Effect of copper incorporation on phase transformation behavior of electroless nickel-phosphorous coating and its effect on the tribological behavior

The microstructural changes of electroless Ni-P-Cu coating at various heat-treatment conditions are investigated to understand its implications on the tribological behavior of the coating. Coatings are heat-treated at temperatures ranging between 200 degrees C and 800 degrees C and for 1-4 h duration. Ni-P-Cu coatings exhibit two-phase transformations in the temperature range of 350-450 degrees C and the resulting microstructural changes are found to significantly affect their thermal stability and tribological attributes. Hardness of the coating doubles when heat-treated at 452 degrees C, due to the formation of harder Ni3P phase and crystalline NiCu. Better friction and wear performance are also noted upon heat treatment of the coating at the phase transformation regime, particularly at 400 degrees C. Wear mechanism is characterized by a mixed adhesive cum abrasive wear phenomena. Heat treatment at higher temperature (600 degrees C and above) and longer duration (4 h) results in grain coarsening phenomenon, which negatively influences the hardness and tribological characteristics of the coating. Besides, diffusion of iron from the ferrous substrate as well as greater oxide formation are noticed when the coating is heat-treated at higher temperatures and for longer durations (4 h).