1995 Else Printed in Great Britain. All rigi Laminar Ceramic Composites T. Chartier. D. merle L. besson LMCTS, URA CNRS 320, ENSCL, 47 av. Albert Thomas, 87065 Limoges, France (Received 18 April 1994; accepted 27 June 1994 abstrac Long fibre-reinforced ceramics exhibit high me- hanical and thermomechanical properties. How- The processing of laminar ceramic composites from ever, the routes available to process the matrix stacking of layers obtained by tape casting is (i. e infiltration or impregnation across a fibre fab- described. In the case of composites for structural ric)are expensive, and only parts with limited applications, the reinforcement mechanisms are thickness can be obtained. Moreover, carbon or briefly reviewed. It is shown that both strength and silicon carbide fibres, which are the most used, ex- toughness can be improved. The evaluation of hibit a limited thermal stability in oxidizing envi- residual stresses allows a strategy for tailoring the ronments mechanical properties of such composites to be By contrast, whisker- or particle-reinforce developed. As an example, results are given in the ramic composites can be processed following clas case of laminar composites with layers made of sical routes. The reinforcing phase may be dis alumina with various zirconia contents persed quasi-isotropically in the matrix, whereas some processing techniques, such as tape casting, allow a preferential orientation of whiskers 1 Introduction Special attention must be focused on laminar composites. Whereas laminar composites with or Designers require materials with higher and higher ganic matrix are well developed, laminar cor performances. Sometimes, materials must be cor- posites with ceramic matrix are relatively new rosion resistant at elevated temperatures while re- These structures provide the advantage for tailor taining high mechanical properties (rupture ing the properties by stacking layers of different strength and toughness ) Ceramic materials, such compositions in a suitable sequence. - Then, it is as alumina, silicon carbide, silicon nitride or mul- possible to produce functionally gradient ceram lite seem to be potential candidates for such appli- ics. Tape casting, which is extensively used for cations. However. these materials. like all the electronic ceramics is well suited to the fabrica monolithic ceramics, are brittle, which limits their tion of ceramic sheets that can be reinforced by use in numerous systems whiskers or particles is to eliminate the flaws that initiate me a In this paper, the different mechanisms that can The usual approach to improve the mechanical catastrophic failure, or at least to reduce their size. briefly reviewed in the first section. Then, the next section will deal with the processing by tape cast- fundamentally different from the conventional ap- ing of laminar ceramic composites. the classical proach. These strategies involve energy dissipative laminated plate theory will be used to predict the mechanisms to develop flaw-tolerant'materials the third ection an Among them are the ceramic ceramic composites. results compared with the experimental data ob In the flaw tolerance approach, the microstructure tained for alumina -zirconia composites. is designed to promote the bridging of the cracks which leads, in the ideal being independent of flaw size. The process must 2 Strengthening and Toughening Mechanisms in produce a material generally with two phe Laminar Ceramic Composites with a controlled heterogeneous microstructure The presence of the second phase, either long Laminar ceramic composites consist of alternate fibres, whiskers or particles(zirconia, for instance) layers reinforced by whiskers or particles. This enhances the mechanical properties type of composite allows the association of two
Journal of the European Ceramic Society 15 (1995) 101-107 0 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0955-2219/95/$9.50 Laminar Ceramic Composites T. Chartier, D. Mlerle & J. L. Besson LMCTS, URA CNRS 320, ENSCI, 47 av. Albert Thomas, 87065 Limoges, France (Received 18 April 1994; accepted 27 June 1994) Abstract The processing of laminar ceramic composites from stacking of layers obtained by tape casting is described. In the case of composites for structural applications, the reinforcement mechanisms are briefly reviewed. It is sh#own that both strength and toughness can be improved. The evaluation of residual stresses allows a strategy for tailoring the mechanical properties (of such composites to be developed. As an example, results are given in the case of laminar composites with layers made of alumina with various zirconia contents. 