A REVIEW OF FUNDAMENTAL COATING ISSUES FOR HIGH-TEMPERATURE COMPOSITES

This review addresses many of the fundamental issues associated with the use of coatings in high temperature aggressive environments with specific emphasis on multi-component composites. A major concern is damage caused by the permeation of oxygen into matrix cracks causing internal oxidation and attack along fiber/matrix interfaces. In addition, many prospective coatings that might be used to protect composites are susceptible to hot corrosion, particularly by small vanadate concentrations that could degrade the coating and, thereby, enhance the permeation of other aggressive species. The mechanical stability of coating systems is also a major consideration in determining performance. Large differences in thermal expansion coefficients between the coating and the composite substrate are not necessarily ameliorated by the practice of grading interfaces. However, the use of functionally graded coatings can be beneficial in reducing interlaminar shear and across-ply strains. Crack management, including the use of sealants, can be an essential part of a functional design, but the thermochemical stability of the glass sealant must be considered at elevated temperatures.