Vickers hardness test, four-point bending test and fracture toughness evaluation by two kinds of indentation (ISB and IF methods) were conducted on reaction sintered and pressure-less sintered silicon carbide ceramics, SiC-R and SiC-PL. The dependences of hardness Hv, bending fracture strength σf and fracture toughness KIC, on the indented load P were investigated. The effects of loading rate and atomosphere on the σf were also studied.
The Hv decreased with increasing P for two SiC materials. The indented shapes were clear comparatively and four cracks initiated from vertices of the indentation for SiC-PL. For SiC-R, however, a part of the indented surface peeled off by the effect of lateral cracks initiated near the surface layer, and the fracture surface was not flat. For the smooth specimens, the mean value of σf of SiC-PL was higher than that of SiC-R. For the specimens pre-cracked by Vickers indentation, σf decreased with increasing P for the both materials, and the mean value and the variation of σf for SiC-R was larger than those for SiC-PL. For SiC-R, the dependences of σf on the strain rate and atomosphere such as in water and in silicon oil were not observed in the present experiment conditions. The KIC by the ISB method and by the IF method were almost identical in the same P condition. The KIC by the IF method decreased with increasing P for SiC-PL, while the KIC increased with increasing P for SiC-R. This might be caused by peeling-off of the indented surface layer of SiC-R.
