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Friction and Wear Effects on a Micro/Nano-scale

E . Santner Bundesanstalt fur Materialforschung und -prufung (BAM), Berlin, Germany

Abstract

Tn this paper are described tribological effects which can be foicizd in micro-tribological systems, and in those macro-systems wliich can be niialysed 27y rnicro-inetliods, e.g., by atomic force microscopy ( A F M ) or related nretliods. Micru-tribo/og~j systeins liazic friction contacts with loads in the microlnairo-izewtoiz range and/or diineiisioiis in tlie iiiicro/nanoinetre range. Experiments on flie microliiano-scale slioiild be easier to ex- plain by tlzeoretical rnodelliq due to their sinipler system structiire. A n exaniple is dis- ciissed of adlzesioiz and friction irieasiireinents between AFM tips and clean, f lat, solid stirfaces in iiltra-lrigli vaciiiiiii, which shows some of tlie special aspects of inicroliiaizci- tribology. Szcrprisiiig friction characteristics on siirfaces with an artificial microstructure can be explained by skilled and carefiil topographical analysis of the frictioii path with an A F M . In micro-sensor con tacfs, 'single wear events' call he detected irsiiig AFM anahysis of tlie contact region. For ceramic compounds, different friction levels for the components of the mnterial can be foiind. Tlie problems, diffictilties, and daiigers of misinterpretation are also discussed.

Keywords

atomic force microscopy, inicro-tribology, naiio-tribology, friction effects, wear

INTRODUCTION

Friction and wear should have the same origin, independent of the scale of investigation. However, some effects found in micro/nano-tribology have no analogy in macro-tribology. Micro/nano sliding contacts are reported with very low friction and no wear. Theoretical models postulate the existence of 'supra- sliding' under certain system conditions. The success of micro-system technology, which is regarded as a key technology of the future with revolutionary im- plications for manufacturing and service industries, medicine, etc., will strongly depend on the existence of such low-wear and low-friction sliding contact cou- ples. This is one important reason for investigating such 'exotic' micro-contacts as atomic force microscope (AFM) tips on Au or Si single crystals in ultra-high

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Figure 1

- m C 0) m .- C 0 ._ c .- t

Friction force of ail \FFI.I tip on Au as a function of external

-25 -20 -15 -10 -5 0 5 10 15 20 External load (nN)

load'

vacuum (UHV). Micro-tribology will lead to explanations of tribological effects in technical systems and can be the basis for an understanding of friction and wear mechanisms from first principles.

ADHESION AND FRICTION OF DRY CONTACTS

The empirical Coulomb law of friction

F,=pF, or p=F,/F,

applied to micro/nano-contact friction can lead to nonsensical results. A friction experiment with an Si AFM tip on an Au single crystal in UHV shows a finite friction force F , even when the external load F , is zero. This means that according to the Coulomb friction law the friction coefficient p becomes infinite. In Figure 1 the results of a UHV friction experiment with decreasing load are shown. Every measuring point is the mean value of the friction signals for ten 7 pm long sliding scans of an AFM tip on a fresh area of an Au single-crystal surface.

From an external load of 17 nN down to zero, the friction falls. However, at zero load there is finite friction. With negative loads (i.e., the AFM tip is

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Figure 2 Force-distance curves for (a) an Si,N4 and (b) an Si tip on an Au single crystal in UHV

100

50

I

5 v

a 0 2 0 U

-50

-1 00

0 500 1000 Distance (nrn)

\ - I

300 nrn al 20 2

1

5 Y

0 U

0

-20

-150 -100 -50 0 50 100 150

Distance (nm)

brought into contact with the Au surface and then retracted with a defined force) there is also friction. The problem of the infinite friction coefficient is re- solved if one takes into consideration the attractive atomic interaction for the acting normal force F,. The JKR (Johnson-Kendall-Roberts) model takes this

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Figure 3 Force-distance curves for an Si tip (a) on a virgin A1 surface and (b) after tribological stressing

-200 0 200 400 600 800 1000 1200

Distance (nm)

effect into account and therefore fits the experiment data well, compared with a linear fit according to the Coulomb law of friction.'

The measuring point marked by a square at about -20 nN load in Figure 1 comes from a so-called 'force-distance' measurement and gives the force neces- sary to retract the AFM tip fully from the Au surface. This type of measurement

~~ ~

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can lead to an understanding of the contribution of adhesion to friction and its dependence on material properties, as long as this value is systematically measured as a function of, for example, binding energy, lattice type, and free surface energy. Force-distance (f-d) curves for an Si and an Si3N4 AFM tip on an Au single crystal are shown in Figure 2.

