Nanoscale Metal-Metal Contact Physics from Molecular Dynamics: The Strongest Contact Size

Abstract

Using molecular dynamics we find that the tensile strength of the contacts between two clean platinum surfaces with nanoscale asperities is strongly size dependent with a maximum strength for contact lengths of approximately 5 nm. This is the first time a strongest size is observed in single crystals. The strengthening with decreasing size down to 5 nm results from a decrease in the initial density of mobile dislocations available for plastic deformation and the subsequent weakening originates from a reduction in the constraint to mechanical deformation inside the contact by the bulk. DOI: 10.1103/PhysRevLett.104.215504 PACS numbers: 62.25.Àg, 62.20.FÀ, 62.20.Qp, 81.07.Lk Processes resulting from the interaction between contacting surfaces including friction and stiction (static friction) are ubiquitous in traditional and emerging technologies including micro-and nanoscale contacting switches for communications [1] and low-power electronics [2]. The performance and reliability of these devices depends on the contact physics that govern the pullout force required to open the switch and on the defect generation that leads to an increase in contact resistance. The interaction between surfaces depends on the materials involved [3], surface chemistry, and roughness as well as operating conditions. In this Letter we focus on clean platinum slabs with nanoscale surface roughness where the interaction is dominated by the metallic bridges that form when surface asperities come into contact. In actual micro-and nanoswitches a large number of nanoscale contacts will form with a size distribution and local forces that depend on surface roughness and chemistry and external closing force [4]. The mechanical response of materials with such nanoscale dimensions is size dependent and the sub-100 nm range remains vastly unexplored. This factor contributes to the current lack of an understanding in the area of nanoscale contacts. Here we report on a molecular dynamics study of the strength of nanoscale contacts (tensile stress required to open the contact) between clean platinum surfaces as a function of their size. Our simulations shed light into the atomic mechanisms that govern size effects in contact physics and indicate that nanocontact experiments between clean surfaces would be a powerful tool to interrogate mechanical properties at extremely small scales that are difficult to explore by other means. A thin layer of adsorbates is always present in real microelectromechanical systems (MEMS) surfaces and contributes to surface adhesion; our work focuses on the interaction via metallic bridges that form due to large local forces and heating caused by electrical currents.Size effects in mechanical properties were first characterized by Hall and Petch in polycrystalline materials and by Brenner in metallic whiskers in the 1950s, with the general observation that smaller is stronger. More recently, size effects in strength have been observed in micropillars [5,6], thin films [7], and nanoindentation [8]. In the case of polycr...

Molecular dynamics characterization of the contact between clean metallic surfaces with nanoscale asperities

“…18 We find that incommensurate (001) contacts exhibit a strong size dependency with the same trends reported for commensurate contacts in Ref. [6]. In all cases studied, we find a strongest contact size of approximately 5 nm.…”

“…Figure 6 shows the initial number of hcp atoms prior This scaling does not indicate an inability to increases the number of dislocations with decreasing size and, thus, we discard this as a possible mechanism of size effects in contact strength not just for commensurate contacts as in Ref. [6] but for incommensurate as well. For initial contacts between commensurate (001) surfaces, the number of defects prior to opening decreases with decreasing contact size and becomes essentially zero for a finite contact size that 14 corresponds to the strongest contact.…”

“…This is continued until the contact opens. 6 Multiple contact simulations are performed by repeating the entire procedure starting from step i) with the structure resulting at the end of step iii). Depending on system size we performed between 1 and 25 closing and opening cycles.…”

“…From a basic science point of view nanoscale contact experiments 4,5 and simulations [6][7][8] between clean surfaces can shed light into the mechanical properties of nanosize materials with sub-100 nm size scales; an important and interesting regime not accessible by other experimental means and that remains essentially unexplored. In this paper we use molecular dynamics (MD) to investigate the strength of the metallic bridges that form when two clean platinum surfaces with nanoscale asperities are brought together focusing on the effect of contact size and repeated contact closing and opening.…”

“…[6][7][8] Our recent molecular dynamics (MD) simulations show that the strength of nanoscale contacts between commensurate surfaces is size-dependent and exhibit a maximum for contact lengths of approximately 5 nm. Despite such advances several questions regarding the behavior of contact with sub-100 nm linear dimensions remain.…”