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*I0)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 465
AlPO4 Coating on Alumina/Mullite Fibers as a Weak Interface in FiberReinforced 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 polyelectrolyte to render them of opposite charge to the AlPO4 particles, and the latter were then coated by electrostatic-attraction dipcoating. A uniform, continuous coating formed after several cycles, 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 deformation of the monazite.7 However, debonding and fiber pullout results have only been reported for monazite-coated, single-crystal-fiber reinforced oxide composites. No appreciable fiber pullout 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 composites. Aluminum orthophosphate (AlPO4) is a refractory compound, the final product of curing aluminum phosphate binder 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 potential 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 Porosity 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 solution. pH plays a critical role in determining particle size, crystallinity, 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 amorphous 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 neutralizing agents. An Al/P molar ratio, 0.86–1.22, was prepared from stoichiometric AlCl3 and H3PO4 solutions with aqueous ammonia, ethylene, and propylene oxide, respectively.30 Urea shifts the solution pH uniformly into the alkaline range by the decomposition at elevated temperatures. Kandori et al. 31 synthesized 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 electrostatic 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 interface as evidenced by fiber pullout. II. Experimental Procedure (1) Synthesis of AlPO4 Nano-Sol AlNO3 9H2O (Caledon Lab. Ltd., Georgetown, Ontario, Canada) 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 solution, and the solution cleared at pH B2.1. Urea was then added (molar ratio, urea/Al31 5 10), and the solution was heated 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 injecting 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.
Journal of the American Ceramic Society-Bao and Nicholson Vol. 89. No. 2 DEsizing fibers A Treating fiber surfaces with PDADMA Electrostatic attraction to coat fibers with AlPO4 particles Nashing coated fibers Drying coated fibers Heat treatment of coated fibers if necessary smission electron spectroscopy image of aluminium ortho- (4) Characterization of the AlPO, Sol, Coating, and the Is coating of desired Fracture Surface of AlPOrCoated Fiber-Reinforced Al2O3 The electrophoretic mobility of AlPO4(Pals Zeta Potential anal zer, BIC, Holtsville, NY) versus pH was measured, adjusting Fig. 1. Flow chart of the electrostatic attraction coating. the latter with 0.01M HNO3 or 0.01M NH4OH solution. The AlPO4 particle size( Capa-700 particle analyzer, Horiba, Kyoto, Japan)was measured at pH 7.5 and also observed by con- St Louis, MO) for 30 min to induce a positive surface charge on ventional tranmission electron spectroscopy (TEM) imaging the fibers. After washing with distilled water to remove excess AlPO4 powder phase evolution(Miniflex X-ray diffractome- PDADMA, the bundle was soaked in 0.05M AlPO4 sol for ter, Rigaku, Tokyo, Japan) was determined after heating to dif- 30 min and then washed with distilled water to remove excess ferent temperatures for 2 h. Differential thermal analysis/ AlPO4 and finally dried at 110C. The coated fiber bundle was thermogravimetric analysis DTA-TGA)(STA 409PC, Netzsch heated at 1100.C for I h to develop bonding between the AlPO GmbH, Hamburg, Germany) was conducted with a tempera and the fibers. The coating cycle was repeated until the desired ure ramp of 10.C/min. Energy-dispersive spectroscopy(EDS) thickness was obtained. Figure 1 illustrates the procedure of was used to analyze the composition of the