Mechanism of phase transition from OLCs with different structures to nPCD at high temperature and high pressure

Abstract

The molecular dynamics (MD) calculation and experimental research methods were used to study the phase transition from onion-like carbon (OLC) with and without a diamond crystal core to nano-polycrystalline diamond (nPCD) under high temperature and high pressure (HTHP) conditions. The results showed that the transformation from OLC to nPCD was mainly a diffusionless transformation at the heating stage and a diffusional transformation at the cooling stage. There were two main paths for diffusionless transformation from the OLC phase to nPCD. The path (PATH1) was that the diamond grew along the graphite crystal direction [002] with the coherent interface formed by the cubic diamond crystal plane (111) parallel to the graphite crystal plane (002). Another path (PATH2) was that the diamond grew along the graphite crystal direction [120] with the coherent interface formed by the cubic diamond crystal plane (11–1) intersecting the graphite crystal plane (002). The initial path of diffusionless transformation of OLC with and without a diamond core was PATH1 and PATH2, respectively. Increasing the PATH1 phase variable can reduce the sintering conditions of the OLC phase to nPCD. The twin boundary content of nPCD can be improved by increasing the grain size consistency of the OLC and the stacking faults density of the OLC structure.