FRACTURE-TOUGHNESS ANISOTROPY AND TOUGHENING MECHANISMS OF A HOT-PRESSED ALUMINA REINFORCED WITH SILICON-CARBIDE WHISKERS

The fracture toughness of a hot-pressed alumina and that of a hot-pres sed alumina/SiC-whisker composite containing 33 vol% SiC whiskers were measured by four-point bending on single-edge precracked bend bars ha ving sharp precracks created by ''bridge indentation.'' Two batches of the composite were examined, one exhibiting a greater degree of whisk er clustering than the other. The fracture toughness of the alumina wa s around 4 MN.m-3/2 whereas that of the composite varied between 5 and 8 MN.m-3/2 depending on microstructural uniformity and crack-propagat ion direction. Crack deflection in combination with a change in fractu re from intergranular to transgranular fracture is proposed as an expl anation of the superior fracture toughness of SiC-whisker-reinforced a lumina as compared to unreinforced alumina. The composite exhibited a variation in fracture toughness with the crack-propagation direction i n identical crack planes. This effect could with good accuracy be desc ribed in terms of crack deflection for the composite with uniform whis ker distribution. However, in the material with whisker clustering the variation of the fracture toughness with crack-growth direction was g reater and could not entirely be explained by crack deflection.