Oxidation behavior of RuAl, PdAl, and ruthenium-palladium aluminides for bond coat applications

Aluminide bond coats are widely used as oxidation-resistant layers in coating systems for gas turbines, where a protective Al₂O₃ thermally grown oxide (TGO) forms to mitigate oxidation of the underlying substrate. Nickel (Ni) aluminide-based bond coats work well for Ni-based superalloys, but these alloys are approaching their limits as the operating temperatures of turbine engines continue to increase. This study investigated the oxidation behavior for a series of ruthenium-palladium (Ru,Pd) aluminides ranging from pure RuAl to PdAl at 1300 °C, a target temperature for next-generation bond coat applications. The results show that the incorporation of Pd into RuAl enables the formation of a stable and uniform Al₂O₃ scale. At low and moderate Pd additions, Kirkendall voids form below the growing TGO; higher Pd concentrations reduce the void formation but are more likely to lead to scale spallation. The addition of hafnium (Hf) as a reactive element slows the TGO growth and pore formation, and enhances its adhesion. With careful composition refinement, (Ru,Pd)-aluminides can be promising bond coat candidates for niobium (Nb)-based alloys due to their compatible coefficient of thermal expansion and improved oxidation resistance at high temperatures.