AlPO4-coated mullitealumina fiber reinforced

Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 3041–3048 AlPO4-coated mullite/alumina reaction-bonded mullite c ich Cerami ring, M May 008 Abstract A precursor 3, Si ethanol susp acid mullite/alum e fib with ≤25% ing th significant fi ce. T 1200 ◦C, the n. Ho thermal agin © 2008 Else Keywords: A comp 1. Introduction Fiber-reinforced ceramic matrix composites (CMC’s) are candidates ance. Curre on non-oxi Oxidation- use in oxid the fracture the bondin from the m been repor layer cand reported ap layer on th pullout on mullite/alu covalently- sintered at ∗ Correspon North, Provo, E-mail ad 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. 0955-2219/$ doi:10.1016/j for structural application due to their damage toler- ntly, most dense fiber-reinforced CMC’s are based de systems which oxidize in air at high temperatures. resistant, fiber-reinforced CMC’s are required for izing atmospheres at high temperatures. To optimize work necessary to break fiber-reinforced CMC’s, g between fiber and matrix must allow fiber pullout atrix via a weak layer therebetween.1 LaPO4 has ted as one of the most successful dense weak idates.2–4 However, Bao and Nicholson5 recently plication of another phosphate, AlPO4, as the weak e fiber surface. They demonstrated significant fiber the fracture surface of hot-pressed, AlPO4-coated mina (NextelTM 720) fiber-reinforced Al2O3. Highly bonded AlPO4 displays poor sintering behavior even 1550 ◦C.5 Thus it should be a stable porous weak ding author. Current address: SII MegaDiamond, 275 West 2230 UT 84604, United States. dress: baoyahua@gmail.com (Y. Bao). 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, – see front matter © 2008 Elsevier Ltd. All rights reserved. .jeurceramsoc.2008.05.032 Yahua Bao ∗, Patrick S. N c Engineering Research Group, Department of Materials Science and Enginee Received 23 February 2008; received in revised form 27 Available online 22 July 2 for reaction-bonded mullite (RBM) is formulated by premixing Al2O ension thereof is stabilized with polyethyleneimine protonated by acetic ina fiber-preforms by electrophoretic infiltration deposition to produc porosity were achieved after pressureless sintering at 1300 ◦C. Pre-coat ber pullout on composite fracture and confers superior damage toleran composite fails in shear due to MREO-based, glassy phase formatio g at 1300 ◦C for 100 h. vier Ltd. All rights reserved. lPO4; Weak layer; Nextel 720 fiber; Reaction-bonded mullite; Ceramic matrix fiber reinforced omposites olson cMaster University, Hamilton, Ontario, L8S 4L7 Canada 2008; accepted 30 May 2008 , mullite seeds and mixed-rare-earth-oxides (MREO). An . The solid in the suspension is infiltrated into unidirectional er-reinforced, RBM green bodies. Crack-free composites e fibers with AlPO4 as a weak intervening layer facilitates he bend strength is ∼170 MPa at 25 ◦C ≤ T ≤ 1100 ◦C. At wever, the AlPO4 coating acts as a weak layer even after osites 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 In this bonded mu electrophor pressureles 2. Experim Reactio DAR alum micron Si genfield, N area >20 m lists comp thanide ox as sinterin (Siral 28 M at 1300 ◦C as mullite- that of mul Anhydr Polyscienc acid was suspension trophoretic zeta poten Holtsville N measured o Sciences, ally presse heated to to SiO2. F to study m was calcul lets. Table 1 Composition Oxide CeO2 La2O3 Nd2O3 Pr6O11 Other 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 m and accomplish electrophoretic-infiltration-deposition ).12,13 The inter-electrode distance was 2 cm and EPID nducted at a constant current of 0.07 mA/cm2. After depo- the c ied i ng. U par gre vert t l sta C. T ede r-po -dri 55, W a fix mple ed b mple ratur ults . 