Microstructure evolution and mechanical performance of Cr-N/Al-Cr multilayer coatings produced by plasma nitriding Cr-coated Al alloy

A two-step treatment was proposed to strengthen the surface mechanical performance of 2024 Al alloys. Nano-scale Cr film was firstly deposited by using magnetron sputtering, then Al alloys deposited with Cr film were plasma nitrided at different temperatures (400-480 °C). Results showed that nitriding treatment transformed the Cr film to Cr(N) and CrN phase. With increasing temperature, the nitride surfaces became denser and the nitrogen content (at. %) increased from 17% to 40% at 480 °C. Low temperature nitriding obtained only single Cr-N layer on the surface of Al substrate. High temperature nitriding stimulated surface Cr-N reaction and inner Cr-Al reaction simultaneously to generate multilayer coatings consisting of outmost Cr-N layer and inner Al-Cr intermetallic layers (Al4Cr and Al18Cr2Mg3). The surface hardness of as-prepared coatings reached the maximum value of 9.21 GPa, which significantly enhanced the Al alloy surface (1.57 GPa). The friction coefficient of Al alloy (0.72) was decreased to 0.51 for samples with multilayer coatings. The multilayer coatings showed a mixture wear mechanism of adhesive wear and abrasive wear, accompanied by oxidative wear. •Cr-N/Al-Cr multilayer coatings could be fabricated by plasma nitriding Cr-coated Al alloys.•Multilayer coatings resulted in considerable enhancement of hardness up to 9.21 GPa for Al alloy surface.•Multilayer coatings improved the wear resistance of Al alloys remarkably.