Study On Properties And CMAS Corrosion Resistance Mechanism Of Rare-earth Tantalate Novel Thermal Barrier Material

The traditional thermal barrier material yttria-stabilized zirconia（YSZ）possesses low service temperature and poor CMAS corrosion resistance.Hence,the novel thermal barrier materials should be developed to improve the aero engine’s ultimate service temperature and performance.This paper prepared the YTa O₄,Gd Ta O₄,and Yb Ta O₄ to research their performances,including microstructure and chemical composition,thermal and mechanical properties,wettability,and CMAS corrosion resistance.On this basis,the modified rare-earth tantalate materials were developed.The main research results are as follows:The solid-state reaction method is used to prepare the YTa O₄,Gd Ta O₄,and Yb Ta O₄,the densification of all original samples is more than 90%,and the ferroelastic domain can be observed on the surface.Gd Ta O₄ and Yb Ta O₄ can keep the phase stability after 150 h sintering at 1350℃,YTa O₄ can keep the phase stability after 50 h sintering at 1350℃,while the phase structure transforms from m phase to m’phase after 100 h sintering.The phase structure of traditional YSZ has transformed from t’phase to m phase after 20 h sintering at 1350℃.The rare earth tantalates show better phase stability than traditional YSZ.The results show that the melting point of YTa O₄,Gd Ta O₄,and Yb Ta O₄is more than 2000℃,and their CTE at 1350℃is 13.5×10^-6 K^-1,14.1×10^-6 K^-1,and 9.2×10^-6 K^-1.The Young’s modulus of YTa O₄,Gd Ta O₄,and Yb Ta O₄ is 92.54 GPa,89.41 GPa,and 88.98 GPa,respectively,which is lower than YSZ（145.04 GPa）.In addition,the load at fracture of YTa O₄,Gd Ta O₄,and Yb Ta O₄ is up to 70%,88%,and 41%of traditional YSZ,respectively.The wettability results show that the change processes of CMAS on the different substrate surfaces are the same,which is decided by the CMAS intrinsic property instead of the types of the substrate.When the temperature rises to 1350℃,the CMAS cylinder shrinks firstly,transforms into the sphere secondly,and spreads on the substrate finally.When the temperature is higher than the melting point of CMAS,the primary spreading process is from 1250℃to 1300℃,lasting about330s.When the temperature reaches 1350℃and lasts for 5min,the equilibrium contact angle of CMAS on YTa O₄,Gd Ta O₄,Yb Ta O₄,and YSZ surface is 14.9°,13.7°,12.8°,and 8.5°,and the spreading radius is 5.16 mm,5.46 mm,5.95 mm,and7.24 mm.The results show that the wettability of the rare-earth tantalates is worse than YSZ.According to the contact angle and spreading radius results,the liquid CMAS can easily adhere to the YSZ surface and destroy its substrate than the rare-earth tantalates.In the same situation,the penetration depth of CMAS on YTa O₄,Gd Ta O₄,and Yb Ta O₄ is 0.9971,0.9824,and 0.9860 times than that of YSZ,the spreading area of CMAS on YTa O₄,Gd Ta O₄,and Yb Ta O₄ is 0.51,0.57,and 0.67times than that on YSZ.In this work,the new CMAS corrosion resistance experiments are designed to assess the CMAS corrosion resistance of YTa O₄,Gd Ta O₄,and Yb Ta O₄ at different corrosion temperatures.The results show that CMAS on the YSZ surface has entirely disappeared after 4 h corrosion at 1250℃,and the reaction layer thickness is～480μm.In comparison,there are residual CMAS on YTa O₄,Gd Ta O₄,and Yb Ta O₄surface after 10 h CMAS corrosion at 1250℃.When the corrosion time is0.5 h and 4 h,there are crystallization phenomena in residual CMAS,and the main crystallization phase is Ca Mg Si₂O₆.Nevertheless,the crystallization phenomenon disappears entirely when the corrosion time rises to 10 h,and the reaction layer thickness of YTa O₄,Gd Ta O₄,and Yb Ta O₄ are 12μm,11μm,and 5μm,respectively.When the corrosion temperature rises to 1350℃,the residual CMAS still exists on three kinds of rare-earth tantalates.The reaction layer thickness of YTa O₄,Gd Ta O₄,and Yb Ta O₄ are 32.4μm,30.8μm,and 16.8μm after 24 h CMAS corrosion at1350℃,which is lower than the reaction layer thickness of YSZ after 4 h CMAS corrosion at 1250℃（～480μm）.The corrosion products and reaction mechanism between CMAS and YTa O₄,Gd Ta O₄,and Yb Ta O₄ are researched by powder mixing experiments.The results show that the main corrosion products between CMAS and three kinds of rare earth tantalates are the same at 1250℃and 1350℃,which is Ca₂Ta₂O₇.The reaction mechanism between CMAS and rare-earth tantalates is that the CMAS elements enter into the lattice,which increases the energy of the rare earth cation and accelerates its migration,so the lattice is destroyed.The first-principles calculation results show that the Ca₂Ta₂O₇ possesses lower cohesive energy and formation enthalpy than Ca Mg Si₂O₆,which explains the formation of Ca₂Ta₂O₇ after the reaction.In addition,Yb Ta O₄ possesses lower cohesive energy,so its stability is best in three kinds of rare earth tantalates.The original Yb Ta O₄ is modified by doping method to prepare(Yb_0.8Gd_0.1Y_0.1)Ta O₄ and(Yb_0.7Gd_0.15Y_0.15)Ta O₄.The high-temperature phase stability,Young’s modulus,microhardness,and CMAS corrosion resistance of these two kinds of modified rare earth tantalate are similar to the original Yb Ta O₄,while the thermal conductivity,CTE,and fracture toughness show an apparent improvement.Compared with the original Yb Ta O₄,the thermal conductivity and fracture toughness of(Yb_0.8Gd_0.1Y_0.1)Ta O₄ are reduced by 65%and increased by13%,respectively,the change of TEC is not obvious.the thermal conductivity,CTE,and fracture toughness of(Yb_0.7Gd_0.15Y_0.15)Ta O₄are reduced by 63%,increased by24%,and increased by 29%,respectively,which is a kind of potential thermal barrier material.