《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_fibrous molithic-41

Availableonlineatwww.sciencedirect.com Part B: engineering ELSEVIER Composites: Part B 37(2006)499-508 www.elsevier.com/locate/composi Processing of Al,O3/Y-TZP laminates from water-based cast tapes A.J. Sanchez-Herencia s.J. Gurauskis C. Baudin Instituto de Ceramica y Vidrio, CSIC, C/Kelsen 5. 28049 Madrid, spain Received 11 March 2005; received in revised form 15 August 2005; accepted 24 August 2005 Available online 4 April 2006 Abstract Laminated structures have been investigated due to their capability for the reinforcement of ceramics. Crack deflexion and bifurcation, surface strengthening and threshold strength are the mechanisms associated to the fracture of laminated ceramic. In all the cases, a precise control of the thickness and composition of the layers is necessary. In this sense, colloidal processing techniques have proved their adequacy for the fabrication f designed layered structures. This paper This paper deals with the fabrication of layered ceramics by stacking water-based cast tapes at room temperature and using low pressures. In order to control the pressing procedure, the engineering strain-stress curves recorded during the stacking of the tapes were analyzed. Afterwards, the sintering conditions have been optimized by adjusting the green density of the tapes to avoid differential sintering and the associated cracks. Monolithic and layered materials free of cracks have been fabricated using optimized processing conditions C 2006 Elsevier Ltd. All rights reserved. Keywords: A. Layered Structures; Tape; E Joining: Sintering 1. Introduction much of the mechanical property benefit of fiber reinforcemen without the associated processing complexity Severe environments imposed by new technologies demand The use of laminar reinforcements is not however a the fabrication of materials with better properties and more completely original idea. For million of years, living creatures tolerance restrictions, in which reliability must be ensured. have designed protective laminar structures with optimized Consequently, ceramics have been proposed as either mechanical properties utilizing the rather limited resources substitutes for currently used materials (e.g. metals and provided by their surroundings [2]. Such is the case of the plastics) or as complements to existing materials (i. e in the mollusk shell, in which a laminar structure composed of form of composites), due to their favorable properties such as alternating brittle but strong aragonite layers and ductile but high temperature hardness and strength, and good thermal and weak organic polymer layers provides a protective shell with chemical stability. However, the intrinsic brittleness of ceramic toughness and strength roughly 10 times that of a correspond- materials has forced to look for new designs and processing ing aragonite single-crystal [3]. Mimicking this concept, Pizyk routes to improve their mechanical behavior while maintaining and Aksay fabricated several laminar metal-ceramic and the low cost and low environmental impact. ceramic-polymer composites that showed improved mechan- One such method for improving the mechanical behavior of ical properties compared to their corresponding monolithic ceramics has been the reduction of the defects in the ceramic materials [4 body through colloidal filtration and processing techniques [1] Layered ceramics were initially developed in the 1960s as a Another method has been the creation of multi-phase result of the necessity of these structures for the packaging of composite architectures composed of ceramic matrices microelectronics. As a consequence of this demand, different reinforced by the addition of particulate, fiber, and/or laminar methods to obtain ceramic multilayers with controlled secondary phases. The use of laminar reinforcement has been thicknesses, as well as handling and lamination possibilities, identified as a simple and inexpensive method of achieving were studied [5,6). These methods were based on the tape casting technique, in which multilayer structures were obtained by the stacking of green tapes followed by subsequent w Corresponding author. Tel: +34 91 735 5840: fax: +3491 735 5843. consolidation by either the application of pressure at a suitable E-lmail address: ajsanchez@icv temperature, or by roll-to-roll compaction 1359-8368/S- see front matter 2006 Elsevier Ltd. All rights reserved. The preparation of tape-cast layered ceramics for structural doi: 10. 1016/ applications was first reported by Mistler [7]. He described an

Processing of Al2O3/Y-TZP laminates from water-based cast tapes A.J. Sa´nchez-Herencia *, J. Gurauskis, C. Baudı´n Instituto de Cera´mica y Vidrio, CSIC, C/Kelsen 5, 28049 Madrid, Spain Received 11 March 2005; received in revised form 15 August 2005; accepted 24 August 2005 Available online 4 April 2006 Abstract Laminated structures have been investigated due to their capability for the reinforcement of ceramics. Crack deflexion and bifurcation, surface strengthening and threshold strength are the mechanisms associated to the fracture of laminated ceramic. In all the cases, a precise control of the thickness and composition of the layers is necessary. In this sense, colloidal processing techniques have proved their adequacy for the fabrication of designed layered structures. This paper deals with the fabrication of layered ceramics by stacking water-based cast tapes at room temperature and using low pressures. In order to control the pressing procedure, the engineering strain–stress curves recorded during the stacking of the tapes were analyzed. Afterwards, the sintering conditions have been optimized by adjusting the green density of the tapes to avoid differential sintering and the associated cracks. Monolithic and layered materials free of cracks have been fabricated using optimized processing conditions. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Layered Structures; Tape; E. Joining; Sintering 1. Introduction Severe environments imposed by new technologies demand the fabrication of materials with better properties and more tolerance restrictions, in which reliability must be ensured. Consequently, ceramics have been proposed as either substitutes for currently used materials (e.g. metals and plastics) or as complements to existing materials (i.e. in the form of composites), due to their favorable properties such as high temperature hardness and strength, and good thermal and chemical stability. However, the intrinsic brittleness of ceramic materials has forced to look for new designs and processing routes to improve their mechanical behavior while maintaining the low cost and low environmental impact. One such method for improving the mechanical behavior of ceramics has been the reduction of the defects in the ceramic body through colloidal filtration and processing techniques [1]. Another method has been the creation of multi-phase composite architectures composed of ceramic matrices reinforced by the addition of particulate, fiber, and/or laminar secondary phases. The use of laminar reinforcement has been identified as a simple and inexpensive method of achieving much of the mechanical property benefit of fiber reinforcement without the associated processing complexity. The use of laminar reinforcements is not, however, a completely original idea. For million of years, living creatures have designed protective laminar structures with optimized mechanical properties utilizing the rather limited resources provided by their surroundings [2]. Such is the case of the mollusk shell, in which a laminar structure composed of alternating brittle but strong aragonite layers and ductile but weak organic polymer layers provides a protective shell with toughness and strength roughly 10 times that of a correspond￾ing aragonite single-crystal [3]. Mimicking this concept, Pizyk and Aksay fabricated several laminar metal–ceramic and ceramic–polymer composites that showed improved mechan￾ical properties compared to their corresponding monolithic materials [4]. Layered ceramics were initially developed in the 1960s as a result of the necessity of these structures for the packaging of microelectronics. As a consequence of this demand, different methods to obtain ceramic multilayers with controlled thicknesses, as well as handling and lamination possibilities, were studied [5,6]. These methods were based on the tape casting technique, in which multilayer structures were obtained by the stacking of green tapes followed by subsequent consolidation by either the application of pressure at a suitable temperature, or by roll-to-roll compaction. The preparation of tape-cast layered ceramics for structural applications was first reported by Mistler [7]. He described an Composites: Part B 37 (2006) 499–508 www.elsevier.com/locate/compositesb 1359-8368/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2006.02.002 * Corresponding author. Tel.: C34 91 735 5840; fax: C34 91 735 5843. E-mail address: ajsanchez@icv.csic.es (A.J. Sa´nchez-Herencia)

