Rate-and-State Effects in Nanoscale Contacts: How Chemical Bonding Induces Frictional Instabilities*

Recent atomic force microscope (AFM) experiments and simulations have found that nanoscale silica-silica contacts exhibit logarithmic ageing for times ranging from 0.1 s to 100 s, consistent with the conventional, empirical rate and state friction (RSF) laws [1-3]. Evidence strongly supports that this nanoscale ageing results from the progressive formation of interfacial siloxane bonds through condensation reactions of surface silanol groups. Here, we discuss the effects of varying the normal pressure [4], the lateral loading rate (pulling speed), and the hold time for small times. We observe the existence of a nanoscale memory distance [5], but with a wide scatter that we attribute to the varying areal density of sites that act to passivate interfacial chemical bonds. We also observe a non-logarithmic tail at short ageing times attributed to the manner in which energy barriers for the chemical reactions are distributed. Related to this, we observe highly regular stick-slip events with distinct load and pulling speed characteristics, and power law scaling between amplitude and slip period. The emerging picture for these contacts is of rate and state behavior, but with significant differences compared to macroscopic systems.

[1] Q. Li et al., Frictional Ageing from Interfacial Bonding and the Origins of Rate and State Friction. Nature 480, 233 (2011).

[2] Y. Liu and I. Szlufarska, Chemical Origins of Frictional Aging. Phys. Rev. Lett. 109, 186102 (2012).

[3] A. Li et al., Effects of Interfacial Bonding on Friction and Wear at Silica/Silica Interfaces. Tribol. Lett. 56, 481 (2014).

[4] K. Tian et al., Load and Time Dependence of Interfacial Chemical Bond-Induced Friction at the Nanoscale. Phys. Rev. Lett. 118, 076103 (2017).

[5] K. Tian et al, The Memory Distance for Friction in Silica-Silica Nanocontacts. Under review (2017).