Effect of heating rate on morphological features of oxidized electrolessnickel- boron coatings

Due to the high hardness and wear resistance, electroless nickel-boron (Ni-B) coatings have found numerous applications. In the present study, the influence of the heating rate on the morphological features of oxidized electroless Ni-B coatings was investigated. The oxidation behavior of electroless coating layers was studied with TGA method under non-isothermal conditions at heating rates 5, 9 and 13 °C/min. The phase compositions, elemental constituents and microstructural characteristics were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) techniques, respectively. Results showed that the morphological features of the oxidized coating were influenced significantly by the heating rate so that under the heating rate of 5 °C/min the morphology of oxidized layer was a porous film with an average pore size of around 2 μm. With increasing the heating rate to 9 °C/min, more recessed pores or intervals were gradually overlaid by a film. Further increasing the heating rate to 13°C/min caused to the formation of polygonal protuberances on the surface.