Some effects of structure and composition on the properties of electron beam vapor deposited coatings for gas turbine superalloys

Abstract

Multi-element overlay coatings applied by electron beam vapor deposition to gas turbine superalloys have been the subject of intensive research and development programs for the past eight years. Such coatings have proven to be superior to diffusion coatings in a number of critical applications primarily because of the ease with which compositional variations can be effected to meet unique protection and mechanical requirements. Overlay coating structural features have been identified (i.e., open columnar structure, vapor source spit marks, flakes and pits), along with present and future methods for their control. Laboratory observations have indicated that in a line-of-sight process, the open columnar structure is the result primarily of shadowing effects and the limited surface diffusion of the depositing atoms. Point defects (spits, flakes, and pits) result from pool behavior and substrate contamination. Methods of minimizing these defects range from in-process controls to post-coating processing. Current development areas for improved processing techniques such as gas randomization and ion plating should result in further improvements and elimination of some present post-coating processing requirements.