Peak Effect versus Skating in High Temperature Nanofriction

TL;DR

This study explores high-temperature nanofriction near melting points, revealing distinct behaviors for plowing and grazing, including friction drops and rises, with implications for metals with non-melting surfaces.

Contribution

It provides new microscopic insights into nanofriction phenomena near melting points, especially the contrasting effects for plowing versus grazing sliding.

Findings

Friction drops near melting point for deep plowing and wear.

Friction increases for grazing, wearless sliding near melting point.

Both phenomena are likely applicable to various metals with non-melting surfaces.

Abstract

The physics of sliding nanofriction at high temperature near the substrate melting point $\Tmelt$ is so far unexplored. We conducted simulations of hard tips sliding on a prototype non-melting surface, NaCl(100), revealing in this regime two distinct and opposite phenomena for plowing and for grazing friction. We found a frictional drop close to $\Tmelt$ for deep plowing and wear, but on the contrary a frictional rise for grazing, wearless sliding. For both phenomena we obtain a fresh microscopic understanding, relating the former to ``skating'' through a local liquid cloud, the latter to linear response properties of the free substrate surface. It is argued that both phenomena should be pursued experimentally, and much more general than the specific NaCl surface case. Most metals in particular possessing one or more close packed non-melting surface, such as Pb, Al or Au(111), that should behave quite similarly.