Low-Temperature Degradation of Zirconia and Implications ......temperature degradation (LTD) of...

ANRV315-MR37-01 ARI 21 May 2007 13:10

Low-TemperatureDegradation of Zirconiaand Implications forBiomedical ImplantsJerome Chevalier,1 Laurent Gremillard,1

and Sylvain Deville2

1Mateis, INSA Lyon, CNRS UMR 5510, 69621 Villeurbanne, France;email: [email protected] Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,California 94720

Annu. Rev. Mater. Res. 2007. 37:1–32

First published online as a Review in Advance onApril 13, 2007

The Annual Review of Materials Research is online athttp://matsci.annualreviews.org

This article’s doi:10.1146/annurev.matsci.37.052506.084250

Copyright c© 2007 by Annual Reviews.All rights reserved

1531-7331/07/0804-0001$20.00

Key Words

Y-TZP, transformation, aging, orthopedics, dental

AbstractThis review describes the mechanisms responsible for low-temperature degradation (LTD) of zirconia ceramics and its detri-mental consequences for biomedical devices. Special emphasis isgiven to the critical issue of zirconia degradation actually observedfor hip prostheses. Experimental methods to accurately measure andpredict LTD in a given zirconia ceramic are presented. Different so-lutions to inhibit LTD or at least reduce its kinetics are reviewed,with the objective of highlighting alternative options for the gener-ation of new zirconia-based biomedical ceramic devices.

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Zirconia: zirconiumdioxide (ZrO2), generallyused in the form of apolycrystalline technicalceramic, stabilized withanother oxide

Y-TZP: yttria-stabilizedtetragonal zirconiapolycrystal

Femoral head: componentof a hip prosthesis in theform of a ball affixed to ametallic femoral stem andbearing against theacetabular cup

LTD: low-temperaturedegradation

INTRODUCTION

From low to high temperatures, zirconia exhibits three allotropic crystal structures:monoclinic (m), tetragonal (t), and cubic (c). A tetragonal-to-monoclinic (t-m) trans-formation occurs at approximately 1000◦C in pure zirconia and is associated with alarge volume increase and shear strains. As a consequence, samples of pure zirconiafracture during cooling from the sintering temperature. Only the addition of oxides[such as yttria (Y2O3)], lowering the t-m transformation temperature, can stabilizethe t phase to a certain extent at room temperature, thus allowing zirconia to be usedas a bulk, structural material. However, zirconia in the t phase remains metastable.Zirconia’s ability to transform under mechanical stress (stress-induced phase trans-formation) represents one of the most remarkable (somewhat unexpected) findingsin the ceramic field.

Indeed, in the 1970s, first Garvie et al. (1) and then Gupta et al. (2) showed thatzirconia exhibits a transformation toughening mechanism that acts to resist crackpropagation. The stress-induced phase transformation involves the transformationof metastable tetragonal crystallites to the m phase at the crack tip, which, accompa-nied by a volumetric expansion, induces compressive stresses. Y-TZP (yttria-stabilizedtetragonal zirconia polycrystal) zirconia ceramics belong to this family of toughenedmaterials. They can exhibit bending strength of more than 1000 MPa, with a tough-ness of approximately 5–10 MPa

√m. Such outstanding properties for oxide ceramics

opened the way to new structural devices and designs not available with standard,more brittle ceramics like alumina. As an example, since the late 1980s, Y-TZP zir-conia has been very popular in the manufacturing of femoral heads for hip prosthesisapplications (3). Y-TZP was preferred to other zirconia ceramics because of its uniquebalance of toughness and strength. Today, more than 600,000 zirconia femoral headshave been implanted worldwide, mainly in the United States and in Europe. Zirconiais also increasingly used in dental applications such as endodontic posts and brack-ets and more recently inlays, crowns, and bridges for restorative dentistry, owing torecent developments in CAD/CAM procedures.

However, the excitement created by these materials was rapidly dampened by thefindings of Kobayashi et al. (4), who discovered a serious limitation of Y-TZP ceram-ics, first for applications near 250◦C. These authors revealed that Y-TZP ceramicscould suffer a slow t-m transformation at the sample surface in a humid atmosphere,followed by microcracking and a loss in strength. Following this work, a series ofpapers attempted to understand the basic micromechanisms of the slow t-m trans-formation [often referred to as aging or low-temperature degradation (LTD)] and toreduce this limiting phenomenon. After more than 20 years of research, the exactmechanism is still under discussion. The increase of internal stresses associated witha penetration of water species inside the lattice is likely to trigger the initiation oftransformation (5). A cascade of events then occurs, the transformation propagatingfirst inside one grain (6) and then invading the surface by a nucleation-and-growth(N-G) mechanism (7) and the core for severe treatments (8).

This review first aims to describe the different processes and sequences of ag-ing. Among the numerous interpretations proposed in the abundant literature, we

2 Chevalier · Gremillard · Deville

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ZTA: zirconia-toughenedalumina

emphasize the most recent and likely models that offer an acceptable coherence be-tween theoretical analysis and multiscale experimental facts. New interpretations ofaging were made possible in the past decade by the development or improvements oftechniques that allow monitoring of LTD from the atomic to the macroscopic scale.This review therefore describes briefly the techniques that may be used to followaging kinetics.

Until the beginning of the present decade, aging kinetics were measured wellabove room or body temperature. Indeed, the transformation rate is the highest atapproximately 250◦C and was considered negligible at 37◦C (9). Therefore, the zir-conia manufacturers assumed that the problem of aging under an in vivo situation wasirrelevant until 2001, when several hundreds of hip prosthesis failures were reportedin a very short period (10, 11). These failures are now attributed to an acceleratedaging of the zirconia femoral head in particular batches. Even if limited in timeand number and clearly identified to be process controlled, these events have had acatastrophic impact on the use of zirconia. More importantly, recent reports suggestthat significant aging even occurs in vivo at the surface of implants processed under“normal” conditions (12). This could lead to critical health and economical issues inthe near future, considering the large number of zirconia femoral heads that have beenimplanted during the past decade. The second objective of this paper is to describethe potential consequences of aging on the integrity of zirconia orthopedic implants.They are assessed on the basis of current knowledge of the aging mechanism and thestudy of retrieved components.

Current state of the art shows the strong variability of zirconia sensitivity to in vivodegradation as a consequence of the strong influence of microstructure and process onLTD. Therefore, here we examine the effects of the most important microstructuralfeatures on LTD. This analysis leads us to propose options to improve the resistanceof zirconia-based ceramics to aging. In particular, the future of zirconia-toughenedalumina (ZTA) composites is discussed, as it represents one trend followed by ceramicmanufacturers.

MECHANISMS OF AGING: FROM PHYSICOCHEMICAL TOMICROMECHANICAL PROCESSES

All the stages of aging have been discussed in the literature. However, a completeoverview from the early stages to the macroscopic consequences is still lacking. Thissection’s objective is to document each stage of the transformation and describe howthey relate to each other. Some fundamentals of the t-m transformation are necessaryto discuss before describing the aging mechanism.

