In Situ Densification Utilizing a Low-Viscosity Wetting Impregnant that Greating Reduces Processing Time to Produce Uniform Density Carbon-Carbon Composites

Abstract

Abstract : High-performance carbon-carbon (C-C) composites possess a unique set of properties that make them desirable materials for high-temperature structural uses such as in rocket propulsion components, hypersonic vehicles, and aircraft brakes. They are stronger and stiffer than steel and less dense than aluminum. In a non-oxidizing environment they maintain their mechanical properties to temperatures above 3000 deg C. In addition, their properties actually improve with heating as the non-ordered carbon is converted to the ordered graphite structure (the process of graphitization). Despite these properties, however, the use of carbon-carbon composites has been limited both because of their high cost and their oxidation at elevated temperatures. A recent development in composite processing at the Propulsion Directorate of the Air Force Research Laboratory (AFRL), Edwards, California-called in situ rapid densification greatly reduces their processing time and their cost. For 30 years, carbon-carbon composites have been manufactured principally by two processes that use different means to place the matrix among the fibers in the preform and thus densify the composite. One process involves infiltrating the composite preform with a hydrocarbon gas which pyrolyzes to produce carbon. The other process involves impregnating the fiber preform with a liquid hydrocarbon such as petroleum or coal tar pitch or a resin followed by carbonization, which removes all non-carbon atoms. Both processes are followed by graphitization in which the partially densified composite is heated to temperatures above 2400 deg C to open up closed porosity in addition to enhancing mechanical properties.