Bhushan B. Nanotribology and Nanomechanics: An Introduction

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950 Bharat Bhushan

icals. The radical could remain on the surface and decompose to generate a short

radical and an alkene. The radical can repeatedly decomposeto ever shorter radicals

and alkenes so long as it remains on the surface.

A summary of the coeﬃcients of friction and the durability of all ﬁlms in vac-

uum is presented in Fig. 17.27.

Environmental Studies

To study theeﬀect of theenvironment,frictiontests were conductedin highvacuum,

argon,dry air (lessthan 2% RH), airwith 30% RH,and air with70% RH;Fig. 17.28.

The applied normal pressure was 50 kPa for HDT/Au, PFTS/Si, ODMS/Si and

ODDMS/Si, which is the same as used in the degradation tests. By comparing the

coeﬃcients of friction in diﬀerent environments, it was found that the friction is

the lowest in argon for the SAMs tested. The intimate contact induced by high vac-

uum leads to high friction.Friction is higher in dry air than in argon.This shows that

oxygenappears to eﬀect SAM performance. Kim et al. [106] studied the thermalsta-

bility of alkylsiloxane SAMs in air. They found that the alkylsiloxane decomposes

at about 200

◦

C, which is much lower than the decomposition temperature of 470

◦

Air (30% RH)

HDT/Au

0.8

0.6

0.4

0.2

High vacuum

Argon

0.8

0.6

0.4

0.2

Dry air

0.8

0.6

0.4

0.2

Air (30% RH)

0.8

0.6

0.4

0.2

Air (70% RH)

0.8

0.6

0.4

0.2

0.8

0.6

0.4

0.2

High vacuum

0.8

0.6

0.4

0.2

High vacuum

0.8

0.6

0.4

0.2

High vacuum

0.8

0.6

0.4

0.2

Argon

0.8

0.6

0.4

0.2

Argon

0.8

0.6

0.4

0.2

Argon

0.8

0.6

0.4

0.2

Dry air

0.8

0.6

0.4

0.2

Dry air

0.8

0.6

0.4

0.2

Dry air

0.8

0.6

0.4

0.2

Air (30% RH)

0.8

0.6

0.4

0.2

Air (30% RH)

0.8

0.6

0.4

0.2

0.8

0.6

0.4

0.2

Air (70% RH)

0.8

0.6

0.4

0.2

Air (70% RH)

0.8

0.6

0.4

0.2

Air (70% RH)

Coefficient of friction

50 1000

Sliding distance (m)

50 1000

Sliding distance (m)

50 1000

Sliding distance (m)

1000

Sliding distance (m)

Coefficient of friction Coefficient of friction Coefficient of friction

PFTS/Si ODMS/Si ODDMS/Si

a) b)

Fig. 17.28. Coeﬃcient of friction data taken in high vacuum, argon and air with diﬀerent

humidity levels for (a) HDT/Au (1.9nm),(b) PFTS/Si (1.8 nm), ODMS/Si (≈ 1.9nm)and

ODDMS/Si (≈2.1 nm) (RH: relative humidity, normal pressure 50 kPa) (after [48])

17 Self-Assembled Monolayers (SAMs) for Controlling Adhesion 951

Coefficient of friction before failure of the film

HDT/Au

0.4

0.3

0.2

0.1

PFTS/Si ODMS/Si ODDMS/Si

High vacuum

Argon

Dry air

Air (30% RH)

Air (70% RH)

Fig. 17.29. Coeﬃcient

of friction comparison

for HDT/Au, PFTS/Si,

ODMS/Si and ODDMS/Si

in high vacuum, argon and air

with diﬀerent humidity lev-

els. Error bars represent ±3σ

based on ﬁve measurements

(normal pressure 50 kPa) (af-

ter [48])

in vacuum reported by Kluth et al. [105]. This diﬀerence could be attributed to the

oxygen in air. Water in the air is found to have signiﬁcant inﬂuence on the friction

of SAMs. A study of humidity eﬀects for alkylsilane on mica substrate performed

by Tian et al. [100] indicated that the water molecules can penetrate the alkylsi-

lane ﬁlm, altering their molecular chain ordering and also detaching the alkylsilane

molecules from the substrate.

