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 constraint of the tetragonal-to-monoclinic phase transformation and its release during crack propagation. In other zirconia ceramics containing the tetragonal phase, the high fracture toughness is associated with ferroelastic domain switching. However, many of these attractive features of zirconia, especially fracture toughness and strength, are compromised after prolonged exposure to water vapor at intermediate temperatures (∼30°–300°C) 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 after 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.

The Tetragonal-Monoclinic Transformation in Zirconia: Lessons Learned and Future Trends

Zirconia-based solid electrolytes: microstructure, stability and ionic conductivity

This paper critically reviews the existing published experimental evidence for ageing mechanisms in zirconia ceramics. The first part of the paper is concerned with the material and environmental variables affecting the kinetics of the transformation. The current ageing mechanisms are presented and discussed and attention is drawn to the contradictions between the theories. Finally, techniques to retard or prevent the tetragonal-monoclinic phase transformation are presented.

Environmental degradation of zirconia ceramics

The solid oxide fuel cell (SOFC) continues to show great promise for the generation of electricity for an increasing range of applications. The present SOFC technology is based on an all-ceramic design, which eliminates the corrosion problems associated with fuel cells containing liquid electrolytes. To obtain good electrochemical performance with the currently used materials, this all-ceramic fuel cell operates at 1000°C. Despite a significant amount of research and several successful demonstrations at the 100 kW level, commercialisation of the technology is not as rapid as anticipated. This is, in part, due to the high operating temperatures required, necessitating the use of expensive materials. As a result of these problems, there has been an effort over the past few years to lower the SOFC operating temperature. This paper will address the issues concerning the development of new materials that can operate at lower temperatures. Many of these issues have been or are being addressed in the research performed at Argonne National Laboratory, and some recent results will be discussed.

Materials for lower temperature solid oxide fuel cells

Current knowledge of toughening mechanisms in zirconia ceramics is reviewed, with particular emphasis on the transformation toughening of yttria-stabilised polycrystalline tetragonal zirconia (Y-TZP). The physics and mechanics of transformation toughening are discussed, but the main aim is to provide a comprehensive understanding of the role of the metastable tetragonal zirconia phase on stress induced transformation toughening. The focus is on the microstructural variables governing the stability of tetragonal zirconia in the context of transformation toughening. The potential for additional toughening contributions from microcracking and ferroelastic toughening is briefly addressed and the coupling of toughening mechanisms is also discussed. Finally, a perspective on the future development of Y-TZP-based high toughness materials is given.

Toughening of yttria-stabilised tetragonal zirconia ceramics

Polycrystalline, tetragonal (t′) zirconia samples containing 3 and 4 mol% yttria were fabricated by annealing pressureless-sintered samples in air at ∼ 2100°C for 15 min. The grain size of these fully tetragonal samples was on the order of 100 to 200 μm. Domain structure of the samples and of a 3-mol%-yttria-doped tetragonal zirconia single crystal was examined by transmission optical microscopy under polarized light and by transmission electron microscopy. The orientations of the domain/colony boundaries were in accord with the predictions of group theory. As-polished surfaces of polycrystalline t′ materials showed no monoclinic phase even after 1000 h at 275°C in air. By contrast, conventionally yttria-doped tetragonal zirconia polycrystalline (Y-TZP) ceramics of grain size >0.5 μm showed substantial transformation. Surface grinding enhanced the resistance to degradation of Y-TZP but decreased that of t′ materials. Even then, the t′ materials exhibited better resistance to degradation than the Y-TZP ceramics. Excellent resistance of the t′ materials to low-temperature aging despite a very large grain size and the opposite effect of grinding on phase stability are all explained on the basis of ferroelastic domain structure of these materials.

