Researchers working with a form of boron nitride discovered a unique mechanism that allows it to become harder than diamond, making it the hardest material known. This would not last however, as this mechanism was extended into a different form of diamond for an even harder material, making boron nitrides reign as the hardness champion short, but sweet.
By Todd Morton | Last updated February 13, 2009 12:30
Boron nitride makes great nanotubes, too.
Diamond has long been regarded as the practical and theoretical hardness champion and has found uses ranging from industrial abrasives to a girl's best friend. Cubic boron nitride, a synthetic material, comes in at second place. That is, until researchers got their hands on an alternate crystal structure of boron nitride and observed a unique atomic bond rearrangement under high stresses. These vaulted it above diamond for raw hardness, but it didn't wind up keeping the crown long.
Alternate crystal structures sharing the same chemical composition, called allotropes, are common in nature. The different atomic bond orientations and planes of symmetry within the crystal give rise to different physical properties. Researchers had investigated the Wurtzite structure of boron nitride (w-BN, a repeating array of hexagons in 3-dimensional space) in the past, but it had inferior properties to the cubic structure of boron nitride mentioned above.
But using a standard hardness test, which indents the material with a small pyramid and measures the force required, researchers recently observed that the w-BN in fact easily exceeded diamond’s hardness value, 114 Gpa (16.5 million pounds/square inch), versus diamond’s 97 Gpa. There also turned out to be surprises in the mechanisms at work to generate that awesome value.
The geometry of the hardness indenter dictates that the stress it puts on the material will not be limited only to one direction relative to a crystal plane in the material. This results in more complicated stresses that allow the bonds to reorder themselves into a slightly different structure; in w-BN's case, this reordering massively increased the material strength. The hardness value increased a whopping 78 percent in the w-BN after the bond-flipping occurred.
Unfortunately for our friend w-BN, the good times did not last. A challenger to its newly claimed hardness title quickly emerged: hexagonal diamond (called lonsdaelite), an allotrope of diamond, shares a similar structure to w-BN. The same bond-flipping process occurred when hardness indentions were performed, and the all-carbon structure outperformed both diamond and w-BN with a strength of 152 GPa, which is 58 percent higher than regular diamond.
The samples tested were small, and the researchers pointed out that the material synthesis techniques are not well-developed. Lonsdaelite is a rare but naturally occurring mineral—if you consider massive meteorites impacting the Earth’s surface to generate the conditions needed for synthesis "natural." The potential for ultra-hard nanocomposites remains within reach, however.
With Valentine’s Day quickly approaching, why not consider the gift that says "This unattractive grey mineral, like my love for you, crushes puny diamonds”? We’re sure she’ll understand.
Physical Review Letters. DOI: 10.1103/PhysRevLett.102.055503