The effect of microstructure on the fatigue behavior at room temperature in air was studied using sialon ceramics with different microstructures prepared by selecting starting raw materials and sintering conditions. Dynamic, static and cyclic fatigue tests were carried out on smooth specimens by the 3-point bending method. Oxide content in the intergranular phase and grain size affectted the dynamic fatigue behavior. Remarkable slow crack growth attributable to the stress corrosion by humidity in air was observed in the sample with elongated grains. In static fatigue tests, the observed lifetime of each sample was in agreement with that predicted from dynamic fatigue properties, and the crack propagated through the intergranular phase. The acceleration of fatigue was observed under cyclic loading. The accelerative effects on cyclic fatigue was significant in the sample with coarse grains. Sialon ceramics with elongated grains prepared by HIP showed both high fracture toughness and high strength, but the fatigue parameter “n” was low and the acceleration of fatigue under cyclic loading was significant because of its coarse grains.
