《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_Mullite-AlPO4-alumina

Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)3041-3048 www.elsevier.com/locate/jeurceramsoc AlPO4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites Yahua bao. patrick s. nicholson Ceramic Engineering Research Group, Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, L&S 4L7 Canada Received 23 February 2008; received in revised form 27 May 2008: accepted 30 May 2008 vailable online 22 July 2008 Abstract A precursor for reaction-bonded mullite(rBm)is formulated by premixing Al]O3, Si, mullite seeds and mixed-rare-earth-oxides(MREO).An ethanol suspension thereof is stabilized with polyethy leneimine protonated by acetic acid. The solid in the suspension is infiltrated into unidirectional mullite/alumina fiber-preforms by electrophoretic infiltration deposition to produce fiber-reinforced, RBM green bodies. Crack-free composites with <% porosity were achieved after pressureless sintering at 1300C. Pre-coating the fibers with alPO4 as a weak intervening layer facilitates significant fiber pullout on composite fracture and confers superior damage tolerance. The bend strength is 170 MPa at 25C<T<1100C. At 200C, the composite fails in shear due to MREO-based, glassy phase formation. However, the AlPOA coating acts as a weak layer even after thermal aging at 1300 C for 100h C 2008 Elsevier Ltd. all rights reserved. Keywords: AlPO4; Weak layer; Nextel 720 fiber; Reaction-bonded mullite; Ceramic matrix composites 1. Introduction layer on fiber surface for crack deflection and fiber pullout. Use of a reaction-bonded matrix with near -zero sintering shrinkage Fiber-reinforced ceramic matrix composites(CMCs) are should avoid the necessity to hot-press candidates for structural application due to their damage toler Mullite is an ideal matrix at elevated temperatures ance. Currently, most dense fiber-reinforced CMC's are based of high temperature strength, low thermal expansion coeffi- on non-oxide systems which oxidize in air at high temperatures. cient and good creep resistance. Due to the volume stability of Oxidation-resistant, fiber-reinforced CMC's are required for mullite/alumina fibers, the matrix sintering shrinkage must be use in oxidizing atmospheres at high temperatures. To optimize low to avoid cracks on pressureless sintering Reaction-bonded the fracture work necessary to break fiber-reinforced CMC's, mullite(RBM) explored as near-zero shrinkage is achieved the bonding between fiber and matrix must allow fiber pullout by mixing alumina, silicon and aluminum precursors. 6-In from the matrix via a weak layer therebetween. LaPO4 has reaction-bonded mullite of composition 3A12O3-2Si, the pre been reported as one of the most successful dense weak cursor Si oxidizes to Sio2 at high temperatures which reacts layer candidates. 2+However, Bao and Nicholson recently with the AlO, to form mullite. Two volume expansion reac- reported application of another phosphate, AlPO4, as the weak tions are involved, i. e, Si-SiO2(+134%), and 3Al2O3- layer on the fiber surface. They demonstrated significant fiber 2SiO2-mullite (+1.3%0). The reaction shrinkage can be the- pullout on the fracture surface of hot-pressed, AlPO4-coated oretically calculated as: mullite/alumina(Nextel 720) fiber-reinforced Al2O3. Highly sintered at 1550oC 5 Thus it should be a stable porous weak 5=1-(1. 1. x1.013 X Vsi P0) 3 covalently-bonded AlPO4 displays poor sintering behavior even (1) VAl2 O3+ Vsi p where po and p are the theoretical-green and fired-densities Corresponding author. Current address: SIl MegaDiamond, 275 West 2230 (VAl2O] and Vsi are the volume fractions of Al2 O3 and Si powder North, Provo, UT 84604, United in the mixture, respectively). Generally, after green process E-mailaddress:baoyahua@gmail.com(YBao ing, the ceramic density is -55%. For 3%o sintering shrinkage, 0955-2219/S-see front matter o 2008 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2008.05.032

Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 3041–3048 AlPO4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites Yahua Bao ∗, Patrick S. Nicholson Ceramic Engineering Research Group, Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, L8S 4L7 Canada Received 23 February 2008; received in revised form 27 May 2008; accepted 30 May 2008 Available online 22 July 2008 Abstract A precursor for reaction-bonded mullite (RBM) is formulated by premixing Al2O3, Si, mullite seeds and mixed-rare-earth-oxides (MREO). An ethanol suspension thereof is stabilized with polyethyleneimine protonated by acetic acid. The solid in the suspension is infiltrated into unidirectional mullite/alumina fiber-preforms by electrophoretic infiltration deposition to produce fiber-reinforced, RBM green bodies. Crack-free composites with ≤25% porosity were achieved after pressureless sintering at 1300 ◦C. Pre-coating the fibers with AlPO4 as a weak intervening layer facilitates significant fiber pullout on composite fracture and confers superior damage tolerance. The bend strength is ∼170 MPa at 25 ◦C ≤ T ≤ 1100 ◦C. At 1200 ◦C, the composite fails in shear due to MREO-based, glassy phase formation. However, the AlPO4 coating acts as a weak layer even after thermal aging at 1300 ◦C for 100 h. © 2008 Elsevier Ltd. All rights reserved. Keywords: AlPO4; Weak layer; Nextel 720 fiber; Reaction-bonded mullite; Ceramic matrix composites 1. Introduction Fiber-reinforced ceramic matrix composites (CMC’s) are candidates for structural application due to their damage toler￾ance. Currently, most dense fiber-reinforced CMC’s are based on non-oxide systems which oxidize in air at high temperatures. Oxidation-resistant, fiber-reinforced CMC’s are required for use in oxidizing atmospheres at high temperatures. To optimize the fracture work necessary to break fiber-reinforced CMC’s, the bonding between fiber and matrix must allow fiber pullout from the matrix via a weak layer therebetween.1 LaPO4 has been reported as one of the most successful dense weak layer candidates.2–4 However, Bao and Nicholson5 recently reported application of another phosphate, AlPO4, as the weak layer on the fiber surface. They demonstrated significant fiber pullout on the fracture surface of hot-pressed, AlPO4-coated mullite/alumina (NextelTM 720) fiber-reinforced Al2O3. Highly covalently-bonded AlPO4 displays poor sintering behavior even sintered at 1550 ◦C.5 Thus it should be a stable porous weak ∗ Corresponding author. Current address: SII MegaDiamond, 275 West 2230 North, Provo, UT 84604, United States. E-mail address: baoyahua@gmail.com (Y. Bao). layer on fiber surface for crack deflection and fiber pullout. Use of a reaction-bonded matrix with near-zero sintering shrinkage should avoid the necessity to hot-press. Mullite is an ideal matrix at elevated temperatures because of high temperature strength, low thermal expansion coeffi- cient and good creep resistance. Due to the volume stability of mullite/alumina fibers, the matrix sintering shrinkage must be low to avoid cracks on pressureless sintering. Reaction-bonded mullite (RBM) explored as near-zero shrinkage is achieved by mixing alumina, silicon and aluminum precursors.6–8 In reaction-bonded mullite of composition 3Al2O3–2Si, the pre￾cursor Si oxidizes to SiO2 at high temperatures which reacts with the Al2O3 to form mullite. Two volume expansion reac￾tions are involved, i.e., Si→SiO2 (+134%), and 3Al2O3– 2SiO2 →mullite (+1.3%). The reaction shrinkage can be the￾oretically calculated as; s = 1 − 1.013 + 1.340 × 1.013 × VSi VAl2O3 + VSi ρ0 ρ 1/3 (1) where ρ0 and ρ are the theoretical-green and fired-densities (VAl2O3 and VSi are the volume fractions of Al2O3 and Si powder in the mixture, respectively). Generally, after green process￾ing, the ceramic density is ∼55%. For 3% sintering shrinkage, 0955-2219/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2008.05.032

