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.