Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 29(2009)1625-1630 www.elsevier.com/locate/jeurceramsoc Creep mechanisms of laminated alumina/zirconia-toughened alumina composites A Morales-Rodriguez A Dominguez-Rodriguez G. de Portu, M. Jimenez-Melendo Departamento de Fisica de la Materia Condensada, Universidad de Sevilla, Aptdo. 1065, 41080 Sevilla, spain Institute of Science and Technology for Ceramics(ISTEC-CNR), Via granarolo 64, Faenza, Italy Received 12 August 2008; received in revised form 19 September 2008: accepted 24 September 2008 Available online 5 November 2008 High-temperature plastic deformation of laminar composites containing alternate layers of Al2O3 and a mixture of 60 vol. Al2O3+40 vol % 3 mol% Y2O3-stabilized tetragonal ZrO,(ZTA) produced by tape casting is investigated in isostrain compression testing at temperatures between 1400 and 1500C. The stress exponent n and the creep activation energy g are close to l and 700 kJ/mol, respectively. Microstructual observations reveal the lack of differential features in the ZTA layers and a general creep damage of the Al2O3 layers, with little microcracking by cavity coalescence even up to strains of 30%. The layer interfaces maintain their initial structural integrity after testing. An isostrain composite creep model predicts correctly the overall mechanical behavior of the laminates, which is dictated by the alumina phase via diffusional creep controlled by ygen grain boundary diffusion. C 2008 Elsevier Ltd. all rights reserved. Keywords: Creep: Al2O3; ZrO2: Composites; Laminates 1. Introduction The creep behavior of alumina/zirconia-toughened alumina ZTA)laminated composites with strong interfaces has bee In the last years, new strategies based on the development of previously evaluated in the isostress condition(stress axis pe ceramic/ceramic lamellar structures have emerged in order to pendicular to layer planes). It was found 0 that the overall improve the mechanical performance of ceramics for structural creep behavior of the laminates was controlled by the softer applications. -These structures provide a unique opportunity phase(ZTA), but with, superior creep resistance than its mono- for tailoring the mechanical properties and meeting apparently lithic counterpart because of the constraints imposed by the contradictory characteristics of structural ceramics. Al2O3-and harder phase through interface bonding In the present study, ZrO2-based ceramics have been preferentially used as starting Al2 O3/ZTA laminated composites were fabricated by warm materials to build up layered microarchitectures because of their pressing and sintering of layers produced by tape casting. The excellent mechanical rge improve- objective of this work is to characterize the high-temperature ments in strength and fracture toughness at room temperature mechanical behavior of these materials, when loaded axially in have been achieved in alumina/zirconia laminar composites compression(stress axis parallel to layer planes)and to generate because of various crack-shielding phenomena related to the the predictive rate equation for correlating microstructural and presence of the layers. It has also been shown that the laminar mechanical data. microarchitecture beneficially influences the high-temperature creep properties of layered ceramics, conjugating the ductility 2. Experimental procedure and creep resistance of the monolithic constituents. Sheets of pure-alumina and 60 vol. Al2O3+40 vol. mol% Y203-stabilized tetragonal Zro2(ztA) were prepare by tape casting; details of the fabrication process can be found Corresponding author. elsewhere Discs with a diameter of 40 mm were cut from E-mail address: melendo@ uses(M. Jimenez-Melendo) the green ceramic sheets and then stacked and warm-pressed 0955-2219 front matter@ 2008 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2008.09.01
Available online at www.sciencedirect.com Journal of the European Ceramic Society 29 (2009) 1625–1630 Creep mechanisms of laminated alumina/zirconia-toughened alumina composites A. Morales-Rodríguez a, A. Domínguez-Rodríguez a, G. de Portu b, M. Jiménez-Melendo a,∗ a Departamento de Física de la Materia Condensada, Universidad de Sevilla, Aptdo. 1065, 41080 Sevilla, Spain b Institute of Science and Technology for Ceramics (ISTEC-CNR), Via Granarolo 64, Faenza, Italy Received 12 August 2008; received in revised form 19 September 2008; accepted 24 September 2008 Available online 5 November 2008 Abstract High-temperature plastic deformation of laminar composites containing alternate layers of Al2O3 and a mixture of 60 vol.% Al2O3 + 40 vol.% 3 mol% Y2O3-stabilized tetragonal ZrO2 (ZTA) produced by tape casting is investigated in isostrain compression testing at temperatures between 1400 and 1500 ◦C. The stress exponent n and the creep activation energy Q are close to 1 and 700 kJ/mol, respectively. Microstructual observations reveal the lack of differential features in the ZTA layers and a general creep damage of the Al2O3 layers, with little microcracking by cavity coalescence even up to strains of 30%. The layer interfaces maintain their initial structural integrity after testing. An isostrain composite creep model predicts correctly the overall mechanical behavior of the laminates, which is dictated by the alumina phase via diffusional creep controlled by oxygen grain boundary diffusion. © 2008 Elsevier Ltd. All rights reserved. Keywords: Creep; Al2O3; ZrO2; Composites; Laminates 1. Introduction In the last years, new strategies based on the development of ceramic/ceramic lamellar structures have emerged in order to improve the mechanical performance of ceramics for structural applications.1–3 These structures provide a unique opportunity for tailoring the mechanical properties and meeting apparently contradictory characteristics of structural ceramics. Al2O3- and ZrO2-based ceramics have been preferentially used as starting materials to build up layered microarchitectures because of their excellent mechanical properties. For example, large improvements in strength and fracture toughness at room temperature have been achieved in alumina/zirconia laminar composites because of various crack-shielding phenomena related to the presence of the layers.4–7 It has also been shown that the laminar microarchitecture beneficially influences the high-temperature creep properties of layered ceramics, conjugating the ductility and creep resistance of the monolithic constituents.8,9 ∗ Corresponding author. E-mail address: melendo@us.es (M. Jiménez-Melendo). The creep behavior of alumina/zirconia-toughened alumina (ZTA) laminated composites with strong interfaces has been previously evaluated in the isostress condition (stress axis perpendicular to layer planes). It was found10 that the overall creep behavior of the laminates was controlled by the softer phase (ZTA), but with, superior creep resistance than its monolithic counterpart because of the constraints imposed by the harder phase through interface bonding. In the present study, Al2O3/ZTA laminated composites were fabricated by warm pressing and sintering of layers produced by tape casting. The objective of this work is to characterize the high-temperature mechanical behavior of these materials, when loaded axially in compression (stress axis parallel to layer planes) and to generate the predictive rate equation for correlating microstructural and mechanical data. 2. Experimental procedure Sheets of pure-alumina and 60 vol.% Al2O3 + 40 vol.% 3 mol% Y2O3-stabilized tetragonal ZrO2 (ZTA) were prepared by tape casting; details of the fabrication process can be found elsewhere.9 Discs with a diameter of 40 mm were cut from the green ceramic sheets and then stacked and warm-pressed 0955-2219/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2008.09.017