3042 Y Bao, PS. Nicholson /Journal of the European Ceramic Sociery 28(2008)3041-3048 the fired density can be 80% theoretical. Thus reaction-bonded- AlPO4 was coated onto the mullite/alumina fibers(Nextel M mullite with less than 20%o porosity and less than 3% shrinkage 720, 3M, St. Paul, MN) by a layer-by-layer electrostatic should not be problem if the SiOz comes from the Si precursor, method. Desized fibers were pre-treated with 0.5 wt%cationic i.e., fiber-reinforced, reaction-bonded-mullite composites can be polyelectrolyte solution(polydiallyldimethylammonium chlo- realistically fabricated by pressureless sintering free of macro ride, Aldrich, M W. 400,000-500,000)to induce a positive racks. The mullite-formation temperature is-1500oC, how- surface charge. The latter attracts the negatively-charged AlPO4 ever, the strength of the mullite/alumina(Nextel 720)fiber nano-particles to pro duce the coating. The coated fibers were degrades severely on heat-treatment >1300C. Thus the mul- heat-treated at 1100 C. AlPO4 was coated for 10 cycles lite matrix sintering temperature must be modified to s1300"C (10 wt% gain) to give an acceptable thickness. The coated for fiber strength retention. Recently, rare earth oxides added to fibers were unidirectionally mounted in a rectangular, plastic RBM reduced the mullite-formation temperature to 1350C. holder(25 mm x mm x 3 mm), the back of which was attached o mixed-rare-earth-oxides(MREO) were added to the rbm a metal plate as cathode to draw particles through the fiber mixture to form mullite 20 m /g(to promote oxidation during sintering). Table 1 Four-point bend testing was performed at 0.10 mm/min in lists composition of mixed-rare-earth-oxides (MREO, Lan- a screw-driven, ultra-hard compression machine(Model 10053 thanide oxide, Molycorp, Fairfield, NJ). These were added &10055, Wykeham Farrance Engineering Ltd, UK) using an as sintering and mullite-formation aids. A mullite precursor alumina fixture with outer span, 20 mm, and inner span, 10 mm. (Siral 28M, SASOL GmbH, Hamburg, German), pre-sintered The sample thickness was 2.0-2.5 mm. Fracture at 1300C for 2 h to form pure mullite, was ground and added observed by SEM and the degree of fiber-pullout determined as mullite-promotion seeds. The molar ratio of Al: Si was set to en samples were tested at room temperature and five at elevated that of mullite temperatures. Anhydrous polyethyleneimine dispersant(PEl, M.W10,000, acid was added to stabilize the RBM-precursor, ethano/ 3. Results and discussion suspension. The optimal addition was determined via the elec- Fig. 1 shows the dta curve for 32 wt% MREo, 22 wt% trophoretic mobility value for the RBM precursors with a Al2O3 and 46 wt% Sio mixture. The endothermic peak around ta potential analyzer(ZetaPALS, Brookhaven Instruments, 1200 C is due to eutectic liquid-phase formation. Mullite phase Holtsville NY). The electrokinetic sonic amplitude(ESA)was can be promoted by formation of the MREO-Al2O3-SiOz eute measured on the mixed suspension(ESA-8000, Matec Applied tic liquid phase. Fig. 2 tracks the phase evolution in the RBM Sciences, Hopkinton, MA). RBM pellets were also uniaxi- matrix versus sintering temperature for a mixture containing ally pressed then cold isostatically pressed at 140 MPa and 7.5 wt% MREO. Mullite appears at 1270 C, and is the major eated to 1175-1200C for 10h in air to oxidize the Si phase at 1300 C. Traces of alumina and silica remain.The sil- to Sio2. Finally they were sintered at 1250-1350C for 2h to study mullite phase formation and shrinkage. Shrinkage was calculated from the change of the diameter of the pel- Table I 16 Composition of the mixed-rare-earth-oxides(MrEO) provided by Molycorp Oxide Concentration(wt%) PrO1 Temperature(C) Fig 1. DTA curve for 32 wt% MREO, 22 wt%AlO3 and 46 wt%SiOz mixture