Tetragonal-to-Monoclinic Transformation in Zirconia

The t-m transformation in zirconia is martensitic in nature. A martensitic transfor-mation is a “change in crystal structure . . . that is athermal, diffusionless and involvesthe simultaneous, cooperative movement of atoms over distances less than an atomic

www.annualreviews.org • Low-Temperature Degradation of Zirconia Implants 3

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diameter, so as to result in a macroscopic change of shape of transformed regions”(13). A consequence of the diffusionless nature of martensitic transformation is theexistence of crystallographic correspondences between the parent and the productphase, described by habit planes and directions (shape strain), as schematically shownin Figure 1. The tetragonal (P42/nmc) to monoclinic (P21/c) transformation occursat ≈950◦C (Ms) on cooling in pure zirconia and is accompanied by a shear strainamplitude of ≈0.16 and a volume expansion of approximately 0.05. The transfor-mation is reversible and occurs at approximately 1150◦C (As) on heating. Table 1summarizes the features of the transformation (14).

Lange (15) first described the thermodynamics of the t-m transformation in zir-conia, considering the simple ideal case of a spherical tetragonal particle in a matrix.The change of total free energy (�Gt−m) due to the transformation is given by

�Gt−m = �Gc + �USE + �US, 1.

where �Gc (<0 when T < Ms) is the chemical free energy (dependent on temperatureand composition), �USE (>0) is the strain energy associated with the transformedparticles (dependent on the surrounding matrix, the size and shape of the particle,and the presence of stresses), and �US (>0) is the change in energy associated withthe surface of the particle (creation of new interfaces and microcracking).

Through a decrease in |�Gc | and an increase in �USE , the addition of alloyingoxides such as Y2O3 in Y-TZP decreases the driving force of the t-m transformationand hence its temperature, as shown in the ZrO2-Y2O3 diagram in Figure 2 (16, 17).It even makes possible, for Y2O3 content above 2 mol.%, the retention of metastabletetragonal particles in dense bodies at room temperatures. �USE is directly related tothe surrounding matrix modulus: High matrix modulus will increase �USE , stabilizingthe t phase. �USE is also directly influenced by applied or internal stresses: Tensilestresses will act to reduce �USE , destabilizing the t phase.

Early Stages of Aging: Stress Accumulation due to Water Penetration

Aging occurs experimentally in zirconia samples, mostly in humid atmosphere or inwater. Therefore, several models currently attempt to explain how the presence ofwater could promote the t-m transformation in zirconia. Early models (18)—basedon the reaction between H2O and Y2O3 to form Y(OH)3 during aging—have beenrejected, and the fundamental role of internal stresses associated with water diffusionin the zirconia lattice has been demonstrated (5, 19).

Several experimental results show that water radicals do indeed penetrate insidethe zirconia lattice during exposure to humidity. Most likely, the oxygen from thewater is located on vacancy sites, and the hydrogen is placed on an adjacent interstitialsite (19). This emphasizes the primary role of oxygen vacancies initially present in

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→Figure 1(a) Schematic of the tetragonal-to-monoclinic (t-m) martensitic transformation, showing habitplane and shape strain. (b) Simple arrangement of the t-m transformation when the free surfacecorresponds to (001)t [the surface uplift is observed by atomic force microscopy (AFM)].


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Tab

le1

Cry

stal

logr

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atur

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tetr

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From

Ref

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alc

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Vol

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1]0.

0344

⎡ ⎣−0.9

537

◦ 0.0

055

◦0.3

005

⎤ ⎦⎡ ⎣−0

.002

6◦ 0

.002

8◦0

.164

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⎤ ⎦0.

1640

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AB

C2

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)[01

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171

−0.9

956⎤ ⎦

⎡ ⎣◦0.1

597

−0.0

007

−0.0

373⎤ ⎦

0.16

400.

1556

0.05

18

BC

A1

(110

)[11

0]0.

0344

⎡ ⎣◦0.0

034

◦0.3

935

−0.9

193⎤ ⎦

⎡ ⎣◦0.0

030

◦0.1

751

◦0.0

186⎤ ⎦

0.17

610.

1683

0.05

18

BC

A2

(110

)[11

0]0.

0344

⎡ ⎣−0.0

168

−0.9

996

−0.0

241⎤ ⎦

⎡ ⎣−0.0

004

−0.0

558

◦0.1

670

⎤ ⎦0.

1761

0.16

830.

0518

CA

B1

(101

)[10

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0027

⎡ ⎣◦0.3

006

−0.9

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−0.0

001⎤ ⎦

⎡ ⎣◦0.1

640

−0.0

026

−0.0

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0.16

400.

1556

0.05

18

CA

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(101

)[10

1]0.

0027

⎡ ⎣−0.9

958

◦ 0.0

915

◦0.0

003

⎤ ⎦⎡ ⎣−0

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a Inp

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eter

s:tp

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=5.

128

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;mph

ase:

a m=

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217

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m=

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8A◦

,βm

=98

.91◦ .

bE

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stem

.


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T0 (F

⇒ T)

T0 (T

⇒ M

)

0 5 10 15 200

500

1000

1500

Tetr

ag

on

al (T

)M

on

oclin

ic (

M)

2500

3000

2000

Liquid (L)

Cubic (F)

T+F

L + F

M+F

Mol.% YO1.5

Tem

per

atu

re (

°C)

3Y-TZP

TetragonalCubicMonoclinic

Figure 2ZrO2-YO1.5 phase diagram(after Reference 16).Metastable phases retainedat room temperature areindicated just above thehorizontal axis. The reddotted lines show thenonequilibriummonoclinic-tetragonal andcubic-tetragonal transitionregions (after Reference 17).

zirconia (20). In Y-TZP, the presence of numerous vacancies due to the trivalentcharacter of Y2O3 makes the diffusion rate of species from the water higher thanin other zirconia ceramics (i.e., CeO2-doped ZrO2). According to Schubert & Frey(19), the penetration of water radicals leads to a lattice contraction, which results inthe formation of tensile stresses in the surface grains that destabilize the t phase byreducing �USE in Equation 2. Martensitic transformation of grains (or part of grains)at the surface can then proceed.

Martensitic Transformation at the Surface of Zirconia During Aging

In zirconia, several lattice correspondences and lattice-invariant shears can be fol-lowed (Table 1). However, during aging the transformation occurs on the surface,


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MAJ:Mehl-Avrami-Johnson

allowing only one lattice correspondance, ABC 1, which means that the at, bt, and ct

axes of the t phase change into, respectively, the am, bm, and cm axes of the m phase (14).Figure 1 displays a simple arrangement of the t-m transformation when the free sur-face corresponds to (001)t. All the t-m interfaces are habit planes and are unchangedduring the transformation. All the interfaces between the variants are of the kind{100}t or {110}t and are jointly elongated in the direction [001]t. For this particularconfiguration, all the volume change is exactly relaxed outside of the free surface, andno internal stresses are generated inside the volume. In fact, as the strain directionsare not exactly [001]t (Table 1), some negligible internal strains are expected. Moreimportantly, in general the angles between the ct axis and the normal vector to thefree surface are randomly distributed. This results in an increase of tensile stressesin the t phase surrounding the transformed zone, accelerating the transformationby a near-to-near mechanism (Figure 3). The figure emphasizes the facts that thetransformation begins mostly at grain corners, where residual tensile stresses are thelargest (21), and that one given grain does not transform all at once but rather pro-gressively as the stress state due to water attack and earlier transformation increases.Some reports (8, 22) show that tensile stresses generated by the transformation mayinduce microcracking at the grain boundaries, easing the diffusion of the water speciesinside the bulk.

