It was a long-held belief that cotunnite, the highest-known pressure phase of the mineral zirconia, was mechanically hard enough to be used for industrial abrasives, and therefore could possibly replace expensive diamond. It turns out, that is not the case.

New Mexico State University physics PhD student Yahya Al-Khatatbeh found the material has a low mechanical hardness and does not qualify as superhard, a property needed when being used for industrial purposes such as drill bits, saws or other cutting tools. His discovery was published in Physical Review B, an American Physical Society journal, earlier this month.

Al-Khatatbeh explains in a synopsis of his research that it has long been speculated that materials synthesized at high pressures provide a promising route to develop materials that can replace diamond as an industrial abrasive. Zirconia is one of these materials. It possesses a very dense high-pressure phase called cotunnite.

Through experiments and theoretical computations, Al-Khatatbeh found that, against common belief, hardness does not necessarily increase in high-pressure phases. The relationship between hardness and incompressibility is more complicated than previously thought.

From a practical point of view, this suggests that scientists and engineers must look for other materials, most likely non-oxides, to replace diamond as an abrasive, said Boris Kiefer, physics professor and Al-Khatatbeh's co-adviser.

Al-Khatatbeh said zirconia's initial structure, its phase at normal conditions, is actually better for mechanical applications because of its hardness. Future testing will include a material called hafnia, which should have properties close to that of zirconia.

Testing the zirconia, compressing it to its cotunnite phase, takes an instrument called a Diamond-Anvil Cell, or DAC. It is a hand-held compression device that allows sub-millimeter materials to undergo extreme pressure between two diamonds. In his experiments, Al-Khatatbeh used diamonds 0.2 to 0.3 millimeters in surface diameter.