Force-distance diagrams show the deflection of the AFM cantilever, which is converted to a force value by multiplying the deflection by the spring constant of the cantilever, as a function of the distance of the undeflected cantilever from the AU surface. The high positive distances correspond to the approach of the cantilever and the tip to the Au surface, zero distance indicates contact without load, and negative distances correspond to indentation of the tip into the Au surface with a given load. The dip to negative force values in the retraction part of the curves, after the tip has been pressed on the Au surface with about 60 nN, is a measure of the adhesion.’ An initial interpretation of the curves would be that the adhesion of Si3N4 to Au is higher than that of Si. But the radius of the Si3N4 tip is much greater than that of the Si tip, and therefore the contact area is different. Another pair of f-d curves for an Si tip on A1 is shown in Figure 3. Figure 3(a) shows the interaction of the Si tip with a virgin A1 surface in UHV, and Figure 3(b) shows the same measurement after the A1 surface has been rubbed by the AFM tip. Whereas in the first case the adhesion is below -5 nN, an adhesion greater than -500 nN is measured when a friction test is performed before the f-d experiment. This behaviour can only be explained by assuming a transfer of A1 to the Si tip during the friction test. In this case, one will measure the adhesion of A1 to Al, which should be higher than that of Si to Al. However, the same type of measurements with an Si tip on mica results in identical f-d curves for virgin surfaces and for those tribologically stressed with the Si tip before the f-d experiment.

FRICTION ON AN ARTIFICIAL SURFACE STRUCTURE

To investigate the correlation between friction and roughness, an Si3N4 ball was rubbed over an artificial surface topography of known structure dimensions (roughness). In this case the structures of equidistant banks on Ti were pro- duced by lithography and had dimensions of 15 pm in width, 300 pm in length, and 0.4 pm in height. The Si3N4 ball was slid with a load of 0.25 N perpendicul- arly over the bank structures. The friction signal for this sliding, given in Figure 4 (overleaf), increases during the movement despite the nominally constant system parameters. The other striking effect of the friction signal is the equidistant dips, which indicate short friction decreases at these points. The distance between them corresponds to the distance of the bank structures. The numbered AFM topography images show the contact area corresponding to the respective part of the friction curve. Examination of these AFM images leads one to an

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SO Sni i t i i rv

Figure 4 Friction signal for an Si3N4 ball sliding over a Ti structure as a function of time, which is correlated with position by sliding velocity. The AFM images show the

wear scars along the sliding path Ti structure (15 x 300 x 0.4 pm), F,. = 0.25 N

I Time (s) ’ ‘!

a’ @ 6

~

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Figure 4 Friction signal for an Si N ball sliding over a Ti structure as a function of time, which is correlated with position by sliding velocity. The AFM images show the

wear scars along the sliding path Ti structure ( 1 5 x 300 x 0.4 pm), F, = 0.25 N

.? .‘f

explanation of the surprising evolution of the friction during one sliding path over the structures under identical conditions. Starting at AFM images (3) and (4), wear scratches can clearly be seen. They also indicate the sliding direction, from upper left to lower right in the images. The lighter stripes show the elevation of the surface structure. The scratches end before the lighter stripes and start again a little distance beyond them in images (3) and (41, except the deepest one (broadest black stripe). This explains the short drops in the friction. Every time the ball is pulled against a structure elevation the asperity peaks of the ball move out of their groove and no longer plough the Ti surface, and the frictional resistance against the sliding is reduced. In the course of the movement, the scratches become broader and deeper as the subsequent images demonstrate. In front of the asperity slopes of the ball, the Ti material is highly plastically de- formed and adheres to the ball. This process continues until the shearing forces on the adherent agglomerates become greater than the strength that the agglomerates can withstand. Then the agglomerated material is stripped and is trans- ferred back to the Ti surface, indicated by the white areas directly beside the black grooves and by the sudden reduction in their width (see images (8), (z), (9), (lo), and (11) in Figure 4). At the end of the sequence, the whole structure is destroyed and the friction signal is therefore high, and the falls are no longer evident.

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Figure 5 AFM images of the sliding path of a Pb-Ag slider on an electrical resistance coating of a position sensor after (a) 5 contacts and (b) 6 contacts

0 Reference point 10 pm

1 0 Reference point 10 prn

SING1 E WEAR EVENTS

Analysis of the path of a Pd-Ag slider3i4 on an electrical resistance coating of a position sensor with an AFM can indicate 'single wear events'. For this purpose, consecutive analyses of the same area of the sliding path have to be made with AFM. To find the same scanning area after each sliding cycle, a reference structure that is not changed by the sliding contact must be present in the AFM images. In Figure 5(a) such a reference point is shown by the arrow, and the horizontal line marks the area where, in the middle of the image, three 'hills' can

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Figure 6 Topography (a) and friction signal (b) of an Sic-TiB2 ceramic compound

0 0

10

5 10 pm

0 0 5 10 pm

be seen. These ‘hills’ have disappeared after one more contact of the slider, as demonstrated in Figure 5(b). At the position where the middle hill was, a hole has now been created. The resistance coating contains Ti02 particles: one of them has been ruptured by the slider, whereas the others have been sheared away, due to a single slider contact.

