Densification processing of carbon-carbon composite, like the design of a woven carbon preform, offers many approaches and alternatives to reach the final composite product. Selection of the process is dependent on final application, preform architecture, and cost considerations.
Densification processing is typically carried out using either a gaseous (chemical vapor deposition) or liquid (resin or pitches) precursor material. Each of the processes entails numerous variations including treatment of the impregnants, temperatures, gas ratios, pressures, and process sequences. The commonality is that the objective of the process is to fill the voids and interfaces of the woven preforms with matrix material as rapidly and cost effectively as possible while meeting the requirements of the application.
The most widely used impregnants for densification are gases (methane) and resins or pitches. The resulting properties of the carbons these precursors form vary significantly, depending on the process parameters and the basic chemical nature of the impregnant.
In almost all instances, the objective of densification is to fill a void or volume formed by the woven structure. Void volumes typically range from 35 to 60 percent. The actual mass that is added to the composite will vary, depending on the selection of the impregnant. For example, pitch precursor materials such as petroleum pitch or coal tar pitch form high density matrices in excess of 2.1 g/cm3 when heat treated to more than 2000C. Alternately, thermosetting type resins such as phenolic or furfural-based resins result in lower specific gravities owing to their inability to form intermediate graphitic crystals. Rapid and cost-effective densification is a result of the volumetric yield the impregnant provides when fully processed. An untreated pitch precursor nominally provides 50 percent carbon yield when pyrolyzed using atmospheric pressure and can provide as high as 90 percent yield when pyrolyzed at pressures exceeding 100 atmospheres. However, the volumetric yield is low when considering the change in density from an impregnant precursor (1.2g/cm3) to the carbon formed (2.1g/cm3) subsequent to pyrolysis and heat treatment. Therefore, repeated impregnation and pyrolysis are required to obtain a low volumetric porosity in the woven structure.