Effect of zirconia addition on pressureless sintering of boron carbide

This paper presents the results of experiments on pressureless sintering of boron carbide with varying addition of zirconia (ZrO2: 0–30 wt.%). Green pellets were densified by sintering at 2275 °C in vacuum for 60 min and characterized by measurement of density, hardness, thermal conductivity and microstructure. Samples prepared with the addition of ≥5 wt.% ZrO2 showed higher densities in the range of 93–96% ρth, compared to 86.63% ρth for boron carbide only. Addition of ZrO2 was found to increase the hardness of sintered samples and regardless of ZrO2 content, the hardness values ranged between 30 and 31.5 GPa. XRD of the sintered pellets showed the presence of ZrB2. Optical microscope as well as electron probe microanalysis (EPMA) showed the presence of two phases, grey matrix with white precipitates. EPMA analysis of second phase revealed the presence of Zirconium in this phase. Fractography of boron carbide with 25% ZrO2 showed the failure to be by mixed fracture (transgranular and intergranular). Thermal conductivity values of the samples measured in the temperature range of 400–1000 °C were marginally higher with the addition of ZrO2.