Probing Friction's Origins

USING A TOOL usually found in a chemist's toolbox, a group of physicists and engineers has shown that surface vibrations play a key role in atomic-scale processes that generate friction. The study broadens understanding of this ubiquitous phenomenon and could lead to new ways to reduce the seemingly inescapable effects of wear and tear that erode nearly all moving parts.

Everyday experience shows that objects that slide, roll, or otherwise move with respect to other objects are subject to friction and wear. Rub your hands together and they get warm. Drive an automobile and the engine parts, tires, and other components heat up and slowly wear out. The same thing happens to knees, hips, and other body joints.

"Friction is so common, yet we still don't understand its origins at the atomic level," says Robert W. Carpick, a professor of mechanical engineering at the University of Pennsylvania.

Scientists who study tribology, the science of friction, wear, and lubrication, have proposed that the vibrational properties of atoms on a material's surface play a role in the fundamental processes that cause friction, Carpick says. He adds that such theories have been explored via computer simulations, but they haven't been verified experimentally.

The challenge in tying surface vibrations to friction experimentally has been coming up with a cleverly designed test that selectively compares the surface vibrations of two materials without interference from other factors, such as differences in the materials' chemical compositions and intermolecular forces.

Carpick and coworkers now report the results of such a test. By using isotopic substitution, a technique generally thought of as a chemist's tool, the team zeroed in on the dependence of friction on surface vibrations. Specifically, Carpick, his former graduate students Rachel J. Cannara and Matthew J. Brukman, and their coworkers measured the friction generated at the nanometer scale by dragging an atomic force microscope tip across a diamond surface that was coated with a layer of hydrogen or, in a second measurement, with a layer of deuterium. They did the same test on a silicon surface. In all cases, the group found that deuterium causes the friction to decrease by roughly 30% (Science 2007, 318, 780).

Explaining the findings, Carpick notes that hydrogen and deuterium bond to surfaces in the same way. But because deuterium is more massive, he says, it vibrates more slowly and transfers energy to the underlying surface more slowly. Thus, it creates less friction. The results suggest that vibrational properties should be considered when designing new types of lubricants, he adds.

Describing the work as "very significant," Lawrence Berkeley National Laboratory Senior Staff Scientist Miquel Salmeron remarks that "this is a beautiful illustration that shows how surface vibrations can control the friction properties of materials."