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Temperature Activates Contact Aging in Silica Nanocontacts
02.12.2019 12:00

Understanding the time evolution of contact strength in silica nano­contacts is of great funda­mental and practical relevance in diverse areas like earth­quake dynamics, wafer bonding mecha­nisms, as well as MEMS applications. The loga­rithmic increase of contact strength with hold time, termed contact aging, can be "quanti­tative" due to defor­mation creep in plastic contacts. An alternative mechanism, termed "quali­tative aging," is the gradual change in inter­facial chemistry, which so far was only observed in the presence of humidity. Here we present nano­scale friction experi­ments of dry silica contacts in ultrahigh vacuum that show a doubling of shear strength with time following a loga­rithmic law. We find that the aging rate scales linearly with tempe­rature, and that shear stress shifts the relevant energy barriers. All-atom MD simu­lations provide a live picture of the bond formation dynamics occurring at the interface. Our experi­ments link contact aging to thermally activated bond formation, show that it exists even in the absence of water molecules, and demon­strate that this atomic aging mecha­nism can stretch over time­scales up to several seconds. Qualitative contact aging is thus highly relevant for a broad variety of material combi­nations and con­ditions.

Publication

Matthias Vorholzer, J. G. Vilhena, Ruben Perez, Enrico Gnecco, Dirk Dietzel, André Schirmeisen: "Temperature Activates Contact Aging in Silica Nanocontacts". Phys. Rev. X 9 (2019), 041045.


Abbildung 1

Fig. 1: Slide-hold-slide experiments performed on SiO2.
  (a) Schematic course of a typical slide-hold-slide procedure, where tip movement is paused for a defined amount of time.
  (b) Exemplary slide-hold-slide curve obtained for silica on native silica at 200 K with thold=2.1 s. The x axis is relative to the stopping point. As indi­cated in the figure, various charac­teristic quantities are evaluated around the stopping point. Especially important are the sliding friction level Fsliding (green), the static friction peak Fstatic (orange), and the contact stiffness k, which can be derived from the slope of the lateral force buildup (purple, extrapolated as dashed line).
  (c) Hold time dependence of the static friction for different tempera­tures between 200 K (yellow) and 300 K (red) measured on the native oxide layer.
  (d) Temperature dependence of the sliding friction ( v=2.5 µm/s) for the native oxide silica (crosses) fitted by a theoretical curve (blue line) ba­sed on the stan­dard model de­scri­bing ther­mally acti­vated fric­tion.

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