Negative friction coefficient

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2013

A training-exercise study initiated a better understanding of how a negative coefficient of friction can be seen at the nanoscale.

KEY CONCEPTS
• The process for moving an AFM tip across a nanosurface can be divided into four steps.
• During this process, a negative friction coefficient was reported for the first time because the friction encountered on the graphite surface increased as the pressure applied by the AFM tip decreased.
• This effect is due to an increase in the oxygen content on the graphite surface that rendered it more hydrophilic.

IN DEALING WITH FRICTION, those in the lubricants industry believe that an increase in the normal load applied in moving an object across a surface is directly proportional to the friction. For example, this means that the more load applied to pushing a box containing paper across a floor leads to an increase in the frictional force that is resisting this motion.

This observation forms the basis for Amontons’ law of friction. Another way to express this relationship is that a positive friction coefficient is seen when two surfaces slide against each other.

Even at the nanoscale, this law seems to work though the relationship between friction and normal load is typically nonlinear. One reason for this statement is the realization that surfaces at the macroscale are not smooth but, rather, exhibit a topography similar to a mountain range with peaks and valleys.

In a previous TLT article, a study on friction at the nanoscale was discussed that showed the thickness of the material studied has a direct effect on the degree of friction (1). The tip of an atomic force microscope (AFM) was moved against atomically thin sheets consisting of materials such as hexagonal boron nitride, graphene, molybdenum disulfide and niobium diselenide in a technique known as friction force microscopy.

The resulting friction generated increases with decreasing atomic thickness. Though not intuitive based on macroscale experience, the reason for this effect is that bending a single layer of atoms with the AFM tip is much easier than doing the same process with multiple layers. This leads to the occurrence of an attractive Van der Waals’ force that causes the single atomic layer to conform more to the tip, leading to greater contact area and higher friction. The degree of friction seen is two to three times higher than with bulk layers of atoms.

Rachel Cannara, team leader at the Center for Nanoscale Science and Technology at the National Institute of Standards and Technology in Gaithersburg, Md., says, “When using the AFM tip to evaluate a nanosurface, the process can be divided into four steps. The tip first indents the surface during the approach. This is followed by sliding the tip along the surface where friction typically increases. In the third step, the tip is retracted but typically lifts the top layer of atoms. In the final step, the tip is allowed to slide on the top layer, which is more easily deformed.”

An image showing the AFM tip lifting the surface layer during the third step is shown in Figure 1.


Figure 1. During the retraction step, an AFM tip is shown lifting the surface layer. In doing such an experiment in the presence of oxygen, a negative coefficient of friction has been reported for the first time. (Courtesy of the Center for Nanoscale Science and Technology, National Institute of Standards and Technology)

STICKY GRAPHITE
A study that was originally started as a training exercise to help an individual understand how to use the AFM tip on a graphite surface turned into a research project, as experimentation showed that a negative friction coefficient is possible. Cannara says, “Originally, we started a training exercise to reproduce known properties of a freshly cleaned graphite surface. But we found that under the right conditions, the friction encountered on the surface increased as the pressure applied by the AFM tip decreased. This effect was seen when the AFM tip retracted from the graphite surface. A negative friction coefficient has not previously been reported.”

Initially, this effect was seen after the researchers exposed the graphite surface to lab air. Cannara says, “Literally, we let the sample sit for a while and then did a friction loop to evaluate the average friction as a function of applied load.”

This change in frictional behavior is caused by the presence of oxygen on the graphite surface. Typically, moving the AFM tip over a surface is done in a nitrogen atmosphere. By introducing 1% oxygen by weight (ambient air), the researchers studied changes in friction by varying the length of time a freshly cleaved graphite surface is exposed to oxygen.

Cannara says, “The oxygen adheres to the graphite surface, rendering it more hydrophilic. Over a 300-hour period, the amount of oxygen that can adhere to the graphite surface can increase to between 3.0 and 3.5 atomic %.”

When the experiments are run in air, water also starts to adhere to the graphite surface, adding to the increased adhesion seen with the AFM tip. Cannara adds, “We think that the water may form a meniscus on the AFM tip that aids its sticking to the graphite surface.”

X-ray photoemission spectroscopy (XPS) was used to evaluate graphite samples exposed to air for various time periods. Atomistic and continuum- based modeling were conducted to determine the reasons for this effect. The findings indicate that the negative friction coefficient is created when the AFM tip retracts a single to a few layers of graphene (two-dimensional sheets of carbon atoms found in graphite), leading directly to an increase in friction even as the applied load is decreased.

Future work will involve development of a complete model to explain how the graphene sheets interact with the AFM tip. Additional information can be found in a recent article (2) or by contacting Cannara at rachel.cannara@nist.gov. 

REFERENCES
1. Canter, N. (2010), “Size Does Matter for Nanoscale Friction,” TLT, 66 (8), pp. 10-11.
2. Deng, Z., Smolyanitsky, A., Li, Q., Feng, X. and Cannara, R. (2012), “Adhesion-Dependent Negative Friction Coefficient on Chemically Modified Graphite at the Nanoscale,” Nature Materials, 11 (12), pp. 1032-1037.


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.