Vickers microindentation hardness of WB4-Ta solid solutions under loads ranging from 0.49-4.9 N.

Superhard Metals: Tungsten Tetraboride Solid Solutions

Richard B. Kaner , University of California-Los Angeles, DMR 1106364

With the shortcomings of traditional cutting tool materials such as tungsten carbide (relatively low hardness) and diamond (expensive), there is an increasing need for new materials to create high-performance, long-lasting tools. This has led to our research into new borides including hard osmium diboride (OsB2) and superhard rhenium diboride (ReB2). In our search for superhard borides that incorporate less expensive transition metals we have recently explored the synthesis and characterization of tungsten tetraboride (WB4), the hardest metallic material known to date as we reported in 2011.

To further enhance the hardness of this inexpensive superhard boride, we have successfully synthesized and fully characterized solid solutions of WB4 with tantalum (Ta). We have demonstrated that by adding just 2.0 at.% Ta to WB4 on a metals basis, its Vickers microindentation hardness (under an applied load of 0.49 N) increases by 18% from 43.3 GPa to 52.8 GPa (top-right Figure).

This hardness increase can be attributed to the defect structure of WB4, in which some tungsten atoms are missing and their positions are likely filled by boron in triangular nets (bottom-right Figure). Thus, the tantalum atoms, when added in a small quantity (~2.0 at.%), likely enhance the hardness due to the atomic size mismatch and valence electron difference between Ta and W.

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Ta Concentration in WB4 (at. %)

0.49 N0.98 N1.96 N2.9 N4.9 N

Crystal structure of WB4.

The Kaner and Tolbert groups are heavily engaged in a variety of outreach activities. In the picture on the left, Prof. Ric Kaner can be seen carrying out an outreach project at Crenshaw High School, which is in a working class neighborhood near downtown Los Angeles. Prof. Kaner went to Crenshaw High to discuss how the chemistry of materials can affect the everyday life of students and to convey the excitement of chemistry to several of their science classes. The trip included undergraduates from the UCLA Student Members of the American Chemical Society for which Prof. Kaner serves as the advisor. The high school students then participated in several demonstrations involving liquid nitrogen and materials ranging from polymers to metals followed by hands-on lab experiments run by our graduate students. These experiments helped engage the students to think about how materials affect them and the extensive possibilities of science.

Superhard Metals: Tungsten Tetraboride Solid Solutions

Richard B. Kaner , University of California-Los Angeles, DMR 1106364