Investigation of the adhesion properties of duplex coating on AZ91 alloy surface by MAO/HiPIMS-PVD methods

This study focuses on enhancing the surface properties and minimizing the mechanical incompatibility between the AZ91 magnesium alloy and the coating by applying CrAlYN/CrYN gradient coatings through the HiPIMS-PVD method, combined with the enlargement of a MgO intermediate layer via the Micro Arc Oxidation (MAO) method. The duplex coating approach was designed to strengthen adhesion and structural integrity between the substrate and the coating. Coating parameters for both MAO and HiPIMS-PVD processes were optimized using a Taguchi L4 orthogonal array, and the coatings were analyzed using SEM, EDS, XRD and scratch testing. MAO coatings exhibited porous and rough surfaces with volcanic morphologies. The best adhesion performance under MAO conditions was achieved with 650 V, 700 Hz, and a Type B solution, resulting in a critical load value of Lc₂ = 24 N. CrAlYN/CrYN gradient coatings alone showed coating thicknesses from 0.83 μm to 1.66 μm with compact and homogeneous microstructures. Adhesive strength was relatively low (Lc₂ = 5–10 N). In the duplex coating configuration, CrAlYN/CrYN coatings were deposited on the MAO-formed MgO layer, significantly increasing adhesion. Critical load values (Lc₂) ranged from 90 to 110 N, with the highest adhesion achieved under low CrY target voltage and current conditions. The findings demonstrate that the MAO/HiPIMS-PVD duplex coating method substantially improves adhesion and surface characteristics of AZ91 alloy, offering a promising solution for advanced surface engineering applications.