distribution of yttria within their structure is different, thus resulting in differing mechanical properties, translucency, and stability. T5Y, with its more uniform yttria distribution and microstructure, exhibits better translucency and aging stability compared to B5Y (as shown in Fig. 6). It also shows the impact of aging on the ceramics, demonstrating that a more balanced distribution of yttria and a careful control of the cubic and tetragonal phases are essential for the long-term performance of dental zirconia ceramics.

Fig. 6
figure 6

a XRD patterns of T5Y and B5Y, b grain size distributions, c translucency, d Biaxial strength with fractured samples [39]

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Furthermore, the granular size and phase distribution within zirconia are critical, as demonstrated in research by Roitero et al., which indicates that nanometric grain sizes, specifically with 1.5 mol% yttria and grain sizes around 110 nm, can enhance mechanical strength and resistance to degradation, shown in Fig. 7. The improvements are likely linked to the synergistic effects of reduced oxygen vacancy concentrations, attributed to nanostructuring and alumina integration [40].

Fig. 7
figure 7

a SEM images and particle distribution showing nano sized in n3Y and n1.5Y samples, b optical images of a 1 mm thick sample, c LTD kinetics of the different zirconia grades [40]

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Sintering conditions, such as temperature and duration, also play a pivotal role in defining the zirconia’s final translucency, with optimal conditions fostering grain growth while minimizing defects and porosity [41]. As the sintering temperature increases, crystal grain growth is promoted (5YSZ > 4YSZ > 3YSZ), and pores are eliminated, resulting in fewer crystal grain boundaries. Consequently, light scattering is reduced, which can increase translucency, shown in Fig. 8. However, too high a sintering temperature may lead to the nucleation of microcracks at the grain boundaries, which can decrease translucency. Therefore, selecting an appropriate sintering temperature is crucial for achieving the optimal optical properties of zirconia [35, 42, 43].

Fig. 8
figure 8

a SEM images of 3YSZ, 4YSZ and 5YSZ, b grain sizes and c translucency parameter as function of sintering temperature [35]

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Studies exploring advanced sintering protocols show how different sintering methods impact the material’s density and translucency (Fig. 9). Conventional sintering (CS) may allow for better-controlled grain growth and higher density, whereas speed sintering (SS) and high speed sintering (HS) may risk incomplete densification and uneven grain growth, which can detrimentally impact the material’s translucency and strength [44].

Fig. 9
figure 9

Influence of sintering protocols on microstructure, translucency, and mechanical strength of yttria-stabilized zirconia ceramics a SEM image, b translucency parameter, c biaxial flexural strength under different sintering protocols [44]

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Manufacturers often issue specific sintering guidelines—temperatures and times—based on the type of zirconia powder used for block production. These recommended parameters play a pivotal role in defining the grain size distribution within the zirconia’s final microstructure. The work by Vult von Steyern et al. underlines the importance of precise control over the sintering process in the production of YSZ ceramics (using four different YSZ-powders: 3YSB-E® (3E), 3YSB- C® (3 C), Zpex4® (4Y), and Zpex®smile (5Y) (Tosoh, Tosoh Corporation, Tokyo, Japan), with a clear indication that even small deviations (± 5%) from recommended sintering temperatures (TR) can lead to significant changes in the material’s physical properties [45, 46]. Specifically, lower sintering temperatures (TL) resulted in decreased strength for some YSZ materials, shown in Fig. 10.

Fig. 10
figure 10

a Phase fraction (%), b average spectral transmittance, c backscattered SEM images of all material groups [45]

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Taken together, these studies collectively highlight the nuanced interplay between zirconia’s microstructural attributes and its optical and mechanical properties. They underscore the importance of controlled yttria distribution, phase balance, grain size, and precise sintering to achieve the desired performance in dental applications. Continued innovation in manufacturing and sintering techniques is critical to advancing the field of dental material science.

4.2 Mechanical properties of zirconia: strength, toughness

Zirconia’s exceptional mechanical properties, particularly its high strength and toughness, are essential for its role in specialized applications, notably dental restorations, where its mechanical reliability is just as important as esthetics. The core of its durability is the transformation toughening mechanism, where the structural change from the tetragonal to the monoclinic phase at microcrack locations leads to energy absorption and compressive stress development. This enhances fracture toughness and is influenced by several factors, including yttria content, phase composition, and sintering conditions that dictate microstructural features like grain size and porosity. Notably, monolithic Y-TZP displays remarkable fracture toughness (5–9 MPa√m), high flexural strength (850–1250 MPa), Vickers hardness (10–12 GPa), and a robust elastic modulus (190–215 GPa) [47, 48]. It also demonstrates exceptional biocompatibility and wear resistance, making it well-suited for a variety of applications beyond dentistry. Innovations in materials processing are crucial, focusing on optimizing yttria content and sintering protocols to maximize these mechanical properties for demanding applications.

