Bhushan B. Nanotribology and Nanomechanics: An Introduction

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980 Bharat Bhushan and Huiwen Liu

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

Thermally treated Si(100)

Z-15

Z-DOL(BW)

Si(100)

Increasing relative humidity

Z-DOL

70 %

Z-15

Fig. 18.15. The inﬂuence of relative humidity (RH) on the friction force, adhesive force,

and coeﬃcient of friction of Si(100), 2.8-nm-thick Z-15 ﬁlm, and 2.3-nm-thick Z-DOL(BW)

ﬁlm at 70 nN, 2 µm/s, and in 22

◦

C air. Schematic (right) shows the change of meniscus

with increasing relative humidity. In this ﬁgure, the thermally treated Si(100) represents the

Si(100) wafer that was baked at 150

◦

C for 1 h in an oven (in order to remove the adsorbed

water) just before it was placed in the 0% RH chamber [21]

The schematic (right) in Fig. 18.15 shows that, because of its high free surface

energy, Si(100) can adsorb more water molecules with increasing relative humidity.

As discussed earlier, for Z-15 ﬁlm in a humid environment, condensed water com-

petes with the lubricant ﬁlm present on the sample surface. Obviously, more water

molecules can also be adsorbed on a Z-15 surface with increasing relative humid-

ity. Thermal adsorbed water molecules in the case of Si(100), along with lubricant

18 Nanoscale Boundary Lubrication Studies 981

molecules in Z-15 ﬁlm, form a bigger water meniscus, which leads to an increase

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

with humidity. But at the very high humidity of RH 70%, large quantities of ad-

sorbed water can form a continuous water layer that separates the tip 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 only be adsorbed at high humidity (≥ RH

45%), which causes an increase in the adhesive force and friction force.

The eﬀect of temperature on friction and adhesion was studied using a thermal

stage attached to the AFM. The friction force was measured at increasing temper-

ature from 22–125

◦

C. The results are presented in Fig. 18.16 [21]. It shows that

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

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

From friction force plot

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. 18.16. The inﬂuence of tempera-

ture on the friction force, adhesive force,

and coeﬃcient of friction of Si(100),

2.8-nm-thick Z-15 ﬁlm, and 2.3-nm-thick

Z-DOL(BW) ﬁlm at 70 nN, at 2 µm/s,

and in RH 40–50 % air. The schematic

(right) shows that, at high temperature,

desorption of water decreases the adhe-

sive 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 [21]

982 Bharat Bhushan and Huiwen Liu

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

Fig. 18.16 indicates that, at high temperature, desorption of water leads to a de-

crease in the friction force, adhesive force, and coeﬃcient of friction for all of the

samples. Besides that, the reduction of surface tension of water also contributes to

the decrease of friction and adhesion. For Z-15 ﬁlm, the reduction of viscosity at

high temperature makes an additional contribution to the decrease of friction. In the

case of Z-DOL(BW) ﬁlm, molecules are more easily oriented at high temperature,

which may also be responsible for the low friction.

Friction force (nN)

Log velocity

Lack of meniscus

reformation decreases

friction with

increasing velocity

Molecularly thick Z-15 film

Reaches

equilibrium

Friction force (nN)

Relative humidity

Meniscus formation

increases friction with

increasing RH

Thick water

film serves

as a lubricant

Friction force (nN)

Temperature

Desorption of water and

decrease of viscosity decrease

friction with increase of

temperature

Fig. 18.17. Schematic show-

ing the change of friction

force of molecularly thick

Z-15 ﬁlms with log veloc-

ity, relative humidity, and

temperature [21]

18 Nanoscale Boundary Lubrication Studies 983

Usinga surfaceforceapparatus,Yoshizawa andIsraelachvili[47,48]haveshown

that a change in the velocity or temperature induces phase transformation (from

crystalline solid-like to amorphous, then to liquidlike) in surfactant monolayers,

which are responsible for the observed changes in the friction force. Stick–slip is

observed in a low-velocity regime of a few µm/s, and adhesion and friction ﬁrst

increase followed by a decrease in the temperature range of 0–50

◦

C. Stick–slip

at low velocity and adhesion and friction curves peaking at some particular tem-

perature (observed in their study), have not been observed in the AFM study. This

suggests that the phase transformation may not happen in this study, because PFPEs

generally have very good thermal stability [10,12].

