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II Surface stability and small-scale testing of zirconia Cover photo: top view of a 3 μm diameter micropillar milled from the polished zirconia surface. A hair from the author is also visualized on the top-left of the image to better appreciate the sample size.

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Surface stability and small-scale testing zirconia III

Outline

Tetragonal polycrystalline zirconia stabilized with 3 mol% of yttria (3Y-TZP) is a popular biocompatible ceramic, showing superior mechanical properties among single oxide ceramics. These properties are partly the consequence of phase transformation: thanks to the presence of the stabilizer, zirconia is composed mainly of a tetragonal metastable form, which can transform to the stable monoclinic phase through a martensitic transformation. The transformation, which involves a net volume increase, can be activated either mechanically by the application of high stresses, or chemically by the diffusion of water species, when the material is exposed to humid environment and moderate temperatures. In the first case, the local transformation at the crack tip under tension hinders its propagation, producing a phenomenon known as transformation toughening, which allows the design of “tough” and damage-tolerant ceramics. On the other hand, the spontaneous and progressive surface transformation in presence of humidity represents an aging phenomenon known as low- temperature degradation (LTD), hydrothermal degradation or aging. Although this phenomenon affects a surface layer of only few micrometers, it is accompanied by a substantial impairment of the surface integrity, which represents a serious issue for the use of 3Y-TZP in load-bearing biomedical applications. It was shown in orthopedics that zirconia femoral heads used in hip-joint arthroplasty may be vulnerable to LTD, leading in some cases to premature failures. Likewise, some recent publications devoted to dental zirconia have highlighted that zirconia is susceptible to LTD if exact processing conditions are not followed.

The first subject of this thesis is the study of reliable solutions to avoid hydrothermal degradation of 3Y-TZP. Two novel methods are proposed, which allow a strong enhancement of the aging resistance through limited changes in the standard processing steps for 3Y-TZP, while maintaining its good mechanical properties. These methods are based on co-doping the material from the surface with CeO2 by two different approaches. One approach includes

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IV Surface stability and small-scale testing of zirconia

infiltration of CeO2 precursors in the pre-sintered porous material. During thermal decomposition and sintering, CeO2 is trapped into the pores and diffused into 3Y-TZP, obtaining a functionally-graded material with decreasing CeO2 content from the surface to the bulk. The co-doping profile has been optimized by studying the pre-sintering, infiltration and sintering parameters in order to avoid a drop in the mechanical properties that is generally found in CeO2-doped zirconia. The second approach derives from the same idea but is applied to the dense material, where surface roughness has been created for adhesion or osseointegration purposes. After pressure infiltration of CeO2 precursors into the surface defects and pores created during the roughening process, a few-micrometers thick co-doped layer is obtained with a diffusion treatment. This treatment helps sealing the surface defects and avoids hydrothermal degradation, without affecting the color or the mechanical properties. It is therefore directly applicable in the manufacturing of dental crowns, abutments and dentures.

The second subject of the thesis focuses on small-scale testing of zirconia near-surface regions. The existence of extrinsic size effects on the mechanical properties has been investigated, and the mechanical response of the degraded and non-degraded state has been compared in bending and compression. By testing micropillars and micro-cantilevers milled with focused ion beam (FIB) from the zirconia surface, it has been proved that mechanical properties are significantly different at the small-scale. Higher strength and strain at failure have been recorded with respect to the bulk state, as a result of the absence of processing defects and the occurrence of significant transformation-induced plasticity. No size effect has been found in terms of strength between small-scale samples of different sizes, whereas the

“yield” stress for activating phase transformation is smaller for samples in the sub-micrometer range. Hydrothermal degradation produces a microcrack network which strongly impairs the mechanical properties of small-scale samples milled inside the degraded layer. In bending, a different response in terms of strength and stiffness has been measured depending on sample orientation, proving that that the induced damage is anisotropic. The degradation effects on the compressive behavior depend on pillar size, since the discrete contribution of a reduced number of weakened grain boundaries and microcracks yield to significant scatter in the response of sub-micrometric pillars.

Keywords: zirconia; 3Y-TZP; infiltration processing; dental materials; low-temperature degradation; micro-pillar; micro-cantilever; small-scale testing; size effect.

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Surface stability and small-scale testing zirconia V

Preface

This dissertation is submitted for the degree of Doctor of Philosophy at the Universitat Politècnica de Catalunya - BarcelonaTech. The research described in this work was carried out by the author during the period from the 1st of September 2010 to the 31st of December 2015 under the supervision of Prof. M. Anglada in the Department of Materials Science and Metallurgical Engineering at the Universitat Politècnica de Catalunya. The work described in this dissertation is original, unless otherwise detailed references are provided.

This thesis is presented as a collection of published articles, which are listed in the next section. Chapter 1 is a review about the state of the art of biomedical grade zirconia and small-scale testing. The aim and scope of the thesis are presented in Chapter 2. The experimental details can be found in each article and supporting information (if present), whereas some additional experimental details about small-scale sample preparation and testing are included in Chapter 3. In Chapter 4, the seven articles included in this thesis are introduced in order of publication date. The full articles appear in the Chapters 5 to 9 and in Annexes A and B. Article VII and Article III are devoted to the infiltration of CeO2

precursors in the pre-sintered porous material. The methodology of infiltration has been developed and the processing parameters have been studied and optimized through a multi- scale characterization of the obtained materials in terms of resistance to LTD, microstructure evolution and mechanical properties under various processing conditions. The method of pressure infiltration, designed for fully dense zirconia where surface roughness is needed, is studied in Article IV and applied to sandblasted and acid-etched materials. Article I is a study of the effect of hydrothermal exposure to standard 3Y-TZP with different grain sizes and Cerium infiltrated 3Y-TZP in terms of indentation crack profiles. This paper details a novel technique for revealing crack profiles, which has been employed in Article III to compare the resistance to crack formation in materials doped under different conditions.

