Fabrication of a nanocrystalline Ni–Co/CoO functionally graded layer with excellentelectrochemical corrosion and tribological performance

Abstract

Nanocrystalline (NC) Ni–Co/CoO functionally graded materials with excellent lubricating,high anti-corrosion and anti-wear performance were fabricated by electrodepositionand subsequent cyclic thermal oxidation and quenching. Transmission electronmicroscopy and energy dispersive x-ray spectroscopy investigations show thatbulk Ni–Co gradient deposits with an average grain size in the range of 13–40 nmdemonstrated a graded structure transition from face-centred cubic to hexagonal closepacked and graded composition changes from Ni-rich to Co-rich regions with theincrease in deposit thickness. X-ray diffraction and x-ray photoelectron spectroscopyanalysis indicated the surface layer of NC Ni–Co graded materials to be mainlycomposed of dense and ultrafine CoO with a (111) preferred orientation. The NCNi–Co/CoO functionally graded materials exhibited significantly enhanced corrosionresistance in both NaOH and NaCl solutions and remarkably improved wearresistance and dry self-lubricating performance when compared with the NC Ni andNi–Co graded deposits under dry sliding wear conditions. The higher corrosion andtribological performance of NC Ni–Co/CoO graded materials can be attributed tothe graded microstructure within the deposits, the anti-corrosion barrier of adense oxide layer and the solid lubrication effect of CoO-rich tribo-surface films.