J.Am. Ceram.Soc.,912124-2129(2008 DOl:10.1l111551-29162008.02441.x C The American Ceramic Society urna Forming of Ceramic Laminates Comprising Thin Layers of a few particles Ilias Nicolaidis, Jonas Gurauskis,Carmen Baudin, Rodrigo Moreno, and A Javier Sanchez-HerenciaT Ceramics Department, Instituto de Ceramica y Vidrio CSIC, 28049 Madrid, Spain In this paper, a water-based colloidal processing method manufacturing multilayers with very thin zirconia lavers tween thick alumina-based layers is presented. The procedure thr volves the dip coating of green tapes in binder-containing slur ries and further lamination and sintering. Dipping studies show hat thickness increases with soaking time a higher limit is achieved, indicating the existence of interaction between slurry and substrate. After sintering, homogeneous, dense, and contin- (1) uous zirconia layers with thicknesses between 3 and 0.8 um alternated with 350 Hm layers of alumina are obtained, thus eaning that the cross section of these thin layers range fror four or five to only one or two grains. Although the where Kc is the fracture toughness of the thin layer, I, and t, are stresses are estimated to be around 2 gPa. indentatic the thicknesses of the thin and thick layers respectively, and oe is do not shown any interaction when passing through the residual compressive stress. From this equati be in pressive layers, indicating the existence of a lower ferred that the magnitude of the threshold stress and conse limit to achieve a threshold stress. quently the capability to stop a crack, for a given compress stress, decreases with the thickness of the layers. In contrast to the benefits of layers under high compression, the associated L. Introduction tensile stresses in the laminates can generate tunneling cracks IE design of laminated ceramics for emerging applications e control of the composition and thickness of In ceramic laminates with strong interfaces the strain differ- the constituent layers For devices such as solid oxide fuel cells ences(Ae)between layers are required to develop the residual (SOFC), thermal barrier coatings(TBC), or corrosion protec stresses. These are achieved using either the differences between the coefficient of thermal expansion"(CTE) or the expansion tive coatings, thin layers are increasingly needed to achieve associated to the l-m zirconia phase transformation during cool higher efficiency. For instance, in the SOFCs. the ohmic cor tribution is large due to the high electrolyte resistance. It has layers can be arisen by using variable amounts of unstabilized ture(700%-800C)drastically decreases with the thickness of irconia inside an alumina matrix. .or adding stabilizers to the the electrol yte dense layer. Consequently the starting designs zirconia compacts. 10, 1& The critical role to the thickness ratios of electrolyte supported cells have changed to the cathode over the residual stresses values can be easily explained using the supported cells and the anode-supported cells with lower simplified model of an infinite plate 6. 12, 19 In a multilayer system thicknesses (30-40 and 10-20 um, respectively). Ceramic lam- with n tensile layers with a thickness ta alternated by(n-1) inates have also demonstrated to be an effective reinforcing compressive layers of thickness Ib, and considering n >>>1, the route to overcome the intrinsic brittleness of ceramics. In this esidual tensile and compressive stresses are interrelated by sense, compressive stresses are usually required as they oppos to the starting crack growth and or may develop a threshold (2) strength to be surpassed by the growing crack.The capability to stop the fracture crack and the development of a threshold trength has been described as a phenomenon associated with From this basic formula it can be deduced that if th< the compressive stresses in ceramic multilayers. The stress then Oa+0, i.e., if thin layers are alternated with thick ones. the needed to extend a crack through a thin lay stresses inside the latter are negligible while the stresses inside b the thins are still important. Indentation test through the cross section of a multilayer has been used elsewhere to evaluate the residual stresses20-2(see references for a scheme).Cracks anal ysis after indentation has shown that on a tensile stressed layer, cracks running perpendicular to the layers direction are longer E. Suvaci-contnbuting editor than parallel ones. Conversely, indentation cracks length layers under compression show the opposite behavior. Hence, fabrication of laminated materials with very thin layers No. 23916 Received October 30. 2007; approved March 6, 2008 s in focus and requires the development of simple and innova This work was supported by Ministerio de Ciencia (Spain) under contract tive processing strategies able to produce defect-free, uniform layers integrated ressed. e-mail: ajsanchez(@ icv. tCurrent techniques developed to fabricate layered ceramics from precur de Aragon-CSIC, Zaragoza. Spain sors or from powder processing techniques-4dip coating has 2124
