Mechanical behaviour at high temperature of spark plasma sintered boron carbide ceramics

Fine-grained and dense boron carbide ceramics are sintered by SPS process. The effect of the decrease of the impurities content by a heat-treatment of the starting powder is studied on the mechanical properties at room temperature (i.e. Vickers hardness and fracture toughness) and at high temperature (up to 1600 degrees C) by bending tests (i.e. elastic modulus, flexural strength). These properties are correlated with microstructural characteristics of both materials. It is shown that heat treatment of the powder decreases the contents of free carbon (exhibiting an onion-like structure) and of secondary oxide phases (silicon and boron-rich) at grain boundaries. It leads to a slight decrease of the hardness, fracture toughness and bending modulus, and an increase of the flexural strength. In addition, no brittle-to-plastic transition is noticed up to 1600 degrees C.