washed powder com- electrostatic attraction coating pared with an AlPO4 standard prepared by heating Al(NO3)3 NHa]HPO4 at 1000.C. AlPO4 pellets were uniaxially pressed (3) Alumina/Mullite Fiber-Reinforced Al,O, Composites at 120 MPa with AlPO4 nano powder preheated at 300C TM-DAR ALO, powder (Taimei Chemicals, Tokyo, Japan) and sintered at 1100% C for 2 h to determine sintering with a low sintering temperature(1300.C)was used to prepare behavior. The relative green and fired density was calculated fiber/Al2O3 composites. Several AlPO4-coated fiber tows were via weight and dimension( theoretical density 2.6 g/cm) The AlPO4 coating morphology and fracture surfaces of the infiltrated with a 10 vol% Al2 O3 aqueous slurry at pH-40 un- fiber composites were examined by scanning electron micros der vacuum and then dried, embedded in alumina in a graph die and hot-pressed at 1250C/20 MPa for I h. a pure TM- copy(SEM). EDS was conducted on the coating. DAR alumina pellet was also hot-pressed for comparison. The density of the hot-pressed composites was measured by the II. Results and discuss Urea in aqueous solution decomposes at elevated temperatures and shifts the pH of the AlPO4 sol from acid to alkali, i.e. CO(NH)2+H20-CO2+ 2NH3 90 05MA3 60 0.25 10 00.050.10.150.20250.3 Particle Size (um) 125 Fig. 2. Aluminium orthophosphate particle size distribution via urea decomposition Fig 4. Electrophoretic mobility of aluminium orthophosphate particles
St. Louis, MO) for 30 min to induce a positive surface charge on the fibers. After washing with distilled water to remove excess PDADMA, the bundle was soaked in 0.05M AlPO4 sol for 30 min and then washed with distilled water to remove excess AlPO4 and finally dried at 1101C. The coated fiber bundle was heated at 11001C for 1 h to develop bonding between the AlPO4 and the fibers. The coating cycle was repeated until the desired thickness was obtained. Figure 1 illustrates the procedure of electrostatic attraction coating. (3) Alumina/Mullite Fiber-Reinforced Al2O3 Composites TM-DAR Al2O3 powder (Taimei Chemicals, Tokyo, Japan) with a low sintering temperature (13001C) was used to prepare fiber/Al2O3 composites. Several AlPO4-coated fiber tows were infiltrated with a 10 vol% Al2O3 aqueous slurry at pHB4.0 under vacuum and then dried, embedded in alumina in a graphite die and hot-pressed at 12501C/20 MPa for 1 h. A pure TMDAR alumina pellet was also hot-pressed for comparison. The density of the hot-pressed composites was measured by the Archimedes method. (4) Characterization of the AlPO4 Sol, Coating, and the Fracture Surface of AlPO4-Coated Fiber-Reinforced Al2O3 Composites The electrophoretic mobility of AlPO4 (Pals Zeta Potential analyzer, BIC, Holtsville, NY) versus pH was measured, adjusting the latter with 0.01M HNO3 or 0.01M NH4OH solution. The AlPO4 particle size (Capa-700 particle analyzer, Horiba, Kyoto, Japan) was measured at pH B7.5 and also observed by conventional tranmission electron spectroscopy (TEM) imaging. AlPO4 powder phase evolution (Miniflex X-ray diffractometer, Rigaku, Tokyo, Japan) was determined after heating to different temperatures for 2 h. Differential thermal analysis/ thermogravimetric analysis (DTA–TGA) (STA 409PC, Netzsch GmbH, Hamburg, Germany) was conducted with a temperature ramp of 101C/min. Energy-dispersive spectroscopy (EDS) was used to analyze the composition of the washed powder compared with an AlPO4 standard prepared by heating Al(NO3)3 (NH4)2HPO4 at 10001C. AlPO4 pellets were uniaxially pressed at 120 MPa with AlPO4 nano powder preheated at 3001C and sintered at 11001–15501C for 2 h to determine sintering behavior. The relative green and fired density was calculated via weight and dimension (theoretical density 2.6 g/cm3 ). The AlPO4 coating morphology and fracture surfaces of the fiber composites were examined by scanning electron microscopy (SEM). EDS was conducted on the coating. III. Results and Discussion Urea in aqueous solution decomposes at elevated temperatures and shifts the pH of the AlPO4 sol from acid to alkali, i.e., COðNH2Þ2 þ H2O ! CO2 þ 2NH3 (1) Desizing fibers Treating fiber surfaces with PDADMA Electrostatic attraction to coat fibers with AlPO4 particles Washing coated fibers Drying coated fibers Heat treatment of coated fibers if necessary Is coating of desired thickness? No Fig. 1. Flow chart of the electrostatic attraction coating. 