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

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lubricant thickness at that point. (The measured thickness is about 2 nm larger than the actual thickness

due to a thin layer of lubricant wetting the tip, Mate et al., 1990). When the sample is withdrawn, the

forces on the tip slowly decrease to zero as a long meniscus of liquid is drawn out from the surface.

Particulate disks were mapped by Mate et al. (1990) and Bhushan and Blackman (1991). The distri-

bution of lubricant across an asperity was mapped by collecting force-vs.-distance curves with the AFM

in a line across the surface. A particulate disk was coated with nominally 20 to 30 nm of lubricant, but

by AFM the average thickness was 2.6 ± 1.2 nm. (The large standard deviation reﬂects the huge variation

of lubricant thickness across the disk.) A large percentage of the lubricant is expected to reside below the

FIGURE 14.37 Critical number of cycles (above which wear increases rapidly) as a function of thickness of sputtered

carbon coatings on magnetic disks at 20 µN load.(From Bhushan, B. and Koinkar, V.N., 1995, Surf. Coat. Technol.

76–77, 655–669. With permission.)

FIGURE 14.38 Wear depth as a function of number of cycles at 60 µN load for uncoated single-crystal silicon and

20-nm-thick coatings deposited by sputtering, ion beam, and cathodic arc carbon processes. (From Koinkar, V.N.

and Bhushan, B., 1997, Proc. Inst. Mech. Eng. Part J: J. Eng. Tribol. 211, 365–372. With permission.)

FIGURE 14.39 Wear depth as a function of normal

load for three tapes in the parallel direction after one

cycle. (From Bhushan, B. and Koinkar, V.N., 1995,

Wear 181–183, 360–370. With permission.)

FIGURE 14.40 Wear depth as a function of number of cycles for (a) MP, (b) BaFe, and (c) ME tapes in different

regions at normal loads indicated in the ﬁgure. Note a higher load used for the ME tape in (c). (From Bhushan, B.

and Koinkar, V.N., 1995, Wear 181–183, 360–370. With permission.)

surface in the pores. In Figure 14.60, we show histograms of lubricant thickness across three regions on

the disk. The light part of the bar represents the hard wall of the substrate and the dark part of the top

is the thickness of the lubricant. Each point on the histogram is from a single force-vs.-distance mea-

surement, with points separated by 25-nm steps. The lubricant is not evenly distributed across the surface.

In regions 1 and 2 there is more than twice as much lubricant than there is on the asperity (region 3).

There are some points on the top and the side of the asperity which have no lubricant coating at all

(Bhushan and Blackman, 1991).

14.8.3 Boundary Lubrication Studies

To study lubricant depletion during microscale measurements, Koinkar and Bhushan (1996b) conducted

nanowear studies using Si

tips. They measured friction as a function of number of cycles for virgin

silicon and silicon surface lubricated with Z-15 and Z-Dol lubricants, Figure 14.61. An area of 1 × 1 µm

was scanned at a normal force of 300 nN. Note that the friction force in a virgin silicon surface decreases

in a few cycles after the natural oxide ﬁlm present on silicon surface gets removed. In the case of Z-15-

coated silicon sample, the friction force starts out to be low and then approaches that of an unlubricated

silicon surface after a few cycles. The increase in friction of the lubricated sample suggests that the

lubricant ﬁlm gets worn and the silicon underneath is exposed. In the case of the Z-Dol-coated silicon

sample, the friction force starts out to be low and remains low during the 100-cycle test. It suggest that

Z-Dol does not get displaced/depleted as readily as Z-15. (Also see Bhushan et al., 1995g.) Microwear

studies were also conducted using the diamond tip at various loads. Figure 14.62 shows the plots of wear

depth as a function of normal load and Figure 14.63 shows the wear proﬁles of worn samples at 40 µN

normal load. The Z-Dol lubricated sample exhibits better wear resistance than unlubricated and Z-15-

lubricated silicon samples. Wear resistance of the Z-15-lubricated sample is little better than that of the

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

a liquid lubricant, debris generated is held by the lubricant and it becomes sticky which moves inside

the wear track and does damage, Figure 14.63.

14.9 Closure

AFM/FFM have been successfully used for measurements of surface roughness, friction, adhesion,

scratching, wear, indentation, detection of material transfer, and lubrication on micro- to nanoscales.

Commonly measured roughness parameters are scale dependent, requiring the need of scale-independent

fractal parameters to characterize surface roughness. A generalized fractal analysis is presented which

allows the characterization of surface roughness by two scale-independent parameters. Measurements of

nanoscale friction of a freshly cleaved, highly oriented pyrolytic graphite exhibited the same periodicity

as that of corresponding topography. However, the peaks in friction and those in corresponding topog-

raphy were displaced relative to each other. Variations in atomic-scale friction and the observed displace-

ment can be explained by the variations in interatomic forces in the normal and lateral directions. Local

variation in microscale friction force is found to correspond to the local surface slope, suggesting that a

ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both

micro- and macroscales which results from the surface preparation and asymmetrical asperities on the

surface. Microscale friction is generally found to be smaller than the macroscale friction as there is less

plowing contribution in microscale measurements.

