Research On Modification Of High Entropy Ceramic Based On Surface Coating

The development of gas turbines toward higher efficiency in recent years has been accompanied by a concomitant increase in inlet temperatures.In order to guarantee the normal service of the base alloy at high temperatures,the preparation of thermal barrier coatings(TBCs)on the surface of the hot end components can significantly increase the operating temperature and extend the service life.The conventional thermal barrier coating material is yttrium oxide stabilized zirconia(YSZ),which has low thermal conductivity,high coefficient of thermal expansion and high toughness properties.However,during long-term operation at temperatures above 1200℃,YSZ undergoes phase changes and sintering problems,which seriously affect the service life of the thermal barrier coating.High-entropy ceramic materials have lower thermal conductivity and better high-temperature phase stability,making them potential materials for thermal barrier coatings,but the coefficient of thermal expansion is lower and the mechanical toughness needs to be improved compared with YSZ.By coating a layer of YSZ material on the surface of high-entropy ceramic particles,the modification of high-entropy ceramic materials can be achieved by combining the respective advantages of both materials.In this study,YSZ was coated on the particle surfaces of two high-entropy ceramics to obtain La2(Zr0.2Ti0.2Hf0.2Ce0.2Sn0.2)2O7@YSZ(L(ZTHCS)O@YSZ)and La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7@YSZ(L(ZTYYN)O@YSZ),by liquid-phase precipitation method to prepare encapsulated structural composite ceramics.The microscopic morphological observation of the particles and high-temperature sintered ceramic sheets of these two composite ceramic materials as well as the analysis of the physical phase composition were carried out.Transmission electron microscopy observation of the generated two ceramic particles showed a core-shell structure.The core material was a high-entropy ceramic material and the shell material was nano-YSZ.Both cladding materials still have a stable phase structure after sintering at 1400℃。In terms of thermophysical properties,the test result of thermal conductivity for L(ZTYYN)O@YSZ at room temperature was 0.709Wm-1·K-1,and the actual theoretical thermal conductivity was calculated to be 1.103W·m-1 K-1 in combination with the porosity of 26.8%of the tested material,which is slightly higher than that of the high entropy ceramic but much lower than the YSZ material.Meanwhile,both L(ZTHCS)O@YSZ and L(ZTYYN)O@YSZ exhibit excellent thermal radiation properties than YSZ.L(ZTYYN)O@YSZ has a thermal expansion coefficient at The coefficient of thermal expansion at 1000℃ is 9.5356×10-6K-1,which is improved compared to the high entropy material.The Vickers hardness of L(ZTHCS)O@YSZ and L(ZTYYN)O@YSZ are HV523.94 and HV661.86 which were measured by Vickers hardness Compared with the two high-entropy ceramics before cladding treatment the Vickers hardness were increased by 145.8%and 172.7%.The modification treatment has significantly improved the hardness of the material.Scanning electron microscopy observed the cracks under the same load,the overall crack length of the clad material was smaller than that of the high-entropy ceramics,and the crack extension showed more deflection and bridging phenomena,and after indentation method calculation.The fracture toughness of L(ZTHCS)O@Y SZ and L(ZTY YN)O@Y SZ are 3.36MPa·m1/2and 3.45MPa·m1/2,which are 60.8%and 43.2%higher than the high-entropy ceramic materials,respectively.It indicates that the cladding material has improved the material toughness.During the service of aero-engines and gas turbines there is corrosion by dust,gravel,volcanic ash,etc.in the environment whose main component is CaO-MgO-Al2O3-SiO2(CMAS),which leads to coating failure.Experiments were conducted on the corrosion experiments of CMAS mixed and coated with La2(Ti0.2Zr0.2Yb0.2Y0.2Nb0.2)2O7,a high-entropy ceramic component of the cladding material,and phase component and SEM analyses were performed.The results show that CMAS can corrode La2(Ti0.Zr0.2Yb0.2Y0.2Nb0.2)2O7,and the corrosion product is apatite phase Ca2La8(SiO4)6O2 at high temperature.CMAS coating on high entropy ceramics will melt at high temperature,penetrate into the loose and porous ceramic structure,and preferentially destroy from the place with more pores.