V.M. Sglaco, M. Bertoldi Acta Materialia 54(2006)4929-4937 depth(um) -DAM -C-AZ 15 E Fig. 7. Thermal expansion coefficient for AZ and AM composites. (depth)(um Fig. 9. Apparent fracture toughness of the AMZ engineered laminate. [29]. The difference between the elastic modulus bounds is The dashed line is used to calculate the final strength less than 7% and 10%, respectively, for mullite and zirconia content below 40%. For the Poisson's ratios the difference less than 1.5%. Therefore, the average of the values reported 4. Experimental in Table I has been used for the evaluation of Eqs. (11)and a-Alumina(ALCOA, A-16SG, Dso=0.4 um) was the (12), with an error range of 5%. The thermal expansion coef- ficient and fracture toughness for AM and AZ composites primary starting material. High-purity mullite(KCM Corp, KM101, D50=0.77 um) and yttria (3 mol%)- were measured on monolithic samples as reported previ- stabilised zirconia(TOSOH, TZ-3YS, D50=0.4 um)pow- ously [29]. The residual stress profile and the T-curve for the aMz ders were chosen as second phases to vary the thermal oefficient with respect to pu re engineered laminate are shown in Fig 8. The applied stress Green layers were produced by tape casting water-based stress(strength) and the crack depth interval are also (Darvan C.R. T. Vanderbilt Inc )as dispersant and acrylic shown in Fig 9. Since T was calculated step by step emulsions(B-1235, DURAMAX")as binder. A lower-T, (Eq.(9)), the corresponding diagram is disc continuous acrylic emulsion(B-1000, DURAMAX was also adde the boundary between layers(Fig 8). One can easily sup- in a 1: 2 by weight ratio with respect to the binder content tinuous and that the discontinuities in Fig. 9 are merely occurrence of cracks in the dried tape. The alumina powder mathematical artefacts dispersion was obtained using a two-stage process [30,31] In order to enhance the electr depth(um) sites on the polymer chains, a slightly acid 00200 400 (pH 4)was used [32]. An optimal dispersant content equal to 1. 5 wt %o with respect to the powder was establish by sta- tic sedimentation. This value corresponds to about 0. 4 mg natter per unit area, which corresponds closely with values suggested by Greenwood et al. [30]for the same material. a ball milling stage using alumina spheres of 6 and 9 mm in diameter was carried out in polyethylene bot tles for 16-24 h to break down the aggregates. Suspensions -400 were ultrasonicated for 10 min before ball milling to reduce the starting viscosity. After adding some drops of concen- trated NH,OH to increase pH, suspensions were filtered with a 40 um polyethylene net and de-aired using a low- vacuum Venturi pump to remove air entrapped during (depth)s(um) the milling stage Acrylic binder emulsion and plasticiser were then added Fig 8. Residual stress profile in the surface region of the AMZ engineered to the suspension and slowly mixed for 30 min to reach good homogeneity, using great care to avoid the formation[29]. The difference between the elastic modulus bounds is less than 7% and 10%, respectively, for mullite and zirconia content below 40%. For the Poisson’s ratios the difference is less than 1.5%. Therefore, the average of the values reported in Table 1 has been used for the evaluation of Eqs. (11) and (12), with an error range of 5%. The thermal expansion coef- ficient and fracture toughness for AM and AZ composites were measured on monolithic samples as reported previ￾ously [29]. The residual stress profile and the T-curve for the AMZ engineered laminate are shown in Fig. 8. The applied stress intensity factor corresponding to the predefined maximum stress (strength) and the crack depth interval are also shown in Fig. 9. Since T was calculated step by step (Eq. (9)), the corresponding diagram is discontinuous at the boundary between layers (Fig. 8). One can easily sup￾pose that the real apparent fracture toughness trend is con￾tinuous and that the discontinuities in Fig. 9 are merely mathematical artefacts. 4. Experimental a-Alumina (ALCOA, A-16SG, D50 = 0.4 lm) was the primary starting material. High-purity mullite (KCM Corp., KM101, D50 = 0.77 lm) and yttria (3 mol.%)- stabilised zirconia (TOSOH, TZ-3YS, D50 = 0.4 lm) pow￾ders were chosen as second phases to vary the thermal expansion coefficient with respect to pure alumina. Green layers were produced by tape casting water-based slurries. Suspensions were prepared using NH4-PMA (Darvan C, R.T. Vanderbilt Inc.) as dispersant and acrylic emulsions (B-1235, DURAMAX) as binder. A lower-Tg acrylic emulsion (B-1000, DURAMAX) was also added in a 1:2 by weight ratio with respect to the binder content as plasticiser to increase green flexibility and to reduce the occurrence of cracks in the dried tape. The alumina powder dispersion was obtained using a two-stage process [30,31]. In order to enhance the electrostatic interaction between the positive charges on the powder surface and the negative sites on the polymer chains, a slightly acid water solution (pH 4) was used [32]. An optimal dispersant content equal to 1.5 wt.% with respect to the powder was establish by sta￾tic sedimentation. This value corresponds to about 0.4 mg/ m2 active matter per unit area, which corresponds closely with values suggested by Greenwood et al. [30] for the same material. A ball milling stage using alumina spheres of 6 and 9 mm in diameter was carried out in polyethylene bot￾tles for 16–24 h to break down the aggregates. Suspensions were ultrasonicated for 10 min before ball milling to reduce the starting viscosity. After adding some drops of concen￾trated NH4OH to increase pH, suspensions were filtered with a 40 lm polyethylene net and de-aired using a low￾vacuum Venturi pump to remove air entrapped during the milling stage. Acrylic binder emulsion and plasticiser were then added to the suspension and slowly mixed for 30 min to reach good homogeneity, using great care to avoid the formation 4 6 8 10 12 0 20 40 60 80 100 AM AZ α (10-6 ˚C) mullite or zirconia content (vol%) Fig. 7. Thermal expansion coefficient for AZ and AM composites. -800 -600 -400 -200 0 200 400 0 5 10 15 20 25 σ res (MPa) (depth)0.5 (µm0.5) 20 100 400 50 200 depth (µm) Fig. 8. Residual stress profile in the surface region of the AMZ engineered laminate. 0 5 10 15 20 0 5 10 15 20 25 T (MPa m0.5) (depth)0.5 (µm0.5) 20 100 400 50 200 depth (µm) Fig. 9. Apparent fracture toughness of the AMZ engineered laminate. The dashed line is used to calculate the final strength. 4934 V.M. Sglavo, M. Bertoldi / Acta Materialia 54 (2006) 4929–4937