Transformation Extension by Nucleation-and-Growth Process

In the above section, we describe how the transformation can start and propagate in-side one grain. If, for some specific orientation relationships, a monoclinic plate cango through two or more tetragonal grains (Figure 4), in most cases grain boundariesmay prevent the extension of a martensitic lath from one grain to another. However,the transformation of the surface of Y-TZP takes place by an apparent N-G mech-anism; i.e., once a grain is transformed, the extension of the transformation occursnot only at random on the surface but also preferentially on the neighboring grains(nucleation is here considered as the transformation of one grain, whereas growth isthe extension to its near neighbors). A typical N-G process observed experimentallyin the literature (7, 23) is shown in Figure 3. Nucleation occurs on the most unsta-ble grains (with less Y2O3 and/or with large size and/or subjected to higher internalstresses) subjected to the highest tensile stresses (either internal or applied). Thenumber of nuclei increases continuously with the stresses, owing to the penetrationof water. At the same time, growth occurs because the transformation of one grainputs its neighbors under tensile stresses, favoring their transformation under the ef-fect of water. Describing a martensitic transformation by an overall N-G process mayappear contradictory, but one must keep in mind that the whole process is controlledby the diffusion of water species.

The most detailed studies (7, 23–25) have shown that LTD kinetics follow sig-moidal laws related to the N-G process. The kinetics were described using Mehl-Avrami-Johnson (MAJ) laws (see, for example, Reference 26):

α = 1 − exp (− (b .t)n) , 2.


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Figure 3Description of the tetragonal-to-monoclinic transformation at different steps and withdifferent techniques. (Central plot) Evolution of the monoclinic fraction versus time measuredby X-ray diffraction along with numerical and analytical modeling. (Upper left corner) Earlystage of the transformation observed by atomic force microscopy. The transformation starts atone grain corner (white arrows) and propagates to the rest of the grain. (Bottom)Nucleation-and-growth process observed by optical interferometry (the circled area indicates amonoclinic spot). (Right side) Growth into the bulk observed by cross-sectional scanningelectron microscopy.


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Figure 4Martensitic lath crossing a grain boundary (AFM relief map). The dashed lines indicate theinterface between monoclinic variants that are growing in a neighboring grain through a grainboundary (white dashed line).

where α is the degree of transformation of the t phase in the surface [e.g., the mono-clinic:(tetragonal + monoclinic) ratio in the volume accessible by the technique usedfor the measure of α], t the time, and n a constant (the MAJ exponent). b is a param-eter dependent on temperature. In the 37◦C–140◦C temperature range, b follows anArrhenius law:

b = b0 · exp[− Q

RT

], 3.


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XRD: X-ray diffraction

SEM: scanning electronmicroscopy

where b0 is a constant, R the gas constant, Q the apparent activation energy, and T theabsolute temperature. Time-temperature equivalences can be derived from Equation3. For example, steam sterilization at 134◦C for 5 h is now proposed to simulate 15–20years at 37◦C (27).

One analytical modeling of zirconia aging kinetics (7) led to an MAJ-type equationfor the degree of transformation versus time. In this model, aging occurs by a constantnucleation of new transformation zones with time and an increase in diameter andheight of existing ones at constant rate (provided that there is enough space to grow).With these features, the monoclinic fraction f versus time t measured by X-raydiffraction (XRD) should be

f = ft0 ·(

1 − exp[−

(π · α2

d · αh · Nr

48 · k · l

)· t4

]), 4.

where αd and αh are the diameter and height growth rates, Nr the nucleation rate, kthe linear expansion accompanying the t-m transformation (1.3%), and l the X-raypenetration depth (5 μm in standard conditions: Cu Kα radiation, θ-2θ configuration).ft0 corresponds to the initial tetragonal fraction before aging [lower than 1, owing tothe presence of up to 20% c phase after a standard sintering condition (Figure 2)]. InEquation 4, αd , αh, and Nr depend on the temperature. The parameter b in Equation2 is thus given by

b =(

π · α2d · αh · Nr

48 · k · l

)1/4

. 5.

This shows that Q is an apparent activation energy that reflects the overall N-Gmechanism but depends on αd , αh, and Nr , which are also thermally activated withdifferent activation energies.

Equation 4 was successfully applied to one zirconia material in References 7 and28 and to the commercial Prozyr� zirconia femoral heads. However, this MAJ-typeequation is valid only in a limited range of N-G rates: Some zirconia follow MAJ ki-netics, but with an n exponent different than 4 (typically between 0.3 to 4). Therefore,numerical simulation based on the same assumptions (constant N-G rates) was alsoproposed to better fit the aging kinetics, whatever the N-G rates (28). Both simulationand experiments show that the exponent of the MAJ laws is controlled not only bythe N-G mechanisms but also—and mainly—by their respective kinetic parameters(Figure 5). Measurements of nucleation and growth rates at the surface and at thevery beginning of aging and the use of numerical simulation allow for the accurateprediction of kinetics for longer aging duration.

In Equation 4, saturation of the surface monoclinic fraction is reached when fapproaches ft0 : All the volume accessible by XRD underwent a t-m transformation.However, aging still proceeds into the volume of the material (8). The transformedlayer can be observed by scanning electron microscopy (SEM) on cross section, asshown in Figure 3. The transformed layer appears rougher as a consequence ofextensive pullout during polishing, owing to the microcracks generated by aging.


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10–2 1 102 104 106 108

αd /NrD2 (m)

0

1

2

3

4

5

n

n max

Simulated points

Experimental points

Figure 5Variations of theexponent n of theMehl-Avrami-Johnson law(Equation 2) with thenucleation and growthrates. αd is the diametergrowth rate, Nr is thenucleation rate, and D isthe initial size of the nuclei(typically the grain size).

EXPERIMENTAL TECHNIQUES TO MEASURE AGING

Because it is a crystallographic change, aging can be easily characterized by techniquessensitive to crystallography or chemical environment. Among them, XRD is mostextensively used (29). It is conducted mostly with a classical Bragg-Brentano geometry,between 27–33◦ (2θ ), with copper Kα radiations. This characterization is based on theequation proposed by Garvie & Nicholson (30) and modified by Toraya et al (31):

Xm = I m111 + I m

111

I m111 + I m

111+ I t

101. 6.

