Tribology for Scientists and Engineers

Understanding engineered surfaces is very important for solving many scientific problems that involve friction, contact mechanics, heat conduction, electric current conduction, and component design. In this chapter, the fundamentals of engineering surfaces and surface texturing are discussed. Various surface layer types are defined, and techniques for generating and characterizing them are presented. Surface roughness measurement techniques to obtain and define roughness parameters using surface profilometry and optical methods are discussed in detail. Surface textures and structures are then classified in terms of various roughness parameters. Finally, experimental results that demonstrate the influence of surface texture on friction are discussed.

Friction is a universal phenomenon which is observed in a great variety of sliding and rolling situations. The study of friction and wear has long been of enormous practical importance, since the functioning of many mechanical, electromechanical, and biological systems depends on the appropriate friction and wear values. In recent decades, this field has received increasing attention as it has become evident that the consumption of resources resulting from high friction and wear is greater than 6 % of the Gross National Product of the USA. In this chapter, various theories, mechanisms, and factors affecting of friction and wear were discussed.

Contact mechanics is a fundamental field of tribology and generally refers to the interaction of solid surfaces. This interaction or contact can occur on many different scales, ranging from nanoscale asperities up to tires on roads and even contact between tectonic plates. This chapter reviews the basic technical information available in predicting the contact area, pressure, stresses, and forces that occur when surfaces interact. The chapter considers different geometries such as spheres and wavy surfaces and also outlines how to consider elastic and plastic deformation. The phenomena of creep and adhesion that are important for many tribological applications, and especially biological contacts, are also discussed. Finally, the chapter concludes by covering methods used to model the complicated situation of contact between rough surfaces that contain many different geometrical features.

In order to conduct an accurate tribological investigation of two materials in sliding contact, a dedicated machine or tribometer is required which can measure both the friction and wear between the materials. A carefully selected tribometer configuration can be used to simulate all the critical characteristics of a certain specific situation or can be used as a quick way to screen various candidate materials before subjecting them to that situation. This chapter focuses on basic experimental methods as well as the most common test configurations. A good knowledge of tribometers will allow the engineer to choose the most appropriate system to fulfil requirements. Additional knowledge of the environment in which the test should be performed will aid the engineer in simulating true in-service conditions which will make the data produced more meaningful. Such conditions (e.g. temperature, humidity, gaseous environment) can then be combined with actual experimental conditions (applied load, sliding speed, contact pressure, etc.) to provide a focused and useful experiment.

The interface temperature between sliding surfaces plays an important role in the tribological performance of the surfaces. Temperature rise at the tribo-contacts can cause microstructural changes and tribo-chemical reactions, which in turn influence the operating wear mechanisms and wear transitions. Flash temperatures that arise at the tip of asperities are much higher than the surface bulk temperatures. Exact estimation of flash temperatures is rather difficult when compared to the surface bulk temperatures. However, calculations of surface bulk temperatures using models that are based on ab initio estimations, at the most, give values that are only indicative but not exact. The parametric uncertainties in these models give rise to either underestimates or overestimates of surface bulk temperatures. Estimation of surface bulk temperatures based on measurements can give temperature values closer to the real ones. This chapter presented a brief historical background on the subject of estimation of temperatures, earlier models based on ab initio calculations, and a new method to estimate surface bulk temperatures based on measurement of temperature at two points along a pin axis away from the sliding interface and in a realistic flow field of air around the pin. Discussion on the issues in the earlier models and comparison of temperature values obtained by these models with those estimated by measurements clearly indicate the importance of undertaking measurements to arrive at better estimates of temperature values that are closer to the real ones.

