Modeling of Gas-Phase Transport and Composition Evolution during the Initial-Stage Sintering of Boron Carbide with Carbon Additions

Densification of B4C during sintering can be aided by removing the native B2O3(condensed) (B2O3(c)) layer present on the starting B4C powder. B2O3 can be removed by adding excess C and holding the powder compact at an intermediate temperature below the normal sintering temperature. This allows time for CO and minor boron gases to diffuse out from the porous compact before the pores close. This process was examined using a computational model based on codiffusion of multiple gas species, which enables prediction of the gas-and condensed-phase composition as a function of time and position in the specimen. The model, described previously elsewhere, was originally applied to the SiC/SiO2 system but has been adapted for the B4C/B2O3 system. The results are used to determine the optimum holding time for complete B2O3(c) removal as a function of key parameters, such as specimen thickness, particle size, temperature, etc. The role of gas-phase transport in residual C and B4C profiles is also examined.