Development of a new mechano-chemical model in boundary lubrication

A newly developed tribochemical model based on thermodynamics of interfaces and kinetics of tribochemical reactions is implemented in a contact mechanics simulation and the results are validated against experimental results. The model considers both mechanical and thermal activation of tribochemical reactions instead of former thermal activation theories. The model considers tribofilm removal and is able to capture the tribofilm behaviour during the experiment. The aim of this work is to implement tribochemistry into deterministic modelling of boundary lubrication and study the effect of tribofilms in reducing friction or wear. A new contact mechanics model considering normal and tangential forces in boundary lubrication is developed for two real rough steel surfaces. The model is developed for real tribological systems and is flexible to different laboratory experiments. Tribochemistry (e.g. tribofilm formation and removal) and also mechanical properties are considered in this model. The amount of wear is calculated using a modified Archard’s wear equation accounting for local tribofilm thickness and its mechanical properties. This model can be used for monitoring the tribofilm growth on rough surfaces and also the real time surface roughness as well as changes in the λ ratio. This model enables the observation of in-situ tribofilm thickness and surface coverage and helps in better understanding the real mechanisms of wear

Modeling of Instabilities and Self-Organization at the Frictional Interface

The field of friction-induced self-organization and its practical importance remains unknown territory to many tribologists. Friction is usually thought of as irreversible dissipation of energy and deterioration; however, under certain conditions, friction can lead to the formation of new structures at the interface, including in-situ tribofilms and various patterns at the interface. This thesis studies self-organization and instabilities at the frictional interface, including the instability due to the temperature-dependency of the coefficient of friction, the transient process of frictional running-in, frictional Turing systems, the stick-and-slip phenomenon, and, finally, contact angle (CA) hysteresis as an example of solid-liquid friction and dissipation. All these problems are chosen to bridge the gap between fundamental interest in understanding the conditions leading to self-organization and practical motivation. We study the relationship between friction-induced instabilities and friction-induced self-organization. Friction is usually thought of as a stabilizing factor; however, sometimes it leads to the instability of sliding, in particular when friction is coupled with another process. Instabilities constitute the main mechanism for pattern formation. At first, a stationary structure loses its stability; after that, vibrations with increasing amplitude occur, leading to a limit cycle corresponding to a periodic pattern. The self-organization is usually beneficial for friction and wear reduction because the tribological systems tend to enter a state with the lowest energy dissipation. The introductory chapter starts with basic definitions related to self-organization, instabilities and friction, literature review, and objectives. We discuss fundamental concepts that provide a methodological tool to investigate, understand and enhance beneficial processes in tribosystems which might lead to self-organization. These processes could result in the ability of a frictional surface to exhibit self-protection and self-healing properties. Hence, this research is dealing with the fundamental concepts that allow the possibility of the development of a new generation of tribosystem and materials that reinforce such properties. In chapter 2, we investigate instabilities due to the temperature-dependency of the coefficient of friction. The temperature-dependency of the coefficient of friction can have a significant effect on the frictional sliding stability, by leading to the formation of hot and cold spots on the contacting surfaces. We formulate a stability criterion and perform a case study of a brake disk. We show that the mechanism of instability can contribute to poor reproducibility of aircraft disk brake tests reported in the literature. Furthermore, a method to increase the reproducibility by dividing the disk into several sectors with decreased thermal conductivity between the sectors is proposed. In chapter 3, we study frictional running-in. Running-in is a transient period on the onset of the frictional sliding, in which friction and wear decrease to their stationary values. In this research, running-in is interpreted as friction-induced self-organization process. We introduce a theoretical model of running-in and investigate rough profile evolution assuming that its kinetics is driven by two opposite processes or events, i.e., smoothening which is typical for the deformation-driven friction and wear, and roughening which is typical for the adhesion-driven friction and wear. To validate our modeling results, we examine experimentally running-in in ultrahigh vacuum friction tests for WC pin versus Cu substrate. We propose to calculate the Shannon entropy of a rough profile and to use it as a simple test for self-organization. We observe, theoretically and experimentally, how Shannon entropy as a characteristic of a rough surface profile changes during running-in, and quantifies the degree of orderliness of the self-organized system. In chapter 4, we investigate the possibility of the so-called Turing-type pattern formation during friction. Turing or reaction-diffusion systems describe variations of spatial concentrations of chemical components with time due to local chemical reactions coupled with diffusion. During friction, the patterns can form at the sliding interface due to the mass transfer (diffusion), heat transfer, various tribochemical reactions, and wear. We present a mathematical model, and solve the governing equations by using a finite-difference method. The results demonstrate a possibility of such pattern-formation. We also discuss existing experimental data that suggest that tribofilms can form in-situ at the frictional interface due to a variety of friction-induced chemical reactions (oxidation, the selective transfer of Cu ions, etc.). In chapter 5, we investigate how interfacial patterns including propagating trains of stick and slip zones form due to dynamic sliding instabilities. These can be categorized as self-organized patterns. We treat stick and slip as two phases at the interface, and study the effects related to phase transitions. Our results show how interfacial patterns form, how the transition between stick and slip zones occurs, and which parameters affect them. In chapter 6, we use Cellular Potts Model to study contact angle (CA) hysteresis as a measure of solid-liquid energy dissipation. We simulate CA hysteresis for a droplet over the tilted patterned surface, and a bubble placed under the surface immersed in liquid. We discuss the dependency of CA hysteresis on the surface structure and other parameters. This analysis allows decoupling of the 1D (pinning of the triple line) and 2D effects (adhesion hysteresis in the contact area) and obtain new insights on the nature of CA hysteresis. To summarize, we examine different cases in frictional interface and observe similar trends. We investigate and discus how these trends could be beneficial in design, synthesis and characterization of different materials and tribosystems. Furthermore, we describe how to utilize fundamental concepts for specific engineering applications. Finally, the main theme of this research is to find new applications of concept of self-organization to tribology and the role played by different physical and chemical interactions in modifying and controlling friction and wear