A Morales-Rodriguez et al /Journal of the European Ceramic Sociery 29(2009)1625-1630 1 mm Fig. 1. SEM micrographs of Al] O3/ZTA multilayer composites (a) Low magnification(dark layers are Al2O3, bright layers are ZTA).(b) Detail of an interface between the Al2O3 layer and the ZTA layer; the latter one is formed by zirconia grains(bright phase)and alumina grains(dark phase) at 80C for 30 min at 30 MPa. The structure was designed to of ZTA (bright phase)215-Am thick; the corresponding volume have alumina layers as outer surfaces. The laminated samples fractions are fAl=0.40 and zTA =0.60. The two types of layers were finally sintered at 1550C for I h in air. After sintering, are well defined with very straight interfaces(Fig. 1(a)and(b)) the density of the samples, measured by Archimedes'method, Alumina grains in the Al2O3 layers have an average grain size d was close to the theoretical density. (taken as the spatial grain size)of 1.2 um. The ZTA layer shows For mechanical testing, rectangular specimens of about the typical equiaxed duplex microstructure of such composites 4 mm x 2 mm x 2 mm were cut from the laminated compacts (Fig. 1(b)), formed by alumina grains(dark phase)of d=0.6 um with the largest dimension(the loading axis) parallel to the layer and slightly smaller zirconia grains of d=0. 4 um. The smaller interfaces. In such configuration, isostrain and thus isostrain-rate size of the Al2O3 grains in the ZTA layer, in comparison to those conditions will apply through the deformation process. Com- in the Al2O3 layer, is due to the presence of the second dis- pressive tests were carried out at constant cross-head speeds persed phase, which produces a fine-grained microstructure that between 5 and 50 um/min(corresponding to initial strain rates is remarkably resistant to coarsening at elevated temperatures strain rates Eo between 2 x 10-and 2 x 10-4s-)and at con- This enhanced microstructural stability of ZTA com stant load(5-120MPa)in air at temperatures between 1400 and been widely exploited to achieve superplasticity 12-1]osites has 1500oC. The recorded data at constant strain rate. load F ver Fig 2 displays ao-Ecurve at 1500C showing several strain sus time t, and at constant load, instantaneous specimen length rates changes which allow the determination of the stress expo l(t) versus time t, were plotted, respectively, as a-E and log nent by using Eq(1). Reasonable steady-state stresses were E-Ecurves, where o= F/So(with So the initial cross-sectional achieved at every rate change, except at the higher strain rate area of the sample)is the nominal stress, E=-In((t)/o(with imposed where the specimen started to fail. At lower temper- lo the initial length of the sample) is the true strain and e the atures, the composites failed at correspondingly lower strain strain rate. Mechanical data were analyzed using the standard high-temperature power law for steady-state deformation l20 E= Ag"d Q where A is a parameter depending on the deformation mecha nism, n the stress exponent, p the grain size exponent, Q the activation energy for flow and RT has the usual meaning The microstructural characterization of the as-received and deformed laminates was carried out using scanning electron 3F0/46×10510x1042.5x104 microscopy(Microscopy Service, University of Sevilla, Spain) To this end, perpendicular sections to the layer interfaces were 40A290 cut from the samples, mechanically polished and then thermally etched at 1350C for 30 min in air to reveal grain boundaries The morphological parameters of the various phases were char- T=1500° acterized by using a semiautomatic image analyzer. 3. Experimental result I(a) shows a low magnification SEM micrograph of (points). Several determinations of the stress exponent n by the as-received laminated composite cross-section, consisting of strain rat re shown. The curve of high-purity monolithic alumina at seven layers of alumina(dark phase)125-um thick and six layers the initia
1626 A. Morales-Rodríguez et al. / Journal of the European Ceramic Society 29 (2009) 1625–1630 Fig. 1. SEM micrographs of Al2O3/ZTA multilayer composites. (a) Low magnification (dark layers are Al2O3, bright layers are ZTA). (b) Detail of an interface between the Al2O3 layer and the ZTA layer; the latter one is formed by zirconia grains (bright phase) and alumina grains (dark phase). at 80 ◦C for 30 min at 30 MPa. The structure was designed to have alumina layers as outer surfaces. The laminated samples were finally sintered at 1550 ◦C for 1 h in air. After sintering, the density of the samples, measured by Archimedes’ method, was close to the theoretical density. For mechanical testing, rectangular specimens of about 4 mm × 2 mm × 2 mm were cut from the laminated compacts with the largest dimension (the loading axis) parallel to the layer interfaces. In such configuration, isostrain and thus isostrain-rate conditions will apply through the deformation process. Compressive tests were carried out at constant cross-head speeds between 5 and 50m/min (corresponding to initial strain rates strain rates ε˙o between 2 × 10−5 and 2 × 10−4 s−1) and at constant load (5–120 MPa) in air at temperatures between 1400 and 1500 ◦C. The recorded data at constant strain rate, load F versus time t, and at constant load, instantaneous specimen length l(t) versus time t, were plotted, respectively, as σ − ε and log ε˙ − ε curves, where σ = F/S0 (with S0 the initial cross-sectional area of the sample) is the nominal stress, ε = −ln [l(t)/l0] (with l0 the initial length of the sample) is the true strain and ε˙ the strain rate. Mechanical data were analyzed using the standard high-temperature power law for steady-state deformation: ε˙ = Aσnd−p exp − Q RT (1) where A is a parameter depending on the deformation mechanism, n the stress exponent, p the grain size exponent, Q the activation energy for flow and RT has the usual meaning. The microstructural characterization of the as-received and deformed laminates was carried out using scanning electron microscopy (Microscopy Service, University of Sevilla, Spain). To this end, perpendicular sections to the layer interfaces were cut from the samples, mechanically polished and then thermally etched at 1350 ◦C for 30 min in air to reveal grain boundaries. The morphological parameters of the various phases were characterized by using a semiautomatic image analyzer. 3. Experimental results Fig. 1(a) shows a low magnification SEM micrograph of the as-received laminated composite cross-section, consisting of seven layers of alumina (dark phase) 125-m thick and six layers of ZTA (bright phase) 215-m thick; the corresponding volume fractions are fAl = 0.40 and fZTA = 0.60. The two types of layers are well defined with very straight interfaces (Fig. 1(a) and (b)). Alumina grains in the Al2O3 layers have an average grain size d (taken as the spatial grain size) of 1.2 m. The ZTA layer shows the typical equiaxed duplex microstructure of such composites (Fig. 1(b)), formed by alumina grains (dark phase) of d = 0.6m and slightly smaller zirconia grains of d = 0.4m. The smaller size of the Al2O3 grains in the ZTA layer, in comparison to those in the Al2O3 layer, is due to the presence of the second dispersed phase, which produces a fine-grained microstructure that is remarkably resistant to coarsening at elevated temperatures.11 This enhanced microstructural stability of ZTA composites has been widely exploited to achieve superplasticity.12–14 Fig. 2 displays a σ − ε curve at 1500 ◦C showing several strain rates changes which allow the determination of the stress exponent by using Eq. (1). Reasonable steady-state stresses were achieved at every rate change, except at the higher strain rate imposed where the specimen started to fail. At lower temperatures, the composites failed at correspondingly lower strain Fig. 2. Stress–strain curve at 1500 ◦C of laminated composite deformed under isostrain condition (points). Several determinations of the stress exponent n by strain rate changes are shown. The curve of high-purity monolithic alumina at the initial strain rate of ε˙o = 2 × 10−5 s−1 is also shown.
