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

Abstract

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.

Microstructural Characterization of a Commercial Hot‐Pressed Boron Carbide Armor Plate

“…Graphitic inclusions were observed to be the most prominent second phase in this commercial boron carbide armor plate. The main benefit of adding carbon is to facilitate the sintering . Sintering of pure boron carbide is difficult due to low self‐diffusivity; and adding free carbon was reported to have substantially enhanced the boron carbide densification .…”

“…The main benefit of adding carbon is to facilitate the sintering. [32][33][34] Sintering of pure boron carbide is difficult due to low self-diffusivity; 35 and adding free carbon was reported to have substantially enhanced the boron carbide densification. 34,36 Carbon additions were also reported to inhibit boron carbide grain growth, which allows retention of relatively fine microstructures.…”

Oxidation effects on spark plasma sintering of molybdenum nanopowders

“…The carbothermic reduction produces the CO or CO 2 gas. Since the gas diffusion is a critical factor for the densification, the low uniaxial pressure of about 15 MPa was used for easy outgassing at the initial and intermediate stages of sintering in the SPS vacuum chamber. At the end of the carbothermic reduction, the pressure has been increased to 60 MPa to enhance the densification of Mo nanopowders.…”

“…Second, the oxide coating of metal powders can be removed by the addition of the carbon powders to starting powders, which results in the reaction of carbon and oxide (carbothermic reaction). This method has been used to remove the surface oxide during the pressure‐assisted or pressure‐less sintering of SiC, B 4 C, and ZrB 2. Since the oxide is removed via the gas phase, the gas diffusion is the rate‐limiting step for the complete removal of the oxide phase .…”

“…This method has been used to remove the surface oxide during the pressure‐assisted or pressure‐less sintering of SiC, B 4 C, and ZrB 2. Since the oxide is removed via the gas phase, the gas diffusion is the rate‐limiting step for the complete removal of the oxide phase . Third, during SPS, surface cleaning from the presence of high‐temperature plasma between particles can be used for an in situ oxide removal .…”