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. CuO particles are observed to the nanoparticles concentration in base lubricants and different normal consistently increase the COF when the contact zone is responsible for the loads. Friction tests were performed added to the formulated oil (this is increase in friction. More tests with and wear measurements were carried due to the high concentration of the different normal loads and also on a out using the three-dimensional particles that was used in this test). disk-on-disk setup were performed surface profile, as shown in Figure 4. However the effect of nanoparticles (results not presented here) to further The results for normal load of 20 N are on wear is mixed and varies with the verify this phenomenon. presented in Figure 5 and show that normal load. At the lower loads of There is another major observation the CuO particles when suspended in 20 and 50 N the nanoparticles were to be made here—there is a critical concentration of the nanoparticles, below which the nanoparticles can’t decrease friction. In other words, as the nanoparticle content is being reduced as the test progresses (Figures 1 and 2) the COF seems to be fairly constant until it reaches a certain point. Beyond this point the nanoparticles can’t decrease the COF anymore and the COF begins increasing rapidly until it reaches the control’s value. In order to measure this critical concentration a test was stopped right before the particles stop performing. An XPS analysis was performed inside the contact zone to measure the Figure 4 | Three-dimensional profile of the wear track for a sample tested with the control stoichiometry of the elements on the dodecane solution at normal load of F=20 N.

22 • JULY 2015 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG capable of reducing the wear (Figure 0.25 1000 Friction Wear 5). However, when the load was increased to 150 N, nanoparticles 0.2 ) m had a negative effect on wear (Figure P ( 6). At higher loads, the film forming 0.15 additives of the 5W20 are more active. 100 COF In addition, nanoparticles are more 0.1 inclined to abrade the contact surfaces at higher loads. In order to investigate 0.05 Width Track Wear this, additional surface analysis was performed on samples tested with 0 10 5W20 at a normal load of 150 N. The DodecaneCuO+Dodecane PAOCuO+ PAOSAE 5W20 CuO+5W20 20 N 20 N 20 N 20 N 20 N 20 N results (not presented here) showed that the presence of the particles in Figure 5 | Friction and wear analysis for CuO nanoparticles suspended in dodacane, PAO and high concentrations can interfere with 5W20 at normal load of 20N. the formation of tribofilms. 0.125 DISCUSSION Friction Wear 1000 In a previous study on CuO 0.12 m) P nanoparticles [2], it was suggested that 0.115 the nanoparticles engage in contact between the surfaces and reduce the 0.11 COF real area of contact between the surfaces. 0.105 However, it was observed that particles can abrade the surfaces based on their 0.1 Width ( Track Wear concentration. In order to further 0.095 100 assess this mechanism a follow up PAOPAO, 50 N CuO+ PAO SAE 5W20 CuO+5W20 SAE 5W20 CuO+5W20 work developed a contact model for 50 N 50 N 50 N 50 N 150 N 150 N nanoparticles in between rough surfaces [7]. The model was able to verify the Figure 6 | Friction and wear analysis for CuO nanoparticles suspended in dodacane, PAO and reduction of the area of contact as a 5W20 at normal load of 20N. key mechanism associated with the performance of nano-lubricants. The The tests on dry CuO and CuO with the surface also affect the contact’s current work is a continuation of the solution in dodecane, PAO and 5W20 tribology. In case of CuO particles, it study with a focus on the synergistic showed some interesting trends. The is an abrasive interaction with the effect of the area of contact reduction particles are more effective at reducing surface. Both of these interactions mechanism with other proposed friction and wear when suspended in affect the overall friction and wear mechanisms for nanoparticles. an inferior lubricant or in a dry form. in the system. The overall friction in The dry friction tests using the CuO The particle’s effect on PAO base oil the system has two components, the particles offered some key observations. is dependent on the normal force friction force as the result of surface- The most important was that there is a (contact pressure) and is desired at to-surface interaction and the friction critical concentration of nanoparticles, higher loads. The particle’s effect on force as the result of particle-to- surface below which the particles can’t sustain fully formulated SAE 5W20 is also interaction, which in equation form is a reduced COF. The dry friction test dependent on the normal force (contact Eq. (1). offers a methodology to measure this pressure). All things considered, it is as critical value. From a practical stand if the CuO particles exhibit competing Foverall = Fsurface+Fparticle (1) point this means that the particle effects on a contact’s tribology. The concentration in the nano- lubricant effect of CuO particles exists in two The Fsurface can be rewritten as As×os should be such that this critical forms: (i) abrasive interaction with the that is the area of contact between the amount is fed into the contact zone. surfaces and (ii) reduction in the area surfaces times the average shear stress Also, the reduced COF value was of contact. The schematic of different between the surface. Substituting this observed to not be directly related to effects of particles is shown in Figure into Eq. (1) yields the overall friction the nanoparticle content in contact as 7. Particles create voids that reduce the as shown in Eq. (2). long as the concentration is above the real area of contact. In addition, the critical value. particle’s lateral or shear interaction Foverall = As os +Fparticle (2)

