The oxidation of carbon becomes quite rapid at temperatures above approximately 500C. The exact rate of oxidation has been shown to depend on the active surface area, the porosity, the degree of crystallinity, the purity, the internal stress, and the absolute temperature. It have been shown that levels of 2-5 percent weight loss during oxidation leads to a substantial degradation of mechanical properties. Although short-term high-temperature applications may only require limited oxidation protection, long-term exposure normally requires that a surface barrier coating be present.
Surface coatings are generally composed of refractory layers designed to separate the oxidizing gases from the carbon composite. Unfortunately, most refractory coatings exhibit coefficients of thermal expansion that differ significantly from the carbon carbon composite substrate. Temperature changes produce stresses in the coating that are sufficient to cause cracking; thus, a glassy layer designed to flow and seal such cracks is also incorporated. Oxidation protection using barrier layer techniques is the subject of a different chapter. However, it is recognized that oxidation of the basic carbon composite structure should be prevented. Appropriate treatments are therefore applied to the matrix to achieve a minimum oxidation rate. Unfortunately, the details of most of these treatments are proprietary.
The density of a carbon composite is particularly important because gaseous penetration of any coating can produce gasification at both internal and external surfaces. A highly porous structure will gasify at a high rate, and it is for this reason that the rate of oxidation of any carbon tends to increase initially because of the expansion of internal surface area. Eventually, however, a decrease in reaction rate tends to occur as the less ordered structure is gasified. The oxidation rate of carbon is strongly dependent on the structure of the carbon layers, less organized parts appearing more reactive than better organized parts. It was stated that less graphitic structures always oxidize first, leaving the more graphitized material.
Gasification is extremely sensitive to purity; many metallic impurities, even is very small amounts, are known to be aggressive oxidation catalysts. Elements such as iron, calcium, lead, copper, vanadium, chromium, manganese, nickel, and cobalt have been shown to increase the rate of gasifcation of carbons. Attempts have been made to improve the oxidation resistance by removing impurities. Acid washing, for instance, has been shown to be beneficial, as has purification of the original feedstock and purification fo the carbon by high-temperature (3000C) halogen treatments. These high-temperature treatments have the additional effect of graphitizing suitable matrices, as well as reducing porosity.
As stated previously, a maximum oxidation resistance is abtained by coating the outer surface of a structure with layer of material specifically designed to stop oxygen from contacting the carbon composite structure. A viscous glass, frequently silicon based, is usually incorporated into this coating. This material is designed to flow at high temperatures and fill any cracks that might develop. An alternative approach is particularly useful at lower temperatures or for conditions in which oxygen has diffused through the outer coating or along cracks. Low-temperature protection can be achieved by adding a boron containing compound to the matrix precursor which on oxidation forms a viscous borate glass that covers the internal surfaces. This type of protection is similar but involves a material less viscous than silicate glass formed at higher temperatures. Other inhibition treatments involve the addition of materials to the matrix than can also produce glass sealants. A JTA quality nuclear graphite contains zirconium diboride and elemental silicon specifically to a glassy phase at about 1200C. Carbon carbon composites can’t usually contain the high amounts of glass-forming materials present in JTA graphite; however, smaller quantities of boron and silicon compounds have been added to the composite in order to provide a source for the continuous generation of new glassy materials.
One of the problems associated with oxidation-resistant additions to organic precursors is that the addition will either removed or modified during the process of carbonization/graphitization. Boron added as fine particulates will change to boron carbide during high-temperature treatment, whereas during polymerization of the original resin, it will oxidize and could to hydrate. cfccarbon.com. Most of these impurity or inhibition reactions may adversely affect mechanical properties of the carbon composite.
An alternative method to improve oxidation resistance at lower temperatures involves protecting active sites of the graphite crystal structure with reactive elements such as halogens and phosphorous compounds. Active sites most prone to oxygen attack are edge sites associated with the plannar graphitic lattice, dislocation sites, and vacancies. Reaction of these sites with elements like the halogen gases suggests the production of stable complexes that prevent oxidation.