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

Abstract

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

Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4

“…However, increasing experimental evidence indicates that hexagonal WB 4 , the hardest TM boride known to date, has an asymptotic (i.e., load-independent) Vickers hardness of H V ≈ 30 GPa [10][11][12], which is close to those of pure γ-boron (~ 30 GPa) and β-boron (~ 26 GPa) but not in that superhard regime [25]. In addition, due to their layered stacking along the caxis of alternating TM layers and B dimers, hexagonal TM tetraborides are structurally unfavorable to form mechanically isotropic superhard materials [22].…”

“…As pointed out by Wang et al [22], the hardness of hexagonal TMB 4 is predominantly determined by the covalently B-B bonded network between the intercalated boron dimers in the parent lattice of transition metals. From this point of view, the primary role that TM lattice plays is a "place-holder" for boron dimers, which limits the hardness of hexagonal TMB x to the level of elemental boron and boron-rich compounds with asymptotic H V ≈ 26-30 GPa [25,26]. However, after an exhausting search of the known structures for TMB x , the orthorhombic TMB 4 (TM = Cr, Mn, and Fe) is found to exhibit a 3D bonding network between TM and boron [15,17,27], which is structurally more favorable than hexagonal TMB 4 for producing mechanically isotropic borides.…”

Pressure-induced zigzag phosphorus chain and superconductivity in boron monophosphide

“…Recently, B and P have been studied extensively in physical, chemical, and material science fields due to their interesting structural properties when pressure is applied45789. Boron has always been recognized as a complex element, both structurally and electronically: its crystalline phases are numerous and inevitably complicated, which is related to its electron deficiency and thus the tendency to form multicenter bonds57.…”

Surface Roughness of Poly-crystalline Cubic Boron Nitride after Rotary Ultrasonic Machining

“…Its value is usually over 8000 HV (over 80 GPa). The diamond hardness reaches the value up to 10,000 HV [21], [22] and [23]. For experiments, PCBN cylinder was used with the following dimensions: diameter 12 mm and length 20 mm.…”