R Bermejo et al. Composites Science and Technology 67(2007)1930-1938 due to the accommodation of strain mismatch by mass ponent as they oppose to crack growth [18, 19] and/or may transport mechanisms, as the temperature decreases, the develop a threshold strength(high reliability)[ll]. On the differences in the Cte (ai) should promote a differential other hand, tensile stresses should be subtracted to the strain between layers. In addition to this strain source, strength of the material, and if they overpass a critical other strain differences should be considered as those due value tunnelling cracks will appear and consequently the to phase transformations(Act[7, 10]or chemical reactions mechanical response will degrade [20]. Following these (Aer)[12]inside one particular layer. Hence, the final strain ideas, thin compressive layers are desirable, as they will cre difference between two given layers A and B, after cooling ate an additional reinforcement as well as diminish the from a reference temperature Trer(above which residual associated residual tensile stress. Moreover, the thickness stresses are negligible) down to a temperature T, may be of the layers are referred to other observations related to the residual stresses, such as edge crack and crack bifurca where AT indicates the difference in temperaure be (1) tion, for which a critical thickness"te"has to be surpassed △E=(x4-xB)△T+△a+Aer [3,16,21 the reference state(Tref) and the actual temperature i en In order to satisfy the particular design requirements the number, thickness and composition of the layers should be In ceramic laminates with strong interfaces the differ- controlled. In this regard, colloidal processing methods tion [7, 14] have been commonly used to develop residual aiming to manufacture reliable laminated structures. Those stresses. Regarding the latter, as zirconia cools down from methods include tape-casting [22-24], centrifugal casting the sintering temperature, it transforms from the tetragonal [25, 26], sequential slip casting [27] and electrophoretic to the monoclinic phase with an expansion of about deposition (EPD)[28, 29] among others.All these vol%. The magnitude of the transformation can be con- approaches are based on the preparation of stable slurries trolled by either adding small amounts of stabilizers like with specific compositions that are piled up by adding a Y2 O3 or Cao among others [lo] or varying the amount layer to a previously formed one Stable slurries that ensure of zirconia included inside the composite [15]. For the case a homogeneous and well-dispersed composition are of alumina-zirconia based ceramics, the zirconia expansion obtained by controlling the interparticle potentials devel inside the alumina matrix has been used as a stress devel- oped within the liquid media [30, 31]. The layer thickness oper to change the material fracture behav In this is controlled by adjusting the processing parameter associ sense, the crack propagation has been varied by tension- ated with the corresponding technique(e.g. casting time, compression states [14], crack bifurcation [16] or threshold blades gap, amount of slurry, etc ength development through compressive residual stres- In this work, multilayered materials composed of thick layers with low residual tensile stresses and thin internal For a multilayer system composed of n layers of compo- layers with high residual compressive stresses are fabricated sition A and thickness ta and (n-1)layers of composition by sequential slip casting, aiming to study the influence of B and thickness tb, the residual stress within each layer can the layers thickness on the fracture behaviour of the lay- be calculated by ered architecture. Processing parameters are optimised △E and layer thickness is controlled by adjusting the casting (2) time of stable slurries. The value of the residual stresses is tailored by varying the thickness of the thin layers. Mate- △Eb rials are tested by the indentation-strength method. M lithic specimens are used to collect data for residual stress evaluation and for comparative purposes where E=E/(I-vi), being E; the Young's modulus and vi the Poisson ratio of a given layer. The stress in one layer 2. Experimental is related to the stress in the adjacent one by Starting powders were submicron-sized Al2O3(HPA 0.5, 0b=-(n-1)hb (4) Condea, USA)with a mean particle size(dso) of 0.3 um, tetragonal zirconia polycrystals(Y-TZP)with 3 mol% of For the case of tb ta, then oa-0, i.e. if thin layers are Y2O3(TZ-3YS, Tosoh, Japan)with dso=0. 4 um, and pure inserted between thick ones, the stresses inside the latter are ZrO2(TZ-0, Tosoh, Japan) with dso=0.3 um. Slurries negligible. This suggests the fabrication of laminar ceram- were prepared to a solid loading of 36.5 vol% by mixing ics with thin layers subjected to high internal compressive starting powders with DI water, containing the desired stresses combined with thick layers exhibiting tensile resid- amount of a commercial acrylic based polyelectrolyte ual stresses whose effect on the final strength of the material (Duramax D-3021, Rohm& Haas, USA). For dispersion, slurries were ultrasonicated using a 400 w sonotrode It has been shown that compressive stresses are usually (UP400S, Hielscher, Germany) with magnetic stirring for beneficial for the mechanical performance of a given com- 2 min, and left stirring for at least 4 h before use. Colloidaldue to the accommodation of strain mismatch by mass transport mechanisms, as the temperature