XRD is fast and nondestructive, allowing the monitoring of LTD evolution on onepiece. It can be used to characterize large zones (a few square millimeters) or smallerzones through the use of microdiffraction (mapping is possible). However, only thesurface is characterized (typical depth of a few micrometers), and thus this method isunable to provide any information when the transformation propagates to the bulk. Itis also not very precise for monoclinic contents lower than 5%, making it unsuitablefor monitoring the beginning of the transformation. It is however possible to improvethe quality of the measure at short aging times by using XRD at grazing incidenceangles [1–5◦ (2θ )]; then, the penetration depth of X rays is even lower (from ∼5 μmat 30◦ to ∼0.3 μm at 1◦ with the copper Kα radiation). The surface:bulk signal ratiois increased, thus apparently increasing the quantity of m phase (but only becausethe analyzed depth is smaller). Through the use of this property, several measures


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AFM: atomic forcemicroscopy

OI: optical interferometry

at different incidence angles give access to the depth profile of the monoclinic con-centration (Figure 6). We know of very few published data on the application ofgrazing angle XRD to the characterization of LTD (32), although it seems to be avery promising technique.

The intensity of a given Raman band is directly proportional to the concentrationof the scattering molecule to which it is associated, allowing quantitative analysis ofthe phases by Raman spectroscopy. In zirconia, the monoclinic content is given by aratio between the intensities of the monoclinic peaks (∼181 and 192 cm−1) and thetetragonal peaks (∼148 and 264 cm−1) (33):

Rm = I m181 + I m

192

I m181 + I m

192 + 0.97(I t148 + I t

264

) . 7.

Several authors have calibrated the technique by comparison with XRD results withvarying dependences: either linear (34), logarithmic (35), or in power law (36). Anagreement has not yet been reached, and systematic studies are needed.

Raman spectroscopy seems to be more sensitive than XRD to smaller traces ofm phase (37). It also allows for discrimination between the t phase and the t′ phase(a t phase that can appear after rapid cooling of large cubic grains), which is moredifficult with XRD. It is therefore particularly appropriate for investigating coatings,for which the t′ phase is present after the process (38). Furthermore, strains lead toa change in the interatomic distances; this translates in Raman spectra by a shift ofthe peaks, and after calibration this effect can be used to measure the stress in thematerial (33, 39). Above all, micro-Raman spectroscopes are now available in confocalconfigurations: Confocal Raman microscopy offers three-dimensional mapping ofboth monoclinic content and stress state, with a micrometer resolution along thethree axes (34). Although less widely used than XRD, Raman spectroscopy is one ofthe most powerful tools for characterizing zirconia LTD.

Surface imaging techniques such as optical interferometry (OI) (7) and atomicforce microscopy (AFM) (23, 40) are more sensitive to the early stages of aging andgive access to topological changes (cf. Figure 3). AFM offers a better spatial resolu-tion in the three directions (it can reach atomic resolution), as compared with OI, butits depth of field is limited to a few micrometers, making difficult its use on nonflatsamples such as femoral heads. Only small areas (typically 50 × 50 μm2) can be ob-served by AFM, making this tool qualitative but not very suitable for any statisticalanalysis. OI permits the observation of larger areas that are not necessarily flat and isa faster and easier technique. These characteristics are emphasized with the new gen-eration of optical interferometers that are fitted with mapping capabilities and thatcan provide local (micrometer scale) information as well as macroscopic information(macroscopic shape) over a several-millimeter square area. Both techniques can benondestructive. They both provide relief maps (where the m phase protrudes from thetetragonal surface) from which the surface monoclinic fraction can be determined (byimage analysis) and the roughness calculated. They also allow the determination ofthe geometry and repartition of the transformed zones and thus are particularly usefulfor quantifying and understanding the first stages of transformation (7, 40). With itsbetter resolution, AFM is very powerful in detecting and characterizing the earliest


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0 10 20 30 40

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0

0.1

0.2

Mea

sure

d

mo

no

clin

ic f

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0 1 2 5

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a

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1.00

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I/Imax

Incident angle:

1°

3°

30°

m(–111)

t (101)

Figure 6Depth profile of themonoclinic concentrationfrom grazing incidentangle XRD. (a) X-raydiffractograms at differentincident angles.(b) Measured monoclinicfraction versus theincident angle (fromEquation 5 and thediffractograms in a).(c) Calculated monoclinicdepth profile.


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Osteolysis: progressivebone resorption

instants of the t-m transformation. A striking example is the application of the marten-sitic transformation theory to zirconia aging by Deville et al. (14, 22) on the basis ofAFM images. In contrast, thanks to its wide field of observation, OI is a much more sta-tistical tool. For example, results of the number and size of monoclinic nuclei obtainedby OI during early stages of aging can be used to predict a whole aging kinetic (7, 28).

However, none of the above-discussed techniques provide information on the ex-tension of LTD into the bulk because only the surface or near-surface region (byRaman) can be investigated. This last stage of aging can be monitored only by tech-niques giving access to the volume. The most common is SEM (cf. Figure 3). Aftercross-sectioning and polishing of an aged sample, the transformed zone exhibits astronger contrast owing to microcracking. This allows direct measurement of thetransformed thickness. Images of the surface can clearly show the uplifted trans-formed zones and the microcracks, but owing to the high depth of field, SEM is notsuitable for the quantification of early stages of aging. However, associated with elec-tron back-scattered diffraction (EBSD) (41), this technique can give useful insight onthe crystallography of the transformation.

Measurement of the microwave dielectric loss tangent has also been applied tocharacterize severe aging (42). Because of differences in loss tangent between thepolymorphs of Y-TZP, and also because of the presence of water in the aged zirconia,transformations of up to 500 μm deep result in a loss tangent roughly proportionalto the thickness of the monoclinic layer. This technique is nondestructive and seemsinteresting, but the literature on the subject is very scarce.

CONSEQUENCES OF AGING ON ORTHOPEDIC IMPLANTS:FROM CONCEPT TO IN VIVO CASE STUDIES

The consequences of the aging process on the long-term performance of zirconia im-plants can be twofold: Aging is indeed associated with roughening and microcracking.This will inevitably impact the wear performance of hip joint heads, as roughen-ing will increase the wear rate of the antagonist part of the prosthesis, whereas thecoupled effect of surface microcracking and wear will generate pullout of zirconiagrains. The biological interaction of small particles with the immune system cellsthen becomes critical. It is indeed currently believed that particles generated at thecontact surfaces enter the periprosthetic tissues, where they trigger macrophages toreact. Macrophages then release proinflammatory cytokines that stimulate osteoclas-tic bone resorption, leading to osteolysis, eventual loosening of the prosthesis, anda need for replacement. The extension of the microcracked, transformed zone alsogenerates defects. Because zirconia is prone to slow crack growth because of stresscorrosion (43), defects generated in stressed regions of the implant may grow withthe transformed zone until reaching a threshold size for slow crack propagation toproceed; the fracture of the implant is then unavoidable. This can apply to bothorthopedic and dental devices.

