Nanotribology and Nanomechanics II

Nanotribology and nanomechanics studies are needed to develop a fundamental understanding of interfacial phenomena on a small scale and to study interfacial phenomena in micro-/nanoelectromechanical systems (MEMS/NEMS), magnetic storage devices, and other applications. Friction and wear of lightly loaded micro-/nanocomponents are highly dependent on surface interactions (few atomic layers). These structures are generally coated with molecularly thin films. Nanotribology and nanomechanics studies are also valuable in the fundamental understanding of interfacial phenomena in macrostructures and provide a bridge between science and engineering. An atomic force microscope (AFM) tip is used to simulate a single-asperity contact with a solid or lubricated surface. AFMs are used to study the various tribological phenomena, which include surface roughness, adhesion, friction, scratching, wear, detection of material transfer, and boundary lubrication. In situ surface characterization of local deformation of materials and thin coatings can be carried out using a tensile stage inside an AFM. Mechanical properties such as hardness, Young’s modulus of elasticity, and creep/relaxation behavior can be determined on micro- to picoscales using a depth-sensing indentation system in an AFM. Localized surface elasticity and viscoelastic mapping of near-surface regions can be obtained with nanoscale lateral resolution. Finally, an AFM can be used for nanofabrication/nanomachining.

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: (1) bare surfaces in contact (dry or interfacial friction), (2) surfaces separated by a thin liquid film (lubricated friction), and (3) 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 dilatancy.

Friction has long been the subject of research: the empirical da Vinci– Amontons friction laws have been common knowledge for centuries. Macroscopic experiments performed by the school of Bowden and Tabor revealed that macroscopic friction can be related to the collective action of small asperities. Over the last 15 years, experiments performed with the atomic force microscope have provided new insights into the physics of single asperities sliding over surfaces. This development, together with the results from complementary experiments using surface force apparatus and the quartz microbalance, have led to the new field of nanotribology. At the same time, increasing computing power has permitted the simulation of processes that occur during sliding contact involving several hundreds of atoms. It has become clear that atomic processes cannot be neglected when interpreting nanotribology experiments. Even on well-defined surfaces, experiments have revealed that atomic structure is directly linked to friction force. This chapter will describe friction force microscopy experiments that reveal, more or less directly, atomic processes during sliding contact.

We will begin by introducing friction force microscopy, including the calibration of cantilever force sensors and special aspects of the ultrahigh vacuum environment. The empirical Tomlinson model often used to describe atomic stick-slip results is therefore presented in detail. We review experimental results regarding atomic friction, including thermal activation, velocity dependence and temperature dependence. The geometry of the contact is crucial to the interpretation of experimental results, such as the calculation of the lateral contact stiffness, as we shall see. The onset of wear on the atomic scale has recently been studied experimentally and it is described here. In order to compare results, we present molecular dynamics simulations that are directly related to atomic friction experiments. The chapter ends with a discussion of dissipation measurements performed in noncontact force microscopy, which may become an important complementary tool for the study of mechanical dissipation in nanoscopic devices.

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.

One of the best materials to use in applications that require very low wear and reduced friction is diamond, especially in the form of a diamond coating. Unfortunately, true diamond coatings can only be deposited at high temperatures and on selected substrates, and they require surface finishing. However, hard amorphous carbon – commonly known as diamond-like carbon or a DLC coating – has similar mechanical, thermal and optical properties to those of diamond. It can also be deposited at a wide range of thicknesses using a variety of deposition processes on various substrates at or near room temperature. The coatings reproduce the topography of the substrate, removing the need for finishing. The friction and wear properties of some DLC coatings make them very attractive for some tribological applications. The most significant current industrial application of DLC coatings is in magnetic storage devices.

In this chapter, the state-of-the-art in the chemical, mechanical and tribological characterization of ultrathin amorphous carbon coatings is presented.

EELS and Raman spectroscopies can be used to characterize amorphous carbon coatings chemically. 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 for those produced using a filtered cathodic arc, contain from a few to about 50 at.% hydrogen. Sometimes hydrogen is deliberately incorporated into the sputtered and ion-plated coatings in order to tailor their properties.

