V M. Sglavo, M. Bertoldi Acta Materialia 54(2006)4929-4937 of air bubbles [31]. The final organic content was about Monolithic bars(thickness M1.5 mm)were produced in 21 vol %. A similar preparation procedure was used for the same way for the measurement of thermal expansion composite slurries, though some modifications of the dis- coefficient, a, elastic modulus, E, and fracture toughness, persion process were introduced in order to obtain limited Kc. The thermal expansion coefficient was measured in thixotropy and high fluidity. AZ suspensions were pre- the range 30-1000C using a silica dilatometer and a heat pared by dispersing the zirconia powder in a slightly acidic ing rate of 2C/min. Elastic modulus was measured using HCI solution and then by adding the alumina powder. four-point bending tests (spans equal to 40 The slurry was ball milled using a high-efficiency mixer 20 mm) with a calibrated extensometer(MTS Systems, (Turbula T2F, W.A. BACHOFEN AG, CH) for 4-8h. USA) to measure the deflection as a function of applied The dispersant was also added in two steps, using an load. Fracture toughness was determined by the conven- amount of 1. 2 wt. with respect to zirconia In the case tional indentation fracture method [1, 21] of AM composites, mullite powder was added after dispers- Sixteen samples were fractured by four-point bending ng alumina for 16 h in the same conditions described for tests. In order to establish the invariance of strength with pure alumina and ball milled for further 24 h All suspen- respect to flaw size, some specimens were also pre-cracked sions were produced with a powder content of 39 vol%. by Vickers indentation using loads ranging from 10 to The volume of powders in the first dispersing stage was 100 N. Three indentations were produced in the centre of higher, ranging from 49 to 51 vol % as the addition of the the perspective tensile surface before applying the bending acrylic emulsions also supplies the solvent(water) to the test Monolithic samples(Azo and AZ40)were also pro- slurry and dilutes the system. Just before casting, slurries duced and tested under the same conditions for were filtered again at 60 um to ensure the elimination of comparison. any bubbles or clusters of flocculated polymer Tape casting was carried out using a double doctor- 5. Results and discussion olade assembly(DDB-1-6, 6 in wide, Richard E Mistler Inc,USA)at a speed of I m/min for a length of about Fig. 10 shows the architecture of the produced AMZ 1000 mm. A composite three-layer film(PETI2/Al7/ ceramic laminate observed using scanning electron micros- LDPE60, BP Europack, Italy) was used as substrate in copy(SEM). One can easily appreciate the perfect adhesion order to make the al of the dried green tape easier. among different layers as hypothesised in the theoretical For this reason the polyethylene hydrophobic side of film was placed side-up. The substrate was placed on a rigid section of this pape The average bending stre measured for the Amz float glass plate in order to ensure a flat surface. The rela- samples was equal to 692 and the standard deviation was tive humidity of the over-standing environment was con- 25 MPa, i.e. lower than 4%. The bending strength of the trolled and set to about 80% during casting and monolithic samples was equal to 418 and 741 MPa for successive drying to avoid fast evaporation of the solvent AZo and AZ40 laminates, respectively. more interestingly and possible cracking of green tapes due to shrinkage stres- standard deviations equal to 43 and 86 MPa were calcu- es. Suspension casting was carried out using two diferent lated, correspondingly It is evident that the strength values blade heights, 250 and 100 um Drops of a 10 wt% wetting measured on the aMz laminate correlate closely to the agent water solution(NH4-lauryl sulphate, code 09887, design value and represent a clear indication of a reliable FLUKA CHEMIE AG, CH) were added to the slurries ("constant")failure stress to help the casting tape to spread on the substrate when The strength variability in brittle materials is often needed, especially in the case of thinner tapes described by the Weibull modulus, which is a stress expo- Green tapes of nominal dimension 60 mm x 45 mm were nent that describes the relation between a failure probabil- punched using a hand-cutter, stacked together and thermo ity function and the applied stress: the higher the strength compressed at 70C under a pressure of 30 MPa for 15 min variability, the lower the Weibull modulus. The strength applied by a universal mechanical testing machine (MTs data measured in this work are graphically represented Systems, model 810, USA). Two 100 um thick poly(ethyl- on a Weibull plot as shown in Fig. ll. The failure proba- ene terephthalate)layers were placed between the laminate bility is evaluated as [1, 21] and the die to make the removal easier. For mechani cal characterisation bars of nominal dimensions 60 mmx F 7.5 mm x 1-2 mm were cut after the thermo-compression and then re-laminated [29] before final thermal treatment where N is the total number of samples for each set and n is to avoid any delamination promoted by localised shear the rank in the ascending ordered distribution. Fitting of stresses developed upon cutting. Samples were finally sin- the strength data shown in Fig. Il, using linear regression, tered at 1600C for 2h. After sintering the edges were allows a calculation of the Weibull modulus that corre- lightly chamfered to remove macroscopic defects and geo- sponds to each strength distribution. Values equal to metrical irregularities. No further polishing and finishing 12t I and 10+ I were obtained for Azo and AZ40 mono- operations were performed on the sample surfaces or edges liths, respectively, similar to other advanced ceramic mate- in order to avoid any artificial reduction of flaw severity. rials. For the engineered AMZ laminate, a Weibullof air bubbles [31]. The final organic content was about 21 vol.