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Inhaltsverzeichnis

Frontmatter

1. Introduction — Measurement Techniques and Applications

Summary
In this introductory chapter, the definition and history of tribology and their industrial significance and origins and significance of an emerging field of micro/nanotribology are described. Next, various measurement techniques used in micro/nanotribological and micro/nanomechanical studies are described. The interest in micro/nanotribology field grew from magnetic storage devices and latter the applicability to emerging field micro/nanoelectromechanical systems (MEMS/NEMS) became clear. A few examples of magnetic storage devices and MEMS/NEMS are presented where micro/nanotribological and micro/nanomechanical tools and techniques are essential for interfacial studies. Finally, reasons why micro/nanotribological and micro/nanomechanical studies are important in magnetic storage devices and MEMS/NEMS are presented. In the last section, organization of the book is presented.
Bharat Bhushan

Scanning Probe Microscopy

2. Scanning Probe Microscopy — Principle of Operation, Instrumentation, and Probes

Summary
Since the introduction of the STM in 1981 and AFMin 1985, many variations of probe based microscopies, referred to as SPMs, have been developed. While the pure imaging capabilities of SPM techniques is dominated by the application of these methods at their early development stages, the physics of probe—sample interactions and the quantitative analyses of tribological, electronic, magnetic, biological, and chemical surfaces have now become of increasing interest. Nanoscale science and technology are strongly driven by SPMs which allow investigation and manipulation of surfaces down to the atomic scale. With growing understanding of the underlying interaction mechanisms, SPMs have found applications in many fields outside basic research fields. In addition, various derivatives of all these methods have been developed for special applications, some of them targeted far beyond microscopy.
This chapter presents an overview of STM and AFM and various probes (tips) used in these instruments, followed by details on AFM instrumentation and analyses.
Bharat Bhushan, Othmar Marti

3. Probes in Scanning Microscopies

Summary
Scanning probe microscopy (SPM) provides nanometer-scale mapping of numerous sample properties in essentially any environment. This unique combination of high resolution and broad applicability has lead to the application of SPM to many areas of science and technology, especially those interested in the structure and properties of materials at the nanometer scale. SPM images are generated through measurements of a tip-sample interaction. A well-characterized tip is the key element to data interpretation and is typically the limiting factor.
Commercially available atomic force microscopy (AFM) tips, integrated with force sensing cantilevers, are microfabricated from silicon and silicon nitride by lithographic and anisotropic etching techniques. The performance of these tips can be characterized by imaging nanometer-scale standards of known dimension, and the resolution is found to roughly correspond to the tip radius of curvature, the tip aspect ratio, and the sample height. Although silicon and silicon nitride tips have a somewhat large radius of curvature, low aspect ratio, and limited lifetime due to wear, the widespread use of AFM today is due in large part to the broad availability of these tips. In some special cases, small asperities on the tip can provide resolution much higher than the tip radius of curvature for low-Z samples such as crystal surfaces and ordered protein arrays.
Several strategies have been developed to improve AFM tip performance. Oxide sharpening improves tip sharpness and enhances tip asperities. For high-aspect-ratio samples such as integrated circuits, silicon AFM tips can be modified by focused ion beam (FIB) milling. FIB tips reach three-degree cone angles over lengths of several microns and can be fabricated at arbitrary angles. Other high resolution and high-aspect-ratio tips are produced by electron beam deposition (EBD) in which a carbon spike is deposited onto the tip apex from the background gases in an electron microscope. Finally, carbon nanotubes have been employed as AFM tips. Their nanometer-scale diameter, long length, high stiffness, and elastic buckling properties make carbon nanotubes possibly the ultimate tip material for AFM. Nanotubes can be manually attached to silicon or silicon nitride AFM tips or “grown” onto tips by chemical vapor deposition (CVD), which should soon make them widely available. In scanning tunneling microscopy (STM), the electron tunneling signal decays exponentially with tip-sample separation, so that in principle only the last few atoms contribute to the signal. STM tips are, therefore, not as sensitive to the nanoscale tip geometry and can be made by simple mechanical cutting or electrochemical etching of metal wires. In choosing tip materials, one prefers hard, stiff metals that will not oxidize or corrode in the imaging environment.
Jason H. Hafner

