Freeze casting for near‐net‐shaping of dense zirconium diboride ceramics

Zirconium diboride (ZrB₂) was formed into dense complex shapes using freeze casting as a near‐net‐shaping technique. Aqueous‐based formulations were compared with nonaqueous (cyclohexane) based formulations in terms of rheological behavior, particle packing in the green body, sintered density, macroscale porosity, and cracking. The influence of particle solids concentration and freezing rate was investigated. The aqueous formulations were found to be deficient in that they produced macroscale porosity that could not be eliminated during sintering resulting in low density and large pores in the final shaped objects. The nonaqueous‐based system was able to produce complex shaped objects with significantly reduced macroscale porosity. The higher concentration of solids in the nonaqueous‐based formulations was primarily responsible for the reduced macroscale porosity and enabled higher sintered densities (up to 90%‐91.5% of theoretical density for fast freezing). The microstructure of the ZrB₂ formed at fast freezing rates and high solids content typically had isolated pores in the order of 5‐10 μm in size, mainly found along grain boundaries (grain sizes between 20 and 50 μm). Although this rapid freezing produced denser components, it tended to produce objects with internal cracks. When slower freezing rates were used, intricate complex shaped objects could be produced without cracks but their density was only between 65% and 80% of theoretical density.