Bhushan, B. “Micro/Nanotribology and Micro/Nanomechanics of Magnetic .” Handbook of Micro/Nanotribology. Ed. Bharat Bhushan Boca Raton: CRC Press LLC, 1999 © 1999 by CRC Press LLC © 1999 by CRC Press LLC 14 Micro/Nanotribology and Micro/Nanomechanics of Magnetic Storage Devices Bharat Bhushan 14.1 Introduction 14.2 Experimental Experimental Apparatus and Measurement Techniques • Test Specimens 14.3 Surface Roughness 14.4 Friction and Adhesion Nanoscale Friction • Microscale Friction and Adhesion 14.5 Scratching and Wear Nanoscale Wear • Microscale Scratching • Microscale Wear 14.6 Indentation Picoscale Indentation • Nanoscale Indentation • Localized Surface Elasticity 14.7 Detection of Material Transfer 14.8 Lubrication Imaging of Lubricant Molecules • Measurement of Localized Lubricant Film Thickness • Boundary Lubrication Studies 14.9 Closure References 14.1 Introduction Micro/nanotribological studies are needed to develop fundamental understanding of interfacial phenom- ena on a small scale and to study interfacial phenomena in micro- and nanostructures used in magnetic storage systems, microelectromechanical systems (MEMS), and other industrial applications (Bhushan, 1992, 1993, 1994, 1995a,b, 1996a, 1997, 1998b). The components used in micro- and nanostructures are very light (on the order of few micrograms) and operate under very light loads (on the order of few micrograms to a few milligrams). As a result, friction and wear (on a nanoscale) of lightly loaded © 1999 by CRC Press LLC micro/nanocomponents are highly dependent on the surface interactions (few atomic layers). These structures and magnetic storage devices are generally lubricated with molecularly thin films. Micro- and nanotribological techniques are ideal to study the friction and wear processes of micro- and nanostruc- tures and molecularly thick lubricant films (Bhushan et al., 1994a–e, 1995a–g, 1997a–c; Koinkar and Bhushan, 1996a,b, 1997a,b, 1998; Sundararajan and Bhushan, 1998). Although micro/nanotribological studies are critical to study micro- and nanostructures, these studies are also valuable in fundamental understanding of interfacial phenomena in macrostructures to provide a bridge between science and engineering. At interfaces of technological applications, contact occurs at multiple asperity contacts. A sharp tip of tip-based microscopes (atomic force/friction force microscopes or AFM/FFM) sliding on a surface simulates a single asperity contact, thus allowing high-resolution measurements of surface inter- actions at a single asperity contacts. AFMs/FFMs are now commonly used for tribological studies (Bhus- han, 1998a). In this chapter, we present the state of the art of micro/nanotribology of magnetic storage devices including surface roughness, friction, adhesion, scratching, wear, indentation, transfer of material detec- tion, and lubrication. 14.2 Experimental 14.2.1 Experimental Apparatus and Measurement Techniques AFM/FFM used in the studies conducted in our laboratory has been described in detail in Chapter 1 of this book. (Also see Ruan and Bhushan, 1993, 1994a–c; Bhushan, 1995a,b, 1998a; Bhushan et al., 1994a–e, 1995a–g, 1997a,c, 1998; Koinkar and Bhushan, 1996a,b, 1997a,b; Sundararajan and Bhushan, 1998.) Briefly, the sample is mounted on a piezoelectric transducer (PZT) tube scanner to scan the sample in the X – Y plane and to move the sample in the vertical ( Z ) direction. A sharp tip at the end of a flexible cantilever is brought in contact with the sample. Normal and frictional forces being applied at the tip–sample interface are measured using a laser beam deflection technique. Simultaneous measurements of surface roughness and friction force can be made with this instrument. For surface roughness and friction force measurements, a microfabricated square pyramidal Si 3 N 4 tip with a tip radius of about 30 nm on a cantilever beam (with a normal beam stiffness of about 0.4 N/m) (Chapter 1) is generally used at normal loads ranging from 10 to 150 nN. A preferred method of measuring friction and calibration procedures for conversion of voltages corresponding to normal and