Researchers at Purdue University in Indiana found solid pieces of metal that slide over each other to display properties resembling fluids rather than solids. Their research — funded by National Science Foundation, U.S. Army and General Motors — appears in the journal Physical Review Letters (paid subscription required).
The team led by materials engineer Srinivasan Chandrasekar (pictured left) offers insights into mechanisms of wear and generation of machined surfaces that can help lengthen the life and improve the performance of common metal parts, such as bearings and engine pistons.
Chandrasekar, with postdoctoral fellows Narayan Sundaram and Yang Guo, observed the behavior of a wedge-shaped piece of steel sliding over a flat piece of copper. It was the first time researchers had directly imaged how sliding metals behave on the scale of 100 microns — 1 micron equals 1 millionth of a meter — to 1 millimeter.
The researchers used specialized lab equipment, including a high-speed camera and a device that applies force to the sliding metals. The behavior was captured in movies that show the flow in color-coded layers just below the surface of the copper specimen. Copper is used to model the mechanical behavior of metals.
With this equipment, the Purdue team found tiny bumps form in front of the steel piece, followed by a swirling spiral movement and then the creation of shallow cracks. The folding and cracking are most pronounced when the steel piece is held at a sharp angle to the copper surface.
Metals are made of groups of crystals called grains. Metal surfaces that have smaller grains may be less susceptible to the folding and crack formation. The experiments were conducted at room temperature and the sliding conditions did not generate enough heat to soften the metal, which made the results more surprising.
“The conventional view is that this requires many cycles of rubbing,” says Chandrasekar, “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.”
The researchers say they are developing models to further study the phenomena and understand the consequences of these fluid-like properties in metals. The findings could lead to improved surface qualities in materials processing.
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