《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_6High-temperature mechanical behaviour of flaw tolerant

Availableonlineatwww.sciencedirect.co ScienceDirect Acta materialia ELSEVIER Acta Materialia 55(2007)4891-4901 High-temperature mechanical behaviour of flaw tolerant alumina-zirconia multilayered ceramics R. Bermejo,, A.J. Sanchez-Herencia", L Llanes, C. Baudin Instituto de Ceramica y Vidrio(CSIC), C/ Kelsen Institut fiir Struktur- tend Funktionskeramik (ISFK), Peter-Thnner StraBe 5, 8700 Leoben, Austria Departamento de Ciencia de los Materiales e Ingenieria Metalirgica, ETSEIB, Universidad Politecnica de Cataluiia (UPC) Io. Diagonal 647, 08028 Barcelona, spain Received 3 April 2007: received in revised form 3 May 2007: accepted 7 May 2007 vailable online 26 June 2007 Abstract The mechanical behaviour of alumina-zirconia multilayered ceramics designed with thin internal compressive layers has been inves- tigated under flexural loading at room and high temperature Youngs modulus and the sintering evolution of each layer have been exper- imentally determined up to 1200C, to account for the residual stress distribution in the layered composite. The fracture behaviour has been assessed by indentation- strength experiments at different temperatures and by a fracture mechanics analysis. Experimental find ings showed that improvement in mechanical properties of the laminate at high temperatures in comparison to the alumina-based mono- lithic material was essentially related to the distinct modes of failure observed as a function of the temperature, in the presence of energy release mechanisms such as crack bifurcation and/or delamination that may be used as a tool for designing tolerant materials at higl temperatures o 2007 Published by Elsevier Ltd on behalf of Acta Materialia Inc Keywords: Layered structures; Residual stresses; High temperature; Toughness; Flaw tolerance 1. Introduction ceramic materials. The so-called flaw elimination approach envisages the production of the highest degree Ceramic materials are the most suitable substitutes for of homogeneity in bulk monophase ceramics with very metals in structural applications that involve high temper- small flaws. However, new strategies fundamentally differ- atures in severe erosive and corrosive environments and/or ent from this conventional approach have emerged aiming under compressive loads. However, the main drawback of to achieve"flaw tolerant'"materials by designing special ceramics is associated with their brittle mode of failure that microstructures that improve the toughness(or apparent implies an extreme variation of the strength of different toughness) of ceramics. One of the most attractive propos- components within the same batch as a function of the flaw als for this latter approach consists of layered architectures size distribution. Therefore, the main requirement for the that combine materials with different properties. As a extension of ceramic use in structural applications is to result, laminates with mechanical behaviour superior to avoid this lack of strength reliability that of the individual constituents can be fabricated. In this In the last three decades, remarkable advances have regard, different fracture mechanics appi roaches been achieved in improving the mechanical behaviour of attempted for the design of ic layered composites. Residual stress free laminates constituted by layers sepa nding author. Address: Instituto de Ceramica y Vidrio rated by weak interfaces between themselves and laminates CSIC), C/Kelsen 5. 28049 Madrid, Spain that combine stiff and high strength external layers with E-mail address: raul bermejo(@mu-leoben at(R. Bermejo) flaw tolerant internal layers have been proposed to 1359-6454530.00 o 2007 Published by Elsevier Ltd on behalf of Acta Materialia Inc. do: 10. 1016/j.actamat. 2007.05.005

High-temperature mechanical behaviour of flaw tolerant alumina–zirconia multilayered ceramics R. Bermejo a,b,*, A.J. Sa´nchez-Herencia a , L. Llanes c , C. Baudı´n a a Instituto de Cera´mica y Vidrio (CSIC), C/ Kelsen 5, 28049 Madrid, Spain b Institut fu¨r Struktur- und Funktionskeramik (ISFK), Peter-Tunner Straße 5, 8700 Leoben, Austria c Departamento de Ciencia de los Materiales e Ingenierı´a Metalu´rgica, ETSEIB, Universidad Polite´cnica de Catalun˜a (UPC), Av. Diagonal 647, 08028 Barcelona, Spain Received 3 April 2007; received in revised form 3 May 2007; accepted 7 May 2007 Available online 26 June 2007 Abstract The mechanical behaviour of alumina–zirconia multilayered ceramics designed with thin internal compressive layers has been inves￾tigated under flexural loading at room and high temperature. Young’s modulus and the sintering evolution of each layer have been exper￾imentally determined up to 1200 C, to account for the residual stress distribution in the layered composite. The fracture behaviour has been assessed by indentation – strength experiments at different temperatures and by a fracture mechanics analysis. Experimental find￾ings showed that improvement in mechanical properties of the laminate at high temperatures in comparison to the alumina-based mono￾lithic material was essentially related to the distinct modes of failure observed as a function of the temperature, in the presence of energy release mechanisms such as crack bifurcation and/or delamination that may be used as a tool for designing tolerant materials at high temperatures. 