4. Noncontact Atomic Force Microscopy and Its Related Topics

Summary
The scanning probe microscopy (SPM) such as the STM and the NC-AFM is the basic technology for the nanotechnology and also for the future bottom-up process. In Sect. 4.2, the principles of AFM such as operating modes and the frequency modulation method of the NC-AFM are fully explained. Then, in Sect. 4.3, applications of NC-AFM to semiconductors that make clear its potentials such as spatial resolution and functions are introduced. Next, in Sect. 4.4, applications of NC-AFM to insulators such as alkali halides, fluorides and transition metal oxides are introduced. At last, in Sect. 4.5, applications of NCAFM to molecules such as carboxylate (RCOO) with R = H, CH3, C(CH3)3 and CF3 are introduced. Thus, the NC-AFM can observe atoms and molecules on various kinds of surfaces such as semiconductor, insulator and metal oxide with atomic/molecular resolutions. These sections are essential to understand the status of the art and the future possibility of NC-AFM that is the second generation of atom/molecule technology.
Seizo Morita, Franz J. Giessibl, Yasuhiro Sugawara, Hirotaka Hosoi, Koichi Mukasa, Akira Sasahara, Hiroshi Onishi

5. Low Temperature Scanning Probe Microscopy

Summary
This chapter is dedicated to scanning probe microscopy, one of the most important techniques in nanotechnology. In general, scanning probe techniques allow the measurement of physical properties down to the nanometer scale. Some techniques, such as the scanning tunneling microscope and the scanning force microscope even go down to the atomic scale. The properties that are accessible are various. Most importantly, one can image the arrangement of atoms on conducting surfaces by scanning tunneling microscopy and on insulating substrates by scanning force microscopy. But also the arrangement of electrons (scanning tunneling spectroscopy), the force interaction between different atoms (scanning force spectroscopy), magnetic domains (magnetic force microscopy), the local capacitance (scanning capacitance microscopy), the local temperature (scanning thermo microscopy), and local light-induced excitations (scanning near-field microscopy) can be measured with high spatial resolution. In addition, some techniques even allow the manipulation of atomic configurations.
Probably the most important advantage of the low-temperature operation of scanning probe techniques is that they lead to a significantly better signal-to-noise ratio than measuring at room temperature. This is why many researchers work below 100 K. However, there are also physical reasons to use low-temperature equipment. For example, the manipulation of atoms or scanning tunneling spectroscopy with high energy resolution can only be realized at low temperatures. Moreover, some physical effects such as superconductivity or the Kondo effect are restricted to low temperatures. Here, we describe the design criteria of low-temperature scanning probe equipment and summarize some of the most spectacular results achieved since the invention of the method about 20 years ago. We first focus on the scanning tunneling microscope, giving examples of atomic manipulation and the analysis of electronic properties in different material arrangements. Afterwards, we describeresults obtained by scanning force microscopy, showing atomic-scale imaging on insulators, as well as force spectroscopy analysis. Finally, the magnetic force microscope, which images domain patterns in ferromagnets and vortex patterns in superconductors, is discussed. Although this list is far from complete, we feel that it gives an adequate impression of the fascinating possibilities of low-temperature scanning probe instruments.
In this chapter low temperatures are defined as lower than about 100K and are normally achieved by cooling with liquid nitrogen or liquid helium. Applications in which SPMs are operated close to 0 °C are not covered in this chapter.
Markus Morgenstern, Alexander Schwarz, Udo D. Schwarz

6. Dynamic Force Microscopy

Summary
This chapter presents an introduction to the concept of the dynamic operational mode of the atomic force microscope (dynamic AFM). While the static, or contact mode AFM is a widespread technique to obtain nanometer resolution images on a wide variety of surfaces, true atomic resolution imaging is routinely observed only in the dynamic mode. We will explain the jump-to-contact phenomenon encountered in static AFM and present the dynamic operational mode as a solution to overcome this effect. The dynamic force microscope is modeled as a harmonic oscillator to gain a basic understanding of the underlying physics in this mode.
Dynamic AFM comprises a whole family of operational modes. A systematic overview of the different modes typically encountered in force microscopy is presented, and special care is taken to explain the distinct features of each mode. Two modes of operation dominate the application of dynamic AFM. First, the amplitude modulation mode (also called tapping mode) is shown to exhibit an instability, which separates the purely attractive force interaction regime from the attractive-repulsive regime. Second, the self-excitation mode is derived and its experimental realization is outlined. While the first is primarily used for imaging in air and liquid, the second dominates imaging in UHV (ultrahigh vacuum) for atomic resolution imaging. In particular, we explain the influence of different forces on spectroscopy curves obtained in dynamic force microscopy. A quantitative link between the measurement values and the interaction forces is established.
Force microscopy in air suffers from small quality factors of the force sensor (i.e., the cantilever beam), which are shown to limit theachievable resolution. Also, the above mentioned instability in amplitude modulation mode often hinders imaging of soft and fragile samples. A combination of the amplitude modulation with the self-excitation mode is shown to increase the quality, or Q-factor, and extend the regime of stable operation, making the so-called Q-control module a valuable tool. Apart from the advantages of dynamic force microscopy as a nondestructive, high-resolution imaging method, it can also be used to obtain information about energy dissipation phenomena at the nanometer scale. This measurement channel can provide crucial information on electric and magnetic surface properties. Even atomic resolution imaging has been obtained in the dissipation mode. Therefore, in the last section, the quantitative relation between the experimental measurement values and the dissipated power is derived.
André Schirmeisen, Boris Anczykowski, Harald Fuchs

