Coatings are often used on critical components in machines and engines to reduce wear and to provide low friction in order to reduce energy losses and the environmental impact. A triboactive coating not only provides this desired performance, it also actively maintains the low friction by a structural or chemical change in a very thin top layer of these already micrometer thin coatings. This so-called tribofilm is often 5-50 nm thick and can be formed either from the coating itself or by a reaction with the counter surface or the surrounding atmosphere, i.e. gas, fuel, oil, etc. The tribofilm will maintain the wanted performance for as long as the system is not chemically disturbed. This thesis provides a detailed overview of the functionality of triboactive low-friction coatings, in many different systems. The majority of the tribofilms discussed, formed in very different environments, are built up by tungsten disulfide (WS2), which is a material similar to graphite, with a lamellar structure where strongly bonded atomic planes may slip over each other almost without resistance. The major difference is that WS2 is an intrinsically triboactive material, while graphite is not. However, graphite and other carbon-based materials can be made triboactive in certain atmospheres or by addition of other elements, such as hydrogen. The remarkable affinity and driving force to form such WS2 low-friction tribofilms, regardless of the initial states of the sulfur and tungsten, and even when the forming elements are present only at ppm levels, is a recurrent observation in the thesis. Addition of an alloying element to sputtered coatings of WS2 can improve its mechanical and frictional properties significantly. Several promising attempts have been made to find good candidates, out of which a few important ones are investigated in this thesis. Their achievable potential in friction reductions is demonstrated. By reducing friction, energy losses can be avoided, which also results in lower particle and exhaust emissions, which directly reduces the environmental impact. Triboactive coatings are shown to be a promising route to significantly improve tribological applications and allow more environmental friendly and energy efficient vehicles.

The study of factors governing cutting tool performance and life is driven by manufacturers’ need to increase economic efficiency in their production facilities. Tooling and process optimization represent an ongoing opportunity for realizing substantial improvements, thus manufacturers continue to focus on promoting design and development of cutting tools and surface treatment technology relevant to machining. The central goal of cutting tool technology is to increase productivity while simultaneously reducing cost and meeting the quality targets of the machined parts. This thesis considers a nano-tribological approach to explain some of the past performance improvements in cutting tools used in dry machining applications and to look for opportunities to make further improvements in this field. The approach considers tribofilms, which are often described as tribo-oxides composed of either the base cutting tool material or freshly cut workpiece material transferred to the tool that have formed on the friction surface through interaction with the environment (air or cutting fluids) and a tribo-oxidation process. In general, the formation of tribofilms plays an important role in friction and wear behaviour by offering thermal protection and/or in-situ lubrication, which is especially important during dry machining of hard workpiece materials. The formation of various tribofilms on cutting tools have been reported through: 1) cutting tools with surface modification and further tribo-oxidation during the cutting process; 2) mass transfer from freshly cut workpiece material due to tool/chip contact in machining; and 3) interaction between the cutting tool surface and the cutting environment. This dissertation presents a novel method to control the formation of tribofilms on the cutting tool surface during machining of hard materials such as Inconel DA 718 and hardened steels (AISI T1 and AISI D2), with coated and uncoated tools. In particular, the frictional conditions experienced by the cutting tool during the initial period of cutting (the running-in stage) are shown to strongly influence whether or not beneficial processes related to adaptability will trigger. Employing a more aggressive cutting speed during the running-in stage noticeably enhances the generation of protective/lubricious tribo-ceramic films on the friction surface. When the cutting speed is subsequently reduced, the accelerated formation of beneficial tribofilms previously initiated is not fully removed and therefore the rate of tool wear is considerably less than if the tool is run at the lower cutting speed for its entire life. In addition, preliminary results are presented regarding tribofilms formation under the influence of the cutting environment, specifically the nature of the cooling medium used, which demonstrates an entirely different avenue to explore in terms of fine-tuning of tribofilms generation. The overall objective of this dissertation is to highlight different approaches to control the tribofilms formed on the cutting tool surface in order to benefit the machining process and to improve cutting tool life, material remove rate and the machined surface quality. Additionally, little work has been found demonstrating the formation of tribofilms on the tool surface through mass transfer from the workpiece material or through interaction with the cooling medium. Thus a secondary objective of this work is to demonstrate the formation of tribofilms through these different means and to investigate the effect of cutting parameters on their formation. / Thesis / Doctor of Philosophy (PhD)

For decades, global regulations and government mandates have driven technological developments to improve vehicle fuel economy. Tribological components found in all automotive engines contain metal-on-metal contact zones that may result in increased friction and wear, reducing overall engine efficiency. Lubricant additives such as antiwear and friction modifying components are added to motor oils to prevent some of the damages that may occur at contact zones and improve friction. The effects of other additive components, such as dispersants, that are prevalent in a lubricant additive package on the anti-wear layer remain relatively unknown. Polyisobutenyl Succinimide (PIBSI) dispersants were evaluated for their interactions with the ZDDP antiwear component. The physical and chemical properties and friction of the tribofilms formed in presence of dispersants were defined revealing a previously unknown structure-activity relationship. Further analysis of ZDDP and dispersants revealed surface and bulk fluid interactions.