FIGURE 14.41 Surface maps showing the worn region (center 2 × 2 µm) after various cycles of wear at (a) 2 µN for

MP (particulate region) and at (b) 20 µN for ME (H = 3.4 GPa, parallel direction) tapes. Note a different vertical

scale for the bottom proﬁle of (b). (From Bhushan, B. and Koinkar, V.N., 1995, Wear 181–183, 360–370. With

permission.)

Wear rates for particulate magnetic tapes, polyester tape substrates, and single-crystal silicon are

approximately constant for various loads and test durations. However, for magnetic disks and magnetic

tapes with a multilayered thin-ﬁlm structure, the wear of the DLC overcoat in the case of disks and

magnetic layer in the case of tapes is catastrophic. Breakdown of thin ﬁlms can be detected with AFM.

Evolution of the wear has also been studied using AFM. We ﬁnd that the wear is initiated at nanoscratches.

Amorphous carbon ﬁlms as thin as 5 nm are deposited as continuous ﬁlms and exhibit some wear life.

Wear life increases with an increase in ﬁlm thickness. Carbon coatings deposited by cathodic arc process

are superior in wear and mechanical properties followed by ion beam and sputtering processes. AFM

has been modiﬁed for nanoindentation hardness measurements with depth of indentation as low as

1 nm. Scratching and indentation on nanoscales are powerful ways of evaluation of the mechanical

integrity of ultrathin ﬁlms. Detection of material transfer on a nanoscale is possible with AFM.

Measurement of lubricant ﬁlm thickness with a lateral resolution on a nanoscale and boundary

lubrication studies have been conducted using AFM. AFM/FFM friction experiments show that lubricants

with polar (reactive) end groups dramatically increase the load or contact pressure that a liquid ﬁlm can

support before solid–solid contact and thus exhibit long durability.

Friction and wear on micro- and nanoscales have been found to be generally smaller compared with

that at macroscales. Therefore, micro/nantribological studies helps deﬁne regimes for ultralow friction

and zero wear.

FIGURE 14.42 Wear depth as a function of normal load (after one cycle) for a PET ﬁlm. (From Bhushan, B. and

Koinkar, V.N., 1995, Tribol. Trans. 38, 119–127. With permission.)

FIGURE 14.43 Wear depth as a function of number of cycles at 1 µN for a PET ﬁlm. (From Bhushan, B. and

Koinkar, V.N., 1995, Tribol. Trans. 38, 119–127. With permission.)

FIGURE 14.44 Surface maps showing the worn region (center 2 × 2 µm) at 1 µN for a PET ﬁlm (a) in the polymer

region, and (b) in the particulate region. The number of cycles are indicated in the ﬁgure. (From Bhushan, B. and

Koinkar, V.N., 1995, Tribol. Trans. 38, 119–127. With permission.)

FIGURE 14.45 Surface maps showing the

worn region (center 2 × 2 µm) after one cycle

of wear at 40 µN load (a) Si(111), (b) PECVD-

oxide-coated Si(111), (c) dry-oxidized Si(111),

and (d) C

-implanted Si(111). (From Bhus-

han, B. and Koinkar, V.N., 1994, J. Appl. Phys.

75, 5741–5746. With permission.)

FIGURE 14.46 Wear depth as a function of (a) normal load (after one cycle), and (b) number of cycles (normal

load = 40 µN) for Si(111) and C

-implanted Si(111). (From Bhushan, B. and Koinkar, V.N., 1994, J. Appl. Phys. 75,

5741–5746. With permission.)

FIGURE 14.47 Tip deﬂection (normal force) as a function of Z (separation distance) curve for an MP tape. The

spring constant of the cantilever used was 0.4 N/m. (From Bhushan, B. and Ruan, J., 1994, ASME J. Tribol. 116,

389–396. With permission.)

FIGURE 14.48 Tip deﬂection (normal force) as a function of Z (separation distance) curve for (a) unlubricated

and (b) lubricated textured thin-ﬁlm rigid disks. The pull-off force is 42 nN for the unlubricated disk and 64 nN

for the lubricated disk calculated from the horizontal distance between points C and D and the cantilever spring

constant of 0.4 N/m. (From Bhushan, B. and Ruan, J., 1994, ASME J. Tribol. 116, 389–396. With permission.)

FIGURE 14.49 (a) Gray-scale plots and (b) line plots of inverted images of indentation marks on the as-received

Si(111) sample at various normal loads. Loads, indentation depths, and hardness values are listed in the ﬁgure. (From

Bhushan, B. and Koinkar, V.N., 1994, Appl. Phys. Lett. 64, 1653–1655. With permission.)