Bhushan B. Nanotribology and Nanomechanics: An Introduction

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394 Bharat Bhushan

Fig. 8.65 [42]. The results measured by these two methods are in good agreement.

Figure 8.65 shows that the presence of the mobile Z-15 lubricant ﬁlm increases the

adhesive force compared to that of Si(100) by meniscus formation. Whereas, the

presence of solid-phase Z-DOL (BW) ﬁlm reduces the adhesive force compared to

that of Si(100) because of the absence of mobile liquid. The schematic (bottom)

in Fig. 8.65 shows the relative size and sources of meniscus. It is well known that

the native oxide layer (SiO

) on top of a Si(100) wafer exhibits hydrophilic proper-

ties, and some water molecules can be adsorbed onto this surface. The condensed

water will form meniscus as the tip approaches the sample surface. The larger ad-

hesive force in Z-15 is not only caused by the Z-15 meniscus, the nonpolarized

Z-15 liquid does not have good wettability and strong bonding with Si(100). Con-

sequently, in the ambient environment, the condensed water molecules from the

environment will permeate through the liquid Z-15 lubricant ﬁlm and compete with

the lubricant molecules present on the substrate. The interaction of the liquid lu-

bricant with the substrate is weakened, and a boundary layer of the liquid lubricant

forms puddles [39,40]. This dewetting allows water molecules to be adsorbed onto

the Si(100) surface as aggregates along with Z-15 molecules. And both of them can

form a meniscus while the tip approaches to the surface. Thus the dewetting of liq-

uid Z-15 ﬁlm results in higher adhesive force and poorer lubrication performance.

In addition, as the Z-15 ﬁlm is quite soft compared to the solid Si(100) surface, and

penetration of the tip in the ﬁlm occurs while pushing the tip down. This leads to

the large area of the tip involved to form the meniscus at the tip–liquid (mixture of

Z-15 and water) interface. It should also be noted that Z-15 has a higher viscosity

than water, therefore Z-15 ﬁlm provides higher resistance to motion and coeﬃcient

of friction. In the case of Z-DOL (BW) ﬁlm, both of the active groups of Z-DOL

molecules are mostly bonded on Si(100) substrate, thus the Z-DOL (BW) ﬁlm has

low free surface energy and cannot be displaced readily by water molecules or read-

ily adsorb water molecules. Thus, the use of Z-DOL (BW) can reduce the adhesive

force.

To study the velocity eﬀect on friction and adhesion, the variation of friction

force, adhesive force, and coeﬃcient of friction of Si(100), Z-15 and Z-DOL(BW)

as a function of velocity are summarized in Fig. 8.66 [42]. It indicates that, for

silicon wafers, the friction force decreases logarithmically with increasing velocity.

For Z-15, the friction force decreases with increasing velocity up to 10µm/s, after

which it remains almost constant. The velocity has a very small eﬀect on the friction

force of Z-DOL (BW); it reduced slightly only at very high velocity. Figure 8.66

also indicates that the adhesive force of Si(100) is increased when the velocity is

higher than 10µm/s. The adhesive force of Z-15 is reduced dramatically when the

velocity increases to 20µm/s, after which it is reduced slightly. The adhesive force

of Z-DOL (BW) also decreases at high velocity. In the tested velocity range, only

the coeﬃcient of friction of Si(100) decreases with velocity, while the coeﬃcients

of friction of Z-15 and Z-DOL (BW) almost remain constant. This implies that the

friction mechanisms of Z-15 and Z-DOL (BW) do not change with the variation of

velocity.

