Microstructures and nano-mechanical properties of multilayer coatings prepared by plasma nitriding Cr-coated Al alloy

A novel surface treatment combined with depositing Cr films followed by plasma nitriding was carried out on the surface of the 2024 Al alloy. Nano-grained Cr films were firstly deposited on the substrate surface by using magnetron sputtering, then plasma nitrided at 490 °C for 8 h under a gas mixture of 25% N₂+75% H₂. Microstructures and mechanical properties of the as-obtained coatings were characterized by using X-ray photoelectron spectrometer (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and nanoindentation. Results showed that a nitride/aluminide multilayer coating was obtained due to N-Cr and Cr-Al diffusion reaction. The outside nitride layer consisted of mainly CrN with nano crystals. The inner aluminide layer was composed of Al₄Cr with nano lath-shaped structure and Al₁₈Cr₂Mg₃ with large rectangle crystals. The nitrided surface showed a remarkable increase in hardness (15.4 GPa) than that of the untreated Cr film (5.2 GPa). The high valves of H/E ratio (0.078) and H³/E² ratio (0.094) indicated the better wear resistance and plastic deformation resistance for the nitride surface.