- surface analysis - routledge · surface analysis 2-3 cos q g g g = solid vapor solid liquid−...

2-1

2.1 Introduction

Bearings are expected to provide relative motion between the components, for example, balls and race, with little or no service for the life of the machine. �e components must be strong enough to carry the load without permanent deformation and smooth enough to allow a lubricant to cover the asperities in the contact zone. Ideally, the lubricant would always separate the two surfaces, to prevent any contact between the mating surfaces. But that is not possible when the relative speed is zero, that is, at start-up and slow down, so bearings are o§en designed to run in the mixed asperity contact/lubrication condi-tion. Ideally, the surfaces would be durable enough to withstand contact. But even when the utmost care is taken, from initial processing of raw materials to �nal �nish, there is no guarantee that the compo-nent is free of damage-inducing surface or subsurface defects. A§er the bearing is assembled, tested, and put into service, what type of damage might occur? Damage can range from mild scoring to gross spallation, and the causes can range from di²cult to avoid contamination like dust (e.g., silica particles) to mechanical instability of the assembly that houses the bearing. Why? Because real bearing materials can deform, transform, form oxides on their surfaces, and accumulate super�cial �lms—for good or for bad—as illustrated by the schematic (a) and real1 (b) cross sections of worn-bearing surfaces in Figure 2.1. Bearing engineers need to be aware that even the best prepared components can be damaged by the various mechanisms detailed in the following chapters. Here we focus on how one uses surface charac-terization methods to discover and identify surface and subsurface damage.

�is chapter describes the methods available to engineers for analyzing surfaces. What surface con-ditions are conducive for the lubricant to lubricate? What techniques can be used to determine the topography, structure chemistry, and mechanical properties of surfaces? �e answers are in the ways that modern scientists are able to “see” surfaces. “Seeing is believing” can be said to underlie all experi-mental science. Until the late nineteenth century, what scientists could see was usually dictated by the optics in a light microscope. With Maxwell’s equations, scientists recognized that visible light was only a small region of the entire spectrum and began to “see” objects with light outside the visible spectrum.

2Surface Analysis

2.1 Introduction ...................................................................................... 2-12.2 Lubrication and Surface Wetting ................................................... 2-22.3 Methods of Surface Analysis........................................................... 2-32.4 Characterization of Surfaces and Subsurfaces ............................. 2-5

Microscopy: Imaging to the Nanoscale • Structure: Texture and Atomic Arrangements • Composition: Mostly Vacuum-Based Techniques • Composition: Optical Techniques

2.5 Mechanical Testing......................................................................... 2-152.6 Summary .......................................................................................... 2-16Acknowledgments ...................................................................................... 2-17References .................................................................................................... 2-17

Irwin L. SingerU.S. Naval Research Laboratory

Copyright Taylor & Francis Group. Do Not Distribute.

2-2 Theory and Practice of Lubrication and Tribology

�e discovery of electron beams and ion beams, combined with an understanding of beam physics, allowed scientists to explore electron and ion beam interactions with surfaces. Today, there are literally hundreds of “beam-in, beam-out” techniques, in which a beam is aimed at an area of the surface and the interaction generates secondary particles or rays that can then be analyzed along with the re¨ected beam. Scientists use these to see not only the topography of surfaces—down to the atomic scale—but also the microstructures and compositions of surfaces from the nanoscale to the mesoscale, from the outermost atomic layer to depths over 1–100 μm below the surface. �e methods of surface analysis are described in Section 2.3 and examples of surface analytical techniques are given in Sections 2.4.1 through 2.4.4. Finally, Section 2.5 covers mechanical testing of surfaces. However, before entering the world of surface characterization, it will be valuable to understand how liquids interact with surfaces.

2.2 Lubrication and Surface Wetting

Liquids must be able to wet the surface to be eµective lubricants. Wetting is determined by the interaction between a liquid, a solid, and the surrounding gas. Consider a droplet on a ̈ at surface (Figure 2.2). �ere are three interfaces: the solid–liquid interface, the liquid–vapor interface, and the solid–vapor interface. Each of these interfaces has an associated surface tension, γ, which represents the energy required to create a unit area of that particular interface, or equivalently, the force per unit length at the interface. By force arguments, the angle q between a liquid drop and a solid surface, the contact angle, is given by

Vapor

(a) (b)

Receding

Advancing

Solid

Solid

Liquidγsolid–vapor γsolid–liquid

γ liq

uid–

vapo

r

θ

θ θ

FIGURE 2.2 Contact angle θ of a droplet on a ¨at surface indicating (a) three interfacial forces, γ, and (b) contact angle hysteresis.

0.1 – 20 nm Tribofilm

Oxide

Deformation layer

Bulk material

(a) (b)

0.1 – 100 µm

20 µm

0.3 – 5 nm

FIGURE 2.1 (a) Schematic cross section of a worn-bearing surface (nonlinear depth scale for emphasis). (b) Optical micrograph of a worn 316L stainless steel surface. (From Rainforth, W.M. et al., Philos. Mag., 66, 621, 1992. With permission.)


Surface Analysis 2-3

cos q g g

g=

−solid vapor solid liquid

liquid vapor

− −

−

(2.1)

where the “gammas” are surface tensions of the three interfaces. According to Equation 2.1, when γ liquid–vapor < γsolid–vapor, cos θ increases and the area of contact between the liquid and the solid interface increases. As the drop spreads and wets the surface, the contact angle approaches zero, hence cos θ approaches 1. Using a contact angle goniometer, Zisman et al. discovered a universal behavior in plots of cos θ vs. the surface tension of (nonpolar) liquid: cos θ increases linearly as the surface energy decreases.2 �ey de�ned the point at which the plot crossed the cos θ = 1 axis as the critical surface ten-sion, an important parameter because it is a characteristic of only the solid and therefore can be used to predict the wettability of the surface. Early studies of wetting were done with model liquids on polished and smooth substrates. Nonideal surfaces can be rough and have a chemically inhomogeneous surface. Contact angle measurements on such surfaces o§en exhibit hysteresis. �e hysteresis is characterized by dynamic contact angle measurements, obtained by subtracting advancing (θa) and receding (θr) contact angles. �e physics behind wetting, surface energy, and contact angle can be found elsewhere.3,4

