Observation of higher stiffness in nanopolycrystal diamond than monocrystal diamond

Abstract

Diamond is the stiffest known material. Here we report that nanopolycrystal diamond synthesized by direct-conversion method from graphite is stiffer than natural and synthesized monocrystal diamonds. This observation departs from the usual thinking that nanocrystalline materials are softer than their monocrystals because of a large volume fraction of soft grainboundary region. The direct conversion causes the nondiffusional phase transformation to cubic diamond, producing many twins inside diamond grains. We give an ab initio-calculation twinned model that confirms the stiffening. We find that shorter interplane bonds along [111] are significantly strengthened near the twinned region, from which the superstiff structure originates. Our discovery provides a novel step forward in the search for superstiff materials.

Nanocrystalline hexagonal diamond formed from glassy carbon

“…Such a challenge is worth addressing because lonsdaleite offers exceptional properties including extreme hardness, potentially exceeding that of cubic diamond17. Further increases in hardness may be possible if lonsdaleite could be prepared in nanocrystalline form, as has been reported for nanocrystalline diamond18. However, experimental measurements of the properties of lonsdaleite have been hampered by an inability to produce pure samples.…”

Large indentation strain-stiffening in nanotwinned cubic boron nitride

“…This sharp contrast of a giant strength hardening in the classic reverse Hall-Petch regime, where a strength softening is normally expected, indicates a new mechanism governing the mechanical property of nt-cBN. It was found recently that twin interfaces in nano-grain diamond (ng-dia) can enhance its elastic constants 21,22 . The reported experiments and calculations showed, however, that the increase of the elastic constants, especially the shear modulus, induced by the twin interfaces in ng-dia is just a few percent 22 , which is too small to explain the nearly 100% increase of hardness observed in nt-cBN 6 .…”

Elasticity and hardness of nano-polycrystalline boron nitrides: The apparent Hall-Petch effect

“…Details of the optics appear elsewhere. 10 This is called Brillouin oscillation 33,34 and we can observe a specific frequency f component, which satisfies Bragg's condition under normal incidence…”

“…The unusual stiffening in diamond stems from local twinned (hexagonal diamond) structures. 10 Since fine-grain cBN also contains wBN, which corresponds to a local twinned structure in cBN, it is important to clarify whether the structure stiffening would occur or not for nanopolycrystalline cBN.…”