Controversy about ultrahard nanotwinned cBN.

Arising from Y. Tian et al. 493, 385-388 (2013)Tian et al. report synthesis of 'nanotwinned' cubic boron nitride ('nt-cBN'). These authors claim that its unprecedented Vickers hardness of 108 GPa is due to nanotwinning, and that hardening of cBN is continuous with decreasing twin thickness down to the smallest size investigated, in contrast to the expected reverse Hall-Petch effect. We demonstrate here that it has been known for tens of years that 'nt-cBN' (refs 2-4) hardens owing to a complex of phenomena associated with its microstructure and defects; we also consider that Tian et al. provide no proof that the hardness of 'nt-cBN' is larger than 85 GPa, which is the previously reported maximum for nanostructured boron nitride. Thus the claim of continuous hardening down to a few nanometres twin thickness is, in our opinion, unjustified. There is a Reply to this Brief Communication Arising by Tian, Y. et al. Nature 502, http://dx.doi.org/10.1038/nature12621 (2013). [ABSTRACT FROM AUTHOR]