Micro/Nanotribology and Its Applications

In this introductory chapter, we present the history of macrotribology and micro/nanotribology and their industrial significance. Next, we present examples of why micro/nanotribological studies are important in magnetic storage devices, microelectromechanical systems (MEMS) and other microcomponents.

Scanning Force Microscopes and Friction Force Microscopes are built in wide variety of designs. They have become welcome additions to industrial laboratories due to their ruggedness and because their measurement principle is, in many respects, a refinement of well established apparatus such as profilometers and tribometers. This article discusses the building blocks of scanning force microscopes.

On the nanometer scale or the molecular level the boundaries between physics, chemistry, biology, medical and engineering science seem to disappear and all the disciplines have a common center of research. A deeper knowledge of the life-processes in biology a more detailed understanding in chemistry, medical science and genetics as well as of the growth of novel materials and its characterization in material science demands tools with highest — in particular cases atomic — lateral resolution.

An atomic force microscope incorporating two optical levers and two quadrant photodiodes was developed to detect the displacement of the tip end with atomic resolution in the x,y and z directions. When observing cleaved mica in air, stick slip action and the trajectory of the tip could be plotted against xyz coordinates in real-time. The sample was rotated with 30 degree increments to verify the operation of the proposed detection system. The instrument enables a truer representation of acquired data in real space and time, as well as an observation of the tip sample interaction with a correct measurement of relative displacements.

Adhesion, wear, and friction are closely related to the interactions and mechanical responses that occur between and within nanometersized surface structures. Thus, besides being interesting in its own right, the study of material properties on this length scale provides the basis for an understanding of phenomenologically well-known macroscopic features on a fundamental level. Experimentally, the nanometer regime has become accessible by the development of scanning tunneling microscopy. Interaction mechanics on the nanometer scale are addressed in this paper from this perspective, both in terms of theoretical concepts and recent experimental work.

An introduction to experimental aspects of atomic force microscopy specific to the liquid-solid interface is given. Emphasis is given to the measurement of forces (e.g. normal, friction, material compliance, the liquid hydrodynamics) acting on the tip in liquids. Specific examples which are discussed include solvation forces and their relation to local lubrication, and the measurement of single asperity contacts in simple liquids. Some general problems which remain outstanding are highlighted.

We have investigated the adsorption of 56,000Mw poly(ethyleneoxide) in an aqueous system (good solvent) to glass using a development of the atomic force microscope technique. A glass particle is glued to a silicon cantilever to give a particle probe surface forces apparatus. The data presented describe the evolution of the adsorbed polymer layer with time and the changes resulting from only allowing one surface to adsorb polymer. We also examine the change of the layer conformation with repeated compressions.

Force gradients between silicon tips and (110) Si surfaces have been measured in Ar controlled atmosphere by means of an Atomic Force Microscope. Non-contact mode techniques have been exploited to investigate long range Van der Waals forces without the influence of meniscus forces. Measurements have been compared to the theoretical predictions for the induced dipole-dipole contribution. Theoretical curves need to be corrected for finite oscillation amplitude effects.

The field ion microscopy (FIM) has been applied to studies of gas adsorption, surface diffusion, transformation, alloy segregation, lattice defects, and many other research fields of surface science[1,2]. FIM studies related to tribology were first carried out by Müller and his co-workers [37]. In their experiments on metal-metal contacts inside FIM, the induced damages were analyzed in atomic detail. Buckley extended FIM studies to adhesion contact using such practical tribology materials, as PTFE etc. [810]. Ohmae et al. conducted experiments on friction contact inside the FIM, and compared friction-induced lattice defects with those analyzed in their study of adhesion contacts [11–16]. These studies have shown that FIM is uniquely suited to the study of microtribology. In FIM, a metallic tip electropolished to an apex radius of 10–100 nm is used as a sample, which may represent a single surface asperity having well-defined surface characteristics. Using a tip and a counter material, adhesion and friction can be studied in ultrahigh vacuum (UHV) environment or under conditions of gas adsorption by back-filling gas species into the chamber.

