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Effect of Size of Multiwalled Carbon Nanotubes Dispersed in Gear Oils for Improvement of Tribological Properties

Effect of Size of Multiwalled Carbon Nanotubes Dispersed in Gear Oils for Improvement of... Hindawi Advances in Tribology Volume 2018, Article ID 2328108, 13 pages https://doi.org/10.1155/2018/2328108 Research Article Effect of Size of Multiwalled Carbon Nanotubes Dispersed in Gear Oils for Improvement of Tribological Properties 1 1 2 Kodanda Rama Rao Chebattina , V. Srinivas , and N. Mohan Rao Department of Mechanical Engineering, GITAM University, Visakhapatnam, India Department of Mechanical Engineering, University College of Engineering, JNTU, Kakinada, India Correspondence should be addressed to V. Srinivas; vsvas1973@yahoo.com Received 21 May 2018; Accepted 23 August 2018; Published 30 October 2018 Academic Editor: Patrick De Baets Copyright © 2018 Kodanda Rama Rao Chebattina et al. is Th is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. eTh aim of the paper is to investigate the effect of size of multiwalled carbon nanotubes (MWCNTs) as additives for dispersion in gear oil to improve the tribological properties. Since long pristine MWCNTs tend to form clusters compromising dispersion stability, they are mildly processed in a ball mill to shorten the length and stabilized with a surfactant before dispersing in lubricant. Investigations are made to assess the effect of ball milling on the size and structure of MWCNTs using electron microscopy and Raman spectroscopy. The long and shortened MWCNTs are dispersed in EP 140 gear oil in 0.5% weight. eTh stability of the dispersed multiwalled carbon nanotubes is evaluated using light scattering techniques. The antiwear, antifriction, and extreme pressure properties of test oils are evaluated on a four-ball wear tester. It is found that ball milling of MWCNTs has a strong eeff ct on the stability and tribological properties of the lubricant. From Raman spectroscopy, it is found that ball milling time of up to 10 hours did not produce any defects on the surface of MWCNTs. eTh stability of the lubricant and the antiwear, antifriction, and extreme pressure properties have improved significantly with dispersion shortened MWCNTs. Ball milling for longer periods produces defects on the surface of MWCNTs reducing their advantage as oil additives. 1. Introduction being extensively studied for their lower friction coefficients, thereby improving antiwear properties. MWCNTs possess Gear oils used in industries and automotive engines are large surface area compared to many inorganic nanomate- often subjected to heavy loads, due to which they experience rials and can be very easily surface-modified. Several novel high temperatures and pressures causing higher friction and studies are made on the eeff ct of dispersion of multiwalled surface damage leading to failure of the system. To prevent carbon nanotubes on the wear and friction characteristics failure, conventional engine and gear oils are dispersed with of lubricants. The length of the nanotube synthesized by existing methods is known to be thousands of times larger extreme pressure (EP) and antiwear (AW) additives that react chemically with the metal surfaces, forming easily sheared than their width and thus limits their functionality for many layers and thereby preventing severe wear and seizure. applications. Owing to their large length to diameter ratios, MWCNTs despite use of surfactant tend to form agglom- Allotropes of carbon such as graphite, fullerenes, carbon nanotubes, and graphene have attracted the interest of the erates faster, thereby leading to them settling in the liquid researchers due to their special properties. The hybridization medium. This aspect is a main challenge in obtaining stable of the atomic orbital of carbons in carbon nanotubes and dispersion in liquid medium. The most common and frugal fullerenes is of type sp2, similar to that of graphite, making a method to reduce the size of MWCNTs is ball milling which perfect hexagonal array of atoms. The sp2C–C bond ofCNTs shortens the length of MWCNTs and obtaining open ends. is considered one of the strongest in solid materials; thereby However, ball milling for extended period produces defects CNTs are expected to yield exceptionally good mechanical and damages the graphite structure rendering it useless for dispersion in lubricants. Studies are made on the effect of properties. Lubricants dispersed with allotropes of carbon are 2 Advances in Tribology ball milling on the structure and defect generation. Pierard reported in the literature which is one of the novel features et. al. [1] studied the eeff ct of ball milling on the structure of the study. The paper also compares chemical and physical of single-walled carbon nanotubes. Raman spectroscopy was routes for dispersion of MWCNTs in lubricant by comparing performance of surface-modified pristine MWCNTs and employed to study the defects produced in various hours of ball milling time. It is found that there exists an optimum time surface-modified ball-milled MWCNTs, respectively. to keep the tubular structure intact without defects. In case of single-walled carbon nanotubes, ball milling times of over 50 2. Experimental hours completely destroy the structure producing amorphous carbon.Dresselhaus et.al.