Tribological and Corrosion Performance of Electroless Ni-P-Al2O3 Composite Coatings on Ti-6Al-4V Alloy

Electroless coatings mark a significant improvement in surface engineering by providing superior uniformity, precision, and functional properties compared with traditional plating processes. Electroless Ni-P-Al2O3 composite coatings were successfully deposited on Ti-6Al-4V, forming a Ni-Ti intermediate (~14–16 µm) that ensured strong interfacial bonding. Al2O3 concentration (0.2–1.4 g/L) strongly influenced microstructure and performance, with 0.4 g/L yielding the most refined coating. AFM revealed a dense, uniform surface with the lowest roughness (~23.4 nm). This composition achieved the highest hardness (464.6 HV0.1), ~157% higher than the substrate and ~53% higher than Ni-P. It also showed superior tribological behavior, reducing wear volume to ~4.46 × 10−7 mm3 and friction coefficient to ~0.32 (~85% and ~50% reductions vs. substrate). Corrosion resistance was maximized at 0.4 g/L, with the lowest corrosion current (2.45 × 10−6 A/cm2) and rate (0.0053 mpy), outperforming both Ni-P and uncoated Ti-6Al-4V due to the compact composite matrix and stable passive film.