<p>The present thesis thoroughly treats a special friction reduction phenomenon that may appear in boundary lubricated tribological contacts, of the type encountered in numerous mechanical components made of steel. The phenomenon involves the formation of a special type of tribofilm that offers very low coefficients of friction. Typically the friction level becomes halved when the film is formed, compared to when it is not formed. Since boundary lubricated mechanical components are so common in all sorts of machinery, the technical and economical potential of this phenomenon is gigantic.</p><p>The tribofilm is produced on the steel surface, resulting in friction coefficient reduction from typically 0.08–0.1 to 0.04–0.06. The tribofilm is formed from the metal in the carbon coating and sulfur in the oil additive. The main film studied was WS₂, which is a well-known low-friction material. It includes easy shearing atomic planes, in the same fashion as the solid lubricants MoS₂ and graphite. Virtually no carbon is present in the tribofilm, despite carbon being the main constituent of both the coating and the additive. No films form on the Me-DLC coated part.</p><p>It was also found that WSi₂-particles could result in the formation of WS₂-containing tribofilms. It was concluded that they, just as the W-DLC film, were sufficiently weak to mill down to very small particles, and chemically reactive in the prevailing tribological conditions. However, WC particles were too stable, both mechanically and chemically, to result in any film formation.</p><p>The chemical driving forces for formation of the tribofilms were analyzed using EkviCalc, a commercial software for thermodynamical calculations based on minimization of Gibbs free energy for a system as a function of temperature and pressure. The simulations indeed confirmed that both WS₂ and MoS₂ should be expected to be stable compounds, coexisting with FeS, in the studied environment. As a spin-off result, the thermodynamical calculations indicated that coatings of the Cr-C type should impose very little tribochemical wear of the uncoated steel surface, and even reduce the forma-tion of FeS (the “traditional” tribofilm) on the steel surface in S-containing environments. </p><p>As a final spin-off, the thermodynamical calculations indicate that the Ti-C coating should be very resistant to tribochemical wear in the S-containing environment. </p>

This thesis includes two parts. The first part reviews the history and fundamentals of surface science and tribology. The second part presents the major research outcomes and contributions. This research explores the aspects of friction, wear, and surface modification for tribological augmentation of surfaces. An effort has been made to study these aspects through gaining insights by fundamental studies leading to specific practical applications in railroads. The basic idea was to surface engineer metals for enhanced surface properties. A micro- and nano- scale experimental approach has been used to achieve these objectives. Novel principles of nano technology are incorporated into the experiments. Friction has the potential to generate sufficient energy to cause surface reactions through high flash temperatures at the interface of two materials moving in relative motion. This allows surface modifications which can be tailored to be tribologically beneficial through a controlled process. The present work developed a novel methodology to generate a functional tribofilm that has combined properties of high hardness and high wear resistance. A novel methodology was implemented to distinguish sliding/rolling contact modes during experiments. Using this method, a super hard high-performance functional tribofilm with “regenerative” properties was formed. The main instrument used in this research for laboratory experiments is a tribometer, using which friction, wear and phase transformation characteristics of railroad tribo-pairs have been experimentally studied. A variety of material characterization techniques have been used to study these characteristics at both micro and nano scale. Various characterization tools used include profilometer, scanning electron microscope, transmission electron microscope, atomic force microscope, X-ray diffractometer, nanoindenter, and X-ray photon spectroscope. The regenerative tribofilms promise exciting applications in areas like gas turbines, automotive industry, compressors, and heavy industrial equipment. The outcome of this technology will be an economical and more productive utilization of resources, and a higher end performance.