[2],Cancado et.al.[3], and Paton .. Materials. In the present study, multiwalled carbon et. al. [4] suggested methods to detect defects and evaluate nanotubes produced by CVD method have been procured purity of carbon nanotubes and graphene. It was proposed from M/sCheap TubesInc., USA. The size of MWCNTsis that the intensity of G-band and the D/G ratio can be highly 20-40 nm in diameter, 25 microns in length, and 95% of useful to determine both the purity and the defect density of purity. All other chemicals purchased are of GR grade. The carbon nanotubes and graphene. Chen et. al. [5] first studied surfactant is AR grade procured from M/s Sigma Aldrich the effect of dispersed of ball-milled and stearic acid modified India Pvt limited. GL4 (EP 140 grade) gear oil is selected as multiwalled carbon nanotubes (MWCNTs) on the stability base lubricant. and thereby improvement in the lubricating properties of liquid paraffin base oil. The friction and wear tests were .. Ball Milling of Multiwalled Carbon Nanotubes. As the conducted on a pin-on-plate wear-testing machine. It is found length of the carbon nanotubes is thousands of times larger that ball-milled MWCNTs could form stable suspensions and than their width, ball milling of MWCNTs is a common improve the antiwear and antifriction properties. Engine oils procedure to generate short and open-ended nanotubes. dispersed with nanomaterials are investigated for tribological Ball milling apparatus consists of tungsten carbide lined property enhancement by several researchers. A detailed bowls containing tungsten carbide lined ball. Arrangement review of the prior art is listed in Table 1. is provided to perform ball milling under argon atmosphere As given in Table 1, most of the studies were done with to prevent oxidation of material during ball milling. The dispersion of larger amounts of MWCNTS in base lubri- speed of rotation of bowls is set at 400 RPM and the ratio of cants or paran ffi ic oils without additives. Only few studies balls to MWCNTs is taken as 10:1 to ensure minimal damage were made on commercial grade formulated lubricating oils. to the tubular structure of the MWCNTs. Higher speeds Further in the studies, due to dispersion of MWCNTs in and ratios will increase the impact and thereby attrition higher concentrations, the viscosity of the lubricant is found of MWCNTs. Ball milling was performed for 4 and 20 to be significantly enhanced. This enhancement in viscosity hours at 400 RPM to avoid damage to the structure. After is the major reason for the improved tribological properties. ∘ ball milling, the MWCNTs are heated in air at 600 Cto Furthermore higher concentrations of any nanomaterials remove amorphous carbon generated during ball milling may lead to faster agglomeration rates and the stability of process. These ball-milled MWCNTs are characterized using the suspension will get compromised. Since MWCNTs have HRSEM and transmission electron microscopy to determine long length ranging from 1 to 25 microns and diameter the average length of the MWCNTs. in nanometers, there is a strong ability of entanglement of individual nanotubes leading to formation of clusters. These .. Electron Microscopy. Figure 1(a) shows HRSEM image of clusters tend to become hard and make the MWCNTs lose pristine long length entangled MWCNTs. Figure 1(b) shows their special properties. Moreover, the stability of nanou fl id MWCNTs ball-milled for 5 hours with a small change in has not been assessed quantitatively in the studies made length of MWCNTs as compared to Figure 1(a). Figures so far. Further the defects produced in MWCNTs due to 2(a) and 2(b) show images of 10- and 20-hour ball-milled ball milling and its consequential effects on tribological MWCNTs. properties also need to be assessed. The present study is aimed It can be seen that the average length of 10-hour ball- at investigating improvements in antiwear, antifriction, and milled MWCNTs is under 4 microns. From Figure 2(b) (TEM extreme pressure properties of formulated EP 140 grade gear image) it can be observed that the length of 20-hour ball- engine oil dispersed with surface-modified and ball-milled milled CNTs has come down to around 150 nm size. MWCNTs. Ball milling is performed in inert gas medium and at a low intensity to prevent damage to the structure of MWCNTs. A simpler surface modification technique is used .. Raman Spectroscopy. Raman spectroscopy is employed to assess the formation of defects during ball millings and to stabilize the MWCNTs in oil medium and the stability of shown in Figure 3 for pristine MWCNTS, 5-, 10-, and 20- the suspensions is investigated over a period of 2 months. The effect of decrease in length of MWCNTs due to ball hour ball-milled MWCNTs. The sp2 structure of MWCNTs causes first order peaks D and G bands that are approximately milling on the tribological properties has been studied. The −1 −1 repeatability of results over a period of 60 days is investigated located at 1350 cm and 1580 cm , respectively. Defect free to observe consistent performance and the average values are MWCNTs due to intact hexagonal graphite structure make reported. The eeff ct of additives in the oil