G de Portu et al. /Composites: Part B 37(2006)556-567 200m ig. 2. Example of tunneling L. The slurry was ball milled for about 60 h then filtered and were obtained. The structure was designed to leave the layers acuum degassed of alumina(A)on the two surfaces(Fig. 5)in order to stimulate Tape-casting was performed with a laboratory tape-casting surface compressive stresses Due to lower thermal expansion bench with a stationary double blade system [55] coefficient and shrinkage during sintering, the external alumina As already reported [57] the types of powder used in this layers underwent residual compressive stresses. work do not lead to an evident green density gradient through To obtain a perfectly symmetrical structure, two A layers the thickness of the tapes were used on each side. This allowed one layer to be removed Sheets of pure alumina(hereinafter designated'A')and of from each side by grinding for a proper machining of the the composite alumina-zirconia with a volume ratio of 60/40 surface after sintering (hereinafter designated 'Az) were produced Final dense samples(98% of the theoretical density)were pproximately 2 mm thick, with layers approxin 3. 2. Production of the laminated structures 180 um. In order to modify the residual stress distribution in the structures the thickness of the Az layers was varied changing the thickness of the green tapes or stacking two layers c Discs with a diameter of 40 mm were cut from the different together(sequence A/2AZ/A/AZ/.). In Fig. 6, the section of The discs were put into a die, heated up to 80C then different architectures are reported To obtain a stress-free material for use as referenc pressed at 30 MPa for 30 min. The sequence and the number of laminated structures containing layers of pure alumina(here the different layers were varied according to the designed inafter referred to as'AA, )were also prepared with the same architectures procedure described before resulting in a material with surface As heating and cooling rates are crucial parameters porosity as the A/AZ one, but with zero( or very low)residual determining residual stresses in the structure [37, 38, the stresses. In addition full dense, pure monolithic alumina(MA) sintering cycle was carefully controlled. was prepared by cold isostatic pressing(pressure 150 MPa)and At the beginning a very low heating rate(3C/h)was used, sintered at 1600"C for lh in order to facilitate the binder removal. Then the heating rate was increased to 30C/h up to the sintering temperature (1550C)and hold for I h. After that the laminated samples 4. Residual stress measurements were cooled down with the same rate to 600 C. The profile of the sintering cycle is reported in Fig. 4 From the model described in paragraph 2, we Samples with a hybrid laminated structure of alumina and the magnitude of residual stresses is proportional to the CtE alumina-zirconia composite(hereinafter referred to as 'A/Az) mismatch between the materials but it also greatly dependsThe slurry was ball milled for about 60 h then filtered and vacuum degassed. Tape-casting was performed with a laboratory tape-casting bench with a stationary double blade system [55]. As already reported [57] the types of powder used in this work do not lead to an evident green density gradient through the thickness of the tapes. Sheets of pure alumina (hereinafter designated ‘A’) and of the composite alumina–zirconia with a volume ratio of 60/40 (hereinafter designated ‘AZ’) were produced. 3.2. Production of the laminated structures Discs with a diameter of 40 mm were cut from the different ceramic green tapes. The discs were put into a die, heated up to 80 8C then pressed at 30 MPa for 30 min. The sequence and the number of the different layers were varied according to the designed architectures. As heating and cooling rates are crucial parameters determining residual stresses in the structure [37,38], the sintering cycle was carefully controlled. At the beginning a very low heating rate (3 8C/h) was used, in order to facilitate the binder removal. Then the heating rate was increased to 30 8C/h up to the sintering temperature (1550 8C) and hold for 1 h. After that the laminated samples were cooled down with the same rate to 600 8C. The profile of the sintering cycle is reported in Fig. 4. Samples with a hybrid laminated structure of alumina and alumina–zirconia composite (hereinafter referred to as ‘A/AZ’) were obtained. The structure was designed to leave the layers of alumina (A) on the two surfaces (Fig. 5) in order to stimulate surface compressive stresses. Due to lower thermal expansion coefficient and shrinkage during sintering, the external alumina layers underwent residual compressive stresses. To obtain a perfectly symmetrical structure, two A layers were used on each side. This allowed one layer to be removed from each side by grinding for a proper machining of the surface after sintering. Final dense samples (w98% of the theoretical density) were approximately 2 mm thick, with layers of approximately 180 mm. In order to modify the residual stress distribution in the structures the thickness of the AZ layers was varied changing the thickness of the green tapes or stacking two layers together (sequence A/2AZ/A/2AZ/.). In Fig. 6, the section of different architectures are reported. To obtain a stress-free material for use as reference, laminated structures containing layers of pure alumina (here￾inafter referred to as ‘AA’) were also prepared with the same procedure described before resulting in a material with surface porosity as the A/AZ one, but with zero (or very low) residual stresses. In addition full dense, pure monolithic alumina (MA) was prepared by cold isostatic pressing (pressure 150 MPa) and sintered at 1600 8C for 1 h. 4. Residual stress measurements From the model described in paragraph 2, we have seen that the magnitude of residual stresses is proportional to the CTE mismatch between the materials but it also greatly depends on Fig. 2. Example of tunneling crack. G. de Portu et al. / Composites: Part B 37 (2006) 556–567 559