Development of High Strength, High Modulus Fibers

The objective of the program was to obtain low cost fibers with tensile strengths greater than 400,000 psi and tensile moduli of 60,000,000 psi. A low cost process for converting concentrated aluminum salt solutions into uniform diameter polycrystalline oxide fibers via thermal decomposition has been developed. The process was used to prepare fibers ranging in composition from 73 alumina, 27 silica to 100 alumina. Fibers in which 0-10 boric oxide was added to the basic alumina-silica composition were also prepared and studied. The properties and microstructure of these fibers were studied with respect to the fiber composition and various thermal decomposition methods used. Electron microscope and X-ray diffraction analyses indicated that grain size, pore size, and pore number could be related to fiber tensile strength and these factors were shown to vary with fiber composition. Modulus of elasticity was found to depend primarily on the amount and composition of the crystalline component of the fibers. Maximum fiber properties of 250,000 psi tensile strength and 28,000, 000 psi tensile modulus were obtained from 85 alumina, 10 SiO2, 5 B2O3 composition fibers in the mullite crystalline form. Gamma alumina fibers with 130,000 psi tensile strength and 14,000,000 psi tensile modulus were obtained. Conversion to alpha alumina resulted in low tensile strength fibers in which excessive grain growth and porosity were apparent.