0 10 20 30 40 50 60 70 80 90 100 0 0.05 0.1 0.15 0.2 0.25 0.3 Particle Size (µm) Cumulative #% 0.1M Al3+ 0.5M Al3+ 0.05M Al3+ Fig. 2. Aluminium orthophosphate particle size distribution via urea decomposition. Fig. 3. Transmission electron spectroscopy image of aluminium orthophosphate from [Al31] 5 0.1M solution. −1.25 −1 −0.75 −0.5 −0.25 0 0.25 0.5 2 3 4 5 6 7 8 9 10 pH Mobility (u/s)/(V/cm) Fig. 4. Electrophoretic mobility of aluminium orthophosphate particles. 466 Journal of the American Ceramic Society—Bao and Nicholson Vol. 89, No. 2
February 2006 AlPO, Coating on Alumina/ Mullite Fibers 1600A 1500°c 1300°C 1100°c 1000°C 1000110012001300140015001600 Sintering Temp.(°C) Fig. 7. Sintering behavior of aluminium orthophosphate 10 obility of pH. The isoelectric point (IEP) is at pH4.7. Nano AlPO4 Grey vertical line: tridymite AlPO4 forms a stable sol with a negative surface charge in the basic (JCPDF 50-54) region Dark vertical line: cristobalite Figure 5 tracks the phase evolution of AlPO4 at high tem- AlPO4 ( JCPDF 11-500) peratures. As-prepared AlPO4 is amorphous, and crystallization commences at >1000C. A mixture of"tridymite, -and cristobalite-type AlPO4 results at 1100C. The tridymite phase Fig. 5. Phase evolution of aluminium orthophosphate transforms to the cristobalite phase on increasing temperature A trace of Al2O3 appears at 1500.C DTA/TGA data(Fig. 6) exhibit an exothermic peak at 1080.C because of the trans- NH3+H2O→NH4+OH formation from amorphous to tridymitic and cristobalitic AlPO4. No weight loss occurs between 800 and 1400C, but 0. 25% weight loss occurs between 1400 and 1500C,sug- u- gesting minor AlPO4 decomposition at >1400"C. The latter ex so plains the trace of AlO3 at 1500C(Fig. 5). Both the dtA AlPOa nucleates and grows Further increase of pH endothermic peak and the 20% weight loss at 5.0 ongly polyelectrolyte area of aluminum phosphate, and clusters (<50 nm)of several adsor bed on the fiber surfaces to attract the AlPO4 particles electrostatically. Figure 8 shows the fiber weight gain versus bution was obtained for [Ar+=0.1M when the ph was ad- APOA coating cycle following PDADMA burnout. The former justed by a NH4OH solution. The pH increases as the urea is linear with the number of coating cycles, i.e. 1. I wt% gain/ decomposes in situ, so AlPOA nano particles nucleate and grow cycle PDADMA. Assuming the APOa coating is uniform on uniformly. When the NH,OH solution controls the ph, AlPO a the fiber surface(diameter 12 um, 50% relative density), the nucleates and grows to a large size around the Nh4OH drops coating thickness in because of locally induced high pH values. Small particles form hean AlPO4 particle size is 0.05 um(from 0. 10M solution) in the low-pH region. Thus, a wide particle size distribution re- AlPOA multilayer coating forms on the adsorbed sults when NHOH is used as the neutralizing agen PDADMa because of the opposite surface charge. No fiber ght gain was detected on dip coating in AlPOa thout PDADMA treatment 1.2 1.0 12.0% 8.0% 70 02004006008001000120014001600 0.0% Fig. 6. Differential thermal analysis and thermogravimetric analysis curves of aluminium orthophosphate. Fig 8. Fiber weight gain versus coating cycle