Early publications, mainly by zirconia femoral head manufacturers, showed nosignificant aging degradation after moderate in vivo duration (9). It was there-fore thought that aging was very limited and well controlled near the ambient


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3Y-TZP: 3 mol.%yttria–stabilized tetragonalzirconia polycrystal

temperature. However, in 1997 the U.S. Food and Drug Administration (FDA) firstreported on the critical effect of the standard steam sterilization procedure (134◦C,2 bars pressure) on the surface roughness of zirconia implants (44). This procedureis now forbidden for zirconia. More importantly, in 2001, the FDA announced thatfirms distributing orthopedic implants were recalling series of Y-TZP hip prosthesesowing to a fracture episode of zirconia ceramic femoral heads. More than 300 failureswere reported during the fracture episode. The cause of the failures was related toan accelerated t-m phase transformation of zirconia in a limited number of batches(11). Although limited in the number of failures and duration, this episode has hada strong negative impact on the application of zirconia in orthopedics. In particular,the main supplier of zirconia implants, Saint Gobain Desmarquest, stopped Prozyr�

activities. The question of the LTD of zirconia in vivo, even under normal processingconditions, thus became salient from 2001 because thousands of femoral heads hadalready been implanted at that time. A series of papers was then published, showingsurface degradation of zirconia hip joint head implants (45–48) or strong osteolysisassociated with the use of zirconia heads (49–51). In contrast, a few papers still showgood clinical results and low wear rates associated with the use of zirconia heads(52–53). Therefore, the correlation between LTD and in vivo behavior is discussedbelow to understand the current state of the art on retrieval analysis.

Zirconia Fractures due to Accelerated LTD: A Striking Example of aProcess-Durability Relationship

Saint Gobain Desmarquest introduced yttria-stabilized zirconia (3Y-TZP) inorthopedics in 1985, as an alternative to alumina. From 1985 to 2000, the Frenchcompany, which was the major zirconia manufacturer, sold more than 350,000 hipjoint heads (under the trade name Prozyr�) worldwide. Within this period, only28 brittle fractures were reported (12, 54), representing a fracture rate of less than0.01% (1 per 10,000 units). Moreover, most of the failures were associated withabnormal use of the hip joint heads (replacement surgery without change of themetallic femoral stem, traumatism, etc.) and were reported in the early period offabrication (before 1993). With the systematic use of proof tests on all implants inthe mid-1990s, the fracture rate even fell to 0.002% (1 per 50,000 units). Zirconiawas therefore considered the second generation of ceramic hip joint heads, providinglow wear rates and high safety (55).

In January 1998, to address the increasing commercial demand of zirconia hip jointheads (100,000 units per year), Saint Gobain Desmarquest introduced a tunnel fur-nace in place of the usual batch furnace (kiln). A tunnel furnace provides a continuoussintering operation and leads to a reduction of processing time for larger series. Allother steps, described in Figure 7, were kept identical. The new hip joint heads were

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→Figure 7Synopsis of the process of zirconia femoral heads. The change from Prozyr� BH- to TH-ballsis emphasized. The sintering thermal cycles are reproduced from Reference 58.


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HIP: hot isostatic pressing

called TH-balls, whereas the first generations of heads sintered in kiln were calledBH-balls. By the end of 2000, two major orthopedic companies (Depuy France andSmith and Nephew) reported an unusual brittle fracture rate for zirconia TH-balls.Only seven batches (a batch representing ≈600 heads) were affected by the high failurerate (11). They were all processed during the first phase of industrialization of TH-balls, and the fractures occurred only months after implantation. In the seven batches,356 fractures were reported, corresponding to a mean failure ratio of 8% (up to 36%for batch TH-93038). This was a dramatic difference from the overall fracture ratesreported in the prior 15 years. The French Medical Agency immediately suspendedsales of the TH-balls (56), and the FDA issued a notice (10). Given these concerns,and because thousands of TH-balls had already been implanted in patients, the manu-facturer, the major customers, and the French Medical Agency independently formedseveral groups of experts to answer a list of the following crucial questions:

1. What was the cause—material, process, or design related—of the failures?2. What difference between BH- and TH-balls led to a dramatic decrease in

safety?3. Why did the companies’ standard quality-control checks (i.e., for density, grain

size, and mechanical strength and including proof testing, accelerated agingtests, etc.) not detect the vulnerability of TH-balls to delayed failure?

4. What was the risk of failure for patients with an implanted TH-ball?Answers to some of these questions may have been kept confidential by the differ-

ent panels of experts and by the companies themselves. However, some papers (12,57) and information disclosed by Saint Gobain Desmarquest (11) now clarify mostpoints and show the importance of the validation of a new process technology forhigh-technology ceramics.

Because the failures did not occur with BH-balls of identical designs, the prob-lem was obviously process related (i.e., involving a change in the sintering furnace).Without going into detail as to the two technologies, the main differences lie in thesintering cycle (58) and atmosphere, giving rise to a difference in microstructure af-ter sintering. According to the case report of Maccauro et al. (57), the thermal cycleapplied on TH-balls likely leads to a lower intermediate density after sintering. Moreimportantly, an uneven radial distribution of density may have occurred owing tothe combination of cold isostatic pressing for the powder compaction and sinteringin the tunnel furnace. The difference was hardly visible via standard quality-controlchecks. Density was measured on the entire ball after the hot isostatic pressing (HIP)step (after HIP no significant change in density was detected). Saint Gobain Desmar-quest tested a sample of every batch for resistance to failure and cyclic fatigue. Thepersistence of very small microcracks inside the core of the heads was not detected.The company also conducted accelerated aging tests on samples of each batch toassess aging sensitivity. Steam sterilization procedures were applied; additionally, thesurface of the balls was inspected via XRD and OI because it was believed that agingwould have deteriorated the bearing surface and consequently the wear behavior.Unfortunately, the zone affected by the process variation was inside the core, wherea taper has to be drilled to fit the ceramic head to the metallic stem. The drillingprocess leads to residual stresses that may be critical for aging resistance. The critical


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Monoclinic spots: surfaceuplifts resulting from thevolume expansionassociated with thetetragonal-to-monoclinictransformation

Explanted head: aprosthetic femoral headretrieved after some monthsor years in vivo

events of 2001 may have been due to a combination of (a) the residual stresses and thepresence of a lower density in the core region, where the taper is drilled, and (b) anunexpected aging rate and the growth of aging-induced defects in a region in whichtensile stresses are maximum (59). In other words, aging was proceeding very fast ina region in which no quality control was possible. In the papers dealing with headsbelonging to a problematic batch (12, 57, 58), the monoclinic content on the bearingsurface was low (less than 5%), indicating a good stability, whereas the surface of thetaper inside the head could reach more than 60% monoclinic content!

Although the above represents a dramatic experience and shows how validationof new processes is crucial, the unfortunate story of Prozyr� TH-balls was unique.Moreover, owing to this critical event, a number of studies have now clearly assessedthe role of density, residual stresses, etc. on aging. Therefore, this provides the ex-perience needed to continue the use of zirconia in biomedical applications, as thematerial is even safer than before.

