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

Abstract

The role of carbon additions and oxygen content on the densification of zirconium diboride (ZrB 2 ) was studied. ZrB 2 with up to 1 wt% added carbon was hot pressed at temperatures of 2000°C and 2100°C. Nominally pure ZrB 2 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 (B 2 O 3 ) 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 (B 4.3 C) in specimens with carbon additions of ≥0.75 wt%. The formation of B 4.3 C was eliminated via a 1 wt% addition of zirconium hydride (ZrH 2 ), 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 lm (2000°C) and 12-16 lm (2100°C) and only appeared to be controlled by temperature, as no trends due to the evaluated carbon or oxygen contents were observed.

Effect of Internal Current Flow During the Sintering of Zirconium Diboride by Field Assisted Sintering Technology

“…The high‐boron phase is located at the multiple grain junctions and inside the enlarged grains. This second phase is observed by other authors when the starting ZrB 2 powder contains carbon, as it is the case of our commercial ZrB 2 powder. The second phase is boron carbide (B 4 C) and is produced through chemical reactions during the sintering process of ZrB 2 .…”

“…(d)]. These inhomogeneities were observed only for the first 0.5 mm from the edge and are relatively common in FAST/SPS specimens . Interestingly, the concentration of the second phase varied as a function of the used setup.…”

Sintering Mechanisms and Kinetics for Reaction Hot‐Pressed ZrB2

“…Relative density as a function of sintering temperature for a ZrB 2 ceramics from this study compared to previous reports …”

“…Harrington et al . reported that commercial ZrB 2 powder with a starting particle size of ~2 μm achieved a relative density of ~90% after ~22 min when hot‐pressed at 2100°C with 32 MPa . Rates reported for monolithic ZrB 2 densified by SPS are higher than RHP with relative densities above ~90% achieved after ~2 min at 1800°C by SPS compared to ~60 min to achieve the same density by RHP .…”

Nanostructured titanium, zirconium and hafnium diborides: the synthesis, properties, size effects and stability

“…Numerous experimental data (see, e.g., Refs 51 and 52) mainly confirm these estimates. Taking hot-pressed (2000 ± 2100 8C) commercially available ZrB 2 powders (H C Starck, Germany) with particles 1.5 ± 3 mm in size as examples, 55 it was shown that small additives of carbon and zirconium hydride favour removal of oxygen and B 2 O 3 and lead to more intense densification and, correspondingly, to higher relative density of samples, which can be as high as 96% ± 99%. However, the grain size is 7 ± 10 (2000 8C) and 12 ± 16 mm (2100 8C).…”