7. Molecular Recognition Force Microscopy

Summary
Atomic force microscopy (AFM), developed in the late eighties to explore atomic details on hard material surfaces, has evolved to an imaging method capable of achieving fine structural details on biological samples. Its particular advantage in biology is that the measurements can be carried out in aqueous and physiological environment, which opens the possibility to study the dynamics of biological processes in vivo. The additional potential of the AFM to measure ultra-low forces at high lateral resolution has paved the way for measuring inter- and intra-molecular forces of bio-molecules on the single molecule level. Molecular recognition studies using AFM open the possibility to detect specific ligand—receptor interaction forces and to observe molecular recognition of a single ligand—receptor pair. Applications include biotin—avidin, antibody—antigen, NTA nitrilotriacetate—hexahistidine 6, and cellular proteins, either isolated or in cell membranes.
The general strategy is to bind ligands to AFM tips and receptors to probe surfaces (or vice versa), respectively. In a force—distance cycle, the tip is first approached towards the surface whereupon a single receptor—ligand complex is formed, due to the specific ligand receptor recognition. During subsequent tip—surface retraction a temporarily increasing force is exerted to the ligand—receptor connection thus reducing its lifetime until the interaction bond breaks at a critical force (unbinding force). Such experiments allow for estimation of affinity, rate constants, and structural data of the binding pocket. Comparing them with values obtained from ensemble-average techniques and binding energies is of particular interest. The dependences of unbinding force on the rate of load increase exerted to the receptor—ligand bond reveal details of the molecular dynamics of the recognition process and energy landscapes. Similar experimental strategies were also used for studying intra-molecular force properties of polymers and unfolding—refolding kinetics of filamentous proteins. Recognition imaging, developed by combing dynamic force microscopy with force spectroscopy, allows for localization of receptor sites on surfaces with nanometer positional accuracy.
Peter Hinterdorfer

Nanotribology and Nanomechanics

8. Micro/Nanotribology and Materials Characterization Studies Using Scanning Probe Microscopy

Summary
A sharp AFM/FFM tip sliding on a surface simulates just one asperity contact. However, asperities come in all shapes and sizes. The effect of radius of a single asperity (tip) on the friction/adhesion performance can be studied using tips of different radii. AFM/FFM are used to study the various tribological phenomena, which include surface/roughness, adhesion, friction, scratching, wear, indentation, detection of material transfer, and boundary lubrication.
Directionality in the friction is observed on both micro- and macroscales and results from the surface roughness and surface preparation. Microscale friction is generally found to be smaller than the macrofriction, as there is less plowing contribution in microscale measurements. The mechanism of material removal on the microscale is studied. Evolution of wear has also been studied using AFM. Wear is found to be initiated at nanoscratches. For a sliding interface requiring near-zero friction and wear, contact stresses should be below the hardness of the softer material to minimize plastic deformation, and surfaces should be free of nanoscratches. Wear precursors can be detected at early stages of wear by using surface potential measurements. Detection of material transfer on a nanoscale is possible with AFM. In situ surface characterization of local deformation of materials and thin coatings can be carried out using a tensile stage inside an AFM.Boundary lubrication studies can be conducted using AFM. Chemically bonded lubricant films and self-assembled monolayers are superior in friction and wear resistance. For chemically bonded lubricant films, the adsorption of water, the formation of meniscus and its change during sliding, viscosity, and surface properties play an important role on the friction, adhesion, and durability of these films. For SAMs, their friction mechanism is explained by a so-called “molecular spring” model.
Bharat Bhushan