1 Introduction Designers require materials with higher and higher performances. Sometimes, materials must be corrosion resistant at elevated temperatures while retaining high mechanical properties (rupture strength and toughness). Ceramic materials, such as alumina, silicon carbide, silicon nitride or mullite seem to be potential candidates for such applications. However, these materials, like all the monolithic ceramics, are brittle, which limits their use in numerous systems. The usual approach to improve the mechanical behaviour is to eliminate the flaws that initiate the catastrophic failure, or a.t least to reduce their size. Recently, new strategies have emerged that are fundamentally different from the conventional approach. These strategies involve energy dissipative mechanisms to develop ‘flaw-tolerant’ materials. Among them are the ceramic-ceramic composites. In the flaw tolerance approach, the microstructure is designed to promote .the bridging of the cracks which leads, in the ideal case, to the strength being independent of flaiw size. The process must produce a material generally with two phases, with a controlled heterogeneous microstructure. The presence of the second phase, either long fibres, whiskers or particles (zirconia, for instance) enhances the mechanical properties. Long fibre-reinforced ceramics exhibit high mechanical and thermomechanical properties. However, the routes available to process the matrix (i.e. infiltration or impregnation across a fibre fabric) are expensive, and only parts with limited thickness can be obtained. Moreover, carbon or silicon carbide fibres, which are the most used, exhibit a limited thermal stability in oxidizing environments. By contrast, whisker- or particle-reinforced ceramic composites can be processed following classical routes. The reinforcing phase may be dispersed quasi-isotropically in the matrix, whereas some processing techniques, such as tape casting, allow a preferential orientation of whiskers. Special attention must be focused on laminar composites. Whereas laminar composites with organic matrix are well developed, laminar composites with ceramic matrix are relatively new. These structures provide the advantage for tailoring the properties by stacking layers of different compositions in a suitable sequence.‘-‘7 Then, it is possible to produce functionally gradient ceramics. Tape casting, which is extensively used for electronic ceramics, is well suited to the fabrication of ceramic sheets that can be reinforced by whiskers or particles. In this paper, the different mechanisms that can be used to reinforce laminar composites will be briefly reviewed in the first section. Then, the next section will deal with the processing by tape casting of laminar ceramic composites. The classical laminated plate theory will be used to predict the composite behaviour in the third section and the results compared with the experimental data obtained for alumina-zirconia composites. 2 Strengthening and Toughening Mechanisms in Laminar Ceramic Composites Laminar ceramic composites consist of alternate layers reinforced by whiskers or particles. This type of composite allows the association of two 101
T. Chartier. D. Merle, J. L. Besson Table 1. Strength to failure of(MPa)(four-point bend test) Table 2. Strength to failure or(MPa)(four-point bend test) and toughness K(MPavm)(SENB) D(SENB) ZrO,(vol %) Forming 0 10 20 30 40 50 Ref. Forming 0 5 10 15 20 Ref. Tape casting or 320 470 560 670 8051 Pressing 459412285 30 K。273346454569 58626257 Extrusion 382390 Ip casting Tape casting KmKK 33544144 46495-4 Slip casting ot 438 2.2 Reinforcement mechanisms at layer interfaces The laminar ceramic-ceramic composites must reinforcing mechanisms: the first one, acting at the fulfill two scale of the microstructure inside the layers, due to the second phases, leading to flaw-tolerant ma- improve the toughness and the work of rup- terials; the second one, acting at a macroscopic cale, due to the interfaces between the layers. The -improve the modulus of rupture in bendir aim is to improve both the toughness and the The first objective is obtained using mechanisms strength involving crack ' clamping'stresses, crack deflec- tion or microcracking, that operate at a macro- 2.1 Reinforcement mechanisms inside the layers scopic scale, thanks to the laminar structure of the material, instead of operating at a local scale(n 2.1.1 Whiskers crostructure) The incorporation of whiskers allows load In the laminar structure, residual stresses develop transfers from the matrix to the whiskers(crack during cooling from the sintering temperature bridging) and crack deflection. An optimum because of the difference in thermal expansion toughening is achieved by loading whiskers by between layers of different compositions. But, internal compressive stresses. Hence, the matrix- contrary to the case of monoliths, these stresses whisker couple must be judiciously chosen with built the whole volume of the layers. The a thermal expansion coefficient mismatch and sign and the magnitude of these stresses may be chemical compositions leading to an appropriate adjusted through the compositions(zro2 amount in Al2O3-ZrO2 composites), but also through the Whisker reinforced composites have already layer thickness. Hence, it is possible to develop been the subject of numerous studies. 