How do liquid lubricants maintain contact with the surface? As stated earlier the lower the sur-face energy, the more easily liquids will wet (spread) on the surface. To increase wetting and reduce “beading,” lubricants are doped with surfactants (surface active agents), compounds that lower the sur-face energy of a liquid. Since metals generally have a high surface energy, due to their strong interatomic bonds, one would expect liquids to wet them easily. However, in ambient environments, metal atoms are not present as metals at the top surface (except for noble metals). Instead, the surface consists of many layers, the topmost being a contaminant �lm consisting of hydrocarbons and condensed water molecules, then an oxide layer, each of which lowers the surface energy.5 To achieve robust, wettable surfaces, proper cleaning procedures must be followed a§er grinding, abrading, and polishing. O§en, a �nal solvent cleaning is su²cient. Occasionally, stronger cleaning processes are necessary, like hot alka-line detergent cleaning, electrocleaning, and acid etching; in other cases, plasma (ion bombardment) cleaning is required to remove the last layer of impurities. Similar cleaning procedures are necessary before coating a surface. Sometimes, however, the surface energy of the metal is much higher than the surface tension of the lubricant, causing the lubricant to spread too thin to provide lubrication. Here, a stable, durable �lm of low surface energy—a barrier �lm—can be deposited on both sides of the contact zone to prevent out-migration of ¨uids.6

Liquid lubricants, in turn, carry additives that form protective tribo�lms on bearings. �e �lms are most important when sliding or rolling speeds are too low for hydrodynamic lubrication. �ese �lms are usually too thin to be seen by the naked eye and, typically are neither smooth nor homogeneous; nonetheless, they can protect the surfaces from wear. �ey have been and still are prime candidates for investigation using modern surface analytical techniques.7

2.3 Methods of Surface Analysis

All “beam-in, beam-out” techniques operate on the same principle: send a beam of photons, electrons, or ions at a surface and collect photons, electrons, or ions from the surface. �ese techniques developed with advances in beam generators combined with basic understanding of beam–surface interactions.8 Perhaps, the earliest and best example of this collaboration is x-ray (Bragg) diµraction. In 1913, William Lawrence Bragg and his father, William Henry Bragg, discovered that x-rays re¨ected oµ crystalline solids produced patterns representative of a lattice structure. For this discovery, they were awarded the Nobel Prize in physics in 1915, the only father–son team to win. Less than 50 years later, the technique unraveled the mysteries of the double helix shape of DNA.9

�e many “beam-in, beam-out” techniques described in this chapter can be understood with the help of the cartoon in Figure 2.3. A beam of photon or electron particles hits a solid layer. �e beam can be



reëcted (specularly) from the surface, can be transmitted, or more likely, will enter the layer but then scatter from atoms below the surface. �e scattering can be elastic, changing the direction but not the energy of the beam, or it can be inelastic, depositing some energy into the atoms. �e energy deposited usually produces secondary particles, which can also be scattered elastically or inelastically, dictated by the physics of interactions of the secondary particles with atoms. �e “beam out” can be the scattered primary beam, or a beam of secondary particles, depending on how the collector, shown on the right, is tuned. Figure 2.3 illustrates several ways in which “beam-in, beam-out” techniques are used to analyze surfaces. For example, an electron beam (in—from the le§) can either reëct specularly from the sur-face or pass into the layer. �e electrons that scatter specularly from the surface can be identi�ed by a detector that collects and counts electrons that have the same energy as the primary beam. �e electrons that enter the layer will be scattered inelastically by atoms and decay within a tear drop–shaped volume beneath the surface.10 Inelastic scattering will excite atoms, and their subsequent decay will produce either x-rays or Auger electrons. �e latter can be identi�ed by electron detectors tuned to Auger elec-tron energies and x-ray detectors tuned to capture the üorescent x-rays. �e depth sensitivity of the technique depends on both the inelastic mean free path (the distance that the primary and secondary beams travel before losing their energy) and the incident and exiting beam angles. Similarly, an incom-ing photon beam can interact with solids and produce electrons, known as “photoelectrons.”

During the past 100 years or so, microscopies and spectroscopies evolved using the “beam-in, beam-out” approach, with beams of electrons, ions, and photons from microwaves to x-rays and even gamma rays. Today, there are hundreds of materials analysis techniques;11 many of them are surface-speci�c and most are available commercially. Figure 2.4 displays a chart with the most widely used, commer-cially available surface analytical techniques;12 all are referred to by acronyms13 that will be spelled out and described in Section 2.4. �e vertical axes give the composition sensitivities and the horizontal axis gives the diameter size of the spot that can be detected. Each technique is represented by an area (bubble) on the chart. �e boundary of the bubble gives the range of compositions and lateral spot size detectible. �e techniques can determine compositions from a few atom percentage (AES, XPS, EDS) to parts per trillion (dynamic SIMS), structures depicting arrangements of atoms with their nearest neigh-bors (STEM/EELS) to defects in centimeter-thick steel plates (RTX), and surface sensitivities that span

Photon

Transmitted photon Transmitted electron

ElectronsAuger electron

Secondary electronBackscattered

electron

Elastically scatteredelectron

Photons

Collector

FIGURE 2.3 Cartoon of the “beam-in, beam-out” method. Electron or photon beams interact with a thin solid layer. �ey can reëct or pass through the layer, or be inelastically scattered, producing secondary particle beams. �e collector determines what particles can be detected and the incident and exiting beam angles control the depth sensitivity.



depths from 1 nm (STEM) to 1 cm (XRR). So, if one needed to identify, for example, a thin boundary �lm in a wear scar 100 μm wide, AES could do the job but XRF or RBS could not.

2.4 Characterization of Surfaces and Subsurfaces

�ere are many books on surface analysis14–17 and several that specialize in applications of surface analy-sis to tribology.18–20 Herein is a survey of many techniques that are used by tribologists today and intro-duce a few others that might be useful in the future.

2.4.1 Microscopy: Imaging to the Nanoscale

�e unaided human eye is capable of resolving features down to tens of micrometers in size and seeing colors over the visible region of the electromagnetic spectrum, roughly from 400 to 700 nm. An optical microscope magni�es features as small as a micrometer in diameter. White light is used to illuminate the feature and the scattered (re¨ected or transmitted) light is collected by lenses to form an image. Alternatively, an image can be formed by scanning/rastering a narrow beam of light over the surface, and the scattered light is collected and displayed as a function of the position of the incident beam on the surface. In both cases, the spatial resolution of the image or spectrum is determined by the diµraction limit of the microscope,21 which is proportional to the size of optical objective and inversely propor-tional to the wavelength of the light. �e light can also be analyzed spectroscopically, qualitatively with a prism, or quantitatively with a spectrometer.