friction forces to force units is described by Ruan and Bhushan (1994a). For roughness measurements, the AFM is generally used in a tapping mode as compared to conventional contact mode, to yield better lateral resolution (Chapter 1; Bhushan et al., 1997c). During the tapping mode, the tip is oscillated vertically on the sample with small oscillations on the order of 100 nm near the resonant frequency of the cantilever on the order of 300 kHz. The tapping tip is only in intermittent contact with the sample with a reduced average load. This minimizes the effects of friction and other lateral forces in roughness measurements for improved lateral resolution and to measure roughness of soft surfaces without small-scale plowing. For roughness and friction measurements, the samples are typically scanned over scan areas ranging from 200 × 200 nm to 10 × 10 µm, in a direction orthogonal to the long axis of the cantilever beam (Bhushan et al., 1994a, c–e, 1995a–g, 1997a,c, 1998; Ruan and Bhushan, 1994a–c; Koinkar and Bhushan, 1996a,b, 1997a,b, 1998; Sundararajan and Bhushan, 1998). The samples are generally scanned with a scan rate of 1 Hz and the sample scanning speed of 1 µm/s, for example, for a 500 × 500 nm scan area. For adhesion force measurements, the sample is moved in the Z -direction until it contacts the tip. After contact at a given load, the sample is slowly moved away. When the spring force exceeds the adhesive force, the tip suddenly detaches from the sample surface and the spring returns to its original position. The tip displacement from the initial position to the point where it detaches from the sample multiplied by the spring stiffness gives the adhesive force. In nanoscale wear studies, the sample is initially scanned twice, typically at 10 nN to obtain the surface profile, then scanned twice at a higher load of typically 100 nN to wear and to image the surface © 1999 by CRC Press LLC simultaneously, and then rescanned twice at 10 nN to obtain the profile of the worn surface. No noticeable change in the roughness profiles was observed between the initial two scans at 10 nN, two profiles scanned at 100 nN, and the final two scans at 10 nN. Therefore, changes in the topography between the initial scans at 10 nN and the scans at 100 nN (or the final scans at 10 nN) are believed to occur as a result of local deformation of the sample surface (Bhushan and Ruan, 1994e). In picoscale indentation studies, the sample is loaded in contact with the tip in the force calibration mode. During loading, tip deflection (normal force) is measured as a function of vertical position of the sample. For a rigid sample, the tip deflection and the sample traveling distance (when the tip and sample come into contact) equal each other. Any decrease in the tip deflection as compared to vertical position of the sample represents indentation. To ensure that the curvature in the tip deflection–sample traveling distance curve does not arise from PZT hysteresis, measurements on several rigid samples including single-crystal natural diamond (IIa) were made. No curvature was noticed for the case of rigid samples. This suggests that any curvature for other samples should arise from the indentation of the sample (Bhushan and Ruan, 1994e). For microscale scratching, microscale wear, and nanoscale indentation hardness measurements, a three-sided pyramidal single-crystal natural diamond tip with an apex angle of 80° and a tip radius of about 100 nm (determined by scanning electron microscopy imaging) is used at relatively higher loads (1 – 150 µN). The diamond tip is mounted on a stainless steel cantilever beam with normal stiffness of about 30 N/m (Chapter 1). For scratching and wear studies, the sample is generally scanned in a direction orthogonal to the long axis of the cantilever beam (typically at a rate of 0.5 Hz) so that friction can be measured during scratching and wear. The tip is mounted on the beam such that one of its edge is orthogonal to the beam axis; therefore, wear during scratching along the beam axis is higher (about two to three times) than that during scanning orthogonal