Journal of the European Ceramic Society 17(1997)299-5 Printed in Great b itin ea s richt reserved PII:s0955-2219(96)00131-8 0955-221997517 Tape-Cast Alumina-Zirconia Laminates Processing and mechanical Properties T. Chartier &t. rouxel LMCTS, URA CNRS 320, ENSCI, 47 Av. Albert Thomas, 87065 Limoges Cedex, France (Received 15 September 1995; revised version received 19 June 1996; accepted 15 July 1996) abstract aim is to improve both the toughness and the strength. This type of composite allows the associ Alumina-zirconia laminar ceramics made from lay- ation of two reinforcing mechanisms, the first act ers of different compositions with diferent stacking ing at the scale of the microstructure, inside the sequences were fabricated by tape casting and com- layers, due to the stress-induced transformation of pared. A phosphate ester dispersant was optimized zirconia particles. the second acting at a macro in MEK/EtOH and an optimum formulation of scopic scale, due to the interfaces between the lay organic components for tape casting was defined. ers. In these laminar structures, residual stresses The fracture resistance, toughness and elastic prop- develop during cooling from the sintering temper erties were characterized. A significant improvement ature because of the differences in thermal expa of both the fracture resistance and the toughness, sions between layers of different compositions from 380 t0 560 Pa and from 3.7 to8 MPavm The sign and the magnitude of these stresses may respectively, was gained between the pressed alu- be adjusted through the compositions, the stacking mina monolith and the tupe-cast AlO -ZrO, comm- sequences, and also through the layer thicknesses posites. The improvement was tentatively related to Hence, it is possible to develop high compressive The presence of residual stresses at both the micro- stresses in thin layers whereas the tensile stresses copic scale( phase transformation toughening )and remain low in the associated thick layers at the macroscopic scale( interface effects). C1996 Tape casting, -4 which is extensively used for Elsevier Science limited electronic ceramics. is well suited to fabricate homogeneous wide and thin ceramic sheets that can be reinforced by zirconia particles. Multilayer 1 Introduction systems are made by stacking green sheets, lami nating, removing the organic components and sin- Engineering applications require improved mechan- tering. Tape casting involves the dispersion of th cal properties of ceramics, particularly fracture ceramic powder in a solvent (typically organic) toughness. A noticeable increase in toughness can with the aid of a dispersant, followed by the addi obtained through ceramic composites. At the tion of binders and plasticizers to ensure the cohe- microscopic scale, composites can be roughly sion, fexibility and workability of the green tape classified into particle-, platelet-and whisker-rein- when the solvent is evaporated forced materials. At the macroscopic scale, com s paper posite materials can be arranged according to nated AlO3-ZrO, composites by tape casting and various configurations, among which are lami- the optimization of the dispersion which is a cru nated composites, consisting of alternate layers cial step. The influence of organic compounds on with different compositions. These structures pro- properties of green tapes, cracking sensitivity, den ide the opportunity for tailoring the properties sity and thermocompression ability will also be by stacking layers of different compositions in a reported. The room temperature fracture char suitable sequence. It is then possible to produce teristics (strength, toughness, fracture path)and functionally gradient ceramics to meet specific elastic properties were investigated to give an requirements- insight into the complex nature of the mechanical The laminated ceramic composites which are behaviour of ceramic laminates and to illustrate studied here consist of alternate layers of alumina the advantages of the composite materials over reinforced by various amount of zirconia. The monoliths. 299
Printed in Great Britain. All rights reserved PII: SO955-2219(96)00131-8 0955-22191971Sl7.00 Tape-Cast Alumina-Zirconia Laminates: Processing and Mechanical Properties T. Chartier & T. Rouxel LMCTS, URA CNRS 320, ENSCI, 47 Av. Albert Thomas, 87065 Limoges Cedex, France (Received 15 September 1995; revised version received 19 June 1996; accepted 15 July 1996) Abstract Alumina-zirconia laminar ceramics made from layers of dtflerent compositions with dtyerent stacking sequences were fabricated by tape casting and compared. A phosphate ester dispersant was optimized in MEWEtOH and an optimum formulation of organic components for tape casting was defined. The fracture resistance, toughness and elastic properties were characterized. A signt$cant improvement of both the fracture resistance and the toughness, from 380 to 560 MPa and from 3.7 to 8 MPadm respectively, was gained between the pressed alumina monolith and the tape-cast A120J-Zr02 composites. The improvement was tentatively related to the presence of residual stresses at both the microscopic scale (phase transformation toughening) and at the macroscopic scale (interface eflects). 0 1996 Elsevier Science Limited. 1 Introduction Engineering applications require improved mechanical properties of ceramics, particularly fracture toughness. A noticeable increase in toughness can be obtained through ceramic composites. At the microscopic scale, composites can be roughly classified into particle-, platelet- and whisker-reinforced materials. At the macroscopic scale, composite materials can be arranged according to various configurations, among which are laminated composites, consisting of alternate layers with different compositions. These structures provide the opportunity for tailoring the properties by stacking layers of different compositions in a suitable sequence. It is then possible to produce functionally gradient ceramics to meet specific requirements. I-9 The laminated ceramic composites which are studied here consist of alternate layers of alumina reinforced by various amount of zirconia.” The aim is to improve both the toughness and the strength. This type of composite allows the association of two reinforcing mechanisms, the first acting at the scale of the microstructure, inside the layers, due to the stress-induced transformation of zirconia particles, the second acting at a macroscopic scale, due to the interfaces between the layers. In these laminar structures, residual stresses develop during cooling from the sintering temperature because of the differences in thermal expansions between layers of different compositions. The sign and the magnitude of these stresses may be adjusted through the compositions, the stacking sequences, and also through the layer thicknesses. Hence, it is possible to develop high compressive stresses in thin layers whereas the tensile stresses remain low in the associated thick layers. Tape casting,’ ‘-I4 which is extensively used for electronic