2007 Published by Elsevier Ltd on behalf of Acta Materialia Inc. Keywords: Layered structures; Residual stresses; High temperature; Toughness; Flaw tolerance 1. Introduction Ceramic materials are the most suitable substitutes for metals in structural applications that involve high temper￾atures in severe erosive and corrosive environments and/or under compressive loads. However, the main drawback of ceramics is associated with their brittle mode of failure that implies an extreme variation of the strength of different components within the same batch as a function of the flaw size distribution. Therefore, the main requirement for the extension of ceramic use in structural applications is to avoid this lack of strength reliability. In the last three decades, remarkable advances have been achieved in improving the mechanical behaviour of ceramic materials. The so-called ‘‘flaw elimination’’ approach envisages the production of the highest degree of homogeneity in bulk monophase ceramics with very small flaws. However, new strategies fundamentally differ￾ent from this conventional approach have emerged aiming to achieve ‘‘flaw tolerant’’ materials by designing special microstructures that improve the toughness (or apparent toughness) of ceramics. One of the most attractive propos￾als for this latter approach consists of layered architectures that combine materials with different properties. As a result, laminates with mechanical behaviour superior to that of the individual constituents can be fabricated. In this regard, different fracture mechanics approaches have been attempted for the design of ceramic layered composites. Residual stress free laminates constituted by layers sepa￾rated by weak interfaces between themselves and laminates that combine stiff and high strength external layers with flaw tolerant internal layers have been proposed to 1359-6454/$30.00 2007 Published by Elsevier Ltd on behalf of Acta Materialia Inc. doi:10.1016/j.actamat.2007.05.005 * Corresponding author. Address: Instituto de Cera´mica y Vidrio (CSIC), C/ Kelsen 5, 28049 Madrid, Spain. E-mail address: raul.bermejo@mu-leoben.at (R. Bermejo). www.elsevier.com/locate/actamat Acta Materialia 55 (2007) 4891–4901

R Bermejo et al. Acta Materialia 55(2007)4891-4901 promote sufficient strength with significant flaw tolerance The temperature effect can be of extreme importance for to delamination and/or crack deflection processes [1-11]. the performance of laminates designed on the basis of Another large family comprises materials designed on residual stresses when the temperature of use approaches the basis of residual stress development in the layers during that of stress relaxation. In this sense, although room tem- cooling from the sintering temperature [12-24], alumina- perature mechanical data in flexure have been reported for zirconia being the most studied layered architecture alumina-yttria tetragonal zirconia polycrystal (YTZP [12, 13, 15, 16, 20-24]. In this system, a broad interval of ther- laminates, claiming high strength and/or toughening and mal strains can be reached owing to differences between the R-curve behaviour, the high-temperature mechanical thermal expansion coefficient of the constituent layers [12- response has only been characterised and compared to that ion, significant impro ers of the laminate are in compres- [13, 15, 16) In de unie als. with the same composition as mation[20,21,24 the layers, by uniaxial compression creep experiments cases, laminates were more creep resistant ements in strength can be achieved when strained in the direction parallel to the interfaces than [22,23]. This is the so-called strengthening approach, simi- when deformed in the perpendicular direction, owing to the lar to that traditionally used in glasses [17, 25]. Moreover, constraints imposed by the more creep resistant constituent R-curve behaviour revealing flaw tolerance is also observed layers. Creep exponents for the alumina/YTZP laminates this type of laminate for flaws embedded in the external were similar to those determined for the monoliths, indicat ayer [23]. On the other hand, a challenging approach is ing coincident mass transport mechanisms. The overall that of using layered ceramics with thin internal layers mechanical behaviour of these laminates was either better under high compressive stresses and thick external layers or fell in between that of the monoliths. An important under low tensile ones [14, 19, 21, 24, 26]. When tested in flex- characteristic of the alumina/YTZP layered ceramics stud- ure, the compressive layers would hinder the crack propa- ied was that the interfaces between layers maintained their gation through the rest of the material yielding as a result a structural integrity after testing sole'critical flaw size, increasing the material reliability In a recent investigation [24], an Al2O3-5 vol % t-ZrO2/ as well as its apparent fracture toughness and work of Al2O3-30 voL m-ZrO2(t= tetragonal and m=mono- acture clinic) layered composite designed with high compressive Extensive work has been reported on the design, pro- internal layers showed a threshold strength under flexural cessing and room temperature mechanical behaviour of loading at room temperature, where the thin compressive ceramic laminates. In contrast, even though the main envis- layers acted as a barrier to crack extension. Moreover, aged applications for ceramics as metal substitutes would the further propagation of the critical flaw throughout involve relatively high temperatures, works aiming to the layered structure took place through deflection/bifurca investigate the mechanical behaviour of ceramic laminates tion mechanisms, yielding as a result an increase in appar- under these conditions are rather scarce [3-6, 9, 12, 13, 15, 16]. ent fracture toughness and fracture energy in comparison Within this context, residual stress free laminates with to a monolithic material that has the same composition weak interfaces between layers, designed for crack deflec- as the external layer, taken as a reference. The purpose of tion and delamination, have been tested at high tempera- the present investigation is to assess the mechanical behav tures. Such experiments showed how the reaction iour of this layered system under high-temperature condi between the layers and/or interactions with the atmosphere tions. Four-point bending tests were performed at at high temperatures can dramatically change the expected different temperatures on indented laminated specimens mechanical behaviour of the laminate. For instance, the and on the reference material for comparative purposes. crack deflection capability and associated"graceful fail- Young,'s modulus over the temperature range of study ure"in flexure(four-point bending) of a laminate consti- and deformation during cooling from the sintering temper tuted by alternated thick Si3 N4 and thin bn layers ature of monoliths of the same compositions as those of the decreased, and appreciable plastic flow occurred