Oxidation behaviour of a boron carbide based material in dry and wet oxygen

The oxidation behaviour of a B4C based material was investigated in a dry a tmosphere O-2 (20 vol.%)-CO2 (5 vol.%)-He and also in the presence of moist ure H2O (2.3 vol%) as boron oxide is very sensitive to water vapour. The ma ss changes of samples consisting of a chemical vapour deposit of B4C on sil icon nitride substrates were continuously monitored in the range 500-1000 d egreesC during isothermal experiments of 20 h. The stability of boron oxide formed by oxidation of B4C was also studied in dry and wet atmospheres to explain the kinetic curves. In both atmospheres, oxidation is diffusion con trolled at 700 and 800 degreesC and enhanced by water vapour. At 900 degree sC and higher temperatures, boron oxide volatilisation and consumption by r eaction with water vapour modifies the properties of the oxide film and the material is no more protected. At 600 degreesC, B4C oxidation is weak but the process remains diffusion controlled in dry conditions as boron oxide v olatilisation is negligible. However, in the presence of water vapour, B2O3 consumption rate is significant and mass losses corresponding to this cons umption and to the combustion of the excess carbon are observed.