Attempts to establish the relationship between adhesion and friction at the contact of solid surfaces has been frustrated by their inevitable roughness. The recent development of nano-tribology, in which a single asperity contact can be modelled in the surface force apparatus (SFA) or the atomic force microscope (AFM), has made possible the simultaneous measurement of friction and adhesion in a sliding experiment. For the case of pure adhesion, continuum mechanics models exist which assist in the interpretation of the measurements. By assuming a ‘Dugdale’ potential, in which the adhesive force is constant in the separation zone, Maugis [1] obtained a solution in closed form to the behaviour in adhesion of elastic spheres. In the limit, when the separation zone is small compared with the size of the contact, the JKR adhesion theory (Johnson, Kendall & Roberts,[2]) is recovered.Recent observations in both the SFA and AFM suggest that the frictional traction in the sliding contact of unlubricated solid surfaces is approximately independent of the contact pressure. This result forms the basis of a continuum model of friction which includes the transition from static to kinetic friction, This model is then extended to accommodate possible interaction between adhesive and frictional forces in terms of an empirical interaction parameter α. The effect of such interaction is for the application of a tangential force to cause a reduction in the apparent adhesion between the surfaces. The value of the parameter α may be found by observing the reduction in the static contact area brought about by sliding.

Boundary lubricating film properties are crucial to the understanding of current lubrication technology as well as in advancing the frontier of lubrication science. This paper reviews the current understanding of boundary lubricating films and presents some new techniques for the measurement of film properties. As the scale of the technologies decreases, devices such as Micro-Mechanical Devices (MMDs) and magnetic hard disk system require an ever decreasing scale of lubrication technology. Monomolecular films, by necessity, are required to provide friction control, wear protection, and system durability. Therefore, film designs are needed to protect such systems. This paper describes some new measurement techniques as well as the conceptual framework for monomolecular film design.

In this paper we report on the application of AFM for analysis of the lubricant morphology on the surface of the magnetic hard disks. Recently AFM was used as a tool for lubricant thickness measurements [1,2], testing of the nanomechanical properties of the lubricant film [3], wear test [4,5] and contaminants characterization on the disk surface [6]. AFM could be used for the imaging of the lubricant on the disk surface when the lubricant film thickness exceeds the disk surface roughness (Figure 1). Combined with AFM friction and modulus mapping, imaging of the lubricant gives valuable information on the surface topography and lubricant distribution at the stiction and wear areas of the disk (Figure 2).

This paper is intended for researchers who already have some knowledge of local probe methods (LPM) and their operational modes and who wish to begin making measurements of materials properties with nanometer-scale lateral resolution. The different possible materials properties measurements are introduced, a brief overview of contact-mechanical theories is given, difficulties in vocabulary are highlighted, LPM are compared with similar instruments, and a list of introductory publications is presented.

This paper is intended for researchers who already have some knowledge of local probe microscopes (LPM) and their operational modes and who wish to begin making measurements of materials properties with nanometer-scale lateral resolution. The difficulties of making quantitative measurements are discussed. The correct choice of LPM configuration and excitation frequency can greatly enhance the signal-to-noise ratio and linearity of response of the microscope to the interaction stiffness. Rheological models of different LPM setups are used to determine the best configuration for a given experiment. Most materials are best studied by means of a transducer placed underneath the sample and excited at frequencies above the highest tip-sample resonance. Common image artifacts are identified.

PMMA-surfaces have been investigated by scanning force microscopy as a function of temperature and imaging conditions. A stand-alone type scanning force microscope was employed together with a heating stage to investigate a model polymer substance, PMMA, as a function of temperature. Contact mode imaging induced wavy structures at higher temperatures, whereas intermittent imaging in the pulsed force mode showed negligible interactions.

Data obtained from indentation testing can provide important information about the near-surface mechanical properties and deformation behavior of solids. Properly executed and interpreted, the technique can provide information regarding hardness, elastic modulus, compressive yield strength (metals), fracture toughness (ceramics) and residual stresses. The apparent simplicity of the test procedure, however, often belies the difficulties of interpreting the data to give quantitative information on specific properties.

