ovember 2006 Oxide Fiber Composites 3311 μm um Fig 3. Fracture surfaces of an alumina/alumina continuous-fiber ceramic composite after 5 h of exposure at 1200.C: (a)uncoated fibers, (b)monazite- coated fibers. ( Courtesy Kristin Keller, AFRL. Reprinted with permission) Although the low hardness of monazite(5GPa2) facilitates plastic option. It can be deposited readily onto tows or woven fabric ccommodation of the misfit, the coating is less effective than C or by chemical vapor deposition or through pyrolysis of organic BN in mitigating these stresses In the latter, the low radial stiffness precursors and is readily oxidized at moderately high tempera- of the coatings allows for elastic accommodation of the misfit with tures. 16, 17 Although straightforward in principle, the approach nly moderate radial pressure and hence low sliding stress has two potential drawbacks: () matrix sintering treatments Three outstanding issues remain. (i Presently, there is no es- must be performed in an inert(non-oxidizing)environment, and tablished method for coating woven fiber cloths or preforms (i once the coating is oxidized, the fibers are unprotected from (distinct from tows). Such capability would circumvent the prob- the surrounding matrix and may be susceptible to bonding lems of weaving coated tows. (ii) The sliding stress of monazite ontact points coated fibers in dense ceramic matrices is considerably higher Preliminary feasibility studies have yielded encouraging re- than that in C- and BN-coated CFCCs, by as much as an order sults. When carbon-coated Nextel"720 fibers were embedded in of magnitude If excessively high, this may compromise compos- a dense calcium aluminosilicate matrix and the carbon subse ite toughness. Relative to SiC-SiC composites, more attention must be directed to thermal expansion mismatch and surfac quently oxidized, significant enhancements in fiber pullout were roughness effects in the oxide systems (ini) Although the issue of ture exposure was also improved. A more recent investigation fiber strength retention has been addressed, an assessment of the has also shown the benefits of combining fugitive coatings with efficacy of monazite coating on Nextel" 720 fibers(the highest orous matrices. For this purpose, composites were fabricated by infiltration of a mullite-20% alumina slurry into a carbon- demonstrated. It will likely require the use of a mullite-based coated Nextel" 720 preform, repeated impregnation and pyr- matrix, to minimize residual stress and allow fiber sliding subse- olysis of an alumina precursor, followed by oxidation of the uent to debonding, while retaining chemical compatibility with carbon. Preliminary results are presented in Fig. 6. With the the fibers. The large difference in thermal expansion coefficients fugitive coating, the composite exhibits significantly greater pull- of alumina (8x 10 K )and 720 fibers(6x 10 K)pr out as well as higher notched strength and fracture energy. The ludes the use of alumina-rich compositions as matrix choices mprovements are attributable to the combined effects of matrix porosity and the interfacial gap formed following carbon re- (2) Fugitive Coatings moval. The long-term stability and role of gap thickness in such Application of fugitive coatings to oxide CFCCs has received ystems is the focus of ongoing investigation. surprisingly little attention. Carbon appears to be the be Al2O3 matrix LaPO4 coating Lapo G300 Al2O3/ZrO2 8 Mullite Sapphire YAG/Al2O3 Sapphire Fig 4. (a) Microstructure and (b) fiber pullout in a dense LaPOa matrix 1200-1000-800-600-400-2000 reinforced with large-diameter sapphire fibers. Courtesy of Janet Radial misfit stress(MPa) Davis, Rockwell Scientific. Reprinted from J. Eur. Ceram. Soc., 19 J B. Davis, D B. Marshall, and P.E. D. Morgan,""Oxide Composites of AlO3 and LaPO4. pp. 2421-2426, 1999, with permission from Elsevier)Although the low hardness of monazite (5 GPa20) facilitates plastic accommodation of the misfit, the coating is less effective than C or BN in mitigating these stresses. In the latter, the low radial stiffness of the coatings allows for elastic accommodation of the misfit with only moderate radial pressure and hence low sliding stress. Three outstanding issues remain. (i) Presently, there is no es￾tablished method for coating woven fiber cloths or preforms (distinct from tows). Such capability would circumvent the prob￾lems of weaving coated tows. (ii) The sliding stress of monazite￾coated fibers in dense ceramic matrices is considerably higher than that in C- and BN-coated CFCCs, by as much as an order of magnitude. If excessively high, this may compromise compos￾ite toughness. Relative to SiC–SiC composites, more attention must be directed to thermal expansion mismatch and surface roughness effects in the oxide systems. (iii) Although the issue of fiber strength retention has been addressed, an assessment of the efficacy of monazite coating on Nextelt 720 fibers (the highest temperature commercially available oxide fiber) has yet to be demonstrated. It will likely require the use of a mullite-based matrix, to minimize residual stress and allow fiber sliding subse￾quent to debonding, while retaining chemical compatibility with the fibers. The large difference in thermal expansion coefficients of alumina (B8 106 K1 ) and 720 fibers (6 106 K1 ) pre￾cludes the use of alumina-rich compositions as matrix choices. (2) Fugitive Coatings Application of fugitive coatings to oxide CFCCs has received surprisingly little attention. Carbon appears to be the best option. It can be deposited readily onto tows or woven fabric by chemical vapor deposition or through pyrolysis of organic precursors and is readily oxidized at moderately high tempera￾tures.16,17 Although straightforward in principle, the approach has two potential drawbacks: (i) matrix sintering treatments must be performed in an inert (non-oxidizing) environment, and (ii) once the coating is oxidized, the fibers are unprotected from the surrounding matrix and may be susceptible to bonding at contact points. Preliminary feasibility studies have yielded encouraging re￾sults. When carbon-coated Nextelt 720 fibers were embedded in a dense calcium aluminosilicate matrix and the carbon subse￾quently oxidized, significant enhancements in fiber pullout were obtained.17 The retention in properties following high-tempera￾ture exposure was also improved. A more recent investigation has also shown the benefits of combining fugitive coatings with porous matrices.34 For this purpose, composites were fabricated by infiltration of a mullite–20% alumina slurry into a carbon￾coated Nextelt 720 preform, repeated impregnation and pyr￾olysis of an alumina precursor, followed by oxidation of the carbon.34 Preliminary results are presented in Fig. 6. With the fugitive coating, the composite exhibits significantly greater pull￾out as well as higher notched strength and fracture energy. The improvements are attributable to the combined effects of matrix porosity and the interfacial gap formed following carbon re￾moval. The long-term stability and role of gap thickness in such systems is the focus of ongoing investigation. Fig. 3. Fracture surfaces of an alumina/alumina continuous-fiber ceramic composite after 5 h of exposure at 12001C: (a) uncoated fibers, (b) monazite￾coated fibers.33 (Courtesy Kristin Keller, AFRL. Reprinted with permission). Fig. 4. (a) Microstructure and (b) fiber pullout in a dense LaPO4 matrix reinforced with large-diameter sapphire fibers.35 (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. 5. Effects of radial misfit stress (from both thermal expansion mis￾match and microstructural roughness) on the sliding stress of several monazite-coated fibers. (Adapted from Davis et al. 24). November 2006 Oxide Fiber Composites 3311