Reverse Hall-Petch effect in ultra nanocrystalline diamond

TL;DR

This study uses atomistic simulations to investigate the mechanical properties of ultra nanocrystalline diamond, revealing a reversal of the Hall-Petch effect at very small grain sizes due to grain boundary effects.

Contribution

It demonstrates the occurrence of the reverse Hall-Petch effect in ultra nanocrystalline diamond through detailed atomistic simulations and scaling relations.

Findings

Ultra nanocrystalline diamond becomes softer at smaller grain sizes.

The reverse Hall-Petch effect is attributed to grain boundary atom concentration.

Simulation results align with experimental observations.

Abstract

We present atomistic simulations for the mechanical response of ultra nanocrystalline diamond, a polycrystalline form of diamond with grain diameters of the order of a few nm. We consider fully three-dimensional model structures, having several grains of random sizes and orientations, and employ state-of-the-art Monte Carlo simulations. We calculate structural properties, elastic constants and the hardness of the material; our results compare well with experimental observations for this material. Moreover, we verify that this material becomes softer at small grain sizes, in analogy to the observed reversal of the Hall-Petch effect in various nanocrystalline metals. The effect is attributed to the large concentration of grain boundary atoms at smaller grain sizes. Our analysis yields scaling relations for the elastic constants as a function of the average grain size.