T. Chartier. T. Rouxel 2 Experimental Procedure green density, (ii)good microstructural homo- geneity, and (iv) good thermocompression ability 2. 1 Starting materials Two parameters were chosen to improve the Tape casting slurries are complex, multicompo urry composition with regards to the crack sensi- nent systems, which contain ceramic powders tivity, density and thermocompression ability of (including sintering aids), solvents, dispersants, the green tapes. The first is the volume solids binders and plasticizers. The starting powders are ratio(X= vol% solids= powder/powder disper 997 wt% purity, 0.5 um grain sizc alumina sant t binder plasticizer) and the sccond is the (P172SB, Pechiney, France)and 97.5 wt% purity, volume binder to plasticizer ratio(r binder/ 0 4 unl grain size zirconia(UPH 12, Criceram, plasticizer). Alunina+10 vol% zirconia slurries were fran prepared and tape cast with volume solids values Tape casting slurries were prepared with an varying from 0 6 to 0.9 with steps of 0.05 and azeotropic mixture of methyl ethyl ketone(MeK) binder/plasticizer values varying from 0.3 to 1. 9 and ethanol (EtOH)(40/60 vol%o), which is a with steps of 0-4. In all cases, the viscosities rather low polarity solvent (dielectric constant of slurries were adjusted to I Pa s by addition of 20). The use of an efficient dispersant is necessary vent to obtain an homogeneous and stable dispersion of ceramic particles in the solvent, and to achieve 2.3 Tape casting a low viscosity with a high ceramic/organic ratio. Tape casting was performed with a laboratory a stable dispersion of deagglomerated particles tape casting bench(Cerlim Equipement Limoges, leads to a dense particle packing and to an homo- france). Slurries were tape cast onto a fixed glass geneous microstructure. According to previous plate with a moving double blade device at a con studies, 5-18 phosphate esters were chosen to dis- stant speed of I m min. The thickness of the perse Al,O3 and ZrO, powders in the MEK/EtoH green tapes was 160 um solvent. The effectiveness of different phosphate sters was evaluated using the viscosity of the 2. 4 Processing of alumina-zirconia laminar ceramic/dispersant/solvent systems composites The binder used is a polyvinyl butyral(PVB) Laminar composites consisting of stacked layers and the plasticizer a mixture of polyethylene gly- of alumina with various zirconia contents(Al,O3 col(PEG)and dibutyl phthalate(DBP) A, AL,O, 5 vol% ZrO,= AZ5. Al 10 vol% ZrO,= AZlO)were fabricated by 2.2 Slurry preparation thermocompression, pyrolysis of the organic com The preparation of slurries is carried out in two ponents and sintering. Two series of composites stages, namely (i) the deagglomeration and disper- were prepared by thermocompression of 19 to 22 sion of powders in the solvent with the aid of the single layers of different compositions with differ dispersant, and (ii) the homogenization of the ent stacking sequences the number of layers slurry with binders and plasticizers. The sequence depends on the stacking sequence. The first series f component addition is critical. The dispersant is devoted to the influence of the processing has to be added before the binders to prevent routes, transformation toughening and interfacial competitive adsorption. The initial adsorption effects. The second series consists of composites of of the binder on the particle surfaces would a and Azio layers with different stacking prevent the dispersant from being adsorbed subse- sequences. Both series are designed according to quently, thereby decreasing its effectiveness. Fur- the schematic drawings of Fig. 1. A/A. aZS/AZ5 thermore, the deagglomeration is morc cfficicnt in and AZ1O/AZ10 arc not true laminar composites a low-viscosity system (i.e. without binders and however these materials were fabricated for com- plasticizers) and the mechanical damage of parison with the properties of laminar composites binder molecules is minimized by this seq and with monolithic materials prepared by dry of addition. The deagglomeration is carried out pressing (i.e. A and AZIO by ultrasonic treatment. The second stage of The thermocompression was performed at homogenization is performed by milling for 24 h. 110 C under a pressure of 60 MPa The slurry is rotated continuously at a slow speed for de-aeration and to prevent settling 2.5 Pyrolysis and sintering All the organic components affect the rheolog Thermal debinding remains one of the most criti cal behaviour of the slurry and therefore affect the cal steps of ceramic processing and requires an properties of the green tapes. An optimized slurry efficient heating cycle to prevent stresses and the should lead to tapes which satisfy the following formation of defects in ceramic parts. According criteria: (i)no cracking during drying, (i1) high to the thermogravimetric analysis of tape-cast300 T. Chartirr. T. Rouxrl 2 