A.J. Sanchez Herencia et aL. / Composites: Part B 37(2006)499-508 experimental device designed for the fabrication of green Gic tapes, which cast multiple layers simultaneously, thus GG (1) Using this device he prepared a trilayered alumina/alumina being Gic the toughness of the interface, Ge the toughness ceramic in which only the two outer layers were doped with of material were the crack penetrate, while Ga and Gp are talc and the inner layer with Mgo. This distribution of dopant he energy release rate of the deflecting and penetrating resulted in a stress free laminate with a coarse-grained, high crack, respectively toughness core layer surrounded by fine-grained, high strength Most of the laminates reported in the outer layer. Mechanical testing revealed that the fracture le reinforcement in the deflection a of he and strength of this laminar material was increased relatively to that Hutchinson. However, the residual constitute of the monoliths with the same composition as that of the particularly significant feature associated with laminated constituent layers. Russo et al. [8)used a similar microstruc- ceramics and play a very important role during fracture of tural engineering concept to tape cast a trilayered material ceramics. These stresses are developed as a consequence of the where the outer, thick layers were made up of a homogeneous thermal strain mismatch between layers of different compo- alumina/aluminum titanate particulate composite, while the sition that occurs during cooling from sintering. This strain inner layer was made of a heterogeneous composite of the same mismatch can be intentionally designed through the use of constituent materials with differing thermal expansion coeffi thickness(around 100 um), the outer layer strength dominated cients(CTE)1I5-181, or through the incorporation of materials fracture at small flaw sizes, while the strength of the inner layer 24]. Despite the differing mechanisms associated with these dominated at large faw sizes. In pioneering work performed by Clegg et al. [9], it was two processes, both generate a differential strain between the layers, Ae, that results in the development of residual stresses shown that a layered structure composed of strong layers The level and sign of the expected residual stresse separated by weak interlayers, formed by a dough rolling evaluated using the simplified model of a symmetric Ic plate technique, could improve mechanical behavior significantly constituted by alternated layers with a biaxial distributio stresses constant across the layers [25]. The level of the stresses manner. The authors showed that by intercalating thin layers in the center of the layers that constitute the layered structure of graphite between thicker layers of Sic, deflection of the are given by crack by the graphite interlayers caused the load-displace- ment curve to display such behavior. The work of fracture △eE of these laminates was two orders of magnitude higher than 1+(Anata/Eanbtb) that of monolithic SiC samples and the apparent toughness was increased from 3.6 MPa m/ in the monolithic to 17.7 MPa min in the laminate. It was also reported that the oo n'a toughening effect of the graphite layers reached a maximum at layer thicknesses of 3 um. Step-wise, graceful failure has where the subscript"a andb refers to the alternating layers, also been produced in alumina/mullite laminates made by ni and ti are the number and thickness of the layers of each co-sintering of extruded alumina sheets of I mm thickness composition, respectively, and E! is given joined with kaolin and resin tapes [10]. The fracture behavior showed once a gain the importance of the relative E- Ei (4) thickness of the layers and the microstructure of the thin internal layer. Step-wise load/displacement behavior result- where E; and v are the Youngs modulus and the Poisson ratio ing from crack deflection by the thin layers was observed of the layers. The strain mismatch, Ae, is calculated as follows for mullite layers with thicknesses in excess of 100 um, while the crack passed undeflected through mullite layers Ae aa)dT+△et ith thicknesses less than 100 um, resulting in non-graceft catastrophic failure. This series of results shows that a where, a; are the coefficients of thermal expansion of the combination of ceramic layers in a structure is capable to layers, Ts is the temperature above which mass transport occurs esult in crack deflection and step-wise fracture, even when in the material, and T the room temperature, and AEt is the the combined layers have the same composition but strain mismatch between the layers due to processes different different microstructure [11-13]. He and Hutchinson [14] from thermal expansion, such as phase transformation settled the energy criteria for the possible propagating path The fracture behavior phenomena associated to the residual material where it could either stresses of laminates are bifurcation [15, 19, 20] surface deflect or penetrate through the interface. The conditions for strengthening [16, 21] and threshold strength [17, 22]. All each possible case were studied in terms of elastic modulus these mechanisms are associated to the development and fracture energy. Their analysis concluded that the residual compressive/tensile stresses in alternated layers impinging crack is likely to be deflected at the interface if Usually, it is desired the relative thickness of the compressed

experimental device designed for the fabrication of green tapes, which cast multiple layers simultaneously, thus eliminating the need for subsequent consolidation operations. Using this device he prepared a trilayered alumina/alumina ceramic in which only the two outer layers were doped with talc and the inner layer with MgO. This distribution of dopant resulted in a stress free laminate with a coarse-grained, high toughness core layer surrounded by fine-grained, high strength outer layer. Mechanical testing revealed that the fracture strength of this laminar material was increased relatively to that of the monoliths with the same composition as that of the constituent layers. Russo et al. [8] used a similar microstruc￾tural engineering concept to tape cast a trilayered material where the outer, thick layers were made up of a homogeneous alumina/aluminum titanate particulate composite, while the inner layer was made of a heterogeneous composite of the same composition. It was observed that, at an optimum surface layer thickness (around 100 mm), the outer layer strength dominated fracture at small flaw sizes, while the strength of the inner layer dominated at large flaw sizes. In pioneering work performed by Clegg et al. [9], it was shown that a layered structure composed of strong layers separated by weak interlayers, formed by a dough rolling technique, could improve mechanical behavior significantly by causing the material to fail in a graceful, step-wise manner. The authors showed that by intercalating thin layers of graphite between thicker layers of SiC, deflection of the crack by the graphite interlayers caused the load–displace￾ment curve to display such behavior. The work of fracture of these laminates was two orders of magnitude higher than that of monolithic SiC samples and the apparent toughness was increased from 3.6 MPa m1/2 in the monolithic to 17.7 MPa m1/2 in the laminate. It was also reported that the toughening effect of the graphite layers reached a maximum at layer thicknesses of 3 mm. Step-wise, graceful failure has also been produced in alumina/mullite laminates made by co-sintering of extruded alumina sheets of 1 mm thickness joined with kaolin and resin tapes [10]. The fracture behavior showed once again the importance of the relative thickness of the layers and the microstructure of the thin internal layer. Step-wise load/displacement behavior result￾ing from crack deflection by the thin layers was observed for mullite layers with thicknesses in excess of 100 mm, while the crack passed undeflected through mullite layers with thicknesses less than 100 mm, resulting in non-graceful, catastrophic failure. This series of results shows that a combination of ceramic layers in a structure is capable to result in crack deflection and step-wise fracture, even when the combined layers have the same composition but different microstructure [11–13]. He and Hutchinson [14] settled the energy criteria for the possible propagating path of a crack in a biphasic material where it could either deflect or penetrate through the interface. The conditions for each possible case were studied in terms of elastic modulus and fracture energy. Their analysis concluded that the impinging crack is likely to be deflected at the interface if Gic Gc ! Gd Gp (1) being Gic the toughness of the interface, Gc the toughness of material were the crack penetrate, while Gd and Gp are the energy release rate of the deflecting and penetrating crack, respectively. Most of the laminates reported in the above paragraphs base the reinforcement in the deflection criteria of He and Hutchinson. However, the residual stresses constitute a particularly significant feature associated with laminated ceramics and play a very important role during fracture of ceramics. These stresses are developed as a consequence of the thermal strain mismatch between layers of different compo￾sition that occurs during cooling from sintering. This strain mismatch can be intentionally designed through the use of constituent materials with differing thermal expansion coeffi- cients (CTE) [15–18], or through the incorporation of materials that undergo volume-displacing phase transformations [19– 24]. Despite the differing mechanisms associated with these two processes, both generate a differential strain between the layers, D3, that results in the development of residual stresses. The level and sign of the expected residual stresses can be evaluated using the simplified model of a symmetric plate constituted by alternated layers with a biaxial distribution of stresses constant across the layers [25]. The level of the stresses in the center of the layers that constitute the layered structure are given by sa ZK D3E0 a 1CðE0 anata=E0 anbtbÞ (2) sb ZKsa nata nbtb (3) where the subscript ‘a’ and ‘b’ refers to the alternating layers, ni and ti are the number and thickness of the layers of each composition, respectively, and E0 i is given by E0 i Z Ei 1Kni (4) where Ei and ni are the Young’s modulus and the Poisson ratio of the layers. The strain mismatch, D3, is calculated as follows D3 Z ðTs Tr ðabKaaÞdT CD3t (5) where, ai are the coefficients of thermal expansion of the layers, Ts is the temperature above which mass transport occurs in the material, and Tr the room temperature, and D3t is the strain mismatch between the layers due to processes different from thermal expansion, such as phase transformation. The fracture behavior phenomena associated to the residual stresses of laminates are bifurcation [15,19,20] surface strengthening [16,21] and threshold strength [17,22]. All these mechanisms are associated to the development of residual compressive/tensile stresses in alternated layers. Usually, it is desired the relative thickness of the compressed 500 A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508

