《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料 Oxide ceramic laminates with highly textured a-alumina interlayers：I. Texture control and laminate formation

J Mater Sci(2006)41:7425-7436 DOI10.1007/10853-0060808y Oxide ceramic laminates with highly textured a-alumina interlayers: I. Texture control and laminate formation Ming wei· Dan Zhi· David G. Brandon Received: 18 August 2005/ Accepted: 29 November 2005/Published online: 20 September 2006 +Business media. Lle Abstract Three kinds of texture-reinforced oxide component, could be used to tailor electronic and ceramic laminates with strongly bonded interfaces have structural properties and obtain some of the singl been fabricated. All three were based on highly textured crystal anisotropy. Such texture could be used to a-alumina interlayers, but each with a different oxide reduce thermal expansion mismatch, and hence the laminate matrix and a correspondingly different ther- risk of microcracking 1, while texture control offers a mal mismatch between the textured interlayers and the useful alternative to aligned second-phase reinforce- matrix. Alginate-based, aqueous gel casting was used to ment [2]. Most recently, it has been reported that the produce a flexible tape for all the compositions studied. formation of texture in a-alumina can effectively lower The highly textured alumina(TA) interlayers were the residual stress and make the stress distribution derived from gel-cast tapes containing aligned alumina narrower 3]. In general, processing to develop texture seed platelets. It has been found that the strongest in ceramics differs significantly from the conventional texture derived from g1 vol of the seed platelets in powder processing of dense, fine-grained and equiaxed the gelled precursor tape. Using the March-Dollase microstructures. Textured ceramics have been fabri model for texture analysis, the oriented volume frac- cated by several techniques: sinter-forging [4, hot- tion in the highly textured interlayers was estimated to pressing [5], slip-casting [6], gel-casting in a magnetic vary from 60 to 80% field 1,7,8, and, most commonly, by templating using seeded grain growth [2, 9-13]. Textured micro- tructures have been reported in a single-phase iron titanate ceramic laminate, containing a predetermine Introduction configuration of the textured and non-textured layer In a conventional ceramic the anisotropic single crystal The design of ceramic composites with layered properties are averaged in the isotropic, randomly macrostructures is also receiving considerable research oriented polycrystal. Texturing a polycrystalline attention because they exhibit decreased sensitivity material, by controlling the orientation distribution of surface defects and have demonstrated non-cata- crystallites with respect to the coordinate system of a strophic failure in some laminate systems [14-16 key feature in multilayer ceramic systems is the ability to deflect propagating cracks and two different mechanisms of crack deflection have previously been of Materials Science and metall employed. Propagating cracks can be deflected either Cambridge, Cambridge CB along weak interfaces with adjacent layers [14, 15]or wei@@gmail.com into layers exhibiting residual biaxial compressive D. G. Brandon stress[17-19. Essentially, the first mechanism depends epartment of Materials Engineering, Technion-Israel on matrix/interface strength ratio and has had varying degrees of success. However, controlling the strength 2 Springer

Abstract Three kinds of texture-reinforced oxide ceramic laminates with strongly bonded interfaces have been fabricated. All three were based on highly textured a-alumina interlayers, but each with a different oxide laminate matrix and a correspondingly different ther￾mal mismatch between the textured interlayers and the matrix. Alginate-based, aqueous gel casting was used to produce a flexible tape for all the compositions studied. The highly textured alumina (TA) interlayers were derived from gel-cast tapes containing aligned alumina seed platelets. It has been found that the strongest texture derived from 9.1 vol% of the seed platelets in the gelled precursor tape. Using the March–Dollase model for texture analysis, the oriented volume frac￾tion in the highly textured interlayers was estimated to vary from 60 to 80%. Introduction In a conventional ceramic the anisotropic single crystal properties are averaged in the isotropic, randomly oriented polycrystal. Texturing a