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Effect of Lubricant with Nanodiamond Particles in Sliding Friction
Effect of Lubricant with Nanodiamond Particles in Sliding Friction
Journal of the Korean Society of Tribologists and Lubrication Engineers. 2015. Aug, 31(4): 183-188
Copyright © 2015, Korean Society of Tribologist and Lubrication Engineers
  • Received : June 01, 2015
  • Accepted : July 31, 2015
  • Published : August 31, 2015
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About the Authors
M. H. Adzaman
Department of Mechanical Engineering of Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia
A. Rahman
Department of Mechanical Engineering of Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia
Y. Z. Lee
School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Korea
yzlee@skku.edu
S. S. Kim
Department of Mechanical Engineering of Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia
sskim@ums.edu.my

Abstract
This paper presents the experimental effects of lubricant with nanodiamond particles in sliding friction. In order to improve the performance of lubricants many additives are used, such as MoS 2 , cadmium chloride, indium, sulfides, and phosphides. These additives are harmful to human health and to the environment, so alternatives are necessary. One such alternative is nanodiamond powder, which has a large surface area. In order to investigate the effect of nanodiamonds in lubricants under sliding friction, they are dispersed in the lubricant at a variety of concentrations (0 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt%, and 1 wt%) using the matrix synthesis method. Friction and wear tests are performed according to the ASTM G99 method using a pin-on-disc tester at room temperature. The specimens used in this experiment are AISI 52100 ball bearings and AISI 1020 steel discs. During the test, lubricant mixed with nanodiamond is supplied constantly to keep the two bodies separated by a lubricant film. To maintain boundary lubrication, the speed is set to 0.18 m/s and a load of 294 N is applied to the disc through the pin. Results are recorded by using workbench software over the test duration of 10 minutes. Experimental results show that when the concentration of nanodiamond increases, the coefficient of friction decreases. However, above a nanodiamond concentration of 0.5 wt%, both the coefficient of friction and wear volume increase. From this experiment, the optimum concentration of nanodiamond showing a minimum coefficient of friction of 0.09 and minimum wear volume of 0.82 nm 2 was 0.5 wt%.
Keywords
1. Introduction
Machinery is nowadays more commonly used than muscle power. Its usage in various sectors in daily life results in higher demands on lubricating oils. Lubricants can reduce the friction and heat generated between two interacting surfaces when the surfaces are moving against each other. At the microscopic level, lubricant can protect equipment surfaces by separating two opposing moving metal surfaces with a very thin layer of oil. This helps extend the life of the equipment or machine and protect it from heavy wear and friction. Further investigation of lubricants can result in reduction of waste and increased profits as assets are protected and kept maximally operational.
Wear is the progressive damage, involving material loss, that occurs on the surface of a component as a result of its motion relative to the adjacent working parts. Abrasive wear is damage to a component surface that arises because of the motion relative to that surface of either harder adjacent parts or hard particles trapped at the interface [1] . Many nano-lubricants are currently available. However, most nano-lubricants use additives such as molybdenum sulfide, cadmium chloride, indium sulfide, and indium phosphide, which are harmful to the environment and to humans [2] . To avoid these harmful effects, an alternative must be found. Nanodiamond (ND) has high hardness, stable chemical properties, and high heat conductivity. There is a need for more research on other nanoparticles that are noncarcinogenic, such as nanodiamond, to replace or reduce the usage of additives currently used in nano-lubricant. An analysis by Y. Y. Wu et al . [3] found that nanoparticles can reduce the friction coefficient between two interacting surfaces. In their research, CuO and TiO 2 were used as additives. The results for both were compared and CuO was found to exhibit good friction-reduction and anti-wear properties. The amount of material lost to wear was reduced by 78.8%. Hsiao et al . [4] used nanodiamond particles synthesized by a detonation process. Their results show that an increase in the additive concentration helped delay the onset of scuffing. However, as the concentration increased further, a point of saturation occurred at 3 wt%, above which the performance decreased. A recent study by Ivanov et al . [5] shows that the application of ND-based additives must be used with caution, taking into consideration the hardness of the friction surfaces. For ‘hard on soft’ steel/steel friction pairs, NDs provide significant reduction in wear and demonstrate a polishing effect. In the ‘hard on hard’ friction pair of steel and WC alloy, the abrasive nature of NDs plays a role, resulting in increased wear of the sliding surfaces.