Application of a Thermodynamically Based Wear Estimation Methodology

Entropic and energy-based approaches are employed for prediction of wear in dry sliding contact between crossed cylinders. The methodology requires measurement or estimation of the temperature rise in the sliding system. The results of experimental tests reported in literature in conjunction with measured degradation coefficients are used to examine the validity of the proposed methodology. The approach presented is shown to be capable of predicting the wear rate for different tribopairs and under different loading conditions

Control over multi-scale self-organization-based processes under the extreme tribological conditions of cutting through the application of complex adaptive surface-engineered systems

This paper features a comprehensive analysis of various multiscale selforganization processes that occur during cutting. A thorough study of entropy production during friction has uncovered several channels of its reduction that can be achieved by various selforganization processes. These processes are (1) self-organization during physical vapor deposition PVD coating deposition on the cutting tool substrates; (2) tribofilm formation caused by interactions with the environment during operation, which consist of the following compounds: thermal barriers; Magnéli phase tribo-oxides with metallic properties at elevated temperatures, tribo-oxides that transform into a liquid phase at operating temperatures, and mixed action tribo-oxides that serve as thermal barriers/lubricants, and (3) multiscale selforganization processes that occur on the surface of the tool during cutting, which include chip formation, the generation of adhesive layers, and the buildup edge formation. In-depth knowledge of these processes can be used to significantly increase the wear resistance of the coated cutting tools. This can be achieved by the application of the latest generation of complex adaptive surface-engineered systems represented by several state-of-the-art adaptive nano-multilayer PVD coatings, as well as high entropy alloy coatings (HEAC)

Control over Multi-Scale Self-Organization-Based Processes under the Extreme Tribological Conditions of Cutting through the Application of Complex Adaptive Surface-Engineered Systems

This paper features a comprehensive analysis of various multiscale selforganization processes that occur during cutting. A thorough study of entropy production during friction has uncovered several channels of its reduction that can be achieved by various selforganization processes. These processes are (1) self-organization during physical vapor deposition PVD coating deposition on the cutting tool substrates; (2) tribofilm formation caused by interactions with the environment during operation, which consist of the following compounds: thermal barriers; Magnéli phase tribo-oxides with metallic properties at elevated temperatures, tribo-oxides that transform into a liquid phase at operating temperatures, and mixed action tribo-oxides that serve as thermal barriers/lubricants, and (3) multiscale selforganization processes that occur on the surface of the tool during cutting, which include chip formation, the generation of adhesive layers, and the buildup edge formation. In-depth knowledge of these processes can be used to significantly increase the wear resistance of the coated cutting tools. This can be achieved by the application of the latest generation of complex adaptive surface-engineered systems represented by several state-of-the-art adaptive nano-multilayer PVD coatings, as well as high entropy alloy coatings (HEAC)

