V Abstract Lubricated tribological contact processes are important in both nature and in many technical applications. Fluid lubricants play an important role in contact processes, e.g. they reduce friction and cool the contact zone. The fundamentals of lubricated contact processes on the atomistic scale are, however, today not fully understood. A lubricated contact process is deﬁned here as a process, where two solid bodies that are in close proximity and eventually in parts in direct contact, carry out a relative motion, whereat the remaining volume is submersed by a ﬂuid lubricant. Such lubricated contact processes are diﬃcult to examine experimentally. Atomistic simulations are an attractive alternative for investigating the fundamentals of such processes. In this work, molecular dynamics simulations were used for studying diﬀerent elementary processes of lubricated tribological contacts. A simpliﬁed, yet realistic simulation setup was developed in this work for that purpose using classical force ﬁelds. In particular, the two solid bodies were fully submersed in the ﬂuid lubricant such that the squeeze-out was realistically modeled. The velocity of the relative motion of the two solid bodies was imposed as a boundary condition. Two types of cases were considered in this work: i) a model system based on synthetic model substances, which enables a direct, but generic, investigation of molecular interaction features on the contact process; and ii) real substance systems, where the force ﬁelds describe speciﬁc real substances. Using the model system i), also the reproducibility of the ﬁndings obtained from the computer experiments was critically assessed. In most cases, also the dry reference case was studied. Both mechanical and thermodynamic properties were studied – focusing on the inﬂuence of lubrication. The following properties were studied: The contact forces, the coeﬃcient of friction, the dislocation behavior in the solid, the chip formation and the formation of the groove, the squeeze-out behavior of the ﬂuid in the contact zone, the local temperature and the energy balance of the system, the adsorption of ﬂuid particles on the solid surfaces, as well as the formation of a triboﬁlm. Systematic studies were carried out for elucidating the inﬂuence of the wetting behavior, the inﬂuence of the molecular architecture of the lubricant, and the inﬂuence of the lubrication gap height on the contact process. As expected, the presence of a ﬂuid lubricant reduces the temperature in the vicinity of the contact zone. The presence of the lubricant is, moreover, found to have a signiﬁcant VI Abstract inﬂuence on the friction and on the energy balance of the process. The presence of a lubricant reduces the coeﬃcient of friction compared to a dry case in the starting phase of a contact process, while lubricant molecules remain in the contact zone between the two solid bodies. This is a result of an increased normal and slightly decreased tangential force in the starting phase. When the ﬂuid molecules are squeezed out with ongoing contact time and the contact zone is essentially dry, the coeﬃcient of friction is increased by the presence of a ﬂuid compared to a dry case. This is attributed to an imprinting of individual ﬂuid particles into the solid surface, which is energetically unfavorable. By studying the contact process in a wide range of gap height, the entire range of the Stribeck curve is obtained from the molecular simulations. Thereby, the three main lubrication regimes of the Stribeck curve and their transition regions are covered, namely boundary lubrication (signiﬁcant elastic and plastic deformation of the substrate), mixed lubrication (adsorbed ﬂuid layers dominate the process), and hydrodynamic lubrication (shear ﬂow is set up between the surface and the asperity). The atomistic eﬀects in the diﬀerent lubrication regimes are elucidated. Notably, the formation of a triboﬁlm is observed, in which lubricant molecules are immersed into the metal surface. The formation of a triboﬁlm is found to have important consequences for the contact process. The work done by the relative motion is found to mainly dissipate and thereby heat up the system. Only a minor part of the work causes plastic deformation. Finally, the assumptions, simpliﬁcations, and approximations applied in the simulations are critically