along with surface- the G-band sharper. Defects in MWCNTs make the G-band modified MWCNTs has resulted in use of lesser amount of peaks wider and shorter. On the other hand, D band peak MWCNTs in the lubricant dispersion compared to studies represents lattice defects and finite crystal size. During defect Advances in Tribology 3 Table 1: Summary of research carried out by authors. Base u fl id, type of nano Authors materials and Experimental apparatus Major findings concentration Mineral oil dispersed with It is foundthat MWCNTs outperformedgraphite with graphite and MWCNTs in Pin on disk tribometer and Bhaumik et al. [6] wear reduced by 70-75% and load bearing capacity concentration 0.1 to 0.6 Four-ball tester increasing by 20%. Wt%. Friction coefficient decreased by 10%, wear reduced by Paraffin oil dispersed with 30-40%. It was proposed that friction-reduction ability Chen et. al. [5] stearic acid modified Ring on plate of nano-lubricant depends on both nano materials and MWCNTs in 0.45% Wt % obtaining stable dispersion of nano-particle in lubricant. The friction coefficients and wear losses measured were lower for either oil or water dispersed with nanotubes Oil and water with Twin-disk machine for with friction coefficients reported as low as 0.063. It was Cornelio et. al. [7] SWCNTs & MWCNTs 0.01 measurements in proposed that decrease of friction and high wear to 0.05 Wt % rolling-sliding contact resistance are due to the formation of an amorphous carbon film transferred from the CNT’s on the surface. Friction coefficient decreased by 14 and 23% for the CuO nanoparticle concentrations of 1.0 and 2.0% wt, Mineral base oil dispersed disk-on-disk friction and respectively. It was suggested that the reduction in the Ghaednia et. al. [8] with CuO (9 nm) in 0.5 to 2 wear test real area of contact due to dispersion of nanomaterials wt% in lubricant is possible mechanism for reduction of friction eTh results indicate that the extreme pressure behavior poly-alpha-olefin(PAO6) of lubricant with nano particles is strongly dependent Hernandez Battez et. dispersed with CuO, ZnO Four-ball tester on the size and hardness of the nanoparticles. Particles al. [9] and ZrO2 in 0.5 to 2% wt with hardness less than surfaces in contact exhibited good EP behavior. Water dispersed with It is found that oxidation of MWCNTs produced defects sodium dodecyl sulfate on the surface leading to formation of better modified oxidized suspension. er Th e is a good improvement in Hu et. al. [10] Four-ball tester MWCNTs in 0.1% Wt (for tribological properties with surfactant modified application in metal MWCNTs compared to pristine MWCNTs indicating working u fl ids) strong influence of stability of suspension. Carbon nano-onions show better tribological poly-alpha-olefin dispersed properties than graphite powder. It is also found that with carbon nano onions Joly-Pottuz et. al.[11] pin-on-flat tribometer tribofilm formed by carbon onions converts wear and graphite powders in particles into ultrafine lubricious iron oxides, thus 0.1% wt preventing further abrasive wear process. A 50% reduction in friction and an increase in weld load by up to 100% are observed with dispersion of Mobil gear 627 and MWCNTs in lubricant. The Tribological performance is Khalil et. al. [12] paraffinic mineral oils in 0.1 Four-ball tribotester attributed to deposition of MWCNTs nanoparticles on to 2 wt% the worn surface resulting in decreasing the shearing stress, thus improving the tribological properties. Raw mineral oil (Sun Oil, Fullerenes have reduced friction by 30% by reducing the Japan) dispersed with Lee et. al. [13] disk-on-disk tribotester metal surface contacts. Further, it is found that volume fullerenes in 0.01 to 0.05% fraction is a key factor to control the friction and wear wt. Commercial gear oil The results indicate dispersion of nano graphite in dispersed with nano lubricant boosted the lubrication characteristics. The graphite (55 nm) in 0.1 to possible reason for improvement of properties is Lee et. al. [14] disk-on-disk tribotester 0.5% wt. Alkyl aryl suggested to be due to nanoparticles acting as ball sulfonate is used as bearing spacers between the friction surfaces reducing dispersant the contact between the plates. 4 Advances in Tribology Table 1: Continued. Base u fl id, type of nano Authors materials and Experimental apparatus Major findings concentration All nano materials improved the tribological properties 4 types of Metal working with MWCNTs giving best results. Tribo-sintering of u fl ids dispersed with TiO , Afour-ball T-02U Pena-Paras et. al. [15] nano materials on the rubbing surfaces during Al O ,CuO and MWCNTs tribotester 2 3 machining is the reason proposed for the performance in 0.01 to 0.1% wt improvement. Nano diamond additives in oils improved the anti- wear Commercial oil dispersed properties and decreased the oil temperature compared Puzyr et. al. [16] with surface modified nano block-on-ring test setup to base oils. eTh anti-wear mechanism of ND additives diamonds in 0.01% wt was attributed to the formation of a hard and porous layer between the contact surfaces. SM gradeengineoil It was found that copper nano particle dispersed dispersed with Cu and Pinondisktester and Sarma et. al. [17] lubricant gave best results on engine tests. 