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

on the friction force of Z-15 ﬁlm is presented in Fig. 18.17.The changing trends are

also addressed in this ﬁgure.

18.4.5 Tip Radius Eﬀect

The tip radius and relative humidity aﬀect adhesion and friction for dry and lubri-

cated surfaces [35,36]. Figure 18.18a shows the variation of single-point adhesive

force measurements as a function of tip radius on a Si(100) sample for several hu-

midities. The adhesive force data are also plotted as a function of relative humidity

for various tip radii. Figure 18.18a indicates that the tip radius has little eﬀect on

the adhesive forces at low humidities, but the adhesive force increases with tip ra-

dius at high humidity. Adhesive force also increases with increasing humidity for

all tips. The trend in adhesive forces as a function of tip radii and relative humidity,

in Fig. 18.18a, can be explained by the presence of meniscus forces, which arise

from capillary condensation of water vapor from the environment. If enough liquid

is present to form a meniscus bridge, the meniscus force should increase with an in-

crease in tip radius, based on (18.2). This observation suggests that the thickness of

the liquid ﬁlm at low humidities is insuﬃcient to form continuous meniscus bridges

andtoaﬀect adhesive forces in the case of all tips.

Figure 18.18a also shows the variation in coeﬃcient of friction as a function of

tip radius at a given humidity and as a function of relative humidity for a given tip

radius on the Si(100) sample. It can be observed that, for RH 0%, the coeﬃcient

of friction is about the same for the tip radii investigated except for the largest tip,

which shows a higher value. At all other humidities, the trend consistently shows

that the coeﬃcient of friction increases with tip radius. An increase in friction with

tip radius at low to moderate humidities arises from increased contact area (i.e.,

higher van der Waals forces) and the higher values of shear forces required for

a larger contact area. At high humidities, similarly to adhesive force data, an in-

crease with tip radius occurs due to both contact area and meniscus eﬀects. It can be

seen that, for all tips, the coeﬃcient of friction increases with humidity to about RH

45%, beyond which it starts to decrease. This is attributed to the fact that, at higher

humidities, the adsorbed water ﬁlm on the surface acts as a lubricant between the

two surfaces [21]. Thus the interface is changed at higher humidities, resulting in

lower shear strength and, hence, a lower friction force and coeﬃcient of friction.

984 Bharat Bhushan and Huiwen Liu

250

200

150

100

0.2

0.15

0.1

0.05

4120816

250

200

150

100

0.2

0.15

0.1

0.05

25 750 50 100

250

200

150

100

0.2

0.15

0.1

0.05

4120816

250

200

150

100

0.2

0.15

0.1

0.05

25 750 50 100

Tip radius (μm)

Adhesive force (nN)

Si(100)

Coefficient of friction

Relative humidity (%)

Adhesive force (nN)

Coefficient of friction

Tip radius (μm)

Adhesive force (nN)

Z-DOL(BW)/Si(100)

Coefficient of friction

Relative humidity (%)

Adhesive force (nN)

Coefficient of friction

15%

45%

65%

15%

45%

65%

15%

45%

65%

0.05

3.8

6.9

9.5

14.5

μm (Si

)

3.8

6.9

9.5

14.5

0.05

μm (Si

)

45%

65%

6.9

14.5

9.5

3.8

0.05

μm (Si

)

45%

65%

6.9

14.5

9.5

3.8

0.05

μm (Si

)

Fig. 18.18. Adhesive force and coeﬃcient of friction as a function of tip radius at several

humidities and as a function of relative humidity at several tip radii on (a) Si(100) and

(b)0.5-nm Z-DOL(BW) ﬁlms [35]

Figure 18.18b shows the adhesive forces as a function of tip radius and relative

humidity on Si(100) coated with 0.5nm-thick Z-DOL(BW) ﬁlm. Adhesive forces

for all the tips with the Z-DOL(BW)-lubricated sample are much lower than those

experiencedon unlubricated Si(100), shown in Fig. 18.18a. The data also show that,

even with a monolayer thickness of the lubricant, there is very little variation in

