Oxide composites of Al203 and LaPO4 2425 fit from, or require a fully dense matrix (e.g, Acknowledgements extremely corrosive environments, need for her metic seal). Previous studies of LaPO/AlO3 Funding for this work was provided by the U. S interfaces have demonstrated that debonding Air Force Office of Scientific Research under con occurs in a variety of cracking configurations tract F49620-96-C-0026 monitored by Dr. A fully del i8 However, damage- Pechenik(work on fully dense matrix composites) tolerant behavior requires sliding and pullout of and the U.S. Office of Naval Research under con- fibers in addition to debonding Fiber sliding after tract N00014-95-C-0057 monitored by Dr. S debonding would be expected to be more difficult Fishman(work on porous matrix composite) in fully dense systems than in composites with porous matrices, because the higher stiffness of the matrix would be less accommodating for the misfit References caused by the sliding motion of any irregularities at the interface 22 I. Prewo, K. M. and Brennan, J. J, High strength silicon Fiber sliding was demonstrated previously in carbon fiber reinforced glass matrix composites. J. Mater pushout experiments involving isolated sapphire 2. Brennan.Jj and Prewo,K.M. Silicon carbide fiber fibers with LaPOa coatings in a fully dense Al2O reinforced glass-ceramic matrix composites exhil matrix.The interface in these experiments con high strength and toughness. Mater. Sci, 1982, tained irregularities in the form of grain boundary 2371-2383 3. Sun, E. Y, Nutt, S.R. and Brennan, J.J., Fiber coatings grooves and cusps with height 50 nm. Sliding for SiC-fiber-reinforced BMAs glass-ceramic composites. occurred without permanent deformation, imply J.Am. Ceran.Soc,1997,80(1),264. ing that the misfitting asperities were accom- 4. Turner, K.R., Speck, J.S. and Evans, A. G, Mechanisms of deformation and failure in carbon-matrix composites modated by elastic strains during sliding. The subject to tensile and shear loading. J. Am. Ceram. Soc. LaPOa-matrix composite examined in the present 1995,78(7),1841-1848 study was prepared using the same starting mate 5. Buckley, J. D, Carbon-carbon, an overview. Am. Ceram rials and nominally identical processing conditions Soc.Bull,1988,67(2),364368 6. Harrison. M.G. Millard. M. L. and szweda. A as the composite used in the previous pushout reinforced ceramic matrix composite member and experiments. 5 The results in Figs. 7 and 8 demon d method for making. U.S. Patent No. 5306554: UK Patent No. 2 strate the feasibility of achieving fiber pullout in 230259(1994) 7. Tu, w.C., Lange, F. F and Evans, A G, Concept for a fully dense systems amage-tolerant ceramic composite withstrong'inter The development of oxide composites with fully faces. J. Am. Ceram Soc., 1996. 79(2). 417-424 dense matrices is presently limited by our ability to 8. Levi, C. G, Yang, J.Y., Dalgleish, B J, Zok, F. w. and densify the matrix under conditions that do not Evans, A.G., Processing and performance of an all-oxid ceramic composite. J. Am. Ceram. Soc., 1998, 81(8) degrade the fibers. Processing temperatures for 2077-2086 composites containing polycrystalline Al2O3 and 9. Lange. F. F, Tu. w-C. and Evans. A. G. Processing of mullite fibers are limited to 1200-1300C(lower amage-tolerant. oxidation-resistant ceramic-matrix com- if pressure is used to aid densification). Such sites by a precursor infiltration and pyrolysis method Mater.Sci.Eng.,1995,A195,145-1 composites require development of either higher 10. Keith, w.P. and Kedward, K. T, Shear damage mechan temperature