discussed, which highlights possible future work. Kurzfassung VII Kurzfassung Geschmierte tribologische Kontaktprozesse sind sowohl in der Natur als auch in vie- len technischen Anwendungen von großer Bedeutung. Fluide Schmiermittel spielen bei vielen Kontaktprozessen eine wichtige Rolle: Sie verringern die Reibung und kühlen die Kontaktzone. Die grundlegenden Mechanismen auf atomistischer Skala von geschmierter Kontaktprozesse sind jedoch heute noch nicht vollständig verstanden. Ein geschmierter Kontaktprozess ist hier deﬁniert als ein Prozess, bei dem zwei feste Körper, die sich in unmittelbarer Nähe und ggf. in direktem Kontakt beﬁnden, eine Relativbewegung ausführen, wobei das restliche Volumen von einem ﬂüssigen Schmiermittel ausgefüllt ist. Solche geschmierten Kontaktprozesse sind experimentell nur schwer in situ zu un- tersuchen. Atomistische Simulationen sind eine attraktive Alternative zu klassischen Experimenten, um die Elementarprozesse von Tribokontakten zu untersuchen. In dieser Arbeit wurden Molekulardynamiksimulationen zur Untersuchung der Elementarprozesse geschmierter tribologischer Kontaktprozesse eingesetzt. Zu diesem Zweck wurde in dieser Arbeit ein vereinfachter, aber realistischer Simulationsaufbau unter Verwendung klassi- scher Kraftfelder entwickelt. Insbesondere wurden die beiden Festkörper vollständig in das ﬂüssige Schmiermittel eingetaucht, so dass das Herausdrücken der Fluidteilchen aus der Kontaktzone realistisch abgebildet wurde. Die Geschwindigkeit der Relativbewegung der beiden Festkörper wurde als Randbedingung aufgeprägt. In dieser Arbeit wurden zwei Fälle betrachtet: i) ein auf Modellsubstanzen basierendes Modellsystem, das ei- ne direkte, aber generische Untersuchung der molekularen Wechselwirkungsmerkmale auf den Kontaktprozess ermöglicht; und ii) reale Stoﬀsysteme, bei denen die Kraftfel- der konkrete reale Stoﬀe beschreiben. Unter Verwendung des Modellsystems i) wurde auch die Reproduzierbarkeit der aus den Computerexperimenten gewonnenen Erkennt- nisse kritisch untersucht. In den meisten Fällen wurde auch der trockene Referenzfall betrachtet. Es wurden sowohl mechanische als auch thermodynamische Eigenschaften des Tribosystems untersucht, wobei der Schwerpunkt auf dem Einﬂuss der Schmierung durch ein Fluid lag. Die folgenden Eigenschaften wurden untersucht: Die Kontaktkräf- te, der Reibungskoeﬃzient, das Versetzungsverhalten im Festkörper, die Spanbildung und die Ausbildung der Furche im Substrat, das Auspressverhalten des Fluids aus der Kontaktzone, die lokale Temperatur und die Energiebilanz des Systems, die Adsorption VIII Kurzfassung von Fluidteilchen an den Festkörperoberﬂächen sowie die Ausbildung eines Triboﬁlms. Systematische Untersuchungen wurden durchgeführt, um den Einﬂuss des Benetzungs- verhaltens, der molekularen Architektur des Schmierstoﬀs und der Schmierspalthöhe auf den Kontaktprozess aufzuklären. Die Ergebnisse zeigen, dass, wie erwartet, das Vorhan- densein eines ﬂuiden Schmiermittels die Temperatur der Kontaktzone reduziert. Darüber hinaus wurde gezeigt, dass das Vorhandensein des Schmierstoﬀs einen erheblichen Ein- ﬂuss auf die Reibung und die Energiebilanz des Prozesses hat. Das Vorhandensein eines Schmierstoﬀs verringert den Reibungskoeﬃzienten im Vergleich zu einem trockenen Fall in der Anfangsphase eines Kontaktprozesses, während noch Schmierstoﬀmoleküle in der Kontaktzone zwischen den beiden Festkörpern vorhanden sind. Dies ist auf eine er- höhte Normalkraft und eine leicht verringerte Tangentialkraft in der Anfangsphase des Kontaktprozesses zurückzuführen. Wenn die Fluidmoleküle mit fortschreitender Kon- taktlaufzeit aus dem Spalt herausgedrückt sind und die Kontaktzone im Wesentlichen trocken ist, wird der Reibungskoeﬃzient durch die Anwesenheit eines Fluids im Vergleich zu einem trockenen Fall erhöht. Dies ist auf das Eindrücken einzelner Fluidmoleküle in die Festkörperoberﬂäche zurückzuführen, wozu Energie aufgewendet werden muss. Durch die Untersuchung des Kontaktprozesses bei unterschiedlichen Spalthöhen wird der gesamte Bereich der Stribeck Kurve mit den molekularen Simulationen abgebildet. Dabei werden die drei Haupt-Reibungsregime der Stribeck Kurve und ihre Übergangs- bereiche abgedeckt: Grenzreibung (signiﬁkante elastische und plastische Verformung des Substrats), Mischreibung (Verhalten adsorbierter Fluidmoleküle an Feststoﬀoberﬂächen