Oxidation behaviour of boron carbide powder

Isothermal oxidation behaviour of powdered boron carbide (B4C) with the fine (1.52 μm), medium (22.5 μm) and coarse (59.6 μm) particle size has been studied in air ranging from 500 to 800 °C. The oxidation rate strongly depends on the particle size of boron carbide and temperature. The smaller particle size the higher oxidation rate of B4C powder due to its larger surface area. When B4C powder is oxidized in air, a B2O3 glass film is formed on the surface of B4C grain which retards the further oxidation reaction. The oxidation kinetics is approximately fitted to the diffusion-controlled rate law which can be described by the Jander's equation. The apparent activation energy for the fine-, medium- and coarse-B4C powders is 209.4 ± 11.4, 212.7 ± 35.8 and 219.2 ± 45.3 kJ mol−1, respectively, slightly varying with the impurity content of B4C powders. The type of rate law suggests that the diffusion of oxygen through the oxide layer is the rate-limiting step in the oxidation reactions. In addition, the change in the oxidation process at higher oxidation fraction might associate with the B2O3 volatilization at higher temperatures.