Le développement de la plaque bipolaire - PB -métallique nécessite une amélioration des propriétés du matériau qui la constitue. L'utilisation de l'acier implique une meilleure compréhension du film passif - FP. En effet, le cahier des charges de la PB demande une bonne conduction électronique ainsi qu'une longue durée de vie. L'amélioration de ces paramètres passe par une meilleure corrélation et compréhension entre les propriétés semi-conductrices et la résistance à la corrosion. La difficulté liée à l'étude de la passivation réside dans les nombreux phénomènes modifiant le comportement du film passif. De nombreux modèles présentés dans la littérature peuvent être utilisés pour caractériser la passivation. L'un des principaux, le Modèle de Défauts Ponctuels de D. D. Macdonald (Point Defect Model - PDM), décrit le FP à partir d'une série de réactions électrochimiques se déroulant à l'interface métal/oxyde et à l'interface oxyde/électrolyte. La réactivité est limitée par le transport de matière à travers l'oxyde. Après une étude bibliographique, ce phénomène semble être un paramètre discriminant dans le choix des modèles. Une méthode de sélection permettant une utilisation de chacun de ces modèles en fonction de leurs spécificités est proposée. Ainsi, cette méthode est basée sur la corrélation entre la mesure du transport de matière à partir des équations du PDM et l'analyse des Spectres d'Impédance Electrochimique (SIE). Le PDM caractérisant le transport de matière indépendamment de la SIE, il devient possible de déterminer le bien-fondé de la prise en compte de celui-ci dans les mesures de spectroscopie d'impédance électrochimique. L'évolution de la densité du porteur de charge majoritaire avec le potentiel de formation de l'oxyde permet d'accéder au coefficient de transport à partir des équations du PDM. Connaissant l'épaisseur de l'oxyde par des mesures XPS, la constante de temps et la fréquence caractéristique peuvent être déterminées. Sur la base de ces deux valeurs, un modèle descriptif d'analyse des spectres est sélectionné en évitant le sur-paramétrage. Cette méthode est appliquée d’abord à un matériau modèle, le chrome pur exposé à un milieu acide (pH 2), à différentes températures (30°C et 80°C). Elle valide la nécessité de la prise en compte du transport de matière à 80°C ainsi que la présence d'une surface composée d'une couche d'oxyde interne et d'une couche d'hydroxyde externe. Dans un second temps, cette méthode est utilisée sur un matériau industriel, un acier de type AISI 316L, à différents pH (1,2 et 3) et à différentes températures (30°C et 80°C). Elle a permis de décrire l'oxyde en surface comme une jonction p-n prenant en compte une couche riche en chrome interne avec un gradient de concentration de fer. Cette méthode a permis de caractériser de manière approfondie l'acier de type AISI 316L. Bien que ne concernant que le substrat, cette étape est déterminante dans l'amélioration des performances des PB métalliques. / Developments in metallic bipolar plate, to apply more widely fuel cells, require an improved of the constitutive material. The use of stainless steel calls for a good understanding of the passive film. The required specifications are for good electrical conductivity and a long life-time. Those two parameters correspond to a correlation between the semiconductive properties and the good corrosion behavior. Nevertheless, the main problems of the passivity lie on the multiplicity of the phenomena that alter the passive film behavior. Numerous models described in the literature can be used to characterize the passivation. The Point Defect Model (PDM) describes the passivation through electrochemical reactions at the metal / oxide and at the oxide / electrolyte interfaces. The reactivity is limited by mass transport through the oxide. From the literature, those phenomena seems to be a discriminating parameter in the choice of a model. The selective method proposed allows us to use each model taking into account their specifics. This methodology is based on the correlation between the mass transport characterization, thanks to the PDM, and the analysis of the Electrochemical Impedance Spectroscopy (EIS). The PDM determines the transport coefficient apart from EIS measurements, so to validate the consideration of the mass transport during the analysis of the electrochemical impedance spectra. The evolution of the main charge carrier density as a function of the oxide formation potential allows us to calculate the transport coefficient from PDM equations. Thanks to the thickness of the oxide (determined by X-ray Photoelectron Spectroscopy), the time constant of the mass transport is determined. Based on this value, a descriptive model is used to analyze the EIS data, avoiding overparametrization. This method is applied first on a model material, pure chromium exposed to acidic solution (pH 2) at several temperatures (30°C and 80°C). It shows that the mass transport has to be taken into account at 80°C and the EIS model considers an inner chromium oxide layer and an outer chromium hydroxide. Secondly, the method is used to characterize an industrial material, AISI 316L stainless steel, at several pH (1, 2 and 3) and at several temperatures (30°C and 80°C). In this case, the oxide is describe as a p-n semiconductor junction with an chromium rich inner layer and an outer iron rich layer. The present methodology permits to deeply characterize the AISI 316L stainless steel. Even if this study concerns the substrate, this step is decisive to improve the performances of the metallic bipolar plates

