The Role of Nanoparticles in Lubricants; Performing Lubricated and Dry Friction Tests
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FELLOWSHIP RESEARCH The Role of Nanoparticles in Lubricants; Performing Lubricated and Dry Friction Tests Hamed Ghaednia* and Dr. Robert L. Jackson (Advisor) Auburn University, Department of Mechanical Engineering, Auburn, Ala. *Ford Motor Co., Powertrain Research and Advanced Engineering Group, Dearborn, Mich. Editor’s Note: This month TLT profiles the 2014 recipient of The E. Elmer Klaus Fellowship, Hamed Ghaednia (Auburn University). The Klaus Fellowship, along with The E. Richard Booser Scholarship, are awarded annually to graduate and undergraduate students, respectively, who have an interest in pursuing a career in tribology. As a requirement for receiving an STLE scholarship, students are given the opportunity to participate in a tribology research project and to submit a report summarizing their research. Hamed Ghaednia graduated with a bachelor’s of science degree in mechanical and chemical INTRODUCTION engineering and a master’s of science degree Nanoparticles when suspended in in mechanical engineering from the Tehran a lubricant can infiltrate small gaps Polytechnic. He recently received his between rough surfaces in contact doctorate in mechanical engineering, with and alter the contact’s tribological an emphasis in tribology, from Auburn performance. Hence, nanoparticles University’s Samuel Ginn College of offer an alternative approach to Engineering’s tribology and lubrication lubrication by introducing third body science minor program. He is currently entities directly into the contact. The working as a tribology research engineer ability of nano-sized particles to pass at the Ford Motor Co. in the Powertrain through conventional filters, penetrate Research and Advanced Engineering Group. into contacts that larger particles You can reach him at [email protected]. cannot, along with the enhanced scale dependent properties of the nano-sized particles have made them a promising new type of lubricant additive. Different nanoparticles have proven and the effect of nanoparticles on wear EXPERIMENTAL APPROACH to reduce the coefficient of friction in is yet to be formalized. Furthermore, Friction and wear in a nano-lubricated base oils up to 25% and 50% in recent the interaction of nanoparticles interface is the result of nanoparticle- studies [1-3]. The dominant enhancing with base oil and additive package to-surface and surface-to-surface mechanisms of nanoparticles are components is not yet fully understood. interactions. The current work aims to uncertain or unknown with few These are the missing links in our investigate both of these interactions exceptions (such as for film forming understanding of the nano-lubricants independently to better understand particles). All of the proposed and hinders the industrialization of the role of the nanoparticles in nano- mechanisms for nanoparticles such as nano-lubricants in major applications. lubricants. Two sets of experiments rolling [4], transfer films [1], formation This work utilized strategically were designed to investigate the system of tribofilms [5], polishing [6] and designed experimental analyses to in depth. reducing the real area of contact [2, evaluate the role of nanoparticles and 7] would justify the friction reduction the lubricant separately in order to 1. Nanoparticles in dry form. In these induced by the nanoparticles. However, bring about a better understanding on experiments CuO particles in a physical understanding that explains some of these questions and study the powder form were used to conduct the synergic effect of these mechanisms role of nanoparticles in lubricants. dry friction experiments. The CuO 20 • JULY 2015 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG nanoparticles used in this work can be obtained in and out of solution 0.8 without any chemical interventions. Therefore, these particles are an ideal 0.7 candidate to perform dry friction tests to understand the explicit role 0.6 of nanoparticles in contact. 0.5 2. Base lubricant’s effect. In these COF experiments CuO particles are 0.4 suspended in (i) dodecane, (ii) polyalphaolefin (PAO) base oil and 0.3 (iii) fully formulated SAE 5W20 engine oil. The friction experiments Dry, w/o Nanoparticle on nano-lubricants with the 0.2 same type and concentration of Dry, with Nanoparticle 0.1 nanoparticles can reveal the role of 0 500 1000 1500 surface-to-surface interactions in Time [s] (Cycle) the system. Figure 1 | Pin-on-disk dry tests performed on CuO particles with the normal load of F=2.0 N. The CuO nanoparticles are coated with sodium oleate, are on average disk methods. Figure 1 shows the during the test and that is why there is 11 nm in diameter and are prepared coefficient of friction (COF) versus time an increase in the COF