3042 Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 the fired density can be 80% theoretical. Thus reaction-bonded￾mullite with less than 20% porosity and less than 3% shrinkage should not be problem if the SiO2 comes from the Si precursor, i.e., fiber-reinforced, reaction-bonded-mullite composites can be realistically fabricated by pressureless sintering free of macro cracks. The mullite-formation temperature is ∼1500 ◦C, how￾ever, the strength of the mullite/alumina (NextelTM 720) fiber degrades severely on heat-treatment >1300 ◦C. 9 Thus the mul￾lite matrix sintering temperature must be modified to ≤1300 ◦C for fiber strength retention. Recently, rare earth oxides added to RBM reduced the mullite-formation temperature to 1350 ◦C.10 So mixed-rare-earth-oxides (MREO) were added to the RBM mixture to form mullite 20 m2/g (to promote oxidation during sintering). Table 1 lists composition of mixed-rare-earth-oxides (MREO, Lan￾thanide oxide, Molycorp, Fairfield, NJ). These were added as sintering and mullite-formation aids. A mullite precursor (Siral 28 M, SASOL GmbH, Hamburg, German), pre-sintered at 1300 ◦C for 2 h to form pure mullite, was ground and added as mullite-promotion seeds. The molar ratio of Al:Si was set to that of mullite. Anhydrous polyethyleneimine dispersant (PEI, M.W. 10,000, Polysciences, Warrington, PA), protonated with glacial acetic acid was added to stabilize the RBM-precursor, ethanol suspension. The optimal addition was determined via the elec￾trophoretic mobility value for the RBM precursors with a zeta potential analyzer (ZetaPALS, Brookhaven Instruments, Holtsville NY). The electrokinetic sonic amplitude (ESA) was measured on the mixed suspension (ESA-8000, Matec Applied Sciences, Hopkinton, MA). RBM pellets were also uniaxi￾ally pressed then cold isostatically pressed at 140 MPa and heated to 1175–1200 ◦C for 10 h in air to oxidize the Si to SiO2. Finally they were sintered at 1250–1350 ◦C for 2 h to study mullite phase formation and shrinkage. Shrinkage was calculated from the change of the diameter of the pel￾lets. Table 1 Composition of the mixed-rare-earth-oxides (MREO) provided by Molycorp Oxide Concentration (wt%) CeO2 49 La2O3 33 Nd2O3 13 Pr6O11 4 Other 1 AlPO4 was coated onto the mullite/alumina fibers (NextelTM 720, 3 M, St. Paul, MN) by a layer-by-layer electrostatic method.5 Desized fibers were pre-treated with 0.5 wt% cationic polyelectrolyte solution (polydiallyldimethylammonium chlo￾ride, Aldrich, M.W. 400,000–500,000) to induce a positive surface charge. The latter attracts the negatively-charged AlPO4 nano-particles to produce the coating. The coated fibers were heat-treated at 1100 ◦C. AlPO4 was coated for 10 cycles (∼10 wt% gain) to give an acceptable thickness. The coated fibers were unidirectionally mounted in a rectangular, plastic holder (25 mm × 5 mm × 3 mm), the back of which was attached to a metal plate as cathode to draw particles through the fiber preform and accomplish electrophoretic-infiltration-deposition (EPID).12,13 The inter-electrode distance was 2 cm and EPID was conducted at a constant current of 0.07 mA/cm2. After depo￾sition, the composite was cold isostatically pressed at 140 MPa then dried in an atmosphere-controlled closed container to avoid cracking. Uncoated fibers were also infiltrated with RBM matrix for comparison. The green composites were heated to 1175 ◦C for 10 h in air to convert the Si to SiO2 then sintered at 1300 ◦C for 2 h. Their thermal stability was determined by heat-treatment for 100 h at 1300 ◦C. The fired density and open porosity were measured by Archimedes’ method in water. Four-point bend testing was performed at 0.10 mm/min in a screw-driven, ultra-hard compression machine (Model 10053 & 10055, Wykeham Farrance Engineering Ltd., UK) using an alumina fixture with outer span, 20 mm, and inner span, 10 mm. The sample thickness was 2.0–2.5 mm. Fracture surfaces were observed by SEM and the degree of fiber-pullout determined. Ten samples were tested at room temperature and five at elevated temperatures. 3. Results and discussion Fig. 1 shows the DTA curve for 32 wt% MREO, 22 wt% Al2O3 and 46 wt% SiO2 mixture. The endothermic peak around 1200 ◦C is due to eutectic liquid-phase formation. Mullite phase can be promoted by formation of the MREO–Al2O3–SiO2 eutec￾tic liquid phase. Fig. 2 tracks the phase evolution in the RBM matrix versus sintering temperature for a mixture containing 7.5 wt% MREO. Mullite appears at 1270 ◦C, and is the major phase at 1300 ◦C. Traces of alumina and silica remain. The sil￾Fig. 1. DTA curve for 32 wt% MREO, 22 wt%Al2O3 and 46 wt%SiO2 mixture

