Continuous pyrolytic graphite composite filaments

Abstract

Advances in aeronautics and astronautics have created a need for new and improved structural and thermally protective materials, which are stable or capable of accommodating ultra high temperatures, mechanically erosive forces, and chemically corrosive environments. New physical forms of materials are thus being developed at an urgent pace, and of these, filamentary reinforced composites are among the most promising. For thermally protective use, carbon and graphite fiber reinforced composites have considerable merit, but current forms of the materials have some deficiencies. Through in-house research by the Air Force Materials Laboratory, continuous pyrolytic graphite filaments have now been synthesized in diameters from 25 to 125 /z and individual lengths up to 2600 ft. They are composed of an inner core of tungsten carbide with an overcoating of a pyrolytic refractory-like highly turbostatic carbon. To obtain these filamentous materials, a novel and versatile laboratory apparatus was designed, built, and successfully operated. The newly developed filamentous materials have potential uses in many future Air Force applications, like ablative and reinforced plastic structures, filament-wound composites, thermal insulation inflatable re-entry structures, thermally protective fabrics, and possibly other important uses. I. Introduction E NVIRONMENTS of very high temperature, high shear force and chemical reactivity have been generated by modern rocketry. Ablative and structural composite materials containing fibrous reinforcements have shown great potential for withstanding such environments. However, currently available fibers may not be entirely adequate for future uses because of the ever increasing material property requirements (thermal stability, strength, capability, etc.), and the associated effects of hyperenvironmental constituents on material performance. Highly refractory materials in bulk form have been made available which meet some of the anticipated future needs. Refractory fiber reinforced composites are preferred to bulk materials, however, because of their inherent versatility and the ability to tailor properties to the specific application requirements. Fibrous refractories for these composites are not presently available in proper compositions, physical structure, or amounts, and thus considerable interest exists in preparing such materials. The objective of this report concerns the preparation of pyrolytic graphite in continuous filament form. The research program designed to fulfill the objective was detailed into the following phases: 1) develop a laboratory apparatus for obtaining continuous pyrolytic graphite filaments; 2) examine the utility of existing chemical vapor-plating processes and improve them in applicable areas; 3) investigate the suitability of various hydrocarbon compounds as the graphite precursor material; and 4) produce continuous lengths of pyrolytic graphite filaments using a tungsten wire substrate.