Investigation of thermal shock resistant in three kinds thermal barrier cerium oxide coating (CeO2) with MCrAlY intermediate layer
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The study aimed to compare the thermal shock behavior of three types of thermal barrier coatings (TBC) containing two and five layers. The substrate of the coatings, like as industrial samples, was selected from IN738LC superalloy. The first type was a two-layer TBC produced from CoNiCrAlY and CeO2 as a bond and top layers, respectively. The second type was a common five-layer TBC with a bond layer of CoNiCrAlY, a top layer of CeO2 and three intermediate layers composed of three mixing kinds of CeO2 + CoNiCrAlY. The third type was composed of a top layer of nano-structured CeO2, and the other four down layers were similar to the second type. To thermal shock test, the samples were kept at 1100 °C for 5 min and quenched in 20–25 °C water. The test was continued until all the samples were destructed. The sample was considered destructed when 20% of the coating surface was detached. To evaluate the microstructure of the samples, SEM and FESEM were used. Finally, the thermal shock lifetime of the five-layer TBC was 1.6 times higher than the two-layer TBC, and by making the top nano-structured layer in five-layer TBC, the lifetime was enhanced approximately 14%.
KeywordsThermal barrier coating (TBC) Functionally graded materials (FGM) Plasma spray Thermal shock Cerium oxide (CeO2)
Increasing high-temperature gas turbines’ efficiency and the performance has always been a concern due to the temperature limitations and physical and mechanical stability of the superalloys [1, 2, 3]. Thermal barrier coatings are made from the materials with a low heat transfer coefficient to transfer a lower amount of heat to the substrate. In this way, operating temperature of the components and their life span could be increased. In the thermal barrier films, the bond provided favor oxidation resistance for the underlying base metal [4, 5, 6, 7].
In new systems, the bond coat is generally one of the MCrAlY (M = Ni and/or Co) coating group. At higher temperatures, such systems developed a thermally grown oxide (TGO) layer in the metal–ceramic interface, which protects the substrate metal surface from oxidation [8, 9]. The top layer provides thermal insulation. This ceramic layer, which is usually made by zirconia (ZrO2), shows lower thermal conductivity. A stabilized zirconia is used in order to prevent the phase transformation of zirconia and also eliminating the volumetric changes [10, 11, 12, 13, 14].
Typically, yttria-stabilized zirconia (YSZ) films are mostly used as thermal barrier. The alumina layer, as a thermally grown oxide (TGO), results high strain energy in this layer, which is due to the thermal unconformity between the TGO and the base metal. The stress created by the excessive growth of TGO is the result of increasing alumina volume due to oxidation aluminum in the bond coat. This strain energy can lead to the TGO distortion, forming and develop cracks, and eventually destroying the thermal barrier coating. Such structures destroyed with the lamination in the coatings [15, 16, 17, 18, 19]. Cerium oxide (CeO2) is one of the lanthanide oxides which are well known for its potential to oxidation–reduction. Cerium oxide nano-particles are the oxide form of rare cerium element that can imitate dismutase and catalase superoxide activity, because of changing their vacancy of surface oxygen and valence array. So, these nano-particles can be used as scavengers of reactive oxygen species (ROS) in the field of thermal coatings [20, 21, 22, 23].
One of the reasons for coating lamination is the differential between the thermal expansion coefficients between layers. One way to reduce this strain is forming the functionally graded material layers (FGM). Functionally graded thermal barrier coatings (FG-TBCs) which have several layers rather than two coats have been proposed to eliminate such problems. In FG-TBC, a gradual change was observed in the layers composition, so this variation makes gradation in the coating layers [function]. This gradual change in the composed of the layers reduces the thermal unconformity strain between layers; this type of coatings has better properties than the dual-layer ones. These properties included higher strength and hardness, greater adhesion of the coating to the substrate and increased the thermal shock resistance. Another method to improve thermal barrier coatings and their quality and life span in different services is adopting nanostructure layers in them. Various researches indicated that in the nano-structured thermal barrier coatings, some properties have been recognized such as strength, hardness, oxidation resistance and wear resistance. The nano-structured films are developed from nano-powders spraying on the surface of the substrate [24, 25, 26]. In these nano-particles, one dimension of the powders could reach less than 100 nm. The experimental relationships in the field of physical and mechanical properties indicated a high potential to improve these properties by reducing the particle size [27, 28, 29].
