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

Recently, amorphous Hf-B-Si-C-N coatings found to demonstrate superior high-temperature oxidation resistance. The microstructure evolution of two coatings, HfBSiCN and HfBSiCN, 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 HfO nanoparticles dispersed in an amorphous SiO-based matrix. The bottom layer in the HfBSiCN coating remains amorphous after annealing while HfBSiCN recrystallized partially showing a nanocrystalline structure of HfB and HfN nanoparticles separated by h-SiN and h-BN boundaries. The HfB and HfN nanostructures form a sandwich structure with a HfB 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 HfO nanoparticles surrounded by SiO quartz boundaries. The high-temperature oxidation resistance of both films is attributed to the particular evolving microstructure consisting of HfO nanoparticles within a dense SiO-based matrix and quartz SiO in front of the oxidized/bottom layer interface acting as a barrier for oxygen and thermal diffusion.