Article VI represents a detailed study of the compressive behavior of zirconia at the small-

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VI Surface stability and small-scale testing of zirconia

scale. Monotonic and cyclic compression tests are performed on micropillars and the behavior is compared with macroscopic samples. Advanced characterization techniques are applied to give an interpretation to the observed plastic effects and size-dependent phenomena. In Article II, the in situ micro-cantilever bending technique is applied to 3Y-TZP to measure its flexural response at the small-scale. The mechanical behavior is compared between micro- cantilevers milled from non-aged surfaces and from inside the degraded layer. In the latter, by testing samples with different orientations with respect to the degraded surface, the anisotropy of the damage induced by hydrothermal degradation is assessed. A similar approach is followed in Article V, where the micropillar compression technique is applied before and after degradation to study the compressive behavior of aged and non-aged pillars. Moreover, interesting insights on the effect of the damage induced by LTD are observed by testing small-scale samples of different sizes and by performing loading/unloading tests at increasing peak stress.

The main results and conclusions of the research work are reviewed in Chapter 10, together with its scientific impact and possible future implications.

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Surface stability and small-scale testing zirconia VII

List of publications

This dissertation is based on the articles contained in the following list, in order of publication date. IF = impact factor; Q = quartile.

Article I. Marro, F. G., Camposilvan, E. & Anglada, M. Revealing crack profiles in polycrystalline tetragonal zirconia by ageing. Journal of the European Ceramic Society 32, 1541–1549 (2012). doi: 10.1016/j.jeurceramsoc.2012.01.013. IF 2.307, Q1 (2/25) in the cathegory: “Materials science, ceramics”.

Article II. Camposilvan, E., Torrents, O. & Anglada, M. Small-scale mechanical behavior of zirconia. Acta Materialia 80, 239–249 (2014). doi:

10.1016/j.actamat.2014.07.053. IF 3.940, Q1 (35/251) in the cathegory “Materials science, multidisciplinary”, and Q1 (1/75) in “Metallurgy & metallurgical engineering”.

Article III. Camposilvan, E., Garcia Marro, F., Mestra, A. & Anglada, M. Enhanced reliability of yttria-stabilized zirconia for dental applications. Acta Biomaterialia 17, 36-46 (2015). doi: 10.1016/j.actbio.2015.01.023. IF 5.684, Q1 (3/76) in the cathegory:

“Engineering, biomedical”, and Q1 (3/32) in “Materials science, biomaterials”.

Article IV. Camposilvan, E., Flamant, Q. & Anglada, M. Surface roughened zirconia:

Towards Hydrothermal stability. Journal of the Mechanical Behavior of Biomedical Materials 47, 95-106 (2015). doi: http://dx.doi.org/10.1016/j.jmbbm.2015.03.017. IF 3.048, Q1 (15/76) in the cathegory: “Engineering, biomedical” and Q2 (10/32) in

“Materials science, biomaterials”.

Article V. Camposilvan, E. & Anglada, M. Micropillar compression inside zirconia degraded layer. Journal of the European Ceramic Society (2015). doi:

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VIII Surface stability and small-scale testing of zirconia

10.1016/j.jeurceramsoc.2015.04.017. IF 2.307, Q1 (2/25) in the cathegory: “Materials science, ceramics”.

Moreover, the following articles are included as annexes for completeness.

Article VI (Annex A). Camposilvan, E. & Anglada, M. Role of size and plasticity in zirconia micropillars compression. Article submitted to Acta Materialia, under review (2015, submission ref. A-15-657).

Article VII (Annx B). Camposilvan, E., Garcia Marro, F., Mestra, Á. & Anglada, M.

Development of a novel zirconia dental post resistant to hydrothermal degradation. IOP Conference Series: Materials Science and Engineering 31, (2012). doi:10.1088/1757- 899X/31/1/012016

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Surface stability and small-scale testing zirconia IX

List of Figures and Tables

Figures

Figure 1.1. Allotropic structures of pure zirconia and transformation temperatures. From left to the right: monoclinic (P21/c), tetragonal (P42/nmc), and cubic (Fm-3m) structures. __________________ 3 Figure 1.2. Phase diagrams for the system ZrO2-YO1.5.The diagram on the left was introduced by Scott et al [14], where the metastable phases retained at room temperature are indicated with horizontal lines at the bottom of the picture. The red dashed lines indicate the T₀ temperatures, which separate the non-equilibrium transition regions between the monoclinic (M) and tetragonal (T) phases and between tetragonal and cubic phases (F = fluorite), while the vertical dashed line represent the composition for 3Y-TZP. The updated diagram on the right was calculated by Fabrichnaya et al [15]. _____________ 4 Figure 1.3. Strength-toughness relation for some PZT and TZP ceramics. Adapted from ref. [17]. ___ 5 Figure 1.4. Formation of oxygen vacancies into the ZrO2 lattice when its doped with Y2O3 [24]. ___ 7 Figure 1.5. In picture a (Laser Scanning Confocal Microscopy) the spray-dried granules are showed, in picture b the particles forming the granules are visualized with TEM. ________________________ 9 Figure 1.6. Top-left: scheme representing current dental CAD/CAM systems for Crown and bridge restorations. a and b: laser and contact scanning, respectively, of the replica tooth. c: soft machining (CAM) of the pre-sintered blank. d: pre-sintered blank (VITA corp.) showing the shape of a milled crown. e: final shape of a sintered abutment (right) and bridge with partial veneering (left).

