Abstract: In this paper the influences of ZrO, and SiC Whiskers on strengthening and toughening effects of HP Si3N4 have been studied. By addition of ZrO2 and SiC whiskers, fracture toughness K IC, of composites increased, but flexural strength decreased simultaneously. In separate addition of 10vol% pure ZrO, and Y ZrO2 containing Y, fracture toughness of the Si3N4 ceramics at room temperature increased from 6.3 to 7.3 and 6.3 to 7.6 MPa m respectively. The improvement of fracture tonghness is due to the toughening effect of microcrack. By the way of addition of 10wt% SiC whiskers, at room temperature KIC of Si3N4 ceramics is increased from 6.3 to 7.1 MPa m . Toughening of SiC whisker was caused by crack deviation and pull-out effect of whiskers as well.

Key words: SiC whisker, zircoma toughening