On dynamic surface instabilities of cracks in brittle crystals

Abstract

We investigated fracture surface instabilities generated during interaction of dynamic cracks with atomistic scale defects in brittle single crystals. We show that the initiation of the surface instabilities is caused by several well-defined variables. We generalized the phenomenon of surface instabilities by cleaving cubic silicon and germanium crystals with a variety of point defects.Well-defined low-speed atomistic-height wedge such as jogs were observed in cubic brittle silicon crystal doped with boron substitutional and oxygen interstitial, but not with phosphorus, and not in cubic germanium crystal doped with gallium. Pile-ups of the jogs are ended with a low speed sub-micron height ridges.A continuum based theoretical model using energy minimization law suggests that the generation of the jog-like surface instabilities in cubic crystals is conceivable due to the proximity of two low energy cleavage planes and depends upon crack speed and local internal chemical strain induced by the defect atom, as well as by the macroscopic asymmetry of the crystal. The jogs are formed to reduce the defect induced local chemical strain energy by creating new surfaces. Based on the current finding, we postulate that the fracture surface of ideal brittle crystals will presumably be stable (flat) at all crack speeds.