Comparative study of plasma nitriding and plasma oxynitriding for optimal wear and corrosion resistance: Influences of gas composition

The phase structures, microstructures, wear properties, and corrosion resistance of plasma nitrided and plasma oxynitrided 4140 steel were studied. The plasma nitriding was conducted on a mixture of nitrogen and hydrogen at 520 °C. Additional oxygen in the form of plain air was added during oxynitriding treatment in a fixed ratio of nitrogen and hydrogen at 520 °C. The influences of gas composition on properties of the nitrided layer and oxynitrided layer were investigated using an optical microscope, scanning electron microscope, profilometer, X-ray diffractometer, X-ray photoelectron spectroscopy, Raman microscope, micro and nano-indenters, a tribometer, and potentiodynamic polarization testing. Results of tests demonstrate that an iron oxide layer with nanocrystalline Fe2–3N phase distributed forms on the surface of plasma oxynitrided specimens. Oxygen addition leads to different phase compositions, microstructures, and thicknesses of surface layer, which result in differences in mechanical, wear, and corrosion properties of specimens. The plasma oxynitrided specimen with optimal properties was obtained at a certain gas composition (N:H:O = 1:1:0.175), which shows better wear property and corrosion resistance than plasma nitrided specimen.