Tuning of the high temperature behaviour of Si-C-N ceramics via the chemical crosslinking of poly(vinylmethyl-co-methyl)silazanes with controlled borane contents

The thermal stability, elemental/phase composition evolution and crystallization behaviour of Si-B-C-N ceramics obtained by the pyrolysis of a series of poly (vinylmethyl-co-methyl)silazanes displaying various boron contents were investigated through annealing in the temperature range 1000-1800 degrees C under nitrogen atmosphere. The increase of the boron content in the early stage of the process involved the nucleation of beta-SiC, inhibited the crystallization of alpha-Si3N4 and modified the activity of the sp2-hybridised carbon phase in the derived ceramics obtained at 1000 and 1400 degrees C. At 1800 degrees C, low boron content Si-B-C-N ceramics gradually evolved toward a major SiC phase mainly formed via the carbothermal reaction of amorphous Si3N4 whereas high boron content Si-B-C-N ceramics led to highly stable materials with a complex microstructure made of SiC, Si3N4 and a BN-rich B(C)N phase that inhibited the activity of sp2-hybridised carbon toward the carbothermal reaction of amorphous Si3N4 and significantly reduced the SiC crystallization process.