Influence of the surface condition on the biaxial strength of a commercial silicon carbide

Abstract

Ceramic materials fail from defects which are distributed on the surface or in the bulk of specimens and components. The more dangerous surface flaws are often caused by improper machining or handling. In this investigation two possible ways to improve the surface quality and, as a consequence, increase the strength of a commercial silicon carbide ceramic are shown.In bending tests which apply a uniaxial stress state on the specimens only cracks perpendicular to the stress direction are dangerous but in biaxial tests cracks of any orientation may become fracture origins. Therefore, for this investigation, the biaxial Ball-on-Three Balls (B3B) test was used for the strength tests. To investigate the influence of surface finishing 6 samples of specimens were machined in different ways and tested with the B3B-test afterwards. It could be shown that a better surface finishing leads to a higher strength.In order to heal surface defects and to find the best healing conditions 5 samples of specimens were heat treated under different conditions and then tested with the B3B-test. For a better comparison of the obtained results all heat treated samples had the same surface finishing (ground with a grit size of 46μm). Microscopic investigations showed that defects are healed by the formation of a glassy layer. The layer fills cracks and pores (at least partially) and causes a significant increase of strength.