Abstract

In-situ electrical resistance measurements of carbon-carbon laminates were obtained in an oxidizing environment at 900 C. Scanning electron and optical microscopy were utilized to interpret the physical damage incurred from oxidation and its impact on the mechanical and electrical properties of the carbon-carbon substrate. The interpretation of the relationships between the percent mass loss, shear modulus, and electrical resistance provided an excellent venue to design future tests. Rescor 780 alumina oxide castable ceramic test fixture used for the electrical resistance measurement was designed and fabricated in our lab.

Microscopy observations of specimens exposed for forty minutes revealed preferential fiber loss in both longitudinal and transverse directions. Rheometry tests revealed that the in-plane shear modulus degraded with increasing oxidation time and mass loss. On the other hand the electrical resistance increased with increasing oxidation time and mass loss. The electrical resistance change is controlled primarily by the bulk electrical resistivity, which is a matrix controlled characteristic. As oxidation time increased, the electrical properties of the specimen approached those of the matrix.

The carbon-carbon specimens used had four constituents; Silicon carbide (SiC) coating at the top and bottom surfaces of the substrate composed of T-300 carbon fibers, carbon matrix, and the oxidation inhibitor boron carbide (B4C) which chemically react with oxygen and become boric oxide (B2O3). Each of these four components contributes to the oxidation characterization of the specimen and can be represented by a simple, parallel electrical resistance model. Consequently, for a given value of either the shear modulus, electrical resistance, or mass loss the other two values can be easily obtained. The results show that the analytical simulation is on average within 4.7% of the averaged experimental value. Correlation between the shear modulus and the electrical resistance is illustrated in Figure 1.

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