Bhushan B. Nanotribology and Nanomechanics: An Introduction

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960 Bharat Bhushan and Huiwen Liu

ﬁlm thickness. In general, the stability and durability of surface ﬁlms decrease in

the following order: chemically reacted ﬁlms, chemisorbed ﬁlms, and physisorbed

ﬁlms. A good boundary lubricant should have a high degree of interaction between

its molecules and the sliding surface. As a general rule, liquids are good lubricants

when they are polar and, thus, able to grip solid surfaces (or be adsorbed). Polar lu-

bricants contain reactive functional groups with low ionization potential, or groups

having high polarizability [1–3]. Boundary lubrication properties of lubricants are

also dependent upon the molecular conformation and lubricant spreading [4–7].

Self-assembled monolayers (SAMs), Langmuir–Blodgett (LB) ﬁlms, and per-

ﬂuoropolyether (PFPE) ﬁlms can be used as boundary lubricants [2,3,8–10]. PFPE

ﬁlms are commonly used for lubrication of magnetic rigid disks and metal evapo-

rated magnetic tapes to reduce friction and wear of a head–medium interface [10].

PFPEs are well suited for this application because of the following properties: low

surface tension and low contact angle, which allow easy spreading on surfaces and

provide a hydrophobic property; chemical and thermal stability, which minimizes

degradation under use; low vapor pressure, which provides low out-gassing; high

adhesionto substrate via organofunctionalbonds; and good lubricity, which reduces

the interfacial friction and wear [10–12]. While the structure of the lubricants em-

ployed at the head–medium interface has not changed substantially over the past

decade, the thickness of the PFPE ﬁlm used to lubricate the disk has steadily de-

creased from multilayer thicknesses to the sub-monolayerthickness regime [11,13].

Molecularly thick PFPE ﬁlms are also being considered for lubrication purposes

of the evolving microelectromechanical systems (MEMS) industry [14]. It is well

knownthat the properties of molecularlythick liquidﬁlms conﬁned to solid surfaces

can be dramatically diﬀerentfrom those ofthe correspondingbulkliquid. In orderto

eﬃciently develop lubrication systems that meet the requirements of the advanced

rigid disk drive and MEMS industries, the impact of thinning the PFPE lubricants

on the resulting nanotribology should be fully understood [15, 16]. It is also im-

portant to understand lubricant–substrate interfacial interactions and the inﬂuence

of the operating environment on the nanotribological performance of molecularly

thick PFPEs.

An overviewof nanotribologicalproperties of SAMs and LB ﬁlms can be found

in many references, such as [17]. In this chapter, we focus on PFPEs. We ﬁrst in-

troduce details of the commonly used PFPE lubricants; then present a summary

of nanodeformation, molecular conformation, and lubricant spreading studies; fol-

lowed by an overview of nanotribological properties of polar and nonpolar PFPEs

studied by atomic force microscopy (AFM) and some concluding remarks.

18.2 Lubricants Details

Properties of two commonly used PFPE lubricants (Z-15 and Z-DOL) are reviewed

here. Their molecular structuresare shownschematicallyin Fig. 18.1. Z-15 has non-

polar

−

end groups, whereas Z-DOL is a polar lubricant with hydroxyl (

−

OH)

18 Nanoscale Boundary Lubrication Studies 961

Z-15

Z-DOL

Fig. 18.1. Schematics of the

molecular structures of Z-15

and Z-DOL. In this ﬁgure

the m/n value, shown in

Table 18.1, equals 2/3

Table 18.1. Typical properties of Z-15 and Z-DOL (data from Monteﬂuous S.P.A., Milan,

Italy)

Z-15 Z-DOL (2000)

Formula CF

−

(CF

−

(CF

−

∗

(CF

−

(CF

−

∗

Molecular weight (Daltons) 9100 2000

Density (ASTM D891) 20

◦

(g/cm

)1.84 1.81

Kinematic viscosity

(ASTM D445) (cSt)

◦

C 148 85

◦

C9034

◦

C25−

Viscosity index (ASTM D2270) 320 −

Surface tension (ASTM D1331)

(dyn/cm) 20

◦

C24 24

Vapor pressure (torr)

◦

C1.6×10

−6

2×10

−5

100

◦

C1.7×10

−5

6×10

−4

Pour point (ASTM D972)

◦

C −80 –

Evaporation weight loss

(ASTM D972)

149

◦

C, 22 h (%) 0.7–

Oxidative stability (

◦

C) − 320

Speciﬁc heat (cal/g

◦

C0.21 –

∗

m/n ∼2/3

962 Bharat Bhushan and Huiwen Liu

end groups. Their typical properties are summarized in Table 18.1; it shows that Z-

15 andZ-DOL havealmost thesame densityand surface tension.But Z-15has larger

molecular weight and higher viscosity. Both of them have low surface tension, low

vapor pressure, low evaporation weight loss, and good oxidative stability [10,12].

