In plasma nitriding of austenite stainless steel experiments, nitrogen penetration depth profiles typically show a characteristic plateau. This is associated with the phase transition to expanded austenite and non-Fickian diffusion of nitrogen involving the trapping of nitrogen by Cr sites, baro-diffusion and concentration dependent diffusion coefficient mechanisms. However, recent studies on thermally annealed post-nitrided austenitic samples have revealed the presence of two plateaus in the nitrogen depth profile curves. This phenomenon is new and has never been considered, and it is of particular interest as it suggests the presence of additional processes distinct from those responsible for the formation of the first plateau during nitriding. This work aims to analyze the appearance, mechanisms and processes responsible for the formation of the second plateau in the nitrogen depth profile during post-nitriding thermal annealing. The primary hypothesis is that the second plateau forms due to nitride formation, which is supported by XRD spectra showing chromium nitride after thermal annealing. The new mathematical model was developed for nitriding with non-Fickian diffusion mechanisms and post-nitrided annealing. The simulations performed with the developed model demonstrated that, under specific conditions, the second plateau emerges, thereby confirming the model's capability to reproduce nitrogen depth profiles exhibiting two plateaus. In the paper, these conditions are analysed in detail, and the influence of various parameters (diffusion coefficient, internal lattice stresses, processing time, etc.) is quantitatively investigated and it reveals that the appearance of the second plateau is very sensitive to these parameters not only during simulations but also in experiments.

Electrical and fast imaging measurements are performed on an experimental device designed to study the dynamics of electric arcs in the DC regime. The work presented here investigates the relationship between electrical fluctuations and material ejection from the electrode surface. Cross-analysis of the electrical and imaging data reveals a significant correlation between electrical fluctuations in the kHz range and particle emission from the electrode surface. Different types of ejection are presented and a study of the dynamics of the ejected particles is carried out through the analysis of a statistically significant number of trajectories.

The present study focused on the use of a duplex surface treatment combining cold-spray deposition and plasma electrolytic oxidation (PEO) to produce an aluminium metal matrix composite coating, including dispersed alpha-Al 2 O 3 particles, with improved tribological properties. Al/alpha-Al 2 O 3 composite coatings were first deposited by cold-spray, with various thickness and proportion in alpha-Al 2 O 3 particles, and, then partially oxidized by PEO under various processing durations and sparking regimes (arcs or soft regime). The feasibility of cold-spraying thick, compact and adherent aluminium coatings containing well-dispersed alpha-Al 2 O 3 particles (up to 14 vol%) was demonstrated in this work. It was also pointed out that the addition of hard alpha-Al 2 O 3 particles into the spray composition tends to densify the deposited aluminium coating. This was related to a stronger peening effect which, in turn, decreases the growth kinetic of the subsequent PEO oxide layer. The presence of dispersed alpha-Al 2 O 3 particles was found to promote the formation of the corundum alumina phase in the PEO oxide layer by triggering the transition to the soft sparking regime earlier. It was also observed that dispersed alpha-Al 2 O 3 particles remain unaffected throughout the PEO oxide layers formed within the conventional arcs sparking regime while they undergo a morphological transformation within the specific soft sparking regime. This was explained by considering the different type of micro-discharges that initiate during each regime. Finally, sliding wear tests revealed that the incorporation of alpha-Al 2 O 3 particles into the cold-sprayed coating resulted in a slight decrease in the friction coefficient and the wear rate of the produced PEO layers.

A short-duration ultrasonic shot peening (USP) is used as a post-treatment to plasma electrolytic oxidation (PEO) coatings produced on a 2017 aluminium alloy under various sparking regimes (arc or soft) and time durations. USP is detrimental on PEO layers produced for short durations and under the arc regime because the typical pancake structure at the topmost surface is partially blasted. In contrast, PEO layers grown under the soft regime not only keep their structural integrity after short duration USP but also experienced a densification of their internal sublayer and a resurfacing of the sponge structure at the external sublayer. Under the specific PEO soft regime, the proposed duplex surface treatment makes the coating harder with a higher fracture toughness compared to the as-produced PEO coating.

