Contact ageing in friction force microscopy experiments
Roland Bennewitz, Johanna Blass, and Christiane Petzold
INM – Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany
Contact ageing is the growth of shear strength in a contact which is at rest. Contact ageing has been quantified by friction force microscopy in slide-hold-slide experiments. Here we will discuss the role of contact ageing for the velocity dependence of nanoscale friction on the example of adhesive friction between surfaces which interact by reversible macromolecular bonds in water. Flexibility in the molecular attachment to the surfaces leads to an increase of the number of bonds with time at rest.
We will also report the absence of contact ageing in some friction force microscopy experiments performed in ultrahigh vacuum. For oxidized silicon tips sliding on Au(111), the contact formation is too fast and the resulting contact too compliant to cause a holding-time dependent static friction peak. For oxidized silicon tips sliding on oxidized silicon in vacuum, no covalent bond formation can contribute to contact ageing as the formation of these bonds requires the presence of water. When removing the passivating oxide layers from tip or surface, tribochemical reactions lead to strong friction and tip wear. Slide-hold-slide experiments for a quantification of contact ageing are not possible under strong wear conditions.