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. 2008.05.032Available 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