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B6O is a possible candidate of superhard materials with a hardness of 45 GPa measured on single crystals. Up to now, densification of these materials was only possible at high pressure. However, recently it was found that different oxides can be utilized as effective sintering additives. In this work the effect of addition of Y2O3/Al2O3 on the densification behaviour as a function of applied pressure, its microstructure evolution, and resulting mechanical properties were investigated. A strong dependence of the densification with increasing pressure was found. The material revealed characteristic triple junctions filled with amorphous residue composed of B2O3, Al2O3 and Y2O3, while no amorphous grain-boundary films were observed along internal interfaces. Mechanical testing revealed on average hardness of 33 GPa, a fracture toughness of 4 MPam1/2, and a strength value of 500 MPa. [ABSTRACT FROM AUTHOR]