dominiert den Prozess) und Flüssigkeitsreibung (Scherströmung im Fluid zwischen den beiden Feststoﬀoberﬂächen). Auf der Basis der Computerexperimente wurden die Ele- mentarprozesse in den drei Reibungsregimen untersucht. Insbesondere wird die Bildung eines Triboﬁlms beobachtet, bei dem die Schmierstoﬀmoleküle in die Metalloberﬂäche eingedrückt werden. Die Bildung eines Triboﬁlms hat wichtige Konsequenzen für den Kontaktprozess – insbesondere im Bereich der Grenzreibung. Ferner konnte gezeigt wer- den, dass die durch die Relativbewegung verrichtete Arbeit größtenteils dissipiert und dadurch das System erwärmt wird. Nur ein kleiner Teil der Arbeit verursacht plastische Verformungen. Abschließend werden die in den Simulationen verwendeten Annahmen, Vereinfachungen und Näherungen kritisch diskutiert, wodurch mögliche zukünftige Ar- beiten aufzeigt werden. Contents IX Contents Abstract V Kurzfassung VII List of Symbols XIII 1 Introduction 1 2 Fundamentals 7 2.1 Tribology and Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.1 History and State of the Art . . . . . . . . . . . . . . . . . . . . . . 9 2.1.2 The Tribosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1.3 Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 The Relevance of Interfaces in Lubricated Tribosystems . . . . . . 17 2.1.5 Tribology on Diﬀerent Scales . . . . . . . . . . . . . . . . . . . . . . 23 2.2 Molecular Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.2.1 Molecular Simulations as Computer Experiments . . . . . . . . . . 24 2.2.2 Molecular Dynamics: Simulation Method . . . . . . . . . . . . . . . 25 2.2.3 Structure of Crystals, Fluids, and Interfaces on the Atomistic Scale 30 2.2.4 Molecular Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.5 Computational Tribology on the Atomistic Level . . . . . . . . . . 40 2.2.6 Simulation Scenario Used in This Work . . . . . . . . . . . . . . . . 43 3 Inﬂuence of Solid-Fluid Interaction on Lubrication 47 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.2.1 Molecular Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.2.2 Simulation Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.2.3 Deﬁnition of Observables . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.3.1 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 56 X Contents 3.3.2 Energy Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.3.3 Temperature Field and Thermodynamic Properties of Lubricant . 66 3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4 Reproducibility of Atomistic Friction Computer Experiments 73 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 4.2 Simulation Scenario and Deﬁnition of Observables . . . . . . . . . . . . . . 76 4.2.1 Force Field and Simulation Setup . . . . . . . . . . . . . . . . . . . 76 4.2.2 Statistical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2.3 Deﬁnition of Observables . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 4.3.1 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.3.2 Thermodynamic Properties . . . . . . . . . . . . . . . . . . . . . . . 88 4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 5 Mechanical Eﬀects of Lubrication on the Nanoscale 95 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.2.1 Simulation Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.2.2 Molecular Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 5.2.3 Deﬁnition of Observables . . . . . . . . . . . . . . . . . . . . . . . . . 100 5.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 5.3.1 Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 5.3.2 Dislocations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.3.3 Groove, Chip, and Contact Zone . . . . . . . . . . . . . . . . . . . . 107 5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6 Molecular Dynamics Simulation of the Stribeck Curve 117 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 6.2.1 Simulation Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 6.2.2 Molecular Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 6.2.3 Deﬁnition of Observables . . . . . . . . . . . . . . . . . . . . . . . . . 124 6.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 6.3.1 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 127 6.3.2 Thermodynamic Properties . . . . . . . . . . . . . . . . . . . . . . . 