Sonia P. Brühl

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.

Wear behavior of plasma nitrided AISI 420 stainless steel

Abstract Martensitic AISI 420 stainless steel was plasma nitrided using a DC N2 – H2 pulsed discharge, after thermal treatment at two different tempering temperatures. Sliding wear behavior was determined by means of a ball-on-disk tribometer at room temperature and without lubrication. Wear volume was calculated from the information provided by an optical profilometer and wear scars were analyzed by Scanning Electron Microscopy. The results indicated that the nitrided samples exhibited improved wear resistance when compared to the untreated specimens, but corrosion resistance was diminished due to the presence of the outer nitrided layer. However, when this layer was removed mechanically considerable improvement in the corrosion resistance was achieved without impairing the wear resistance of the whole system, which was 200 % higher compared to the wear resistance of the non-nitrided specimens.

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.

Behavior of the pulsed ion nitrided AISI 4140 steel/CVD TiN coatings as tribological pair

Abstract The aim of the present investigation was to determine the effect of pulsed plasma nitriding process on the wear behavior of an AISI 4140 steel. Wear tests were performed using a ball-on-disc configuration with a linear sliding speed of 0.1 m s−1, 5 N load and 700 m sliding distance. The tests were carried out at room temperature in air, without lubrication, by employing CVD TiN coated balls of 6 mm in diameter. The surface morphology and topography of the wear scars of samples and balls have been determined by using both scanning electron microscopy and three-dimensional stylus profilometry. Different wear mechanisms were detected such as abrasion, adhesion and oxidational wear for the pairs under study. Results have showed an improved wear resistance of the nitrided samples compared to the AISI 4140 steel samples.

Generation of residual deformations by pulsed ion implantation

The generation of residual deformations on AISI 316 stainless steel submitted to pulsed ion implantation by means of a plasma focus device was investigated. It is shown that the residual deformation can be attributed to thermal stresses induced in the target by the strong and fast heating process due to the accelerated ions. A numerical simulation is presented as an attempt to explain the generation of these deformations. The model is based on the computation of the temperature distribution in the implanted zone and the calculation of the induced thermal stresses. The mechanical response of the whole specimen is then evaluated using the finite-element method. Numerical results are compared with those measured by means of a holographic method and reasonable agreement is obtained.

Tribological behaviour of nitrided and nitrocarburized carbon steel used to produce engine parts

Purpose – The purpose of this study is to select a proper surface treatment to enhance wear resistance of engine camshafts. The camshaft is a relevant part of a diesel engine which works under torsion, fatigue and wear efforts. They are usually manufactured by casting, forging or machining from forged bar of low alloy steels, and in most cases, the machined surfaces are quenched and tempered by induction heating. After that, in many cases, to withstand the efforts imposed on the active surfaces and improve tribology and fatigue properties, the industry used for decades, thermochemical technologies such as salt bath or gaseous nitriding and nitrocarburizing processes. Design/methodology/approach – This paper studied the effects of plasma nitriding and plasma nitrocarburizing, on the tribological behaviour of the steel SAE 1045HM3 proposed to produce camshafts. After the plasma treatments, the change in surface roughness was measured; the modified layers were studied by X-ray techniques and its thickness by...

Evaluation of residual deformations generated by a pulsed ion implanter using interferometric phase measurement

Abstract A Fourier transform method of holographic fringe pattern analysis is applied to measure surface residual deformations generated by a pulsed ion implanter. The technique uses a fixture that makes it possible to remove the specimen and put it back into the same position after being implanted. The phase information from interferograms extracted by means of the Fourier transform method is unwrapped using an algorithm based on cellular automata. Results computed from the application of a numerical model are compared with those determined experimentally and a reasonable agreement is obtained.

Power voltage source synchronized with a spectrometer for real-time spectroscopic studies of pulsed discharges

The evolution of the species generated in a plasma may be characterized through real-time optical emission spectroscopy. Nevertheless, for experimental situations in which the light emission intensity is insufficient for acquisition times of several tens of milliseconds (typical resolution time of optical multichannel analyzers), some complementary instrumentation has to be incorporated. In this paper, we present the design of an experiment to study the spectroscopic evolution of a plasma during a switch-on of a dc discharge, and/or during its switch-off (afterglow). The solution consisted of a periodic discharge generated by a dc square wave voltage source, synchronized with the optical multichannel analyzer of the spectrometer through its TTL (transistor-transistor logic) level pulse. The periodicity of the discharge under similar physical conditions allows accumulation the spectroscopic information for each cycle, making possible a time resolution of tens of milliseconds by increasing the resulting intensity profile of the spectra.

Holographic interferometry applied to the study of residual deformations induced by a pulsed ion implanter

Abstract The application of holographic interferometry to study the residual deformations induced in steel specimens by a pulsed ion implanter is presented. The proposed technique is non-destructive and non-contact. It is based on the use of a fixture which makes it possible to remove the specimen and to put it back into the same position after being implanted. A numerical model to calculate the residual deformations is evaluated by comparing with the results obtained holographically. The experimental behavior correlates with that predicted by the numerical computation results.

Carbon Based Coatings Deposited on Nitrided Stainless Steel: Study of Thermal Degradation

Amorphous hydrogenated carbon (DLC) coatings have a high hardness depending on the relative amount of sp3/sp2 bondings. They also exhibit an extremely low friction coefficient and are chemically inert. However, these coatings have some disadvantages which limit their applications. For instance, adhesion is poor when they are deposited on metallic substrates and they are also unstable at high temperatures, degrading into graphite and loosing hardness. In this work, DLC coatings were deposited on precipitation hardening stainless steel (PH Corrax) which was plasma nitrided before the coating deposition. The samples were submitted to annealing treatments for an hour at different temperatures from 200 to 600 °C, together with a control group, which was only coated but not nitrided. After each annealing cycle, Raman Spectroscopy, nanoindentation and microscopy were used to check film properties. It was demonstrated that the nitriding pre treatment improved not only adhesion but also the thermal stability of the DLC, slowing degradation and preventing delamination.