The surface properties of metals and alloys become important when these materials are used especially for tribological applications. Some basic concepts involved during wear of metals and alloys are briefly discussed in this chapter. Delamination theory of adhesive wear which is dominating wear mechanism for most metals and alloys is discussed. Most of the tribological joints are exposed to environmental oxygen when used in atmospheric conditions. Oxidation becomes problematic for such and high-temperature sliding applications when oxygen source is readily available at the interface. The debris formation mechanism and oxidation during sliding are included in this chapter. Information on oxidation and tribological behavior of 60NiTi is reviewed as it is a potential alloy for tribo-element applications. A brief description on phase transformation and high-temperature tribology of metallic materials is also included. The wear of materials at the interface depends on the interfacial strength of the sliding materials. In high-temperature oxidative wear, wear performance can be determined by the type of oxides formed on the sliding surfaces.

Ceramic materials are well suited for tribological applications due to their superior hardness, high wear resistance, good chemical resistance, stability at high temperatures, etc. Ceramic pairs are commonly used in extreme environmental applications, such as high loads, high speeds, high temperatures and corrosive environments. This present chapter briefly discusses the friction and wear behaviour of ceramics and ceramic matrix composites. Friction of ceramics depends largely on fracture toughness besides normal load, sliding speed, temperature, etc. Wear mechanisms in ceramics involve fracture, tribo-chemical effects and plastic flow. In case of ceramic matrix composites, the incorporation of the secondary phase into ceramic matrix results in the improvement of both mechanical properties and friction performance. In nano-ceramics, reduction in microstructural scale yields significant improvements in wear resistance. Tribological behaviour of ceramics in biological environment is also highlighted.

Metal matrix composites (MMCs) are an important class of engineering materials that are increasingly replacing a number of conventional materials in the automotive, aerospace, marine, and sports industries due to their lightweight and superior mechanical properties. In MMCs, nonmetallic materials are embedded into the metals or the alloys as reinforcements to obtain a novel material with attractive engineering properties, such as improved ultimate tensile strength, ductility, toughness, and tribological behavior. In this chapter, an attempt has been made to summarize the tribological performance of various MMCs as a function of several relevant parameters. These parameters include material parameters (size, shape, volume fraction, and type of the reinforcements), mechanical parameters (normal load and sliding speed), and physical parameters (temperature and the environment). In general, it was shown that the wear resistance and friction coefficient of MMCs are improved by increasing the volume fraction of the reinforcements. As the normal load and sliding speed increase, the wear rate of the composites increases and the friction coefficient of the composites decreases. The wear rate and friction coefficient decrease with increasing temperature up to a critical temperature, and thereafter both wear rate and friction coefficient increase with increasing temperature. The nano-composites showed best friction and wear performance when compared to micro-composites.

Optimization of coefficient of friction in association with minimum wear is a general requirement to reduce energy consumption due to friction and wear losses. Lubrication such as solid and liquid is utilized to meet low friction and wear demands. Extreme environmental conditions such as space and high-temperature applications limit their usefulness. There is a need to design newer class of coatings for such applications. Design and selection parameters of coatings and their tribology are discussed in this chapter. These parameters include scale-dependent failure modes (nano- and micrometer length scale), state of stress at the interface, material properties, and chemical interactions at the interface. The requirements for selection of coating for friction applications are included. Tribology of low-friction coatings such as graphite, molybdenum disulfide, diamond-like carbon, chromium-based coatings, and polymeric coatings is discussed. Effects for service conditions such as load, nitrogen, humidity, and temperature for selected coatings are listed. Knowledge of the interfacial phenomena plays very important role in selection and development of coatings for tribological applications.

