High-temperature strength and plastic deformation behavior of niobium diboride consolidated by spark plasma sintering

Bulk niobium diboride ceramics were consolidated by spark plasma sintering (SPS) at 1900 degrees C. SPS resulted in dense specimens with a density of 98% of the theoretical density and a mean grain size of 6 mu m. During the SPS consolidation, the hexagonal boron nitride (h-BN) was formed from B2O3 on the powder particle surface and residual adsorbed nitrogen in the raw diboride powder. The room-temperature strength of these NbB2 bulks was 420 MPa. The flexural strength of the NbB2 ceramics remained unchanged up to 1600 degrees C. At 1700 degrees 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 degrees C signs of plastic deformation were observed. Fractographic analysis revealed a number of etching pits and step-like surfaces suggestive of high-temperature deformation. The temperature dependence of the flexural strength of NbB2 bulks prepared by SPS was compared with data for monolithic TiB2, HfB2 and ZrB2. Our analysis suggested that the thermal stresses accumulated during SPS consolidation may lead to additional strengthening at elevated temperatures.