Table the stacking sequence [5, 24, 47]. Usually, the tapes are of colloidal processing methods used for the fabrication of ceramic fabricated with thermoplastic binders and plasticizers in an organic media and pressed at a temperature close to the melting Method temperature(20-120C)of the tape additives. Nowadays, the tendency in production methods is to use water-based AlO AlTiOs [81 formulations, for economic reasons as well as in order to AlgO LaPO4 41 avoid environmental and safety problems derived from the use Al2O3ZnO2[15,23,2446 MoSi2/AlO3(40 systems for tape casting makes tapes to be more prone to YPO,/ZrO2AlO3 [411 cracking during drying, because of the evaporation of water is B.C/graphite [12] slower than that of organics. In order to overcome this problem, SiC/SiC [121 the optimization of the slurry in terms of high solid content is required. High solids content reduces the amount of water to be Al2O3ZrO2[21,28,30,42 Al2O,/LaPO4 [431 evaporated and, consequently the tendency of the tape to AlyOJAl2TiOs [44) cracking [48]. With water-based tapes, the studies related to the Sic/graphite[291 lamination process are not as extensive as in the case of A⊥2OZrO2[31,32 organic-based tapes. Some works describing the fabrication of SiC/TiC 145 compacts from lamination of water-based tapes can be found in SiC/graphite [461 the literature [51, 52]. In these cases, the interfaces should be completely removed by the applied pressure [53] In this paper, a method to obtain multilayer ceramics from layer to be as thin as possible in order to minimize the residual water-based green tapes using a gluing agent for stacking and tensile stresses developed in the adjacent layer. pressing at room temperature is described. The criteria for The control of the residual stresses and, consequently, the selecting the mechanical conditions to ensure the joining reinforcing mechanism, is achieved by a strict control of the between tapes are presented. After this the green density of the composition and thickness of the layers that have to be layers is adjusted in order to avoid cracks associated with the designed for an optimum behavior. In this sense, the fabrication differential sintering between the layers. This method of of multilayer ceramics by colloidal processing techniques has adjusting the green density of the layers has been reported to been widely used due its versatility and reliability. These ensure the fabrication of metal-ceramic graded materials with methods have the advantages that allow to strictly control the very different sintering behavior [54, 55]. Following an composition and, depending on the technique, the thickness of optimized procedure, an alumina-alumina/zirconia crack free the layers. The colloidal processing techniques described for multilayer ceramic has been obtained the fabrication of laminated ceramic include tape-castin [6, 11, 18, 24], centrifugal casting [26, 27], sequential Sp- 2. Experimental asting [17, 28-30), electrophoretic deposition(EPD)[31, 32] and others (e.g. [33-35]). All of them are based on the preparation of stable slurries with specific compositions that 2. 1. Preparation of tapes are piled up by adding a layer to a previously formed one. Tapes were cast from stable slurries of high purity a- Stable slurries that ensure a homogenous and well-dispersed Al2O3 and Y-TZP powders in deionized water as dispersing composition are obtained by controlling the interparticle media. Table 2 shows the main characteristics(particle size potentials developed within the liquid media [36-38]. The thickness is controlled by controlling the processing parameter used specific surface area and density) of the starting powders associated to the technique(casting time [30,31], blades gap Slurries were prepared by mixing the powders with the [6], amount of slurry [26], etc. ) Finally, the green layers that water containing a 0.8 wt%, referred to solids, of a form the laminate are co-sintered polyelectrolyte(Dolapix CE schimmer and Schwarz illustrates the versatility of three of the above- Germany) used as dispersant. After mixing, the slurries were mentioned colloidal processing methods for the fabrication of ball milled during 4 h in alumina jars using alumina ball laminated ceramics. Studies dealing with the processing and behavior of laminar ceramics for non-structural Table 2 applications (e.g. microelectronics packaging) are beyond the Characteristics of the powders used for preparing the tapes scope of this work Tape casting is one of the methods that more extensively has Powder(manufacturer) dso(um) SSA (g m2) p(g/cm attractive due to its suitability for mass production and its USA/( Condea HPA 0.5, 0.3 design ability for different layered structures by varying the Tos H ol Y 203)(123Y5, 0.4 individual layer composition and thickness as well as

layer to be as thin as possible in order to minimize the residual tensile stresses developed in the adjacent layer. The control of the residual stresses and, consequently, the reinforcing mechanism, is achieved by a strict control of the composition and thickness of the layers that have to be designed for an optimum behavior. In this sense, the fabrication of multilayer ceramics by colloidal processing techniques has been widely used due its versatility and reliability. These methods have the advantages that allow to strictly control the composition and, depending on the technique, the thickness of the layers. The colloidal processing techniques described for the fabrication of laminated ceramic include tape-casting [6,11,18,24], centrifugal casting [26,27], sequential slip￾casting [17,28–30], electrophoretic deposition (EPD) [31,32], and others (e.g. [33–35]). All of them are based on the preparation of stable slurries with specific compositions that are piled up by adding a layer to a previously formed one. Stable slurries that ensure a homogenous and well-dispersed composition are obtained by controlling the interparticle potentials developed within the liquid media [36–38]. The thickness is controlled by controlling the processing parameter associated to the technique (casting time [30,31], blades gap [6], amount of slurry [26], etc.). Finally, the green layers that form the laminate are co-sintered. Table 1 illustrates the versatility of three of the above￾mentioned colloidal processing methods for the fabrication of structural laminated ceramics. Studies dealing with the processing and behavior of laminar ceramics for non-structural applications (e.g. microelectronics packaging) are beyond the scope of this work. Tape casting is one of the methods that more extensively has been used for producing multilayer ceramics. It is very attractive due to its suitability for mass production and its design ability for different layered structures by varying the individual layer composition and thickness as well as the stacking sequence [5,24,47]. Usually, the tapes are fabricated with thermoplastic binders and plasticizers in an organic media and pressed at a temperature close to the melting temperature (20–120 8C) of the tape additives. Nowadays, the tendency in production methods is to use water-based formulations, for economic reasons as well as in order to avoid environmental and safety problems derived from the use of organics [48–50]. Unfortunately, the use of water-based systems for tape casting makes tapes to be more prone to cracking during drying, because of the evaporation of water is slower than that of organics. In order to overcome this problem, the optimization of the slurry in terms of high solid content is required. High solids content reduces the amount of water to be evaporated and, consequently the tendency of the tape to cracking [48]. With water-based tapes, the studies related to the lamination process are not as extensive as in the case of organic-based tapes. Some works describing the fabrication of compacts from lamination of water-based tapes can be found in the literature [51,52]. In these cases, the interfaces should be completely removed by the applied pressure [53]. In this paper, a method to obtain multilayer ceramics from water-based green tapes using a gluing agent for stacking and pressing at room temperature is described. The criteria for selecting the mechanical conditions to ensure the joining between tapes are presented. After this the green density of the layers is adjusted in order to avoid cracks associated with the differential sintering between the layers. This method of adjusting the green density of the layers has been reported to ensure the fabrication of metal–ceramic graded materials with very different sintering behavior [54,55]. Following an optimized procedure, an alumina–alumina/zirconia crack free multilayer ceramic has been obtained. 2. Experimental 2.1. Preparation of tapes Tapes were cast from stable slurries of high purity a￾Al2O3 and Y-TZP powders in deionized water as dispersing media. Table 2 shows the main characteristics (particle size, specific surface area and density) of the starting powders used. Slurries were prepared by mixing the powders with the water containing a 0.8 wt%, referred to solids, of a polyelectrolyte (Dolapix CE 64, Zschimmer and Schwarz, Germany) used as dispersant. After mixing, the slurries were ball milled during 4 h in alumina jars using alumina balls. Table 1 Examples of colloidal processing methods used for the fabrication of ceramic laminates Method Layers compositions Tape casting Al2O3/Al2O3 [7,11] Al2O3/Al2TiO5 [8] Al2O3/LaPO4 [41] Al2O3/ZrO2 [15,23,24,46] Mullite/SiC [39] MoSi2/Al2O3 [40] YPO4/ZrO2/Al2O3 [41] B4C/graphite [12] SiC/SiC [12] Slip casting Al2O3/Al2O3 [13] Al2O3/ZrO2 [21,28,30,42] Al2O3/LaPO4 [43] Al2O3/Al2TiO5 [44] SiC/graphite [29] Electrophoretic deposition Al2O3/ZrO2 [31,32] SiC/TiC [45] SiC/graphite [46] Table 2 Characteristics of the powders used for preparing the tapes Powder (manufacturer) d50 (mm) SSA (g m2 ) r (g/cm3 ) a-Al2O3 (Condea HPA 0.5, USA) 0.3 9.5 3.88 ZrO2 (3 mol% Y2O3) (TZ3YS, TOSOH, Japan) 0.45 6.7 6.04 A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508 501

