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).2898-2904Y. 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.