Methods and systems for modifying a surface of a polymer with a shielded plasma are provided. The surface may be modified to create a surface with increased crosslinking and/or a particular mechanical property, such as a coefficient of friction. A shielding arrangement is used to modify the plasma to which the polymer surface is exposed, thereby providing a surface with the desired mechanical properties. In one aspect, a single source that provides multiple species of plasma particles is advantageously used instead of having to switch or move in multiple sources. The extent of crosslinking is evaluated using a surface force microscope to determine a frictional property that is correlated to the crosslinking, e.g., via calibrated values determined from reference surfaces.