A Morales-Rodriguez et al / Journal of the European Ceramic Society 29(2009 )1625-1630 1.2 (MPa)= Fig3. Creep curve at constant load plotted as log i-8 for laminated com- Fig. 5. SEM micrograph of a multilayer composite deformed at 1500C up to posite deformed under isostrain condition. Several determinations of the stress a final strain of 30% Controlled cavitation occurred in the Al2O3 layers.The stress axis is vertical exponent n by stress changes are shown. structural integrity, indicating an excellent interlayer adhesion rates. The measured stress exponent is close to unity, indicating(Fig. 5). No measurable change in grain size or shape during test a Newtonian creep process. Fig. 2 also shows the o-E curve ing was observed. The ZTA layers appear almost unchanged-a at Eo=2 x 10-s-of high-purity monolithic Al2 O3 with an consequence of its superplastic behavior, as noted above, indi- average grain size d=1.8 um, similar to that of the alumina lay- cating that this phase deforms preferentially by grain boundary ers. The monolith failed with very little plastic deformation after sliding. On the other hand, the Al2O3 layers show extensive attaining a maximum stress omax of 55 MPa. creep damage consisting of cavities distributed homogeneously Fig 3 displays a constant load creep test conducted at 1500C throughout the layer, initiated at two-grain boundaries parallel to showing several load changes for determining the stress expo- the stress direction(Fig. 5). Except for the most severe testing nent n(Eq(I); the corresponding stresses o were calculated conditions(higher stresses or strain rates, and lower tempera using the original cross-sectional area of the sample. Again, tures), no microcrack development by cavity coalescence was observed, thus resulting in a damage-tolerant regime. This dam (characterized by negative slopes in compression constant load age mode has been reported previously in pure-alumina at low stresses, where the final failure occurs by the coalescence of a controlled degradation of the specimen can be observed. creep damage at large strains 15-17 The measured stress exponent is n=1.1tO. 1, equal to that determined from constant strain rate tests(Fig. 2). The acti- 4. Discussion vation energy for flow Q, measured from temperature changes △T=±50° C during testing(Fig.4) by using Eq.(1),was The creep behavior of the laminates deformed in isostrain Q=710+40kJ/mol, regardless of stress level and temperature. conditions is characterized by a stress exponent n and an acti- After testing, the laminates showed barrelling due to friction vation energy g of about 1 and 700kJ/mol, respectively. In effects, although the layer interfaces still retained their initial order to elucidate the creep mechanism operating in the lam inates, the mechanical behavior of the composites must be compared to that of the constituent phases, alumina and ZTA, which exhibit very different creep behavior. Monolithic ZTA shows very large tensile elongations(>500%)and, correspond ingly, low flow stresses. At 1500C, for example, the flow stress of ZTA composites o, with composition and grain size similar to those in the present laminates is below 10 MPa at 8=2 x 10->5-1. ZTa deforms primarily by grain boundary sliding, as in superplastic metals and metallic alloys, charac- terized by a stress exponent n=2 and an activation energy 1450114001450140011450 0=600-750kJ/mol. The lack of differential microstructural features of the ZTA layers in the strained laminates with respect 0=50 MPa to the as-received ones(Fig. 5)agrees with this behavior. On the other hand. monolithic alumina is much more resistant 60 than ZTA, with a marked brittle behavior, 10, I5(Fig. 2). For grain sizes larger than I um, it shows a stress exponent n=l and an Fig. 4. Creep curve plotted as log i-e for a multilayer showing several deter- activation energy @=500-800kJ/mol, with very limited grain tions of the activation energy g by temperature changes. boundary sliding.20-22The large scatter in the reported values of
A. Morales-Rodríguez et al. / Journal of the European Ceramic Society 29 (2009) 1625–1630 1627 Fig. 3. Creep curve at constant load plotted as log ε˙ − ε for laminated composite deformed under isostrain condition. Several determinations of the stress exponent n by stress changes are shown. rates. The measured stress exponent is close to unity, indicating a Newtonian creep process. Fig. 2 also shows the σ − ε curve at ε˙o = 2 × 10−5 s−1 of high-purity monolithic Al2O3 with an average grain size d = 1.8m, similar to that of the alumina layers. The monolith failed with very little plastic deformation after attaining a maximum stress σmax of 55 MPa. Fig. 3 displays a constant load creep test conducted at 1500 ◦C showing several load changes for determining the stress exponent n (Eq. (1)); the corresponding stresses σ were calculated using the original cross-sectional area of the sample. Again, steady-states regimes were attained after every load change (characterized by negative slopes in compression constant load tests), except in the last section of the creep curve where a controlled degradation of the specimen can be observed. The measured stress exponent is n = 1.1 ± 0.1, equal to that determined from constant strain rate tests (Fig. 2). The activation energy for flow Q, measured from temperature changes T = ±50 ◦C during testing (Fig. 4) by using Eq. (1), was Q = 710 ± 40 kJ/mol, regardless of stress level and temperature. After testing, the laminates showed barrelling due to friction effects, although the layer interfaces still retained their initial Fig. 4. Creep curve plotted as log ε˙ − ε for a multilayer showing several determinations of the activation energy Q by temperature changes. Fig. 5. SEM micrograph of a multilayer composite deformed at 1500 ◦C up to a final strain of 30%. Controlled cavitation occurred in the Al2O3 layers. The stress axis is vertical. structural integrity, indicating an excellent interlayer adhesion (Fig. 5). No measurable change in grain size or shape during testing was observed. The ZTA layers appear almost unchanged – a consequence of its superplastic behavior, as noted above, indicating that this phase deforms preferentially by grain boundary sliding. On the