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Experimental studies in nano-tribology

Atomic Force Microscopy (AFM) [1] has become a standard tool for obtaining images of surfaces with nanometre resolution. It was realised quite quickly [2] that AFM could also be used to study the frictional forces acting between the AFM tip and the surface. What is unique about AFM for tribology work ? Essentially the AFM approximates a single asperity contact [3] so that the basic mechanics of sliding can be investigated under a wide variety of environmental conditions. Furthermore, since the surface can be imaged, information concerning surface wear and roughness are also obtained.

Our early work concentrated on using AFM at solid-liquid interfaces [4]. By measuring the relation between friction, load and adhesion at the asperity [3] one can verify the single asperity nature of the contact and the atomic scale variations of frictional slip-stick. Such experiments are also vital to study the important problem of understanding the physical basis of AFM imaging i.e. how do we relate the observed "atomic" pictures of a surface to the forces acting at the contact. We have demonstrated that solvation forces (i.e. the forces arising from the ordering of structure of liquid molecules near a surface) can be measured using AFM [4,5]. Such forces can be viewed as the squeezing and displacement of liquid molecules as the AFM tip and a surface approach each other. Such effects are clearly important in boundary layer lubrication [6]. More recently we have begun to look at the dynamical response of the liquid squeezed between two surfaces i.e. at the local viscosity [7]. The data appear to show that the viscosity of the solvation layers nearest the surface is higher than that of the bulk even for tip radii ~100 nm, although more work is required here because of the difficulty in accounting for the hydrodynamic damping of the AFM cantilever itself.

We also study friction and wear under ultra-high vacuum (UHV) conditions, which ensures that we know the chemical condition of the surfaces. The aim is to test point contact theories which relate load, friction, adhesion and the contact area. Studies of atomic scale mechanics require an estimate of the contact area if significant statements about the physics are to be made. To date there has been no means of finding the tip-surface contact area in AFM and we are approaching this problem by simultaneous measurements of the mechanical compliance and the local conductivity [8,9]. The results are encouraging with the conductivity [9] and compliance [10,11] changing with load in a manner calculated for an elastic contact using a continuum mechanics approach. Variations in the friction force with load are also consistent with the continuum model, even for contact radii down to ~ 3 nm.

To summarise, our research aims to investigate short range forces, localised conduction and nanoscale mechanical properties using AFM. These experiments are conducted at well defined solid-liquid or solid-vacuum (UHV) interfaces. Apparatus and expertise for such work has been established within the Department and related collaborations are maintained with Hitachi (MERL, Ibaraki, Japan), IBM (Zurich, Switzerland), ICI (Runcorn, UK), and the Chemistry Department, Cambridge.

Relevant/recent publications

  1. Binnig, G., Quate, C. F. and Gerber Ch. 'Atomic force microscope' Phys. Rev. Lett., 56, pp930-933 (1986).
  2. Mate, C, M., McClelland, G. M., Erlandsson, R. and Chiang, S. 'Atomic-Scale friction of a tungsten tip on a graphite surface' Phys. Rev. Lett, 59, pp1942-1945 (1987).
  3. O'Shea, S. J., Welland, M. E. and Wong, T. M. H. 'The influence of frictional forces on atomic force microscope images' Ultramicroscopy, 52, pp55-64 (1993).
  4. O'Shea, S. J., Welland, M. E. and Rayment, T. 'Solvation forces near a graphite surface measured with an atomic force microscope ' Appl. Phys. Lett., 60, pp2356-2358 (1992).
  5. O'Shea, S. J., Welland, M. E. and Pethica, J. B. 'Atomic Force Microscopy of local compliance at solid-liquid interfaces' Chem .Phys. Lett., 223 pp336-340 (1994).
  6. O'Shea, S. J., Welland, M. E., and Rayment, T. 'Atomic force microscope study of boundary layer lubrication' Appl. Phys. Lett., 61, pp2240-2242 (1992).
  7. O'Shea, S. J., Lantz, M. A. and Welland, M. E. 'AFM at Liquid-Solid Interfaces' in Micro and Nano-tribology and its applications, (ed. Welland M. E. and Bhushan, B.) Kluwer, Dordrecht (1997).
  8. O'Shea, S. J., Atta, R. M. and Welland, M. E. 'Characterisation of tips for conducting AFM' Rev. Sci. Instrum., 66, pp2508-2512 (1995).
  9. Lantz, M. A., O'Shea, S. J. and Welland, M. E. and Johnson, K. L. 'An atomic force microscope study of contact area and friction on NbSe2' J. Vac. Sci. Technol. B, 13, pp1945-1953 (1995).
  10. O'Shea, S. J., Atta, R. M. and Welland, M. E. 'Characterisation of tips for conducting AFM' Rev. Sci. Instrum., 66, pp2508-2512 (1995).
  11. Lantz, M. A., O'Shea, S. J., Hoole, A. C. F. and Welland, M. E. 'Lateral stiffnesss of the tip-sample contact in frictional force microscopy' Appl. Phys. Lett., 70, pp970-972 (1997).

Contact Details

For further information on the work summarised above contact Professor M E Welland at Cambridge University Engineering Department, Trumpington St, Cambridge, CB2 1PZ, UK. Tel: 01223 332676, Fax: 01223 332662. E-mail: mew10@eng.cam.ac.uk
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