A summary of friction coeﬃcients observed before lubricant ﬁlm failure in var-

ious environments is presented in Fig. 17.29. The data in Fig. 17.29 are average

values based on ﬁve measurements.

17.5 Closure

Exposure of devices to a humid environment results in condensates of water vapor.

Condensed water or a pre-existing ﬁlm of liquid results in the formation of concave

meniscus bridges between the hydrophilic mating surfaces. The negative Laplace

pressure present in the meniscus results in an adhesive force which depends on the

roughness of the interface, the surface tension and the contact angle. The adhesive

forcecan be signiﬁcantin an interfacewith ultrasmoothsurfaces,and it can beon the

same order as the external load if the latter is small, such as in micro/nanodevices.

Surfaces with high hydrophobicity can be produced by surface treatment. In many

applications, hydrophobic ﬁlms are expected to provide low adhesion, friction and

wear. Because of the small clearances inherent to micro/nanodevices, these ﬁlms

should be molecularly thick. Liquid ﬁlms with low surface tension or hydrophobic

solid ﬁlms can be used. Ordered molecular assemblies with high hydrophobicity

can be engineered by chemically grafting various polymer molecules with suitable

functional head groups and nonpolar surface terminal groups.

The adhesion, friction and wear properties of SAMs with alkyl, biphenyl and

perﬂuoroalkyl spacer chains and diﬀerent surface terminal (

−

and

−

)and

952 Bharat Bhushan

headgroups (

−

SH,

−

OH, and P

−

),studied usingan AFM, are reported

in this chapter. It was found that the adhesive force varies linearly with the W

value

of the SAM, which indicatesthat capillary condensation of water plays an important

role in the adhesion of SAMs on the nanoscale at ambient conditions. SAMs with

long high-compliance carbon spacer chains exhibit the smallest adhesive and fric-

tion forces.The friction data areexplainedusing a molecular springmodel, in which

the local stiﬀness and intermolecular force govern frictional performance. Results

from stiﬀness and friction characterizations of a micropatterned sample with dif-

ferent structures on it support this model. Perﬂuoroalkylsilane SAMs exhibit lower

surface energies, higher contact angles and lower adhesive forces than alkylsilane

SAMs. The substrate had little eﬀect. The coeﬃcients of friction for various SAMs

were comparable.

The inﬂuence of the relative humidity on the adhesion and the friction of SAMs

is dominated by the thickness of the adsorbed water layer. At higher humidity, water

increases friction due to the increased adhesion caused by the meniscus eﬀect in the

contact zone. As the temperature is increased, in the case of Si(111), the desorption

of the adsorbed water layer and the reduced surface tension of water reduces the ad-

hesive and friction forces. Decreases in adhesionand friction with temperaturewere

found for PFTS/Si, ODMS/Si and ODDMS/Si. PFTS showed the lowest adhesion

at low temperature (≈ 70

◦

C). Diﬀerences among the SAMs were small at high tem-

perature (≈ 100

◦

C). Increases in adhesive force and friction with sliding velocity

were observed for PFTS/Si, ODMS/Si and ODDMS/Si. A peak in the coeﬃcient

of friction appeared for PFTS.

PFTS/Si showed better wear resistance than ODMS/Si. ODDMS/Si showed

better wear resistance than ODMS/Si due to the chain length eﬀect. SAM wear

behavior is mostly determined by the molecule–substrate bond strength. The long

chain molecules of ODP/Al and ODDMS/Si showed higher critical loads for fail-

ure.

