Abstract To modify the surface structure of AISI 1005 steel and its properties without any dimensional loss, different plasma surface treatments were performed at low temperatures (500 ºC) in this study. The samples were subjected to single plasma treatments including: nitriding (N5% and N3%), carburizing (CE) and ferritic nitrocarburizing (NC) and to duplex treatments of nitriding followed by carburizing (N5%+CE and N3%+CE) and ferritic nitrocarburizing followed by carburizing (NC+CE). The gas mixture used for these treatments was varied as follows: nitriding (5%N2+95%H2 and 3%N2+97%H2), carburizing (5%CH4+95%H2) and ferritic nitrocarburizing (5%N2+1.5%CH4+93.5%H2). A microstructural characterization of the samples was carried out using optical and scanning electron microscopy in addition to XRD analysis. Microhardness testing was also performed. The XRD analysis showed a stabilization of the outermost cementite layer for all of the carburizing treatments. The results show that a greater hardness increase was achieved for the nitriding treatment as well as a more regular compound layer. However, a greater depth of hardening was obtained in samples with NC+CE and N5%+CE, which extended to the hardened depth to 800 µm.

In this work, plasma carburizing of sintered pure iron at low temperature was investigated. Samples were carburized under a dc abnormal glow discharge in two mixed atmospheres (5%CH4 + 95%H2 and 20%CH4 + 80%H2) at 500 °C and 700 °C for 3 and 6 hours. The influence of these parameters was investigated by Scan Electron Microscopy (SEM), X-ray Diffraction (DRX), Raman spectroscopy and microhardness measurements. There is a correlation between temperature and layer morphology, and a layer of polycrystalline cementite was obtained at 500 °C and 700 °C on a ferrite substrate. Furthermore, the carburized layer thickness increased with an increase in the treatment time and the methane percentage. The topography is very dependent on the treatment temperature.

In this work, stearic acid (SA) was degraded in an Ar-O2and Ar-H2post-discharge environment created by a plasma reactor with a microwave source and in an Ar, Ar-H2and Ar-O2DC (Direct Current) discharge environment created in a cathode-anode confined system. The afterglow region is useful for understanding the role of the chemically active species (O, O2, H and H2). In contrast, the discharge region allows the observation of the effects of chemically active species, charged species (ions and electrons) and photons. The influence of these species on the grafting and etching of SA was evaluated by measuring the mass variation, mass variation rate and chemical composition. The results showed that when only chemically active oxygen species are present, the SA is preferentially grafted. However, when both photons and charged species are present, the SA is more efficiently etched. When the Ar-H2and Ar environments are utilized; the SA is not efficiently degraded.

Stearic acid (CH3(CH2)16COOH) was treated with Ar and Ar-O2 (10%) pulsed DC discharges created by a cathode-anode confined system to elucidate the role of oxygen in plasma cleaning. The treatment time (5 to 120 minutes) and plasma gas mixture (Ar and Ar-O2) were varied, and the results showed that the mass variation of stearic acid after Ar-O2 plasma exposure was greater than that of pure Ar plasma treatment. Thus, compared to Ar*, active oxygen species (O and O2, in all states) enhance the etching process, regardless of their concentration. During the treatments, a liquid phase developed at the melting temperature of stearic acid, and differential thermal analyses showed that the formation of a liquid phase was associated with the breakage of bonds due to treatment with an Ar or Ar-O2 plasma. After treatment with Ar and Ar-O2 plasmas, the sample surface was significantly modified, especially when Ar-O2 was utilized. The role of oxygen in the treatment process is to break carbonaceous chains by forming oxidized products and/or to act as a barrier again ramification, which accelerates the etching of stearic acid.

Samples of unalloyed iron were sintered in the presence of an abnormal glow discharge using a confined anode-cathode configuration. The samples were placed on a holder acting as the discharge anode inside an AISI 1020 steel hollow cathode. Heating of the outer cathode was obtained by the bombardment of ions and energetic neutral particles. As a consequence of the confined geometry, the sample was efficiently heated mainly by radiation from the cathode. In order to evaluate the results of the sintering process by plasma, samples of unalloyed iron were also sintered in a resistive furnace using the same thermal cycle. The porosity, dimension and morphology of the pores were characterized by means of four basic parameters: area fraction, mean diameter, shape factor and elongation factor. The results show that the shape factor depends on the pore size, a characteristic that was not observed for elongation factor. For samples sintered in plasma reactor or conventional furnace, no significant difference in the porosity or in the pores morphology was observed.

Results of polypropylene removal in injected metallic parts by using plasma technology are presented. The samples were injected with 55.18% Vol. of unalloyed iron powder, 23.57% Vol. of paraffin and 21.25% Vol. of polypropylene. The paraffin was previously removed in hexane bath, followed by a treatment in abnormal glow discharge of argon and hydrogen for the removal of the polypropylene. The electric discharge was generated in two configurations: a confined anode-cathode with the samples placed on the anode and a confined grid (anode)-cathode system with the sample placed on a holder at floating potential inside the grid. In the first geometry, electrons bombarded the sample surface, while in the second, they are collected by the grid. The samples were characterized by mass loss measurements and scanning electron microscopy. The results showed that the hydrogen discharge was more efficient for both configurations, which is attributed to the high reactivity of hydrogen atoms produced in the discharge. In addition, when plasma debinding was performed at 400 ºC for 10 min, by using the confined anode-cathode configuration, the polypropylene removal was total, while in the confined grid (anode)-cathode geometry the removal was approximately 32%. The higher polypropylene removing observed in the anode-cathode configuration is attributed to electron bombardment of the surface of the sample.

Samples of unalloyed iron powder were compacted and sintered in an abnormal glow discharge, generated in a gas mixture of 80% Ar + 20% H2 by using a pulsed power source. The samples were placed on a holder, acting as the discharge cathode, and were heated by the bombardment of ions, strongly accelerated in the cathode sheath. Sintering was performed at temperatures of 1173, 1273 and 1373 K for 30 min, varying the voltage applied to the cathode from 400 to 700 V and pressure ranging from 470 to 2650 Pa. It is shown that the kinetic energy of ions striking the sample surface increased approximately three times, when the voltage changed from 400 to 700 V, with a corresponding reduction of surface porosity. The surface sealing is related to the ion bombardment, which produced a high mobility of surface atoms and consequent enhanced diffusion as well as sputtering and condensation on the concave surface, resulting in an activation of surface sintering.