Sphere indentation tests were performed to clarify damage properties on surface of Advanced Pore-Free SiC (APF-SiC) with excellent damage tolerance. The ring crack initiation strength properties were discussed from fracture mechanics viewpoint based on FEM analysis and detailed observations of growth behaviors of the ring crack and the cone crack. As compared to conventional SiC, the initiation strength was nearly equal to regardless of sphere sizes. However, its scatter was increased with increasing sphere sizes. Clusters are formed by uniting some in multi-micro cracks which occur near surface with increasing the contact load, one of that from which the ring crack occurs. Therefore, the strength and the scatter are controlled by the cluster size and its distribution, and the size depends on the generating crack density. It can be explained by the mechanics model which is expressed by the relationship between strength ratio and crack density based on the energy equilibrium theory.
