Effect of sintering techniques on the microstructure and mechanical properties of niobium borides

This study reports on the consolidation of niobium boride powders by cold pressing/pressureless sintering (PS, with or without Co addition) and spark plasma sintering (SPS) techniques. Niobium boride powders (containing NbB, Nb3B4 and NbB2 phases) which were mechanochemically synthesized from Nb2O5, B2O3 and Mg blends and purified by HCl leaching were used as precursor material. The effects of different sintering techniques on the consolidation behaviour, microstructure, densification rate and mechanical properties (microhardness, wear characteristics, elastic modulus, indentation response and fracture toughness) of the bulk products were investigated. PS with Co addition resulted in superior properties than those yielded after PS without Co addition and SPS. Bulk niobium boride products having relative density of 93–98%, microhardness of 24–28 GPa, wear volume loss of 2.8–4.5 × 10−4 mm3, elastic modulus of 508–552 GPa and fracture toughness of 4.05–4.74 MPa m1/2 were obtained by Co (2 or 5 wt.%) activated PS at 1500 °C.