Gas nitriding is known as a convenient process to improve the wear resistance of steel components. A precipitation-free hardening by low-temperature processes is established to retain the good corrosion resistance of stainless steel. In cases of thermal spray coatings, the interstitial solvation is achieved without an additional surface activation step. The open porosity permits the penetration of the donator media and leads to a structural diffusion. An inhomogeneous diffusion enrichment occurs at the single spray particle edges within the coating’s microstructure. A decreasing diffusion depth is found with increasing surface distance. The present study investigates an adjusted process management for low-temperature gas nitriding of high velocity oxy-fuel-sprayed AISI 316L coatings. To maintain a homogeneous diffusion depth within the coating, a pressure modulation during the process is studied. Additionally, the use of cracked gas as donator is examined. The process management is designed without an additional surface activation step. Regardless of surface distance, microstructural investigations reveal a homogeneous diffusion depth by a reduced processing time. The constant hardening depth allows a reliable prediction of the coatings’ properties. An enhanced hardness and improved wear resistance is found in comparison with the as-sprayed coating condition.

Active screen plasma nitrocarburizing (ASPNC) increases the surface hardness and lifetime of austenitic stainless steel without deteriorating its corrosion resistance. Using an active screen made of carbon opens up new technological possibilities that have not been exploited to date. In this study, the effect of screen power variation without bias application on resulting concentrations of process gas species and surface modification of AISI 316L steel was studied. The concentrations of gas species (e.g., HCN, NH3, CH4, C2H2) were measured as functions of the active screen power and the feed gas composition at constant temperature using in situ infrared laser absorption spectroscopy. At constant precursor gas composition, the decrease in active screen power led to a decrease in both the concentrations of the detected molecules and the diffusion depths of nitrogen and carbon. Depending on the gas mixture, a threshold of the active screen power was found above which no changes in the expanded austenite layer thickness were measured. The use of a heating independent of the screen power offers an additional parameter for optimizing the ASPNC process in addition to changes in the feed gas composition and the bias power. In this way, an advanced process control can be established.

Le micro-fraisage (diamètre fraise < 1 mm) permet l’usinage précis de structures en 3D, à des dimensions micrométriques, dans desmatériaux d’ingénierie, se plaçant aux frontières de deux mondes : d’une part, le fraisage traditionnel appelé « fraisage macro » et d’autre part,la micro-fabrication et ses techniques dites de « salle blanche ».L'étude innovante porte sur le micro-fraisage d’aciers inoxydables 316L avec des micro-fraises cylindriques en carbure de tungstèneavec un équipement industriel (machine outil commercialisée et non optimisée) permet d’accentuer les nombreuses difficultés technologiquesliées à la mise en oeuvre du micro-fraisage et d’effectuer directement un transfert de compétences vers l’industrie. L’acier 316L(biocompatible, réputé de difficilement usinable) n’a jamais été étudié en micro-fraisage.L’étude aborde, au travers de neufs ratios caractéristiques du micro-fraisage, les problématiques de choix de moyens et de méthodespour caractériser la technique du micro-fraisage.Après analyses des paramètres de l’étude et des caractérisations des usinages, la définition géométrique optimale d’une micro-fraiseinnovante est proposée. Sa tenue en service est validée par des tests en usinage dans l’acier 316L, répondant ainsi, à une problématique decoupe négative à basse vitesse de coupe avec des effets d’échelle du matériau.Une originalité de l’étude est d’aborder l’effet de la population inclusionnaire visant à améliorer l’usinabilité. En comparant lesrésultats obtenus par micro-fraisage de 2 nuances d’acier 316L, la population inclusionnaire de l’acier 316L n’est pas identifiée comme unfacteur améliorant l’usinabilité à l’échelle de la coupe micro. / The micro-milling ( tool diameter < 1 mm) target the precise machining of 3D structures to micrometric dimensions, in engineeringmaterials, to be placed at the borders of two worlds : the one hand , the traditional milling called "macro milling" and other hand, the microfabricationand its so-called "clean room" techniques.The innovative study focuses on the micro-milling of 316L steel with carbide micro end mills with industrial equipment (machine toolmarketed unoptimized) can caricature the many technological challenges related to the implementation of the micro-milling and make a directtransfer of skills to the industry. 316L steel (biocompatible, reputed difficult to machine) has never been studied in micro-milling.The study looks at ratios through new features of the micro-milling, the problems of choice of means and methods to characterizemicro-milling.After analysis study parameters and machined parts, the optimal geometric definition of an innovative micro end mill is proposed.Service behavior is validated by testing machining in 316L steel, responding to a question of negative cutting with low cutting speeds andscale effects of the material.An originality of the study is to address the effect of the inclusion population to improve machinability. Comparing the resultsobtained by micro-milling two 316L steel grade, the inclusion population of 316L steel is not identified as a factor improving themachinability cutting at micro scale.