Y Bao, P.S. Nicholson /Journal of the European Ceramic Society 28(2008)3041-3048 3043 (a)100% 1270° : 2.0% 15.0% Seeds(wt%) 2(° (b)34 300% Fig. 2. Phase evolution of the reaction-bonded mullite containing 7.5 wt%o MREO 200%自 ◆ Bulk density ica is totally consumed >1300C but the alumina trace persists The mullitization temperature was further decreased by addin -Apparent densityt15.0% mullite"seeds"(Fig 3). With 0.5 wt% seeds, the mullitization process was complete at 1270C. However, the sintering shrink- age(6-8%)was still too large(should be <x3% to avoid matrix cracks around the volume stable fibers). Fig 4 tracks the effect of mullite seed content on the shrinkage, density and open porosity, for rbm with 7.5 wt%o mreo, a small amount of mullite seed 20 can prompt the formation of mullite. However, when seed con- Seeds(wt%) centration is high, it serves as refractory inclusion and retards Fig 4 Influence of mullite seed addition on the sintering shrinkage, density and the rBm sintering. Fig. 5 shows the dilatometric measurement open porosity for RBM sintered at 1300C for 2h. curve for rBM with 7.5 wt% MREO and 5.0 wt% seeds. Lengt expansion occurs due to oxidation of Si metal powder and increase of temperature, RBM sintering shrinkage takes place reaches maximum(<3%)at 170C, close to the eutectic point and final length change is <3%.When(seeds)=5wt%, the RBM of MREO-Al2o3-SiO2. Formation of MREO-Al2o3-Sioz iq- shrinkage is <3% and open porosity, < 20%. This composition is uid phase prompts the sintering shrinkage and a steep drop in optimum and is used in the EPID processing length was detected at 1170-1200oC Si metal is completely oxi- dized into silica when soaked at 1200 C for 10h. With further Si particles are negatively charged. These particles tend to hetero-coagulate. A dispersant was added to induce a com- 0.5wt% seeds mon surface charge sign and stabilize the mixed suspension 1300c PEl, protonated with acetic acid, adsorbs on both surfaces ren- 1250° 2e( 1200 ▲ mullite口:Sio2◆:Al2O3 Sinering Time(min. Fig. 5. Dilatometric measurement of rbM with 7.5 wt% MREO and 5. 0wt% Fig. 3. Effect of mullite seed addition on the mullitization temperature

Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 3043 Fig. 2. Phase evolution of the reaction-bonded mullite containing 7.5 wt% MREO. ica is totally consumed >1300 ◦C but the alumina trace persists. The mullitization temperature was further decreased by adding mullite “seeds” (Fig. 3). With 0.5 wt% seeds, the mullitization process was complete at 1270 ◦C. However, the sintering shrink￾age (6–8%) was still too large (should be <∼3% to avoid matrix cracks around the volume stable fibers). Fig. 4 tracks the effect of mullite seed content on the shrinkage, density and open porosity, for RBM with 7.5 wt% MREO. A small amount of mullite seeds can prompt the formation of mullite. However, when seed con￾centration is high, it serves as refractory inclusion and retards the RBM sintering. Fig. 5 shows the dilatometric measurement curve for RBM with 7.5 wt% MREO and 5.0 wt% seeds. Length expansion occurs due to oxidation of Si metal powder and reaches maximum (<3%) at 1170 ◦C, close to the eutectic point of MREO–Al2O3–SiO2. Formation of MREO–Al2O3–SiO2 liq￾uid phase prompts the sintering shrinkage and a steep drop in length was detected at 1170–1200 ◦C. Si metal is completely oxi￾dized into silica when soaked at 1200 ◦C for 10 h. With further Fig. 3. Effect of mullite seed addition on the mullitization temperature. Fig. 4. Influence of mullite seed addition on the sintering shrinkage, density and open porosity for RBM sintered at 1300 ◦C for 2 h. increase of temperature, RBM sintering shrinkage takes place and final length change is <3%. When [seeds] = 5 wt%, the RBM shrinkage is <3% and open porosity, <20%. This composition is optimum and is used in the EPID processing. Alumina particles in ethanol are positively charged whereas Si particles are negatively charged. These particles tend to hetero-coagulate. A dispersant was added to induce a com￾mon surface charge sign and stabilize the mixed suspension. PEI, protonated with acetic acid, adsorbs on both surfaces ren￾Fig. 5. Dilatometric measurement of RBM with 7.5 wt% MREO and 5.0 wt% seeds

3044 Y Bao, PS. Nicholson /Journal of the European Ceramic Sociery 28(2008)3041-3048 E 的04 0.2 si Mullite Seeds PEI concentration (wt%) Fig. 6. Effect of PEl on mobility dering them the same sign. Fig. 6 illustrates the influence of PEI on the RBM-precursor-particle-mobility in ethanol. When [PEI]>0.15 wt%, all particles are positively charged. No change of particle mobility is observed for [PEI] >0.4 wt%. Fig. 7 shows the electrokinetic sonic amplitude(ESA)values for a 2.5 vol% mixed suspension of alumina, Si, MREO and mullite seeds(3Al2O3-2Si +5 wt% mullite seeds +7.5 wt %o MREO)ver us PEI concentration. The eSa value increases with Pel and plateaus at 0.3 wt%o Thus 0.5 wt% Pei was used toensure well-dispersed suspensions for the EPID process. As PEI also absorbs on the AlPO4-coated fiber surface, it renders the fibers repulsive to the particles. The fiber adsorption of PEl causes particles passing through them to be repelled as they pass, i. e, particles are"streamed Fig 8 shows the morphology of an AlPO4-coated Nextel 720 fiber-reinforced composite prepared by EPID(current density 0.07 mA/cm") pressurelessly sintered at 1300C for 2h. Sin- Fig 8. Morphology of fiber/RBM composite prepared by EPID tered fiber-reinforced composites have -25% open porosity and 25 vol% fiber Macro cracks do not occur and particles are well for 2h. It is a mixture of alumina, cristobalite and mullite, sug infiltrated into the fiber preform. After polishing, the weakly- gesting non-uniform distribution of the mREo in the matrix. bonded AlPO4 coating polished away and grooves appeared MREO promotes matrix mullite formation via low temperature around the fibers. An uncoated Nextel 720/RBM composite eutectics formed with alumina and silica. As the MREO pow was employed for matrix phase analysis due to the very similar der has high density, it also may sediment during fiber preform XRD patterns for AlPO4 and Sio2 Fig 9 is an XRD pattern for uncoated Nextel 720 Fiber/RBM composite sintered at 1300oC Mullite D SiO2 05 Fig 9. X-ray diffraction pattern of fiber/RBM composite prepared by EPID and Fig. 7. Effect of PEl on ESA of 2.5 vol% suspension sintered at1300°C

