V.M. Sglavo, M. Bertoldi/ Composites: Part B 37(2006)481-489 respectively. The applied stress intensity factor corresponding to emulsions (B-1235, DURAMAX )as binder. A lower-Tg the predefined maximum stress(strength)and the cracks depth acrylic emulsion(B-1000, DURAMAX )was also added in a interval are also shown in Fig 9. Since, Kci was calculated step 1: 2 by weight ratio with respect to the binder content as by step Eq (9), the corresponding diagram is discontinuous at the plasticizer to increase green flexibility and to reduce cracks boundary between layers, this reflecting the discontinuities in the occurrence in the dried tape. The slurries were obtained using a Pres diagram(Fig 8). One can easily suppose that the real apparent two-stage process as detailed elsewhere [27, 28]. All suspen fracture toughness trend is continuous and that the discontinuities sions were produced with a powder content of 39 vol% and a in Fig. 9 correspond to mathematical artifacts only final organic content around 21 vol%. Just before casting, slurries were filtered at 60 um to ensure the elimination of any bubble or cluster of flocculated polymer 4. Production and properties of the designed laminates Tape casting was carried out using a double doctor-blade assembly(DDB-1-6, 6 in wide, Richard E Mistler Inc, USA) Ceramic laminates corresponding to the materials designed at a speed of I m/min for a length of about 1000 mm.A in the previous section were produced and characterized As composite three-layer film(PET12/AI7/LDPE60, BP Euro- previously pointed out, the thermal expansion coefficient as pack, Italy) was used as substrate in order to make the removal required for the development of the residual stress profile was of the dried green tape easier. For this reason the polyethylene tailored by considering composites in the alumina/mullite and hydrophobic side of the film was placed side-up. The substrate alumina/zirconia systems for the production of the single was placed on a rigid float glass plate in order to ensure a flat laminae surface. The relative humidity of the over-standing environ Alpha-alumina(ALCOA, A-16SG, D5o=0.4 um) was ment was controlled and set to about 80% during casting and considered as the fundamental starting material. High purity successive drying to avoid fast evaporation of the solvent and mullite(KCM Corp, KM101, D5o=0. 77 um)an possible cracking of green tapes due to shrinki (3 mol%) stabilised zirconia (TOSOH, TZ-3YS, Dso- Suspensions casting was carried out using two different blade 0.4 um) powders were chosen as second phases to vary the heights, 250 and 100 um, respectively. Drops of a 10 wt% thermal expansion coefficient with respect to pure alumina. Green laminae were produced by tape casting water-based FLUKA CHEMIE AG, CH)were added to the slurries to help slurries. Suspensions were prepared by using NHA-PMA the casting tape spreading on the substrate when needed, (Darvan C, R.T. Vanderbilt Inc. as dispersant and acrylic particularly in the case of thinner tape x(um) Green tapes of nominal dimension 60X45 mm were 2050100200 punched using a hand-cutter, stacked together and thermo- compressed at 70C and 30 MPa for 15 min applied by a ,≈500MPa universal mechanical testing machine(MTS Systems, mod 810, USA). Two 100 um thick PET layers were placed between the laminate and the die to make the removal easier for mechanical characterisation bars of nominal dimensions 60X 6x 1-2 mm were cut after the thermo-compression and then re-laminated [29] before final thermal treatment to avoid any delamination promoted by localised shear stresses developed upon cutting. Samples were finally sintered at 1600C for 2 h. After sintering the edges were slightly chamfered to remove macroscopic defects and geometrical irregularities. No furthe x(um) polishing and finishing operations were performed on the sample surfaces or edges to avoid any artificial reduction of flaws severity. Gr as 400 mpa Alumina(AMO) and AZ40 monolithic samples(thickness =1.5 mm) were produced in the same way for the measurement of thermal expansion coefficient, a, elastic modulus, E, and fracture toughness, Kc. The thermal expansion coefficient was measured in the range 30-1000C by using a silica dilatometer and a heating rate of 2C/min. elastic modulus was measured by 4-points bending tests(spans equal to 40 and 20 mm) by using a calibrated extensometer Fracture oughness was measured by the conventional indentation fracture method [301 The structure of the az-1 and AM-1 laminates is shown as Fig. 9. Apparent nt fracture toughness for AZ-1(a) and AM-1(b)engineered an example in Fig. 10. The perfect adhesion between layers is evident as well as the absence of any edge crackrespectively. The applied stress intensity factor corresponding to the predefined maximum stress (strength) and the cracks depth interval are also shown in Fig. 9. Since, K