Plasma electrolytic oxidation of aluminum (PEO) was performed with dispersed multi-walled carbon nanotubes (MWCNTs) into the electrolyte. The effect of the carbon nanotubes concentration, as well as that of the electrical processing conditions, on the PEO process was investigated. From the point of view of the electrical response of the process, results show that the “soft” sparking regime appears earlier in presence of MWCNTs. The process time at which the transition to the “soft” regime occurs decreases linearly with the increase in the MWCNTs concentration. Over a concentration threshold, depending on the applied pulse bipolar current waveform, the PEO process is inhibited. The dispersion of MWCNTs in the electrolyte results in thicker but more porous alumina layers. More specifically, the proportion of the α-alumina phase relative to the γ-alumina phase decreases with the increase in the MWCNTs concentration. Results are explained by considering the increased coating growth kinetics and lower heat accumulation from MWCNTs addition. Finally, the present study gives new insights into the right management of the PEO process under “soft” sparking regime with dispersed carbon-based nanoparticles in the electrolyte.

In the present study, the feasibility of the elaboration of alumina (Al2O3) - zirconia (ZrO2) composite coatings on steel was explored by combining cold-spray deposition and plasma electrolytic oxidation (PEO). More specifically, two different configurations were investigated. The first configuration consisted in successively cold-spraying an aluminium layer first and a zirconium layer second, giving rise to a Zr / Al metallic multilayer coating that was then partially oxidized by PEO. In this configuration, microstructural characterizations evidenced the formation of a ZrO2 / Zr / Al composite multilayer coating on steel. The second configuration consisted in simultaneously cold-spraying aluminium and zirconium into a (Zr, Al) metallic composite monolayer prior to its partial oxidation by PEO. In this case, results evidenced the formation of a (ZrO2, Al2O3) / (Zr, Al) composite multilayer coating on steel. Finally, the present study demonstrated that the duplex surface treatment combining cold-spray deposition and PEO method is viable as a new route to produce ceramic/metal composite coatings on various metallic alloys.

Plasma nitriding is used to increase the durability of steels. The wettability of nitrided surfaces is poorly studied. The ability of nitriding in modifying the wetting and evaporation of sessile droplets on austenitic stainless steel is studied. Transferred plasma for nitriding provides independent substrate biasing giving the opportunity to tailor the surface before and during the nitriding treatments. It was shown that a cleaning treatment (Ar-H2 plasma) produces topographical modifications of the surface by selective sputtering of the grains giving contact angles between 80 and 90 degrees. Similar results are obtained when the nitriding treatments are carried out with a rather high bias voltage. It is explained using the skewness (Ssk) topographical roughness parameter which reveals the asymmetry of the profile. The more the Ssk is, the higher the contact angle is. During nitriding treatments, the very high internal stresses leads to the formation of spikes at the grain boundaries. Pinning of the three-phase contact line on peaks localized at grain boundaries are responsible for high contact angles. For nitriding treatments carried out with lower bias, surface contamination occurs and a discontinuous oxynitride layer is formed. The nanostructure formed by these oxides gives contact angles up to 120 degrees.

Incorporation of carbon nanotubes (CNTs) into ceramic layers during plasma electrolytic oxidation (PEO) is promising for direct synthesis of composite protective coating on lightweight metallic alloys. In the present study, complementary characterization techniques were used in order to definitively assess the presence of CNTs through the thickness of a PEO alumina coating formed in a silicate-based electrolyte containing dispersed multi-walled carbon nanotubes (MWCNTs). Cross-checked results helped in evidencing that MWCNTs are partially incorporated in the thickness of the PEO alumina coating, and more precisely through the porous outer sublayer. They were mainly found in the form of scaffolds that line the interior walls of pores. To a lesser extent, some individual MWCNTs were found interlocked in the fine grain structure of alumina. The analysis of the specific Raman bands of MWCNTs also highlighted that incorporation is associated with an alteration of the structural integrity of the MWCNTs. It was also established that the increase in the amount of dispersed MWCNTs results in a thicker but more porous CNTs-enriched PEO coating. Based on the collected results, some precisions regarding the incorporation mechanisms were proposed.