ceramics, is well suited to fabricate homogeneous wide and thin ceramic sheets that can be reinforced by zirconia particles. Multilayer systems are made by stacking green sheets, laminating, removing the organic components and sintering. Tape casting involves the dispersion of the ceramic powder in a solvent (typically organic) with the aid of a dispersant, followed by the addition of binders and plasticizers to ensure the cohesion, flexibility and workability of the green tape when the solvent is evaporated. This paper describes the processing of laminated A&O,-Zr02 composites by tape casting and the optimization of the dispersion which is a crucial step. The influence of organic compounds on properties of green tapes, cracking sensitivity, density and thermocompression ability will also be reported. The room temperature fracture characteristics (strength, toughness, fracture path) and elastic properties were investigated to give an insight into the complex nature of the mechanical behaviour of ceramic laminates and to illustrate the advantages of the composite materials over monoliths. 299
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-cast
300 T. Chartirr. T. Rouxrl 2 Experimental Procedure 2.1 Starting materials Tape casting slurries are complex, multicomponent 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 homogeneous microstructure. According to previous studies,‘5m’8 phosphate esters were chosen to disperse 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 glyco1 (PEG) and dibutyl phthalate (DBP). 2.2 Slurry preparation The preparation of slurries is carried out in two stages, namely (i) the deagglomeration and dispersion 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 subsequently, thereby decreasing its effectiveness. Furthermore, 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 components and sintering. Two series of composites were prepared by thermocompression of 19 to 22 single layers of different compositions with different 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 comparison 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 critical 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 homogeneity, and (iv) good thermocompression ability. Two parameters were chosen to improve the slurry composition with regards to the crack sensitivity, density and thermocompression ability of the green tapes. The first is the volume solids ratio (X = ~01% solids = powder/powder + dispersant + 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 constant speed of 1 m min ‘. The thickness of the green tapes was 160 pm
Tape-cast alumina-zirconia laminates processing and mechanical properties FIRST SERIES PRESSED LAMINATED (21 LAY Load roller A210 A/A AZ5/AZ5 AZ10/AZ10AZ10/A25 Support rolle A25 +10 vol Zr02 SECOND SERIES 3A/A210A Fig. 2. Chevron-notched(CN)specimens used for the fracture 5A10 10A210 toughness and fracture energy determination 2A10 3AAz1o】A 3n5/3 2/2/2n02/2/2 span length and a cross-head speed of 0.1 mm Fig. I. Schematics of the two series of laminar composites min-l. The toughness was measured by four-point bending of4×3×50mm( height x width length)chevron-notched(CN) specimens( with a 60 V-notch angle, and a 0.15 mm samples, the pyrolysis was carried out with heat- width, inner and outer span of 20 and ing at rate of 1C min up to 120C, then with respectively, and cross-head speeds between 0-001 heating at rate of 0.lC min up to 550C with and 0.I mm min. The V-notch is cut perpendicu dwell time of 4 h. The samples were sintered in an lar to the laminae. However, two different orienta electric furnace with MoSi, heating elements, with tions of the V, relative to the orientation of the heating at rate of 5C min up to 1600C with laminae, were investigated to make the crack dwell time of 3 h propagate normal to the layers(N specimens)or transverse to them (T specimens). The mode I 2.6 Characterization fracture toughness Kic is then calculated from the Rheological measurements were performed using maximum load measured on the experimental a rotating cylinder viscometer(Rotovisco RV12, curves, using the polynomial equation of munz Haake)at a shear rate of 28s". The shear rate et al. 2 Chevron-notched specimens were used as evaluated according to the gap between because of their suitability for fracture toughnes he casting support and the moving blade, and to determination in brittle materials. Sharpness of the casting speed. the notch is not critical and in the absence of the The apparent densities of green samples, exclud- R-curve effect, Kc values are relatively indepen ing organic phases, were determined by measuring dent of the initial crack length and agree with the ht (mcal ) of samples values obtained from straight-through crack speci before and after calcination, respectively. The mens with notch thickness as low as 66 um(single apparent density is expressed by the Mcal/V ratio. edge notched beam specimens ). The elastic mod- Evaporation of solvent can causc visible crack- uli were measured by means of an ultrasonic tech ing of tapes. The tape shrinkage, and then the nique, using 10 MHz piezo-electric transducers in shrinkage rate, during drying were measured by a contact with specimens. Youngs modulus(E)and laboratory-made detector using a laser system.20 the shear modulus (G)are calculated from the ers in the green composite was detected using an shear(V) wave velocities according to 2O)and The presence of any delaminations between lay- measured values for the compressional (V)and ultrasonic method. 2 (3v2-4V3) The mechanical testing was conducted in bend E P and G=pV (1) ing on an INSTRon 8562 testing machine equipped with a differential measurement device, oy means of a Linear Variable Displacement where p is the specific mass, as measured by Transducer(LvdT)and mechanical contact with Archimedes mcthod using distilled watcr Poissons he specimen, to accurately measure the specimen ratio (v)is then given by deflection. Fracture tests were performed in three- point bending,on3×4×25mm( height width x length) rectangular bars with a 20 mm