18 Nanoscale Boundary Lubrication Studies 985

adhesive forces with tip radius at a given humidity. For a given tip radius, the vari-

ation in adhesive forces with relative humidity indicates that these forces slightly

increase from RH 0% to RH 45%, but remain more or less the same with further

increases in humidity. This is seen even with the largest tip, which indicates that

the lubricant is indeed hydrophobic;there is some meniscus formation at humidities

higher than RH 0%, but the formation is very minimal and does not increase even

up to RH 65%. Figure 18.18b also shows the coeﬃcient of friction for various tips

at diﬀerent humidities for the Z-DOL(BW)-lubricated sample. Again, all the val-

ues obtained with the lubricated sample are much lower than the values obtained

on unlubricated Si(100), shown in Fig. 18.18a. The coeﬃcient of friction increases

with tip radius for all humidities, as was seen on unlubricated Si(100), due to an

increase in the contact area. Similarly to the adhesive forces, there is an increase in

friction from RH 0% to RH 45%, due to a contribution from an increasing number

of menisci bridges. However, thereafter very little additional water ﬁlm forms, due

to the hydrophobicityof the Z-DOL(BW) layer,and, consequentially, the coeﬃcient

of friction does not change, even with the largest tip. These ﬁndings show that even

a monolayer of Z-DOL(BW) oﬀers good hydrophobic performance of the surface.

18.4.6 Wear Study

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

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

ured, Fig. 18.19 [21]. As observedearlier, frictionforce and coeﬃcient of friction of

Z-15 is higher than that of Si(100), and Z-DOL(BW) has the lowest values. During

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

during the initial few cycles then remain constant. This is related to the removal of

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

friction show an increase during the initial few cycles and then approach higher and

stable values. This is believed to be caused by the attachment of the Z-15 molecules

to the tip. The molecular interaction betweenthese attached molecules on the tip and

molecules on the ﬁlm surface is responsible for an increase in the friction. However,

after severalscans, this molecular interaction reaches equilibrium, and after that, the

friction force and coeﬃcient of friction remain constant. In the case of Z-DOL(BW)

ﬁlm, the friction force and coeﬃcient of friction start out low and remain low dur-

ing the entire test for 100 cycles. This suggests that Z-DOL(BW) molecules do not

become attached or displaced as readily as Z-15.

Koinkar and Bhushan [20,34] conducted wear studies using a diamond tip at

high loads. Figure 18.20 shows the plots of wear depth as a function of normal

force, and Fig. 18.21 shows the wear proﬁles of the worn samples at 40 µN nor-

mal load. The 2.3nm-thick Z-DOL(BW)-lubricated sample exhibits better wear re-

sistance than unlubricated and 2.9nm-thick Z-15-lubricated silicon samples. Wear

resistance of a Z-15-lubricated sample is little better than that of the unlubricated

sample. The Z-15-lubricated sample shows debris inside the wear track. Since Z-15

is a liquid lubricant,debrisgenerated is held bythe lubricantand theybecome sticky.

986 Bharat Bhushan and Huiwen Liu

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. 18.19. Friction force and coeﬃcient of friction versus number of sliding cycles for

Si(100), 2.8-nm-thick Z-15 ﬁlm, and 2.3-nm-thick Z-DOL(BW) ﬁlm at 70nN, 0.8 µm/s, and

in ambient air. Schematic (bottom) shows that some liquid Z-15 molecules can be attached

on the tip. The molecular interaction between the attached molecules on the tip with the Z-15

molecules in the ﬁlm results in an increase of the friction force with multiple scanning [21]

18 Nanoscale Boundary Lubrication Studies 987

20 30 40 50 60 70

Si(100)

Z-DOL(BW)

Z-15

Wear depth (nm)

Normal load (

μN)

Fig. 18.20. Wear depth as

a function of normal load us-

ing a diamond tip for Si(100),

2.9-nm-thick Z-15 ﬁlm, and

2.3-nm-thick Z-DOL(BW)

after one cycle [20]

2.0

1.5

1.0

0.5

Si(100)

0.5

1.0

2.0

1.5

2.0

1.5

1.0

0.5

Z-15/Si(100)

0.5

1.0

2.0

1.5

2.0

1.5

1.0

0.5

Z-DOL(BW)/Si(100)

0.5

1.0

2.0

1.5

4μN

1cyc.

6.0 nm

4μN

1cyc.

6.0 nm

4μN

1cyc.

6.0 nm

Fig. 18.21. Wear proﬁles for

Si(100), 2.9-nm-thick Z-15

ﬁlm, and 2.3-nm-thick Z-

DOL(BW) ﬁlm after wear

studies using a diamond tip.