Amorphous carbon coatings deposited by different techniques exhibit different mechanical and tribological properties. Thin coatings deposited by filtered cathodic arc, ion beam and ECR-CVD hold much promise for tribological applications. Coatings of 5 nm or even less provide wear protection. A nanoindenter can be used to measure DLC coating hardness, elastic modulus, fracture toughness and fatigue life. Microscratch and microwear tests can be performed on the coatings using either a nanoindenter or an AFM, and along with accelerated wear testing, can be used to screen potential industrial coatings. For the examples shown in this chapter, the trends observed in such tests were similar to those found in functional tests.

Reliability of various micro- and nanodevices requiring relative motion as well as magnetic storage devices requires the use of hydrophobic and lubricating films to minimize adhesion, stiction, friction, and wear. In various applications, surfaces need to be protected from exposure to the operating environment, and hydrophobic films are of interest. The surface films should be molecularly thick, well-organized, chemically bonded to the substrate, and insensitive to environment. 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.

In this chapter, we focus on self-assembled monolayers (SAMs) with high hydrophobicity and good nanotribological properties. SAMs are produced by various organic precursors. We first present a primer to organic chemistry, followed by an overview of selected SAMs with various substrates, spacer chains, and terminal groups in the molecular chains and an overview of nanotribological properties of SAMs. The contact angle, adhesion, friction, and wear properties of SAMs having various spacer chains with different surface terminal and head groups (hexadecane thiol, biphenyl thiol, perfluoroalkylsilane, alkylsilane, perfluoroalkylphosphonate, and alkylphosphonate) on various substrates (Au, Si, and Al) are surveyed. Chemical degradation mechanisms and environmental effects are studied. Based on the contact angle and nanotribological properties of various SAM films by atomic force microscopy (AFM) it is found that perfluoroalkylsilane and perfluorophosphonate SAMs exhibit attractive hydrophobic and tribological properties.

Boundary films are formed by physisorption, chemisorption, and chemical reaction. A good boundary boundary film physisorption chemisorption lubricant should have a high degree of interaction between its molecules and the solid surface. As a general rule, liquids are good lubricants when they are polar and thus able to grip solid surfaces perfluoropolyether (PFPE) (or be adsorbed). In this chapter, we focus on various perfluoropolyethers (PFPEs) and nanodeformation ionic liquid films. We present a summary of nanodeformation, molecular conformation, and lubricant perfluoropolyether (PFPE) nanotribological property spreading studies, followed by an overview of the nanotribological properties of polar and nonpolar atomic force microscopy (AFM) PFPEs and ionic liquid films studied by atomic force microscopy (AFM), and chemical tribotest apparatus degradation studies using a high-vacuum tribotest apparatus. In this chapter, we focus on PFPE and ionic liquid (IL) film nanodeformation ionic liquid films. We first present a summary of nanodeformation, molecular conformation, and lubricant spreading studies and an overview of nanotribological and electrical studies of commonly used polar and nonpolar PFPE and ionic liquid films using AFM and chemical degradation studies using a high-vacuum tribotest apparatus.

Nature has developed materials, objects, and processes that function from the macroscale to the nanoscale. The emerging field of biomimetics allows one to mimic biology or nature to develop nanomaterials, nanodevices, and processes which provide desirable properties. Hierarchical structures with dimensions of features ranging from the macroscale to the nanoscale are extremely common in nature to provide properties of interest. There are a large number of objects including bacteria, plants, land and aquatic animals, and seashells with properties of commercial interest. Certain plant leaves, such as Lotus leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical roughness of their leaf surfaces. The self-cleaning phenomenon is widely known as the “Lotus effect”. These surfaces with high contact angle and low contact angle hysteresis with a self-cleaning effect also exhibit low adhesion and drag reduction for fluid flow. In this article, the theoretical mechanisms of the wetting of rough surfaces are presented followed by the characterization of natural leaf surfaces. The next logical step is to realize superhydrophobic surfaces based on understanding of the leaves. Next, a comprehensive review is presented on artificial superhydrophobic surfaces fabricated using various fabrication techniques and the influence of micro-, nano- and hierarchical structures on superhydrophobicity, self-cleaning, low adhesion, and drag reduction. An aquatic animal, such as a shark, is another model from nature for the reduction of drag in fluid flow. The artificial surfaces from the shark skin have been created, and the influence of structure on drag reduction efficiency is discussed. Furthermore, oleophobic surfaces can be used as a biomimetic coating that prevents contamination of the underwater parts of ships by biological and organic contaminants, including oil. The article discusses the wetting behavior of oil droplets on various superoleophobic surfaces.