Research by Martin Trunec et al. delves into optimizing 2Y-TZP ceramics, demonstrating that fine-tuning the microstructure through methods like gelcasting and pressureless sintering can result in ceramics that rival the biaxial strength of 3Y-TZP while offering significantly enhanced fracture toughness, shown in Fig. 11c, d. This is largely due to an increased transformation toughening effect, with these advancements being made without adding alumina and maintaining a critical grain size of approximately 100 nm, shown in Fig. 11a, b [49].

Fig. 11
figure 11

TEM micrograph of a raw (SZ-2Y) and b calcined (SZ-2Y-c) zirconia powders, indicating nanoscale character with around 100 nm most crystallites, c Weibull plots of the best strength, d total length of indentation cracks obtained for SZ-2Y, SZ-2Y-c, and SZ-3Y samples [49]

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Additionally, the work of Bettina Osswald and Frank Kern presents a novel approach to enhancing the toughness of TZP materials without compromising low-temperature degradation (LTD) resistance, through the co-stabilization method using 1.5 mol% ytterbia and 1.5 mol% praseodymia, followed by hot-pressing the powder at temperatures between 1300–1400 °C under 60 MPa axial pressure [50]. The result was an ultrafine microstructure with grain sizes ranging from 130 to 200 nm (as shown in Fig. 12a), exhibiting high toughness (11–12 MPa√m) and moderate strength (600–950 MPa), as shown in Fig. 12b, c. The study reveals that the mechanical properties of this co-stabilized TZP, including a combination of moderate strength and high toughness, are due to the transformation toughening mechanism. This mechanism is evident from the nonlinear stress–strain curves and the presence of transformation bands before fracture. The transformation stress, determined from these bands, was found to increase with the sintering temperature, being 100–250 MPa lower than the bending strength.

Fig. 12
figure 12

a SEM images after sintered at different temperatures, b fracture toughness of 1.5Yb-1.5Pr-TZP, c 3-pt bending strength and the residual strength of HV10 indented samples as well as micrographs of specimen sintered at different temperature [50]

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These studies collectively highlight the intricate interplay between zirconia’s microstructural attributes and its performance. By achieving a balance between mechanical properties and translucency, zirconia can better mimic the natural appearance of teeth, making it an increasingly preferred material for dental restorations that require both strength and esthetic appeal. Moving forward, the latest trends in research and development within this field are expected to focus on further refining zirconia’s properties to meet the evolving demands of dental applications.

5 Addressing the challenges

5.1 Low-temperature degradation (LTD) process of zirconia

Low-temperature degradation (LTD), also known as aging, adversely affects zirconia ceramics’ (ZrO2) mechanical properties and longevity by facilitating a phase transformation from tetragonal to monoclinic at relatively low temperatures (typically between 200 and 300 °C) when exposed to humid environments. This process is driven by the dissociative adsorption of water molecules on the zirconia surface, which fills oxygen vacancies and encourages yttrium ion precipitation, destabilizing the tetragonal phase (Fig. 13d) [51]. Unlike the beneficial transformation in toughening mechanisms (Fig. 13a), LTD’s spontaneous phase change leads to surface degradation, microcracking, and strength reduction due to a volume increase between 3 and 5%, thereby increasing brittleness [15, 17, 52,53,54]. To mitigate LTD, effective strategies encompass modifying the chemical composition of zirconia, refining manufacturing techniques to improve material density and uniformity, and implementing surface treatments designed to block moisture infiltration [20, 55]. Despite these efforts, LTD has been observed in 3Y-TZP surfaces within just six months in oral environments, with the degradation increasing significantly over time (Fig. 13). Additionally, the dislodgement of monoclinic particles under abrasive stress raises concerns about the potential release of zirconia nanoparticles into the body, emphasizing the importance of addressing LTD for the material’s durability and safety [56].

