Oxidation behavior of amorphous and nanocrystalline SiBCN ceramics – Kinetic consideration and microstructure

In this study, the structural evolution of SiBCN ceramics during crystallization and its effects on oxidation behavior involving different atomic units or formed phases in amorphous or crystalline SiBCN ceramics were analyzed. The amorphous structure has exceptionally high oxidation activity but presents much better oxidation resistance due to its synchronous oxidation of atomic units and homogeneous composition in the generated oxide layer. However, the oxidation resistance of SiBCN ceramic will degrade during the continual crystallization process, especially for the formation of the nanocapsule-like structure, due to heterogeneous oxidation caused by the phase separation. Besides, the activation energy and rate-controlling mechanism of the atomic units and phases in SiBCN ceramics were obtained. The BNC_x (E_a ​= ​145 ​kJ/mol) and SiC_(2-x) (E_a ​= ​364 ​kJ/mol) atomic units in amorphous SiBCN structure can be oxidized at relatively lower temperatures with much lower activation energy than the corresponding BN(C) (E_a ​= ​209 ​kJ/mol) and SiC (E_a ​= ​533 ​kJ/mol) phases in crystalline structure, and the synchronous oxidation of the SiC_(2-x) and BNC_x units above 750 ​°C changes the oxidation activation energy of BNC_x (E_a ​= ​332 ​kJ/mol) to that similar to SiC_(2-x). The heterogeneous oxide layer formed from the nanocapsule-like structure will decrease the activation energy SiC (E_a ​= ​445 ​kJ/mol) and t-BN (E_a ​= ​198 ​kJ/mol).