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/B ₂O₃ 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.