In this study, contamination by oxygen species in nitrogen plasmas produced by the active screen system used for plasma nitriding has been investigated by optical emission spectroscopy in the spectral range of 200–900 nm. Temporal evolution of emission intensity of different species (N2, N2+, NO, OH, H, NH, and Fe) was monitored, as well as electrical characteristics (current and voltage) of the discharge produced by a pulsed unipolar power supply. In nitrogen plasma, it was found that the emission of oxygen-containing species (NO and OH) decreases with time, while Fe emission intensity increases. Such behavior is observed only when the discharge is initiated immediately after venting the reactor. Starting from the hypothesis of contamination of the reactor walls by water vapor, we propose an explanation based on the synergistic effect between the temperature and the reactive nitrogen created by the plasma. Such a long decay of NO emission was not observed in N2–H2 gas mixtures. After decontamination, the fourth positive system of N2 could be observed. Such a not commonly observed system can be used as a tool to control reactor cleanliness in pure nitrogen discharge for nitriding applications.

The dry etching process of Fe, Cr and Fe-Cr alloys under a chlorine-based plasma is studied. The objective is to create new surface functionalities. The approach combines an experimental study of an ICP (Inductively Coupled Plasma) reactor with the development of a multi-scale etching model including kinetic, sheath and surface models. The results from plasma etching of substrates made of Fe, Cr and Fe-Cr alloys are presented. Optical emission spectroscopy and interferometry measurements show strong modifications of the plasma when Fe or Cr samples are present in the reactor. It is shown that Fe is easier to etch than Cr. The study highlights the role of chemical etching by the formation of volatile products such as FeCl3. The Cr content in Fe-Cr alloys has a strong impact on both the lateral and vertical etch rates, as well as on the roughness along the profile. For Fe-Cr alloys, the experimental and calculated values of etch rate are very similar. The concept of hard zones is introduced to get a better agreement between simulation results and experimental ones. This good agreement demonstrates the capability of the developed simulator to implement new phenomena.

In this study, different configurations of hard micro-patterns on AISI 316LM samples are obtained by a dry etching step followed by plasma nitriding. A "ball-on-plane" tribometer was used for low velocity reciprocal dry friction tests in order to characterise the influence of configurations on asperity lifetime, friction coefficient and wear mechanisms. The distance between asperities plays a key role in friction and wear behaviours by acting on contact pressure and development of debris. For a given ball dimension, pattern configuration determines the debris management at the contact and along the wear track. With 100 mu m spacing between asperities, most part of the debris are simply trapped. At 300 mu m, wear mechanisms are more complex with a debris recycling by the ball.

Expanded austenite, also known as S phase, m phase or gamma(N) phase forms during low temperature (<420 degrees C) nitriding of austenitic materials containing chromium. In this communication, annealing of a nitrided sample is used as a tool to understand the nature of expanded austenite. Starting from an AISI 316LN sample plasma nitrided for 4 hat 410 degrees C, the decomposition of expanded austenite is studied by conversion electron Mossbauer spectroscopy (CEMS) and X ray diffraction (XRD) for different annealing times at 400 degrees C. For the as nitrided sample, the CEMS spectra are analysed using two hyperfine field distributions (HFD). HFD A is attributed to a nitrogen enriched expanded austenite, HFD B is attributed to a bcc environment without or with very low nitrogen content. Both the local deformation of the bcc lattice and the presence of other elements as iron neighbouring atoms explain the broad range of hyperfine field in HFD B. The vanishing of HFD A, after 2 h of annealing, is explained by the formation of short-range order (SRO) between chromium and nitrogen and by the redistribution of nitrogen in excess. For the 3 h of annealing, a bcc phase is observed by XRD. We propose to describe this phase as martensite. An interplay between local internal stress and local nitrogen content is proposed to explain the unexpected formation of such a martensitic phase in a strongly gamma stabilizing environment. From the microscopic point of view expanded austenite can be considered to be constituted of at least two different environments: a gamma environment supersaturated in nitrogen and a martensitic environment without nitrogen. (C) 2019 Elsevier B.V. All rights reserved.

