Jérôme Chevalier

A critical comparison of methods for the determination of the aging sensitivity in biomedical grade yttria-stabilized zirconia

Since the recent failure events of two particular series of zirconia femoral heads for total hip replacement prosthesis, a large decrease in the use of zirconia ceramics for orthopaedic implants has been observed. In spite of the biomedical success of this material during the last 10 years, this decrease in use was required for safety reasons, until the cause of the failures is known. It has been shown that these failures were related to the low temperature hydrothermal degradation (also known as aging). Thus, it is crucial to better understand the aging behavior, in order to be able to assess its importance and then control it if required. In this study, various techniques relevant to assess the hydrothermal degradation sensitivity of biomedical grade yttria-stabilized zirconia are discussed and compared. The expected outputs of conventional methods, that is, X-ray diffraction and scanning electron microscopy are examined. More recent methods like optical interferometry and atomic force microscopy are presented, with their respective benefits and drawbacks. An up-to-date comparison of these different techniques is provided, and their use for ensuring the long-term reliability of a particular batch of zirconia in terms of aging degradation is demonstrated.

Low-temperature ageing of zirconia-toughened alumina ceramics and its implication in biomedical implants

Changes in crystalline phases resulting from low-temperature ageing of different yttria doped and non-doped zirconia-toughened alumina composites and nanocomposites were investigated under controlled humidity and temperature conditions in autoclave. A classical powder mixing processing route and a new modified colloidal processing route were used to process the composites. Different compositions ranging from 2.5 wt.% zirconia in a matrix of alumina to pure zirconia (3Y-TZP) were studied. It was observed that Al2O3+yttria stabilised ZrO2 composites exhibited significant ageing. However, ageing was much slower than traditionally observed for Y-TZP ceramics, due to the presence of the alumina matrix. Ageing was clearly limited for zirconia content beyond 25 wt.%. On the other side of the spectrum, Al2O3+2.5 wt.% ZrO2 initially presented a monoclinic fraction but did not show any ageing degradation. These composites seem to represent the best choice between slow crack growth and ageing resistance.

Microstructure development in calcium hexaluminate

Abstract Calcium hexaluminate (CA 6 ) was prepared from alumina and calcium carbonate powders. The influence of processing method and firing temperature on calcium hexaluminate grain morphology was studied. A significant correlation was found between grain morphology and green density, porosity distribution and presence of agglomerates. Platelet grains were observed for low green densities and large pores, while more equiaxed grains were found when green density was increased. A model is proposed for the formation of equiaxed or platelet grains. The model is based on the number of contact areas between alumina and calcium carbonate grains in green specimens as well as on the free space available for calcium hexaluminate to grow.

Martensitic transformation in zirconia: Part I. Nanometer scale prediction and measurement of transformation induced relief

We investigate by atomic force microscopy (AFM) the surface relief resulting from martensitic tetragonal to monoclinic phase transformation induced by low temperature autoclave aging in ceria-stabilized zirconia. AFM appears as a very powerful tool to investigate martensite relief quantitatively and with a great precision. The crystallographic phenomenological theory is used to predict the expected relief induced by the transformation, for the particular case of lattice correspondence ABC1, where tetragonal c axis becomes the monoclinic c axis. A model for variants spatial arrangement for this lattice correspondence is proposed and validated by the experimental observations. An excellent agreement is found between the quantitative calculations outputs and the experimental measurements at nanometer scale yielded by AFM. All the observed features are explained fully quantitatively by the calculations, with discrepancies between calculations and quantitative experimental measurements within the measurements and calculations precision range. In particular, the crystallographic orientation of the transformed grains is determined from the local characteristics of transformation induced relief. It is finally demonstrated that the strain energy is the controlling factor of the surface transformation induced by low temperature autoclave treatments in this material.

Double-torsion testing a 3Y-TZP ceramic

Abstract Double torsion tests were performed to study subcritical crack growth of a 3Y-TZP ceramic. A significant variation of stress intensity factor K 1 with crack length was observed. In order to take into account this variation a correction factor is introduced into the well-known analytical expression of stress intensity factor in mode I. This has enabled us to obtain accurate crack growth parameters i.e. an accurate measurement of (i) the three propagation stages of the subcritical crack growth and (ii) a threshold value equal to K IO = 3.5 MPa√m. Analytical expression of K 1 based on a compliance analysis was shown to be unsatisfactory for slow crack growth analysis because of the influence of the unbroken ligament on the compressive side arising from the curved crack front.