Fig. 13
figure 13

a Schematic illustration of the low-temperature degradation process. Analysis of the prostheses at different times of clinical use b SEM images of MS + and MS- area after 5 years, c landmarking of the areas of the zirconia crown, d time evolution percentage measured using Raman spectroscopy [56]

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An effective approach to improving hydrothermal stability and translucency involves co-doping Y-TZP ceramics with oxides like Al2O3 and La2O3. These dopants, particularly those with a larger ionic radius than Zr4+ such as La3+, can alter the grain boundary chemistry, stabilizing the tetragonal phase essential for mechanical robustness. The dopants’ tendency to segregate at grain boundaries changes the kinetics of phase transformations from nucleation-driven to growth-driven, potentially slowing the LTD process [22, 57,58,59]. Fei Zhang’s research highlights the advantages of adding La2O3 to Al2O3-doped 3Y-TZP, achieving an optimal balance between translucency and hydrothermal stability without compromising mechanical strength [60]. Specifically, the introduction of 0.2 mol% La2O3 in conventional Al2O3-doped 3Y-TZP resulted in a unique combination of high translucency (42% increase compared to conventional 0.25 wt.% alumina-doped 3Y-TZP) and superior hydrothermal stability (no transformation up to 120 h of hydrothermal aging at 134 °C), while maintaining excellent mechanical properties. However, beyond a critical concentration, the dopants segregated from the particles is detrimental effect of accelerating LTD (Fig. 14).

Fig. 14
figure 14

Characterization of yttria-stabilized zirconia (YSZ) doped with various dopants a microstructural analysis, showing added dopants are segregated at the grain boundaries, b translucency parameter, c mechanical strength [60]

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This comprehensive overview emphasizes the necessity of addressing LTD through a multifaceted approach that includes careful material selection and dopant incorporation to ensure the durability and safety of zirconia ceramics in dental and other applications.

5.2 Discoloration issue of zirconia

Another issue that cannot be overlooked when using zirconia in dental applications is the discoloration observed in dental zirconia ceramics upon exposure to ultraviolet (UV) irradiation. Kurihara et al.’s study examines the discoloration of dental zirconia ceramics under ultraviolet (UV) irradiation under UVC light at 254 nm wavelengths (Fig. 15) [61]. This discoloration varies with the yttria content; 3 mol% yttria-stabilized zirconia (3YZ) exhibits more significant color change than 5 mol% yttria-stabilized zirconia (5YZ), attributed to the excitation of electron-trapping oxygen vacancies [62]. Interestingly, this discoloration is reversible through heat treatment, suggesting a temporary effect but emphasizing the need for dental practitioners to manage UV exposure in zirconia-based restorations carefully.

Fig. 15
figure 15

Effects of UV irradiation on the color and UV transmittance of 3YZ and 5YZ Zirconia a non-colored specimens, b tooth-colored specimens, c representative transmittance spectra, d transmittance at 254 nm UV light [61]

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5.3 Advanced manufacturing techniques

The manufacturing technique significantly affects zirconia’s performance in dental applications [41, 45]. Wang et al. have introduced a novel one-step flash sintering (FS) method that uses an electric field to densify alumina–zirconia ceramics at much lower temperatures than conventional methods [63]. This technique, distinct from traditional sintering, employs amorphous precursor powders instead of crystalline ones, achieving densification at furnace temperatures between 849–882 °C, which is significantly lower than standard sintering temperatures that exceed 1500 °C. The applied electric field reduces the viscosity of the amorphous powder, thereby enabling liquid-phase sintering at reduced temperatures. Moreover, the applied electric field generates Joule heating, which promotes the phase transition from metastable to stable, thereby facilitating faster atomic migration and the overall sintering process (Fig. 16a). Yuqing Lu and colleagues investigated the impact of surface finishing and layer orientation of the printing layers on the fatigue behavior of 3Y-TZP fabricated by stereolithography (SLA), compared to traditionally milled zirconia, shown in Fig. 16b [64]. Their research reveals that 3D printing technique could produce zirconia with high fatigue strength when the printing layers are parallel to the tensile surface and the surfaces are polished, recommending that design and printing strategies should consider load orientations for clinical applications. This finding is crucial for understanding how to design and build 3D-printed dental restorations to withstand the mechanical stresses experienced in the oral environment.

Fig. 16
figure 16

a Diagrammatic drawing particle rearrangement during crystallization [63], b illustration of the printing layer orientations in dental zirconia restorations and associated 3-point bending test setup [64]

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Advanced CAD/CAM technology has revolutionized the creation of dental crowns by employing layered zirconia with varying levels of translucency [5]. This method accurately replicates the natural gradation of human teeth, from the more transparent cutting edge to the opaque base, ensuring a more lifelike appearance. Techniques such as multi-slurry tape-casting 3D printing further enable the production of prosthetics tailored to individual dental anatomy and aesthetic desires [65]. The paper describes the successful use of this technology to create a translucent graded dental crown, demonstrating the printer’s capability to fabricate complex structures with precise control over material properties, as shown in Fig. 17. The dental market continues to expand with the introduction of new CAD/CAM materials designed to meet the growing demands for aesthetic and durable restorations. However, it’s crucial to note that the layer thickness can vary considerably across different manufacturers’ products. Additionally, despite the technological advancements, clinical evidence supporting the use of these specific graded CAD/CAM blanks remains sparse.

