La-doped Modified Yttrium Aluminum Garnet Thermal Barrier Coating Material And Its Resistance To CMAS Corrosion

The application and development of thermal barrier coatings（Thermal Barrier Coatings,TBCs）have led the aviation industry into a new level.Nowadays,the developmental level of aero-engine is determined by the improvement of the comprehensive performance of coating materials.The turbine blades serve above 1200℃during the operation of the aircraft,causing CaO-MgO-Al₂O₃-SiO₂（CMAS）in the environment to penetrate into the coating and react with YSZ,resulting in coating failure.Therefore,the thermal barrier coating material should not only have high bearing temperature,but also have good corrosion resistance to CMAS.Yttrium aluminum garnet(Y₃Al₅O₁₂,YAG)is an ideal thermal barrier coating material.The ceramic materials with four components(La_xY_1-x)₃Al₅O₁₂（x=0,0.1,0.2,0.3）obtained by doping La instead of Y element were synthesized by chemical co-precipitation method according to the crystal structure of YAG.The microstructure,mechanical properties,thermophysical properties and thermal corrosion resistance to CMAS of the four-component ceramic materials were studied,and the following conclusions were obtained:（1）The phase of(La_xY_1-x)₃Al₅O₁₂（x=0,0.1,0.2,0.3）precursor powder calcined at 1150℃for2 h was studied.The results showed that the calcined powders with undoped and doping content x=0.1 were YAG phase.When x=0.2,the calcined powders were mainly YAG phase,and some LaAlO₃ phase and YAlO₃（YAP）phase appeared;when x=0.3,the calcined powders were composed of YAP phase and a small amount of LaAlO₃ phase.（2）The phase structure and microstructure of(La_xY_1-x)₃Al₅O₁₂（x=0,0.1,0.2,0.3）ceramic blocks sintering at 1600℃for 6 h were studied.The undoped ceramic block still exhibited the YAG phase;But the doped(La_xY_1-x)₃Al₅O₁₂（x=0.1,0.2,0.3）ceramic blocks mainly comprised YAG phase and a small quantity of LaAlO₃ phase.There were very legible grain boundaries in(La_xY_1-x)₃Al₅O₁₂（x=0,0.1,0.2,0.3）ceramic blocks and smaller LaAlO₃ grains near the YAG grains of the ceramic blocks doped with La element.（3）The mechanical properties of(La_xY_1-x)₃Al₅O₁₂（x=0,0.1,0.2,0.3）ceramic blocks were studied.It is found that the hardness and fracture toughness of ceramic block doped with La are significantly higher than that of undoped YAG ceramic block.When x=0.2,its hardness can reach the maximum value of 18.027 GPa,and x=0.1,the fracture toughness was the highest,about 2.887MPa.m^1/2.（4）The thermophysical properties of ceramic blocks(La_xY_1-x)₃Al₅O₁₂（x=0,0.1,0.2,0.3）were studied.It was found that the thermal expansion coefficient of the ceramic block gradually increased with increasing the doped La proportion.When the doping content x=0.3,the thermal expansion coefficient of the ceramic is 9.79×10^-6 K^-1 at 1000℃.The thermal conductivity of the doped ceramic block is higher than that of the undoped ceramic block.（5）The CMAS corrosion behavior of(La_xY_1-x)₃Al₅O₁₂（x=0,0.1,0.2,0.3）ceramic blocks at1250℃was studied.It was found that the corrosion section of ceramic blocks was divided into three layers:the top CMAS residual layer,the middle CMAS infiltration layer,and the bottom layer was the matrix ceramic layer.CMAS almost completely penetrated into the undoped ceramic block,while the penetration depth of the La-doped ceramic block was small and CMAS mainly remained in the top layer.With the gradual increase of the doping content of La,the depth of CMAS penetration layer gradually decreased.In the process of CMAS corrosion for 5 h-10 h,with the increase of doping content,the increase of CMAS penetration depth gradually decreases.When the doping content x=0.3,the depth of the penetration layer hardly increases.（6）The corrosion products of undoped ceramic blocks are anorthite（CaAl₂Si₂O₈）,spinel（MgAl₂O₄）and apatite（Ca₄Y₆O（SiO₄）₆）;In addition to the above materials,another kind of apatite material（CaLa₄O（SiO₄）₃）also existed in the corrosion products of ceramic blocks doped with La,and the content of CaLa₄O（SiO₄）₃ gradually increased with increasing the doped La proportion.The apatite phase CaLa₄O（SiO₄）₃ has a high melting point and a dense structure,which can effectively suppress the further reaction of the ceramic block and fill the corrosion channels of CMAS,thus hindering the penetration of CMAS.