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A testing technique has been developed for measuring the tensile properties of ceramic matrix composite laminates at temperatures up to 1500°C in air. Using this technique, the tensile properties of unidirectional silicon carbide (SiC) fiber reinforced reaction bonded silicon nitride (SiC/RBSN) composite specimens were measured at 25, 1300, and 1500°C. The composite consists of nearly 30 vol % of aligned 142 μm, chemically vapor deposited SiC fibers (Textron SCS-6). The specimens used were straight section flat specimens with bonded end tabs. A specially designed gripping system was used to minimize bending moment and assured that failure always occurred in the gage section. The specimen extension was measured using a high-temperature capacitive extensometer.
The material displayed metal-like stress-strain behavior at 25, 1300, and 1500°C and failed in a noncatastrophic manner. Final fracture at 25 and 1300°C was always preceded by matrix cracking at approximately 0.10% strain. At 1500°C the material was more ductile than at 1300°C, and there was no definite indication of matrix cracking before final fracture. The mechanical properties of the composite decreased considerably with increasing test temperature. The modulus values at 1300 and 1500°C were 20 and 26% lower than that measured at 25°C. The ultimate strengths were 70% lower.
The failure mechanisms in the material were examined by use of the scanning electron microscope. The roles of matrix, fiber, and interfacial cracking were determined by examining the fracture surfaces. Strong fiber bonding was seen at 25 and 1500°C, with cracks running through the fibers, rather than circumventing them. At 1300°C the fiber/matrix interface was weaker, with cracks running both parallel and perpendicular to the fiber axis. Also at 1300°C, the matrix contained many secondary cracks.
Manager of research laboratory, Instron Corporation, Canton, MA
Material scientist, Propulsion directorate, U.S. Army Aviation and Technology Activity-AVSCOM, NASA-LEWIS Research Center, Cleveland, OH
Stock #: CTR10190J