Generally, a single-crystalSi(100)wafer with a nativeoxidelayer was usedas a sub-

strate for deposition of molecularly thick lubricant ﬁlms for nanotribological char-

acterization.Z-15 and Z-DOL ﬁlmscan be deposited directlyonto the Si(100) wafer

by the dip-coating technique. The clean silicon wafer is vertically submerged into

a dilute solution of lubricant in hydrocarbonsolvent (HT-70) for a certain time. The

silicon wafers are pulled up vertically from the solution with a motorized stage at

a constant speed for deposition of the desired thicknesses of Z-15 and Z-DOL lu-

bricants. The lubricant ﬁlm thickness obtained in dip coating is a function of the

concentration and pull-up speed, among other factors. The Z-DOL ﬁlm is bonded

to the silicon substrate by heating the as-deposited Z-DOL samples in an oven at

150

◦

C for about 30minutes.The native oxide layer of Si(100) wafer reacts with

the

−

OH groups of the lubricants during thermal treatment [18–21]. Subsequently,

ﬂuorocarbon solvent (FC-72) washing of the thermally treated specimen removes

looselyabsorbed species, leavingthe chemically bondedphase on the substrate. The

chemical bonding between Z-DOL molecules and silicon substrate is illustrated in

Fig. 18.2. The bonded and washed Z-DOL ﬁlm is referred to as Z-DOL(BW) in this

chapter. The as-deposited Z-15 and Z-DOL ﬁlms are mobile-phase lubricants (i.e.,

liquid-like lubricants), whereas the Z-DOL(BW) ﬁlms are fully bonded soft solid

phase (i.e., solid-like) lubricants. This will be further discussed in the next section.

Z-DOL molecules

Silica

Silicon

wafer

Fig. 18.2. Schematic of

Z-DOL molecules that

are chemically bonded on

Si(100) substrate surface

(which has native oxide) after

thermal treatment at 150

◦

for 30min

18 Nanoscale Boundary Lubrication Studies 963

18.3 Nanodeformation, Molecular Conformation,

and Lubricant Spreading

Nanodeformation behavior of Z-DOL lubricants was studied using an AFM by

Blackman et al. [22,23]. Before bringing a tungsten tip into contact with a molecu-

lar overlayer,it was brought into contact with a bare clean-silicon surface, Fig. 18.3.

As the sample approaches the tip, the force initially is zero, but at point A the force

suddenly becomes attractive (top curve), which increases until point B, where the

sample and tip come into intimate contact and the force becomes repulsive. As the

sample is retracted, a pull-oﬀ force of 5×10

−8

N (point D) is required to overcome

adhesion between the tungsten tip and the silicon surface. When an AFM tip is

brought into contact with an unbonded Z-DOL ﬁlm, a sudden jump into adhesive

contact is also observed. A much larger pull-oﬀ force is required to overcome the

adhesion. The adhesion is initiated by the formation of a lubricant meniscus sur-

rounding the tip. This suggests that the unbonded Z-DOL lubricant shows liquid-

like behavior. However, when the tip was broughtinto contact with a lubricant ﬁlm,

which was ﬁrmly bonded to the surface, the liquid-like behavior disappears. The

initial attractive force (point A) is no longer sudden, as with the liquid ﬁlm, but,

rather, gradually increases as the tip penetrates the ﬁlm.

Accordingto Blackmanet al. [22,23],if the substrateand tip were inﬁnitely hard

with no compliance and/or deformation in the tip and sample supports, the line for

B to C would be vertical with an inﬁnite slope. The tangent to the force–distance

curve at a given point is referred to as the stiﬀness at that point and was determined

Wire deflection (nm)

Tip-sample separation distance (nm)

010203040

Force (nN)

100

Clean Si(100)

Unbonded perfluoropolyether

Bonded perfluoropolyether

100

Fig. 18.3. Deﬂection (normal

load) as a function of tip–

sample separation-distance

curves comparing the behav-

ior of clean Si(100) surface

to a surface lubricated with

free and unbonded PFPE lu-

bricant, and a surface where

the PFPE lubricant ﬁlm was

thermally bonded to the sur-

face [22]

