Thermal Activation of Nanoscale Wear

Nanoscale wear tracks on ionic crystals are created by reciprocating single asperity scratch tests using atomic force microscopy. The wear characteristics are analyzed by the scratch depth as a function of surface temperature from 25 to 300 K. The average wear depth shows a nonmonotonic behavior as a function of temperature, with a transition between two different regimes characterized by the occurrence of quasiperiodic ripple formation. A thermally activated bond breaking model quantitatively explains the wear data in the low temperature, nonripple regime, but fails above the temperature threshold. This discrepancy is resolved with a geometric separation of the ripple mounds from the troughs, leading to full agreement with Arrhenius kinetics over the full temperature range.

Figure 1

Experimental setup, typical topography images of the wear trench and time dependent friction force on a NaCl(001) crystal surface. (a) All experiments have been performed under UHV conditions by atomic force microscopy. To produce wear scars, a straight line perpendicular to the long cantilever axis has been repeatedly scanned. (b) Topography scan of the three trenches produced at $T=296.2\mathrm{K}$. (c) Typical changes of the average friction force during wear experiments at different temperatures using a normal force of 20 nN and a scan velocity of $1\mu \mathrm{m}/\mathrm{s}$.

Figure 2

Temperature dependence of the wear depth on ionic crystals. (a),(b) Schematic representations of the surface structures in regimes without (a) and with (b) ripple formation. In (b) a maximum wear depth $\mathrm{\Delta }{h}_{\max }$ is defined in addition to the average wear depth $\mathrm{\Delta }{h}_{\text{average}}$. (c),(e) temperature dependence of the wear depth for NaCl (c) and KBr (f) measured using normal loads of 20 and 10 nN, respectively, and a fixed scanning velocity $v=1\mu \mathrm{m}/\mathrm{s}$. The black spheres represent the average depth $\mathrm{\Delta }{h}_{\text{average}}$ along the trenches, while the red triangles represent the maximum wear depth $\mathrm{\Delta }{h}_{\max }$ found inside the trenches for temperatures during which ripple formation was observed. The dashed black lines show the fitting results in the ripple free region, which have been derived from the first seven data points of NaCl and the first five data points of KBr according to Eq. (2). For NaCl we used the parameters $\alpha =3.63\times {10}^{-5}$ and $\beta =0.04\mathrm{eV}$, while KBr can be described by $\alpha =1.78\times {10}^{-5}$ and $\beta =0.01\mathrm{eV}$. (d),(f) Number of ripples formed on NaCl and KBr.

Figure 3

Topography images of wear scars on a NaCl(001) crystal surface induced at different temperatures together with the corresponding height cross sections along the center of the trenches measured at different temperatures of (a) $T=255.1\mathrm{K}$, (b) $T=195.4\mathrm{K}$, (c) $T=112.6\mathrm{K}$.

Figure 4

Velocity dependent wear experiments on KBr at room temperature with a scan size of 400 nm. (a) The depth of the wear scar as a function of sliding velocity and (b) wear rate as a function of sliding velocities from $250\mathrm{nm}/\mathrm{s}$ to $10\mu \mathrm{m}/\mathrm{s}$. The solid lines represent fitting results by Eq. (2) with parameters $\alpha =4.18\times {10}^{-5}$ and $\beta =0.01\mathrm{eV}$ for maximum wear depth and $\alpha =3.02\times {10}^{-5}$ and $\beta =0.01\mathrm{eV}$ for the average wear depth.