The present thesis thoroughly treats a special friction reduction phenomenon that may appear in boundary lubricated tribological contacts, of the type encountered in numerous mechanical components made of steel. The phenomenon involves the formation of a special type of tribofilm that offers very low coefficients of friction. Typically the friction level becomes halved when the film is formed, compared to when it is not formed. Since boundary lubricated mechanical components are so common in all sorts of machinery, the technical and economical potential of this phenomenon is gigantic. The tribofilm is produced on the steel surface, resulting in friction coefficient reduction from typically 0.08–0.1 to 0.04–0.06. The tribofilm is formed from the metal in the carbon coating and sulfur in the oil additive. The main film studied was WS2, which is a well-known low-friction material. It includes easy shearing atomic planes, in the same fashion as the solid lubricants MoS2 and graphite. Virtually no carbon is present in the tribofilm, despite carbon being the main constituent of both the coating and the additive. No films form on the Me-DLC coated part. It was also found that WSi2-particles could result in the formation of WS2-containing tribofilms. It was concluded that they, just as the W-DLC film, were sufficiently weak to mill down to very small particles, and chemically reactive in the prevailing tribological conditions. However, WC particles were too stable, both mechanically and chemically, to result in any film formation. The chemical driving forces for formation of the tribofilms were analyzed using EkviCalc, a commercial software for thermodynamical calculations based on minimization of Gibbs free energy for a system as a function of temperature and pressure. The simulations indeed confirmed that both WS2 and MoS2 should be expected to be stable compounds, coexisting with FeS, in the studied environment. As a spin-off result, the thermodynamical calculations indicated that coatings of the Cr-C type should impose very little tribochemical wear of the uncoated steel surface, and even reduce the forma-tion of FeS (the “traditional” tribofilm) on the steel surface in S-containing environments. As a final spin-off, the thermodynamical calculations indicate that the Ti-C coating should be very resistant to tribochemical wear in the S-containing environment.

This thesis includes two parts. The first part reviews the history and fundamentals of surface science and tribology. The second part presents the major research outcomes and contributions. This research explores the aspects of friction, wear, and surface modification for tribological augmentation of surfaces. An effort has been made to study these aspects through gaining insights by fundamental studies leading to specific practical applications in railroads. The basic idea was to surface engineer metals for enhanced surface properties. A micro- and nano- scale experimental approach has been used to achieve these objectives. Novel principles of nano technology are incorporated into the experiments. Friction has the potential to generate sufficient energy to cause surface reactions through high flash temperatures at the interface of two materials moving in relative motion. This allows surface modifications which can be tailored to be tribologically beneficial through a controlled process. The present work developed a novel methodology to generate a functional tribofilm that has combined properties of high hardness and high wear resistance. A novel methodology was implemented to distinguish sliding/rolling contact modes during experiments. Using this method, a super hard high-performance functional tribofilm with “regenerative” properties was formed. The main instrument used in this research for laboratory experiments is a tribometer, using which friction, wear and phase transformation characteristics of railroad tribo-pairs have been experimentally studied. A variety of material characterization techniques have been used to study these characteristics at both micro and nano scale. Various characterization tools used include profilometer, scanning electron microscope, transmission electron microscope, atomic force microscope, X-ray diffractometer, nanoindenter, and X-ray photon spectroscope. The regenerative tribofilms promise exciting applications in areas like gas turbines, automotive industry, compressors, and heavy industrial equipment. The outcome of this technology will be an economical and more productive utilization of resources, and a higher end performance.

