Abstract
The crystallisation kinetics of amorphous precursor-derived ceramics of composition Si26C41N33 is investigated as a function of temperature and nitrogen partial pressure using X-ray diffractometry. Isothermal annealing at a pressure of 1 bar leads to simultaneous crystallisation of Si3N4 and SiC, while only crystalline SiC is formed with annealing at a reduced pressure of 1 mbar. Rate constants of crystallisation are determined using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism. For temperatures below 1700°C, crystallisation rates are significantly higher for annealing at 1 mbar compared to 1 bar. For an explanation of the results, a model is proposed, which is based on diffusion-controlled nucleation and growth of crystalline Si3N4 and SiC in an amorphous matrix combined with thermal decomposition of Si3N4 at high temperatures.
Acknowledgements
The authors thank P. Gerstel (MPI Stuttgart) for supplying the precursor ceramics and E. Ebeling for ceramographic preparation. Funding by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.