fibers(e.g. eutectic or single crystal isms in a woven. nicalon reinforced ceramic matrix com- posite.J. Am. Ceram Soc., 1997, 80(2), 357-364 fibers)or methods, currently being examined, for I1. Cooper, R. F. and Hall, P. C,Reactions be promoting densification of the matrix at lower thetic mica and simple oxide compounds with cation temperatures to oxidation-resistant ceramic composite. J. Am. Ce 12. Morgan, P. E. D and Marshall, D. B, Functional 5 Conclusions 13. Cinibulk netoplumbite compounds as a fiber pating for oxide- oxide composites. Ceram. Eng. and S An oxide composite consisting of woven Al2O3 fibers and a porous matrix of Al2O3 and LaPO 14. Petuskey, WT(private communication) was found to exhibit much greater nonlinear 15. Morgan, P. E D. and Marshall, D. B, Ceramic compo- sites of monazite and alumina. J. Am. Ceram. Soc. 1995. response and notch insensitivity than other porous 78(6),1553-1563 matrix composites. The enhanced properties were 16. Morgan, P.E. D. Marshall,DB and Housley,R. M attributed to weak bonding between the fibers and High temperature stability of monazite-alumina the LapOa phase, which allowed extensive fiber ites. J. Mat Sci. Eng王D, and Housley, R. M pullout. Debonding in multilayered composites of zirconia and The feasibility of achieving fiber pullout in fully LaPO4.J.Am. Ceran.Soc.,1997,80(7),1677-1683 R. M dense Al]O3-LaPO4 composites was demonstrated aI. D P. E Cheung, J. T, High temperature stability of the AlO3 using a hot pressed composite with sapphire fibers LaPOa system J. Am. Ceram Soc., 1998, 81(4), 951-956®t from, or require a fully dense matrix (e.g., extremely corrosive environments, need for her￾metic seal). Previous studies of LaPO4/Al2O3 interfaces have demonstrated that debonding occurs in a variety of cracking con®gurations in fully dense systems.15±18 However, damage￾tolerant behavior requires sliding and pullout of ®bers in addition to debonding. Fiber sliding after debonding would be expected to be more dicult in fully dense systems than in composites with porous matrices, because the higher sti€ness of the matrix would be less accommodating for the mis®t caused by the sliding motion of any irregularities at the interface.22 Fiber sliding was demonstrated previously in pushout experiments involving isolated sapphire ®bers with LaPO4 coatings in a fully dense Al2O3 matrix.15 The interface in these experiments con￾tained irregularities in the form of grain boundary grooves and cusps with height 50 nm. Sliding occurred without permanent deformation, imply￾ing that the mis®tting asperities were accom￾modated by elastic strains during sliding. The LaPO4-matrix composite examined in the present study was prepared using the same starting mate￾rials and nominally identical processing conditions as the composite used in the previous pushout experiments.15 The results in Figs. 7 and 8 demon￾strate the feasibility of achieving ®ber pullout in fully dense systems. The development of oxide composites with fully dense matrices is presently limited by our ability to densify the matrix under conditions that do not degrade the ®bers. Processing temperatures for composites containing polycrystalline Al2O3 and mullite ®bers are limited to 1200±1300C (lower if pressure is used to aid densi®cation). Such composites require development of either higher temperature ®bers (e.g. eutectic or single crystal ®bers) or methods, currently being examined, for promoting densi®cation of the matrix at lower temperatures. 