at high constituent layers were determined in order to analyse the temperature(1400C)owing to oxidation [5]. Also the evolution of the distribution of residual stresses through capability for crack deflection at the interfaces disappeared the laminate with temperature. The effect of temperature at 1300C in laminates consisting of alumina/MoSi2+ on the mechanical response of the multilayered material Mo2Bs and alumina/TiC-MoSi2+Mo2 Bs layers tested in is discussed in terms of the variation with the temperature flexure(three-point bending), indicating that the strength of:(i)the elastic properties of the layers, (ii) the residua of the bonding between layers increased with temperature stress profile and (iii) the apparent fracture toughness [3, 6]. To overcome the degradation of mechanical behav iour in laminates designed for crack deflection caused by 2. Experimental the strengthening of the interfaces alumina based lami- nates formed by dense alumina layers linked by porous alu- 2. 1. Materials mina interfaces have been proposed. Full crack deflection and graceful failure have been observed at testing temper- A laminar ceramic consisting of alternating layers of atures up to1200°C[4] Al2O3-5 vol t-ZrO2, named ATZ, and Al,O3-30 vol%

promote sufficient strength with significant flaw tolerance to delamination and/or crack deflection processes [1–11]. Another large family comprises materials designed on the basis of residual stress development in the layers during cooling from the sintering temperature [12–24], alumina– zirconia being the most studied layered architecture [12,13,15,16,20–24]. In this system, a broad interval of ther￾mal strains can be reached owing to differences between the thermal expansion coefficient of the constituent layers [12– 16,22,23] and/or different levels of zirconia phase transfor￾mation [20,21,24]. When the external layers of the laminate are in compres￾sion, significant improvements in strength can be achieved [22,23]. This is the so-called strengthening approach, simi￾lar to that traditionally used in glasses [17,25]. Moreover, R-curve behaviour revealing flaw tolerance is also observed in this type of laminate for flaws embedded in the external layer [23]. On the other hand, a challenging approach is that of using layered ceramics with thin internal layers under high compressive stresses and thick external layers under low tensile ones [14,19,21,24,26]. When tested in flex￾ure, the compressive layers would hinder the crack propa￾gation through the rest of the material yielding as a result a ‘‘sole’’ critical flaw size, increasing the material reliability as well as its apparent fracture toughness and work of fracture. Extensive work has been reported on the design, pro￾cessing and room temperature mechanical behaviour of ceramic laminates. In contrast, even though the main envis￾aged applications for ceramics as metal substitutes would involve relatively high temperatures, works aiming to investigate the mechanical behaviour of ceramic laminates under these conditions are rather scarce [3–6,9,12,13,15,16]. Within this context, residual stress free laminates with weak interfaces between layers, designed for crack deflec￾tion and delamination, have been tested at high tempera￾tures. Such experiments showed how the reaction between the layers and/or interactions with the atmosphere at high temperatures can dramatically change the expected mechanical behaviour of the laminate. For instance, the crack deflection capability and associated ‘‘graceful fail￾ure’’ in flexure (four-point bending) of a laminate consti￾tuted by alternated thick Si3N4 and thin BN layers decreased, and appreciable plastic flow occurred at high temperature (1400 C) owing to oxidation [5]. Also the capability for crack deflection at the interfaces disappeared at 1300 C in laminates consisting of alumina/MoSi2+ Mo2B5 and alumina/TiC–MoSi2+Mo2B5 layers tested in flexure (three-point bending), indicating that the strength of the bonding between layers increased with temperature [3,6]. To overcome the degradation of mechanical behav￾iour in laminates designed for crack deflection caused by the strengthening of the interfaces, alumina based lami￾nates formed by dense alumina layers linked by porous alu￾mina interfaces have been proposed. Full crack deflection and graceful failure have been observed at testing temper￾atures up to 1200 C [4]. The temperature effect can be of extreme importance for the performance of laminates designed on the basis of residual stresses when the temperature of use approaches that of stress relaxation. In this sense, although room tem￾perature mechanical data in flexure have been reported for alumina–yttria tetragonal zirconia polycrystal (YTZP) laminates, claiming high strength and/or toughening and R-curve behaviour, the high-temperature mechanical response has only been characterised and compared to that of monolithic materials, with the same composition as the layers, by uniaxial compression creep experiments [13,15,16]. In all cases, laminates were more creep resistant when strained in the direction parallel to the interfaces than when deformed in the perpendicular direction, owing to the constraints imposed by the more creep resistant constituent layers. Creep exponents for the alumina/YTZP laminates were similar to those determined for the monoliths, indicat￾ing coincident mass transport mechanisms. The overall mechanical behaviour of these laminates was either better or fell in between that of the monoliths. An important characteristic of the alumina/YTZP layered ceramics stud￾ied was that the interfaces between layers maintained their structural integrity after testing. In a recent investigation [24], an Al2O3–5 vol.% t-ZrO2/ Al2O3–30 vol.