Structure of superhard tungsten tetraboride: a missing link between MB2 and MB12 higher borides

Abstract

Superhard metals are of interest as possible replacements with enhanced properties over the metal carbides commonly used in cutting, drilling, and wear-resistant tooling. Of the superhard metals, the highest boride of tungsten--often referred to as WB4 and sometimes as W(1-x)B3--is one of the most promising candidates. The structure of this boride, however, has never been fully resolved, despite the fact that it was discovered in 1961--a fact that severely limits our understanding of its structure-property relationships and has generated increasing controversy in the literature. Here, we present a new crystallographic model of this compound based on refinement against time-of-flight neutron diffraction data. Contrary to previous X-ray-only structural refinements, there is strong evidence for the presence of interstitial arrangements of boron atoms and polyhedral bonding. The formation of these polyhedral--slightly distorted boron cuboctahedra--appears to be dependent upon the defective nature of the tungsten-deficient metal sublattice. This previously unidentified structure type has an intermediary relationship between MB2 and MB12 type boride polymorphs. Manipulation of the fractionally occupied metal and boron sites may provide insight for the rational design of new superhard metals.

Keywords: Rietveld refinement; borides; neutron diffraction; superhard; tungsten tetraboride.

Fig. 1.

Plot of average nanoindentation hardness versus displacement for WB₄, indicating superhardness (hardness above 40 GPa) from ∼60 nm displacement to ∼250 nm. The average hardness over this range is 40.9 ± 1.1 GPa with a maximum value of 41.7 ± 1.3 GPa at 95.25 nm. The shaded area represents the 95% confidence interval. (Inset) The full hardness curve from 0 to 850 nm. The average value of hardness from 60 to 850 nm is 39.7 ± 0.8 GPa. Numbers following the ± sign represent SDs.

Fig. 2.

Comparison of the various proposed structures of WB₄. (A) The structure of WB₄ by Romans and Krug (15). (B) The structure of W_1.83B₉ according to Nowotny et al. (16). (C) The structure of “W_1-xB₃” following Rosenberg and Lundström (17) and Zeiringer et al. (14) Green spheres represent boron atoms and gray spheres represent tungsten atoms. Partial occupancy is indicated by partial sphere filling. Bonds are shown to clarify the spatial arrangement only.

Fig. 3.

(A) SEM image of a sectioned W¹¹B₁₂ ingot in backscattered electron (compositional) mode indicating compositional uniformity of WB_4.2 (bright) grains. (B) Backscattered electron SEM image of a fractured ingot of an arc-melted sample in the ratio W:B of 1:12. Light regions are the tungsten-containing phase.

Fig. 4.

(A) Neutron and (B) X-ray powder diffraction patterns for the highest boride of tungsten. Red points indicate observed data; the green line represents the fit against the final model. The difference between the two is shown beneath (magenta line). The background has been subtracted for clarity. (C) The best fit to the neutron diffraction data without the inclusion of the trigonal boron clusters. (D) Three-dimensional Fourier difference map (yellow) from the neutron refinement overlaid on the boron-deficient model structure lacking interstitial boron. Please see SI Appendix for enlarged plots.

Fig. 5.

The proposed structure of the highest boride of tungsten.

Fig. 6.

(A) Occurrence of a cuboctahedron at the intersection of three unit cells. (B) Overlay of the UB₁₂ structure type on WB_4.2, showing a close similarity.