Transient adhesion and conductance phenomena in initial nanoscale mechanical contacts between dissimilar metals

Abstract

We report on transient adhesion and conductance phenomena associated with tip wetting in mechanical contacts produced by the indentation of a clean W(111) tip into a Au(111) surface. A combination of atomic force microscopy and scanning tunneling microscopy was used to carry out indentation and to image residual impressions in ultra-high vacuum. The ∼7 nm radii tips used in these experiments were prepared and characterized by field ion microscopy in the same instrument. The very first indentations of the tungsten tips show larger conductance and pull-off adhesive forces than subsequent indentations. After ∼30 indentations to a depth of ∼1.7 nm, the maximum conductance and adhesion forces reach steady state values approximately 12 × and 6 × smaller than their initial value. Indentation of W(111) tips into Cu(100) was also performed to investigate the universality of tip wetting phenomena with a different substrate. We propose a model from contact mechanics considerations which quantitatively reproduces the observed decay rate of the conductance and adhesion drops with a 1/e decay constant of 9-14 indentation cycles. The results show that the surface composition of an indenting tip plays an important role in defining the mechanical and electrical properties of indentation contacts.

Electromigration and the structure of metallic nanocontacts

“…Here, we focus on ways to obtain information on individual atomic hops. This has become possible using scanning probe techniques (Hoffmann et al, 2008;Paul et al, 2013;and Landman and Luedtke, 1991).…”

Indentation-formed nanocontacts: an atomic-scale perspective

“…In these indentation experiments, the maximal conductance of the junction decreases dramatically by a factor of B12 Â from its initial value. 119 This happens at the same 1/e decay rate as the adhesive force (11-14 indentation cycles in this experiment).…”

“…118 There is good experimental evidence that the surface of a clean tungsten indenter undergoes progressive wetting during multiple indentations with gold substrates. 119 In Fig. 11 we show the apex of a clean W(111) indenter prepared by FIM and the first 25 load-displacement curves measured while indenting pristine locations of a Au( 111) crystal (loading direction only).…”

Microstructures and Mechanical Properties of Mg-9Al/Ti Metallurgical Bonding Prepared by Liquid-Solid Diffusion Couples

“…As can be seen in Figure 4a, there are a large number of nanostructures at the interface. It is reported in the literature that these nanostructures will accelerate the diffusion of adatoms and improve the wettability of the interface [21][22][23]. The interface of Mg-9Al matrix and Ti rob obtained after 60 min at 700 °C was analyzed using EDS through line scan in Figure 5.…”

“…Temperature dependence of Gibbs free energy of formation of various Ti-A1 compounds, data from ref [21]…”