In-depth investigation of the evolution of microstructure and its influence on the mechanical properties of medium-phosphorus electroless nickel coatings after thermomechanical treatments

Abstract

To understand and further improve the corrosion resistance of a medium electroless phosphorus nickel coating (9 wt% P) under high-temperature and corrosive conditions, the microstructural evolution of the coating after various treatments, including thermal and mechanical methods (such as Hammer Peening), was extensively studied. Complementary analytical techniques, including SEM, EDS, in situ and ex situ XRD, and micro-indentation, were employed for detailed analysis. The transformation of the deposit from its amorphous state to a distinct structure comprising Ni, Ni₃P, and NiO due to thermal treatment (ranging from 20 to 800 °C) was examined. The evolution of microstructure with temperature and annealing duration was discussed, correlating with alterations in mechanical properties, particularly micro-hardness. At temperatures exceeding 310 °C, a phase transition occurred, characterized by co-precipitation of Ni and Ni₃P, leading to a significant change in the coating's mechanical behavior. With further temperature elevation, nickel diffused toward the surface, initiating NiO formation at 500 °C. The coating's oxidation behavior during isothermal treatment at varied temperatures (up to 800 °C) was also explored. This investigation was supported by thermodynamic calculations. Additionally, simplified kinetic simulations with the Dictra module from Thermo-Calc were proven to be able to reproduce the oxidation behavior. Hammer peening treatment enhanced the coating's hardness in its as-deposited state by introducing residual stresses that affected the precipitation kinetics during subsequent heat treatment. However, this hardening effect was no longer evident after the thermal treatment.