Zirconias

Zirconias are ceramic materials composed primarily of zirconium dioxide (ZrO₂), often with stabilizing additives such as yttria (Y₂O₃) or magnesia (MgO). Pure zirconia has a high melting point, low thermal conductivity, and excellent mechanical strength, but it is also prone to cracking during cooling from the melt. The addition of stabilizers suppresses the high-temperature cubic phase, resulting in a mixed-phase microstructure that is resistant to thermal shock and hydrogen embrittlement.

The most common commercial form is 3 mol% yttrium‑stabilized zirconia (3Y‑SZ), which shows a notable transformation‑toughening effect. As the crystal lattice changes from tetragonal to monoclinic under stress, the material expands slightly, which closes cracks and increases toughness. This makes yttria‑stabilized zirconia suitable for high‑strength structural applications. Other stabilizing systems, such as 7 mol% or 8 mol% yttria‑stabilized zirconia, are more suitable for use as sound‑absorbing panels or cone‐shaped trim, while magnesia‑stabilized zirconia is preferred for thin, castable components that require high flexural strength.

Zirconias are produced by powder processing: raw materials are mixed, calcined, and then shaped by pressing or injection. Sintering at temperatures above 1500 °C causes densification, resulting in a material with grain sizes that can be slowly enlarged to improve toughness or kept small for optical clarity. After sintering, grains may be further processed by polishing or glazing, depending on the end use.

Applications of zirconias include dental crowns, bridges, and fillings, due to their biocompatibility, stain resistance, and translucency. In engineering, they function as high‑temperature structural components, thermal barrier coatings in gas turbines, wear‑resistant parts in marine and automotive industries, and components in solid‑oxide fuel cells. Other uses include nuclear waste immobilization, sensors, and refractive elements in optical devices. Despite their excellent mechanical properties, high‐cost production and susceptibility to low‑temperature degradation in humid environments limit some applications, prompting ongoing research into improved stabilizing chemistries and processing techniques.