Fracture and property relationships in the double diboride ceramic composites by spark plasma sintering of TiB ₂ and NbB ₂

Bulk titanium diboride–niobium diboride ceramic composites were consolidated by spark plasma sintering (SPS) at 1950°C. SPS resulted in dense specimens with a density exceeding 98% of the theoretical density and a multimodal grain size ranging from 1 to 10 µm. During the SPS consolidation, the pressure was applied and released at 1950 and 1250°C, respectively. This allowed obtaining a two-phase composite consisting of TiB ₂ and NbB ₂ . For these ceramics composites, we evaluated the flexural strength and fracture toughness and room and elevated temperatures. Room-temperature strength of thus produced bulks was between 300 and 330 MPa, at 1200°C or 1600°C an increase in strength up to 400 MPa was observed. Microstructure after flexure at elevated temperatures revealed the appearance of the needle-shape subgrains of NbB ₂ , an evidence for ongoing plastic deformation. TiB ₂ –NbB ₂ composites had elastic loading stress curves at 1600°C, and at 1800°C fractured in the plastic manner, and strength was ranged from 300 to 450 MPa. These data were compared with a specimen where a (Ti,Nb)B ₂ solid solution was formed during SPS to explain the behavior of TiB ₂ –NbB ₂ ceramic composites at elevated temperatures.