% m-ZrO2 (t = tetragonal and m = mono￾clinic) layered composite designed with high compressive internal layers showed a threshold strength under flexural loading at room temperature, where the thin compressive layers acted as a barrier to crack extension. Moreover, the further propagation of the critical flaw throughout the layered structure took place through deflection/bifurca￾tion mechanisms, yielding as a result an increase in appar￾ent fracture toughness and fracture energy in comparison to a monolithic material that has the same composition as the external layer, taken as a reference. The purpose of the present investigation is to assess the mechanical behav￾iour of this layered system under high-temperature condi￾tions. Four-point bending tests were performed at different temperatures on indented laminated specimens and on the reference material for comparative purposes. Young’s modulus over the temperature range of study and deformation during cooling from the sintering temper￾ature of monoliths of the same compositions as those of the constituent layers were determined in order to analyse the evolution of the distribution of residual stresses through the laminate with temperature. The effect of temperature on the mechanical response of the multilayered material is discussed in terms of the variation with the temperature of: (i) the elastic properties of the layers, (ii) the residual stress profile and (iii) the apparent fracture toughness. 2. Experimental 2.1. Materials A laminar ceramic consisting of alternating layers of Al2O3–5 vol.% t-ZrO2, named ATZ, and Al2O3–30 vol.% 4892 R. Bermejo et al. / Acta Materialia 55 (2007) 4891–4901

ermejo et al. Acta Materialia 55(2007)4891-4901 m-ZrO2, referred to as AMZ, as well as the corresponding under displacement control, at a rate of 0.05 mm min- ATZ and AMZ monoliths were fabricated by slip casting using a universal testing machine model Instron 8562 following a procedure described elsewhere [27]. Samples Great Britain) with an electrical furnace. Mechanical test were sintered at 1550C for 2 h using heating and cooling ing was performed at different temperatures and after dif- rates of 5C min. As a result, symmetrical laminates ferent thermal histories, i.e. room temperature after with four thin AMZ layers sandwiched between five thick sintering, 800C reached on heating(before m-t trans- ATZ layers as well as monolithic specimens of composi- formation), 1200C reached on heating, 800C reached tions ATZ and AMZ were obtained. After sintering, spec- on cooling from 1200C (after the m-t transformation imens were polished with diamond paste down to I um for on heating and before the reverse transformation on cool- SEM observation. The density of the sintered ATZ and ing), and 650C cooling(after t-m transformation). AMZ monolithic samples was measured by the archimedes All the fractured specimens were inspected by reflected method in water. Additionally, an XRD analysis was also light optical microscopy and ng electron microscopy carried out in the monoliths for composition and phase(DSM-950, Zeiss, Germany) to determine the type, size and identification. Finally, bars of approximately 3. 6 mm x location of the failure-controlling flaws The load-displace- 3.2 mm x 45 mm were diamond machined for mechanical ment curves were recorded using the software coupled to characterisation the testing set-up, and the engineering stress was calculated using the load values and the dimensions of the specimens 2. 2. Elastic properties evaluation and the spans, assuming linear elastic behaviour. Since the elastic properties of the laminate vary through the different Youngs modulus, E; of both ATZ and AMZ monoliths layers, the failure stress for the indented specimens, or, was was evaluated between room(20C) and high (1250C) calculated using the following equation temperature using the impulse excitation technique (IET) e8, following the guidelines provided by ASTM E 1876- GR,E.(-yna 99 and ENV-843-2. Dimensional changes in sintered monolithic samples(10 mm length and cross section of where Ei is the Young's modulus of the corresponding 5x5mm)were determined on heating up to 1250C layer, M is the moment for the case of four-point bending and on cooling using a dilatometer(D1-24, Adamel Lhom- tests(M=Fa, where F is the applied load at failure and a argy, France)with alumina rod and calibrated with a plat- the distance between inner and outer spans), yna is the po- inum standard of similar size as that of the testing sition of the neutral axis and EI the flexural rigidity of the composite calculated for bending perpendicular to the lay ers considering the corresponding Youngs modulus of 23. Residual stress estimation each layer [29-31]. n order to evaluate the residual stress profile within the 3. Results and discussion tridimensional multilayered structure owing to the thermal strain mismatch between adjacent layers, a three-dimen- 3. 1. Microstructural characterisation sional(3D)finite element model developed elsewhere [20] was implemented using the FE code ANSYS 10.0. Pris- Densities of the sintered atz and amz monoliths were matic bar-shaped specimens were modelled as representa-.5% and 98.5% of the theoretical values, respectively tive of the specimens utilised in the experiments for The XRD analysis revealed a 95.5 vol. of alumina and material characterisation, and the elastic and thermal prop 4.5 vol. of tetragonal zirconia(t-ZrO2) in the ATZ com- erties previously determined were introduced as a function pacts, while for the AMz samples a 70 vol % of alumina of the temperature and a 30 vol% of monoclinic zirconia (m-ZrO2)were detected [32]. a phase analysis of the zirconia content in 2.4. Flexural tests the AMz compacts indicated that most of the zirconia had transformed into the monoclinic phase, and only 5- Indentation tests were performed using a Vickers inden- 10% remained as the tetragonal phase in the sintered com ter (Microtest, Spain) with a displacement rate of pacts [32]. Further microstructural observations in the bulk 0. 1 mm min up to reach a maximum load of 100N and AMZ compacts showed small microcracks in the alumina- holding time of 10 s. Three indentations were placed in zirconia grain boundaries, as observed by other authors the middle of one of the outer layers along the longitudinal [33; the volume expansion associated with the zirconia direction of the sample with an offset separation of 2 mm to transformation from tetragonal to monoclinic phase on avoid any crack interaction. Indented samples were frac- cooling led to the formation of radial microcracks which tured in a fully articulated alumina four-point bending emerged from the transformed zirconia grains in the alu- device(the loading axis normal to the layer plane and the mina matrix. Regarding the laminar composite, uniform indented layer in tension) with inner and outer spans of layer thickness was obtained for the ATZ and AMZ layers, 20 mm and 40 mm, respectively. Tests were carried out resulting in 530+ 10 um and 100+5 um, respectively

