Microstructural development during liquid phase sintering of silicon carbide ceramics

The microstructures of liquid phase sintered alpha-SiC ceramics have been characterised by analytical electron microscopy including electron energy filtered imaging. The materials were fabricated with different additions of Al(2)O(3) and/or Y(2)O(3) and densified by pressureless sintering oi hot isostatic pressing (HIP). Y, Al-garnet and alpha-Al(2)O(3) partitioned from the liquid phase sintering medium during pressureless sintering leaving only thin intergranular films of residual glass. The formation of alpha-Al(2)O(3) was promoted by the incorporation of Al(2)O(3) from the surrounding SiC/Al(2)O(3) 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, Y(2)Si(2)O(7) partitioned from the liquid phase sintering medium in the absence of Al. The applied high pressure during HIP resulted in a limited decomposition of the alpha-SiC whereby graphite and SiO(2) formed. (C) 1997 Elsevier Science Limited.