1 s and C is prom id p ver M at 13 TA c form mullite <1300 ◦C.11 paper, AlPO4-coated, Nextel 720 fiber/reaction- llite composites are fabricated by constant-current etic-infiltration-deposition (EPID) and subsequently s-sintered. ental n-bonded mullite was prepared with submicron TM- ina powder (Taimei Chemicals, Tokyo, Japan) and metal powder (Atlantic Equipment Engineers, Ner- J) which was pre-ball-milled in ethanol to a surface 2/g (to promote oxidation during sintering). Table 1 osition of mixed-rare-earth-oxides (MREO, Lan- ide, Molycorp, Fairfield, NJ). These were added g and mullite-formation aids. A mullite precursor , SASOL GmbH, Hamburg, German), pre-sintered for 2 h to form pure mullite, was ground and added promotion seeds. The molar ratio of Al:Si was set to lite. ous polyethyleneimine dispersant (PEI, M.W. 10,000, es, Warrington, PA), protonated with glacial acetic added to stabilize the RBM-precursor, ethanol . The optimal addition was determined via the elec- mobility value for the RBM precursors with a tial analyzer (ZetaPALS, Brookhaven Instruments, Y). The electrokinetic sonic amplitude (ESA) was n the mixed suspension (ESA-8000, Matec Applied Hopkinton, MA). RBM pellets were also uniaxi- d then cold isostatically pressed at 140 MPa and 1175–1200 ◦C for 10 h in air to oxidize the Si inally they were sintered at 1250–1350 ◦C for 2 h ullite phase formation and shrinkage. Shrinkage ated from the change of the diameter of the pel- of the mixed-rare-earth-oxides (MREO) provided by Molycorp Concentration (wt%) 49 33 13 4 1 prefor (EPID was co sition, then dr cracki for com The to con therma 1300 ◦ Archim Fou a screw & 100 alumin The sa observ Ten sa tempe 3. Res Fig Al2O3 1200 ◦ can be tic liqu matrix 7.5 wt% phase Fig. 1. D omposite was cold isostatically pressed at 140 MPa n an atmosphere-controlled closed container to avoid ncoated fibers were also infiltrated with RBM matrix ison. en composites were heated to 1175 ◦C for 10 h in air he Si to SiO2 then sintered at 1300 ◦C for 2 h. Their bility was determined by heat-treatment for 100 h at he fired density and open porosity were measured by s’ method in water. int bend testing was performed at 0.10 mm/min in ven, ultra-hard compression machine (Model 10053 ykeham Farrance Engineering Ltd., UK) using an ture with outer span, 20 mm, and inner span, 10 mm. thickness was 2.0–2.5 mm. Fracture surfaces were y SEM and the degree of fiber-pullout determined. s were tested at room temperature and five at elevated es. and discussion hows the DTA curve for 32 wt% MREO, 22 wt% 46 wt% SiO2 mixture. The endothermic peak around due to eutectic liquid-phase formation. Mullite phase oted by formation of the MREO–Al2O3–SiO2 eutec- hase. Fig. 2 tracks the phase evolution in the RBM sus sintering temperature for a mixture containing REO. Mullite appears at 1270 ◦C, and is the major 00 ◦C. Traces of alumina and silica remain. The sil- urve 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 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 mulliti mullite “se process wa age (6–8% cracks arou mullite see for RBM w can promp centration the RBM s curve for R expansion reaches ma of MREO– uid phase p length was dized into Fig. 3. Ef nfluence of mullite seed addition on the sintering shrinkage, density and rosity for RBM sintered at 1300 ◦C for 2 h. e of temperature, RBM sintering shrinkage takes place al length change is <3%. When [seeds] = 5 wt%, the RBM age is <3% and open porosity, <20%. This composition is m and is used in the EPID processing. mina particles in ethanol are positively charged whereas ticles are negatively charged. These particles tend to -coagulate. A dispersant was added to induce a com- zation temperature was further decreased by adding eds” (Fig. 3). With 0.5 wt% seeds, the mullitization s complete at 1270 ◦C. However, the sintering shrink- ) was still too large (should be <∼3% to avoid matrix nd the volume stable fibers). Fig. 4 tracks the effect of d content on the shrinkage, density and open porosity, ith 7.5 wt% MREO. A small amount of mullite seeds t the formation of mullite. However, when seed con- is high, it serves as refractory inclusion and retards intering. Fig. 5 shows the dilatometric measurement BM with 7.5 wt% MREO and 5.0 wt% seeds. Length occurs due to oxidation of Si metal