HIGHLIGHTS
• Researchers visualized real-time atomic friction for the first time by having an electrochemically etched tungsten probe side-tip make contact with a tungsten substate.
• An in situ transmission electron microscope was used to generate images at the interface between the two surfaces. 
• The mobile tip followed a zig-zag sliding pathway when interacting with the substrate. 
• Friction is present even if the surface involved is clean and smooth. 

This column has devoted many articles to researchers working to develop techniques for minimizing friction. As new technologies continue to be developed, gaining a better understanding about the origin of friction is becoming even more important.

The fourth revolution is leading to the wider use of robotics in manufacturing. Robots have their own set of challenges in controlling friction as was discussed in a previous TLT article.¹ The basic task of griping of a wet object occurs due to elastohydrodynamic friction. Human beings contain a high concentration of sensor receptors that allow gripping to occur easily. In contrast, robots do not have that internal ability to grip an object. Research in this article describes the use of a modified tribometer that models elastohydrodynamic friction for various materials and geometries. This should allow robots to conduct specific tasks on wet-patterned surfaces.”

Tribologists understand how friction occurs when two surfaces pass in close proximity to each other on a macroscopic level. But it would be very helpful to understand how the atoms present in each surface interact with each other on the nanoscale to gain a better idea of the origin and mechanism for friction.

Evaluation of friction at the atomic level has been done previously through measurement of force. Dr. Guofeng Wang, CNG Faculty Fellow and professor of mechanical engineering and materials science at the University of Pittsburgh in Pittsburgh, Pa., says, “Force measurement is conducted through the use of atomic force microscopy (AFM)-based technologies. However, the physical sliding scenario between the contacts and buried surface/interface deformation during friction have proven to be elusive due to the lack of direct interface observation in the AFM-based experiments.”

Two surfaces interacting with each other may appear to be flat at the macroscopic level, but this is not the case at the atomic level. Surfaces at the atomic level are not flat but contain atoms that are situated above the others in what are known as asperities. Friction at the atomic scale is taking place as asperities on different surfaces encounter each other in space. Dr. Xiang Wang, postdoctoral research associate at Pacific Northwest National Laboratory and formerly a graduate student at the University of Pittsburgh, says, “The biggest challenge is to achieve atomic-scale contact between asperities with a well-defined interface.”

Instrumentation examining the atomic scale continues to improve, and now researchers have used a technique known as in situ transmission electron microscopy (TEM) to generate images of the interface structure between asperities in different surfaces that encounter each other. Guofeng Wang says, “Our work represents the first time that atomic-scale friction is visualized and not just estimated by simulations.”



Single tungsten asperity
Guofeng Wang, Xiang Wang and Dr. Scott Mao, adjunct professor of mechanical and materials engineering at the University of Pittsburgh, and their colleagues utilized their experience in working with atomic-scale in situ TEM studies on the deformation of nanocrystals to prepare a single crystal with a specific orientation as the substrate for their study. The material used was tungsten, and a nanosized crystal with [001] zone analysis was selected as the single asperity candidate. 

To assess the atomic-scale friction produced from this single tungsten asperity, the researchers produced an electrochemically etched tungsten probe side-tip to be the mobile asperity to make contact with the tungsten substrate. Xiang Wang says, “Tungsten was selected for this study because this refractory metal exhibits high strength and good thermal stability, preventing severe deformation of the asperity and surface thermal diffusion during the friction process. These two phenomena would potentially interfere with determining the mechanism for atomic-scale friction.”

The two asperities slid with each other and were then separated by controlling the lateral and longitudinal movement of the probe, which was driven by a scanning tunneling microscope piezo-system. Xiang Wang says, “A nanocontact was established to form a friction pair between the tip and the substrate by precisely tuning the separation distance and alignment. Friction was generated by driving the lateral motion of the tungsten tip, and the scanning velocity was controlled through the tip motion step (approximately 0.004 nanometer).”

Real-time atomic friction was visualized by the researchers through the use of TEM.

Xiang Wang says, “The interface energy landscape formed by the interaction between the tip and the substrate provides energy barriers for the movement of the mobile tip, leading it to choose a zig-zag stick-slip sliding pathway. As the tip overcomes one energy barrier, the accumulated energy would be released, making the tip atoms trapped in another potential well with low strain energy. The zig-zag pathway gives the lowest energy barrier for movement of the mobile tip.”

The researchers used molecular dynamic simulation to confirm the zig-zag pathway of the discrete stick-slip behavior observed empirically. Guofeng Wang says, “The importance of the modeling work was that in a controlled environment, the same zig-zag stick-slip frictional pattern found in the empirical studies was obtained.”

Future work will concern analysis of the detailed structural evolution of the interfacial structure as friction occurs. A second objective will be to determine how the crystal orientation relation between asperities affects friction and what the effect of contact area is on friction.”

Guofeng Wang says, “We have now seen atomic-scale friction for the first time and have found that friction is present even if the surface is clean and smooth. Our work also will include evaluating other material systems including metals more commonly used in machinery applications.”

Additional information can be found in a recent article² or by contacting Guofeng Wang at guw8@pitt.edu or Mao at sxm2@pitt.edu.

REFERENCES
1. Canter, N. (2021), “Better understanding of elastohydrodynamic friction in robotics and haptics,” TLT, 77 (8), pp. 16-17. Available here.
2. Wang, X, Liu, Z., He, Y., Tan, S., Wang, G. and Mao, S. (2022), “Atomic-scale friction between single-asperity contacts unveiled through in situ transmission electron microscopy,” Nature Nanotechnology, 17 (7), pp. 737-745.