502 A.J. Sanchez Herencia et aL. / Composites: Part B 37(2006)499-508 Two compositions were formulated containing 95 vol% of Table 3 -Al2 O3 and 5 vol% of Y-TZP (named A-5YTZP) and Starting solid loading of the slurries and green density of the tapes used to 60 vol% of a-Al2O3 and 40 vol% of Y-TZP(named bricate the final compacts A-40YTZP) For each composition, two slurries with different Sample Starting solid loading Green density (th. %) solid content loading(solid content of 47 and 50 vol%)were (vol%) A-5YTZP(1) As tape casting additive a water-based polymeric emulsion A-5YTZP(2) 6.2±0.1 (Mowilith DM 765 E Celanese, Spain ) with a T of-6C and A-40YTZP(I) 55.1±0.1 solid content 50 vol%, particle size 0.05-0.15 um was added in A-4OYTZP(2) 53.5±0.1 a concentration of 5 wt% referred to solids. Tapes were cast using a moving carrier with a blades gap of 500 um. Full details of slurry preparation and tape casting procedure ar and minimum values. For lower pressures(up to 20 MPa) given elsewhere [53, 56] five samples were analyzed, in this cases error bars After casting the green ceramic tapes were dried in air for corresponded to the standard deviations. Green densities of 4 h, to further drying at 60C for 48 h. The final thickness of obtained specimens were determined by the Archimedes the green tapes obtained varied between 480 and 520 um. method in mercury, using five dry pieces fabricated under to those of 2. 2. Lamination of tapes dry green tapes. Reported values are the average of the five values and errors are the standard deviations Relative green Round shaped tapes(diameters 0 26 mm and 0 60 mm) densities were calculated as percent of the calculated were used to avoid heterogeneous stress distribution within the theoretical density for each composition, using 3.99 g/cm ecimens during pressing [56]. Green densities of the single for a-Al2O3(ASTM 42-1468)and 6.10 g/em' for Y-TZP tapes were determined using the geometrical method Reported (ASTM 83-113) values are the average of those obtained for four discs of 26 mm diameter processed under nominally identical Laminated and monolithic compacts with seven layers were a)100 obtained by piling up tapes with a gluing agent between them and further pressing. Before lamination some of the tapes were subjected tower pretreatment that consists of dipping the ceramic tape in distilled water during 1 min. The gluing agent consisted in a 5 wt%o dilution of the binder used in the formulation of the starting slurries(Mowilith DM 765 E). Full details of the gluing agent selection can be found elsewhere 王王王 [56 Symmetrical laminated samples were stacked in a way to keep the A-5YTZP composition tapes as outer layers. Obtained green laminates were co unlax 00.050.100.150.200250.300 a universal testing machine(Microtest SA, Spain) with steel compression plates. In order to avoid friction with the plates, the stacked pieces were placed between two sheets (b)300 of polypropylene film. The pressure was applied using a load frame displacement rate of 0.05 mm/min. The load and the displacement of the load frame were recorded during the essing process and engineering stress-apparent strain curves were calculated assuming uniaxial compression sing the dimensions of the pieces. The safe pressure interval was determined by analyzing the curves correspond ing to nominally identical samples, adjusted to third degree represent the behavior of the samples under the pressure 50 samples a Corresponding to two individual(@ 26 mm) The curves 00.020.040060080.100.12 each composition and pre-treatment recorded for samples pressed up to the maximum stress Fig. 1. Characteristic behavior of dry and wet samples made of stacked A values(=90 MPa). The corresponding stress-strain behavior 5YTZP(1)tapes during pressing. (a) Average polynomial curves for each was represented by the average of the corresponding treatment(error bars represent the upper and lower curves).(b) Derivative of polynomial fits and error bars indicated the maximum the average polynomial curves at low pressures

Two compositions were formulated containing 95 vol% of a-Al2O3 and 5 vol% of Y-TZP (named A-5YTZP) and 60 vol% of a-Al2O3 and 40 vol% of Y-TZP (named A-40YTZP). For each composition, two slurries with different solid content loading (solid content of 47 and 50 vol%) were prepared. As tape casting additive a water-based polymeric emulsion (Mowilith DM 765 E Celanese, Spain), with a Tg of K6 8C and solid content 50 vol%, particle size 0.05–0.15 mm was added in a concentration of 5 wt% referred to solids. Tapes were cast using a moving carrier with a blades gap of 500 mm. Full details of slurry preparation and tape casting procedure are given elsewhere [53,56]. After casting the green ceramic tapes were dried in air for 24 h, to further drying at 60 8C for 48 h. The final thickness of the green tapes obtained varied between 480 and 520 mm. 2.2. Lamination of tapes Round shaped tapes (diameters : 26 mm and : 60 mm) were used to avoid heterogeneous stress distribution within the specimens during pressing [56]. Green densities of the single tapes were determined using the geometrical method. Reported values are the average of those obtained for four discs of 26 mm diameter processed under nominally identical conditions. Laminated and monolithic compacts with seven layers were obtained by piling up tapes with a gluing agent between them and further pressing. Before lamination some of the tapes were subjected to ‘wet’ pretreatment that consists of dipping the ceramic tape in distilled water during 1 min. The gluing agent consisted in a 5 wt% dilution of the binder used in the formulation of the starting slurries (Mowilith DM 765 E). Full details of the gluing agent selection can be found elsewhere [56]. Symmetrical laminated samples were stacked in a way to keep the A-5YTZP composition tapes as outer layers. Obtained green laminates were cold uniaxially pressed using a universal testing machine (Microtest SA, Spain) with steel compression plates. In order to avoid friction with the plates, the stacked pieces were placed between two sheets of polypropylene film. The pressure was applied using a load frame displacement rate of 0.05 mm/min. The load and the displacement of the load frame were recorded during the pressing process and engineering stress–apparent strain curves were calculated assuming uniaxial compression using the dimensions of the pieces. The safe pressure interval was determined by analyzing the curves correspond￾ing to nominally identical samples, adjusted to third degree polynomials, and the average of the curves was used to represent the behavior of the samples under the pressure. The curves corresponding to two individual (: 26 mm) samples of each composition and pre-treatment were recorded for samples pressed up to the maximum stress values (z90 MPa). The corresponding stress–strain behavior was represented by the average of the corresponding polynomial fits and error bars indicated the maximum and minimum values. For lower pressures (up to 20 MPa) five samples were analyzed, in this cases error bars corresponded to the standard deviations. Green densities of obtained specimens were determined by the Archimedes method in mercury, using five dry pieces fabricated under nominally identical conditions, and compared to those of the dry green tapes. Reported values are the average of the five values and errors are the standard deviations. Relative green densities were calculated as percent of the calculated theoretical density for each composition, using 3.99 g/cm3 for a-Al2O3 (ASTM 42-1468) and 6.10 g/cm3 for Y-TZP (ASTM 83-113). Table 3 Starting solid loading of the slurries and green density of the tapes used to fabricate the final compacts Sample Starting solid loading (vol%) Green density (th.%) A-5YTZP(1) 50 59.1G0.1 A-5YTZP(2) 47 56.2G0.1 A-40YTZP(1) 50 55.1G0.1 A-40YTZP(2) 47 53.5G0.1 Fig. 1. Characteristic behavior of dry and wet samples made of stacked A- 5YTZP(1) tapes during pressing. (a) Average polynomial curves for each treatment (error bars represent the upper and lower curves). (b) Derivative of the average polynomial curves at low pressures. 502 A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508