C;i was calculated step by step Eq. (9), the corresponding diagram is discontinuous at the boundary between layers, this reflecting the discontinuities in the sres diagram (Fig. 8). One can easily supposethatthe real apparent fracture toughness trend is continuous and that the discontinuities in Fig. 9 correspond to mathematical artifacts only. 4. Production and properties of the designed laminates Ceramic laminates corresponding to the materials designed in the previous section were produced and characterized. As previously pointed out, the thermal expansion coefficient as required for the development of the residual stress profile was tailored by considering composites in the alumina/mullite and alumina/zirconia systems for the production of the single laminae. Alpha–alumina (ALCOA, A-16SG, D50Z0.4 mm) was considered as the fundamental starting material. High purity mullite (KCM Corp., KM101, D50Z0.77 mm) and yttria (3 mol%) stabilised zirconia (TOSOH, TZ-3YS, D50Z 0.4 mm) powders were chosen as second phases to vary the thermal expansion coefficient with respect to pure alumina. Green laminae were produced by tape casting water-based slurries. Suspensions were prepared by using NH4-PMA (Darvan Cw, R.T. Vanderbilt Inc.) as dispersant and acrylic emulsions (B-1235, DURAMAXw) as binder. A lower-Tg acrylic emulsion (B-1000, DURAMAXw) was also added in a 1:2 by weight ratio with respect to the binder content as plasticizer to increase green flexibility and to reduce cracks occurrence in the dried tape. The slurries were obtained using a two-stage process as detailed elsewhere [27,28]. All suspen￾sions were produced with a powder content of 39 vol% and a final organic content around 21 vol%. Just before casting, slurries were filtered at 60 mm to ensure the elimination of any bubble or cluster of flocculated polymer. Tape casting was carried out using a double doctor-blade assembly (DDB-1-6, 6 in. wide, Richard E. Mistler Inc., USA) at a speed of 1 m/min for a length of about 1000 mm. A composite three-layer film (PET12/Al7/LDPE60, BP Euro￾pack, Italy) was used as substrate in order to make the removal of the dried green tape easier. For this reason the polyethylene hydrophobic side of the film was placed side-up. The substrate was placed on a rigid float glass plate in order to ensure a flat surface. The relative humidity of the over-standing environ￾ment was controlled and set to about 80% during casting and successive drying to avoid fast evaporation of the solvent and possible cracking of green tapes due to shrinkage stresses. Suspensions casting was carried out using two different blade heights, 250 and 100 mm, respectively. Drops of a 10 wt% wetting agent water solution (NH4-lauryl sulphate, code 09887, FLUKA CHEMIE AG, CH) were added to the slurries to help the casting tape spreading on the substrate when needed, particularly in the case of thinner tapes. Green tapes of nominal dimension 60!45 mm were punched using a hand-cutter, stacked together and thermo￾compressed at 70 8C and 30 MPa for 15 min applied by a universal mechanical testing machine (MTS Systems, mod. 810, USA). Two 100 mm thick PET layers were placed between the laminate and the die to make the removal easier. For mechanical characterisation bars of nominal dimensions 60! 6!1–2 mm were cut after the thermo-compression and then re-laminated [29] before final thermal treatment to avoid any delamination promoted by localised shear stresses developed upon cutting. Samples were finally sintered at 1600 8C for 2 h. After sintering the edges were slightly chamfered to remove macroscopic defects and geometrical irregularities. No further polishing and finishing operations were performed on the sample surfaces or edges to avoid any artificial reduction of flaws severity. Alumina (AM0) and AZ40 monolithic samples (thickness z1.5 mm) were produced in the same way for the measurement of thermal expansion coefficient, a, elastic modulus, E, and fracture toughness, KC. The thermal expansion coefficient was measured in the range 30–1000 8C by using a silica dilatometer and a heating rate of 2 8C/min. Elastic modulus was measured by 4-points bending tests (spans equal to 40 and 20 mm) by using a calibrated extensometer. Fracture toughness was measured by the conventional indentation fracture method [30]. The structure of the AZ-1 and AM-1 laminates is shown as an example in Fig. 10. The perfect adhesion between layers is evident as well as the absence of any edge cracks. Fig. 9. Apparent fracture toughness for AZ-1 (a) and AM-1 (b) engineered laminates. V.M. Sglavo, M. Bertoldi / Composites: Part B 37 (2006) 481–489 487