Friction at Atomic-Scale Surface Steps: Experiment and Theory
  Experiments performed by friction force microscopy at atomic-scale surface steps on graphite, MoS$_2$, and NaCl in ambient conditions are presented. Both step-down and step-up scans exhibit higher frictional forces at the edge, but distinguish in their load dependence: While the additional frictional force due to the step edge increases linearly with load if the tip has to jump a step up, it remains constant for downward jumps. This phenomena represents a universal effect that can be explained in terms of a modified Prandtl-Tomlinson model featuring a Schoebel-Ehrlich barrier at steps. 
Hendrik Holscher, Daniel Ebeling, and Udo D. Schwarz