other hand, the Al2O3 layers show extensive creep damage consisting of cavities distributed homogeneously throughout the layer, initiated at two-grain boundaries parallel to the stress direction (Fig. 5). Except for the most severe testing conditions (higher stresses or strain rates, and lower temperatures), no microcrack development by cavity coalescence was observed, thus resulting in a damage-tolerant regime. This damage mode has been reported previously in pure-alumina at low stresses, where the final failure occurs by the coalescence of creep damage at large strains.15–17 4. Discussion The creep behavior of the laminates deformed in isostrain conditions is characterized by a stress exponent n and an activation energy Q of about 1 and 700 kJ/mol, respectively. In order to elucidate the creep mechanism operating in the laminates, the mechanical behavior of the composites must be compared to that of the constituent phases, alumina and ZTA, which exhibit very different creep behavior. Monolithic ZTA shows very large tensile elongations (>500%) and, correspondingly, low flow stresses. At 1500 ◦C, for example, the flow stress of ZTA composites18,19 with composition and grain size similar to those in the present laminates is below 10 MPa at ε˙ = 2 × 10−5 s−1. ZTA deforms primarily by grain boundary sliding, as in superplastic metals and metallic alloys, characterized by a stress exponent n = 2 and an activation energy Q = 600–750 kJ/mol.14 The lack of differential microstructural features of the ZTA layers in the strained laminates with respect to the as-received ones (Fig. 5) agrees with this behavior. On the other hand, monolithic alumina is much more resistant than ZTA, with a marked brittle behavior8,10,15 (Fig. 2). For grain sizes larger than 1m, it shows a stress exponent n = 1 and an activation energy Q = 500–800 kJ/mol, with very limited grain boundary sliding.20–22 The large scatter in the reported values of
A Morales-Rodriguez et al /Journal of the European Ceramic Sociery 29(2009)1625-1630 Q is probably due to the highly extrinsic character of this oxide; In this study, the laminates were deformed in isostrain co even a very few at ppm of impurities/dopants change drastically ditions. French et al. 32have developed a creep model for duplex the mechanical behavior. 22-25 The creep of large-grained alu- microstructures by assuming an isostress(alternate plates of mina has been usually associated with a Coble diffusion creep each phase aligned perpendicular to the applied stress)or isos- mechanism,20-22,26 where the transport of matter along grain train(plates are parallel to the applied stress)model; the latter boundaries is both the deformation and rate-controlling mech- one coincides with the configuration of the present laminates. In anism. The steady-state strain rate in the Coble model is given this case, the overall(applied) composite stress oc is given by by the following equation he relation- 150σg2 (2) Oc=fAIOAl+ fzTAOZTA where o al and azTA are the stresses supported by the Al2O3 where 52 is the atomic volume, k is the Boltzmanns constant, and ZTA layers, respectively, and fAl and fzta the correspond 8 is the grain boundary width and Dgd is the grain boundary ing volume fractions. This model has been successfully used to diffusion coefficient, determined by the slower ionic specie explain the high-temperature mechanical behavior of laminated he diffusion behavior in alumina is a far from simple matter metal33 and ceramic,34 composites. Owing to the higher creep today(see, for example, the recent reviews by Heuerz7and Hard- strength of the alumina phase in comparison with ZTA, Eq (3) ing et al. ).Reported aluminum and oxygen diffusivities exhibit can be simplified to a considerable scatter, both in activation energy and in absolute magnitude of the coefficient, due probably to small variations ac= fAIOAl in impurity content which significantly influence the diffusion kinetics. It must be noted, however, that oxygen diffuses slower That is, the strain rate of the laminate will be controlled by the than aluminum both in the bulk and along(sub)grain boundaries alumina phase. This is consistent with the value n=1 found in when measured on the same suite of crystals.29,30 The creep rate the laminates, also reported in monolithic alumina. 20-22In the of monolithic alumina(Eq (2) should be thus associated with isostress configuration, where the softer(ZTA) phase would be oxygen diffusion along grain boundaries. Fig. displays the oxy- the rate-controlling phase, a stress exponent n=2 was found. 10 gen diffusivities in bulk(D')and along grain boundaries(Deb) Regarding the creep activation energy, both alumina and ZTA measured by Prot et al. 2in undoped and yttria-doped alumina monoliths have Q values in the same range, which precludes a (although contaminated with >1000 at ppm Si), along with the quantitative comparison corresponding activation energies. Recent data by Nakagawa et Assuming that the alumina phase controls the overall al. on high-purity and yttria-doped alumina bi-crystals are also mechanical behavior of the laminates, the diffusion coefficient plotted in Fig. 6. Despite the scatter, the experimental data sug- Dgb in Eq(2)can be deduced from the laminate creep data gests that the activation energy for grain boundary diffusion is with o =OAI=0c/0.40=2.500c(Eq(4);8 is assumed to be larger than for bulk diffusion I nm, as usual, and S2=2.20 x 10-m. These apparent dif fusion coefficients are reported in Fig. 6. The good agreement between both sets of data(diffusion and creep) confirms the T(C) 17001600 1500 1400 validity of the previous hypothesis, i. e, the composite creep rate is primarily controlled by the Al2O3 layers 921 kJ/mol [29 Despite the apparent good agreement with pristine alumina (Fig. 6), several features suggest that the deformation of the 627 kJ/mol 31 laminates may be related to grain boundary diffusion in doped alumina rather than in pure-alumina 1016E800029 729 KJ/mol [313 (i) EDX(energy dispersive X-ray )/SEM linescans performed layers(Fig. 7) show the presence of yttrium and zirconium in these layers in an approximate 590 kJ/mol [291 ratio(alumina layer ZTA layer)1: 4 and 1: 9, respectively; he aluminum ratio is 2: 1, in agreement with the amount 636kmol[29 62 kJ/mol [31 of alumina in the two types of layers. Such a cation diffu sion from ZTA layers towards Al2O3 layers is due to the 505.2545.6586.0 no data on zirconium diffusion in alumina, but lesage et 104T(K) alhave reported that yttrium diffuses rapidly in poly crystalline alumina, reaching depths larger than 100 um(the undoped(solid lines)and, s in bulk (D')and along grain boundaries(Ds)in same order as the layer thickness in the laminates)after dif- Fig. 6. Oxygen difft yttrium-doped(dashed lines)alumina deduced fror fusion experiments, along with the corresponding activation energies.29,31 fusion of 17 h at 1408C. This unintentional doping of the Apparent diffusion coefficients deduced from creep data(Eq (2)are also plotted alumina layers explains the damage mode exhibited by the na layers(Figs. 5 and 7). In high-purity alumina
