Thermal shock resistance of polycrystalline cubic boron nitride

The effect of thermal shock on the flexural strength has been investigated experimentally. It was found that the variation in flexural strength with quench temperature was influenced by the CBN grain size. Polycrystalline material containing small CBN grains showed a discontinuous drop in measured flexural strength above a material dependent critical quench temperature difference, ΔTc. The sharp decrease in measured strength is accompanied by unstable crack propagation. Material containing a significantly larger CBN grain size exhibited a gradual decrease in strength above the critical quench conditions. The experimental observations agreed with an established theory developed for thermal shock of alumina. The theoretically calculated critical temperatures agree well with the observed experimental data for each material when a flaw size equal to the CBN grain size is employed.