9. Surface Forces and Nanorheology of Molecularly Thin Films

Summary
In this chapter, we describe the static and dynamic normal forces that occur between surfaces in vacuum or liquids and the different modes of friction that can be observed between (i) bare surfaces in contact (dry or interfacial friction), (ii) surfaces separated by a thin liquid film (lubricated friction), and (iii) surfaces coated with organic monolayers (boundary friction).
Experimental methods suitable for measuring normal surface forces, adhesion and friction (lateral or shear) forces of different magnitude at the molecular level are described. We explain the molecular origin of van der Waals, electrostatic, solvation and polymer mediated interactions, and basic models for the contact mechanics of adhesive and nonadhesive elastically deforming bodies. The effects of interaction forces, molecular shape, surface structure and roughness on adhesion and friction are discussed.
Simple models for the contributions of the adhesion force and external load to interfacial friction are illustrated with experimental data on both unlubricated and lubricated systems, as measured with the surface forces apparatus. We discuss rate-dependent adhesion (adhesion hysteresis) and how this is related to friction. Some examples of the transition from wearless friction to friction with wear are shown.
Lubrication in different lubricant thickness regimes is described together with explanations of nanorheological concepts. The occurrence of and transitions between smooth and stick—slip sliding in various types of dry (unlubricated and solid boundary lubricated) and liquid lubricated systems are discussed based on recent experimental results and models for stick—slip involving memory distance and dilatency.
Marina Ruths, Alan D. Berman, Jacob N. Israelachvili

10. Friction and Wear on the Atomic Scale

Summary
Friction is an old subject of research: the empirical da Vinci—Amontons laws are common knowledge. Macroscopic experiments systematically performed by the school of Bowden and Tabor have revealed that macroscopic friction can be related to the collective action of small asperities. During the last 15 years, experiments performed with the atomic force microscope gave new insight into the physics of single asperities sliding over surfaces. This development, together with complementary experiments by means of surface force apparatus and quartz microbalance, established the new field of nanotribology. At the same time, increasing computing power allowed for the simulation of the processes in sliding contacts consisting of several hundred atoms. It became clear that atomic processes cannot be neglected in the interpretation of nanotribology experiments. Experiments on even well-defined surfaces directly revealed atomic structures in friction forces. This chapter will describe friction force microscopy experiments that reveal, more or less directly, atomic processes in the sliding contact.
We will begin by introducing friction force microscopy, including the calibration of cantilever force sensors and special aspects of the ultra-high vacuum environment. The empirical Tomlinson model largely describes atomic stick-slip results and is therefore presented in detail. We review experimental results regarding atomic friction. These include thermal activation, velocity dependence, as well as temperature dependence. The geometry of the contact plays a crucial role in the interpretation of experimental results, as we will demonstrate, for example, for the calculation of the lateral contact stiffness. The onset of wear on atomic scale has recently come into the scope of experimental studies and is described here. In order to compare the respective results, we present molecular dynamics simulations that are directly related to atomic friction experiments. We close the chapter with a discussion of dissipation measurements performed in noncontact forcemicroscopy, which may become an important complementary tool for the study of mechanical dissipation in nanoscopic devices.
Enrico Gnecco, Roland Bennewitz, Oliver Pfeiffer, Anisoara Socoliuc, Ernst Meyer

11. Nanoscale Mechanical Properties — Measuring Techniques and Applications

Summary
The first part of this chapter describes local (at the scale of nanometers) measurements of mechanical properties. It includes detailed state-of-the-art presentation and in-depth analysis of experimental techniques, results, and interpretations.
After a short introduction, the second part describes local mechanical spectroscopy using coupled Atomic Force Microscopy and ultrasound. This technique allows us to map quickly not only spatial distribution of the elasticity but anelastic properties as well. At one point in the sample, semi-quantitative measurements can be made as a function of the temperature. On the nanometer scale, results have close similitudes to bulk measurements and interpretable differences. Local elasticity and damping were measured during phase transition of polymer samples and shape-memory alloys.
The third part describes the “nano-Swiss cheese” method of measuring the elastic properties of such tubular nanometer size objects as carbon nanotubes and microtubules. It is probably the only experiment in which properties of single-wall nanotube ropes were measured as a function of the rope diameter. We extended this idea to biological objects, microtubules, and successfully solved major experimental difficulties. We not only measured the temperature dependency of microtubule modulus in pseudo-physiological conditions but also estimated shear modulus using the same microtubule with several lengths of suspended segments.
The fourth section demonstrates the scanning nanoindentation technique as applied to human bone tissue. This instrument allows performing topography scans and indentation tests using the identical tip. The available surface scan allows a high positioning precision of the indenter tip on the structure of interest. For very inhomogeneous samples, such as bone tissue, this tool provides a probe to detect local variations of the mechanical properties. The indentation test supplies quantitative parameters like elastic modulus and hardness on the submicron level. Local mechanical properties of compact and trabecular bone lamellae were tested under both dry and pseudo-physiological conditions.
Finally, last part is given to a discussion of future prospects and conclusions.
Andrzej J. Kulik, András Kis, Gérard Gremaud, Stefan Hengsberger, Philippe K. Zysset, Lásló Forró

