J.Am. Ceram Soc.,861962-6402003) urna Mullite-Aluminum Phosphate Laminated Composite Fabricated by tape castin 8 Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 Mullite-aluminum phosphate (3Al,O3 2SiO,/AlPO4)lami- II. Experimental Procedure nated composites were fabricated by tape casting. a density of 1.56 g/cm, which corresponds to 61% Commercial mullite(KM 101, Kyoritsu Ceramics Materials density, and a bending strength of 1.5 MPa after Co. Ltd, Nagoya, Japan) powder was used. Aluminum phos- at 1600 C for 10 h. The aluminum phosphate functioned as a phate was synthesized by the organic, steric entrapment meth- porous, weak, and chemically stable interphase which was able od. To synthesize AlPO4, aluminum nitrate nonahydrate to deflect cracks in a laminated composite. To increase the [AI(NO3)3 9H2O, 98+%, Aldrich Chemical, Inc, Milwaukee, strength of the weak interphase material, 10 and 30 vol% of wI and diabasic ammonium phosphate [(NH4)2. HPO4, Fisher mullite were added Scientific, Fair Lawn, NJ were used as the Al and P source, respectively. The nitrates are first mixed in the distilled water. After 30 min of mixing, 5 wt% poly( vinyl alcohol)(PVA, 205S, Celanese, Ltd, Dallas, TX) solution was added to the solution followed by another 50 min of mixing. The solution was ther A LUMINUM PHOSPHATE(AlPO4)has long been known for its heated at 200% and 400C to remove the water. The partiall imilar structure and analogous polymorphic transformations dehydrated cake was dried at 150C overnight and finally to silica. The a(tetragonal)-B(cubic)cristobalite transforma- calcined at900°C tion, on heating, is known to be clearly first order, with a volum The laminated composites were made by the -casting hange of +4.44%. AlPO, is chemically inert, thermally stable ocess. A mixture of 60 wt% ethanol(ethyl alcohol USP, AAPER (mp=2000C), electrically neutral, and highly covalent. These ALCOL and Chemical, Shelbyville, KY)and 40 wt% methyl ethyl properties make AlPO4 an attractive candidate material for high ketone(99.8%, Fisher Scientific) was used as a solvent. Phosphate temperature applications. AlPO, is known to be difficult to sinter ester(Emphos PS-21A, Witco Chemicals, Houston, TX) was the dispersant. The binder was poly( vinyl butyral)(Butvar B90 because of its high degree of covalency. There are some reports Solutia Chemicals, New Milford, CT). Poly(ethylene glycol) concerning the volatilization of P2Os from AIPO. 3,5,6 Gitzen er(300NF, FCC grade, Union Carbide Chemicals and Plastics Co al. fabricated AlPO -bonded, alumina castables and noted that the inc, Danbury, CT) and dibutyl phthalate(99%, Aldrich)were used as plasticizers. A conventional tape-casting machine with double doctor blades was used. The speed of casting was I cm/s Laminated ceramic composites have been made by tape casting, The procedures for making laminated composites were as follows rolling, slip casting, and electrophorectic deposition, etc. Some powder solvent(48-h ball milling)- powder solvent typical laminated composite systems that have been fabricated clude Al, O, /ZrO2, Al,O, /LaPO4, Al,ZrO,/YPO4, etc plasticizer binder (24-h ball milling)- deairing(rotated at slow peed without balls)- casting cutting and laminating Crack deflection in the Al2O3/Zro2 laminated composite was thermocompression(80 C/(1 h)-345 MPa uniaxial pressing) attributed to residual stresses at the interface 7,9 Morgan et al. 8 binder removal(l°C/ min to I50°C→0.1° min to600°C/(2h) suggested that a monazite ( LapOahalumina interface is weak →CP(413.7MPa)→ sintering(l600°C/10h) enough to produce interfacial debonding when a crack approaches The particle size of synthesized AlPO4 powder before and after t. However, no laminated system has been made to date using I h attrition milling was measured using a centrifugal, automatic AlPOa as a crack deflecting interphase particle size distribution analyzer(Model CAPA-700, Horiba, In this study, the chemical stability and physical and mechanical Kyoto, Japan ). The specific surface area was measured by seve perties of AlPO4 were studied. Mullite-aluminum phosphate point BET analysis from nitrogen gas adsorption(Model ASAP (, 2SiO2/AlPO) laminated composites were investigated to 2400, Micrometrics, Norcross, GA). The sample was dried at see whether AlPO, could function as a crack-deflecting interphase 150 C overnight to remove the moisture before BET analysis. The n a ceramic composite system bulk density of the sintered material was measured by the Archimedes method (ASTM C373). To check for possible vola tilization of po. from the 2AIPO three different kinds of AlPOa were acid-treated, heated, and nalyzed in a Perkin-Elmer Model Plasma Il, inductively coupled asma(ICP) analyzer. The chemical compatibility between mul lite and alpo checked by a rigaku x F W. Zok--contributing editor Model D-Max automated diffractometer, Rigaku/USA, Danvers, MA). The two powders were mixed by 24-h ball milling, sintered 1600°C(10h),and 二二 eceived June 6, 2002; approved June 12, 2003. diffraction(XRD). The microstructures of the lami ites were examined by scanning electron microsc mnv ise M. Mode S-530, Hitachi, Osaka, Japan)Mullite–Aluminum Phosphate Laminated Composite Fabricated by Tape Casting Dong-Kyu Kim* and Waltraud M. Kriven** Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 Mullite–aluminum phosphate (3Al2O3