Forming of Ceramic Laminates Comprising Thin Layers of a Few Particles Ilias Nicolaidis,z Jonas Gurauskis,y Carmen Baudı´n, Rodrigo Moreno, and A. Javier Sa´nchez-Herenciaw Ceramics Department, Instituto de Cera´mica y Vidrio—CSIC, 28049 Madrid, Spain In this paper, a water-based colloidal processing method for manufacturing multilayers with very thin zirconia layers between thick alumina-based layers is presented. The procedure involves the dip coating of green tapes in binder-containing slurries and further lamination and sintering. Dipping studies show that thickness increases with soaking time until a higher limit is achieved, indicating the existence of interaction between slurry and substrate. After sintering, homogeneous, dense, and continuous zirconia layers with thicknesses between 3 and 0.8 lm alternated with 350 lm layers of alumina are obtained, thus meaning that the cross section of these thin layers range from four or five to only one or two grains. Although the residual stresses are estimated to be around 2 GPa, indentation cracks do not shown any interaction when passing through the compressive layers, indicating the existence of a lower thickness limit to achieve a threshold stress. I. Introduction THE design of laminated ceramics for emerging applications requires the control of the composition and thickness of the constituent layers. For devices such as solid oxide fuel cells (SOFC), thermal barrier coatings (TBC), or corrosion protective coatings, thin layers are increasingly needed to achieve higher efficiency. For instance, in the SOFCs, the ohmic contribution is large due to the high electrolyte resistance. It has been shown that ohmic polarization at intermediate temperature (7001–8001C) drastically decreases with the thickness of the electrolyte dense layer.1 Consequently the starting designs of electrolyte supported cells have changed to the cathode supported cells and the anode-supported cells with lower thicknesses (30–40 and 10–20 mm, respectively).2 Ceramic laminates have also demonstrated to be an effective reinforcing route to overcome the intrinsic brittleness of ceramics.3–6 In this sense, compressive stresses are usually required as they oppose to the starting crack growth and/or may develop a threshold strength to be surpassed by the growing crack.6–9 The capability to stop the fracture crack and the development of a threshold strength has been described as a phenomenon associated with the compressive stresses in ceramic multilayers.6,10,11 The stress needed to extend a crack through a thin layer under compression (sthr) is given by sthr ¼ Kc ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p t2 2 1 þ 2t1 t2 s þ sc 1 1 þ t1 t2 2 p sin1 1 1 þ 2t1 t2 0 BB@ 1 CCA 0 BB@ 1 CCA (1) where KC is the fracture toughness of the thin layer, t1 and t2 are the thicknesses of the thin and thick layers respectively, and sc is the residual compressive stress. From this equation it can be inferred that the magnitude of the threshold stress and consequently the capability to stop a crack, for a given compressive stress, decreases with the thickness of the layers. In contrast to the benefits of layers under high compression, the associated tensile stresses in the laminates can generate tunneling cracks that reduce the integrity of ceramics,12,13 independently of the use or application. In ceramic laminates with strong interfaces the strain differences (De) between layers are required to develop the residual stresses. These are achieved using either the differences between the coefficient of thermal expansion6,14 (CTE) or the expansion associated to the t–m zirconia phase transformation during cooling.15,16 Regarding the last, variable differential strain between layers can be arisen by using variable amounts of unstabilized zirconia inside an alumina matrix,9,17 or adding stabilizers to the zirconia compacts.10,18 The critical role to the thickness ratios over the residual stresses values can be easily explained using the simplified model of an infinite plate.6,12,19 In a multilayer system with n tensile layers with a thickness ta alternated by (n1) compressive layers of thickness tb, and considering n 1, the residual tensile and compressive stresses are interrelated by sa ¼ sb tb ta (2) From this basic formula it can be deduced that if tb ta then sa-0, i.e., if thin layers are alternated with thick ones, the stresses inside the latter are negligible while the stresses inside the thins are still important. Indentation test through the cross section of a multilayer has been used elsewhere to evaluate the residual stresses20–22 (see references for a scheme). Cracks analysis after indentation has shown that on a tensile stressed layer, cracks running perpendicular to the layers direction are longer than parallel ones. Conversely, indentation cracks length on layers under compression show the opposite behavior. Hence, fabrication of laminated materials with very thin layers is in focus and requires the development of simple and innovative processing strategies able to produce defect-free, uniform layers integrated in complex structures. Among the processing techniques developed to fabricate layered ceramics from precursors or from powder processing techniques23,24 dip coating has E. Suvaci—contributing editor This work was supported by Ministerio de Educacio´n y Ciencia, (Spain) under contract MAT 2006-01038. w Author to whom correspondence should be addressed. e-mail: ajsanchez@icv.csic.es z Current address: Rolls Royce Fuel Cell Systems Ltd., Loughborough, U.K. y Instituto de Ciencia de Materiales de Arago´n—CSIC, Zaragoza, Spain. Manuscript No. 23916. Received October 30, 2007; approved March 6, 2008. Journal J. Am. Ceram. Soc., 91 [7] 2124–2129 (2008) DOI: 10.1111/j.1551-2916.2008.02441.x r 2008 The American Ceramic Society 2124
