Interfacial Mechanics
eBook - ePub

Interfacial Mechanics

Theories and Methods for Contact and Lubrication

Jane Wang, Dong Zhu

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eBook - ePub

Interfacial Mechanics

Theories and Methods for Contact and Lubrication

Jane Wang, Dong Zhu

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Understanding the characteristics of material contact and lubrication at tribological interfaces is of great importance to engineering researchers and machine designers. Traditionally, contact and lubrication are separately studied due to technical difficulties, although they often coexist in reality and they are actually on the same physical ground. Fast research advancements in recent years have enabled the development and application of unified models and numerical approaches to simulate contact and lubrication, merging their studies into the domain of Interfacial Mechanics.

This book provides updated information based on recent research progresses in related areas, which includes new concepts, theories, methods, and results for contact and lubrication problems involving elastic or inelastic materials, homogeneous or inhomogeneous contacting bodies, using stochastic or deterministic models for dealing with rough surfaces. It also contains unified models and numerical methods for mixed lubrication studies, analyses of interfacial frictional and thermal behaviors, as well as theories for studying the effects of multiple fields on interfacial characteristics. The book intends to reflect the recent trends of research by focusing on numerical simulation and problem solving techniques for practical interfaces of engineered surfaces and materials.

This book is written primarily for graduate and senior undergraduate students, engineers, and researchers in the fields of tribology, lubrication, surface engineering, materials science and engineering, and mechanical engineering.

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CRC Press

1 Introduction

1.1 Significance of the Topics

“Interface” is a general term that concerns a boundary between two different materials, which can possibly be in any combination of solid(s), liquid(s), and gas(es). Interfacial phenomena are complex in nature. Relevant studies on interfacial behaviors have been in a wide range from different aspects, often multi-scale and interdisciplinary that may involve many branches of science and engineering. Great efforts have been made in order to understand interfacial mechanics, physics, and chemistry in different fields. However, so far, there has not been a well-developed branch of science that covers all the different types of interfacial phenomena in a general sense. Interfacial mechanics, in fact, is still an evolving field of study generally in its infancy.
In this book, our discussion will focus on a special type of interface system, called “tribological interface”, or “interface” in short, that consists of two solid body surfaces in contact and possible relative motion with or without fluid(s) in between. This specific type of interface system is widely seen in reality and extremely important in science and engineering practice. Figure 1.1 gives a sketch showing two solid surfaces in contact and relative motion, but, in reality, fluid(s) (often acts as lubricant), boundary films and a small quantity of debris/particles may also be observed in this interface system. For commonly used metallic materials, thin oxide films, as well as some other surface layers and coatings, may often exist in engineering reality. Because the boundary and oxide films are usually extremely thin, and the size and quantity of possible debris very small, they are often ignored in most interfacial analyses. Therefore, a basic model used in this book is constructed with two solid bodies having smooth or rough surfaces in contact and possible relative motion with or without fluid lubricant in between.
FIGURE 1.1 Schematic of a tribological interface system.
Power and motion are transmitted through interfaces, which are often lubricated in one way or another, at surface contact locations of various components that are basic elements of all kinds of vehicles, industrial machineries, engineering equipment, and scientific devices. As is well known, a large portion of energy produced globally is dissipated through frictional loss that occurs mainly due to the interaction of surfaces of machine components. Also, it has been found that, today, roughly more than 70%–80% of mechanical component failures take place or originate at surfaces due to severe contact, rubbing, and insufficient lubrication. Machine performance, loading capacity, efficiency, durability, and reliability appear to be macro-scale events that always need to be well controlled and optimized in engineering practice. However, they are dependent strongly upon micro- and nano-scale interfacial characteristics. A deep understanding of the nature of such interfacial mechanisms, therefore, is vital to components design and product development. In the 21st century, critical issues associated with energy shortage, environmental pollution, and global warming, as well as their impact to economic growth and stability, impose increasingly strong challenges to researchers and engineers, thus striving for better efficiency and reliability becomes more urgent than ever before.
In reality, solving interfacial problems may involve multi-scale multidisciplinary science and engineering, including continuum mechanics, rheology, mechanical design theory, materials science, thermal dynamics, physics, chemistry, and others. Associated studies may span from macro- to micro- all the way down to nano- and subnano-scales, as depicted in Figure 1.2.
For such complicated multi-scale problems, however, continuum mechanics-based contact and lubrication analyses are fundamental, providing basic information, such as interfacial pressure due to solid contact and/or hydrodynamics, lubricant film thickness or gap, surface deformation, friction/traction, temperature, and subsurface strain, and stress distributions, which are necessary for performance, efficiency, and failure predictions. Also, it serves as a foundation of further in-depth investigations in the areas of interfacial physics, chemistry, and materials failure mechanisms.
FIGURE 1.2 Multi-scale system problems.
It is well known that material failures found in mechanical components can generally be categorized into two types: structure (or bulk) failures and surface failures. For structure failures, predicting and preventing technologies have been much better developed. For example, not long ago, people still relied on photoelasticity, strain gauge experiments, and destructive tests to evaluate structure strengths. In the last few decades, as the FEM/CAD technologies have been well developed with commercial software packages readily available, system and component structure strength can now be quickly and accurately predicted by using computers. Product development cycles have been greatly shortened, and photoelasticity and destructive tests can be avoided in most cases. Today structure failures have been significantly reduced in engineering reality.
On the other hand, however, the majority of component failures fall in the surface failure category. These include sliding and fretting wear, scuffing, pitting due to contact fatigue, and others. Unfortunately, so far, interfacial sciences and problem-solving technologies are still mostly premature, and few commercial software packages are conveniently available for engineers to accurately evaluate surface strength of various components.
In fact, today modeling and analyzing contact and lubrication problems, predicting interface performance, and evaluating surface strength and life often appear to be a bottleneck in advanced product design and development. Moreover, critical problems found during laboratory and field tests, machine operations, and customer warranty services are usually associated with malfunction and failure of interface systems, statistically much more than any other problems. There are great challenges, together with vast opportunities, for further development in the field of interfacial sciences, and the importance of understanding these interfaces can never be overestimated.

1.2 Tribological Interface Systems

The interface in contact with relative motion is often referred to as a tribological interface. The word “tribology” originated from the Greek word, tribos, which means rubbing between two surfaces or simply refers to a science about “rubbing” (Halling, 1971). Tribology, in other words, is a branch of science that develops theories and technologies for improving functionality, efficiency, and durability of the surfaces in contact and relative motion.
It is important to note that a tribological interface usually involves two solid surfaces in contact with or without lubricating media in between that can possibly be liquid(s), solid(s), gas(es) or their combination(s). Also, the two solid bodies may have surface oxide layers, adsorbed layers, boundary layers, non-uniform layers due to various fabrication processes and surface treatments. Within the solid bodies, there may be impurities, particles, fibers, inclusions, defects, voids, and other types of inhomogeneity. Typically, ultrathin oxide and bou...

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