H13 tool steel is widely used in the mold and die industry. Due to tighter geometric tolerances and higher quality expectations, the use of hard machining has increased over the years. Hard machining refers to the machining of materials in their hardened state. The challenges with hard machining are rapid tool wear and maintaining a high surface integrity of the machined surface. Surface integrity is measured in terms of surface roughness, residual stresses, presence of surface and subsurface cracks, and the quality of the developed microstructure. In order to minimize wear and improve product quality, researchers are working on the development of different tool coatings. Some of the recent tool coatings function by adapting to their environment using heat to form thin layers of oxides, referred to as ―tribo-films‖, on the surface of the tool. If engineered properly, these tribofilms can prolong tool life and improve the surface integrity of a hard machined surface. A titanium based nano multi-layered coating (TiAlCrSiYN/TiAlCrN) has been developed by researchers at the MMRI. The tribological performance of two different coatings TiAlCrSiYN/TiAlCrN and TiAlCrN were tested in a hard machining metal cutting process. The impact of these coatings on tool wear, Cutting process (Chips) and Surface Integrity (Quality of machined surface) was assessed. This research involves characterizing the coating to understand how the formation of different oxide films (tribofilms) effect tool wear and surface integrity. The generation of these tribofilms is sensitive to coating composition and cutting condition (temperature/pressure). Next, an in-depth characterization of the chips produced during machining was carried out as part of studying the effect of different tribological conditions between the tool and workpiece. The chip's hardness, oxidation, chip formation mechanism and topography as the chip slid against the cutting tool surface was studied. Also, the Surface integrity of the machined part was investigated, considering its microstructure, residual stresses and surface roughness. Lastly, tests were performed in an attempt to accelerate the generation of beneficial tribofilms. Results indicate significant improvement in wear life and surface integrity of the machined surface due to the generation of tribo-films in this machining application. / Thesis / Master of Applied Science (MASc)

Les fullerènes de bisulfure métallique de type ( M eS2 , où Me= Mo et W) rencontrent un intérêt croissant du fait de leurs pouvoirs anti usure et réducteur de frottement en régime de lubrification limite. Les propriétés tribologiques de ces nanoparticules, dépendent à la fois de leurs caractéristiques intrinsèques (structure, morphologie, taille, ... ), des conditions de sollicitations (nature des surfaces, pression, température, ... ) ainsi que du cocktail d'additifs présent dans une formulation d'huile moteur. La compréhension de l'origine de ces propriétés passe obligatoirement par une parfaite connaissance du mode d'action des nanoparticules. L'objectif de ce travail de thèse est d’identifier les paramètres pouvant avoir une influence sur le comportement des nanoparticules à l’échelle nanométrique et de faire le lien entre ce comportement, les mécanismes de lubrification des nanoparticules, et leurs propriétés tribologiques. Afin de répondre à cet objectif nous avons adopté une approche multi échelle qui consiste dans un premier temps à étudier le comportement de fullerènes individuels (IF - M eS2 , ou Me= Mo et W) en cours de sollicitation. Ainsi grâce à une méthodologie expérimentale originale couplant la technique de nano indentation à une observation in situ dans un microscope électronique à transmission haute résolution (HRMET), nous avons visualisé pour la toute première fois et en temps réelle comportement de nanoparticules individuelles d’if- M eS2 (Me= Mo et W) sollicitées en compression et/ou en cisaillement dans un contact dynamique. Cette étude a permis d'identifier l'influence des caractéristiques intrinsèques des fullerènes sur leur réponse à l'échelle nanométrique et d'estimer des pressions de contact pour lesquelles le fullerène s'exfolie, roule ou glisse dans le contact. Nous nous sommes ensuite intéressés aux mécanismes de lubrification des fullerènes en dispersion dans une base lubrifiante, en condition de lubrification limite. En se basant sur des analyses XPS et des observations MEB et MET des tribofilms et des débris d'usure générés à l'issu d'essais de frottement réalisés dans trois contacts de nature différente (acier, alumine et DLC), nous avons clairement montré que les propriétés lubrifiantes des nanoparticules d'IF - M eS2 (Me= Mo et W) dépendaient à la fois de leurs caractéristiques intrinsèques et de la nature des surfaces frottantes. Ainsi un lien a été établi entre le comportement des fullerènes