Diamond loses title of world’s hardest material

A material known as lonsdaleite has displaced diamond as the “world’s hardest material”.

A material known as lonsdaleite has displaced diamond as the "world's hardest material".

Though diamond lost its title of the "world's hardest material" some time ago, to man-made nanomaterials of slightly greater toughness, now, a rare natural substance looks likely to leave them all far behind - at 58 percent harder than diamond.

According to a report in New Scientist, Zicheng Pan at Shanghai Jiao Tong University and colleagues simulated how atoms in two substances believed to have promise as very hard materials would respond to the stress of a finely tipped probe pushing down on them.

The first, wurtzite boron nitride has a similar structure to diamond, but is made up of different atoms.

The second, the mineral lonsdaleite, or hexagonal diamond is made from carbon atoms just like diamond, but they are arranged in a different shape.

Only small amounts of wurtzite boron nitride and lonsdaleite exist naturally or have been made in the lab so until now no one had realised their superior strength.

The simulation showed that wurtzide boron nitride would withstand 18 percent more stress than diamond, and lonsdaleite 58 percent more.

If the results are confirmed with physical experiments, both materials would be far harder than any substance ever measured.

Rare mineral lonsdaleite is sometimes formed when meteorites containing graphite hit Earth, while wurtzite boron nitride is formed during volcanic eruptions that produce very high temperatures and pressures.

If confirmed, however, wurtzite boron nitride may turn out most useful of the two, because it is stable in oxygen at higher temperatures than diamond.

This makes it ideal to place on the tips of cutting and drilling tools operating at high temperatures, or as corrosion resistant films - on the surface of a space vehicle, for example.

Paradoxically, wurtzite boron nitride's hardness appears to come from the flexibility of the bonds between the atoms that make it up. When the material is stressed some bonds re-orientate themselves by about 90 degrees to relieve the tension.

Although diamond undergoes a similar process, something about the structure of wurtzite boron nitride makes it nearly 80 percent stronger after the process takes place, according to study co-author Changfeng Chen at the University of Nevada, Las Vegas, an ability diamond does not have.