964 Bharat Bhushan and Huiwen Liu

by ﬁtting a least-squares line through the nearby data points. For silicon, the de-

formation is reversible (elastic), since the retracting (outgoing) portion of the curve

(C to D) follows the extending (ingoing) portion (B to C). For bonded lubricant

ﬁlm, at the point where the slope of the force changes gradually from attractive to

repulsive, the stiﬀness changes gradually, indicating compression of the molecular

ﬁlm. As the load is increased, the slope of the repulsive force eventually approaches

that of the bare surface. The bonded ﬁlm was found to respond elastically up to the

highestloads of 5 µN that couldbe applied.Thus, bonded lubricantbehavesas a soft

polymer solid.

Figure 18.4 illustrates two extremes for the conformation on a surface of a lin-

ear liquid polymer without any reactive end groups and at submonolayer cover-

ages [4,6]. At one extreme, the molecules lie ﬂat on the surface, reaching no more

than their chain diameter δ above the surface. This would be the case if a strong

attractive interaction exists between the molecules and the solid. On the other ex-

treme, when a weak attraction exists between polymer segments and the solid,

the molecules adopt a conformation close to that of the molecules in the bulk,

with the microscopic thickness equal to about the radius of gyration R

. Mate and

Novotny [6] used AFM to study conformation of 0.5–1.3 nm-thick Z-15 molecules

Bulk conformation

Low molecular weight

Side view

High molecular weight

Flat conformation

~2R

side view

Side view

Expanded

top view

Expanded

top view

Fig. 18.4. Schematic representation of two extreme liquid conformations at the surface of the

solid for low and high molecular weights at low surface coverage. δ is the cross-sectional

diameter of the liquid chain, and R

is the radius of gyration of the molecules in the bulk [6]

18 Nanoscale Boundary Lubrication Studies 965

on clean Si(100) surfaces. They found that the thickness measured by AFM is

thicker than that measured by ellipsometry, with the oﬀset ranging from 3–5nm.

They found that the oﬀset was the same for very thin submonolayer coverages. If

the coverage is submonolayer and inadequate to make a liquid ﬁlm, the relevant

thickness is then the height (h

) of the molecules extended above the solid sur-

face. The oﬀset should be equal to 2h

, assuming that the molecules extend the

same height above both the tip and silicon surfaces. They therefore concluded that

the molecules do not extend more than 1.5–2.5nm above a solid or liquid surface,

much smaller than the radius of gyration of the lubricants, which ranges between

3.2and7.3nm, and to the approximate cross-sectional diameter of 0.6–0.7nmfor

the linear polymer chain. Consequently, the height that the molecules extend above

the surface is considerably less than the diameter of gyration of the molecules and

only a few molecular diameters in height, implying that the physisorbed molecules

on a solid surface have an extended, ﬂat conformation. They also determined the

disjoining pressure of these liquid ﬁlms from AFM measurements of the distance

needed to break the liquid meniscus that forms between the solid surface and the

AFM tip. (Also see [7].) For a monolayer thickness of about 0.7nm, the disjoining

pressure is about 5 MPa, indicating strong attractive interaction between the liquid

molecules and the solid surface. The disjoining pressure decreases with increasing

ﬁlm thickness in a manner consistent with a strong attractive van der Waals interac-

tion between the liquid molecules and the solid surface.

Rheological characterization shows that the ﬂow activation energy of PFPE lu-

bricants is weakly dependent on chain length and is strongly dependenton the func-

tional end groups [25]. PFPE lubricant ﬁlms that contain polar end groups have

lowermobility than thosewith nonpolarend groupsof similar chainlength [26]. The

mobility of PFPE also depends on the surface chemical properties of the substrate.

The spreading of Z-DOL on amorphous carbon surface has been studied as a func-

tion of hydrogen or nitrogen content in the carbon ﬁlm, using scanning microel-

lipsometry [24]. The diﬀusion coeﬃcient data presented in Fig. 18.5 is thickness-

dependent. It shows that the surface mobility of Z-DOL increased as the hydrogen

content increased, but decreased as nitrogen content increased. The enhancement

of Z-DOL surface mobility by hydrogenation may be understood from the fact that

the interactions between Z-DOL molecules and the carbon surface can be signif-

icantly weakened, due to a reduction of the number of high-energy binding sites

on the carbon surface. The stronger interactions between the Z-DOL molecules and

carbon surface, as the nitrogen content in the carbon coating increases, leads to the

lowering of Z-DOL surface mobility.

