Careful friction tests for a nanolubricant in the elastohydrodynamic lubrication regime showed that the presence of nanoparticles reduces friction. By using surface analyses and molecular dynamics simulation, the effectiveness of different interactions between the system components was explored. Based on the results, a novel friction reduction mechanism was deduced in which the nanoparticles can induce an obstructed flow in the thin film between lubricated surfaces.
It has been recently reported that nanoparticle additives can improve the lubricity of lubricants.1-2 However, most of the studies were aimed at the performance of the nanoparticles in the boundary lubrication regime in which substantial contact occurs between the surfaces. This work focuses on the performance of nanoparticle additives on thin film elastohydrodynamic lubrication (EHL) in which no contact occurs between the lubricated surfaces. EHL3 and thinfilm lubrication4 have been the topic of extensive studies in the past 60 years. The hypothesis of the current work is that nanosized particles can infiltrate into the small gaps between surfaces in the EHL regime and when the size of the gap is comparable to the particle's size and the film pressure is high, particles may affect the friction properties of the contact pairs.
The nanolubricant used in the experiments contains silver nanoparticles with an average size of 7 nm suspended in polyethylene glycol (PEG 600), and polyvinylpyrollidone (PVP) is used as the coating agent to stabilize the suspension. The nanolubricant has 3 mM silver and 1.5 mM of PVP in PEG 600 and the control lubricant consists of 1.5 mM of PVP in PEG 600. Friction tests were performed with a submerged pin-on-disk friction test setup and an electrical contact resistant sensor (ECR) was used to detect contact between conductive surfaces. Upon completion of the tests, the profiles of the pins and the disks were measured in order to accurately estimate the contact pressure (using the general Hertzian contact solution). The results plotted in Figure 1 show the contact pressure increases. The coefficient of friction (COF) first decreases and then increases, which is a typical behavior of lubricating pairs in the EHL regime. The results indicate that the nanoparticle additives reduce friction in the EHL regime.
SURFACE ANALYSIS AND MOLECULAR DYNAMICS SIMULATIONS
The contact system consists of three components: particles, surfaces and the lubricant. Therefore, surface analysis and molecular dynamics (MD) simulations were performed to investigate the possible interaction of the components in search of the dominant mechanism responsible for reducing friction.
Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) spectra were recorded, both inside and outside the contact groove. Results indicated that there is slightly more Ag inside the wear groove than outside the groove. Figure 2 shows the possible scenarios of the nanoparticles/surface interaction, which are (1.) the nanoparticles deposit on the surface which includes (i) particles deposit in the valleys, (ii) particles coalesce to form nanostructures or (iii) particles bond to the surface (Figure 2(a)) or as illustrated in Figure 2(b), (2.) particles loosely adhere to the surface. The next step was to study the configuration of the silver particles on the surface and detect possible chemical bonding between the nanoparticles and the surfaces. Ar sputter etching was used to drill -12.5 nm into the surface (slightly over the average size of a particle) for this purpose. AES results before and after the drilling show that the Ag signal completely vanished after drilling (see Figure 3). Figure 2 schematically shows the effect of drilling on particles on the surface (see dashed blue line). In the first scenario (see Figure 2 (a)), it is assumed that the nanoparticle deposit on the surface and inside the valleys. In this scenario, drilling does not remove all of the nanoparticles. In contrast, in the second scenario in which individual nanoparticles are loosely adhered to the surface (see Figure 2 (b)), drilling removes the Ag nanoparticles, which is the phenomenon observed in our tests. Therefore, the surface analyses suggest that Ag resides strictly on the surface in the form of individual nanoparticles loosely adhered to the surface. Hence particles don't change the roughness of the surface by forming nanostructures.
Next, molecular dynamics (MD) simulations were utilized to study the system. Simulations were performed on a smaller system containing a nanoparticle with a diameter (D) of either -24 or 37 Â suspended within the gap (G) of 54 Â filled with lubricant molecules confined between two rigid walls. The geometries in the MD simulations were chosen to have the similar ratio of particle size over gap size as the experiments. Control lubricant (without nanoparticle) or nanolubricant systems were initially equilibrated under isobaric-isothermal (NPT) ensembles at T=300 K and atmospheric pressure without the walls. These systems were brought into contact with the two walls and subjected to compression. The walls were sheared in opposite directions until the system reached the steady-state condition. Figure 4 exhibits the calculated COF versus pressure for the control and nanolubricant cases. The general trend of the friction results from the MD simulations, and the experiments are similar. By monitoring the motion of lubricant molecules, we found that the presence of a nanoparticle forces the medial layers of the lubricant's molecules to move along with the nanoparticle, promoting an obstructed flow (plug flow). This would result in sliding of layers of the lubricant's molecules over one another adjacent to the walls. In other words, shearing occurs over only a few layers of lubricant adjacent to the walls. This effect is similar to the thin-film lubrication where surface forces and high pressure promote layering of the molecules and sliding of layers over one another, which, in turn, leads to a reduction in the COE
In summary, a new friction reduction mechanism was proposed for nanolubricants in the thin-film EHL regime. Through careful friction tests, it was demonstrated that nanoparticles reduce the COF in the thin-film EHL regime. The proposed mechanism suggests a new method to enhance friction performance in the EHL regime, which is the addition of nano-sized particles to infiltrate into the gaps and enhance local lubricity.
This material is based upon work supported by the
For a closer look at Hamed Ghaednla's poster abstract, be sure to check out his short video presentation in the December digital version of TLT (available at www.stle.org)
1. Martin, J.M., and Ohmae, N. (2008),
2. Ghaednia, H., Jackson, R.L., and Khodadadi, J.M. (2013), "Experimental Analysis of Stable Cuo Nanoparticle Enhanced Lubricants,"
3. Greenwood, J. (1988), "Film Thicknesses in Circular Elastohydrodynamic Contacts," Proceedings of the Institution of Mechanical Engineers, Part C:
4. Gao, J., Luedtke, W, and Lman, U. (1995), " Nano-Elas tohydrodynamics : Structure, Dynamics, and Flow in Nonuniform Lubricated Junctions," Science, 270(5236), pp. 605-608.
Jeyhoon M. Khodadadi,
Hamed Ghaednia received bachelor's of science degrees in mechanical and chemical engineering from Tehran Polytechnic, along with a master's of science in mechanical engineering from Tehran Polytechnic in 2010 while working as a graduate assistant in the
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