to the beam axis. For wear studies, typically an area of 2 × 2 µm is scanned at various normal loads (ranging from 1 to 100 µN) for a selected number of cycles (Bhushan et al., 1994a,c,d, 1995a–e, 1997a, 1998; Koinkar and Bhushan, 1996a, 1997b). For nanoindentation hardness measurements the scan size is set to zero and then the normal load is applied to make the indents (Bhushan et al., 1994b). During this procedure the diamond tip is continuously pressed against the sample surface for about 2 s at various indentation loads. Sample surface is scanned before and after the scratching, wear, or indentation to obtain the initial and the final surface topography, at a low normal load of about 0.3 µN using the same diamond tip. An area larger than the scratched worn or indentation region is scanned to observe the scratch or wear scars or indentation marks. Nanohardness is calculated by dividing the indentation load by the projected residual area of the indents (Bhushan et al., 1994a–d, 1995a–e, 1997a,b, 1997a; Koinkar and Bhushan, 1996a, 1997b). From the image of the indent, it is difficult to identify the boundary of the indentation mark with great accuracy. This makes the direct measurement of contact area somewhat inaccurate. A nano/picoindentation tech- nique with the dual capability of depth sensing as well as in situ imaging is most appropriate (Bhushan et al., 1996). This indentation system provides load–displacement data and can be subsequently used for in situ imaging of the indent. Hardness value is obtained from the load–displacement data. Young’s modulus of elasticity is obtained from the slope of the unloading curve. This system is described in detail in Chapter 7 in this book. The force modulation technique is used to obtain surface elasticity maps (Maivald et al., 1991; DeVecchio and Bhushan, 1997; Scherer et al., 1997). An oscillating tip is scanned over the sample surface in contact under steady and oscillating loads. The oscillations are applied to the cantilever substrate with a bimorph, consisting of two piezoelectric transducers bonded to either side of a brass strip, which is located on the substrate holder, Figure 14.1. For measurements, the tip is first bright in contact with a sample under a static load of 50 to 300 nN. In addition to the static load applied by the sample piezo, a small oscillating (modulating) load is applied by a bimorph generally at a frequency (about 8 kHz) far below that of the natural resonance of the cantilever (70 to 400 kHz). When the tip is brought in contact with the sample, the surface resists the oscillations of the tip, and the cantilever deflects. Under the same applied load, a stiff area on the sample would deform less than a soft one; i.e., stiffer surfaces cause greater deflection amplitudes of the cantilever, Figure 14.2. The variations in the deflection © 1999 by CRC Press LLC amplitudes provide a measure of the relative stiffness of the surface. Contact analyses (Bhushan, 1996b) can be used to obtain quantitative measure of localized elasticity of soft and compliant samples (DeVec- chio and Bhushan, 1997). The elasticity data are collected simultaneously with the surface height data using a so-called negative lift mode technique. In this mode, each scan line of each topography image (obtained in tapping mode) is retraced with the tapping action disabled and with the tip lowered into steady contact with the surface. A variant of this technique, which enables one to measure stiffer surfaces, has been used to measure the elastic modulus of hard and rigid surfaces quantitatively (Scherer et al., 1997). This latter technique engages the tip on the top of the sample which is then subjected to oscillations at the frequencies near FIGURE 14.1 Schematic of the bimorph assembly used in AFM for operation in tapping and force modulation modes. FIGURE 14.2 Schematics of the motion of the cantilever and tip as a result of the oscillations of the bimorph for an infinitely stiff sample, an infinitely compliant sample, and an intermediately compliant sample. The thin line represents the cantilever at the top of the cycle; and the thick line corresponds to the bottom of the cycle. The dashed line represents the position of the tip if the sample was not present or was infinitely compliant. d c , d s , and d b are the oscillating (AC) deflection amplitude of the cantilever, penetration depth, and oscillating (AC) amplitude of the bimorph, respectively. (From DeVecchio, D. and Bhushan, B., 1997, Rev. Sci. Instrum. 