1.6, 18-23Some high compressive stresses in thin layers whereas esults concerning SiCw reinforced mullite are re- the tensile stresses remain low in the associated ported in Table I thick layers. The propagation of the crack in mode I being impeded by the compressive stresses 2.1.2 Particles propagation modes I and Ill can occur, leading Zirconia-toughened ceramics are the best example crack deflection at the interface and to an increase of particle-reinforced materials. 4-30 The most of the surface of rupture. An example is given effective toughening mechanism in ZrO2-contain- by Chartier and coworkers'for alumina-zirconia ing ceramics is the stress-induced transformation composites of small Zro, particles in their metastable tetrago Crack deflection at the layer interface can be nal state. Both toughness and strength can be in- favoured by a weak interlayer bond. Clegg et al. lo creased, although a compromise between these have fabricated Sic-graphite laminar composites two effects must be accepted 3 Other mechanisms, that, although the onset of microcracking appears that is, crack deflection and microcracking around at 500 MPa, exhibit an ultimate strength three monoclinic times higher, an apparent toughness(calculated ally at the expense of strength from the ultimate strength) of 15 MPavm and a The transformation of zirconia particles is also work of fracture of 4625 Jm. For comparison, d to develop compressive surface stresses, in- the modulus of rupture, the toughness and the creasing the modulus of rupture. work of fracture of monolithic sic were 500 MPa Some results obtained on zirconia-reinforced 3.6 MPavm and 25 Jm2, respectively. alumina are given in Table 2 Toughness can also be enhanced by deflection
102 T. Chartier, D. Merle. J. L. Besson Table 1. Strength to failure q (MPa) (four-point bend test) and toughness Kc (MPadm) (SENB) SC, (vol. %) Forming 0 10 20 30 40 50 Ref Tape casting of 320 470 560 670 940 805 1 Kc 2.73 3.46 4.54 5.69 6.85 6.5 Extrusion of 407 18 moulding Kc 4.6 Slip casting cf 452 18 KC 444 Slip casting of 438 20 KC 4.6 reinforcing mechanisms: the first one, acting at the scale of the microstructure, inside the layers, due to the second phases, leading to flaw-tolerant materials; the second one, acting at a macroscopic scale, due to the interfaces between the layers. The aim is to improve both the toughness and the strength. 2.1 Reinforcement mechanisms inside the layers 2.1. I Whiskers The incorporation of whiskers allows load transfers from the matrix to the whiskers (crack bridging) and crack deflection. An optimum toughening is achieved by loading whiskers by internal compressive stresses. Hence, the matrixwhisker couple must be judiciously chosen with a thermal expansion coefficient mismatch and chemical compositions leading to an appropriate bonding. Whisker reinforced composites have already been the subject of numerous studies.‘,6,‘8-23 Some results concerning Sic, reinforced mullite are reported in Table 1. 2.1.2 Particles Zirconia-toughened ceramics are the best example of particle-reinforced materials.2”30 The most effective toughening mechanism in ZrO,-containing ceramics is the stress-induced transformation of small ZrO, particles in their metastable tetragonal state. Both toughness and strength can be increased, although a compromise between these two effects must be accepted.31 Other mechanisms, that is, crack deflection and microcracking around monoclinic particles, increase toughness but generally at the expense of strength. The transformation of zirconia particles is also used to develop compressive surface stresses, increasing the modulus of rupture. Some results obtained on zirconia-reinforced alumina are given in Table 2. Table 2. Strength to failure vf (MPa) (four-point bend test) and toughness Kc (MPadm) (SENB) ZrOz (vol. %) Forming 0 5 10 15 20 Ref: Pressing flf 459 412 285 30 KC 5.8 6.2 6.2 5.7 Slip casting q 382 390 370 30 KC 5.7 5.5 4.9 Tape casting uf 335 441 444 3 KC 4.6 4.9 5.4 2.2 Reinforcement mechanisms at layer interfaces The laminar ceramic-ceramic composites must fulfill two requirements: -improve the toughness and the work of rupture, -improve the modulus of rupture in bending. The first objective is obtained using mechanisms involving crack ‘clamping’ stresses, crack deflection