1628 A. Morales-Rodríguez et al. / Journal of the European Ceramic Society 29 (2009) 1625–1630 Q is probably due to the highly extrinsic character of this oxide; even a very few at.ppm of impurities/dopants change drastically the mechanical behavior.22–25 The creep of large-grained alumina has been usually associated with a Coble diffusion creep mechanism,20–22,26 where the transport of matter along grain boundaries is both the deformation and rate-controlling mechanism. The steady-state strain rate in the Coble model is given by the following equation: ε˙ = 150 π σΩ kTd3 δDgb (2) where Ω is the atomic volume, k is the Boltzmann’s constant, δ is the grain boundary width and Dgb is the grain boundary diffusion coefficient, determined by the slower ionic specie. The diffusion behavior in alumina is a far from simple matter today (see, for example, the recent reviews by Heuer27 and Harding et al.28). Reported aluminum and oxygen diffusivities exhibit a considerable scatter, both in activation energy and in absolute magnitude of the coefficient, due probably to small variations in impurity content which significantly influence the diffusion kinetics. It must be noted, however, that oxygen diffuses slower than aluminum both in the bulk and along (sub)grain boundaries when measured on the same suite of crystals.29,30 The creep rate of monolithic alumina (Eq. (2)) should be thus associated with oxygen diffusion along grain boundaries. Fig. 6 displays the oxygen diffusivities in bulk (Dl ) and along grain boundaries (Dgb) measured by Prot et al.29 in undoped and yttria-doped alumina (although contaminated with >1000 at.ppm Si), along with the corresponding activation energies. Recent data by Nakagawa et al.31 on high-purity and yttria-doped alumina bi-crystals are also plotted in Fig. 6. Despite the scatter, the experimental data suggests that the activation energy for grain boundary diffusion is larger than for bulk diffusion. Fig. 6. Oxygen diffusivities in bulk (Dl ) and along grain boundaries (Dgb) in undoped (solid lines) and yttrium-doped (dashed lines) alumina deduced from diffusion experiments, along with the corresponding activation energies.29,31 Apparent diffusion coefficients deduced from creep data (Eq.(2)) are also plotted (squares). In this study, the laminates were deformed in isostrain conditions. French et al.32 have developed a creep model for duplex microstructures by assuming an isostress (alternate plates of each phase aligned perpendicular to the applied stress) or isostrain (plates are parallel to the applied stress) model; the latter one coincides with the configuration of the present laminates. In this case, the overall (applied) composite stress σc is given by the relation32: σc = fAlσAl + fZTAσZTA (3) where σAl and σZTA are the stresses supported by the Al2O3 and ZTA layers, respectively, and fAl and fZTA the corresponding volume fractions. This model has been successfully used to explain the high-temperature mechanical behavior of laminated metal33 and ceramic9,34 composites. Owing to the higher creep strength of the alumina phase in comparison with ZTA, Eq. (3) can be simplified to: σc = fAlσAl (4) That is, the strain rate of the laminate will be controlled by the alumina phase. This is consistent with the value n = 1 found in the laminates, also reported in monolithic alumina.20–22 In the isostress configuration, where the softer (ZTA) phase would be the rate-controlling phase, a stress exponent n = 2 was found.10 Regarding the creep activation energy, both alumina and ZTA monoliths have Q values in the same range, which precludes a quantitative comparison. Assuming that the alumina phase controls the overall mechanical behavior of the laminates, the diffusion coefficient Dgb in Eq. (2) can be deduced from the laminate creep data with σ ≡ σAl = σc/0.40 = 2.50σc (Eq. (4)); δ is assumed to be 1 nm, as usual, and Ω = 2.20 × 10−29 m3. These apparent diffusion coefficients are reported in Fig. 6. The good agreement between both sets of data (diffusion and creep) confirms the validity of the previous hypothesis, i.e., the composite creep rate is primarily controlled by the Al2O3 layers. Despite the apparent good agreement with pristine alumina (Fig. 6), several features suggest that the deformation of the laminates may be related to grain boundary diffusion in dopedalumina rather than in pure-alumina: (i) EDX (energy dispersive X-ray)/SEM linescans performed across the alumina layers (Fig. 7) show the presence of yttrium and zirconium in these layers in an approximate ratio (alumina layer:ZTA layer) 1:4 and 1:9, respectively; the aluminum ratio is 2:1, in agreement with the amount of alumina in the two types of layers. Such a cation diffusion from ZTA layers towards Al2O3 layers is due to the elevated temperatures during sintering (1550 ◦C). There is no data on zirconium diffusion in alumina, but Lesage et al.35 have reported that yttrium diffuses rapidly in polycrystalline alumina, reaching depths larger than 100 m (the same order as the layer thickness in the laminates) after diffusion of 17 h at 1408 ◦C. This unintentional doping of the alumina layers explains the damage mode exhibited by the alumina layers (Figs. 5 and 7). In high-purity alumina, two