68, 4498–4505. With permission.) © 1999 by CRC Press LLC the cantilever resonances, up to several megahertz, by a PZT beneath the sample. These sample oscillations create oscillations in the tip. The resonance frequencies of these tip oscillations depend on the surface elasticity. The high-frequency technique is useful for stiffer materials (like metals and ceramics) without the need for special tips, but requires the extra piezo and driving equipment and it is more complicated in its theory and application. All measurements are carried out in the ambient atmosphere (22 ± 1°C, 45 ± 5% RH, and Class 10,000). 14.2.2 Test Specimens In this chapter, data on various head slider materials, magnetic media and silicon materials with and without various treatments are presented. Al 2 O 3 –TiC (70/30 wt%) and polycrystalline and single-crystal (110) Mn–Zn ferrite are commonly used for construction of disk and tape heads. Al 2 O 3 –TiC, a single- phase material, is also selected for comparisons with the performance of Al 2 O 3 –TiC, a two-phase material. A α -type SiC is also selected which is a candidate slider material because of its high thermal conductivity and attractive machining and friction and wear properties. Two thin-film rigid disks with polished and textured substrates, with and without a bonded perfluo- ropolyether, are selected. These disks are 95 mm in diameter made of Al–Mg alloy substrate (1.3 mm thick) with a 10-µm-thick electroless plated Ni–P coating, 75-nm-thick (Co 79 Pt 14 Ni 7 ) magnetic coating, 20-nm-thick amorphous carbon or diamondlike carbon (DLC) coating (microhardness ~ 1500 kg/mm 2 as measured using a Berkovich indenter), and with or without a top layer of perfluoropolyether lubricant with polar end groups (Z-Dol) coating. The thickness of the lubricant film is about 2 nm. The metal particle (MP) tape is a 12.7 mm wide and 13.2 µm thick — poly(ethylene terephthalate (PET) base thickness of 9.8 µm, magnetic coating of 2.9 µm with Al 2 O 3 and Cr 2 O 3 particles, and back coating of 0.5 µm. The barium ferrite (BaFe) tape is a 12.7-mm-wide and 11-µm-thick (PET base thickness of 7.3 µm, magnetic coating of 2.5 µm with Al 2 O 3 particles, and back coating of 1.2 µm). Metal-evaporated (ME) tape is a 12.7-mm-wide tape with 10-µm-thick base, 0.2-µm-thick evaporated Co–Ni magnetic film, and about 10-nm-thick perfluoropolyether lubricant and a backcoat. PET film is a biaxially oriented, semicrystalline polymer with particulates. Two sizes of nearly spherical particulates are generally used: submicron (~0.5 µm) particles of typically carbon and larger particles (2 to 3 µm) of silica. Virgin single-crystal and polycrystalline silicon samples and thermally oxidized (under both wet and dry conditions) plasma-enhanced chemical vapor deposition (PECVD) oxide-coated and ion-implanted single-crystal pins of orientation (111) are measured. Thermal oxidation of silicon pins was carried out in a quartz furnace at temperatures of 900 to 1000°C in dry oxygen and moisture-containing oxygen ambients. The latter condition was achieved by passing dry oxygen through boiling water before entering the furnace. The thicknesses of the dry oxide and wet oxides are 0.5 and 1 µm, respectively. PECVD oxide was formed by the thermal oxidation of silane at temperatures of 250 to 350°C and was polished using a lapping tape to a thickness of about 5 µm. Single-crystal silicon (111) was ion implanted with C + ions at 2 to 4 mA cm –2 current densities, 100 keV accelerating voltage, and at a fluence of 1 × 10 17 ion cm –2 . 