Harder than nails

UCLA scientists have discovered a new approach to designing super-hard materials, which are used in drills, scratch-resistant coatings for precision instruments and watch faces.

Diamond is the hardest material known, because its carbon atoms form very short covalent bonds. Most of the diamond used in the world is synthetic and very expensive. Diamond powder is used for oil drills and machines that build roads and cut holes in mountains. Diamond cannot be used, however, to cut steel without ruining the diamond blade.

Cubic boron nitride is a diamond substitute used to cut steel. It is made synthetically under very high-temperature, high-pressure conditions, but it is even more expensive than diamond.

One way to make super-hard materials that are ‘ultra-incompressible,’ or resistant to shape deformation, is to look for metals that are already incompressible and try to make them hard by combining them with short, covalent bonds. .

In 2005, the UCLA research team combined the relatively soft element osmium, the most incompressible metal known, with small covalent-bond forming atoms to make a material that is almost as incompressible as diamond, yet is so hard that it scratches sapphire, which is ranked 9 on a hardness scale of 1 to 10.

They found that if they combined boron with osmium, the osmium atoms were pushed apart by only 10 per cent from where they were in osmium metal. They then searched for a transition metal that would give an expansion of less than 10 per cent, and settled on rhenium. From this they made rhenium diboride, which is as incompressible as diamond in one direction, and only slightly more compressible in the other.

At low applied forces, the hardness of rhenium diboride is equivalent to cubic boron nitride, the second-hardest material known. At higher applied forces, rhenium diboride is a bit below that. It is hard enough to scratch diamond, and much harder than osmium diboride.

While other super-hard materials, including diamond and cubic boron nitride, are made under expensive, high-pressure conditions, rhenium diboride is made in a simple process without applying pressure.