Investigation on the phase transformation of electroless Ni-B coating after dry sliding against alumina ball

The purpose of the work was to evaluate the possibility of tribochemical reactions during dry sliding of electroless Ni-B coating against an alumina ball. The effect of probable tribochemical products on the tribological behavior of Ni-B/Al2O3 tribosystem was investigated. Wear experiments were performed in a ball-on-disc configuration pressing alumina balls towards rotating disc specimens. Wear tests were carried out at normal loads of 5, 20, 40 and 60 N at the sliding distance of 100 m. Electroless Ni-B coatings with a mean thickness of 50-60 μm were applied on the mild carbon steel substrates. The worn surfaces of Ni-B coatings were studied with Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-Ray Diffractometer (XRD) and X-ray Photoelectron Spectrometer (XPS). The results showed that the structure of the worn surfaces of the coatings changed from amorphous to the crystalline. The dominant wear mechanism in the present tribosystem was combination of adhesive and abrasive wear. As the load was increased the friction coefficient and its fluctuations were decreased. The wear rate of alumina balls changed from increasing to decreasing at the load of 40 N. XPS and XRD analysis revealed the phase transformation and production of Ni-Al compounds on the worn surface of electroless Ni-B coating. It was elucidated that tribochemical reactions between the mating surfaces were responsible for tribological behavior of Ni-B coating and unexpected wear behavior of alumina balls.