Interfacial processes underlying the temperature-dependence of friction and wear of calcite single crystals

Hypothesis: This study posits that thermal effects play a substantial role in influencing interfacial processes on calcite, and consequently impacting its mechanochemical properties. Experiments: This work interrogates the temperature-dependence of friction and wear at nanoscale contacts with calcite single crystals at low air humidity (≤ 3–10 % RH) by AFM. Findings: Three logarithmic regimes for the velocity-dependence of friction are identified. BelowT_c ∼ 70 °C, where friction increases with T, there is a transition from velocity-weakening (W₁) to velocity-strengthening friction (S₁). AboveT_c ∼ 70 °C, where friction decreases with T, a second velocity-strengthening friction regime (S₀) precedes velocity-weakening friction (W₁). The low humidity is sufficient to induce atomic scale changes of the calcite cleavage plane due to dissolution-reprecipitation, and more so at higher temperature and 10 % RH. Meanwhile, the surface softens above T_c –likely owing to lattice dilation, hydration and amorphization. These interfacial changes influence the wear mechanism, which transitions from pit formation to plowing with increase in temperature. Furthermore, the softening of the surface justifies the appearance of the second velocity-strengthening friction regime (S₀). These findings advance our understanding of the influence of temperature on the interfacial and mechanochemical processes involving calcite, with implications in natural processes and industrial manufacturing.