October 20 Tailored Residual Stresses in Ceramic laminates 2829 depth(um) the initial viscosity. After adding some drops of concentrated NH4OH to increase pH, suspensions were filtered through a 40 m polyethylene filter and de-aired using a low-vacuum Venturi pump to remove air entrapped during the milling stage Acrylic binder emulsion and plasticizer were then added to the dispersion and slowly mixed for 30 min to obtain the nec- essary homogeneity, applying great care to avoid the formation of new air bubbles. The final organic content was about 21 vol%. A similar preparation procedure was used for composite slurries, although some modifications of the dispersion process re introduced in order to obtain limited thixotropy and high fluidity. In this case, mullite powder was added after dispersing alumina for 16 h in the same conditions described for pure alu- mina and then ball milled for a further 24 h. All suspension were produced with a powder content of 39 vol %. It should be noted that the volume of powders in the first dispersing stage depth(um) was obviously higher, ranging from 49 to 51 vol%, as the ae 100 dition of the acrylic emulsions supplies also solvent(water )to the slurry and thus dilutes the system. Just before casting, slur ries were filtered again at 60 um to ensure the elimination of any bubbles or clusters of flocculated polymer. A flow chart of the overall process is shown in Fig. 5. Tape casting was conducted using a double doctor-blade as- 皇 sembly(DDB-1-6, 6 in wide, Richard E Mistler Inc, Yardley, PA)at a speed of I m/min for a length of about 1000 mm. A omposite three-layer film(PET12/Al7/LDPE60, BP Europack Vicenza, Italy) was used as a substrate in order to make the re- moval of the dried green tape easier. For this reason the poly ethylene hydrophobic side of the film was placed side-up. The (depth)o5(umo.5) substrate was placed on a rigid float glass sheet in order to en- sure a flat surface and properly fixed with adhesive tape to the sign residual stress profile(a) and corresponding apparent borders. The relative humidity of the over-standing environment was controlled and set to about 80% during casting and drying line shows the construction used for the calculation to avoid the excessively quick evaporation of the solvent and (σr≈400MPa) possible cracking of green tapes because of shrinkage stresses Casting of suspensions was performed using two different blade Table ll Ceramic powders used in this work heights, 250 and 100 um, respectively. Drops of a 10 wt% wet ting agent water solution (NHa-lauryl sulphate, code 09887 BET specific Purity Fluka Chemie AG, Buchs, Switzerland) were added to the slur- Material Code ries to help the casting tape spread on the substrate when re- - Ale O3 A-16SG. ALCOA 8.6 quired, particularly in the case of thinner tapes 3AlO32SiO2 KM1Ol, KCM 8.4 Green tapes of nominal dimension 60 mm x 45 mm wer punched using a hand-cutter, stacked together and thermo-com pressed at 70C using a pressure of 30 MPa for 15 min applied by a universal mechanical testing machine (MTS Systems minneapolis, MN, mod. 810). Two 100 um thick PET layers 1000, DURAMAX) was also added in 1: 2 by weight ratio with were placed between the laminate and the die to make the re- espect to the binder content as plasticizer to increase the green moval easier For the mechanical characterization bars of nom- flexibility and to reduce cracks occurrence in the dried tape. Th inal dimensions 60 mm x 7.5 mm 1.5 mm were cut after the organic ingredients used in this work are listed in Table lll. thermo-compression stage and then re-laminated2before ther The alumina powder suspension was obtained by using a two- mal treatment to avoid any delamination promoted by localized stage process. In order to enhance the electrostatic interaction shear stresses developed upon cutting between the positive charges on the powder surface and the Bars(6 mm x 48 mm) of the AM engineered composite were negative sites on the polymer chains, an initially slightly acid obtained after sintering. The edges were slightly chamfered to water solution (pH=4) was used. An optimum dispersant remove macroscopic defects and geometrical irregularities. No content equal to 1.5 wt% with respect to the powder was es- further polishing and finishing operations were performed on tablished by static sedimentation. This value corresponds to he sample surfaces or edges to avoid any artificial reduction of about 0. 4 mg/m of active matter per unit area, corresponding in he severity of flaws. Monolithic bars(thickness a 1.5 mm) turn to values suggested by greenwood et al.- for the same were produced in the same way for the measurement of thermal material. a ball milling stage using alumina spheres of 9 expansion coefficient, a, elastic modulus, E, and fracture tough mm nominal diameter was carried out in polyethylene bottles ness, Kc. The thermal expansion coefficient was measured in the for 16-24 h to break down all powder aggregates. The suspen range 30-1000.C by using a silica dilatometer and a heating sion was ultrasonicated for 10 min before ball milling to redu te of 2C/ lastic modulus was measured by four-points Table lll. Organic Ingredients Used for the Preparation of the Slurries NH4-PMA Darvan C.R.t. vanderbilt Inc Dispersant 7.5-90 High-Tg acrylic emulsion B-1235 DURAMAX Low-Tg acrylic emulsion B-1000, DURAMAX Plasticizer 55.01000, DURAMAXs ) was also added in 1:2 by weight ratio with respect to the binder content as plasticizer to increase the green flexibility and to reduce cracks occurrence