The Tetragonal-Monoclinic Transformation in Zirconia: Lessons Learned and Future Trends Je´roˆ me Chevalier and Laurent Gremillard w University of Lyon, INSA-Lyon, MATEIS, Villeurbanne FR-69621, France Anil V. Virkar Department of Material Science & Engineering, University of Utah, Salt Lake City, Utah 84112 David R. Clarke School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 Zirconia ceramics have found broad applications in a variety of energy and biomedical applications because of their unusual combination of strength, fracture toughness, ionic conductivity, and low thermal conductivity. These attractive characteristics are largely associated with the stabilization of the tetragonal and cubic phases through alloying with aliovalent ions. The large concentration of vacancies introduced to charge compensate of the aliovalent alloying is responsible for both the exceptionally high ionic conductivity and the unusually low, and temperature independent, thermal conductivity. The high fracture toughness exhibited by many of zirconia ceramics is attributed to the con- straint of the tetragonal-to-monoclinic phase transformation and its release during crack propagation. In other zirconia ce- ramics containing the tetragonal phase, the high fracture tough- ness is associated with ferroelastic domain switching. However, many of these attractive features of zirconia, especially fracture toughness and strength, are compromised after prolonged expo- sure to water vapor at intermediate temperatures (B301–3001C) in a process referred to as low-temperature degradation (LTD), and initially identified over two decades ago. This is particularly so for zirconia in biomedical applications, such as hip implants and dental restorations. Less well substantiated is the possibility that the same process can also occur in zirconia used in other applications, for instance, zirconia thermal barrier coatings af- ter long exposure at high temperature. Based on experience with the failure of zirconia femoral heads, as well as studies of LTD, it is shown that many of the problems of LTD can be mitigated by the appropriate choice of alloying and/or process control. I. Introduction Z IRCONIA has been one of the most important ceramic mate- rials for well over a century but the discovery of trans- formation toughening in 1975 1 heralded new visions for new high-performance applications of zirconia, ranging from bearing and wear applications to thermal barrier coatings (TBCs) to, most recently, biomedical applications. The subsequent discov- ery that zirconia could also be toughened by ferroelastic switch- ing 2 gave further confidence in the application of zirconia ceramics in critical applications. Nevertheless, despite the suc- cess of zirconia in many new applications, it has become appar- ent that certain zirconia compositions can also have an Achilles heel, namely their propensity to low-temperature degradation (LTD) in the presence of moisture. This is a kinetic phenomenon in which polycrystalline tetragonal material slowly transforms to monoclinic zirconia over a rather narrow but important tem- perature range, typically room temperature to around 4001C, depending on the stabilizer, its concentration, and the grain size of the ceramic. The transformation occurs by a nucleation and growth process and typically begins at the surfaces of polycrys- talline ceramics. It has all the characteristics of being an iso- thermal martensite. Also, although there continues to remain some uncertainty as to the precise mechanism by which moisture causes destabilization of the tetragonal phase, the observation that the kinetics of LTD are similar to those of oxygen vacancy diffusion suggests that the transformation occurs by the in- diffusion of a moisture species with an activation energy similar to that of oxygen vacancy diffusion. In practical terms, LTD is, in effect, an alternative to crack propagation, stress-induced transformation for the transformation from metastable tetrag- onal to monoclinic (t–m) zirconia (see Fig. 1). In this feature article we describe the role of phase trans- formations responsible for the impressive combination of me- chanical properties of zirconia, their relationship to equilibrium and metastable phase diagrams, and the phenomenon of LTD. We include the effects of transformations at free surfaces be- cause these affect the surface finish that is important for many Feature D. J. Green—contributing editor w Author to whom correspondence should be addressed. e-mail: laurent.gremillard@ insa-lyon.fr Manuscript No. 26208. Received April 27, 2009; approved 1 July 2009. J ournal J. Am. Ceram. Soc., 92 [9] 1901–1920 (2009) DOI: 10.1111/j.1551-2916.2009.03278.x r 2009 The American Ceramic Society