8 Nanotribology, Nanomechanics and Materials Characterization 395

125

100

0.15

0.10

0.05

1 1000100100.1

70 nN, 22°C, RH 45–55%

Friction force (nN)

Si(100)

Z-DOL (BW)

Z-15

Velocity (μm/s)

From friction force plot

Adhesive force (nN)

Si(100)

Z-DOL (BW)

Z-15

Coefficient of friction

Si(100)

Z-DOL (BW)

Z-15

Z-DOL (BW)

Si(100)

Increasing velocity

0.4

μm/s

240 μm/s

Si(OH)

Z-15

Z-DOL

Fig. 8.66. The inﬂuence of velocity on the friction force, adhesive force and coeﬃcient of

friction of Si(100) and Z-15 and Z-DOL (BW) ﬁlms at 70 nN, in ambient air. The schematic

(right) shows the change of surface composition (by tribochemical reaction) and formation

of meniscus while increasing the velocity [42]

The mechanisms of the eﬀect of velocity on the adhesion and friction are ex-

plained based on the schematics shown in Fig. 8.66 (right) [42]. For Si(100), tri-

bochemical reactions play a major role. Although, at high velocity, the meniscus is

broken and does not have enough time to rebuild, the contact stresses and high ve-

locity lead to tribochemical reactions of the Si(100) wafer (which has native oxide

(SiO

)), and Si

tip with water molecules, forming Si(OH)

. The Si(OH)

is re-

movedand continuouslyreplenished during sliding. The Si(OH)

layer between the

tip and Si(100) surface is known to be of low shear strength and causes a decrease

396 Bharat Bhushan

in friction force and coeﬃcient of friction [10,17]. The chemical bonds of Si−OH

between the tip and the Si(100) surface induce large adhesive force. For Z-15 ﬁlm,

at high velocity the meniscus formed by condensed water and Z-15 molecules is

broken and does not have enough time to rebuild; therefore, the adhesive force and

consequently friction force is reduced. The friction mechanisms for the Z-15 ﬁlm

still is shearing the same viscous liquid even at high velocity range, thus the coeﬃ-

cient of friction of Z-15 does not change with velocity. For Z-DOL (BW) ﬁlm, the

surface can adsorb few water molecules in ambient condition, and at high velocity

these molecules are displaced, which is responsible for a slight decrease in friction

force and adhesive force. Koinkar and Bhushan [40,40] have suggested that, in the

case of samples with mobile ﬁlms, such as condensed water and Z-15 ﬁlms, align-

ment of liquid molecules (shearthinning)is responsiblefor the drop in friction force

with increasing scanning velocity. This could be another reason for the decrease in

friction force for Si(100) and Z-15 ﬁlm with velocity in this study.

To study the eﬀect of relative humidity on friction and adhesion, the variation

of friction force, adhesive force, and coeﬃcient of friction of Si(100), Z-15, and

Z-DOL (BW) as a function of relative humidity is shown in Fig. 8.67 [42]. It shows

that, for Si(100) and Z-15 ﬁlm, the friction force increases with relative humidity up

to 45%, and then shows a slight decrease with further increases in the relative hu-

midity. Z-DOL (BW) has a smaller frictionforce than Si(100) andZ-15 in the whole

testing range and its friction force shows an apparent relative ncrease when the rel-

ative humidity is higher than 45%. For Si(100), Z-15 and Z-DOL (BW), their adhe-

sive forces increase with relative humidity, and their coeﬃcients of friction increase

with relative humidity up to 45%, after which they decrease with further increases

of relative humidity. It is also observed that the eﬀect of humidity on Si(100) de-

pends on the history of the Si(100) sample. As the surface of Si(100) wafer readily

adsorb water in air, without any pretreatment the Si(100) used in our study almost

reaches to its saturate stage of adsorbedwater, and is responsible for a smaller eﬀect

during increasing relative humidity. However,once the Si(100) wafer was thermally

treated by baking at 150

◦

C for 1hour, a bigger eﬀect was observed.

