The effect of oxygen and sintering parameters on Spark Plasma Sintering (SPS) of B4C

This project was carried out in two parts:

In the first part, presence of oxygen in boron carbide (B4C) powder can result in deterioration of mechanical properties of the sintered sample. In this study, the potential effects of reaction between boron oxide (B2O3) and free carbon (C) as common impurities of commercial B4C powder were discussed. The emission of different gas species during B4C sintering, due to the reaction between B2O3 and C, was predicted. The importance of C/B2O3 ratio was also discussed and the ideal ratio was determined. It was shown that both B2O3 and C can be consumed completely by using the B4C powder with the ideal C/B2O3 ratio. It was also predicted that the B4C powder in carbon deficit region is highly prone to coarsening and the sintered product can have low density and hardness. It was suggested to adjust the C/B2O3 ratio in B4C powder slightly in carbon rich region, to keep the system away from carbon deficit region. The results of thermodynamic modeling were then applied in selected B4C sintering experiments. It was shown that thermodynamic modeling can provide critical clues to design more effective approaches and sintering parameters based on the sintering technique adopted.

In the second part, the effects of sintering temperature, heating rate, and holding time on the density and hardness of the spark plasma sintered B4C were investigated. Experimental results are compared with the predictions from computational thermodynamics. It is explained how the choice of sintering parameters can affect the mechanical properties of the sintered samples. The fundamental mechanisms of how the sintering parameters affect the properties of the sintered B4C are discussed with the sintering experiments and the predictions from the CALPHAD (Calculation of Phase Diagrams) approach. The effect of the number of graphite foil layers to pack the powder was also investigated. It is proposed that increasing the number of graphite foil layers may increase the driving force for the C-B2O3 reaction to proceed. Higher density and hardness is thus achieved with the removal of free carbon and B2O3 from the sample.