Molecularly thick ﬁlms may be sheared at very high shear rates, on the order of

–10

−1

during sliding, such as during magnetic disk drive operation. During

such shear, lubricant stability is critical to the protection of the interface. For proper

lubricant selection, viscosity at high shear rates and associated shear thinning need

to be understood. Viscosity measurements of eight diﬀerent types of PFPE ﬁlms

show that all eight lubricants display Newtonian behavior and their viscosity re-

mains constant at shear rates up to 10

−1

[27,28].

966 Bharat Bhushan and Huiwen Liu

D(h) (10

–12

/s)

Film thickness (nm)

a-C:H (50%)

a-C:H (10%)

a-C

a-C:N (10%)

a-C:N (50%)

6810

Fig. 18.5. Diﬀusion coeﬃcient D(h) as a function of lubricant ﬁlm thickness for Z-DOL on

diﬀerent carbon ﬁlms [24]

18.4 Boundary Lubrication Studies

With the development of AFM techniques, studies have been carried out to investi-

gate the nanotribological performance of PFPEs. Mate [29,30], O’Shea et al. [31,

32], Bhushan et al. [15, 33], Koinkar and Bhushan [20, 34], Bhushan and Sun-

dararajan [35], Bhushan and Dandavate [36], and Liu and Bhushan [21] used an

AFM to provide insight into how PFPE lubricants function at the molecular level.

Mate [29,30] conducted friction experiments on bonded and unbonded Z-DOL and

found that the coeﬃcient of friction of the unbonded Z-DOL is about two times

largerthan thebonded Z-DOL (also see [31,32]). Koinkar and Bhushan [20,34] and

Liu and Bhushan [21] studiedthe friction and wear performanceof a Si(100) sample

lubricated with Z-15, Z-DOL, and Z-DOL(BW) lubricants. They found that using

Z-DOL(BW) could signiﬁcantly improve the adhesion, friction, and wear perfor-

mance of Si(100). Theyalso discussed the lubrication mechanisms on the molecular

level. Bhushan and Sundararajan[35] and Bhushan and Dandavate[36] studied the

eﬀect of tip radius and relative humidity on the adhesion and friction properties of

Si(100) coated with Z-DOL(BW).

In this section, we review, in some detail, the adhesion, friction, and wear prop-

erties of Z-15 and Z-DOL at various operating conditions (rest time, velocity, rela-

tive humidity, temperature, and tip radius). The experiments were carried out using

a commercial AFM system with pyramidal Si

and diamond tips. An environ-

mentally controlled chamber and a thermal stage were used to perform relative hu-

midity and temperature eﬀect studies.

18 Nanoscale Boundary Lubrication Studies 967

18.4.1 Friction and Adhesion

To investigate the friction properties of Z-15 and Z-DOL(BW) ﬁlms on Si(100),

the curves of friction force versus normal load were measured by making friction

measurementsat increasing normalloads [21].The representativeresults of Si(100),

Z-15, and Z-DOL(BW) are shown in Fig. 18.6. An approximately linear response

of all three samples is observed in the load range of 5–130nN. The friction force of

solid-like Z-DOL(BW) is consistently smaller than that for Si(100), but the friction

force of liquid-like Z-15 lubricant is higher than that of Si(100). Sundararajan and

Bhushan [37] have studied the static friction force of silicon micromotors lubricated

with Z-DOL by AFM. They also found that liquid-like lubricants of Z-DOL signif-

icantly increase the static friction force, whereas solid-like Z-DOL(BW) coatings

can dramatically reduce the static friction force. This is in good agreement with

the results of Liu and Bhushan [21]. In Fig. 18.6, the nonzero value of the friction

force signal at zero external load is due to the adhesive forces. It is well known that

the following relationship exists between the friction force F and external normal

load W [2,3]

F = μ(W + W

) , (18.1)

where μ is the coeﬃcient of friction and W

is the adhesive force. Based on this

equation and the data in Fig. 18.6, we can calculate the μ and W

values. The coeﬃ-

cients of friction of Si(100),Z-15, and Z-DOL are 0.07, 0.09, and 0.04, respectively.

Based on (18.1), the adhesive force values are obtained from the horizontal inter-

cepts of the curves of friction force versus normal load at a zero value of friction

force. Adhesive force values of Si(100), Z-15, and Z-DOL are 52 nN, 91nN, and

34nN, respectively.

