X.. Jin Current Opinion in Solid State and Materials Science 9(2005)313-316 1500 Fig. 5. Morphology and shape memory effect of 8Ce-05Y-TZP ceramics manufactured by superfine particle and sintered at ambient, which shows 1.2% recovery strain upon pre-strained at room temperature followed by heat ove550°C 7. Stabilization of metastable tetragonal phase Table 1 Interfacial energies (Jm-) for monoclinic and tetragonal ZrO2under metastable tetragonal phase stabilization in zirconia was Interface o nditions reported by Garvie[4, "5) recently presented by Shukla and Seal [*5] Incoherent 146 The tetragonal phase can be stabilized at room tempera Partially coherent 0.73 0.55 ture in an isolated, single, strain free nanoparticle below crit- Coherent 0.29 0.22 ical size of 10 nm on account of the surface energy difference etween tetragonal and monoclinic phases(Table 1) besides of tetragonal structure in an isolated, strain free, spherically to a large specific surface area on the nanoscale[*4,**5, 37) shaped ZrOz nanocrystallite is because of the generation of size can be schematically shown as Fig. 6. The critical size size effecr en ion vacancies as a result of the"nanoparticle An schematic phase diagram including the effect of grain excess o the mechanism of tetragona increases to 33 nm owing to aggregation of ZrO2 nanocrys- phase stabilization in nanocrystalline ZrOz appears to be tallies [4. Various factors such as hydrostatic stra the same as that in doped ZrO2 at roo nergy, non-hydrostatic strain energy, structural similarity, undoped ZrOz at higher temperature". The excess oxygen ion vacancies may correlate with the excess volume,a significantly affect the tetragonal phase stability at room parameter used in the evaluation the contribution of surface temperature [38-42]. It is suggested that the stabilization layer to the whole Gibbs free energy by dilation model [44, 45]. The contribution becomes significant as the grain size of ZrO, nanoparticle below the critical value. The con cept that the presence of vacancies are essential for stabilize tion of the tetragonal phase was not accepted by all and more evidence is required [7] Tetragonal 8. Conclusions Characteristics(thermodynamics, kinetics and crystal Monoclinic graphics) of t-m martensitic transformation, related mechanism of transformation toughening and stabilization of metastable tetragonal phase at lower temperatures have been briefly reviewed. Combination of toughness and new functions makes TZP very attractive. Attention will be con- 1/2nm tinuously paid to the mechanism of t- m transformation in bulk as well as in nanocrystalline such as the role of Fig. 6. Schematic presentation of the effect of nanocrystallinite size on the vacancies in the stabilization of metastable tetragonal t- m martensitic transformation temperature under ambient. phase and the structural similarities between the evolving7. Stabilization of metastable tetragonal phase A detail review about mechanisms of room temperature metastable tetragonal phase stabilization in zirconia was recently presented by Shukla and Seal [**5]. The tetragonal phase can be stabilized at room tempera￾ture in an isolated, single, strain free nanoparticle below crit￾ical size of 10 nm on account of the surface energy difference between tetragonal and monoclinic phases (Table 1) besides to a large specific surface area on the nanoscale [*4,**5,37]. An schematic phase diagram including the effect of grain size can be schematically shown as Fig. 6. The critical size increases to 33 nm owing to aggregation of ZrO2 nanocrys￾tallines [*4]. Various factors such as hydrostatic strain energy, non-hydrostatic strain energy, structural similarity, foreign surface oxides, water vapor and anionic impurities, significantly affect the tetragonal phase stability at room temperature [38–42]. It is suggested that the stabilization of tetragonal structure in an isolated, strain free, spherically shaped ZrO2 nanocrystallite is because of the generation of excess oxygen ion vacancies as a result of the ‘‘nanoparticle size effect’’ [43]. Therefore, ‘‘the mechanism of tetragonal phase stabilization in nanocrystalline ZrO2 appears to be the same as that in doped ZrO2 at room temperature and undoped ZrO2 at higher temperature’’. The excess oxygen ion vacancies may correlate with the excess volume, a parameter used in the evaluation the contribution of surface layer to the whole Gibbs free energy by dilation model [44,45]. The contribution becomes significant as the grain size of ZrO2 nanoparticle below the critical value. The con￾cept that the presence of vacancies are essential for stabiliza￾tion of the tetragonal phase was not accepted by all and more evidence is required [*7]. 8. Conclusions Characteristics (thermodynamics, kinetics and crystallo￾graphics) of t ! m martensitic transformation, related mechanism of transformation toughening and stabilization of metastable tetragonal phase at lower temperatures have been briefly reviewed. Combination of toughness and new functions makes TZP very attractive. Attention will be con￾tinuously paid to the mechanism of t ! m transformation in bulk as well as in nanocrystalline such as the role of vacancies in the stabilization of metastable tetragonal phase and the structural similarities between the evolving 500 400 300 200 100 Strain (%) Temperature (˚C) 0 1 2 3 4 5 0 500 1000 1500 2000 Stress (MPa) Fig. 5. Morphology and shape memory effect of 8Ce–0.5Y–TZP ceramics manufactured by superfine particle and sintered at ambient, which shows 1.2% recovery strain upon pre-strained at room temperature followed by heating above 550 C. 1443 K 300 K 1/12 nm-1 Temperature 1/D Tetragonal Monoclinic Fig. 6. Schematic presentation of the effect of nanocrystallinite size on the t ! m martensitic transformation temperature under ambient. Table 1 Interfacial energies (J m2 ) for monoclinic and tetragonal ZrO2 under different conditions reported by Garvie [*4,**5] Interface Monoclinic Tetragonal Incoherent 1.46 1.1 Partially coherent 0.73 0.55 Coherent 0.29 0.22 X.-J. Jin / Current Opinion in Solid State and Materials Science 9 (2005) 313–318 317