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A new set of simulations of the contact in an atomic force microscope challenge our thinking
One of the central issues in tribology is correctly determining the true area of contact between two surfaces. In large-scale contacts, the real contact area is made up of a number oF contacting asperities. One of the central ideas behind using atomic force microscopy (AFM) to study friction is that the sharp tips used in this technique model a single-asperity contact. Moreover, it was thought that the contact mechanics could be described using analytical theories developed by Hertz, which showed that the area of contact was proportional to (Load)^sup 2/3^. This theory has been modified to include the efFects of adhesion between the contacting surfaces but still yields analytical formulae with sublinear dependences of contact area on load.
In this case, if the lateral force is proportional to the real contact area, where the proportionality constant is defined as the interfacial shear strength, this will result in the lateral (frictional) force being a sublinear function of load, and this has indeed been observed in a number of AFM experiments. However, in some cases, deviations have been observed from such...