Low-temperature pressureless sintering B4C with Si aids: Is it feasible?

The feasibility of fabricating dense boron carbide–SiC biparticulate composites by low-temperature pressureless sintering of B 4 C with Si aids was investigated. Firstly, it was found with 6 types of powder compacts (Si aids in the range 5–30 vol%) that low-temperature conventional pressureless sintering is unfeasible, with hardly any densification even after 1 h at 1600 °C (well above Si melting point) with as much as 30 vol% Si aids (excess Si). Secondly, it was also found with some of these same compacts during a more detailed study ( i.e. , sintering conditions of 1400–2000 °C for 15–60 min) that fast pressureless sintering is also unfeasible because either the densification is poor/insufficient or undesirable melting with phase decomposition occurs. Thirdly, it was found with powders of the model composition B 4 C with 20 vol% Si aids that fast pressure-assisted sintering is feasible at only 1400 °C for 15 min provided that a certain threshold pressure (75 MPa) is applied, which results in a superhard (∼30 GPa), fine-grained boron carbide–SiC biparticulate composite. Finally, these findings are justified considering that molten Si is a short-lived transient phase that reacts rapidly with B 4 C to form SiC and B 12 (C,Si,B) 3 , so external pressure is needed for it to be spread quickly and efficiently throughout the entire porous microstructure before being consumed (thus contributing to densification by enhanced pore filling and particle rearrangement).