Will Polycrystalline Platinum Tip Sliding on a Gold(111) Surface Produce Regular Stick-Slip Friction?

Friction measurements by an atomic force micro-scope (AFM) frequently showed regular stick-slip friction signalswith atomic-scale resolutions. Typically, for an AFM metal tipsliding on a metal crystal surface, the microstructure of the tipmade from the thermally evaporated metal coating on a siliconcantilever was polycrystalline. Our detailed molecular dynamics-(MD) simulations of a polycrystalline Pt tip (R= 10 nm in radius)sliding on an Au(111) surface revealed how the geometry of the polycrystalline tip took effect on the friction behavior at the contactinterface. We found that the apex of the Pt tip with multiple grains near the edge of contact could induce severe plastic deformationsof the gold substrate, leading to irregular stick-slip frictions upon sliding. Simulation results showed that in order to achieve a clearstick-slip friction signal with single atomic slips, the apex of the Pt tip must adopt a single crystalline protrusion without anyneighboring grains involved in the metal contact. We showed that such a single crystalline protrusion, which presumably could beachieved during initial run-in or wear-out of high-energy Pt atoms in the neighboring grains, was passivated by a large number ofgold atoms due to metal adhesion in the contact periphery. Using such a crystalline protrusion tip, we demonstrated that the stick-slip friction produced was very"tolerant"to the adhesion of a large number of gold atoms on the tip apex. We further showed thatAFM tip mass used in MD simulations also played an important role in determining the transition between friction regimes, whichcould be well explained by the Prandtl-Tomlinson thermal activation model.