leg attachment pad gecko foot smart adhesion The leg attachment pads of several creatures, including many insects, spiders, and lizards, are capable of attaching to a variety of surfaces and are used for locomotion. Geckoes, in particular, have hairy attachment the largest mass and have developed the most complex hairy attachment structures capable of smart smart adhesion adhesion – the ability to cling to different smooth and rough surfaces and detach at will. These microscale hair animals make use of about three million microscale hairs (setae) (about 14,000 mm⁻²) that nanoscale spatula branch off into hundreds of nanoscale spatulae (about three billion spatula on two feet). This so-called division of contacts provides high dry adhesion. This multiple-level hierarchically structured surface construction provides the gecko with the compliance and adaptability to create a large real area of contact with a variety of surfaces. Modeling of the gecko attachment system as a hierarchical hierarchical spring model spring model has provided insight into the adhesion enhancement generated by this system. van der Waals forces are the primary mechanism utilized to adhere to surfaces, and capillary forces are a secondary effect that can further increase the adhesion force. Preload applied to the setae increases adhesive force. Although a gecko is capable of producing of the order of 20 N of adhesive force, it retains the ability to remove its feet from an attachment surface at will. The adhesive strength of gecko setae is dependent on orientation; maximum adhesion occurs at 30°. During walking, a gecko is able to peel its foot from surfaces by changing the angle at fibrillar structure which its setae contact the surface. Manmade fibrillar structures capable of replicating gecko adhesion superadhesive tape wall-climbing robot have the potential for use in dry superadhesive tapes and treads for wall-climbing robots for various applications. These structures can be created using micro/nanofabrication techniques or self-assembly.

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 1,000 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.

Micro-/nanoelectromechanical systems (MEMS/NEMS) micro-/nanoelectromechanical system (MEMS/NEMS) need to be designed to perform expected functions in short durations, typically in the millisecond to picosecond range. The expected life of devices for high-speed contacts can vary from a few hundred thousand to many billions of cycles, e.g., over a hundred billion cycles micromirror device (DMD) digital for digital micromirror devices (DMDs), which puts serious requirements on materials. The surface-area-to-volume ratio in MEMS/NEMS is large, and in systems involving relative motion, surface forces such as adhesion, friction, and meniscus and viscous forces become very large compared with inertial and electromagnetic forces. There is a need for fundamental understanding of adhesion, friction friction/stiction, stiction wear, lubrication, and the role of surface contamination and environment, all on the nanoscale. Most mechanical properties are known to be scale dependent, therefore the properties of nanoscale structures need to be measured. For bioMEMS/bioNEMS, bioMEMS/bioNEMS adhesion between biological molecular layer molecular layers and the substrate, and friction and wear of biological layers, can be important. Component-level studies are required to provide a better understanding of the tribological phenomena occurring in MEMS/NEMS. The emergence of the fields of nanotribology nanotribology and nanomechanics nanomechanics, and atomic force microscopy (AFM) atomic force microscopy (AFM)-based techniques, has provided researchers with a viable approach to address these problems. The emerging field of biomimetics biomimetics holds promise for the development of nanomaterial biologically inspired biologically inspired nanomaterials and nanotechnology products. One example is the design of surfaces with superhydrophobicity roughness-induced roughness-induced superhydrophobicity, self-cleaning, and low adhesion based on the so-called lotus effect. This chapter presents an overview of nanoscale adhesion nanoscale adhesion, friction, and wear wear studies of materials and lubrication lubrication for MEMS/NEMS and bioMEMS/bioNEMS, and component-level studies of stiction phenomena in MEMS/NEMS devices, MEMS/NEMS deviceas well as hierarchical nano-structured surfaces for superhydrophobicity, self-cleaning self-cleaning, and low adhesion.

To be able to accurately design structures and make reliability predictions in any field, it is first necessary to know the mechanical properties mechanical property nanostructure mechanical property of the materials that make up the structural components. The devices encountered in the fields of microelectromechanical systems (MEMS) MEMS and nanoelectromechanical systems (NEMS), NEMS stress strength Young’s modulus are necessarily very small, and so the processing techniques and the microstructures of the materials used in these devices may differ significantly from bulk structures. Also, the surface-area-to-volume ratios in such structures are much higher than in bulk samples, and so surface properties become much more important. In short, it cannot be assumed that the mechanical properties measured for a bulk specimen of a material will apply when the same material is 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 been performed will then be summarized, in particular the values obtained for polycrystalline silicon (polysilicon).