Investigation Of Multi-element Rare Earth High-entropy Modification Of La₂Zr₂O₇-based Ceramic Thermal Protective Coatings

Pyrochlore lanthanum zirconate（P-La₂Zr₂O₇）is considered as one of the novel materials of thermal protective coating,which is due to the good phase stability and sintering resistance.However,the application of P-La₂Zr₂O₇ in the field of surface protection of hot end components is seriously restricted as a result of the poor plastic toughness and the vulnerability at high temperature.In recent years,rare earth high entropy modification provides a new way to improve the comprehensive performance of La₂Zr₂O₇-based coating.Nevertheless,the relevant research is still in its infancy,and there are still some problems to be studied and solved,such as the difficulty in the design high entropy pyrochlore phase materials for coatings,and the unclear transformation law of microstructure in complex high-temperature environment（such as long-term isothermal,thermal coupling impact,hot corrosion,etc.）.Therefore,in this paper,the high entropy modified La₂Zr₂O₇-based ceramic coating with pyrochlore phase was designed,based on the phase regulation principle of lanthanum zirconate decided by cation radius ratio and the principle of configuration entropy design of high entropy materials.In addition,the the transformation law of microstructure,thermal and mechanical properties of high entropy modified,binary rare earth modified and unmodified La₂Zr₂O₇-based ceramic coatings were systematically compared and studied.Furthermore,the mechanism of rare earth high entropy modification on the phase and microstructure stability of La₂Zr₂O₇-based coating in complex environment was also investigated.The multi-element rare earth co-doping modified La₂Zr₂O₇-based（RE-LZ）thermal protective surface materials,including the(La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇ obtained by quaternary rare earth co-doping high entropy modification,and(La_1/3Sm_1/3Yb_1/3)₂Zr₂O₇ and(La_1/3Gd_1/3Y_1/3)₂Zr₂O₇ obtained by binary rare earth co-doping modification,were designed,in terms of the phase regulation principle of lanthanum zirconate decided by cation radius ratio and the principle of configuration entropy design of high entropy materials.The RE-LZ coatings were prepared by spray granulation,solid-state sintering and atmospheric plasma spraying,based on La₂Zr₂O₇-YSZ coating.Each above modified La₂Zr₂O₇-based ceramic layer was mainly cubic pyrochlore phase,which the rare earthes all doped at La³⁺position.Due to the small radius characteristics of doped rare earth ions,the crystal plane spacings of rare earth co-doping modified ceramic layers decreased.Among them,the high entropy modified La₂Zr₂O₇-based ceramic layer had the smallest crystal plane spacing and the highest degree of structural disorder.The thermal conductivity of the multi rare earth co-doping modified La₂Zr₂O₇-based ceramic layer was significantly reduced by 16～33%within the range of 1073 K to 1273 K,which was due to the lattice distortion and increase of lattice disorder caused by rare earth doped ions.Meanwhile,the plastic toughness of modified La₂Zr₂O₇-based coating were significantly improved,and solid particle erosion resistance at 45°and 90°were increased by66%and 63%at most,respectively,which was due to rare earth doped ions the improving the ability of lattice deformation.Among them,the(La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇ high entropy ceramic layer presented the highest degree of lattice distortion and disorder,which was due to the disordered arrangement of rare earth ions in the lattice.Forthermore,the high entropy modified La₂Zr₂O₇-based ceramic coating presented the best thermal insulation performance and solid particle erosion resistance.The multi-element rare earth co-doping modified could effectively improve the microstructure stability of La₂Zr₂O₇-based coating under isothermal and hot corrosion conditions.It was due to the doped rare earth ions could promote the formation of anion Frenkel defect pair in La₂Zr₂O₇ lattice,and meanwhile increased the Gibbs free energy of hot corrosion reaction.Among them,the(La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇-YSZ coating presented the most obvious improvement.Compared with the unmodified coating,the thicknesses of thermally grown oxide layer and hot corrosion layer in modified coating were reduced by 28%and 80%,respectively.It was due to the competitive strengthening of ionic bond between rare earth ions and oxygen ions（RE-O bond）,showing a high entropy stabilization effect.It not only reduced the oxygen ion conductivity of the ceramic layer,which improved the oxidation resistance of the coating in the isothermal environment,but also reduced the hot corrosion reaction rate of the coating.Moreover,the high entropy corrosion products of(La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇ ceramic layer could refine the corrosion reaction layer,further hinderred the penetration of molten salt.Thus,the high entropy modified coating possed the best hot corrosion resistance,due to the reduction of hot corrosion reaction rate and the obstruction of molten salt penetration.The multi rare earth co-doping modification could effectively improve the thermal expansion coefficient of La₂Zr₂O₇-based ceramic layer,by regulating the bond strength of RE-O bond in La₂Zr₂O₇ lattice.It could improve the thermal expansion matching between different ceramic layers,avoiding the accumulation of thermal stress in the coating under the impact of thermal coupling.Meanwhile,it inhibitted crack growth and improved the structural stability of the coating.Moreover,the(La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇-YSZ coating obtained by high entropy modification improved its thermal expansion coefficient and fracture toughness due to the disorder arrangement of rare earth ions and great lattice deformation ability.Therefore,in the ceramic layers of(La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇-YSZ coating,the thermal stress accumulation was the smallest,and the cracks were difficult to propagate,which mean the structural stability of(La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇-YSZ coating under thermal mechanical coupling was the best.Compared with the unmodified La₂Zr₂O₇–YSZ coating,the thermal mechanical coupling impact resistance of high entropy modificated coating was increased by 57%.