Wear and friction characteristics of electroless Ni-B-W coatings at different operating temperatures

Sodium borohydride reduced electroless nickel alloy coatings have high wear resistance and low coefficient of friction. The present work investigates the deposition and tribological behavior of a ternary variant of the borohydride reduced coating i.e. Ni-B-W. Electroless Ni-B-W coatings were deposited on AISI 1040 steel substrates. In order to improve the mechanical properties of the deposits, they were heat treated at 350 degrees C for 1 h. The coatings in their as-deposited and heat treated conditions were characterized by scanning electron microscope, energy dispersive x-ray analysis and x-ray diffraction techniques. Ni-B-W coatings are amorphous in their as-deposited state while they become crystalline on heat treatment. In fact a high microhardness of Ni-B-W coatings is also observed in as-deposited condition. The microhardness further improves on heat treatment. Tribological behavior of the heat treated coatings with varying load (10-50 N), sliding speed (0.25-0.42 m s(-1)) and operating temperature (25 degrees C-500 degrees C) were evaluated on a pin-on-disc type test setup while the wear mechanisms were also studied. Tribological behavior of Ni-B-W coatings is enhanced at 500 degrees C operating temperature in comparison with 100 or 300 degrees C due to formation of protective oxide scales and microstructural changes due to in-situ heat treatment effect.