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 1wt% added carbon was hot pressed at temperatures of 2000 degrees C and 2100 degrees C. Nominally pure ZrB2 hot pressed at 2100 degrees 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 0.5wt% enabled densification to proceed to >96.5% of theoretical at 2000 degrees C. Raman spectroscopy revealed the formation of boron carbide (B4.3C) in specimens with carbon additions of 0.75wt%. The formation of B4.3C was eliminated via a 1wt% 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 7-10m (2000 degrees C) and 12-16m (2100 degrees C) and only appeared to be controlled by temperature, as no trends due to the evaluated carbon or oxygen contents were observed.