Slippery but Tough: The Rapid Fracture of Lubricated Frictional Interfaces

We study the onset of friction for rough contacting blocks whose interface is coated with a thin lubrication layer. High speed measurements of the real contact area and stress fields near the interface reveal that propagating shear cracks mediate lubricated frictional motion. While lubricants reduce interface resistances, surprisingly they significantly increase the energy dissipated G during rupture. Moreover, lubricant viscosity affects the onset of friction but has no effect on G. Fracture mechanics provide a new way to view the otherwise hidden complex dynamics of the lubrication layer.