Lubricants are substances used to minimize the friction and wear of moving parts. Additionally, they can serve to distribute heat, remove contaminants, and improve the efficiency and lifetime of mechanical systems. Lubricants can generally be categorized as liquid, solid, or gaseous. Liquid lubricants consist of base oils such as natural oils, mineral (petroleum) oils, and synthetic oils with combinations of additives that further enhance the properties of the lubricants. Solid or dry lubricants are generally powders or semisolids in the form of a grease or solid–liquid suspension. Gaseous lubricants have a much lower viscosity than liquid or solid lubricants and utilize gasses such as air under pressure. The selection of an appropriate lubricant for a mechanical system requires a thorough understanding of the rheology of lubricants, the effects of additive combinations, and the knowledge of lubrication theory. Lubrication theory is linked to numerous fields of expertise outside of tribology, and without this interdisciplinary aspect, the progression of lubricants and lubrication technologies within the vast array of applications may not have reached the necessary levels of success. The use of liquid lubricants is ubiquitous in most applications, ranging from automotive fluids, to industrial oils, and process oils. Within the lubrication industry, there are over 10,000 different lubricants used around the world. This chapter explores the many aspects of lubricants and lubrication technologies including lubrication fundamentals, rheology of liquid lubricants, liquid lubricant additives, and liquid lubrication theory.

Lubricants are extensively used between contacting surfaces to reduce friction and wear. Typically, liquid lubricants are used to achieve low friction and wear. However, these lubricants are not effective in elevated temperature applications or vacuum environments. For these reasons, solid lubricants are utilized to meet these operational needs, where liquid lubrication is impractical. Solid lubricants are only effective as long as they are present in the tribo-interface. Therefore, it is desirable to provide a constant supply of solid lubricant material to the contacting surface. This is often achieved by incorporating solid lubricants as a second phase in the base material. These composite materials have the ability to achieve low friction and wear at the contact surfaces without any external supply of lubrication during sliding. Metal matrix composites reinforced with lamellar solid lubricant particles such as graphite are being used as self-lubricating materials for various engineering applications. In this chapter, the tribological behavior of metal and ceramic matrix composites reinforced with graphite particles has been reviewed. More specifically, copper–graphite, nickel–graphite, magnesium–graphite, silver–graphite, aluminum–graphite, silicon nitride–graphite, and alumina–graphite composites are studied. The influence of various environmental and mechanical parameters on the friction coefficient and wear rate is discussed. It was found that the amount of graphite released on the worn surface forms a thin transfer film on the contact surfaces. This transfer film reduces the overall friction coefficient and wear rate. The presence of the graphite-based transfer film increases the seizure resistance and enables the contacting surfaces to run under boundary lubrication without galling. The formation and retention of this transfer film on the sliding surface as well as its composition, area fraction, thickness, and hardness are important factors in controlling the friction and wear behavior of the material. The effectiveness of the transfer film also depends on the nature of the sliding surface, the test condition, environment, and the graphite content in the composite.

The purpose of this chapter is to give the reader a basic understanding of particles in sliding contact. First, we will describe granular flows (the flow of inelastic particles that transfer momentum primarily through collisions) from a tribology perspective, including modeling and experiments that have been conducted inside and outside of the tribology community. Second, slurry flow (particles in gas or liquids) tribosystems will be discussed including models and experiments related to the flow of particles in fluids. And finally, we conclude with a section on powder lubrication (soft particles which coalesce under load and coat surface asperities), where thick and thin film powder lubrication is discussed along with select modeling and experimental approaches.

Over the last seven decades, extreme operating conditions encountered in many industrial and engineering applications—particularly those within the aerospace industry—have driven the evolution of more advanced commercial lubricants. While many long-standing lubrication techniques utilize liquid or grease-type lubricants, new tribological applications have developed over the last 70 years that have led to the development of lubricants derived from solid materials and coatings with self-lubricating properties. Many tribological applications require two surfaces to slide over one another in relative motion, resulting in friction and wear, such as in cutting and forming operations, gears, bearings, and engine parts. Increasingly, more of these applications are operating in extreme environments (such as high vacuum, microgravity, high/low temperatures, extreme pressure, space radiation, and corrosive gas environments) that are beyond the tolerable and usable domain of liquid and grease-based lubricants. This has propelled the development of dry/solid lubricants that are nonvolatile and can withstand such extreme environmental conditions. In this chapter, a review of the state of solid lubrication and the utilization of solid lubricants as powder transfer films, thin film coatings, colloidal mixtures, and composite matrices are presented for the field of tribology.

