Correlating tribological performance with phase transformation behavior for electroless Ni-(high)P coating

The present study attempts to systematically assess the effect of varying the heat treatment temperature together with the duration of heat treatment on the tribological performance of EN coating and also correlating the same with the microstructural observations. High phosphorous nickel coating is developed in-house over steel substrates. The coatings are subjected to heat treatment at temperatures ranging from 200 to 800 °C for different durations (1-4 h). Suitable microstructural evaluations are carried out to study the morphology, phase structure and composition of the coatings. Hardness and friction-wear tests are carried out in a micro-hardness tester and pin-on-disc tribo-tester respectively. It is observed that heat treatment temperature as well as duration has profound impact on the microstructure of the coating. Iron diffusion and oxide formation is one of the major phenomena occurring at higher heat treatment temperatures. Hardness and wear resistance is found to increase with heat treatment. However, increased temperature (above 600 °C) and duration (4 h) is not found to have much effect on friction and wear behavior of the coating. The oxide layer formed due to high temperature gives rise to a damaged surface with cracks and delamination due to growth stresses. However, the damage is limited to the top surface of the coating which mainly comprises of oxides of nickel and iron. Hardness and wear resistance is not found to be correlated even through the dominant wear mechanism is abrasive in nature.