A.J. Sanchez-Herencia et al /Composites: Part B 37(2006)499-508 23. Sintering and characterization observation were chemically etched with HF (40 vol%) for 15min(20°C Large green samples (0 60 mm)were machined into bars( approximately50mm×7mm×4.lmm) and the surfaces were smoothed with sandpaper. Binder burn out 3 Results and discussion and sintering were performed in single thermal treatment cycle. The binder burn out was carried out with heating at 3.1 Stacking of green tapes the rate of 1 C/min up to 600C, with a dwell time of Right after this the sintering was carried out by Two series of tapes of each composition were cast sing different solid loading in the starting slurries table 3 increasing the temperature with heating rate of 5 C/min up shows how, for each composition, the green density to 1550C with a dwell time of 2 h. The dilatometer curves increases with the dlid loading he slurry. The slurry were recorded using green monolithic samples (5 mm X 5 mm)in a dilatometer with alumina support tion capability with the largest alumina content presented better compac (Setaram, Setsys-16/17, France) and corrected for alumina In order to identify the defects associated with the stacking expansion. Density values of sintered pieces were deter- ocedure, with no interferences of sintering related effects, the mined by the Archimedes method in distilled water and stacking of green tapes of the same composition was first relative densities were calculated as described for green investigated. Once optimized, the same method was applied to densities fabricate the laminate Cross-sections of the sintered samples were polished using The engineering stress-strain curves corresponding to the diamond paste down to I um. Microstructural observations compression of six wet(solid line)and six dry(dotted line) were carried out using a reflected light optical microscopy A-5YTZP stacked tapes is shown in Fig. 1. It is clearly Carl-Zeiss H-Pl, Germany) and scanning electron microscopy observed the extreme differences between the behavior (Zeiss DSM-950, Germany). Samples for tunnel type crack wet and dry samples. Fig. la shows that the stacked (c) 2. Micrographs of the dry and w les made of stacked A-5YTZP()tapes after pressing(a) Macroscopic aspect of a dry sample pressed at 30 MPa(b) scopic aspect of a wet sample pressed at 90 MPa. Cross-sections corresponding to dry samples(c)and wet samples(d)after pressing at 10 MPa

2.3. Sintering and characterization Large green samples (: 60 mm) were machined into bars (approximately 50 mm!7 mm!4.1 mm) and the surfaces were smoothed with sandpaper. Binder burn out and sintering were performed in single thermal treatment cycle. The binder burn out was carried out with heating at the rate of 1 8C/min up to 600 8C, with a dwell time of 30 min. Right after this the sintering was carried out by increasing the temperature with heating rate of 5 8C/min up to 1550 8C with a dwell time of 2 h. The dilatometer curves were recorded using green monolithic samples (5 mm! 5 mm!4 mm) in a dilatometer with alumina support (Setaram, Setsys-16/17, France) and corrected for alumina expansion. Density values of sintered pieces were deter￾mined by the Archimedes method in distilled water and relative densities were calculated as described for green densities. Cross-sections of the sintered samples were polished using diamond paste down to 1 mm. Microstructural observations were carried out using a reflected light optical microscopy (Carl-Zeiss H-P1, Germany) and scanning electron microscopy (Zeiss DSM-950, Germany). Samples for tunnel type crack observation were chemically etched with HF (40 vol%) for 15 min (20 8C). 3. Results and discussion 3.1. Stacking of green tapes Two series of tapes of each composition were cast using different solid loading in the starting slurries. Table 3 shows how, for each composition, the green density increases with the solid loading in the slurry. The slurry with the largest alumina content presented better compac￾tion capability. In order to identify the defects associated with the stacking procedure, with no interferences of sintering related effects, the stacking of green tapes of the same composition was first investigated. Once optimized, the same method was applied to fabricate the laminate. The engineering stress–strain curves corresponding to the compression of six wet (solid line) and six dry (dotted line) A-5YTZP stacked tapes is shown in Fig. 1. It is clearly observed the extreme differences between the behavior of wet and dry samples. Fig. 1a shows that the stacked wet Fig. 2. Micrographs of the dry and wet samples made of stacked A-5YTZP(1) tapes after pressing. (a) Macroscopic aspect of a dry sample pressed at 30 MPa. (b) Macroscopic aspect of a wet sample pressed at 90 MPa. Cross-sections corresponding to dry samples (c) and wet samples (d) after pressing at 10 MPa. A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508 503

A.J. Sanchez- Herencia et al./Composites: Part B 37(2006)499-508 Table 4 ensity of A-5YTZP samples obtained by pressing stacked wet tapes at different pressures Pressure(MPa) Density (g/cm) 2.38±0.01 980±04 39±0.01 84士04 058 2.40±0.01 2.43±0.01 1000±04 2.44±0.001 Density is indicated as the percentage of a simple tape density tapes sustained higher pressures than the dry ones and also 00050.100.150200.250300.35 maintain an increasing slope for all the pressures. From about 30 MPa, increasing strains did not lead to stress increase in the dry samples, and variability increased. This is the typical behavior for a fractured sample where the stress is released by the opening of cracks. The maximum 80 allowed pressure for working without cracking can be evaluated by analyzing the maximum in the derivative of the stress-strain curves in the low pressure regime(Fig. 1b) which will correspond to the inflexion point for the stress- strain behavior. From figure, the strain limit is 0.07, which corresponds to a pressure of 13 MPa On the other hand, the wet sample does not present a maximum in the derivative curve, meaning that no fracture nor important structural changes happened during the pressing process within the range studied for these samples (up to 00.050.100.150.200.250.300.35 90 MPa) These changes revealed in the strain-stress curves can be clearly observed in the microscopical analysis after pressing. Fig 3. Strain-stress behavior f Ariz el An+or TzP( laye amp Fig. 2a shows clearly the macroscopic failure of a Polynomial fit curve of experimental curves(error bars represent the standard obtained using dry tapes after pressed at 30 MPa deviation).(b) Derivative of the polynomial fit curve failure is a consequence of the low deformation capability of dry samples and did not occur in samples obtained using wet tapes even after pressed at 90 MPa(Fig. 2b). Fig. 2c will be the one that allows the achievement of a maximum and d show a micrograph for the cross-section of samples interface contact without producing microstructural changes obtained with dry and wet tapes pressed below the joining inside the individual layers. In order to determine this pressure(10 MPa). In this picture, the fact that dry tapes pressure, the green density of the compacts was compared developed cracks across the tapes before joining to other with that of the single tapes. Stacked tapes shall be joined can be also clearly observed. On the other hand, when no porosity is between layers, at this point the density did not develop these cracks perpendicular to of the stack of tapes should be the same that the density of layers direction. In these samples, the lack of complete a single tape. On Table 4, it is indicated the green density joining was revealed by the presence of linear arrangements of A-5YTZP tapes as a function of the single tape density of pores along the interfaces (Fig. 2d). From these obtained. It can be observed that the zero interlayer porosity complementary observations (strain-stress curves and was achieved for pressure values over 15 MPa. microscopy after pressing) it can be determined from From the above-discussed results. it has been stated for tructural point of view that water-based green tapes can be monolithic samples that piling up and pressing wetted water- safely pressed and joined at relatively higher pressures if based tapes the fabrication of defect-free compacts at room they are pre-treated by soaking in water before stacking and temperature is possible. The same process was followed to pressing. fabricate four monoliths and two laminates. The four Once established that appropriate pre-treatment of the monolithic compacts obtained by stacking the four different tapes(wetting) generate a wide safe pressure interval, tapes described in Table 3 were: two A-5YTZP compacts with optimized pressure for adhesion between the layers was green densities of 59. 4 and 56 1 th. and two A-40YTZF studied by analyzing the evolution of the green density of compacts with green densities of 55.1 and 53.5 th. % the compacts after pressing. In principle, optimum pressure Laminates were composed by alternating A-5YTZP

