Boron suboxide ultrahard materials

B6O is a possible candidate as a superhard material with a hardness of 45 GPa measured on single crystals. The first dense superhard B6O-materials were produced under high pressure (1 GPa). However, recently it was found that different oxides can be utilized as an effective sintering additive allowing the reproducible densification at 50-80 MPa pressure at temperatures 1800-1900 degrees C. The resulting materials have similar hardness as the pure B6O materials but strongly increased fracture toughness. This article summarizes the state of the art of B6O materials - the densification, microstructure and resulting room and high temperature properties. It compares the B6O materials with boron carbide and cBN based materials. (C) 2012 Elsevier Ltd. All rights reserved.