142 6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 7 Discussion and Outlook 149 7.1 Critical Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Contents XI 7.2 Possible Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 8 Conclusions 155 Literature 159 Appendix 203 A The Lennard-Jones Model System 203 B Details on Statistical Uncertainty Analysis 207 B.1 Calculation of Local Thermodynamic Properties . . . . . . . . . . . . . . . 207 B.2 Comparison of Uncertainty Analysis to Results from Chapter 3 . . . . . . 208 C Details on Stribeck Curve Simulations 215 C.1 Model for Solid-Fluid Interactions . . . . . . . . . . . . . . . . . . . . . . . . 215 C.2 Computational Aspects of Simulations . . . . . . . . . . . . . . . . . . . . . 216 C.3 Force Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 C.4 Temperature and Pressure in the Lubrication Gap . . . . . . . . . . . . . . 218 Authorship Declaration 219 Supervised Student Theses 223 Curriculum Vitae 225 Acknowledgment 227 List of Symbols XIII List of Symbols Latin symbols a,b parameters A area a acceleration b slip length b Burgers vector c parameter d depth E total energy F force F embedding function g(r) radial distribution function h ﬁlm thickness h height ^H Hamilton operator K total kinetic energy k parameters kB Boltzmann constant L dislocation length L load l length M particle mass N number of components N number of particles N number of simulations n number of moles p pressure p momentum psvapor pressure XIV List of Symbols Q heat q charge R roughness parameter R radius r position vector r distance between particles rc cut-oﬀ radius T temperature U internal energy u potential energy v velocity V volume V total potential energy W work x composition vector x spatial coordinate y spatial coordinate z spatial coordinate Z observable variable Superscripts 0 friction coeﬃcient oﬀset 0 reference point F ﬂuid I indenter i particle index i component index j particle index k particle index l particle index N normal S substrate s interphase SL solid-liquid SV solid-vapor T tangential VL vapor-liquid List of Symbols XV * reduced with respect to LJ units system ~ solid-ﬂuid interaction arithmetic mean ′moving coordinate system Greek symbols contact angle or angle parameter _ shear rate coupling coeﬃcient surface tension  phase space  diﬀerence or interval Z signiﬁcance measure for Z  normalized standard deviation " Lennard-Jones energy parameter 0 electric constant  shear viscosity  dimensionless ﬁlm thickness  kinematic viscosity  coeﬃcient of friction i chemical potential of component i angle wave function interaction function  density ~ electron density  Lennard-Jones size parameter  surface roughness  standard deviation time shear stress  angle XVI List of Symbols Abbreviations AFM atomic force microscope bcc body centered cubic BL boundary lubrication BZS Boltzmann-Zuse society DXA dislocation extraction algorithm EAM embedded atom method EMD equilibrium molecular dynamics fcc face centered cubic HL hydrodynamic lubrication LJ Lennard-Jones LNG liquid natural gas MD molecular dynamics ML mixed lubrication NB non-bonded NEMD non-equilibrium molecular dynamics NVE microcanonical ensemble NVT canonical ensemble sc simple cubic TraPPE transferable potentials for phase equilibria 1 Introduction 1 1Introduction Tribological processes and the functionality of lubricants in such are of great importance in nature as well as in many technical applications, for example in production processes like cutting and grinding, in machine parts such as bearings and gears, in micro-devices such as micro-pumps, and also in bone joints. Fluid lubricants play a crucial role in such contact processes. The main tasks of lubrication are (a) the reduction of friction and (b) the cooling of the contact zone. The underlying elementary processes in tribological contacts occur on the nanoscale. Today, these elementary processes are still not fully understood. This is, on one hand, due to the fact that a large number of physical eﬀects and mechanisms are relevant in an interrelated and complex way and, on the other hand, due to the fact that contact processes are diﬃcult to study experimentally. The actual contact zone is often very small (on the nanoscale) and experimentally hard to monitor. This is due to the geometrical setup, i.e. two solid bodies enfolding the lubricated contact zone. Moreover, in many applications, extreme conditions prevail in the