Phase Transformation of Zirconia

A reverse t m transformation and related volume increase during the course of cooling the zirconia parts from the processing temperature, which exceeds the reported temperature for unconstrained transformation of 1174 6 °C [177], results in significant strains, which can be only accommodated by the formation of cracks. Thus, the fabrication of large parts of pure zirconia is not possible due to spontaneous failure on cooling. 

There exists a size interval for zirconia particles, where the tetragonal particles can be transformed by stress. If the particles are less than critical size they will not transform, but if they are larger than the critical size then they will transform spontaneously. The spontaneous transformation of overcritical partides facilitates the additional toughening mechanism known as microcraddng. On cooling through the transformation temperature, the volume expansion of 3-5% is 

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No phase transformation of zirconia was detected, using x-ray diffraction analysis, in the hot-pressed sinter at any of the HAP/PSZ ratios used in this study. 

E. Dow Whitney, Electrical resistivity and diffusionless phase transformation of zirconia at high temperatures and ultrahigh pressures, J. Electrochem, Soc. 112(1), 91-94 (1965). 

Figure 14.20 Microcrack process zone around a crack in an alumina-zirconia duplex ceramic, where the stress-induced phase transformation of zirconia particles induces microcracking. From Ref. [21],...

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

Fig. 6 Kinetics of the lamellar to hexagonal transformation of zirconia at different temperatures. Inset shows the temperature variation of the percentage of the hexagonal (H) phase in a fixed period of 2 h.

Budiansky B. and Truskinovsky L., On the Mechanics of Stress-Induced Phase Transformation in Zirconia, J. Mech. Phys. Solids 41, 1445 (1993). 

G.M. Woken, Diffusionless Phase Transformations in Zirconia and Hafnia, Journal of the American Ceramics Society, 46 9, pp.418-422 (1963). 

Figure 4.4 Hysteresis during monocline-tetragonal phase transformation of non-doped zirconia (Heimann, 2010b).

In room-temperature applications, phase transformation-toughened zirconia ceramics are some of the toughest stmctural ceramics available. However, one drawback of these materials is that the phase-transformation process ceases to occur as the temperature rises above a few hundred degrees. 

Characteristics of phase transformation of sol-gel derived alumina, zirconia and titania... 

Figure 7.16 Hysteresis during the monocline-tetragonai phase transformation of undoped zirconia.

An important toughening mechanism involves a phase transformation in zirconia (ZrOi). The best-known example is zirconia-toughened alumina (ZTA), which contains 10-20 vol% of fine ZrOi, particles. At elevated temperatures the equilibrium structure of Zr02 is tetragonal (t) and at low temperatures it is monoclinic (m). On cooling ZTA from the high temperatures required for fabrication the t —> m transformation may occur in the zirconia particles. This transformation is accompanied by an increase in volume of about 3%. 

H. (2001) Phase transformation of a zirconia ceramic head after total hip arthroplasty. I. Bone Joint Sure. Br., 83-B, 996-1000. 

In addition to improvements in experimental techniques to detect and simulate phase transformation in zirconia, clinical studies and retrieval studies have noted the tendency of certain components to exhibit surface roughening in vivo, presumably as a consequence of local phase transformation. In vivo changes in zirconia surface roughness are by no means universal, and observations of phase transformation are highly variable in zirconia from retrieval to retrieval. However, in several population-based studies of zirconia articulating against UHMWPE, a gradual increase in wear rates has been observed over time. 

Interestingly, the SiOC matrix was found to be effective in suppressing the corrosion induced phase transformation of the tetragonal oxide phase into monoclinic ZrOj/HfOj, which is a well known problem in the case of zirconia and hafnia materials exposed to hydrothermal conditions. Thus, the excellent behavior of the SiZrOC/SiHfOC-based ceramic nanocomposites relies on a unique synergistic effect related to the reinforcing ZrO /HfO phase and the SiOC matrix protecting against corrosion induced phase transformation of the oxide nanoparticles (Linck, 2012). 

Electrocatalytic reactions, such as the transformation of O2 from the zirconia lattice to oxygen adsorbed on the film at or near the three-phase-boundaries, which we denote by 0(a), have been found to take place primarily at these three phase boundaries.5 8 This electrocatalytic reaction will be denoted by ... 

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