Superhard

Superhard is a term used in materials science to describe materials that exhibit exceptionally high hardness, typically defined as having a Vickers hardness above about 40 gigapascals (GPa). The term is often used to distinguish materials that can maintain their hardness under high contact stress, high temperatures, and wear. Superhard materials usually form strong covalent bonds in three-dimensional networks, giving high elastic moduli and high melting points. The most famous example is diamond, which has the highest known hardness and is widely used in cutting and grinding tools. Synthetic diamonds, produced by high-pressure, high-temperature (HPHT) methods or chemical vapor deposition (CVD), are common in industry. Cubic boron nitride (cBN) is another well-known superhard material, offering high hardness and chemical inertness, particularly when cutting ferrous metals, where diamond can react.

Other candidates include boron nitride in the wurtzite structure (w-BN) and various transition metal borides and carbides that have been investigated for high hardness. Polycrystalline forms, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN), are optimized for tools that require a combination of hardness, toughness, and thermal stability.

Measuring superhardness relies on indentation testing (Vickers or Knoop) and nanoindentation; hardness is only one aspect of performance. Fracture toughness, thermal stability, and chemical resistance also influence tool life. The field continues to explore new ultrahard materials and composites aimed at surpassing existing benchmarks while maintaining manufacturability.