The tensile behavior of Nextel 720 fibers at elevated temperature was compared with room temperature results for both bare and monazite-coated fibers. While coated and uncoated fibers have nearly identical tensile strengths and Weibull moduli at room temperature, differences in response were seen at elevated temperature. Coated fibers tested at 1200°C were found to have a 40% drop in strength. Uncoated fibers at high temperature exhibited 55% less strength than at room temperature. However, the tensile strength distribution for uncoated fibers tested at high temperature exhibited two distinct populations, indicating two different failure mechanisms. One population showed a 50% drop while the other showed a 64% drop. The coating was thus found to have a protective effect in terms of short-duration high-temperature exposure. Further, the effect of soaking on strength was investigated by thermally soaking coated and uncoated fibers in air at 1200°C for 100 hours prior to tensile testing at elevated temperature. In this case, the long duration of thermal exposure appeared to eliminate the beneficial effects of the coating. Soaked fibers, both coated and uncoated, were found to have nearly identical strengths at 1200°C—a reduction of about 60%.

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