Increased demand to protect the environment from mineral oil based lubricants has necessitated replacing them with products derived from natural resources. In addition to this a combination of environmental, health, and economic challenges has also renewed interest in the development and use of green lubricants over the last two decades. This chapter gives an overview of green lubricants beginning from their ancient use to the challenges prospective of the lubricant industry. The review includes structural and chemical properties of important vegetable oils and chemical modification of vegetable oils. Tribological behavior of products based on vegetable oils, green additives, and ionic liquids under varied testing conditions is also presented. This study also gives an insight into the testing procedures and sustainability aspects of green lubricants and additives.

Nanotribology is a study on friction phenomena occurring at nanometer scale. The distinction between nanotribology and conventional tribology is primarily due to the effect of surface forces in the determination of the adhesion and friction behavior of the system. Commercial bearings and lubricating oils reduce friction in the macroscopic machines; however, the tribological issues on small devices such as microelectromechanical systems and nanoelectromechanical systems require other solutions. Their high surface-to-volume ratio leads to severe adhesion and friction issues, which dramatically reduce their reliability and lifetime. This chapter reviews the basic concepts for handling the adhesion and friction issues at nanoscale. A brief summary on analytical models of single-asperity contact as well as the basic concepts on the surface forces occurring at nanometer gap are discussed in the first two sections, followed by three case studies: (1) experimental measurements on adhesion and friction at single-asperity contact, (2) experimental measurements on adhesion at multi-asperity contact, and (3) biomimetics: controlling nano-adhesion and nano-friction.

Study of surface properties is of significance in tribology since interaction between objects takes place at surface level and also surface properties are different than the bulk of the material. This chapter discusses about different surface probe techniques which are used to characterize the surface at nanoscale. Depending upon the interaction of the probe (electron/mechanical) with the sample, different techniques are developed. In case electron probe, scanning electron microscope and transmission electron microscope are used to study surface as well as subsurface information. In case of mechanical probes, atomic force microscope and nanoindentation techniques are widely used in nanoscale tribology. Each of the above techniques is discussed in this chapter with the working principle to application along with the limitations. Finally, recent developments in real-time tribological studies or in situ techniques are discussed.

In biotribology and human tribology, tribological theories are applied to biological systems and human interactions, respectively. This chapter focuses on the human tribology fields, slip and fall accidents and hand-object interaction, and the biotribology fields, ocular and oral tribology. Slip and fall accidents are caused by low friction between the shoe and floor surface, frequently due to a fluid contaminant. Experimental methods of evaluating shoe-floor friction are most relevant to human slips when mimicking the dynamics of human stepping and the environmental conditions (using common shoes, floors, and/or contaminants). The tribological mechanisms affecting shoe-floor friction are adhesion, hysteresis, boundary lubrication, and hydrodynamic lubrication. Modeling efforts have shown that certain floor roughness parameters correlate well with shoe-floor friction, that adhesion and hysteresis can be simulated with finite element analysis, and that models using Reynolds equation can simulate the hydrodynamic effects. Skin friction is essential for everyday activities such as gripping and manipulating objects. The friction of the outermost layer of the skin, the stratum corneum, is modulated by hydration allowing the body to optimize skin friction through perspiration. Increasing skin friction leads to improved performance by increasing grip strength and fine motor speed. The tribological interaction in the eye, with or without a contact lens, contributes significantly to comfort. Eye discomfort and the disorder dry eye syndrome occur when abnormalities occur to any of the three tear film layers that protect and lubricate the eye. Low friction is essential to the comfort of contact lens. Experiments and models have implicated that adhesion, hysteresis, boundary lubrication, and elastohydrodynamic lubrication may all contribute to eye lens friction. Tooth wear due to abrasion and corrosion is a major threat to healthy teeth and dental restorative surfaces. Tribological principles such as two-body wear, three-body wear, and corrosion along with innovative modeling and experimental techniques have revealed personal risk factors and the effects of behavior on dental wear. Another application of oral tribology is creaminess perception in the mouth. Creaminess of a food is largely dependent on its ability to lubricate the mouth surfaces. Replication of this sensation using low-fat alternatives to traditional high-fat creamy foods has the potential to achieve reductions in obesity. This chapter demonstrates the applications of tribology to biological systems.

