Indentation Fatigue of Silicon Carbide and Silicon Nitride

Abstract

The fatigue failure mechanism of ceramic material has been hardly made clear under cyclic loading. In this study, we examine the damage of the surface area in ceramics under local cyclic compressive loading by using the cyclic indentation method (indentation fatigue), and discuss its mechanism. Four kinds of materials (transgranular and intergranular fracture type) were used for the experiment. The indentation fatigue test was carried out by indenting repeatedly onto the same point of the specimen surface by using a Vickers hardness tester. The median/radial crack length, the number of cycles to chipping and the size of the chipping area were measured, and the lateral crack growth rate was computed. The fracture surface in the chipping area was also observed by a scanning electron microscope. The median/radial crack growth was slightly accelerated by cyclic loading in any material. During cyclic indentation, the lateral crack propagated, which resulted in the chipping phenomenon. Such damage accumulation remarkably occurred in the transgranular fracture type material rather than in the intergranular fracture type one. Striations were observed in the chipping area and the striation spacing approximately corresponded to the size of the chipping area divided by the cycles to chipping, i.e., the lateral crack propagation rate. The observed zigzag propagation of the lateral crack is considered to be attributable to the difference between the stress fields under loading and unloading. The reason why the lateral crack growth and the resultant chipping behavior are suppressed in intergranular fracture type materials is most likely that crack branching and microcracking easily occur in those materials so that the stress concentration at the crack tip is relieved.