Comparative study of CrAlSiN monolayer and CrN/AlSiN superlattice multilayer coatings: Behavior at high temperature in steam atmosphere

Protective coatings that retard ionic diffusion at high temperature in the presence of steam are of interest for the energy industry. A CrAlSiN monolayer and two CrN/AlSiN coatings of periodic layers of different thicknesses deposited on P92 steel by cathodic arc evaporation (CAE) were studied. Their protection capabilities under the working conditions of ultrasupercritical steam turbines (at 650 °C in 100% steam atmosphere) up to 2000 h were assessed. Several characterization techniques were used to achieve this objective, such as gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy with an energy dispersive detector (SEM-EDS), field emission scanning electron microscopy (FE-SEM) and glow discharge optical emission spectrometry (GDOES). After 2000 h, the oxidation resistance of the coated samples showed a significant improvement compared to that of the bare substrate. This behavior is explained by the formation of a protective outer layer of chromia (Cr₂O₃), which prevents or reduces the inward and outward diffusion of oxygen and metallic cations, respectively, leading to a reduction of the oxidation rate at working conditions. Oxidation kinetics of the coated samples follows a parabolic trend at the first stages of oxidation, according with the development of a protective oxide on the surface during the thermal treatment. However, after a certain time an apparent change in the kinetics of the oxide growth is observed, that has been related to the detachment of the oxidized macrodroplets of the evaporated materials, which are characteristic of the CAE technique. Nevertheless, this phenomenon does not decrease the oxidation resistance of the coatings.