14.3 Surface Roughness Solid surfaces, irrespective of the method of formation, contain surface irregularities or deviations from the prescribed geometric form. When two nominally flat surfaces are placed in contact, surface roughness causes contact to occur at discrete contact points. Deformation occurs in these points, and may be either elastic or plastic, depending on the nominal stress, surface roughness, and material properties. The sum of the areas of all the contact points constitutes the real area that would be in contact, and for most materials at normal loads, this will be only a small fraction of the area of contact if the surfaces were perfectly smooth. In general, real area of contact must be minimized to minimize adhesion, friction, and wear (Bhushan, 1996a,b, 1998c). Characterizing surface roughness is therefore important for predicting and understanding the tribo- logical properties of solids in contact. The AFM has been used to measure surface roughness on length © 1999 by CRC Press LLC scales from nanometers to micrometers. Roughness plots of a glass–ceramic disk measured using an AFM (lateral resolution of ~15 nm), noncontact optical profiler (lateral resolution ~1 µm), and stylus profiler (lateral resolution of ~0.2 µm) are shown in Figure 14.3a. Figure 14.3b compares the profiles of the disk obtained with different instruments at a common scale. The figures show that roughness is found at scales ranging from millimeter to nanometer scales. The measured roughness profile is dependent on the lateral and normal resolutions of the measuring instrument (Bhushan and Blackman, 1991; Oden FIGURE 14.3 © 1999 by CRC Press LLC et al., 1992; Ganti and Bhushan, 1995; Poon and Bhushan, 1995a,b). Instruments with different lateral resolutions measure features with different scale lengths. It can be concluded that a surface is composed of a large number of length of scales of roughness that are superimposed on each other. Surface roughness is most commonly characterized by the standard deviation of surface heights, which is the square roots of the arithmetic average of squares of the vertical deviation of a surface profile from its mean plane. Due to the multiscale nature of surfaces, it is found that the variances of surface height and its derivatives and other roughness parameters depend strongly on the resolution of the roughness- measuring instrument or any other form of filter, hence not unique for a surface (Ganti and Bhushan, 1995; Poon and Bhushan, 1995a,b; Koinkar and Bhushan, 1997a); see, for example, Figure 14.4. Therefore, a rough surface should be characterized in a way such that the structural information of roughness at all scales is retained. It is necessary to quantify the multiscale nature of surface roughness. A unique property of rough surfaces is that if a surface is repeatedly magnified, increasing details of roughness are observed right down to nanoscale. In addition, the roughness at all magnifications appear quite similar in structure, as qualitatively shown in Figure 14.5. That statistical self-affinity is due to similarity in appearance of a profile under different magnifications. Such a behavior can be characterized by fractal analysis (Majumdar and Bhushan, 1990; Ganti and Bhushan, 1995; Poon and Bhushan, 1995a,b; Koinkar and Bhushan, 1997a). The main conclusions from these studies are that a fractal characterization of surface roughness is scale independent and provides information of the roughness structure at all length scales that exhibit the fractal behavior. Structure function and power spectrum of a self-affine fractal surface follow a power law and can be written as (Ganti and Bhushan model) (14.1) FIGURE 14.3 Surface roughness plots of a glass–ceramic disk (a) measured using an AFM (lateral resolution ~ 15 nm), NOP (lateral resolution ~ 1 µm), and stylus profiler (SP) with a stylus tip of 0.2-µm radius (lateral resolution ~ 0.2 µm), and (b) measured using an AFM (~150 nm), SP (~0.2 µm), and NOP (~1 µm) and plotted on a common scale. (From Poon, C.Y. and Bhushan, B., 1995, Wear 190, 89–109. With permission.) SC DD τητ () = − () − () 2342 , © 1999 by CRC Press LLC (14.2a) and (14.2b) The fractal analysis allows the characterization of surface roughness by two parameters D and C , which are instrument independent