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.

Refractory Diborides of Zirconium and Hafnium

The effect of crystal anisotropy on the formation of grain-boundary microcracks is analyzed, by considering a planar array of hexagonal grains as a model of a polycrystalline ceramic. The stress singularities at triple-grain junctions are analyzed by an asymptotic method as well as by a numerical solution, and the critical size of a grain-boundary defect is investigated by a crack analysis. It is found that elastic anisot-ropies can significantly increase the stress levels near triple points, which results in a smaller critical grain size for microcracking.

Microcracking in Ceramics Induced by Thermal Expansion or Elastic Anisotropy

Grain size-microcracking trends are examined in the three pseudobrookite oxides MgTi2O5, Fe2TiO5, and Al2TiO5. These trends are toward smaller critical grain sizes, lower elastic moduli, and lower strengths with increased thermal expansion anisotropy. Extension of an energy criterion relates the micro-cracking transition to the inverse square of the maximum difference in single-crystal thermal expansion coefficients, Δαmax-2.

Grain Size/Microcracking Relations for Pseudobrookite Oxides

Recent advances in Li1+xAlxTi2−x(PO4)3 solid-state electrolyte for safe lithium batteries

An approximate analysis is presented of the toughening induced by the incorporation of second-phase particles subject to microcracking. It is demonstrated that the toughening becomes appreciable for a narrow size distribution of particles of appropriate size. The magnitude of the toughening is determined by the mismatch strain (due to thermal contraction incompatibility) and the microfracture resistance of the particle/matrix interface. Implications for designing optimally tough ceramics are presented.

Toughening of Ceramics by Circumferential Microcracking

The laser flash technique was used to study the effect of grain size on the thermal diffusivity of polycrystalline MgTi2O5 from 25° to 800°C. Microcracking decreased the thermal diffusivity by as much as a factor of two with the decrease increasing with increasing grain size. When specimens were heated then cooled, the thermal diffusivity exhibited a horizontal flat figure-eight hysteresis. This hysteresis, which appeared to be a function of the thermal history, was attributed to a balance between crack healing, or closure, at high temperatures and the growth of existing cracks or the formation of new cracks during cooling.

J. J. Cleveland

Papers

Grain Size/Microcracking Relations for Pseudobrookite Oxides

Effects of Grain Size and Microcracking on the Thermal Diffusivity of MgTi2O5

Grain size/microcracking relations for pseudobrookite oxides