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°C. SPS resulted in dense specimens with a density of 98% of the theoretical density and a mean grain size of 6 μm. 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°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 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.