Tape-cast alumina-zirconia laminates: processing und mechanical properties 301 FIRST SERIES PRESSED LAMINATED (21 LAYERS) A AZ10 AIA AiWAZ5 A21 OlAzlO AZ1 OIAZS A= Al203 A25= Al203+ 5VOl% 2fO2 AZ1 0 = A1203 + 10 VOl% LfD2 SECOND SERIES 3lAIAZl OIIA 2A 3(A/AZlOl/A 2A 7/5/7 3n 513 2/2/211012/212 Fig. 1. Schematics of the two series of laminar composites tested. samples, the pyrolysis was carried out with heating at rate of 1°C min’ up to 120°C then with heating at rate of O.l”C min’ up to 550°C with a dwell time of 4 h. The samples were sintered in an electric furnace with MoSi, heating elements, with heating at rate of 5°C min’ up to 1600°C with a dwell time of 3 h. 2.6 Characterization Rheological measurements were performed using a rotating cylinder viscometer (Rotovisco RV12, Haake) at a shear rate of 28 s&. The shear rate was evaluated according to the gap between the casting support and the moving blade, and to the casting speed. The apparent densities of green samples, excluding organic phases, were determined by measuring the volume (V) and weight (Mea,,) of samples before and after calcination, respectively. The apparent density is expressed by the M,,,,/V ratio. Evaporation of solvent can cause visible cracking of tapes. The tape shrinkage, and then the shrinkage rate, during drying were measured by a laboratory-made detector using a laser system.20 The presence of any delaminations between layers in the green composite was detected using an ultrasonic method.2 The mechanical testing was conducted in bending on an INSTRON 8562 testing machine equipped with a differential measurement device, by means of a Linear Variable Displacement Transducer (LVDT) and mechanical contact with the specimen, to accurately measure the specimen deflection. Fracture tests were performed in threepoint bending, on 3 X 4 X 25 mm (height X width X length) rectangular bars with a 20 mm / Load roller Fig. 2. Chevron-notched (CN) specimens used for the fracture toughness and fracture energy determinations. span length and a cross-head speed of 0.1 mm min-‘. The toughness was measured by four-point bending of 4 X 3 X 50 mm (height X width X length) chevron-notched (CN) specimens (Fig. 2), with a 60” V-notch angle, and a 0.15 mm notch width, inner and outer span of 20 and 40 mm respectively, and cross-head speeds between 0.00 1 and 0.1 mm min’. The V-notch is cut perpendicular to the laminae. However, two different orientations of the V, relative to the orientation of the laminae, were investigated to make the crack propagate normal to the layers (N specimens) or transverse to them (T specimens). The mode I fracture toughness K,, is then calculated from the maximum load measured on the experimental curves, using the polynomial equation of Munz et ~1.~’ Chevron-notched specimens were used because of their suitability for fracture toughness determination in brittle materials. Sharpness of the notch is not critical and in the absence of the R-curve effect, K,, values are relatively independent of the initial crack length and agree with the values obtained from straight-through crack specimens with notch thickness as low as 66 pm (single edge notched beam specimens).2’ The elastic moduli were measured by means of an ultrasonic technique, using 10 MHz piezo-electric transducers in contact with specimens. Young’s modulus (E) and the shear modulus (G) are calculated from the measured values for the compressional (V,) and shear (V,) wave velocities according to:22 and G = pV,’ (1) where p is the specific mass, as measured by Archimedes method using distilled water. Poisson’s ratio (v) is then given by:23 E v=__l 2G
T. Chartier T. Rou./ 3 Results and discussion VISCOSITY (mPa. s) 3.1 Tape casting of layers 1500 30vo.%Ak23 3.1. I Selection of the parameters of phosphate esters The effectiveness of the different phosphate esters HLB 1< HLB 2< HLB 3 HLB 4 was evaluated using the viscosity of the alumina HLB 2 dispersant/solvent systems. The phosphate esters used were prepared by reaction between phospho- B3HB4 cid and an ethoxylate. The ethoxy 500 obtained by condensation of ethylene oxide in alcohol. The esters contain a combination of mono- and diesters and the remains of the ethoxylate not combined with the phosphoric acid. The chemical ESTER/AL2O3(wt %) structure of a typical phosphate ester is shown in Fig. 4. Apparent viscosity of a 30 volo alumina susper Fig. 3. The infuence of four parameters was stud versus dispersant concentration for phosphate esters different hlb value ied, namely (i)the molecular structure(aliphatic or aromatic), 5( i) the monoester/diester ratio, (ii) the degree of phosphatization and (iv) the the unreacted ethoxylate disturbs ceramic/disper Hydrophile/Lipophile Balance(HLB). The addition sant interactions and increases the viscosity, and of phosphate ester to alumina results in lowering (iv) a high hLB because adsorption on charged the pH, indicating that the dispersant partially dis- ceramic particles is favoured by more hydrophilic phosphate ester, the alumina dispersants(Fig 4) powder exhibited a slightly negative surface The most efficient phosphate ester, fulfilling all positive after addition of phosphate ester suggest- cialized( Beycostat C213, CECA, France s mmer- MEK/EtOH. The surfacc reversed to these characteristics, was synthesized and ce g that the H*ions liberated on dissociation were adsorbed onto alumina particles. Phosphate esters 3. 1.2 Selection of the slurry formulation act by a combination of electrostatic and steric For Al2O3 +10 vol% ZrO2 green sheets, cracking repulsion. The steric hindrance prevents contact develops for a volume solids ratio(X) higher than between particles. The double layer, which may be 0. 75 and for a binder/plasticizer ratio (r) higher due to net charge on the particle surface and/ than 0-7(Table 1). Cracking depends on shrinkage or charges coming from the dissociation of the rate, which depends itself on the composition of adsorbed polymer, provides repulsion by a poten- the slurry. The maximum shrinkage rate decreases tial energy barrier at larger distances when the volume solids ratio decreases. The high The best dispersion (i.e. the lowest viscosity) is content of organic phase slows down the motion achieved with a phosphate ester having (i)an of particles, then reduces the shrinkage rate aliphatic molecule, (ii)a high diester concentra- Binders are polymeric molecules which adsorb tion, as diesters contain two lipophilic tails, each on the particle surfaces and form organic bridges able to extend into the solvent for steric sta biliza I'able 1. Cracking during drying (C: cracking, N C:non tion. (ii)a high degree of phosphatization because cracking) and green density, excluding the organic phase of alumina+10 vol zirconia tapes (theoretical density = 4. 19 g MONOESTER Composition Cracking Green densit OH HH HH Y HO-P-0-C-C-0)(C)-C-H HH HYDROPHILE LIPOPHILE < CRACKING LIMIT DIESTER OH 2.64 RO-P-OR RACKING LIMIT 9 4 Fig. 3. Typical chemical structure of a phosphate ester
