Diffraction Study on the Atomic Structure and Phase Separation of Amorphous Ceramics in the Si-(B)-C-N System. 2. Si-B-C-N Ceramics

The atomic structure and phase separation of amorphous Si-B-C-N ceramic powder samples obtained by thermolysis of boron-modified polysilazanes were investigated using X-ray and neutron diffraction in the wide-angle and small-angle scattering regimes. The short-range order of the Si-B-C-N ceramics corresponds to that of Si-C-N ceramics consisting of two separated amorphous phases: an amorphous graphite-like phase and amorphous Si3+(1\4)xN4-xCx (x = 0-4). The evaluation of the total pair correlation functions revealed that the boron atoms are bonded to nitrogen with B-N bond lengths typical for h-BN. These units are incorporated in the graphite-like phase not statistically, but within regions of BN-rings which are less extended than the regions formed by C-rings. X-ray diffraction phase analysis and small-angle scattering showed that the addition of up to 10 at. % of boron to Si-C-N ceramics significantly influences the high-temperature behavior of the ceramics: coarsening of the separated amorphous phases, the crystallization of Si3N4 and its decomposition are retarded, and evolution of a nanocrystalline SiC phase is observed.