NH3 þ H2O ! NHþ 4 þ OH (2) Thus, the pH shifts from acid to alkali and AlPO4 particles nucleate uniformly, i.e. the pH gradually increases with time so AlPO4 nucleates and grows. Further increase of pH produces a white precipitate. Figure 2 tracks the particle size distribution for different initial [Al31] and [HPO4 2]. The mean particle size increases from 30 to 100 nm as [Al31] increases from 0.05 to 0.5M. The size distribution is narrow for [Al31]r0.1M and wide for [Al31] 5 0.5M. Figure 3 shows a TEM image of the AlPO4 precipitate for [Al31] 5 0.10M. Most particles are B20 nm. They agglomerate after drying because of the ultra-high surface area of aluminum phosphate, and clusters (B50 nm) of several particles (B20 nm) form in the sol. A wider particle size distribution was obtained for [Al31] 5 0.1M when the pH was adjusted by a NH4OH solution. The pH increases as the urea decomposes in situ, so AlPO4 nano particles nucleate and grow uniformly. When the NH4OH solution controls the pH, AlPO4 nucleates and grows to a large size around the NH4OH drops because of locally induced high pH values. Small particles form in the low-pH region. Thus, a wide particle size distribution results when NH4OH is used as the neutralizing agent. Figure 4 shows the electrophoretic mobility of AlPO4 versus pH. The isoelectric point (IEP) is at pHB4.7. Nano AlPO4 forms a stable sol with a negative surface charge in the basic region. Figure 5 tracks the phase evolution of AlPO4 at high temperatures. As-prepared AlPO4 is amorphous, and crystallization commences at 410001C. A mixture of ‘‘tridymite,’’- and ‘‘cristobalite’’-type AlPO4 results at 11001C. The tridymite phase transforms to the cristobalite phase on increasing temperature. A trace of Al2O3 appears at 15001C. DTA/TGA data (Fig. 6) exhibit an exothermic peak at B10801C because of the transformation from amorphous to tridymitic and cristobalitic AlPO4. No weight loss occurs between 8001 and 14001C, but B0.25% weight loss occurs between 14001 and 15001C, suggesting minor AlPO4 decomposition at 414001C. The latter explains the trace of Al2O3 at 15001C (Fig. 5). Both the DTA endothermic peak and the 20% weight loss at o3001C suggest dehydration of AlPO4. EDS defines the Al/P molar ratio between 1.00 and 1.05, i.e. close to stoichiometric AlPO4. Figure 7 tracks the sintering behavior of AlPO4 bulk ceramics. The green density is B43%, and even though sintered at 15501C, the density is still low (B60%). AlPO4 particles have a negative surface charge at pH45.0. PDADMA is a strongly cationic polyelectrolyte. It is preadsorbed on the fiber surfaces to attract the AlPO4 particles electrostatically. Figure 8 shows the fiber weight gain versus AlPO4 coating cycle following PDADMA burnout. The former is linear with the number of coating cycles, i.e. 1.1 wt% gain/ cycle PDADMA. Assuming the AlPO4 coating is uniform on the fiber surface (diameter 12 mm, 50% relative density), the coating thickness increases 0.08 mm after pre-treatment. The mean AlPO4 particle size is B0.05 mm (from 0.10M solution); thus an AlPO4 multilayer coating forms on the adsorbed PDADMA because of the opposite surface charge. No fiber weight gain was detected on dip coating in AlPO4 sol without PDADMA treatment. Grey vertical line: tridymite AlPO4 (JCPDF 50-54) Dark vertical line: cristobalite AlPO4 (JCPDF 11-500) A: alumina 10 20 30 40 50 60 2θ (°C) Intensity (arb.) 300°C 1000°C 1100°C 1300°C 1500°C 1600°C A A A A A Fig. 5. Phase evolution of aluminium orthophosphate. 60 70 80 90 100 0 200 400 600 800 1000 1200 1400 1600 Temp. (°C) DTA (uV/mg) −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 TGA (wt%) Fig. 6. Differential thermal analysis and thermogravimetric analysis curves of aluminium orthophosphate. 40% 60% 80% 100% 1000 1100 1200 1300 1400 1500 1600 Sintering Temp. (°C) Relative Denstiy Fig. 7. Sintering behavior of aluminium orthophosphate. 0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 0 2 4 6 8 10 12 Coating Cycle Weight Gain (wt%) Fig. 8. Fiber weight gain versus coating cycle. February 2006 AlPO4 Coating on Alumina/Mullite Fibers 467
