Effects of Y and Ho doping on microstructure evolution during oxidation of extraordinary stable Hf-B-Si-Y/Ho-C-N films up to 1500 C

Hard, and optically transparent amorphous Hf6B12Si29Y2C2N45 and Hf5B13Si25Ho3C2N48 films were prepared by reactive pulsed dc magnetron co-sputtering and annealed up to 1500 degrees C in air. The evolved microstructures were studied by X-ray diffraction and transmission electron microscopy to understand the Y- and Ho- doping effects on thermal stability and oxidation behavior. A three-layered microstructure developed in both annealed films. A fully oxidized layer formed at the top surface consisting of cubic fluorite Hf(Y/Ho)O2 nanoparticles embedded in an amorphous SiOx-based matrix. The oxide layer is about 36 % thinner compared to undoped films with similar composition [1]. A recrystallized structure formed at the bottom of both films composed mainly of Hf(Y/Ho)N and alpha-/beta-Si3N4. All Hf(Y/Ho)N in the middle layer was oxidized producing vertically oriented Hf(Y/Ho)O2 nanocolumns surrounded by Si3N4 nanocrystalline. The Y- and Ho- doping found to stabilize the cubic fluoride oxide structure and promoted its (1 1 1) columnar texture. The oxidation mechanism of Si3N4 nanodomains occurs via formation of beta-SiO2 first followed by its transformation to amorphous SiOx. It is suggested that substitution of Hf4+ with Y3+ and Ho3+ ions within the Hf(Y/Ho)O2 formed an anion vacancy defect structure to preserve charge neutrality affecting the oxidation mechanism in the doped films.