Tribological Behaviors of Surface-Modified Layers on Cr12MoV Steel by Plasma Nitriding and Composite Ion Sulfurization

Composite sulfide layers were prepared on the nitrided surface of Cr12MoV steel by means of plasma S-C-N composite treatment so as to improve the friction and wear behavior of the steel. Thus the Cr12MoV steel block was surface modified by plasma nitriding and plasma nirtiding plus composite ion sulfurization at optimized processing parameters. The friction and wear behaviors of the surface-modified layers sliding against SAE52100 steel at dry- and oil-immersed or oil-dripping conditions were evaluated on an MM-200 test rig in a ring-on-block configuration. The morphologies of the worn surfaces of the surface-modified layers were observed on a scanning electron microscope. It was found that a porous sulfide layer was generated on the steel surface after the composite ion sulfurization. The porous sulfide layer was capable of adsorbing and storing lubricating oil, which contributed to greatly reducing the friction coefficient and wear mass loss. The composite sulfide layer showed the best wear-resistance under the oil-dripping condition, because the boundary lubricating film functioned to reduce the friction and wear, while the composite sulfide layer functioned to increase the antiwear and load-carrying capacity of the film in this case.