polycrystalline material, by controlling the orientation distribution of crystallites with respect to the coordinate system of a component, could be used to tailor electronic and structural properties and obtain some of the single crystal anisotropy. Such texture could be used to reduce thermal expansion mismatch, and hence the risk of microcracking [1], while texture control offers a useful alternative to aligned second-phase reinforce￾ment [2]. Most recently, it has been reported that the formation of texture in a-alumina can effectively lower the residual stress and make the stress distribution narrower [3]. In general, processing to develop texture in ceramics differs significantly from the conventional powder processing of dense, fine-grained and equiaxed microstructures. Textured ceramics have been fabri￾cated by several techniques: sinter-forging [4], hot￾pressing [5], slip-casting [6], gel-casting in a magnetic field [1, 7, 8], and, most commonly, by ‘templating’, using seeded grain growth [2, 9–13]. Textured micro￾structures have been reported in a single-phase iron titanate ceramic laminate, containing a predetermined configuration of the textured and non-textured layers [8]. The design of ceramic composites with layered macrostructures is also receiving considerable research attention because they exhibit decreased sensitivity to surface defects and have demonstrated non-cata￾strophic failure in some laminate systems [14–16]. A key feature in multilayer ceramic systems is the ability to deflect propagating cracks and two different mechanisms of crack deflection have previously been employed. Propagating cracks can be deflected either along weak interfaces with adjacent layers [14, 15] or into layers exhibiting residual biaxial compressive stress [17–19]. Essentially, the first mechanism depends on matrix/interface strength ratio and has had varying degrees of success. However, controlling the strength M. Wei (&) Æ D. Zhi Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK e-mail: dr.m.wei@gmail.com D. G. Brandon Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel J Mater Sci (2006) 41:7425–7436 DOI 10.1007/s10853-006-0808-y 123 Oxide ceramic laminates with highly textured a-alumina interlayers: I. Texture control and laminate formation Ming Wei Æ Dan Zhi Æ David G. Brandon Received: 18 August 2005 / Accepted: 29 November 2005 / Published online: 20 September 2006
Springer Science+Business Media, LLC 2006

J Mater Sci(2006)41:7425-7436 of such ceramics is technologically difficult, and fur- Experimental procedure thermore weak interfaces are known to compromise certain properties. The use of residual stress patterns Powder preparation associated with thermal expansion mismatch within ceramic laminate materials therefore offers an attrac- A high purity, submicron a-Al2O3 powder(Ceralox, tive alternative mechanism to improve fracture HPA-0.5) and a known volume percent of a-Al2O3 behaviour seed platelets(10-15 um diameter, ELF Atochem) Q In the present research, texture-reinforced oxide were used to prepare the TA layers. The oxide matrix amic laminates have been prepared by an alginate- precursor powder compositions and their sources are based, gel-cast, doctor-blade process, in which tape listed in Table 1. The rBao powder mixture was lay-ups to develop the laminated structure. Three attrition-milled in acetone with 3 mm-diameter high laminate systems were studied; all based on strongly purity Al2O, beads(99.9%, Union Process) for 6-8 h bonded interfaces between the matrix composition and at 500-600 rpm. After drying in a ventilation hood, the highly textured alumina(TA)interlayers. The control powder mixture was sieved with a 60-mesh sieve. The bonded and textured interlayers is independent of, and attrition milling was monitored by scanning electa of mechanical anisotropy by the presence of strongly particle size of the rBao powder before and af quite different from, the principle of reinforcement by microscopy(LEO 982)and laser particle size analysis weak-interface crack deflection, which is used to inhibit (HORIBA, LA-910) through-thickness crack propagation in many aligned ceramic composites. Gel casting and lamination In this paper, the texture and microstructure of the alumina interlayers were optimized by controlling the Water-based gel casting [2] was used to fabricate both volume fraction and alignment of alumina seed plate- the textured and texture-free layers. An in situ sol-gel lets in a precursor