The formation of an oxygen diffusion barrier can be generated by deposition of suitable coatings. However, in doing this care must be taken to avoid thermal mismatch between carbon and the protective coating as this may lead to the formation of cracks within the latter and this may have a detrimental effect. Oxygen can diffuse through any cracks that may form and oxidation can take place leading to the formation of further cracks and the fragmentation of the protective layer. Silicon carbide and silicon nitride have thermal expansion coefficients that are close to that for carbon and therefore they represent a potential choice as protective coatings against oxidation; while they have been applied as coatings to minimize or prevent the oxidation of C-C composite, as yet there is no information on whether they can have any effect on the catalytic oxidation of the material. The use of surface treatments with phosphoric acid has been used by Labruquere et al. to protect C-C composites; from the data they presented, it was observed that the treated samples lost about 2% carbon after one hour at 650C in air, in comparison to the unprotected samples that lost about 12%. It was suggested that at low temperatures the phosphorus species reacted with the catalytic impurities to form alkali and alkaline-earth orthophosphates to make the catalysts inactive. At temperatures above 600C, the phosphorus-bearing species was reported to attach itself to active carbon sites and prevent access of oxygen to these sites. It was also reported the catalytic oxidation could not be prevented above about 900C. At these temperatures, the phosphorus-bearing species lost adherence with the carbon fibers and could not provide any protection.
It is clear that further research is required to reduce or even eliminate the effect of catalysis of C-C composite brakes. C-C materials are also being considered for hypersonic flight vehicle development for temperatures up to 1950C and for reuseable launch vehicles so the development of oxidation protection systems is likely to gain importance in the near future. In addition, the understanding of the mechanism of catalysis will be useful in emerging fields where there is a desire to enhance the catalytic effect. For example, the catalysis of the oxidation of carbon is currently attracting the attention of researchers working in the field of fuel-cell technology. Jiang and Irving developed a hybrid direct carbon fuel cell which combined a molten alkali metal carbonate fuel cell with a solid-oxide fuel cell and reported an increase in the electrochemical performance due to catalysis of the oxidation of carbon. Similar observations have been reported by Kouchachnili and Ikura. Another area where the oxidation of carbon is becoming of interest concerns its use to reduce the amount of particle emission of carbon soot from diesel engines.
Conclusions:
The catalytic of oxidation of C-C composite aircraft brakes is of great concern as it can enhance the oxidation rate by an order of magnitude and thus degrade the material. The decomposition of the de-icing chemicals which are based on acetates and formats of sodium, potassium and calcium leads to formation of the respective carbonates. Based on the results of the various investigations, it is generally agreed that the potassium-based species leads to the highest catalytic effect followed by sodium and then calcium. Based on observed kinetic data, the effect of the sodium- and potassium- based catalysts, the rate of oxidation at 600C increases by at least one order of magnitude. In the case of the alkali metals, the carbonates in the presence of carbon tend to decompose to the monoxides. Evidence in the literature has shown that the subsequent redox reaction between the monoxides and the peroxides is the mechanism of catalysis. In the case of calcium, the reversible reaction between the carbonate and the oxide appears to be responsible for the catalysis. Most of the investigations to reduce or eliminate the catalytic oxidation of C-C composites have focused on doping the material or improving its quality, in addition to the use of protective coatings. Doping with phosphorus have been successful in stopping the catalytic activity of calcium salts by formation of C-O-PO3 and C-PO3 bonds. The use of boron as a dopant has shown a variety of effects; boron catalyses the conversion to graphite and also enters the lattice as a substitutional atom resulting in the reduction of the Fermi level. Both of these effects inherently increase the oxidation resistance of the material. However, there is controversy concerning the addition of boron as a substitutional atom in the carbon lattice owing to the observation that additions of small amounts of up to 2% by weight can have a catalytic effect on the oxidation of carbon, while levels of 5% have in inhibiting effect. This dual behaviour of substitutional boron is rather strange and in particular the catalytic effect since this in not compatible with the reported decrease in the Fermi energy. In addition, the formation of B2O3 around the carbon tends to block it from oxygen. Based on the current state-of-the-art, the current authors propose the use additives that will react with the alkali carbonates/monoxides and prevent the redox reaction between the monoxides and the peroxides.