The surface failure of fluorapatite single crystals

Abstract

Single crystals of various material are used for basic studies of friction and wear because of their simplicity. Single pass sliding enables one to sort out the effects influencing wear such as anisotropy and environment. A review of the literature is given for studies with metallic single crystals, diamond, magnesium oxide, and sapphire. Results are presented for studies on fluorapatite single crystals. Frictional anisotropy was found to be similar to that in MgO single crystals even though fluorapatite and MgO have different lattice structures. A new finding is the fact that repeated sliding at contact stress states above a threshold value causes catastrophic surface damage after the second pass even though very little damage appeared on the first pass. Most previous studies report the results of single passes. In practical sliding systems where repeated sliding is always occurring, wear of brittle materials must be dominated by the catastrophic mechanisms found on the second pass rather than by the events that occurred during the first pass.