12. Nanomechanical Properties of Solid Surfaces and Thin Films

Summary
Instrumentation for the testing of mechanical properties on the submicron scale has developed enormously in recent years. This has enabled the mechanical behavior of surfaces, thin films, and coatings to be studied with unprecedented accuracy. In this chapter, the various techniques available for studying nanomechanical properties are reviewed with particular emphasis on nanoindentation. The standard methods for analyzing the raw data obtained using these techniques are described, along with the main sources of error. These include residual stresses, environmental effects, elastic anisotropy, and substrate effects. The methods that have been developed for extracting thin-film mechanical properties from the often convoluted mix of film and substrate properties measured by nanoindentation are discussed. Interpreting the data is frequently difficult, as residual stresses can modify the contact geometry and, hence, invalidate the standard analysis routines. Work hardening in the deformed region can also result in variations in mechanical behavior with indentation depth. A further unavoidable complication stems from the ratio of film to substrate mechanical properties and the depth of indentation in comparison to film thickness. Even very shallow indentations may be influenced by substrate properties if the film is hard and very elastic but the substrate is compliant. Under these circumstances nonstandard methods of analysis must be used. For multilayered systems many different mechanisms affect the nanomechanical behavior, including Orowan strengthening, Hall—Petch behavior, image force effects, coherency and thermal stresses, and composition modulation.
The application of nanoindentation to the study of phase transformations in semiconductors, fracture in brittle materials, and mechanical properties in biological materials are described. Recent developments such as the testing of viscoelasticity using nanoindentation methods are likely to be particularly important in future studies of polymers and biological materials. The importance of using a range of complementary methods such as electron microscopy, in situ AFM imaging, acoustic monitoring, and electrical contact measurements is emphasized. These are especially important on the nanoscale because so many different physical and chemical processes can affect the measured mechanical properties.
Adrian B. Mann

13. Computational Modeling of Nanometer-Scale Tribology

Summary
Friction and wear have long been acknowledged as limiting factors to numerous applications and many areas of technology, which has lead to significant interest in understanding and controlling these processes. Current interest in microscale and nanoscale machines with moving parts add to this interest, especially as the mechanisms that lead to friction at the atomic-scale can sometimes be quite distinct from the mechanisms that dominate at the macroscale.
This chapter presents a review of the applications of computational modeling methods to atomic-scale and nanometer-scale tribology. It includes a discussion of computational modeling methods frequently employed in these studies, with some analysis of the conditions under which these methods are best applied. This is followed by a review of the findings of computational studies of nanometer-scale indentation, friction, and lubrication.
In this chapter, a relatively complete discussion of the contribution that molecular dynamics and related simulations are making in the area of nanotribology is presented. The examples discussed above make it clear that these approaches are providing exciting insights into friction, wear, and related processes at the atomic scale that could not have been obtained in any other way. Furthermore, the synergy between these simulations and new and experimental techniques such as the surface force apparatus and proximal probe microscopes is producing a revolution in our understanding of the origin of friction at its most fundamental atomic level.
Seong-Jun Heo, Susan B. Sinnott, Donald W. Brenner, Judith A. Harrison

14. Mechanics of Biological Nanotechnology

Summary
One of the most compelling areas to be touched by nanotechnology is biological science. Indeed, we will argue that there is a fascinating interplay between these two subjects, with biology as a key beneficiary of advances in nanotechnology as a result of a new generation of single molecule experiments that complement traditional assays involving statistical assemblages of molecules. This interplay runs in both directions, with nanotechnology continually receiving inspiration from biology itself. The goal of this chapter is to highlight some representative examples of the exchange between biology and nanotechnology and to illustrate the role of nanomechanics in this field and how mechanical models have arisen in response to the emergence of this new field. Primary attention will be given to the particular example of the processes that attend the life cycle of bacterial viruses. Viruses feature many of the key lessons of biological nanotechnology, including self assembly, as evidenced in the spontaneous formation of the protein shell (capsid) within which the viral genome is packaged, and a motor-mediated biological process, namely, the packaging of DNA in this capsid by a molecular motor that pushes the DNA into the capsid. We argue that these processes in viruses are a compelling real-world example of nature’s nanotechnology and reveal the nanomechanical challenges that will continue to be confronted at the nanotechnology-biology interface.
Rob Phillips, Prashant K. Purohit, Jané Kondev