Advances in computer-based modeling and simulation methodologies and capabilities, coupled with the emergence and development of high-resolution experimental techniques, allow investigations of tribological phenomena with unprecedented atomic-scale spatial and temporal resolution. We focus here on molecular dynamics simulations of formation and properties of interfacial junctions, and on nanoelastohydrodynamics in sheared lubricated junctions. Simulations predict that upon approach of a metal tip to a surface a jump-to-contact instability occurrs and that subsquent nanoindetation leads to plastic deformation of the gold surface. Retraction of the tip from the surface results in formation of a connective junction, or wire, of nano-scale dimensions, whose elongation mechanism consists of a series of plastic stress-accumulation and stress-relief stages which are accompanied by structural order-disorder transformations. These transformations involve multiple-glide processes. The yield-stress of a gold nanowire is predicted to be ~ 3 GPa, which is an order of magnitude larger than that of bulk gold. The nearly ideal mechanical response of nanowires is portrayed also in the ability to manipulate them reversibly in elongation-compression cycles. Comparisons of the structural, mechanical and electrical properties of nanowires generated via elongation of junctions of initial very different dimensions, confirms that nanowires of similar nature are formed, irrespective of the history of the junctions. Shearing the junction occurs via an atomic-scale stick-slip mechanism characterized by a similar critical-stress. The elongation process is reflected in hysteresis in the force versus tip-to-surface distance records, and in oscillatory behavior of the force. Measurements of room-temperature electronic transport in such pulled gold nanowires reveal periodic conductance quantization steps, in units of 2e2/h. Simulations of atomic-scale structure, dynamics, flow, and response characteristics of a thin film molecular hexadecane lubricant, confined and sheared by topographically nonuniform solid gold surfaces sliding at a relative velocity of 10 m/s, are described. The simulations reveal nano-scale processes which include: spatial and temporal variations in the density and pressure of the lubricant, particularly in the region confined by the approaching asperities, accompanied by asperity-induced molecular layering transitions which are reflected in oscillatory patterns in the friction force; dynamical formation of elasto-plastic, or glassy, states of the lubricant in the interasperity zone; drastic asperity deformations mediated by the lubricant, leading to microstructural transformations of the nonuniform bounding solid surfaces; molecular trapping and formation of intermetallic junctions; and onset of cavitated zones in the lubricating fluid after the asperity-asperity collision process. The simulations extend micro-elastohydrodynamic continuum investigations into the nano-scale regime, and provide molecular-scale insights into the fundamental mechanisms of ultrathin film lubrication phenomena under extreme conditions, which are of significance for modern technologies.

Experimental studies of the dynamics of molecularly thin films using the Surface Force Apparatus (SFA) reveal a rich variety of behavior [1–13]. As the film thickness decreases, the relaxation times and viscosities of simple fluids increase by many orders of magnitude. At small enough thicknesses, films enter a solid state that is capable of resisting static shear forces [1,3,11–13]. When the yield stress of these films is exceeded, motion occurs through intermittent stick-slip events rather than smooth sliding [3–5,8,9]. Stop/start experiments [3, 5] and the response to oscillatory shear [10] reveal that films store memory of their previous sliding history for very long times.

We present a constitutive relation for the friction between surfaces lubricated by grafted monolayer films. A linear instability analysis is used to determine the boundary line in the dynamical phase diagram, separating stick-slip motion from steady sliding. The nature of the transition between stick-slip and steady sliding is studied and is found to be in very good agreement with experiments.

New developments based heavily on integrated-circuit-related technologies have led to rapid progress in the development of microdynamical systems. These systems are based upon the science, technology, and design of moving micromechanical devices, and are a subclass of what is known in the USA as microelectromechanical systems (MEMS). Recent rapid progress gives promise for new designs of integrated sensors, actuators, and other devices that can be combined with on-chip microcircuits to make possible high-performing, compact, portable, low-cost engineering systems. Mechanical materials for some of the microdynamical systems that have thus far been demonstrated consist of deposited thin films of polycrystalline silicon, silicon nitride, aluminum, polyimide, and tungsten among other materials. To make mechanical elements using thin-film processing, microstructures are freed from the substrate by etching a sacrificial layer of silicon dioxide. First demonstrated as a means to produce electrostatically driven, doubly supported beam bridges, this sacrificial-layer technology has proven very versatile and been used to make, among other structures, laterally vibrating doubly folded bridges, gears, springs, and impacting microvibromotors. Recently, micromirrors that consist of multiply hinged plates which fold out of the surface plane in which they are formed reaching vertical heights of mm dimensions have been demonstrated. The polycrystalline-silicon cross section for these mirrors is thinner than two micrometers. The mirrors can be moved using electrostatic comb drives or microvibromotors. Continued research on the mechanical properties of the electrical materials forming microdynamic structures (which previously had exclusively electrical uses), on the scaling of mechanical design, on tribological effects, on coatings, and on the effective uses of computer aids is now underway and promises to provide the engineering base that will exploit this promising technology.

