The influence of the mode of plasma generation on the plasma nitriding behaviour of a micoralloyed [i.e. microalloyed] steel

It is well understood that the introduction of nitrogen into metallic surfaces improves the mechanical properties of engineering components, such as hardness, wear and fatigue as well as the corrosion resistance. In addition to conventional techniques such as salt bath and gas nitriding there are currently several methods which utilise plasma environments for mass transfer, e.g. D C plasma nitriding, R F plasma nitriding and plasma immersionion implantation (PI3). It was the main aim of this investigation to study the plasma nitriding behaviour of a commercially available chromium bearing microalloyed steel, MAXIMA™ , in the hot rolled and air cooled condition. The ultimate goal was to assess the possibility of using it as an alternative to conventional quenched and tempered nitriding steels, such as En19(AISI 4140) which was used as a reference in this investigation. In order to identify themost suitable gas mixture for the nitriding of MAXIMA™ , a comprehensive investigation of DC plasma nitriding at 450°C was performed by varying the hydrogen concentration in the nitriding atmosphere and examining the structure and the properties of the surface layers. A detailed study of RF plasma nitriding and PI3 was undertaken using MAXIMA™ steel. In order to gain an insight into the influence of different modes of plasma generation on the formation of the hardened layer and the mass transfer mechanisms involved, a comparison was also made with DC plasma nitriding. All the experiments were conducted on MAXIMA™ steel for five hours at the temperatures of 350, 400,450, 500 and 550°C. Optical, scanning electron and atomic force microscopy, in conjunction with microhardness and surface roughness measurements were employed to characterise the nitrided surfaces. X-ray diffraction, both in normal and glancing angle configurations and glow discharge optical emission spectroscopy were also utilised to identify the phases that formed on the surface of nitrided specimens and the concentration profiles of different elements. It was found that DC plasma nitriding of MAXIMA™ at all temperatures developed cases with higher surface hardness and roughness, and a slightly thinner compound layer and diffusion zone than those obtained from Enl9 steel. A comparative study between PI3 and RF plasma nitriding revealed that the PI3 process produced surface layers with more favourable properties, such as higher hardnesses and thicker cases, than RF plasma nitriding. Unlike DC plasma nitriding, for the conditions employed no compound layer was formed on the surface of the samples treated by either PI3 or RF plasma nitriding. The best case, in terms of hardness and depth, was produced by PI3 at 350°C and by DC plasma nitriding at higher temperatures. At all temperatures, RF plasma nitriding produced the shallowest cases with the lowest hardnesses. It has been argued that the mode of plasma generation plays a critical role in nitrogen mass transfer. More specifically, the operating pressure and ion energy have been found to be the key parameters in determining the total nitrogen adsorption on the surface. In DC plasma nitriding mass transfer is primarily controlled by sputtering and re-deposition.The moderate ion energy in DC plasma nitriding causes sputtering of the cathodic surfaces. Sputtered species undergo collision with the nitrogen in the vicinity of the surface because of low mean free path for collision (few millimetres) which is a direct result of the relatively high operating pressure of a DC plasma. In stark contrast, in PI3 and RF plasma nitriding, the mean free path for collision of the ions and atoms is in the order of several centimetres because of low operating pressure, hence there will be no collisions near the sample and no re-deposition of sputtered material will take place. Owing to extremely high ion energies involved in PI3, ion implantation is the dominant mass transfer mechanism and sputtering of surface atoms is considerably lower than DC plasma nitriding. The lack of significantly energised ions in RF plasma nitriding eliminates the possibility of implantation and the main mass transfer mechanism is due to the adsorption of low energy neutral species which are incapable of inducing and sputtering