FRACTURE-TOUGHNESS ASSESSMENT OF SILICON CARBIDE-BASED CERAMICS AND PARTICULATE-REINFORCED COMPOSITES

The fracture toughness of sintered silicon carbide (alpha-SiC) and silicon carbide reinforced with particulate titanium diboride (TiB2/SiC) has been evaluated using specimens in bending containing chevron notches and through-thickness precracks at ambient and elevated temperatures in air and in vacuum. Fracture toughness values measured from through-thickness precracked test-pieces are lower at all test temperatures. The particulate reinforcement has been shown to toughen the matrix significantly at room temperature only. At the test temperature of 1200-degrees-C the difference in toughness between the two materials is reduced and increasing the temperature to 1600-degrees-C further reduces this difference, to the extent that the two materials have values of fracture toughness which are indistinguishable. This provides strong evidence that the dominant toughening mechanism in the composite is the effect of thermal residual stresses which are relieved as the temperature is increased. Fractographic observations suggest that the bonding between the SiC and TiB2 particulate is relatively weak because interfacial decohesion of particles is observed at all test temperatures. Nevertheless, surface roughness measurements indicate that there may also be a contribution to the toughness from increased crack deflection in the composite material at room temperature only.