Moisture-Induced Delayed Alumina Scale Spallation on a Ni(Pt)Al Coating (Preprint)

摘要

Delayed scale failure was examined for samples of a Ni(Pt)Al-coated CMSX4 single crystal superalloy, cyclically oxidized at 1150 degrees C for 2000 hr. One sample exhibited accentuated coating grain boundary wrinkling, initiating local alumina scale spallation to bare metal, resulting in a final weight loss of 3.3 milligrams/(square centimeter). Spallation under ambient conditions was monitored with time after cooldown and was found to continue for times up to 24 hr, producing up to 0.05 milligrams/(square centimeter) additional loss for each hold, and accumulating 0.7 milligrams/(square centimeter) (20% of the total) over the course of the test. After test termination, water immersion produced an additional 0.15 milligrams/(square centimeter) loss. (A duplicate sample produced much less wrinkling and time dependent spalling, maintaining a net weight gain). The results are consistent with the general phenomena of moisture-induced delayed spallation (MIDS) of mature, distressed alumina scales formed on oxidation resistant MAl alloys. Relative ambient humidity is discussed as the factor controlling adsorbed moisture, reaction with the substrate, and hydrogen effects on interface strength.