Formation kinetics of crystalline Si3N4/SiC composites from amorphous Si–C–N ceramics

The crystallization kinetics of amorphous precursor-derived bulk ceramics of composition Si26C41N33 are investigated in the temperature range between 1500 and 1645 °C using X-ray diffractometry. Isothermal annealing in nitrogen leads to crystallization of microcrystalline α-Si3N4 and nanocrystalline SiC. At 1500 °C only Si3N4 is crystallized, while with increasing temperature a growing amount of additional SiC and a decreasing amount of Si3N4 is formed in a simultaneous crystallization process, until at 1645 °C only SiC is observed. Crystallization can be described according to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism, assuming a three-dimensional, diffusion controlled grain growth with a time dependent nucleation rate. The determined rate constants of crystallization are temperature dependent, but equal for both crystallized phases. An Arrhenius behavior with a very large activation enthalpy of about 12.5 eV is observed, where the enthalpies of diffusion and nucleation both contribute substantially to that value. Two kinetic processes are identified which may influence phase composition and microstructure of the crystallized material in order to produce tailor-made composites: (a) the direct crystallization process of amorphous Si26C41N33 and (b) a transformation process where already crystallized α-Si3N4 reacts with carbon, embedded in the residual amorphous Si–C–N matrix, to SiC at high temperatures.