Other Clinical, Aging-Related Experiences with 3Y-TZP Ceramics

Despite 20 years of clinical experience regarding zirconia femoral heads and morethan 600,000 heads implanted, the literature concerning retrieval analysis of zirconiafemoral heads and clinical follow-up is sparse. Moreover, some results show excellentbehavior of some heads after several years in vivo (52, 53), whereas others show poorfollow-up results (49–51), with severe wear and osteolysis around the implant. A fewcase studies report surface degradation of zirconia implants, which may be related toaging (45–48).

Haraguchi et al. (45) reported for the first time two cases of surface degradation(roughening and microcracking) caused by phase transformation. The zirconia usedin this work—presumably processed by Kyocera—followed ISO criteria. They mea-sured monoclinic contents of 20% and 30% in both heads after only 3 and 6 years,respectively, associated with a strong increase of roughness (from 6 nm to 120 nm).From the photomicrographs provided by Haraguchi et al., small surface domes ofsome dozen of microns were present on the pole of the heads. Such domes werelikely the monoclinic spots described in Figure 3, which were observed in previousworks with accelerated in vitro tests (7, 23). Craters of the same size were visibleat the equator (region of wear contact) as a consequence of extensive pullout of themonoclinic spots. More recent papers from Moore and coworkers (46, 47) confirmthe degradation of surface properties for some zirconia heads by means of nanoin-dentation hardness measurements on explanted heads that had undergone variousdegrees of phase transformation in service (ranging from 0% to 78% monocliniccontent). The hardness dropped from 18 GPa to 11 GPa for high transformationratios, which indicates extensive microcracking on the transformation sites. Limitedinformation was given on the origin of the heads, their microstructural features, ortheir clinical history, making an extended analysis of the results difficult. However,one head exhibited approximately 78% of m phase after only 62 months of implan-tation. On the status of zirconia as a biomaterial, Clarke et al. (12) also report on theresults of a collaborative research, between two Japanese groups and a U.S. group,


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on two explanted heads, one retrieved at eight years and the other at ten years. TheY-TZP ball retrieved at eight years appeared badly transformed (more than 70%monoclinic), with surface craters in the bearing region. The damage was even visi-ble to the eye. As a counterpoint, the ball with the longest time in vivo (ten years)showed minimal phase transformation and a high-quality bearing surface with verylow residual surface stresses as well as no change in surface roughness. These twodifferent behaviors reported by the same team show the strong variability of zirconiaheads with regard to LTD resistance. Figure 8 summarizes the results reported so

400 80 120 200

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Santos (47) Caton (52)

Haraguchi (45) Hernigou (50) Masonis (58)

Chevalier (48)Clarke (12) Maccauro (57)

160

Simulated on a Prozyr® head, batch 87

Figure 8Monoclinic fraction measured at the surface of retrieved heads versus implantation duration.The two circled points were in fact measured inside the taper of two TH-heads affected by thefailure episode. These experimental points are compared with the kinetics predicted fromaccelerated aging in autoclave in a batch of Prozyr� BH-balls (27).


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far. Many processing factors may affect the LTD resistance, and it seems that, evenin the framework of ISO standards, some heads behave well, whereas others showstrong surface degradation.

Up-to-date clinical follow-up reports also appear quite controversial. Some au-thors report a low wear rate at middle term follow-up (4 to 6 years) on 22.22-mm headsprocessed at the end of the 1990s (52, 53). In contrast, other researchers report poorclinical results after a mean follow-up of 5 to 12 years (49–51) on 28-mm heads pro-cessed from 1988 to 1995. By poor clinical results, we mean an especially low survivalrate along with aseptic loosening and progressive osteolysis, most probably owingto high wear rates and particle debris generation in the body. Especially interestingis the French study from Hernigou et al. (50), which compares 10 years of follow-up of stainless steel-polyethylene, alumina-polyethylene, and zirconia-polyethyleneconfigurations. Although in the first years the zirconia-polyethylene group comparedwell with the two other combinations, Hernigou et al. noticed a strong increase ofwear rate after 5 years. More importantly, the wear rate continuously increased up to12 years (which was the end of the clinical experience). The analysis of three explantedheads reveals high monoclinic content, associated with an increase of roughness ofthe head and progressive lack of sphericity of the polyethylene cup. In their reporton the poor 8-year survival rate of zirconia-polyethylene prostheses, Allain et al. (49)reveal the presence of submicron zirconia grains small enough to be scavenged bymacrophages and to contribute to the inflammatory process and osteolysis. Thesebad clinical experiences may therefore be related to the LTD of zirconia, which canappear in vivo.

This leads us to propose a plausible scenario of zirconia degradation and conse-quential implant failure, schematically described in Figure 9. After bedding-in regime(that corresponds to an early decrease of wear rate due to polyethylene polishing bythe femoral head), a steady state (constant wear rate) is generally observed with metalor alumina heads. In the case of zirconia, bedding-in regime is followed by a sig-nificant increase in wear rate (50), correlated to the increases in monoclinic contentand roughness of the head that occur during aging (27). Zirconia grain pullout in thebearing region and polyethylene debris may then lead to premature osteolysis.

INFLUENCE OF PROCESSING AND MICROSTRUCTUREON AGING

Aging can be controlled. For a given zirconia ceramic, density and grain size, as well asthe homogeneity of the phase distribution and the residual stress state on the surface,are of primary importance.

Perhaps the most important parameter to limit aging is the density. Indeed, lowdensity—especially the presence of open porosity—offers water molecules easy accessto the bulk of the material, resulting in aging not only on the surface of the zirconiapiece but on all the internal surfaces (pore and crack surfaces). Under such conditions,the material rapidly loses its cohesion, and the decrease in mechanical properties isdramatic. Figure 10 shows a cross-sectional SEM micrograph of a pressed zirconiapellet after aging. This piece presents a density gradient, with an average density


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0 3 6 9 12 15

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Monoclinic fraction

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Decrease in

PE roughness

Increase

in ZrO2

roughness

Zirconia and PE debris

Inflammatory response

Aseptic loosening

Osteolysis

t-zirconia

m-zirconia

Figure 9Suggested scenario ofaseptic loosening triggeredby aging of a zirconia head.(a) Monoclinic fraction andwear rate versus time (27,50). After bedding-inregime (that corresponds toan early decrease of wearrate due to polyethylenepolishing by the zirconiahead), the increase of wearrate is correlated to theincrease of roughness of thehead that occurs duringaging. (b) Schematic of apolyethylene (PE)cup–zirconia head systemafter a few years in vivo.Note the asymmetrical wearof the PE cup and theextensive pullout of zirconiagrains in the bearing region.

of 5.7 (93% of theoretical density), and was aged in autoclave at 134◦C for 10 h(corresponding to roughly 30 years in vivo). The whole surface is covered by a 5-μm-thick homogeneous monoclinic layer. In the bulk, only the zone with the highestdensity (Figure 10) did not suffer from aging. Everywhere else, water was able toinfiltrate through the open porosities and lead to aging inside the material. This effect


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Figure 10Cross-sectional SEM micrograph after aging of a pressed zirconia pellet with a porositygradient [from the bottom-left corner (high porosity) to the upper-right corner (high density)].The starting powder was spray dried, with granule sizes of approximately 100 μm. Thetetragonal zones are seen in dark gray, whereas the monoclinic ones are seen in white becauseof the high contrast due to extensive microcracking. (a) General view. (b) Aging around thegranules in a medium-density area. The idealized view below shows the dense, tetragonalgranules in black and the transformed, monoclinic zone surrounding them in white. (c) Agingon the surface of porosities (dashed white circle) in a dense zone close to the surface of the pellet.The surface itself is also covered by a constant-thickness monoclinic layer. (d ) Homogeneousmonoclinic layer at the surface of the fully dense zone. (e) Generalized aging in thelower-density zone. The granules are completely separated from each other by a thickmonoclinic layer, and the material has no remaining cohesion.

is hardly documented in the literature (60, 61). Indeed, no XRD investigation canreveal it because XRD is limited to the outer surface, and cross-sectional observationsare necessary.

