Effect of Monazite Coating on Tensile Properties of Nextel 720 Fibers, Tows and Minicomposites

Abstract

The tensile strengths of Nextel™ 720 fibers as well as tows and minicomposites made of these fibers were measured to study the effects of monazite coating at room temperature (RT) and 1200°C (HT). Tests were conducted on as-received materials (called unsoaked) as well as materials that had been thermally soaked at HT for 100 hours. Weibull analysis of fiber strength data shows close agreement between the Weibull modulus obtained from the statistical fracture distribution and the modulus obtained from a study of the gage length dependence of strength. The monazite coating does not change the RT strength of unsoaked single fibers, but has a beneficial effect in terms of strength retention under HT testing. This beneficial effect of the coating disappears for soaked samples. The results for tows and minicomposites also indicate that for the unsoaked condition, tensile failure appears to be driven by fiber surface flaws and the presence of the monazite coatings clearly improves both RT and HT strength of fibers, tows and minicomposites. However, the beneficial effects of the coating disappear after long term exposure to HT.