Thermochemische Randschichthärteverfahren ermöglichen eine ausscheidungsfreie Einlagerung von Kohlenstoff bzw. Stickstoff innerhalb des austenitischen Mischkristalls. Im Zusammenhang mit einer Randschichtbehandlung thermisch gespritzter Schichtsysteme stellen die charakteristischen Strukturmerkmale eine bislang weitgehend unerforschte Einflussgröße für die Beurteilung von Diffusionsprozessen dar. Bei der Verarbeitung von randschichtgehärtetem Pulver durch Verfahren des thermischen Spritzens ist die Phasenstabilität des Spritzzusatzwerkstoffs von übergeordneter Bedeutung. Die beiden Möglichkeiten einer Verfahrenskombination werden für hochgeschwindigkeitsflamm- und atmosphärisch plasmagespritzte Schichtsysteme des Werkstoffs EN 1.4404 durch systematische Prozess- und Parametervariation eingehend betrachtet. Für die einzelnen Schichtsysteme werden Einflussfaktoren struktur- und prozessspezifisch sowie in Abhängigkeit vom Anreicherungsmedium erfasst und im Kontext der Massivwerkstoffreferenz eingeordnet. Die daraus abgeleiteten allgemeingültigen Aussagen zu verfahrenstechnischen Wechselwirkungseffekten ermöglichen eine anwendungsorientierte Verfahrensauswahl bzw. Entwicklungsstrategie. / Thermochemical surface hardening enables a precipitation-free solvation of carbon or nitrogen on interstices of the austenitic crystal lattice. However, the interplay of the diffusion mechanisms with the structural properties of thermal spray coatings has not yet been understood. Thermal spraying of surface-hardened powders is a further opportunity, where the phase stability of the feedstock material is of crucial importance. A process and parameter study is conducted on high velocity oxy-fuel and atmospheric plasma spraying of AISI 316L considering both basic concepts. Structural and process-specific influence factors are examined for the different coating systems in comparison to the bulk material reference. Correlation effects are determined allowing for an application-oriented process selection or development strategy.

Today's economic and ecological directives demand for highly sustainable machine parts by low production cost and energy consumption. Consequently, it is crucial to guarantee a long service life by protecting all components against wear and corrosion. However, hydraulic components always include stressed surfaces, which suffer from heavy loads at high relative speeds. To prevent fretting, coating processes like thermal spraying or hard chrome have a long history in the field of hydraulics. New additive laser-based processes like EHLA and LPBF offer the potential to apply new coatings without environmentally hazardous substances such as chromium or to manufacture complex parts with new functionalities. So far, additively manufactured surfaces with relative movements are post-processed to obtain surface qualities similar to subtractive methods, as the tribological properties of additive surfaces have not been investigated to date. Therefore, this paper investigates the frictional behavior of 316L surfaces produced by laser-based EHLA and LPBF processes using a disc-disc tribometer.

A solution-hardening of AISI 316L stainless-steel powder was conducted. The expansion of the crystal lattice and a strong increase in the nanoindentation hardness confirm the successful diffusion of carbon and nitrogen in the interstices. A multiphase state of the powder feedstock with phase fractions of the metastable S-phase (expanded austenite) mainly at the particle’s edge, and the initial austenitic phase within the core was found. Thermal spraying using high velocity oxy-fuel (HVOF) and atmospheric plasma spraying (APS) prove the sufficient thermal stability of the Sphase. Microstructural investigations of the HVOF coating reveal the ductility of the S-phase layer, while the higher heat load within the APS cause diffusion processes with the initial austenitic phase. The lattice expansion and the nanoindentation hardness decrease during thermal spraying. However, the absence of precipitates ensures the sufficient heat stability of the metastable S-phase. Even though further efforts are required for the thermochemical treatment of powder feedstock, the results confirm the feasibility of the novel powder treatment approach.

