The influence of microstructure on the mechanical properties of polycrystalline diamond: a literature review

Polycrystalline diamond (PCD) is an extremely high-performance cutting tool material used in the machining of rock, high-strength, non-ferrous metal alloys and carbon-fibre-reinforced composites. It is favoured for its exceptional hardness and wear resistance which results in at least an order of magnitude improvement in performance over previous technologies in almost all metrics. However, PCD suffers from unpredictable brittle fracture and degradation at high temperature during service which limits its capabilities in cutting applications. The literature on the link between its microstructure and its mechanical properties, including strength, toughness and flaw size distribution as measured by pseudo-static tests, is investigated. The conclusions of the seminal paper on this topic are re-examined in the light of modern ceramics research and an alternative explanation is put forth for the strength–grain size relationship published in this paper. All known literature values for strength and toughness vs. grain size and binder content are collated showing no overall trend in strength with binder content but moderate trends in all other combinations. The common claim of weak grain boundaries is brought into question in the light of the lack of any evidence of this fracture mode being evident in pseudo-static tests. The industrial literature on wear testing and failure modes of PCD in service and service-like tests is examined to bridge the gap between pseudo-static and dynamic, application-based experiments. Six main failure modes are recorded and summarised with intergranular fracture being the most conspicuously absent from the pseudo-static tests. It is suggested that the temperature generated by friction in dynamic tests causes the weakening of grain boundaries, resulting in a transition from transgranular to intergranular fracture and a call for further research in this area is made.