In this paper, we present the results of plasma nitriding treatments on austenitic stainless steel substrates previously coated with a patterned silicon oxide layer. For this purpose, masks were made by PECVD for the deposition of a silicon oxide layer on polished austenitic AISI 316L samples. For the final nitriding treatment, we used a multi-dipolar plasma providing independent substrate polarization. The interactions between expanded austenite and fixed silicon oxide mask in different shapes (circular and square dots) are observed by atomic force microscopy (AFM) on the same area before and after the nitriding treatment. After this thermochemical treatment, we obtain strong distortions of the dots, in particular at the edges of the larger size dots. The role of elastic deformation, due to the expanded austenitic phase formed by the diffusion of nitrogen under the mask is of primary importance.

Plasma-assisted diffusion treatments are widely used in manufacturing for surface hardening of ferrous and no ferrous materials. In particular, tribological properties of austenitic stainless steels (AAS) are improved by the formation of the so-called expanded austenite containing large amount of nitrogen or carbon. Depending on the conditions, it is commonly observed that the superficial content in nitrogen or carbon reaches a stationary value only for long times (some hours) during plasma assisted diffusion treatments. This phenomenon is difficult to observe during plasma assisted nitriding of ferritic steels due to the formation of iron nitrides with well defined composition. Low temperature nitriding of AAS offers the chance to reach a very wide range of superficial nitrogen content. In this communication, we will present and compare the time evolution of the superficial content in interstitial elements for different plasma assisted diffusion treatments: nitriding, carburizing and nitrocarburizing. These treatments were performed on AISI 316L in a distributed electron cyclotron resonance plasma reactor. In such a system, the sample holder can be independently heated and biased and so sputtering due to ion bombardment of the surface is limited. The interstitial element contents are measured by Glow Discharge Optical Emission Spectroscopy. For nitriding, the nitrogen superficial content exponentially increases to reach a stationary value at 9 h of treatment. This behaviour is modelled by using gain and loss terms source. Diffusion and recombination of nitrogen atoms on the surfaces are taken into account to describe the observed exponential law. Such non constant superficial nitrogen content has important consequences on the nitrogen distribution in the depth of the nitrided layer. For carburizing, carbides formation occurs very early in the process and the superficial carbon content can be considered as constant. For nitrocarburizing, the nitrogen superficial content also increases slowly with the treatment time, but the range of variation is much lower as compared to nitriding. Finally, a discussion on the role of different surface mechanisms will be given in order to optimize this kind of process.

Upon nitriding of a Fe-3%Cr alloy at 550 degrees C during 48 h at a nitriding potential K-N = 5 atm(-1/2), finely dispersed CrN grains precipitate. Considering that the nitrides developing within the nitrided layer adopt a cubic, rock-salt structure, Transmission Electron Microscopy (TEM) investigations revealed the Bain orientation relationships (also called Baker-Nutting) existing between the matrix and the precipitates. Given the latter and the structures of both ferrite (bcc) and nitrides (fcc), we have shown that group theory predicts the existence of three nitrides variants, orthogonal to each other, developing in ferrite along the < 001 >(alpha-Fe) directions, which is in agreement with our experimental observations. The nitrides adopt a very thin platelet-like morphology, energetically corresponding to an absolute extremum. Diffraction patterns obtained along the [001](alpha-Fe) result from the simultaneous diffraction of the ferritic matrix and the three nitrides variants. The platelet-like nitride precipitates give birth to diffuse intensity lines, originating from the two families developing along the (100) and (010) ferrite planes, as well as possible appearance of additional reflections, located at the forbidden positions {100}(alpha-Fe) and {110}(crN) of the remaining variant, driving the interpretation of such diffraction patterns complicated. Foil thickness has proven to play a key role in the occurrence of this phenomenon. When the geometric diffraction conditions are favorable (ie. very thin foil along the zone axis and very fine precipitates platelets), TEM analysis permitted to attribute these extra reflections to the nodes located in the first layer of the third variant's reciprocal space.