m-ZrO2, referred to as AMZ, as well as the corresponding ATZ and AMZ monoliths were fabricated by slip casting following a procedure described elsewhere [27]. Samples were sintered at 1550 C for 2 h using heating and cooling rates of 5 C min1 . As a result, symmetrical laminates with four thin AMZ layers sandwiched between five thick ATZ layers as well as monolithic specimens of composi￾tions ATZ and AMZ were obtained. After sintering, spec￾imens were polished with diamond paste down to 1 lm for SEM observation. The density of the sintered ATZ and AMZ monolithic samples was measured by the Archimedes method in water. Additionally, an XRD analysis was also carried out in the monoliths for composition and phase identification. Finally, bars of approximately 3.6 mm · 3.2 mm · 45 mm were diamond machined for mechanical characterisation. 2.2. Elastic properties evaluation Young’s modulus, Ei, of both ATZ and AMZ monoliths was evaluated between room (20 C) and high (1250 C) temperature using the impulse excitation technique (IET) [28], following the guidelines provided by ASTM E 1876- 99 and ENV-843-2. Dimensional changes in sintered monolithic samples (10 mm length and cross section of 5 · 5 mm2 ) were determined on heating up to 1250 C and on cooling using a dilatometer (DI-24, Adamel Lhom￾argy, France) with alumina rod and calibrated with a plat￾inum standard of similar size as that of the testing specimens. 2.3. Residual stress estimation In order to evaluate the residual stress profile within the tridimensional multilayered structure owing to the thermal strain mismatch between adjacent layers, a three-dimen￾sional (3D) finite element model developed elsewhere [20] was implemented using the FE code ANSYS 10.0. Pris￾matic bar-shaped specimens were modelled as representa￾tive of the specimens utilised in the experiments for material characterisation, and the elastic and thermal prop￾erties previously determined were introduced as a function of the temperature. 2.4. Flexural tests Indentation tests were performed using a Vickers inden￾ter (Microtest, Spain) with a displacement rate of 0.1 mm min1 up to reach a maximum load of 100 N and holding time of 10 s. Three indentations were placed in the middle of one of the outer layers along the longitudinal direction of the sample with an offset separation of 2 mm to avoid any crack interaction. Indented samples were frac￾tured in a fully articulated alumina four-point bending device (the loading axis normal to the layer plane and the indented layer in tension) with inner and outer spans of 20 mm and 40 mm, respectively. Tests were carried out under displacement control, at a rate of 0.05 mm min1 , using a universal testing machine model Instron 8562 (Great Britain) with an electrical furnace. Mechanical test￾ing was performed at different temperatures and after dif￾ferent thermal histories, i.e. room temperature after sintering, 800 C reached on heating (before m ! t trans￾formation), 1200 C reached on heating, 800 C reached on cooling from 1200 C (after the m ! t transformation on heating and before the reverse transformation on cool￾ing), and 650 C cooling (after t ! m transformation). All the fractured specimens were inspected by reflected light optical microscopy and scanning electron microscopy (DSM-950, Zeiss, Germany) to determine the type, size and location of the failure-controlling flaws. The load–displace￾ment curves were recorded using the software coupled to the testing set-up, and the engineering stress was calculated using the load values and the dimensions of the specimens and the spans, assuming linear elastic behaviour. Since the elastic properties of the laminate vary through the different layers, the failure stress for the indented specimens, rRi , was calculated using the following equation: rRi ¼ EiM EI ðy ynaÞ ð1Þ where Ei is the Young’s modulus of the corresponding layer, M is the moment for the case of four-point bending tests (M = Fa, where F is the applied load at failure and a the distance between inner and outer spans), yna is the po￾sition of the neutral axis and EI the flexural rigidity of the composite calculated for bending perpendicular to the lay￾ers considering the corresponding Young’s modulus of each layer [29–31]. 3. Results and discussion 3.1. Microstructural characterisation Densities of the sintered ATZ and AMZ monoliths were 99.5% and 98.5% of the theoretical values, respectively. The XRD analysis revealed a 95.5 vol.% of alumina and 4.5 vol.% of tetragonal zirconia (t-ZrO2) in the ATZ com￾pacts, while for the AMZ samples a 70 vol.% of alumina and a 30 vol.% of monoclinic zirconia (m-ZrO2) were detected [32]. A phase analysis of the zirconia content in the AMZ compacts indicated that most of the zirconia had transformed into the monoclinic phase, and only 5– 10% remained as the tetragonal phase in the sintered com￾pacts [32]. Further microstructural observations in the bulk AMZ compacts showed small microcracks in the alumina– zirconia grain boundaries, as observed by other authors [33]; the volume expansion associated with the zirconia transformation from tetragonal to monoclinic phase on cooling led to the formation of radial microcracks which emerged from the transformed zirconia grains in the alu￾mina matrix. Regarding the laminar composite, uniform layer thickness was obtained for the ATZ and AMZ layers, resulting in 530 ± 10 lm and 100 ± 5 lm, respectively. R. Bermejo et al. / Acta Materialia 55 (2007) 4891–4901 4893