July 2008 Forming of ceramic Laminates been extensively used in the sol-gel technology to produce thin 4.8 mm/s. Coated tapes were left to dry at room conditions onto ceramic, glass, and metallic sub- overnight and further weighted to calculate the mass variation onsolidation of a coating layer by dipping is per unit area. Reported data were the average of three mea achieved by the deposition and evaporation of the liquid, al surements. Direct observation of the cross-section of coated though it can also be assisted by an external force like filtration reen samples were made by scanning electron microscopy or electrophoretic deposition. The thickness of the films (FEG-SEM Hitachi S-4700, Tokyo, Japan) with the samples fabricated by dip coating can vary from nanometers to a few embedded in epoxy resin and soft polished with sandpaper tens of micrometers depending on substrate parameters(wett Multilayered materials with five thick layers of Al2O3 ability, roughness, and porosity), sol/suspension parameters Y-TZP and four very thin layers of Tz-0 were prepared by (viscosity, solid content, binders content, and surface tension), lamination and pressing of two coated tapes alternating with and specific processing parameters (withdrawal rate, soaking three uncoated tapes. The stack was pressed at 18 MPa using flat time, temperature, and humidity). For a Newtonian slurry and parallel plates as reported before. Figure I shows the thickness of the layer(h)can be calculated by the Landau- overall scheme of the lamination process Samples were sintered at 1550C/2 h(heating and cooling ates of 5C/min). Cross sections were polished with diamon h=094-(ny) paste down to I um and thermally etched for SEM character YLV(p-g) ization and indentation tests. Vickers indentations were per formed with loads of 49 and 98n for 15s. Indentations were performed on the three intermediate thick layers at different where n is the viscosity of the slurry, v is the withdrawal rate, distances from the zirconia compressed thin layers. Cracks par YLy is the liquid-vapor surface tension, p is the density of the allel and perpendicular to the layers, were measured by optical Q, Previous work demonstrated that layered ceramics with thin icroscopy in well-defined indentations. To calculate the Kic lurry, and g is the gravity ve indentations were done. Finally the indented cross sections d thick alternating layers can be fabricated by a water-based of the multilayers were coated with gold for SEM observation of colloidal route involving the dip co The thickness of the coating on the green tape was different on each face of the tape due to the different porosity between the top and he bottom surfaces of the tape. In addition to dipping there is II. Results and discussion of the tape as in the slip casting. t- through the open porosity Figure 2 shows the optical micrographs for cross sections of dry In this paper, the manufacture of multilayered ceramics com- (a) and wet(b) tapes dip-coated using a 30 vol% slurry at a ithdrawal rate of 3.7 mm/s. On one hand, it can be observed ers)and thick layers with very low tensile residual stresses is that dry tapes yield much thicker layers than wet ones as a con studied. The method implies the saturation of the tape with wa- sequence of the filtration phenomena described in a previou filtration contribution to the coating thickness is avoided and thicknesses between opposite sides due to the different porosity nly dip-coating parameters must be taken into account. The dip-coated tapes are used to build up a multilayer material that different thickness is more significant as the solid content of the alternates thin zirconia layers(from 3 um to <1 um)with thick slurries decreases. On the other hand, wet saturated substrates alumina ones(350 um). The interaction of a crack produced by yield similar thicknesses in both sides. Table I summarizes the Vicker indentation with the thin layer under compression is als microscopy for dry and wet tapes coated with 30, 15, and 7.5 vol% slurries. All he dry tapes show a thicker coating than wet ones as well as the thickness difference between sides the objective of the work is to obtain very thi sive layers, further studies were devoted to optimize the The substrates to be coated were produced by the doctor blade 15 and 7.5 vol%o slurries using water-saturated substrates. This nethod using concentrated aqueous suspensions of a mixture of pretreatment assures that coatings thicknesses are almost Al2O,(HPAO5, Ceralox, dso=0.35 um and Ss=9.5 m/g)with homogenous on both sides and do not have any contribution 5 vol% of tetragonal zirconia polycrystalline with 3 mol% y due to filtration effects (Y-TZP, TZ3YS, Tosoh, Japan, dso=0.4 um and Ss=6.5 m-/ To evaluate the influence of soaking time and withdrawal rate g), which was added to avoid exaggerated grain growth of n the thickness of the coating, disks were dip coated using a 15 water using a 0.8 wt%(on a dry powder basis) of a polyelec- rolyte(Dolapix CE-64 Zschimer-Schwanz, Burgstadt, Germa- Drying Pressing ny). Slurries were ball milled for 4 h using alumina jar and balls tape casting a 5 wt%(with regard to solids) of binder was added to the suspension and stirred for 30 min before casting The binder was a polyacrylate/styrene emulsion (DM765E, ufacture of these green tapes has been described elsewhere Dry substrates were obtained by punching out disks of 2.5 cm in diameter from the green tapes after complete drying at 60C weight gain was observed. Coatings were prepared by dippin either the dry or wet green substrates into pure zirconia (Tz 75-15vol% Tosoh, Japan, dso=0.3 um, Ss=14.6 m"/g)slurries which were 18 Mp prepared to solids contents of 30, 15, and 7. 5 vol% containing 0.8 wt% of dispersant and 5 wt% of binder. Dipping studies were carried out for soaking times of 0, 60, 180, 360, and 900s Fig. 1. Scheme of the dip-coating and piling processing steps that (0, 1, 3, 6, and 15 min) and withdrawal rates of 1.8, 2.7, 3.7, and brication of the multilayered ceram