or microcracking, that operate at a macroscopic scale, thanks to the laminar structure of the material, instead of operating at a local scale (microstructure). In the laminar structure, residual stresses develop during cooling from the sintering temperature because of the difference in thermal expansions between layers of different compositions. But, contrary to the case of monoliths, these stresses built up in the whole volume of the layers. The sign and the magnitude of these stresses may be adjusted through the compositions (ZrO, amount in Al,O,-ZrO, composites), but also through the layer thickness. Hence, it is possible to develop high compressive stresses in thin layers whereas the tensile stresses remain low in the associated thick layers. The propagation of the crack in mode I being impeded by the compressive stresses, propagation modes I and III can occur, leading to crack deflection at the interface and to an increase of the surface of rupture. An example is given by Chartier and coworkers3,5 for alumina-zirconia composites. Crack deflection at the layer interface can be favoured by a weak interlayer bond. Clegg et al.” have fabricated Sic-graphite laminar composites that, although the onset of microcracking appears at 500 MPa, exhibit an ultimate strength three times higher, an apparent toughness (calculated from the ultimate strength) of 15 MPadm and a work of fracture of 4625 Jm-*. For comparison, the modulus of rupture, the toughness and the work of fracture of monolithic Sic were 500 MPa, 3.6 MPadm and 25 Jm-‘, respectively. Toughness can also be enhanced by deflection
Laminar ceramic composites of the growing crack at a porous layer in laminar 3. 1 Slurry formulation composites consisting of a sequence of dense and The composition of the slurry and the drying orous layers y stage greatly affect the microstructure and hence Folsom et al. have devcloped composites with the properties of the green tapes(dcnsity and alumina and carbon fibre-reinforced epoxy resin strength). The rheological behaviour of the slurry layers in which alumina sheets provide high an important parameter for the homogeneity strength, high stiffness and abrasion resistance, and the microstructure of the green tape, and for whereas the fibre-reinforced sheets provide high the reproducibility of the process. Tape-casting toughness and flaw tolerance. They suggest that slurries are complex, multicomponent systems composites for high-temperature applications which contain ceramic powders(including sinter could be designed in a similar way, replacing the ing aids), solvents, dispersants, binders and plasti poxy resin by an inorganic glass cizers. The control of such a system requires the The second objective is reached by choosing for knowledge of the role of each component the outer layers a material with a thermal expan- sion coefficient smaller than the inner layer one. In 3. I Powder this configuration, the outer layers sustain com- Powders of fine particle size are used to obtain pressive stresses 2,5 thin films, smooth surfaces, high densities and small critical flaw size. The particle size distribu tion greatly influences the rhcology of the slurry 3 Processing of Alumina-Zirconia Laminar The starting powders are 99. 7 wt% purity, 0.5 um Composites grain size alumina (P172SB, Pechiney, France) and 97.5 wt% purity, 0-4 um grain size zirconia Laminar composites consisting in a regularly al-(UPH 12, Criceram, France) ternate stacking of (2n+ 1)layers made of alu mina with various zirconia content(Al2O3 3.1.2 Solvent A12O3+ 5 vol. ZrO2=AZ5; AL,O3+ 10 vol. The solvents ensure the dissolution of all organi ZrO2= AZl0)were fabricated by tape casting, components(i.e. dispersant, binders and plasticiz- stacking, thermocompression, pyrolysis of organic ers) to give an homogeneous slurry. Interactions compo were between the solvent and the solid surfaces influ chosen to put the outer layers in a compressive ence the further adsorption of the dispersant and state. The layer thickness was adjusted by super- of the binders, and then the stability and the rheo- posing a variable number of individual identic logical behaviour of the slurry. Mixtures of organic tapes. So the AZiO/AZS/AZ10 composite with n= solvents, and particularly binary systems, are pre- 2 was fabricated by stacking five layers, each con- ferred for solubility reasons: the various organic sisting in three identical tapes whereas the additives have their higher solubility in different AZIO/AZ5/AZ10 composite with n 10 was fab- solvents. The solvent used is an azeotropic mi ricated with 21 alternate tapes ture of methyl ethyl ketone(MEK)and ethanol I ape casting is the prominent process for pro-(40/60) ducing homogeneous wide and thin(25-1000 um) ceramic sheets with