The influence of impurities on the formation of protective aluminium oxides on RuAl thin films

• High-temperature oxidation RuAl intermetallic thin films were studied.
• Microstructural analysis with and without impurities in the RuAl film.
• Effect of impurities content in the oxidation kinetics was evaluated.
• Stress evaluation of the oxide grown on RuAl with and without impurities.

Single-phase RuAl is a promising candidate for protective coating materials in applications that demand oxidation resistance at temperatures above 600 °C in air. The main advantage of this system over other B2-aluminides emerges from the adherence to α-Al2O3 oxide scale, formed at the surface under thermal cycling conditions. In particular, the presence of impurities and reactive elements may play a crucial role in tailoring the thermo-mechanical properties of the protective oxide. The influence of Cr and Fe impurities in the isothermal oxidation of RuAl thin films deposited on austenitic stainless steel was studied in air at 900 °C. The oxidation kinetics was analysed using an Arrhenius model, whereas microstructural and stress analyses were performed on α-Al2O3 using scanning transmission electron microscopy and X-ray diffraction, respectively. The oxidation behaviour of RuAl was affected by the presence of impurities diffused from austenitic stainless steel substrate. Cracking in the α-Al2O3 layer was observed in the absence of impurities as a result of thermal tensile stresses generated in the oxide scale. On the contrary, compressive stresses were developed after Fe (∼62 at.%) and Cr (∼20 at.%) diffusion into the RuAl film, which enhanced the activation energy of the oxide formation, mainly due to the energy barriers produced at grain boundaries. These findings highlight the potential tailoring of RuAl stability and performance at high temperatures through bottom-up diffusion of impurities and reactive elements from different substrates.