5E22 STEM/EELS STEM/

EDSAES

SEM FIBSPM

TEM/STEM

SEM/EDS

XPS/ESCA

XRD

FTIR XRF

Lexes

TOF-SIMS

LA-ICPMS TXRF

Dynamic SIMS

RBS

XRRRTX

Imagingtechniques

Raman1E22

1E21

1E20

1E19

1E18

1E17

Physical limit for 0.3 nm sampling depth

Physical limit for 3 nm sampling depth

Physical limit for 30 nm sampling depth1E16

Copyright©2010 evans analytical group LLC

1E15

1E14

1E13

1E120.1 nm 1 nm 10 nm 100 nm

Analytical spot size1 µm 10 µm 100 µm 1 mm 1 cm

10 ppt

100 ppt

1 ppb

10 ppb

100 ppb

1 ppm

10 ppm

100 ppm

0.1 at%

1 at%

10 at%

100 at%

Det

ectio

n ra

nge

(Ato

ms/

cm3 )

FIGURE 2.4 Chart of surface analytical techniques. (Chart Courtesy of Evans Analytical Group®, http://www.eaglabs.com/techniques/analytical_techniques/.)



Many of the advances in science over the last �ve centuries can be attributed to developments in optical instruments operating with visible light. In the last 50 years or so, dozens of new “white light” and ultraviolet–visible–infrared (UV/VIS/IR) light microscopies have been developed. Furthermore, during the last 25 years, nanotechnologies have allowed us to break through the barrier of the “far-�eld” diµraction limit, which has limited the resolution of light microscopes to a micrometer. Today, with ever-increasing understanding of the physics of “near-�eld” techniques, numerous microscopes have been developed that allow us to see nanoscale features with IR and VIS light.22

Digital cameras and VIS light microscopes are indispensible for examining worn surfaces at magni-�cations from 1× to 100×. Figure 2.5 gives three examples of visible light images of wear features. Photo (a) shows surface fatigue of both the rollers and race in the tapered roller bearing,* photo (b) depicts galling of a high temperature alloy steel, and photo (c) reveals both impressions and particle detachment caused by impacting an abrasive-covered steel. Components can be inspected for defects and debris with a low-magni�cation (1× to 50×) stereo microscope or a high-magni�cation (50× to 1000×) reëcting microscope, the smallest size being about 1 μm, limited only by the diµraction of VIS light. Lighting can be used in many ways. Oµ-axis (oblique) lighting can bring out vertical features on ät surfaces. Transmitted light can illuminate buried structures and subsurface damage in transparent ceramics or plastics and semitransparent materials, like �ber-reinforced epoxies. Polarized transmitted light of pho-toelastic materials can depict stress �elds in materials. Two examples of photos in transmitted light are shown in Figure 2.6. �e photo on the le§ displays both the bulk structure and worn surface of a G10 composited burned by molten Al; the photo24 on the right reveals the stress contours produced by a cyl-inder pressed against a photoelastic ̈ at. Surface topography can be enhanced using Nomarski objectives, dark �eld, oblique, or back-lit illumination. Confocal microscopes create in-focus images by computer-aided image reconstruction of 3D features that would be blurred in a standard microscope. Even simpler, 3D features can be captured as a stereoscopic image by photographing an object twice, at an angle about 4° apart; an example of a stereo image taken in a scanning electron microscope (SEM) will be given later.

Optical pro�lometers provide the most detailed images on reëctive surfaces, capturing both topography and height variations, by a variety of techniques: interferometry, confocal microscopy, chromatic aberration, laser triangulation, and others. Depending on the technique used, these non-contacting pro�lometers can measure and image 3D features over lateral scales from micrometers to meters and vertical scales from below 1 nm to many centimeters. Contact (stylus) pro�lometers can also produce images of 3D surface features, but are more invasive (scratch surfaces, move around debris, etc.) and usually take longer to acquire images than optical pro�lometers. On the positive side, their spatial resolution is limited mainly by the size (radius) of the stylus’ tip, and they are more

* Photo (a) is Figure 55 from Reference 23.

2.0 mm

(a) (b) (c)60.0 µm

FIGURE 2.5 (a) Damaged cone and rollers of taper roller bearing, due to heavy load and inadequate lubrication; (b) severe plastic deformation (galling) of Nitronic 60™ oscillating against a ät of the same material at 485°C; and (c) damage to AISI 4340 steel surface a§er impacting 100 times on a platen covered by a layer of coarse, but mobile, alumina abrasives. (Photo (a) from Bearing failures and their causes, SKF Publication PI 401 E, Figure 55; photos (b) and (c) are courtesy of P. Blau, ORNL, Oak Ridge, TN.)



reliable than optical pro�lometers when the reëctivity is poor or the surfaces are transparent or have semitransparent layers. Optical pro�lometers can produce signi�cant height-quanti�cation artifacts on poorly reëctive or transparent surface, both of which can be avoided by depositing a thin reëctive coating on the surface.

Scanning probe microscopy (SPM), in particular atomic force microscopy (AFM), can operate in a contact mode like a stylus pro�lometer, imaging surfaces with nanometer resolution laterally and verti-cally. �e stylus probe, with a tip of radius 20 nm or less, is attached near the end of a cantilever. �e tip, loaded to around 1 nN, is guided across a surface by a piezoelectric positioning element. �e location of the tip, both lateral and vertical coordinates, is displayed either as a true 3D or pseudo-3D map, the latter being a 2D image with the z-coordinate color coded. SPMs can also be used in more gentle modes, by either cruising above the surface while sensing the attractive van der Waals forces between the tip and the surface or tapping the surface. In both cases, the tip is set oscillating at or near the resonant frequency of the cantilever. In the former, noncontact mode, the tip-to-surface distance is changed to maintain a constant resonance frequency, providing a topographic image of the surface. In the tapping mode, the initial vibration amplitude is increased to several hundred nanometers to allow the tip to intermittently touch the surface. A servo motor is then used to adjust the tip height to maintain a con-stant vibration amplitude, and this height pro�le becomes a force map of the surface.25

�e SEM magni�es up to 100,000 times, showing features with dimensions at the nanometer scale. A beam of high-energy electrons (from 0.5 to 30 keV) is rastered across the sample and either secondary or backscattered electrons are collected. Image contrast can o§en be improved by operating at voltages below 5 kV, but with a loss of lateral resolution. For samples that are nonconducting or have nonconduc-tive features (oxide debris or insulating second phases), thin conductive �lms of carbon or gold reduce “charging” artifacts. Figure 2.7 shows a stereo image of a Vickers indent in a ceramic.26 Stereo images of 3D features bring out the shape of indent impressions, surface bulges, and detached chips. An SEM must be housed in a vacuum chamber to operate the electron gun. However, an environmental SEM (ESEM), with diµerentially pumped sections, can operate at relatively high pressure (0.001 atm), allowing a range of investigations, from in situ SEM analysis of atmospheric eµects on friction on ceramics27 to analysis of articular cartilage.28

