Performance issues associated with the present generation of CC coating systems are coating spallation due to CTE mismatch, borate glass corrosion of the outer coating, moisture sensitivity of the borate glasses, and the high oxygen permeability of the borate glasses.
Coating spallation is often a problem because very few coatings can match the altrlow CTE of CC composites made with high-performance fibers. The traditional ways of avoiding spallation are to maximize coating adherence and minimize coating thickness. Often, pure CTE mismatch failures are caused by shear failure of the CC rather than by a lack of coa sting adhesion. Regardless of whether spallation is caused by a lack of coating adhesion or cohesive failure of the CC substrate, converting a thin layer of the CC surface to a carbide is beneficial. The rough, hard surface promotes adhesion, and a gradual transition from carbon to carbide distributes the strain mismatch rather than isolating it at a discrete interface.
Glass-forming inner layers are generally thin, no more than approximately 50 um. Thin outer coatings have been tried, but because of rapid degradation by the borate sealant glasses, the outer coatings are usually at least 200 um thick. Corrosion of the outer SiC and Si3N4 coating is a result of destructive oxidation caused by dissolution of the protective SiO2 that normally maintained on the silicon-based ceramics. This corrosion reflects the high oxygen permeability of the borate glasses both in terms of excessive glass formation by rapid oxidation of the inner layer and outer coating corrosion that again results from the nonprotective nature of the borate glasses. Thick outer coating eventually develop cavities that allow gross oxidation of the inner layer and the attack of the CC substrate. Thin outer coatings fail rapidly by massive coating dissolution.
The moisture sensitivity of B2O3 and most borate glasses in well-known. Hydrolysis under ambient conditions in moist air produces swelling and converts coherent and adherent glass layers into loosely bonded boric acid particulate. The moisture attack of the glass that eventually forms beneath an outer coating during long-term oxidation can result in a coating spallation due to a lack of adherence. Subsequent heating to rapidly release the moisture can also be disruptive, and exposure of the glass to moisture at high temperatures makes the glass susceptible to vaporization by the formation of volatile HBO2. The spallation occurred during the final moisture exposure because of the degradation of borate glass that had formed beneath the SiC coating. Boric acid can be seen in the outer coating cracks. cfccarbon.com
The relatively high rate at which oxygen permeates borate glasses is not only the cause of outer coating corrosion and the generally negative effects associated with excessive glass formation, but also represents an inherent limitation of the current approach of using borate glasses to seal cracks in outer coatings. The oxygen permeability of B2O3 as function of temperature is compared with the permeabilities of other materials. Using this type of information in conjunction with a model of a cracked outer coating with B2O3 filling the cracks, an analysis showed that under ideal conditions even the smallest cracks can allow significant oxidation of the CC substrate over the hundreds of hours of operation required for many important applications.
It is known that chemical modifications of the borate glasses can improve moisture resistance. Furthermore, it may be possible to vary the physical arrangement and chemical of the coating to prevent excessive glass formation. Limiting the glass formation would benefit both the moisture and corrosion problems and could allow thinner outer coatings for better adherence. The inherent limitation of high oxygen permeability of the cracked coatings combined with the realization that ideal coating protection is never established or maintained has led to the approach of using coatings within the CC composites to enhance oxidation resistance. Internal coatings aid the overall oxidation resistance of the CC when the external coating is intact and prevent rapid catastrophic oxidation if the external coating develops a major flaw.