Zirconia - an overview

A historical perspective is provided through bench marks in the development of zirconia. The crystal structures of the three polymorphs-cubic, tetragonal and monoclinic-are reviewed. The martensitic nature of the monoclinic-tetragonal transformation is substantiated. Enhancement of strength and toughness through the phase transformation in partially stabilized zirconia, which led to important applications as extrusion dies and tool bits, is discussed. The process of stabilization, which enabled refractory applications of zirconia, is covered, followed by a discussion of the defect structure and consequent ionic conductivity of doped zirconia. The use of stabilized zirconia in oxygen sensors, fuel cells, etc., is based on its high oxygen ion conduction. Cubic zirconia, because of its high refractive index, is widely used as jewellery. The state of knowledge in each area is reviewed, and gaps in present understanding are highlighted for future studies