302 T. Churtier. T. Rouse1 3 Results and Discussion 3.1 Tape casting of layers 3. I. 1 Selection of’ the parameters oj’phosphate esters The effectiveness of the different phosphate esters was evaluated using the viscosity of the alumina/ dispersant/solvent systems. The phosphate esters used were prepared by reaction between phosphoric acid and an ethoxylate. The ethoxylate is obtained by condensation of ethylene oxide in alcohol, The esters contain a combination of monoand diesters and the remains of the ethoxylate not combined with the phosphoric acid. The chemical structure of a typical phosphate ester is shown in Fig. 3. The influence of four parameters was studied, namely (i) the molecular structure (aliphatic or aromatic),15 (ii) the monoester/diester ratio, (iii) the degree of phosphatization, and (iv) the Hydrophile/Lipophile Balance (HLB). The addition of phosphate ester to alumina results in lowering the pH, indicating that the dispersant partially dissociates. Without phosphate ester, the alumina powder exhibited a slightly negative surface charge in MEWEtOH. The surface reversed to positive after addition of phosphate ester suggesting that the H’ ions liberated on dissociation were adsorbed onto alumina particles. Phosphate esters act by a combination of electrostatic and steric repulsion. The steric hindrance prevents contact between particles. The double layer, which may be due to net charge on the particle surface and/ or charges coming from the dissociation of the adsorbed polymer, provides repulsion by a potential energy barrier at larger distances. The best dispersion (i.e. the lowest viscosity) is achieved with a phosphate ester having (i) an aliphatic molecule, (ii) a high diester concentration, as diesters contain two lipophilic tails, each able to extend into the solvent for steric stabilization, (iii) a high degree of phosphatization because MONOESTER R I 0,H 7 lyi 7 c;’ ’ HO-P-0-(C-C-0)-U-C-H c; AA “A’A i._ _I I J HYDROPHILE LIPOPHILE DIESTER OIH RO-P-OR t; Fig. 3. Typical chemical structure of a phosphate ester. VISCOSITY 1mPa.s) 2.000 ,,500_ -2 3Ovol.% A’2O3 1 HLE 1 0.8 0.7 c. 2.56 x7 0.9 0.7 c. 2.60 Yl o-7 0.3 N.C. 2.64 Y2 o-7 o-7 N.C. 2.48 Y3 o-7 1.1 C. 2.45 Y4 0.7 I.5 C. 2.43 Y5 o-7 I .9 C. 2.40
Tape-cast alumina-zirconia laminates: processing and mechanical properties Table 2. Composition of the retained tape casting slurry Table 3. Physical properties of the various grades tested. E and G are measured normal to the direction of the layering Function Volv Poissons ratio is equal to 0- 24 for all the grades Alumina t Zirconia Ceramic powder 306 Material E(GPa) G(GPa Solvent Dispersant PVB A 143 PEG 300 Plasticizer Plasticizer AZo 880 AZS/AZ4 9.06 AZ 342 AZIO/AZ5 between them. The shrinkage rate increases when 7/5/7 362 the binder/plasticizer ratio increases because the 3/15/3 high binder content increases the links between 2/2/2102/2/2 particles and then the shrinkage rate. In order to avoid cracking during drying, the formulation of the slurry has to be defined to achieve a low way that compressive residual stresses develop cizer-rich organIc phase 6 igh amount of plasti- the outer layers. These residual stresses were shrinkage rate i.e. with a n the of the azio induced upon cooling from high temperature system, the shrinkage rate does not exceed a value (1400C), as a result of different thermal expan of I um s sion coefficients between the layers. The classical The apparent densities of dried tapes, excluding plate theory(plane stress hypothesis) was used to the organic phase, were determined for various calculate the normal stresses in each layer of the volume solids and binder/plasticizer ratios. When two-phase composite. 4 In its main lines, the cal- the volume solids ratio decreases, the organic phase culation is based on the assumption that a cross- prevent particles from packing together, therefore section before deformation remains a cross-section he density decreases. A low binder/plasticizer after deformation and that layers remain plane ratio leads to green tapes with higher densiti and parallel to each other throughout deformation When the binder/plasticizer ratio decreases, the This results in the following form for the elastic low-viscosity plasticizer-rich organic phase allows displacement vector at any point (x, x2, x3) a good packing of ceramic particles l{(x1,x2) 3. 1.3 Thermocompression ability u=2=2(x1,x2 The thermocompression behaviour of green sheets is sensitive to x and Y. delaminations were numerous in samples with low quantities of organic where (1, 2) subscripts refer to the in-plane axis phases (X4 to X7)(Table 1). a lot of delamina- and (3)is normal to the layering The studied lam tions were observed in samples with a high quan- inates are constructed such that they have com tity of plasticizer (Y1). Plasticizers are low plete symmetry of individual lamina thickness and molecular weight species which can act as a lubri- properties about the middle plane of the laminate cant between the individual layers, limiting the Furthermore, no texture was introduced by the bond strength during thermocompression tape casting process, so that the studied laminates The final formulation of tapc casting slurries can be considered as perfectly orthotropic. Hence, was defined in order to tape cast non-cracked in the case of pure linear elasticity, the thermal tapes with a high green density, which do not lead induced normal(N, and N2) and tangential (T12 to delamination during thermocompression of forces, per unit length, are given by: 4 laminated composites. This formulation (Table 2) was applied to the tape casting of pure alumina N1「A1A1201e and of AlO3+5 vol%o ZrO, with similar green tape A21A220 characteristics and a good thermocompression 00 AbLE ability As= 2Gd 3. 2 Laminar composites I du: du 3. 2. I Design and e sical f monoliths and com posites are given in Table 3. These physical prop- One recalls that eqns(4)only stand for the elastic erties were used to design the composites in such a part of the strain components(elastic gtotal-gthermal)
Tape-cast alumina-zirconia laminates: processing and mechanical properties 303 Table 2. Composition of the retained tape casting slurry Component Function Vol’% _ Alumina + Zirconia MEKlethanol Phosphate ester PVB PEG 300 DBP Ceramic powder 30.6 Solvent 57.6 Dispersant 0.8 Binder 4.6 Plasticizer 2.9 Plasticizer 3.5 between them. The shrinkage rate increases when the binder/plasticizer ratio increases because the high binder content increases the links between particles and then the shrinkage rate. In order to avoid cracking during drying, the formulation of the slurry has to be defined to achieve a low shrinkage rate, i.e. with a high amount of plasticizer-rich organic phase. In the case of the AZ10 system, the shrinkage rate does not exceed a value of 1 pm ss’. The apparent densities of dried tapes, excluding the organic phase, were determined for various volume solids and binder/plasticizer ratios. When the volume solids ratio decreases, the organic phase prevent particles from packing together, therefore the density decreases. A low binder/plasticizer ratio leads to green tapes with higher densities. When the binder/plasticizer ratio decreases, the low-viscosity plasticizer-rich organic phase allows a good packing of ceramic particles. 3.1.3 Thermocompression ability The thermocompression behaviour of green sheets is sensitive to X and Y. Delaminations were numerous in samples with low quantities of organic phases (X4 to X7) (Table 1). A lot of delaminations were observed in samples with a high quantity of plasticizer (Yl). Plasticizers are low molecular weight species which can act as a lubricant between the individual layers, limiting the bond strength during thermocompression. The final formulation of tape casting slurries was defined in order to tape cast non-cracked tapes with a high green density, which do not lead to delamination during thermocompression of laminated composites. This formulation (Table 2) was applied to the tape casting of pure alumina and of A&0,+5 vol% ZrO, with similar green tape characteristics and a good thermocompression ability. 