tape. Meanwhile, three types of reaction based on ion exchange between water-soluble matrix layer were used in three laminate systems: sodium alginate and polyvalent cations yields an 1. A reaction-bonded aluminum oxide (RBAO) insoluble, cation cross-linked alginate gel near-zero residual thermal stress in the fully sin- Nanalginate n/2Ca--nNa+ Can/2 alginate (1) tered, single-phase alumina/TA laminate 2. An alumina-toughened zirconia (ATZ), designed A 0.15 M Ca solution of calcium nitrate-4-hydrate for minimum tetragonal zirconia grain-size and (AR purity, Riedel-de Haen), served as the gelling with controlled residual tension in the zirconia solution. The seeded alumina layers were prepared by matrix layers of the laminate first mixing the alumina powder with alginate and a 3. A reaction-bonded(zircon-based) mullite(RBM), dispersant in distilled water by ball milling in a plastic designed for maximum mullite conversion and bottle for at least 12 h(99.9% alumina balls, 5 mn residual compression in the mullite matrix layers of diameter--Union Process). The alumina seed platelets the laminate were then incorporated and ball-milled for a further 5 h. The homogeneously mixed slip was transferred to This paper reports the processing methodology and a second plastic bottle to remove the alumina balls, and texture control for these three texture-reinforced then rolled slowly on the ball mill for about 2 h to ceramic laminates. It will be followed by a further remove trapped air bubbles. The de-gassed slips were report on microstructural development and mechanical poured into the tape caster and cast with the gelling behaviour of the three laminate systems. solution onto a Mylar sheet substrate(Dupont) which Table 1 Precursor powders for RBAO. ATZ and rBM ayer type Material Powder characteristic and source Composition(wt% RBAO 99.36%,45-90mm Miller Thermal. In AlO3 TZ3Y TSK, Y-stabilized tetragonal ZrO2(75.2)Y2O3(4.2) s Another zircon powde ZrO+a-AlO3, <100 n Al2O3(20) submicron Rami submicron (5 mm) was used for AlO3 Ceralox, HPA-05 2 Springer

of such ceramics is technologically difficult, and fur￾thermore weak interfaces are known to compromise certain properties. The use of residual stress patterns associated with thermal expansion mismatch within ceramic laminate materials therefore offers an attrac￾tive alternative mechanism to improve fracture behaviour. In the present research, texture-reinforced oxide ceramic laminates have been prepared by an alginate￾based, gel-cast, doctor-blade process, in which tape lay-ups to develop the laminated structure. Three laminate systems were studied; all based on strongly bonded interfaces between the matrix composition and highly textured alumina (TA) interlayers. The control of mechanical anisotropy by the presence of strongly bonded and textured interlayers is independent of, and quite different from, the principle of reinforcement by weak-interface crack deflection, which is used to inhibit through-thickness crack propagation in many aligned ceramic composites. In this paper, the texture and microstructure of the alumina interlayers were optimized by controlling the volume fraction and alignment of alumina seed plate￾lets in a precursor tape. Meanwhile, three types of matrix layer were used in three laminate systems: 1. A reaction-bonded aluminum oxide (RBAO), designed for minimum sintering shrinkage and a near-zero residual thermal stress in the fully sin￾tered, single-phase alumina/TA laminate. 2. An alumina-toughened zirconia (ATZ), designed for minimum tetragonal zirconia grain-size and with controlled residual tension in the zirconia matrix layers of the laminate. 3. A reaction-bonded (zircon-based) mullite (RBM), designed for maximum mullite conversion and residual compression in the mullite matrix layers of the laminate. This paper reports the processing methodology and texture control for these three texture-reinforced ceramic laminates. It will be followed by a further report on microstructural development and mechanical behaviour of the three laminate systems. Experimental procedure Powder preparation A high purity, submicron a-Al2O3 powder (Ceralox, HPA-0.5) and a known volume percent of a-Al2O3 seed platelets (10–15 lm diameter, ELF Atochem) were used to prepare the TA layers. The oxide matrix precursor powder compositions and their sources are listed in Table 1. The RBAO powder mixture was attrition-milled in acetone with 3 mm-diameter high￾purity Al2O3 beads (99.9%, Union Process) for 6–8 h