Bulk niobium diboride ceramics were consolidated by spark plasma sintering (SPS) at 1900°C. SPS resulted in dense specimens with a density of 98% of the theoretical density and a mean grain size of 6 lm. During the SPS consolidation, the hexagonal boron nitride (h-BN) was formed from B 2 O 3 on the powder particle surface and residual adsorbed nitrogen in the raw diboride powder. The roomtemperature strength of these NbB 2 bulks was 420 MPa. The flexural strength of the NbB 2 ceramics remained unchanged up to 1600°C. At 1700°C an increase in strength to 450 MPa was observed, which was accompanied by the disappearance of the secondary h-BN phase. Finally, at 1800°C signs of plastic deformation were observed. Fractographic analysis revealed a number of etching pits and steplike surfaces suggestive of high-temperature deformation. The temperature dependence of the flexural strength of NbB 2 bulks prepared by SPS was compared with data for monolithic TiB 2 , HfB 2 and ZrB 2 . Our analysis suggested that the thermal stresses accumulated during SPS consolidation may lead to additional strengthening at elevated temperatures. K E Y W O R D Shigh-temperature strength, niobium diboride, spark plasma sintering

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Hard boron suboxide (B 6 O) 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 B 6 O 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·m 1/2 fracture toughness is achieved by controlling the amount of glassy phase boron oxide (B 2 O 3 ) 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, B 6 OxB 4 C (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 B 6 O10 wt %B 4 C composite material with a hardness above 40 GPa and a fracture toughness of 4.8 MPa·m 1/2 were obtained.