Interface Properties of Ceramic Composites at High Temperature
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Abstract For high-temperature performance of ceramic composites, interfaces are designed to provide toughness through debonding while resisting thermal oxidation in aggressive environments. Thus, the evaluation of interfacial properties at high temperatures is of critical importance. In recent work at USC, interfacial properties were measured at high temperatures by single fiber pushout tests. Six advanced ceramic composites were selected to perform pushout testing at 20-1000°C. Variation in interface designs and effects of thermal history were evaluated with respect to interface failure strength. At higher temperatures, the average interfacial bond strength was often higher. SEM observations were correlated with pushout measurements to evaluate the interfacial behavior.
1. Introduction It is widely recognized that the mechanical behavior of ceramic matrix composites is dependent not only on the intrinsic properties of the constituents, but also largely on the fiber/matrix interface."12 Thus, the properties of the interfacial bond should be tailored to maximize composite performance. In order to design the interface effectively, engineers must find some way to measure and evaluate the interfacial properties correctly. Single fiber push-out tests have some advantage in obtaining interfacial properties. This testing technology affords a simple screening test and makes it possible to probe specific fibers in the composites. In the test, an indentor that is housed in a Scanning Electron Microscope is used to apply load on the fiber. Continuous measurement of the applied load as a function of indenter tip displacement can be performed, with the monitoring of dynamic, high-magnification images. By measuring the applied force F, the fiber length H (the thickness of the specimen), and the fiber diameter D, the applied interfacial shear 4 stress t may be obtained as F 7rDH where F is the maximum applied force capable of debonding and pushing a fiber out of the opposite side of the specimen, and T gives the interfacial shear strength ts. When F is the applied force which causes the friction sliding of a fiber in the matrix after the interfacial debonding, T becomes the friction shear stress tf. Much work 5 8 has been done to quantify interfacial properties in ceramic matrix composite using single fiber pushout test in recent years. However, most push-out tests were performed at room temperature. For composites designed for use at higher temperature or in thermally oxidative environments, it is important to evaluate the interfacial properties by push-out testing at elevated temperatures. When exposed to oxygen at high temperature, some fibers oxidize and react with the matrix, leading to
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Mat. Res. Soc. Symp. Proc. Vol. 586 © 2000 Materials Research Society
weak composites. Under vacuum or inert atmospheres, interface oxidation cannot occur. Unfortunately, there have been few attempts to measure interface properties at high temperature. Marscher and co-workers 9 studied interfacial strength as a function of temperatur
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