at 500–600 rpm. After drying in a ventilation hood, the powder mixture was sieved with a 60-mesh sieve. The particle size of the RBAO powder before and after attrition milling was monitored by scanning electron microscopy (LEO 982) and laser particle size analysis (HORIBA, LA-910). Gel casting and lamination Water-based gel casting [2] was used to fabricate both the textured and texture-free layers. An in situ sol-gel reaction based on ion exchange between water-soluble sodium alginate and polyvalent cations yields an insoluble, cation cross-linked alginate gel: Nanalginate þ n=2Ca2þ ! nNaþ þ Can=2 alginate ð1Þ A 0.15 M Ca2+ solution of calcium nitrate-4-hydrate (AR purity, Riedel-deHae¨n), served as the gelling solution. The seeded alumina layers were prepared by first mixing the alumina powder with alginate and a dispersant in distilled water by ball milling in a plastic bottle for at least 12 h (99.9% alumina balls, 5 mm diameter—Union Process). The alumina seed platelets were then incorporated and ball-milled for a further 5 h. The homogeneously mixed slip was transferred to a second plastic bottle to remove the alumina balls, and then rolled slowly on the ball mill for about 2 h to remove trapped air bubbles. The de-gassed slips were poured into the tape caster and cast with the gelling solution onto a Mylar sheet substrate (Dupont) which Table 1 Precursor powders for RBAO, ATZ and RBM layers * Another zircon powder (~5 mm) was used for comparison Layer type Material Powder characteristic and source Composition (wt%) RBAO Al Globular, 99.36%, 45–90 mm, Miller Thermal, Inc. 20–40 Al2O3 Submicron, Ceralox, HPA-0.5 60–80 ATZ TZ3Y20A TSK, Y-stabilized tetragonal ZrO2 + a-Al2O3, < 100 nm ZrO2 (75.2) Y2O3 (4.2) Al2O3 (20) RBM ZrSiO4* submicron, Rami Submicron, Ceralox, HPA-0.5 54.5 Al2O3 45.5 7426 J Mater Sci (2006) 41:7425–7436 123

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

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

J Mater Sci(2006)41:7425-7436 parallel-beam assembly (PW1710 based). The data SEM. Surface preparation for the EBSD sample is were recorded for 20 values from 40 to 154 using Cu critical since the diffraction information originates in a Kan radiation with a step size of 0.05 and a 3-s scan 20 nm layer at the surface, corresponding to the pen time per step. The lower limit of 40 was chosen to etration depth for back-scattered electrons. In our ensure a constant irradiated volume for all data. The process, the sample surface was polished, etched, and integrated intensities for all reflections were calcu- ultrasonically cleaned before EBSD examination. The lated using PhILLIPs APD software EBSD data were collected sequentially by positioning The March-Dollase function [28, P,(n), was used to the focussed electron beam on each grain individually estimate the probability of observing diffraction from The normal to the sample surface was tilted 70 to the the hkl planes at a given angle incident beam and the eBsd pattern was captured at a beam voltage of 20 kV. The individual crystal orien Pr(n)=(cos2n+r-lsin2n)-3/2 (2) tations were analyzed using commercial software to generate discrete pole figures for each localized where n is the angle between the basal plane(001) and microtexture data se the(hkl) planes. The parameter r is the ' compaction ratio'(final thickness/initial thickness) in the original March model. For the present purpose, r was used to Results and discussion indicate the degree of texture: r= l for a random sample and r< 1 for a textured sample. The March- Texture control in c-axis TA interlayers Dollase function satisfies the following normalized equation (29] A cross-section of gel-cast green tape is shown in Fig. 1 and demonstrates alignment of the alumina seed platelets by the gel casting process. Good alignment of Phk(n,中) sin dude=2 (3) the seed alumina platelets is a key factor in attaining high particle packing density in the green bodies, and the aligned, high aspect ratio platelets did not prevent where Phkl(l, o)is the polar axis density function, o is ially and cold-isostatically pressed compacts. High the in-plane rotation angle, and n is the tilt angle about sintered densities were ob ainable the TA samples an axis in the plane of diffraction and perpendicular to after sintering at 1550C for 2 h, despite the pres the is g axis. P(n), is therefore a multiple of the ran- of the aligned platelets and the limitations of dom distribution(MRD)in an isotropic polycrystal In sureless