Journal of the American Ceramic Society-Bao and Nicholson Vol. 89. No. 2 10 um (a)&(b), AIPOa coating with 6.0% weight gain (c)&(d), AlPOa coating with 11.0% weight gain Fig 9. Aluminium orthophosphate coating morphology on fibers Figure 9 shows the AlPO4-coated fiber surface morphology tially peeled, but the fiber surface is smooth, suggesting no in- for 6.0 and 11.0 wt% gain, respectively. The coating is smooth terfacial reaction has occurred. Figure 1l(a)shows an SEM and uniform for the former, but rough for the latter. All fiber image of a cross section of the AlPO4-coated fibers with 11.0 shows an se Coated with a porous AIPOa layer. Figure 10 wt% gain embedded in epoxy. There is no bridging between fil- filaments were oating the fibers in 10 cycles(11.0 wt% gain), followed bp mage of an AlPO4 coating synthesized by aments. After polishing, the AlPO4 coating is grooved. Contrast between the AlPO4 coating and fibers is low because of the close heating at 1300.C for 2 h. Neither grain growth nor isolated tomic number of the element therein. Figures 1l(c)and (d) pheroidization occured. The coating was porous and has pa show the eds line-scanning results for P and al across the fiber and the AlPO4 coating, for the line shown in Figure 11(b). Here, the AlPOa coating with thickness 2 um is clear. The AlPOa-coated fibers were used to produce flber-rein- forced AlO3 composites by hot-pressing. An SEM image of a fracture surface is shown in Fig. 12. The Al,O3 matrix is almost dense with a composite open-porosity of 14%. Fiber pullout is clearly evident on the fracture surface(Fig. 12(a)). Recently Lee et al. fabricated a monazite-coated, AlO3-fiber-reinforced Al2O3 composite but observed no fiber pullout following hot pressing at 1250C. They observed pullout from a less-dense matrix after hot-pressing at 1200.C In the present work, AlPO- coated fibers exhibited pullout after hot-pressing at 1250C be- ause the AlPO4 sinters so sluggishly that it is porous with a lov mechanical strength even after sintering at > 1300C. Thus the AIPO. C ating guarantees crack deflection and the fiber/AlPO/matrix interface, resulting in fiber pullout with 5 ums IV. Conclusions AlPO4 nano particles have been synthesized by co-precipitation Fig 10. Aluminium orthophosphate coating after sintering at 1300oC farand HPOa- with urea at 95oC On decomposition, the urea releases OH and shifts the ph to alkali whereas amor-
Figure 9 shows the AlPO4-coated fiber surface morphology for 6.0 and 11.0 wt% gain, respectively. The coating is smooth and uniform for the former, but rough for the latter. All fiber filaments were coated with a porous AlPO4 layer. Figure 10 shows an SEM image of an AlPO4 coating synthesized by dip coating the fibers in 10 cycles (11.0 wt% gain), followed by heating at 13001C for 2 h. Neither grain growth nor isolated spheroidization occured. The coating was porous and has partially peeled, but the fiber surface is smooth, suggesting no interfacial reaction has occurred. Figure 11(a) shows an SEM image of a cross section of the AlPO4-coated fibers with 11.0 wt% gain embedded in epoxy. There is no bridging between filaments. After polishing, the AlPO4 coating is grooved. Contrast between the AlPO4 coating and fibers is low because of the close atomic number of the element therein. Figures 11 (c) and (d) show the EDS line-scanning results for P and Al across the fiber and the AlPO4 coating, for the line shown in Figure 11 (b). Here, the AlPO4 coating with thickness B2 mm is clear. The AlPO4-coated fibers were used to produce fiber-reinforced Al2O3 composites by hot-pressing. An SEM image of a fracture surface is shown in Fig. 12. The Al2O3 matrix is almost dense with a composite open-porosity of B14%. Fiber pullout is clearly evident on the fracture surface (Fig. 12(a)). Recently Lee et al. 39 fabricated a monazite-coated, Al2O3-fiber-reinforced Al2O3 composite but observed no fiber pullout