The shear-driven transformation mechanism from glassy carbon to hexagonal diamond

Hexagonal diamond, a potentially superhard material, forms from a glassy carbon precursor at pressuresof ~100 GPa at the relatively low temperature of 400 C. The formation mechanism of the hexagonaldiamond phase was investigated by performing microstructural analysis on cross-sections of therecovered samples. Three distinct structures have been observed, a graphitic region near the centre of thesample with low density, a hexagonal diamond region at the edge of the sample with high density, and amixed region containing significant proportions of both the graphitic structure and hexagonal diamond.The hexagonal diamond was more likely to occur at greater radial distance from the centre of the samplewith some evidence for greater amounts also near the diamond anvil faces. The observed distribution ofthe hexagonal phase correlates well to regions of greatest shear strain expected from modelling studiesof strainfields in diamond anvil cells. Thefindings support the proposition that shear strain plays animportant role in the formation of hexagonal diamond, and that it may be a driving force for the naturaloccurrence of hexagonal diamond in the shear zone of meteorite impact craters.J. E. B. would like toacknowledge the Australian Research Council (ARC) (FT130101355)and J. E. B. and D. G. M. acknowledge funding under the ARC Dis-covery Project scheme (DP140102331 and DP150103487). The au-thors acknowledge the facilities, and the scientific and technicalassistance, of the Australian Microscopy&Microanalysis ResearchFacility at the RMIT Microscopy&Microanalysis Facility, at RMITUniversity