Oxidation of carbon-carbon composite and coatings on CC composite

Carbon-carbon composite are being considered for aerospace applications because of their light weight and excellent mechanical properties. Depending upon the application, carbon-carbon composite could be used for periods ranging from a few hours to a few thousand hours at temperatures in the interval between 600C and 2200C. A major problem in using such materials in oxidizing environments is that carbon reacts with oxygen forming gaseous the carbon oxides. Two approaches are being examined to protect carbon-carbon composites in oxidizing environments, in particular, the use of inhibitors to slow the reaction rates, and the use of coatings where a barrier develops between the composite and gases which limits the reaction rate. It has been proposed that boron and phosphorus additives which form B2O3 and P2O5, can inhibit the oxidation of carbon. Furthermore, several protective coatings are being considered to prevent oxygen from reacting with carbon. These coatings rely on the development of xide films which provide protection by inhibiting oxygen diffusion. A typical coating under consideration is silicon carbide which upon oxidation forms a silica film that acts as a diffusion barrier. A problem arises in attempting to use coatings to protect carbon-carbon, since thermal expansion mismatch between the composite and the coating causes the coating to crack. Frequently, composite systems contain additives which upon oxidation form liquids that can seal such cracks.

Another problem encountered in using carbon-carbon composites arises from variability in fabricating these materials. For example, the SIC conversion layers often penetrate deeper through the matrix than through the fibers. The effect of such variations in coating microstructure on coating performance has not been documented.

This program has been concerned with the oxidation of carbon-carbon composites at temperatures between 300 and 1500C. Inhibition effects, the influence of water vapor, and the behavior of relevant coatings have also been investigated. The approach utilized has consisted of thorough documentation of the as-processed materials using optical metallography, scanning electron microscopy, and transmission electron microscopy. Oxidation of uninhibited and inhibited carbon-carbon was studied in dry oxygen over r range of temperature between 300 and 1300C. The effect of water vapor on the oxidation of such materials was then studied. Finally, oxidation of coated specimens was examined at temperatures between 800 and 1500C.

In the following sections of this report an assessment of the literature for carbon-carbon is presented. The experimental procedures are then described. In the results and discussion section the oxidation of carbon-carbon composites is documented and described, inhibition of the oxidation process and effects of water vapor are explained, and coating degradation on carbon-carbon composites is documented along with the identification of coating characteristics which result in premature failure.


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