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Penn Research Shows Mechanism Behind Wear at the Atomic Scale

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An illustration of a silicon AFM tip sliding over a diamond surface, with a TEM image of the tip inset. (Image Source: University of Pennsylvania; Art: Felice Macera)

Wear is a fact of life. As surfaces rub against one another, they break down and lose their original shape. With less material to start with and functionality that often depends critically on shape and surface structure, wear affects nanoscale objects more strongly than it does their macroscale counterparts.  Worse, the mechanisms behind wear processes are better understood for things like car engines than nanotech devices. But now, researchers at the University of Pennsylvania’s School of Engineering and Applied Science have experimentally demonstrated one of the mechanisms behind wear at the smallest scale: the transfer of material, atom by atom, from one surface to another.  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.  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.