controlled thicknesses and 3.1.3 Dispersant smooth surfaces. Tape casting, basically, consists in Dispersants are necessary to obtain a good deag depositing a suspension composed of the ceramic glomeration and dispersion of the ceramic parti- powder and organic components on a support, cles in the solvent, and to stabilize the tape-casting either by spreading under a blade(doctor blade ensions with a high ceramic-organic ratio. A process)or by coating. Typical applications are stable dispersion of deagglomerated particles leads substrates of AlO,(or AIN) for thick- and thin- to a dense particle packing and to a homogeneous film circuitry and BaTiO3 for multilayer capa microstructure tors, which represent the two principal markets in The best stabilization is given by a combination electronic ceramics. Tape casting is now used on a of electrostatic and steric repulsion (referred as larger scale to produce thin ceramic sheets and electrosteric stabilization ) The steric hindrance multilayer structures of various materials for difi- prevents contact between particles and the double erent applications. The evaporation of the solvents layer, which may be due to net charge on the par leads to a dried green tape with sufficient strength ticle surface and/or charges associated with the and fexibility to be handled and cut to the proper adsorbed polymer, providing repulsion by a po shape. After removal of organic components, tential energy barrier at larger distances green sheets or multilayer systems made by stack he dispersant used is a phosphate ester ing and laminating green sheets are sintered costat C213, CECA, france)
Laminar ceramic composites 103 of the growing crack at a porous layer in laminar composites consisting 0.f a sequence of dense and porous layers9 Folsom et al.” have developed composites with alumina and carbon fibre-reinforced epoxy resin layers in which alumina sheets provide high strength, high stiffness and abrasion resistance, whereas the fibre-reinforced sheets provide high toughness and flaw tolerance. They suggest that composites for high-temperature applications could be designed in a similar way, replacing the epoxy resin by an inorganic glass. The second objective is reached by choosing for the outer layers a material with a thermal expansion coefficient smaller than the inner layer one. In this configuration, the outer layers sustain compressive stresses.2,5 3 Processing of Alumina-Zirconia Laminar Composites Laminar composites consisting in a regularly alternate stacking of (2n + 1) layers made of alumina with various zirconia content (Al,O, = A; Al,O, + 5 vol.% ZrO, := AZ5; Al,O, + 10 vol.% ZrO, = AZlO) were fabricated by tape casting, stacking, thermocompression, pyrolysis of organic components and sintering. The sequences were chosen to put the outer layers in a compressive state. The layer thickness was adjusted by superposing a variable number of individual identical tapes. So the AZlO/AZYAZlO composite with n = 2 was fabricated by stacking five layers, each consisting in three identical tapes whereas the AZlO/AZS/AZ 10 composite with n = 10 was fabricated with 21 alternate tapes. Tape casting is the prominent process for producing homogeneous wide and thin (25-1000 pm) ceramic sheets with controlled thicknesses and smooth surfaces. 32 Tape casting, basically, consists in depositing a suspension composed of the ceramic powder and organic components on a support, either by spreading under a blade (doctor blade process) or by coating. Typical applications are substrates of Al,O, (or .AIN) for thick- and thinfilm circuitry and BaTQ for multilayer capacitors, which represent the two principal markets in electronic ceramics. Tape casting is now used on a larger scale to produce thin ceramic sheets and multilayer structures of various materials for different applications. The evaporation of the solvents leads to a dried green tape with sufficient strength and flexibility to be handled and cut to the proper shape. After removal of organic components, green sheets or multilayer systems made by stacking and laminating green sheets are sintered. 3.1 Slurry formulation The composition of the slurry and the drying stage greatly affect the microstructure and hence the properties of the green tapes (density and strength). The rheological behaviour of the slurry is an important parameter for the homogeneity and the microstructure of the green tape, and for the reproducibility of the process. Tape-casting slurries are complex, multicomponent systems, which contain ceramic powders (including sintering aids), solvents, dispersants, binders and plasticizers. The control of such a system requires the knowledge of the role of each component. 