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, Hf(7)B(23)Si(22)C(6)N(40) and Hf(6)B(21)Si(19)C(4)N(47), 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(2) nanoparticles dispersed in an amorphous SiO(x)-based matrix. The bottom layer in the Hf(6)B(21)Si(19)C(4)N(47) coating remains amorphous after annealing while Hf(7)B(23)Si(22)C(6)N(40) recrystallized partially showing a nanocrystalline structure of HfB(2) and HfN nanoparticles separated by h-Si(3)N(4) and h-BN boundaries. The HfB(2) and HfN nanostructures form a sandwich structure with a HfB(2) 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(2) nanoparticles surrounded by SiO(2) quartz boundaries. The high-temperature oxidation resistance of both films is attributed to the particular evolving microstructure consisting of HfO(2) nanoparticles within a dense SiO(x)-based matrix and quartz SiO(2) in front of the oxidized/bottom layer interface acting as a barrier for oxygen and thermal diffusion.