The effect of adding different amounts of sodium saccharin on the mechanical properties and co-deposition characteristics of Ni-Co-Al2O3 composite coatings

This study investigates how saccharin concentration regulates the microstructure, mechanical response, and particle co-deposition of Ni–Co–Al₂O₃ composite coatings electrodeposited from a modified Watts bath. Coatings prepared with 0–0.02 M saccharin were characterized using SEM/EDS, white-light interferometry, XRD, nanoindentation, zeta-potential/size analysis, sedimentation tests, and contact-angle measurements. Saccharin reduced surface roughness from 0.99 to 0.58 μm, whereas excessive addition (0.02 M) produced arc-like nodules and slightly increased roughness. Al₂O₃ incorporation decreased sharply with saccharin (13.53 → 1.72 vol%), while coating thickness remained nearly constant (∼22–23 μm). XRD confirmed an FCC structure with (111) preferred orientation, and crystallite size decreased from 18.7 to ∼11.6 nm, resulting in microhardness and elastic modulus increases from 474 HV to ∼580 HV and from 142 to > 200 GPa. Mechanistically, saccharin altered particle charge, suspension stability, surface wettability, and the electrical double layer (EDL), collectively suppressing particle co-deposition, while higher current density partly compensated this inhibition. Overall, strengthening was dominated by grain refinement, whereas dispersion strengthening by the ∼300 nm Al₂O₃ particles was negligible. These findings clarify additive–particle interactions and support the design of environmentally friendly chromium-free coatings.