Wear, 168 (1993) 1-5

Microtribology today and tomorrow

R. Kaneko Kaneko Laboratory, NTT Interdisciplinary Research Laboratories, Midori-cho, Musashino-shi, Tokyo 180 (Japan)

(Received September 16, 1992; accepted October 15, 1992)

Abstract

Very lightweight sliding parts are beginning to be used in magnetic recording and micromechanical systems which require wear rates that are almost zero. The wear of sliding surfaces subjected to light loads is primarily the result of surface interaction forces rather than of load or weight. New tools, such as scanning probe microscopes (SPMs) and computer simulation of molecular dynamics, have advanced our scientific understanding of micro- tribology. The SPM is also used to evaluate the microwear process and solid surface properties. The SPM and a surface force apparatus have also been used to investigate very thin liquid lubricant films on solid surfaces, Microtribology is expanding in many areas but it remains a somewhat haphazard field of knowledge. Almost all practical sliding surfaces have defects, damaged layers, and contaminants - i.e. they are neither well defined nor homogeneous. Untangling the complicated phenomena related to microtribology will require much more knowledge in both engineering and basic science.

1. What is ‘microtribology’

Although microtribology is commanding attention in research fields ranging from basic physics to industrial production, researchers and engineers have images of microtribology that differ according to each interest of function. This is partly because microtribology is still in its infancy and is not established as a generalized concept.

It was magnetic recording technology which brought about microtribology. The partnership between tribology and magnetic recording has a half-century history of progress achieved by ‘gold panning’ engineering data rather than by scientific study. Klaus and Bhushan mentioned in their 1985 review that most magnetic media systems are lubricated by lubrication technology which was developed by an ‘Edisonian’ process [l]. Wear of l-10 nm must be considered in high density magnetic recording. Also the atomic- or molecular-level properties of surfaces will affect the friction and wear on this scale. Considering the lack of scientific knowledge in these fields, I proposed the specific study of ‘mi- crotribology’ in 1986 [2]. At the same time, my co- workers and I started to evaluate magnetic recording surfaces using a scanning probe microscope (SPM) [3]. However, our data obtained from experiments on sur- faces with many defects and contaminants were difficult to understand. The surfaces were far from well defined and the data were confusing and seemingly random. We soon realized that physics and chemistry was nec-

essary to interpret the data but we had very little knowledge about physical and chemical phenomena on a microtribological scale.

A film head for magnetic recording has a weight of the order of 1 pg and a motor rotor in a micromechanical system weighs less than 1 pg. As such very lightweight sliding parts begin to be applied in ‘real world’ systems, the commonsense view that the role of a bearing is to support a load no longer holds: the wear on micro- tribological sliding surfaces is primarily the result of surface interaction forces instead of the result of the load [4]. The head-media interface of floppy disk drives must be good for more than 10 million revolutions without error. This means that a high-recording-density floppy disk must wear at a rate of one atomic layer per 10-100 km of sliding range. The same, almost no- wear condition is demanded of hard disks. Although it will of course be extremely difficult to make practical devices that are perfectly wear resistant, I think that this should be the final goal of microtribological en- gineering.

In conventional tribology, the friction between solid surfaces frequently is a result of surface destruction. Because rapid, deep wear occurs when heavy loads and large masses act on solid surfaces, the properties of bulk materials are important in these cases. In contrast, under light loads and small masses, the physical and chemical properties of the surfaces are the primary factors. The wear mechanism under very light loads is the critical subject in microtribology. If we can control

0043-1648/93/$6.00 0 1993 - Elsevier Sequoia. All rights reserved

R. Kaneko J Microtribology today and tomorrow

Tribolosv

Conventional Micro

Bulk Material ---- Surface

Heavy Load - Light Load

Large Mass - Small Mass

7 a Wear No Wear

(Inevitable) Fig. 1. Conventional tribology and microtribology.

the surface interaction forces under these conditions, the wear rate will be drastically decreased. Micro- tribology is compared with conventional tribology in Fig. 1.

A flagellum of a certain bacterium rotates at over 10 000 rev min-’ completely without wear [5,6]. Al- though the bearing mechanism of the flagellum is still unclear, because we know little about biomicrotribology, I expect that we will be able to learn new microtri- bological systems from biology.