FRICTION CONTRASTS

Ceramic compounds have been developed at BAM, Germany, with the aim of minimising their friction properties. One of these sintered ceramic compounds

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Figure 7 AFM topographical measurement of TiN droplets. The structures visible are images of the cantilever and the AFM tip

0 0

is Sic-TiB,. The topography and the friction against an Si3N4 AFM tip are shown in Figure 6 . The topographic image shows clearly the two material components. These two components are also visible in the 'friction image' which is the result of an AFM scan in the 'lateral force mode' (LFM) over the same surface area as the topographic image. In the friction image, in addition to the components, the grain boundaries of each can be seen. The contrast in the friction image is caused by differences in the friction level of the material components and structures. Such friction differences in the microstructure may also be the reason for friction signal fluctuations in technical sliding contacts, because the micro- contact situation changes rapidly during sliding.

ARTEFACTS AND OTHER PROBLEMS

The contrast in the topographic and friction images corresponds to definite cantilever deflection signals due to surface structures and friction differences. This deflection of the APM cantilever is converted to a force by the spring constant of the cantilever. The measurement and calculation of this spring constant, however, is quite difficult, because all dimensions of the cantilever, which are in the inicrometre or sub-micrometre range, must be known very precisely for each cantilever. This is especially a problem for the spring constant for drilling of the cantilever for friction measurement. Therefore, one can measure differences of the friction force quite well but it is, in general, not presently possible to give ab- solute values for friction. Another problem arises from the detecting principle,

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using the deflections of a laser beam and measuring the light intensity with seg- mented photodetectors. The laser beam may not always point at the same position on the cantilever, it may not be exactly circular, and cross-talk of tlie topography signal over the friction signal may occur. The consequence of this is that one must be very careful in tlie interpretation of AFM/LFM images. An example of an AFM artefact is shown in Figure 7, which is the scan result over a TiN coating with droplets.

CONCLUSIONS

Atomic force microscopy and its derivatives offer exciting new investigation methods for measuring basic friction and wear effects, which could be valuable for the investigation of both the fundamentals of friction and wear, and the tribology of micro/nano-systems. A highly developed, and developing, industry will benefit from this, because these methods of investigation can help in the continuous development and improvement of many products - whether in micro- or macro-technology - by a systematic a p p r ~ a c h . ~ , ~

For fundamental research, micro-tribology poses some questions regard- ing definitions in tribology. For example, one can regard the AFM tip as a single sliding asperity for which, in the LFM, the friction may be measured. But if this asperity slides against a steep-sided asperity on the counter-body, the bending force on the tip which is taken as tlie measure of friction is not tlie tangential force acting against the movement, but contains components of the normal force. This is not equivalent to macroscopic tribological contacts. Maybe ad- vanced macro-tribological contacts of the future will be atomically flat and there will be no problems, but it may be that the problems will be nevertheless present, but on another scale.

References

1. Polaczcyk, C., Schneider, T., Schiifer, J., and Santner, E., 'Microtribological behaviour of Au(001) studied by

2. Klol3, H., nnd Santner, E., 'Micrntribology and w e x modelling', /'rot-. 1111. Workshop Mi.r./mifiin/ B ~ W J I I J J ~ I $ P V / I

3. Santner, E., Polaczyk, Ch., Schneider, Th., and Schnfer, J., 'Mikrotribologie Analyst. \'on Reibsystemen auf ele-

4. Schnfer, J., and Santner, E., 'Quantitative w e x andysis using atomic force microscopy', Wcnr, 222 (1998) 74-H3. 5. Bhushan, B., Prij ir i l iks n ~ ~ d App/~:izftori if TriMqgy, Wiley, New York, 1999. 6. Bhushm, EL, Trih/os!/ Issiics nrrd Oppir~ i i~~ i1J i~s 111 MEMS, Kluwer Academic, Dordrecht, 1998.

AFM/LFM', Si~rf. Sri., 402-404 (1998) 454-8.

C i i n t ~ ~ i Mnh'r inls, ed. H. Dettel, S. Hogmark, and J.v. Stebut, Holzhau/Erzgeb., 13-17 October 1997, pp. 253-61.

mentme Prozesse', Mn1i 'r in/~ir i i~ir~i~, 39, 7/8 (1997) 293-6.

This paper was first presented at the 12th International Colloquium on Tribology, Technische Akademie Esslingen, Germmy.

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