Low-Temperature Aging of t′-Zirconia: The Role of Microstructure on Phase Stability

Large-grained (100- to 200-μm), yttria-doped, polycrystalline t'-zirconia ceramics were fabricated by heat-treating presintered samples at temperatures 2100°C. Polarized light microscopy revealed the ferroelastic domain structure in the t' samples. XRD showed that no monoclinic phase was detected on as-polished, ground and fracture surfaces, or on surfaces while under a tensile stress as high as 400 MPa. By contrast, relative changes occurred in the tetragonal peak intensities, which were attributed to ferroelasatic domain switching. The higher toughness of 3-mol%-Y2O3-doped t' samples (7.7 MPa · m1/2) compared to that of 8 mol% Y2O3 cubic samples (2.4 MPa · m1/2) was explained in part by ferroelastic domain switching.

Fabrication, Microstructural Characterization, and Mechanical Properties of Polycrystalline t'-Zirconia

As-received, yttria-doped (4.2 wt% Y{sub 2}O{sub 3}) single crystals of zirconia were heated to {ge}2100{degrees}C in air. Cube-shaped samples with faces perpendicular to (100) axes on the basis of the pseudocubic symetry were cut from the crystals. X-ray and electron diffraction indicate that the crystals are polydomain with (001) axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (perpendicular to the cube faces). The cube-shaped crystals were tested in compression at temperatures as high as 1400{degrees}C. X-ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along (100) on the basis of the pseudocubic symmetry were fractured in three-point bending at temperatures as high as 1000{degrees}C. X-ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000{degrees} was {approximately}12 and {approximately}8 MPa {center dot} m{sup 1/2}, respectively. Preliminary experiments on polycrystalline tetragonal zirconia samples containing 5.4 wt% Y{sub 2}O{sub 3} and sintered at {>=}2100{degrees}C also showed no evidence of the monoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples wasmore » typically 7.7 NPa {center dot} m{sup 1/2}. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness.« less

Ferroelastic Domain Switching in Polydomain Tetragonal Zirconia Single Crystals

Al2O3–ZrO2 and MgO–ZrO2 layered composites were fabricated in such a way that the outer layers of bar-shaped specimens consisted of the oxide (Al2O3 or MgO) and unstabilized ZrO2, while the bulk consisted of the oxide and partially stabilized ZrO2. During cool-down from the sintering or the annealing temperature, the outer layers expanded because of the tetragonal → monoclinic transition in ZrO2, thereby creating compressive stresses in the outer layers and tensile stresses in the bulk. Residual stresses were determined using a strain gage technique in which a strain gage was mounted on one face while the opposing face was incrementally ground off. Measurement of the strain as a function of thickness permitted the evaluation of residual stresses using pertinent equations from simple beam theory.

Measurement of Residual Stresses in Oxide–ZrO2 Three‐Layer Composites

Ferroelastic materials, which are characterized by the existence of a hysteresis between strain and stress, can exhibit enhanced toughness relative to the paraelastic, prototypic phase. Domain switching in the near crack tip stress field provides the requisite energy absorption mechanism and thus a toughening mechanism. Experimental evidence (X-ray diffraction) is presented on three different types of ferroelastics; namely, tetragonal (t') zirconia, gadolinium molybdate (GMO) and its terbium (TMO) and dysprosium (DMO) analogs, and lead zirconate titanate (PZT). It is shown that ferroelastic domain switching occurs in these materials in the vicinity of a crack tip. The fracture toughness of the ferroic phase is shown to be two to three times that of the nonferroic phase in all three materials. The results of the present study demonstrate that ferroelastic domain switching is viable toughening mechanism.

Jan Fong Jue

Papers

Low-Temperature Aging of t′-Zirconia: The Role of Microstructure on Phase Stability

Fabrication, Microstructural Characterization, and Mechanical Properties of Polycrystalline t'-Zirconia

Ferroelastic Domain Switching in Polydomain Tetragonal Zirconia Single Crystals

Measurement of Residual Stresses in Oxide–ZrO2 Three‐Layer Composites

The role of ferroelasticity in toughening of brittle materials