Additive manufacturing (AM) has the potential to overtake the conventional production of pipe systems of stainless steel grade 316L intended for bioprocessing applications by utilising powder bed fusion laser beam (PBF-LB) technologies. However, challenges such as high surface roughness need to be overcome and corrosion properties improved to meet industry standards. This can be done by applying surface post-treatments to the printed parts. Therefore, the first aim of this thesis was to investigate the ability of different post-treatments of PBF-LB 316L samples, including tumbling, electropolishing, chemical milling, Hirtisation®, and DryLyte®, to reduce the surface roughness. One set of samples was left untreated, as-built set, and used as reference. The second aim was to try to correlate the surface roughness with the corrosion properties of the various post-treated samples. Analytical techniques used included needle profilometry, light optical microscopy (LOM), critical pitting temperature (CPT), cyclic potentiodynamic polarisation (CPDP) and immersion in 1M NaCl and 1M NaOH for two different durations for each solution. While some results were expected, others were surprising. Even though tumbled samples resulted in the lowest surface roughness, the results for immersion and CPDP curves tests indicated the lowest corrosion resistance compared to the other post-treatments. On the other hand, electropolishing resulted in the lowest surface roughness reduction among the post-treated samples. However, these samples had comparatively high corrosion resistance in the CPT and CPDP tests. The DryLyte® samples were tested using two DryLyte® medias where one of them resulted in the second lowest surface roughness and showed highest corrosion resistance in all three corrosion tests even though the process parameters are still to be developed. The results for samples post-treated by Hirtisation® and chemical milling were inconsistent and need further investigations. The reference as-built set showed high corrosion resistance in its untreated state, indicating post-treatments having minor impact on its performance from a corrosion perspective. / Additiv tillverkning (AM) har möjlighet att konkurrera ut konventionell produktion av rörsystem i rostfritt stål typ 316L avsett för bioprocessapplikationer genom att använda pulverbäddsinfusionslaserstråleteknik (PBF-LB). För tekniken kvarstår dock utmaningar för att uppfylla branschstandarder då tillverkningen medför material med hög ytråhet och sämre korrosionsegenskaper än traditionella metoder. Detta kan åstadkommas genom olika typer av efterbearbetning av de printade ytorna. Ett syfte med denna studie var att utvärdera hur olika typer av efterbehandling av PBF-LB 316L (trumling, elektropolering, kemisk etsning, Hirtisation® och DryLyte®) påverkar ytråheten. Som referensmaterial användes obearbetade printade ytor. Studiens andra huvudmål var att undersöka möjligheten för korrelation mellan ytråhet efter behandling och korrosionsegenskaperna. Analyser gjordes med hjälp av nålprofilometri, ljusoptisk mikroskopi (LOM), bestämning av kritisk gropfrätningstemperatur (CPT), framtagning av cykliska potentiodynamiska polarisationskurvor (CPDP) samt nedsänkning i 1M NaCl och 1M NaOH under två olika tidsperioder per lösning. Resultaten var delvis väntade och delvis överraskande. Trots att de trumlade proverna resulterade i lägst ytråhet, visade de elektrokemiska studierna (CPDP) i de olika lösningarna på att lägre korrosionsbeständighet jämfört med övriga efterbehandlingar. Å andra sidan resulterade elektropolering resulterade i den lägsta minskningen i ytråhet bland de efterbehandlade proverna. Dessa prover visade dock på en jämförelsevis hög korrosionsbeständighet i både CPT och CPDP testerna. DryLyte®-proverna studerades med två olika DryLyte®-medier, varav en av dem gav näst lägst ytråhet samt visade högst korrosionsbeständighet i samtliga korrosionstester trots att processparametrarna inte ännu är optimerade för denna nya teknik. Resultaten för prover som efterbehandlats med Hirtisation® och kemisk etsning var tvetydiga och behöver studeras vidare. De obehandlade referensproverna indikerade hög korrosionsbeständighet, vilket tyder på att efterbehandlingar hade minimal påverkan ur ett korrosionsperspektiv.

Electron beam powder bed fusion (PBF-EB) is an additivemanufacturing (AM) method based on layer-by-layer melting of apowder bed. The technology is industrialized in certain applicationsbut still considered as immature and is not as widely used as laserbeam-based systems (PBF-LB). PBF-EB can offer several benefits overPBF-LB such as process cleanliness, thermal efficiency, fast beam speed,higher power and energy transfer, low residual stresses in built partsand a good signal environment for process monitoring. This can beadded on top of the general benefits of AM such as geometricalfreedom, manufacturing efficiency, easy design revisions, short leadtimes and so on. This suggests that PBF-EB holds potential as atechnology for the sustainable production of materials andcomponents. This thesis investigates how PBF-EB can be furtherdeveloped to create new and unique materials features. This isachieved by introducing innovative methods for material processingand by further developing the PBF-EB process itself. The thesisintroduces a charge-free heating method for PBF-EB and the resultssuggest an enhanced processability of difficult-to-process materialsand powders. A method for building multi-materials in PBF-EB isintroduced and demonstrated by the manufacturing of direct andlamellar transitions between different alloys. Methods for processmonitoring and powder bed resistivity evaluation are proposed andxiidemonstrated. It is concluded that the results presented in this thesisenabled new PBF-EB processing modes, increased the knowledge ofthe process, and introduced a new material group by demonstratingthat ceramics can be processed at high temperatures (~1600C). / <p>Vid tidpunkten för framläggningen av avhandlingen var följande delarbeten opublicerade: delarbete 2 och delarbete 4 (inskickat).</p><p>At the time of the defence the following papers were unpublished: paper 2 and paper 4 (submitted).</p>

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