The objective of this study is to investigate the effect of nanodiamond particles in sliding friction when they are present at different concentrations (0 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt%, 1 wt%) in the base oil. This coefficient of friction is evaluated, as well as wear losses to the interacting surfaces in sliding friction caused by nanodiamond particles in the lubricant.
2. Experimental Details
- 2-1. Nano-lubricant Preparation
Nanodiamond powder (ITC, USA) has an almost-rectangular shape, as shown in Fig. 1 using TEM. The nano-lubricant was prepared by Neomond Company. The nanodiamond powder was mixed with base oil lubricant (SK Supermar 13TP) at concentrations of 0.1 wt%, 0.3 wt%, 0.5 wt%, and 1 wt%.
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TEM images of the diamond particles: (a) 50k magnification, (b) 250k magnification [2].
Using dispersion technology, which is a matrix synthesis method, the diameter of the concentrated nanodiamond was reduced from 500 nm to 20 nm dispersed nanodiamond, as shown in Fig. 2 . This method chemically modified the surface of the nanodiamond, with nano-dispersion control by interfacial adhesion control.
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Nanodiamond diameter before and after dispersion, as measured by dynamic light scattering (DLS).
The dispersion stability was measured by Turbiscan for a month; no precipitation occurred. The result is shown in Fig. 3 .
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Dispersion stability result using Turbiscan.
- 2-2. Experimental Design
The experiment was done using a ball-on-disk type tester according to the ASTM G 99 method. Before the experiment, the ball for the pin was immersed in acetone and subjected to ultrasonic vibration for 5 minutes to remove impurities on the ball’s surface. The disc was cleaned using acetone. The load applied to the specimen was 294 N. Testing was conducted for 10 minutes at a rotational velocity of 0.18 m/s for each concentration, as illustrated in Fig. 4 .
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Schematic diagram of ball-on-disc tester.
After the test, the wear track was inspected using a roughness tester (Mitutoyo SJ-410). The wear volume was calculated from the results.
The specimens used in the experiment were an AISI 52100 bearing steel ball and AISI 1020 steel. The properties and condition for the experiment are given in Tables 1 and 2 . Each concentration was tested 4 times and averages were taken for accurate data.
Chemical composition and properties of specimens
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Chemical composition and properties of specimens
Experimental conditions for friction and wear test
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Experimental conditions for friction and wear test
3. Results and Discussion
- 3-1. Relationship between COF and nanodiamond concentration
The coefficient of friction (COF) between the AISI 1020 and AISI 52100 specimens with different concentrations of nanodiamond in the lubricant are shown in Figs. 5 and 6 . The COF decreases as the concentration of the nanodiamond increases. However, as the concentration increases to more than 0.5 wt%, the coefficient of friction increases. The reduction of the COF can be explained by the physical shape of the nanodiamond. Nanodiamond has an almost-spherical shape, so that it acts as a roller allowing the two surfaces to slide across each other with ease [6] . At 0.5 wt%, the nanodiamond was able to perform the rolling action well, thereby separating the surfaces and preventing them from abrading each other, because of the optimization of the dispersed nanodiamond quantity in the lubricant. With the nanodiamond additive, the COF was reduced by 14% compared with base oil.
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Result of COF vs time shows that 0.5 wt% of nanodiamond has the lowest COF.
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Standard deviation for COF on each nanodiamond concentration in the lubricant.
- 3-2. Relationship between wear behavior and nanodiamond concentration
The wear volume was obtained from the wear track on the disc surface. The areas of the four points were measured and averaged. The area obtained was then multiplied by the circumference of the wear track to obtain the volume.
