Classification and Generations of Dental Zirconia

Abstract

Zirconium oxide (ZrO2) is polymorphic (temperature dependent) structure; zirconia can take three crystallographic forms at ambient pressure. Under normal conditions, pure zirconia is monoclinic (m). At (1170°C), the substance converts to a tetragonal crystal structure (t), then to a cubic crystal structure (c) at (2370°C), and finally to a fluorite structure above (2370°C), melting at (2716°C). During the heating and cooling cycles, the Zirconium oxide ceramic undergoes a hysteretic, martensitic t- m transformation, which is reversible at 950°C upon cooling. For dental applications, various types of zirconium-dioxide (zirconia) materials are available. These materials have a variety of chemical compositions, crystal configurations, manufacturing processes, and important variations in their mechanical and optical properties. Numerous generations of zirconia materials have been developed, ranging from the use of zirconia crystals as reinforcement elements in zirconia toughened alumina (ZTA) to partially stabilized zirconia (PSZ) and the conventional (3Y-TZP) to the appearance of new translucent zirconia materials such as cubic stabilized zirconia (CSZ).