Development of Oxidation Protection Coatings for Gamma Titanium Aluminide Alloys
Metallic material systems play a key role in meeting the stringent weight and durability requirements for reusable launch vehicle (RLV) airframe hot structures. Gamma titanium aluminides (gamma-TiAl) have been identified as high-payoff materials for high-temperature applications. The low density and good elevated temperature mechanical properties of gamma-TiAl alloys make them attractive candidates for durable lightweight hot structure and thermal protection systems at temperatures as high as 871 C. However, oxidation significantly degrades gamma-TiAl alloys under the high-temperature service conditions associated with the RLV operating environment. This paper discusses ongoing efforts at NASA Langley Research Center to develop durable ultrathin coatings for protecting gamma-TiAl alloys from high-temperature oxidation environments. In addition to offering oxidation protection, these multifunctional coatings are being engineered to provide thermal control features to help minimize heat input into the hot structures. This paper describes the coating development effort and discusses the effects of long-term high-temperature exposures on the microstructure of coated and uncoated gamma-TiAl alloys. The alloy of primary consideration was the Plansee alloy gamma-Met, but limited studies of the newer alloy gamma-Met-PX were also included. The oxidation behavior of the uncoated materials was evaluated over the temperature range of 704 C to 871 C. Sol-gel-based coatings were applied to the gamma-TiAl samples by dipping and spraying, and the performance evaluated at 871 C. Results showed that the coatings improve the oxidation resistance, but that further development is necessary.
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 ef"icieney 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.
Inhomogeneous Boundary Value Problems for Compressible Navier-Stokes Equations: Well-Posedness and Sensitivity Analysis
In this paper compressible, stationary Navier–Stokes equations are considered. A framework for analysis of such equations is established. In particular, the well-posedness for inhomogeneous boundary value problems of elliptic-hyperbolic type is shown. Analysis is performed for small perturbations of the so-called approximate solutions that take form (1.12). The approximate solutions are determined from Stokes problem (1.11). The small perturbations are given by solutions to (1.13). The uniqueness of solutions for problem (1.13) is proved, and, in addition, the differentiability of solutions with respect to the coefficients of differential operators is shown. The results on the well-posedness of nonlinear problems are interesting on their own and are used to obtain the shape differentiability of the drag functional for incompressible Navier–Stokes equations. The shape gradient of the drag functional is derived in the classical and useful for computations form; an appropriate adjoint state is introduced to ...