J.Am.Cera.Soc.,892]465-4702006) Dol:10.11551-2916.2 c 2005 The American Ceramic Society urna AlPO4 Coating on Alumina/Mullite Fibers as a Weak Interface in Fiber Reinforced Oxide Composites Yahua bao and patrick s. Nicholson**t Ceramic Engineering Research Group, Department of Materials Science and Engineering. McMaster Universi Hamilton ON. Canada LSS 4L7 A sol of amorphous aluminum orthophosphate(AlPO4)nano oating at articles(30-100 nm)was synthesized by co-precipitation of igh temperatures(>1200.C) limits application as a AF and HPOZ with urea at 95C. A narrow particle size porous AlPO 4 has potential as a weak layer between oxide fibers distribution was achieved when JAF*I<.1M. The isoelectric and an oxide matrix in oxide com ratures int of the synthesized powder is pH- 4.7. Alumina/mullite fAr+ and fibers (Nextel 720) were pretreated with cationic polyelectro- tion. ph plays a critical role in determining particle size, cry lyte to render them of opposite charge to the AlPOa particles allinity and tion Submicron variscite(AlPO4 2H2 nd the latter were then coated by electrostatic-attraction di ating a uniform continuous coating formed after several cy amorphous AlPO4 precipitates with a large surface area.29-31 les, and its effectiveness as the weak layer between the fibers Kandori et al.- synthesized spherical, amorphous AlPO4 at pH nd the AlO3 matrix of a hot-pressed composite was reported. 2 and 100 C. Vogel and Marcelin reported a mixed amor- Significant fiber pullout was noted phous precipitate of alumina-aluminum phosphate under basic conditions. But Goldshmid and Rubinreported that the molar ratio of Al/P is controlled by the ph, i.e. an average molar ratio L. Introduction (Al/P)of 0.62 results in acidic solutions and 1.0 in alkali. The precipitate chemical composition is also determined by neutral TONAZITE(LaPOa or Cepo), with a bend strength of 100- vine cometric AlCls and Ha POa solutions with aqueous ve de- and mullite at <1600C,and serves as a weak layer between nia, ethylene, and propylene oxide, respectively. 30 Urea shifts oxide fibers and oxide ceramic matrices: The weak interface the solution ph uniformly into the alkaline range by mation of the monazite. However, debonding and fiber pullout ized amorphous, spherical AlPOa particles of Al/P molar ratic results have only been reported for monazite-coated, single-crys- 0.96-1.03 with urea. Depending on the urea concentration, the out was noted for monazite-coated, polycrystalline-fibers in a 560 nm dense oxide matrix. Thus, another compound was explored as Oxide fibers have been coated with monazite, ZrO2, etc by the weak layer for polycrystalline-fiber-reinforced oxide com- electrostatic attraction.3 +-38 A uniform coating of nano particles develops on fibers In the present work, AlPO4 nano particles Aluminum orthophosphate(AlPO4)is a refractory com- were laid onto alumina mullite fibers layer by layer via the elec pound, the final product of curing aluminum phosphate bind trostatic attraction protocol. The treated fibers were matrixed in er at elevated temperatures. 3-16 Its structure is analogous to AlO, by hot-pressing(1250C for I h), and the composites were the polymorphs of silica. It is chemically inert and thermally fractured to determine whether the AlPO4 serves as a weak i stable with Alos and mullite at 1600° with a melting terface as evidenced by fiber pullout int of 2000C. it forms a limited solid solution with AlPO4 laminates were fabricated at 1600Q19Q, and mullite/ O2. Recently, damage-tolerant AlO3/Al wherein the AlPO4 functioned as a weak layer. It is listed as one of the II. Experimental Procedure tential weak interfacial compounds with the formula APO4(A (1) Synthesis of AlPO Nano-Sol metal ions), but there are no reports on such an application. AINO3-9H20(Caledon Lab Ltd, Georgetown, Ontario,Can- Hay and boakye noted that there is no strength degradation ada) was dissolved in distilled water, and a white precipitate on formation of amorphous AlPOa between Al,, fibers and formed on adding equimolar (NH4)2HPO4(Caledon Lab Ltd. LaP1+O4(x>0)coating at 1300.C. However, Kerans et al. The precipitate dissolved when dropped with 0. 