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


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

×

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

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

,

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

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

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


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

, 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



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


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


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

4000 (NOP) 3.7 1.29 7.9

×

10

-5

AFM = atomic force microscope; NOP = noncon-
tact optical profiler.

© 1999 by CRC Press LLC

Majumdar and Bhushan (1991) and Bhushan and Majumdar (1992) have reported relationships for
cumulative size 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 atomic-
scale friction of a freshly cleaved, highly oriented pyrolytic graphite (HOPG) exhibited the same period-

© 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 are shown by lighter color. (From Ruan, J.
and Bhushan, B., 1994,

J. Appl. Phys.

76, 5022–5035. With permission.)

© 1999 by CRC Press LLC

humidity should be controlled during the experiments. Care also should be taken to ensure that tip
radius does not change during the experiments.

14.4.2.1 Head Slider Materials

Al

2

O
3
–TiC is a commonly used slider material. In order to study the friction characteristics of this two

the coefficient of friction of Al
2
O
3
–TiC composite is 0.03. Local variation in friction force also arises from
the scratches present on the Al
2
O
3
–TiC surface. A good correspondence between surface slope (also shown
in Figure 14.10) and friction force at scratch locations is observed. (Reasons for this correlation will be
discussed later.) Thus, local friction values of two-phase materials can be measured. Ruan and Bhushan
(1994c) reported that local variation in the coefficient of friction of cleaved HOPG was significant, which
arises from structural changes occurring during the cleaving process. The cleaved HOPG surface is largely
atomically smooth but exhibits line-shaped regions in which 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. They reported that FFM can be
used to image and identify compositional domains with a resolution of ~0.5 nm. These measurements
suggest that friction measurements can be used for structural mapping of the surfaces. FFM measurements
can also be used to map chemical variations, as indicated by the use of the FFM with a modified FFM
tip to map the spatial arrangement of chemical functional groups in mixed monolayer films (Frisbie
et al., 1994). Here, sample regions that had stronger interactions with the functionalized FFM tip exhibited
larger friction.
Surface roughness and coefficient of friction of various head slider materials were measured by Koinkar
and Bhushan (1996a). For typical values, see Table 14.2. Macroscale friction values for all samples are
higher than microscale friction values; the reasons are presented in the following subsection.
FIGURE 14.9 Schematic of surface topography and fric-
tion force maps shown in Figure 14.8. The oblate triangles

correlation was found between adhesive forces measured by the AFM and the coefficient of macroscale
static friction. They also reported that adhesive force increased almost linearly with an increase in the
tip radius. (Also see Sugawara et al., 1993; Bhushan et al., 1998).
14.4.2.2 Magnetic Media
Bhushan and co-workers measured 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 disks, indicating that most of the lubricant (although partially thermally bonded) is
squeezed out from between the rubbing surfaces at high interface pressures, consistent with liquids being
poor boundary lubricant (Bowden and Tabor, 1950). Coefficient of friction values on a microscale are
much lower than those on the macroscale. When measured for the small contact areas and very low loads
used in microscale studies, indentation hardness and modulus of elasticity are higher than at the mac-
roscale (data to be presented later). This reduces the real area of contact and the degree of wear. In
addition, the small apparent areas of contact reduces the number of particles trapped at the interface,
and thus minimizes the “plowing” contribution to the friction force (Bhushan et al., 1995d,f).
Miyamoto et al. (1991b) reported the coefficient of friction of an unlubricated disk with amorphous
carbon and SiO
2
overcoats against the diamond tip to be 0.24 and 0.36, respectively. The coefficients of
friction of disks lubricated with 2-nm-thick perfluoropolyether lubricant films were 0.08 for functional
lubricant (with hydroxyl end groups, Z-Dol) on SiO
2
overcoat, 0.10 for functional lubricant on carbon
overcoat, and 0.19 for nonpolar lubricant (Krytox 157FS L) on carbon overcoat. They found that the
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, respectively, and of unlubricated textured disks and with 2.5-nm-thick lubricant film were

noted that there is no resemblance between the friction force maps and the corresponding roughness
maps; e.g., high or low points on the friction force map do not correspond to high or low points on the
roughness map. By comparing the slope of roughness profiles taken in the tip sliding direction and
friction force map, we observe a strong correlation between the two. (For a clearer correlation, see gray-
scale plots of surface roughness slope and friction force profiles for FFM tip sliding in either directions
in Figures 14.13 and 14.15).
TABLE 14.2 Surface 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)
Coefficient of friction
Scratch Depth
at 60 µN (nm)
Wear Depth at
60 µN (nm)
Hardness (GPa)
Macroscale
b
Nano at
2 mN MicroSample σ P-V
a
Microscale Initial Final
Al
2
O
3
0.97 9.9 0.03 0.18 0.2–0.6 3.2 3.7 24.8 15.0
Al
2
O

