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 (γTiAl) have been identified as high-payoff materials for high-temperature applications. The low density and good elevated temperature mechanical properties of γ-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 γ-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 γ-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 γ-TiAl alloys. The alloy of primary consideration was the Plansee alloy γ-Met, but limited studies of the newer alloy γ-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 γ-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. INTRODUCTION γ-TiAl intermetallic alloys are candidate materials for many high-temperature structural applications because of their high specific strengths and excellent high-temperature properties. These applications include airframe hot structures and thermal protection systems for reusable hypersonic flight vehicles which re-enter the earth’s atmosphere under extreme temperature and environmental conditions hundreds of times during the vehicle’s life. However, these alloys are prone to both oxidation and oxygen embrittlement when exposed to these severe service conditions. Therefore, development of coatings to prevent environmental damage is critical, especially for thin-gage sheet and foil products where a significant portion of the material cross-section can be severely affected. Approved for public release; distribution is unlimited Many challenges exist in the development of a useful coating. It must be thin and lightweight in order to minimize structural weight. Ease of application is important because the coating process must allow coverage of large acreage material and be compatible with in-field repair. The coating process must not adversely impact the mechanical properties of the alloy, and the coating must be chemically compatible with the alloy throughout the service life. In addition, the coating can also be used to modify the surface of the structure to enhance its thermal control characteristics. For many applications, a high emittance coating can significantly reduce the amount of heat that is absorbed by the structure. Also, in hot flowing-air environments where gaseous species (such as oxygen and nitrogen) have been dissociated into their atomic form, the recombination of these atoms at the surface can cause significant increases in heat input into the structure. Catalytic efficiency, or recombination efficiency, is a measure of the propensity of a material to promote this recombination and can vary significantly for different materials. Generally metallic materials have a relatively high catalytic efficiency, and coatings that reduce the efficiency of this recombination can greatly reduce the structural temperature. Multifunctional, multilayer coatings are being developed at NASA Langley Research Center (LaRC) to meet the oxidation protection and thermal control requirements for γ-TiAl hot structures. One technique that is attractive for applying these coatings is a sol-gel process because of its ability to produce very thin, lightweight coatings, and the wide range of coating chemistries that can be produced. In addition, sol-gel coatings are simple to apply, offering the potential for easy scale-up and field repair. This paper discusses the work at NASA LaRC to develop ultrathin, lightweight, sol-gel based coatings that offer environmental protection and thermal control for candidate titanium aluminide alloys. Testing was performed on a γ-TiAl based alloy with elemental additions to increase room temperature ductility. Oxidation performance was evaluated using thermogravimetric analysis (TGA), and the oxidation products formed were evaluated using scanning electron microscopy (SEM), wavelength dispersive spectroscopy (WDS) and X-ray diffraction (XRD). The oxidation performance of the uncoated γ-Met was evaluated over the temperature range 704°C to 871°C. Several methods for applying the solgel coatings were investigated, and coating performance was evaluated at 871°C.