contact zone regarding pressure and temperature, which makes in situ experiments diﬃcult. Molecular dynamics simulation are an interesting alternative to classical laboratory experiments for studying processes on the nanoscale. On one hand, they are physically- based and provide great predictive capabilities and, on the other hand, they provide a great level of insight into the studied process. In molecular simulations, matter is modeled on the atomistic level based on the molecular interactions, which are described by mathematical models – so-called force ﬁelds. The time evolution of the system, i.e. trajectories of the particles are computed by solving Newton ’s equation of motion. Molecular dynamics simulations have today become an indispensable tool in many sci- entiﬁc disciplines, e.g. physics, chemistry, biology, and material sciences. Molecular dynamics simulations are therein often used to elucidate the inner workings and funda- mentals of processes and mechanisms that are hard to study experimentally. Solving and backtracking the particle motion on the atomistic level provides enormous insights. This is not only due to the fact that the atomistic structure can be analyzed in great detail, but also due to the fact that the motion and interactions of particles on 2 1 Introduction the atomistic scale are linked to macroscopic properties by statistical mechanics. Hence, molecular simulations provides simultaneous access to both the atomistic structure and macroscopic properties, which is very beneﬁcial for relating the simulation results to laboratory experiment data (usually from the macroscopic level). The beneﬁts of pre- dicting and analyzing atomistic trajectories are reﬂected in the famous quote of Richard P. Feynman stating that a great deal "can be understood in terms of the jigglings and wigglings of atoms." [120]. MD simulation actually provides access to monitoring this ’jigglings and wigglings’ of the atoms within diﬀerent setups and situations, e.g. a tribological system, using relatively few physical assumptions. Therefore, MD simulation has become a popular tool in tribology1. Hence, MD simulation has been used for several years for investigating contact processes. However, many of the mechanisms of interest (lubrication, dislocations, local structural change, plastic deformation, topography change, etc.) occur on relatively large length scales compared to the size of atoms. Accordingly, large and computationally demanding simulations (approx. 107- 108particles) are required to capture these properties and phenomena. Furthermore, complex, i.e. computationally intensive interaction models are required for realistically modeling speciﬁc substances. For these reasons, molecular simulation that provides a realistic kinematic and kinetic picture of tribological contact processes and captures essential physical features has only been accessible for some years. There are numerous studies available in the literature, in which processes like nanoin- dentation and nanoscratching were investigated using MD simulation. Mostly, dry con- tacts were studied – focusing on mechanical properties. Thermal properties and eﬀects caused by the thermo-mechanical coupling have been only few times studied so far. Most importantly, there are comparatively few MD simulation studies available addressing the inﬂuence of lubrication on contact processes. In most studies that consider the presence of a ﬂuid lubricant, only very thin liquid ﬁlms consisting of a few molecular layers were modeled. Hence, no ﬂuid bulk phase was considered. Yet, this ﬂuid bulk phase is crucial for establishing a physically realistic picture of the behavior of ﬂuid particles in contact processes, e.g. regarding the squeeze-out from the contact zone and its cooling eﬀects. In order to realistically model the ﬂuid ﬂow and the thermal balance (including heat conduction) of a contact process consisting of two solids and a lubricant, the contact must be submersed in a ﬂuid. In this work, lubricated contact processes were studied by MD simulation, where the contact was fully submersed in a ﬂuid lubricant. This – computationally challenging – simulation setup was used for studying diﬀerent aspects of lubricated contact processes 1A detailed literature review on applications of MD simulation in nanotribology is presented sec- tion 2.2.5 1 Introduction 