15. Mechanical Properties of Nanostructures

Summary
Knowledge of the mechanical properties of nanostructures is necessary for designing realistic MEMS/NEMS devices. Microelectromechanical systems (MEMS) refer to microscopic devices that have a characteristic length of less than 1mm but more than 1 µm and combine electrical and mechanical components. Nanoelectromechanical systems (NEMS) refer to nanoscopic devices that have a characteristic length of less than 1 µm and combine electrical and mechanical components.
Elastic and inelastic properties are needed to predict deformation from an applied load in the elastic and inelastic regimes, respectively. The strength property is needed to predict the allowable operating limit. Some of the properties of interest are hardness, elastic modulus, bending strength, fracture toughness, and fatigue strength.
Structural integrity is of paramount importance in all devices. Load applied during the use of devices can result in component failure. Cracks can develop and propagate under tensile stresses, leading to failure. Atomic force microscopy and nanoindenters can be used satisfactorily to evaluate the mechanical properties of micro/nanoscale structures for use in MEMS/NEMS.
The most commonly used materials are single-crystal silicon and silicon-based materials, e.g., SiO2 and polysilicon films deposited by low-pressure chemical vapor deposition. An early study showed silicon to be a mechanically resilient material in addition to its favorable electronic properties. Single-crystal SiC deposited on large-area silicon substrates is used for high-temperature micro/nanosensors and actuators. Amorphous alloys can be formed on both metal and silicon substrates by sputtering and plating techniques, providing more flexibility in surface-integration. Electroless deposited Ni-P amorphous thin films have been used to construct microdevices, especially using the so-called LIGA techniques. Micro/nanodevices need conductors to provide power, as well as electrical/magnetic signals to make them functional. Electroplatedgold films have found wide applications in electronic devices because of their ability to make thin films and process simply. Use of SiC, Ni-P, and Au films, together with silicon and silicon-based materials, opens up new design opportunities for MEMS/NEMS devices.
This chapter presents a review of mechanical property measurements on the nanoscale of various materials of interest and stress and deformation analyses of nanostructures.
Bharat Bhushan

16. Scale Effect in Mechanical Properties and Tribology

Summary
A model, which explains scale effects in mechanical properties and tribology is presented. Mechanical properties are scale dependent based on the strain gradient plasticity and the effect of dislocation-assisted sliding. Both single asperity and multiple asperity contacts are considered. The relevant scaling length is the nominal contact length — contact diameter for a single-asperity contact, and scan length for multiple-asperity contacts. For multiple asperity contacts, based on an empirical power-rule for scale dependence of roughness, contact parameters are calculated. The effect of load on the contact parameters and the coefficient of friction is also considered. During sliding, adhesion and two- and three-body deformation, as well as ratchet mechanism, contribute to the dry friction force. These components of the friction force depend on the relevant real areas of contact (dependent on roughness and mechanical properties), average asperity slope, number of trapped particles, and shear strength during sliding. Scale dependence of the components of the coefficient of friction is studied. A scale dependent transition index, which is responsible for transition from predominantly elastic adhesion to plastic deformation has been proposed. Scale dependence of the wet friction, wear, and interface temperature has been also analyzed. The proposed model is used to explain the trends in the experimental data for various materials at nanoscale and microscale, which indicate that nanoscale values of coefficient of friction are lower than the microscale values due to an increase of the three-body deformation and transition from elastic adhesive contact to plastic deformation.
Bharat Bhushan, Michael Nosonovsky

Moleculary-Think Films for Lubrication

17. Nanotribology of Ultrathin and Hard Amorphous Carbon Films

Summary
Diamond material and its smooth coatings are used for very low wear and relatively low friction. Major limitations of the true diamond coatings are that they need to be deposited at high temperatures, can only be deposited on selected substrates, and require surface finishing. Hard amorphous carbon, commonly known as diamond-like carbon or DLC coatings, exhibit mechanical, thermal, and optical properties close to that of diamond. These can be deposited with a large range of thicknesses by using a variety of deposition processes on a variety of substrates at or near room temperature. The coatings reproduce substrate topography, avoiding the need of post-finishing. Friction and wear properties of some DLC coatings can be very attractive for tribological applications. The largest industrial application of these coatings is in magnetic storage devices.
The prevailing atomic arrangement in the DLC coatings is amorphous or quasi-amorphous with small diamond, graphite, and other unidentifiable micro- or nanocrystallites. Most DLC coatings, except those produced by filtered cathodic arc, contain from a few to about 50 at% hydrogen. Sometimes hydrogen is deliberately incorporated in the sputtered and ion plated coatings to tailor their properties.
EELS and Raman spectroscopies can be successfully used for chemical characterization of amorphous carbon coatings. The prevailing atomic arrangement in the DLC coatings is amorphous or quasi-amorphous with small diamond (sp 3), graphite (sp 2) and other unidentifiable micro- or nanocrystallites. Most DLC coatings except those produced by filtered cathodic arc contain from a few to about 50 at% hydrogen. Sometimes hydrogen is deliberately incorporated in the sputtered and ion plated coatings to tailor their properties.
Amorphous carbon coatings deposited by various techniques exhibit different mechanical and tribological properties. The nanoindenter can be successfully used for measurement of hardness, elastic modulus, fracture toughness, and fatigue life. Microscratch and microwear experiments can be performed using either a nanoindenter or an AFM. Thin coatings deposited by filtered cathodic arc, ion beam, and ECR-CVD hold a promise for tribological applications. Coatings as thin as 5 nm or even thinner provide wear protection. Microscratch, microwear, and accelerated wear testing, if simulated properly, can be successfully used to screen coating candidates for industrial applications. In the examples shown in this chapter, trends observed in the microscratch, microwear, and accelerated macrofriction wear tests are similar to that found in functional tests.
In this chapter, the state-of-the-art of recent developments in the chemical, mechanical, and tribological characterization of ultrathin amorphous carbon coatings is presented.
Bharat Bhushan