1M HNO3 SO- excluded AlPOa as the weak layer as a result of the inherent lution, and the solution cleared at pH 2. binding behavior of AlPO. The latter is highly covalent and dded(molar ratio, urea/APt= 10), and the hard to sinter. Thus, AlPOa ceramics are porous after sintering d with stirring for several hours at 95C. An it 1300-1600C, with a bend strength of 1-10 MPa.Po- When pH-65, the sol was cooled and cer rosity in such fiber coatings provides a preferred crack path se three times at 4000 rpm. AlPO4 sol was also prepared by inject matrix cracks are deflected along the fibers that pull out as the ng 0.5 mL/min of 5 vol% NH,OH to adjust the ph to 6.5 fiber-reinforced composites strain. Densification of a porous Washed 0.05M AIPOa sol prepared from 0.10M AF*/HPO4 with urea was brought to pH7.5 for coating lein-contributing editor (2) Coating AlPO onto Alumina/Mullite Fibers Alumina/mullite fibers(Nextel" 720, 3M, St. Paul, MN) were Manuscript No 20513. Received May 4, 2005: approved August 29. 2005. desized at 600C for I h in air and then x0.5 g of the fiber Author to whom corespondence should be addressed. e-mail: nicholson a mcmaster. a bundle(length 10 cm) was soaked in 0.5 wt% cationic polydiallyldimethylammonium chloride(PDADMA, Aldrich 465AlPO4 Coating on Alumina/Mullite Fibers as a Weak Interface in Fiber￾Reinforced Oxide Compositesz Yahua Bao and Patrick S. Nicholson**w Ceramic Engineering Research Group, Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7 A sol of amorphous aluminum orthophosphate (AlPO4) nano particles (30–100 nm) was synthesized by co-precipitation of Al31 and HPO4 2
with urea at 951C. A narrow particle size distribution was achieved when [Al31]r0.1M. The isoelectric point of the synthesized powder is pHB4.7. Alumina/mullite fibers (Nextelt 720) were pretreated with cationic polyelectro￾lyte to render them of opposite charge to the AlPO4 particles, and the latter were then coated by electrostatic-attraction dip￾coating. A uniform, continuous coating formed after several cy￾cles, and its effectiveness as the weak layer between the fibers and the Al2O3 matrix of a hot-pressed composite was reported. Significant fiber pullout was noted. I. Introduction MONAZITE (LaPO4 or CePO4), with a bend strength of 100– 200 MPa,1,2 is thermodynamically stable with alumina and mullite at o16001C,3,4 and serves as a weak layer between oxide fibers and oxide ceramic matrices.5–11 The weak interface between alumina and monazite is associated with plastic defor￾mation of the monazite.7 However, debonding and fiber pullout results have only been reported for monazite-coated, single-crys￾tal-fiber reinforced oxide composites. No appreciable fiber pull￾out was noted for monazite-coated, polycrystalline-fibers in a dense oxide matrix.12 Thus, another compound was explored as the weak layer for polycrystalline-fiber-reinforced oxide com￾posites. Aluminum orthophosphate (AlPO4) is a refractory com￾pound, the final product of curing aluminum phosphate bind￾er at elevated temperatures.13–16 Its structure is analogous to the polymorphs of silica.17 It is chemically inert and thermally stable with Al2O3 and mullite at 16001C18–20, with a melting point of B20001C.17 It forms a limited solid solution with SiO2. 