3.1.1 Powder Powders of fine particle size are used to obtain thin films, smooth surfaces, high densities and small critical flaw size. The particle size distribution greatly influences the rheology of the slurry. The starting powders are 99.7 wt% purity, 0.5 pm grain size alumina (P172SB, Ptchiney, France) and 97.5 wt% purity, 0.4 ,um grain size zirconia (UPH 12, Criceram, France). 3.1.2 Solvent The solvents ensure the dissolution of all organic components (i.e. dispersant, binders and plasticizers) to give an homogeneous slurry. Interactions between the solvent and the solid surfaces influence the further adsorption of the dispersant and of the binders, and then the stability and the rheological behaviour of the slurry. Mixtures of organic solvents, and particularly binary systems, are preferred for solubility reasons: the various organic additives have their higher solubility in different solvents. The solvent used is an azeotropic mixture of methyl ethyl ketone (MEK) and ethanol (40/60). 3. I. 3 Dispersan t Dispersants are necessary to obtain a good deagglomeration and dispersion of the ceramic particles in the solvent, and to stabilize the tape-casting suspensions with a high ceramic-organic ratio. A stable dispersion of deagglomerated particles leads to a dense particle packing and to a homogeneous microstructure. The best stabilization is given by a combination of electrostatic and steric repulsion (referred as electrosteric stabilization). The steric hindrance prevents contact between particles and the double layer, which may be due to net charge on the particle surface and/or charges associated with the adsorbed polymer, providing repulsion by a potential energy barrier at larger distances. The dispersant used is a phosphate ester (Beycostat C213, CECA, France)
T. Chartier. D. Merle..L. besson 3. 1.4 Binder is performed by milling for 24 h. The slurry is After evaporation of the solvent, the binders pro- kept rotating at a slow speed for deairing and to vide the strength to the green tape, to enable han- prevent settling dling and subsequent processing steps like punch ing. Binders are polymeric molecules which adsorb 3.3 Tape casting on the particle surfaces and form organic bridges Tape casting is performed with a laboratory tape. between them. The slurry should exhibit a shear casting bench (Cerlim Equipement, Limoges, thinning behaviour caused by an alignment of the France). Slurries were tape cast onto a fixed glass binder molecules at high shear rate. plate with a moving double blade device at a con The binder used is a poly ( vinyl butyral)(PVB) stant speed of 1 m min 3.1.5 Plasticizer Most binders require the addition of plasticizer 4 Evaluation of Residual Stresses in alu to improve the flexibility and workability of the Zirconia Laminar Composites green tape. Plasticizers are low molecular weight species which can reduce the glass transition tem- The determination of the distribution of the resid- perature of the binder at room tcmperature or ual stresses allows the appropriate choice of the below, leading to a better plasticity. The addition layer composition and thickness and of the stack of plasticizers modifies the mechanical properties ing sequence, for an optimum reinforcement of the green tape, the strain to failure increases Models have previously been developed for or but the strength decreases The plasticizer used is a mixture of poly(eth ganic compositesor glass-to-metal seals lene glycol)(PEG)and dibutyl phthalate(DBP) During cooling, the difference in deformation due to the different thermal expansion coefficients The slurry must be adjusted, not only for tape of the layers, is accommodated by creep, as long casting but taking into account other processing as the temperature is high enough. Below a certain n ), pyrolysis of organic compe Temperature, that will be called the joiningtem nents and sintering (table 3) perature, the different layers become bonded to- gether and internal stresses appear. For laminar composites with symmetrical stack- 3.2 Slurry preparation ing(odd number of layers)and with a low thick The preparation of slurries is carried out in two ness compared with the plate size, the problem is stages,namely () deagglomeration and dispersion that of an in-plane stress state with the stress nor- of powders in the solvent with the aid of the mal to the surface equal to zero (03=0(Fig. 1) dispersant, and (ii homogenization of the slurry In the case of a solid with an isotropic, linear with binders and plasticizers. The sequence of elastic behaviour. the mechanical strains are ex component addition is critical. The dispersant has pressed as follow to be added before the binders to prevent com petitive adsorption. The initial adsorption of the I +y (1) binder on the particle surfaces would prevent the E dispersant from being subsequently adsorbed thereby decrcasing its effectiveness. Furthermore the deagglomeration is more efficient in a low vis- cosity system(i. e without binders and plasticizers) E by this sequence of addi tion. The deag tion is done by ultrasonic E 2 treatment. The stage of homogenization Table 3. Composition asting shi Function where e is the strain tensor o the stress tensor y Alumina+zirconia Ceramic powder 30 6 the Poisson ratio and e the Young modulus MEK/ethanol 57-6 In a loading configuration which is symmetrical Phosphate ester In resp erections 1 and 2. an PEG 30 Plasticizer and2 are equivalent(orthotropic system)、t如m DBP Plasticize 3.5
