ultrahard

Ultrahard is an informal descriptor used in materials science to refer to materials that exhibit extremely high hardness, typically exceeding the conventional threshold for superhard materials. In practice, ultrahard materials are those with hardness values high enough to resist penetration and deformation in extreme wear conditions, often assessed by Vickers hardness. The widely cited boundary for superhardness is 40 GPa; materials in the ultrahard category frequently reach or exceed tens of GPa, with diamond commonly cited as the standard reference, around 70–100 GPa in Vickers measurements depending on orientation and testing method. Cubic boron nitride is the closest synthetic competitor, with reported hardness near 40–60 GPa. Other materials that have been proposed as ultrahard include certain boron nitride phases such as wurtzite BN, ultra-dense carbon nitride phases, and various boron-rich carbides and borides; some claims remain contested or controversial.

Synthesis and properties: Ultrahard materials are usually produced by high-pressure, high-temperature synthesis or specialized deposition techniques such as chemical vapor deposition for diamond coatings. Their exceptional hardness stems from strong covalent bonds and stiff crystal lattices, with orientation effects often leading to anisotropic hardness in crystalline materials. Toughness tends to be a separate concern; ultrahard materials can be brittle and difficult to integrate into tools without careful design.

Applications and research: They are used for cutting, grinding, and wear-resistant coatings in industrial tooling, where high hardness improves tool life and performance. Research continues to discover new ultrahard phases, improve synthesis methods, and balance hardness with fracture toughness and thermal stability.