By Rick C. Hodgin
Thursday, February 19, 2009 00:01
Chicago (IL) – Diamonds are made from carbon atoms that have undergone a process aligning them in an isometric-hexoctahedral crystal lattice. Clear to yellow, sometimes flawless, diamonds are also the second most stable form of carbon (behind graphite). However, diamonds are no longer the hardest natural substance — at least not according to New Scientist. That honor now falls to lonsdaleite, which is 58% harder than diamonds.
Lonsdaleite is a form of hexagonal diamond made from the same carbon atoms as a regular isometric-hexoctahedral diamond, except that the carbon atoms are arranged into a different shape — which also causes it to have a yellow to brown translucent color. In nature, Lonsdaleite is extremely rare and often impure, making it somewhat less hard than diamonds. Lonsdaleite has been observed formed in meteorites containing graphite which have impacted on the Earth.
Pure lonsdaleite created in laboratories is up to 58% harder than diamonds. Researchers from the University of Nevada and the University of Shanghai have reported that in simulations, the material resists indentation at pressures of 152 GPa, whereas diamonds typically break apart at 97 GPa.
Another competing hard substance which also steals the crown away from diamonds is wurtzite boron nitride, which is 18% harder. Wurtzite boron nitride is formed naturally during volcanic eruptions under high temperature and pressure, though for commercial uses it is formed via detonation. Wurtzite boron nitride’s strength is derived from its atomic bonds which actually bend up to 90 degrees under stress. This relieves and distributes tension over a wider area. It can be created in either metal-like plate or regular crystal structures.
According to UK abrasives, Wurtzite boron nitride “is the next step in the boron nitride generation which includes a well-known superabrasive – cubic boron nitride, as well as, a graphite similar in structure – hexagonal boron nitride. This new material has a polycrystalline structure that provides superior cutting properties and, moreover, sharpens itself during cutting. Polycrystalline structure means plenty of crystals are bonded chemically together that work at the same time. In this case the material removal rate and finishing level of the Wurtzite Boron Nitride cannot be compared to even the cutting characteristics of other well-known materials.”
Wurtzite boron nitride might actually prove to be of greater commercial interest in specialty applications because, even though it is less hard than lonsdaleite, it is stable in oxygen at higher temperatures than diamonds, making it suitable for many dry-cut applications, and is relatively easy to manufacture en masse.
Still, given that diamonds are almost unimaginably plentiful, they aren’t likely to be supplanted for most applications any time soon.