�e transmission electron microscope (TEM) can achieve even higher magni�cation, as small as a single row of atoms. �e lateral resolution is improved over SEM because the beam is transmitted

(a) (b)

FIGURE 2.6 (a) G10 epoxy, burned by molten Al, is illuminated by back lighting, showing �ber weave within and on top of translucent epoxy. (b) Stresses produced by a cylinder pressed against a photoelastic ̈ at, with both normal and tangential loading, are made visible by polarized light. Main images are magni�ed areas from insets in upper right. (Photo (a) from author; photo (b) is a public domain image from http://en.wikipedia.org/wiki/File:Kontakt_Spannungsoptik.JPG.)



through an ultrathin specimen, eliminating most of the secondary and backscattered electrons that broaden the electron-illuminated region. When a �nely focused beam is rastered across a thin section, the technique is called scanning TEM (STEM) or high-resolution TEM (HRTEM). In the past, the prep-aration of thin sections—as much an art as a skill—was done by mechanical grinding and chemical and/or ion etching in several time-consuming steps. Figure 2.8 shows a HRTEM image of a cross- sectional, tribomechanically crystallized layer of an amorphous Pb-Mo-S coating.29 �e image gave direct evi-dence that single layers of ordered MoS2 (area marked by downward arrows) were created by sliding on the surface. Also seen are subsurface patches of basal MoS2 sheets, marked by upward arrows and the letter “s.” Today, thin sections can be made in one step by focused ion beam (FIB) etching in a SEM. Areas to be thinned are identi�ed in the microscope, then thin rectangular sections are etched with the FIB and removed with a manipulator for TEM analysis.30,31

Microscopy can also be used to examine subsurface features of materials. One can access the near subsurface top down, to depths from 1 to 500 nm, by chemical or electrochemical etching, by ion etch-ing, or by mechanical or chemomechanical polishing (recognizing that each of the etching process men-tioned earlier can leave artifact in the surface). Figure 2.9 shows both pseudo-3D (gray scale as height) and 3D interferometric pro�lometry images of a polished 304 stainless steel surface that was ion-etch 100 nm deep. �e etched surface reveals large austenitic grains; the height diµerence from grain to grain is a result of the diµerent sputtering rates of grains in diµerent orientations.32 A more common way to examine the subsurface is by cross-sectioning, that is, cutting perpendicular to the surface fol-lowed by polishing, to expose the entire subsurface, for example, the cross-sectional image of a worn surface on the right in Figure 2.1. A variant of cross-sectioning that magni�es the near subsurface is

6 µm 6 µm

FIGURE 2.7 SEM stereo image of a Vickers indent (at 4.9 N) in sintered SiC a§er heating in vacuum to 900°C. Note that the ceramic deformed plastically inside the indent but chipped at the tensile-stressed edges. (From Singer, I.L., Surf. Coat. Technol., 33, 487, 1987. With permission.)

10 nm

s s

FIGURE 2.8 Cross-sectional HRTEM micrograph of a wear track on an amorphous Pb-Mo-S a§er 1000 cycles (sliding direction parallel to image plane). Sliding created and li§ed sheets of basal planes MoS2 (area marked by downward arrows). Also, subsurface patches of basal MoS2 sheets are marked by upward arrows and the letter “s.” (From Dunn, D.N. et al., MRS Proceedings Volume 522, Fundamentals of Nanoindentation and Nanotribology, Moody, N.R. et al., Eds., MRS, Pittsburgh, PA, 1998, pp. 451–456; Wahl, K.J. et al., Wear, 230, 175, 1999. With permission.)



called taper sectioning. �is is performed by polishing the substrate at a shallow angle, θ, nearly parallel to the surface (typically 1°–5°), which magni�es the cross-sectional view by sin−1 (θ).

2.4.2 Structure: Texture and Atomic Arrangements

X-ray diµraction (XRD) is the most commonly used technique to identify the structure of materials. Specularly reëcted x-rays produced intense peaks at angles that depend on the lattice spacing (Bragg’s law), allowing identi�cation of crystal structure, crystallite (grain) size, lattice strain, and preferred ori-entations of grains. Although x-rays can penetrate tens of micrometers into solids, the technique can be made very surface-sensitive by aiming the incoming x-ray beam at a small incident angle (0.3°–3°) to the surface. Below a critical angle, the incident x-ray is con�ned to a thin layer near the surface, typically, 10–100 nm, and the reëcted x-ray conveys the layer’s structure. �e technique is known as grazing incidence x-ray diµraction (GIXD or GID).

Electrons possess the same wavelike behavior as x-rays, and a variety of electron diµraction tech-niques are used to study structures of single atomic layers at and below the surface. �e TEM/STEM, mentioned earlier, is able to image structures as well as produce electron diµraction patterns (TED) at the nanometer scale. Besides identifying structures, these electron techniques are capable of determin-ing the location and extent of plastic deformation down to the 10 nm scale. Low-energy electron diµrac-tion (LEED), performed in ultrahigh vacuum chambers, impinges low-energy electrons (20–200 eV) onto well-ordered crystalline surface. Spot patterns of backscattered diµracted electrons depict the symmetry of the atomic surface and adsorbed species. Reëction high-energy electron diµraction (RHEED) is a similar surface structural technique, but uses a more focused, higher-energy electron beam (10–30 keV). �e incident beam hits the surface at a shallow angle; the scattered electrons project a diµraction pattern onto a screen. Electron backscatter diµraction (EBSD) operates like SEM, but is able to produce “Kikuchi” diµraction patterns that identify texture or preferred orientation of crystalline or polycrystalline material.

Finally, two very powerful, but specialized, techniques are electron energy loss spectroscopy (EELS) and x-ray absorption near-edge structure (XANES), also called near-edge x-ray absorption �ne structure (NEXAFS). Both exploit the energy lost by inelastically scattered electrons to measure element-speci�c, pair-distance distribution functions, that is, interatomic distances. By calculation-intense analysis, both techniques are capable of determining the precise location of nearest neighbors, the ultimate atomic

Z Range: 188.8 nm

0(a) (b)

0.050.0

100.0150.0

200.00.20.00.20.4

–50.0–100.0

–150.0–200.0–150.0

–100.0–50.00.0

50.0100.050.0

010

320

5

138 276

40

0

–40

–80

–120

50 µm

µm

X (µm)

Z (µm)

Y (µm)

µm

FIGURE 2.9 (a) Pseudo-3D (grayscale as height) and (b) 3D interferometric pro�lometry images of a polished 304 stainless steel surface that was ion-etch 100 nm deep. �e etched surface revealed large austenitic grains; the height diµerence from grain to grain is a result of the diµerent sputtering rates of grains in diµerent orientations. Spikes on le§ image are noise artifacts. (Interferometric pro�les shown here were taken on samples investigated in Singer, I.L., J. Vac. Sci. Technol., 18, 175, 1981.)



structure technique. High-resolution EELS (HREELS) is an even more precise EELS technique, but it must be performed in an ultrahigh vacuum chamber.