3.2 Laminar composites 3.2. I Design Some physical properties of monoliths and composites are given in Table 3. These physical properties were used to design the composites in such a Table 3. Physical properties of the various grades tested. E and G are measured normal to the direction of the layering. Poisson’s ratio is equal to 0.24 for all the grades Material E (GPa) G (GPa) (YXJ IJllV~ (IO” “C 1) A 355 143 8.69 Al A 363 146 AZ10 341 137 8.80 AZSIAZS 349 141 9.06 AZlOlAZlO 342 138 AZIOIAZS 71517 362 146 311513 349 141 212121 Io/21212 377 152 way that compressive residual stresses develop in the outer layers. These residual stresses were induced upon cooling from high temperature (14OO”C), as a result of different thermal expansion coefficients between the layers. The classical plate theory (plane stress hypothesis) was used to calculate the normal stresses in each layer of the two-phase composite.24 In its main lines, the calculation is based on the assumption that a crosssection before deformation remains a cross-section after deformation and that layers remain plane and parallel to each other throughout deformation. This results in the following form for the elastic displacement vector at any point (x,, x2, x3): [ Ul = r&x,, x2) ii = u2 = l&x,, x2) (3) u3 = &X3> where (1,2) subscripts refer to the in-plane axis and (3) is normal to the layering. The studied laminates are constructed such that they have complete symmetry of individual lamina thickness and properties about the middle plane of the laminate. Furthermore, no texture was introduced by the tape casting process, so that the studied laminates can be considered as perfectly orthotropic. Hence, in the case of pure linear elasticity, the thermally induced normal (N, and N,) and tangential (T,?) forces, per unit length, are given by:24 Ed with A,, = - vEd 1 - v2’ A,, = A,, = ~ 1 --I and E.. = ! %! + ?!! ” 2 [ aXj aXi 1 ’ (4) A,, = 2Gd One recalls that eqns (4) only stand for the elastic part of the strain components (.?lastic = E”‘~’ -E~~~““~‘)
T. Chartier, T, rouxel so that the normal force ni in a layer of composi Table 4. Calculated values for the residual stresses in the tion(I) is expressed by inner and outer layers of the two-phase composites N}=Ah1(elh-a△T)+A2(E12-a4△T)(5) AZl0AZ57/573/153222/102/22 where a' is the isotropic thermal expansion coeffi- n outer (MPa)-118 cient of material (I) au inner(MPa) I In the absence of external forces, (1)and(2) equivalent axes and En = ey N=(A1+A2)(el1-a2△T) (6) Results of this first-order calculation are sum- marized in table 4. According to the observa The overall equilibrium of the system in the(1) tions, the most questionable hypothesis of the direction gives, for a composite consisting of n model consists in taking u, as a function of (x, ayers of composition (I)and p layers of composi- x,), that the laminate sides did not form flat tion(m) surfaces after cooling from the joining tempera nN!+PN!=0 ture, hence that shear strains and stresses(Eu, and o13)develop upon cooling. Estimation of shear u, being independent of x, so are en and o. this effects is currently under survey allows N, to be expressed as a function of n N,=oondx,=ond (8) 3.2.2 Mechanical properties The A/A material without interface shows better Substituting in eqn (7)gives mechanical properties(o =408 MPa, KIC =5.5 nond+ pold=0 MPavm), compared to the same alumina sinter in the same conditions but pre by dry pr It is further assumed that a perfect interfacial ing(U= 380 MPa, KIc =4. 5 MPavm)(Figs 5a bonding exists bctween the layers, so an addi- tional kinematic hypothesis is now considered: uh FRACTURE STRESS (MPa i, and since the laminates consist of layers of identical length and width, it becomes: al= ell FIRST SERIES Extracting the strain from eqn (6) and taking 500 eqn( 8) for the normal force expression gives the following boundary condition +GAt lid A+4+a"47(10 By writing eqns(9)and (10)for each of the lami- possible calculate the normal residual stresses in the various layers of the composite, For instance, in the case of A/A A25JAZ5 AZ10JAZ10 AZ10JAZ5 the 2/2/2/10/2/2/2 grade, otherwise modelled by a IV/In/I/I/l/mI/IV sequence, the following set of equations is obtained FRACTURE STRESS (MPa Three boundary conditions SECOND SE +al△T H+a△r 400 E Ell g I a4△T(11) ol+a△T rY+a1△T 100 One equilibrium equation 3/15/32/2/2/10/2/2/2 5o1+2(o出+o})=0(12) This gives from the data of Table 3: o=all Fig. 5. Fracture resistance(3-point bend test) of the compos- 28 MPa and all =ali=-49 MPa es from. (a) the firs st series
304 T. Chrfirr so that the normal force Nf in a layer of composition (I) is expressed by : A’; = A;, (E;, - cu’AT) + A’,, (& - #‘AT) (5) where a? is the isotropic thermal expansion coefficient of material (I). In the absence of external forces, (I) and (2) are equivalent axes and E,, = E??: N; = (A;, + A;?) @f’ - a’AT) (6) The overall equilibrium of the system in the (1) direction gives, for a composite consisting of n layers of composition (I) and p layers of composition (II): nlv; + pN;’ = 0 (7) ul being independent of .x~, so are E,, and (T,,. This allows N, to be expressed as a function of (T, ,: N, = I; a,,& = a,,d Substituting in eqn (7) gives: (8) no# + pa\f# = 0 (9) It is further assumed that a perfect interfacial bonding exists between the layers, so an additional kinematic hypothesis is now considered: U\ ZZ u\‘, and since the laminates consist of layers of identical length and width, it becomes: E:, = E:\. Extracting the strain from eqn (6) and taking eqn (8) for the normal force expression gives the following boundary condition: Of ,d’ + &AT = af/d" A',, + A;: A” f A” + cu”AT (f0) II II By writing eqns (9) and (10) for each of the laminate composites, it is possible to calculate the normal residual stresses in the various layers of the composite. For instance, in the case of the 2/212/10121212 grade, otherwise modelled by a IV/IIIHI/I/IIIIIIiIV sequence, the following set of equations is obtained: Three boundary conditions a;, -I- a’AT = a;; + cu”AT CT;; -t a”AT = a;;’ + c&AT (11) One equilibrium equation 5a;, + 2(a;; + al,!’ + a;;) = 0 (12) This gives from the data of Table 3: gf’ = G:\’ = 28 MPa and al: = CT\: = 49 MPa. T. ~ou.~el Table 4. Calculated values for the residual stresses in the inner and outer layers of the two-phase composites AZlWAZS 7/M 3/15/j _ ,/7/‘/l _ i.af2lJ2Q - rrll ,,ULel (MPa) -118 -40 50 49 ~II inner WPaf 130 29 20 28 - ___- ..~._____ Results of this first-order catculation are summarized in Table 4. According to the observations, the most questionable hypothesis of the model consists in taking uI as a function of (x,, s7), that the laminate sides did not form flat surfaces after cooling from the joining temperature, hence that shear strains and stresses (E’~ and c’~) develop upon cooling. Estimation of shear effects is currently under survey. The A/A material without interface shows better mechanical properties (a, = 408 MPa, K,, = 5.5 MPa-\jm), compared to the same alumina sintered in the same conditions but prepared by dry pressing (Us = 380 MPa, K,, = 4.5 MPadm) (Figs 5a FRACTURE STRESS (MPa) ___ FIRST SERIES I ! ~ 5oo/ j :, __e 400 300 200 100 n - A A/A AZ51AZ5 AZlOlAZlO AZlOIAZ5 (4 FRACTURE STRESS IMPa) 5001-----_ ; SECOND SERtES 400 I 300 I 200 100 0 A/A 715/7 3/l 513 2/2/2/1012/2/2 @I Fig. 5. Fracture resistance (3-point bend test) of the composites from. (a) the first series (b) the second series