3 to 5% TiO2 nano particles in engine test rig increase in thermal efficiency of the engine is observed. 0.025 to 0.1% wt Lubricant dispersed with WS nanoparticles gave EP 140 Transmission oil higher weld load and load wear index (LWI) than that dispersed with WS and of lubricant dispersed with MoS nanoparticles. The 2 2 Srinivas et. al. [18] Four-ball tester MoS nano materials in reason for better performance of WS nano particles is 2 2 0.5% wt. attributed to their lower hardness resulting in better deposition on the rubbing surfaces under load. The surface modification of multi walled carbon tubes CI 4 Engine oil dispersed plays prominent role in the improving stability and Srinivas et.al. [19] Four-ball tester with long MWCNTs thereby anti-wear and anti-friction properties of engine oils. Poly alpha olefin (PAO6) dispersed with carbon Block on-ring tribometer carbon-coated copper nanoparticles decreases wear and Viesca et. al. [20] coated nano particles in 0.5 and four-ball tester increases the load-carrying capacity of polyalphaolefin to 2% wt Table 2: Raman Spectra characteristics of MWCNTs. 󸀠 󸀠 MWCNTs Intensity of D band, ID Intensity of G band, IG Intensity of G band, IG Ratio of ID and IG Pristine 1536 2100 1594 0.731 5 hour ball milled 1261 1708 1082 0.738 10 hour ball milled 1410 1855 974 0.76 20 hour ball milled 1413 1266 774 1.11 formation, due to breaking of the 2D translational symmetry .. Surface Modification of MWCNTs. Pristine MWCNTs the D band peak will increase and become wider. Another tend to agglomerate and form large particles clusters in 󸀠 −1 peak G band which can be seen at 2700 cm Raman Shift liquid medium. Moreover, aer ft the ball milling process, represents amorphous defects in MWCNTs. the MWCNTs tend to be compressed by the balls forming Table 2,provides the intensities ofD,G,andG bands. larger aggregates of entangled MWCNTs. To disentangle In case of pristine MWCNTs, 5- and 10-hour ball-milled them and make them stable in the lubricant medium, MWCNTs, there is no significant difference in the intensities it is required to modify the surface of MWCNTs with of D, G, and G bands with ratio of intensities of D and G a surfactant to create stearic repulsions between individ- bands remaining marginally the same. In case of 20-hour ual nanotubes. To stabilize the nanoparticles in the liquid ball-milled MWCNTs, a significant decrease in intensity of medium, a surfactant SPAN 80 (Sorbitan monooleate) is Gand G bandsisobserved with increaseofintensity of D used to modify surface of MWCNTs during the prepa- band indicating mild destruction of graphite structure and ration of oil. SPAN 80 is a nonionic surfactant with a formation of amorphous defects. In all cases, the peaks of all hydrophilic-lipophilic balance of 4.6 which is ideally suitable bands are sharp indicating either no defects or mild defect (in for oils. It adsorbs on the surface of MWCNTs reduc- case of 20-hour ball-milled MWCNTs). ing their surface energy, thereby preventing agglomeration Advances in Tribology 5 (a) (b) Figure 1: HRSEM images of pristine and 5-hour ball-milled MWC NTS. (a) Pristine MWCNTS, (b) 5-hour ball-milled MWCNTS. (a) (b) Figure 2: HRSEM images of 10-hour and 20-hour ball-milled MWC NTs. (a) 10-Hour ball-milled MWCNTs, (b) 20-hour ball-milled MWCNTs. and settling of nanoparticles. To prepare surface-modiefi d coats the surfactant on to the surface of the MWC- MWCNTs, SPAN 80 and multiwalled carbon nanotubes NTs. (both pristine and ball-milled) are taken in the ratio of 2:1 and ultrasonicated in a solvent for 30 minutes, .. FTIR Spectroscopy. The surface-modified MWCNTs are which creates a mechanochemical reaction. This reaction characterized for functional groups on the surface using 6 Advances in Tribology 1000 1500 2000 2500 3000 -1 Raman Shift (cm ) 5 Hour BM MWCNTs Pristine MWCNTs 10 Hour BM MWCNTs 20 Hour BM MWCNTs Figure 3: Raman spectroscopy of pristine and ball-milled MWCNTs. 4000 3500 3000 2500 2000 1500 1000 500 400 cm-1 Figure 4: FTIR spectroscopy of pristine MWCNTs. Fourier transforms infrared spectroscopy as shown in Fig- for a period of 60 days is monitored by light scattering tech- ures 4 and 5. Figure 4 shows pristine MWCNTs with no niques. Ball-milled MWCNTs when dispersed in lubricating characteristic peak detected. Figure 4 shows the FTIR image oils could form better stable suspension compared to long of surface-modiefi d MWCNTs with characteristics peaks MWCNTs and gave better tribological properties. −1 The stability of the lubricants dispersed with MWCNTs between 1463 and 1486 cm wavelength indicating lipophilic is evaluated using light scattering techniques by means of groups attached to the surface. zeta sizer (Horiba SZ 100). The zeta potential of the samples, an indicator of dispersion stability of MWCNTs in the . . Preparation and Evaluation of Stability of Lubricants lubricating oil medium, has been analyzed over a period of 60 with MWCNTs. The surface-modified long and ball-milled days. A zeta potential value of± 40 indicates good stability. MWCNTs in 0.5 weight percent are dispersed in lubricating As the viscosity of the gear oil is high, the oil samples are oil by processing it in a probe ultrasonicator (Hielscher diluted using toluene before charging it