Fluid-like flow detected in metals at room temperature

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2012

Researchers found that sliding a copper metal surface against a hard-steel wedge causes the formation of folds.

KEY CONCEPTS
• By changing the rake angle to such an extreme that metal-chip formation ceases, researchers have shown that copper metal will exhibit fluid-like flow when it slides against a hard-steel wedge.
• Folds in the copper metal are formed, which shows copper displays non-laminar flow during this sliding process.
• This folding phenomenon is a new mechanism that suggests metal wear may occur more rapidly than currently believed.

RESEARCH DETERMINING THE SOURCE OF METAL FAILURE has focused on finding mechanisms for the formation of cracks. One approach is to detect the sources of metal wear, a prelude to final failure.

In a previous TLT article, the concept of fracture fatigue entropy was described as a way to predict metal failure (1). This new material property measures the degree of entropy that builds up in metals used over time. Through experimentation, a fracture fatigue entropy value is obtained that was found to be constant no matter how the experimental conditions are varied. This means that metal failure can be accurately predicted based on this parameter no matter the level of wear or damage seen on a specific machine.

In machining of metals, much is known about how a cutting tool interacts with a workpiece to generate a metal chip. Yang Guo, postdoctoral research associate at Purdue University in West Lafayette, Ind., says, “We have been studying the (fundamental) mechanics of metal cutting using in situ, high-resolution imaging.”

One area of interest to Guo and his fellow researchers at Purdue has been to change the rake angle to such an extreme that metal-chip formation ceases and the experiment moves into a sliding regime where one metal surface slides over a second metal. In this sense, the research has implications for most applications where machinery is involved, whether it is a bearing or an engine piston.

In the course of this research, a new phenomenon was observed when two metal surfaces slide against each other at room temperature. This phenomenon suggests that metal wear may occur more rapidly.

FOLDING
Working with Srinivasan Chandrasekar, professor of industrial engineering at the Center for Materials Processing and Tribology at Purdue, Guo along with Narayan Sundaram, postdoctoral research associate, have observed metal flow in a similar fashion to a fluid at room temperature when two surfaces are sliding against each other.

The researchers prepared an annealed workpiece from 99.99% OFHC (oxygen-free, high conductivity) copper alloy. Sundaram says, “We picked copper for this study because it is a good model material that is not too hard and tends not to adhere too readily to other surfaces.”

This workpiece was then slid against a hard-steel wedge prepared from high-speed Mo-Max M42 cobalt steel. Guo says, “We utilized a high-speed PCO Dimax camera with a CMOS sensor, coupled to a Nikon Optihphot optical microscope to image the deformation near the sliding interface as it occurred.”

In conducting the experiments, the researchers adjusted the rake angle or indenter angle of the two sliding surfaces. This is the angle between the face of the hard-steel wedge relative to the workpiece.

They found that at an indenter angle of -60 degrees, a series of bumps were formed on the surface of the copper workpiece that eventually turned into folds or cracks. Figure 2 shows a frame from the high-speed image sequence used by the researchers to film the interactions between the two surfaces at -60 degrees.


Figure 2. Sliding copper metal against a hard-steel wedge leads to the formation of folds that indicates metal can flow in a similar fashion to a fluid at room temperature. (Courtesy of Narayan Sundaram and Yang Guo, Purdue University)

The observations were made at the mesoscale, which ranges from 100 microns to 1 millimeter. The bumps have a maximum amplitude between 100 and 200 microns.

The formation of folds is surprising because it shows that the copper displays non-laminar flow in sliding. After another sliding pass, the researchers saw that the surface folds sheared off the copper workpiece as needle-shaped lamellae.

The implication of this observation is that folding can accelerate the rate of metal wear. The indenter angle is important in observing this effect. At other angles such as -70 degrees, only minor bumps are seen, and the flow of the copper workpiece against the hard-steel wedge is mainly laminar. Even fewer perturbations were observed when the indenter angle was changed to -80 degrees.

The mechanism for the formation of folds was determined using plane-strain finite element analysis (FEA). Sundaram says, “We simulated the mechanics of the sliding process to see if we could explain what was going on.”

Sundaram indicated that different copper grains have different degrees of hardness. He says, “The softer grains on the free surface exhibit localized deformation as the steel wedge approaches. This produces upward flow in the copper metal, leading to the formation of bumps. The bumps come into contact with each other or the wedge face, leading to the formation of a fold.” In effect, the localized deformation process can be described as necking, which is also seen when a piece of metal is stretched.

Guo points out that a ductile metal such as copper is required to observe fluid-like flow. He adds, “If a very hard metal is used, then a crack will form instead of a fold.”

The researchers consider the folding phenomenon to be a new mechanism for wear. Guo says, “In conventional wear, damage to metal surfaces occurs over many sliding cycles that eventually nucleate a crack. However, the formation of bumps and then folds causes cracks to form on the metal surface in a single cycle.”

The initial work was done dry in the absence of a lubricant. Guo reports that the folding has also been seen when a lubricant is used.

Future work will involve trying to duplicate the effect with a different metal such as low carbon steel instead of copper. Additional information can be found in a recent article (2) or by contacting Chandrasekar at chandy@purdue.edu. 

REFERENCES
1. Canter, N. (2011), “Predicting Metal Failure,” TLT, 67 (7), pp. 10-11.
2. Sundaram. N., Guo, Y and Chandrasekar, S. (2012), “Mesoscale, Folding, Instability and Disruption of Laminar Flow in Metal Surfaces,” Physical Review Letters, 109 (10), 106001.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.