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 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 phenomena 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 nanostructures 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 interactions at a single asperity contacts AFMs/FFMs are now commonly used for tribological studies (Bhushan, 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 detection, 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 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 Si3N4 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 Hz and the sample scanning speed of 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 × µm is scanned at various normal loads (ranging from 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 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 technique 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 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 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 FIGURE 14.1 modes Schematic of the bimorph assembly used in AFM for operation in tapping and force modulation 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 dc, ds, and db 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.) 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 (DeVecchio 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 © 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 Al2O3–TiC (70/30 wt%) and polycrystalline and single-crystal (110) Mn–Zn ferrite are commonly used for construction of disk and tape heads Al2O3–TiC, a singlephase material, is also selected for comparisons with the performance of Al2O3–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 perfluoropolyether, 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 (Co79Pt14Ni7) magnetic coating, 20-nm-thick amorphous carbon or diamondlike carbon (DLC) coating (microhardness ~ 1500 kg/mm2 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 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 Al2O3 and Cr2O3 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 Al2O3 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 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 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 µm Single-crystal silicon (111) was ion implanted with C+ ions at to mA cm–2 current densities, 100 keV accelerating voltage, and at a fluence of × 1017 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 tribological properties of solids in contact The AFM has been used to measure surface roughness on length © 1999 by CRC Press LLC FIGURE 14.3 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 © 1999 by CRC Press LLC FIGURE 14.3 Surface roughness plots of a glass–ceramic disk (a) measured using an AFM (lateral resolution ~ 15 nm), NOP (lateral resolution ~ 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.) 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 roughnessmeasuring 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) () S τ = Cη( © 1999 by CRC Press LLC D− ) τ ( −2 D ) , (14.1) 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 () Pω = c1η( ω( D− 5−2 D ) ) , (14.2a) and c1 = ( ) [( Γ − 2D sin π − D 2π )] C (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 to 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 © 1999 by CRC Press LLC FIGURE 14.6 Structure functions for the roughness data measured at various scan sizes using AFM (scan sizes: ì àm, 10 ì 10 àm, 50 ì 50 àm, and 100 ì 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) D C (nm) (AFM) 10 (AFM) 50 (AFM) 100 (AFM) 250 (NOP) 4000 (NOP) 0.7 2.1 4.8 5.6 2.4 3.7 1.33 1.31 1.26 1.30 1.32 1.29 9.8 × 10-4 7.6 × 10-3 1.7 × 10-2 1.4 × 10-2 2.7 × 10-4 7.9 × 10-5 AFM = atomic force microscope; NOP = noncontact optical profiler 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 m2, 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, nanoasperities 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) © 1999 by CRC