3 on the nanoscale such as its cooling capabilities and its inﬂuence on the coeﬃcient of friction. In this work, a simpliﬁed scenario is considered, that yet comprises essential features, such that central elementary processes of a lubricated contact process become observable and accessible via the computer experiment. In this simpliﬁed scenario, two solid bodies carry out a relative motion (ﬁrst indentation, then scratching, and eventually retraction) – at constant velocity. The two solid bodies are fully submersed in a ﬂuid lubricant. Both solid bodies are single crystals. The two solids are composed of diﬀerent materials, the so-called substrate is relatively soft, whereas the so-called indenter is relatively hard. This terminology is adapted from experimental nanoindentation and atomic force microscopy (AFM) [123] and used throughout this work. The indenter used in this work has a somewhat spherically-curved contact surface (sphere, cylinder, spherical cap), whereas the substrate is atomistically ﬂat. All three components (substrate, indenter, and ﬂuid lubricant) are atomistically mod- eled using classical force ﬁelds. Two diﬀerent modeling cases were considered: (a) a simpliﬁed yet realistic and generic model system, where all atomistic interactions are modeled by the Lennard-Jones potential; (b) real substance systems, where the atom- istic interaction potentials were chosen such that speciﬁc components are modeled, e.g. iron for the substrate and decane for the lubricant. Both approaches have diﬀerent advantages, e.g. the model system is computationally signiﬁcantly cheaper and more extensive and systematic studies are therefore feasible using this approach. Furthermore, the model system chosen in this work, the so-called Lennard-Jones model system , is a widely used system such that a large body of knowledge of the individual elements and properties are available, e.g. on the wetting and adsorption properties of the ﬂuid on the solid surfaces as well as on the thermodynamic properties of the ﬂuid and the solid bulk phases (details on the Lennard-Jones model system are given in Appendix A). Moreover, the insights into the elementary processes obtained from the model system are generic and (with some limitations) generally applicable. These limitations are, however, not a priori exactly known. Hence, the transferability of the results and insights from the model system to a given speciﬁc use case has restrictions. The real substance case, on the other hand, is evidently well applicable to the speciﬁc use case it is designed for. However, it is in general unfeasible to transfer the results from a speciﬁc real substance case modeling speciﬁc components to other use cases, since it is not a priori known to what extent certain aspects of the results and ﬁndings are a result of speciﬁc features of the considered component models. Therefore, the complementary approach, i.e. using in parts a generic model system and in other parts real substance models, is a good compromise between these conﬂicting objectives. Thus, general insights are obtained in this work from a generic, computationally cheap, and strongly simpliﬁed model sys- 4 1 Introduction tem. And, speciﬁc insights are obtained from a selection of real substance systems. This complementary approach additionally enables an assessment of the transferability of the ﬁndings obtained from the model system to a given real substance system. Hence, the methodology itself can be assessed by the combination of the approaches. This work is (in most parts) a cumulative dissertation, which summarizes results from work previously published in scientiﬁc peer-reviewed journals [348, 356, 358, 359]. These four papers correspond to the chapters 3 - 6. The Appendix ’Authorship Declaration’ provides an overview of the four papers and the corresponding contributions from the author of this thesis. The state of the art and the scope of each study is presented at the beginning of each chapter. Additionally, chapter 2 presents an introduction into both the physical aspects of tribology and lubrication as well as molecular dynamics simulation as a computer experiment method. Moreover, chapter 7 presents a critical discussion on the results and simulation methodology as well as possible future improvements. The chapters 2 and 7 were speciﬁcally constructed for this thesis. All