The Study on Crack Healing Behaviour of Silicon Nitride Ceramics.

Abstract

Crack healing behaviour of monolithic Si3N4 and Si3N4/SiC composite have been investigated, systematically. In these ceramics, 8 wt.% Y2O3 was used as a sintering additive. The semicircular crack (average diameter was 110 μm) was made in the center of specimen using Vickers hardness indenter. Some of the cracked specimens were heat-treated at 1 300°C for 1 hour in air or in argon gas atmosphere. Bending strength was measured at room and elevated temperature up to 1 300°C in air by 4-point bending test system. Bending strength was measured at room and elevated temperature up to 1 300°C in air by 4-point bending test system. Bending strength of cracked specimen reduced to about 50% of that of smooth specimen. However, bending strength of pre-cracked and heat treated specimen recovered completely up to that of smooth specimem. And in most specimens tested, failure occured from the outside of pre-cracked area even at 1 300°C. By the systematical investigation, it is concluded that crack healing due to oxidation results the recovery in bending strength since precrack was filled in oxides, such as Y2Si2O7 and SiO2, which were produced by heat treatment in air. Crack healing behaviours in the both ceramics were also investigated, as a function of heat treatment temperature and times, initial crack size and bending test temperature. Based on the these results, it was concluded that crack healed part in Si3N4/SiC composite ceramics has enough strength up to 1 300°C.