2.4.3 Composition: Mostly Vacuum-Based Techniques

Before the advent of modern “dry” surface analytical techniques, metallurgists relied on wet chemical methods to determine the microstructure of bearing material and detect impurities on the surface.33 �ese methods, while not quantitative nor low spot size, are easy to use and can be performed in the �eld. Spot test kits, with reactive reagents, can detect speci�c elements found in stainless steels, tool steels, high alloy steels, and many others. A drop of solution is placed on the sample, and the color spot is used to identify the alloy. �e tests are sensitive to a microgram.

Modern surface analytical techniques, as mentioned earlier, depend on beam-in, beam-out methods. �e three most commonly used surface analytical techniques are (1) energy-dispersive spectroscopy (EDS), also known as energy-dispersive x-ray analysis (EDX), (2) Auger electron spectroscopy (AES), and (3) x-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical anal-ysis (ESCA). When an atom is bombarded by photons (x-ray) or electrons with energies in the range of 2–50 keV, a core state electron can be ejected leaving behind a hole, an unstable electronic structure. �e hole can be �lled in two ways. An electron from a higher orbit can �ll the hole and the diµerence in energy released as a photon (x-ray), the basis for x-ray ¨uorescence (XRF) analysis. Or, the core hole can be �lled by an outer shell electron and a second electron is ejected with kinetic energy equal to the energy diµerence between the outer shell and the core; this is a three electron process, known as an Auger process, named a§er one of its discoverers, Pierre Auger. In both cases, the energy of the emitted x-ray or electron is characteristic of the atom and thus serves as a unique �ngerprint of the material. Which process is more likely to occur during electron excitation? Typically, Auger yields are higher at low excitation energies (0.5–10 keV) and for lower atomic number elements, while XRF dominates at high electron energies and higher atomic number elements.

Compositions of surface layers from 1 to 5 monolayers thick can be identi�ed using electron and x-ray sources in ultrahigh vacuum chambers. AES uses an electron beam in the 1–10 keV range to excited atoms, which decay preferentially by Auger electron emission.14–17 Auger electrons have energies from 10 eV to several thousands of electrovolts. At these energies, the electron mean free path is small, 0.3–3 nm. �erefore, Auger spectra represent the atomic compositions of atoms in the near-surface layer, making AES a very surface-sensitive technique. All atoms, except for H and He, emit Auger electrons. Furthermore, the kinetic energy of the Auger electron is sensitive to how the atom bonds with its neigh-bors, providing chemical “�ngerprinting” as well as elemental identi�cation. Although XPS relies on x-rays to excite atoms, it is very much like AES. X-ray-induced photoelectrons are emitted in the same energy range as the electron-induced Auger electron, and therefore, XPS and AES have similar depth sensitivities. �e chemical information obtained from XPS is directly related to the binding energy of the photoelectron and, therefore, the chemical state of the atom is more easily identi�ed than that from the three electron Auger process. �e main advantage of AES is that its electron beam is much smaller (hundreds of nanometers) than the x-ray beam (5–10 μm at the present time), so the spatial resolution of scanning Auger microscopy (SAM) is much better than that of XPS. AES and XPS can be used in conjunction with ion beam sputtering (ion beam erosion) to obtain compositions vs. depth information.

Figure 2.10 shows an example of Auger sputter depth pro�ling, with chemical �ngerprinting of both Ti and C as a function of depth below the surface of a Ti+-implanted bearing steel.34 Sputter depth pro-�ling, however, can be misinterpreted if the following caveats are not taken into account: �rst, prefer-ential sputtering—a very common process—o§en makes it impossible to quantify compositions below the surface.35 As an example, S sputters up to four times faster than Mo in a MoS2 matrix, giving S/Mo ratios of 0.5 instead of the bulk ratio of 2.36 Second, sputtering breaks bonds, reducing the compounds being interrogated at the surface, for example, sulfates (SO4)2− are reduced to sul�tes (SO3)2− and even to sul�des (S)2−.37,38



Energy-dispersive x-ray spectroscopy (EDS) is the analytical arm of the SEM. EDS can detect ele-ments from B up (due to detector limitations), and unlike AES and XPS, EDS does not require ultrahigh vacuum; SEM/EDS can be performed on large and not pristinely clean samples. EDS takes advantage of fast, solid-state detectors to energy resolve the XRF emitted by excited atoms. Collection times are fast (minutes) and energy resolution is su²cient to quantify elemental compositions to several percent. �e depth sensitivity of EDS in an SEM depends on the depth over which the electrons decay yet are suf-�ciently energetic to excite atoms, typically 0.5–5 μm, depending on electron beam energy and atomic mass of the near-surface atoms. While nowhere as depth sensitive as AES, it can be much faster and simpler for qualitatively identifying local compositions than AES depth pro�ling. �e two combined, however, are an unbeatable tool for investigating the wear of coated surfaces: the SEM locates features; EDS assesses the local compositions and thereby identi�es candidate spots for AES depth pro�ling.

Figure 2.11 shows an example of how several surface analytical techniques were combined to study the wear behavior of a MoS2 coating.39 In this study, EDS was used to located features in the wear track according to composition, for example, oxygen-rich and oxygen-depleted areas. �en, AES depth pro-�les were taken to identify the layered compositions that gave rise to the features. �e three depth pro-�les identify thick MoS2 debris at the edge of the track; a very thin, hence worn layer of MoS2 on the track; and a nearly bare patch of track, with oxidized Fe and a very thin layer of MoS2.

