SI3N4-AL2O3 SI3N4-Y2O3 COUPLE DIFFUSION SYSTEM/

The behaviour of the additives Al2O3 and Y2O3 on the microstructure of Si3N4 and Si3N4-ZrO2 doped materials was studied using couple diffusi on systems (Si3N4-Al2O3/Si3N4-Y2O3). After heat treatments, the line p rofile for Al and Y was determined. The secondary intergranular phases were identified by XRD and TEM analysis. The influence of the chemica l composition of the intergranular glassy phase in the final microstru cture after sintering was observed. The drastic effect of the Y2O3 and ZrO2 on the alpha-beta Si3N4 transformation, beta-Si3N4 average grain size, density and on the crystalline secondary grain boundary phases has been investigated. The main conclusions are that the presence of z irconia drastically decreases the viscosity of the glassy phase develo ped in the Si3N4-Al2O3 system; when both Y2O3 and ZrO2 are present in this system devitrification of grain boundary amorphous phase takes pl ace; when zirconia is present in the Si3N4-Y2O3 system no unwanted qua ternary crystalline phases are formed.