Quantitative EFTEM study of precursor-derived Si–B–C–N ceramics

Ceramic materials derived from a boron modified polysilazane were investigated by means of energy-filtering transmission electron microscopy (EFTEM). After cross-linking of the polymer and subsequent thermolysis, a coarse powder with average composition Si24.0B8.0C44.0N24.0 is obtained. For further investigation, monolithic particles with sizes of several millimeters were heat treated in crucibles under a flowing nitrogen atmosphere at 1800 °C for 10 h. During thermolysis, the particles developed internal cracks on the macroscopic scale. At the crack surfaces, a layer of pure carbon was found. In the crack-free region, the material is composed of Si3N4 and SiC nano crystallites which are embedded in a turbostratic BNC-matrix. Quantitative electron spectroscopic imaging (ESI) shows an atomic ratio of the elements B:C:N of 1.0:4.0:1.1 in this matrix. In the vicinity of the cracks, silicon nitride locally decomposes with formation of silicon carbide because of its reaction with excess carbon. A detailed EFTEM study of the phase distribution near the crack surfaces showed that the first Si3N4 crystallites occur at a distance of approx. 5 μm from the carbon covered crack surface. In additional experiments the composition of the BNC-layers as a function of the distance from the crack surface was investigated.