Reproduced from Miyazaki et al.[25]. _________________________________________________ 10 Figure 1.7. Fracture toughness and aging sensitivity of Y-TZP as a function of yttria content.

Toughness data for cubic and monoclinic zirconia is shown together with the toughness of a zirconia thermal barrier coating (TBC). Aging sensitivity (solid line) is indicated by the monoclinic fraction after 3 hours at 134ºC in water vapor [7]. _______________________________________________ 11 Figure 1.8. Surface phase transformation of 3Y-TZP zirconia aged at 230ºC in air [38], as observed for different grain sizes._____________________________________________________________ 13 Figure 1.9. First steps of LTD by Chevalier et al. [7]. The figure shows the diffusion of water species (a) and the consequent variation of lattice parameters (b). __________________________________ 14

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XIV Surface stability and small-scale testing of zirconia Figure 1.10. (a) Degradation kinetics in terms of monoclinic content vs. time measured on 3Y-TZP by XRD for different degradation temperatures. Curves have been fitted with the MAJ law and n=36.

Adapted from ref. [41]. (b) TTT curves at 30% monoclinic content for TZP with 3 different yttria contents (ref. [44]) and for 3Y-TZP with smaller grain size measured by XRD. Adapted from ref. [7].

(c) TTT curves for 2,8 mol% Y-TZP thermal barrier coating at the beginning and end of transformation measured by Raman spectroscopy. Adapted from ref. [45]. ____________________ 15 Figure 1.11. Growing of the transformed layer and extension of the degradation to the underlying layer due to water penetration and consequent rising of tensile stresses. Reproduced from ref. [7]. __ 15 Figure 1.12. LTD kinetics of 10Ce-TZP, 3Y-TZP, Mg-PSZ, alumina-toughened zirconia (A80Z3Y), and zirconia-toughened alumina (A10Z0Y) measured at 134 ºC and expected at 37 ºC (from Chevalier et al. [7]). _______________________________________________________________________ 17 Figure 1.13. Zirconia–ceria phase diagram, from Yashima et al. [51]. Dashed lines represent the temperatures for non-equilibrium transformation ________________________________________ 17 Figure 1.14. Correlation of three points bend strength and CeO2 contents in terms of YO1,5 content, from Lin et al. [47]. _______________________________________________________________ 19 Figure 1.15. Map of flexural strength (grey lines, MPa), fracture toughness (black lines, MPa·m) and composition of various Y-Ce-TZP samples. The dotted line indicates the composition boundary between stable and unstable t phase after 1000 h hydrothermal aging at 200ºC, while the dashed square indicates the optimum composition region according to Lin et al. [46]. _______________________ 19 Figure 1.16. From left to right: MTS Nanoindenter XP with the main parts indicated, nanoindenter simplified scheme and typical load-displacement curve obtained with nanoindentation. __________ 22 Figure 1.17. Torsional response of copper wires of diameter 2a in the range 12-170 µm. Q is the torque, a the wire radius and k the twist per unit length. If the constitutive law were independent of strain gradients, the plots of normalized torque Q/a³ vs. ka would all lie on the same curve. Reprinted from ref. [66]. ____________________________________________________________________ 24 Figure 1.18. Compression behavior at room temperature for pure Ni microsamples having a <134>

orientation. A: stress-strain curves for pillars of different sizes and for bulk material. B, C: SEM images of 20 µm and 1 µm diameter pillars, respectively, after testing. D: dependence of the yield strength (σ_ys) on the inverse of the square root of the sample diameter for Ni₃Al-Ta. The transition from bulk to size-limited behaviour is predicted by the linear fitting at approx. 42 µm. From Uchic et al. [71] _________________________________________________________________________ 25 Figure 1.19. A: micro-pillar of 43 µm diameter milled by the lathe automated milling procedure into the surface of a Ni₃(Al,Hf) single crystal. On the top of the pillar the fiducial mark can be appreciated.

B: micro-pillar of 2.3 µm diameter milled into Ni superalloy (UM-F19) single crystal. ___________ 26 Figure 1.20. (a) Low lattice resistance (fcc metals and LiF) and (b) Intermediate lattice resistance (bcc metals) normalized shear yield stress vs. pillar diameter and power-law fit. Reproduced from ref. [76].

_______________________________________________________________________________ 27

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Surface stability and small-scale testing zirconia XV Figure 1.21. a) Extrinsic and intrinsic size effects for Ni and Ni-4%W, reproduced from Ref. [75]. b) opposite behavior of nanocrystalline Ni after Rinaldi et al. [78] and Jang and Greer [79]. _________ 28 Figure 1.22. a) and b): GaAs micropillar before and after compression, showing the presence of slip bands. Reproduced from [80]. Right: yield stress vs. pillar diameter for GaAs, with the failure mode indicated. From ref. [81]. ___________________________________________________________ 28 Figure 1.23. Left: yield stress vs. pillar diameter for Si micro-pillars of different sizes, tested ex-situ (triangles) and in-situ (diamonds). The circled pillars showed cracking, which is reflected in (a) picture and curve, meanwhile smaller pillars showed plastic deformation, reflected in (b) picture and curve. (a) and (b) pillars are 400 and 310 nm, respectively. Reprinted from [82]. ________________________ 29 Figure 1.24. a) Stress-strain curves for spinel micropillars deformed at 25 ºC (filled diamonds), 200 ºC (open squares) and 400 ºC (open diamonds). b) Shear yield stress y vs. temperature for <001>

oriented bulk crystals (triangles, from ref. [84]) and micropillars (squares) at the shear strain indicated.

