Surface Cracks of Solid-phase-sintered Silicon Carbide Ceramics and Their Influeces on Material Strength

The effects of surface cracks on bending strength of Solid-phase-sintered Silicon Carbide Ceramic (SSiC) were systematically investigated in aspects of crack size and inclined angle. The relation between the surface cracks and material strength revealed that the Vickers crack size is about 10 mu m after 0.5 N indentation load that begins to dominant the fracture of SSiC test bars. Vickers cracks no larger than 3 mu m are safe and do not harm to the general bending strength. It is found that inclined Knoop crack after 5 N or above is easier to deflect from the original direction and dominates the sample fracture process. And the relationship between the flexure strength sigma and inclined angle theta is found to correspond with sigma(proportional to)(sin theta)(-1/2). When the indentation load is smaller than 1-2 N, Knoop crack plays a similar effect as intrinsic defect that they randomly dominate the sample fracture process; and this randomness makes the impact of inclined angle on the flexure strength irregularly.