Influence of Microstructure on Fatigue Fracture Characteristics of Al2O3

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Fatigue fracture characteristics under static and cyclic loading were investigated for three kinds of Al2O3, with microstructures controlled by sintering additives (MgO, SiO2+CaO) and particle size. The microstructures were: fine equi-axed grains (Material M, MgO added), coarse columnar grains (Material SC, SiO2+CaO added) and fine columnar grains (Material SC-f, SiO2+CaO added). The crack propagation rates da/dN of Material SC was accelerated in water. The measurements of R-curve behavior and COD under cyclic loading showed that the grain bridging effect of Material SC was reduced further than that of Material SC-f. In addition, surface analysis by EDX on fatigue fracture surfaces in Material SC showed that Si and Ca, the components of the sintering additives, were decreased markedly at SCG sites under cyclic loading in water. Consequently, it is concluded that the degradation of fatigue strength in Material SC under cyclic loading in water was caused by the activation of a hydration reaction at grain boundaries in the crack tip and a marked decrease in the grain bridging effect (i.e., an increase in Ktip).

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