Cyclic Fatigue Strength of Crack-healed Si3N4/SiC Composite Ceramics for Spring

Crack-healing behavior of silicon nitride ceramics and high temperature strength of crack-healing zone have been investigated previously systematically in our last work. In this paper, the cyclic fatigue strength behavior of crack-healed ceramics was investigated in detail. Two kinds of the compositions of Si3N4 and SiC with different sintering additive were tested. One contains the 8wt.% Y2O3 as a sintering additive, and was called SNS-Y8 specimen in this paper, another contains the 5wt.% Y2O3 and 3wt.% Al2O3 as a sintering additive, and was called SNS-Y5A3 specimen in this paper. The semicircular crack (average diameter is 100μm and 200μm) was made in the center of specimen using Vickers hardness indenter. Cracked specimen were heattreated at 1200°C-1300°C for 1 hour in the air. Cyclic fatigue strength test was conducted at room temperature. Crack-healed material has shown the outstanding fatigue strength behavior and attains almost the same strength as smooth specimen do. The surface state of the specimen which did not fail for cyclic fatigue experiments of 2×106 cycles was observed by SEM., any new crack was not found on the surface of crack-healed specimen which survived fatigue test of 2×106 cycles.