Synthesis of B6O powder and spark plasma sintering of B6O and B6O–B4C ceramics

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Hard boron suboxide (B6O) is a difficult-to-consolidate ceramic material that requires extreme processing conditions to achieve sintering activation and is only consolidated readily at high pressures above 4 GPa. In this contribution, for the first time, we report the consolidation of hard and tough laboratory-synthesized B6O by the spark-plasma sintering (SPS) technique. The density of SPS-sintered specimens of >= 98% is reported, and an optimal combination of 34 GPa hardness and 4 MPa·m1/2 fracture toughness is achieved by controlling the amount of glassy phase boron oxide (B2O3) with an appropriate SPS set up. The effects of the type of protection used, i.e., graphite die lined only with graphite foil, BN coating, or tantalum foil, on the phase compositions and properties of bulk were studied. Finally, composites of boron suboxide and boron carbide, B6O–xB4C (x = 0, 3, 5, 10, 20, 40 wt %) were fabricated by SPS, and a significant improvement in the mechanical properties was achieved. Results showed that dense B6O–10 wt %B4C composite material with a hardness above 40 GPa and a fracture toughness of 4.8 MPa·m1/2 were obtained.
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