So far, extensive researches have been done to compare the thermal shock behavior of TBC and FG-TBC coatings, as well as the conventional thermal barrier coatings versus the nano-structured coatings. For example, in a study, in order to compare the mechanical and microstructural properties of dual-layer TBC coatings in both the conventional and the nano-structured types, the samples of these coatings were applied on the Inconel substrate using the atmospheric plasma spray method. The properties of the coatings were investigated by the adhesion test of coatings, as well as the phase and microstructural analysis of the coating sections. The results indicate the improvement of the nano-structured TBC coating properties compared to the conventional TBC coating . In another study, the comparison of the conventional and the nano-structured dual-layer TBC coatings was performed in terms of thermal cyclic behavior of them. Increments in the life span of the nano-structured coatings compare to the conventional coatings were reported as the results of this investigation . In another research, in order to compare the thermal shock behavior of the dual-layer TBC coatings in both conventional and nano-structured types, the thermal shock test was performed on these specimens in the cycles at 950 °C for 5 min, followed by the rapid quench in water. The results of this test show that the life span of the nano-structured coating is approximately 1.5 times of the conventional coating. To evaluate the quality and the durability of the nano-structured thermal barrier coatings, most researchers have used a nano-structured ceramic layer in the dual-layer TBC thermal insulation coatings [32, 33].
In this study, the nano-structured coatings in the top ceramic layer of five-layer FG-TBC thermal insulation coating were used to investigate the quality and the life span of the nano-structured thermal barrier coatings. In order to analyze the properties of the nanostructure layer in this coating, in addition to the five-layer nanostructure thermal barrier coating (FG-TBC), the five-layer conventional thermal barrier coating and the dual-layer conventional thermal barrier coating (TBC) were evaluated. By performing a thermal shock test on these three types of the thermal barrier coatings, it is possible to compare the fracture strength of these coatings in the shock test.
2 Experimental activities
Properties of IN738LC superalloy
Proportion by mass (%)
Applied parameters of plasma spraying for the coating of each layer
Argon flow rate (SCFH)
Hydrogen gas flow rate (SCFH)
Argon powder carrier gas
Spray distance (cm)
Powder feed rate (Lbs/h)
To thermal shock test, for each coating type, triplicate samples were produced, and each sample was kept in a furnace at 1100 °C for 5 min and then rapidly quenched in water at 20–25 °C. After bringing out and drying the samples, they were put again in the furnace, and the steps mentioned above were repeated. The repeating cycles were continued until all the samples were destructed. The fracture cycle number was reported when the sample was destroyed. The sample was considered destructed when it delaminated, and 20% of its coating surface was destroyed. After destructing the sample, the test was stopped, and the number of applied thermal shock cycles was reported as a fracture cycle number. After destroying all the samples based on the aforementioned criterion and recording their fracture cycle numbers, the test was accomplished.
3 Results and discussion
In sample B and C, the presence of the intermediate layers created a moderate concentration incline between the top and bond layers. Through the layers, although it cannot be possible to determine the interface of the layers clearly, the zone of each layer can approximately be distinguished. As it can be seen in the FESEM micrographs of the coating surfaces, the coated surface of sample A and B are similar, because of the similarity in their top layers, but the coated surface of sample C is different compared to the others, because of its top nano-structured ceramic layer.
Fracture cycle number of the samples determined by the thermal shock test
Types of coating A
Types of coating B
Types of coating C
In the common plasma sprayed ceramic coating, the crack grew through the inter-splat boundaries. However, in the nano-structured coating, because the nano-domains pinned the inter-splat boundaries, the crack growth was prevented by the domains, and its growth direction deviated. So, it was expected that the nano-structured TBC compared the common one can tolerate more thermal cycles. The results obtained from this study also confirmed that the creation of the top nano-structured ceramic layer in FG-TBC enhanced the thermal shock resistance of the coating.
In other researches similar to this study, the same results were mentioned. For example, in study , to compare the thermal shock behavior of a common two-layer TBC and a nano-structured FG-TBC, a thermal shock test was done by keeping the sample at 950 °C for 5 min and quenching them in water. The obtained results were shown that the lifetime of the nano-structured coating was approximately 1.6 times more than the common coating. In addition, in research , thermal shock resistance was evaluated for a common TBC and a nano-structured FG-TBC. The samples were investigated in a thermal shock test by keeping them at 1020 °C and quenching in water. Its results mentioned that the thermal shock resistance of the nano-structured TBC was higher than the common one.
The thermal shock lifetime of the five-layered FG-TBC was approximately 1.6 times higher than the two-layer TBC. The observed increment in the lifetime confirmed that the creation of FGM structure in TBC enhanced quality and lifetime of the coating. Moreover, the developing top nano-structured ceramic layer was increased around 14% of the thermal shock lifetime of the films. Enhancements indicated that creating top nano-structured ceramic layer improved the quality and lifetime of the coating.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no known conflicts of interest.
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