Electroless MAO-Ni-W-Sn-P quaternary coatings on AZ91D magnesium alloy: Enhanced wear, scratch resistance and microhardness

Micro-arc oxidation (MAO) followed by direct electroless nickel (DEN) deposition was employed to develop duplex Ni-based coatings on as-cast AZ91D magnesium alloy with enhanced mechanical and tribological performance. Binary MAO-Ni-P, ternary MAO-Ni-W-P, and MAO-Ni-Sn-P, and quaternary MAO-Ni-W-Sn-P coating systems were comparatively investigated to elucidate the individual and combined effects of W and Sn additions. The morphology and elemental composition of the coatings were characterised using optical microscopy and field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FESEM-EDS), while X-ray diffraction (XRD) was employed to evaluate their structural characteristics. XRD revealed broadened FCC-Ni-related diffraction features, consistent with poorly crystalline or amorphous-nanocrystalline Ni-based deposits, with no distinct crystalline W- or Sn-rich phases resolved. Mechanical and tribological performance was evaluated using microhardness, progressive-load scratch testing, and dry-sliding pin-on-disc wear testing. MAO treatment increased the surface microhardness of AZ91D from approximately 91 to 426 HV, while subsequent electroless deposition produced further substantial increases. MAO-Ni-W-P exhibited the highest microhardness of approximately 891 HV. In contrast, the quaternary MAO-Ni-W-Sn-P coating exhibited approximately 775 HV, corresponding to approximately 8.5 times the hardness of the untreated substrate. Despite its lower hardness than MAO-Ni-W-P, MAO-Ni-W-Sn-P exhibited superior overall wear resistance, with an improvement of up to 75% relative to untreated AZ91D. The quaternary coating also demonstrated adequate adhesion under progressive-load scratching, with a critical scratch load of 3 N. The results demonstrate that optimum wear performance is not governed solely by maximum hardness but by the combined effects of coating composition, resistance to deformation and damage, interfacial integrity, and frictional response. Overall, the MAO-assisted Ni-W-Sn-P coating provides a promising duplex surface-engineering strategy for improving the dry mechanical and tribological performance of AZ91D magnesium alloy.