2SiO2/AlPO4) lami￾nated composites were fabricated by tape casting. AlPO4 had a density of 1.56 g/cm3 , which corresponds to 61% of theoret￾ical density, and a bending strength of 1.5 MPa after sintering at 1600°C for 10 h. The aluminum phosphate functioned as a porous, weak, and chemically stable interphase which was able to deflect cracks in a laminated composite. To increase the strength of the weak interphase material, 10 and 30 vol% of mullite were added. I. Introduction ALUMINUM PHOSPHATE (AlPO4) has long been known for its similar structure and analogous polymorphic transformations to silica.1 The
(tetragonal) 3 (cubic) cristobalite transforma￾tion, on heating, is known to be clearly first order, with a volume change of 4.44%.2 AlPO4 is chemically inert, thermally stable (mp  2000°C3 ), electrically neutral, and highly covalent. These properties make AlPO4 an attractive candidate material for high￾temperature applications.4 AlPO4 is known to be difficult to sinter because of its high degree of covalency.4 There are some reports concerning the volatilization of P2O5 from AlPO4. 3,5,6 Gitzen et al. 5 fabricated AlPO4-bonded, alumina castables and noted that the castables gave excellent serviceability in temperature ranges to 1870°C. Laminated ceramic composites have been made by tape casting, rolling, slip casting, and electrophorectic deposition, etc. Some typical laminated composite systems that have been fabricated include Al2O3/ZrO2, 7 Al2O3/LaPO4, 8 Al2O3/ZrO2/YPO4, 9 etc. Crack deflection in the Al2O3/ZrO2 laminated composite was attributed to residual stresses at the interface.7,9 Morgan et al. 8 suggested that a monazite (LaPO4)–alumina interface is weak enough to produce interfacial debonding when a crack approaches it. However, no laminated system has been made to date using AlPO4 as a crack deflecting interphase. In this study, the chemical stability and physical and mechanical properties of AlPO4 were studied. Mullite–aluminum phosphate (3Al2O3
2SiO2/AlPO4) laminated composites were investigated to see whether AlPO4 could function as a crack-deflecting interphase in a ceramic composite system. II. Experimental Procedure Commercial mullite (KM 101, Kyoritsu Ceramics Materials Co. Ltd., Nagoya, Japan) powder was used. Aluminum phos￾phate was synthesized by the organic, steric entrapment meth￾od.10,11 To synthesize AlPO4, aluminum nitrate nonahydrate [Al(NO3)3
9H2O, 98%, Aldrich Chemical, Inc., Milwaukee, WI] and diabasic ammonium phosphate [(NH4)2
HPO4, Fisher Scientific, Fair Lawn, NJ] were used as the Al and P source, respectively. The nitrates are first mixed in the distilled water. After 30 min of mixing, 5 wt% poly(vinyl alcohol) (PVA, 205S, Celanese, Ltd., Dallas, TX) solution was added to the solution, followed by another 50 min of mixing. The solution was then heated at 200° and 400°C to remove the water. The partially dehydrated cake was dried at 150°C overnight and finally calcined at 900°C. The laminated composites were made by the tape-casting process. A mixture of 60 wt% ethanol (ethyl alcohol USP, AAPER ALCOL and Chemical, Shelbyville, KY) and 40 wt% methyl ethyl ketone (99.8%, Fisher Scientific) was used as a solvent. Phosphate ester (Emphos PS-21A, Witco Chemicals, Houston, TX) was the dispersant. The binder was poly(vinyl butyral) (Butvar B90, Solutia Chemicals, New Milford, CT). Poly(ethylene glycol) (300NF, FCC grade, Union Carbide Chemicals and Plastics Co. Inc., Danbury, CT) and dibuthyl phthalate (99%, Aldrich) were used as plasticizers. A conventional tape-casting machine with double doctor blades was used. The speed of casting was 1 cm/s. The procedures for making laminated composites were as follows: powder solvent (48-h ball milling) 3 powder solvent plasticizer binder (24-h ball milling) 3 deairing (rotated at slow speed without balls) 3 casting 3 cutting and laminating 3 thermocompression (80°C/(1 h) 3 34.5 MPa uniaxial pressing) 3 binder removal (1°C/min to 150°C 3 0.1°C/min to 600°C/(2 h)) 3 CIP (413.7 MPa) 3 sintering (1600°C/(10 h)). The particle size of synthesized AlPO4 powder before and after 1 h attrition milling was measured using a centrifugal, automatic, particle size distribution analyzer (Model CAPA-700, Horiba, Kyoto, Japan). The specific surface area was measured by seven￾point BET analysis from nitrogen gas adsorption (Model ASAP 2400, Micrometrics, Norcross, GA). The sample was dried at 150°C overnight to remove the moisture before BET analysis. The bulk density of the sintered material was measured by the Archimedes method (ASTM C373). To check for possible vola￾tilization of P2O5 from the reaction 2AlPO4 3 Al2O3 P2O5, three different kinds of AlPO4 were acid-treated, heated, and analyzed in a Perkin-Elmer Model Plasma II, inductively coupled plasma (ICP) analyzer. The chemical compatibility between mul￾lite and AlPO4 was checked by a Rigaku X-ray diffractometer (Model D-Max automated diffractometer, Rigaku/USA, Danvers, MA). The two powders were mixed by 24-h ball milling, sintered at 1600°C/(10 h), and analyzed for existing phases by X-ray diffraction (XRD). The microstructures of the laminated compos￾ites were examined by scanning electron microscopy (SEM, Model S-530, Hitachi, Osaka, Japan). F. W. Zok—contributing editor Manuscript No. 186902. Received June 6, 2002; approved June 12, 2003. *Member, American Ceramic Society. **Fellow, American Ceramic Society. 1962 journal J. Am. Ceram. Soc., 86 [11] 1962–64 (2003)