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

The coefficients of the piezospectroscopic tensor for a 0.05 wt% ruby are evaluated by direct measurement of the frequency shift for uniaxial compression along the three principal crystallographic directions and for loading in the three principal shear directions. Although the numerical value of the trace of the tensor is the same as previously reported, the piezospectroscopic coefficients along the c and m directions are slightly different. Contrary to expectation from the rotational symmetry of sapphire about the c axis, the coefficient along the a direction was found to be different from that along the m direction. It also varies nonlinearly with strain in the a direction. The difference is attributed to the applied stress breaking the rotational symmetry about the c axis and is interpreted in terms of the relative motion of the Cr 3+ ion with respect to the nearest-neighbor oxygen ions. An additional finding is that there is a significant difference in the piezospectroscopic tensor for the RI and R2 lines. This will facilitate a more complete determination of the stress tensor from fluorescence measurements than has hitherto been possible

Determination of the Piezospectroscopic Coefficients for Chromium‐Doped Sapphire

Mechanical and chemical consequences of the residual stresses in plasma sprayed hydroxyapatite coatings.

Fracture toughness of alumina and ZTA ceramics: microstructural coarsening effects

A procedure is described for the evaluation of the fracture behaviour of polycyrstalline ceramics using an in-situ stress analysis involving either fluorescence or Raman microprobe spectroscopy. The full details of the experimental set-up, calibration procedures and stress calculations are shown, and their reliability discussed in some detail. Stress maps were obtained along the wake of cracks stably propagated in six selected polycrystalline ceramics with representative microstructures. A conventional fracture mechanics assessment was concurrently performed to relate microscopic stress data to the macroscopic fracture resistance of the materials. It is demonstrated that the most effective toughening mechanism in polycrystalline ceramics takes place in the crack wake and arises from both elastic and frictional shielding tractions operated by crystallites which are left unfractured behind the crack tip. Copyright © 1999 John Wiley & Sons, Ltd.

In situ study of fracture mechanisms in advanced ceramics using fluorescence and Raman microprobe spectroscopy

In cooling sintered composites of Al2O3ZrO2 from their fabrication temperature residual stresses are created as a result of both the difference in thermal expansion between the two phases and the crystallographically anisotropic thermal expansion of the Al2O3 phase. In this work the first and second moments of the residual stress distribution have been measured as a function of volume fraction of zirconia from 0.01 to 0.90. The measurement technique used is piezo-spectroscopy based on the optical fluorescence from Cr3+ dopants in the alumina phase. For zirconia volume fractions up to 0.35 the average stress accurately fits the predictions given by the upper Hashin bound and this fit provides a value of the average thermal strain in the composites. Using this value, the effective medium approximation produces an excellent description of the average stress over the entire volume fraction. It is also shown that the fluorescence broadening due to stress fluctuations lies close to the predicted upper and lower Hashin bounds modified by the restrictions imposed by the principle of maximum entropy. The measured moments and those predicted by stochastic stress analysis compare well suggesting that the stochastic analysis provides a reliable method of calculating residual stress in composites.

J.D. French

Papers

Residual stresses in Al2O3ZrO2 composites: A test of stochastic stress models