R Bermejo et al. Acta Materialia 55(2007)4891-4901 3.2. Young s modulus, thermal expans d residual stress evolution with temperature ng. I shows the Youngs modulus of the AMz an ATZ monolith, Young's modulus varied following the clas- 兰 sical relationship(1% every 100C)found by Watchman et al. for a large number of ceramics [34] ranging from 9390 GPa at room temperature to 344 GPa at 1200C AT7 and did not show hysteresis in the heating-cooling cycle AMZ For the AMz specimens, the elastic modulus decreased from room temperature(≈290GPa)upto900°C 2 25050075010001250 221 GPa)during heating, this decrease(22.6% for every 100C) being significantly higher than for ATZ. The rela tive low value determined at room temperature(as com- Fig. 2. Dilatometry curves for the ATZ and AMz monoliths. Circles that obtained with the rule of mixtures correspond to the different temperatures(indicated by th considering 400 GPa for alumina [34] and 244 GPa for flexural tests were carried out monophase monoclinic zirconia [35] may be related to the presence of microcracks caused by the high content of monoclinic zirconia in these compacts, as discussed heating and cooling can be attributed to experimental above. From about 860C, values started to increase, first uncertainties). This behaviour, together with the observed moothly and then sharply from 950C up to 1150C. The variation of Youngs modulus proves that no significant initial increase might be associated with stiffening of the microstructural changes occurred during the thermal material caused by closure of the microcracks whereas cycling of the sintered ATZ compacts. Conversely, the vol the sharp increase has to be attributed to the transforma- ume changes associated with the reversible zirconia phase ion of zirconia from the monoclinic(Ex 164 GPa at transformation, from monoclinic to tetragonal during 950C)to the tetragonal phase(E N 220 GPa at 1200C) heating above 1150C and below 725C on cooling, con- 65]. The effect of the inverse transformation was observed ditioned the dimensional changes of the AMZ monoliths. on cooling where, after a slight increase, an abrupt decrease The expected differences in thermal strain of the layers, occurred at the temperature of the tetragonal to monoclinic referred to as Ae, which will condition the residual stress zirconia phase transformation(725C). After this sharp state in the layered structure, can be drawn from the decrease, the Youngs modulus continued rising as the tem- dimensional changes experienced by the monoliths perature fell to room temperature, where a similar value to(Fig. 2). At high temperature, where mass transport mech the initial room temperature was achieved anisms are active, the strain mismatch between layers The dilatometer curves during heating and cooling of caused by differences in thermal expansion would be the sintered atz and Amz monoliths are shown in accommodated. Therefore, the strain mismatch between Fig. 2. For ATZ, both curves were monotonous and with- layers at room temperature can be derived by taking out any significant hysteresis(the small differences between 1200C as zero point for both materials and considering the dimensional changes from 1200C to room ture. In doing so, the thermal strain mismatch between AE=EAMZ-EATZ 0.00212 [271, where EAMz and Eat refer to the thermal strains of the amz and atz mono- ATZ liths from Fig. 2. as discussed in the introduction. the mechanical esponse of the layered material at a given temperature will AMZ be determined by the residual stress field within the layers which, in turn, would be associated with two features: ( E modulus heating the elastic properties at the given temperature, i.e. ET E modulus cooling and (ii) the thermal strain mismatch between layers namely AE(T). Hence, an evaluation of the residual stress 020040060080010001200 state at every testing temperature was carried out. In doing modulus variation with temperature of the AtZ and esidual stress calculation over the temperature range The corresponding elastic and thermal properties of each curves s. The abrupt change in slope on the heating and cooling layer, l.e. Young s modulus and thermal strain, were intro- AMz is due to the m - t and t zirconia phase transformations, respectively. duced as a function of temperature

3.2. Young’s modulus, thermal expansion and residual stress evolution with temperature Fig. 1 shows the Young’s modulus of the AMZ and ATZ monoliths as a function of temperature. For the ATZ monolith, Young’s modulus varied following the clas￾sical relationship (1% every 100 C) found by Watchman et al. for a large number of ceramics [34], ranging from 390 GPa at room temperature to 344 GPa at 1200 C and did not show hysteresis in the heating–cooling cycle. For the AMZ specimens, the elastic modulus decreased from room temperature (290 GPa) up to 900 C (221 GPa) during heating, this decrease (2.6% for every 100 C) being significantly higher than for ATZ. The rela￾tive low value determined at room temperature (as com￾pared to that obtained with the rule of mixtures considering 400 GPa for alumina [34] and 244 GPa for monophase monoclinic zirconia [35]) may be related to the presence of microcracks caused by the high content of monoclinic zirconia in these compacts, as discussed above. From about 860 C, values started to increase, first smoothly and then sharply from 950 C up to 1150 C. The initial increase might be associated with stiffening of the material caused by closure of the microcracks whereas the sharp increase has to be attributed to the transforma￾tion of zirconia from the monoclinic (E  164 GPa at 950 C) to the tetragonal phase (E  220 GPa at 1200 C) [35]. The effect of the inverse transformation was observed on cooling where, after a slight increase, an abrupt decrease occurred at the temperature of the tetragonal to monoclinic zirconia phase transformation (725 C). After this sharp decrease, the Young’s modulus continued rising as the tem￾perature fell to room temperature, where a similar value to the initial room temperature was achieved. The dilatometer curves during heating and cooling of the sintered ATZ and AMZ monoliths are shown in Fig. 2. For ATZ, both curves were monotonous and with￾out any significant hysteresis (the small differences between heating and cooling can be attributed to experimental uncertainties). This behaviour, together with the observed variation of Young’s modulus proves that no significant microstructural changes occurred during the thermal cycling of the sintered ATZ compacts. Conversely, the vol￾ume changes associated with the reversible