Summarizing what I have mentioned so far, I would like to propose the following concepts for microtribology:

(1) the final goal of microtribology is to create practical no-wear devices;

(2) very light loads can be used in microtribological systems;

(3) surface properties are important for microtri- bology;

(4) not only engineering but also surface science is important for understanding microtribology;

(5) new knowledge will be obtained from biology. Microtribology treats phenomena on scales ranging

from Qngrstriims to the bulk material, so we can approach the ideal ‘no-wear’ condition from the direction of both atomic-level physics and from that of conventional engineering. Microtribology thus includes both molec- ular tribology and atomic-scale tribology.

2. Approach from physics point of view

The physics of solids has supported the progress of electronics and physicists are now extending this field to the physics of surfaces. The invention of SPMs, such as a scanning tunnelling microscope (STM) [7] and an atomic force microscope (AFM) [8], has advanced our knowledge about the physics of surfaces. An SPM is used not only to observe surfaces but also to manipulate individual atoms and modify surfaces at the atomic

level. Using xenon atoms, Eigler and Schweizer drew the letters of ‘IBM’ [9]. The force which removes atoms from a surface is the origin of friction and wear and an SPM is a powerful tool for exploring these forces. Mate et al. [lo] used a frictional force microscope (FFM) to observe the atomic-scale friction between a tungsten tip and a graphite surface.

The atomic-level analysis of surfaces has also been progressing. Burnham et al. [ll] showed that forces between two bodies can be due to patch charges. Computer simulation with molecular dynamics can be used to trace the movement of each atom between contacting or sliding surfaces. Landmann et al. [12] simulated the atomic wetting and fracturing that occurs when a nickel tip contacts and then detaches from a gold surface. In addition, Hirano and Shinjo [13] in- dicated the existence of a sliding condition that generates no frictional force.

The improvements in experimental techniques and analyses have advanced the study of atomic-level mi- crotribology on well-defined solid surfaces but it is difficult to apply these studies directly to ‘real world’ surfaces. Even a single crystal in the real world has a strength which is 10e4 to lo-’ times less than that of the perfect crystal, because real crystals have many dislocations or cracks, or both. Furthermore, many materials in the real world are polycrystalline or amor- phous, and almost all practical sliding surfaces are contaminated, not well defined and not homogeneous. The surface science constructed in the ideal world is far from the microtribology of the real world. Therefore, it is important for microtribology to blaze the trail between science and engineering.

The case of gas film lubrication analysis is much closer to the real world. The dynamics of molecules in the gas phase molecules can be almost completely expressed with equations, because gas molecules have no defects and are almost homogeneous. In gas film lubrication under submicrometre or less clearances, however, flows in the gas films cannot be described by a continuum flow model. Therefore, instead of the conventional Reynolds equation, a generalized lubri- cation equation based on the Boltzmann equation was introduced as an exact solution [14]. A method for rapidly calculating this generalized lubrication equation was also developed [15] and this equation has been used in designing practical bearings.

3. Microwear process and properties of solid surfaces

Figure 2 shows wear marks on a silicon-containing and fluorinated lO%Si-C surface [16]. These wear marks were made by repeated scanning scratches with a dia-

R. Kaneko / Microtn’bology today and tomorrow 3

(4

Fig. 2. Scanning-scratching wear marks on fluorinated lO%Si-C [16]. The tip radius was 0.1 urn, the load was 20 PN and the feed was 10 nm: (a) 1 cycle; (b) 6 cycles; (c) 36 cycles.

mond tip of radius 0.1 pm. One scanning-scratch con- sisted of 100 line-scratches with a span of 1 pm and feed of 10 nm under a load of 20 pN. One to two

atomic layers of the surface were scooped out in each scanning-scratch and the wear particles around the wear marks were easily removed by an additional scanning- scratch. Destruction of the surface was the main wear process in this case. Figure 3 shows the wear marks produced when the same scanning-scratching method was used on a polycarbonate surface. In this case, however, the load was 200 nN. The surface first formed a plateau-like upheaval, then formed projections and, finally, was worn down. The wear marks of the first and second stages show the plastic deformation that led to destruction [17,18].

Although SPMs are beginning to be used to evaluate very thin layers of solid surfaces, very little data have been obtained so far. Furthermore, almost solid surfaces in air adsorb water and are contaminated by organic materials. Surface finishings, such as polishing, grinding, cutting and etching, produce surface layers whose prop- erties differ from those of the bulk material. We have to use such complicated surfaces as the ‘real’ sliding surfaces. There is no easy way to understand the wear mechanisms occurring on ‘real’ surfaces. Much more data and many more discussions will be needed.