The schematic (left) in Fig. 8.67 shows that Si(100), because of its high free

surface energy, can adsorb more water molecules while increasing relative humid-

ity [42]. As discussed earlier, for the Z-15 ﬁlm in the humid environment, the con-

densed water from the humid environmentcompetes with the lubricant ﬁlm present

on the sample surface, and the interaction of the liquid lubricant ﬁlm with the sili-

con substrate is weakened and a boundary layer of the liquid lubricant forms pud-

dles. This dewetting allows water molecules to be adsorbed onto the Si(100) sub-

strate mixed with Z-15 molecules [39, 40]. Obviously, more water molecules can

be adsorbed onto the Z-15 surface with increasing relative humidity. The higher

number of adsorbed water molecules in the case of Si(100), along with the lubri-

cant molecules in the Z-15 ﬁlm case, form a larger water meniscus, which leads to

an increase of friction force, adhesive force, and coeﬃcient of friction of Si(100)

and Z-15 with humidity. However, at the very high humidity of 70%, large quan-

tities of adsorbed water can form a continues water layer that separates the tip

8 Nanotribology, Nanomechanics and Materials Characterization 397

200

175

150

125

100

0.15

0.10

0.05

20 8060400

70 nN, 2 μm/s, 22°C

Friction force (nN)

Si(100)

Z-DOL (BW)

Z-15

Relative humidity (%)

From friction force plot

Adhesive force (nN)

Si(100)

Z-DOL (BW)

Z-15

Coefficient of friction

Si(100)

Z-DOL (BW)

Z-15

Z-DOL (BW)

Si(100)

Increasing relative humidity

Thermally treated Si(100)

Z-DOL

70 %

Z-15

Fig. 8.67. The inﬂuence of relative humidity on the friction force, adhesive force, and coeﬃ-

cient of friction of Si(100) and Z-15 and Z-DOL (BW) ﬁlms at 70 nN, 2 µm/s, and in 22

◦

air. The schematic (left) shows the change of meniscus while increasing the relative humid-

ity. In this ﬁgure, the thermally treated Si(100) represents the Si(100) wafer that was baked at

150

◦

C for 1hour in an oven (in order to remove the adsorbed water) just before it was placed

in the 0% RH chamber [42]

and sample surface and acts as a kind of lubricant, which causes a decrease in

the friction force and coeﬃcient of friction. For the Z-DOL (BW) ﬁlm, because

of its hydrophobic surface properties, water molecules can be adsorbed at humid-

ity higher than 45%, and this causes an increase in the adhesive force and friction

force.

To study the eﬀect of temperature on friction and adhesion,the variation of fric-

tion force, adhesive force, and coeﬃcient of friction of Si(100), Z-15 and Z-DOL

(BW) as a function of temperature are summarized in Fig. 8.68 [42]. It shows that

increasing temperature causes a decrease of the friction force, adhesive force and

398 Bharat Bhushan

125

100

0.15

0.10

0.05

150100500

70 nN, 2 μm /s, RH 45–55%

Friction force (nN)

Si(100)

Z-DOL (BW)

Z-15

Temperature (°C)

From friction force plot

Adhesive force (nN)

Si(100)

Z-DOL (BW)

Z-15

Coefficient of friction

Si(100)

Z-DOL (BW)

Z-15

Si(100)

Increasing temperature

25 °C

125 °C

Z-15

Z-DOL (BW)

Z-DOL

Fig. 8.68. The inﬂuence of temperature on the friction force, adhesive force, and coeﬃcient

of friction of Si(100) and Z-15 and Z-DOL (BW) ﬁlms at 70 nN, at 2µm/s,andinRH40–

50% air. The schematic (right) shows that, at high temperature, desorption of water decreases

the adhesive forces. And the reduced viscosity of Z-15 leads to the decrease of coeﬃcient of

friction. High temperature facilitates orientation of molecules in Z-DOL (BW) ﬁlm, which

results in a lower coeﬃcient of friction [42]

coeﬃcient of friction of Si(100), Z-15 and Z-DOL (BW). The schematic (right) in

Fig. 8.68 indicates that, at high temperature, desorption of water leads to a decrease

of the friction force, adhesive forces and coeﬃcient of friction for all of the samples.

