More durable metal parts in machines and improved surface quality of metal surfaces — just about a couple of benefits that have emerged out of a new study in which scientists studied the sliding of one metal on another in microscopic detail1.
Sliding metal surfaces are common in engineering components, such as bearings and engine pistons. Sliding contributes to the degradation of surfaces by wear, and is intimately linked to the phenomenon of friction. In addition, various sliding type processes like polishing and machining are commonly used to create components with ultra-fine grained surfaces of high strength.
There is a general notion that solids — unlike liquids and gases — do not alter their shape under a little stress. But Narayan Sundaram and co-workers were surprised to find that the response of solids is more fluid-like than believed.
Their experiment involved a copper metal piece sliding against a hard steel wedge. The deformation of the copper was studied live using high-speed, high-resolution imaging. "This was the first direct and live observation of sliding at the length scale of 100 micrometers to a few millimeters (the 'mesoscale')," Sundaram, an IIT Roorkee graduate now a postdoctoral fellow at Purdue University's Center for Materials Processing & Tribology, told Nature India .
The researchers found that for some wedge angles, the surface of the copper ahead of the wedge develops bumps, which then interact to form folds and vortex-like features. The fold is 'ironed-out' into a shallow surface crack after it passes the tip of the wedge. "The overall flow pattern bears a striking resemblance to what is seen in fluids and is highly meandering," says Sundaram. "This is a totally unexpected and surprising observation."
According to the researchers, the studies showed that fluid-like flow and deformation in metal can occur when they are made to slide under room temperature and low speeds. Here, the heat produced is too small to soften the metal and the observed formation of bumps and folds "is intimately connected to the grain size of the material."
Using computer simulations the researchers showed that the cause of fold formation and fluid-like flow is because ordinary metals are polycrystalline (made up of multiple crystals or 'grains', as they are commonly called) with different crystal orientations exhibiting different mechanical properties. For instance, one crystal on the surface might be softer than a neighboring crystal, deform more and produce a 'bump' on the surface. The bump grows in size like a 'neck' when a uniform metal rod is stretched beyond a point.
From the engineering point, their work overwrites the prevailing notion that cracks are formed after many cycles of sliding. "When folding occurs, cracks can be formed even in a single act of sliding. This is not related to accumulation of the deformation. Thus, it's a new mechanism of wear," the authors say. "Because of folding, the use of machines to prepare surfaces requires additional care," they report.
Sundaram said the work could lead to improved surface quality in machining processes and metals processing. "Better understanding of wear mechanisms could help improve the durability of metal components," he added.
The scientists, including Srinivasan Chandrasekar, an IIT Madras graduate now a professor at Purdue University, are exploring other possible consequences of fluid-like flow patterns in metals since their experimental system provides a unique way for engendering such flow patterns.
"We know that the average size of metal grains should play a key role in controlling the extent of folding and fluid-like flow, and a systematic study of this effect is also underway," Sundaram said.