18. Self-Assembled Monolayers for Controlling Adhesion, Friction and Wear

Summary
Reliability of micro- and nanodevices, as well as magnetic storage devices require the use of lubricant films for the protection of sliding surfaces. To minimize high adhesion, friction, and because of small clearances in the devices, these films should be molecularly thick. Liquid films of low surface tension or certain hydrophobic solid films can be used. Ordered molecular assemblies with high hydrophobicity can be engineered using chemical grafting of various polymer molecules with suitable functional head groups, spacer chains and nonpolar surface terminal groups.
The classical approach to lubrication uses multi-molecular layers of liquid lubricants. Boundary lubricant films are formed by either physisorption, chemisorption, or chemical reaction. The physiosorbed films can be either monomolecularly or polymolecularly thick. The chemisorbed films are monomolecular, but stoichiometric films formed by chemical reaction can be multilayered. A good boundary lubricant should have a high degree of interaction between its molecules and the sliding surface. As a general rule, liquids are good lubricants when they are polar and thus able to grip on solid surfaces (or be adsorbed).
In this chapter, we focus on self-assembled monolayers (SAMs) for high hydrophobicity and/or low adhesion, friction, and wear. SAMs are produced by various organic precursors. We first present a primer to organic chemistry followed by an overview on suitable substrates, head groups, spacer chains, and end groups in the molecular chains and an overview of tribological properties of SAMs. The adhesion, friction, and wear properties of SAMs, having alkyl and biphenyl spacer chains with different surface terminal and head groups, are surveyed. The friction data are explained using a molecular spring model in which the local stiffness and intermolecular force govern its frictional performance. Based on the nanotribological studies of SAM films by AFM, they exhibit attractive hydrophobic and tribological properties.
Bharat Bhushan, Huiwen Liu

19. Nanoscale Boundary Lubrication Studies

Summary
Boundary films are formed by physisorption, chemisorption, and chemical reaction. With physisorption, no exchange of electrons takes place between the molecules of the adsorbate and those of the adsorbant. The physisorption process typically involves van der Waals forces, which are relatively weak. In chemisorption, there is an actual sharing of electrons or electron interchange between the chemisorbed species and the solid surface. The solid surfaces bond very strongly to the adsorption species through covalent bonds. Chemically reacted films are formed by the chemical reaction of a solid surface with the environment. The physisorbed film can be either monomolecularly or polymolecularly thick. The chemisorbed films are monomolecular, but stoichiometric films formed by chemical reaction can have a large film thickness. In general, the stability and durability of surface films decrease in the following order: chemically reacted films, chemisorbed films, and physisorbed films. A good boundary lubricant should have a high degree of interaction between its molecules and the sliding surface. As a general rule, liquids are good lubricants when they are polar and, thus, able togrip solid surfaces (or be adsorbed). In this chapter, we focus on PFPEs. We first introduce details of the commonly used PFPE lubricants; then present a summary of nanodeformation, molecular conformation, and lubricant spreading studies; followed by an overview of nanotribological properties of polar and nonpolar PFPEs studied by atomic force microscopy (AFM) and some concluding remarks.
Bharat Bhushan, Huiwen Liu

