Production of single-phase intermetallic films from Ru-Al multilayers

Single-phase RuAl is a promising candidate for protective coating materials which are used in applications that demand high oxidation resistance at elevated temperatures. The principal advantage of this system over other B2-aluminides emerges from the coefficient of thermal expansion (CTE), which is almost identical to that of α-Al2O3. The adherence of the alumina to RuAl is improved during thermal cycling conditions by decreasing the CTE mismatch between RuAl and its protective oxide. The present work describes the microstructural and physical characterisation carried out on RuAl thin films formed from multilayers (MLs) possessing very small periods of about ~ 2 nm. Stainless steel was selected as substrate due to its technical relevance. XRD and TEM analysis of the as-sputtered films showed the presence of Ru, Al and RuAl phases, whereas atom probe tomography revealed the presence of both intermixed and multilayered zones. In-situ XRD measurements during the annealing displayed the direct formation of the single-phase RuAl from the MLs. Moreover, scratch tests confirmed the good adhesion of the films to the substrate.