Universal Aging Mechanism for Static and Sliding Friction of Metallic Nanoparticles Michael Feldmann, 1 Dirk Dietzel, 1,* Antoni Tekiel, 2 Jessica Topple, 2 Peter Grütter, 2 and André Schirmeisen 1,† 1 Institute of Applied Physics, Justus-Liebig-Universität Giessen, 35392 Giessen, Germany 2 Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada (Received 29 February 2016; revised manuscript received 10 May 2016; published 8 July 2016) 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. DOI: 10.1103/PhysRevLett.117.025502 The force needed to initiate sliding of an object is usually higher than the force needed to sustain its motion, which is termed static and sliding friction [1,2]. Early investigators have attributed this difference to the inevitable surface roughness between interfaces and the number of true contact points where their asperities meet [3]. Upon external shear these asperities exhibit a transient creeplike motion leading to static and kinetic friction [4–7]. It is commonly believed that the threshold for sliding initiation is higher than for sustaining sliding, which is caused by aging phenomena, i.e., an increase of asperity interaction with time [8–12]. Friction of such rough interfaces can be well described in the framework of rate and state theories [13,14], where contact aging is assigned to the not further specified state variable Θ. This parameter is usually interpreted as the number (or overall area) of contact points, but may also be related to other interface processes [15]. These models anticipate that 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. However, the atomistic interpretation of contact aging remains difficult, especially since nanoscale contact aging has been studied only in a few experiments up to now. Studied examples range from atomic stick slip on graphite [16], aging of diamond-silicate contacts [17], to nano- particle friction [18,19]. One unresolved question is whether contact aging is solely responsible for the static friction threshold. Also, it remains unknown how to connect contact aging to individual stick-slip events, which are inevitably present during sliding. Our goal was to analyze a model friction system, which shows stick-slip behavior and exhibits aging without wear or other irreversible interface changes over time. For this we performed experiments of sliding metallic nanoparticles on a flat surface in ultrahigh vacuum (UHV) conditions. Stick- slip motion of the particles during sliding is clearly resolved for a large variation of sliding speeds and our analysis of a large set of individual stick-slip events shows that the slip force increases systematically with the stick-time during continuous sliding. This is complemented by slide-hold- slide measurements [17] of the static friction force. We show that static and sliding friction can essentially be described by the same universal law, which is based on thermally activated contact breaking combined with logarithmic con- tact aging. This allows us to close the gap between static and sliding friction over 5 orders of magnitude in time scale. All nanomanipulation experiments have been performed using antimony nanoparticles prepared under UHV con- ditions by thermal evaporation onto freshly cleaved HOPG [Fig. 1(a)]. The crucible of a conventional Knudsen cell containing antimony was heated up to 450 °C and the evaporation time was about 7 min at a rate of approximately 6.5 Å= min. Directly afterwards, the sample was transferred to the UHV atomic force microscope (AFM; type: JEOL JSPM-4500A) without breaking the vacuum. All measure- ments were done at room temperature and the average sliding friction values were in quantitative agreement with values of superlubric sliding found before [20], thus indicat- ing an atomically clean interface [21]. The nanomanipula- tion measurements were performed using the “tip on side” mode, where the AFM tip was placed directly beside the nanoparticle [20–22]. The manipulation sequences were live monitored by an integrated scanning electron microscope, which induced no apparent changes in friction. The adhesion between tip and nanoparticle was typically strong enough to enable pushing as well as pulling of the particle [Fig. 1(b)]. This allows measuring the lateral force signals PRL 117, 025502 (2016) PHYSICAL REVIEW LETTERS week ending 8 JULY 2016 0031-9007=16=117(2)=025502(5) 025502-1 © 2016 American Physical Society