RAMAN-SPECTRA OF SIYAION GLASSES AND CERAMICS

Silicon nitrides are ceramics that exhibit excellent properties for application at high temperatures in advanced heat engines as well as in oxidizing environments. Their densification with Y2O3 and Al2O3 as sintering aids leads to a two-phase material, called β′-Sialon, containing β′-Si3N4 grains where Si and N are partially substituted by Al abd O, and a mainly glassy intergranular phase which controls the high temperature behaviour of these ceramics. Nitrogen glasses, with compositions close to that of the grain boundary phase, have been synthesized to study the intrinsic properties of this secondary phase. Raman spectra obtained from both SiYAlON ceramics and glasses are reported and compared. Raman scattering spectroscopy appears to be an appropriate technique to investigate crystallization of nitrogen glasses. Further, this technique is very sensitive to the occurrence of free silicon in silicon nitride based materials. Residual silicon, resulting from incomplete nitridation or from decomposition during processing, leads to a narrow and strong Raman band located at about 520 cm-1. © 1990.