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Refractory Diborides of Zirconium and Hafnium
Author(s) -
Fahrenholtz William G.,
Hilmas Gregory E.,
Talmy Inna G.,
Zaykoski James A.
Publication year - 2007
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2007.01583.x
Subject(s) - materials science , zirconium , ceramic , hafnium , sintering , microstructure , zirconium diboride , carbothermic reaction , ceramic matrix composite , refractory metals , solid solution , chemical engineering , composite material , metallurgy , carbide , engineering
This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB 2 ) and hafnium diboride (HfB 2 ) 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 ZrB 2 and HfB 2 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 B 2 O 3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi 2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB 2 and HfB 2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re‐entry, and rocket propulsion.