Surface texturing the friction faces of a mechanical seal reduces friction as well as leakage and wear, where dimples machined on the surface enhance the formation of a full lubricating film in the seal interface. Numerical simulation is used to determine the optimal surface texturing for the investigated operating conditions of the seal. The geometry of the dimples introduced in the model assume a perfect shape, but machining causes the dimple shapes to be imperfect in terms of the roughness in the dimples, absence of sharp angles, deformed boundaries, and so on. The effect of the real geometry must be considered to confirm that surface texturing will provide the desired results. In the present work, surface texturing is performed using low-temperature plasma coupled with a thermo-chemical surface treatment on stainless steel sealing rings. The real dimple shapes are analysed and considered within the hydrodynamic lubrication model. The influence of different types of defects is studied. It is shown that there is a limit above which surface imperfections dispel the texture's positive effects. Controlling the dimple shapes is important when performing surface texturing.

AISI 316 LM samples were plasma nitrided at a temperature of 380°C for different times between 0.5 and 8 h in a 85%N2- 15%H2 gas mixture. Different experimental techniques such as: optical microscopy (OM), X- ray diffraction (XRD ) and glow discharge optical emission spectroscopy (GDOES) were used to characterize the expanded austenite layer formed at the surface of AISI 316 LM stainless steel. The microscopical observations revealed the presence of the expanded austenite with a mean layer thickness between 1.90 and 4.31 μm. The growth kinetics of expanded austenite was also investigated. In addition, both the compressive stresses in the expanded austenite layer and the compositional strains were estimated by means of a simple mechanical model based on the XRD results.

This communication focus on the evaporation of sessile water droplets on different states of austenitic stainless steel surfaces: mirror polished, mirror polished and aged and patterned by sputtering. The evolution of the contact angle and of the droplet diameter is presented as a function of time at room temperature. For all the surface states, a constant diameter regime (CCR) is observed. An important aging effect on the contact angle is measured on polished surfaces due to atmospheric contamination. The experimental observations are compared to a quasi-static evaporation model assuming spherical caps. The evolution of the droplet volume as a function of time is almost linear with the evaporation time for all the observed surfaces. This is in accordance with the model prediction for the CCR mode for small initial contact angles. In our experiments, the evaporation time is found to be linearly dependent on the initial contact angle. This dependence is not correctly described by the evaporation model

Surface patterning, also known as surface texturation or surface structuration, is part of the surface engineering that consists in the production of a "patterned" surface with some regular array of surface height features on the size scale of micrometers to some nanometers. Patterned surfaces of metals have many potential applications and particularly in the thermal management for heat exchangers for instance. Robust and efficient surface patterning manufacturing methods are existing but alternative cheap and flexible technologies are needed to satisfy the vast demand for emerging applications. Plasma technologies such as nitriding and etchning by ion bombardment are well adapted technologies for that purpose. This communication will focus on the evaporation od sessile water droplets on differentstates of metallic surfaces modified by plasma treatments. We will present the time evolution of the contact angle and of the droplet diameter as a function of time for different temperatures ranging from ambiant to 120 °C. Different surface states of austenitic stainless steel AISI 316L were investigated: mirror polished, nitrided with a resulting honeycom-like structure and patterned by ion sputtering using masks. Two different regimes for the evaporation were observed at low temperature : a constant diameter regime and a quasi-constant contact angle regime. The data are well described by theoretical models for evaporation on conventional surfaces. This is not the case for patterned surfaces, for which many small transition regimes, corresponding to local jump of the triple line, were observed.