and unique for each surface. D (ranging from 1 to 2 for surface profile) primarily relates to relative power of the frequency contents and C to the amplitude of all frequencies. η is the lateral resolution of the measuring instrument, τ is the size of the increment (distance), and ω is the frequency of the roughness. Note that if S ( τ ) or P ( ω ) are plotted as a function of τ or ω , respectively, on a log–log plot, then the power law behavior would result in a straight line. The slope of line is related to D and the location of the spectrum along the power axis is related to C . Figure 14.6 presents the structure function of a thin-film rigid disk measured using AFM, noncontact optical profiler (NOP), and stylus profiler (SP). A horizontal shift in the structure functions from one scan to another arises from the change in the lateral resolution. D and C values for various scan lengths are listed in Table 14.1. We note that fractal dimension of the various scans is fairly constant (1.26 to 1.33); however, C increases/decreases monotonically with σ for the AFM data. The error in estimation FIGURE 14.4 Scale dependence of standard deviation of surface heights for a glass–ceramic disk, measured using AFM, SP, and NOP. FIGURE 14.5 Qualitative description of statistical self-affinity for a surface profile. P c D D ω η ω () = − () − () 1 23 52 , c DD C 1 52 2 2 = − () π− () [] π Γ sin . © 1999 by CRC Press LLC of η is believed to be responsible for variation in C . These data show that the disk surface follows a fractal structure for three decades of length scales. Majumdar and Bhushan (1991) and Bhushan and Majumdar (1992) developed a fractal theory of contact between two rough surfaces. This model has been used to predict whether contacts experience elastic or plastic deformation and to predict the statistical distribution of contact points. For a review of contact models, see Bhushan (1996b, 1998c). Based on the fractal model of elastic–plastic contact, whether contacts go through elastic or plastic deformation is determined by a critical area which is a function of D , C , hardness, and modulus of elasticity of the mating surfaces. If the contact spot is smaller than the critical area, it goes through the plastic deformations and large spots go through elastic deformations. The critical contact area for inception of plastic deformation for a thin-film disk was reported by Majumdar and Bhushan (1991) to be about 10 –27 m 2 , so small that all contact spots can be assumed to be elastic at moderate loads. The question remains as to how large spots become elastic when they must have initially been plastic spots. The possible explanation is shown in Figure 14.7. As two surfaces touch, the nanoasperities (detected by AFM-type of instruments) first coming into contact have smaller radii of curvature and are therefore plastically deformed instantly, and the contact area increases. When load is increased, nanoas- perities in the contact merge, and the load is supported by elastic deformation of the large-scale asperities or microasperities (detected by optical profiler type of instruments) (Bhushan and Blackman, 1991). FIGURE 14.6 Structure functions for the roughness data measured at various scan sizes using AFM (scan sizes: 1 × 1 µm, 10 × 10 µm, 50 × 50 µm, and 100 × 100 µm), NOP (scan size: 250 × 250 µm), and SP (scan length: 4000 µm), for a magnetic thin-film rigid disk. (From Ganti, S. and Bhushan, B., 1995, Wear 180, 17–34. With permission.) TABLE 14.1 Surface Roughness Parameters for a Polished Thin-Film Rigid Disk Scan size (µm x µm) σ (nm) DC (nm) 1 (AFM) 0.7 1.33 9.8 × 10 -4 10 (AFM) 2.1 1.31 7.6 × 10 -3 50 (AFM) 4.8 1.26 1.7 × 10 -2 100 (AFM) 5.6 1.30 1.4 × 10 -2 250 (NOP) 2.4 1.32 2.7 × 10 -4 4000 (NOP) 3.7 1.29 7.9 × 10 -5 AFM = atomic force microscope; NOP = noncon- tact optical profiler. [...]