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 densification 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 rate 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% Y2O3
and 2.95 wt% AIN. Materials without phase transformation showed a
homogeneous microstructure with equiaxed grains, whereas
microstructures with elongated grains were developed from SIC powders
with a high initial alpha/beta-ratio (>1:9) when phase transformation
occurs. Since liquid phase sintered silicon carbide reveals
predominantly an intergranular fracture mode, the grain size and shape
has a significant influence on the mechanical properties. The toughness
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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.