FATIGUE BEHAVIOR OF SILICON-NITRIDE CERAM ICS PREPARED FROM BETA-PHASE POWDER

Beta(beta)-Si3N4 powder containing 1 mol% equimolar Y2O3-Nd2O3 was gas -pressure-sintered under three combined conditions of temperature, N-2 gas pressure and time. Cyclic fatigue testing of these materials was performed using smooth specimens in ambient air. The fatigue crack gro wth diagram (K-Imax-da/dN diagram) was calculated from the fatigue lif etime distribution data by using the statistical method proposed by Fe tt and Munz. These materials had a microstructure of ''in-situ composi tes'' as a result of exaggerated grain growth of some beta-Si3N4 grain s during firing, and showed high fracture toughness. The crack growth rates of the materials were retarded by high fracture toughness. The t hreshold K-Imax value of crack growth was recognized in crack growth r ate diagrams of these materials. It was deduced that the characteristi c fatigue behavior of Si3N4 materials prepared from beta-phase powder was due to high fracture toughness caused by in-situ composite microst ructure.