Effect of lactic acid on internal stress and mechanical properties of electrodeposited Ni-P alloys for ultraprecision manufacturing

Achieving Ni-P alloy coatings with ultra-low internal stress, excellent mechanical properties, and high current efficiency is critical for meeting the stringent demands of ultra-precision manufacturing. This study aims to explore the effects of lactic acid concentration and current density on the internal stress, mechanical properties, and current efficiency of Ni-P coatings electrodeposited on copper substrates under direct current (DC) conditions. The lactic acid concentration was varied from 39.6 to 48.4 g L⁻¹. Theoretical calculations and electrochemical tests were employed to investigate the solvation behavior of Ni²⁺ in the plating bath and the subsequent influence on the electrodeposition process. Results indicated that lactic acid forms a stable solvation structure with Ni²⁺, enhancing the polarization during deposition, modifying the nucleation mechanism from instantaneous to progressive nucleation, and ultimately affecting the coating properties. Under the optimized conditions of 44 g L⁻¹ lactic acid concentration and 1.6 A dm⁻² current density, the Ni-P coating achieved an ultra-low internal stress of + 0.029 MPa, a high Vickers hardness of 707 HV, a ultimate tensile strength of 1537 MPa, a current efficiency of 95.28 %, and a phosphorus content of 1.92 at%. This study provides valuable insights for the development of high-performance Ni-P coatings tailored for ultra-precision applications.