into the zeta sizer to UP400S) for about 30 minutes. The stability of the nanou fl id improve the transmittance of the oil so that accurate values %T Intensity (Arbitrary unit) Advances in Tribology 7 1471.78cm-1, 12.31%T 981.33cm-1, 910.80cm-1, 15.16%T 936.72cm-1, 15.78%T 1463.06cm-1, 12.30%T 618.26cm-1, 15.84%T 960.11cm-1, 14.64%T 1486.72cm-1, 12.12%T 665.99cm-1, 15.86%T 13 729.43cm-1, 15.23%T 12 718.65cm-1, 15.17%T 2000 1800 1600 1400 1200 1000 800 600 400 cm-1 Figure 5: FTIR spectroscopy of surface-modified MWCNTs. (a) (b) (c) (d) Figure 6: Zeta potential variation of nanou fl ids during 60 days. (a) Lubricants with long MWCNTs immediately aeft r preparation, (b) lubricant with long MWCNTs after 60 days, (c) lubricant with 5-hour ball-milled MWCNTs immediately aeft r preparation, (d) lubricant with 5-hour ball-milled MWCNTs aeft r 60 days. can be obtained. Higher values of zeta potential values are This can be attributed to lower agglomeration rates due to found when the oil samples are dispersed with ball-milled short length of MWCNTs. MWCNTs. The variation of zeta potential immediately after preparation and 60 days aer ft preparation is shown in Figures . Physicochemical Properties of Test Oils. The gear oils 6and 7. are manufactured by blending base stocks and additive Figures 6(a) and 6(b) show the variation of zeta potential components to meet the requirements of standards. Basic of lubricant dispersed with long MWCNTs and 5-hour ball- physicochemical properties should be in compliance with milled MWCNTs, respectively. As can be seen the zeta poten- international standards for statutory purposes. The main tial with long MWCNTs is much less indicating low stability. physicochemical properties to be assessed for a gear oil are With vfi e-hour ball-milled MWCNTs there is an improve- viscosity, viscosity index, pour point, ash fl point, total acid ment in the stability compared to stability of long MWCNTs. number, and copper strip corrosion resistance. All the prop- Figures 7(a) and 7(b) show the variation of zeta potential erties for test oils are evaluated in 5 replicable experiments of lubricant dispersed with 10-hour and 20-hour ball-milled and the average values are reported. MWCNTs. As can be seen there is a significant improvement As can be observed from Table 3, the eeff ct of ball milling in the stability with 20-hour MWCNTs showing best stability. and surface modification has practically no effect on the %T 8 Advances in Tribology (a) (b) (c) (d) Figure 7: Zeta potential variation of nanou fl ids. (a) Lubricants with 10-hour ball-milled MWCNTs immediately aeft r preparation, (b) lubricant with 10-hour ball-milled MWCNTs aeft r 60 days, (c) lubrica nt with 20-hour ball-milled MWCNTs immediately aeft r preparation, (d) lubricant with 20-hour ball-milled MWCNTs aeft r 60 days. physicochemical properties of the test oils. The viscosity and “load wear index” which indicates the behavior of lubricant viscosity index are unchanged with dispersion of nanomate- in resisting the aforementioned weld conditions. The test is rials. Surface modification has no effect on pour point, flash used to determine the load carrying properties of a lubricant point, and total acid number of the test lubricants. at high test loads usually encountered in gears. In this test on four-ball tester a series of 10 tests of 10-second duration are carried with varying load under the following conditions: . . Tests for Tribological Properties. The test oils are tested for temperature of oil: room temperature, improvement in tribological properties on a four-ball tester. speed of rotation: 1760 RPM, The weight percentage of surface-modified and ball-milled duration: 10 s, MWCNTs is maintained as 0.5 wt %. The standard code of load applied: 32 kgf to weld load. the tests, a rotating steel ball, is pressed against three steel A total of 10 readings are considered in the test and the balls firmly held together under a load and immersed in corrected load is calculated for all ten readings. The load lubricant. The test parameters of load, duration, temperature, wear index is a single parameter that shows the overall EP and rotational speed are set in accordance with standard test behavior in a range between well below seizure and welding procedure. is calculated from the corrected load. In wear test done as per ASTM D 4172, the average scar LD diameter formed on the bottom of three balls shows the Corrected load= (1) ability of the lubricant to prevent wear. A larger diameter where L is the applied load, kgf, X is the average scar indicates poor antiwear behavior. The test is carried out for diameter on the worn balls in mm and Hertz scar diameter, one hour at a load of 40 kgf and speed of 1200 RPM with −2 1/3 and𝐷 =8.73×10 (L) in mm. The load wear index is temperature of oil maintained at 75 C. calculated from the expression LWI = (A/10) (kgf), where A is The friction test is carried out to find the friction coef- the sum of the corrected loads determined for the ten applied ficient oer ff ed by the lubricant as per ASTM D 5183 code. loads immediately preceding the weld load. Initially the balls are subjected to “wear in” for one hour at a load of 40 kgf and speed of 600 RPM with temperature of oil maintained at 75 C. After “wear