G de Portu et al. /Composites: Part B 37(2006)556-567 557 clampedby the compressive layers, crack extension through of large EM=(a2-a1)dT (1) stresses and catastrophic failure only occurs at a well defined, hreshold stress The increase in mechanical properties of layered architec- where, al and a2 are the thermal expansion coefficient of the tures can be also achieved through different strategies such as two materials and To is the temperature at which elastic stress introduction of weak interfaces[29,30), containment of develops due to thermal strain mismatch and T is the room martensitic transformation [31] or existence of porous layers ature [32]. However, the approach, which involves the stimulation of Considering a perfectly symmetrical architecture, far away compressive stresses [13, 33-35] is one of the most promising from the free surface, the residual stress ores depend from the one. if also contact damage phenomena are considered. Such ratio between the thickness ti and t2 of the materials I and 2, respectively structures are normally obtained by stacking alternating layers The layer with the lower thermal expansion coefficient sidual stresses can also be introduced by other means such as given by ) underg oes residual biaxial compressive stress of materials with different cte that will translate in residual stresses during cooling in the sintering stage. In some cases, phase transformation [27, 31]. In case of well bonded interface the increase in fracture toughness and fracture energy is 1+(1Eh2E) achieved through the existence of residual stresses and the crack deflection caused by the elastic mismatch between the while in the layer with greater thermal expansion coefficien dissimilar materials [8 (material 2)experiences a biaxial tensile stress given by: Some interesting reviews on laminated composites have been published in the past [6,7]. The aim of this work is to (3) illustrate the recent achievements we have got in the development and characterization of laminated ceramic and v is the Poissons ratio of the relative materials modulus In Eq( 2)E=E/(I-v), where E is the Y structures obtained from oxides system In the model, the role of thermal-physical properties in developing residual stresses appears evident. From the theoretical analysis laminated structures with proper residual 2. Theoretical background stress distribution can be designed. The experimental evidence of the type of stress associated to different layers is reported in With reference to the structural performance/reliability of Fig. I the laminated structures, different authors have emphasized As a matter of fact Eqs.(2)and ()are valid as first that several toughening mechanisms are operative in these approximation. They assume a constant value within each layer structures. Some authors have suggested the importance of and do not account for edge effects. In fact the actual situation obtaining a structure with low interface fracture energy, as this is more complicated and a correct theoretical model for the would enable the crack to be deflected along those weak prediction of the residual stress in different layers should take interfaces [29, 30]. Other authors evidenced the role played by into account the position of each layer(outer or inner),the elastic modulus mismatch, shrinkage and coefficient of thermal position in the single layers, the numbers of layers and the expansion( CTE)between different layers in generating volume fraction of the laminae of each material in the whole residual stresses in the laminated structure [36-39]. The structure(the latter requisite implies a size effect) stresses have been shown to improve both mechanical [40, 41] However, a deep discussion of the models is not in the aim and tribological properties of the composite [34] of this paper. If the observed toughening is actually related to res tresses, detailed understanding of the nature of these stresses, 3. Processing their distribution and entity are essential. Such residual stresses yield higher strength values, apparent toughness and wear Lamination of different thin ceramic layers to form thick resistance than found in monolithic materials of the same specimens is a relatively simple and inexpensive process, composition. Green et [37-39] performed theoretical which has shown interesting results and can be considered a calculations to determine the influence and magnitude of the valid alternative to more sophisticated processes [1,2,42 different thermo-physical parameters affecting residual Several processing routes have been explored for the stresses preparation of these composites including electrophoretic Several models have been proposed 18-13] to predict the deposition [43-46), sequential slip casting[11,31,47)and stress amount and distribution in laminated structures tape casting[12, 48-51] In the case of rigidly bonded layers of two different Among the ceramic laminated composites that can be naterials, the laminated structure suffer a mismatch strain produced, one of the most studied system is the alumina- represented by [9] zirconia one. As discussed in a previous paragraph, in order‘clamped’ by the compressive layers, crack extension through the compressive layers requires