Green Tribology is a new area of tribology studying ecological approaches, methods, and applications in tribology. The major premise of green tribology is minimization of friction and wear, and reduction or elimination of lubrication. It includes eco-friendly materials and coating, green lubricants (biodegradable lubricants) and biomimetic surfaces used in tribological application. Using sustainable chemistry and engineering principles, biomimetic approaches and surface texturing are other concerns of green tribology. The area also takes degradation of surfaces, coatings, and components into consideration during design stages. The tribology of renewable sources of energy can also be considered as a relatively new challenge. We discuss areas and principles of green tribology and current developments in biomimetic materials and surfaces.

Metal/alloys have been used in orthopedic and dental implants for many years. In a physiological environment, they are susceptible to materials degradation affecting its performance and lead to gradual and early failures due to the continuous exposure to the variation in in vivo mechanical and chemical conditions. Tribocorrosion is a new area of research that links tribology and corrosion, which can be defined as the irreversible material degradation process resulting from the synergistic interaction of wear and corrosion phenomena on surfaces subjected to a relative contact movement in biological environments. This book chapter describes some fundamental aspects about this new research area and focused on its significances in biomedical application, namely, orthopedics and dentistry.

Total joint arthroplasty (TJA) is one of the major successes of the twentieth century changing the lives of millions of people, both younger and older generations. However, critical problems like wear, loosening, and osteolysis still remain in the process of realizing its capabilities to the fullest extent. The change in the lifestyle of the people has further aggravated the seriousness of the problem. Thus, the present chapter is focused on the “Wear of biomedical implants,” which introduces the various possibilities, causes and concerns, critical issues and solutions to the above-said problems enabling a neophyte to understand the current scenario. It briefly discusses the basic tribological aspects involved in a typical synovial joint, its structure, and wear mechanism involved. The discussion then turns towards highlighting the primary causes and concerns of wear debris, various tools and techniques to estimate the wear in specific relevance to artificial joint materials. It also highlights the various implant materials and techniques for reduction of wear in the implants.

This chapter revisits tribology tests in metal cutting in order to obtain new fundamental knowledge on friction and to understand which technical modifications and operating parameters need to be developed and implemented in order to obtain good estimates of the coefficient of friction. The methodology draws from the development of new equipment and testing procedures focused on the interaction between surrounding medium, surface roughness and freshly formed surfaces to the independent determination of the coefficient of friction.

The assessment of the coefficient of friction obtained from experimentation with two of the most commonly utilized simulative tribology tests in metal working (pin-on-disc and ring compression tests) against that determined in orthogonal metal cutting conditions allows concluding that the former, performed in dry friction conditions with adequate control of surface morphology and under a protective shield of Argon, is capable of modeling contact with friction in close agreement with real metal cutting.

The identification of operative testing conditions that are capable of merging the estimates of the coefficient of friction provided by the different tribology tests ensures a unified view of tribologists and metal cutting experts on the accuracy, reliability, and validity of simulative tribology tests for metal cutting applications.

Tribological losses at interfaces can cost important recourses such as time and money. Knowledge of interface chemistry is vital to understand fundamentals of tribological parameters. Sliding of two surfaces provide favorable thermodynamic parameters for chemical reactions to take place at these interfaces. The study of these reactions primarily studied under special fields of tribology, i.e., tribo-chemistry and tribo-corrosion. It is difficult to separate these two fields. Usually, study of tribo-corrosion deals with surface deterioration due to the synergism of tribological factors, electrical stimulus, and corrosion. The thermodynamics approach can be utilized to understand the tribo-chemical reactions. This chapter provides different approaches taken to study these two fields. In this chapter, some of the mechanisms responsible for and applications of tribo-chemical interactions are discussed for example tribo-emission, tribo-chemical polishing, tribo-chemistry of magnetic media drive. Tribo-corrosion of coatings and metallic materials is briefly discussed. The case study of complex tribo-chemistry in sugarcane roller mill is included.

