Bhushan B. Handbook of Micro/Nano Tribology, Second Edition

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Bhushan, B. “Micro/Nanotribology and Micro/Nanomechanics of Magnetic...”

Handbook of Micro/Nanotribology.

Ed. Bharat Bhushan

Boca Raton: CRC Press LLC, 1999

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

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

–

plane and to move the sample in the vertical (

) 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

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 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

-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

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

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.

and

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.)

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

–TiC (70/30 wt%) and polycrystalline and single-crystal

(110) Mn–Zn ferrite are commonly used for construction of disk and tape heads. Al

–TiC, a single-

phase material, is also selected for comparisons with the performance of Al

–TiC, a two-phase material.

-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

) magnetic coating,

20-nm-thick amorphous carbon or diamondlike carbon (DLC) coating (microhardness ~ 1500 kg/mm

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

and Cr

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

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

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

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

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.)

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