in”, the used lubricating 3. Results and Analysis oil is discarded and balls are cleaned. Fresh lubricant sample is taken in the ball cup with the same worn test balls in place. All the tests are conducted in ten repeatable trails over a The test is again started under the above conditions with load period of 60 days to ascertain the influence of nanoparticles in varying from an initial load of 10 kgf and increasing by 10 kgf terms of repeatable and reproducible results, and the average at the end of each successive 10 min interval until there is a values are reported. The results of wear test conducted as sharp rise in the Frictional Torque which indicates incipient per ASTM D4172 are as given below for dieff rent test oils. seizure. Thisiscalled seizure load whichisan important From Table 4, it can be observed that the average wear scar factor in determining the effectiveness of the lubricant. of lubricant with MWCNTsismuchlessthan that ofbase ASTM D 2783 specifies the extreme pressure properties lubricant. Further, with dispersion of long MWCNTs, the of lubricant in terms of weld load which is the ultimate load range of wear scar over 60 days of testing is higher compared at which the lubricant evaporates due to high pressure and to other oils due to poor lubricant suspension. temperature resulting in all the four balls welded to each Table 5 shows the results of friction test in terms of other. The standard also specifies another parameter called average friction coefficient and seizure load. With dispersion Advances in Tribology 9 Table 3: Physicochemical properties of test oils. ∘ ∘ Viscosity at 40 C Viscosity at 100 C Flash Point Pour Point Total Acid Number Copper strip Test oil Viscosity index ∘ ∘ (cSt) (cSt) C C (mg KOH/g) Corrosion test Base oil (EP140 gear oil) 408.4 27.9 94 194 -3 0.25 2a Base oil +0.5% pristine 409.1 27.75 93 192 -3 0.26 2a MWCNTs Base oil + 0.5% 10 hour 410.2 28.6 95 194 -3 0.25 2a ball milled MWCNTs Base oil + 0.5% 20 hour 411.5 28.4 95 194 -3 0.26 2a ball milled MWCNTs 10 Advances in Tribology Table 4: Results of wear test conducted as per ASTM D 4172 at 40 kgf load. Test Oils Average wear scar in microns Base lubricant (EP 140 transmission oil) 352.353 Base lubricant + long MWCNTs 347.071 Base lubricant + 5 hour Ball milled MWCNTs 335.085 Base lubricant + 10 hour Ball milled MWCNTs 317.954 Base lubricant + 20 hour Ball milled MWCNTs 323.547 Table 5: Results of friction test conducted as per ASTM D 5183. Test oil seizure load, kgf Average coefficient of friction Base lubricant (EP 140 transmission oil) 220 0.0901 Base lubricant + long MWCNTs 230 0.0839 Base lubricant +5 hour Ball milled MWCNTs 240 0.0784 Base lubricant + 10 hour Ball milled MWCNTs 260 0.0746 Base lubricant + 20 hour Ball milled MWCNTs 240 0.0783 0.1 of ball-milled MWCNTs, there is a good improvement in both seizure load and the friction coefficient. 0.095 The 10 sets of results of repeatable friction characteristics are plotted in graphs in Figure 8. In case of lubricant dispersed 0.09 with long MWCNTs, although the performance improved during the initial days, there is steady decrease in the per- 0.085 formance characterized by increase in friction coecffi ient due 0.08 to poor stability over a period of time. Shortened MWCNTs due to their better stability gave a consistence performance 0.075 over a period of 60 days with lubricant dispersed 10-hour ball-milled MWCNTs giving best performance. The variation 0.07 of friction torque with time is plotted in Figure 9. From the graph it can be seen that the effect of nanomaterials is 0.065 more significant at higher loads. In boundary lubrication 0.06 regime there is significant contact between surfaces separated by a thin lubricant film. Increase of normal load would 0.055 sweep the lubricant out of the contact region, reducing the lubricant lm fi thickness between surfaces and increasing the 0.05 chance of contact between surfaces in motion. Due to short 123456789 10 length, the ball-milled MWNTs dispersed in lubricant could Base lubricant effortlessly glide and roll between the two contact surfaces in motion like spacers. This increases the pressure limits of the Base Lubricant +pristine MWCNTs lubricant, thereby significantly reducing friction coefficient Base lubricant +0.5% 5 Hr and improving the seizure load. Lubricant dispersed with ball milled MWCNTs ball-milled MWCNTs has shown consistent performance on Base lubricant +0.5 %10 Hr the torque-time plot compared to base lubricant. Ball milled MWCNTs The extreme pressure properties, namely, last nonseizure Base lubricant+0.5 % 20 Hr load, weld load, and load wear index of test oils under Ball milled MWCNTs consideration, are summarized in Table 6. With dispersion of MWCNTs, there is an improvement Figure 8: Variation of friction coefficient in 10 repeatable tests in last nonseizure load, weld load, and load wear index. A carried out during 2 months. graph showing the variation of wear scar diameter with load is shown in Figure 10. The region between points 0 to 1 is normally designated as antiwear region in which all the lubricants exhibited similar behavior. Points 1 and 1 indicated gear oil comes into play. In this region, the pressures and the last nonseizure load (LNSL) up to which the wear scar temperature