page_title: Superhard material
section_title: Other boron-rich superhard materials
hierarchical_section_title: Superhard material / Other boron-rich superhard materials
caption_attribution_description: English: Structure unit of B4C
context_page_description: A superhard material is a material with a hardness value exceeding 40 gigapascals 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.
  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 of this material. However, diamond has several limitations for mass industrial application, including its high cost and oxidation at temperatures above 800 °C. In addition, diamond dissolves in iron and forms iron carbides at high temperatures and therefore is inefficient in cutting ferrous materials including steel. Therefore, recent research of superhard materials has been focusing on compounds which would be thermally and chemically more stable than pure diamond.
context_section_description: Other hard boron-rich compounds include B₄C and B₆O. Amorphous a-B₄C has a hardness of about 50 GPa, which is in the range of superhardness. It can be looked at as consisting of boron icosahedra-like crystals embedded in an amorphous medium. However, when studying the crystalline form of B₄C, the hardness is only about 30 GPa. This crystalline form has the same stoichiometry as B₁₃C₃, which consists of boron icosahedra connected by boron and carbon atoms. Boron suboxide (B₆O) has a hardness of about 35 GPa. Its structure contains eight B₁₂ icosahedra units, which are sitting at the vertices of a rhombohedral unit cell. There are two oxygen atoms located along the (111) rhombohedral direction.
caption: Boron carbide
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