J Mater Sci(2006)41:7425-7436 7427 was supported on an inclined glass plate to drain away low heating rate(1C/min) and low temperature hold excess gelling solution and inhibit premature gelling of (600C, 4-10 h). The green RBAO/TA laminated the slip beneath the doctor blade. The thickness of the samples were placed on a porous alumina tile to ensure cast tape was controlled by adjusting both the caster free oxygen access beneath the sample during reaction velocity and the gap between the extended doctor bonding. blade and the casting plate. Strong, flexible green tapes The alumina-toughened zirconia/textured 100-200 um in thickness were obtained with a doctor (ATZ/TA)and reaction-bonded mullite/t blade gap of 250 um alumina(RBMTA)laminates were sintered The non-textured oxide layers(RBAO, ATZ, and tionally. Dissociation of zircon and its reaction wit RBM) were cast by an identical gel-casting process, but alumina to form zirconia and mullite only occurs above without incorporation of the seed platelets. The slip 1400C [24], so the heating cycle selected for the RBM compositions for the TA layers and the three types of laminates was 5/min up to 1400C, 2/min from 1400 non-textured oxide layers are listed in Table 2. to1600C, followed by holding for 5 h at 1600C and After casting, the tapes were detached from the then cooling to room temperature at 5C/min. The Mylar sheet, by inverting the substrate in a distilled effect of the zircon particle size on the phase evolution water bath. The tapes were then washed to remove of the RBM layers was evaluated by X-ray diffraction excess cation, and 1.25"diameter disks were stamped phase analysis and the final density of the laminate from the washed tapes using a polished steel punch composite samples was determined by Archimedes (Buehler). Green laminate specimens were pressed density measurements from a stacked lay-up of the textured and non-textured disks in a stainless steel die(also 1. 25"in diameter) using a single filter paper top and bottom to prevent Texture characterization dhesion and allow excess water to escape. The spec imens were uniaxially pressed in the die at -80 Mpa for The chemical homogeneity and composition of indi- 20 min and subsequently cold isostatically pressed vidual grains were determined from carbon-coated (CIP)at 250-300 MPa for 30 min. polished and thermally etched samples using conven- tional scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDS-JEOL 840). The microstructure of cross-sections taken from laminated Residual water was removed by drying in a 2.45 GHz samples was also examined by diffraction contrast microwave oven(EM-S301, SANYO)operated at 100- transmission electron microscopy (TEM-JEOL 2000- 150 W for 30-60 min. The progress of microwave FX)at 200 kV. TEM samples were prepared by dia- drying was assessed by weighing the samples. The small mond-sectioning perpendicular to the original quantity of organic additives (<1 vol% of dispersant the precursor tape, followed by mechanical grinding, and alginate) was removed during the sintering/reac- dimpling, and precision ion polishing(PIPS-Gatan tion-bonding cycle and not as a separate burnout Model 691) treatment The Harris method [25-27 was used to quantify orted for monolithic RBAo the degree of crystallographic alignment(texture)in ceramic processing [20-23], a somewhat complex all specimens. X-ray diffraction spectra were taken heating cycle is required to sinter the BAOTa from polished surfaces prepared parallel to the laminates to high density(avoiding swelling)and with direction of tape casting. Reference diffraction data 100% conversion to oxide. Oxidation of the aluminum were obtained from randomly oriented high purity metal particles at low temperatures(below the melting alumina powder(HPA-0.5, Ceralox). All X-ray data point) is essential and was accomplished by an initially were collected on an X-ray diffractometer with Table 2 Slip comp osition for Tape gel casting of textured and Matrix powder AlO, platelet Na alginate non-textured oxide layers TA 25-30 1-6 0.15-0.2 0.5-0.6 66-72 RBAO 000 0.5-0.6 0.2-0.25 06-0.7 DISPEX A40. Allied RBM 0.2-0.25 0.5-0.6 Colloid, england 2 Springerwas supported on an inclined glass plate to drain away excess gelling solution and