MICROSTRUCTURAL DEVELOPMENT DURING LIQUID-PHASE SINTERING OF SILICON-CARBIDE CERAMICS

The microstructures of liquid phase sintered alpha-SiC ceramics have b een characterised by analytical electron microscopy including electron energy filtered imaging. The materials were fabricated with different additions of Al2O3 and/or Y2O3 and densified by pressureless sinterin g oi hot isostatic pressing (HIP). Y, Al-garnet and alpha-Al2O3 partit ioned from the liquid phase sintering medium during pressureless sinte ring leaving only thin intergranular films of residual glass. The form ation of alpha-Al2O3 was promoted by the incorporation of Al2O3 from t he surrounding SiC/Al2O3 powder bed. The thickness of analysed Al- and O-rich glassy films at SiC/SiC grain boundaries was estimated to be 1 .4-1.5 nm by Fresnel out-of-focus imaging. The crystallisation of Y-, Al-, Si- and O-rich liquids was suppressed during HIP, however, Y2Si2O 7 partitioned from the liquid phase sintering medium in the absence of Al. The applied high pressure during HIP resulted in a limited decomp osition of the alpha-SiC whereby graphite and SiO2 formed. (C) 1997 El sevier Science Limited.