been extensively used in the sol–gel technology to produce thin functional coatings onto ceramic, glass, and metallic substrates.25,26 The consolidation of a coating layer by dipping is achieved by the deposition and evaporation of the liquid, although it can also be assisted by an external force like filtration or electrophoretic deposition.26–28 The thickness of the films fabricated by dip coating can vary from nanometers to a few tens of micrometers depending on substrate parameters (wettability, roughness, and porosity), sol/suspension parameters (viscosity, solid content, binders content, and surface tension), and specific processing parameters (withdrawal rate, soaking time, temperature, and humidity).29–31 For a Newtonian slurry, the thickness of the layer (h) can be calculated by the Landau– Levich equation.32 h ¼ 0:94 ðZ vÞ 2=3 g 1=6 LV ðr gÞ 1=2 (3) where Z is the viscosity of the slurry, v is the withdrawal rate, gLV is the liquid–vapor surface tension, r is the density of the slurry, and g is the gravity. Previous work demonstrated that layered ceramics with thin and thick alternating layers can be fabricated by a water-based colloidal route involving the dip coating of green tapes.33 The thickness of the coating on the green tape was different on each face of the tape due to the different porosity between the top and the bottom surfaces of the tape.28 In addition to dipping there is also a filtration process of the slurry through the open porosity of the tape as in the slip casting.34 In this paper, the manufacture of multilayered ceramics combining very thin layers of pure zirconia (without phase stabilizers) and thick layers with very low tensile residual stresses is studied. The method implies the saturation of the tape with water before immersion into the slurry. In this way, any possible filtration contribution to the coating thickness is avoided and only dip-coating parameters must be taken into account. The dip-coated tapes are used to build up a multilayer material that alternates thin zirconia layers (from 3 mm to o1 mm) with thick alumina ones (350 mm). The interaction of a crack produced by Vicker indentation with the thin layer under compression is also studied. II. Experimental Procedure The substrates to be coated were produced by the doctor blade method using concentrated aqueous suspensions of a mixture of Al2O3 (HPA05, Ceralox, d50 5 0.35 mm and Ss 5 9.5 m2 /g) with 5 vol% of tetragonal zirconia polycrystalline with 3 mol% yttria (Y–TZP, TZ3YS, Tosoh, Japan, d50 5 0.4 mm and Ss 5 6.5 m2 / g), which was added to avoid exaggerated grain growth of alumina. Suspensions were prepared to 50 vol% solids in DI water using a 0.8 wt% (on a dry powder basis) of a polyelectrolyte (Dolapix CE-64 Zschimer-Schwarz, Burgsta¨dt, Germany). Slurries were ball milled for 4 h using alumina jar and balls. For tape casting a 5 wt% (with regard to solids) of binder was added to the suspension and stirred for 30 min before casting. The binder was a polyacrylate/styrene emulsion (DM765E, Celanese, Spain) with solid content of 50 vol%, particle size ranging between 0.05 and 0.15 mm, and Tg 5 61C. The manufacture of these green tapes has been described elsewhere.19 Dry substrates were obtained by punching out disks of 2.5 cm in diameter from the green tapes after complete drying at 601C. Substrates were immersed in DI water until saturation, i.e., no weight gain was observed. Coatings were prepared by dipping either the dry or wet green substrates into pure zirconia (TZ-0, Tosoh, Japan, d50 5 0.3 mm, Ss 5 14.6 m2 /g) slurries which were prepared to solids contents of 30, 15, and 7.5 vol% containing 0.8 wt% of dispersant and 5 wt% of binder. Dipping studies were carried out for soaking times of 0, 60, 180, 360, and 900 s (0, 1, 3, 6, and 15 min) and withdrawal rates of 1.8, 2.7, 3.7, and 4.8 mm/s. Coated tapes were left to dry at room conditions overnight and further weighted to calculate the mass variation per unit area. Reported data were the average of three measurements. Direct observation of the cross-section of coated green samples were made by scanning electron microscopy (FEG-SEM Hitachi S-4700, Tokyo, Japan) with the samples embedded in epoxy resin and soft polished with sandpaper. Multilayered materials with five thick layers of Al2O3– Y–TZP and four very thin layers of TZ-0 were prepared by lamination and pressing of two coated tapes alternating with three uncoated tapes. The stack was pressed at 18 MPa using flat and parallel plates as reported before.8,19 Figure 1 shows an overall scheme of the lamination process. Samples were sintered at 15501C/2 h (heating and cooling rates of 51C/min). Cross sections were polished with diamond paste down to 1 mm and thermally etched for SEM characterization and indentation tests. Vickers indentations were performed with loads of 49 and 98 N for 15 s. Indentations were performed on the three intermediate thick layers at different distances from the zirconia compressed thin layers. Cracks parallel and perpendicular to the layers, were measured by optical microscopy in well-defined indentations. To calculate the KIC five indentations were done. Finally the indented cross sections of the multilayers were coated with gold for SEM observation of the cracks. III. Results and Discussion Figure 2 shows the optical micrographs for cross sections of dry (a) and wet (b) tapes dip-coated using a 30 vol% slurry at a withdrawal rate of 3.7 mm/s. On one hand, it can be observed that dry tapes yield much thicker layers than wet ones as a consequence of the filtration phenomena described in a previous work.28 