y changes with temperature at -5 x 106 PaK-1 (ref. [84]) and -3.17 x 106 PaK-1 (micro-pillars) ___ 29 Figure 1.25. Normalized shear yield stresses and power-law fits for several ceramic systems. In (a) the results of refs. [82], [85] are showed, while in (b) a summary of metal and ceramic systems is represented, together with the results of Korte and Clegg [76], from which the charts are reproduced.30 Figure 1.26. 2 µm TiC micro-pillar before and after testing, with the correspondent stress-strain curve, which is magnified in the last picture. From [89]. ________________________________________ 31 Figure 1.27. 0.8 µm TiC micro-pillar compressed around 6,6% strain, with the correspondent stress/strain curve. Slip bands are clearly visible. From [89]. ________________________________ 32 Figure 1.28. The dislocation shielding concept is outlined in (a), while (b) is the formula which relates K_Ic with the number of shielding dislocations N, the shear modulus µ, yield strength σ_ys, the Burger’s vector b and the Poisson’s ratio , from [93]. (c) Fit of 22 ceramic materials according to the model proposed. (d) Arrested crack in silicon single crystal with N = 14 shielding dislocations [94]. Pictures reproduced from Gerberich et al. [92]. _________________________________________________ 33 Figure 1.29. A) Pseudoelastic effect in tetragonal zirconia. B) Thinning of the loading-unloading cycles (“training”). Reproduced from Lai et al. [99]. ______________________________________ 34 Figure 1.30. Left: in-situ fracture testing of silicon cantilevers by Johansson et al. [100]. Right: FIB machined micro-cantilevers by McCarthy et al. [101]. _____________________________________ 35 Figure 1.31. Left: fracture strength and effective volume of polysilicon cantilevers. Right: Fracture strength vs. surface-to-volume ratio of the polysilicon cantilevers. Reproduced from ref. [102]. ____ 35 Figure 1.32. Left: micro-cantilevers at 15º intervals in (110) single crystal copper face, optical micrograph. Experimental (points) and literature data (lines) about Young’s modulus of copper single crystal, with both techniques proposed in ref. [104]. ______________________________________ 36 Figure 3.1. Left: dual beam Zeiss Neon 40 (top) and image of the chamber, where SEM and FIB columns are visible. The specimen (half of a disc, polished both on top and cross-section) is mounted on a special holder and tilted 54º. Right: two-steps milling of a pillar (scale bar = 3 µm). _________ 43

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XVI Surface stability and small-scale testing of zirconia Figure 3.2. Final shape of cantilevers milled from the zirconia surface region with two different procedures. ______________________________________________________________________ 44 Figure 3.3. a) Micro-compression scheme for a MTS Nanointenter XP. b) Flat-punch marks during microscope-to-indenter calibration on aluminum single crystal. c) SEM image of the home made flat punch tip obtained by FIB assisted cutting of a Berkovich diamond tip. _______________________ 45 Figure 6.1. Detailed milling procedure for horizontal and vertical cantilevers. The different trenches and cuts are shown. (a): T1; (b): T2; (c) and (d): T3h; (e) and (f): T4h (final shape for horizontal cantilevers); (g), (h) and (i):, T3v (final shape for vertical cantilevers). ________________________ 80 Figure 7.1. Microstructure of materials with different pre-sintering conditions. a: 1100L, b: 1100H, c:

1150L, d: 1150H, e: 1200L f: 1200H, g: 1300H, h: AS. Scale bar = 1 μm. _____________________ 97 Figure 7.2 (Figure S2 in the article). XRD normalized spectra after artificial aging, in the region 25- 75 deg., for selected conditions: AS, 1300L, 1200L, 1150L, 1100L. Additionally, one extra spectra is included for the AS non-aged material (i.e. the base material). ______________________________ 97 Figure 7.3. SEM micrographs of the AS (a), S1450 (b), S1550 (c) and, S1600 (d) microstructures.

Scale bar = 1 μm. Samples have been infiltrated at pre-sintered condition 1150º. ________________ 98 Figure 7.4. XRD normalized spectra after artificial aging, in the region 27.5-32 deg. ____________ 98 Figure 8.1. FIB trench on EIT material infiltrated at ambient pressure. To the right, a degraded spot is highlighted. _____________________________________________________________________ 113

Tables

Table 1.1. Mechanical properties of different ceramics for biomedical applications. From ref. [26] ... 10 Table 7.1. Summary of properties of all materials sintered at 1450 ºC: biaxial strength, CeO₂ content near the surface, IF toughness in the surface region, grain size and monoclinic content after 30 h of artificial aging (mean values ± SD). Letters indicate the statistically significant differences. ... 96 Table 10.1. Summary of properties of all materials sintered at 1450 ºC: solution concentration, apparent porosity, CeO₂ content near the surface, biaxial strength, IF toughness in the surface region, grain size and monoclinic content after 30 h of artificial aging (mean values ± SD). Letters indicate the statistically significant differences (ANOVA with Tuckey test, p = 0.05). Adapted from the supporting information of Article III. ... 126 Table 10.2. Comparison of failure stress (σ*) and failure strain (ε*) for micropillars and 2 x 2 x 4 mm bulk specimens. ... 130

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Surface stability and small-scale testing of zirconia XVII

Glossary of terms

3Y-TZP: TZP stabilized with 3% mol content of Y2O3

AFM: Atomic Force Microscopy AS: As-sintered

ATZ: Alumina-Toughened Zirconia B3B: Ball-on-three-balls strength test Ce-TZP: TZP stabilized with CeO2

CIP: Cold isostatic pressing

CSM: Continuous Stiffness Measurement EDS: Energy-Dispersive X-ray Spectroscopy EPMA: Electron Probe Micro Analysis FGM: Functionally-Graded Material FIB: Focused Ion Beam

