The Role of Graphite Crystal Structure and Microstructure on the Shock-induced Graphite to Diamond Phase Transformation

Due to diamond’s scientific utility, the formation of diamond from shock-compressed graphite has been of broad interest since the 1960s. Despite continued research, studies on the shock-induced graphite to diamond phase transformation have reported a broad range of seemingly contradictory findings. In this work, we performed plate-impact shock compression experiments to better understand the effects of initial graphite crystal structure and microstructure on the shock-induced graphite to diamond phase transformation. Three graphite types studied were, listed by decreasing internal order: ZYB-grade highly oriented pyrolytic graphite (HOPG), ZYH-grade HOPG, and as-deposited pyrolytic graphite (PG). Wave profile transmission experiments showed rapid transformation for all graphite types though the PG transformation stress (46 GPa) was more than twice that of HOPG (22 GPa). Real time in-situ X-ray diffraction measurements during shock compression highlighted a likely cause for the different observed transformation stresses—additional c-axis compression was required to overcome the internal crystallographic disorder of PG and induce transformation to the cubic diamond structure. HOPG instead transformed to the rare hexagonal diamond polymorph, likely following a martensitic mechanism. Front-surface impact experiments showed that shock-formed hexagonal diamond has a larger longitudinal modulus than shock-formed or single crystal cubic diamond. These results suggest that hexagonal diamond is likely stronger than cubic diamond, making it a potential ultra-hard alternative to cubic diamond for numerous scientific and industrial applications.