Effects of pressure on densification behaviour, microstructures and mechanical properties of boron carbide ceramics fabricated by hot pressing

Effects of pressure, from ordinary (30 MPa) to high pressure (110 MPa), on densification behaviour, microstructures and mechanical properties of boron carbide ceramics sintered by hot pressing are investigated. With increasing pressure, the relative density sharply increases within 30-75 MPa, slowly increases within 75100 MPa and finally stagnates. For samples within 75-100 MPa, densification begins at approximately 1000 degrees C, and the dominant densification process ends before the soaking stage. High relative densities of 98.49% and 99.76% are achieved. For samples within 30-50 MPa, densification begins at approximately 1500 degrees C, and the soaking stage (initial 20 min) is still important for the dominant densification process. The final relative densities are only 87.90% and 92.32%. The above -mentioned differences are derived from contributions of pressure, and the dominant densification mechanism under high pressure is plastic deformation. The average grain size of the samples slightly increases with increasing soaking time. The grain size under higher pressure is larger than that under lower pressure at corresponding periods because grains grow easily with reduced pores. Vickers hardness and fracture toughness increase as grain size decreases in fully dense samples. However, when the samples do not achieve full density, relative density becomes more influential than grain size in hardness and toughness. A soaking time of 30 min is enough for samples under 100 MPa. Prolonging the soaking time has deleterious effects on mechanical properties. The relative density, grain size, hardness and fracture toughness of the samples under 100 MPa for 30 min are 99.73%, 1.96 mu m, 37.85 GPa and 3.94 MPa m(1/2), respectively.