MAJ: Mehl-Avrami-Johnson

MIP: Mercury Intrusion Porosimetry PSZ: Partially Stabilized Zirconia SEM: Scanning Electron Microscopy TBCs: Thermal Barrier Coatings

TEM: Transmission Electron Microscopy THA: Total Hip Arthroplasty

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XVIII Surface stability and small-scale testing of zirconia

TZP: Tetragonal Polycrystalline Zirconia XRD: X-ray diffraction

Y-Ce-TZP: TZP stabilized with Y2O3 and CeO2

ZTA: Zirconia-Toughened Alumina

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Surface stability and small-scale testing of zirconia 1 / 184

Chapter 1 Introduction

1.1. Overview: zirconia-based ceramics

Zircon gems were known since ancient times, and zirconia (Zirconium oxide) was used during centuries as a pigment for ceramics, but the material itself could not be processed due to catastrophic phase transformation during cooling of sintered parts. Nevertheless, since about 90 years ago, sane zirconia parts could be obtained at room temperature thanks to the addition of stabilizers starting [1]. The seminal discovery of transformation toughening in 1975 [2], paved the way to their wider use because of their excellent strength, fracture toughness, hardness and chemical stability.

These materials are used in high refractory and thermal shock resistant parts like valves and liners for engines and in foundry, extrusion dyes and thermal barrier coatings (TBCs), wear resistant component like blades, moulds and bearings and as high-temperature ionic conductors, e.g. solid electrolytes in fuel cells and in gas sensors [3].

The first paper concerning the use of zirconia to manufacture ball heads for Total Hip Arthroplasty (THA), was introduced by Christel et al. in 1988 [4]. In the 90’s, biomedical grade 3% molar yttria-tetragonal polycrystalline zirconia (3Y-TZP) became an alternative to alumina as structural bioinert ceramic in orthopedics because it has substantially higher fracture toughness and strength. In fact, 3Y-TZP exhibits some of the best mechanical properties among single-phase oxide ceramics: flexural strength above 1000 MPa, fracture toughness around 5 MPa·m and hardness above 12 GPa. The use of 3Y-TZP in orthopedics paved the way to new implant designs that were not possible with alumina, which is more

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2 / 184 Surface stability and small-scale testing of zirconia

brittle. Examples of medical applications are smaller 3Y-TZP femoral heads, 3Y-TZP knees and more recently a large range of dental devices (crowns, bridges, abutments and dental implants), where the translucency and easy color modification for matching teeth shade offered by TZP is of great interest [3], [5].

Fracture toughness in 3Y-TZP is related to the transformation toughening mechanism, that is, the local tetragonal to monoclinic (t–m) martensitic phase transformation activated by stress and accompanied by a volume increase that occurs in front of the crack tip. Since the surrounding elastic material does not transform, this induces a net compressive residual stress which develops on the crack flanks and hinder crack growth [2].

In spite of these excellent properties, 3Y-TZP has a strong drawback: poor resistance to hydrothermal degradation also referred to as Low Temperature Degradation or aging (LTD) [6]. This phenomenon consists in the spontaneous and progressive t–m phase transformation which happens at the surface when exposed to humid environment and moderate temperature.

The kinetics of LTD depends on the alloying element, its concentration and other microstructural parameters, but in general this process affects only near-surface regions [7], with the transformed surface layer growing progressively in the depth direction [8]. The phase transformation is accompanied by the formation of microcracks and surface roughening, which reduce the hardness [9] and induce grain pull-out [3]. Although the kinetics of transformation is maximal at temperatures around 250ºC, there is now evidence that LTD is also possible at lower temperatures, even at human body temperature. This was considered as a minor issue by femoral heads manufacturers until an unusual high rate of failures for some batches of 3Y-TZP femoral heads was reported in 2001 during a short period of time [10].

During the posterior investigation, it was clearly established that these failures had their origin in a change in the processing steps of the balls that decreased their resistance to LTD.

These failures had a strong negative impact on the implementation of zirconia in hip arthroplasty. Additionally, there are now many reports on explanted femoral heads which show that even in prosthesis processed under normal conditions, t–m transformation occurs and the surface is degraded in contact with human body fluids. As a consequence, 3Y-TZP is currently not anymore used in the manufacturing of femoral heads prosthesis in Europe and United States [11] and new nanocomposites, mainly based on the alumina-zirconia system, have been developed instead.

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.2. Phase diag ere the metasta cture. The red between the m while the vert ulated by Fabri

ragonal Zirc retained in a se is monoc sformation t nt of a crack

idely used s O3) and Ceri

2-Y2O3. to the final m e major type

d zirconia equently use

lized zircon uch ceramic

grams for the s able phases ret dashed lines monoclinic (M)

tical dashed lin ichnaya et al [1

conia Polycr a metastable clinic. This to the mono

tip during it

tabilizers ar ium oxide (

microstructu s [7]:

(FSZ): the ed in oxygen

ia (PSZ): c cs are genera

system ZrO2-Y tained at room

indicate the T ) and tetragon ne represent th 15].

rystals (TZP e state down type of cera oclinic phas

ts propagati

Su

re Calcium o (CeO2). Figu

ure achieved

e crystallog n sensors an consists of ally obtained

YO1.5.The di m temperature T0 temperature nal (T) phases he composition

P): With allo n to room te amic exhibi se is locally

ion.

urface stability

oxide (CaO ure 1.2 show

d, zirconia c

graphic pha d fuel cell e nano-sized d with the ad

agram on the are indicated w s, which separ

and between n for 3Y-TZP.

oy stabilizat emperature its exception

triggered b

y and small-sca

), Magnesiu ws the phase

ceramics can

se is most lectrolytes.