3044 Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 Fig. 6. Effect of PEI on mobility. dering them the same sign. Fig. 6 illustrates the influence of PEI on the RBM-precursor-particle-mobility in ethanol. When [PEI] >0.15 wt%, all particles are positively charged. No change of particle mobility is observed for [PEI] >0.4 wt%. Fig. 7 shows the electrokinetic sonic amplitude (ESA) values for a 2.5 vol% mixed suspension of alumina, Si, MREO and mullite seeds (3Al2O3–2Si +5 wt% mullite seeds +7.5 wt% MREO) ver￾sus PEI concentration. The ESA value increases with PEI and plateaus at [PEI] >0.3 wt%. Thus 0.5 wt% PEI was used to ensure well-dispersed suspensions for the EPID process. As PEI also absorbs on the AlPO4-coated fiber surface, it renders the fibers repulsive to the particles. The fiber adsorption of PEI causes particles passing through them to be repelled as they pass, i.e., particles are “streamed”. Fig. 8 shows the morphology of an AlPO4-coated Nextel 720 fiber-reinforced composite prepared by EPID (current density 0.07 mA/cm2) pressurelessly sintered at 1300 ◦C for 2 h. Sin￾tered fiber-reinforced composites have ∼25% open porosity and ∼25 vol% fiber. Macro cracks do not occur and particles are well infiltrated into the fiber preform. After polishing, the weakly￾bonded AlPO4 coating polished away and grooves appeared around the fibers. An uncoated Nextel 720/RBM composite was employed for matrix phase analysis due to the very similar XRD patterns for AlPO4 and SiO2. Fig. 9 is an XRD pattern for uncoated Nextel 720 Fiber/RBM composite sintered at 1300 ◦C Fig. 7. Effect of PEI on ESA of 2.5 vol% suspension. Fig. 8. Morphology of fiber/RBM composite prepared by EPID. for 2 h. It is a mixture of alumina, cristobalite and mullite, sug￾gesting non-uniform distribution of the MREO in the matrix. MREO promotes matrix mullite formation via low temperature eutectics formed with alumina and silica.11 As the MREO pow￾der has high density, it also may sediment during fiber preform Fig. 9. X-ray diffraction pattern of fiber/RBM composite prepared by EPID and sintered at 1300 ◦C

Y Bao, P.S. Nicholson /Journal of the European Ceramic Society 28(2008)3041-3048 (a) (a) (b) Fig. 10. Effect of AlPO4 coating on fracture surface of fiber/RBM composites Fig. 11. Porous AlPO4 coating after sintering at 1300C for 2h tested at R.T.(a) without AlPO4 coating and (b) with AlPO4 coating infiltration. In fact, a yellow layer was noted on the base of a green body after infiltration. A non-uniform distribution, or, less-MREO-present-than-designed will locally retard formation of mullite at1300°C Fig. 10 compares the effect of AlPO4-coating on fiber pullout, i.e., without AlPO4 on the fibers, strong bonds form between the fibers and the rBm matrix so that a planar fracture sur- face results. However, when AlPO4 is coated on the fibers significant fiber pullout is observed. The high covalent bond ing level in AlPO4 retards its sinterability so AlPO4 ceramics remain very porous even when sintered at 1550C. Thus the inherently-porous AlPO4 coating on Nextel 720 fiber surface, serves as a porous weak layer for crack deflection and fiber pu out. Fig. I l show porous AlPO4 coating attached to the matrix after the fiber pullout, indicating that the AlPO4/fiber bondin is also weak and cracks defect therefrom. Fig. 12 illustrates 200m the fiber bridging effect. Though the crack opening distance is M150 um, the fibers still bridge across it. Fiber pullout reaches 150um(10 times the fiber diameter). The pullout length Fig 12 Fiber bridging and pullout across a matrix crack in AlPO4-coated is shorter on fracture at 1100C (Fig. 13). Fig. 14 shows the fiber/RBM composite tested at room temperature

Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 3045 Fig. 10. Effect of AlPO4 coating on fracture surface of fiber/RBM composites tested at R.T. (a) without AlPO4 coating and (b) with AlPO4 coating. infiltration. In fact, a yellow layer was noted on the base of a green body after infiltration. A non-uniform distribution, or, less-MREO-present-than-designed will locally retard formation of mullite at 1300 ◦C. Fig. 10 compares the effect of AlPO4-coating on fiber pullout, i.e., without AlPO4 on the fibers, strong bonds form between the fibers and the RBM matrix so that a planar fracture sur￾face results. However, when AlPO4 is coated on the fibers, significant fiber pullout is observed. The high covalent bond￾ing level in AlPO4 retards its sinterability so AlPO4 ceramics remain very porous even when sintered at 1550 ◦C.5 Thus the inherently-porous AlPO4 coating on Nextel 720 fiber surface, serves as a porous weak layer for crack deflection and fiber pull￾out. Fig. 11 show porous AlPO4 coating attached to the matrix after the fiber pullout, indicating that the AlPO4/fiber bonding is also weak and cracks deflect therefrom. Fig. 12 illustrates the fiber bridging effect. Though the crack opening distance is ∼150m, the fibers still bridge across it. Fiber pullout reaches ∼150m (>10 times the fiber diameter). The pullout length is shorter on fracture at 1100 ◦C (Fig. 13). Fig. 14 shows the Fig. 11. Porous AlPO4 coating after sintering at 1300 ◦C for 2 h. Fig. 12. Fiber bridging and pullout across a matrix crack in AlPO4-coated fiber/RBM composite tested at room temperature.

