*Member, American Ceramic Society.
Refractory Diborides of Zirconium and Hafnium
Article first published online: 24 APR 2007
Journal of the American Ceramic Society
Volume 90, Issue 5, pages 1347–1364, May 2007
How to Cite
Fahrenholtz, W. G., Hilmas, G. E., Talmy, I. G. and Zaykoski, J. A. (2007), Refractory Diborides of Zirconium and Hafnium. Journal of the American Ceramic Society, 90: 1347–1364. doi: 10.1111/j.1551-2916.2007.01583.x
D. Green—contributing editor
At UMR, portions of this work were funded by the Air Force Office of Scientific Research (F49620-03-1-0072 and FA9550-06-1-0125), the National Science Foundation (DMR-0346800), and the Air Force Research Laboratory (FA8650-04-C-5704). At NSWCCD the work was funded by the Office of Naval Research on several contracts monitored by Dr. Steve Fishman.
- Issue published online: 10 MAY 2007
- Article first published online: 24 APR 2007
- Manuscript No. 22451. Received November 6, 2006; approved January 10, 2007.
This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.