Microstructure and thermal properties of hafnium-doped highentropy oxide(Y_(0.25)Ho_(0.25)Er_(0.25)Yb_(0.25))_(2)O_(3)coatingsOA

The development of thermal/environmental barrier coatings(T/EBCs)is significantly constrained by the stringent material requirements for their topcoats.This study explores the implications of hafnium oxide(HfO_(2))doping on the crystal structure and thermophysical properties of high-entropy(Y_(0.25)Ho_(0.25)Er_(0.25)Yb_(0.25))_(2)O_(3)oxide,with the goal of designing and selecting novel topcoat materials.(RE_(1−x)Hfx)_(2)O_(3+x)X_(1−x)(RE=Y_(0.25)Ho_(0.25)Er_(0.25)Yb_(0.25);X=oxygen vacancy)coatings with 5,10,20,and 50 mol%hafnium oxide were deposited by atmospheric plasma spraying(APS).Systematic investigations were conducted on their hardness,Young’s modulus,phase composition,phase stability,thermal conductivity,and coefficient of thermal expansion.At lower HfO_(2)doping contents(5-20 mol%),the(RE_(1−x)Hfx)_(2)O_(3+x)X_(1−x)coatings retained the bixbyite structure,whereas the higher HfO_(2)doping level(50 mol%)induced a phase transition to the fluorite structure.The structural evolution is attributed to the ordered arrangement of oxygen anions and vacancies in the crystal structure,which enhances the phase stability and simultaneously reduces the thermal conductivity but increases the hardness,Young’s modulus and thermal expansion coefficient.The thermophysical properties of the(RE_(1−x)Hfx)_(2)O_(3+x)X_(1−x)coatings were strongly dependent on the HfO_(2)doping content.The establishment of composition-structure-property relationships in the(Y_(0.25)Ho_(0.25)Er_(0.25)Yb_(0.25))_(2)O_(3)-HfO_(2)system provides critical insights for the optimization of T/EBC materials in extreme environments.