Adhesive wear can cause machine failure, particulate-matter air pollution and many other societal woes. Yet despite its impact on the economy and health, the physics behind wear has been poorly understood.

Experimental researchers have developed countless models of the adhesive wear process and debris formation. But in computational simulations based on the inter-atomic forces that act between the two surfaces, the surfaces instead become smoother as they rub against each other, and no sustained debris is produced. The challenge has been to reconcile the simulations and reality.

A new study led by École Polytechnique Fédérale de Lausanne (EPFL), published in the journal Nature Communications, has now reproduced the production of debris particles by adhesive wear in a computer simulation based on the atomic interactions between two surfaces. Their findings could pave the way toward diminishing energy and material losses.

“A team of Swiss scientists recently estimated that 20% of noxious particulate matter in the air comes from brake pads in our vehicles and road surface wear," says Ramin Aghababaei, a researcher at EPFL's Computational Solid Mechanics Laboratory. "And any machine with moving parts is subject to adhesive wear, which ultimately shortens its lifespan.”

Using simulations, EPFL researchers offer new insights into what happens when seemingly smooth surfaces rub against each other. Image credit: © Ramin Aghababaei 2016.Regardless of scale, the process is always the same: two surfaces—the gears of a watch, brake pads against their disks, an artificial joint against its socket—rub against each other. Short-ranged atomic forces acting between the surfaces, known as van der Waals forces, stick the surfaces to each other. If the adhesion is strong enough, small fragments of one of the surfaces can be peeled off, eventually leading to the formation of tiny debris particles.

What Aghababaei and his fellow researchers found was that, though the contact surfaces may appear to be perfectly smooth, at the microscopic level they are covered in tiny bumps. “Our simulation is the first to reproduce the generation of debris particles through adhesive wear," he says.

“What we found is that debris is only formed when the contact between the microscopic asperities on the material surfaces exceeds a critical length," Aghababaei adds. "When the contact is smaller, the materials instead become smoother as they rub against each other.”

One reason their discovery has eluded researchers for so long is the sheer size of the computer simulation it would take to capture asperities exceeding the critical length in an atomic-scale simulation. To make their discovery, the researchers had to run their simulation considering a model material with properties such that the simulation could capture the critical length scale for debris formation.

“We hope that our finding will spark some new activity in the experimental community and in the industries for which wear is important,” concludes Jean-François Molinari, the study’s senior author.