On the nanoscale, wear is mainly understood through two processes, fracture and plastic deformation. Fracture is where large pieces of a surface break off at once, like when the point of a pencil snaps off in the middle of a sentence. Plastic deformation is what happens when the surface changes shape or compresses without breaking, like when the edge of knife gets dull or bent.   

These mechanisms typically affect thousands or millions of atoms at a time, whereas nanoscale wear often proceeds through a much more gradual process. Determining the mechanisms behind this more gradual process is key to improving such devices. 

One wear mechanism that had been hypothesized for the nanoscale is a process known as atomic attrition. There, atoms from one surface are transferred to the other surface via a series of individual bond-forming and bond-breaking chemical reactions. Other researchers have attempted to test this process by putting two surfaces in contact and sliding one against the other.

Those previous investigations involved Atomic Force Microscopes. Using an AFM involves dragging a very sharp tip mounted on a flexible cantilever over a surface while a laser aimed at the cantilever precisely measures how much the tip moves. By using the tip as one of the surfaces in a wear experiment, researchers can precisely control the sliding distance, sliding speed and load in the contact. But the AFM doesn’t visualize the experiment at all; the volume of atoms lost from the tip can only be inferred or examined after the fact, and the competing wear mechanisms, fracture and plastic deformation can’t be ruled out.  

The Penn team’s breakthrough was to conduct AFM-style wear experiments inside of a transmission electron microscope, or TEM, which passes a beam of electrons through a sample (in this case, the nanoscale tip) to generate an image of the sample, magnified more than 100,000 times. 

By modifying a commercial mechanical testing instrument that works inside a TEM, the researchers were able to slide a flat diamond surface against the silicon tip of an AFM probe. By putting the probe-cantilever assembly inside the TEM and running the wear experiment there, they were able to simultaneously measure the distance the tip slid, the force with which it contacted the diamond and the volume of atoms removed in each sliding interval.     
While this new microscopy method can’t image individual atoms moving from the silicon tip to the diamond punch, it enabled the researchers to see the atomic structure of the wearing tip well enough to rule out fracture and plastic deformation as the mechanism behind the tip’s wear. Proving that the silicon atoms from the tip were bonding to the diamond and then staying behind involved combining the visual and force data into a mathematical test. 

Now that they could measure the volume of atoms removed, the distance the tip slid and the force of the contact for each experimental test, the researchers could calculate the rate at which the silicon-diamond bonds form under different conditions and compare that to predictions based on reaction rate theory, a theory that is routinely used in chemistry.

The math behind the atomic attrition mechanism could eventually be applied in a fundamental way. This fundamental, predicative understanding of wear could vastly improve nanomechanical design, increasing functionality and decreasing costs.

This story is reprinted from material from
University of Pennsylvania, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.