Many authors have pointed out the important influence of grain size on ag-ing. Although the existence of critical grain size—below which no transformationis possible—is still debated, the beneficial effect of reducing the grain size on the ag-ing kinetics is widely recognized. Practically, the effect of grain size on LTD has alsobeen demonstrated on explanted heads (48). However, decreases in the grain size also


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lead to a decrease in toughness and slow crack propagation threshold, mostly becauseof less efficient phase transformation toughening. A relationship between grain size,density, and aging was observed (61) but still needs to be refined.

3Y-TZP sintered at usual temperatures typically contains up to 15% c phase. Forlong sintering times or high sintering temperatures, bimodal microstructures canappear; the cubic grains are larger than the tetragonal ones and pump the Y2O3

out of the t phase (62–64). The tetragonal grains neighboring the c phase are thusdepleted in Y2O3 and act as preferential nucleation sites for aging (65).

It is now accepted that microscopic tensile stresses in a grain trigger the t-mtransformation (19). Macroscopical tensile stresses can play the same role (66, 67).Thus, machining has to be carefully controlled. Rough polishing induces compressionstresses that delay aging, but the surface state is not suitable for wear applications;fine polishing leads to an acceptable roughness but removes the compressive stressesand allows tensile stresses to appear along the residual scratches. Thus, fine polishingwithout any scratches is needed.

To summarize, LTD is accelerated by coarse grains, low density, and c-t phasepartitioning. Sintering conditions have to be tailored to avoid these characteristics.This can necessitate the use of such tools as HIP or flash sintering (also called sparkplasma sintering). Machining has to be carefully controlled so as to limit the apparitionof residual tensile stresses on the surfaces exposed to aging. However, LTD can beslowed down, but never completely suppressed, in 3Y-TZP. Other zirconia-based orzirconia-containing materials can offer increased aging resistance.

Increasing the Y2O3 content improves resistance to LTD (68). However, increas-ing the stabilizer content inhibits the t-m transformation and thus decreases themechanical properties of the material (69). Furthermore, the proportion of c phaseincreases with increasing Y2O3 content. This results again in a loss of mechanicalproperties and also in an Y2O3 diffusion out of the tetragonal grains neighboring thec phase, which may trigger a preferential t-m transformation around the cubic grains(65).

Doping Y-TZP with other oxides is a more promising way. Over the many ox-ides tested, only a few provide a satisfactory balance between aging resistance andmechanical properties. These are antimony, bismuth, neptunium, niobium, cerium(Ce), aluminum, and silicon oxides. Doping with antimony (70), bismuth (71) andneptunium (72) oxides is beneficial for both mechanical properties, and LTD resis-tance may be detrimental for biocompatibility. An addition of 5% Nb2O5 to 3Y-TZP(73) decreases sensitivity to LTD and improves toughness without decreasing fracturestress. Ce doping in systems in which either Ce or Y is the main stabilizer has beenthe most studied. Adding Ce to a Y-TZP increases the grain size (74–76) but im-proves aging resistance (76–77), and the mechanical properties are influenced moreby the grain size than by the amount of Ce. Because Ce-TZP is biocompatible (it isalready used for dental implants), Ce-Y-TZP could prove an interesting biomedicalgrade of zirconia. The presence of small alumina quantities (between 0.15% and 3%)slows down LTD (78, 79) and increases the temperature at which transformation isthe fastest (80). The influence of silica is more controversial. Neither Lange et al.(18) nor Schmauder & Schubert (21) noted any effect. However, slow cooling after


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sintering silica-doped 3Y-TZP allows the formation of rounded zirconia grains (81,82) and thus a decrease of the internal stresses that results in an improvement inaging resistance (83). In addition, commercial powders containing small amounts ofalumina and silica (such as Tosoh TZ3Y-E) are less sensitive to aging.

Because aging degradation starts as a surface phenomenon, surface treatments(cementation and nitridation) may help prevent aging and retain the good mechanicalproperties of 3Y-TZP (84–86). Such surface treatments can completely suppress agingof the surface (84); its mechanical properties are completely retained even after severeaging. However, between 600◦C and 800◦C, subsurface aging can occur (87), resultingin the appearance of large internal stresses. Whether subsurface aging also takes placeat low temperatures remains to be confirmed.

Perhaps the most promising future of zirconia as a biomaterial is through its useas reinforcement in zirconia-toughened alumina (ZTA), which may offer an excep-tional balance between slow crack growth resistance and aging resistance (43, 88).Controlling the size and homogeneity of the repartition of the zirconia grains iscrucial (89, 90). Indeed, above a critical amount (∼16 vol.%), geometrical percola-tion of zirconia grains leads to a continuous path between zirconia particles, makingthe composite sensitive to aging (88). Below this critical amount, only the largestzirconia grains (or the zirconia clusters in a nonhomogeneously dispersed material)undergo aging; the smallest ones are stabilized by the stiff alumina matrix (decreaseof �USE in Equation 1). LTD can thus be completely suppressed in a ZTA contain-ing fine-grained, well-dispersed zirconia particles below the geometrical percolationthreshold.

Optimal mechanical properties are obtained with only 10 vol.% intergranular zir-conia particles (91). Alumina-zirconia nanocomposites containing 1.7 vol.% of nano-metric intragranular zirconia particles offer even better crack propagation threshold(∼5 MPa · m1/2) and toughness (∼6.5 MPa · m1/2) (to compare with 2.5 and 4 for atypical alumina and 3 and 5.5 for a typical 3Y-TZP), but they are more difficult toprocess (92). In the latter material, the reinforcement is achieved not through phasetransformation of zirconia but through the development in the alumina matrix of ahigh compressive stress field (100–150 MPa) that results from the thermal stressesappearing upon cooling from the sintering temperature. The relative effect of theresidual stress intensity factor is higher for lower applied stress intensity factors,resulting in a high threshold:toughness ratio similar to those observed in covalentmaterials. As a result, this material is less sensitive to delayed failure by slow crackpropagation.

To summarize, ZTA ceramics can offer very high mechanical properties as wellas complete stability. They lead the way to the manufacturing of more demandingdesigns, e.g., 22.22-mm femoral heads, knee prostheses, or dental implants.

