Heat Treatments Modify the Tribological Properties of Nickel Boron Coatings

The effects of annealing temperature on the tribological properties of electroless nickel–boron coatings have been investigated. The coatings were annealed in a tube furnace under a flow (0.0094 N m3/min) of oxygen gas at temperatures of 250, 400, 550, and 700 °C for 3 h. Using scanning electron microscopy, images of the annealed coatings documented changes in surface morphology. From this it was seen that the higher annealing temperatures produced marked changes, moving from the nodular structure of nickel–boron coatings to a flaked surface morphology. The chemical effect of the annealing temperature was studied via X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS data indicated that after annealing at the temperatures of 550 and 700 °C, an accumulation of boron oxide species could be seen at the surface as well as a complete loss of nickel signal. An analysis of Raman spectra collected across the surface further identified the predominant species to be boric acid. The tribological response of the coatings was studied with a pin-on-disk tribometer with 440C stainless steel balls run against the coatings in ambient air. It was seen that the as received sample and the sample annealed at 250 °C samples exhibited modest friction properties, while the 400 °C sample had increased friction due to wear debris from the ball. The 550 and 700 °C samples showed remarkably low friction coefficients between 0.06 and 0.08, attributable to the presence of boric acid. The wear tracks were analyzed using scanning white light interferometry and from this data wear rates were obtained for the coatings ranging from 10–8 to 10–7 mm3/Nm.