80% of the Earth’s original forests have been cleared or destroyed. 23 The CuO particles affect both of particles can reduce or increase the the friction components. They directly COF depending on the base oil. That induce a friction force by scratching the is because particles when suspended surface, Fparticle. They also affect the in an inferior lubricant can better friction force between the surfaces by influence the contact tribology through reducing the area of contact, As. These the indirect effect. Whereas, in case of two effects are competing and one can a superior lubricant the particle’s direct dominate the system depending on the effect may overshadow that. normal force, the lubricant type (os) and the particle content. In the case of CONCLUSION dry tests or inferior lubricants, os is so In this work strategically designed large that the first term dominates the experiments were used to investigate equation and the friction induced by the role of nanoparticles in a contact’s the particles is negligible. That is why tribology and the effect of nanoparticles the only effect that can be seen is the on fully formulated oils. The positive effect of the particles on the experiments were run using the dry area of contact. Hence, the results are CuO particles powder form and CuO in agreement with the reduction in the particles suspended in dodecane, PAO area of contact mechanism. However and SAE 5W20 oil. when a superior lubricant is used, The dry experiments showed that such as the PAO, both of the terms Figure 7 | Schematic showing various roles the nanoparticles even absent of the contribute to the overall friction. Then of nanoparticles in a contact’s tribology. lubricants can reduce both friction the results are mixed, less decisive and and wear. The behavior of the dry dependent on the conditions. Similar of contact between the surfaces. It nanoparticles was in agreement with arguments stand for the effect of might be possible to control the area the reduction in the contact area nanoparticles on wear. of contact and tune the effect of the mechanism. The dry experiments also Expanding this discussion to the lubricant on the system. This is more presented a practical way of measuring other types of nanoparticle additives, of a general effect induced by particles the critical concentration of particles one can say nanoparticle additives have on the contact. The overall effect of the below, which particles can’t maintain a a dual effect on the tribology of contact. nanoparticle additives on friction and reduced COF. The studies on the effect They have a direct effect defined by wear is determined by adding the two of CuO particles in dodecane and PAO the interaction between the particles effects together. This explains some suggested that the particles have a with the surfaces. This effect could be of the common contradictions in the dual effect on the contact’s tribology. plowing, deposition of transfer films or nanoparticle research, such as why CuO particles can affect the tribology rolling (see Figure 7). This interaction hard particles can decrease wear. That through direct interaction with the is dependent on the character of the is because the reduction in wear is the surfaces or by reducing the area of individual particles. The particles also result of the particles indirect effect on contact. This is why the nanoparticle have an indirect effect caused by the the area of contact. effectiveness depends on the type of the nanoparticles reducing the real area Another contradiction is why some lubricant and the normal load.

REFERENCES 5. Li, B., et al. (2006), “Tribochemistry and antiwear mechanism 1. Greenberg, R., et al. (2004), “The effect of WS 2 nanoparticles of organic–inorganic nanoparticles as lubricant additives,” on friction reduction in various lubrication regimes,” Tribology Tribology Letters, 22 (1), pp. 79-84. Letters, 17 (2), pp. 179-186. 6. Zeng, T. and Sun, T. (2005), “Size effect of nanoparticles 2. Ghaednia, H., Jackson, R.L. and Khodadadi, J.M. (2013), in chemical mechanical polishing-a transient model,” “Experimental analysis of stable CuO nanoparticle enhanced Semiconductor Manufacturing, IEEE Transactions on, 18 (4), pp. lubricants,” Journal of Experimental Nanoscience, pp. 1-18. 655-663. 3. Ghaednia, H., et al. (2013), “The effect of nanoparticles on 7. Ghaednia, H. and Jackson, R.L. (2013), “The Effect of thin film elasto-hydrodynamic lubrication,” Applied Physics NanoParticles on the Real Area of Contact, Friction and Wear,” Letters, 103 (26), p. 263111. Journal of Tribology, 135 (4), p. 041603. 4. Tao, X., Jiazheng, Z. and Kang, X. (1996), “The ball-bearing 8. Clary, D.R. and Mills, G. (2011), “Preparation and thermal effect of diamond nanoparticles as an oil additive,” Journal of properties of CuO particles,” The Journal of Physical Chemistry Physics D: Applied Physics, 29 (11), p. 2932. C, 115 (5), pp. 1767-1775.

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