3312 of the American Ceramic Society--Zok Fugitive C 610 No coating Al2Og-LaPOA 2 Fig. 7. (a) Microstructure and (b) fiber pullout in a p aPO4/ Janet Davis, Rockwell Scientific. Reprinted from J. 02 0.6 1.0 19, J.B. Davis, D.B. Marshall, and P.E. D Morgan, "Oxide Compos o of Al2O3 and LaPO4, pp. 2421-2426. 1999, with permission fr Displacement(mm) Elsevier) some sense)all three principal toughening schemes: porous matri- Notched tensile behavior of a porous mullite-alumina matrix d monazite coati ngs. The initial pro- ed by Nextel 720 fibers, showing the effects cessing steps are identical to those used to produce porous-matrix ( Courtesy J H. Weaver) FCCs with an interfacial gap(described above). Following oxi dation of the carbon, a monazite precursor is repeatedly impre (3) Hybrid Concepts nated and pyrolyzed, thereby filling the interface gaps for One approach that obviates the problems associated with weav occupied by carbon as well as between the matrix particles ing of coated tows and simplifies processing involves a hybrid- (Fig. 8). If successful, this approach could provide an effective ization of the coated-fiber and the porous-matrix schemes Here, route to fabricating coated fiber composites with virtually any woven uncoated preforms are infiltrated with a monazite pre- architecture and configuration. A critical assessment of perform- cursor solution containing fine alumina particles. Following pyrolysis, a layer of monazite is formed on the fibers as well as composite exhibits extremely large pullout lengths(> 100R, (1) Microstructural Concll e between the alumina particles. The resulting matrix consists of a porous two-phase mixture of LaPOa and Al2O3( Fig. 7(a). The II. Matrix-Enabled damage tolerance When introduced in the mid-1990s, the porous-matrix co Ivity in typ cept was motivated principally by two factors: (i) the lack of a performance characteristics appear to be a consequence of ()the suitable suite of coatings for oxide fibers, and ( ii) the expectation monazite coating enabling crack deflection, and (i) the low of reduced manufacturing costs resulting from the absence of matrix stiffness reducing the radial constraints on the fiber oatings. Although the concept has proven to be an effective hence reducing the sliding resistance. Contrary to other reports lternative to fiber coatings for enabling damage tolerance, it of fiber strength degradation following exposure to acidic pre- as several inherent limitations: (i)CFCCs with two-dimension cursors, the reported combination appears to be innocuous. It al (2D) fiber architectures exhibit low thermal conductivit has been suggested that the alumina buffers the solution, strength, and fracture resistance in the through-thickness direc- making the fibers less susceptible to reaction with the precur ion;(ii)regardless of fiber architecture, these composites are non-hermetic;(iii) they are expected to have lower compressive strengths than the dense matrix counterparts, because of the re- In addition to the combined porous-matrix/coated fiber scheme. a second hybrid duced constraint on fiber microbuckling: and(iv) they are more Mullite Monazite 720 10 um fiber μm Fig 8. Scanning electron micrographs of an oxide continuous-fiber ceramic composite(using backscatter electron imaging). Monazite is present within the interface gap produced by arbon as well as between matrix particles. Monazite precursor provided by Janet Davis, Rockwell(3) Hybrid Concepts One approach that obviates the problems associated with weav￾ing of coated tows and simplifies processing involves a hybrid￾ization of the coated-fiber and the porous-matrix schemes. Here, woven uncoated preforms are infiltrated with a monazite pre￾cursor solution containing fine alumina particles.35 Following pyrolysis, a layer of monazite is formed on the fibers as well as between the alumina particles. The resulting matrix consists of a porous two-phase mixture of LaPO4 and Al2O3 (Fig. 7(a)). The composite exhibits extremely large pullout lengths (  100R, with R being the fiber radius; Fig. 7(b)) and virtually no detect￾able notch sensitivity in typical specimen configurations. These performance characteristics appear to be a consequence of (i) the monazite coating enabling crack deflection, and (ii) the low matrix stiffness reducing the radial constraints on the fiber, hence reducing the sliding resistance. (Contrary to other reports of fiber strength degradation following exposure to acidic pre￾cursors, the reported combination appears to be innocuous. It has been suggested that the alumina buffers the solution, making the fibers less susceptible to reaction with the precur￾sors and the decomposition products formed during precursor pyrolysis).29,35 In addition to the combined porous-matrix/coated fiber scheme, a second hybrid approach has emerged, using (in some sense) all three principal toughening schemes: porous matri￾ces, fugitive coatings, and monazite coatings.34 The initial pro￾cessing steps are identical to those used to produce porous-matrix CFCCs with an interfacial gap (described above). Following oxi￾dation of the carbon, a monazite precursor is repeatedly impreg￾nated and pyrolyzed, thereby filling the interface gaps formerly occupied by carbon as well as between the matrix particles (Fig. 8). If successful, this approach could provide an effective route to fabricating coated fiber composites with virtually any architecture and configuration. A critical assessment of perform￾ance and durability of this class of composite has yet to be made. III. Matrix-Enabled Damage Tolerance (1) Microstructural Concept When introduced in the mid-1990s,5–8 the porous-matrix con￾cept was motivated principally by two factors: (i) the lack of a suitable suite of coatings for oxide fibers, and (ii) the expectation of reduced manufacturing costs resulting from the absence of coatings. Although the concept has proven to be an effective alternative to fiber coatings for enabling damage tolerance, it has several inherent limitations: (i) CFCCs with two-dimension￾al (2D) fiber architectures exhibit low thermal conductivity, strength, and fracture resistance in the through-thickness direc￾tion; (ii) regardless of fiber architecture, these composites are non-hermetic; (iii) they are expected to have lower compressive strengths than the dense matrix counterparts, because of the re￾duced constraint on fiber microbuckling; and (iv) they are more susceptible to wear.36 Fig. 6. Notched tensile behavior of a porous mullite–alumina matrix reinforced by Nextelt 720 fibers, showing the effects of a fugitive carbon coating. (Courtesy J. H. Weaver). Fig. 7. (a) Microstructure and (b) fiber pullout in a porous LaPO4/ Al2O3 matrix reinforced with Nextelt 610 alumina fibers. (Courtesy of Janet Davis, Rockwell Scientific. Reprinted from J. Eur. Ceram. Soc., 19, J.B. Davis, D.B. Marshall, and P.E.D. Morgan, ‘‘Oxide Composites of Al2O3 and LaPO4,’’ pp. 2421–2426, 1999, with permission from Elsevier). Fig. 8. Scanning electron micrographs of an oxide continuous-fiber ceramic composite (using backscatter electron imaging). Monazite is present within the interface gap produced by removal of the fugitive carbon as well as between matrix particles. (Monazite precursor provided by Janet Davis, Rockwell Scientific). 3312 Journal of the American Ceramic Society—Zok Vol. 89, No. 11