are very high and the additives are supposed formed is uniform. to withstand these extremities. MWCNTs due to their good The region above last nonseizure load is called extreme mechanical properties could form a barrier between the pressure zone in which the efficacy of additives in the surfaces withstanding the extremities to improve weld load. Friction coefficient Advances in Tribology 11 Table 6: Results of EP test conducted as per ASTM D 2783. Test oil Last non seizure load, kgf weld load, kgf load wear index Base lubricant (EP 140 transmission oil) 120 250 64.82 Base lubricant + long MWCNTs 120 315 69.54 Base lubricant +5 hour Ball milled MWCNTs 140 315 71.23 Base lubricant + 10 hour Ball milled MWCNTs 140 400 75.65 Base lubricant +20 hour Ball milled MWCNTs 120 250 72.81 2.5 1.5 0.5 0 5000 10000 15000 20000 Time Base Lubricant (EP 140 Transmission oil) Base Lubricant +0.5 % 10 Hr BM MWCNTs Base lubricant+0.5 % 5 hr BM MWCNTs Base Lubricant +0.5 % 20 Hr BM MWCNTs Base lubricant+0.5 % pristine MWCNTs Figure 9: Variation of friction torque with time during friction test. There is a reduction in the wear scar with dispersion of formation during ball milling, exhibited lower performance pristine and ball-milled MWCNTs. The effect of dispersion in EP region. This can be attributed to the formation of defects of shortened MWCNTs is found to be quite visible in this on the surface (Figure 3 and Table 2) leading to loss of special region. Lubricant dispersed with 10-hour ball-milled MWC- properties of MWCNTs. NTs improved the LNSL and could reduce the wear scars even in the extreme pressure regions. Lubricant dispersed 4. Conclusions with ball-milled MWCNTs due to decrease of the friction coefficient offered lower wear scars on the test balls. Further (1) The ball milling of multiwalled carbon tubes prior to after last nonseizure load, it can be observed that the wear scar dispersion in lubricant plays an important role in improve- diameter with dispersion of nanomaterials is significantly ment of stability, antiwear, antifriction, and extreme pressure low, resulting in improvement in load wear index as well properties of gear oil. as weld load. This is due to better dispersion owing to (2) Ball milling could shorten the MWCNTs making them short length. Here the ball milling timing of MWCNTs plays stable in the lubricant for a period of more than 60 days. an important role in defining the optimum length with (3) From Raman spectroscopy it can be observed that ball minimum damage to the tube structure. The optimum length milling time of up to 10 hours did not produce any defects on for attaining good stability and tribological properties can the surface of MWCNTs but 20-hour ball milling produced be assessed as 1 to 5 microns, which can be attained with mild defects on the surface. 10 hours of ball milling at 400 RPM speed. 20-hour ball- milled MWCNTs could improve the load wear index by (4) Long MWCNTs, although surface-modiefi d, when performing well in the antiwear region but, due to defect dispersed in lubricants exhibited poor stability and could Frictional Torque (N-m) 12 Advances in Tribology 10.00 1.00 0.10 50 500 Load applied in Kgf Base Lubricant Base lubricant + 0.5 % long MWCNTs Base lubricant+0.5 % 5 hour Ball milled MWCNTs Base lubricant+0.5 % 10 hour Ball milled MWCNTs Base lubricant+0.5 % 20 hour Ball milled MWCNTs hertz line Figure 10: Variation of wear scar diameter with applied load during EP test. marginally improve the antifriction and extreme pressure LWI: Load wear index properties. MWCNTs: Multiwalled carbon nanotubes SL: Seizure load (5) The physicochemical properties remain unaltered with dispersion of surface-modified and ball-milled MWC- X: Average scar diameter. NTs. (6) There is a good improvement in the wear scar diam- Conflicts of Interest eters and friction coefficients with dispersion of shortened MWCNTs as short-length MWCNTs could slide between The authors declare that they have no conflicts of interest. mating surfaces reducing contact. (7) The load wear index and weld load of lubricants dispersed with shortened MWCNTs have improved signifi- Acknowledgments cantly as the performance of short MWCNTs is good in both The authors gratefully acknowledge the support received antiwear and extreme pressure region. from Hindustan Petroleum Corporation Ltd., India, for (8) At an optimum length of MWCNTs in the range of 1 conducting the tests. The authors acknowledge the assistance to 3 microns, the lubricant could give best results while too from IIT Madras, IIT Kharagpur, and Osmania University, short MWCNTs due to formation of defects despite forming Hyderabad, in characterization. The authors sincerely thank good suspension could not give best performance. the management of GITAM deemed University, India, for the support extended. Nomenclature 𝐷 : Hertz scar diameter References EP: Extreme pressure AW: Antiwear [1] N. Pierard, A. Fonseca, J.