signal. in a chloroform solution according for the CuO nanoparticles dry test in In order to prove that the to [8]. The solution was applied comparison to the control test. It should performance of the particles on on the surfaces and dried using a be noted that the tests were done by a the surface is solely governed by heating lamp. This would leave dry pin-on-disk setup with a 1 Hz frequency. their concentration, another dry CuO particles in powder form on the Therefore the time axis is also the cycle test was run. In this experiment the surface for dry tests. Moreover, the axis in these tests. Figure 1 shows the nanoparticles were deposited on the same solution can be used to make results for a normal load of 2.0 N. In surface in powder form and the test the nano-lubricants. This was done by this test the dry CuO nanoparticles was run at a normal load of 2.0 N. As applying the CuO-chloroform solution reduce the COF significantly in the was expected, the dry CuO particles to the base lubricant and evaporating first 400 cycles. However the COF then stopped reducing the COF at about 400 the chloroform using heat and vacuum increases to the value of the control after cycles (see Figure 2). while stirring the solution. All the 400 cycles. However, this time the test was CuO nano-lubricants were prepared The hypothesis was that the stopped and dry CuO nanoparticles at a concentration of 5.0%wt of CuO nanoparticles are pushed out of contact were reapplied to the surface to account particles to highlight the particles’ effect in the experiments. It should be noted 0.8 that this is a very high concentration Test Stopped, CuO Nanoparticles of nanoparticles, which is far from the 0.7 Surface Originally Coated Added to the Surface with CuO Nanoparticles Test Stopped, CuO Nanoparticles in the Pile-up were Pushed into the Contact optimum concentration for reducing 0.6 friction and wear. This relatively high concentration was chosen for the 0.5 purpose of intensifying the nanoparticle 0.4 to surface interaction effect in the COF test. The CuO nano-lubricant in fully 0.3 formulated SAE 5W20 oil is also 0.2 prepared as mentioned above and also consists of 5.0%wt of CuO nanoparticles 0.1 coated with sodium oleate. 0 0 200 400 600 800 1000 1200 1400 Time [s], (Cycles) RESULTS The experiments were carried out Figure 2 | COF in a pin-on-disk dry test performed on CuO particles. The test was stopped when using the pin-on-disk and disk-on- the nanoparticles stopped performing to add particles to the contact and resume the experiment. “Calcium Sulfonate Complex Grease,” an STLE University Webinar by Wayne Mackwood (Chemtura Corp.), July 22, 12-1 pm CDT. Register at www.stle.org. 21 dodecane can reduce both friction and wear significantly. In the case of PAO base oil, it is observed that the control PAO Figure 3 | XPS surface analysis marginally outperforms the nano- inside the groove of a sample lubricant at a normal load of 20 N tested with the dry nanoparticles right before the particles stop (see Figure 5). However, when the test performing to measure the was repeated at a higher normal load critical nanoparticle of F=50 N, the nano-lubricant yields concentration. a lower COF value (see Figure 6). This shows that the performance of the CuO particle in PAO depends on the normal force. The particles are increasing friction at a lower normal load of F=20 N while they decrease friction for the particles pushed out of the surface. The measurement showed at F=50 N. A possible explanation is contact zone. The test was resumed, that there is 1.6 at% of Cu on the that at higher loads there is a higher and as the results show in Figure surface right before the particles stop possibility of particles engaging in 2, the particles decreased the COF performing (see Figure 3). The XPS contact. The effect of particles on wear for another 400 cycles. Afterwards, analysis on the control sample did is also mixed and the experiments do the COF began increasing again. not show any trace of Cu element. not produce a clear trend. This time the test was stopped and a Additionally, surfaces were drilled Next, a set of experiments were small portion of the nanoparticles in a depth of 12.5 nm using Ar sputter conducted that focused on the effect the pile-up around the edges of the cleaning. Analysis of the drilled surface of CuO nanoparticle additives on fully contact were pushed back into contact didn’t show any trace of Cu, which formulated SAE 5W20. The friction zone. The test was resumed and it was means that Cu is strictly present on the tests were performed at normal loads observed that a small amount of the surface only. of 20, 50 and 150 N. The friction and particles could decrease the COF again. The next round of experiments were wear results are presented in Figures 5 This test proved that the reduction of done using the CuO particles in various and 6.