Improvement of fracture toughness in hot isostatically pressed mixtures of ultrafine and coarse-grained SiC ceramics

Laser synthesised, spherical beta-SiC powder with a particle size of 20 nm was mixed with coarse-grained (10 microns), blocky shaped alpha-SiC powder with an aspect ratio of 2-3 and sintering additives. Due to the large particle size, coarse-grained powders had extremely low sinteractivity but mixtures with ultrafine powders could be densified at low temperatures, the resultant microstructure showing a bimodal grain-size distribution. High fracture toughness values are explained by the formation of grain bridges. During cracking, large grains are near the crack tip and in contact with both sides of the fracture surface which leads to interlocking of the grains. During crack propagation, the large grains are pulled out completely and friction forces contribute to the work of fracture. 15 refs.