J. Feugeas

Nitrogen implantation of AISI 304 stainless steel with a coaxial plasma gun

The application of a plasma focus (PF) device operated at 1 kJ and 25 kV as a pulsed ion implanter is described. Samples of AISI 304 stainless steel implanted with nitrogen with this device show a reduction of wear of 42 times with respect to the implanted ones, with a reduction, at the same time of the friction coefficient. X‐ray diffraction and x‐ray photoelectron spectroscopy analyses of nitrided samples show the Fe2N precipitate formation, in an almost homogeneous ∼0.4‐μm‐thick superficial layer. The feasibility of the use of the PF as an implanter arises from these results as well as the possibility of designing and constructing similar systems that could perform the whole process (e.g., nitriding) in short time intervals (∼10 s).

Ion nitriding of stainless steels. Real time surface characterization by synchrotron X-ray diffraction

Abstract Three types of steels with different Cr and Ni contents were ion nitrided by means of 100 Hz glow discharge in a N 2 -H 2 gas mixture using a special reactor installed in the SAXS beamline of the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. The studied materials were austenitic (AISI 304 and DIN WNr 1.4882) and ferritic (SAE HNV3) steels. The experimental setup allowed the study of the structural evolution in situ and in real time of steel surfaces during the ion nitriding process itself, using the X-ray diffraction technique. The use of a well-collimated and intense X-ray beam of the synchrotron radiation source allowed successive short exposure diffraction patterns to be taken. A very fast ion nitriding process was verified for the three studied steels. The measurements concerning the nitriding kinetics of both austenitic steels indicated the formation of an S phase. Nevertheless, while in the AISI 304 steel, the S phase was later replaced by a compound layer of γ′-Fe 4 N, in the DIN WNr 1.4882 steel, the S phase remained until the end of the nitriding process. In both cases, CrN was formed during the process. Instead, in the ferritic SAE HNV3 steel, the S and CrN phases were not observed.

Thermal effect of ion implantation with ultra-short ion beams

Abstract Ion implantation with plasma guns operated in the detonation mode presents several differences from normally used low current ion implanter systems. The most important differences are the high power of the beams generated with the plasma guns owing to their pulsed nature on one hand, and the plasma environment in which the target is immersed during the process of implantation on the other hand. Both effects were studied in this work. The temperature profiles and their evolutions during and after nitrogen implantation in pure titanium, stainless steel and copper were investigated by using the finite differences method. The calculation for nitrogen ion implantation (fluence of 10 13 cm −2 and pulse time of 400 ns) in pure titanium, shows melting layers of 20 ns after the first 200 ns of implantation, with a fast cooling after the end of implantation. Thermal gradients of 1000 K μm −1 and a heating rate of 5 K ns −1 were also observed. Optical spectroscopy observations (real time spectroscopy) of the implantation region show a highly activated nitrogen plasma. Both effects can be of extreme importance in several applications such as, for example, titanium nitriding because of an extra temperature assisted absorption by the getter effect.

A study by emission spectroscopy of the active species in pulsed DC discharges

From the optical emission of and radiative states in the negative glow of DC pulsed discharges at pressures 1 - 4 Torr (133 - 532 Pa) and current densities , it has been determined that the vibrational excitation of the state deviates from a Boltzmann distribution and strong intensities of the second positive system (especially from v = 1) were also observed in the afterglow. Such strong emission is interpreted in terms of the growing of the vibrational distribution in the post-discharge region as the gas temperature is reduced. The ) states are mainly produced by electron collisions on ions under discharge conditions and quickly disappear in the afterglow as a result of - electron recombination.

Hydrogen permeation modification of 4140 steel by ion nitriding with pulsed plasmas

Abstract It is widely known that the hydrogen in steel produces embrittlement. This effect may cause the failure of the elements (confining walls, mechanical parts, etc.) whose surfaces are in contact with this gas or with processes in which hydrogen is continuously generated. In this work it is shown that the ion nitriding of the surface of AISI 4140 is a good mechanism to act as a barrier against hydrogen permeation in its bulk. The ion nitriding was performed using a square wave DC glow discharge. The development of a compound layer of iron nitrides was observed as the cause of the hydrogen permeation reduction. For equal duration of treatment, thicker compound layers were developed in higher discharge/post-discharge ratios in the square wave of the applied voltage onto the sample (cathode), with a greater reduction of hydrogen permeation coefficient as a consequence. Nevertheless, the permeation was not reduced to zero in any of the treatment conditions used. The results of the analysis of the permeation tests and the image of the photomicrographs showed that the existence of cracks, fractures, failures, etc. in the compound layer (pre-existing in the AISI 4140 steel) could be the cause of the residual hydrogen permeation. This can be attributed to the movement of the hydrogen through these defects diffusing through the original α-Fe phase of the non-treated steel.