5 Conclusions An oxide composite consisting of woven Al2O3 ®bers and a porous matrix of Al2O3 and LaPO4 was found to exhibit much greater nonlinear response and notch insensitivity than other porous matrix composites. The enhanced properties were attributed to weak bonding between the ®bers and the LaPO4 phase, which allowed extensive ®ber pullout. The feasibility of achieving ®ber pullout in fully dense Al2O3±LaPO4 composites was demonstrated using a hot pressed composite with sapphire ®bers. Acknowledgements Funding for this work was provided by the U.S. Air Force Oce of Scienti®c Research under con￾tract F49620-96-C-0026 monitored by Dr. A. Pechenik (work on fully dense matrix composites) and the U.S. Oce of Naval Research under con￾tract N00014-95-C-0057 monitored by Dr. S. Fishman (work on porous matrix composite). References 1. Prewo, K. M. and Brennan, J. J., High strength silicon carbon ®ber reinforced glass matrix composites. J. Mater. Sci., 1980, 15(2), 463±468. 2. Brennan, J. J. and Prewo, K. M., Silicon carbide ®ber reinforced glass±ceramic matrix composites exhibiting high strength and toughness. J. Mater. Sci., 1982, 17(8), 2371±2383. 3. Sun, E. Y., Nutt, S. R. and Brennan, J. J., Fiber coatings for SiC-®ber-reinforced BMAS glass±ceramic composites. J. Am. Ceram. Soc., 1997, 80(1), 264. 4. Turner, K. R., Speck, J. S. and Evans, A. G., Mechanisms of deformation and failure in carbon±matrix composites subject to tensile and shear loading. J. Am. Ceram. Soc., 1995, 78(7), 1841±1848. 5. Buckley, J. D., Carbon±carbon, an overview. Am. Ceram. Soc. Bull., 1988, 67(2), 364±368. 6. Harrison, M. G., Millard, M. L. and Szweda, A., Fiber reinforced ceramic matrix composite member and method for making. U.S. Patent No. 5 306 554; UK Patent No. 2 230 259 (1994). 7. Tu, W. C., Lange, F. F. and Evans, A. G., Concept for a damage-tolerant ceramic composite with `strong' inter￾faces. J. Am. Ceram. Soc., 1996, 79(2), 417±424. 8. Levi, C. G., Yang, J. Y., Dalgleish, B. J., Zok, F. W. and Evans, A. G., Processing and performance of an all-oxide ceramic composite. J. Am. Ceram. Soc., 1998, 81(8), 2077±2086. 9. Lange, F. F., Tu, W.-C. and Evans, A. G., Processing of damage-tolerant, oxidation-resistant ceramic-matrix com￾posites by a precursor in®ltration and pyrolysis method. Mater. Sci. Eng., 1995, A195, 145±150. 10. Keith, W. P. and Kedward, K. T., Shear damage mechan￾isms in a woven, nicalon reinforced ceramic matrix com￾posite. J. Am. Ceram. Soc., 1997, 80(2), 357±364. 11. Cooper, R. F. and Hall, P. C., Reactions between syn￾thetic mica and simple oxide compounds with application to oxidation-resistant ceramic composite. J. Am. Ceram. Soc., 1993, 76(5), 1265±1273. 12. Morgan, P. E. D. and Marshall, D. B., Functional inter￾faces in oxide±oxide composites. J. Mat. Sci. Eng., 1993, A162(1±2), 15±25. 13. Cinibulk, M. K., Magnetoplumbite compounds as a ®ber coating for oxide±oxide composites. Ceram. Eng. and Sci. Proc., 1994, 15(5), 721±728. 14. Petuskey, W.T. (private communication). 15. Morgan, P. E. D. and Marshall, D. B., Ceramic compo￾sites of monazite and alumina. J. Am. Ceram. Soc, 1995, 78(6), 1553±1563. 16. Morgan, P. E. D., Marshall, D. B. and Housley, R. M., High temperature stability of monazite-alumina compo￾sites. J. Mat. Sci. Eng., 1995, A195, 215±222. 17. Marshall, D. B., Morgan, P. E. D. and Housley, R. M., Debonding in multilayered composites of zirconia and LaPO4. J. Am. Ceram. Soc., 1997, 80(7), 1677±1683. 18. Marshall, D. B., Morgan, P. E. D., Housley, R. M. and Cheung, J. T., High temperature stability of the Al2O3- LaPO4 system. J. Am. Ceram Soc., 1998, 81(4), 951±956. Oxide composites of Al2O3 and LaPO4 2425