In addition, coating formed on dry tapes show different thicknesses between opposite sides due to the different porosity between the upper and the bottom surfaces of the tape. The different thickness is more significant as the solid content of the slurries decreases. On the other hand, wet saturated substrates yield similar thicknesses in both sides. Table I summarizes the layer thicknesses measured directly by optical microscopy for dry and wet tapes coated with 30, 15, and 7.5 vol% slurries. All the dry tapes show a thicker coating than wet ones as well as the thickness difference between sides. As the objective of the work is to obtain very thin compressive layers, further studies were devoted to optimize the 15 and 7.5 vol% slurries using water-saturated substrates. This pretreatment assures that coatings thicknesses are almost homogenous on both sides and do not have any contribution due to filtration effects. To evaluate the influence of soaking time and withdrawal rate on the thickness of the coating, disks were dip coated using a 15 Dip Coating Drying Pressing 7.5-15 vol% Slurry 18 Mpa . Coated Multilayer Green tape green tape Fig. 1. Scheme of the dip-coating and piling processing steps that allows the fabrication of the multilayered ceramics. July 2008 Forming of Ceramic Laminates 2125
2126 Journal of the American Ceramic Society--Nicolaidis et a/ Vol 91. No. 7 Zro Fig. 2. Optical micrograph of the cross section of green coating obtained by dipping dry (a) and wet(b)tapes in a 30 vol% slurry vol% solids Tz-0 slurry with soaking times from 0 to (Fig. 4(b)) slurries were named as 15M, while multilayer ithdrawal rates of 1. 8. 2.7.3.7. and 4.8 obtained from 7.5 vol% coatings(Fig. 4(c) were named as the mass per unit area of deposited material versus the 75M. Figure 5 shows the cross-sectional microstructure of time for withdrawal rates of 1.8 and 4.8 mm/s. A similar trend laminates 15M(Fig. 5(a)and 7. 5M(Fig. 5(b) that includes a was observed for the other rates but only those two are shown close-up to the thin-layer area. within any laminate the zirconia with the soaking time until a limit is achieved for times longer joined to the tape, showing very sharp interfaces. The binder than 180 s. It can be also observed that for the same soaking used in the dipping slurry formulation ensures a good adhesion pected (Eq.(3). However, when withdrawal starts just after amples. the multilayered structure cannot be easily appreciated immersion, that is, zero soaking time. the thinnest coating cor- a so general view, due to the large difference in thickness responds to the highest withdrawal rate in opposition to the between the thick layers of Al,O/Y-TZP, with an average Landau-Levich equation. Assuming that all samples have a thickness of 350+20 and the thinner Tzo layers with similar green density, the larger thickness obtained for larger I um in thickness. Neither discontinuities, nor delaminations soaking times suggests that there exists a time depending int or cracks were appreciated in the thin layers after sintering, even action between the binder-containing slurry and the substrate, when alumina and zirconia have very different sintering behav because no filtration takes place with saturated wet substrate ior. Some experiments performed at short soaking times Hence there is a second contribution to the total thickness re- below 60 s in Fig 3)generated thin layers composed of a lated to soaking time until a limit value is achieved. Longer row of just one particle, but some discontinuities occurred along soaking times do not produce further deposition of particles. the layer This limit is reached later when coatings are built-up at lower The porosity of the different layers in the multilayer was very ithdrawal rate low, indicating that a high densification level was achieved. The According to these results a constant soaking time of 180 s average thickness measured in the four thin layers was 0.8+0.1 was then maintained in further experiments. The lowest and 2.8+0.5 um for the laminates 7.5 and 15M, respectively. withdrawal rate (1.8 mm/s)was used to reach thinner coatings. It can be observed that 7. 5M layers contain only one or two Figure 4 shows the SEM microstructure of the cross sections grains, while the 15M layer is composed by three to five grains of green tapes coated with zirconia slurries with 30, 15, and of variable sizes 7.5 vol% solids( Figs. 4(aHc)respectively). These pictures dem- Considering the martensitic transformation of zirconia and onstrate the evident increase of thickness of the zirconia layer the thicknesses measured in the laminates the residual stresses (bright phase) with solids loading. The thickness of the green were estimated to be a2 GPa in the zirconia layer whereas the coating varied between 14-15 and 3.54 and 0.5-1 um for the associated tensile stress in the alumina layers range between 2 slurries with solids contents of 30, 15, and 7.5 vol%, respectively The last coating involves the presence of only three or four pa ticles in the layer, so that thickness uniformity has a high relative 0.014 error since the variation of only one particle leads to a relative error of 25%0-33% 0.013 After piling up the coated and uncoated green tapes under 0012 low pressure, samples were sintered at 1550C/2 h. Multilayers fabricated using the green tapes coated with the 15 vol% 0.00g Table l. Summary of the Weigth Gain by Area and Thickness 0.007 Measured on Dry and Wet Samples Dip Coated in Slurries with 0.006 Different Solid Contents at a Withdrawal Rate of 3.7 mm/s 8 mm/s C005 I5 vol% 7. 5 vol% 0003 Tape pretreatment Dry Wet Dry Wet 0.002 Weight gain(mg/mm)0.390.070.110.0067 0.0036 Thickness (um Soaking time(s) Bottom 21012443.530.5<1 9711583.36.2<1 Fig 3. Variation of mass per unit area with soaking time for coatings prepared with a 15 vol% slurry at two different withdrawal rates
