Corrosion and wear study of Ni-P-PTFE-Al2O3 coating: the effect of heat treatment

In the present work, Ni-P-PTFE-Al2O3 nanodispersion coatings were developed on mild steel. These coatings were heat treated and were exposed to 3.5% NaCl solution. Thus obtained as plated, heat treated and exposed coatings were investigated for their corrosion and wear behavior together with other tribological properties. SEM/EDAX and X-ray diffractometry were used to analyze composition and structural changes of the coatings. Corrosion parameters of the coated samples were obtained from electrochemical polarization and immersion tests. Microhardness, wear resistance and friction coefficient of the coatings were also measured. The results showed that heat treatment at 200°C has little effect on the coating whereas treatment at 400°C results in enhanced crystallinity due to formation of intermetallics phases and possibly aluminum phosphide. Beyond this temperature at 600°C coarsening of the grains occurs which reduces the number of hardening sites. These structural changes result in the observance of highest hardness and lowest wear rate in case of coatings treated at 400°C. Corrosion resistance of the coated samples was observed to decrease with the heat treatment presumably due to the various physical and structural changes of the coating components. Similar changes were observed on their exposure to the corrosive media.