Cracking diamond anvil cells by compressed nanographite sheets near the contact edge

Uniformly cold-compressed nanographite sheets in diamond anvil cells (DAC) are found to transform from soft into hard phase at about 17 GPa using molecular dynamics simulations. The hard phase can reach the compressive strength of about 150 GPa. Finite element analyses show that high stress concentrations occur along the boundary of interface on the diamond-anvil culets contacted with the nanographite sheets. The concentrated compressive stress can exceed the strength of diamond in a ring region with the width about $0.2μm$⁠, when the average pressure in the graphite sample is 17 GPa as in [W. L. Mao et al., Science 302, 425 (2003)]. Within the narrow ring, superhard carbon phase can be formed from the nanographite sheets, which leads to cracking of the DAC near the contact edge.