tapes sustained higher pressures than the dry ones and also maintain an increasing slope for all the pressures. From about 30 MPa, increasing strains did not lead to stress increase in the dry samples, and variability increased. This is the typical behavior for a fractured sample where the stress is released by the opening of cracks. The maximum allowed pressure for working without cracking can be evaluated by analyzing the maximum in the derivative of the stress–strain curves in the low pressure regime (Fig. 1b), which will correspond to the inflexion point for the stress– strain behavior. From figure, the strain limit is 0.07, which corresponds to a pressure of 13 MPa. On the other hand, the wet sample does not present a maximum in the derivative curve, meaning that no fracture nor important structural changes happened during the pressing process within the range studied for these samples (up to 90 MPa). These changes revealed in the strain–stress curves can be clearly observed in the microscopical analysis after pressing. Fig. 2a shows clearly the macroscopic failure of a sample obtained using dry tapes after pressed at 30 MPa. This failure is a consequence of the low deformation capability of dry samples and did not occur in samples obtained using wet tapes even after pressed at 90 MPa (Fig. 2b). Fig. 2c and d show a micrograph for the cross-section of samples obtained with dry and wet tapes pressed below the joining pressure (10 MPa). In this picture, the fact that dry tapes developed cracks across the tapes before joining to other tapes can be also clearly observed. On the other hand, wet tapes did not develop these cracks perpendicular to the layers direction. In these samples, the lack of complete joining was revealed by the presence of linear arrangements of pores along the interfaces (Fig. 2d). From these complementary observations (strain–stress curves and microscopy after pressing) it can be determined from a structural point of view that water-based green tapes can be safely pressed and joined at relatively higher pressures if they are pre-treated by soaking in water before stacking and pressing. Once established that appropriate pre-treatment of the tapes (wetting) generate a wide safe pressure interval, optimized pressure for adhesion between the layers was studied by analyzing the evolution of the green density of the compacts after pressing. In principle, optimum pressure will be the one that allows the achievement of a maximum interface contact without producing microstructural changes inside the individual layers. In order to determine this pressure, the green density of the compacts was compared with that of the single tapes. Stacked tapes shall be joined when no porosity is between layers, at this point the density of the stack of tapes should be the same that the density of a single tape. On Table 4, it is indicated the green density of A-5YTZP tapes as a function of the single tape density obtained. It can be observed that the zero interlayer porosity was achieved for pressure values over 15 MPa. From the above-discussed results, it has been stated for monolithic samples that piling up and pressing wetted water￾based tapes the fabrication of defect-free compacts at room temperature is possible. The same process was followed to fabricate four monoliths and two laminates. The four monolithic compacts obtained by stacking the four different tapes described in Table 3 were: two A-5YTZP compacts with green densities of 59.4 and 56.1 th.% and two A-40YTZP compacts with green densities of 55.1 and 53.5 th.%. Laminates were composed by alternating A-5YTZP Fig. 3. Strain–stress behavior of A-5YTZP(2)/A-40YTZP(1) layered sample (diameter :Z60 mm) up to the selected fabrication pressure (18 MPa). (a) Polynomial fit curve of experimental curves (error bars represent the standard deviation). (b) Derivative of the polynomial fit curve. Table 4 Density of A-5YTZP samples obtained by pressing stacked wet tapes at different pressures Pressure (MPa) Density (g/cm3 ) rpiece/rsingle tape 3 2.38G0.01 98.0G0.4 5 2.39G0.01 98.4G0.4 10 2.40G0.01 98.8G0.3 15 2.43G0.01 100.0G0.4 18 2.44G0.001 100.0G0.1 Density is indicated as the percentage of a simple tape density. 504 A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508

A.J. Sanchez-Herencia et al /Composites: Part B 37(2006)499-508 a)000长x 0.0 0.10 0.15 0.20 A-5YTzP(1)「D=59.4%) A-40YTZP(2)(TD=535% 8009001000110012001300140015 T(C) (b)0.00 -2.0x10 -0.10 40×x104与 0.15 A5YTZP(2)(TD=56.1% -025 800900100011001200130014001500 T(°c) Sintering behavior of A-5YTZP and A-40YTZP monolithic samples. Dilatometric cuves and sintering kinetics for(a) samples with different densities(PA- and(b)samples with similar densities(PA-SYTZP(2)=PA-40YTZP(D)) and A-40YTZP tapes with different green densities (PA- the sintering behavior is very influenced by the packing in 5YTZP()+PA-40YTZP(2) and similar green densities (PA- the green stage. Fig. 4 shows the linear shrinkage and the 5YTZP(2)-PA-40YTZP(I sintering rate curves recorded for the monoliths of the same Fig. 3 shows the strain-stress curves for the layered compositions as those of the layers in the laminates. In material (PA-5YTZP()+PA-40YTZP(2). It can be seen that Fig. 4a, curves for materials with very different green strains as high as 30% can be achieved by working with densities are plotted. It can be seen that sintering started at wet tapes and no inflexion point is detected indicating that similar temperature for both materials, and that the the sample maintains its structural integrity in all the range dilatometric curves run clos to1150°C. After this of pressures. Very similar behavior was observed for the temperature, they shrank in a different way giving a final laminate made with tapes with similar green densities. For shrinkage of 17% for A-5YTZP(I) and 22% for the monolithic and laminated materials described above, a A-40YTZP(2). Also the maximum sintering rate wa ressure of 18 MPa was selected using a 5 wt% dilution of achieved at different temperatures (1400Cfor binder as a gluing agent, as it has been appointed from A-40YTZP(2)and 1460C for A-5YTZP(I). On the other Table 4 hand, the samples with similar green densities(Fig. 4b showed different sintering behavior at low temperatures but 3. 2. Sintering optimization of the laminate finally they achieved similar shrinkages at 1350C, and continued together until a final shrinkage around 20% was The fabrication of multilayer ceramics from powder reached. Also the maximum sintering rate was located at processing techniques involves the co-sintering of green close temperatures(1410C for A-40YTZP(1) and 1425C joined layers. This is a critical point in order to obtain crack for A-5YTZP(2)). The differences in the shrinkage and free layers. Different compositions sinter in different ways sintering rate of the different layers are made evident in the and stresses can be developed between layers if shrinkage microstructure of the samples. Fig. 5a shows a tunneling levels and rates are very different. In this sense, crack developed in a A-40YTZP(2)layer during sintering of