This paper focuses mainly on the development of dry etching for structures of high aspect ratios, which will offer the potential to manufacture micro-sensors, micro-engines, micro-turbines, micro-actuators, and electronic circuits onto a single IC silicon chip. This technology is based on the highly anisotropic and selective dry etching of Simonocrystals. The suitability of reactive ion etching for the fabrication of micro electro mechanical systems (MEMS) has been evaluated by characterising the change of lateral dimensions vs. depth in etching deep structures in silicon. Fluorine, chlorine -and bromine-containing gases have provided the basis for this investigation. A conventional planar RIE (Reactive Ion Etching) reactor has been used, in some cases with magnetic field enhancement or with the ICP (Inductive Coupled Plasma) Source and low substrate temperatures. For reactive ion etching based on Cl₂ or Cl₂/HBr plasma, a slightly “positive” (top wider than bottom) slope is achieved when structures are etched with a depth of several 10 pm, whereas a “negative” slope is obtained when etching with an SF₆/CCl₂F₂ based plasma. Pattern transfer with vertical walls is obtained for reactive ion etching based on SF₆ (with O₂ added) when maintaining the substrate at Iow temperature (in range ≈-100–). Further optimization of plasma chemistries and reactive ion etching procedures should result in runout depths of the order of 0.1 µm/100 gm in Si as well as in organic materials. Etching processes are demonstrated in the realisation in Si microturbine. Axles or stators (nonmoving parts) are etched into the original Si-wafer. The movable parts (rotors, beams, etc.) are prepared from electrochemically etched Si-membranes with defined thicknesses. Then all movable parts are created lithographically on the SiN_xO_y surface. This is followed by dry etching the mono-crystalline Si-membrane down to the SiN_xO_y sacrificial layer on the back side of the membrane by an RIE-process. The wafer with the movable parts is flipped onto the wafer with the already etched axle and then positioned and centred. The SiN_xO_y sacrificial layer is then dissolved by a chemical wet or vapour etch process. Subsequent bonding with a Pyrex glass wafer seals the parts. The topic of lithography (masked ion beam lithography, MIBL), which delivers high resolution and large focus depths as well as e-beam lithography with tunnel-tips, is also addressed in this paper.

Characteristic for MEMS structures is the high surface to volume ratio. In combination with the extreme smooth surfaces, this can easily lead to dominant adhesion forces between contacting structures. In the past years, stiction problems have been successfully reduced by avoiding contact between the slender structures and the substrate. In micromotor applications, contact between moving and fixed parts can not be avoided, or even is essential to the functioning of the device. Reduction of adhesion forces, to obtain low enough friction is needed in these devices.

With MEMS or micromachines, where atoms of opposing surfaces locate close enough to interact one another, microtribology is a key technology which ensures functioning, reliability and long life of MEMS. Microtribology of MEMS is characterized by a relative motion of small microcomponents with small masses under very light loads, in which cases interfacial forces are essential rather than body forces[1, 2]. Adsorption of water molecules is one of the most troublesome impediments to proper functioning when MEMS is used in an ordinary atmosphere. Strong stiction due to adsorbed water molecules[3] causes enough resistance to stop relative motions. Bhushan showed a model of interfacial liquid with different levels [4]. Binggeli and Mate studied the influence of water vapor on nanotribology[5, 6]. However, the thickness of adsorbed water molecules, for example, in conditions of 50% relative humidity is about four or five layers. Therefore, adsorption behavior of water molecules on polymers should be studied on a finer scale.

Increasing the amount of information that can be stored in a thin magnetic film in hard disk drives requires that, in the near future, the recording head be brought within 20 nm of the magnetic film while moving at speed in excess of 10 m/s. With such high shear rates and with only room for a few atomic and molecular layers of protecting material, it is essential to obtain a good understanding of the molecular origins of tribological phenomena occurring in a disk drive. This paper examines the tribology of disk drives with an emphasis on how lubrication affects the tribological performance at the molecular level. The topic covered include: 1) how capillary and disjoining pressures determine the distribution of lubricants and meniscus forces at the contacting interfaces; 2) how lubricants and adsorbed water vapor affect adhesive forces and friction coefficients; 3) the interaction and mobility of lubricant molecules on disk surfaces; and 4) the stability and degradation of lubricants during the sliding process.

In the low flying height magnetic disk drive, where the head is flying just 20–30 nm above the rotating disk surface, the possibility of mechanical damage to the head significantly increases. In fact, chipping and microcracks of the alumina of the thin-film head is a frequently encountered problem. The complex layered structure of the thin-film head contains many materials and interfaces. Interaction with the disk surface and thermal expansion of the head materials causes high mechanical stresses in the head alumina, which may result in a crack initiation and growth.