The Nature of Friction
By Canter, Neil
Friction remains an enormous challenge for those of us in the lubricant industry to overcome. From a macroscale standpoint, we know that when one object slides on a second object, friction occurs, leading to a loss of energy through the generation of heat. Friction can cause mechanical components to unnecessarily heat up, and to wear out prematurely. A myriad of different lubricant types and technologies are available to deal with the friction problem in applications ranging from air compressors to worm gears. But there is still a lack of understanding of how friction is occurring on the atomic level.
In a previous TLT article, development of a solid lubricant that produced a near-frictionless carbon coating on a metal surface was described. The lubricant exhibited coefficient of friction values that are less than 0.01.1
STLE member Robert Carpick, associate professor in the department of mechanical engineering and applied mechanics at the University of Pennsylvania and Penn Director of the Nanotechnology Institute, says, “We know that friction converts translational kinetic energy into vibrational energy, which generates heat. But we do not know how this occurs.”
Evaluation of the vibrational properties of atoms may provide an indication of how friction occurs at the atomic level in a loaded nanoscale contact. Such a study has been explored at the theoretical level through computer simulations, according to Carpick but not experimentally examined until now.
Isotopic effect
The key to better understanding the role of vibrational energy is to understand that while they are held in place by bonds, atoms are constantly shifting and moving through space with tiny amplitudes. Many of us who are chemists evaluate these bond stretches and bends through the use of infrared analytical tools such as FT-IR. Different atomic combinations exhibit vibrational energy frequencies at different positions in the infrared spectrum, which enables chemists to obtain compositional information about specific molecules.
Carpick and his research team went about studying friction by deliberately applying a specific type of atom as a single layer onto a surface and then measuring how effective the atom is in resisting the motion of the tip of an atomic force microscope (AFM) as it transverses across that same surface. Carpick says, “We decided the best approach was to compare the vibrational energy generated by atoms that exhibited the same chemical behavior but differed in their atomic weight.” In other words, isotopes of the same element were used.
Carpick felt this was the way to examine vibrations because it eliminated the possibility that different chemical forces could occur when the AFM tip interacted with the different single layers of atoms on the surface of a substrate. The best prospect for seeing an isotopic effect was to use the low-mass element, hydrogen, and its isotope, deuterium.
In order to carry out these experiments, a detailed procedure was undertaken to deposit either hydrogen or deuterium atoms on a single- crystal diamond surface and on a silicon surface. The technique used on diamond was the hot filament process. A wet chemical procedure was employed with the silicon substrate.
Carpick explains, “The hot filament process was needed on diamond because of the difficulty in working with this surface. A major challenge is that almost any substance contacting clean diamond stays on the surface and is difficult to remove. For this reason, we needed to eliminate the major sources of contaminants (such as oxygen, metals and water) prior to applying hydrogen or deuterium on the surface.”
The challenge with preparing the silicon surface was to ensure the stability of the single layer of hydrogen or deuterium. Carpick adds, “Silicon-hydrogen and silicon-deuterium bonds slowly oxidize when exposed to oxygen and water. The surface was stored and tested under an atmosphere of dry nitrogen to prevent its decomposition.”
Checking that the single layer of atoms was really remaining bonded to the surface presented a second challenge. Carpick indicates that a number of measurement techniques, including infrared spectroscopy, were used to map the trillions of atoms per square centimeter of surface to make sure the pure, single layer of hydrogen or deuterium was present.
The experimental setup in which the AFM tip interacts with the sample is shown in Figure 3. The tip is on a cantilever that is in mechanical contact with the surface. Force measurements are obtained by reflecting a laser spot from the cantilever into a quadrant detector.
Carpick says, “We pushed the AFM tip back and forth across the single layer of hydrogen or deuterium atoms. Data was then collected over a range of applied normal loads.”
The kinetic energy generated by the movement of the AFM tip across the single layer of hydrogen or deuterium atoms was absorbed by these atoms. This in turn led the atoms to vibrate in order to dissipate the energy.”
Data collected over a range of loads clearly shows there is a difference in the amount of friction generated by the hydrogen and deuterium atoms. Surfaces with deuterium exhibited a 30% drop in friction regardless of the conditions as compared to hydrogen.
Carpick says, “Hydrogen and deuterium atoms on the surface are vibrating and can slow the AFM tip down. The reason that hydrogen generates more friction is because hydrogen atoms are colliding more frequently with the AFM tip due to their smaller size as compared to deuterium. In contrast, deuterium has a larger mass and, as a result, a lower natural vibration frequency. This means that vibration energy is dissipated at a slower rate than with hydrogen.
Carpick adds, “As a crude analogy, a sailing ship in rough seas will encounter rough waves very quickly that force it to slow down. A sea with slower moving waves will not be slowed down as much.”
Implications
At this point, it is difficult to figure out how these findings will impact the development of new lubricants. Carpick says, “We do not know the consequences of this finding on a macroscopic level. Insight has been gained in our work into the contribution vibrational properties have to the dissipation of frictional forces in a very specific case, and only at the nanometer scale. The vibrational properties of other molecular groups should be studied to determine their impact in reducing friction.”
Carpick believes that the friction phenomenon is a rich, complex puzzle. Vibration is just one aspect of the answer. Future work will focus on the evaluation of other systems using hydrogen/deuterium and by refining the isotopie purity of samples. Besides hydrogen, other low mass elements will be evaluated.
Additional details on this work can be found in a recently published paper.2
‘We know that friction converts translational kinetic energy into vibrational energy which generates heat. But we do not know how this occurs.’
The best prospect for seeing an isotopic effect was to use the low mass element, hydrogen, and its isotope, deuterium.
References
1. Canter, N. (2005), “Developing a NearFrictionless Carbon Coating,” Tribology and Lubrication Technology, 61 (10), pp. 12-14.
2. Cannara, R., Brukman, M., Cimatu, K., Sumant, A., Baldelli, S. and Carpick, R. (2007), “Nanoscale Friction Varied by Isotopie Shifting of Surface Vibrational Frequencies,” Science, 318 (5851), pp. 780-783.
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
Copyright Society of Tribologists and Lubrication Engineers Feb 2008
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