Experimental Procedure 2.1 Starting materials Tape casting slurries are complex, multicompo￾nent systems, which contain ceramic powders (including sintering aids), solvents, dispersants, binders and plasticizers. The starting powders are 99.7 wt% purity, 0.5 pm grain size alumina (P172SB, Pechiney, France) and 97.5 wt% purity, 0.4 ,um grain size zirconia (UPH 12, Criceram, France). Tape casting slurries were prepared with an azeotropic mixture of methyl ethyl ketone (MEK) and ethanol (EtOH) (40/60 vol%), which is a rather low polarity solvent (dielectric constant = 20). The use of an efficient dispersant is necessary to obtain an homogeneous and stable dispersion of ceramic particles in the solvent, and to achieve a low viscosity with a high ceramic/organic ratio. A stable dispersion of deagglomerated particles leads to a dense particle packing and to an homo￾geneous microstructure. According to previous studies,‘5m’8 phosphate esters were chosen to dis￾perse A&O, and ZrO, powders in the MEWEtOH solvent. The effectiveness of different phosphate esters was evaluated using the viscosity of the ceramic/dispersant/solvent systems. The binder used is a polyvinyl butyral (PVB) and the plasticizer a mixture of polyethylene gly￾co1 (PEG) and dibutyl phthalate (DBP). 2.2 Slurry preparation The preparation of slurries is carried out in two stages, namely (i) the deagglomeration and disper￾sion of powders in the solvent with the aid of the dispersant, and (ii) the homogenization of the slurry with binders and plasticizers. The sequence of component addition is critical. The dispersant has to be added before the binders to prevent competitive adsorption.17 The initial adsorption of the binder on the particle surfaces would prevent the dispersant from being adsorbed subse￾quently, thereby decreasing its effectiveness. Fur￾thermore, the deagglomeration is more efficient in a low-viscosity system (i.e. without binders and plasticizers) and the mechanical damage of the binder molecules is minimized by this sequence of addition. The deagglomeration is carried out by ultrasonic treatment.” The second stage of homogenization is performed by milling for 24 h. The slurry is rotated continuously at a slow speed for de-aeration and to prevent settling. 2.4 Processing of alumina-zirconia laminar composites Laminar composites consisting of stacked layers of alumina with various zirconia contents (Al,O, = A, A&O, + 5 ~01% ZrOz = AZ5, A&O3 + 10 ~01% ZrO, = AZlO) were fabricated by thermocompression, pyrolysis of the organic com￾ponents and sintering. Two series of composites were prepared by thermocompression of 19 to 22 single layers of different compositions with differ￾ent stacking sequences. The number of layers depends on the stacking sequence. The first series is devoted to the influence of the processing routes, transformation toughening and interfacial effects. The second series consists of composites of A and AZ10 layers with different stacking sequences. Both series are designed according to the schematic drawings of Fig. 1. AIA, AZ5iAZ5 and AZlO/AZlO are not true laminar composites: however these materials were fabricated for com￾parison with the properties of laminar composites and with monolithic materials prepared by dry pressing (i.e. A and AZlO). The thermocompression was performed at 110°C under a pressure of 60 MPa. 2.5 Pyrolysis and sintering All the organic components affect the rheologi- Thermal debinding remains one of the most criti￾cal behaviour of the slurry and therefore affect the cal steps of ceramic processing and requires an properties of the green tapes. An optimized slurry efficient heating cycle to prevent stresses and the should lead to tapes which satisfy the following formation of defects in ceramic parts. According criteria: (i) no cracking during drying, (ii) high to the thermogravimetric analysis of tape-cast green density, (iii) good microstructural homo￾geneity, and (iv) good thermocompression ability. Two parameters were chosen to improve the slurry composition with regards to the crack sensi￾tivity, density and thermocompression ability of the green tapes. The first is the volume solids ratio (X = ~01% solids = powder/powder + disper￾sant + binder + plasticizer) and the second is the volume binder to plasticizer ratio ( Y = binder/ plasticizer). Alumina+ 10 vol”% zirconia slurries were prepared and tape cast with volume solids values varying from 0.6 to 0.9 with steps of 0.05 and binder/plasticizer values varying from 0.3 to 1.9 with steps of 0.4. In all cases, the viscosities of slurries were adjusted to 1 Pa s by addition of solvent. 2.3 Tape casting Tape casting was performed with a laboratory tape casting bench (Cerlim Equipement, Limoges, France). Slurries were tape cast onto a fixed glass plate with a moving double blade device at a con￾stant speed of 1 m min ‘. The thickness of the green tapes was 160 pm