Improved wear resistance of alternating amorphous and crystalline layers in electrodeposited Ni[sbnd]P multilayers

Abstract

Nickel‑phosphorus (NiP) multilayer coatings are expected to show enhanced wear resistance due to the alternating sequence of individual single layers with different properties and to be used as alternatives to conventional hard chromium layers. NiP multilayer coatings with different thicknesses and properties of the sublayers were electrodeposited from a single electrolyte with alternating pulse currents. The NiP multilayer coatings showed enhanced wear resistances and lower internal stresses. The investigation of the individual NiP layers electrodeposited at low current densities (2 A dm−2) had a high P content (≈14%), while the NiP layer deposited at 8 A dm−2 had a medium P content (≈5%). The P content strongly affects various properties of the layers, e.g. crystallographic structure, corrosion resistance, internal stress, hardness and wear resistance. XRD measurements showed that the layers with high P content were amorphous, while NiP layers with medium P content were crystalline. The amorphous layers have a lower hardness and a higher corrosion resistance, while the crystalline ones show a comparatively higher hardness and lower corrosion resistance. AFM measurements before and after etching cross sections of the NiP multilayer coatings revealed this interesting interplay between P content, hardness and corrosion resistance of the individual NiP layers. The wear resistance of the NiP multilayers is improved in comparison to the amorphous single layers, while the internal stress is lower in the multilayers compared to the crystalline layer.