tetragonal ddition of C

left was intro with horizonta rate the non-eq

tetragonal and The updated

tion, the tetr even though nal fracture y the high t

ale testing of z

um oxide (M e diagram fo

n be conven

tly cubic. T

precipitates CaO or MgO

oduced by Sco al lines at the b equilibrium tra nd cubic phase diagram on th

ragonal phas h the equilib

toughness tensile stres

irconia

MgO), for the

niently

These

s in a O.

ott et al bottom ansition es (F = he right

se can brium when ses in

(23)

Surfac

conte tough Y-TZ conte abou been incre trans Hann stren phase also ceram tough

Figur

1.3.

diffu less trans

ce stability and

From no ent has a st hness of Y- ZP, which c ent. Neverth ut 2 MPa·√m

shown that eased, but th ition from nink et al. [ ngth is limit

e transforma evidenced i mics is show

hening, LTD

re 1.3. Strength

. The t–

The t–m usionless and

than an ato formed regi

d small-scale t

ow on in thi trong influe -TZP ceram

can reach v heless, the fr m. However t by the des here is limit the strength [16] propos ted by flaw

ation, which in the Figur wed. In any D resistance

h-toughness re

–m trans

phase trans d involves t omic diame ions [18].

esting of zirco

is manuscri ence on the mics. As show

values as hi racture toug r, high tough

stabilization over which h controlled sed that, for size, where h occurs at a re 1.3, wher case, when is also affec

elation for som

sformatio

sformation i the simultan eter. Theref

onia

ipt, only TZ e tetragonal

wn in Figur igh as 5-6 ghness of the

hness does n of the tetr h the fractur d by the fla r materials w eas, above t a stress low re the streng n the stabiliz cted, as it w

me PZT and TZ

on

in zirconia i neous, coop fore, it prod

ZP materials phase stab re 1.7, the f MPa·√m, d e cubic and

not mean n ragonal pha re toughnes aw size to t with KIc le this value, i wer than the

gth-toughne zer content will be discus

ZP ceramics. A

is martensiti erative mov duces a ma

s will be of bility and th

fracture toug decreases wi

monoclinic necessarily h

se, the stren s starts to d transformati ess than app

it is limited fracture stre ess relation

is optimized ssed in secti

Adapted from r

ic. This mea vement of at acroscopic c

f interest. T herefore on

ghness of te ith increasin c phases is a

high strengt ngth of TZP drop. This m

ion-limited prox. 8 MP d by stress-a

ess. This be for various d for transfo ion 1.6.

ref. [17].

ans that is a atoms over d change of s

5 / 184

The Y2O3

fracture etragonal ng Y2O3

as low as th. It has P can be marks the strength.

Pam the activated havior is zirconia ormation

athermal, distances shape of

(24)

To explain the behavior of zirconia ceramics, it is essential to understand the role of the stabilizers. For doing so, it is necessary to know the thermodynamics of the t–m phase transformation. Considering the simple ideal case of a spherical tetragonal particle in a matrix, the change of total free energy (Gt−m) due to its transformation is given by:

(1.1)

where Gc (< 0 when T < Ms) is the chemical free energy change (dependent on temperature and composition), USE (> 0) is the strain energy change associated with the transformed particles (dependent on the surrounding matrix and its stiffness, the size and shape of the particle, and the presence of stresses), and US (> 0) is the change in energy associated with the surface of the particle (creation of new interfaces and micro-cracking).

Through a decrease in |Gc| and an increase in USE, the addition of alloying oxides such as Y2O3 in Y-TZP decreases the driving force of the t–m transformation and hence its temperature, with the result of the metastable phase diagram of Figure 1.2, allowing the retention of metastable tetragonal particles at room temperature. At the same time, a stiffer matrix can stabilize the tetragonal phase reducing USE. Moreover, the driving force for the transformation is not the same for the bulk and the surface of the material, because both US

and USE are different, especially when it is considered the possibility of accommodating the volume change with a surface uplift [7].

Some authors [19], [20] have argued that stabilization of the tetragonal phase at room temperature is a direct consequence of the presence of oxygen vacancies introduced by the aliovalent alloying elements rather than by the aliovalent dopant itself. It has been shown computationally that the tetragonal phase can be produced with lower energy than the monoclinic phase by introducing oxygen vacancies and without any aliovalent ions into the unit cell [21]. Then, all the phase diagrams for different stabilizers would roughly collapse onto one when plotted as a function of concentration of oxygen vacancies, so it can be understood how the concentration of oxygen vacancies is a crucial factor. However, this is not the complete explanation of the behavior of TZP ceramics as shown by Li et al. [20], [22], [23], who studied the effect of the various aliovalent dopants and determined the local atomic structures. The key concept in zirconia stabilization is that Oxygen atoms tend to be overcrowded around the small Zr atom, reducing the stability of the structure with the 8-fold coordination. By relieving the overcrowding with aliovalent doping and vacancy generation,

t m C SE S

G_ G U U

      

(25)

Surfac

the te zirco

overs the c partia (near

tetrah wher There overc it for phase bifur

Figur

1 Tetra bonds tetrago

ce stability and

etragonal an onia structur

It was co sized trivale case of unde

ally relieved r Zr) and the

With tet hedra form re the CeO8

efore, Ce d crowding an rms an order e has been a rcated tetrah

re 1.4. Formati

agonality: feat s, where O1 an onal layer stru

d small-scale t

nd cubic stru e and oxyge

oncluded th ent dopants ersized dop d, oversized e additional travalent do with Zr. C

8 polyhedra dilates the nd so impro red substitut associated in hedral bondin

ion of oxygen

ture of tetrago d O2 are Oxyg ucture and is an

esting of zirco

uctures are s en vacancies

at the vacan (Y³⁺, Gd³⁺) ants (Fe³⁺, d dopants a dilatation of opants like

e forms a r a is bigger lattice beca ving tetrago tion and inc n the latter c ngs in the la

vacancies into

nal Zirconia th gens of differen n indicator of t

2 3( Y O ZrO

onia

stabilized. W s are generat

ncies are as ), and with t Ga³⁺). Whi are more eff

f the cation Ce⁴⁺ and G random sub

than for pu ause of his onal stability creases the t

case to the c ayer-like stru

o the ZrO2 latt

hat consists of nt layers in the the instability o

' 2) 2 _Zr O  Y 

With trivalen ted for charg

sociated wi two dopant le in both c fficient due

network.