Current distribution during the breakdown in a coaxial electrode system

The breakdown stage in a coaxial electrode system (similar to Mather‐type plasma focus) was studied in hydrogen for a wide range of pressures through the fast discharge of the energy stored in a parallel plate capacitor. A peak current Iep of 18.5 kA was reached in 60 ns for a 20 kV capacitor charge voltage, giving the typical initial current rise time obtained in normal plasma focus discharges. Thin current sheaths (≲2 mm thick) in the insulator region with current density ≳10 kA/cm2 were measured. These current sheaths always showed a well‐defined filamentary structure with the number of filaments depending linearly on Iep , with an observed current limit per filament If depending on the pressure P as I2f∼1/P.

Pulsed ion implantation of nitrogen in pure titanium

Abstract High current, short length ion beam pulses appear to be a new alternative for surface property modification of solids, due to the combined effect of ion implantation with induced fast heating-cooling which this process presents. The repetitive pulsed nitrogen implantation (with a low energy plasma focus) of pure titanium with different pulse lengths (300 and 400 ns), and fluences per pulse ranging between 1.4 × 1014 and 1 × 1015cm−2, with total accumulated fluences between 7 × 1014 and 1.6 × 1016 showed a surface heating effect with important compositional and physical changes in the layers close to the surface. XPS analysis showed TiN0.8 formation independent of the total range of fluences used, with an increase in the superficial microhardness, when short pulse lengths were used. A correlation between the N/Ti concentration ratio, the binding energy difference (Ti2p3/2 -N1s) and the x value in the stoichiometry of the TiN, compound formed was observed for the long pulse length case.

The influence of the insulator surface in the plasma focus behavior

The insulator (Pyrex glass pipe in our system) surface alteration suffered due to successive plasma focus discharges was found to be responsible for the improvement in stabilization in the plasma focus behavior. The development of microscopic conductive sites (∼1 μm in size), observed on the insulator surface due to the accumulation of successive discharges, increases the efficiency, by a metal‐insulator‐metal process, on current sheath buildup with an increment in current density during the breakdown. The influence of the surface in the early stage of the discharge, and its correlation with the intensity of the pinch, was studied by analyzing the Si concentration on the surface of the targets of AISI 304 exposed to the plasmas and ion beams generated in the discharge for different experimental situations.

Time evolution of Cr and N on AISI 304 steel surface during pulsed plasma ion nitriding

Abstract Chromium concentration on steels plays an important role in ion nitriding processes. In this work, the time variation of the concentration of the elements in the near-surface region of the AISI 304 (18% of Cr) stainless steel, during pulsed ion nitriding, was studied. The techniques used were the real time and in situ X-ray diffraction by Synchrotron radiation. Auger spectroscopy, Nuclear Reaction Analysis, and Conversion Electron Mossbauer Spectroscopy. Ion nitriding was performed with a 100 Hz square wave pulsed glow discharge, with different treatment times, in an atmosphere of 80% N2 and 20% H2 mixture, under a total pressure of 5.6 mbar. The high Cr concentration in the near surface layers (∼17 A) and the intermediate ‘S’ phase formation were explained through the nitrogen ion sputtering during the ion nitriding process.

The time variations of N2 active species in pulsed N2-H2 dc discharges

Time-varying intensities of N2 and N2+ bands under discharge and post-discharge conditions of a pulsed dc 2-H2 diode plasma have been analysed. It is shown that, at a constant discharge current, the discharge voltage and the substrate temperature continuously decrease with the introduction of H2 into N2. As a consequence the degree of ionization ne/[N2] keeps a nearly constant value. Under such conditions, a V-V excitation of N2(X,v) by H2(X,v) explains small increases in intensity of the N2 second positive system and N2+ first negative system in the discharge with about 5% H2 in N2. In the afterglow, the N2 and N2+ band intensities decrease with the amount of H2 let into N2 but a vibrational excitation of N2(C,v´) radiative states has been observed in the N2-H2 afterglow, just like in pure N2, indicating that pooling reactions of N2(X,v) vibrational ground-state molecules are always active in the post-discharge regime.