Catalytic oxidation of C-C composites & kinetics and mechanism of catalysis

C-C composite discs in aircraft have been reported to reach temperatures around 1000C during normal landing operations and up to 1400C in extreme cases such as an aborted take-off. Exposure to such high temperatures leads to thermal oxidation which is reported to be the main cause of degradation of C-C composites. Whilst material loss occurs as a result of friction, according to Chang and Rusnak, at least 60% of the total weight loss is due to thermal oxidation. In another study by Chang on the correlation between wear and oxidation of C-C composites, good agreement has been reported between the activation energy for the oxidation of carbon and for material loss due to wear on non-frictional surfaces. Thus it was proposed that mechanism of the wear loss is due to oxidation. In addition to the oxidation of the carbon matrix and fibers, this process also attacks the fiber-matrix interface and weakens the overall integrity of the composite. As with graphite, C-C composites are thought to start oxidizing at a temperature of around 400C. Oxidation becomes progressively more severe with further temperature increase. As can be expected, studies on the kinetics of the oxidation of graphite, coal and coke have been conducted extensively in the past. The problem of C-C composite oxidation is also beginning to attact attention owing to the problems that are being addressed by the aerospace industry. The potential use of this type of material in alternative high-temperature applications is likely to yield even more investigations in the future. A comprehensive study of the kinetics of the oxidation of C-C composites is indeed a complex task due to the various forms in which such a material can be manufactured and due to the variation in the resulting microstructure.

The oxidation of carbon can lead to formation of CO and CO2 and mixtures of these and is summarized by the following reactions:

2C+O2→2CO  (1)

C+O2→CO2   (2)

In the presence of water, additional reactions as shown below may occur:

C+H2O(g) → CO+H             (3)

C+2H2O(g) →CO2+2H2                (4)

The use by the present authors of chemical thermodynamic data as compiled by Turkdogan, has shown that reactions 3 and 4 are possible only at temperatures above 667C and 623 C respectively. As suggested by Savage these reactions are probably not relevant for the case of C-C composite brakes as they are typically used in air and not in steam; moreover, the frictional heat is thought to lead to evaporation of any water that may be present. However, according to Blanco, the presence of adsorbed water within the C-C composite structure can be highly detrimental as it can enhance the polarity of active sites that contribute to carbon degradation during oxidation. This observation was further supported by evidence presented by Duvivier et al. who investigated the effect of 5% water vapour in air on the oxidation of C-C composite disc samples in the presence of K2CO3 catalyst at temperatures between 500C and 650C. Their results showed that the presence of water vapour could enhance the kinetics of oxidation by an order of magnitude.

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