Phosphorus-Induced Changes in Microstructure, Optical, and Tribological Behavior of Electrodeposited Ni-P Coatings

This work establishes a map between deposition, structure, and properties that enables the design of Ni-P coatings for advanced surface engineering applications. The coatings were electrodeposited on 316L stainless steel substrates using electrolytes of different phosphorus contents, achieved by systematically varying the phosphorous acid (H3PO3) concentrations. The influence of phosphorus content and intrinsic pH on elemental composition, cathodic current efficiency (CCE), thickness, microstructure, surface topography, crystalline structure, optical properties, and tribological behavior was investigated. The incorporation of phosphorus follows the H3PO3 concentration increase in a non-linear trend, achieving a maximum value of 22.17 at.% P at the highest bath concentration. The CCE presented an opposite trend, decreasing from approximately 96% to 40%, due to intense activity of hydrogen evolution reactions, and evidencing indirect phosphorus incorporation mechanisms. A transition from crystalline to amorphous structures was observed as the phosphorus content increased, being accompanied by grain refinement and significant roughness reduction to a minimum Sa = 8 ± 1 nm at ~15 at.% P. The optical properties, such as diffuse reflectivity and CIE Lab* color coordinates, were strongly correlated to surface roughness and microstructural evolution, demonstrating the influence of phosphorus through structural changes. Tribological behavior of the coatings revealed a complex interplay between composition, roughness, and wear mechanisms. The lower and more stable coefficients of friction were observed for high phosphorus coatings, although their durability depended on the balance between brittleness and grain refinement. The results demonstrate the combined role of phosphorus concentration and intrinsic pH changes as an effective tool for tailoring the structural, optical, and tribological properties of electrodeposited Ni-P coatings.