104 T. Chartier, D. Merle, J. L. Besson 3. I .4 Binder After evaporation of the solvent, the binders provide the strength to the green tape, to enable handling and subsequent processing steps like punching. Binders are polymeric molecules which adsorb on the particle surfaces and form organic bridges between them. The slurry should exhibit a shear thinning behaviour caused by an alignment of the binder molecules at high shear rate. The binder used is a poly(viny1 butyral) (PVB). 3.1.5 Plasticizer Most binders require the addition of plasticizers to improve the flexibility and workability of the green tape. Plasticizers are low molecular weight species which can reduce the glass transition temperature of the binder at room temperature or below, leading to a better plasticity. The addition of plasticizers modifies the mechanical properties of the green tape, the strain to failure increases but the strength decreases. The plasticizer used is a mixture of poly(ethylene glycol) (PEG) and dibutyl phthalate (DBP). The slurry must be adjusted, not only for tape casting but taking into account other processing parameters such as shaping (cutting, punching, thermocompression), pyrolysis of organic components and sintering (Table 3). 3.2 Slurry preparation The preparation of slurries is carried out in two stages, namely (i) deagglomeration and dispersion of powders in the solvent with the aid of the dispersant, and (ii) 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. The initial adsorption of the binder on the particle surfaces would prevent the dispersant from being subsequently adsorbed, 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 done by ultrasonic treatment. The second stage of homogenization Table 3. Composition of a tape casting slurry Component Function vol. % Alumina + zirconia Ceramic powder 30.6 MEWethanol Solvent 57.6 Phosphate ester Dispersant 0.8 PVB Binder 4.6 PEG 300 Plasticizer 2.9 DBP Plasticizer 3.5 is performed by milling for 24 h. The slurry is kept rotating at a slow speed for deairing and to prevent settling. 3.3 Tape casting Tape casting is performed with a laboratory tapecasting 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. 4 Evaluation of Residual Stresses in AluminaZirconia Laminar Composites The determination of the distribution of the residual stresses allows the appropriate choice of the layer composition and thickness and of the stacking sequence, for an optimum reinforcement. Models have previously been developed for organic composites33 or glass-to-metal seals.34 During cooling, the difference in deformation, due to the different thermal expansion coefficients of the layers, is accommodated by creep, as long as the temperature is high enough. Below a certain temperature, that will be called the ‘joining’ temperature, the different layers become bonded together and internal stresses appear. For laminar composites with symmetrical stacking (odd number of layers) and with a low thickness compared with the plate size, the problem is that of an in-plane stress state with the stress normal to the surface equal to zero (a33 = 0) (Fig. 1). In the case of a solid with an isotropic, linear elastic behaviour, the mechanical strains are expressed as follow: El1 = - (all - vu*21 E 1 &22 = - (g22 - V~ll> E l+v 812 = - (T E l2 &33 = - --k l-v 41 + &22) 813 = &23 = 0 where E” is the strain tensor, d the stress tensor, v the Poisson ratio and E the Young modulus. In a loading configuration which is symmetrical in respect to directions 1 and 2, and the axes 1 and 2 are equivalent (orthotropic system), then: a11 = 022 (2)
