The role of oxidation in the wear of plasma nitrided and PVD TiN coated steel

Abstract

The wear behaviour of plasma nitrided En40B steel has been investigated under dry rolling contact with applied sliding traction. Particular attention has been paid to the examination of the tribochemical reaction, i.e. oxidation, of the nitrided surface during the wear process. X-ray diffraction, optical and scanning electron microscopes have been employed to study the wear debris, worn surfaces and subsurfaces. Experiments revealed that, as a result of the hardening effect induced by plasma nitriding, wear of the nitrided steel occurred in a mild mode via oxidation of the contact surfaces and the subsequent removal of the oxide products, and the overall wear rate was dominated by the rate of oxidation of the contact surfaces. However, the unnitrided steel was worn via surface and subsurface shear deformation, leading to severe wear. In order to further improve the wear resistance of the steel, a thin, hard titanium nitride (TiN) coating was deposited on top of the nitrided steel by ion plating. Such duplex treated steel possessed superior resistance to both subsurface shear deformation and tribo-oxidation, thus ensuring minimisation of the wear rate.