vol% solids TZ-0 slurry with soaking times from 0 to 900 s and withdrawal rates of 1.8, 2.7, 3.7, and 4.8 mm/s. Figure 3 plots the mass per unit area of deposited material versus the soaking time for withdrawal rates of 1.8 and 4.8 mm/s. A similar trend was observed for the other rates but only those two are shown for simplicity. It is observed that the coating thickness increases with the soaking time until a limit is achieved for times longer than 180 s. It can be also observed that for the same soaking time coatings are thicker as withdrawal rate increases, as expected (Eq. (3)). However, when withdrawal starts just after immersion, that is, zero soaking time, the thinnest coating corresponds to the highest withdrawal rate in opposition to the Landau–Levich equation. Assuming that all samples have a similar green density, the larger thickness obtained for larger soaking times suggests that there exists a time depending interaction between the binder-containing slurry and the substrate, because no filtration takes place with saturated wet substrates. Hence, there is a second contribution to the total thickness related to soaking time until a limit value is achieved. Longer soaking times do not produce further deposition of particles. This limit is reached later when coatings are built-up at lower withdrawal rate. According to these results a constant soaking time of 180 s was then maintained in further experiments. The lowest withdrawal rate (1.8 mm/s) was used to reach thinner coatings. Figure 4 shows the SEM microstructure of the cross sections of green tapes coated with zirconia slurries with 30, 15, and 7.5 vol% solids (Figs. 4(a)–(c) respectively). These pictures demonstrate the evident increase of thickness of the zirconia layer (bright phase) with solids loading. The thickness of the green coating varied between 14–15 and 3.5–4 and 0.5–1 mm for the slurries with solids contents of 30, 15, and 7.5 vol%, respectively. The last coating involves the presence of only three or four particles in the layer, so that thickness uniformity has a high relative error since the variation of only one particle leads to a relative error of 25%–33%. After piling up the coated and uncoated green tapes under low pressure, samples were sintered at 15501C/2 h. Multilayers fabricated using the green tapes coated with the 15 vol% (Fig. 4(b)) slurries were named as 15M, while multilayers obtained from 7.5 vol% coatings (Fig. 4(c)) were named as 7.5M. Figure 5 shows the cross-sectional microstructure of laminates 15M (Fig. 5(a)) and 7.5M (Fig. 5(b)) that includes a close-up to the thin-layer area. Within any laminate the zirconia layers do not show relevant thickness changes and are perfectly joined to the tape, showing very sharp interfaces. The binder used in the dipping slurry formulation ensures a good adhesion of the coating to the substrate.8,19 In the case of the 7.5M samples, the multilayered structure cannot be easily appreciated in a so general view, due to the large difference in thickness between the thick layers of Al2O3/Y–TZP, with an average thickness of 350720 mm, and the thinner TZ0 layers with o1 mm in thickness. Neither discontinuities, nor delaminations or cracks were appreciated in the thin layers after sintering, even when alumina and zirconia have very different sintering behavior.19 Some experiments performed at short soaking times (below 60 s in Fig. 3) generated thin layers composed of a row of just one particle, but some discontinuities occurred along the layer. The porosity of the different layers in the multilayer was very low, indicating that a high densification level was achieved. The average thickness measured in the four thin layers was 0.870.1 and 2.870.5 mm for the laminates 7.5 and 15M, respectively. It can be observed that 7.5M layers contain only one or two grains, while the 15M layer is composed by three to five grains of variable sizes. Considering the martensitic transformation of zirconia and the thicknesses measured in the laminates, the residual stresses were estimated to be 2 GPa in the zirconia layer whereas the associated tensile stress in the alumina layers range between 2 Fig. 2. Optical micrograph of the cross section of green coating obtained by dipping dry (a) and wet (b) tapes in a 30 vol% slurry. Table I. Summary of the Weigth Gain by Area and Thickness Measured on Dry and Wet Samples Dip Coated in Slurries with Different Solid Contents at a Withdrawal Rate of 3.7 mm/s Solids content 30 vol% 15 vol% 7.5 vol% Tape pretreatment Dry Wet Dry Wet Dry Wet Weight gain (mg/mm2 ) 0.39 0.07 0.11 0.0067 0.047 0.0036 Thickness (mm) Bottom 210 12 44 3.5 30.5 o1 Top 97 11 5.8 3.3 6.2 o1 Fig. 3. Variation of mass per unit area with soaking time for coatings prepared with a 15 vol% slurry at two different withdrawal rates. 2126 Journal of the American Ceramic Society—Nicolaidis et al. Vol. 91, No. 7