inhibit premature gelling of the slip beneath the doctor blade. The thickness of the cast tape was controlled by adjusting both the caster velocity and the gap between the extended doctor blade and the casting plate. Strong, flexible green tapes 100–200 lm in thickness were obtained with a doctor blade gap of 250 lm. The non-textured oxide layers (RBAO, ATZ, and RBM) were cast by an identical gel-casting process, but without incorporation of the seed platelets. The slip compositions for the TA layers and the three types of non-textured oxide layers are listed in Table 2. After casting, the tapes were detached from the Mylar sheet, by inverting the substrate in a distilled water bath. The tapes were then washed to remove excess cation, and 1.25¢¢ diameter disks were stamped from the washed tapes using a polished steel punch (Buehler). Green laminate specimens were pressed from a stacked lay-up of the textured and non-textured disks in a stainless steel die (also 1.25¢¢ in diameter), using a single filter paper top and bottom to prevent adhesion and allow excess water to escape. The spec￾imens were uniaxially pressed in the die at ~80 Mpa for 20 min and subsequently cold isostatically pressed (CIP) at 250–300 MPa for 30 min. Sintering Residual water was removed by drying in a 2.45 GHz microwave oven (EM-S301, SANYO) operated at 100– 150 W for 30–60 min. The progress of microwave drying was assessed by weighing the samples. The small quantity of organic additives (<1 vol% of dispersant and alginate) was removed during the sintering/reac￾tion-bonding cycle and not as a separate burnout treatment. As previously reported for monolithic RBAO ceramic processing [20–23], a somewhat complex heating cycle is required to sinter the RBAO/TA laminates to high density (avoiding swelling) and with 100% conversion to oxide. Oxidation of the aluminum metal particles at low temperatures (below the melting point) is essential and was accomplished by an initially low heating rate (1 C/min) and low temperature hold (600 C, 4–10 h). The green RBAO/TA laminated samples were placed on a porous alumina tile to ensure free oxygen access beneath the sample during reaction bonding. The alumina-toughened zirconia/textured alumina (ATZ/TA) and reaction-bonded mullite/textured alumina (RBM/TA) laminates were sintered conven￾tionally. Dissociation of zircon and its reaction with alumina to form zirconia and mullite only occurs above 1400 C [24], so the heating cycle selected for the RBM laminates was 5/min up to 1400 C, 2/min from 1400 to1600 C, followed by holding for 5 h at 1600 C and then cooling to room temperature at 5 C/min. The effect of the zircon particle size on the phase evolution of the RBM layers was evaluated by X-ray diffraction phase analysis and the final density of the laminate composite samples was determined by Archimedes density measurements. Texture characterization The chemical homogeneity and composition of indi￾vidual grains were determined from carbon-coated, polished and thermally etched samples using conven￾tional scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS—JEOL 840). The microstructure of cross-sections taken from laminated samples was also examined by diffraction contrast transmission electron microscopy (TEM-JEOL 2000- FX) at 200 kV. TEM samples were prepared by dia￾mond-sectioning perpendicular to the original plane of the precursor tape, followed by mechanical grinding, dimpling, and precision ion polishing (PIPS—Gatan, Model 691). The Harris method [25–27] was used to quantify the degree of crystallographic alignment (texture) in all specimens. X-ray diffraction spectra were taken from polished surfaces prepared parallel to the direction of tape casting. Reference diffraction data were obtained from randomly oriented high purity alumina powder (HPA-0.5, Ceralox). All X-ray data were collected on an X-ray diffractometer with a Table 2 Slip composition for gel casting of textured and non-textured oxide layers (vol%) * DISPEX A40, Allied Colloid, England Tape Matrix powder Al2O3 platelet Dispersant* Na alginate Water TA 25–30 1–6 0.15–0.2 0.5–0.6 66–72 RBAO 32–38 0 0.15–0.2 0.5–0.6 62–68 ATZ 25–30 0 0.2–0.25 0.6–0.7 70–75 RBM 28–32 0 0.2–0.25 0.5–0.6 68–72 J Mater Sci (2006) 41:7425–7436 7427 123