The behavior was captured in movies that show the flow in color-coded layers just below the surface of the copper specimen. Copper is commonly used to model the mechanical behavior of metals.

It has been known that little pieces of metal peel off from sliding surfaces. 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.

—Srinivasan Chandrasekar

The findings are detailed in an open access paper published in the 7 September issue of Physical Review Letters; a viewpoint commentary (de Beer and Müser) about the work is published in the journal Physics.

Sliding metal interfaces are important for the physics of wear, friction, machining, and generation of graded and nano-grained structures. Using high resolution in situ imaging and direct flow measurement in a relatively unstudied regime (≈100
m–1 mm) in prior sliding studies, we report the formation of mesoscale folds and primitive vortical structures at a free surface in sliding of annealed oxygen-free high conductivity copper. We deduce a mechanism for the formation of such folds using simulations. The phenomenon shows remarkable similarities with Kelvin-Helmholtz-type flow instabilities in fluids. Importantly, while such instabilities have been conjectured to exist in sliding interfaces at the nanoscale, our experiments show folding in metals at the mesoscale and away from the interface itself. The occurrence of folds impacts many applications, including surface generation processes and tribology. In particular, it may limit the quality of metal surfaces produced by repeated sliding treatments suggested in the past.

—Sundaram et al.

The team 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 on the scale of 100 microns to 1 millimeter, known as the mesoscale.

The observations show how tiny bumps form in front of the steel piece, followed by the swirling vortex-like 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.

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 researchers are developing models to further study the phenomena and understand the wide-ranging consequences of such fluid-like flow in metals, Chandrasekar said. The findings might also lead to improved surface quality in materials processing.

We need to explore what role grain size plays. 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.

—Srinivasan Chandrasekar

In their commentary on the work, de Beer and Müser concluded that improving the surface quality of machined metals lies in a better control of the grain geometry and its impact angle with the metal. This insight could enable the designing of future machining equipment in a more targeted and controlled fashion than presently possible.

Schematic sketch of the temporal and spatial evolution of the folding instability: The wedge, moving from right to left, displaces the material in the machined solid, causing a compressive stress in front of the wedge. (1) Due to this compressive stress, the grains with a larger deformability already form bumps several hundreds of microns in front of the wedge. (2) When different bumps are close enough, they often interact to form a fold. (3) Under the wedge the fold is flattened to finally form a tear (4) when it reappears from underneath the wedge. Thin red lines represent streak lines indicating laminar flow away from the surface. Credit: de Beer and Müser. Image: APS/Stonebraker. Click to enlarge.

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

Resources

• Narayan K. Sundaram, Yang Guo, and Srinivasan Chandrasekar (2012) Mesoscale Folding, Instability, and Disruption of Laminar Flow in Metal Surfaces. Phys. Rev. Lett. 109, 106001 doi: 10.1103/PhysRevLett.109.106001

• Sissi de Beer and Martin H. Müser (2012) Viewpoint: Surface Folds Make Tears and Chips. Physics 5, 100 doi: 10.1103/Physics.5.100