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 ana 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