Researchers have discovered a swirling, fluidlike behavior in a solid piece of metal sliding over another, providing insights into the mechanisms of wear and generation of machined surfaces that could help improve the durability of metal parts. Numerous mechanical parts, from bearings to engine pistons, undergo such sliding.

“We see phenomena normally associated with fluids, not solids,” said Srinivasan Chandrasekar, a Purdue University professor of industrial engineering who is working with postdoctoral research associates Narayan Sundaram and Yang Guo.

As reported in the September 7 issue of Physical Review Letters (10.1103/PhysRevLett.109.106001), the researchers observed what happens when a wedge-shaped piece of steel slides over a flat piece of copper. It was the first time researchers had directly imaged how sliding metals behave at the mesoscale.

The observations—using a microscope and high-speed camera—show how tiny bumps form in front of the steel piece, followed by the swirling vortexlike movement and then the creation of shallow cracks. The folding and cracking were most pronounced when the steel piece was held at a sharp angle to the copper surface.

The researchers hypothesize that the folding and cracking are due in part to a phenomenon similar to “necking,” which happens as a piece of metal is stretched.

The findings were surprising because the experiment was conducted at room temperature and the sliding conditions did not generate enough heat to soften the metal.

“It has been known that little pieces of metal peel off from sliding surfaces,” Chandrasekar said. “The conventional view is that this requires many cycles of rubbing, but what we are saying is that when you have surface folding you don’t need too many cycles for these cracks to form. This can happen very quickly, accelerating wear.”

Metal surfaces that have smaller grains may be less susceptible to the folding and crack formation.

“We need to explore what role grain size plays,” Chandrasekar said. “We think there should be some grain size below which this folding mechanism might be less active. We need to explore why—under what conditions—solid metals behave like fluids.”

This frame from a high-speed camera sequence reveals fluidlike behavior in a solid piece of metal sliding over another (300 μm). The white line is the manually identified surface and superimposed colored lines are streak lines produced from velocity measurements.Purdue University School of Industrial Engineering image/N. Sundaram and Y. Guo.