Press LLC FIGURE 14.52 Nanoindentation marks generated on a polished, unlubricated thin-film rigid disk The normal load used in the indentation, the indentation depths, and the hardness values are indicated in the figure (From Bhushan, B et al., 1994, Proc Inst Mech Eng Part J: J Eng Tribol 208, 17–29 With permission.) © 1999 by CRC Press LLC FIGURE 14.53 Images with nanoindentation marks generated on a polished, unlubricated thin-film rigid disk at 140 µN (a) before subtraction and (b) after subtraction (From Bhushan, B et al., 1994, Proc Inst Mech Eng Part J: J Eng Tribol 208, 17–29 With permission.) © 1999 by CRC Press LLC FIGURE 14.54 Nanohardness and elastic modulus as a function of residual indentation depth for Si(100) and 100-nm-thick coatings deposited by sputtering, ion beam, and cathodic arc processes (From Kulkarni, A.V and Bhushan, B., 1997, J Mater Res 12, 2707–2714 With permission.) © 1999 by CRC Press LLC FIGURE 14.55 (a) Surface height and elasticity maps for an MP tape A (σ = 6.72 nm and P-V = 31.7 nm) σ and P-V refer to standard deviation of surface heights and peak-to-valley distance, respectively The gray scale on the elasticity map is arbitrary (From DeVecchio, D and Bhushan, B., 1997, Rev Sci Instrum 68, 4498–4505 With permission.) © 1999 by CRC Press LLC FIGURE 14.56 Surface height and elasticity maps for a scan size 10 ì 10 àm for three tape formulations: Tape A (σ = 15.1 nm and P-V = 171 nm), tape B (σ = 10.4 nm and P-V = 136 nm), tape C (σ = 14.4 nm and P-V = 117 nm) (From DeVecchio, D and Bhushan, B., 1997, Rev Sci Instrum 68, 4498–4505 With permission.) © 1999 by CRC Press LLC FIGURE 14.57 Tip deflection as a function of sample traveling distance curves for a natural diamond (a) right after the tip has been indented into as-deposited fullerene film and (b) after the tip has been scanned over the diamond surface for a short time (From Ruan, J and Bhushan, B., 1993, J Mater Res 8, 3019–3022 With permission.) References Andoh, Y., Oguchi, S., Kaneko, R., and Miyamoto, T (1992), “Evaluation of Very Thin Lubricant Films,” J Phys D: Appl Phys 25, A71–A75 Anonymous (1988), “Properties of Silicon,” EMIS Data Reviews Series, No 4, INSPEC, The Institution of Electrical Engineers, London Bhushan, B (1992), Mechanics and Reliability of Flexible Magnetic Media, Springer-Verlag, New York Bhushan, B (1993), “Magnetic Recording Surfaces,” in Characterization of Tribological Materials (W.A Glaeser, ed.), pp 116–133, Butterworth-Heinemann, Boston Bhushan B (1994), “Tribology of Magnetic Storage Systems,” in Handbook of Lubrication and Tribology, Vol 3, pp 325–374, CRC Press, Boca Raton, FL Bhushan, B (1995a), “Nanotribology and Its Applications to Magnetic Storage Devices and MEMS,” in Forces in Scanning Probe Methods (H.J Guntherodt, D Anselmetti, and E Meyer, eds.), Vol E 286, pp 367–395, Kluwer Academic, Dordrecht, The Netherlands Bhushan, B (1995b), “Micro/Nanotribology and Its Application to Magnetic Storage Devices and MEMS,” Tribol Int 28, 85–95 Bhushan, B (1996a), Tribology and Mechanics of Magnetic Storage Devices, 2nd ed., Springer-Verlag, New York Bhushan, B (1996b), “Contact Mechanics of Rough Surfaces in Tribology: Single Asperity Contact,” Appl Mech Rev 49, 275–298 Bhushan, B (1997), Micro/Nanotribology and Its Applications, E330, Kluwer Academic Publishers, Dordrecht, Netherlands © 1999 by CRC Press LLC TABLE 14.6 Chemical Structure, Molecular Weight, and Viscosity of Perfluoropolyether Lubricants Lubricant Fomblin Z-25 Fomblin Z-15 Fomblin Z-03 Fomblin Z-DOL Fomblin AM2001 Fomblin Z-DISOC Fomblin YR Demnum S-100 Krytox 143AD a Formula CF3–O–(CF2–CF2–O)m–(CF2–O)n–CF3 CF3–O–(CF2–CF2–O)m–(CF2–O)n–CF3 (m/n ~2/3) CF3–O–(CF2–CF2–O)m–(CF2–O)n–CF3 HO–CH2–CF2–O–(CF2–CF2–O)m–(CF2–O)n–CF2–CH2–OH Piperonyl–O–CH2–CF2–O–(CF2–CF2–O)m–(CF2–O)n–CF2–O–piperonyla O–CN–C6H3–(CH3)–NH–CO–CF2–O–(CF2–CF2–O)n–(CF2–O)m–CF2–CO– NH–C6H3–(CH3)–N–CO CF3 | CF3–O–(C–CF2–O)m(CF2–O)n–CF3 (m/n~40/1) | F CF3–CF2–CF2–O–(CF2–CF2–CF2–O)m–CF2–CF3 