The results from nanotribological characterization studies of SAMs deposited

on Si and Al substrates are summarized in Table 17.15 [47]. SAMs deposited on Si

Table 17.15. Summary of nanotribological characterization studies of SAMs on Si and Al

substrates

SAM property Friction force Adhesive force Wear

Substrate Hard High Low Low

Soft Low Low High

Chemical structure Linear chain molecule High Low High

Ring molecule High High Low

Backbone Fluorocarbon backbone Low Low Low

Hydrocarbon backbone Low High High

Chain length Long backbone chain High High

Short backbone chain Low Low

17 Self-Assembled Monolayers (SAMs) for Controlling Adhesion 953

and Al substrates show low friction and low adhesion, both of which are desirable

for MEMS/NEMS applications.

Based on studies in high vacuum (2×10

−7

Torr), the friction coeﬃcients for

SAMs have been found to follow the order (from low to high): ODDMS/Si,

PFTS/Si, HDT/Au, ODMS/Si. HDT on Au is less durable than perﬂuoroalkylsi-

lane and alkylsilane on Si due to its weak interfacial bonding. PFTS/Si is more

durable than ODMS/Si. This indicates that ﬂuorinating alkylsilane can improve

durability. ODDMS/Si is more durable than ODMS/Si and PFTS/Si because of the

chain length eﬀect. SAM friction is higher in high vacuum than in argon because of

the intimate contact involved. Based on studies in argon and air with various rela-

tive humidities, oxygen can increase the friction and durability of SAMs. The water

molecule can detach SAM molecules from the substrate, resulting in high friction

and low durability.

Insummary,nanotribologicalstudies of SAMﬁlms using AFMdemonstratethat

they exhibit attractive hydrophobic and tribological properties. Fluorinated SAMs

appear to be the most performant.SAM ﬁlms should ﬁnd application in manyﬁelds,

including micro/nanodevices.

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(2003)

Nanoscale Boundary Lubrication Studies

Bharat Bhushan and Huiwen Liu

Summary. Boundary ﬁlms are formed by physisorption, chemisorption, and chemical re-

action. With physisorption, no exchange of electrons takes place between the molecules of

the adsorbate and those of the adsorbant. The physisorption process typically involves van

der Waals forces, which are relatively weak. In chemisorption, there is an actual sharing of

electrons or electron interchange between the chemisorbed species and the solid surface. The

solid surfaces bond very strongly to the adsorption species through covalent bonds. Chem-

ically reacted ﬁlms are formed by the chemical reaction of a solid surface with the environ-

ment. The physisorbed ﬁlm can be either monomolecularly or polymolecularly thick. The

chemisorbed ﬁlms are monomolecular, but stoichiometric ﬁlms formed by chemical reac-

tion can have a large ﬁlm thickness. In general, the stability and durability of surface ﬁlms

decrease in the following order: chemically reacted ﬁlms, chemisorbed ﬁlms, and physisor-

bed 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). In this chapter, we focus on per-

ﬂuoropolyethers (PFPEs). We ﬁrst introduce details of the commonly used PFPE lubricants;

then present a summary of nanodeformation, molecular conformation, and lubricant spread-

ing studies; followed by an overview of nanotribological properties of polar and nonpolar

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

18.1 Introduction

Boundary ﬁlms are formed by physisorption, chemisorption,and chemical reaction.

With physisorption, no exchange of electrons takes place between the molecules

of the adsorbate and those of the adsorbant. The physisorption process typically

involves van der Waals forces, which are relatively weak. In chemisorption, there

is an actual sharing of electrons or electron interchange between the chemisorbed

species and thesolid surface. Thesolid surfaces bondvery stronglyto the adsorption

species through covalent bonds. Chemically reacted ﬁlms are formed by the chem-

ical reaction of a solid surface with the environment. The physisorbed ﬁlm can be

either monomolecularly or polymolecularly thick. The chemisorbed ﬁlms are mo-

nomolecular, but stoichiometric ﬁlms formed by chemical reaction can have a large