July 2008 Forming of ceramic Laminates 2127 Fig 4. Scanning electron micr pictures of the cross section of green tapes coated by in slurries with solid contents of 30 vol%(a),15 vol%(b)and 7.5 vol%(c)for g times of 180 s nd 14 MPa for the 7.5 and 15M samples, respectively. 9, 10. 8 587 MPa that is consistent to other values calculated for similar These residual tensions should not have a significant effect on laminates. Figure 6 shows an indentation performed on the the crack length of indentations performed on the layer of alumina of a 15M layer. No special difference is ob- umina layers. The experimental indentations confirm this erved between the length of parallel and perpendicular cracks prediction and no differences between parallel and perpendicular Moreover, the crack path seems not to be affected by the pres- lengths of the cracks could be appreciated. The Kic calculated ence of the zirconia layer. This is an indicative of two important according to the anstis form was of 3. 1 +0.4 MPa-m' /2 for sample 15M and 2.9+0.2 for sample 7.5M. All these data were delamination nor deflection is appreciated. Figures 7(a) and (b) cluded in Eq(I)to calculate a threshold strength of around shows the close up to the area of intersection between crack and compressive layers for the materials with two different thickness ratio. Except a small change in the direction through the inter- face, no significant change in the spite of the very high compressive stress level. This is in agreement with Eq (1), which establishes that the lower is the thickness of the thin compressive layer, the lower is the thresh- old stress required by the crack to pass through. Previous works with layers thicker than 10 um have shown that layers under ompression can stop a crack or change the growing direction. 8 This is the first demonstration that below a certain limit thick ness the reinforcement of the ressive layers is not relevant but further fracture studies with bending and tensile test are quired to co 2.5 um Dipping of water saturated green tapes into binder containing slurries yield to homogeneous coatings of different thicknesses 250pm 2.5 Fig. 5. Mutilayered ceramics with alternative thick alumina layers (dark phase)and thin zirconia layers(bright lines).(a) From 15 ve Fig. 6. Indentation on a alumina layer. No significant difference coating slurries with an average thickness of about 2.8 um(4-5 grains) tween crack lengths can be observed. It should be noted that crack b) From 7.5 vol coating slurries with an average thickness of about teraction with the ZrOz layer do not provide any effect on the crack 0.8 um(1-2 grains)
and 14 MPa for the 7.5 and 15M samples, respectively.9,10,18 These residual tensions should not have a significant effect on the crack length of indentations performed on the alumina layers. The experimental indentations confirm this prediction and no differences between parallel and perpendicular lengths of the cracks could be appreciated. The KIC calculated according to the Anstis formula35 was of 3.170.4 MPa m1/2 for sample 15M and 2.970.2 for sample 7.5M. All these data were included in Eq. (1) to calculate a threshold strength of around 587 MPa that is consistent to other values calculated for similar laminates.10 Figure 6 shows an indentation performed on the layer of alumina of a 15M layer. No special difference is observed between the length of parallel and perpendicular cracks. Moreover, the crack path seems not to be affected by the presence of the zirconia layer. This is an indicative of two important facts. Firstly that joining between layers is coherent and neither delamination nor deflection is appreciated. Figures 7(a) and (b) shows the close up to the area of intersection between crack and compressive layers for the materials with two different thickness ratio. Except a small change in the direction through the interface, no significant change in the growing behavior is observed in spite of the very high compressive stress level. This is in agreement with Eq. (1), which establishes that the lower is the thickness of the thin compressive layer, the lower is the threshold stress required by the crack to pass through. Previous works with layers thicker than 10 mm have shown that layers under compression can stop a crack or change the growing direction.18 This is the first demonstration that below a certain limit thickness the reinforcement of the compressive layers is not relevant, but further fracture studies with bending and tensile test are required to corroborate this idea. IV. Conclusions Dipping of water saturated green tapes into binder containing slurries yield to homogeneous coatings of different thicknesses Fig. 4. Scanning electron microscopy pictures of the cross section of green tapes coated by dipping in slurries with solid contents of 30 vol% (a), 15 vol% (b) and 7.5 vol% (c) for soaking times of 180 s. Fig. 5. Mutilayered ceramics with alternative thick alumina layers (dark phase) and thin zirconia layers (bright lines). (a) From 15 vol% coating slurries with an average thickness of about 2.8 mm (4–5 grains). (b) From 7.5 vol% coating slurries with an average thickness of about 0.8 mm (1–2 grains). Fig. 6. Indentation on a alumina layer. No significant difference between crack lengths can be observed. It should be noted that crack interaction with the ZrO2 layer do not provide any effect on the crack growth. July 2008 Forming of Ceramic Laminates 2127
