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.
"We can watch the whole process live to see what happens while the surfaces are in contact," Jacobs said. "Then, after each pass, we use the TEM like a camera and take an even higher magnification picture of the tip. We can trace its outline and see how
|Contact: Evan Lerner|
University of Pennsylvania