Fracture toughness and strength of SiC-whisker-reinforced Si sub 3 N sub 4 composites

The fracture toughness (K{sub IC}), strength, and elastic moduli of hot-pressed Si{sub 3}N{sub 4} reinforced with SiC whiskers were measured at room temperature over a range of SiC content from 0 to 20 wt%. The K{sub IC} was determined from cracks produced by Vickers indentation, the elastic moduli were determined by an ultrasonic technique, and the strength was measured by four-point bending. Although K{sub IC} increases from 4 to 7 MN {center dot} m{sup {minus}3/2} with the addition of 20 wt% SiC, the corresponding increase in strength is relatively small, i.e., from 375 to 550 MPa. Scanning electron microscopy reveals that the small increase in strength is partly related to processing-induced critical flaws.