Tape-cast alumina-circonia laminates: processing and mechanical properties and 9a). This suggests that tape casting leads to tip healing processes(blunting)at relatively low a more homogeneous green state, with small com- testing rates. In conjunction, the work of fracture paction defects becomes larger and larger as the testing rate The fracture resistance (o) of both series decreases increases with the zirconia content(Fig. 5). A fur Works of fracture(WOF)of 1 9 x 104, 2. X ther improvemcnt is achicved by combining AZIo 10+ and 40x 104 J were dctcrmincd by comput ued AZ5 layers, probably due to interfacial ing the area below the load-deflection curves cor resses inl this latter case. Curiously, material lls responding to cross-head speeds of 0.1, 0.01 from the second series show lower fracture resis- 0-001 mm min, respectively. An estimation of tances than pure alumina( Fig 5b). The propagat- the intrinsic fracture surface energy (yi) can be ing crack is deflected several times in or near layer obtained from the griffith/Irwin similarity rela interfaces for composite materials(Fig. 6) tionship:y =2E. From our experimental data y Results of the Cn tests are depicted in Fig. 7. equals 10-7, 19 and 25.9 J m for cross-head Figure 7a shows the load/deflection curves obtained speeds of 0.1, 0-01 and 0-00l mm min, respec on the A, A/A and AZ1O/AZl0 with a deflection tively. This very simple calculation shows that rate of 0 01 mm min- together with results work of fracture is significantly higher than 2y S obtained on the aziO/AZ10 with different deflec-. where s is the minimum fracture surface area as tion rates. A and A/a curves are almost superim- defined by the chevron geometry(S= 4. 7 mm) posed; these materials exhibit unstable crack This would support either the occurrence of vis propagation. A/A fractured specimens show a fat coplastic energy dissipation, i. e. Effective >yi and/or fracture path, essentially transgranular, whereas crack deflection(Sermective S) AZIO/AZ10 specimens fractured with a mixed transgranular/ intergranular path giving rise to a rough fracture surface, as shown in Fig. 8. Stable or semi-stable crack propagations were observed in this latter case, with a significant influence of the cross-head speed on the maximum load E peak, i.e. the lower the displacement rate, the a 04Z1 higher the fracture toughness. This behaviour is tentatively attributed to the occurrence of crack FRACTURE PATH AND FRACTURE SURFACE Deflection'u’(mm) 2 Chevron notch tests performed in bending on(a)spec- from the second series with a cross-head Fig. 6. Fracture path in a 2/2/2/10/2/2/2 specimen fractured in speed of nin and two layers to crack front orienta- ns in the case of the 2/2/2/10/2/2/2 grad
Tape-cast alumina-zirconia laminates: processing and mechanical properties 305 and 9a). This suggests that tape casting leads to a more homogeneous green state, with small compaction defects. The fracture resistance (a,) of both series increases with the zirconia content (Fig. 5). A further improvement is achieved by combining AZ10 and AZ5 layers, probably due to interfacial stresses in this latter case. Curiously, materials from the second series show lower fracture resistances than pure alumina (Fig. 5b). The propagating crack is deflected several times in or near layer interfaces for composite materials (Fig. 6). Results of the CN tests are depicted in Fig. 7. Figure 7a shows the load/deflection curves obtained on the A, A/A and AZlO/AZlO with a deflection rate of 0.01 mm min.’ together with results obtained on the AZlOiAZlO with different deflection rates. A and A/A curves are almost superimposed; these materials exhibit unstable crack propagation. A/A fractured specimens show a flat fracture path, essentially transgranular, whereas AZlO/AZlO specimens fractured with a mixed tramgranular/ intergranular path giving rise to a rough fracture surface, as shown in Fig. 8. Stable or semi-stable crack propagations were observed in this latter case, with a significant influence of the cross-head speed on the maximum load peak, i.e. the lower the displacement rate, the higher the fracture toughness. This behaviour is tentatively attributed to the occurrence of crack FRACTURE PATH AND FRACTURE SURFACE Fig. 6. Fracture path in a 2/212/10/2/2/2 specimen fractured in bending. tip healing processes (blunting) at relatively low testing rates. In conjunction, the work of fracture becomes larger and larger as the testing rate decreases. Works of fracture (WOF) of 1.9 X lOA, 2.2 X lOA and 4-O x lOA J were determined by computing the area below the load-deflection curves corresponding to cross-head speeds of 0.1, 0.01 and 0.001 mm min-‘, respectively. An estimation of the intrinsic fracture surface energy (7;) can be obtained from the Griffith/Irwin similarity relationship: yi = s+ From our experimental data yi equals 10.7, 19 and 25.9 J mm2 for cross-head speeds of 0.1, 0.01 and 0.001 mm min’, respectively. This very simple calculation shows that work of fracture is significantly higher than 2yiS where S is the minimum fracture surface area as defined by the chevron geometry (S = 4.7 mm’). This would support either the occurrence of viscoplastic energy dissipation, i.e. ?/e~~tive >yi and/or crack deflection (Seffectiue > S). 32 "0 0 02 0 04 0.06 0 08 O.! 0.!2 0.14 0.16 Deflection ‘u’ (mm) (a) 32 a 4 0 I,, i 0 0.01 0.02 0.03 0.04 0.05 0.06 Deflection ‘u’ (mm) @I Fig. 7. Chevron notch tests performed in bending on (a) specimens of the AZlOiAZlO grade with different loading rates and (b) specimens from the second series with a cross-head speed of 10 pm min ’ and two layers to crack front orientations in the case of the 21212/10121212 grade
