Microstructure and thermal stability of coated Si3N4 and SiC

Microstructure of coatings obtained by treating Si₃N₄ and SiC in Cr powders at 1273–1623 K has been studied employing XRD, SEM, AES and TEM. In accordance with thermodynamic calculations and kinetic consideration, the coatings have layered structures and contain metal-rich silicides and metal-rich nitrides or carbides. The microstructure of the coatings has been found to depend on the treatment conditions. The kinetics of the coatings growth obeys a parabolic growth law, the activation energies being close to the activation energies for self-diffusion of the corresponding metals. Thermal stability of the coated and uncoated Si₃N₄ and SiC in Fe-, Ni- and Co-based matrices has been studied and the coatings have been found to considerably improve the stability of Si₃N₄- and SiC-metal interfaces.