3046 Y Bao, PS. Nicholson /Journal of the European Ceramic Sociery 28(2008)3041-3048 g. 13. Fracture surface of AlPO4- -fiber/RBM composite with AlPO4 weak layer tested at 1100C Fig. 15. A morphology of an AlPO4-coated-fiber/RBM composite tested at 200°C exposure at 1100C in air, the composite embrittles due localized matrix densification and increased bonding of fibers 175MPa therewith. Although the RBM matrix is still with -20%0 poros ity, the composite thermal stability can be significantly increased z150 with AlPO4 weak layer coating. Fig. 16 shows the load/cross- head-displacement curve for an AlPO4-coated Nextel 720/RBM 1100°c composite after heat-treatment at 1300C for 100h. The com- posite still exhibits damage tolerance with a bend strength of 160 MPa. The AlPO4 coating is still porous(Fig. 17). Almost no AlPO4 grain growth occurs during heat-treatment at 1300C for 100h. But the fiber pullout length is shorter(Fig. 18). Short fiber pullout length should be mostly due to the severe fiber strength Cross-head displacement (mm) degradation on thermal aging at 1300C. The effect of thermal aging on interfacial bonding needs to be further evaluated. Fig. 14. 4-point bending strength of AlPOa-coated- fiber/RBM composites ested at R1.and1l00°C riven and Lee proposed phase transformation causes weakening in mullite/cordierite laminates with B-cristobalite load/cross-head-displacement curves for the composite tested (SiO2) as the interface.Thea←阝,“ cristobalite,APO4 room temperature and 1100C. The composite exhibits dam age tolerance at both temperatures. The ultimate bend strength of the composite is 175+20 MPa and 170+25 MPa at room 160MPa temperature and 1100C, respectively. At 1200C, the MREO- induced MREO-Al2O3-Sio2 glassy phase occurs in the matrix, thus the composite fails in shear at 1200C(Fig. 15). The ulti mate bend strength of RBM and AlPO4-coated fiber/RBM is listed in table 2 Antti et al. reported thermal degradation of commercial fiber reinforced porous aluminosilicate matrix composites. After The bend strengths of RBM and AlPOA-coated Nextel 720 fiber/RBM Sample Bend strength(MPa) 1100°C 1200°C Cross-head Displacement(mm) RBM 105±15 AlPOA-coated fiber/RBM 175±20 Shear failure Fig. 16. Load/cross-head displacement curve for AlPOA-coated composite tested at room temperature after heat-treatment at 1300C for 100h

3046 Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 Fig. 13. Fracture surface of AlPO4-coated-fiber/RBM composite with AlPO4 weak layer tested at 1100 ◦C. Fig. 14. 4-point bending strength of AlPO4-coated-fiber/RBM composites tested at R.T. and 1100 ◦C. load/cross-head-displacement curves for the composite tested at room temperature and 1100 ◦C. The composite exhibits dam￾age tolerance at both temperatures. The ultimate bend strength of the composite is 175 ± 20 MPa and 170 ± 25 MPa at room temperature and 1100 ◦C, respectively. At 1200 ◦C, the MREO￾induced MREO–Al2O3–SiO2 glassy phase occurs in the matrix, thus the composite fails in shear at 1200 ◦C (Fig. 15). The ulti￾mate bend strength of RBM and AlPO4-coated fiber/RBM is listed in Table 2. Antti et al.14 reported thermal degradation of commercial fiber reinforced porous aluminosilicate matrix composites. After Table 2 The bend strengths of RBM and AlPO4-coated Nextel 720 fiber/RBM composites Sample Bend strength (MPa) 25 ◦C 1100 ◦C 1200 ◦C RBM 105 ± 15 70 ± 10 17 ± 5 AlPO4-coated fiber/RBM 175 ± 20 170 ± 25 Shear failure Fig. 15. A morphology of an AlPO4-coated-fiber/RBM composite tested at 1200 ◦C. exposure at 1100 ◦C in air, the composite embrittles due to localized matrix densification and increased bonding of fibers therewith. Although the RBM matrix is still with ∼20% poros￾ity, the composite thermal stability can be significantly increased with AlPO4 weak layer coating. Fig. 16 shows the load/cross￾head-displacement curve for an AlPO4-coated Nextel 720/RBM composite after heat-treatment at 1300 ◦C for 100 h. The com￾posite still exhibits damage tolerance with a bend strength of 160 MPa. The AlPO4 coating is still porous (Fig. 17). Almost no AlPO4 grain growth occurs during heat-treatment at 1300 ◦C for 100 h. But the fiber pullout length is shorter (Fig. 18). Short fiber pullout length should be mostly due to the severe fiber strength degradation on thermal aging at 1300 ◦C. The effect of thermal aging on interfacial bonding needs to be further evaluated. Kriven and Lee 15 proposed phase transformation causes weakening in mullite/cordierite laminates with -cristobalite (SiO2) as the interface. The ↔, “cristobalite”, AlPO4 Fig. 16. Load/cross-head displacement curve for AlPO4-coated-fiber/RBM composite tested at room temperature after heat-treatment at 1300 ◦C for 100 h.