A. Morales-Rodriguez er al. Journal of the European Ceramic Society 29(2009)1625-1630 (ii)The values of the apparent diffusion coefficient Dgbdeduced from Eqs. (2)and (4)( Fig. 6)are, in fact, an upper limit of the actual values because the alumina layers support an addi- from the con by the rigid interface bonding for the strain to be the same in the soft and hard layers. This additional stress, not taken into account in the simple composite creep model(Eq (3)), should be superimposed on the applied stress(Eq (4)), thus giving lower apparent diffusivities than those plotted in Fig. 6. 5. Conclusions Fully dense AlO3/ZTA(60 vol %o Al2O3 +40 vol. %03 mol% Y203-stabilized tetragonal ZrO2) layered composites with uni- form layers and strong interfaces have been produced starting from sheets obtained by tape casting Mechanical tests were performed in compression in air at constant strain rate and constant load between 1400 and 1500oc with the loading axis parallel to the layer interfaces (isostrain configuration). After testing, the layer interfaces maintain their structural integrity. The creep parameters, stress exponent n and activation energy @, are close to l and 700 kJ/mol, respectively Except for the most severe deformation conditions, the laminates exhibit a damage-tolerant regime, characterized by an extensive grain boundary cavitation without coalescence into microcracks, reaching strains of up to 30%o without failure An analysis based on a duplex creep model demonstrates 0255075100125150 that the alumina phase dominates the overall creep process of Position (um the laminates, accumulating more of the applied load. It is shown that diffusional creep controlled by oxygen grain boundary diffu Fig.7.EDX/SEM linescan across the alumina layer in the laminate, showing sion is consistent with mechanical data. An unintentional doping the presence of yttrium and zirconium in the layer. of the nominally high-purity alumina layers with yttrium and zi conium has been detected by EDX/SEM measurements, caused modes have been reported 5-At high stresses, th by the elevated temperatures of the laminate fabrication proces is dictated by the growth of several cracks by coa- lescence of two-grain boundary cavities, with very small Acknowledgments failure strains(typically 20%) In the laminates, the stress No. MAT2000-1117. CNR and CSIC are also acknowledged carried by the alumina layers oAI is not very far from the for the financial support provided to ISTEC-CNR and Depar maximum stress omax supported by high-purity monolithic tamento de Fisica de la Materia Condensada(Universidad de lumina; for example, at 1500C and Eo=2 x 10-5s-1, Sevilla), in the framework of bilateral agreement oAl= 2.5, 0c 2 35 MPa and omax=55 MPa(Fig. 2), and lus a premature failure of the laminate would be expected. References Because of the unintentional doping, however, such a com- parison must be done with cation-doped alumina, not with alLDB. d pristine one. Yoshida et al. have reported a decrease in Ceram Soc. BulL. 1992. 71, 969 creeprate higher than one order of magnitude in 0.045 mol% 2. Moya, J.S., Layered ceramics. Adv: Mater, 1995. 7. 185-189 han. H. liller, G.A., Unique opportunities for Y2O3-doped Al2O3 owing to segregation of yttrium at the microstructural engineering with duplex and laminar ceramic composites. alumina grain boundaries. A similar effect has been found J.Am. Ceram.Soc.,1992,75,1715-1728 with other dopant cations 22-24 Therefore, the stress level 4. Marshall, D. B, Ratto, J.J. and Lange, E. E, Enhanced fract at which the alumina layers are submitted in the lami- in layered microcomposites of Ce-ZrO2 and Al2 03. JAm nate corresponds to a low-stress damage-tolerant regime as experimentally observed 5. Oeschner, M., Hillman, C and Lange, F. F, Crack bifurcation in laminar ceramic composites. J. Am. Ceram Soc., 1996, 79, 1834-1838
A. Morales-Rodríguez et al. / Journal of the European Ceramic Society 29 (2009) 1625–1630 1629 Fig. 7. EDX/SEM linescan across the alumina layer in the laminate, showing the presence of yttrium and zirconium in the layer. failures modes have been reported.15–17 At high stresses, the fracture is dictated by the growth of several cracks by coalescence of two-grain boundary cavities, with very small failure strains (typically 20%). In the laminates, the stress carried by the alumina layers σAl is not very far from the maximum stress σmax supported by high-purity monolithic alumina; for example, at 1500 ◦C and ε˙o = 2 × 10−5 s−1, σAl = 2.5, σc 35 MPa and σmax = 55 MPa (Fig. 2), and thus a premature failure of the laminate would be expected. Because of the unintentional doping, however, such a comparison must be done with cation-doped alumina, not with pristine one. Yoshida et al.25 have reported a decrease in creep rate higher than one order of magnitude in 0.045 mol% Y2O3-doped Al2O3 owing to segregation of yttrium at the alumina grain boundaries. A similar effect has been found with other dopant cations.22–24 Therefore, the stress level at which the alumina layers are submitted in the laminate corresponds to a low-stress damage-tolerant regime, as experimentally observed. (ii) The values of the apparent diffusion coefficient Dgb deduced from Eqs.(2) and (4)(Fig. 6) are, in fact, an upper limit of the actual values because the alumina layers support an additional in-plane stress arising from the constraint imposed by the rigid interface bonding for the strain to be the same in the soft and hard layers.36 This additional stress, not taken into account in the simple composite creep model (Eq. (3)), should be superimposed on the applied stress (Eq. (4)), thus giving lower apparent diffusivities than those plotted in Fig. 6. 5. Conclusions Fully dense Al2O3/ZTA (60 vol.% Al2O3 + 40 vol.