Friction Study of a Ni Nanodot-patterned Surface

Nanoscale frictional behavior of a Ni nanodot-patterned surface (NDPS) was studied using a TriboIndenter by employing a diamond tip with a 1 μm nominal radius of curvature. The Ni NDPS was fabricated by thermal evaporation of Ni through a porous anodized aluminum oxide (AAO) template onto a Si substrate. Surface morphology and the deformation of the NDPS were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), before and after friction/scratch testing. SEM images after scratching clearly showed that, similar to what was assumed at the macroscale, the frictional force is proportional to the real area of contact at the nanoscale. It was found that adhesion played a major role in the frictional performance, when the normal load was less than 20 μN and plastic deformation was the dominant contributor to the frictional force, when the normal load was between 60 μN and 125 μN. Surprisingly, a continuum contact mechanics model was found to be applicable to the nanoscale contact between the tip and the inhomogeneous Ni NDPS at low loads. The coefficient of friction (COF) was also found to depend on the size of the tip and was four times the COF between a 100 μm tip and the Ni NDPS. Finally, the critical shear strength of the Ni nanodots/Si substrate interface was estimated to be about 1.24 GPa.