Deformation- and rupture-controlled friction between PDMS and a nanometer-scale SiOx single-asperity

This work investigates the friction between polydimethylsiloxane (PDMS) and silicon oxide (SiOx) in single asperity sliding contact by atomic force microscopy (AFM). Two friction dependences on the normal force are identified: a tensile regime and a compressive regime of normal forces. In the compressive regime, friction is governed by the shear deformation and rupture of junctions between PDMS and SiOx. In this case, the shear strength tau asymptotic to 10 MPa is comparable with the cohesive strength of PDMS under compressive loading. In contrast, friction in the tensile regime is also affected by the elongation of the junctions. The single SiOx-asperity follows a stick-slip motion on PDMS in both normal force regimes. Statistical analysis of stick-slip as a function of the normal force allows determining the necessary amount of energy to break a SiOx/PDMS junction. Friction between a SiOx-asperity and a PDMS surface can be rationalized based on an energy criterion for the deformation and slippage of nanometer-scale junctions.