When EDS is used with thin sections in the TEM/STEM, the depth sensitivity is controlled by the section’s thickness, which can be as thin as 10 nm. Elemental maps generated by EDS complement secondary and backscattered electron maps. While EDS is a fast way to identify the composition of a surface, it cannot easily resolve overlapping emission energies, for example, the severe overlap of the Mo(Lα) and S(Kα) spectra at 2300 eV. Wavelength-dispersive x-ray spectroscopy (WDS) uses diµraction gratings to detect the precise wavelength of emitted x-rays and is therefore much better at quantifying

0

0 5 50 100 150 200

420

O

270C

O515

C270

380420

Ti

Fe650

Ti

10 20Ion milling time (min)

Depth (nm)

Aug

er p

eak-

to-p

eak

inte

nsiti

es (a

rb.u

nit)

30 40

FIGURE 2.10 Auger sputter depth pro�le of Ti-implanted 52100 steel. Oxide layer (le§) is shown with a more expanded depth scale than the subsurface (right). �e chemical states of both Ti and C can be inferred from the line shapes of Ti(LMM) spectra (360–430 eV) and C(KLL) spectra (250–280 eV): Ti oxide on surface, Ti carbide at oxide–steel interface, metallic Ti as C concentration falls, and an Fe carbide in the bulk of the steel. (From Singer, I.L. et al., Nucl. Instrum. Methods, 182/183, 923, 1981. With permission.)



composition and avoiding overlapping/interfering peaks. However, WDS is much slower than EDS. Low-energy x-ray emission spectrometry (LEXES) is a variant of WDS. �e electron probe microana-lyzer (EPMA) is an SEM/EDS/WDS machine designed for more quantitative analysis. Electrons can also stimulate light emission, known as cathodoluminescence (CL), in many nonmetallic and semicon-ducting materials. �e light, covering the UV/VIS/NIR range, can be analyzed spectroscopically and mapped to complement SEM and EDX maps.

Finally, there are several üorescence techniques for measuring wear that do not require vacuum chambers for either beam in or beam out. �e �rst is the aforementioned XRF. It relies on the same detection scheme as EDS, but uses x-rays instead of electrons to generate the üorescence. Because x-rays penetrate 10 times deeper than electrons, the composition is obtained to depths from 10 to 50 μm. And, since electron beams are not used, the technique does not require a vacuum chamber. In fact, handheld XRF devices are available. Two other XRF techniques are ion-induced x-ray analysis (IIX) and particle (alpha or proton)-induced x-ray spectroscopy (PIXE). As the names imply, they rely on ions and protons, respectively, to generate XRF. Both are available commercially and used in real-time monitoring of engine wear. PIXE can even be performed with a small radioactive source, which, while not as e²cient as more intense electron, x-ray, or particle beam sources, is very compact and requires low power. A device built on this technique, the alpha proton x-ray spectrometer (APXS), was used by the Sojourner rover to analyze rocks during the 1997 Path�nder mission on Mars.

�e second is thin-layer activation (TLA), a related technique that relies on gamma-ray üorescence and, therefore, also does not need a vacuum chamber.40 �e surface to be studied is “labeled” with a radioactive isotope marker, and then a gamma-ray detector monitors the loss of material from the com-ponent or the increase in gamma-ray activity in the engine oil. TLA has been used to monitor wear (and corrosion) of engines and other machinery. It can be performed under engine-operating conditions in real time, is noncontacting, and can be made sensitive to submicrometer levels.

Position across track

Mo + S

EDS

Line

EDS

inte

nsity O

Fe

2

13

45

6

Sputtering time (min)0

C

FeS O

S

Fe

CO

SO

MoC

Fe

Nor

mal

ized

inte

nsity

(%)

Mo

Point 3

Mo

0

12

24

36

48

600

12

24

36

48

600

12

24

36

48

60S

Mo

Point 2

Point 1

C O

FeLegend

S

Mo

O

Fe

C

3 6 9 12 15 18 21 24

FIGURE 2.11 (Top le§) SEM image of a heavily worn track on MoS2-coated steel. (Bottom le§) EDS line scans depict the relative concentrations (thickness) of Fe(Kα), O(Kα), and Mo(Lα) + S(Kα) collected along line indicated on SEM image, indicating compositions and thicknesses. (Right) Auger sputter depth pro�les at three of the six circles indicated in the SEM image. Circles indicate spots where Auger sputter depth pro�les were taken. Point 1 was taken on a debris äke along the edge of the track; the debris is a mixture of Mo-S-Fe-O; point 2, an area of the track with a MoS2 layer thinner than the original coating; point 3, mainly the oxidized wear track (Fe oxide), with a thin cap of MoS2. (From Ehni, P.D. and Singer, I.L., Appl. Surf. Sci., 59, 45, 1992. With permission.)



Secondary ion mass spectrometry (SIMS) uses an ion beam to sputter remove surface layers, then col-lects and identi�es the removed ion species with a mass spectrometer. While more di²cult to quantify than AES or XPS, SIMS can detect impurities at parts per trillion. SIMS can be used for composition vs. depth pro�ling (dynamic SIMS) or simply to determine the composition of surfaces (static SIMS) with minimal damage. Techniques that detect the backscattered or forward scattered primary ions are called ion-scattering spectroscopies (ISS). Low-energy ion-scattering spectroscopy (LEIS) is sensitive to the outermost atomic layer and can give both composition and structure. Medium-energy ion scat-tering (MEIS) and high-energy ion scattering (HEIS), known in practice as Rutherford backscattering spectroscopy (RBS), use higher-energy beams, from 100 keV to over 1 MeV, to probe compositions and structure of surface and subsurface layers.

In glow discharge optical spectroscopy (GDOS) and glow discharge mass spectrometry (GDMS), surfaces are bombarded by inert gas atoms in a glow discharge, producing sputtered atoms in either an excited atomic state or an ionized state, respectively. In GDOS, the excited atoms decay by emitting radiation, whose spectrum can be detected with an optical emission spectrometer (OES). Emission peaks in the spectrum occur at wavelengths characteristic of the sputtered atoms; peak intensities help determine atomic concentrations. In GDMS, the ionized atoms are collected in a mass spec-trometer, and concentrations for most elements can be found down to the subparts per billion range. Furthermore, in GDMS, unlike the related SIMS technique, quanti�cation is nearly matrix independent.

RBS, which relies on the energy loss of an ion passing through a material, is one of the few nonde-structive techniques capable of composition pro�ling to depths of a micrometer or more. It is also highly quantitative, thanks to decades of precision measurements of nuclear stopping powers and scattering cross sections. RBS can also be used in the “channeling mode” to quantify defects in nearly perfect crystals. Two related nuclear scattering techniques are particularly valuable for detecting hydrogen in surfaces and �lms. Nuclear reaction analysis (NRA) relies on a nuclear reaction between the incoming ion and a targeted nucleus, for example, the proton in hydrogen. �e ion resonantly excites the nucleus, which decays and emits ionizing radiation. A reaction used to pro�le hydrogen is

15 121H C 4 965 MeVN + → + + ( )a g .

with a resonance at 6.385 MeV. �e concentration pro�les of H vs. depth is obtained by detecting the energy of the emitted gamma ray as a function of the 15N incident beam energy and computing the depth from the known energy loss of 15N in the material. Elastic recoil detection (ERD), also referred to as forward recoil scattering, uses a relatively low-energy (2 MeV) 4He beam to depth pro�le hydrogen, but requires a foil sample from 1 to 10 μm thick.

