Nanoscale wear evolution on a polystyrene/poly (n-butyl methacrylate) blend

We have investigated the formation of surface nanostructures caused by early-stage wear of polystyrene (PS) and poly(n-butyl methacrylate) (PnBMA) thin films as pure materials, or mixed in the form of a 1:1 blend. To this end, atomic force microscopy (AFM) was used to repeatedly scratch the sample surfaces, measure the accompanying friction forces, and image the resulting features. In the very first stage ordered ripples are formed in all cases. As the process goes on, hillocks are nucleated on the crests of the ripples, and progressively released in the form of nanoplastics while the ripples become wider and less regular. On the blend surface the more compliant PnBMA presents more corrugated ripple structures and larger friction oscillations than PS in the beginning, but the scenario becomes again more complex as the wear test is repeated and the original 'rim and hole' geometry of the blend is disrupted. Quite noticeably, the wear damage is reduced if the surfaces are scraped forth and back and not only in one direction. The influence of the scan pattern (distance between scan lines) and of the normal force (well below and above the force threshold of about 50 nN leading to surface ripples in the first stage) have been also addressed.