Duplex electroless Ni-P/Ni-Cu-P coatings: Preparation, evaluation of microhardness, friction, wear, and corrosion performance

The current study focuses on the development of duplex Ni-P/Ni-Cu-P coatings by the electro­less deposition method. Coatings are developed on mild steel substrates with Ni-Cu-P as the outer layer and Ni-P as the inner layer and vice versa. The coated samples are heat-treated at temperatures ranging between 200 to 800 °C during 1 and 4 h. Coated samples are characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The effect of heat treatment tempera­ture and its time duration on the hardness, friction and wear behaviour of both coatings are evaluated and compared. This would help in understanding how heat treatment influences the duplex system of coatings and helps in identifying the suitable condition of heat treatment for optimal performance of the coating. It is observed that heat treatment has a positive influence over the coating performance, which is the best when treated under optimal temperature and time duration conditions. The corrosion behaviour of the coatings is also assessed with the help of electrochemical techniques, viz. potentiodynamic polari­zation and electrochemical impedance spectroscopy. The results show that the duplex coat­ings can provide substantial protection to the mild steel substrates. Heat treatment is also found to have a significant influence on the corrosion behaviour of duplex coatings.