Laminar ceramic composites and Table modulus, Poisson ratio and thermal expan- of pseudo-laminar layers with a thickness of 160 um in the green state 1+p 341 120×10 In each layer I, the total deformation after sin- AZS/AZ 0241 -125×104 tering, is the sum of an elastic component and of AZ10AZ10325 02421215×104 a thermal c rigid bonding between the layers, the total defor- mation will be the same for all the layers, then in compressive(oil) and tensile(oi D stresses with the thickness ratio(x= dodi)of the layers, taking d1+ar△T= constant(3) V,=V2=v, is given by eqns( 8 )and(9) EEla2-a1)Arx where AT is the difference in temperature between G1(x) 2E1+E2 the actual temperature, To, and the joining tem- perature The force balance requires(in normal stresses oil(r)=-2 gi(x) ∑o1d=0 (4) Figure 2 illustrates these changes for an A/AZS composite. The characteristics of the constitutive where d, is the thickness of the Ith layer. That gives, for a symmetrical composite with 2n+ materials are given in Table 4 I alternate layers of type: 1 and 2 The compressive stress increases when the dy/d1 ratio increases, whereas the tensile stress de- (n+1)ar1d1+nn1d2=0 (5) creases. In the case of brittle materials, which From eqns (3)and(5), it comes high stresses, the thickness of the layers in tension must nE1E24(a2-a1)△T n(1-n)E2d2+(n+1)(1 DEid,(6 be high enough to avoid catastrophic failure This result is consistent with amateau me and ing who observed a decrease in crack frequency n the inner layer (under tension)in a Al O3+ y G (n+1)E1E2d(a2-a1)△T vol. SiCw(20/0/20)when d2/d, increases from 1 n(1-n)E2d2+(n+1)(1-n2)Ed to 4 The magnitude of these stresses depends on the thermal expansion coefficient mismatch(that must be small enough to avoid delamination) and on the relative thickness of the layers AAZ5 In order to limit the sensitivity to surface flaws, the composite is tailored to develop compressive residual stresses in the outer layers. In the case of a symmetrical laminate, this is obtained by choos- ing the layer compositions so that the thermal ex pansion of the odd layers is smaller than the even one es(a, <a,) dyd For a three layer composite (n= 1), the change d OM(MPal Fig. Z. Compressive stresses ped in alumina Fig. 1. In-plane stress stat a ith a stress normal to the layers and tensile ones(oi)in containing 5 vol% urface equal to 0(o33 a laminar composite with a zirconia layers versus the thick io(x= d2/d1)in
Luminar ceramic composites 105 and l-v & 11 =- E g11 = 822 l+v & 12 = - E a12 In each layer 1, the total deformation after sintering, is the sum of an elastic component and of a thermal component. :In the case of a perfectly rigid bonding between the layers, the total deformation will be the same for all the layers, then: 1 - v, 4 =E o-f1 ,t aI AT = constant (3) I where AT is the difference in temperature between the actual temperature, T,, and the joining temperature, q. The force balance requires (in normal stresses): cd,& = 0 (4) where dI is the thickness of the Ith layer. That gives, for a symmetrical composite with 2n + 1 alternate layers of type 1 and 2: (n + I)a:,dl + n4,d2 = 0 (5) From eqns (3) and (5), it comes: a’ - nE,E2d2(cx2 - a,) AT ” - n(l - vJE,d, + (n + l)(l - v2)Eld, (6) and 4, = - (n + l)S,E,d,(cu, - al) AT n(1 - vJE,d:! + (n + l)( 1 - v2)E,d, (7) The magnitude of these stresses depends on the thermal expansion coefficient mismatch (that must be small enough to avoid delamination) and on the relative thickness of the layers. In order to limit the siensitivity to surface flaws, the composite is tailored to develop compressive residual stresses in the outer layers. In the case of a symmetrical laminate, this is obtained by choosing the layer compositions so that the thermal expansion of the odd layers is smaller than the even ones (ai < cy2). For a three layer com:posite (n = l), the change dr Fig. 1. In-plane stress state with a stress normal to the surface equal to 0 (us3 = 0) :in a laminar composite with a symmetrical stacking. Table 4. Young modulus, Poisson ratio and thermal expansion coefficient of ‘pseudo-laminar composites’ consisting of 21 identical layers with a thickness of 160 km in the green state E (GPa) V aAT A/A 341 0.234 -120 x lOA AZ5lAZ5 328 0.241 -125 X lOA AZlO/AZlO 325 0.242 -121.5 x lOA in compressive (a:,) and tensile (4,) stresses with the thickness ratio (x = d2/dl) of the layers, taking v1 = v2 = v, is given by eqns (8) and (9): d,*(x) = E,E,(cu, - q) AT x (1 - v) 2E, + E2x (8) Figure 2 illustrates these changes for an A/AZ5 composite. The characteristics of the constitutive materials are given in Table 4. The compressive stress increases when the d2/dl ratio increases, whereas the tensile stress decreases. In the case of brittle materials, which have a high sensitivity to cracking under tensile stresses, the thickness of the layers in tension must be high enough to avoid catastrophic failure. This result is consistent with Amateau & Messing2 who observed a decrease in crack frequency in the inner layer (under tension) in a Al,O, + y vol.% Sic, (20/O/20) when d,ld, increases from 1 to 4. 0,: (MPa) 10 ddd, Fig. 2. Compressive stresses (a\,) developed in alumina layers and tensile ones (~$1) in alumina containing 5 vol.% zirconia layers versus the thickness ratio (x = d2/dl) in a A/AZ5 composite