Superhard Material

What’s super hard material?

A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high bond covalency. As a result of their unique properties, these materials are of great interest in many industrial areas including, but not limited to, abrasives, polishing and cutting tools, disc brakes, and wear-resistant and protective coatings.

The way to find the new superhard materials

In the first approach, researchers emulate the short, directional covalent carbon bonds of the diamond by combining light elements like boron, carbon, nitrogen, and oxygen.

The second approach incorporates these lighter elements (B, C, N, and O), but also introduces transition metals with high valence electron densities to provide high incompressibility. In this way, metals with high bulk moduli but low hardness are coordinated with small covalent-forming atoms to produce superhard materials. Tungsten carbide is an industrially-relevant manifestation of this approach, although it is not considered super hard. Alternatively, borides combined with transition metals have become a rich area of superhard research and have led to discoveries such as ReB2, OsB2, and WB4.

Classification of superhard materials 

Superhard materials can be generally classified into two categories: intrinsic compounds and extrinsic compounds. The intrinsic group includes diamond, cubic boron nitride (c-BN), carbon nitrides, and ternary compounds such as B-N-C, which possess an innate hardness. Conversely, extrinsic materials are those that have super hardness and other mechanical properties that are determined by their microstructure rather than composition. An example of extrinsic superhard material is a nanocrystalline diamond known as aggregated diamond nanorods.

Diamond is the hardest known material to date, with a Vickers hardness in the range of 70–150 GPa. Diamond demonstrates both high thermal conductivity and electrically insulating properties, and much attention has been put into finding practical applications for this material. The properties of individual natural diamonds or carbonado vary too widely for industrial purposes, and therefore synthetic diamonds became a major research focus.

Synthetic diamond

The high-pressure synthesis of diamonds in 1953 in Sweden and in 1954 in the US made possible by the development of new apparatus and techniques, became a milestone in the synthesis of artificial superhard materials. The synthesis clearly showed the potential of high-pressure applications for industrial purposes and stimulated growing interest in the field. 

PDC cutter is a kind of super-hard material that compacts polycrystalline diamond with a tungsten carbide substrate. Diamond is the key raw material for PDC cutters. Because natural diamonds are difficult to form and take a long time, they are too expensive, and costly for industrial application, in this case, Synthetic diamond has played a great role in the industry. 

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