Fig. 17
figure 17

a Composition and dimension gradient tooth, b the benchmark dental crown model, c hardness Vickers, d flexural strength of different Y2O3 and zirconia powder composition [65]

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6 Conclusion

6.1 Main findings and conclusion

This review examines the intricate relationship between the microstructural features of zirconia and its implications for both aesthetic and mechanical performance, drawing on recent significant studies. The main findings highlight several key aspects:

  1. 1.

    Traditionally, the addition of yttria to zirconia was thought to improve translucency at the expense of mechanical strength. However, recent studies indicate that the relationship between yttria content and the mechanical properties of yttria-stabilized zirconia (YSZ) is more complex than previously believed. Some YSZ compositions exhibit comparable mechanical strengths despite varying levels of yttria. This suggests that the interplay between yttria content, mechanical properties, and translucency is more complex than previously thought, highlighting the need for further research to optimize zirconia’s composition for dental applications.

  2. 2.

    The enhancement of zirconia’s mechanical strength and toughness is intricately linked to controlling its microstructure, which is significantly influenced by the yttria concentration and the sintering techniques utilized. The precise regulation of these microstructural characteristics is a key factor in determining the material’s overall properties and its effectiveness in applications. Therefore, adhering to Manufacturers provide detailed sintering guidelines is crucial as they directly impact the grain size distribution within the zirconia’s final microstructure, a critical factor in determining the material’s overall properties and performance.

  3. 3.

    Low-temperature degradation (LTD) presents a notable challenge, potentially undermining zirconia’s mechanical properties and longevity via spontaneous phase transformations. Addressing this issue requires careful optimization of the microstructure and chemical composition.

  4. 4.

    Zirconia’s susceptibility to aesthetic changes under ultraviolet light and certain environmental conditions, which can impact color stability, necessitates research focused on understanding and mitigating discoloration without compromising the material’s strength.

  5. 5.

    Advancements in manufacturing techniques, such as flash sintering and 3D printing, are pioneering methods to refine zirconia’s microstructure, thereby enhancing its aesthetic and mechanical qualities. These innovations aim to develop dental restorations with better translucency, strength, and environmental degradation resistance.

The ongoing research and development in these areas are critical to addressing the current limitations, ensuring zirconia’s status as a preferred material for durable and esthetically appealing dental restorations.

6.2 Research limitations and suggestions for future research

For the utilization of zirconia in dental applications, various issues still need to be considered.

  1. 1.

    It stresses the necessity of evaluating zirconia’s mechanical properties under conditions that closely replicate the oral environment, such as through thermal cycling and artificial aging, to ensure its durability and functionality.

  2. 2.

    Ongoing research is necessary to enhance zirconia’s biocompatibility and long-term stability, ensuring it remains safe and effective when used within the human body.

  3. 3.

    The development of zirconia composites with additional functionalities like antimicrobial properties, self-healing capabilities, and resistance to tartar deposition is highlighted as an essential research area.

  4. 4.

    The challenge of balancing toughness, resistance to low-temperature degradation, and translucency is noted. Achieving the optimal mix of these properties is crucial for the material’s clinical success.

  5. 5.

    The potential of advanced manufacturing techniques, including CAD/CAM technologies and 3D printing, to improve customization, aesthetics, and fit of dental prosthetics is significant.

  6. 6.

    Understanding how different manufacturing processes affect zirconia’s microstructure, and thus its mechanical and aesthetic properties, is crucial. Comparative studies could lead to better production practices.

  7. 7.

    The text concludes with the importance of long-term clinical studies to investigate zirconia’s biocompatibility and the potential release of particles, which would offer deeper insights into its safety and efficacy.

While zirconia ceramics have significantly advanced dental materials, several challenges remain. Future research should explore advanced surface modification techniques, such as laser surface texturing and plasma spraying, to enhance the adhesion and bioactivity of zirconia. The development of hybrid materials combining zirconia with other biocompatible substances could provide enhanced mechanical properties and aesthetic qualities. Additionally, the use of bioactive coatings, such as hydroxyapatite and bioactive glass, may further improve the osseointegration and longevity of zirconia-based implants. Exploring these emerging technologies and novel materials will be crucial in overcoming the current limitations and advancing the field of dental materials.