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Journal Club Forum for October 15th: Experimental Solid Mechanics at the Nanoscale

Ravi-Chandar's picture

Browsing through past themes, I realize that this is the third time the topic of experimental mechanics at the nanoscale has been highlighted in the jClub forum. There are many problems that require experimental research at the nanoscale that we can do this a few more times without being repetitive. I would like to focus this discussion on the fundamental issue of friction.

Few subjects have been studied more extensively than friction, yet answers to some of the most fundamental questions in the field remain unclear; the origin of friction at small scales is incomplete and hotly debated. There are arguments to indicate that friction should not exist at all, and many theoretical responses to these arguments (He et al. 1999, Goto, 2002, de Gennes, 2006); certainly measurements and common experience indicate that friction exists! The most familiar, and yet remarkable, result concerning friction is that the shear force required to slide is proportional to the normal force and independent of the contact area. This result, established a long time ago by Leonardo da Vinci, Amontons and Coulomb, was essentially explained by Bowden and Tabor for macroscopic contact as being governed by the plastic deformation of asperities. For a recent demonstration of measurements of this at the microscale, see Hommola et al 1990; the frictional stress in this length scale is about 0.001G (where G is the shear modulus). A similar result appears at much smaller scales in experiments using an Atomic Force Microscope (AFM) (see Carpick et al. 2004 and earlier references therein). However, when the contact dimensions are under a few nanometers, the frictional stress is about 0.02 almost equal to the theoretical shear strength of a bonded interface clearly suggesting an adhesive type of failure.

As a result, while frictional stress may be considered to be independent of the contact area, it is indeed different at the different scales, with clearly different mechanisms operating at the two extremes. What happens at the mesoscales? Is friction scale dependent? What mechanisms operate in this scale? Is the transition abrupt or smooth? Hurtado and Kim (1999a,b) examined scale effects in the friction of single asperity contacts. They developed a dislocation-based model that explains the transition from concurrent slip at the nanoscale, where no dislocations are generated, to a single dislocation assisted slip at the mesoscale to the cooperative glide of piled-up dislocations at the microscale. This model predicts a gradual transition in the frictional strength from the high values at the nanoscale to small values at the microscale.

There is a gap in the measurements over contact length in the range of 10-8 < a < 10-5 m due to the non overlapping load ranges of the instruments available for nanoscale experiments (scanning probe microscopes vs surface force apparatus). Here comes the role of experimental mechanics: while there are many tools from the atomic scale on upwards, and also at macroscopic length scales, we need measurements of friction in this range of length scales indicated above. It would be nice to have some flexibility in the selection of contacting materials as well. Both Kim and his colleagues at Brown University (Li and Kim, 2008) and our group at the University of Texas at Austin (Xu et al., 2008) have been looking into this in recent years. What both groups have found is that friction is quantized: the transition from the nanoscale to the larger scale occurs in discrete step(s) as the scale of contact goes above ~30 nm. Questions of whether this is due to roughness of contact surfaces or hydrophilicity of the surfaces are being discussed in this literature.

There is also a large literature associated with MEMs applications, self-assembled monolayers and other applications that I have not highlighted here, but a simple search should reveal these easily. In this forum, I have focused on one of the issues related to scale dependence of friction. Perhaps others would like to chime in!

Carpick., R. W., Agrait, N., Ogletree, D. F. and Salmeron, M. 1996, “Measurement of the interfacial shear strength and adhesion of nanometer-sized contact”, Langmuir 12, 3334-3340.

Carpick, R.W., Flater, E.E., Sridharan, K., Ogletree, D.F. and Salmeron, M. 2004, “Atomic-scale friction and its connection to fracture mechanics”, JOM 56(10), 48-52.

de Gennes, P.G. 2006, “Friction between two misoriented crystalline monolayers”, Comptes Rendus Physique, 7(2), 267—271

Goto, M., Watanabe, K., and Honda, F. 2002, “Super lubricity of diamond sliding on the Ag monolayer/Si(111) and the effect of gas adsorption”, Surface Science, 507-510, 922-7

He, G., Muser, M.H., and Robbins, M.O. 1999, “Adsorbed layers and the origin of static friction”, Science, 284, 1650-2.

Homola, A.M., Israealachvili, J. N., McGuiggan P. M., and Gee, M. L., 1990, “Fundamental studies in tribology: The transition from interfacial friction of undamaged molecularly smooth surfaces to normal friction and wear”, Wear, 136, 65-83.

Hurtado, J.A., and Kim, K. S., 1999a, “Scale effects in friction of single asperity contacts. I. From concurrent slip to single-dislocation -assisted slip”, Proc. R. Soc. Lond. A 455, 3363-3384.

Hurtado, J.A., and Kim, K. S., 1999b, “Scale effects in friction of single asperity contacts. II. Multiple-dislocation-cooperated slip, Proc. R. Soc. Lond. A 455, 3363-3384.

Krim, J. 2002, “Resource Letter: FMMLS-1: Friction at macroscopic and microscopic length scales”, American Journal of Physics, 70, 890-897.

Li, Q.Y., and Kim, K.S. 2008, “Micromechanics of friction: effects of nanometre-scale roughness” Proceedings of the Royal Society A-Mathematical Physical and Engineering Sciences, 464, 1319-1343.

Xu, D., Liechti, K.M., and Ravi-Chandar, K., 2008, “On scale dependence in friction: Transition from intimate to monolayer lubricated contact”, Journal of Colloid and Interface Science, 318, 507-519.


