Liquid phase assisted densification of superhard B6O materials

B6O-based materials are known as some of the hardest materials after diamond and cubic boron nitride with a hardness of 45 GPa measured on single crystals. Several attempts were made to produce B6O materials by hot pressing, but without success. Based on thermodynamic considerations the possibility of the use of sintering additives was discussed and the developed concepts were validated by densification of the materials using FAST (field assisted sintering technique)/SPS methods and analysing the microstructure and properties of the resulting materials. Two groups of materials were found to be suitable for the densification: transition metals which form borides with B6O, the elements of the first to fourth main groups of the periodic table and the rare earths (Sc, Y, and lanthanides) which are in equilibrium in the oxide form with B6O and form a liquid phase during densification at 1700–1900 °C. Superhard dense B6O materials were produced and their properties investigated.