17 Recently, damage-tolerant Al2O3/AlPO4 and mullite/ AlPO4 laminates were fabricated at 16001C19,20, wherein the AlPO4 functioned as a weak layer. It is listed as one of the po￾tential weak interfacial compounds with the formula APO4 (A, metal ions),21 but there are no reports on such an application. Hay and Boakye22 noted that there is no strength degradation on formation of amorphous AlPO4 between Al2O3 fibers and a LaP11xO4 (x40) coating at 13001C. However, Kerans et al. 23 excluded AlPO4 as the weak layer as a result of the inherent binding behavior of AlPO4. The latter is highly covalent and hard to sinter. Thus, AlPO4 ceramics are porous after sintering at 13001–16001C, with a bend strength of 1–10 MPa.19,24 Po￾rosity in such fiber coatings provides a preferred crack path so matrix cracks are deflected along the fibers that pull out as the fiber-reinforced composites strain.25 Densification of a porous coating at high temperatures (412001C) limits application as a weak layer,26 but AlPO4 is sluggish to sinter. Thus, inherently porous AlPO4 has potential as a weak layer between oxide fibers and an oxide matrix in oxide composites at high temperatures. AlPO4 forms on co-precipitation of Al31 and PO4 3
in solu￾tion. pH plays a critical role in determining particle size, crys￾tallinity, and composition. Submicron variscite (AlPO4 2H2O) forms at pHB1.5–2.5 at 901–1001C.27–29 When the pH44, amorphous AlPO4 precipitates with a large surface area.29–31 Kandori et al. 32 synthesized spherical, amorphous AlPO4 at pH 2 and 1001C. Vogel and Marcelin29 reported a mixed amor￾phous precipitate of alumina–aluminum phosphate under basic conditions. But Goldshmid and Rubin33 reported that the molar ratio of Al/P is controlled by the pH, i.e. an average molar ratio (Al/P) of 0.62 results in acidic solutions and 1.0 in alkali. The precipitate chemical composition is also determined by neutral￾izing agents. An Al/P molar ratio, 0.86–1.22, was prepared from stoichiometric AlCl3 and H3PO4 solutions with aqueous ammo￾nia, ethylene, and propylene oxide, respectively.30 Urea shifts the solution pH uniformly into the alkaline range by the de￾composition at elevated temperatures. Kandori et al. 31 synthe￾sized amorphous, spherical AlPO4 particles of Al/P molar ratio 0.96–1.03 with urea. Depending on the urea concentration, the mean diameter of the spherical particles ranges from 36 to 560 nm. Oxide fibers have been coated with monazite, ZrO2, etc. by electrostatic attraction.34–38 A uniform coating of nano particles develops on fibers. In the present work, AlPO4 nano particles were laid onto alumina/mullite fibers layer by layer via the elec￾trostatic attraction protocol. The treated fibers were matrixed in Al2O3 by hot-pressing (12501C for 1 h), and the composites were fractured to determine whether the AlPO4 serves as a weak in￾terface as evidenced by fiber pullout. II. Experimental Procedure (1) Synthesis of AlPO4 Nano-Sol AlNO3 9H2O (Caledon Lab. Ltd., Georgetown, Ontario, Can￾ada) was dissolved in distilled water, and a white precipitate formed on adding equimolar (NH4)2HPO4 (Caledon Lab. Ltd.). The precipitate dissolved when dropped with 0.1M HNO3 so￾lution, and the solution cleared at pH B2.1. Urea was then added (molar ratio, urea/Al31 5 10), and the solution was heat￾ed with stirring for several hours at 951C. An AlPO4 sol formed. When pHB6.5, the sol was cooled and centrifugally washed three times at 4000 rpm. AlPO4 sol was also prepared by inject￾ing 0.5 mL/min of 5 vol% NH4OH to adjust the pH to 6.5. Washed 0.05M AlPO4 sol prepared from 0.10M Al31/HPO4 2
with urea was brought to pHB7.5 for coating. (2) Coating AlPO4 onto Alumina/Mullite Fibers Alumina/mullite fibers (Nextelt 720, 3M, St. Paul, MN) were desized at 6001C for 1 h in air and then B0.5 g of the fiber bundle (length B10 cm) was soaked in 0.5 wt% cationic polydiallyldimethylammonium chloride (PDADMA, Aldrich, 465 Journal J. Am. Ceram. Soc., 89 [2] 465–470 (2006) DOI: 10.1111/j.1551-2916.2005.00751.x r 2005 The American Ceramic Society L. Klein—contributing editor **Fellow, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: nicholsn@mcmaster.ca z Patent applied for. Manuscript No. 20513. Received May 4, 2005; approved August 29, 2005.