Overlay coatings for gas turbine airfoils

The degradation modes associated with gas turbine operation under a variety of conditions are reviewed as a preliminary to defining the needs of protective airfoil coatings and processing techniques. These requirements and present coating procedures are briefly described as an introduction to the need for overlay type coatings in general and electron beam (EB) evaporation processed coatings in particular. A discussion of the features of production EB coaters is followed by a detailed description of the individual coating processing steps involved. While most are identical to other coating procedures, a few are significantly different and are discussed in detail. Current overlay coating compositions and their use and experience are reviewed. Future directions in overlay coating processing and composition including electron beam-physical vapor deposition applied ceramics are discussed.