Temperature Dependence of Strengthening by Whisker Reinforcement: SiC Whisker-Reinforced Alumina in Air

The high fracture toughness of alumina reinforced with 20 vol% SiC whiskers is maintained at temperatures up to {approx} 1,100{degree}C in air. The fracture strength values ({sigma}{sub f}) decrease slowly with increasing temperature to 1,100{degree}C; however, when subjected to an applied stress (= 2/3 {sigma}{sub f}) for periods of up to 1,000 h in air at 800{degree}, 1,000{degree}, or 1,100{degree}C, the subsequent fracture strength values (retained strengths) at these temperatures increase with increasing exposure times. On the other hand, above 1,100{degree}C, the fracture strength values decrease while the apparent toughness value increases. At 1,200{degree}C, samples first subjected to applied stresses of {le} 310 MPa for up to {approx} 240 h exhibit a loss in retained strengths with increasing exposure time. The temperature dependence of the mechanical behavior is shown to be related to oxidation reactions in these composites with creep phenomena contributing to the loss in strength and increased toughness above 1,100{degree}C.