Influence of the alpha/beta-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete den sification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AIN as sintering additives. Studies of the phase transformation from beta to alpha-SiC reveals a dependency on the initial beta-content and the sintering atmosphere. The transformation r ate decreases with an increasing beta-content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure beta-SiC starting powders when the additive system consists of 10.34 wt% Y2 O3 and 2.95 wt% AIN. Materials without phase transformation showed a homoge neous microstructure with equiaxed grains, whereas microstructures with elo ngated grains were developed from SIC powders with a high initial alpha/bet a-ratio (>1:9) when phase transformation occurs. Since liquid phase sintere d silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical propert ies. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing. (C) 1999 Kluwer Academic Publishers.