Room and high temperature flexural failure of spark plasma sintered boron carbide

Dense (95-98.6%) bulk boron carbide prepared by Spark Plasma Sintering (SPS) in Ar or N2 atmospheres were subject to three-point flexural tests at room and at 1600 °C. Eight different consolidation conditions were used via SPS of commercially available B4C powder. Resulting specimens had similar grain size not exceeding 4 µm and room-temperature bending strength (σ25 °C) of 300-600 MPa, suggesting that difference in σ25 °C is due to development of secondary phases in monolithic boron carbide ceramics during SPS processing. To explain such difference the composition of boron carbide and secondary phases observed by XRD and Raman spectroscopy. The variation in intensity of the Raman peak at 490 cm−1 of boron carbide suggests modification of the boron carbide composition and a higher intensity correlates with a higher room-temperature bending strength (σ25 °C) and Vickers hardness (HV). Secondary phases can modify the level of mechanical characteristics within some general trends that are not dependent on additives (with some exceptions) or technologies. Namely, HV increases, σ25 °C decreases, and the ratio σ1600 °C/σ25 °C (σ1600 °C - bending strength at 1600 °C) is lower when fracture toughness (KIC) is higher. The ratio σ1600 °C/σ25 °C shows two regions of low and high KIC delimited by KIC=4.1 MPa m0.5: in the low KIC region, boron carbide specimens are produced in nitrogen.