powder and ximum (<3%) at 1170 ◦C, close to the eutectic point Al2O3–SiO2. Formation of MREO–Al2O3–SiO2 liq- rompts the sintering shrinkage and a steep drop in detected at 1170–1200 ◦C. Si metal is completely oxi- silica when soaked at 1200 ◦C for 10 h. With further Fig. 4. I open po increas and fin shrink optimu Alu Si par hetero fect of mullite seed addition on the mullitization temperature. mon surfac PEI, proton Fig. 5. Dilato seeds. e charge sign and stabilize the mixed suspension. ated with acetic acid, adsorbs on both surfaces ren- metric measurement of RBM with 7.5 wt% MREO and 5.0 wt% 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 2.5 vol% m seeds (3Al2 sus PEI co plateaus at well-disper absorbs on repulsive t particles pa particles ar Fig. 8 sh fiber-reinfo 0.07 mA/cm tered fiber- ∼25 vol% fi infiltrated i bonded Al around the was emplo XRD patte uncoated N F ig. 8. Morphology of fiber/RBM composite prepared by EPID. . It is a mixture of alumina, cristobalite and mullite, sug- non-uniform distribution of the MREO in the matrix. promotes matrix mullite formation via low temperature cs formed with alumina and silica.11 As the MREO pow- s high density, it also may sediment during fiber preform electrokinetic sonic amplitude (ESA) values for a ixed suspension of alumina, Si, MREO and mullite O3–2Si +5 wt% mullite seeds +7.5 wt% MREO) ver- ncentration. The ESA value increases with PEI and [PEI] >0.3 wt%. Thus 0.5 wt% PEI was used to ensure sed suspensions for the EPID process. As PEI also the AlPO4-coated fiber surface, it renders the fibers o the particles. The fiber adsorption of PEI causes ssing through them to be repelled as they pass, i.e., e “streamed”. ows the morphology of an AlPO4-coated Nextel 720 rced composite prepared by EPID (current density 2) pressurelessly sintered at 1300 ◦C for 2 h. Sin- reinforced composites have ∼25% open porosity and ber. Macro cracks do not occur and particles are well nto the fiber preform. After polishing, the weakly- PO4 coating polished away and grooves appeared fibers. An uncoated Nextel 720/RBM composite yed for matrix phase analysis due to the very similar rns for AlPO4 and SiO2. Fig. 9 is an XRD pattern for extel 720 Fiber/RBM composite sintered at 1300 ◦C F for 2 h gesting MREO eutecti der ha ig. 7. Effect of PEI on ESA of 2.5 vol% suspension. Fig. 9. X-ray sintered at 13 diffraction pattern of fiber/RBM composite prepared by EPID and 00 ◦C. Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 3045 Fig. 10. Effec tested at R.T. infiltration. a green bo less-MREO of mullite a Fig. 10 c i.e., withou the fibers a face result significant ing level in remain ver inherently- serves as a out. Fig. 11 after the fib is also wea the fiber br ∼150�m, ∼150�m is shorter o t of AlPO4 coating on fracture surface of fiber/RBM composites (a) without AlPO4 coating and (b) with AlPO4 coating. In fact, a yellow layer was noted on the base of dy after infiltration. A non-uniform distribution, or, -present-than-designed will locally retard formation t 1300 ◦C. ompares the effect of AlPO4-coating on fiber pullout, t AlPO4 on the fibers, strong bonds form between nd the RBM matrix so that a planar fracture sur- s. However, when AlPO4 is coated on the fibers, fiber pullout is observed. The high covalent bond- AlPO4 retards its sinterability so AlPO4 ceramics y porous even when sintered at 1550 ◦C.5 Thus the porous AlPO4 coating on Nextel 720 fiber surface, porous weak layer for crack deflection and fiber pull- show porous AlPO4 coating attached to the matrix er pullout, indicating that the AlPO4/fiber bonding k and cracks deflect therefrom. Fig. 12 illustrates idging effect. Though the crack opening distance is the fibers still bridge across it. Fiber pullout reaches (>10 times the fiber diameter). The pullout length n 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, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 Fig. 13. Frac weak layer te Fig. 14. 