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 aslayer 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