sintering. As expected, under the same the present case of axisymmetric texture Eg 3 simp sintering conditions lower final densities were associ fies to ated with higher initial platelet contents Phkl(n)sin ndn After normalizing the polar density function Pnkdn) 多3 and fitting the March-Dollase function, the oriented volume fraction was calculated as the volume fraction of crystallites oriented in directions for which P>I F (greater than random). The oriented volume fraction was thus defined as: Oriented=/(P(m)-1)sin ndn where is given by P(1)=1. Fig. 1 SEM image of a cross-section of gel-cast textured green Microtexture was also investigated using electron tape showing the distribution of seed platelets(marked by back-scatter diffraction(EBSD) in the LEO 982-FEG- arrows)in the tap 2 Springer

parallel-beam assembly (PW1710 based). The data were recorded for 2h values from 40 to 154 using Cu Ka1 radiation with a step size of 0.05 and a 3-s scan time per step. The lower limit of 40 was chosen to ensure a constant irradiated volume for all data. The integrated intensities for all reflections were calcu￾lated using PHILLIPS APD software. The March–Dollase function [28], Pr(g), was used to estimate the probability of observing diffraction from the hkl planes at a given angle: PrðÞ¼ð g r 2 cos2 g þ r
1 sin2 gÞ
3=2 ð2Þ where g is the angle between the basal plane (001) and the (hkl) planes. The parameter r is the ‘compaction ratio’ (final thickness/initial thickness) in the original March model. For the present purpose, r was used to indicate the degree of texture: r = 1 for a random sample and r < 1 for a textured sample. The March– Dollase function satisfies the following normalized equation [29] Z 2p 0 Zp=2 0 Phklðg;/Þsin gdgd/ ¼ 2p ð3Þ where Phklðg; /Þ is the polar axis density function, / is the in-plane rotation angle, and g is the tilt angle about an axis in the plane of diffraction and perpendicular to the is / axis. Pr(g), is therefore a multiple of the ran￾dom distribution (MRD) in an isotropic polycrystal. In the present case of axisymmetric texture Eq. 3 simpli- fies to Z p=2 0 PhklðgÞsin gdg ¼ 1 ð4Þ After normalizing the polar density function Phkl(g) and fitting the March–Dollase function, the oriented volume fraction was calculated as the volume fraction of crystallites oriented in directions for which P > 1 (greater than random). The oriented volume fraction was thus defined as: Voriented ¼ Z g1 0 ðPðgÞ
1Þsin gdg ð5Þ where g1 is given by P(g1) = 1. Microtexture was also investigated using electron back-scatter diffraction (EBSD) in the LEO 982-FEG￾SEM. Surface preparation for the EBSD sample is critical since the diffraction information originates in a 20 nm layer at the surface, corresponding to the pen￾etration depth for back-scattered electrons. In our process, the sample surface was polished, etched, and ultrasonically cleaned before EBSD examination. The EBSD data were collected sequentially by positioning the focussed electron beam on each grain individually. The normal to the sample surface was tilted 70o to the incident beam and the EBSD pattern was captured at a beam voltage of 20 kV. The individual crystal orien￾tations were analyzed using commercial software to generate discrete pole figures for each localized, microtexture data set. Results and discussion Texture control in c-axis TA interlayers A cross-section of gel-cast green tape is shown in Fig. 1 and demonstrates alignment of the alumina seed platelets by the gel casting process. Good alignment of the seed alumina platelets is a key factor in attaining high particle packing density in the green bodies, and the aligned, high aspect ratio platelets did not prevent the achievement of high green densities in the uniax￾ially and cold-isostatically pressed compacts. High sintered densities were obtainable in the TA samples after sintering at 1550 C for 2 h, despite the presence of the aligned platelets and the limitations of pres￾sureless sintering. As expected, under the same sintering conditions lower final densities were associ￾ated with higher initial platelet contents. Fig. 1 SEM image of a cross-section of gel-cast textured green tape showing the distribution of seed platelets (marked by arrows) in the tape 7428 J Mater Sci (2006) 41:7425–7436 123

