Fracture and property relationships in the double diboride ceramic composites by spark plasma sintering of TiB2 and NbB2

Bulk titanium diboride-niobium diboride ceramic composites were consolidated by spark plasma sintering (SPS) at 1950 degrees C. SPS resulted in dense specimens with a density exceeding 98% of the theoretical density and a multimodal grain size ranging from 1 to 10m. During the SPS consolidation, the pressure was applied and released at 1950 and 1250 degrees C, respectively. This allowed obtaining a two-phase composite consisting of TiB2 and NbB2. 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 330MPa, at 1200 degrees C or 1600 degrees C an increase in strength up to 400MPa was observed. Microstructure after flexure at elevated temperatures revealed the appearance of the needle-shape subgrains of NbB2, an evidence for ongoing plastic deformation. TiB2-NbB2 composites had elastic loading stress curves at 1600 degrees C, and at 1800 degrees C fractured in the plastic manner, and strength was ranged from 300 to 450MPa. These data were compared with a specimen where a (Ti,Nb)B-2 solid solution was formed during SPS to explain the behavior of TiB2-NbB2 ceramic composites at elevated temperatures.