Several methods have been investigated in order to develop the means to prevent or limit the catalytic oxidation of C-C composite brakes. The proposed protection mechanisms are based on three different approaches; (1) improvement of the quality of the C-C composite material, (2) addition to the composite of compounds that can reduce the rate of oxidation and (3) the use of coatings or physical barriers to restrict access to the C-C composite.
The improvement of the quality of the material has concentrated on the elimination of defects; this approach essentially reduces the amount of porosity and the number of active carbon sites. In addition, the conversion of amorphous carbon to the crystalline graphitic structure which is less prone to reaction with oxygen may be another possible means to enhance the oxidation resistance. However, porosity is one of the features of the manufacturing method and it cannot be fully eliminated, while further heat-treatment to induce more graphitization is likely to introduce defects in the carbon fibers and reduce their strength. Treatment of the material to improve its quality was used by Ehrburger and Lahaye who observed that a lower catalytic effect could be achieved if the material was heat-treated at high temperatures up to 2623C.
The introduction of chemical groups to the C-C composite aims to decrease the rate of carbon oxidation by reducing the rate of the mobility of the catalysts or by forming a barrier between the composite and oxygen. Phosphorus compounds were thermally deposited by Wu and Radovic by impregnating C-C composite samples using methl-phosphoric acid or phosphorus oxychloride and heating at around 600C. The phosphorus deposits were shown to almost completely suppress the catalytic effect of calcium, and to partially suppress that of potassium. By using XRD, SEM, X-ray photoelectron spectroscopy and temperature-desorption studies, the authors showed that one of the oxygen atoms from the phosphorus-based compounds was bonded onto the carbon active sites. The inhibition effect was due to a combination of site blockage whereby C-O-PO3 and C-PO3 groups formed a bridge-bond with the carbon through the oxygen atom and this led to the formation of a physical barrier whereby metaphosphates prevented access of the catalysts to the carbon active sites. The bridge-bond was observed to remain stable up to temperatures of 1000C. According to Wu and Radovic, the catalytic activity of calcium carbonate during the initial stages of oxidation was due to the initial interfacial contact with the carbon surface; it was suggested that the presence of the C-O-PO3 and C-PO3 groups prevented optimal interfacial contact and therefore stopped the calcium catalytic activity. This was not the case with potassium which is much more mobile than calcium and is therefore able to re-disperse and maintain interfacial contact with the carbon for the duration of the oxidation reaction.
Boron has been reported to be another element that can be used for inhibiting the catalytic oxidation of C-C composites. Wu and Radovic investigated its effect by impregnating C-C samples with solutions of B2O3 in water followed by heating at 2500C. The effect of this treatment was to reduce the oxide to elemental boron which had previously been reported to catalyse the conversion of amorphous carbon to graphite. By using XRD measurements, Wu and Radovic demonstrated that the boron-doped samples had undergone more graphitization and also exhibited three-dimensional order. According to the authors, substitutional boron can also enter the graphite crystal structure of the carbon fibers to make it more ordered. This has the effect of increasing the crystallite dimension which when lowers the electron density and prevents the chemisorption of oxygen. Indeed, it has been reported that the presence of boron in the carbon lattice re-distributes the π electrons in the grapheme plane and leads to reduction of the Fermi level of carbon and to inhibition of the desorption of CO and CO2. While there is acceptance for the reduction in the Fermi level of carbon, it is not clear how inhibition of the CO and CO2 desorption would take place. The role of sub-stitutional boron in the carbon lattice is rather controversial because studies have shown that boron itself may exhibit both aninhibiting and a catalytic effect on the oxidation of carbon. Further investigation by Karra et al. has shown that at low levels of substitutional boron, a catalytic effect on the oxidation rate is exhibited by boron itself. On the other hand, at levels of substitution of 5% by weight, an inhibiting effect has been observed. Clearly the catalytic effect of boron is not compatible with the observation of the redistribution of the π–electrons in the grapheme layer and the lowering of the Fermi level. In their study, Thrower and Jones only considered the effect onπ–electrons and paid no attention to any possible effects on the σ–electrons, while the substitutional boron was considered to reduce the number of electrons in the structure as it has one electron less than carbon. The controversial behaviour of substitutional boron warrants further investigation to understand its role in the oxidation of carbon.