Mössbauer spectroscopy is a spectroscopic technique based on the recoil-free, resonant absorption and emission shi§s of gamma rays emitted by atoms in solids. It is particularly useful for identifying compositions of iron-containing specimens and, as with PIXE, data collection of Mössbauer spectra has also been carried out on Mars.41 Gamma rays have very long path lengths in solids, which excludes their use for surface-sensitive experiments. However, the same resonant shi§s are detectable in emitted x-rays and electrons, whose mean free paths in solids are tens of micrometers and 1–10 nm, respectively. While not applicable for studying most elements, the latter two techniques, conversion x-ray and conversion electron Mössbauer spectroscopy (CXMS and CEMS), have been exploited to study tribological coatings and wear.42

2.4.4 Composition: Optical Techniques

Raman and IR spectroscopies are used to investigate mainly nonmetallic materials, including organic and inorganic coatings and surface �lms, plastic and ceramic bearings, and lubricants. Both



techniques can identify compounds based on vibrational and rotational modes of molecules. In IR spectroscopy, the light directly excites molecules at frequencies characteristic of their structure. It can be done by sweeping the frequency/wavelength of a monochromatic beam vs. time, or by using a broadband light source and an interferometer to measure all wavelengths at once, then performing a Fourier transform on the IR spectrum (FTIR). FTIR has been a standard practice in industry and the military for condition monitoring of lubricants.43 In Raman spectroscopy, the electric �eld of the laser (monochromatic) light is inelastically scattered by molecular, vibronic or electronic excitations, resulting in small shi§s, positive and negative, in the wavelength of the exiting light. �e measured shi§ between the incoming and outgoing photon energy, typically reported in units of wavelength or frequency, corresponds to the excited mode.

Laser Raman spectroscopy is an excellent tool for analyzing wear scars and small features. It is com-monly combined with an optical microscope, so locating features is easy. Its spatial resolution is about 1 μm in diameter by several micrometers in depth, as determined by the collection optics. It has also been combined with a tribometer that allows in situ, real-time optical and Raman analysis of slid-ing contacts.44 Raman is useful for detecting many inorganic compounds45 used as solid lubricants, like MoS2 and diamond-like carbon (DLC). Moreover, it can be very sensitive, due to resonant Raman eµects,46 and detect �lms as thin as 10 nm.47 Sensitivity to selected compounds can also be enhanced by selecting appropriate laser wavelengths. Raman can be made many orders of magnitude more sensi-tive by enhancing the electric �eld in the neighborhood of the Raman interaction, as occurs when the light excites surface plasmons. Surface-enhanced Raman spectroscopy (SERS) has been achieved by adding colloidal nanoparticles of Ag or Au to surfaces. Another SERS approach is to take Raman spec-tra in the vicinity of a metallic (usually silver- or gold-coated) AFM or STM tip. �e latter approach is known as tip-enhanced Raman spectroscopy (TERS) and has been shown to have sensitivity down to the single molecule level. TERS is one of many near-�eld scanning optical microscopy (NSOM/SNOM) techniques that breaks the far-�eld resolution (diµraction) limit to investigate nanostructures with UV/VIS/IR light.

An IR microscope allows one to locate areas on a sample, then to acquire IR spectra. Infrared micro-scopes operate in either transmission or reëction mode. �e spatial resolution is typically diµraction limited to 20 μm or larger, determined by the wavelength of the blackbody radiation and the objective apertures. Spectra of samples must be normalized to that of a “control” sample to eliminate the inü-ence of the instrument and gases. In the lab, IR spectra of lubricants can be obtained by spreading the lubricant on an IR-transmitting salt crystal and acquiring spectra in the transmission mode. Attenuated total reëctance (ATR) IR is an alternative technique for studying thin lubricant �lms. ATR is based on a special behavior of light in matter known as total internal reëction: When a ray of light enters a trans-parent medium at or below a critical angle with respect to its surface, the light cannot pass through the surface and, instead, reëcts and becomes totally internally reëcted. A �lm is deposited onto the ATR crystal. A beam of infrared light is passed through a bevel edge of the crystal, then undergoes internal reëction, producing an evanescent wave that penetrates typically 0.5–2 μm into the �lm. �e technique allows for in situ studies of tribo�lm formation during boundary lubrication, as shown recently by Piras et al.48 for zinc dialkyldithiophosphate (ZnDTP) against Fe and Sasaki et al.49 for tricresyl phosphate against Fe.

Several instruments have been developed around the AFM and IR light sources that allowed IR spec-troscopy to break through the diµraction limit (tens of microns) and measure IR absorption at the micro- and nanoscale.50,51 In photothermal microspectroscopy (PTMS), IR light is beamed onto a sur-face. If the radiation is absorbed by the sample, heat is generated locally and the surface expands and is detected by the tip of an AFM. Topographical AFM maps are acquired at discrete IR frequencies or by sweeping the IR frequencies, giving IR absorption maps with the spatial resolution of the AFM. A related technique is scanning thermal microscopy (S�M), in which the AFM tip is replaced by a sub-miniature temperature sensor like a thermocouple or a resistance thermometer.



2.5 Mechanical Testing

Understanding the mechanical properties of surfaces is crucial to bearing design, from run-in to steady-state performance. For a given bearing contact geometry (see Chapters 4 and 13), the loading conditions are conservatively estimated from bulk elastoplastic properties of the counterfaces. But these properties do not usually account for changes in surface mechanical properties that arise during rolling or slid-ing contact, for example, changes in surface hardness or in fracture toughness. Nor are bulk properties su²cient to account for behaviors of coated or surface-engineered bearings. For these, it is necessary to characterize the deformation and fracture resistance of thin layers.

�e most common techniques for characterizing mechanical properties of surfaces are indentation and scratch testing. In both techniques, a hard tip is pushed into and along a surface, respectively, while the deformation response is recorded by sensors; later, the scratch tracks are examined with a micro-scope. Indentation testing is o§en performed quasi-statically, that is, a load is applied to the surface, held for a period of seconds, then removed; this method will be described in detail later. Alternatively, dynamic indentation testing is performed by impacting the tip quickly, either once or repetitively, and monitoring the rebound response. �is technique is referred to as rebound “hardness” testing, but that is a misnomer since it mainly measures elastic response whereas hardness usually refers to plastic response. One such device is a scleroscope, in which a diamond-tipped or hardened steel tip is allowed to fall from a known height, and the higher the rebound, the higher the elastic “hardness” of the material. A more modern, electronic rebound tester, the Leeb rebound tester, measures the rebound velocities and determines the rebound “hardness” from the energy lost during impact.

