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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.

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several derivative alloys at 103.5 MPa (15 KSI) and 1093'C (2000'F). Rupture lives of the modified SC alloys were significantly affected by both alloy purity and heat treatment. Critical aspects of vacuum induction refining, exothermic casting technology, alloy development and heat treatment, which contributed to this new class of turbine blades, are reviewed.

A commercially available, hot isostatically pressed Si 3 N 4 containing 4 wt% yttria was exposed to 982°C for up to 75 h in a burner rig pressurized to 500 kPa. Synthetic sea salt added to the flame (5 ppm) resulted in formation of a sodium magnesium silicate corrosion product. A 33% reduction in room-temperature strength occurred after 5 h exposure. This is thought to be due to modification of the near-surface grain boundary phase and relief of surface compressive stresses. Exposures to longer times resulted in continued strength reduction, up to 46% at 75 h. Strength also decreased when salt concentration was increased, as shown by exposures using 2 and 10 ppm sea salt. In tests at 100 and 300 kPa with other variables held constant, postcorrosion strengths were similar to those after 500 kPa exposure.