CONCLUDING REMARKS

Although in the 1990s 3Y-TZP ceramics were considered very promising materials forbiomedical applications, long-term follow-up is needed to address the critical problemof aging in vivo and its negative impact on orthopedic implant durability. Even if in


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most cases the zirconia head still presents high mechanical resistance, aging leadsto increased wear and particle release in the body, which triggers an inflammatoryresponse and aseptic loosening. As 600,000 patients to date have been implanted withzirconia hip joint heads, a careful explant analysis must be conducted, with a specialemphasis on LTD-microstructure relationships. However, most zirconia implantswere processed at a time when aging was not yet fully understood. Methods to assess apriori the aging sensitivity of a given zirconia ceramic have been developed and shouldlead to safer implants. In the meantime, new zirconia or zirconia-based materials thatovercome the major drawback of the standard 3Y-TZP are now available.

The present review describes aging and its consequences on biomedical implants.It does not cover applications that may also be concerned, such as solid oxide fuelcells or thermal barrier coatings. Owing to specific thermochemical environments,the results obtained on biomedical grade zirconia may not be fully extended, andfurther analysis is required.

SUMMARY POINTS

1. After more than 20 years of research, aging still remains a key issue forzirconia components. Aging in zirconia orthopedic devices is so far the moststriking example.

2. Aging occurs by a water-assisted martensitic phase transformation, whichpropagates at the surface by a nucleation-and-growth mechanism and theninvades the bulk.

3. Aging results in roughening and microcracking. For hip joint heads it leadsto increased wear and the release of wear debris in the body. In the mostextreme cases, it leads to failure of the component.

4. Explanted head analysis and clinical follow-up clearly demonstrate the detri-mental effect of aging on implant durability. The extent of implant degra-dation may vary from one head to another as a consequence of the strongimpact of processing on aging sensitivity.

5. During the past decade, improved techniques have been developed to mon-itor and predict aging. Therefore, it is now possible to predict aging priorto surgery.

6. New zirconia or zirconia-based materials that overcome the major drawbackof the standard 3Y-TZP are now available.

FUTURE ISSUES

1. To date, approximately 600,000 zirconia femoral heads have been implanted.Large-scale, long-term clinical studies and explant analyses are necessary toassess the risks associated with the use of zirconia in vivo.


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2. Orthopedics is a striking example of the negative impact of low-temperaturedegradation in the presence of water on the long-term performance of zirco-nia devices. With the increasing use of Y-TZP in other applications (dental,coatings, solid oxide fuel cells), a careful understanding and control of agingin different thermochemical conditions will be necessary.

3. Alternative zirconia-based materials have been successfully developed on thelaboratory scale. However, only in vivo studies will confirm their superiorityover 3Y-TZP.

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Annual Review ofMaterials Research

Volume 37, 2007Contents

MATERIALS CHARACTERIZATION

Low-Temperature Degradation of Zirconia and Implications forBiomedical ImplantsJerome Chevalier, Laurent Gremillard, and Sylvain Deville � � � � � � � � � � � � � � � � � � � � � � � � � � � � �1

Single-Molecule Micromanipulation TechniquesK. C. Neuman, T. Lionnet, and J.-F. Allemand � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 33

Spin-Polarized Scanning Tunneling Microscopy of MagneticStructures and Antiferromagnetic Thin FilmsWulf Wulfhekel and Jürgen Kirschner � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 69

Microscale Characterization of Mechanical PropertiesK. J. Hemker and W. N. Sharpe, Jr. � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 93

Three-Dimensional Atom-Probe Tomography: Advances andApplicationsDavid N. Seidman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �127

The Study of Nanovolumes of Amorphous Materials Using ElectronScatteringDavid J. H. Cockayne � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �159

Nanoscale Electromechanics of Ferroelectric and Biological Systems:A New Dimension in Scanning Probe MicroscopySergei V. Kalinin, Brian J. Rodriguez, Stephen Jesse, Edgar Karapetian,

Boris Mirman, Eugene A. Eliseev, and Anna N. Morozovska � � � � � � � � � � � � � � � � � � � � � � �189

AFM and Acoustics: Fast, Quantitative Nanomechanical MappingBryan D. Huey � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �351

Electron Holography: Applications to Materials QuestionsHannes Lichte, Petr Formanek, Andreas Lenk, Martin Linck,

Christopher Matzeck, Michael Lehmann, and Paul Simon � � � � � � � � � � � � � � � � � � � � � � � � � �539

Three-Dimensional Characterization of Microstructure by ElectronBack-Scatter DiffractionAnthony D. Rollett, S.-B. Lee, R. Campman, and G.S. Rohrer � � � � � � � � � � � � � � � � � � � � � � �627

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Atom Probe Tomography of Electronic MaterialsThomas F. Kelly, David J. Larson, Keith Thompson, Roger L. Alvis,

Joseph H. Bunton, Jesse D. Olson, and Brian P. Gorman � � � � � � � � � � � � � � � � � � � � � � � � � � � �681

Electron Holography: Phase Imaging with Nanometer ResolutionMartha R. McCartney and David J. Smith � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �729

FERROELECTRICS AND RELATED MATERIALS, David R. Clarkeand Venkatraman Gopalan, Guest Editors

Atomic-Level Simulation of Ferroelectricity in Oxides: Current Statusand OpportunitiesSimon R. Phillpot, Susan B. Sinnott, and Aravind Asthagiri � � � � � � � � � � � � � � � � � � � � � � � � �239

Ferroelectric Domain BreakdownMichel Molotskii, Yossi Rosenwaks, and Gil Rosenman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �271

Local Structure of Ferroelectric MaterialsT. Egami � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �297

Terahertz PolaritonicsT. Feurer, Nikolay S. Stoyanov, David W. Ward, Joshua C. Vaughan,

Eric R. Statz, and Keith A. Nelson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �317

Spiral Magnets as MagnetoelectricsT. Kimura � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �387

Universal Domain Wall Dynamics in Disordered Ferroic MaterialsW. Kleemann � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �415

Defect–Domain Wall Interactions in Trigonal FerroelectricsVenkatraman Gopalan, Volkmar Dierolf, and David A. Scrymgeour � � � � � � � � � � � � � � � � �449

Influence of Electric Field and Mechanical Stresses on the Fracture ofFerroelectricsGerold A. Schneider � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �491

Strain Tuning of Ferroelectric Thin FilmsDarrell G. Schlom, Long-Qing Chen, Chang-Beom Eom, Karin M. Rabe,

Stephen K. Streiffer, and Jean-Marc Triscone � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �589

Ferroelectric Epitaxial Thin Films for Integrated OpticsBruce W. Wessels � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �659

Index

Cumulative Index of Contributing Authors, Volumes 33–37 � � � � � � � � � � � � � � � � � � � � � � � �769

Errata

An online log of corrections to Annual Review of Materials Research chapters (if any,1997 to the present) may be found at http://matsci.annualreviews.org/errata.shtml

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