Velocity-weakening and -strengthening friction at single and multiasperity contacts with calcite single crystals

Although earthquakes are one of the most notorious natural disasters, a full understanding of the underlying mechanisms is still lacking. Here, nanoscale friction measurements were performed by atomic force microscopy (AFM) on calcite single crystals with an oxidized silicon tip to investigate the influence of roughness, contact aging, and dry vs. aqueous environment. In dry environments, smooth and rough calcite surfaces yielding single- and multiasperity contacts, respectively, exhibit velocity-weakening (beta(D)lnV) or neutral friction at slow sliding velocities and velocity-strengthening friction (alpha(D)lnV) at higher velocities, while the transition shifts to slower velocities with an increase in roughness. The origin of the velocity-weakening friction is determined to be contact aging resulting from atomic attrition of the crystalline surface. Friction measurements in aqueous environment show evidence of pressure solution at sufficiently slow sliding velocities, which not only significantly reduces friction on single-and multiasperity contacts but also, eliminates atomic attrition and thereby, velocity-weakening friction. Importantly, the friction scaling law evolves from logarithmic (beta(D)lnV) into linear (alpha V-PS), deviating from commonly accepted rate-and-state friction (RSF) laws; this behavior extends over a wider range of velocities with higher roughness. Above a transition velocity, the scaling law remains logarithmic (alpha(W) lnV). The friction rate parameters alpha(D), beta(D), alpha(PS), and alpha(W) decrease with load and depend on roughness in a nonmonotonic fashion, like the adhesion, suggesting the relevance of the contact area. The results also reveal that parameters and memory distance differ in dry and aqueous environments, with implications for the understanding of mechanisms underlying RSF laws and fault stability.

Automated summary

Nanoscale friction measurements were conducted on calcite single crystals to study the influence of surface roughness, contact aging, and different environments. The results showed that friction behavior varied significantly between dry and aqueous environments, with water environment even eliminating velocity-weakening friction caused by atomic attrition. Friction rate parameters and memory distance also exhibited differences between dry and aqueous environments, with implications for understanding fault stability and friction laws.