Universal Aging Mechanism for Static and Sliding Friction of Metallic Nanoparticles

Abstract

The term "contact aging" refers to the temporal evolution of the interface between a slider and a substrate usually resulting in increasing friction with time. Current phenomenological models for multiasperity contacts anticipate that such aging is not only the driving force behind the transition from static to sliding friction, but at the same time influences the general dynamics of the sliding friction process. To correlate static and sliding friction on the nanoscale, we show experimental evidence of stick-slip friction for nanoparticles sliding on graphite over a wide dynamic range. We can assign defined periods of aging to the stick phases of the particles, which agree with simulations explicitly including contact aging. Additional slide-hold-slide experiments for the same system allow linking the sliding friction results to static friction measurements, where both friction mechanisms can be universally described by a common aging formalism.

Time Strengthening of Crystal Nanocontacts

“…These results triggered the development of so-called "rate-and-state" theories [2,3], according to which, in addition, the kinetic friction F kin should decrease logarithmically with the sliding velocity, if so-called contact ageing is present. An investigation of these effects on the nanoscale has begun only recently using atomic force microscopy (AFM) [4][5][6][7][8][9][10]. The logarithmic velocity weakening of the friction has been clearly recognized when silicon tips slide on SiO 2 and NaCl surfaces in the range of few nanometers per second to few microns per second driving speeds and at low temperatures of a few tens of degrees kelvin [11,12].…”

Friction vs. Area Scaling of Superlubric NaCl-Particles on Graphite

“…Thus it might seem plausible to correlate the stick-slip pattern with the interface conditions between the nanparticle and the substrate. Crucially these slip-lengths are also similar to that of amorphous Sb-nanoparticles sliding on HOPG [8], where the slip lengths have been attributed to insufficient relaxation of the soft spring within a single lattice constant. Therefore, it is difficult to conclude anything from these slip lengths about lattice parameters in the interface.…”

“…Consequently, one strategy to achieve structural lubricity is to prepare and analyze well defined model systems under ultra high vacuum (UHV) conditions, which are more tractable theoretically. Here, especially metallic nanoparticles prepared by thermal evaporation on layered materials with van der Waals interaction between layers, like for example, highly oriented pyrolithic graphite (HOPG) or MoS 2 , have proven to be suitable [5][6][7][8][9][10]. At the same time, researchers strove to establish structural lubricity also under ambient conditions.…”

“…Additionally, a number of studies have emphasized that additional effects can become relevant, if the interfaces cannot be assumed to be completely rigid. In this case, contact aging or enhanced static friction can occur [7,8,25,26]. Depending on the elastic properties of the materials additionally pseudo-commensurate states can appear [23] and superlubricity can even break down completely once a critical system size is reached and dislocations can be accommodated within the interface [10,21,27].…”

Atomistic modeling of tribological properties of Pd and Al nanoparticles on a graphene surface

“…The term “contact aging” [ 6 ] is related to time-dependent atomic reconstructions at the interface between the slider and the substrate, which usually lead to an increase in friction with time. Modern rate-and-state models for rough contacts predict that aging does not only influence the transition from static contact to sliding, but affects the overall sliding dynamics.…”