Microstructural and topographical studies of DC-pulsed plasma nitrided AISI 4140 low-alloy steel

The influence of DC-pulsed plasma nitriding time on the surface properties of AISI 4140 low-alloy steel has been investigated. The samples were nitrided in an industrial equipment using a gas mixture consisting of 25% N₂ + 75% H₂ and the DC-pulsed glow discharge time was varied between 1 and 28 h. Optical and scanning electron microscopy as well as X-ray diffraction, electron probe microanalysis and microhardness measurements have been used to study the ion-nitrided surfaces. It was found that the compound layer thickness does not follow a parabolic law with treatment time. An equation has been derived according to the experimental data that can predict the thickness under the nitriding conditions studied in the present investigation. Furthermore, when the nitriding time is increased, the compound layer passes from a dual phase (ε-Fe_2-3N + γ′-Fe₄N) to a monophase γ′-Fe₄N. The topographical evolution and roughness, studied by atomic force microscopy (AFM), have shown that all the roughness parameters increase with the ion nitriding time. The rate of increase is higher during the first hours of nitriding, which could be related to the relative intensity of sputtering and redeposition. The microformations developed during nitriding show a conical aspect with different morphologies. Their growth direction changes from a perpendicular orientation in the center of the sample to an inclination close to the sample edge, as a consequence of the change of the direction of the electric field lines close to the edges.