à l'échelle nanométrique et leur mode d'action dans un contact tribologique. Enfin, l'influence de la mise en dispersion des nanoparticules sur leurs propriétés tribologiques a été étudiée. Les propriétés tribologiques des nanoparticules dans une huile moteur ont été également évaluées. Deux approches expérimentales de type 'Bottom up' et 'Top dawn'ont été adoptées afin d'évaluer les interactions entre les nanoparticules et l'ensemble des additifs présents dans une huile complétement formulée. L'influence de la température sur les propriétés tribologiques des nanoparticules a été également abordée. / Inorganic Fullerene-(IF) like nanoparticles made of metal dichalcogenides ( M eS2 , Me= Mo and W)continue to attract an increasing interest as friction modifiers and anti-wear additives in liquid lubricant. Their efficiency as lubricant additive strongly depends on intrinsic properties of the nanoparticles (structure, morphology, size ... ), tribological conditions (nature of rubbing surface, pressure, temperature ... ) and also on the package of additives present in the full y formulated engine oil. Thus the control and the optimization of these properties require a perfect knowledge of the lubrication mechanisms of these nanoparticles. The aim of this work is to identify the parameters which influence the behavior of the nanoparticles at the nano-scale and to establish a correlation between this behavior, the lubrication mechanisms of nanoparticles and their tribological properties observed at macro-scale. For this aim, we have chosen a multi-scale approach, which firstly consists in studying the behavior of individual fullerenes (IF- M eS2, Me= Mo and W) during mechanical solicitation. Therefore, thanks to a new in situ TEM technique including nanoindentation, we have visualized the behavior of individual fullerenes in real time during nana-compression and nano-sliding tests. These results allowed us to identify the influence of the intrinsic characteristics of nanoparticles on their response at the nano-scale and to estimate critical values of pressure for rolling, sliding, exfoliation and failure of individual IF - M eS2 particles (Me= Mo and W).Secondly, we focused on the lubrication mechanisms of fullerenes when they are dispersed in base oil in boundary lubrication. The tribofilms and the wear particles obtained after friction tests at three different rubbing surfaces (steel, alumina and DLC), were studied using XPS analyses, SEM and TEM observations. We have clearly shown that the lubricating properties of nanoparticles depend both on their intrinsic properties and on the nature of the contact. Thereby, a correlation between the behavior of single nanoparticles at nano-scale and their lubricating properties under boundary lubrication was established. Finally, the influence of the dispersant on the tribological properties of the nanoparticles was investigated. The tribological properties of nanoparticles in fully formulated engine oil were also evaluated. Two experimental methods based on a 'Bottomup' and a 'top-dawn' approach were adopted to evaluate the interactions between nanoparticles and all the additives in fully formulated oil. The influence of the temperature on the tribological properties of the nanoparticles was also discussed.

Les revêtements de carbone amorphe hydrogéné (a-C:H) avec incorporation de silicium et d’oxygène (a-C:H:Si:O) sont une catégorie de lubrifiants solides, de la famille des Diamond-Like Carbon (DLC), présentant aussi bien de bonnes propriétés mécaniques que tribologiques et une bonne stabilité thermique. Bien qu’il soit établi que le comportement tribologique de ces couches est moins dépendant de l’environnement que celui des couches a-C:H, sans éléments d’addition, l’origine physicochimique de ce comportement reste à élucider. Ce travail de thèse s’inscrit dans le cadre une collaboration internationale entre le Laboratoire de Tribologie et Dynamique des Systèmes de l’Ecole Centrale Lyon et le département de Génie Mécanique et Mécanique Appliquée de l’Université de Pennsylvanie, et a pour objectifs d’apporter des réponses à ces questions ouvertes. Un large éventail de techniques expérimentales complémentaires, notamment la nanoindentation, la microscopie à force atomique (AFM), la microscopie à mesure de force (FFM), la microscopie optique et électronique, le Raman, la spectroscopie de photoélectron X (XPS) et la spectroscopie de structure près du front d’absorption de rayons X (NEXAFS) a été mis en oeuvre pour non seulement établir une carte d’identité mécanique, structurale et chimique du revêtement initial, mais aussi comprendre les modifications structurelles induites par le frottement. Afin de contrôler l’environnement au cours des essais tribologiques, nous avons utilisé d’une part un tribomètre linéaire alternatif, que nous avons équipé d’un système de