IntroductionZirconium dioxide (ZrO₂) ceramics have superior properties such as high hardness, high thermal expansion coefficient and low thermal conductivity, which are widely used in high-temperature components like aircraft engines, gas turbines and combustion power plants. However, ZrO₂ ceramics in the use can be CMAS (i.e., CaO, MgO, Al₂O₃ and SiO₂) and other small particles corrosion, resulting in the decrease of ZrO₂ ceramics performance. The problem of CMAS corrosion becomes a challenge. The technologies used to protect the CMAS corrosion of ZrO₂ ceramics are the use of a non-permeable protective layer, the use of a top layer of laser glazing treatment, a thermal spray nanostructure coating and a rare-earth co-doped coating. Although these methods have a certain anti-CMAS corrosion effect, they still have some shortcomings of complex preparation process and high cost. To further investigate and improve the high-temperature corrosion resistance of ZrO₂ ceramics, Al₂O₃-ZrO₂ prestressed ceramics were prepared with Al₂O₃ as a coating and ZrO₂ as a matrix. The flexural strength at different temperatures and the behavior of CMAS penetration and element diffusion were investigated via comparative analysis of Al₂O₃-ZrO₂ prestressed ceramics and ZrO₂ ceramics. The effect of prestressed enhancement, the CMAS penetration and the corrosion mechanism of Al₂O₃-ZrO₂ prestressed ceramics against CMAS at a high temperature were analyzed.MethodsThe thermal corrosion experiment was carried out in a high-temperature sintering furnace. The samples of ZrO₂ ceramics and Al₂O₃-ZrO₂ prestressed ceramics with CMAS powder were heated at different temperatures (i.e., 1 200, 1 250, 1 300, 1 350 ℃ and 1 400 ℃), respectively, for 10 h, and then cooled in the furnace. In the analysis of corrosion resistance of ZrO₂ ceramics and Al₂O₃-ZrO₂ prestressed ceramics, the mechanical properties of ZrO₂ ceramics and Al₂O₃-ZrO₂ prestressed ceramics before and after corrosion were tested by a model C45 machine controlled electronic universal testing machine. During the test, the span of 30 mm and the loading rate of 0.5 mm /min were set, and the data of 5 samples in each group were collected to obtain the three-point flexural strength of the samples affected by corrosion. The corrosion and morphology characteristics of the sample in a macro-scale were determined by a model KEYENCE VHX-970F optical microscope. The depth of CMAS corrosion of ZrO₂ ceramics and the change of the state of Al₂O₃ coating at different corrosion temperatures were analyzed via adjusting different magnifications. The microstructure of the sample was characterized by a model QUANTA 250FEG scanning electron microscope in order to further analyze the corrosion mechanism of CMAS. The morphology and distribution area of corrosion products generated by Al₂O₃-ZrO₂ prestressed ceramics were determined, and the corrosion mode of ZrO₂ ceramics was analyzed. Also, the element composition ratio and distribution of the sample cross-section were analyzed by an EDS scanning system. The phase of Al₂O₃ ceramic particles corroded by CMAS was analyzed by a model Rigaku SmartLab SE X-ray diffractometer.Results and discussionBefore corrosion, the flexural strength of Al₂O₃-ZrO₂ prestressed ceramics reaches (1 038±41) MPa, which is 35.6% higher than that of ZrO₂ ceramics. After CMAS corrosion, the flexural strength of Al₂O₃-ZrO₂ prestressed ceramics decreases with the increase of corrosion temperature, but it is still higher than that of ZrO₂ ceramics. This is because the residual stress formed during the cooling process enhances the strength of ZrO₂ ceramics after sintering Al₂O₃ coating and ZrO₂ ceramics.In the high-temperature service environment, the EDS analysis results show that CMAS exists an element diffusion in ZrO₂ ceramics. Based on the analysis of the cross section of ZrO₂ ceramics after corrosion, the corrosion mode of ZrO₂ ceramics is a grain boundary corrosion. The penetration depth of CMAS gradually increases from 24 μm to 143 μm with the increase of corrosion temperature, thus bringing a serious harm to ZrO₂ ceramic components. Al₂O₃ coating in Al₂O₃-ZrO₂ prestressed ceramics effectively blocks a direct contact between CMAS and ZrO₂ ceramics, avoids the diffusion of elements Ca, Mg, Al and Si in CMAS to ZrO₂ ceramics, and effectively protects the microstructure of ZrO₂ ceramics. The results indicate that Al₂O₃ coating can improve the strength and CMAS corrosion resistance of ZrO₂ ceramics.Based on the analysis of the corroded section of Al₂O₃-ZrO₂ prestressed ceramics as well as the analysis of EDS and XRD results, Al₂O₃ coating reacts with CMAS at a high temperature. The high melting point compounds calc feldspar (CaAl₂Si₂O₈), spinel (MgAl₂O₄) and a small amount of calc aluminite (Ca₂Al₂SiO₇) are generated. The diffusion channel is clogged and the penetration depth of CMAS is greatly reduced due to these substances with a high melting point and a good chemical stability as well as the increased viscosity of molten CMAS, thus having a positive effect on preventing CMAS corrosion. In addition, the compressive stress in the Al₂O₃ coating can also inhibit the expansion of cracks, prevent the deep oxidation effect entering through cracks, and improve the service life of ZrO₂ ceramic components in CMAS corrosion environment.ConclusionsThe mechanism of resistance of Al₂O₃-ZrO₂ prestressed ceramics to CMAS corrosion at a high temperature was investigated, and ZrO₂ ceramics were taken as a reference sample. The results showed that in Al₂O₃-ZrO₂ prestressed ceramics, Al₂O₃ coating with compressive stress could effectively improve the flexural strength of ZrO₂ ceramics, and help to slow down the action of CMAS corrosion, thus having the superior resistance to CMAS corrosion. This was mainly attributed to that the compressive stress of the surface layer of the material could improve the strength and toughness of the ceramic, effectively inhibiting the crack propagation; and at high temperatures, Al₂O₃ coating in Al₂O₃-ZrO₂ prestressed ceramics could react with CMAS, producing a high melting point calcium feldspar, spinel and a small amount of calcium-aluminum feldspar at the interface, and forming a stable and dense reaction layer, which inhibited the continuous penetration of CMAS melt.