But Natalia Dubrovinskaia, a crystallographer at the University of Bayreuth in Germany, notes that measuring the properties of superhard materials is problematic because it requires the use of an even harder material for reference. The Vickers hardness test, for instance, which the new study’s authors used to measure the hardness of nano-twinned boron nitride, gauges how a material responds to pressure from the point of a pyramid-shaped piece of diamond called an indenter. As increasing force (as measured in newtons) is applied to the diamond pyramid, the material’s ability to resist indentation levels off at its so-called asymptotic value (as measured in gigapascals). But the test is predicated on the idea that the diamond will do the indenting, and not the other way around. “If the indenter is softer than the material under probe, it is absolutely meaningless,” Dubrovinskaia says.
The pursuit of superhard materials is not just a quest to set records. Boron nitride already finds use in cutters that can slice through extremely tough materials, and Dubrovinskaia cites drilling for resource extraction as another application. “We still need really superhard materials to explore deeper and deeper into Earth’s interior,” she says. In some respects, such as stability at high temperatures, boron nitride is superior to diamond.
As such, she notes, “it probably would be a breakthrough in the field” if researchers proved that polycrystalline boron nitride boasted hardness values over 100 gigapascals. “The paper doesn’t provide any proof that the material is so hard,” she cautions. The data in the new study only show how the nano-twinned boron nitride responded to indentation loads with up to seven newtons of force. “But they report in the paper that they loaded at higher loads in this material and they obtained a lot of cracks around the imprint,” Dubrovinskaia notes. If the hardness measures dropped off at higher loads, she says, the true value for the boron nitride samples might be closer to 80 or 85 gigapascals. That measure would jibe with numbers she and her colleagues reported in 2007 for another high-pressure, high-temperature synthesis of nanostructured boron nitride. In that work, published in Applied Physics Letters, Dubrovinskaia and her colleagues presented data from Vickers testing with loads of up to 10 newtons.