soufflage de gaz permettant de changer rapidement l’environnent au cours des essais, et d’autre part un tribomètre analytique à environnement contrôlé autorisant des expérimentations tant sous vide poussé qu’à pression élevée de gaz. Ainsi, nous avons pu montrer que le coefficient de frottement augmente avec le taux de vapeur d’eau dans l’environnement et cela est réversible lorsqu’on diminue brusquement l’humidité. En outre, la vapeur d’eau protège la couche de l’usure alors que la présence d’oxygène la favorise. Grace aux observations en microscopie électronique, nous avons pu prouver que le comportement tribologique des couches a- C:H:Si:O, lors d’un frottement contre de l’acier 100Cr6, est essentiellement contrôlé par la formation de jonctions adhésives dans l’interface. Sous vide poussé ou à faible pression de gaz (<1 mbar de vapeur d’eau, <10 mbar d’oxygène ou <50 mbar d’hydrogène), la rupture de ces jonctions adhésives a lieu dans l’acier, résultant en un transfert de matériau de l’acier vers l’a-C:H:Si:O s’accompagnant d’un coefficient de frottement élevé (μ≈1.2). Au delà de ces pressions seuils de gaz, les jonctions adhésives se rompent du côté du a-C:H:Si:O, le transfert de matière s’opérant alors dans la direction opposée, du revêtement vers l’acier. Des analyses NEXAFS ont révélé que ce phénomène s’expliquait par une réaction dissociative entre les éléments du gaz environnant et les liaisons carbone C–C contraintes, favorisée par la sollicitation mécanique en extrême surface de l’a-C:H:Si:O. Ceci résulte en une diminution drastique du coefficient de frottement à des valeurs d’un ordre de grandeur inférieures à celles obtenues dans la configuration précédente. L’ensemble de ces résultats nous a ainsi permis de développer un modèle expérimental expliquant les mécanismes fondamentaux d’interaction entre l’environnement et les lubrifiants solides du type a-C:H:Si:O. / Like Carbon (DLC) coatings that exhibit outstanding mechanical properties, thermal stability and tribological performance. It is well established that the frictional and wear performances of a-C:H:Si:O are less dependent on environment than that of pure hydrogenated amorphous carbon (a-C:H). However the fundamental mechanisms accounting for such excellent tribological behavior of a-C:H:Si:O are still not fully understood. The present work, which is part of a collaboration between the Laboratoire de Tribologie et Dynamique des Systèmes of Ecole Centrale de Lyon and the department of Mechanical Engineering and Applied Mechanics of University of Pennsylvania, consists in using a multi-scale, multidisciplinary and multi-technique experimental approach for understanding the influence of environment on the tribological response of a-C:H:Si:O. A wide rang of complementary techniques, including nanoindentation, Atomic Force Microscopy (AFM), Friction Force Microscopy (FFM), optical and electron microscopy, Raman, X-ray Photoelectron Spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy (NEXAFS), have thus been used to fully characterize the structure, composition and mechanics of the studied material, as deposited as well as after tribological testing. Control of the environment has been achieved first thanks to an open air linear reciprocating tribometer that we equipped with a gas blowing system, thus allowing a quick change of the sliding environment, and a environment-controlled analytical tribometer operating from high vacuum to elevated pressures of desired gases. We were able to evidence the strong influence of the amount of water vapor in the environment on the friction behavior of a- C:H:Si:O, with a reversible behavior when abruptly changing the environment. Contrary to water vapor, oxygen promotes the wear of a-C:H:Si:O. SEM observations revealed that while sliding a-C:H:Si:O against 52100 steel, the frictional response is controlled by the build-up and the release of adhesive junctions within the interface. Under high vacuum and below a threshold pressure of water vapor (1 mbar), oxygen (10 mbar) and hydrogen (50 mbar), adhesive junctions are released in the steel, resulting in a transfer of material from steel to a-C:H:Si:O and in a high coefficient of friction (μ≈1.2). However, as the gas pressure is increased above the threshold, the adhesive junctions break on the a-C:H:Si:O side, leading to a material transfer in the opposite direction, from the a-C:H:Si:O to the steel. NEXAFS spectroscopy revealed that a dissociative reaction occurs between the gaseous species and the strained C–C atoms in the near surface region ofa-C:H:Si:O, thus resulting in a drastic decrease of the steady state coefficient of friction by at least an order of magnitude. In light of these observations, an analytical model has been proposed to describe the fundamental interaction mechanisms between the environment and the a-C:H:Si:O/steel tribopairs.