CF | CF3–CF2–CF2–O–(C–CF2–O)m–CF2–CF3 | F Molecular Weight (Daltons) Kinematic Viscosity cSt(mm2/s) 12800 9100 3600 2000 2300 1500 250 150 30 80 80 160 6800 1600 5600 250 2600 — 3,4-Methylenedioxybenzyl FIGURE 14.58 AFM (right) and the enlarged region and cross-sectional view of the particulate rigid disk surface and the AFM tip (left) during lubricant film thickness measurement (From Mate, C.M., 1992, Phys Rev Lett 68, 3323–3326 With permission.) © 1999 by CRC Press LLC FIGURE 14.59 The force perpendicular to the surface acting on the tip as a function of the particulate disk sample position, as the sample is first brought into contact with the tip (x) and then pulled away (+) The sample is moved with a velocity of 100 nm/s, and the zero sample position is defined to be the position where the force on the tip is zero when in contact with the sample A negative force indicates an attractive force (From Bhushan, B and Blackman, G.S., 1991, ASME J Tribol 113, 452–458 With permission.) Bhushan, B (1998a), “Micro/Nanotribology Using Atomic Force/Friction Force Microscopy: State of the Art,” Proc Inst Mech Eng Part J: J Eng Tribol 212, 1–18 Bhushan, B (1998b), Tribology Issues and Opportunities in MEMS, Kluwer Academic, Dordrecht, Netherlands Bhushan, B (1998c), “Contact Mechanics of Rough Surfaces in Tribology: Multiple Asperity Contact,” Tribol Lett 4, 1–35 Bhushan, B and Blackman, G.S (1991), “Atomic Force Microscopy of Magnetic Rigid Disks and Sliders and Its Applications to Tribology,” ASME J Tribol 113, 452–458 Bhushan, B and Majumdar, A (1992), “Elastic-Plastic Contact Model for Bifractal Surfaces,” Wear 153, 53–64 Bhushan, B., Gupta, B.K., Van Cleef, G.W., Capp, C., and Coe, J.V (1993a), “Sublimed C60 Films for Tribology,” Appl Phys Lett 62, 3253–3255 Bhushan, B., Ruan, J., and Gupta, B.K (1993b), “A Scanning Tunneling Microscopy Study of Fullerene Films,” J Phys D: Appl Phys 26, 1319–1322 © 1999 by CRC Press LLC FIGURE 14.60 Histograms of lubricant thickness distribution at three different regions of a particulate disk (From Bhushan, B and Blackman, G.S., 1991, ASME J Tribol 113, 452–458 With permission.) © 1999 by CRC Press LLC FIGURE 14.61 Friction force as a function of number of cycles using Si3N4 tip at a normal force of 300 nN for unlubricated and lubricated samples in ambient environment Arrows in the figure indicate significant changes in the friction force because of removal of surface or lubricant film (From Koinkar, V.N and Bhushan, B., 1996, J Vac Sci Technol A 14, 2378–2391 With permission.) FIGURE 14.62 Wear depth as a function of normal force using diamond tip for unlubricated and lubricated samples after one cycle (From Koinkar, V.N and Bhushan, B., 1996, J Vac Sci Technol A 14, 2378–2391 With permission.) Bhushan B and Venkatesan, S (1993c), “Friction and Wear Studies of Silicon in Sliding Contact with Thin-Film Magnetic Rigid Disks,” J Mater Res 8, 1611–1628 Bhushan, B., and Koinkar, V.N (1994a), “Tribological Studies of Silicon for Magnetic Recording Applications,” J Appl Phys 75, 5741–5746 Bhushan, B and Koinkar, V.N (1994b), “Nanoindentation Hardness Measurements Using Atomic Force Microscopy,” Appl Phys Lett 64, 1653–1655 Bhushan B., Koinkar, V.N., and Ruan, J (1994c), “Microtribology of Magnetic Media,” Proc Inst Mech Eng Part J: J Eng Tribol 208, 17–29 © 1999 by CRC Press LLC FIGURE 14.63 Wear maps for unlubricated and lubricated samples after microwear using diamond tip Normal force used and wear depths are listed in the figure (From Koinkar, V.N and Bhushan, B., 1996, J Vac Sci Technol A 14, 2378–2391 With permission.) 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single-phase materials Multiphase materials have more material flaws than the single-phase material The... plastic deformation of the tape surface Similar behavior was observed on all tapes Magnetic tape coating is made of magnetic particles and polymeric binder Any movement of the coating material can