3
N
4
1.88 (1.00–2.82) 0.07 (0–0.16)
SiC 0.21 (0.13–0.32) 0.030 (0–0.09)
From Miyamoto, T. et al., 1990, ASME J. Tribol. 112, 567–572. With
permission.
FIGURE 14.10 Gray-scale plots of surface topography (σ = 1.12 nm), slope of the roughness profiles taken along
the sliding direction (the horizontal axis) (mean = –0.003, σ = 0.015), and friction force map (mean = 28.5 nN, σ =
4.0 nN; Al
2
O
3
grains: mean = 24.8 mN, σ = 1.85 nN and TiC grains: mean = 32.7 nN, σ = 2.6 nN) for a Al
2
O
3
–TiC
slider for a normal load of 950 nN.
© 1999 by CRC Press LLC
To further verify the relationship between surface roughness slope and friction force values, to eliminate
any effect resulting from nonuniform composition of disk and tape surfaces, Bhushan and Ruan (1994a)
measured a polished natural (IIa) diamond. Repeated measurements were made along one line on the
surface. Highly reproducible data were obtained, Figure 14.16. Again, the variation of friction force
correlates to the variation of the slope of the roughness profiles taken along the sliding direction of the
tip. This correlation has been shown to hold for various magnetic disks, magnetic tapes, polyester tape
substrates, silicon, graphite and other materials (Bhushan et al., 1994a,c–e, 1995a–d, 1997a, 1998; Ruan
and Bhushan, 1994b,c).
We now examine the mechanism of microscale friction, which may explain the resemblance between
the slope of surface roughness profiles and the corresponding friction force profiles (Bhushan and Ruan,

(µN)
NOP
AFM
250 × 250 µm
a
1 × 1 µm
a
10 × 10 µm
a
1 × 1 µm
a
10 × 10 µm
a
Mn–Zn
Al
2
O
3
–TiCFerrite
Polished,
unlubricated
disk
2.2 3.3 4.5 0.05 0.06 — 0.26 21/100
Polished,
lubricated disk
2.3 2.3 4.1 0.04 0.05 — 0.19 —
Textured,
lubricated disk
4.6 5.4 8.7 0.04 0.05 — 0.16 —
MP tape 6.0 5.1 12.5 0.08 0.06 0.19 — 0.30/50

J., 1994, ASME J. Tribol. 116, 389–396. With permission.)
© 1999 by CRC Press LLC
FIGURE 14.13 Gray-scale plots of the slope of the surface roughness and the friction force maps for a textured and lubricated thin-
film rigid disk. Arrows indicate the tip sliding direction. Higher points are shown by lighter color.
© 1999 by CRC Press LLC
FIGURE 14.14 (a) Surface roughness map (σ = 7.9 nm), (b) slope of the roughness profiles taken along the sample
sliding direction (mean = –0.006, σ = 0.300), and friction force map (mean = 5.5 nN, σ = 2.2 nN) of an MP tape
at a normal load of 70 nN. (From Bhushan, B. and Ruan, J., 1994, ASME J. Tribol. 116, 389–396. With permission.)
© 1999 by CRC Press LLC
FIGURE 14.15 Gray-scale plots of the slope of the roughness and the friction force maps for an MP tape. Arrows indicate the tip sliding
direction. Higher points are shown by lighter color.
© 1999 by CRC Press LLC
FIGURE 14.16 Surface roughness map (σ = 15.4 nm), slope of the roughness map (mean = –0.052, σ = 0.224),
and the friction force map (σ = 2.1 nN) of a polished natural (IIa) diamond crystal. (From Bhushan, B. and Ruan,
J., 1994, ASME J. Tribol. 116, 389–396. With permission.)
FIGURE 14.17 Schematic illustration showing the effect of an
asperity (making an angle θ with the horizontal plane) on the
surface in contact with the tip on local friction in the presence of
“adhesive” friction mechanism. W and F are the normal and fric-
tion forces, respectively. S and N are the force components along
and perpendicular to the local surface of the sample at the contact
point, respectively.
© 1999 by CRC Press LLC
(14.5)
if µ
0
tan θ is small. For a symmetrical asperity, the average coefficient of friction experienced by the FFM
tip traveling across the whole asperity is
(14.6)
if µ

()
+µ
()
µ−
20 0
0
1tan tan
~ tan ,
θθ
θ
µ=µ+µ
()
=µ +
()
−µ
()
µ+
()
ave 1 2
0
2
0
22
0
2
2
11
1
tan tan
~ tan ,

d
are the surface energies of the tip and sample
surfaces, and D
0
~ 0.2 nm (Israelachvili, 1992). As a consequence of the strong 1/D
2
dependence, the tip
should experience a much weaker van der Waals force on the top of a summit as compared with that of
a valley. Mate (1993a) reported that a separation change ∆D of 5 nm would give a variation in the van
der Waals force by a factor of 5 if the distance of closest approach, approximately the amount of roughness
separation between the two surfaces, is 4 nm. Another component of the attractive adhesive force in the
presence of liquid film is the meniscus force. The meniscus force for a sphere on a flat in the presence
of liquid is
(14.9)
where γ
l
is the surface tension of the liquid. Meniscus force is generally much stronger than the van der
Waals force. Thus, the contribution of adhesion mechanism to the friction force variation is relatively
small for samples used in this study. Furthermore, the correlation between the surface and friction force
profiles is poor; therefore, an adhesion mechanism cannot explain the topography effects. The ratchet
mechanism already quantitatively explains 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 corresponds to the tip sliding from the left
toward the right (or the sample sliding from the right to the left). We again note a general correspondence
between the surface roughness slope and the friction profiles. The middle figures in Figures 14.13 and
14.15 correspond to the tip sliding from the right toward left. We note that generally the points that have
high friction force and high slope in the left-to-right scan have low friction and low slope as the sliding
direction is reversed (Meyer and Amer, 1990; Grafstrom et al., 1993; Overney and Meyer, 1993; Bhushan