following hotpressing at 12501C. They observed pullout from a less-dense matrix after hot-pressing at 12001C. In the present work, AlPO4- coated fibers exhibited pullout after hot-pressing at 12501C because the AlPO4 sinters so sluggishly that it is porous with a low mechanical strength even after sintering at 413001C. Thus the AlPO4 coating guarantees crack deflection and debonding at the fiber/AlPO4/matrix interface, resulting in fiber pullout with crack tolerance. IV. Conclusions AlPO4 nano particles have been synthesized by co-precipitation of Al31 and HPO4 2 with urea at 951C. On decomposition, the urea releases OH and shifts the pH to alkali whereas amorFig. 9. Aluminium orthophosphate coating morphology on fibers. Fig. 10. Aluminium orthophosphate coating after sintering at 13001C for 2 h. 468 Journal of the American Ceramic Society—Bao and Nicholson Vol. 89, No. 2
February 2006 AlPO, Coating on Alumina Mullite Fibers Q 5 um (c) (um) (c)element P, ( d)element Al Fig. 11. Cross section and energy dispersive spectroscopy(EDS) examination of the aluminium orthophosphate- coated fibers(embedded in epoxy) Scanning electron microscopy images(a, b); EDS line scanning for element P(c) and Al(d phous stoichiometric AlPO4 precipitates. The latter has a nar- tic phase Al2O3 /mullite fiber bundles, pre-treated with cationic row particle size distribution of mean size 30-100 nm when polyelectrolyte to induce a positive surface charge have been [AP]=0.05-01M. The IEP of this material is pH <4.7. dipped in the AlPOA sol at pH 7.5(negative particle surface morphous AlPO4 crystallizes to"tridymitic"and"cristobal- charge), and a uniform and continuous coating formed by tic" phases at 1100C, gradually changing to a 100% cristobal- lectrostatic attraction. AlPOa-coated, fiber-reinforced AL,O3 Fig 12. Fracture surface of AlPOa coated fiber/Al2O3 composite, hot-pressed at 1250C for I h
phous stoichiometric AlPO4 precipitates. The latter has a narrow particle size distribution of mean size 30–100 nm when [Al31] 5 0.05–0.1M. The IEP of this material is pH B4.7. Amorphous AlPO4 crystallizes to ‘‘tridymitic’’ and ‘‘cristobalitic’’ phases at 11001C, gradually changing to a 100% cristobalitic phase. Al2O3/mullite fiber bundles, pre-treated with cationic polyelectrolyte to induce a positive surface charge, have been dipped in the AlPO4 sol at pH B7.5 (negative particle surface charge), and a uniform and continuous coating formed by electrostatic attraction. AlPO4-coated, fiber-reinforced Al2O3 Fig. 11. Cross section and energy dispersive spectroscopy (EDS) examination of the aluminium orthophosphate-coated fibers (embedded in epoxy). Scanning electron microscopy images (a, b); EDS line scanning for element P (c) and Al (d). Fig. 12. Fracture surface of AlPO4 coated fiber/Al2O3 composite, hot-pressed at 12501C for 1 h. February 2006 AlPO4 Coating on Alumina/Mullite Fibers 469
Journal of the American Ceramic Society-Bao and Nicholson Vol. 89. No. 2 composites were hot-pressed at 1250.C, and fiber pullout was D E C Corbridge, Phosphorus: An Outline of its Chemistry, Bioch detected on the fracture surfaces. The latter s AlPOA is ethnology, pp. 140-5. Elsevier, New York, 19 atisfactory as a"weak layer"for the mullite/AlO3(Nextel" Sp:.masP(B如mms+m 720)-fiber-reinforced, Al,O3 matrix composit ID. K. Kim and W. M. Kriven "Mullite-Aluminum I ape Casting. Acknowledgment D. H. Kuo and w.M. Kriven, " "Fracture of Multilayer Oxide Composites, would like to thank prof. J. Barbier and Prof. D. S. wilkinson for ased Fiber-Reinforced Ceramic-Matrix Composites: Prin- ions regarding the phosphate compound as a weak layer in fiber- ciples and Materials, pp. 89-154. Kluwer Academic, Norwell. MA. 2003 reinto "R.S. Hay and E. E. Boakye, Monazite Coatings on Fibers: I, Effect of References 2R. J Kerans, R S Hay, T. A Parthasarathy, and M. K.Cinibulk,"Interface Design for Oxidation-Resistant Ceramic