Ge⁴⁺ formin bstitution wi ure ZrO2 bu

oversized y. The beha

etragonality cation order ucture [22].

tice when its do

f a different st e lattice. It cor of the tetragon

3O_O^x V_O^

 

nt dopants, t ge compens

th the Zr ca cations shar cases, Oxyg

to the posit

ng solid sol ith a decrea ut smaller t

dimension, vior of Ge i y. The stabil ing and the

oped with Y2O

trength and len rresponds so w nal phase.

the cations a sation:

ations in the aring one va

gen overcrow tion of the

lutions, two ase in tetrag than in pur decreasing is the oppos lization of te full develop

O3 [24].

ngth of Zr-O1

with the anisotr

7 / 184

adopt the

(1.2) e case of acancy in wding is vacancy

o sets of gonality¹,

re CeO2. g oxygen

site since etragonal pment of

and Zr-O2

ropy of the

(26)

1.4. Processing of zirconia-based ceramics

Diverse techniques can be employed to process dense zirconia based ceramics with the desired shape, integrity and dimensional tolerances. These techniques can be summarized in two main categories: wet and dry processing.

1.4.1. Slip casting

Among wet processing techniques, slip casting is the one traditionally used for the fabrication of pottery, which need the preparation of an adequate suspension (slip) of the powder. This can be formed by particles of different constituents or already pre-alloyed. The suspension is poured into a mould with the desired shape, in which the liquid is partially absorbed giving some mechanical strength to the “green” piece, which is then allowed to dry.

After drying, the piece can be fired and, depending on the firing technique and temperature, the desired density can be achieved. Despite of the versatility of this process and the reduced waste of material, up to now it is only occasionally employed in the manufacturing of biomedical implants, even though the interest toward slip casting has been recently rising due to the possibility to create engineered surfaces for bone adhesion and regeneration.

1.4.2. Dry processing

The dry processing route involves the employment of powders with tailored characteristics of particle size, composition, flowability and strength after pressing. Even though there is not only one way to process powders and current research is addressed to evaluate possible modifications and improvements to the method, the following describes a general route followed by most of the companies. Powders are constituted of spray-dried spherical agglomerates, generally around 60 m in size, of smaller particles usually of some tens of nanometers, with crystallites of 20-30 nm (see Figure 1.5). These agglomerates, containing also a binder designed for giving the green strength, are usually pressed isostatically in a deformable mould to obtain the green piece, which can be manipulated. The green piece is then pre-sintered in air, obtaining a porous machinable preform with a reasonable mechanical strength.

(27)

Surfac

Figur the pa

elimi and Desig the p typic the s produ non-c blank 1300 perfo at the used perfo to ob steril

ce stability and

re 1.5. In pictu articles forming

During p inated and b

dwell time gn/Compute piece during cally applied shape is sta uction of de contact meth k [25]. After 0 ºC and 15 orm the sinte e end in ord as it is; ofte ormed. Whe btain very sm

lization and

d small-scale t

ure a (Laser Sc g the granules

pre-sintering burn out prod

[5]. The p er Aided M g full sinter d to total jo andard, the ental crown hods, and th r soft machi 550 ºC dep

ering in two der to minim en strict tole en the piece mooth surfa packaging t

esting of zirco

canning Confo are visualized

g, which is ducts must b pre-sintered Manufacturin ring, to the

int arthropla part is des and bridges he CAM cen ining the pa

ending on o or more ste

mize the re erances are is designed aces and red the part is re

onia

ocal Microscop d with TEM.

commonly be allowed t d blank can ng (CAD/CA e desired sh

asty prosthe signed with s a replica o

ntre machin arts are sinte the time an eps, includin esidual poro

expressed a d for joint re

duce the pre eady for bein

py) the spray-d

performed to escape by n be soft m AM), taking hape. In the

esis, dental a compute of the part is nes the shrin ered at temp nd atmosph ng a Hot Iso sity. Once f and hard ma

eplacement, esence of re ng implante

dried granules

around 110 y choosing a machined vi g into accou CAD/CAM posts and a er. On the scanned, ei nkage-compe

eratures tha here used. S ostatic Press fired, the pi achining and , careful pol esidual stres

d.

are showed, in

00 ºC, the b a correct hea

ia Compute unt the shrin M system, w abutments, i

other hand, ither with co ensated part at can range Some manu sing (HIP) t iece in only d polishing h

lishing is pe sses. After c

9 / 184

n picture b

binder is ating rate er Aided nkage of which is i.e. when , for the ontact or t into the between ufacturers treatment y seldom has to be erformed cleaning,

(28)

10 / 184

Figure 1.

a and b: la blank. d:

abutment

1.5.

