  We present experimental and theoretical results for the surface topography of
a plastically deformed metallic (aluminum) block. When a hard spherical body
(here a steel-, silica glass- or silicon nitride ball) with a smooth surface is
indented in a metal block with a nominally flat, but still rough, surface, a
spherical-cup-like indentation result due to plastic flow. The surface
roughness in the indented region is, however, not entirely flattened. The long
wavelength (macroasperity) content of the roughness result from the roughness
on the original (aluminum) surface, but now plastically deformed. The roughness
at short length scale, in the plastically deformed macroasperity contact
regions, result from the roughness on the hard ball, and from inhomogeneous
plastic flow.
  We model the contact mechanics using the boundary element method, combined
with a simple numerical procedure to take into account the plastic flow. The
theory can semi-quantitatively describe the modification of the roughness by
the plastic flow. Since the fluid leakage of metallic seals in most cases is
determined by the long wavelength roughness, we predict that the leakage can be
estimated based on the elastoplastic contact mechanics model employed here.
  The plastic deformations of surfaces of some glassy polymers is very
different from what we observed for aluminum, which we attribute to strong
work-hardening and to inhomogeneous plastic flow for the polymers. Thus the
numerical procedure to account for the plastic flow proposed here cannot be
applied to glassy polymers in general.