Applications

20. Micro/Nanotribology and Micro/Nanomechanics of Magnetic Storage Devices

Summary
A magnetic recording process involves relative motion between a magnetic medium (tape or disk) against a stationary or rotating read/write magnetic head. For ever-increasing, high areal recording density, the linear flux density (number of flux reversals per unit distance) and the track density (number of tracks per unit distance) should be as high as possible. The size of a single bit dimension for current devices is typically less than 1000 nm2. This dimension places stringent restrictions on the defect size present on the head and medium surfaces.
Reproduced (read-back) magnetic signal amplitude decreases with a decrease in the recording wavelength and/or the track width. The signal loss results from the magnetic coating thickness, read gap length, and head-to-medium spacing (clearance or flying height). It is known that the signal loss as a result of spacing can be reduced exponentially by reducing the separation between the head and the medium. The need for increasingly higher recording densities requires that surfaces be as smooth as possible and the flying height (physical separation or clearance between a head and a medium) be as low as possible. The ultimate objective is to run two surfaces in contact (with practically zero physical separation) if the tribological issues can be resolved. Smooth surfaces in near contact lead to an increase in adhesion, friction, and interface temperatures, and closer flying heights lead to occasional rubbing of high asperities and increased wear. Friction and wear issues are resolved by appropriate selection of interface materials and lubricants, by controlling the dynamics of the head and medium, and the environment. A fundamental understanding of the tribology (friction, wear, and lubrication) of the magnetic head/medium interface, both on macro- and micro/nanoscales, becomes crucial for the continued growth of this more than $ 60 billion a year magnetic storage industry.
In this chapter, initially, the general operation of drives and the construction and materials used in magnetic head and medium components are described. Then the micro/nanotribological and micro/nanomechanics studies including surface roughness, friction, adhesion, scratching, wear, indentation, and lubrication relevant to magnetic storage devices are presented.
Bharat Bhushan

21. Micro/Nanotribology of MEMS/NEMS Materials and Devices

Summary
The field of microelectromechanical systemsMEMS/NEMS nanoelectromechanical systemshas expanded considerably over the last decade. The length scale and large surfaceto-volume ratio of the devices result in very high retarding forces such as adhesion and friction that seriously undermine the performance and reliability of the devices. These tribological phenomena need to be studied and understood at the micro- to nanoscales. In addition, materials for MEMS/NEMS must exhibit good microscale tribological properties. There is a need to develop lubricants and identify lubrication methods that are suitable for MEMS/NEMS. Using AFM-based techniques, researchers have conducted micro/nanotribological studies of materials and lubricants for use in MEMS/NEMS. In addition, component level testing has also been carried out to aid in better understanding the observed tribological phenomena in MEMS/NEMS.
Macroscale and microscale tribological studies of silicon and polysilicon films have been performed. The effects of doping and oxide films and environment on the tribological properties of these popular MEMS/NEMS materials have also been studied. SiC film is found to be a good tribological material for use in high-temperature MEMS/NEMS devices. Hexadecane thiol self-assembled monolayers and bonded perfluoropolyether lubricants appear to be well suited for lubrication of microdevices under a range of environmental conditions. DLC coatings can also be used for low friction and wear. Surface roughness measurements of micromachined polysilicon surfaces have been made using an AFM. The roughness distribution on surfaces is strongly dependent on the fabrication process. Roughness should be optimized for low adhesion, friction, and wear. Adhesion and friction of microstructures can be measured using novel apparatuses. Adhesion and friction measurements on silicon-on-silicon confirm AFM measurements that hexadecane thiol and bonded perfluoropolyether films exhibit superior adhesion and friction properties. Static friction force measurements of micromotors have been performed using an AFM. The forces are found to vary considerably with humidity. A bonded layer of perfluoropolyether lubricant is found to satisfactorily reduce the friction forces in the micromotor.
AFM/FFM-based techniques can be satisfactorily used to study and evaluate micro/nanoscale tribological phenomena related to MEMS/NEMS devices. This chapter presents a review of macro- and micro/nanoscale tribological studies of materials and lubrication studies for MEMS/NEMS and component-level studies of stiction phenomena in MEMS/NEMS devices.
Bharat Bhushan

22. Mechanical Properties of Micromachined Structures

Summary
To be able to accurately design structures and make reliability predictions, in any field it is necessary first to know the mechanical properties of the materials that make up the structural components. In the fields of microelectromechanical systems (MEMS) MEMSand nanoelectromechanical systems (NEMS), nanoelectromechanical systemsthe devices are necessarily very small. The processing techniques and microstructures of the materials in these devices may differ significantly from bulk structures. Also, the surface-area-to-volume ratio in these structures is much higher than in bulk samples, and so the surface properties become much more important. In short, it cannot be assumed that mechanical properties measured using bulk specimens will apply to the same materials when used in MEMS and NEMS. This chapter will review the techniques that have been used to determine the mechanical properties of micromachined structures, especially residual stress, strength, and Young’s modulus. The experimental measurements that have beenperformed will then be summarized, in particular the values obtained for polycrystalline silicon (polysilicon).
Harold Kahn

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