Scratching the surface

A microscopic model describing frictional forces between a tip and a surface near its melting point predicts different behaviour for grazing friction compared with ploughing friction

Credit: © 2007 Nature Materials

On a macroscopic scale, the physics of friction is well understood: skiers do their utmost to minimize it whereas racecar drivers hang on to it desperately during high-speed cornering. Yet, the physical mechanisms governing nanoscale friction are not clear. A quantitative microscopic model of friction at high temperatures is particularly important for predicting how nanoscale structures will respond to indentation.

Now, Tatyana Zykova-Timan and colleagues¹ of the Scuola Internazionale Superiore di Studi Avanzati and the International Centre for Theoretical Physics in Italy and the Swiss Federal Institute of Technology in Zurich use molecular dynamics to model the temperature dependence of frictional forces between a diamond tip and the non-melting (100) surface of NaCl. They predict that near the bulk melting point (T_M) of NaCl, frictional forces decrease when a sharp tip penetrates and ploughs through the surface (ploughing friction) but increase when a flat tip merely grazes the surface (grazing or wearless friction).

Zykova-Timan and colleagues chose NaCl(100) surfaces to simplify their analysis as other surfaces can spontaneously melt near T_M and 'jump' to contact approaching tips. However, similarly sharp changes of friction forces near T_M can also be expected in metals such as Pb, Al or Au.