J Mater Sci(2006)41:7425-7436 7429 The linear shrinkage ratios(the ratio of the sintering Ka) shrinkage of the compact perpendicular and parallel to the original seed platelets) were also measured, as shown in Fig. 2. Increasing the initial platelet content leads to increased constraint in the plane of the platelets, so that more densification occurs normal to the platelets. Thus, platelet alignment is accompanied by anisotropic densification, as has been reported fo an alumina-platelet-reinforced Ce-ZrO2/Al2O3 com- The volume of dramatically during sintering through growth of the aligned seed platelets at the expense of the surround um ing, randomly oriented, fine Al2O, particles [2]. The microstructure of a cross-section of a sintered alumina compact with 4.8 vol% initial seed platelet content is (b) shown in Fig. 3 and the microstructural anisotropy of the monolithic ta is evident in both the micrograph in Fig 3a and the SEM image in The seed platelets have acted as templates for both sintered grain morphology and crystalline texture development, so that the strong 0001 texture(c-axis perpendicular to the sample surface and normal to the tape-casting direction) is accompanied by morpholog ical anisotropy(flattened grains parallel to the surface) Figure 3c shows a bright-field TEM image taken from a cross-section of TA layer, in which a grain boundary between two Ta grains is accurately parallel to both basal planes(see inserted SAD pattern) with no sign of 10 Hm any glassy phase or second-phase precipitation X-ray diffraction patterns from a green body and c sintered (1550C, 2 h) samples with different initial latelet contents are shown in Fig 4. The 00.6 and 00. 12 peaks for the sintered, platelet-containing zA.[210 sintered textured alumina (TA)with 4.8 vol% initia (b) SEM micrograph of the cross-section of fully sir with 4.8 vol% initial platelets:(c) bright-field TEM basal grain boundary between two Ta grains Platelet Content(vol%) les are much more prominent than those from either Fig 2 Linear shrinkage ratios for the textured alumina(TA) the green body or a sintered, platelet-free sample samples with different initial seed platelet contents Similarly, the near-basal-plane peaks, such as 10.10 and

The linear shrinkage ratios (the ratio of the sintering shrinkage of the compact perpendicular and parallel to the original seed platelets) were also measured, as shown in Fig. 2. Increasing the initial platelet content leads to increased constraint in the plane of the platelets, so that more densification occurs normal to the platelets. Thus, platelet alignment is accompanied by anisotropic densification, as has been reported for an alumina-platelet-reinforced Ce–ZrO2/Al2O3 com￾posite [30]. The volume of textured (aligned) crystals increased dramatically during sintering through growth of the aligned seed platelets at the expense of the surround￾ing, randomly oriented, fine Al2O3 particles [2]. The microstructure of a cross-section of a sintered alumina compact with 4.8 vol% initial seed platelet content is shown in Fig. 3 and the microstructural anisotropy of the monolithic TA is evident in both the optical micrograph in Fig. 3a and the SEM image in Fig. 3b. The seed platelets have acted as templates for both sintered grain morphology and crystalline texture development, so that the strong 0001 texture (c-axis perpendicular to the sample surface and normal to the tape-casting direction) is accompanied by morpholog￾ical anisotropy (flattened grains parallel to the surface). Figure 3c shows a bright-field TEM image taken from a cross-section of TA layer, in which a grain boundary between two TA grains is accurately parallel to both basal planes (see inserted SAD pattern) with no sign of any glassy phase or second-phase precipitation. X-ray diffraction patterns from a green body and sintered (1550 C, 2 h) samples with different initial platelet contents are shown in Fig. 4. The 00.6 and 00.12 peaks for the sintered, platelet-containing sam￾ples are much more prominent than those from either the green body or a sintered, platelet-free sample. Similarly, the near-basal-plane peaks, such as 10.10 and 1.0 1.5 2.0 2.5 3.0 0 5 10 15 20 Linear Shrinkage Ratio (Y/X Shrinkage Ratio) Platelet Content (vol%) X X Y Fig. 2 Linear shrinkage ratios for the textured alumina (TA) samples with different initial seed platelet contents Fig. 3 (a) Optical micrograph of the cross-section of fully sintered textured alumina (TA) with 4.8 vol% initial platelets; (b) SEM micrograph of the cross-section of fully sintered TA with 4.8 vol% initial platelets; (c) bright-field TEM image of a basal grain boundary between two TA grains J Mater Sci (2006) 41:7425–7436 7429 123