zirconia phase transformation, from monoclinic to tetragonal during heating above 1150 C and below 725 C on cooling, con￾ditioned the dimensional changes of the AMZ monoliths. The expected differences in thermal strain of the layers, referred to as De, which will condition the residual stress state in the layered structure, can be drawn from the dimensional changes experienced by the monoliths (Fig. 2). At high temperature, where mass transport mech￾anisms are active, the strain mismatch between layers caused by differences in thermal expansion would be accommodated. Therefore, the strain mismatch between layers at room temperature can be derived by taking 1200 C as zero point for both materials and considering the dimensional changes from 1200 C to room tempera￾ture. In doing so, the thermal strain mismatch between the layers of the sintered laminates at 25 C is De = eAMZ eATZ  0.00212 [27], where eAMZ and eATZ refer to the thermal strains of the AMZ and ATZ mono￾liths from Fig. 2. As discussed in the introduction, the mechanical response of the layered material at a given temperature will be determined by the residual stress field within the layers which, in turn, would be associated with two features: (i) the elastic properties at the given temperature, i.e. E(T), and (ii) the thermal strain mismatch between layers, namely De(T). Hence, an evaluation of the residual stress state at every testing temperature was carried out. In doing so, a 3D finite element analysis was carried out for the residual stress calculation over the temperature range. The corresponding elastic and thermal properties of each layer, i.e. Young’s modulus and thermal strain, were intro￾duced as a function of temperature. Fig. 1. Young’s modulus variation with temperature of the ATZ and AMZ monoliths. The abrupt change in slope on the heating and cooling curves of the AMZ is due to the m ! t and t ! m zirconia phase transformations, respectively. Fig. 2. Dilatometry curves for the ATZ and AMZ monoliths. Circles correspond to the different temperatures (indicated by the squares) where flexural tests were carried out. 4894 R. Bermejo et al. / Acta Materialia 55 (2007) 4891–4901

R Bermejo et al. Acta Materialia 55(2007)4891-4901 4895 Table I Maximum residual stress values in the ATZ and AMZ layers of the laminate at the testing temperatures indicated by the circles in Fig. 2 Residual stress(MPa) 800°C↑ heating 800°C↓( cooling) 650°C↓( cooling Gres.max in ATZ layers +105 +70 +154 A minus sign indicates that the layer is under compression. In order to compare the residual stress distribution in the AMz and ATZ layers at the temperatures of study men,across the layers. The results showed a biaxial stress =150 e inate rT (see full squares in Fig. 2), the maximum stress developed in the laminate was determined at the centre of the speci L.650°C L.800°C distribution within the aTz and AMz layers parallel to the layer plane, which was constant far from the edges 9100 Results are listed in table l It is inferred from the referred table that there is a strong influence, not only of the temperature, but also of the pre-50 vious thermal cycle, on the level of residual stresses within Indent. pop-t the layers. In this regard, if the temperature of 800C is reached on heating from room temperature, the internal 0.000.030.060.090.120.150.18 AMZ layers would be in compression and the atZ ones in tension as it occurs in the " as sintered" laminate. In Displacement [ mm] contrast, cooling from a temperature over 1150C results Fig. 3. Stress-displacement curves corresponding to the indentation- in ATZ layers under compression and AMZ layers under strength tests in the laminates and in the reference monolith at several nsion. For specimens tested oling from 1200 oC temperatures. The"pop-in" events indicate the initial growth of the dentation cracks efore reaching the temperature for the t-m phase trans the ATz/AMZ interface. Curves are shifted ong the displacement axis for clearer observation formation(a725C), residual stresses would be solely due to thermal expansion mismatch between the layers, being The first thin AMZ layer, with high compressive stresses significantly smaller than strain differences caused by the at room temperature (Table 1), acted as a barrier to crack phase transformation(Fig. 1). However, once transforma- propagation yielding a threshold strength, characteristic of tion occurs on further cooling, the previous residual stress this layered configuration. The effect of the tensile residual state turns into tensile stress in the ATZ layers and into stresses in the outer ATZ layers was reflected by the fact compressive stress in the AMz ones (Table 1), the magni- that the first pop-in events, corresponding to the propaga tude of such stresses being mainly affected by the thermal tion of the indentations up to the first ATZ/AMZ interface, strain between layers coming from the zirconia phase trans- occurred at a stress levels significantly lower than those for ormation. As a consequence, the response of the layered the ATZ monolithic specimens pecimens under flexure will change as a function of the As temperature increases, a decrease of strength is thermal history, as it will be discussed below expected in the materials associated with the decrease in Youngs modulus(Fig. 1)and fracture energy. However, 3.3. Indentation -strength tests at different temperatures the stress levels of the first fracture events(280 MPa)in the layered specimens tested at 800C on heating were Fig 3 shows the stress-displacement response of the lam- higher than those corresponding to the specimens tested inate specimens at the temperatures of study. The corre- at room temperature(x45 MPa). Therefore, in the former ponding characteristic curve for Atz monolithic specimens, the detrimental effect of temperature on specimens at room temperature is also represented for com- strength was counterbalanced by the decrease in tensile parative purposes. The variations in Young's modulus with stresses in the outer layer(Table 1). Nevertheless, compres- temperature( Fig. I)are reflected in these curves by a change sive residual stresses were still active in the Az layers and in the slope. Differences