Defect-mediated atomic reconstruction of nickel-phosphorus via multi-cycle Ti implantation

Nickel-Phosphorus (Ni-P) alloys are widely employed in precision molding because of their excellent machinability, hardness, and corrosion resistance. Nevertheless, their high-temperature performance is often limited by nickel diffusion, grain precipitation, and coating delamination, which lead to rapid surface degradation. In this work, a multi-cycle titanium (Ti) ion implantation method was developed to regulate the surface structure of Ni-P coatings. The influence of implantation cycles and subsequent annealing was investigated in terms of microstructure, mechanical behavior, adhesion, and tribological performance. The results show that Ti implantation generates a modified surface layer of about 90 nm, within which annealing promotes the formation of a dense TiO₂ layer and a nickel-rich interfacial zone. This stratified structure effectively suppresses Ni₃P precipitation, stabilizes grain refinement, and improves oxidation resistance. Both Monte Carlo and molecular dynamics simulations were quantitatively correlated with energy dispersive spectroscopy (EDS) results, confirming the vacancy-assisted Ti diffusion mechanism. With increasing implantation cycles, the coatings exhibited a hardness enhancement of up to 43 %, reduced scratch penetration depth to 40–45 nm, and superior wear resistance. These improvements highlight the potential of defect-engineered Ti-Ni-P coatings for extending service life and maintaining stability of molds under demanding thermal conditions.