4. Very thin liquid lubricant films on solid surfaces

The ideal state of a monolayer lubricant is one in which part of the lubricant molecule adsorbs onto the solid surface and the rest of the molecule is free to slide on the surface. To adsorb onto a solid surface, a molecule must contain reactive functional groups with a low ionization potential or groups with interactive electrons. An STM was used to observe the configu- ration, adsorption and mobility of several types of lubricant molecule with benzene rings on highly oriented pyrolitic graphite (HOPG) and molybdenum disulphide (MO&) [19]. A 1 u b ricant molecule with a benzene ring lying flat on HOPG was anchored but there was no anchoring effect when the benzene rings were inclined (Fig. 4). A benzene ring did not anchor on an MoS, surface but the sulphur atom of diphenylsulphone did (Fig. 5). The inert long chains (Rf) were free to shift across the HOPG and MoS, surfaces, which is indicative of low frictional characteristics.

The STM imaging technique can be used to evaluate the fundamental properties of lubricants on a well- defined surface. However, a ‘real’ sliding surface has many defects and is contaminated. This makes the adsorption mechanism complicated. A recent study [20] showed that water molecules are held tightly to defects on an MoS, surface. Another study [21] showed that the oxygen atom of the diphenylsulphone group anchors onto a sputtered carbon surface. This study also sug- gested that adsorbed water may contribute to this

:I K. h’aneko I Microtribology today and tomorrow

Fig. 3. Scanning-scratching wear marks on polycarbonate The tip radius was 0.1 pm, the load was 200 nN and the was 10 nm: (a) 1 cycle; (b) 2 cycles; {c) 8 cycles.

WI. feed

anchoring effect. These studies imply that the adsorption mechanism of a lubricant on a practical solid surface differs from that on a well-defined, defect-free surface.

nm 2r

v...., : 1st Scan

- : 2nd Scan

3rd Scan

Benzene rings

F a 0 F nm

F-G-F H W F-G-F

i I

F

/F F\

Rf =F G-G-O

Fig. 4. Molecular shift (on HOPF) of a lubricant with benzene rings [19].

The ST&I technique used to observe molecules on a solid surface still has problems in assigning atoms and molecules. This is because the tunnelling mechanism from a tip to a solid surface, through the molecule on

the solid surface, is unclear. Furthermore, the defects and ~ntami~ants on a ‘real’ surface make it difkult to solve this problem. We need much more knowledge.

Liquid films can be defined better than can soEd surfaces. Israelachvili [22] used a surface forces ap- paratus (SPA) to measure the adhesion forces and frictional forces between surfaces in very thin liquid films, Ultrathin films often behave more like a solid than like a normal liquid.

5. Extension of mierotriboiogy

The final goal of rni~t~~l~ is to achieve (no- wear’ conditions. However, in contrast to this, micro- tribology can also be extended to atomic-level and nanometre-level machining and modification. The scratch marks shown in Fig. 2 are the result of wear testing but this experiment also shows the possibility of mi~roma~hin~g. SPMs are being used in fundamental studies of ultrahigh density recording. For example, pits 0.3 pm in diameter were made on an organic dye

R. Kaneko I Microtribology today and tomorrow 5

nm ,

I -.

k-

1.6 --- : 1st Scan

,r _‘b

- : 2nd Scan --_-( ‘>-_.

- : 3rd Scan /’ i

0.8 -

0.4 0.8 1.2 1.6 nm 2

: i

Rf =F

F

Fig. 5. Molecular shift (on MO&) of a lubricant with diphen- 9

ylsulphone group [19]. 10

surface with very short (2 ns) pulses [23]. Electric charge recording with a resolution of less than 0.1 pm was demonstrated by Barrett and Quate [24]. Point magnetic recording has been used for submicrometre magnetizing and its detection [25]. These recording methods are applications of local surface modification and micro- tribology is an important technology for improving these ultrahigh density recording techniques.

6. Microtribology tomorrow

Tribology needs information from a wide area of science and technology. Conventional tribology is based on these kinds of information but, as pointed out in this paper, we have very little of the kinds of knowledge needed for microtribology. We must obtain much more data if we are to untangle the complicated microtri-

bological phenomena. We need not only engineering data but also basic physical and chemical knowledge.

Microtribology, which is closely related to the tech- nologies of data storage devices and micromechanical systems, has become a subject of attention for scientists. It is a field that is developing in many areas and that is now an interdisciplinary activity. Tomorrow’s micro- tribology will require researchers and engineers in widely divergent fields to exchange information.

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