Tungsten tetraboride, an inexpensive superhard material

Abstract

Tungsten tetraboride (WB 4 ) is an interesting candidate as a less expensive member of the growing group of superhard transition metal borides. WB 4 was successfully synthesized by arc melting from the elements. Characterization using powder X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) indicates that the as-synthesized material is phase pure. The zeropressure bulk modulus, as measured by high-pressure X-ray diffraction for WB 4 , is 339 GPa. Mechanical testing using microindentation gives a Vickers hardness of 43.3 AE 2.9 GPa under an applied load of 0.49 N. Various ratios of rhenium were added to WB 4 in an attempt to increase hardness. With the addition of 1 at.% Re, the Vickers hardness increased to approximately 50 GPa at 0.49 N. Powders of tungsten tetraboride with and without 1 at.% Re addition are thermally stable up to approximately 400°C in air as measured by thermal gravimetric analysis.dispersion hardening | indentation hardness | intrinsic hardness | nano-indentation hardness | solid solutions I n many manufacturing processes, materials must be cut, formed, or drilled, and their surfaces protected with wearresistant coatings. Diamond has traditionally been the material of choice for these shaping operations, due to its superior mechanical properties (e.g., hardness > 70 GPa) (1, 2). However, diamond is rare in nature and difficult to synthesize artificially due to the need for a combination of high temperature and high pressure. Industrial applications of diamond are thus generally limited by cost. Moreover, diamond is not a good option for high-speed cutting of ferrous alloys due to its graphitization on the material's surface and formation of brittle carbides, which leads to poor cutting performance (3). Other hard or superhard (hardness ≥ 40 GPa) substitutes for diamond include compounds of light elements such as cubic boron nitride (4) and BC 2 N (5) or transition metals combined with light elements such as WC (6), HfN (7), and TiN (8). Although the compounds of the first group (B, C, or N) possess high hardness, their synthesis requires high pressure and high temperature and is thus nontrivial (9, 10). On the other hand, most of the compounds of the second group (transition metal-light elements) are not superhard although their synthesis is more straightforward.To overcome the shortcomings of diamond and its substitutes, we have been pursuing the synthesis of dense transition metal borides, which combine high hardness with synthetic conditions that do not require high pressure (11,12). For example, arc melting and metathesis reactions have been used to synthesize the transition metal diborides OsB 2 (13, 14), RuB 2 (15), and ReB 2 (16-20). Among these, rhenium diboride (ReB 2 ) with a hardness of approximately 48 GPa under a load of 0.49 N has proven to be the hardest (16, 21). The boron atoms are needed to build the strong covalent metal-boron and boron-boron bonds that are responsible for the high hardness of these materials (12). Because of this, it is expected th...

Exploring the high-pressure behavior of superhard tungsten tetraboride

“…17 Our shear modulus derived from the isotropic model is 249 GPa, comparable with the measured shear modulus of ReB 2 (223-276 GPa) 22,25 and nearly twice the value reported from theoretical calculations (104-129 GPa). 16 Although many assumptions went into calculating this shear modulus, the high value seems reasonable given the similar hardnesses of ReB 2 and WB 4 , and the known correlation between shear modulus and hardness.…”

“…GPa, which is only slightly higher than the value of 553.8 GPa derived from nanoindentation measurements, 17 but lower than the measured Young's modulus of ReB 2 (642-671 GPa). 29…”

“…Specifically, 1) both tungsten and boron are relatively inexpensive, 2) the lower metal content in the higher borides reduces the overall cost per volume of production and 3) the higher boron content lowers the overall density of the compound, which could prove to be beneficial in applications where light-weight is a critical asset. 17 Recently, Gu et al 18 synthesized WB 4 and they measured hardness values as high as 46.2 GPa and a bulk modulus of 304 ± 10 GPa by fitting the second-order Birch-Murnaghan equation of state (EOS). With an exceptionally-high first derivative K 0 ' of 15.3 ± 5.7, they obtained an extremely low value of the zero pressure bulk modulus K 0 of 200 ± 40 GPa using the third-order Birch-Murnaghan equation of state.…”

“…17 From high pressure X-ray diffraction results, a bulk modulus of 339 ± 3 GPa was obtained using a second order finite strain EOS. This value was 10% higher than the value reported by Gu et al 18 and was close to some of the value reported by Liu et al 19 In order to clarify the elastic moduli of WB 4 with higher accuracy and to further examine the lattice distortions of WB 4 under elevated pressure, we have undertaken a more complete experimental study of the pressure-dependent compression behavior of WB 4 using synchrotron-based angle-dispersive X-ray diffraction in the diamond anvil cell.…”

Pressure effect on structural, elastic, and thermodynamic properties of tetragonal B4C4

“…[1][2][3][4] Traditionally, diamond is a good choice for above operations due to its high Vickers hardness beyond 70 GPa. However, its oxidizable property leads to the formation of iron carbides at high temperature for high-speed cutting of ferrous materials including steel, and the high cost and difficulty for synthesizing diamond artificially further limit its industrial applications.…”