Synthetic Diamond Strength Enhancement Through High Pressure/High Temperature Annealing

Abstract

ABSTRACTAnnealing of low inclusion, single crystals of synthetic Type I diamond at 1200–1700°C and 50–60kbar, was observed to increase average crystal compressive fracture strength by as much as 20%. The increase in strength is associated with the healing of lattice dislocations, coalescence of small (< 400 nm) metal inclusions, and dissociation of aggregatenitrogen. Photoluminescence and FTIR spectroscopic data indicate H3 and “A”-type defect centers are unstable relative to NVx and “C”-type defect centers under the conditions of these experiments. Dissociation rate is greater in the (111) growth sector than in the (100). Changes in residual lattice strain of the samples could not be detected using Raman spectroscopy and optical microscopy. A model reaction of the type, 2V + N2V = NV2 + NV, in which N = nitrogen and V = vacancy (i.e., N2V = H3 center) is proposed which provides a qualitative thermodynamic understanding of the observations reported in this study consistent with previous results of other researchers. In addition, photoluminescence examination of smaller crystals produced from a quench of early stage non-equilibrium growth reveal a lower concentration of aggregate nitrogen than those quenched from late stage growth suggesting that nitrogen may aggregate via in situ annealing processes. Ultimately, understanding mechanisms to improve synthetic single crystal diamond strength will lead to improved tools which utilize such crystals (e.g., drill bits and saws for cutting stone and concrete) which constitute a market of ≈ $2b worldwide.