Boron suboxide compounds are of interest because of their low densities coupled with high hardness. In the present study, we have attempted to determine the nature of the BxO phases that occur in the field defined by pressures of zero to 1.5 GPa, temperature between 1200-degrees and 1700-degrees-C, and the compositional range 2/3 less-than-or-equal-to x less-than-or-equal-to 24. Amorphous boron powder and boric acid B2O3 were the starting reactants for all the runs. The processing of the specimens was carried out in a controlled atmosphere furnace, a hot-pressing assembly, and in a piston-cylinder high-pressure apparatus within quasi-hydrostatic and inductively heated cell assemblies. After processing at elevated temperature and pressure, for compositions over the range 2/3 less-than-or-equal-to x less-than-or-equal-to 6, B2O3 (identical to the hexagonal starting material) and B6O (R3BARm) were the dominant phases present. For the compositions 6 less-than-or-equal-to x less-than-or-equal-to 24, B6O and rhombohedral B were the primary phases identified. In general, the hardness of the processed composites was dominated by the occurrence of B6O (approximately equivalent to B4C). However, there is some suggestion of particularly high values of hardness on a very localized scale in specimens near the B22O composition.
