Extraordinary low friction of Ag film on the Si(111) as the function of substrate temperature and film thickness

Abstract

The tribological characteristics of a few monolayer(ML)-thick Ag films on Si(111) substrates have been studied as a function of substrate temperature and film thickness in ultra high vacuum (UHV) environment. The frictional coefficient of less than 0.0027, which is the detection limit of the tribometer, is observed at a film thickness of Ag thicker than 0.9 ML. After sliding with the ultra-low frictional coefficient, no wear scar is observed on the slid track by scanning electron microscopy (SEM), and still an equivalent amount of Ag remains. On the other hand, at a thickness less than 0.7 ML, a frictional coefficient higher than 0.1 and a distinct wear scar due to adhesive wear is observed. At a substrate temperature higher than 400 K, the ultra-low frictional behavior is not observed and the frictional coefficient increases with increasing substrate temperature. The atomic force microscopy (AFM) observation showed a large wear scar and wear debris of a few μm diameter after sliding at 600 K. From Auger electron spectroscopy (AES) analysis, it was suggested that a part of Ag in the wear scar is removed during sliding at elevated temperatures. The two dimensional (2D) surface structure of absorbed Ag on Si(111) is modified by annealing at 723 K for 120 s: the rearrangement temperature to form the well-known Si(111)-√3 × √3-Ag R 30 0 periodic structure (superstructure) of Ag on Si substrate. The friction coefficient increases to 0.15 on the annealed surface. However, the surface damage is small and the AES analysis shows that Ag still remains in the wear scar. The Ag atoms are assumedly fixed to the more stable sites on the annealed Si substrate than that without annealing (as-deposited state), and partially loose the mobility between the contact surfaces. As a result, the degree of contribution of the topmost atomic layers of the surfaces i.e. the chemical composition or surface structure to friction is very distinct for this solid/solid sliding contact.