For Z-15 ﬁlm, the reduction of viscosity at high temperature also makes a contri-

bution to the decrease of friction force and coeﬃcient of friction. In the case of the

8 Nanotribology, Nanomechanics and Materials Characterization 399

Z-DOL (BW) ﬁlm, molecules are more easily oriented at high temperatures, which

may be partly responsible for the low friction force and coeﬃcient of friction.

As a brief summary, the inﬂuence of velocity, relative humidity, and temperature

on the frictionforce of mobile Z-15 ﬁlm is presented in Fig. 8.69 [42].The changing

trends are also addressed in this ﬁgure.

To study the durability of lubricant ﬁlms at nanoscale, the friction of Si(100),

Z-15, and Z-DOL (BW) as a function of the number of scanning cycles are shown

Friction force (nN)

Log velocity

Lack of meniscus

reformation decreases

friction with

increasing velocity

Molecularly thick Z-15 film

Reaches to

equilibrium

Friction force (nN)

Relative humidity

Meniscus formation

increases friction with

increasing RH

Thick H

film serves

as a lubricant

Friction force (nN)

Temperature

Desorption of water and

decrease of viscosity decrease

friction with increase of

temperature

Fig. 8.69. Schematic shows

the change of friction force of

molecularly thick Z-15 ﬁlms

with log velocity, relative

humidity, and temperature.

The changing trends are also

addressed in this ﬁgure [42]

400 Bharat Bhushan

0 50 75 10025 125

0.15

0.10

0.05

70 nN, 0.4 μm/s, 22°C, RH 45–55%

Friction force (nN)

Si(100)

Z-DOL (BW)

Z-15

Number of cycles

Coefficient of friction

Si(100)

Z-DOL (BW)

Z-15

Increasing scan number

Z-15

Fig. 8.70. Friction force and coeﬃcient of friction versus number of sliding cycles for Si(100)

and Z-15 and Z-DOL (BW) ﬁlms at 70 nN, 0.8 µm/s, and in ambient air. The schematic

(bottom) shows that some liquid Z-15 molecules can be attached onto the tip. The molecular

interaction between the attached molecules onto the tip with the Z-15 molecules in the ﬁlm

results in an increase of the friction force with multiple scanning [42]

8 Nanotribology, Nanomechanics and Materials Characterization 401

in Fig. 8.70 [42]. As observed earlier, the friction force and coeﬃcient of friction

of Z-15 are higher than that of Si(100), with the lowest values for Z-DOL(BW).

During cycling, the friction force and coeﬃcient of friction of Si(100) show a slight

decrease during the initial few cycles and then remain constant. This is related to

the removal of the top adsorbed layer. In the case of the Z-15 ﬁlm, the friction force

and coeﬃcient of friction show an increase during the initial few cycles and then ap-

proach higher,stable values. This is believedto be caused by the attachment of Z-15

molecules to the tip. The molecular interaction between these attached molecules on

the tip and molecules on the ﬁlm surface is responsible for an increase in the fric-

tion. However, after several scans, this molecular interaction reaches an equilibrium

and the friction force and coeﬃcient of friction then remain constant. In the case

of the Z-DOL (BW) ﬁlm, the friction force and coeﬃcient of friction start low and

remain low during the entire test for 100cycles. This suggests that Z-DOL (BW)

molecules do not become attached or displaced as readily as Z-15.

8.6.2 Self-Assembled Monolayers

For lubrication of MEMS/NEMS, another eﬀective approach involves the deposi-

tion of organized and dense molecular layers of long-chain molecules. Two com-

mon methods to produce monolayers and thin ﬁlms are the Langmuir–Blodgett

(LB) deposition and self-assembled monolayers (SAMs) by chemical grafting of

molecules. LB ﬁlms are physically bonded to the substrate by weak van der Waals

attraction, while SAMs are chemically bonded via covalent bonds to the substrate.

