Coarsening of nano-crystalline SiC in amorphous Si-B-C-N

The formation of nano-crystalline SiC is studied in various amorphous precursor derived Si-B-C-N bulk ceramics at temperatures between 1600 and 1800 °C. The formation process of SiC can be described by a very rapid crystallization (<15 min) of nano-sized particles with diameters between 2 and 7 nm which are embedded in an amorphous Si-B-C-N matrix. During further annealing of the material up to 40 h, particle growth due to coarsening takes place, which leads to maximum crystallite diameters of 30 nm. The kinetics of coarsening can be described by the Lifshitz-Slyozov-Wagner model. The product of the rate constant of coarsening, kc, and of the temperature, T, follows an Arrhenius behaviour with an activation enthalpy of about 8 eV (770 kJ/mol), which is approximately the activation enthalpy of self-diffusion in Si-B-C-N, indicating diffusion controlled crystallite growth. The kinetics of coarsening is fastest for the ceramics with a low concentration of Si and N in the amorphous matrix.