%. A similar preparation procedure was used for composite slurries, though some modifications of the dis￾persion process were introduced in order to obtain limited thixotropy and high fluidity. AZ suspensions were pre￾pared by dispersing the zirconia powder in a slightly acidic HCl solution and then by adding the alumina powder. The slurry was ball milled using a high-efficiency mixer (Turbula T2F, W.A. BACHOFEN AG, CH) for 4–8 h. The dispersant was also added in two steps, using an amount of 1.2 wt.% with respect to zirconia. In the case of AM composites, mullite powder was added after dispers￾ing alumina for 16 h in the same conditions described for pure alumina and ball milled for further 24 h. All suspen￾sions were produced with a powder content of 39 vol.%. The volume of powders in the first dispersing stage was higher, ranging from 49 to 51 vol.%, as the addition of the acrylic emulsions also supplies the solvent (water) to the slurry and dilutes the system. Just before casting, slurries were filtered again at 60 lm to ensure the elimination of any bubbles or clusters 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 Europack, 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 rela￾tive humidity of the over-standing environment was con￾trolled 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 stres￾ses. Suspension casting was carried out using two different blade heights, 250 and 100 lm. 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 to spread on the substrate when needed, especially 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￾compressed at 70 C under a pressure of 30 MPa for 15 min applied by a universal mechanical testing machine (MTS Systems, model 810, USA). Two 100 lm thick poly(ethyl￾ene terephthalate) layers were placed between the laminate and the die to make the removal easier. For mechani￾cal characterisation bars of nominal dimensions 60 mm · 7.5 mm · 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 sin￾tered at 1600 C for 2 h. After sintering the edges were slightly chamfered to remove macroscopic defects and geo￾metrical irregularities. No further polishing and finishing operations were performed on the sample surfaces or edges in order to avoid any artificial reduction of flaw severity. 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 toughness, KC. The thermal expansion coefficient was measured in the range 30–1000 C using a silica dilatometer and a heat￾ing rate of 2 C/min. Elastic modulus was measured using four-point bending tests (spans equal to 40 mm and 20 mm) with a calibrated extensometer (MTS Systems, USA) to measure the deflection as a function of applied load. Fracture toughness was determined by the conven￾tional indentation fracture method [1,21]. Sixteen samples were fractured by four-point bending tests. In order to establish the invariance of strength with respect to flaw size, some specimens were also pre-cracked by Vickers indentation using loads ranging from 10 to 100 N. Three indentations were produced in the centre of the perspective tensile surface before applying the bending test. Monolithic samples (AZ0 and AZ40) were also pro￾duced and tested under the same conditions for comparison. 5. Results and discussion Fig. 10 shows the architecture of the produced AMZ ceramic laminate observed using scanning electron micros￾copy (SEM). One can easily appreciate the perfect adhesion among different layers as hypothesised in the theoretical section of this paper. The average bending strength measured for the AMZ samples was equal to 692 and the standard deviation was 25 MPa, i.e. lower than 4%. The bending strength of the monolithic samples was equal to 418 and 741 MPa for AZ0 and AZ40 laminates, respectively. More interestingly, standard deviations equal to 43 and 86 MPa were calcu￾lated, correspondingly. It is evident that the strength values measured on the AMZ laminate correlate closely to the design value and represent a clear indication of a reliable (‘‘constant’’) failure stress. The strength variability in brittle materials is often described by the Weibull modulus, which is a stress expo￾nent that describes the relation between a failure probabil￾ity function and the applied stress: the higher the strength variability, the lower the Weibull modulus. The strength data measured in this work are graphically represented on a Weibull plot as shown in Fig. 11. The failure proba￾bility is evaluated as [1,21] F ¼ n  0:5 N ; ð13Þ where N is the total number of samples for each set and n is the rank in the ascending ordered distribution. Fitting of the strength data shown in Fig. 11, using linear regression, allows a calculation of the Weibull modulus that corre￾sponds to each strength distribution. Values equal to 12 ± 1 and 10 ± 1 were obtained for AZ0 and AZ40 mono￾liths, respectively, similar to other advanced ceramic mate￾rials. For the engineered AMZ laminate, a Weibull V.M. Sglavo, M. Bertoldi / Acta Materialia 54 (2006) 4929–4937 4935