... friction properties of magnetic media including polished and textured thin-film rigid disks, MP, BaFe and ME tapes, and PET tape substrate For typical values of coefficients of friction of polished and textured, thin-film rigid disks and MP, BaFe and ME tapes, PET tape substrate, see Table 14.4 In the case of magnetic disks, similar coefficients of friction are observed for both lubricated and unlubricated... coefficient of friction of a 4-nm-thick lubricant film was about twice that of a 2-nm-thick film Mate (1993a) measured the coefficient of friction of unlubricated polished and textured disks and with a lubricant film with ester end groups (Demnum SP) against a tungsten tip with a tip radius of 100 nm The coefficients of friction of unlubricated polished disks and with 1.5-nm-thick lubricant film were 0.5 and 0.4,... distribution of the contact spots, portions of the real area of contact in elastic and plastic deformation modes, and the load–area relationships 14.4 Friction and Adhesion 14.4.1 Nanoscale Friction Ruan and Bhushan (1994b) measured friction on the nanoscale using FFM They reported that atomicscale friction of a freshly cleaved, highly oriented pyrolytic graphite (HOPG) exhibited the same periodicity as that of. .. the coefficient of friction is more than an order of magnitude larger Meyer et al (1992) and Overney et al (1992) also used FFM to measure structural variations of a composite surface They measured friction distribution of mixed monolayer films produced by dipping into a solution of hydrocarbon and fluorocarbon molecules The resulting film consists of discrete islands of hydrocarbon in a sea of fluorocarbon... Roughness (σ and P-V distance), Micro- and Macroscale Friction, Microscratching/Wear, and Nano- and Microhardness Data for Various Samples Surface Roughness nm (1 × 1 µm) Sample Al2O3 Al2O3–TiC Polycrystalline Mn–Zn ferrite Single–crystal (110) Mn–Zn ferrite SiC (α-type) Coefficient of friction Macroscale Hardness (GPa) b Final Scratch Depth at 60 µN (nm) Wear Depth at 60 µN (nm) Nano at 2 mN Micro 0.18... the normal and lateral directions 14.4.2 Microscale Friction and Adhesion Friction and adhesion of magnetic head sliders, magnetic media, virgin, treated and coated Si(111) wafers, and graphite on a microscale have been measured by Kaneko et al (1988, 1991a), Miyamoto et al (1990, 1991a,c), Mate (1993a,b), Bhushan et al (1994a–c,e, 1995a–g, 1997c, 1998), Ruan and Bhushan (1994a–c), Koinkar and Bhushan... reported that friction and adhesion forces are a function of tip radius and relative humidity (also see Koinkar and Bhushan, 1996b) Therefore, relative © 1999 by CRC Press LLC © 1999 by CRC Press LLC FIGURE 14.8 Gray-scale plots of (a) surface topography and (b) friction force maps of a 1 70× 1 nm area of a freshly cleaved HOPG showing the atomic-scale variation of topography and friction Higher points... 7.2 Microscale Initial Peak-to-valley distance Obtained using silicon nitride ball with 3 mm diameter in a reciprocating mode at a normal load of 10 mN, reciprocating amplitude of 7 mm, and average sliding speed of 1 mm/s Initial coefficient of friction values were obtained at first cycle (0.007 m sliding distance) and final values at a sliding distance of 5 m a b TABLE 14.3 Mean Values and Ranges of Adhesive... the variation of friction Since the local friction force is a function of the local slope of sample surface, the local friction force should be different as the scanning direction of the sample is reversed Figures 14.13 and 14.15 show the gray-scale plots of slope of roughness profiles and friction force profiles for a lubricated textured disk and an MP tape, respectively The left side of the figures... and/ or wear between the head and tape, in addition to the deterioration of the tape itself With disks, they did not notice any deformation under a 100 nN normal load © 1999 by CRC Press LLC FIGURE 14.19 Surface roughness maps of an MP tape at applied normal load of (a) 10 nN and (b) 100 nN Location of the change in surface topography as a result of nanowear is indicated by arrows (From Bhushan, B and . Bhushan, B. Micro/ Nanotribology and Micro/ Nanomechanics of Magnetic .” Handbook of Micro/ Nanotribology. Ed. Bharat Bhushan Boca. 14 Micro/ Nanotribology and Micro/ Nanomechanics of Magnetic Storage Devices Bharat Bhushan 14.1 Introduction 14.2 Experimental Experimental Apparatus and