Theoretical and experimental study of organic additive effects in Ni–P electrodeposition from lactate-nickel sulfamate system for ultra-precision electroforming

Achieving ultra-low internal stress, excellent mechanical properties, and high current efficiency simultaneously is essential for applying the Ni–P alloy electrodeposition process to ultra-precision electroforming. In this study, potential additives suitable for the lactate-nickel sulfamate system—thiourea and sodium saccharin—were screened through quantum chemical calculations, electrochemical measurements, and the bent-cathode method, conducted in a constant-current mode using annealed high-purity copper foil as the substrate, with an initial additive concentration of 1.44 mmol L⁻¹. The effects of these additives on the internal stress and mechanical properties of Ni–P coatings were further investigated using combined experimental characterization and theoretical analysis. Thiourea (0.24–1.44 mmol L⁻¹) caused pronounced fluctuations in internal stress and deteriorated mechanical performance, whereas sodium saccharin (1.44–3.36 mmol L⁻¹) led to a much more stable deposition process, with the concentration ranges of the additives determined by the minimum internal stress achieved in the coatings. Based on the corrosion current density, the coating with the lowest internal stress deposited using sodium saccharin exhibits approximately 57.3 % higher corrosion resistance than that with thiourea. At an optimized sodium saccharin concentration of 2.88 mmol L⁻¹, the Ni–P coating exhibited ultra-low internal stress (−4.13 MPa), excellent mechanical properties (hardness of 708.4 HV_0.1 and ultimate tensile strength of 1495.2 MPa), and current efficiency exceeding 95 %. This study provides a feasible electrodeposition strategy for fabricating high-performance Ni–P coatings suitable for ultra-precision electroforming applications.