Chemical–mechanical planarization (CMP) is one of the most important process steps in making integrated circuits. Tribology plays significant roles in material removal, polishing, and planarization. This chapter provides an introduction to CMP and discusses evolution, roles, and effects of tribology in this important industrial process. Tantalum and copper CMP are discussed here as two examples in understanding mechanisms of CMP.

The ability to produce a variety of shapes from a block of metal at high rates of production has been one of the real technological advances of the current century. This transition from hand-forming operations to mass-production methods has been an important factor in the great improvement in the standard of living, which occurred during the period. With these forming processes, it is possible to mechanically deform metal into a final shape with minimal material removal. The use of metal forming processes is widely spread over many different industries. In metal forming processes, friction forces between metal and forming tools play an important role because of their influence on the process performance and on the final product properties. In many instances, this frictional behavior is often taken into account by using a constant coefficient of friction in the simulation of metal forming processes. Several different types of instruments are constructed to measure the coefficient of friction for different materials. In this chapter, the fundamental concept of forming processes and the influence of friction in metal forming are discussed. A case study on the influence of friction based on surface texture during metal forming is also presented.

Proper tribological practices are extremely important from the view point of reliable and maintenance free operation of machine components. Friction, wear, and lubrication are three main domains of tribology. Proper interfacing among these domains is essential to have a better performance of machine components. Bearings are most critical machine elements which imparts constrained relative motion between two machine components. Bearings either have sliding contact or rolling contact. In the present chapter different types of bearings have been discussed. The main focus of this chapter is on hydrostatic/hybrid fluid-film journal bearings. The components of the hydrostatic/hybrid bearing system that have significant impact on bearing performance have been described. The analysis of fluid-film journal bearings and their performance characteristics parameters have been discussed. The chapter also discusses some current research trends in the design of hydrostatic/hybrid journal bearings. In concluding section the chapter discusses some of the results pertaining to the current issues of research, which needs to be considered for an accurate and realistic design of hydrostatic/hybrid journal bearing system, such as the number of recesses, type of restrictor, shape of recess, influence of turbulence and flexibility, etc.

This chapter addresses some of the basic mechanical and chemical issues affecting tribology in a broad range of macroscale applications such as space; automotive; rail transport; earthmoving, mining, and mineral processing; marine equipment; and gas and steam turbines. Many of the featured system’s successes rely on understanding and solving tribological issues involving friction, lubrication, and wear of mating components. In many instances, solutions require a multidisciplinary approach to achieve the desired efficiency, reliability, compliance, and safety necessary to ensure economical and feasible operation. The presented topics were chosen to illustrate the broad range and importance that tribology plays beyond the traditional industrial and manufacturing applications, as well as to illustrate the numerous encounters of tribology in our lives.

This chapter focuses on microscale applications involving basic mechanical and biological issues affecting tribology in magnetic storage devices, microelectromechanical devices, flexible media, and biomedical systems. Similar to the factors influencing macroscale applications—described in the earlier chapter—microscale applications of tribology rely on understanding and solving complex tribological issues involving friction, adhesion, lubrication, and wear of components in relative motion. Although many of the presented solutions are multidisciplinary in approach, they also depend on a thorough understanding of the smaller length-scales to achieve the desired efficiency, reliability, compliance, and safety necessary to ensure economical and practical operation. The presented topics are chosen to further illustrate the broad range and importance that tribology plays in lesser known applications which people encounter in their daily lives from computer hard drives to printers and from contact lenses to basketballs.