Y Bao, PS. Nicholson /Journal of the European Ceramic Society 28(2008)3041-3048 3047 modulus of AlPO4 is 57GPa, So, assuming the porosity is approximately that of sintered AlPO4, i.e., 30%0, the coating elastic modulus is v29 GPa. This low value explains why si nificant fiber pullout occurs from the matrix. It can be concluded that the high covalent bonding(poor sinterability)in AlPO4 is key to its performance on the fiber pullout. The latter results from both the low elastic modulus of porous AlPO4 coating and the weak bonding between the fibers and the alpO4 coating Reaction-bonded mullite was formed at <1300oC via incor- poration of 7.5 wt% mixed-rare-earth-oxides into an Al2O3-Si mixture. Inclusion of 5 wt%o mullite seeds decreased the sinter ng shrinkage to <2%o and give a density v2.6 g/cm with open porosity <20%0. A PEl dispersant produced a stable ethanol sus- Fig 17. Porous AlPO4 coating around fiber after heat-treatment at 1300C for pension of the reaction-bonded mullite precursor. Crack free, 100h unidirectional fiber-reinforced RBM composites with 25 vol% fibers and 25%o porosity were achieved by EPID followed by transformation occurs at 220C with 4.6% volume change. pressureless sintering at 1300 C. AlPO4 was coated onto mul- Microcracks due to the phase transformation are minimized lite/alumina fibers and the fiber/RBM composites exhibited the coating is porous. The AlPO4 coating should be pure superior damage tolerance with significant fiber pullout between B-cristobalite"phase at 1100C, however, it will still serve room temperature and 1100C. The ultimate bend strength of the a weak layer between the fibers and matrix. Thus, phase- composites was 170 MPa At 1200C, the composite failed in transformation weakening in porous AlPO4 coating can be shear due to glassy phase formation in the matrix. The composite ing attached to the matrix suggest weak bonding between the still displayed damage tolerance with fiber pullout after thermal aging at 1300C for 100h, and testing at room temperature fibers and the AlPO4 coating even after heating to high temper- It is concluded AlPO4 is an effective oxidation-resistant, weak atures. Therefore, an approaching crack will deflect along the fiber/AlPO4 surface. layer between the fibers and matrix of oxide -fiber/oxide-matrix CMCS The interfacial sliding resistance depends on the AlPO coating elastic modulus. A low elastic modulus significantly promoting fiber Acknowledgement decreases the fiber/coating sliding resistance, promoting fiber pullout. 6. I Elastic modulus is a function of porosity, Yahua bao would like to thank prof, d.s. wilkinson and prof Ep=E(-19+09f) J. Barbier for fruitful discussions where E and Ep are the elastic modulus of the fully dense and References porous materials, respectively and fp the porosity. The elastic 1. Committee on Advanced Fibers for High-Temperature Ceramic Compos- ites, National Research Council, Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. NMAB-494, National Academies on,DC,1998 2. Morgan, P. E. D. and Marshall, D. B, Ceramic composites of mon zite and alumina. Journal of the American Ceramic Society, 1995, 78(6 3. Marshall, D B, Davis, J.B., Morgan, P. E D, Waldrop, J.R. and Porter, J. R ies of La-monazite as an interphase in oxide composites. Zeitschrift Fur Metallkunde,1999,9012),1048-1052 4. Kerans, R.J., Hay, R.S., Parthasarathy, T. A and Cinibulk, M. K. Interface Ceramic Society.2002,85(11).2599-2632. 5. Bao, Y and Nicholson, P S, AlPOA coating on alumina/mullite fibers as a Ceramic Society, 2006, 89(2), 465-470 6. Wu, S.X. and Claussen, N, Fabrication and properties of low-shr reaction-bonded mullite. Journal of the American Ceramic Sociery 4(10),2460-2463 7. Wu, S and Claussen, N, Reaction bonding and mechanical properties of mullite/silicon carbide composites Joumal of the American Ceramic Soci Fig. 18. Fiber pullout after heat-treatment at 1300" for 100h, fractured at R.T. ery,1994,7711).2898-2904

Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 3047 Fig. 17. Porous AlPO4 coating around fiber after heat-treatment at 1300 ◦C for 100 h. transformation occurs at 220 ◦C with 4.6% volume change. Microcracks due to the phase transformation are minimized as the coating is porous. The AlPO4 coating should be pure “-cristobalite” phase at 1100 ◦C, however, it will still serve as a weak layer between the fibers and matrix. Thus, phase￾transformation weakening in porous AlPO4 coating can be neglected. The smooth pullout fiber surface and AlPO4 coat￾ing attached to the matrix suggest weak bonding between the fibers and the AlPO4 coating even after heating to high temper￾atures. Therefore, an approaching crack will deflect along the fiber/AlPO4 surface. The interfacial sliding resistance depends on the AlPO4- coating elastic modulus. A low elastic modulus significantly decreases the fiber/coating sliding resistance, promoting fiber pullout.16,17 Elastic modulus is a function of porosity18; Ep = E(1 − 1.9fp + 0.9f 2 p ) where E and Ep are the elastic modulus of the fully dense and porous materials, respectively and fp the porosity. The elastic Fig. 18. Fiber pullout after heat-treatment at 1300 ◦C for 100 h, fractured at R.T. modulus of AlPO4 is 57 GPa,19 so, assuming the porosity is approximately that of sintered AlPO4, i.e., ∼30%,5 the coating elastic modulus is ∼29 GPa. This low value explains why sig￾nificant fiber pullout occurs from the matrix. It can be concluded that the high covalent bonding (poor sinterability) in AlPO4 is key to its performance on the fiber pullout. The latter results from both the low elastic modulus of porous AlPO4 coating and the weak bonding between the fibers and the AlPO4 coating. 4. Summary Reaction-bonded mullite was formed at <1300 ◦C via incor￾poration of 7.5 wt% mixed-rare-earth-oxides into an Al2O3–Si mixture. Inclusion of 5 wt% mullite seeds decreased the sinter￾ing shrinkage to <2% and give a density ∼2.6 g/cm3 with open porosity <20%. A PEI dispersant produced a stable ethanol sus￾pension of the reaction-bonded mullite precursor. Crack free, unidirectional fiber-reinforced RBM composites with 25 vol% fibers and 25% porosity were achieved by EPID followed by pressureless sintering at 1300 ◦C. AlPO4 was coated onto mul￾lite/alumina fibers and the fiber/RBM composites exhibited superior damage tolerance with significant fiber pullout between room temperature and 1100 ◦C. The ultimate bend strength of the composites was ∼170 MPa. At 1200 ◦C, the composite failed in shear due to glassy phase formation in the matrix. The composite still displayed damage tolerance with fiber pullout after thermal aging at 1300 ◦C for 100 h, and testing at room temperature. It is concluded AlPO4 is an effective oxidation-resistant, weak layer between the fibers and matrix of oxide-fiber/oxide-matrix CMC’s. Acknowledgement Yahua Bao would like to thank Prof. D.S. Wilkinson and Prof. J. Barbier for fruitful discussions. References 1. Committee on Advanced Fibers for High-Temperature Ceramic Compos￾ites, National Research Council, Ceramic Fibers and Coatings: Advanced Materials for the Twenty-First Century. NMAB-494, National Academies Press, Washington, DC, 1998. 2. Morgan, P. E. D. and Marshall, D. B., Ceramic composites of mon￾azite and alumina. Journal of the American Ceramic Society, 1995, 78(6), 1553–1563. 3. Marshall, D. B., Davis, J. B., Morgan, P. E. D., Waldrop, J. R. and Porter, J. R., Properties of La-monazite as an interphase in oxide composites. Zeitschrift Fur Metallkunde, 1999, 90(12), 1048–1052. 4. Kerans, R. J., Hay, R. S., Parthasarathy, T. A. and Cinibulk, M. K., Interface design for oxidation-resistant ceramic composites. Journal of the American Ceramic Society, 2002, 85(11), 2599–2632. 5. Bao, Y. and Nicholson, P. S., AlPO4 coating on alumina/mullite fibers as a weak interface in fiber-reinforced oxide composites. Journal of the American Ceramic Society, 2006, 89(2), 465–470. 6. Wu, S. X. and Claussen, N., Fabrication and properties of low-shrinkage reaction-bonded mullite. Journal of the American Ceramic Society, 1991, 74(10), 2460–2463. 7. Wu, S. and Claussen, N., Reaction bonding and mechanical properties of mullite/silicon carbide composites. Journal of the American Ceramic Soci￾ety, 1994, 77(11), 2898–2904.

