Development of protective coatings for high-temperature metallic materials

Metallic material systems with potential for high-temperature operations are critical for many land-based and space-based systems. Advanced alloys with improved elevated temperature properties and/or reduced densities offer improved structural efficiency and longer service life compared to more conventional alloys. However, in extreme operating environments, these alloys require coatings for environmental protection and thermal control. We discuss some results from a program to develop ultrathin, lightweight, protective coatings applied via sol-gel techniques for some emerging high-temperature alloys. The coatings were designed to reduce oxidation, increase emittance, and reduce the catalytic efficiency for recombination of dissociated hot-gas species for the candidate materials. The alloys considered in this study include PM1000 (an oxide dispersion strengthened Ni-based alloy), 602CA Ni-based alloy, and a gamma titanium aluminide alloy. Inconel 617, a Ni-based alloy, was included as a reference. Microstructural analysis and oxidation weight gain results indicated that the coatings significantly reduced oxidation damage during extended high-temperature exposures for these alloys. In addition, one coating system was shown to improve the emittance of Inconel 617. A substantial reduction in the recombination of atomic nitrogen and oxygen at the surface of Inconel 617 substrates in a hot flowing airstream was also observed.