In scratch testing, a sharp tip is loaded against the surface as the sample moves transverse to the nor-mal load. �e earliest scratch tester was a sclerometer, an instrument used by mineralogists to measure the width of a scratch made by a diamond under �xed load (not to be confused with a scleroscope). Modern scratch testers are instrumented to record normal and tangential forces, and o§en have an acoustic sensor to detect brittle fracture events.52 A§er testing, the scratched tracks are examined in a microscope, then the wear patterns are associated with diµerent mechanisms of failure.53 Scratch test-ing is most valuable for investigating the limits of decohesion of the coating–substrate interface, what is o§en referred to imprecisely as the “adhesion” of the coating. �e combined normal and tangential force of the tip applies increasing stresses to the interface, causing cracking, crack growth, delamina-tion, and eventually spallation. Scratch tests can be used for quality control of coated bearings by, for example, assigning a cutoµ critical load for delamination. In addition, scratch testing has provided a deeper understanding of coating–substrate mechanics.54 By correlating the behavior of the force curves, acoustic emissions, and scratch patterns, numerous failure modes have been postulated, investigated, and modeled. Scratch testing has also been used to characterize ceramics, polymers, ceramic-coated polymers, and boundary-lubricant �lms. For thin (<1 μm) coatings, scratch tests are performed by scan-ning force microscopy (SFM), which uses AFM-like devices as nanoscratch testers. �ey can be oper-ated at high loads, simultaneously scratching the surface and measuring the scratch track depth during deformation, then retracted and rerun at much lower loads to pro�le the depth of the permanently deformed track.

Quasistatic indentation testing is categorized in terms of the load range applied or depth penetrated. Macroindentation tests are usually performed with commercial machines designed for selected hard-ness ranges of metals and alloys, for example, the Rockwell scales HRA-HRG; the procedures are well documented in ASTM or ISO55 standards. �e tests are performed with either a sharp diamond tip (Rockwell, Vickers, and Knoop) or a hard spherical tip (Rockwell, Brinell); loads are in the kilogram-force range and penetration depths around 0.1–1 mm. Most tests have been designed for engineering metals; the Shore durometer test is used on polymers and elastomers.

Microindentation tests are carried out at lower loads, typically 0.01–10 N (1–1000 gf) and require a sharp-tipped diamond indenter (Vickers, Knoop, or Berkovich) that penetrates only 10–100 μm into



the surface. �e geometry of the tip determines the depth of penetration at a given load. Vickers and Berkovich have symmetric pyramidal geometries with the same projected area, but the Berkovich is a three-sided pyramid and the Vickers, a four-sided pyramid. Knoop is an asymmetric (length to width ratio = 7:1) four-sided pyramid, ätter than the Vickers, so it penetrates less than the Vickers at the same projected area. Traditionally, microindentation tests were used to �nd the hardness of the surface, where the hardness is computed as the ratio of the maximum load to the projected impression area. A skilled operator measured the projected area of the impression with a light microscope. Modern microindentation (and nanoindentation) machines are instrumented to collect force vs. time vs. depth (FTD) data, a method known as depth-sensing indentation (DSI). �e DSI approach is used to compute parameters associated with the material’s deformation response—like hardness, elastic modulus, and viscoelasticity—from FTD data.56 But it, like the traditional impression imaging approach, cannot take into account the “pileup” of material along the rim of the impression. Pileup, however, can be measured using 3D pro�lometry to compute the actual volume of material displaced by the indenter, above and below the original surface. Finally, the indentation technique has many other uses besides hardness testing.56 It can be used to evaluate the fracture toughness of ceramics and hard coatings, known as indentation fracture toughness; stereo micrographs, like that in Figure 2.7, were used to assess the toughness of sintered SiC subjected to several surface engineering treatments. It can be performed with spherical indenters on so§ coatings or with sharp diamond tips, like cube-corner indenters, on hard coatings. It is also used to investigate fracture toughness of coatings and the durability of the coating–substrate interface.57 DSI can also be used to obtain the viscoelastic properties of polymers and elastomers.58

Nanoindentation testing is also performed by DSI, but at loads from nN to tens of mN and is thus able to obtain mechanical properties of nanometer-thin �lms and structures as well as image surface mechanical properties like stiµness and hardness.59 Such testers are built with very sensitive dis-placement (nm) and load (μN) sensors, and require isolation from vibrations, thermal üctuations, and airöw. In addition to acquiring FTD data, they are capable of outputting 3D and pseudo-3D (2D images with the third dimension color coded) spatial maps of topography, as well as normal and lateral (friction) forces. �e most sensitive ones are based on SPMs or AFMs, which can sense individual surface atoms, measure the “spring constants” of molecules, and image areas at a magni-�cation 1000× greater than light microscopes. SPMs can be used to obtain mechanical properties of tribo�lms, transfer �lms, and liquid layers. Researchers have also exploited the nanometer sensitiv-ity of SPMs as a near-�eld detector to avoid the diµraction limit on photon optics and extended the property domain of SPMs to include magnetic, chemical, and thermal properties of surfaces,60 as mentioned earlier.

Finally, while indentation testing has been used extensively to extract the elastic modulus of coatings, other techniques can o§en do a better job. For example, the elastic modulus of a surface layer can be obtained with good accuracy using surface acoustic waves (SAWs). And since the depth sensitivity of the wave is the acoustic wavelength in the surface, that is, inversely proportional to the frequency, sub-micron �lms can be probed with frequencies in the 1–50 GHz range. Surface Brillouin scattering (SBS) from SAWs in this range of frequencies is capable of determining elastic moduli of hard coatings,61 some thinner than 10 nm.62

2.6 Summary

As tribology continues to look toward new materials and new bearing designs to meet the needs of the engineering community, tribologists will have to rely more on surface analytical techniques to solve problems. �ese approaches to understanding surface and subsurface damage were pioneered by surface scientists who possessed equipment like AES, XPS, and SIMS to analyze thin solid and liquid �lms and by materials specialists who had access to XRD, SEMs, and TEMs. Today, all these techniques and more are available (at a price) to all who need to obtain direct evidence about materials



successes as well as failures. And, with continued development in both analysis and technology at the nanoscale, tribologists will have to become more versed in the alphabet soup of techniques, even as they continue to rely on their optical microscopes to “see” what is happening in their moving mechanical assemblies.

Acknowledgments

�e author wishes to thank NRL and ONR for support, and the following people and organizations for contributing �gures: �e Evans Analytical Group; W.M. Rainford, She²eld University; SKF Corporation; P. Blau, ORNL and DOE.

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