The adhesive forces of these samples were also measured using a force calibra-

tion plot (FCP) technique. In this technique, the tip is brought into contact with the

Friction force (nN)

2μm /s, 22°C, RH 45– 55%

Z-15, μ = 0.09

Si (100), μ = 0.07

Z-DOL(BW), μ = 0.04

Normal load (nN)

–100 100 150500–50

Fig. 18.6. Curves of friction

force versus normal load

for Si(100), 2.8-nm-thick

Z-15 ﬁlm, and 2.3-nm-thick

Z-DOL(BW) ﬁlm at 2 µm/s,

and in ambient air sliding

against a Si

tip. Based on

these curves, the coeﬃcient

of friction μ and adhesion

force of W

can be calcu-

lated [21]

968 Bharat Bhushan and Huiwen Liu

sample and the maximum force needed to pull the tip and sample apart is measured

as the adhesive force. Figure 18.7 shows the typical FCP curves of Si(100), Z-15,

and Z-DOL(BW) [21]. As the tip approaches the sample within a few nanome-

ters (point A), an attractive force exists between the tip and the sample surfaces.

The tip is pulled toward the sample, and contact occurs at point B on the graph. The

adsorption of water molecules and/or presence of liquid lubricant molecules on the

sample surface can also accelerate this so-called snap-in, due to the formation of

meniscus of the water and/or liquid lubricant around the tip. From this point on,

the tip is in contact with surface, and as the piezo extends further, the cantilever is

further deﬂected. This is represented by the slope portion of the curve. As the piezo

retracts, at point C the tip goes beyond the zero deﬂection (ﬂat) line, because of the

attractive forces, into the adhesive force regime. At point D, the tip snaps free of the

–40

–80

–40

–80

–40

–80

0 400

500

300

200

100

0 400

500

300

200

100

0 400

500

300

200

100

Z-position (nm)

Cantilever deflection (nN)

Z-DOL(BW)

Z-15

Si(100)

Trace

Retrace

Fig. 18.7. Typical force cali-

bration plots of Si(100),

2.8-nm-thick Z-15 ﬁlm, and

2.3-nm-thick Z-DOL(BW)

ﬁlm in ambient air. The adhe-

sive forces can be calculated

from the horizontal distance

between points C and D, and

the cantilever spring constant

of 0.58 N/m [21]

18 Nanoscale Boundary Lubrication Studies 969

adhesive forces and is again in free air. The adhesive force (pull-oﬀ force) is deter-

mined by multiplying the cantilever spring constant (0.58N/m) by the horizontal

distance between points C and D, which correspondsto the maximum cantilever de-

ﬂection toward the samples before the tip is disengaged. Incidentally, the horizontal

shift between the loading and unloading curves results from the hysteresis of the

PZT tube.

The adhesive forces of Si(100), Z-15, and Z-DOL(BW) measured by FCP and

plots of friction force versus normal load are summarized in Fig. 18.8 [21]. The re-

sults measured by these two methods are in good agreement. Figure 18.8 shows that

the presence of mobile Z-15 lubricant ﬁlm increases the adhesive force compared to

that of Si(100). In contrast, the presence of solid-phase Z-DOL(BW) ﬁlm reduces

the adhesive force compared to that of Si(100). This result is in good agreement

with the results of Blackman et al. [22] and Bhushan and Ruan [38]. Sources of ad-

hesive forces between the tip and the sample surfaces are van der Waals attraction

and long-rangemeniscusforces [2,3,16].The relativemagnitudesof the forces from

the two sourcesare dependenton variousfactors, includingthe distance between the

tip and the sample surface, their surface roughness, their hydrophobicity, and rela-

tive humidity [39]. For most surfaces with some roughness, meniscus contribution

dominates at moderate to high humidities.

100

Z-DOL(BW)Z-15Si(100)

22 °C,

RH 45 – 55%

Adhesive force (nN)

Force

calibration

plot

Friction

force plot

Z-DOL

Z-15

Fig. 18.8. Summary of the

adhesive forces of Si(100),

2.8-nm-thick Z-15 ﬁlm, and

2.3-nm-thick Z-DOL(BW)

ﬁlm measured by force cali-

bration plots and friction

force versus normal load in

ambient air. The schematic

(bottom)showstheeﬀect of

meniscus formed between

the AFM tip and the sample

surface on the adhesive and

friction forces [21]