On the nature of grain boundaries in nanocrystalline diamond

The atomic structures of a few representative large-unit-cell grain boundaries thought to largely determine the behavior of nanocrystalline diamond are determined via Monte-Carlo simulation. In these highly disordered grain boundaries up to 80% of the C atoms exhibit local sp(2) bonding. However, because the three-coordinated C atoms are poorly connected to each-other, graphite-like electrical conduction through the grain boundaries is unlikely without "bridging" impurities. Surprisingly, based on their fracture energies, the high-energy, large-unit-cell boundaries are more stable against brittle decohesion into free surfaces than low-energy ones and perhaps even the perfect crystal. (C) 1999 Acta Metallurgica Inc.