Because of the choice of chain length and terminal linking group that SAMs of-

fer, they hold great promise for boundary lubrication of MEMS/NEMS. A num-

ber of studies have been conducted to study tribological properties of various

SAMs [38,41,43,129–135].

Bhushanand Liu [41]studied the eﬀect of ﬁlm complianceon adhesion and fric-

tion.Theyused hexadecanethiol(HDT), 1,1,biphenyl-4-thiol(BPT),and crosslinked

BPT (BPTC) solventdepositedon Au(111)substrate,Fig. 8.71a.The averagevalues

and standard duration of the adhesive force and coeﬃcient of friction are presented

in Fig. 8.71b. Based on the data, the adhesive force and coeﬃcient of frictions of

SAMs are less than those of corresponding substrates. Among various ﬁlms, HDT

exhibits the lowest values. Based on stiﬀness measurements of various SAMs, HDT

was the most compliant,followed by BPT and BPTC. Based on friction and stiﬀness

measurements, SAMs with high-compliance long carbon chains exhibit low fric-

tion; chain compliance is desirable for low friction. Friction mechanism of SAMs

is explained by a so-called molecular spring model (Fig. 8.72). According to this

model, the chemically adsorbed self-assembled molecules on a substrate are just

like assembled molecular springs anchored to the substrate. An asperity sliding on

the surface of SAMs is like a tip sliding on the top of molecular springs or a brush.

The molecular spring assembly has compliant features and can experience orienta-

tion and compression under load. The orientation of the molecular springs or brush

under normal load reduces the shearing force at the interface, which in turn reduces

the friction force. The orientation is determined by the spring constant of a single

402 Bharat Bhushan

Adhesive force (nN)

HDT BPT BPTC

Coefficient of friction

0.08

0.06

0.04

0.02

Materials

HDT BPT BPTC

Hexadecane thiol

(HDT)

1,1'–biphenyl–4–thiol

(BPT)

Cross-linked 1,1'–

biphenyl–4–thiol

(BPTC)

Au(111)

SSSS

Biphenyl

–(C

)

–

Au(111)

Alkyl

–(CH

)

–

Au(111)

Fig. 8.71. (a) Schematics

of the structures of hex-

adecane thiol and biphenyl

thiol SAMs on Au(111) sub-

strates, and (b) adhesive force

and coeﬃcient of friction of

Au(111) substrate and vari-

ous SAMs

8 Nanotribology, Nanomechanics and Materials Characterization 403

Substrate

Fig. 8.72. Molecular spring

model of SAMs. In this ﬁg-

ure, α

<α

, which is caused

by the further orientation un-

der the normal load applied

by an asperity tip [41]

molecule as well as the interaction between the neighboring molecules, which can

be reﬂected by packing density or packing energy. It should be noted that the orien-

tation can lead to conformational defects along the molecular chains, which lead to

energy dissipation.

The SAMs with high-compliance long carbon chains also exhibit the best wear

resistance [41,130]. In wear experiments, the wear depth as a function of normal

loadcurves showa critical normalload. A representativecurveis shownin Fig. 8.73.

Below the critical normal load,SAMs undergoorientation;at the critical load SAMs

wear away from the substrate due to weak interface bond strengths, while above the

critical normal load severe wear takes place on the substrate.

Bhushan et al. [43], Kasai et al. [131], and Tambe and Bhushan [134] stud-

ied perﬂuorodecyltricholorosilane(PFTS), n-octyldimethyl (dimethylamino) silane

(ODMS) (n = 7), and n-octadecylmethyl(dimethylamino)silane (ODDMS) (n = 17)

vapor deposited on Si substrate, and octylphosphonate(OP) and octadecylphospho-

nate (ODP) on Al substrate, Fig. 8.74a. Figure 8.74b presents the contact angle,

adhesive force, friction force, and coeﬃcient of friction of two substrates and with

Decrease of surface height (nm)

Normal load (μN)

–1

123456

Critical load

Fig. 8.73. Illustration of

the wear mechanism of

SAMs with increasing normal

load [130]