Edge flaking in diamond

The edge toughness of single crystal synthetic diamond was investigated. The diamond plates were indented using spherical diamonds on {1 0 0} surfaces near 〈100〉 edges and on {1 1 0} surfaces near 〈110〉 edges. The strain fields were examined qualitatively using birefringent imaging and finite element modeling. The edge toughness was found to be approximately 1500 N mm−1. A value of approximately 300 N mm−1 was expected from trends in the literature [Morrel R, Gant AJ. Edge chipping of hard materials. J Refract Met Hard Materials 2001;19:293-301]. The flakes formed were deeper and wider than those for other materials [Almond EA, McCormick NJ. Constant-geometry edge-flaking of brittle materials. Lett Nature 1986;321:53-55]. The dominance of the (1 1 1) cleavage plane has an effect on the geometry of the edge flake but is not exclusively responsible for the higher edge toughness. This is apparent since the edge toughness of polycrystalline diamond is also higher than predicted (2500 N mm−1 compared to 900 N  mm−1). The high edge toughness is therefore attributed to the low Poisson ratio and high Young's modulus in diamond. These findings are substantiated by the finite element models presented.