Shear-promoted graphite-to-diamond phase transition at the grain boundary of nanocrystalline graphite

Abstract

High pressure has traditionally been considered essential for the transformation of graphite into diamond. However, reducing the transition pressure required for this graphite-to-diamond (G2D) conversion holds significant appeal in both scientific research and engineering applications. In this study, we conducted large-scale molecular dynamics (MD) simulations using an environment-dependent interaction potential (EDIP) to examine the shear deformation of nanocrystalline graphite (n-graphite) with a grain size of approximately 6.5 nm. We discovered that the G2D transition pressure in n-graphite can be reduced to 2–3 GPa, significantly lower than the ∼90 GPa uniaxial stress required in crystalline graphite. This reduction is primarily due to concentrated local shear stresses at grain boundaries (GBs), which induce substantial rotations of graphite layers. These rotations facilitate the initial formation of diamond bonds at sites of pre-existing imperfections at the GBs, assisted by shear. Once initiated at the GBs, the G2D transition rapidly propagates within grains aligned parallel to the shear components, resulting in the formation of nanocrystalline diamond. Our findings underscore the critical roles of GBs and shear stress in enabling the G2D transition in n-graphite.

Keywords:

• Graphite
• diamond
• grain boundaries
• phase transition
• molecular dynamics

Acknowledgments

Part of the simulations was performed on the HPC cluster (Pronghorn) at the University of Nevada, Reno.

Disclosure statement

The authors declare no competing financial interests.

Additional information

Funding

The author(s) reported there is no funding associated with the work featured in this article.

Notes on contributors

Dezhou Guo

Dezhou Guo received his bachelor’s degree from the Beijing Institute of Technology in 2011, and his Ph.D degree in the University of Nevada, Reno in 2020. He is now the professor at the Beijing Institute of Technology. His research focuses on superhard materials, energetic materials, and surface chemistry using first-principles and molecular dynamics simulations.

Kun Luo

Kun Luo received his bachelor’s degree from Wuhan University of Science and Technology in 2010 and his Ph.D. from Yanshan University in 2017. He is now a postdoctoral fellow at Iowa State University. His research centers on investigating the atomic mechanisms of structural changes in materials to comprehend their novel properties.

Qi An

Qi An received his bachelor’s degree from the University of Science and Technology of China in 2002, and his Ph.D degree from Caltech in 2012. He is an Associate Professor in the Department of Materials Science and Engineering at the Iowa State University. His research focuses on quantum mechanical data-based computational materials science. He is particularly interested in machine learning force field, molecular dynamics simulations, quantum mechanics simulations, reactive force field, and applying these methods in advanced materials design.