Formation of magnetite overlayer on nitrocarburized low-alloy steel by plasma postoxidation

Duplex surface treatments of steel products are known to enhance fatigue strength, tribological properties, and corrosion resistance. In combined nitriding/coating deposition or nitriding/oxidation processes, a precise control of all steps of process parameters is necessary to get the required surface architecture. The process parameters are specific for different steel grades, which require detailed investigations and surface treatment of different steel-grade workpieces in separate charges. The samples made of steel grades AISI C 1045, 4140, and H 11 were salt bath and pulse plasma-nitrided, and untreated and surface treated samples were plasma-postoxidized. The Vickersʹs method was used for surface microhardness and depth profiling measurements, while the microstructure and surface zone morphology were examined by optical microscopy and scanning electron microscopy (SEM). The surface zone phase composition was analyzed by X-ray diffraction (XRD) method using CuKα radiation, and ball-grinding technique was used to measure the compound zone thickness. It was demonstrated that a low-friction, chemically stable, single-phase magnetite Fe3O4 superficial layer can be formed by plasma postoxidation of various steel-grade substrates in the same batch. Based on experiments performed on different steel-grade substrates, a source drain model was proposed to predict the phase composition of nitrided and postoxidized selected steel-grade samples.