Composites. "J.Am. Ceram. Soc., 85 [lll SmR上0bm甲 (RPO, "B. Chem. Soc. Jpm.53图4:289 Chang, and Z Jie, "Die Sintering and Properties of AIPOr-CrPO Hikichi, T. Nomura, Y. Tanimura, S. Suzuki, and M. Miyamoto, "Sintering J B. Davis. J.P. A. Lofvan G. Evans. E. Bischoff and M. L. Emiliani and Properties of Monazite-Type CePO4, J. Am. Ceram. Soc., 73[12]3594-6 "Fiber Coating Concepts for Brittle- Matrix Composites. "J. Am. Ceram. Soc., 76 1249-57(1993) 3Y. Hikichi, K. Hukuo and J " Solid-State Reactions Between Rare M. K. Cinibulk, T. A. Parthasarathy, K.A. Keller, and T I. Mah. ""Porous M. Housley, and J. T Cheung. Soe85Ill2703-10(2002) r-LaPO System, J. Am. Ceram. Soc. 23. G. Morales, R. R. Clemente, and E Matijevic. "The Mechanism of Pre- P E. D. Morgan and D. B. Marshall, ""Functional Interfaces for Oxide/Oxide 151, sssn of Colloidal Variscite(AlPO4-2H20) Particles, " J. Colloid Interf. Sci Mat. Sci ASmt,162[-2]1525(1993) E. P. Katsanis and E. Matijevic.""Preparation and Pro es of Uniform m1AmCm.SC,间1Bs(19s Composites of Monazite and Colloidal Aluminum Phosphate Particles. Colloid Surf- 5. 43-53(198 R. F. Vogel and G. Marcelin, ""Preparation of Stoichiometric Aluminum B. Marshall, J. B. Davis, P. E. D. Morgan, J. R. Waldrop, and J. R. Porter. rties of La-Monazite as an Interphase in Oxide Comp is,D B. Marshall, and P E D Morgan. ""Oxide Composites of AlyO and LaPO4. J. Eur. Ceram. Soc., 19(13-14 2421-6(1999) "D. H. Kuo. W. M. Kriven and T. J Macki pherical Aluminum Phosphate Particles. " J. Colloid Control of Interfacial Prope Interf. Sci., 182, 425-30 m飞Nm( tngs in Oxide-Oxide Composites 32K Kandori. T Imazato.A Aluminum Phosphate Particles,"Colloid Polym. Sci, 274, 2904(1996) hawla H. Liu, J. Janczak-Rusch, and S. Sambasivan "Microstructure of Soluble Species and P Composites, J /. Eur. Ceram. Soc. 20 (15)551- 2000 on Fiber/Alumina Matrix 咄且、Ax2MWm图My E.E. Boakye, K. A Keller, and R. S. Hay. "Evaluation Short Alumina Fibers Using Layer-by-Layer Assembly Technique. d Monazite Coatings Using Nextel" 720-Fiber-Rein- Mater Sci Eng. A Struct, 364[1-21324-32(2004 Blackglas (TM) Minicomposites "J. Am. Ceram. Soc. 84 [7] 1526-3 X. Gu P. A. Trusty, E. G. BD湖mbm static At E. Cassidy, "Phosphate Bonding-Then and Now. Am. Ceram. Soc. Bull utter, and C. B. Ponton. ""Deposition of Zirconia 55443-3(197 ls on Woven Fibre Preforms Using a Dip-Coating Technique, " J. Eur. Ceran J H. Morris, P. G. Perkins, A E.A. Rose, and w.E. Smith, ""Chemistry and Soc,206675-84(2000 Binding Properties of Aluminum Phosphates,"Chem. Soc. Rev. 6 3C. Kaya,E.G.Butler, ASelcuk, ARBoccaccini, and M.H.Lewis. "Mullite (Nextel" 720) Fibre-Reinforced Mullite Matrix Composites Exhibiting Favour Il, Warm-Temperature Process for Alumina Ceramics, "J.A. Ceram. V Bothe and P w. Brown. "Low-Temperature Format opposites Fabricated by Hot-Pressing. "J. Ceran. Soc. Jpr. 112 [1] 29-34 2004)
composites were hot-pressed at 12501C, and fiber pullout was detected on the fracture surfaces. The latter suggests AlPO4 is satisfactory as a ‘‘weak layer’’ for the mullite/Al2O3 (Nextelt 720)-fiber-reinforced, Al2O3 matrix composites. Acknowledgment The authors would like to thank Prof. J. Barbier and Prof. D. S. Wilkinson for fruitful discussions regarding the phosphate compound as a weak layer in fiberreinforced composites. References 1 Y. Hikichi, K. Hukuo, and J. Shiokawa, ‘‘Strength and Thermal-Expansion of Sintered Rare-Earth Orthophosphate (RPO4),’’ B. Chem. Soc. Jpn., 53 [5] 1457–8 (1980). 2 Y. Hikichi, T. Nomura, Y. Tanimura, S. Suzuki, and M. Miyamoto, ‘‘Sintering and Properties of Monazite-Type CePO4,’’ J. Am. Ceram. Soc., 73 [12] 3594–6 (1990). 3 Y. Hikichi, K. Hukuo, and J. Shiokawa, ‘‘Solid-State Reactions Between RareEarth Ortho-Phosphate and Oxide,’’ B. Chem. Soc. Jpn., 53 [5] 1455–6 (1980). 4 D. B. Marshall, P. E. D. Morgan, R. M. Housley, and J. T. 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