| B4C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates (TCS) or constant contraction rates (RCS). Weight losses after sintering were attributed to evaporation of B 2O3 coatings on B4C grains, which dominated the densification process. Densification started at 1860°C, the boiling point of B2O3, and continued sluggishly until 2130°C, at which point B2O3 had been largely extracted, and sintering ensued by solid state processes. In the temperature range 1860 to 2130°C, extensive grain coarsening occurred via solution and precipitation of B4C from the surfaces of small grains to the surfaces of large grains through the B2O3 liquid. This coarsening restricted the maximum possible density which could be obtained. Rapid heating through this temperature range facilitated higher terminal densities. 3 wt% carbon doping in the form of phenolic resin resulted in more dense sintered compacts, devoid of extensive coarsening. Carbon reacted with B2O3 to form B4C and CO gas, thereby extracting the B2O 3 coatings, permitting sintering to start at 1300°C. Specimens without carbon additions, exposed to a dilute H2 atmosphere (5%H2 --95%He), showed higher maximum densities, though not as high as carbon-doped specimens fired in He. The percent of theoretical density achieved without additives in He was 92.7%, without additives in dilute H2 was 93.7%, and with carbon additives was 98.6%. Hardness increases and indentation fracture toughness decreases with decreasing grain sizes in sintered B 4C were attributed to the effects of more rapid strain hardening associated with dislocation pileups at grain boundaries. |
