Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Abstract

Zirconium diboride (ZrB 2 ) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B 2 O 3 and ZrO 2 ) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B 4 C were evaluated as additives to facilitate the removal of ZrO 2 . Reactions were proposed based on thermodynamic analysis and then confirmed by X‐ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as‐received ZrB 2 (surface area ∼1 m 2 /g) or attrition‐milled ZrB 2 (surface area ∼7.5 m 2 /g) with WC and/or B 4 C as a sintering aid. ZrB 2 containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B 4 C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.