Noble Metal Coatings for Increased Turbopump Rotor Temperature and Life

Current superalloy tururbopump rotor materials are limited in both maximum use temperature and environmental stability. Extended use at elevated temperatures leads to stress corrosion cracking, followed by turbine blade separation. Ultramet previously fabricated an iridium coating on a forged Astroloy Mk-51 turbopump turbine rotor by chemical vapor deposition (CVD). Iridium was selected as the coating material for two reasons. First, its high temperature envionmental stability has been demonstrated in test-firing of iridium-lined rhenium thrust chambers at temperatures above 4000F for more than 15 hours without failure. Second, iridium can be applied by CVD at ~900F, low enough that the coating process will not affect the condition of the forging or cause dimensional changes. Even though the turbine disk had been tested previously and was severely work hardened, the iridium-coated rotor survived 10 starts totaling 4,106 seconds of hot-fire time at 60,000-65,000 rpm with only isolated areas of coating failure, which were attributed to the external work hardening of the rotor surface during fabrication by the manufacturer and/or a coating/substrate thermal expansion mismatch. In this Phase I program, Ultramet proposes to apply innovative materials and processing to turbopump protection. By expanding the materials studied to include iridium, platinum, and palladium and developing new processing techniques, success is virtually ensured. Each material offers outstanding high temperature environmental stability, with the latter two possibly providing better mechanical integrity due to a closer CTE match.