Microstructure evolution in amorphous Hf-B-Si-C-N high temperature resistant coatings after annealing to 1500 °C in...

Recently, amorphous Hf-B-Si-C-N coatings found to demonstrate superior high-temperature oxidation resistance. The microstructure evolution of two coatings, Hf₇B₂₃Si₂₂C₆N₄₀ and Hf₆B₂₁Si₁₉C₄N₄₇, annealed to 1500 °C in air is investigated to understand their high oxidation resistance. The annealed coatings develop a two-layered structure comprising of the original as-deposited film followed by an oxidized layer. In both films, the oxidized layer possesses the same microstructure with HfO₂ nanoparticles dispersed in an amorphous SiO_x-based matrix. The bottom layer in the Hf₆B₂₁Si₁₉C₄N₄₇ coating remains amorphous after annealing while Hf₇B₂₃Si₂₂C₆N₄₀ recrystallized partially showing a nanocrystalline structure of HfB₂ and HfN nanoparticles separated by h-Si₃N₄ and h-BN boundaries. The HfB₂ and HfN nanostructures form a sandwich structure with a HfB₂ strip being atomically coherent to HfN skins via (111)-Hf monolayers. In spite of the different bottom layer structure, the oxidized/bottom layer interface of both films was found to exhibit a similar microstructure with a fine distribution of HfO₂ nanoparticles surrounded by SiO₂ quartz boundaries. The high-temperature oxidation resistance of both films is attributed to the particular evolving microstructure consisting of HfO₂ nanoparticles within a dense SiO_x-based matrix and quartz SiO₂ in front of the oxidized/bottom layer interface acting as a barrier for oxygen and thermal diffusion.