between the fracture behaviour at thus, the effectiveness of the layered configuration in arrest room temperature of the indented monolithic and layered ing the crack propagation was not lost, as evidenced by the pecimens are also apparent. The former presented linear "pop-in"events in the stress-displacement curves(Fig 3) stress-displacement behaviour up to fracture, which In fact, post-mortem examinations of the specimens occurred at a maximum stress value of 145+10 MPa, and showed crack bifurcation at the first AMz layer, as the failure was catastrophic as it corresponds to brittle mate- occurred in the specimens tested at room temperature rials. In contrast, laminate failures were preceded by three [24]. Moreover, additional"pop-in"events were observed pop-in"events associated with the initial growth of the in the 800C T test, corresponding to energy dissipation three indentation cracks up to the ATZ/ AMZ interface, as mechanisms occurring after the indentation cracks reached evidenced in a recent work [24]. the first ATZ/AMZ interface as it is discussed below

In order to compare the residual stress distribution in the AMZ and ATZ layers at the temperatures of study (see full squares in Fig. 2), the maximum stress developed in the laminate was determined at the centre of the speci￾men, across the layers. The results showed a biaxial stress distribution within the ATZ and AMZ layers parallel to the layer plane, which was constant far from the edges. Results are listed in Table 1. It is inferred from the referred table that there is a strong influence, not only of the temperature, but also of the pre￾vious thermal cycle, on the level of residual stresses within the layers. In this regard, if the temperature of 800 C is reached on heating from room temperature, the internal AMZ layers would be in compression and the ATZ ones in tension, as it occurs in the ‘‘as sintered’’ laminate. In contrast, cooling from a temperature over 1150 C results in ATZ layers under compression and AMZ layers under tension. For specimens tested on cooling from 1200 C before reaching the temperature for the t ! m phase trans￾formation (725 C), residual stresses would be solely due to thermal expansion mismatch between the layers, being significantly smaller than strain differences caused by the phase transformation (Fig. 1). However, once transforma￾tion occurs on further cooling, the previous residual stress state turns into tensile stress in the ATZ layers and into compressive stress in the AMZ ones (Table 1), the magni￾tude of such stresses being mainly affected by the thermal strain between layers coming from the zirconia phase trans￾formation. As a consequence, the response of the layered specimens under flexure will change as a function of the thermal history, as it will be discussed below. 3.3. Indentation – strength tests at different temperatures Fig. 3 shows the stress–displacement response of the lam￾inate specimens at the temperatures of study. The corre￾sponding characteristic curve for ATZ monolithic specimens at room temperature is also represented for com￾parative purposes. The variations in Young’s modulus with temperature (Fig. 1) are reflected in these curves by a change in the slope. Differences between the fracture behaviour at room temperature of the indented monolithic and layered specimens are also apparent. The former presented linear stress–displacement behaviour up to fracture, which occurred at a maximum stress value of 145 ± 10 MPa, and the failure was catastrophic as it corresponds to brittle mate￾rials. In contrast, laminate failures were preceded by three ‘‘pop-in’’ events associated with the initial growth of the three indentation cracks up to the ATZ/AMZ interface, as evidenced in a recent work [24]. The first thin AMZ layer, with high compressive stresses at room temperature (Table 1), acted as a barrier to crack propagation yielding a threshold strength, characteristic of this layered configuration. The effect of the tensile residual stresses in the outer ATZ layers was reflected by the fact that the first pop-in events, corresponding to the propaga￾tion of the indentations up to the first ATZ/AMZ interface, occurred at a stress levels significantly lower than those for the ATZ monolithic specimens. As temperature increases, a decrease of strength is expected in the materials associated with the decrease in Young’s modulus (Fig. 1) and fracture energy. However, the stress levels of the first fracture events (80 MPa) in the layered specimens tested at 800 C on heating were higher than those corresponding to the specimens tested at room temperature (45 MPa). Therefore, in the former specimens, the detrimental effect of temperature on strength was counterbalanced by the decrease in tensile stresses in the outer layer (Table 1). Nevertheless, compres￾sive residual stresses were still active in the AMZ layers and thus, the effectiveness of the layered configuration in arrest￾ing the crack propagation was not lost, as evidenced by the ‘‘pop-in’’ events in the stress–displacement curves (Fig. 3). In fact, post-mortem examinations of the specimens showed crack bifurcation at the first AMZ layer, as occurred in the specimens tested at room temperature [24]. Moreover, additional ‘‘pop-in’’ events were observed in the 800 C › test, corresponding to energy dissipation mechanisms occurring after the indentation cracks reached the first ATZ/AMZ interface, as it is discussed below. Table 1 Maximum residual stress values in the ATZ and AMZ layers of the laminate at the testing temperatures indicated by the circles in Fig. 2 Residual stress (MPa) 20 C 800 C › (heating) 1200 C 800 C fl (cooling) 650 C fl (cooling) rres,max. in ATZ layers +105 +40 – 26 +70 rres,max. in AMZ layers 695 245 – +154 415 A minus sign indicates that the layer is under compression. Fig. 3. Stress–displacement curves corresponding to the indentation– strength tests in the laminates and in the reference monolith at several temperatures. The ‘‘pop-in’’ events indicate the initial growth of the indentation cracks up to the ATZ/AMZ interface. Curves are shifted along the displacement axis for clearer observation. R. Bermejo et al. / Acta Materialia 55 (2007) 4891–4901 4895