Hardness of as-synthesized cubic boron nitride (111) bulk single crystals

The cubic boron nitride is the second most important superhard materials. To date, the effect of crystallographic anisotropy of the {111} plane on the Knoop microhardness of cubic boron nitride single crystals has not been experimentally obtained. Herein, we measure the detailed Knoop hardness and Vickers hardness for the {111} plane of the amber cubic boron nitride. We find that the crystal orientation dependence of Knoop hardness for amber samples is remarkable and has an obvious regular pattern. The Knoop hardness for amber samples shows a minima about 5° from ⟨110⟩ directions, which is different from the traditional viewpoint that the minimum hardness is on ⟨110⟩ directions. Raman spectra of cubic boron nitrides are measured and help us to explain the hardening of samples.