Y Bao, PS. Nicholson /Journal of the European Ceramic Sociery 28(2008)3041-3048 8. Holz, D, Pagel, S, Bowen, C, Wu, S and Claussen, N, Fabrication of 14. Antti, M.-L.. Lara-Curzio, E and Warren, R, Thermal degradation of ar European Ceramic Sociery, 1996. 16(2), 255-260. Ceramic Society,2004,24(3),565-578. 9. Petry, M. D and Mah, T I, Effect of thermal exposures on the strengths of 15. Kriven, w.M. and Lee, s.J., Toughening of mullite/cordierite laminated xtel M 550 and 720 fila composites by transformation weakening of beta-cristobalite interphases 1999,82(10,2801-2807 Journal of the American Ceramic Sociery, 2005, 88(6). 1521-1528 10. Mechnich. P, Schneider. H. Schmucker, M. and Saruhan. B, Accelerated 16. Hsueh, C-H. Becher, P. F and Angelini, P. Effects of interfacial films or bonding of mullite. Journal of the American Ceramic Society, 1998, thermal stresses in whisker-reinforced ceramics. Journal of the American 81(7),1931-1937 Ceramic Society, 1988, 71(11). 929-933 11. Kim, H. Sand Nicholson, P. S, Use of mixed-rare-earth oxide in the prepa- 17. Kerans, R. J, Viability of oxide fiber coatings in ceramic composites for ation of reaction-bonded mullite at <1300C Joumal of the Americ accommodation of misfit stresses. Journal of the American Ceramic Sociery, Ceramic Society,2002,85(7),1730-1734. 1996,79(6,1664-1668 12. Bao, Y and Nicholson, P. S, Electrophoretic Infiltration deposition for 18. Kingery, W.D., Bowen, H.K. and Uhlmann, D R, Introduction to Ceramics. fiber-reinforced ceramic composites under constant current. Journal of the John Wiley Sons, New York, 1976 American Ceramic Sociery, 2007, 90(4), 1063-107 19. Hanada, T, Bessyo, Y. and Soga, N. Elastic-constants of amorphous 13. Y Bao, Strong, Damage-Tolerant Oxide-Fiber/Oxide-Matrix Composite thin-films in the systems SiO2-AlO and AlPO4-Al2O3. Journal of Non- Ph. D. Thesis, McMaster University, Hamilton, Ontario, Canada 2006 Crystalline Solids, 1989, 113(2-3), 213-220

3048 Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 8. Holz, D., Pagel, S., Bowen, C., Wu, S. and Claussen, N., Fabrication of low-to-zero shrinkage reaction-bonded mullite composites. Journal of the European Ceramic Society, 1996, 16(2), 255–260. 9. Petry, M. D. and Mah, T. I., Effect of thermal exposures on the strengths of NextelTM 550 and 720 filaments. Journal of the American Ceramic Society, 1999, 82(10), 2801–2807. 10. Mechnich, P., Schneider, H., Schmucker, M. and Saruhan, B., Accelerated reaction bonding of mullite. Journal of the American Ceramic Society, 1998, 81(7), 1931–1937. 11. Kim, H. S. and Nicholson, P. S., Use of mixed-rare-earth oxide in the prepa￾ration of reaction-bonded mullite at ≤1300 ◦C. Journal of the American Ceramic Society, 2002, 85(7), 1730–1734. 12. Bao, Y. and Nicholson, P. S., Electrophoretic Infiltration deposition for fiber-reinforced ceramic composites under constant current. Journal of the American Ceramic Society, 2007, 90(4), 1063–1070. 13. Y. Bao, Strong, Damage-Tolerant Oxide-Fiber/Oxide-Matrix Composites. Ph.D. Thesis, McMaster University, Hamilton, Ontario, Canada 2006. 14. Antti, M.-L., Lara-Curzio, E. and Warren, R., Thermal degradation of an oxide fibre (Nextel 720)/aluminosilicate composite. Journal of the European Ceramic Society, 2004, 24(3), 565–578. 15. Kriven, W. M. and Lee, S. J., Toughening of mullite/cordierite laminated composites by transformation weakening of beta-cristobalite interphases. Journal of the American Ceramic Society, 2005, 88(6), 1521–1528. 16. Hsueh, C.-H., Becher, P. F. and Angelini, P., Effects of interfacial films on thermal stresses in whisker-reinforced ceramics. Journal of the American Ceramic Society, 1988, 71(11), 929–933. 17. Kerans, R. J., Viability of oxide fiber coatings in ceramic composites for accommodation of misfit stresses. Journal of the American Ceramic Society, 1996, 79(6), 1664–1668. 18. Kingery, W. D., Bowen, H. K. and Uhlmann, D. R.,Introduction to Ceramics. John Wiley & Sons, New York, 1976. 19. Hanada, T., Bessyo, Y. and Soga, N., Elastic-constants of amorphous thin-films in the systems SiO2-Al2O3 and AlPO4-Al2O3. Journal of Non￾Crystalline Solids, 1989, 113(2–3), 213–220