A friction-wear correlation for four-ball extreme pressure lubrication

A first-ever friction-wear model for Four-Ball Extreme Pressure (EP) Lubrication test (ASTM D2783) is presented in this work. The model considers the rate of entropy generation and dissipation within the lubricated tribosystem to establish the friction-wear correlations for 12 lubricating oils comprising minerals, esters and other formulated oils. The correlations can be used to calculate the probability to pass/fail in the EP lubrication. The probability has similar trend as load-wear index from ASTM D2783 method. Besides, the friction-wear correlations allows quick estimation of EP performance of an unknown lubrication, upon comparing with that of an established one. The methods demonstrated here will help researchers or lubricant technologist to characterize the EP behavior quickly without over-relying on tribotester

Mechano-Chemical Modelling of Boundary Lubrication

Boundary lubrication is known to be significantly important in the design of machine parts. The decrease in the efficiency of the system as well as its durability when operating in boundary lubrication conditions highlights the importance of this regime. Boundary lubrication involves many different physical, chemical and mechanical phenomena which make it difficult to understand the real mechanisms of friction, wear and lubrication. Tribochemistry is undoubtedly one of the most important processes occurring in boundary lubrication. Modelling such a complicated process needs a robust physical and chemical modelling framework that is capable of capturing different phenomena. The majority of the modelling attempts in boundary lubrication covers the contact mechanics of rough surfaces with different numerical approaches. Despite the importance of the tribochemistry and its effect in reducing friction and wear of boundary lubricated contacts, there is no comprehensive modelling framework that considers tribochemistry into the boundary lubrication models. In this work, tribochemistry was implemented into the deterministic contact mechanics simulation for elastic-perfectly plastic contact of rough surfaces. A tribochemical model for the growth of the ZDDP antiwear additive was developed based on the thermodynamics of interfaces that combines formation and removal of the tribofilm. The tribochemical model was then coupled with the contact mechanics model which was developed based on potential energy principles. A modification to Archard’s wear equation was proposed which accounts for the role of ZDDP tribofilm in reducing the wear. This model was proposed based on the experimental observations of ZDDP in reducing wear. The numerical framework was then validated against experiments. The wear prediction capability of the model was validated against experiments from Mini-Traction Machine in a rolling/sliding contact. The model is able to predict changes in the topography of the surfaces and this was validated with experiments on a Micro Pitting Rig (MPR). The model shows a good potential in capturing the behaviours in boundary lubrication and opens new ways for further developments and testing the effect of different parameters in tribochemistry and wear. It can give insights in better understanding the real mechanisms of tribochemistry and also help in optimizing boundary lubricated contacts

Green Tribology

This chapter provides an overview of Green tribology, which is a new direction in the development of tribology, a new interesting area for scientific researches and a new way to turn tribology into a friend of ecological environment and saving energy. Green tribology is considered as well as close area with other “green” disciplines like green engineering and green chemistry. In the chapter, the various aspects of green tribology such as the concept, perspectives, role and goal, main principles, primary areas, challenges and directions of the future development have been discussed. It was clarified that green tribology can be defined as an interdisciplinary field attributed to the broad induction of various concepts such as energy, materials science, green lubrication, and environmental science. The most important role and goal of green tribology is improvement of efficiency by minimizing wear and friction in tribological processes to save energy, resources and protect environment, and consequently, improve the quality of human life. The twelve principles and three areas of green tribology were analyzed. Observation of these principles can greatly reduce the environmental impact of tribological processes, assist economic development and, as a result, improve the quality of life. The integration of these areas remains the major challenge of green tribology and defines the future directions of research in this field. This work also presents a rather detailed analysis of the most important effect in green tribology—the “zero-wear” effect (selective transfer effect). It was established that the “zero-wear” effect is due to self-organization in frictional interaction in tribological systems, which is the consequence of the complex tribo-chemical reactions and physico-chemical processes occurred in the area of frictional contact, that lead to the manifestation of unique tribological characteristics: super-antifrictional (friction coefficient ~ 10−3) and without wear (intensity wear ~10−15). This condition of tribo-system was provided by a protective nanocrystalline servovite film made of soft metal with unusual combination of mechanical properties