Preparation of the electroless Ni–P and Ni–Cu–P coatings on engine cylinder and their tribological behaviors under bio-oil lubricated conditions

Two electroless Ni–P and Ni–Cu–P coatings on engine cylinder were prepared with chemical plating nickel technology. The coatings were characterized by Energy Dispersive X-ray spectrometer (EDX), Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD). The friction and wear behaviors of cylinders with these two coatings were compared with those of the cylinder without coating under bio-oil lubricated conditions. It was found that the cylinders after coating had significant improvement of tribological properties; in addition, the Ni–Cu–P coating had higher wear resistance and lower friction coefficient than those of Ni–P coating. Under the bio-oil lubricated conditions, the evolution of friction coefficient with sliding speed agreed with the classical Stribeck curves, namely, there was a transition from boundary lubrication (BL) to mixed lubrication (ML) to hydrodynamic lubrication (HL) as sliding speed increased. The lower friction coefficient in ML regimes than that in BL regimes depended on both of the adsorbed oil film and the tribo-film on the worn surfaces. The better tribological performances of Ni–Cu–P coating than those of Ni–P coating were ascribed to the formation of a tribo-film with CuO. Therefore, the Ni–Cu–P coating showed a very potential for preventing engine cylinders from the corrosiveness of bio-oil.