2128 Journal of the American Ceramic Society--Nicolaidis et a/ Vol 91. No. 7 Fig. 7. Close up to the intersection of the crack growing through the alumina layer with the pure zirconia thin layer on the 15M(a) and 7. 5M depending on the solids content of the slurry, the withdrawal R rate and the immersion time. The thickness dependence on the Processing optimisation and Behaviour of Layered Ceram immersion time indicates that a binding mechanism different to Compressive Layers, "Compos. Sci. Technol, 67[9] 1930-8 the deposition-evap on is de between particles in the M. G. Pontin. M. P. Rao, A. J. Sanchez-Herencia, and F. F. Lange, slurry and substrate. Homogeneous and reliable coatings on the green tapes were piled, pressed and cosintered for the manufac- formation to Obtain a Threshold Strength, J. Am. Ceram Soc., 85[12]3041-8 ng of multilayered ceramics that combine very thick layers R. Bermejo, Y. Torres. C. n. A.J. Sanchez-Herencia, J. Pascual. sandwiching layers as thin as 0. 8 um(i.e, two or three partick old surer diameter). Considering that the minimum layer thickness reached is about twice or three times the average particle diam- eter, it can be expected that this simple route could be applied to nanometric layers using aqueous based diluted suspensions of sign in Alumina-Alumina/ Zirconia Layered Composites, "Scr. Mater. 38[1]1-5 nanosized particles. This combination of different thicknesses is 14M. Oechsner. C. Hillman. and F. F. Lange. "Crack Bifurcation in Laminar applicable in technologies such as SOFC or TBCs as they re- Ceramic Composites."J.Am. Ceram Soc. 79[7] 1834-8(1996) quire designs that comprise very thin layers. The low thickness IR. A. Cutler. J. D. Bright, A.V. Virkar, and D. K. Shetty, "Strength Im- of the layers allows the fabrication of crack-free layers of mono- provement in Transformation-Toughened Alumina by Selective Phase-Transfor- estimated to be about 2 GPa. Although this high stress level tic Modulus in rigid Al,O /ZrO, Ceramic Laminates, "Ser. Mater. 17)1095- hould be enough to stop the crack no interaction has been ob- served between thin layers and indentation cracks, showing that V. Virkar, J. L. Huang, and R. A. Cutler, ""Strengthening of Oxide Ce- a critical thickness has to be surpassed to reach an eff ramics by Transformation-Induced Stresses, J. Am. Ceram. Soc., 70[3] 164-70 inforcement. This indicate that achievement of an effective re- inforcement by compressive layers the thickness of the thin layer Layered Composites for Crack Bifurcation,"J. Am. Ceram Soc. 82(6)1512-8 hould balance the residual tensile stress associated to the thick layer and the critical thickness. (9A.J. Sanchez-Herencia, J. Gurauskis, and C. Baudin, "Processing of Al2O,/ 2D. H. Park, Y. G. Jung, and U. Paik, "Evaluation of Residual Stress in A. v. Akkaya, "Electrochemical Model for Performance Analysis of a Tubular Moon. M. G. Pontin, and F. F. Lange. Crack Interactions in Laminar Ceramics that Exhibit a Threshold Strength. "J. Am. Ceram. Soc., 87[9 1694- J. w. Kim. A. V. Virkar, K. Z Fung, K. Mehta, and S. C. Singhal. ""polar- ture Behaviour of an Al-Ox-ZrOz Multi-Layered Ceramic with J. Clegg, K. Kendall. N. M. Alford, T. w. Button, and J. D. birchall Residual Stresses Due to Phase Transformations. Fatigue Fract. Eng. Mal 292]455-7 Will A. Mitterdorfer. C. Kleinlogel, D. Perednis, and L. J. Gauckler Thin Electrolytes for Second-Generation Solid Oxide Fuel Cells. Tandon, and V. M. Sglavo, ""Crack Arrest and Multiple 24A.J. Sanchez-Herencia "Water Based Colloidal Processing of Ceramic Lam hrough the Use of Designed Residual Stress Profiles, Science, 283[5406]1295-7(199 A. Villegas, M. Aparicio, and A Duran, " Thick SokGel Coatings Base bM. P. Rao, A.J. Sanchez-Herencia, G.E. Beltz, R. M. McMeeking, and F.F. Lange, " Laminar Ceramics that Exhibit a Threshold Strength, Science, 286[5437 2Y. Castro, A Duran. R. Moreno, and B. Ferrari. "Thick Sol-Gel Coatings 102-5(1999) tion, Adv. Mater. 14 [7]50 R. Tandon and D J. Green, "Crack Stability and T-Curves Due to Macroscopic rinker and A. J. Hurd. "Fundamentals Residual Compressive Stress Profiles,"J. Am. Ceran. Soc., 74 [8]1981-6(199 nchez-Herencia. and C. Baudin. ""Alumina-Zirconia 2>MGPontin.FF.Lange,AJ Layered Ceramics Fabricated by Stacking Water Processed Green Ceramic of Unfired Tape Porosil (2007) Ceran.Soe,88pl2945802005)
depending on the solids content of the slurry, the withdrawal rate and the immersion time. The thickness dependence on the immersion time indicates that a binding mechanism different to the deposition–evaporation is developed between particles in the slurry and substrate. Homogeneous and reliable coatings on the green tapes were piled, pressed and cosintered for the manufacturing of multilayered ceramics that combine very thick layers sandwiching layers as thin as 0.8 mm (i.e., two or three particle diameter). Considering that the minimum layer thickness reached is about twice or three times the average particle diameter, it can be expected that this simple route could be applied to nanometric layers using aqueous based diluted suspensions of nanosized particles. This combination of different thicknesses is applicable in technologies such as SOFC or TBCs as they require designs that comprise very thin layers. The low thickness of the layers allows the fabrication of crack-free layers of monoclinic zirconia which very high compressive stresses have been estimated to be about 2 GPa. Although this high stress level should be enough to stop the crack no interaction has been observed between thin layers and indentation cracks, showing that a critical thickness has to be surpassed to reach an effective reinforcement. This indicate that achievement of an effective reinforcement by compressive layers the thickness of the thin layer should balance the residual tensile stress associated to the thick layer and the critical thickness. References 1 A. V. Akkaya, ‘‘Electrochemical Model for Performance Analysis of a Tubular SOFC,’’ Int. J. Energy Res., 31 [1] 79–98 (2007). 2 J. W. Kim, A. V. Virkar, K. Z. Fung, K. Mehta, and S. C. Singhal, ‘‘Polarization Effects in Intermediate Temperature, Anode-Supported Solid Oxide Fuel Cells,’’ J. Electrochem. Soc., 146 [1] 69–78 (1999). 3 W. J. Clegg, K. 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