November 2002 Interface Design for Oxidation-Resistant Ceramic Co 2607 the change in chemistry, and probably elastic properties, may environmental resistance have been studied. Periodic matrix introduce some ambiguity in interpretation of results. cracks, nonlinear load displacement, and hysteresis during unload- Porous-matrix CMCs without fiber coatings can have attractive reload cycles have been observed, from which debond energies properties via distributed damage mechanisms, because cracks and the average friction(T)have been estimated. o However, full deflect around fibers without need for a coating (see Section confidence in validity of the results for property prediction in a full Tv(9). Matrix pore volume fractions at which significant tough- CMC has not been established. 100 ening is observed range from >30% to 15%.98,9%Hence, porous Oxide/oxide microcomposites have been fabricated and tested matrices complicate evaluation of fiber coatings, because the to evaluate the effectiveness of monazite (LapO,)and hibonite porous matrix and the coating can contribute to toughening. CaAl12Ojg)as interlayers in sapphire reinforced/Al,O,matrix matrix composites may be necessary for complete understanding as the control composites, the fractography and fracture strengths of damage mechanisms in coated-fiber composites with imper- were compared. For interlayer thicknesses of 0.3-0.5 um, bot fectly densified matrices-usually the case interlayers showed evidence of crack deflection; however debond lengths in hibonite-coated specimens were limited to just a smal (4) Micro- and Mini-CMCs fraction of the fiber diameter. Monazite-coated specimens showed Use of micro- or mini-CMCs for more-rapid evaluation has multiple matrix cracks and extensive debonding at the coating atrix interface. In both cases, the load-displacement curves were cylindrical matrix reinforced with one fiber, while a mini-CMC almost linear to failure, therefore, there was no unload-reload ses one or multiple fiber tows(200-3000 fibers/tow and up to hysteresis from which to measure interfacial friction. Failure four tows). The mechanical behavior of a mini-CMC is more strength was the only measurable mechanical parameter. The difficult to interpret, but it includes the statistical nature of fiber extent of nonlinearity in tension of specimens of any type with fracture and is more representative of a real composite. These high fiber modulus, straight fibers d low matrix volume fractio micro- and mini-CMCs are easier to fabricate than full cMcs. and must be small. The evaluation of the results was based on the relaxed sintering constraints on matrix densification can allow hypothesis that, even if the coating and matrix volume fraction is denser matrices to be more easily made. 4 03 Most such tests very low, there is severe degradation in apparent fiber strength if ave been limited to carbon and bn fiber/matrix interfaces and there is no mechanism to deflect cracks. The matrix and coating ostly CVI-SiC matrices. Effects of fiber surface treatments or crack at relatively low strain, and, unless the crack deflects, it ac coating procedures on interface properties and evaluation of as a large flaw in the fiber. In this experiment, composite strengths Sapph I mm Hibonite TM-DAR :bonded Su √ atrix dislodge AlumIt 2 2 CMC-Control 1. 18 GPa 0 1.18 G -1 Eiber-1450.C,2h 2 2.33 GPa m-lI. Fiber-Hibonite 3 Fiber CMCMonazite 2.25 GPa m=103 m=577 -10.500.51 1.5 1-0.500.511.5 Ln I Stress, GPa I Ln Stress, GPa Fig. 10. Single-filament sapphire fiber reinforced/Al,O, matrix microcomposites tested in tension (a) cracks deflect within the hibonite interface but by matrix regions that fell off during the test; (c)and(d) ites with coatings have almost the same mean strengths as the control composite tes with coatings, but the Weibull modulus is higher, about the same as the coated fibers. Results imply that the matrix is not sufficiently dense for evaluation of the coatings, because even the control samples have high microcomposite strength.the change in chemistry, and probably elastic properties, may introduce some ambiguity in interpretation of results. Porous-matrix CMCs without fiber coatings can have attractive properties via distributed damage mechanisms, because cracks deflect around fibers without need for a coating (see Section IV(9)). Matrix pore volume fractions at which significant tough￾ening is observed range from 30% to 15%.98,99 Hence, porous matrices complicate evaluation of fiber coatings, because the porous matrix and the coating can contribute to toughening. Therefore, better understanding of damage mechanisms in porous￾matrix composites may be necessary for complete understanding of damage mechanisms in coated-fiber composites with imper￾fectly densified matrices—usually the case. (4) Micro- and Mini-CMCs Use of micro- or mini-CMCs for more-rapid evaluation has received increasing attention.100,101 A micro-CMC is defined as a cylindrical matrix reinforced with one fiber, while a mini-CMC uses one or multiple fiber tows (200–3000 fibers/tow and up to four tows). The mechanical behavior of a mini-CMC is more difficult to interpret, but it includes the statistical nature of fiber fracture and is more representative of a real composite. These micro- and mini-CMCs are easier to fabricate than full CMCs, and relaxed sintering constraints on matrix densification can allow denser matrices to be more easily made.102,103 Most such tests have been limited to carbon and BN fiber/matrix interfaces and mostly CVI-SiC matrices. Effects of fiber surface treatments or coating procedures on interface properties100 and evaluation of environmental resistance have been studied.101 Periodic matrix cracks, nonlinear load displacement, and hysteresis during unload– reload cycles have been observed, from which debond energies and the average friction () have been estimated.100 However, full confidence in validity of the results for property prediction in a full CMC has not been established.100 Oxide/oxide microcomposites have been fabricated and tested to evaluate the effectiveness of monazite (LaPO4) and hibonite (CaAl12O19) as interlayers in sapphire reinforced/Al2O3 matrix composites.99 Using sapphire monofilaments in an Al2O3 matrix as the control composites, the fractography and fracture strengths were compared. For interlayer thicknesses of 0.3–0.5 m, both interlayers showed evidence of crack deflection; however debond lengths in hibonite-coated specimens were limited to just a small fraction of the fiber diameter. Monazite-coated specimens showed multiple matrix cracks and extensive debonding at the coating/ matrix interface. In both cases, the load–displacement curves were almost linear to failure; therefore, there was no unload–reload hysteresis from which to measure interfacial friction.99 Failure strength was the only measurable mechanical parameter. The extent of nonlinearity in tension of specimens of any type with high fiber modulus, straight fibers, and low matrix volume fraction must be small. The evaluation of the results was based on the hypothesis that, even if the coating and matrix volume fraction is very low, there is severe degradation in apparent fiber strength if there is no mechanism to deflect cracks. The matrix and coating crack at relatively low strain, and, unless the crack deflects, it acts as a large flaw in the fiber. In this experiment, composite strengths Fig. 10. Single-filament sapphire fiber reinforced/Al2O3 matrix microcomposites tested in tension: (a) cracks deflect within the hibonite interface but debond lengths are very short, much less than a fiber diameter, because of the roughness; (b) debonds present at the monazite/matrix interface are revealed by matrix regions that fell off during the test; (c) and (d) microcomposites with coatings have almost the same mean strengths as the control composites with no coatings, but the Weibull modulus is higher, about the same as the coated fibers. Results imply that the matrix is not sufficiently dense for evaluation of the coatings, because even the control samples have high microcomposite strength.99 November 2002 Interface Design for Oxidation-Resistant Ceramic Composites 2607