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 BNCx (Ea =145 kJ/mol) and SiC(2-x) (Ea= 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) (Ea = 209 kJ/mol) and SiC (Ea = 533 kJ/mol) phases in crystalline structure, and the synchronous oxidation of the SiC(2-x) and BNCx units above 750 degrees C changes the oxidation activation energy of BNCx (Ea = 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 (Ea = 445 kJ/mol) and t-BN (Ea= 198 kJ/mol).