We have used the newly developed interfacial force microscope (EFM) to study the adhesive and mechanical properties of a Au substrate. We show that a W probe interacting with a Au surface results in plastic deformation under very small repulsive loads and transfers Au to the probe surface in a galling interaction upon removal. However, when the Au surface is passivated by a self-assembling monolayer of n-docosanethiol molecules (CH3(CH2)21SH), the substrate is able to elastically support large repulsive loads with no attractive interactions. Within this elastic region, the load/deformation relationship closely follows the classic Hertzian model for a rigid punch deforming an elastic half space. As a result, we are able to determine the maximum values for the shear-stress threshold for plastic deformation to be approximately 1 GPa in agreement with theoretical determinations. Above this value, the substrate permanently deforms. Remarkably, however, the strong binding of the molecular film to the Au surface permits it to maintain its passivating properties even after gross plastic damage.