Phase evolution and crystallization in Si-B-C-N ceramics derived from a polyborosilazane precursor: microstructural characterization

Amorphous Si-B-C-N ceramic powder samples obtained by thermolysis of polyborosilazane {B[C2H4Si(H)NH]3}n were isothermally annealed at different temperatures (1400-1800 °C) and hold-times (3, 10, 30, 100 h). Scanning electron microscopy (SEM) of annealed powders as well as polished cross sections of large powder particles from selected samples were carried out to study surface morphology, crystallization and associated microstructural changes. Microstructural and phase evolution were additionally investigated using high-resolution transmission electron microscopy (HRTEM) and energy filtering TEM (EFTEM). Higher surface areas of the powders were found to promote vapor-phase decomposition reactions resulting in SiC whisker growth. In coarser powders the influence of surface is manifested as a skin-core effect, where the 'skin' has undergone a higher degree of decomposition accompanied by an increased SiC crystal growth, compared to the 'core.' Crystallization of SiC occurs already at 1400 °C, although Si3N4 crystallization occurs only at 1700 °C, after more than 3 h of annealing.