Explorations for fundamental mechanisms of friction and wear can lay foundation for their real-world applications, which are hindered by the complex contacting surfaces at the macroscale under various environmental conditions. In situ transmission electron microscope (TEM) techniques demonstrate the ability to conduct frictional studies for single-asperity contacts at the microscale and to link frictional behaviors with dynamically physical events near the interface, providing well-defined interface under well-defined conditions. In this work, we conducted systematically investigations on friction and wear mechanisms for submicron-sized single-asperity metallic contacts via in situ TEM triboprobes.

First, we developed an in situ TEM approach to conduct friction tests. Taking advantage of the magnetic field inside TEM, a unique Lorentz-force-actuated method was developed via a commercial electromechanical holder. The friction force was obtained by tracking the elastic deflection of the fabricated cantilever from in situ video, with the normal force controlled by the built-in transducer of the holder. The transition processes from static to dynamic friction involved the elastic deformation, plastic deformation, and fracture near the interface for the elastic contact (W-W), the elastic-plastic contact (Ag-W), and the elastic-plastic contact with strong adhesion (Al-W), respectively. Experimental evidences demonstrated that the relative motion at the interface was not always commencing with the maximum friction force. And we propose that the arrival of maximum friction force was accompanied by the interfacial failure.

Then, we conducted friction tests for single-asperity worn contacts to explore effects of adhesion and contact-area evolution on the friction law (the linear relationship between friction force and external normal force). For the elastic contact (W-W pair), the static coefficient of friction (SCOF) increased from 0.1 to 0.4 with the gradual wear of the natural oxide layer under constant normal forces, which was ascribed to the increase of adhesive forces at the interface. For a series of friction tests with increasing normal forces, the SCOF kept an almost constant value for contacts hardly with adhesive forces, while exhibiting a gradually decreasing trend for contacts with strong adhesion. The adhesive forces at the interface would not influence the linear relationship between the friction force and the normal force. For the elastic-plastic contact (Ag-W pair), the Ag asperity could experience plastic deformation before sliding, resulting in a contact area increment. The relative increment of contact area at sliding inception to that value at the initial point of friction would lead to a high value of SCOF. For a series of friction tests, the relative increment of the contact area decreased with increasing normal forces, leading to a decreasing trend of the SCOF and a nonlinear relationship between the friction force and normal force.

Finally, we explored the adhesive wear mechanisms for single-asperity elastic-plastic contacts (Al-W pair). Under low normal stress, at the early stage of sliding, dislocations emerged from the sliding interface serving as heterogeneous dislocation sources and accumulated to form a dislocated interface which would further stop the dislocations from penetrating. With the sliding distance increasing, the dislocated interface could evolve to a new grain boundary, which replaced the Al-W interface and served as a new sliding interface. Under high normal stress, cracks were generated and propagated, resulting in a piece of wear debris.

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