The wear rate showed different results. Generally, there is no direct correlation between friction and wear. As the concentration of the nanodiamond increased, the wear rate became more severe. The friction-modifying additives reduced the tribo-film thickness and caused surface polishing [7] . This can be seen at 0.1 wt%, 0.3 wt%, and 1 wt% of nanodiamond from Fig. 7 . The nanodiamond did not fully cover both of the interacting surfaces, so that the rolling action gave rise to high stress or grinding abrasion. The results at 0.1 wt% and 0.3 wt%, where the wear increases but the COF decreases, were because the ball and the nanodiamond, which are hard materials, were pressed on the disc surface, which is a soft material. Plastic deformation occurred on the soft material in a phenomenon called ‘microploughing, which can be seen in Fig. 9 . The decrease in COF with the increase in wear can be explained by the nearly spherical shape of the nanodiamond.
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Average wear volume for each concentration.
At 0.5% nanodiamond concentration, the rolling action occurred properly, since all the nanodiamond particles were dispersed at an optimum distance. As a result, the wear rate was reduced to a minimum.
- 3-3. Role of Nanodiamond in Sliding Friction
Three-body abrasion is a form of abrasive wear that occurs when hard particles become embedded in a soft surface. The nanodiamond act as a roller when the two surfaces are interacting with each other. Due to its hardness, the nanodiamond can withstand a heavy load. The rolling action of the nanodiamond helps the lubricant by avoiding severe metallic contact between the two sliding surfaces [8] . Fig. 8 shows the rolling action when the two surfaces slide against each other at the microscopic level.
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Drawing of wear mechanism of three-body abrasion.
- 3-4. Microscopic Observation of Worn Surface
The SEM images in Fig. 9 shows how the nanodiamond affect the surfaces of AISI 1020. The wear scar surface generated during sliding between the two bodies can be seen. All the images were taken at 2000 × magnification.
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(a), (b), (c), (d) and (e) Morphology of AISI 1020 disc surface after sliding with lubricant containing nanodiamond additive at concentrations of 0 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt% and 1 wt%, respectively. The images were obtained by SEM under 2k magnification (ZEISS SEM). Red arrows show the direction of sliding and the smaller arrow shows that some tearing occurs on AISI 1020.
Comparing the micrographs, we can see that the wear track becomes more severe as the nanodiamond concentration increases. From Fig. 9(d) we can see that the wear is decreasing, and the SEM image shows some scratches on the surface of the disc. This demonstrates that abrasion occurred. Normally, it is hard to differentiate between an adhesive and abrasive through SEM images. However, these images show clearly that more abrasion took place.
4. Conclusions
In order to investigate the effect of nanodiamond in lubricants under sliding friction, nanodiamond was dispersed in a base lubricant using the matrix synthesis method, at concentration of 0 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt%, and 1 wt%. Friction and wear tests were carried out according to the ASTM G99 method using pin-on-disc tester at room temperature. The specimen used in this experiment was an AISI 52100 ball bearing and an AISI 1020 steel disc. During the test, the lubricant mixed with nanodiamond was supplied constantly to keep the two bodies separated by a lubricant film. The speed used was 0.18 m/s to maintain the boundary lubrication and the load applied on the disc through the pin was 294 N. The test duration was 10 minutes; all results were recorded using software. The experimental results show that when the concentration of nanodiamond increases, the coefficient of friction decreases. However, above 0.5 wt% concentration of nanodiamond, the coefficient of friction increases, as does the wear volume. The experiments showed that the optimum concentration of nanodiamond of 0.5 wt% yielded the minimum friction coefficient, 0.09, and the minimum wear volume, 0.82 nm 3 . The role of the nanodiamond was to act as a roller between two surfaces to avoid severe wear.
Acknowledgements
This experimental work was supported by Sungkyunkwan University (SKKU), South Korea and Universiti Malaysia Sabah (UMS), Malaysia.
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