A force controlled tribometer for pre-sliding measurements at the nanometer scale

Abstract

In the pre-sliding friction regime, interfaces partially stick and partially slip. The pre-slip is thought to be locally initiated at regions of the interface where the ratio of shear stress to normal stress exceeds a critical value. The displacements involved in pre-slip can be limited to the nanoscale, especially for stiff interfaces. Furthermore, little is known experimentally about the interplay between surface topography, pre-sliding behavior and wear. In this work, we introduce a pre-sliding tribometer that enables the study of how the pre-sliding friction at various types of ball-on-flat interfaces evolves as a function of wear. Polytetrafluoroethylene-on-silicon (PTFE-on-Si) pre-sliding measurements covering interfacial displacements up to 50 nm, conducted with the new instrument, show good agreement with Mindlin theory predictions, without adjustable parameters.

Molecular Probing of the Microscopic Pressure at Contact Interfaces

“…Preslipping is believed to be locally initiated at points where the ratio of tangential to normal force surpasses a threshold value, represented by the CoF. 25,53,54 When localized preslipping takes place, the "slipping" contact gets smaller, leading to the remaining contact area bearing an increased normal force. 6,51 Increasing the normal load per contact area, therefore, requires greater tangential load locally to break the contact due to a nonlinear relationship between diminishing contact area and tangential load.…”

“…The differences suggest that contacts exhibit diverse frictional behaviors, for instance, in the preslipping regime. Preslipping is believed to be locally initiated at points where the ratio of tangential to normal force surpasses a threshold value, represented by the CoF. ,, When localized preslipping takes place, the “slipping” contact gets smaller, leading to the remaining contact area bearing an increased normal force. , Increasing the normal load per contact area, therefore, requires greater tangential load locally to break the contact due to a nonlinear relationship between diminishing contact area and tangential load . Therefore, the initial local pressure distribution and the application of lateral forces over time critically influence the evolution of the contact area and, thus, the critical shear stress to initiate the slipping in the presliding stage.…”