and A-40YTZP tapes with different green densities (rA- 5YTZP(1)srA-40YTZP(2)) and similar green densities (rA- 5YTZP(2)zrA-40YTZP(1)). Fig. 3 shows the strain–stress curves for the layered material (rA-5YTZP(1)srA-40YTZP(2)). It can be seen that strains as high as 30% can be achieved by working with wet tapes and no inflexion point is detected indicating that the sample maintains its structural integrity in all the range of pressures. Very similar behavior was observed for the laminate made with tapes with similar green densities. For the monolithic and laminated materials described above, a pressure of 18 MPa was selected using a 5 wt% dilution of binder as a gluing agent, as it has been appointed from Table 4. 3.2. Sintering optimization of the laminate The fabrication of multilayer ceramics from powder processing techniques involves the co-sintering of green joined layers. This is a critical point in order to obtain crack free layers. Different compositions sinter in different ways and stresses can be developed between layers if shrinkage levels and rates are very different. In this sense, the sintering behavior is very influenced by the packing in the green stage. Fig. 4 shows the linear shrinkage and the sintering rate curves recorded for the monoliths of the same compositions as those of the layers in the laminates. In Fig. 4a, curves for materials with very different green densities are plotted. It can be seen that sintering started at similar temperature for both materials, and that the dilatometric curves run close up to 1150 8C. After this temperature, they shrank in a different way giving a final shrinkage of 17% for A-5YTZP(1) and 22% for A-40YTZP(2). Also the maximum sintering rate was achieved at different temperatures (1400 8C for A-40YTZP(2) and 1460 8C for A-5YTZP(1)). On the other hand, the samples with similar green densities (Fig. 4b) showed different sintering behavior at low temperatures but finally they achieved similar shrinkages at 1350 8C, and continued together until a final shrinkage around 20% was reached. Also the maximum sintering rate was located at close temperatures (1410 8C for A-40YTZP(1) and 1425 8C for A-5YTZP(2)). The differences in the shrinkage and sintering rate of the different layers are made evident in the microstructure of the samples. Fig. 5a shows a tunneling crack developed in a A-40YTZP(2) layer during sintering of Fig. 4. Sintering behavior of A-5YTZP and A-40YTZP monolithic samples. Dilatometric cuves and sintering kinetics for (a) samples with different densities (rA- 5YTZP(1)srA-40YTZP(2)) and (b) samples with similar densities (rA-5YTZP(2)zrA-40YTZP(1)). A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508 505

A.J. Sanchez Herencia et aL. / Composites: Part B 37(2006)499-508 ←,→4 -6n Fig. 6. Scheme of the stresses developed during sintering into the layers. (a Fig. 5. Micrographs of the polished cross-section of A-5YTZP/A-40YTZP The layer subjected to tensile stress generates a crack, (b)crack opening and minates with(a)different densities(PA. 5YTZP()*PA- 40YTZP2)and(b)similar tress state changes as the sintering goes forward and (c)tensile stresses extends crack in the adjacent layers a multilayer with PA-5YTZP(1+PA-40YTZP(). As it can be Tunneling cracks has been described to appear due to the observed in Fig. 4, this layer shrank more than tensile stresses that associated layers will generate when a A-5YTZP(1). This differential shrinkage would differential strain between layers is present [28,57]. In the develop inside the layers a stresses system as the one laminates here described differential strain can arise from two represented in Fig. 6, where the A-40YTZP(2)layer is different mechanism, one the differential shrinkages and other under tension while the A-5YTZP(1) is under compression. the difference in the thermal expansion coefficients between If tension stress overcomes the tensile fracture strength layers. To check the fact that cracks were developed during of the layer at the given temperature, a tunneling crack sintering and not during cooling from the sintering temperature will be formed, as schematized in Fig. 6a. As the due to differences in the thermal expansion of the layers, a differential shrinkage continues, two processes would close up to the cracks was made by SEM. Fig. 7 shows the occur. First, the opening of the crack in the layer cross-section of a sample polished and chemically etched (HF (Fig. 6b)and, second, the change in the stresses state in 15 min). In Fig. 7a, the crack in the A-40YTZP(2) layer is the adjacent layers with tension instead of compression in presented. It can be observed how the grains located at the edge the crack tip(Fig. 6c). This can be enough to produce small of the crack had round and smooth surfaces, indicating that cracks on the adjacent layers, as extensions of the main crack underwent a thermal treatment after been created. If crack, which will propagate under the combination of cracks would be generated during cooling, due to thermal and compression; thus, their path will be not residual stresses, sharp edges would be observed. Moreover, a On the other hand, the multilayer shown in Fig. 5b low density linear zone extending into A-5YTZP()layers was will not stand sintering stresses because of the similar observed(Fig. 7b). This observation is justified at the light of shrinkage levels of both layers through the whole the tensile residual stresses, schemed in Fig. 6, that will oppose temperature interval to the sintering shrinkage in a very narrow zone giving an

a multilayer with rA-5YTZP(1)srA-40YTZP(2). As it can be observed in Fig. 4, this layer shrank more than A-5YTZP(1). This differential shrinkage would develop inside the layers a stresses system as the one represented in Fig. 6, where the A-40YTZP(2) layer is under tension while the A-5YTZP(1) is under compression. If tension stress overcomes the tensile fracture strength of the layer at the given temperature, a tunneling crack will be formed, as schematized in Fig. 6a. As the differential shrinkage continues, two processes would occur. First, the opening of the crack in the layer (Fig. 6b) and, second, the change in the stresses state in the adjacent layers with tension instead of compression in the crack tip (Fig. 6c). This can be enough to produce small cracks on the adjacent layers, as extensions of the main crack, which will propagate under the combination of tension and compression; thus, their path will be not straight. On the other hand, the multilayer shown in Fig. 5b will not stand sintering stresses because of the similar shrinkage levels of both layers through the whole temperature interval. Tunneling cracks has been described to appear due to the tensile stresses that associated layers will generate when a differential strain between layers is present [28,57]. In the laminates here described differential strain can arise from two different mechanism, one the differential shrinkages and other the difference in the thermal expansion coefficients between layers. To check the fact that cracks were developed during sintering and not during cooling from the sintering temperature due to differences in the thermal expansion of the layers, a close up to the cracks was made by SEM. Fig. 7 shows the cross-section of a sample polished and chemically etched (HF 15 min). In Fig. 7a, the crack in the A-40YTZP(2) layer is presented. It can be observed how the grains located at the edge of the crack had round and smooth surfaces, indicating that crack underwent a thermal treatment after been created. If cracks would be generated during cooling, due to thermal residual stresses, sharp edges would be observed. Moreover, a low density linear zone extending into A-5YTZP(1) layers was observed (Fig. 7b). This observation is justified at the light of the tensile residual stresses, schemed in Fig. 6, that will oppose to the sintering shrinkage in a very narrow zone giving an Fig. 5. Micrographs of the polished cross-section of A-5YTZP/A-40YTZP laminates with (a) different densities (rA-5YTZP(1)srA-40YTZP(2)) and (b) similar densities (rA-5YTZP(2)zrA-40YTZP(1)). Fig. 6. Scheme of the stresses developed during sintering into the layers. (a) The layer subjected to tensile stress generates a crack, (b) crack opening and stress state changes as the sintering goes forward and (c) tensile stresses extends crack in the adjacent layers. 506 A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508

