Bhushan B. Handbook of Micro/Nano Tribology, Second Edition

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FIGURE 3.27 Diagram of UHV tribometer by Gellman (1992). The sample mounted to the tribometer is ≈1 cm

in diameter. The support bar is mounted onto a liquid nitrogen reservoir. (From Gellman, A.J. (1992), J. Vac. Sci.

Tec hol. A 10, 180–187. With permission.)

FIGURE 3.28 Adhesion coefﬁcient vs. sulfur coverage by McFadden and Gellman (1995a). Experimental conditions:

temperature = 300 K; normal force = 40–60 mN; contact time before separation = 15–20 s; separation speed = 50 µm/s.

At each coverage the standard deviation of the adhesion coefﬁcient calculated from ten measurements is plotted. (From

McFadden, C.F. and Gellman, A.J. (1995), Tribol. Lett. 1, 201–210. With permission.)

We conclude this discussion with some TDS and IRS studies of ﬂuorinated hydrocarbons on Cu(111)

surfaces by Meyers et al. (1996). Fluorinated hydrocarbons are of interest because of their thermal stability

and, therefore, are useful for higher-temperature lubrication. All of the ethers were found to adsorb

molecularly and reversibly on the Cu(111) surfaces, exhibiting ﬁrst-order desorption kinetics. These

studies found that ﬂuorination of perﬂuoropolyalkyl ethers (PFPAE) reduced the desorption energy. The

general reason for this behavior was felt to be that bonding occurred with the oxygens on the PFPAEs.

Fluorine is thought to weaken this interaction. In addition, ﬂuorination changed the adsorbate–adsorbate

FIGURE 3.29 AES spectrum of air-exposed copper (111)

surface reintroduced to vacuum chamber and annealed to

1000 K to remove weakly adsorbed species showing the

primary contaminants sulfur and carbon. (From McFad-

den, C. F. and Gellman, A. J. (1995), Tribol. Lett. 1, 201–210.

With permission.)

FIGURE 3.30 Static friction coefﬁcient vs. Auger peak

ratios as the contaminated copper (111) surfaces are

gradually cleaned by ion bombardment. For each con-

tamination level the average and standard deviation of

the friction coefﬁcient calculated from ten measurements

are presented. (From McFadden, C. F. and Gellman, A. J.

(1995), Tribol. Lett. 1, 201–210. With permission.)

interaction from repulsive to attractive. FTIR studies shows that the diethyl ethers are oriented with their

molecular axes parallel to the Cu(111) surface.

We conclude this section with some elegant experiments performed by Krim et al. (Sokoloff et al.,

1993; Mak et al., 1994; Krim et al., 1995; Daly and Krim, 1996) for which Widom and Krim (1986; also

Krim and Widom, 1988) originally adapted an apparatus for sliding monolayer and thicker physisorbed

FIGURE 3.31 The effects of oxygen and chlorine adsorption on static friction for a metal–metal contact as a function

of coverage. (From Wheeler, D. R. (1976), J. Appl. Phys. 47, 1123–1130. With permission.)

FIGURE 3.32 Friction force vs. time for three types

of sliding behavior observed during shearing of inter-

faces between ethanol-charged nickel (100) surfaces, by

Gellman (1992). The loads on the interface are all

10 mN at the beginning of each trace and shearing

begins at the points marked with the arrows. Temper-

ature = 120 K, load = 10 mN, velocity = 10 µm/s. (From

Gellman, A.J. (1992), J. Vac. Sci. Techol. A 10, 180–187.

With permission.)

ﬁlms on silver substrates. The true contact area is known in these studies and there is no wear of the

surfaces, which are silver deposited with a (111) face on a quartz crystal microbalance. These experiments

were designed to investigate the nature of the friction forces. Slipping of physisorbed ﬁlms, e.g., ethane

and ethylene, on the silver surfaces causes shifts in the frequency and amplitude of the quartz crystal

vibrations, and ﬁlm characteristic slip times are obtained from shifts in the quality factor, Q, of the

oscillator circuit. Friction results from energy losses due to the interactions at an interface. Typically, it

is assumed that these losses occur through phonons, vibrations of the lattice which dissipate the energy

(Sokoloff et al., 1993), although recently Persson (1993), followed by Sokoloff (1995), demonstrated that

electronic excitations could also be an energy dissipation mechanism. In these experiments the average

friction force per unit area is related to the slip time by

(3.17)

where F

is the friction force, ρ and ν are the ﬁlm density and velocity, respectively, and τ is the slip time.

