Softening of ultra-nanocrystalline diamond at low grain sizes

TL;DR

This study uses atomistic simulations to analyze how ultra-nanocrystalline diamond softens at smaller grain sizes, revealing the role of interfacial atoms and providing scaling formulas for its properties.

Contribution

It introduces a quantitative model describing the softening of ultra-nanocrystalline diamond as grain size decreases, based on atomistic simulation data.

Findings

Softening observed with decreasing grain size.

Interfacial atoms contribute to softening.

Scaling formulas for cohesive energy and bulk modulus.

Abstract

Ultra-nanocrystalline diamond is a polycrystalline material, having crystalline diamond grains of sizes in the nanometer regime. We study the structure and mechanical properties of this material as a function of the average grain size, employing atomistic simulations. From the calculated elastic constants and the estimated hardness, we observe softening of the material as the size of its grains decreases. We attribute the observed softening to the enhanced fraction of interfacial atoms as the average grain size becomes smaller. We provide a fitting formula for the scaling of the cohesive energy and bulk modulus with respect to the average grain size. We find that they both scale as quadratic polynomials of the inverse grain size. Our formulae yield correct values for bulk diamond in the limit of large grain sizes.