Densification Behavior and Microstructure Evolution of Hot-Pressed HfB2

Densification behavior and microstructure evolution of hot-pressed HfB2 were studied. When unmilled HfB2 was hot pressed at 2200 degrees C, the resulting ceramics contained open porosity (85.8% (th.)). In contrast, attrition-milled HfB2 containing similar to 0.7 wt% WC-Co milling contamination could be hot pressed to > 98% density at temperatures as low as 1900 degrees C. The addition of either boron carbide (4 wt%) or carbon (2 wt%) improved densification and reduced the temperature necessary to reach full density to 1750 degrees and 1850 degrees C, respectively. Full density (> 99%) was achieved for additive free, attrition-milled HfB2 at temperatures of 1950 degrees C or higher. Fully dense HfB2 was also produced with the addition of 1 wt% carbon, although 2.1 vol% residual carbon remained in the microstructures after densification. The combination of carbon additions and WC impurities, introduced during milling, resulted in the formation of (Hf,W)C-0.98 solid solution inclusions. (Hf,W)C-0.98 formation suggested that carbon reacted with HfO2 impurities, which were present on particle surfaces from powder processing. The improved densification behavior with the addition of boron carbide or carbon suggests that both additives increase hafnium and/or boron mobility.