Abstract
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 SiOx-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 SiOx-based matrix and quartz SiO2 in front of the oxidized/bottom layer interface acting as a barrier for oxygen and thermal diffusion.
Highlights
Both annealed Hf-B-Si-C-N films exhibit a two-layered structure comprising the original as-deposited film at the bottom and an oxidized layer at the top
The bottom layer in the Hf6B21Si19C4N47 film remains amorphous while that in the Hf7B23Si22C6N40 film is partially crystallized exhibiting a nanocomposite structure of HfB2 and HfN nanoparticles surrounded by h-Si3N4 and h-BN boundary phases
The HfB2 and HfN nanoparticles found to form a sandwich structure with a HfB2 strip core being atomically coherent to HfN skins via a (111) Hf monolayer
Summary
Both annealed Hf-B-Si-C-N films exhibit a two-layered structure comprising the original as-deposited film at the bottom and an oxidized layer at the top.
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