T. Chartier. T rouxel Fig 8. SEM micrographs of the fracture surfaces from CN specimens of (a)A/A and(b)AZIO/AZIO Highcr magnification vicws of the fracture path(c)A/A and (d)AZIO/AZIO A cross-head speed of 0.01 mm min was Youngs moduli of A and AZ10 in order to com selected to compare the materials of the second pare with the effective moduli of the diferent series by Chevron-Notched (CN)tests(Fig. 7b). composites(Fig. 10). It is observed that Youngs Under the chosen testing condition, the CN speci- modulus values measured parallel to the layering mens of this series failed in purely brittle mode (which should be close to the Voigt expression) (unstable crack propagation) with maximum load agree well with the prediction, whereas values peaks slightly higher than in the case of AZ10. In measured normal to the layering (corresponding addition, the incidence of the layer orientation to the reuss model) fall far outside the bounds with respect to the propagation direction was This statement confirms the presence of an interfa investigated on the 2/2/2/10/2/2/2 grade, by testing cial effect, affecting the elastic properties, i.e. the a V-notched CN specimen oriented in such a way ultrasonic wave propagation in the present case that the crack propagated parallel to the layer Possible sources for this effect include composition interfaces. This figure shows that the reinforcement heteroge es within the layer surfaces(chemical is much more significant normal to the layering alteration)and changes in the interatomic bond yalues for Kic are summarized in Fig 9, together ing distances(stored elastic energy or thermally ith the values previously obtained on the same induced residual stresses materials by the Single Edge Notched Bear (SENB) method. CN tests give lower values fo Kc than SENB tests as is expected. This is tenta- 4 Conclusion tively attributed to a sharper crack tip in the first case. Poissons ratios being identical for all stud- Laminar composites proved to have promising d materials, the theoretical prediction of Voigt mechanical properties with fracture strength and and Reuss(Hill bounds) were drawn from the toughness up to 1 5 to 2 times higher for the
306 T. Chartier, T. ROLLWI 1 mm (4 20 urn (b) (4 Fig. 8. SEM micrographs of the fracture surfaces from CN specimens of (a) A/A and (b) AZlOiAZlO. Higher magnification views of the fracture path (c) A/A and (d) AZlOiAZlO. A cross-head speed of 0.01 mm min ’ was selected to compare the materials of the second series by Chevron-Notched (CN) tests (Fig. 7b). Under the chosen testing condition, the CN specimens of this series failed in purely brittle mode (unstable crack propagation) with maximum load peaks slightly higher than in the case of AZlO. In addition, the incidence of the layer orientation with respect to the propagation direction was investigated on the 2/2/2/10/2/2/2 grade, by testing a V-notched CN specimen oriented in such a way that the crack propagated parallel to the layer interfaces. This figure shows that the reinforcement is much more significant normal to the layering. Values for K,, are summarized in Fig. 9, together with the values previously obtained on the same materials by the Single Edge Notched Beam (SENB) method.’ CN tests give lower values for K,, than SENB tests as is expected. This is tentatively attributed to a sharper crack tip in the first case. Poisson’s ratios being identical for all studied materials, the theoretical prediction of Voigt and Reuss (Hill bounds) were drawn from the Young’s moduli of A and AZ10 in order to compare with the effective moduli of the different composites (Fig. 10). It is observed that Young’s modulus values measured parallel to the layering (which should be close to the Voigt expression) agree well with the prediction, whereas values measured normal to the layering (corresponding to the Reuss model) fall far outside the bounds. This statement confirms the presence of an interfacial effect, affecting the elastic properties, i.e. the ultrasonic wave propagation in the present case. Possible sources for this effect include composition heterogeneities within the layer surfaces (chemical alteration) and changes in the interatomic bonding distances (stored elastic energy or thermally induced residual stresses). 4 Conclusion Laminar composites proved to have promising mechanical properties with fracture strength and toughness up to 1.5 to 2 times higher for the
Tape-cast alumina-zirconia laminates: processing and mechanical properties K1c(MPavm) K1c(MPavm FIRST SERIES (SENB) SECOND SERIES ICNI AZ5IAZ5 AZ10/A210 AZ10JAZ5 AAZ10/A210757 15/32/2210/2/2/2 Fig9. Fracture toughness of the diffcrent grades(a)SENB rcsults from Rcf. 2.(b )CN results(cross-head speed=0-01 mm min ') cast materials. The slurry formulation was opti- mized in order to tape cast defect-free samples 80 with a high green density and a good thermocom pression ability 四m5 2/2/2/10/2/2/2 370 360 3/15/ R eferences 350 Hill bounds 340Az10 Amateau, M. F.& Messing, G. L, Laminated ceramic omposites. In international Encyclopedia of Composite 2. Chartier, T, Besson, J. L.& Boch, P, Mechanical prop- 320 erties of ZrO -Al,O, laminated composites. In Advance E ics, Science and Technology of Zirconia Ill. E vol. 24, eds S. Somiya, N. Yamamoto& H. Yanagida m. Ceram Soc., Westerville, Ohio, 1988, pp. I131-1138 3. Boch, P, Chartier, T& Huttepain, M, Tape casting of 00.1020.30.40.50.60.70.80.9 12OyZro, laminated composites, J. Am. Ceram. Soc. Fig. 10. Youngs moduli of the different laminates. The Hill 9(8)(1986)191-19 bounds were drawn for comparison. Voigt and Reuss's curves 4. Chartier, T.& Besson, J. L,, Behaviour of ZrOx-AlO3 are almost superposed due to rather close Young,'s moduli for laminated composites loaded by various A and AZIo rrangements. In Science of Ceramics, vol. 14. ed. D. Tay- lor. The Institute of Ceramics, UK, 1988, pp. 639-644 5. Russo. C.J. Harmer, M. P. Chan, h. M.& miller, G A. Design of laminated ceramic composite for improved composite than for the bulk components. This strength and toughness. J. Am. Ceram. Soc.. 75(12) remarkable improvement is assumed to be essen (1992)3963400 tially related to the presence of thermally induced 6. Harmer, M.P, Chan. H, M.& Miller, G. A, Unique pportunities for microstructural engineering with dupley residual stre which, according to a and laminar ceramic composites. J, Am. Cerum. Soc plane stress model, can be as high as 130 MPa 75(7)(1992)1715-1728 Crack deflection is essentially a consequence of 7. Plucknett. K. P. Caceres. C. H.& Wilkinson. D...Tape sting of fine alumina/zirconia powders for co the residual stresses fabrication, J. Am. Ceram. Soc., 77(8)(1994)2137-2144 Phosphate esters are highly effective dispersants 8. Plucknett, K. P,, Caceres, C. H, Hughes. C.& wilkin- for alumina in MEK/Etoh solvent but their on.D.S, Processing of tape-cast laminates prepared effectiveness is strongly dependent on their charac 77(8)(1994)2145-2153. Powders.J. Am. Ceran. Soc teristics. Four parameters were varied and an 9. Takebe H. Morinaga, K. Fabrication and mechanical improved phospha aving (i)an aliph properties of lamellar AlO3 ceramics. J, Ceram. Soc. Jpn molecule, (ii) a high diester concentration, (ii)a Inter.,96(1988)l122-1128 10. Chartier. T. Merle. D. Besson.I. I. I aminar ceramic high degree of phosphatization and (iv) a high composites. J. Eur. Ceram. Soc., 15(1995)101-10 Hydrophile/ Lypophile Balance was synthesized 11. Mistler, R. E, Shanefield, D. J.& Runk. R. B. Tape The organic components in a slurry play a sting of ceramics. In Ceramic Processing before Firing. ds g. Y. Onada L. L. Hench, John wiley Sons prominent role on the green properties of tape New York. 1978. 411-418
