Polycrystalline γ-boron: As hard as polycrystalline cubic boron nitride

The Vickers hardness of polycrystalline γ-boron (γ-B) was measured using a diamond indentation method. The elastic properties of polycrystalline γ-B were determined using an ultrasonic measurement method. Under a loading force of up to 20 N our test gave an average Vickers hardness in the asymptotic hardness region around 30.3 GPa. We also measured the hardness and elastic properties of polycrystalline β-B and cubic boron nitride–Ti3SiC2 (PcBN) for comparison. The hardness and elastic properties of polycrystalline γ-B were found to be very close to those of PcBN.