Dynamic equilibrium of the surface oxide film during plasma oxynitrocarburising and its effect on performances

Abstract

In order to investigate the effect of oxide film on performances, evolution of surface oxide film during PNCO (Plasma Oxynitrocarburising) was studied. The microstructures, surface morphology, phase constitutions, cross-sectional hardness profile, and wear performances were evaluated by scanning electron microscope (SEM), X-ray diffraction (XRD), microhardness tester and wear tester. The research results showed that nanoparticles oxide film formed on the surface of samples by oxidation could be simultaneously decomposed by reduction, thus a dynamic equilibrium between the oxidation and reduction could be obtained under appropriate processing condition, resulting in a similar surface morphology of an oxide film and with the same thickness of 1–2 μm during PNCO process, while the compound layer thickness increases with treating time. Meanwhile, it is found that the maximum hardness of 760 HV0.05 is not obtained on the utmost surface, and almost the same friction coefficient of 0.45 and wear rate of 1.38 mg·(N·m)−1 was obtained under light load due to the oxide layer on the top surface; while better wear resistance can be obtained for longer time PNCO-treated samples under heavy load due to thicker compound layer. Therefore, the study provides a significant guide for plasma surface modification technology.

Keywords

• Oxide films
• Dynamic equilibrium
• Microstructure
• Phase constitutions
• Hardness