Most of the coatings used to protect C/C composite are fabricated by chemical vapor deposition (CVD). CVD coatings are made by passing a source gas and a reductant together over a heated substrate. The source gases for SiC are usually trichloromethylsilane (TMS) and H2 at 1000-1400C. The sources for Si3N4 are TMS and ammonia at 1000-1400C. The temperature of the deposition process can be lowered by using Plasma assisted CVD. The plasma is generated by using radio frequency radiation. Lowering deposition temperatures may reduce cracking induced by cool down from the process temperature. Coatings of SiC can also be formed on carbon using gases that fix a silicon activity in the gas whereby the carbon is siliconized.
Adhesion to the substrate is a problem with all coatings. The thermal expansion mismatch between a coating and the substrate is a critical parameter. A large mismatch leads to a coating that is no adherent. Graded coatings have been developed with compositions ranging from the substrate to the protective coating phase. These coatings may have better adherence to the substrate.
The mechanical properties of a coating are also important. Ceramic coatings undergo little plastic deformation to relieve expansion stresses while metal coatings/substrates can deform plastically. The tensile behavior of coatings depends on the deposition microstructure. The amount of the reductant affects the morphology of the CVD coating. Low supersaturations result in columnar grains with domed tops. High amounts of reductant cause a structure composed of fine equiaxed grains. The strength of CVD SiC is reduced from that expected of the bulk.
Coating systems chosen to protect CC composite for long times depend on the intended service temperature. Roughly the service temperature intervals for coatings are low temperature. (T<1400C), intermediate temperature (1400C<T<1800C), and high temperature (T>1800). These three regimes will be discussed based on work by Strife and Sheehan.
The low temperature regime has received the most attention commercially. Most efforts deal with coating the CC composite with SiC, Sialon, or Si3N4. The SiC and Si3N4 coating, however, have significant thermal expansion mismatches with CC composite. The net result is the thermal cycling results in cracking of the coating. The cracking is a serious problem because oxygen can diffuse rapidly down the cracks and react with unprotected CC composite. The oxidation of unprotected CC composite is catastrophic at these temperatures and, therefore, the cracks must be sealed. The composite is sealed internally or externally. One external method uses TEOS to deposit silica in cracks and fissures that are present in the coating. The purpose of the glass is to fill the cracks that open during thermal cycling. The requires the glass to have a low enough viscosity to wet the coating-substrate system. Too low a viscosity, however, will result in the glass flowing off the surface by gravitational or convectional forces. One can see from this discussion that the glass chemistry is critical. Borate glasses can also be used to seal the coatings both internally and externally. Borate glasses have viscosities and wetting characteristics that make them attractive additions to the coating protection system. Two disadvantages of the borate glasses are moisture sensitivity and fluxing of silica scales. Moisture sensitivity refers to the swelling of the glass when water is absorbed into the glass network. Glass swelling may disrupt the coating. Fluxing of the silica by reaction with the borate glasses causes a break down in the protective mechanism of the coating. The fluxing may be lowered by additions of SiO2 to the borate glasses. Borates can seal the coating internally by adding the glass formers to the CC composite. When oxygen diffusing through cracks encounters the glass formers a borate glass forms. There is a volume expansion that causes the borates to fill cracks and pores.