Effect of Carbon and Oxygen on the Densification and Microstructure of Hot Pressed Zirconium Diboride



The role of carbon additions and oxygen content on the densification of zirconium diboride (ZrB2) was studied. ZrB2 with up to 1 wt% added carbon was hot pressed at temperatures of 2000°C and 2100°C. Nominally pure ZrB2 hot pressed at 2100°C achieved relative densities >95.5%. Carbon and oxygen analysis indicate that oxygen removal was facilitated by the reduction of oxides with carbon or the removal of boria (B2O3) as a vapor. Therefore, by removing oxides from the particle surfaces, carbon additions of ≥0.5 wt% enabled densification to proceed to >96.5% of theoretical at 2000°C. Raman spectroscopy revealed the formation of boron carbide (B4.3C) in specimens with carbon additions of ≥0.75 wt%. The formation of B4.3C was eliminated via a 1 wt% addition of zirconium hydride (ZrH2), as a source of zirconium, resulting in the formation of carbon as the only residual second phase. Grain sizes were in the range of 7–10 μm (2000°C) and 12–16 μm (2100°C) and only appeared to be controlled by temperature, as no trends due to the evaluated carbon or oxygen contents were observed.