4-po tested at R.T. load/cross- at room tem age toleran of the com temperatur induced M thus the co mate bend listed in Ta Antti et fiber reinfo Table 2 The bend st composites Sample RBM AlPO4-coated A morphology of an AlPO4-coated-fiber/RBM composite tested at . re at 1100 ◦C in air, the composite embrittles due to ed m ture surface of AlPO4-coated-fiber/RBM composite with AlPO4 sted at 1100 ◦C. Fig. 15. 1200 ◦C exposu localiz int bending strength of AlPO4-coated-fiber/RBM composites and 1100 ◦C. head-displacement curves for the composite tested perature and 1100 ◦C. The composite exhibits dam- ce at both temperatures. The ultimate bend strength posite is 175 ± 20 MPa and 170 ± 25 MPa at room e and 1100 ◦C, respectively. At 1200 ◦C, the MREO- REO–Al2O3–SiO2 glassy phase occurs in the matrix, mposite fails in shear at 1200 ◦C (Fig. 15). The ulti- strength of RBM and AlPO4-coated fiber/RBM is ble 2. al.14 reported thermal degradation of commercial rced porous aluminosilicate matrix composites. After rengths of RBM and AlPO4-coated Nextel 720 fiber/RBM Bend strength (MPa) 25 ◦C 1100 ◦C 1200 ◦C 105 ± 15 70 ± 10 17 ± 5 fiber/RBM 175 ± 20 170 ± 25 Shear failure therewith. ity, the com with AlPO head-displa composite posite still 160 MPa. T AlPO4 grai 100 h. But t pullout len degradation aging on in Kriven weakening (SiO2) as Fig. 16. Loa composite tes atrix densification and increased bonding of fibers Although the RBM matrix is still with ∼20% poros- posite thermal stability can be significantly increased 4 weak layer coating. Fig. 16 shows the load/cross- cement curve for an AlPO4-coated Nextel 720/RBM after heat-treatment at 1300 ◦C for 100 h. The com- exhibits damage tolerance with a bend strength of he AlPO4 coating is still porous (Fig. 17). Almost no n growth occurs during heat-treatment at 1300 ◦C for he fiber pullout length is shorter (Fig. 18). Short fiber gth should be mostly due to the severe fiber strength on thermal aging at 1300 ◦C. The effect of thermal terfacial bonding needs to be further evaluated. and Lee 15 proposed phase transformation causes in mullite/cordierite laminates with �-cristobalite the interface. The �↔�, “cristobalite”, AlPO4 d/cross-head displacement curve for AlPO4-coated-fiber/RBM ted at room temperature after heat-treatment at 1300 ◦C for 100 h. 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 coat “�-cristoba as a weak transforma neglected. ing attache fibers and t atures. The fiber/AlPO The int coating ela decreases t pullout.16,1 Ep = E(1 where E an porous ma Fig. 18. Fiber modulus of AlPO4 is 57 GPa,19 so, assuming the porosity is approximately that of sintered AlPO4, i.e., ∼30%,5 the coating elastic mod nificant fibe that the hig key to its p from both t the weak b 4. Summa Reactio poration of mixture. In ing shrinka porosity <2 pension of unidirectio fibers and pressureles min r da emp sites ue to splay at 13 nclu etwe s. wled ua B ier f nce mitte Natio ing is porous. The AlPO4 coating should be pure lite” phase at 1100 ◦C, however, it will still serve layer between the fibers and matrix. Thus, phase- tion weakening in porous AlPO4 coating can be The smooth pullout fiber surface and AlPO4 coat- d to the matrix suggest weak bonding between the he AlPO4 coating even after heating to high temper- refore, an approaching crack will deflect along the 4 surface. erfacial sliding resistance depends on the AlPO4- stic modulus. A low elastic modulus significantly he fiber/coating sliding resistance, promoting fiber 7 Elastic modulus is a function of porosity18; − 1.9fp + 0.9f 2p ) d Ep are the elastic modulus of the fully dense and terials, respectively and fp the porosity. The elastic lite/alu superio room t compo shear d still di aging It is co layer b CMC’ Ackno Yah J. Barb Refere 1. Com ites, pullout after heat-treatment at 1300 ◦C for 100 h, fractured at R.T. Materials Press, Wa 2. Morgan, azite and 1553–156 3. Marshall, Properties Fur Meta 4. Kerans, R design for Ceramic S 5. Bao, Y. an weak inte Ceramic S 6. Wu, S. X reaction-b 74(10), 24 7. Wu, S. an mullite/si ety, 1994, ulus is ∼29 GPa. This low value explains why sig-