Technical Comments

Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"

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Science  07 Dec 2007:
Vol. 318, Issue 5856, pp. 1550
DOI: 10.1126/science.1147650

Abstract

Chung et al. (Reports, 20 April 2007, p. 436) reported the synthesis of superhard rhenium diboride (ReB2) at ambient pressure. We show that ReB2, first synthesized at ambient pressure 45 years ago, is not a superhard material. Together with the high cost of Re, this makes the prospect for large-scale industrial applications of ReB2 doubtful.

Research on superhard materials (those with hardness higher than 40 GPa) is driven by both scientific and practical objectives: the desire to understand their structure and bonding, which determine unique properties of these materials, and the demand of modern technologies for robust materials with superior properties. Chung et al. (1) recently reported the synthesis of superhard rhenium diboride (ReB2) at ambient pressure. They reported a very high hardness value of 48 GPa and suggested impressive future applications and competitiveness of this material in a row of superhard materials. We question the validity of these claims.

Synthesis of pure ReB2 by reaction of rhenium with amorphous boron at high temperatures (1200 to 1500°C) and ambient pressure and its crystal structure were reported 45 years ago (2). It is not a novel material, and its previous development and discovery was not discussed in the Chung et al. article with sufficient emphasis, especially given that one of the three synthesis methods they reported is extremely similar to that used by LaPlaca and Post (2).

The ReB2 Vickers hardness (HV) of 48.0 (±5.6) GPa was estimated not in the asymptotic-hardness region [Fig. 1; reprinted from figure 2 in (1)], as recommended for hard and superhard materials (3), but at a very small load that is inappropriate for this class of solids. For soft materials in hardness testing, plastic deformation can be assumed, and the results can be easily interpreted. But for superhard materials, indentation is no longer controlled by plastic deformation alone, and issues such as brittle cracking and deformation of the indenting tip come into play. These effects change the hardness of a material with load, and attempting to infer the hardness of a material above the asymptotic leveling is not informative (3). This problem was discussed in detail at the International Workshop on Advanced Superhard Materials (Villetaneuse, France, 10 to 12 December 2003); the recommendations were published as a letter to the scientific community (3). For comparison, in our recent study of a superhard boron nitride nanocomposite, hardness reached 145 GPa at low loads, but we reported the asymptotic-hardness value of 83 GPa (4) (Fig. 2).

Fig 1.

(A) “Hv of ReB2 plotted as a function of applied load measured at room temperature, using a four-sided pyramidal diamond indenter tip. The average hardness increases from 30 to 48 GPa as the applied load decreases from 4.9 to 0.49 N (error bars, ±1 SD). (B) An ingot of ReB2 creates a scratch on the surface of a natural diamond parallel to the (100) plane.” [reprinted from figure 2 in (1)].

Fig. 2.

The load dependence of the Vickers hardness for the sample of superhard BN nanocomposite synthesized at 20 GPa and 1870 K. The inset shows a scanning electron microscopy image of a typical indentation produced by 20-s loading.

As seen in Fig. 1, the hardness of ReB2 in the asymptotic-hardness region reaches only 30.1 (±1.3) GPa (1), so this phase cannot be considered a superhard one. There are many carbides, nitrides, and borides with similar Vickers hardness [WC, 26 to 28 GPa (5); SiC, 27 to 31 GPa (6); TiB2, 33 GPa (6); ZrB2, 35 GPa (6)].

Demonstration of the ability of ReB2 to scratch a diamond surface is also problematic. The optical microscopy image presented by Chung et al. (Fig. 1B) gives the impression that this was not a true scratch, but rather a smearing of ReB2 on the surface of the diamond crystal. If the authors wanted to prove that the ReB2 indeed scratched the diamond, they should have provided more robust evidence such as an AFM (atomic force microscopy) map of the scratched area. However, even a proven scratch itself does not confirm superior hardness of ReB2, because it is well known that materials much softer than diamond can damage its surface (7). A scratch test is more of a quick field test for identifying minerals and cannot be considered a reliable scientific method in general.

The claim about prospective applications and competitiveness of ReB2 is questionable from the point of view of both the functional properties of ReB2 and its commercial value. First, in hardness ReB2 (HV ≈ 30 GPa) cannot even compete with commercially available polycrystalline cBN (HV > 40 GPa), which is successfully used for machining ferrous steels instead of diamond. Second, the cost of raw materials, particularly rhenium [which is six times as expensive as platinum and nine times as expensive as gold (8)], is much higher than that of other precursors for superhard materials synthesis. Thus, the prospect of producing hard-tool inserts from ReB2 seems unrealistic.

Methods of producing superhard coatings at ambient or very low pressure on an industrial scale, including chemical and physical vapor deposition, are well known. The search for alternatives to high pressure–high temperature methods of bulk hard material synthesis continues to be a worthy task.

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