A.J. Sanchez Herencia et al. Composites: Part B 37(2006)499-508 aspect similar to a crack partially healed into the A-5YTZP(I) Fig. 8 is the main conclusion of this work: dense multilayer ceramics with strong joining between layers and free of defects can be obtained by stacking water-based tapes and pressure less sintering. The use of wet tapes and a glue agent, with the same composition that the tape binder, avoids defects in the stacks due to the pressure and allows the use of higher pressures than with dry tapes. By controlling the solid content of the startin slurries, it is possible to control the final green density of the tapes and consequently the shrinkage and sintering rate of the layers that compose the laminate. The differential sintering between layers develops tunneling cracks that extend into the tapes that supposedly are under compression. If tapes with the appropriate green density are selected, stresses due to sinterin ell as the associated defects can be avoided This work has been supported by Comunidad de madrid project GRMAT 0707-2004 and by CiCYT project MAT2003- 00836 and partly by the Europan Communitys Human Potential programme under contract HPRN-CT-2002-00203 (SICMAC). Jonas Gurauskis acknowledges the financial upport provided by the Europan Communitys Human Potential programme under contract HPRN-CT-2002-00203 Fig. 7. SEM micrographs at the cross-section of laminated samples obtained (SICMAC) from tapes with different green densities showing(a) the aspect of tunneling crack in A-40YTZP(2) layer and( b) a close up to the low density zone into A-5YTZP(1)layer. References [1] Lange FF. Powder processing science and technology for increased liability. J Am Ceram Soc 1989:72(1): 3-15 [2 Weiner S, Addadi L. Design strategies in mineralized biological materials. J Mater Chem 1997: 7(5): 689-702. [3] Aksay IA, Sarikaya M. Bioinspired processing of composite materials. In: Ceramics toward the 21st century. Tokyo: The Ceramic Society of Japan 1991.p.136-9 [4] Pyzik AJ, Aksay IA. Microdesign of B4C-Al cermets. In: Proceeding of 'oPamic and metal matrix composites. New York, NY: Pergamon Press, [5] Schwartz B, Wilcox DL. Laminated ceramics Ceram Age 1967: 83(6 Ceram Soc Bull 1972: 51(5): 482-5 [7] Mistler RE. High-strength alumina substrates produced by a multiple- layer casting technique. Am Ceram Soc Bull 1973: 52(11): 850-4 [8 Russo C, Harmer MP, Chan HM, Miller GA Desig eramic composite for improved strength and toughness. J Am Ceram Soc 1992;75(12)3396-400 [9] Clegg wJ, Kendall K, Alford NM, Button Tw. Birchall JD. A simple way to make tough ceramics. Nature 1990: 347(6292): 455-7. 58m [10] Katsuki H, Ichinose H, Shiraishi A, Takagi H, Hirata Y. Preparation and fracture characteristic of laminated alumina mullite composite. Nippon Seram Kyo Gak1993:101(9):1068-70. 8. General view of a crack free A-5YTZP(2)A-40YTZP(1) laminate [11 Davis JB, Kristoffersson A, Carlstrom E, Clegg wJ. Fabrication and crack fuced by gluing, piling up and pressing wet layers obtained by a water- deflection in ceramic laminates with porous interlayers. J Am Ceram Soc 2000;83(10):2369-74

aspect similar to a crack partially healed into the A-5YTZP(1) layer. 4. Conclusions Fig. 8 is the main conclusion of this work: dense multilayer ceramics with strong joining between layers and free of defects can be obtained by stacking water-based tapes and pressure less sintering. The use of wet tapes and a glue agent, with the same composition that the tape binder, avoids defects in the stacks due to the pressure and allows the use of higher pressures than with dry tapes. By controlling the solid content of the starting slurries, it is possible to control the final green density of the tapes and consequently the shrinkage and sintering rate of the layers that compose the laminate. The differential sintering between layers develops tunneling cracks that extend into the tapes that supposedly are under compression. If tapes with the appropriate green density are selected, stresses due to sintering as well as the associated defects can be avoided. Acknowledgements This work has been supported by Comunidad de Madrid project GRMAT 0707-2004 and by CICYT project MAT2003- 00836 and partly by the Europan Community’s Human Potential Programme under contract HPRN-CT-2002-00203 (SICMAC). Jonas Gurauskis acknowledges the financial support provided by the Europan Community’s Human Potential Programme under contract HPRN-CT-2002-00203 (SICMAC). References [1] Lange FF. Powder processing science and technology for increased reliability. J Am Ceram Soc 1989;72(1):3–15. [2] Weiner S, Addadi L. Design strategies in mineralized biological materials. J Mater Chem 1997;7(5):689–702. [3] Aksay IA, Sarikaya M. Bioinspired processing of composite materials. In: Ceramics toward the 21st century. Tokyo: The Ceramic Society of Japan; 1991. p. 136–9. [4] Pyzik AJ, Aksay IA. Microdesign of B4C-Al cermets. In: Proceeding of ceramic and metal matrix composites. New York, NY: Pergamon Press; 1989. p. 169–80. [5] Schwartz B, Wilcox DL. Laminated ceramics. Ceram Age 1967;83(6): 40–4. [6] Ettre K, Castles GR. Pressure-fusible tapes for multilayer structures. Am Ceram Soc Bull 1972;51(5):482–5. [7] Mistler RE. High-strength alumina substrates produced by a multiple￾layer casting technique. Am Ceram Soc Bull 1973;52(11):850–4. [8] Russo CJ, Harmer MP, Chan HM, Miller GA. Design of a laminated ceramic composite for improved strength and toughness. J Am Ceram Soc 1992;75(12):3396–400. [9] Clegg WJ, Kendall K, Alford NM, Button TW, Birchall JD. A simple way to make tough ceramics. Nature 1990;347(6292):455–7. [10] Katsuki H, Ichinose H, Shiraishi A, Takagi H, Hirata Y. Preparation and fracture characteristic of laminated alumina mullite composite. Nippon Seram Kyo Gak 1993;101(9):1068–70. [11] Davis JB, Kristoffersson A, Carlstrom E, Clegg WJ. Fabrication and crack deflection in ceramic laminates with porous interlayers. J Am Ceram Soc 2000;83(10):2369–74. Fig. 8. General view of a crack free A-5YTZP(2)/A-40YTZP(1) laminate produced by gluing, piling up and pressing wet layers obtained by a water￾based tape casting. Fig. 7. SEM micrographs at the cross-section of laminated samples obtained from tapes with different green densities showing (a) the aspect of tunneling crack in A-40YTZP(2) layer and (b) a close up to the low density zone extension into A-5YTZP(1) layer. A.J. Sa´nchez-Herencia et al. / Composites: Part B 37 (2006) 499–508 507

508 A.J. Sanchez Herencia et aL. / Composites: Part B 37(2006)499-508 [12] Tariolle S Reynaud C. Thevenot F, Chartier T, Besson JL. Preparation, [35] Koh YH, Kim HW, Kim HE Mechanical properties of three-layered microstructure and mechanical properties of SiC-SiC and B4C-BC monolithic silicon nitride-fibrous silicon nitride/boron nitride monolith laminates. J Solid State Chem 2004: 177(2): 487- J Am Ceram Soc2002;85(11):2840-2. lamellar Al2O3 ceramics. Nippon Seram Kyo Gak 1988: 96(12): 1149-56 [14] He MY, Hutchinson Jw. Crack deflection at an interface between [37] Gutierrez CA, Moreno R Interparticle dissimilar elastic-materials. Int J Solids Struct 1989: 25(9): 1053-67 J Am Ceram Soc 2003: 86(1): 59-64 [15] Oechsner M, Hillman C, Lange FF. Crack bifurcation in laminar ceramic [38] Gutierrez CA, Moreno R. Inter ntials of aqueous Al2O3 es. J Am Ceram Soc 1996 79(7): 1834-8 suspensions. Br Ceram Trans 2003: 102(5): 219-24 [16 Yeh CH, Hon MH. 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