Sliding Metals Show Fluid Behavior

When metals slide across each other, their surfaces form tiny bumps, swirls, vortexes and cracks. Researchers at Purdue University who discovered the surprising behavior say further study could lead to more durable surfaces for metal parts.

The scientists at Purdue's School of Industrial Engineering captured the fluid-like effect with a high-speed camera as they slid an angled piece of steel across a flat copper surface. Their work is reported in the Sept. 7 issue of Physical Review Letters and discussed in a separate article in Physics.

The team believes their work is the first to directly record in real time the behavior of sliding metal surfaces at the mesoscale (100 microns to 1 millimeter). The tests were performed at low speed (1 millimeter per second) and room temperature and did not generate enough heat to soften either metal, according to the scientists.

Metallurgists know that small bits of metal peel off metal surfaces that rub against each other, presumably because of surface roughness. The Purdue research shows that flaking also can be caused by variations in a metal's grain, in which case wear can occur very quickly.

The team likens the phenomenon to the "necking" that occurs when metal is stretched. They suspect that relatively large grains are more susceptible than finer-grain surfaces folding and cracking. The researchers speculate there may be grain sizes below which the fluidlike behavior is less active.

The Purdue research was funded by the National Science Foundation, U.S Army and General Motors Co.