TRIBOLOGICAL CHARACTERISTICS OF ELECTROLESS Ni-B-Mo COATINGS AT DIFFERENT OPERATING TEMPERATURES

Electroless nickel coatings containing Mo possess higher thermal stability in comparison with the binary alloy variants. In a quest to achieve enhanced thermal stability of Ni-B coatings, Mo is incorporated to obtain a ternary Ni-B-Mo coating. The coatings are deposited on AISI 1040 steel and characterized using energy dispersive X-ray (EDX) analysis, X-ray diffraction method and scanning electron microscope. The coatings are observed to lie in the mid-B range with amorphous structure in as-deposited condition. On heat treatment, precipitation of crystalline Ni and its borides is observed. The typical cauliflower-like surface morphology of the deposits could be observed in scanning electron micrographs. Microhardness measurements reveal the enhanced thermal stability of Ni-B-Mo coatings. Tribological behavior of Ni-B-Mo coatings at room and elevated temperatures (100 degrees C, 300 degrees C and 500 degrees C) is observed on a pin-on-disc type tribo-tester by varying the applied normal load (10-50 N) and rotational speed of the counterface disc (60-100 rpm). The purpose of the present work is to observe the tribological behavior and associated tribo-mechanisms at different temperatures under dry sliding condition. In general, the wear of the coatings increases with an increase in applied normal load and speed at room temperature, 100 degrees C and 300 degrees C. At 500 degrees C, the wear increases with load but with speed it first increases up to 80 rpm and then decreases. The COF does not show a similar behavior like the wear with varying load and speed at different temperatures. Instead, it is controlled by the accompanying wear mechanisms, formation of oxide debris and oxide layers of Ni and Mo. The worn surface of the coatings is examined using scanning electron microscope and EDX analysis. Back scattered images of wear tracks of Ni-B-Mo coatings at the highest levels of load (50 N) and speed (100 rpm) at different temperatures further reveal the oxide formation and tribochemical reactions.