W properti rapid lo ceramic stable if more ma

Table 1.1

Z osseoco physical (ZrN) a

.6. Top-left: sc aser and conta pre-sintered b (right) and bri

A mater

Why to use ies are very ook at Tabl cs used as b

f compared atter of conc

1. Mechanical p

Zirconia ha onductive. T

l and chemi are able to

cheme represen act scanning, re

blank (VITA idge with parti

rial of ch

e zirconia in good amon le 1.1 can biomaterials with metal cern.

properties of d

as been pr The interactio

ical treatmen reduce plaq

nting current d espectively, of

corp.) showin ial veneering (

hoice for

n biomedica ng ceramics

give an ide . Besides, b ls, where io

different ceram

oved to be on with soft nts. Some s que on imp

Su

dental CAD/C f the replica to ng the shape o (left). Reprodu

biomedi

al implants?

s, both in te ea of the m

being an ox on release d

mics for biome

e biocompa ft tissues (m

studies have plant and su

urface stability

CAM systems f oth. c: soft ma of a milled cr uced from Miy

cal impla

? It was alr erms of stren main mecha

xide ceramic due to corro

dical applicati

atible in vi mainly fibrob e shown that urrounding

y and small-sca

for Crown and achining (CAM rown. e: final yazaki et al.[25

ants

ready stated ngth and w anical prope c, its chemi osion is bec

ons. From ref.

itro and in blastic) can b t zirconia an tissues, ind

ale testing of z

d bridge restor M) of the pre-s

shape of a s 5].

d that mecha wear resistan

erties for va ical state is coming mor

f. [26]

n vivo and be modified nd its deriv dicating that

irconia

rations.

intered intered

anical nce. A arious s very

e and

it is d with

atives t they

(29)

Surfac

shoul Reso to be in ae 1.6.

phase for d invol out micro the fr Figur prom LTD

Figur cubic Aging

ce stability and

ld have an orption of pe e colored to

sthetical app . Low-t

Low tem e is converte details on L

lves various and may a ostructures.

fracture toug re 1.7). Th motes transfo D.

re 1.7.Fracture and monoclin g sensitivity (so

d small-scale t

important eri-implant b

match natur plications [2 temperat

mperature d ed into mon LTD mechan

s steps and le also induce

Unfortunat ghness are at his happens ormation tou

e toughness an nic zirconia is

olid line) is ind

esting of zirco

role in sof bone is prob ral teeth tint 27].

ture degr

degradation noclinic by t

nism and k eads to surfa e loss of s

ely, the Y-T t the same ti s because t ughening, is

nd aging sensit shown togethe dicated by the

onia

ft tissue he bably avoide t represents

radation

(LTD) is t the diffusion kinetics). Th face roughen strength aft TZP compos

ime those th the transfor s in general

tivity of Y-TZ er with the tou monoclinic fr

ealing and i ed as well. F

a beneficial

(LTD)

the phenom n of water-d his degradat ning, micro er very lon sitions that hat are the m

rmability of l also respo

ZP as a function ughness of a zi raction after 3 h

implant suc Finally, the l property co

menon by w derived spec

tion is a co and macro-c ng exposur

show the be most suscept

f the tetrag nsible for th

n of yttria con irconia therma hours at 134ºC

ccess at bon capacity of ompared to

which the te cies (see App omplex proc cracking, gr re times or est performa tible to LTD gonal phase the driving f

ntent. Toughne al barrier coati C in water vapo

11 / 184

ne level.

f zirconia titanium

etragonal pendix 2 cess that rain pull- r altered ances for D [7] (see e, which

force for

ess data for ing (TBC).

or [7].

(30)

A complete and accurate description of the phenomenon of LTD is still missing, however many reports have qualitatively argued on possible explanations. The most accepted one is that transformation is likely to be due to the increase of internal stresses and reduction of vacancies associated with the penetration of water species, which have been often recognized as hydroxyl ions [28] or water molecules [29]. More recently, a variation of this mechanism has been proposed, where Oxygen ion migrates filling a vacancy site and Hydrogen ion is located in an adjacent interstitial site [30]. Finally, after secondary ion mass spectroscopy studies of samples exposed to isotopically enriched water, it was found that moisture diffusion into zirconia occurs by parallel but independent diffusion of H and O ions, while the transport of either water molecules or hydroxyl ions was excluded [31].

It has been shown that the strong variability of zirconia sensitivity to in vivo degradation is a consequence of the strong influence of microstructure and processing conditions on LTD [32]. Many variables play a role in LTD and can modify the resistance of the material to this phenomenon. The most important are:

 Density and porosity distribution: the presence of interconnected porosity around the original granules may allow percolative transport of the external moisture inside the body and accelerate LTD [7]. Moreover, the presence of regions with non uniform density may lead to stress concentration, damage and slow crack propagation [33].

 Phase separation: the presence of the cubic phase is likely to allow the formation of locally Yttrium depleted tetragonal (t) grains close to the cubic ones; these t grains are less stable and act as nucleation sites for the t–m transformation [34].

 Residual stresses: the local stress state seems to play a dominant factor governing grain stability, so everything that leads to a state of tensile stress into the material, both at the micro- and macroscopic level, may affect negatively LTD. Stresses can be induced, for example, after heavy machining or careless polishing [35], while grain coarsening and thermal anisotropy contribute in generating thermal stresses [36].

 Grain size: there seems to be a critical value of tetragonal grain size Dc below which the water-enhanced transformation does not occur, and it becomes more active with increasing grain size [36] (see Figure 1.8). However, this grain size is strongly related to composition and therefore is different for each zirconia ceramic [37].

Surfa all‐sca

an

d sma

Surfa all‐sca

ace sta ale tes

ability sting o

y

of zirco

onia

Outline

Preface

List of publications

Contents

List of Figures and Tables

Glossary of terms

Chapter 1 Introduction