MICROSTRUCTURE AND THERMAL-STABILITY OF COATED SI3N4 AND SIC

Microstructure of coatings obtained by treating Si3N4 and SiC in Cr powders at 1273-1673 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 coating 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.