Effect of Monazite Coating on Tensile Properties of Nextel 720 Fibers, Tows and Minicomposites
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U11.43.1
Effect of Monazite Coating on Tensile Properties of Nextel 720 Fibers, Tows and Minicomposites Devdas M. Pai1, Sergey Yarmolenko1, Jagannathan Sankar1, Balasubramanian Kailasshankar1, Christopher Murphy1, Edwardo Freeman1, and Larry P. Zawada2 1
Center for Advanced Materials and Smart Structures, North Carolina A&T State University, Greensboro, NC 27411, U.S.A. 2 Metals, Ceramics and NDE Division (AFRL/MLLN), Air Force Research Laboratory, WrightPatterson Air Force Base, OH 45433, U.S.A. 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 1200ºC (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. INTRODUCTION The flaw-tolerance of ceramic matrix composites (CMCs) can be improved by applying coatings to the fibers that promote crack deflection and fiber pullout at the fiber-matrix interface [1]. Carbon and boron nitride (BN) are the traditionally used fiber coating materials; however, they suffer the problems of oxidation in the presence of water and also at intermediate temperatures [2]. This has prompted research into oxide coatings (as a substitute for carbon or BN) that are stable with the matrix and fiber at high temperatures. A leading oxide coating under investigation is monazite (LaPO4), which is stable at high temperatures with many common oxides including alumina [3]. There is also some evidence to suggest that monazite-alumina interphase boundaries deflect cracks [4]. Among oxide fibers, Nextel™ 720, an alumina fiber with about 15% mullite [5] offers greatly improved creep resistance owing to its content of mullite grains that are five times larger than the alumina grains in Nextel™ 610. Effective use of these fibers in CMCs requires a weak interface between fiber and matrix. Various aspects of monazite coated Nextel™ 720 fibers have been studied – these include the effect of different liquid precursors on coating characteristics [6] and on fiber strength [7], and the effect of coating deposition temperature on fiber strength [8]. The fibers are used in tow (multiple fiber groupi
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