Evaluation of surface-modified 20CrMo by plasma nitriding coupled with ion sputtering and SMAT

Plasma nitriding treatment is a well-established method of improving the wear and corrosion properties of steel materials by the formation of a unique composite structure with a hard surface layer and a tough interior. However, the toughness of the nitrided layer that arises from the nitriding treatment is substantially lower than the underlying substrate. This paper describes the combined treatment of ion sputtering and SMAT on a nitrided steel sample which leads to the decomposition of the compound layer and the formation of a nanostructured diffusion layer. Optical microscopy, transmission electron microscopy, X-ray diffraction and microhardness testing were used to investigate the structure and mechanical properties of the modified surface layer in comparison with those of the nitrided sample. The result indicates that a nanostructured diffusion surface layer with satisfactory hardness and wear properties was formed on the treated sample relative to that of the conventional nitrided sample. Especially, the toughness of surface layer was greatly improved by this combined treatment. This method demonstrates the technological significance of SMAT in improving traditional processing techniques. It also provides a new approach for preparation of a nitrided surface layer with high strength and high toughness.

► The compound layer of nitrided sample was decomposed by means of argon sputtering.
► A nanostructured diffusion layer was formed in nitrided sample by means of SMAT.
► The hardness of nitrided sample was increased by about 10% after argon sputtering and SMAT.
► The compressive residual stress was induced by SMAT, which may improve the fatigue lifetime.
► The toughness of surface layer was greatly improved by combined treatment.