% 3 mol% Y2O3-stabilized tetragonal ZrO2) layered composites with uniform layers and strong interfaces have been produced starting from sheets obtained by tape casting. Mechanical tests were performed in compression in air at constant strain rate and constant load between 1400 and 1500 ◦C with the loading axis parallel to the layer interfaces (isostrain configuration). After testing, the layer interfaces maintain their structural integrity. The creep parameters, stress exponent n and activation energy Q, are close to 1 and 700 kJ/mol, respectively. Except for the most severe deformation conditions, the laminates exhibit a damage-tolerant regime, characterized by an extensive grain boundary cavitation without coalescence into microcracks, reaching strains of up to 30% without failure. An analysis based on a duplex creep model demonstrates that the alumina phase dominates the overall creep process of the laminates, accumulating more of the applied load. It is shown that diffusional creep controlled by oxygen grain boundary diffusion is consistent with mechanical data. An unintentional doping of the nominally high-purity alumina layers with yttrium and zirconium has been detected by EDX/SEM measurements, caused by the elevated temperatures of the laminate fabrication process. Acknowledgments The authors would like to thank the Ministerio de Ciencia y Tecnología (Spain) for the financial support through the Project No. MAT2000-1117. CNR and CSIC are also acknowledged for the financial support provided to ISTEC-CNR and Departamento de Física de la Materia Condensada (Universidad de Sevilla), in the framework of bilateral agreement. References 1. Marshall, D. B., Design of high-toughness laminar zirconia composites. Am. Ceram. Soc. Bull., 1992, 71, 969–973. 2. Moya, J. S., Layered ceramics. Adv. Mater., 1995, 7, 185–189. 3. Harmer, M. P., Chan, H. M. and Miller, G. A., Unique opportunities for microstructural engineering with duplex and laminar ceramic composites. J. Am. Ceram. Soc., 1992, 75, 1715–1728. 4. Marshall, D. B., Ratto, J. J. and Lange, F. F., Enhanced fracture toughness in layered microcomposites of Ce–ZrO2 and Al2O3. J. Am. Ceram. Soc., 1991, 74, 2979–2987. 5. Oeschner, M., Hillman, C. and Lange, F. F., Crack bifurcation in laminar ceramic composites. J. Am. Ceram. Soc., 1996, 79, 1834–1838
A Morales-Rodriguez et al /Journal of the European Ceramic Sociery 29(2009)1625-1630 6. De Portu, G, Micele, L, Guicciardi, S, Fujimura, S, Pezzotti, G. and 21. Cannon, R. M, Rhodes, w. H. and Heuer, A. H, Plastic deformation of Sekiguchi, Y. Effect of residual stresses on the fracture behaviour of notched fine-grained alumina(Al2O3). 1. Interface-controlled diffusional creep. J. laminated composites loaded in flexural geometry. Compos. Sci. TechnoL., Am Ceram.Soc.,1980,63,46-53. 2005,65,1501-15 22. Yoshida, H, Yamamoto, T. and Sakuma, T. The influence of lutetium- 7. Lube, T, Pascual, J, Chalvet, F and de Portu, G, Effective fracture tough doping effect on diffusional creep in polycrystalline AlzO3. J. Eur Ceram. ness in Al203-Al2 03/ZrO2 laminates. J. Eur Ceram. Soc.. 2007 Soc2003,23,1795-1801 1449-1453. 23. Li, Y.-Z.. Wang. C, Chan. H. M, Rickman. J M., Harmer. M. P, Chabala. 8. Jimenez-Melendo, M.. Clauss, C, Dominguez. Rodriguez, A, Sanchez J. M, Gavrilov, K. L and Levi-Setti, R, Codoping of alumina to enhance Herencia, A. J and Moya, J. S, Microstructure and high-temperature creep resistance. J. Am. Ceram. Soc., 1999, 82, 1497-1504. mechanical behavior of alumina/alumina-yttria-stabilized tetragonal zirco- 24. Cho, J, Harmer, M. P, Chan, H M, Rickman, J. M. and Thompson, A M es..Am.Cerm.Soc.,1997,80,2126-2130 Effect of yttrium and lanthanum on the tensile creep behaviour of aluminum 9. Jimenez-Melendo, M, Clauss, C, Dominguez-Rodriguez, A, de Portu, G oxide. J. Am. Ceram Soc. 1997. 1013-1017 Roncari, E and Pinasco, P, High temperature plastic deformation of multi- 25. Yoshida, H, Ikuhara, Y and Sakuma, T, High-temperature creep resistance layered YTZP/ZTA composites obtained by tape casting. Acta Mater., 1998 in rare earth-doped fine-grained Al]O3. J Mater Res, 1998, 13, 2597- 46,3995-4004. 2601 10. Jimenez-Melendo, M, Gutierrez-Mora, F and Dominguez-Rodriguez, A, 26. Chokshi, A H, Diffusion creep in oxide ceramics. J. Eur Ceram Soc. Effect of layer interfaces on the high-temperature mechanical properties of 2002,22,2469-2478. alumina/zirconia laminate composites. Acta Mater, 2000, 48, 4715-4720 27. Heuer, A H, Oxygen and aluminium diffusion in a-Al2O3: how much do 11. Lange, FF and Hirlinger, M. M, Grain growth in two-phase ceramics we really understand? J. Eur Ceram Soc., 2008, 28, 1495-1507 AlO3 inclusions in ZrOz. J. Am. Ceram. Soc.. 1987, 7 28. Harding, J H, Atkinson, K.J. Wand Grimes, R W Experiment and theory 12. Chen, I.W. and Xue, L.A., Development of superplastic structural ceramics of diffusion in alumina. J. A. Ceram. Soc.. 2003 86. 554-559 J.Am.Cerm.Soc.,1990,73,2585-2609 29. Prot, D, Le Gall, M., Lesage, B, Huntz, A. M. and Monty, C, Self-diffusion 13. Flacher, O, Blandin, J.J. and Plucknett, K. P, Effects of zirconia additions in a-Al2O. IV. Oxigen grain-boundary self-diffusion in undoped and yttria- I the superplasticity of alumina-zirconia composites. Mater. Sci. Eng doped alumina polycrystals. Phil. Mag. A, 1996, 73, 935-949 1996,A221,102-112. 30. Fielitz,P, Borchardt, G, Ganschow,S,Bertram, R and Markwitz, A, 26Al 14. Owen, D. M, Characteristics of superplastic deformation in dual phase tracer diffusion in titanium doped single crystalline a-Al2O3. Solid State alumina-zirconia composites. Mater. Sci. Forum, 1997, 243-245, 405-410 Tonics,2008,179,373-379 15. Robertson, A. G, wilkinson, D S. and Caceres, C. H, Creep and creep 31. Nakagawa, T, Sakaguchi, L, Shibata, N, Matsunaga, K, Mizoguchi, T, fracture in hot-pressed alumina. J. Am. Ceram. Soc., 1991, 74, 915-921 Yamamoto,T, Haneda, H and Ikuhara, Y, Yttrium doping effect on oxygen 16. Wilkinson, D.S. Caceres, C H. and Robertson, A. G. Damage and fracture grain boundary difusion in a-Al2O3. Acta Mater, 2007, 55, 6627-6633 mechanisms during high-temperature creep in hot-pressed alumina. J Am. 32. French, J D, Zhao, J, Harmer, M. P, Chan, H M. and Miller, G A. Creep Ceran.Soc.,1991,74.922-933. of duplex microstructures. J. Am. Ceram. Soc., 1994, 77, 2857-286 17. Dalgleish, D. J, Slamovich, E. B and Evans, A. G. Duality in the creep 33. Grishaber, R B, Sergueeva, A V, Mishra, R.S. and Mukherjee, A.K., Lam- rupture of a polycrystalline alumina. J. Am. Ceram Soc., 1985, 68, 575-581 inated metal composites-high temperature deformation behavior. Mater 18. Kellet, B. J and Lange, F. F, Hot forging characteristics of transformation- Sci. Eng. A, 2005, 403 toughened Al3/Zr02 composites. J Mater Res, 1988, 3, 545-551 34. Wang, J, Taleff, E. M. and Kovar, D, High-temperature deformation of 19. Wakai, F. and Kato, H, Rheological flow in superplastic fine-grained Al2O3/Y-TZP particulate laminates. Acta Mater, 2004, 52, 4685-4693 ceramic composites In Ultrastructure Processing of Advanced Materials, 35. Lesage, B, Le Gall, M, Loudjani, M. and Huntz, A M, Yttrium diffusion ed J. D. Mackenzie and D. R. Ulrich. wiley and Sons, New York, 1988, pp. in a alumina. Defect. Diffus. Forum, 1993, 95-98, 1061-1064 36. Wang, J, Taleff, E. M. and Kovar, D, Superplastic deformation of 20. Kottada, R. S. and Chokshi, A. H, The high temperature tensile and Al2O3/Y-TZP particulate composites and laminates. Acta Mater, 2004, 52, compressive deformation characteristics of magnesia doped alumina. Acta 5485-5491 Mater,2000,48,3905-3915
