Initial wear in nanometer-scale contacts on polystyrene

The initial wear in nanometer-scale contacts between Si3N4 tips with 20 nm nominal radius and polystyrene surfaces was studied in air using a scanning force microscope. Polystyrene films had molecular weights (MW) 24 k and 210 k and were cast from toluene solution and dried in vacuum. Film thicknesses were greater than 0.5 mu m to avoid film-substrate interfacial effects. The starting surfaces were smooth (roughness of less than or equal to 0.8 nm root mean square over 4 mu m x 4 mu m areas) except for a few small pits (<80 nm wide and 10 nm deep) at low molecular weight which had no apparent influence on the abrasion process. Wear was produced on 4 mu m x 4 mu m areas by raster scanning the Si3N4 tip under applied loads of 10-210 nN at speeds in the range 80-320 mu m s(-1). The abraded area develops characteristic parallel ridges oriented perpendicular to the scan direction. For 24 k MW polystyrene, the roughness of the abraded areas increases with the number of abrasion cycles and, at constant load, the ridge spacing is proportional to scan speed. 210 k MW polystyrene is more resistant to wear and has smaller ridge spacing than 24 k MW polystyrene. Some samples were also heated at about 25 K above the glass transition temperature. After cooling, heated samples of both molecular weights show enhanced resistance to wear. The qualitative features of the observed wear correlate with the MW dependence of the tensile, flexure, and impact strengths of polystyrene. (C) 1997 Elsevier Science S.A.