Transition from normal to severe wear in PCD during high-speed cutting of a ductile material

The observation of a 10-fold increase of tool wear during the high-speed cutting of ductile Cu-Pb alloys for journal bearings was the motivation of an in-depth analysis of the alloy and PCD-tools used in the corresponding industrial process. The high wear rate was found in the softer alloy batches and could be traced to the amount of skinpass reduction, which, while increasing hardness, diminishes toughness and the corresponding cutting energy, all other parameters being equal. At low wear rates, wear occurred by removal of small diamond particles by a brittle process. At high wear rates, clear evidence of plastic deformation was found. Parallel, needle-like inclusions were observed inside the diamond single grains, which may either represent deformation twins or martensitically formed lonsdaleite (hexagonal diamond). A review of recent literature and a qualitative analysis of the thermodynamics and mechanics of polymorphism in diamond confirm the possibility of such transformation and provide arguments against the occurrence of graphitisation as a wear mechanism under the present conditions.