the application of larger stresses and catastrophic failure only occurs at a well defined, threshold stress. The increase in mechanical properties of layered architec￾tures can be also achieved through different strategies such as introduction of weak interfaces [29,30], containment of martensitic transformation [31] or existence of porous layers [32]. However, the approach, which involves the stimulation of compressive stresses [13,33–35] is one of the most promising one, if also contact damage phenomena are considered. Such structures are normally obtained by stacking alternating layers of materials with different CTE that will translate in residual stresses during cooling in the sintering stage. In some cases, residual stresses can also be introduced by other means such as phase transformation [27,31]. In case of well bonded interface, the increase in fracture toughness and fracture energy is achieved through the existence of residual stresses and the crack deflection caused by the elastic mismatch between the dissimilar materials [8]. Some interesting reviews on laminated composites have been published in the past [6,7]. The aim of this work is to illustrate the recent achievements we have got in the development and characterization of laminated ceramic structures obtained from oxides systems. 2. Theoretical background With reference to the structural performance/reliability of the laminated structures, different authors have emphasized that several toughening mechanisms are operative in these structures. Some authors have suggested the importance of obtaining a structure with low interface fracture energy, as this would enable the crack to be deflected along those weak interfaces [29,30]. Other authors evidenced the role played by elastic modulus mismatch, shrinkage and coefficient of thermal expansion (CTE) between different layers in generating residual stresses in the laminated structure [36–39]. These stresses have been shown to improve both mechanical [40,41] and tribological properties of the composite [34]. If the observed toughening is actually related to residual stresses, detailed understanding of the nature of these stresses, their distribution and entity are essential. Such residual stresses yield higher strength values, apparent toughness and wear resistance than found in monolithic materials of the same composition. Green et al. [37–39] performed theoretical calculations to determine the influence and magnitude of the different thermo-physical parameters affecting residual stresses. Several models have been proposed [8–13] to predict the stress amount and distribution in laminated structures. In the case of rigidly bonded layers of two different materials, the laminated structure suffer a mismatch strain represented by [9] 3M Z ð T0 T ða2Ka1ÞdT (1) where, a1 and a2 are the thermal expansion coefficient of the two materials and T0 is the temperature at which elastic stress develops due to thermal strain mismatch and T is the room temperature. Considering a perfectly symmetrical architecture, far away from the free surface, the residual stress sres depend from the ratio between the thickness t1 and t2 of the materials 1 and 2, respectively. The layer with the lower thermal expansion coefficient (material 1), undergoes residual biaxial compressive stress given by sres1 ZK 3ME0 1 1 C t1E0 1=t2E0 2
(2) while in the layer with greater thermal expansion coefficient (material 2) experiences a biaxial tensile stress given by: sres2 ZKsres1 t1 t2 (3) In Eq. (2) E0 ZE/(1Kn), where E is the Young’s modulus and n is the Poisson’s ratio of the relative materials. In the model, the role of thermal–physical properties in developing residual stresses appears evident. From the theoretical analysis laminated structures with proper residual stress distribution can be designed. The experimental evidence of the type of stress associated to different layers is reported in Fig. 1. As a matter of fact Eqs. (2) and (3) are valid as first approximation. They assume a constant value within each layer and do not account for edge effects. In fact the actual situation is more complicated and a correct theoretical model for the prediction of the residual stress in different layers should take into account the position of each layer (outer or inner), the position in the single layers, the numbers of layers and the volume fraction of the laminae of each material in the whole structure (the latter requisite implies a size effect). However, a deep discussion of the models is not in the aim of this paper. 3. Processing Lamination of different thin ceramic layers to form thick specimens is a relatively simple and inexpensive process, which has shown interesting results and can be considered a valid alternative to more sophisticated processes [1,2,42]. Several processing routes have been explored for the preparation of these composites including electrophoretic deposition [43–46], sequential slip casting [11,31,47] and tape casting [12,48–51]. Among the ceramic laminated composites that can be produced, one of the most studied system is the alumina– zirconia one. As discussed in a previous paragraph, in order G. de Portu et al. / Composites: Part B 37 (2006) 556–567 557