Martensitic transformation in zirconia Part II. Martensite growth

Though the martensitic transformation in zirconia has been the object of a very large number of studies for the last decades, qualitative and quantitative observations of the formation and growth of relief induced by low temperature treatments has hardly ever been reported. In the first part of the study (Martensitic transformation in zirconia, Part I), we have demonstrated the excellent agreement between the atomic force microscopy quantitative observations and the outputs of the calculations derived from the phenomenological theory of martensitic transformation. The intermediate stages of transformation were nonetheless not considered. In this second part, the growth mechanisms of monoclinic phase resulting from the martensitic transformation in ceria-stabilized zirconia (10 mol% CeO2) are investigated. Surface transformation is induced by aging treatments in water vapor at 413 K. The observations are rationalized by the recent analysis proposed for the crystallographic ABC1 correspondence choice, where the ct axis transforms to the cm axis. Three growth modes are observed and interpreted in terms of transformation strains accommodation. Microcracks formation is observed, explaining grain pop-out where the crystallographic disorientation between two adjacent grains is the largest. The influence of grain boundary paths on the surface relief features is demonstrated. Overall, our results strongly support the non-existence of a critical grain size for low temperature transformation, confirmed by the classical thermodynamics theory applied to this particular case.

Microstructural Investigation of the Aging Behavior of (3Y‐TZP)–Al2O3 Composites

The low-temperature autoclave aging behavior of zirconia-toughened alumina composites processed by a classical powder mixing processing route was analyzed using atomic force microscopy (AFM), scanning electron microscopy, and X-ray diffraction (XRD). The transformation was evaluated in terms of nucleation and growth, assessed by XRD. The time–temperature equivalency of the transformation was used to measure an apparent activation energy of the nucleation stage of the transformation of 78 kJ/mol. The microstructural features influencing the transformation were identified, and the influence of the alumina matrix on the transformation was investigated. Transformation progression grain by grain was observed by AFM. Transformation does not only occur in zirconia agglomerates but also in isolated zirconia grains. The matrix could partially inhibit the transformation. This behavior could be rationalized considering the constraining effect of the alumina matrix, shape strain accommodation arguments, and microstructural homogeneity effects.

Crack propagation and fatigue in zirconia-based composites

This paper reviews existing published studies on crack propagation behavior of zirconia-based composites. The first part of the paper is concerned with slow crack growth (SCG) under static loading. SCG in zirconia ceramics is shown to be a consequence of stress corrosion by water molecules at the crack tip. The influence of transformation toughening on SCG is discussed in terms of a stress intensity factor acting to reduce the net driving force for propagation. This proposition is in agreement with results obtained on 3Y-TZP and Mg-PSZ ceramics. A master curve is proposed which could be applied roughly to all zirconia ceramics. The influence of zirconia addition to alumina ceramics (ZTA ceramics) is also discussed. The second part of the paper deals with SCG under cyclic loading. A mechanical degradation of all zirconia-based composites is observed by a decrease of crack shielding. This degradation of zirconia-based composites under cyclic loading leads to increased velocities as compared to the static fatigue case. A master curve is also obtained, as in the case of static fatigue. Cyclic fatigue results are interpreted in terms of stress corrosion at the crack tip assisted by a decrease of the reinforcement.

Creep behaviour of alumina, zirconia and zirconia-toughened alumina

Creep behaviour of various alumina, zirconia and zirconia-toughened alumina is investigated. A very large scale of creep rates and creep behaviours is observed. Creep rate depends on the grain size, on the purity and on the composition of the glassy phase present in grain boundaries. Cavitation and microcracking by grain boundary sliding have been identified as the main creep mechanisms.

Study of the residual stress field around Vickers indentations in a 3Y-TZP

Indentation cracks are often used as initial flaws in ceramics for different mechanical tests because of their unique advantages. The residual stresses around the indent due to the elastic/plastic contact must generally be relaxed, which can be conducted by heat treatment. The stress field around Vickers indentation and conditions of annealing have been analysed on a Y-TZP material. Different heat treatments from room temperature up to 1200 °C have been conducted. The residual stress field has been characterized by different methods. First, stable crack propagation in the residual stress field has been conducted by applying a bending stress to the indented specimens. The apparent toughness has also been measured by conducting fast fracture on indented specimens. Finally the residual stress present around the indentation has been measured by making micro-indentations and by measuring the crack lengths. The results show that at intermediate temperatures up to 600 °C an apparent stress relaxation occurs due to the tetragonal to monoclinic phase transformation which induced a superimposed compressive stress. The higher temperature of 1200 °C effectively leads to a real stress relaxation without healing the crack.