A team of researchers at the Purdue University has found that when solid metals were slid one over another, a swirling fluid-like behavior occurred. This discovery provides new clues to the property of wear in materials and may help develop more durable metal parts. Sliding occurs in many types of machinery, including engine pistons and bearings.
The team consisted of professor of industrial engineering, Srinivasan Chandrasekar, and postdoctoral research associates, Yang Guo and Narayan Sundaram. They utilized a high-speed camera and a microscope to study the phenomena.
The swirling fluid-like behavior has been observed in fluids, but not in solids. The sliding between metals resulted in formation of folds, bumps, cracks and vortex-like features on the surface of the metal. These formations occurred at normal room temperature.
Conventionally it has been considered that many cycles of rubbing cause metals to peel off. However, the researchers hypothesize that when surface folding exists cracks occur very quickly, which speeds up wear.
In a specialized laboratory, the team slid a wedge-shaped piece of steel over a copper piece. A high speed camera was used to capture the material behavior in video form. Force was applied to the slide the metals. The captured video revealed the flow that occurred in the copper material below its surface. The flow was shown as color-coded layers. Tiny bumps formed before the steel piece. This was followed by a swirling vortex-like movement, which was followed by the formation of shallow cracks.
When the steel piece and the copper surface were at sharp angles more folding and cracking occurred. This may be partly due to the "necking" phenomenon that occurs when metal is stretched.
Grains, which are crystal groups make up metals. Surfaces of metals that have small grains may be able to withstand folding and crack formation better. More study on the function of grain size and the conditions under which solid metals behave like fluids have to be performed.
The study has been published in the journal Physical Review Letters.