Tape-cast alumina-zirconia laminates: processing and mechanical properties 301 Klc (MPa\Tm) Klc (MPaViE) 1 A A/A AZSIAZS AZlO/AZlO AZlOIAZS A A/A AZlO/AZlO 71517 311513 2/2/2/1oi2/2/ (4 04 Fig. 9. Fracture toughness of the different grades (a) SENB results from Ref. 2, (b) CN results (cross-head speed = 0.01 mm min ‘). 390 380 370 340 ( / dL_ 0 --IfQ %k ; 0 A/A 7/5/-l 4 A Hill bounds - AZ10 t 360 350 330 320 310 t 300”“““““““““’ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 10. Young’s moduli of the different laminates. The Hill bounds were drawn for comparison. Voigt and Reuss’s curves are almost superposed due to rather close Young’s moduli for A and AZIO. composite than for the bulk components. This remarkable improvement is assumed to be essentially related to the presence of thermally induced residual stresses which, according to a simple plane stress model, can be as high as 130 MPa. Crack deflection is essentially a consequence of the residual stresses. Phosphate esters are highly effective dispersants for alumina in MEIUEtOH solvent, but their effectiveness is strongly dependent on their characteristics. Four parameters were varied and an improved phosphate ester having (i) an aliphatic molecule, (ii) a high diester concentration, (iii) a high degree of phosphatization and (iv) a high Hydrophile/Lypophile Balance was synthesized. The organic components in a slurry play a prominent role on the green properties of tapecast materials. The slurry formulation was optimized in order to tape cast defect-free samples with a high green density and a good thermocompression ability. References 1. 2. 3. 4. 5. 6. I. 8. 9. 10. 11. Amateau, M. F. & Messing, G. L., Laminated ceramic composites. In International Encyclopedia of Composites, vol. 3, ed. S. M. Lee. VCH New York, 1990, pp. 11-16. Chartier, T., Besson, J. L. & Both, P., Mechanical properties of Zr02-A120, laminated composites. In Advances in Ceramics, Science and Technology of Zirconia III, vol. 24, eds S. Somiya, N. Yamamoto & H. Yanagida. Am. Ceram. Sot., Westerville, Ohio, 1988, pp. 1131-l 138. Both, P., Chartier, T. & Huttepain, M., Tape casting of Al,O,/ZrO, laminated composites, J. Am. Ceram. Sot., 69(8) (1986) 191-192. Chartier, T. & Besson, J. L., Behaviour of ZrO?-A&O, laminated composites loaded by various mechanical arrangements. In Science of Ceramics, vol. 14, ed. D. Taylor. The Institute of Ceramics, UK, 1988, pp. 639-644. Russo, C. J., Harmer, M. P., Chan, H. M. & Miller, G. A. Design of laminated ceramic composite for improved strength and toughness. J. Am. Ceram. Sot., 75( 12) ( 1992) 3396-3400. Harmer, M. P., Chan, H. M. & Miller, G. A.. Unique opportunities for microstructural engineering with duplex and laminar ceramic composites. J. Am. Ceram. Sot.. 75(7) (1992) 1715-1728. Plucknett, K. P., Caceres. C. H. & Wilkinson, D. S.. Tape casting of fine alumina/zirconia powders for composite fabrication. J. Am. Ceram. Sot., 77(8) (1994) 2137-2144. Plucknett, K. P., Caceres, C. H.. Hughes, C. & Wilkinson, D. S., Processing of tape-cast laminates prepared from fine alumina/zirconia powders. J. Am. Ceram. Sot., 77(8) (1994) 2145-2153. Takebe, H. & Morinaga, K., Fabrication and mechanical properties of lamellar A&O, ceramics. J. Ceram. Sac. Jpn Inter., 96 (1988) 1122-l 128. Chartier, T., Merle, D. & Besson, J. L., Laminar ceramic composites. J. Eur. Ceram. Sot., 15 (1995) 101-107. Mistler, R. E., Shanefield, D. J. & Runk, R. B., Tape casting of ceramics. In Ceramic Processing &fore Firing, eds G. Y. Onada & L. L. Hench, John Wiley & Sons, New York, 1978. 411418
T. Chartier. T. rouxel 12. Shanefield. D dvanced 18. MacKinnon R. J. Blum. J. B. Particle size distribu ceramics In Enc rials Science and Engi tion effects on tape casting of barium titanate. In ed. M 13. Williams, J. C. Doctor blade process. In Treatise on J. A. Mangels. 1984, 150-15 Material Science and Technologr, vol 9, Ceramics Fabri- 19. Chartier, T, Jorge, E,& Boch, P, Ultrasonic deagglom esses, ed. f..Y wang. Academic Press ation of Al,O, and BaTiO, for tape casting. J. Phvs. Il New york,1976.173197 (1991)689 14. Chartier, T, Tape casting. In Encvelopedia of Advanced 20 Streicher, E, Chartier, T& Boch P, Study of cracking Materials, eds D, Bloor, R. J, Brook, M.C. Flemings and microstructural evolution during drying of tape-cast S. Mahajan. Pergamon Press, 1994, 2763 aluminium nitride sheets. J. Mat. Sc, 26(1991)1659 15. Chartier, T, Streicher. E& Boch, P. Phosphate esters as 21, Munz, D, Bubsey, R. T.& Shannon, J. L Jr, Fracture dispersants for the tape casting of alumina. J.A toughness determination of Al,O, using four-point-bend Ceran.Bw.,66(1987)1653-1655 specimens with straight-through and chevron notches. J 16. Mikeska, K.& Cannon. W.R, Dispersant for tape Am. Ceran.Soc.63(1980)3442 Ceramics. 22, McSkimmin, H. J. Fisher, E rement of ultrasonic wave velocities for solids American Ceramic Society, Columbus OH. 1984, 164183 (1960)627 7. Morris, J. R.& Cannon, W.R., Rheology and comp 23. Poisson, S. Journal de I'Ecole Polytechnique. Cahier nent interactions in tape casting slurries. In materials n°15,1809,266 Research Sociery Symposium Proceeding. MaL. Res Soc 24. Christensen, R. M. Mechanics of Composite Materials. Pittsburgh, PA. USA 60(1986)135-142 Laminates. Wiley and Sons. New York. 1979. chap. V
308 T. Churtier, T. Rouxel 12. Shanefield, D. J.. Tape casting for forming advanced 18. MacKinnon. R. J. & Blum, J. B., Particle size distribuceramics. In Encyclopedia of’ Muterials Science and Engi- tion effects on tape casting of barium titanate. In neering, ed. M. B. Bever. 1984,48554858. Advances in Ceramics, vol. 9, Forming in Ceramics. ed. 13. Williams, J. C.. Doctor blade process. In Treatise OH J. A. Mangels. 1984, 150-157. Material Science and Technolog,v, vol. 9, Ceramics Fahri- 19. Chartier, T., Jorge, E. & Both, P.. Ultrasonic deagglomcation Processes. ed. F. F. Y. Wang. Academic Press. eration of A&O, and BaTiO, for tape casting. J. Phys. 111 New York, 1976. 173 197. (1991) 689. 14. Chartier, T., Tape casting. In Encyclopedia of’ Advanced Materials, eds D. Bloor, R. J. Brook, M. C. Flemings & S. Mahajan. Pergamon Press, 1994, 2763. 15. Chartier, T., Streicher, E. & Both, P.. Phosphate esters as dispersants for the tape casting of alumina. J. Am. Ceram. Bull., 66 (1987) 1653-1655. 16. Mikeska, K. & Cannon, W. R., Dispersant for tape casting pure barium titanate. In Advances in Ceramics, vol. 9, Forming in Ceramics, ed J. A. Mangels. The American Ceramic Society, Columbus, OH, 1984, 164 183. 17. Morris, J. R. & Cannon, W. R., Rheology and component interactions in tape casting slurries. In Materials Reseurch Society Symposium Proceeding. Mat. Res. Sot. Pittsburgh, PA, USA, 60 ( 1986) 135~~ 142. 20. Streicher, E., Chartier. T. & Both. P., Study of cracking and microstructural evolution during drying of tape-cast aluminium nitride sheets. J. Mat. Sc., 26 (1991) 1659. 21. Munz, D., Bubsey, R. T. & Shannon, J. L. Jr.. Fracture toughness determination of Al?O, using four-point-bend specimens with straight-through and chevron notches. J. Am. Gram. Sot., 63 (1980) 3442. 22. McSkimmin. H. J. & Fisher, E. S., Measurement of ultrasonic wave velocities for solids. J. Appl. Phys.. 31 (1960) 627. 23. Poisson, S., Journal de I’Ecole Polytechnique, Cahier no 15, 1809, 266. 24. Christensen, R. M., Mechunics oj Composite Muteriuls, Laminates. Wiley and Sons. New York. 1979. chap. V