Xiaodong Li's picture

Thanks a lot. This is an excellent topic. My group has just published a paper in Acta Mater (online available). Your comments and suggestions are very welcome.

Surface wear of coatings occurring at extremely low loads and in nanocontacts is of great importance for the development and the reliability of structural/functional nanocomponents in micro/nanoelectromechanical systems. To date, appropriate tools for mapping the nanoscale wear of thin coatings are still lacking. In this study, a new method combining atomic force microscopy (AFM) and digital image correlation (DIC) techniques has been developed and applied for the determination and visualization of the nanoscale wear of a gold coating. It has been shown that the initiation and development of nanowear, which is usually difficult to detect directly from AFM topographical images, can be efficiently revealed by monitoring the correlation coefficient change in DIC analysis. A linear relation between the correlation coefficient and the wear depth is found and may be used to quantify the nanowear. The nanowear of gold coating is dominated by material removal without any plastic deformation.

Z. H. Xu, M. A. Sutton and X. D. Li, Mapping nanoscale wear field by combined atomic force microscopy and digital image correlation techniques, Acta Materialia



Yong Zhu's picture

Dr. Ravi,

Thanks for leading this exciting theme. I think adhesion/friction and mechanical properties of 1D nanostructures are two important issues among others for nanoscale experimental mechanics. A rich literature has been reported on mechanical property measurement of 1D nanostructures, see Xiaodong’s theme of May 2007. However, few experiments have reported adhesion and friction between nanostructures and substrate or between nanostructures themselves in spite of the importance for nanodevice applications. There might be lots of challenges for such experiments. One I can think of is high resolution force measurement. Perhaps you and others can point out other challenges.

I do have a question about the paper by Li and Kim. It reported friction strengthening as a result of the asperity flattening for the M-PSA. Is it possible that in addition to increasing the true contact area, the flattening also increases the shear strength according to the model in your paper?


In this context the latest Nature article on adhesion might be of interest.  See 
Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape

Carlos G. Camara1,2,
Juan V. Escobar1,2,
Jonathan R. Hird1
Seth J. Putterman1

Rui Huang's picture

Nanoindentation tests have been used widely for characterizing mechanical properties of thin films and nanostructures. Since it involves contact, I would think frcition/adhesion has to be considered in the data analysis of the indentation tests. However I have not seen systematic studies (experimentally or theoretically) on the friction effect. I vaguely recall that someone had shown that friction has negligible effect for thin films, but the same conclusion may not hold for nanowires and other 1D nanostructures. In a recent study on silicon nanolines , we found that friction plays a critical role in the indentation tests as the nanolines buckled and slid along the surface of the indenter tip. The analysis of friction in this case turns out to be challenging because the contact area is not well defined and multiple contacts occurred at the same time. In addition, the buckling instability of the high-aspect-ratio nanolines adds more into the challenge. So far we have only limited success in modeling through elementary treatments of friction and buckling; see M. K. Kang, B. Li, P.S. Ho, R. Huang, Buckling of Single-Crystal Silicon Nanolines under Indentation. Journal of Nanomaterials, Vol. 2008, Article ID 132728.


Dear All,


I just accidentally discovered this forum as was searching for Timoshenko and goodier's book on web!!!.

I was attracted by the title and then as I entered I came across an old friend Prof. Li! Hi Xiaodong!.

To introduce my work briefly - I have been interested in Nanoindentation for last 15 years and have been building an instrument for doing mechanics inside a Transmission Electron Microscope. I belive that I have sorted out most of the experimental issues and ready to take up challenging contact mechanics problem at the scale Dr. Ravi had mentioned and could observe the dynamics in real time. It has very high stiffness to overcome the attractive instabilites at close to the contact.  

 The following refences will being the capability of my instrument,

  M.S.Bobji, J.B. Pethica and B.J.Inkson, 2005, Indentation mechanics of Cu-Be quantified by an in situtransmission electron microscopy mechanical probe Journal of MaterialsResearch, 20(10), 2726 – 2732

    Bobji MS, Ramanujan CS, Pethica JB, and Inkson BJ., 2006,  A miniaturized TEM nanoindenter for studying material deformation in situ, Measurement Science & Technology 17 (6),1324-1329.

 I guess this should broadly will fall into the scales mentioned by Dr. Ravi. 

 Friction if seen as energy dissipation problem then the insitu experiments should give me a direct answer through the observed microstructural changes, I hope.

 If every one here agress we can look into the friction at all scales from scratch and I would certainly like to see how an experiment could be designed to cover the wide time scales that seems to me is the key to understanding of friction. 

hoping for a nice discussion. 





Yanfei Gao's picture

Besides the methods mentioned in Prof. Ravi-Chandar's blog, there is another way to bridge the scale gaps in, for example, AFM (Carpick's work) and SFA (Homla's work). We recently used a three-dimensional nanoindentation system to examine friction behavior. See Y.F. Gao, B.N. Lucas, J.C. Hay, W.C. Oliver, and G.M. Pharr, Scripta Mater. v55, p653, 2006. The so-called friction stress is about 0.01G, and the contact size is about 100nm.

Regarding friction model, many AFM experimentalists and molecular theories argue that incommensurate interface will have vanishing friction. From mechanics point of view, the friction can be viewed as interface dislocation activities -- a work I presented at GRC Thin Film 2008. If anyone is interested along this line, my contact info can be found at:

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