"We are measuring changes in volume that are one thousand times smaller than can be seen using other techniques for wear detection."
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.
"If atomic attrition is what's happening," Carpick said, "then the rate at which those bonds are formed and the dependence on contact stress the force per unit area is well-established science. That means we can apply chemical kinetics, or reaction rate theory, to the wear process."
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 more force the atoms are under, the more likely they are to form a bond with an atom on the opposing surface, so the wear rate should accelerate exponentially with additional stress," Jacobs said. "Seeing that in the experimental data was a smoking gun. The trend in the data implies that we can predict the rate of wear of the tip, knowing only the stress levels in the contact, as long as this wear mechanism is dominant."
For now, those predictions can only be made about the wear of silicon on diamond in a vacuum, though the selection of those two materials was not accidental. They are common in nanoscale devic
|Contact: Evan Lerner|
University of Pennsylvania