in the dried tape. The organic ingredients used in this work are listed in Table III. The alumina powder suspension was obtained by using a two￾stage process.25 In order to enhance the electrostatic interaction between the positive charges on the powder surface and the negative sites on the polymer chains, an initially slightly acid water solution (pH 5 4) was used.26 An optimum dispersant content equal to 1.5 wt% with respect to the powder was es￾tablished by static sedimentation. This value corresponds to about 0.4 mg/m2 of active matter per unit area, corresponding in turn to values suggested by Greenwood et al. 27 for the same material. A ball milling stage using alumina spheres of 6 and 9 mm nominal diameter was carried out in polyethylene bottles for 16–24 h to break down all powder aggregates. The suspen￾sion was ultrasonicated for 10 min before ball milling to reduce the initial viscosity. After adding some drops of concentrated NH4OH to increase pH, suspensions were filtered through a 40 mm polyethylene filter and de-aired using a low-vacuum Venturi pump to remove air entrapped during the milling stage. Acrylic binder emulsion and plasticizer were then added to the dispersion and slowly mixed for 30 min to obtain the nec￾essary homogeneity, applying great care to avoid the formation of new air bubbles.28 The final organic content was about 21 vol%. A similar preparation procedure was used for composites slurries, although some modifications of the dispersion process were introduced in order to obtain limited thixotropy and high fluidity. In this case, mullite powder was added after dispersing alumina for 16 h in the same conditions described for pure alu￾mina and then ball milled for a further 24 h. All suspensions were produced with a powder content of 39 vol%. It should be noted that the volume of powders in the first dispersing stage was obviously higher, ranging from 49 to 51 vol%, as the ad￾dition of the acrylic emulsions supplies also solvent (water) to the slurry and thus dilutes the system. Just before casting, slur￾ries were filtered again at 60 mm to ensure the elimination of any bubbles or clusters of flocculated polymer. A flow chart of the overall process is shown in Fig. 5. Tape casting was conducted using a double doctor-blade as￾sembly (DDB-1-6, 6 in. wide, Richard E. Mistler Inc., Yardley, PA) at a speed of 1 m/min for a length of about 1000 mm. A composite three-layer film (PET12/Al7/LDPE60, BP Europack, Vicenza, Italy) was used as a substrate in order to make the re￾moval of the dried green tape easier. For this reason the poly￾ethylene hydrophobic side of the film was placed side-up. The substrate was placed on a rigid float glass sheet in order to en￾sure a flat surface and properly fixed with adhesive tape to the borders. The relative humidity of the over-standing environment was controlled and set to about 80% during casting and drying to avoid the excessively quick evaporation of the solvent and possible cracking of green tapes because of shrinkage stresses. Casting of suspensions was performed using two different blade heights, 250 and 100 mm, respectively. Drops of a 10 wt% wet￾ting agent water solution (NH4-lauryl sulphate, code 09887, Fluka Chemie AG, Buchs, Switzerland) were added to the slur￾ries to help the casting tape spread on the substrate when re￾quired, particularly in the case of thinner tapes. Green tapes of nominal dimension 60 mm  45 mm were punched using a hand-cutter, stacked together and thermo-com￾pressed at 701C using a pressure of 30 MPa for 15 min applied by a universal mechanical testing machine (MTS Systems, Minneapolis, MN, mod. 810). Two 100 mm thick PET layers were placed between the laminate and the die to make the re￾moval easier. For the mechanical characterization bars of nom￾inal dimensions 60 mm  7.5 mm  1.5 mm were cut after the thermo-compression stage and then re-laminated29 before ther￾mal treatment to avoid any delamination promoted by localized shear stresses developed upon cutting. Bars (6 mm  48 mm) of the AM engineered composite were obtained 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 the severity of flaws. Monolithic bars (thickness 1.5 mm) were produced in the same way for the measurement of thermal expansion coefficient, a, elastic modulus, E, and fracture tough￾ness, KC. The thermal expansion coefficient was measured in the range 301–10001C by using a silica dilatometer and a heating rate of 21C/min. Elastic modulus was measured by four-points − 400 − 300 − 200 − 100 0 100 200 0 5 10 15 20 25 residual stress, σres (MPa) (depth)0.5 (µm0.5) 0 5 10 15 20 25 (depth)0.5 (µm0.5) 20 100 400 50 200 depth (µm) 0 2 4 6 8 10 12 apparent fracture toughness, KC (MPa m0.5) 20 100 400 50 200 (b) depth (µm) (a) * Fig. 4. Design residual stress profile (a) and corresponding apparent fracture toughness (b) for the monolithic engineered laminate. The dashed tangent line shows the construction used for the calculation of the failure stress (sf 400 MPa). Table II. Ceramic Powders Used in this Work Material Code, producer BET specific area (m2 /g) Purity (%) a-Al2O3 A-16SG, ALCOA 8.6 499.8 3Al2O3  2SiO2 KM101, KCM Corp. 8.4 499.7 Table III. Organic Ingredients Used for the Preparation of the Slurries Substance Name, producer Function Tg (1C) pH Active matter (wt%) NH4-PMA Darvan Cs , R. T. Vanderbilt Inc. Dispersant 7.5–9.0 25.0 High-Tg acrylic emulsion B-1235, DURAMAXs Binder 14 8.3 46.5 Low-Tg acrylic emulsion B-1000, DURAMAXs Plasticizer 26 9.4 55.0 October 2005 Tailored Residual Stresses in Ceramic Laminates 2829