  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 energy dissipated,
$\Gamma$, during rupture. Moreover, lubricant viscosity affects the onset of
friction but has no effect on $\Gamma$. Fracture mechanics provide a new way to
view the otherwise hidden complex dynamics of the lubrication layer.
