Fundamental aspects of friction and wear contacts in <100> surfaces

Unexpected friction and wear transitions occur in transition metals associated with dislocation emission, dislocation storage, and oxide break-through phenomena. Both normal nanoidentation and nanoscratch evaluations of conical diamond tips driven into tungsten {100} single crystal surfaces have been conducted. In terms of initiating plasticity under the contact, this represents a high Peierl's barrier for dislocation motion in transition metals. Both quasi-equilibrium and kinetic aspects are reported along with current but speculative ideas on multiple friction and wear transitions. Preliminary results show that yielding under contacts can produce a 250 nm displacement excursion. Ramifications are seen in terms of friction coefficients which can double during the near-instantaneous yield excursion but then continue to triple from about 0.05 to 0.15 in the pile-up phase in front of the sliding contact. Implications of how nanotribological issues such as adhesion connect through this mesoscale activity to macroscopic friction and wear are discussed.