Crystallization and creep behavior of Si–B–C–N ceramics

Amorphous Si–B–C–N ceramics were prepared from suitable boron-modified polyvinylsilazanes by solid state thermolysis under an argon atmosphere. The amorphous Si–B–C–N ceramics were further annealed at 1800 °C by applying different nitrogen pressures and holding times. The present work reports for the first time on high temperature creep studies coupled with a comprehensive structural characterization of the samples via X-ray diffraction, transmission electron microscopy and solid state NMR methods. It is shown that, depending on the actual annealing conditions, SiC and Si3N4 nanocrystallites are formed which are distributed in an amorphous B–C–N matrix consisting of amorphous carbon and boron nitride domains. The high temperature mechanical properties of the various annealed Si–B–C–N samples and of a representative amorphous specimen were examined by compression creep experiments. Constant load experiments were carried out at 1400 °C which showed that an improved creep resistance exists for the annealed Si–B–C–N samples, containing SiC and Si3N4 nanocrystallites. The smaller creep rates of the nanocrystalline ceramic are attributed primarily to the densification of the amorphous structure upon sample annealing. In addition, load change experiments were performed on the present ceramic materials. The high temperature viscosities derived were found to be substantially higher than for instance those discussed for fused silica. Finally, the anelastic behavior was examined by load release experiments. Here, a quantification of the experimental data was possible on the basis of the Kohlrausch–Williams–Watts equation.