Therefore, a longer slip time implies a smaller friction force. Based on electronic effects alone (Persson,

1994) ethane on silver is expected to have a longer slip time than ethylene and thus a higher friction.

Krim’s results are consistent with this prediction, although the ability of the theory to predict hard

numbers is limited. Monolayer oxygen adsorption on the surface increases the slip time and thus lowers

the friction coefﬁcient substantially, consistent with the electronic excitation model. Although these

experiments are not completely deﬁnitive at this time, they open new areas of investigation for probing

the little-understood energy loss mechanisms in friction.

3.6.3 Atomic Effects in Metal–Insulator Contacts

We now discuss a number of results where monolayer effects have been observed in metal–insulator

friction experiments performed by Pepper (1976, 1979, 1982). First, we discuss static friction experiments

performed with a copper ball on the basal plane of a diamond ﬂat. The diamond surface is known to be

terminated by hydrogen atoms. Pepper (1982) found that the hydrogen could be removed by either

FIGURE 3.33 Coefﬁcients of friction measured at interfaces between nickel (100)-c(2×2)–S surface modiﬁed by

adsorption of ethanol at coverages from 0 to 300 ML. (From Gellman, A. J. (1992), J. Vac. Sci. Technol. A 10, 180–187.

With permission.)

=ρν τ

electron bombardment or heating in ultrahigh vacuum. Following removal of the hydrogen, the LEED

pattern for the surface changed from a 1 × 1 to 1 × 2 pattern. The interesting feature is shown in the

static friction results for the two situations given in Figure 3.34. We can see that removal of the hydrogen

caused an increase in µ

. Pepper found that exposing the surface to excited hydrogen could cause

readsorption. Following hydrogen readsorption the static friction was again reduced, although not to its

original value. Pepper also performed core-level ionization energy loss spectroscopy on the diamond

surface and found changes in the electronic structure of the valence band. Therefore, these experiments

showed results sensitive to the electronic structure of the surface.

In Figure 3.35 we show similar studies performed by Pepper (1979) for copper and nickel balls on the

basil plane of sapphire with chlorine and oxygen as adsorbates. Again the balls were cleaned by sputtering

and the sapphire was cleaned by heating as veriﬁed by AES. In both cases, we see that oxygen increased

the interfacial friction and chlorine decreased it compared to the clean metal. For the increased shear

strength case, one cannot identify the locus of failure, whether at the interface or in the bulk of one of

the materials. Unfortunately, the wear scar area was too small to analyze. In any event, again monolayer

adsorption was detectable in friction experiments. Perhaps even more surprising were effects shown in

Figure 3.36. Here dynamic friction experiments were performed by Pepper (1976) with a polycrystalline

sapphire pin on a polycrystalline iron disk in ultrahigh vacuum. We see the same behavior reproduced

with dynamic friction. After removing the oxygen or chlorine atmospheres, the friction coefﬁcients

returned to their original values.

FIGURE 3.34 Copper–diamond static friction coefﬁcient as a function of diamond-annealing temperature. (From

Pepper, S. V. (1982), J. Vac. Sci. Technol. 20, 643–646. With permission.)

3.7 Concluding Remarks

We have attempted to present a description of issues and techniques in the physics of surfaces of interest

in nanotribology. This ﬁeld of research is exceedingly difﬁcult with the effects of a wide variety of materials

and phenomena to understand. It is further complicated by the inability to observe the interactions

directly, resulting in conclusions from inference. It is clear that there is a great deal of research necessary

to obtain a comprehensive understanding of tribology at the nanoscale. It is hoped that the demonstra-

tions in this chapter and in this volume indicate that there is the possibility of gaining rational under-

standing leading to the design of better materials in this technologically important ﬁeld.

FIGURE 3.35 Static friction coefﬁcient after exposure to gas (µ

) ratio to static friction coefﬁcient of clean contact

(µ

), plotted vs. exposure to oxygen or chlorine for (A) nickel and (B) copper. (From Pepper, S. V. (1979), J. Appl.

Phys. 50, 8062–8065. With permission.)

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