1630 A. Morales-Rodríguez et al. / Journal of the European Ceramic Society 29 (2009) 1625–1630 6. De Portu, G., Micele, L., Guicciardi, S., Fujimura, S., Pezzotti, G. and Sekiguchi, Y., Effect of residual stresses on the fracture behaviour of notched laminated composites loaded in flexural geometry. Compos. Sci. Technol., 2005, 65, 1501–1506. 7. Lube, T., Pascual, J., Chalvet, F. and de Portu, G., Effective fracture toughness in Al2O3–Al2O3/ZrO2 laminates. J. Eur. Ceram. Soc., 2007, 27, 1449–1453. 8. Jiménez-Melendo, M., Clauss, C., Domínguez-Rodríguez, A., SánchezHerencia, A. J. and Moya, J. S., Microstructure and high-temperature mechanical behavior of alumina/alumina–yttria-stabilized tetragonal zirconia multilayer composites. J. Am. Ceram. Soc., 1997, 80, 2126–2130. 9. Jiménez-Melendo, M., Clauss, C., Domínguez-Rodríguez, A., de Portu, G., Roncari, E. and Pinasco, P., High temperature plastic deformation of multilayered YTZP/ZTA composites obtained by tape casting. Acta Mater., 1998, 46, 3995–4004. 10. Jiménez-Melendo, M., Gutiérrez-Mora, F. and Domínguez-Rodríguez, A., Effect of layer interfaces on the high-temperature mechanical properties of alumina/zirconia laminate composites. Acta Mater., 2000, 48, 4715–4720. 11. Lange, F. F. and Hirlinger, M. M., Grain growth in two-phase ceramics: Al2O3 inclusions in ZrO2. J. Am. Ceram. Soc., 1987, 70, 827–830. 12. Chen, I. W. and Xue, L. A., Development of superplastic structural ceramics. J. Am. Ceram. Soc., 1990, 73, 2585–2609. 13. Flacher, O., Blandin, J. J. and Plucknett, K. P., Effects of zirconia additions on the superplasticity of alumina–zirconia composites. Mater. Sci. Eng., 1996, A221, 102–112. 14. Owen, D. M., Characteristics of superplastic deformation in dual phase alumina-zirconia composites. Mater. Sci. Forum, 1997, 243–245, 405–410. 15. Robertson, A. G., Wilkinson, D. S. and Cáceres, C. H., Creep and creep fracture in hot-pressed alumina. J. Am. Ceram. Soc., 1991, 74, 915–921. 16. Wilkinson, D. S., Cáceres, C. H. and Robertson, A. G., Damage and fracture mechanisms during high-temperature creep in hot-pressed alumina. J. Am. Ceram. Soc., 1991, 74, 922–933. 17. Dalgleish, D. J., Slamovich, E. B. and Evans, A. G., Duality in the creep rupture of a polycrystalline alumina. J. Am. Ceram. Soc., 1985, 68, 575–581. 18. Kellet, B. J. and Lange, F. F., Hot forging characteristics of transformationtoughened Al2O3/ZrO2 composites. J. Mater. Res., 1988, 3, 545–551. 19. Wakai, F. and Kato, H., Rheological flow in superplastic fine-grained ceramic composites. In Ultrastructure Processing of Advanced Materials, ed. J. D. Mackenzie and D. R. Ulrich. Wiley and Sons, New York, 1988, pp. 671–680. 20. Kottada, R. S. and Chokshi, A. H., The high temperature tensile and compressive deformation characteristics of magnesia doped alumina. Acta Mater., 2000, 48, 3905–3915. 21. Cannon, R. M., Rhodes, W. H. and Heuer, A. H., Plastic deformation of fine-grained alumina (Al2O3). I. Interface-controlled diffusional creep. J. Am. Ceram. Soc., 1980, 63, 46–53. 22. Yoshida, H., Yamamoto, T. and Sakuma, T., The influence of lutetiumdoping effect on diffusional creep in polycrystalline Al2O3. J. Eur. Ceram. Soc., 2003, 23, 1795–1801. 23. Li, Y.-Z., Wang, C., Chan, H. M., Rickman, J. M., Harmer, M. P., Chabala, J. M., Gavrilov, K. L. and Levi-Setti, R., Codoping of alumina to enhance creep resistance. J. Am. Ceram. Soc., 1999, 82, 1497–1504. 24. Cho, J., Harmer, M. P., Chan, H. M., Rickman, J. M. and Thompson, A. M., Effect of yttrium and lanthanum on the tensile creep behaviour of aluminum oxide. J. Am. Ceram. Soc., 1997, 1013–1017. 25. Yoshida, H., Ikuhara, Y. and Sakuma, T., High-temperature creep resistance in rare earth-doped fine-grained Al2O3. J. Mater. Res., 1998, 13, 2597– 2601. 26. Chokshi, A. H., Diffusion creep in oxide ceramics. J. Eur. Ceram. Soc., 2002, 22, 2469–2478. 27. Heuer, A. H., Oxygen and aluminium diffusion in -Al2O3: how much do we really understand? J. Eur. Ceram. Soc., 2008, 28, 1495–1507. 28. Harding, J. H., Atkinson, K. J. W. and Grimes, R. W., Experiment and theory of diffusion in alumina. J. Am. Ceram. Soc., 2003, 86, 554–559. 29. Prot, D., Le Gall, M., Lesage, B., Huntz, A. M. and Monty, C., Self-diffusion in -Al2O. IV. Oxigen grain-boundary self-diffusion in undoped and yttriadoped alumina polycrystals. Phil. Mag. A, 1996, 73, 935–949. 30. Fielitz, P., Borchardt, G., Ganschow, S., Bertram, R. and Markwitz, A., 26Al tracer diffusion in titanium doped single crystalline -Al2O3. Solid State Ionics, 2008, 179, 373–379. 31. Nakagawa, T., Sakaguchi, I., Shibata, N., Matsunaga, K., Mizoguchi, T., Yamamoto, T., Haneda, H. and Ikuhara, Y., Yttrium doping effect on oxygen grain boundary difusión in -Al2O3. Acta Mater., 2007, 55, 6627–6633. 32. French, J. D., Zhao, J., Harmer, M. P., Chan, H. M. and Miller, G. A., Creep of duplex microstructures. J. Am. Ceram. Soc., 1994, 77, 2857–2865. 33. Grishaber, R. B., Sergueeva, A. V., Mishra, R. S. and Mukherjee, A. K., Laminated metal composites—high temperature deformation behavior. Mater. Sci. Eng. A, 2005, 403, 17–24. 34. Wang, J., Taleff, E. M. and Kovar, D., High-temperature deformation of Al2O3/Y-TZP particulate laminates. Acta Mater., 2004, 52, 4685–4693. 35. Lesage, B., Le Gall, M., Loudjani, M. and Huntz, A. M., Yttrium diffusion in alumina. Defect. Diffus. Forum, 1993, 95–98, 1061–1064. 36. Wang, J., Taleff, E. M. and Kovar, D., Superplastic deformation of Al2O3/Y-TZP particulate composites and laminates. Acta Mater., 2004, 52, 5485–5491.