decreases, the differences in the CTE (ai) should promote a differential strain between layers. In addition to this strain source, other strain differences should be considered as those due to phase transformations (Det) [7,10] or chemical reactions (Der) [12] inside one particular layer. Hence, the final strain difference between two given layers A and B, after cooling from a reference temperature Tref (above which residual stresses are negligible) down to a temperature Ti, may be expressed as De ¼ ðaA aBÞDT þ Det þ Der ð1Þ where DT indicates the difference in temperature between the reference state (Tref) and the actual temperature Ti. In ceramic laminates with strong interfaces the differ￾ences in a [11,13] as well as the zirconia phase transforma￾tion [7,14] have been commonly used to develop residual stresses. Regarding the latter, as zirconia cools down from the sintering temperature, it transforms from the tetragonal to the monoclinic phase with an expansion of about 4 vol%. The magnitude of the transformation can be con￾trolled by either adding small amounts of stabilizers like Y2O3 or CaO among others [10] or varying the amount of zirconia included inside the composite [15]. For the case of alumina–zirconia based ceramics, the zirconia expansion inside the alumina matrix has been used as a stress devel￾oper to change the material fracture behaviour. In this sense, the crack propagation has been varied by tension– compression states [14], crack bifurcation [16], or threshold strength development through compressive residual stres￾ses [11,17]. For a multilayer system composed of n layers of compo￾sition A and thickness ta and (n 1) layers of composition B and thickness tb, the residual stress within each layer can be calculated by ra ¼ DeE0 a 1 þ E0 anta E0 bðn1Þtb ð2Þ rb ¼ DeE0 b 1 þ E0 bðn1Þtb E0 anta ð3Þ where E0 i ¼ Ei=ð1 miÞ, being Ei the Young’s modulus and mi the Poisson ratio of a given layer. The stress in one layer is related to the stress in the adjacent one by rb ¼ ra nta ðn 1Þtb ð4Þ For the case of tb ta, then ra ! 0, i.e. if thin layers are inserted between thick ones, the stresses inside the latter are negligible. This suggests the fabrication of laminar ceram￾ics with thin layers subjected to high internal compressive stresses combined with thick layers exhibiting tensile resid￾ual stresses whose effect on the final strength of the material is negligible. It has been shown that compressive stresses are usually beneficial for the mechanical performance of a given com￾ponent as they oppose to crack growth [18,19] and/or may develop a threshold strength (high reliability) [11]. On the other hand, tensile stresses should be subtracted to the strength of the material, and if they overpass a critical value tunnelling cracks will appear and consequently the mechanical response will degrade [20]. Following these ideas, thin compressive layers are desirable, as they will cre￾ate an additional reinforcement as well as diminish the associated residual tensile stress. Moreover, the thickness of the layers are referred to other observations related to the residual stresses, such as edge crack and crack bifurca￾tion, for which a critical thickness ‘‘tc’’ has to be surpassed [13,16,21]. In order to satisfy the particular design requirements the number, thickness and composition of the layers should be controlled. In this regard, colloidal processing methods have proved to be useful to tailor the mentioned variables aiming to manufacture reliable laminated structures. Those methods include tape-casting [22–24], centrifugal casting [25,26], sequential slip casting [27] and electrophoretic deposition (EPD) [28,29] among others. All these approaches 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 homogeneous and well-dispersed composition are obtained by controlling the interparticle potentials devel￾oped within the liquid media [30,31]. The layer thickness is controlled by adjusting the processing parameter associ￾ated with the corresponding technique (e.g. casting time, blades gap, amount of slurry, etc.). In this work, multilayered materials composed of thick layers with low residual tensile stresses and thin internal layers with high residual compressive stresses are fabricated by sequential slip casting, aiming to study the influence of the layers thickness on the fracture behaviour of the lay￾ered architecture. Processing parameters are optimised and layer thickness is controlled by adjusting the casting time of stable slurries. The value of the residual stresses is tailored by varying the thickness of the thin layers. Mate￾rials are tested by the indentation-strength method. Mono￾lithic specimens are used to collect data for residual stress evaluation and for comparative purposes. 2. Experimental Starting powders were submicron-sized Al2O3 (HPA 0.5, Condea, USA) with a mean particle size (d50) of 0.3 lm, tetragonal zirconia polycrystals (Y-TZP) with 3 mol% of Y2O3 (TZ-3YS, Tosoh, Japan) with d50 = 0.4 lm, and pure ZrO2 (TZ-0, Tosoh, Japan) with d50 = 0.3 lm. Slurries were prepared to a solid loading of 36.5 vol% by mixing starting powders with DI water, containing the desired amount of a commercial acrylic based polyelectrolyte (Duramax D-3021, Rohm& Haas, USA). For dispersion, slurries were ultrasonicated using a 400 W sonotrode (UP400S, Hielscher, Germany) with magnetic stirring for 2 min, and left stirring for at least 4 h before use. Colloidal R. Bermejo et al. / Composites Science and Technology 67 (2007) 1930–1938 1931