The normal force used and

wear depths are listed in the

ﬁgure [20]

988 Bharat Bhushan and Huiwen Liu

50 75 100

Z-DOL/Si(100)

BUW (2.3/1.7 nm)

BUW (1.0/2.0 nm)

BW (1.0 nm)

BW (2.3 nm)

Friction force (nN)

Number of cycles

Fig. 18.22. Friction force

as a function of number of

cycles using a Si

tip at

a normal load of 300 nN

for Z-DOL(BW) and Z-

DOL(BUW) ﬁlms with dif-

ferent ﬁlm thicknesses [34]

The debris moves inside the wear track and does damage, Fig. 18.20. These results

suggest that using Z-DOL(BW) can improve the wear resistance of the substrate.

To study the eﬀect of the degree of chemical bonding, the durability tests were

conducted on both fully bonded and partially bonded Z-DOL ﬁlms. Durability re-

sults for Z-DOL(BW) and Z-DOL bonded and unwashed (Z-DOL(BUW), a par-

tially bonded ﬁlm that contains both bonded and mobile phase lubricants) with

diﬀerent ﬁlm thicknesses are shown in Fig. 18.22 [34]. Thicker ﬁlms, such as

Z-DOL(BUW), with a thickness of 4.0nm (bonded/mobile = 2.3nm/1.7nm)ex-

hibit behavior similar to 2.3 nm-thick Z-DOL(BW) ﬁlm. Figure 18.22 also indicates

that Z-DOL(BW) and Z-DOL(BUW) ﬁlms with a thinner ﬁlm thickness exhibit

a higher friction value. Comparing 1.0 nm-thick Z-DOL(BW) with 3.0 nm-thick

Z-DOL(BUW) (bonded/mobile = 1.0nm/2.0nm), the Z-DOL(BUW) ﬁlm exhibits

a lower and stable friction value. This is because the mobile phase on the surface

acts as a source of lubricant replenishment. A similar conclusion has also been re-

ported by Ruhe et al. [19], Bhushan and Zhao [13], and Eapen et al. [49]. All of

them indicate that using partially bonded Z-DOL ﬁlms can dramatically reduce the

friction and improve the wear life.

18.5 Closure

Nanodeformationstudies haveshown thatfully bonded Z-DOL lubricants behaveas

polymer coatings, while the unbonded lubricants behave like liquids. AFM studies

have shown that the physisorbed nonpolar molecules on a solid surface have an

extended, ﬂat conformation. The spreading property of PFPE is strongly dependent

on the molecular end groups and substrate chemistry.

Using solid-like Z-DOL(BW) ﬁlm can reduce the friction and adhesion of

Si(100), while using mobile lubricant of Z-15 shows a negative eﬀect. Si(100) and

Z-15 ﬁlm show apparent time eﬀects. The friction and adhesion forces increase as

a result of growth of meniscus up to an equilibrium time, after which they remain

18 Nanoscale Boundary Lubrication Studies 989

constant. Using Z-DOL(BW) ﬁlm can prevent time eﬀects. High velocity leads to

the rupture of meniscus and prevents its reformation, which leads to a decrease of

friction and adhesive forces of Z-15 and Z-DOL(BW). The inﬂuence of relative hu-

midity on the friction and adhesion is dominated by the amount of the adsorbed

water molecules. Increasing humidity can either increase friction through increased

adhesion by the water meniscus, or reduce friction through an enhanced water-

lubricating eﬀect. Increasing temperature leads to desorption of the water layer,

decrease of the water surface tension, decrease of viscosity, and easier orientation

of the Z-DOL(BW) molecules. These changes cause a decrease of the friction force

and adhesion at high temperature. During cycling tests, the molecular interaction

between the Z-15 molecules attached to the tip and the Z-15 molecules on the ﬁlm

surface causes the initial rise of friction. Wear tests show that Z-DOL(BW) can im-

prove the wear resistance of silicon. Partially bonded PFPE ﬁlm appears to be more

durable than fully bonded ﬁlms.

These results suggest that partially/fully bonded ﬁlms are good lubricants for

micro/nanoscaledevicesoperatingin diﬀerentenvironmentsandunder varyingcon-

ditions.

References

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B. Bhushan (CRC, Boca Raton 2001) pp.1413–1513

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thick boundary layers of perﬂuoropolyether lubricants for magnetic thin-ﬁlm rigid disks,

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