-F. Colomer et al., “Ball milling effect L: Load applied on the structure of single-wall carbon nanotubes,” Carbon,vol. 42,no. 8-9, pp.1691–1697, 2004. LNSL: Last nonseizure load Mean scar diameter in mm Advances in Tribology 13 [2] M.S. Dresselhaus, A.Jorio,A.G. Souza Filho,and R.Saito, [18] V.Srinivas,R. N.Thakur,A.K.Jain,and M.Saratchandra “Defect characterization in graphene and carbon nanotubes Babu, “Tribological studies of transmission oil dispersed with using Raman spectroscopy,” Philosophical Transactions of the molybdenum disulfide and tungsten disulfide nanoparticles,” Royal Society A, vol.368,pp.5355–5377, 2010. Journal of Tribology, vol.139,no.4,pp.041301–041306, 2017. [3] L. G. Canc¸ado,A. Jorio, E.H.M. Ferreira et al., “Quantifying [19] V. Srinivas, Rao. Kodanda Rama, and C. N. Mohan Rao, defects in graphene via Raman spectroscopy at different excita- “Lubricating and physico-chemical properties of CI-4 plus tion energies,” Nano Letters, vol. 11, no. 8, pp. 3190–3196, 2011. engine oil dispersed with surface modified multi-walled carbon nanotubes,” Tribology-Materials, Surfaces & Interfaces,vol. 12, [4] K. 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Toro, “Tribological properties of carbon nanotubes as lubricant additive in oil and water for a wheel-rail system,” Journal of Materials Research and Technology,vol.5,no. 1, pp. 68–76, 2016. [8] H. Ghaednia, R. L. Jackson, and J. M. Khodadadi, “Experimen- tal analysis of stable CuO nanoparticle enhanced lubricants,” Journal of Experimental Nanoscience,vol.10,no.1,pp.1–18,2013. [9] A. Hernand ´ ez Battez, R. Gonzalez ´ , D. Felgueroso et al., “Wear prevention behaviour of nanoparticle suspension under extreme pressure conditions,” Wear,vol. 263,no. 7-12,pp. 1568– 1574, 2007. [10] Y. Hu, Y. Peng, and H. Wang, “Tribological behaviors of surfactant-functionalized carbon nanotubes as lubricant addi- tive in water,” Tribology Letters, vol.25,no.3, pp. 247–253,2006. [11] L. J.Pottuz, B.Vacher, N.Ohmae, J.M.Martin,and T.Epicier, “Anti-wear and friction reducing mechanisms of carbon nano- onions as lubricant additives,” Tribology Letters, vol.30,no.1, pp.69–80,2008. [12] W. Khalil,A. Mohamed, M.Bayoumi,and T. A.Osman, “Tribo- logical properties of dispersed carbon nanotubes in lubricant,” Fullerenes, Nanotubes and Carbon Nanostructures,vol.24,no. 7, pp. 479–485, 2016. [13] J. Lee, S. Cho, Y. Hwang, C. Lee, and S. H. Kim, “Enhancement of lubrication properties of nano-oil by controlling the amount of fullerene nanoparticle additives,” Tribology Letters,vol. 28,no. 2, pp. 203–208, 2007. [14] C. Lee, Y. Hwang, Y. Choi, J. Lee, C. Choi, and J. Oh, “A study on the tribological characteristics of graphite nano lubricants,” International Journal Of Precision Engineering and Manufacturing, vol.10, no.1,pp.85–90,2009. [15] L. Pena-P ˜ aras ´ , J. Taha-Tijerina, A. Garc´ıa et al., “Antiwear and extreme pressure properties of nanofluids for industrial applications,” Tribology Transactions,vol.57, no.6, pp.1072– 1076, 2014. [16] A. P. Puzyr, A. E. Burov, G. E. Selyutin, V. A. Voroshilov, and V. S. Bondar, “Modified nanodiamonds as antiwear additives to commercial oils,” Tribology Transactions,vol.55, no.1,pp.149– 154, 2012. [17] P. K. Sarma, V. Srinivas, V. D. Rao, and A. K. Kumar, “Exper- imental study and analysis of lubricants dispersed with nano Cu and TiO in a four-stroke two wheeler,” Nanoscale Research Letters, vol.6,pp.233–243, 2011. 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Effect of Size of Multiwalled Carbon Nanotubes Dispersed in Gear Oils for Improvement of Tribological Properties

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Copyright © 2018 Kodanda Rama Rao Chebattina et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Advances in Tribology Volume 2018, Article ID 2328108, 13 pages https://doi.org/10.1155/2018/2328108 Research Article Effect of Size of Multiwalled Carbon Nanotubes Dispersed in Gear Oils for Improvement of Tribological Properties 1 1 2 Kodanda Rama Rao Chebattina , V. Srinivas , and N. Mohan Rao Department of Mechanical Engineering, GITAM University, Visakhapatnam, India Department of Mechanical Engineering, University College of Engineering, JNTU, Kakinada, India Correspondence should be addressed to V. Srinivas; vsvas1973@yahoo.com Received 21 May 2018; Accepted 23 August 2018; Published 30 October 2018 Academic Editor: Patrick De Baets Copyright © 2018 Kodanda Rama Rao Chebattina et al. is Th is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. eTh aim of the paper is to investigate the effect of size of multiwalled carbon nanotubes (MWCNTs) as additives for dispersion in gear oil to improve the tribological properties. Since long pristine MWCNTs tend to form clusters compromising dispersion stability, they are mildly processed in a ball mill to shorten the length and stabilized with a surfactant before dispersing in lubricant. Investigations are made to assess the effect of ball milling on the size and structure of MWCNTs using electron microscopy and Raman spectroscopy. The long and shortened MWCNTs are dispersed in EP 140 gear oil in 0.5% weight. eTh stability of the dispersed multiwalled carbon nanotubes is evaluated using light scattering techniques. The antiwear, antifriction, and extreme pressure properties of test oils are evaluated on a four-ball wear tester. It is found that ball milling of MWCNTs has a strong eeff ct on the stability and tribological properties of the lubricant. From Raman spectroscopy, it is found that ball milling time of up to 10 hours did not produce any defects on the surface of MWCNTs. eTh stability of the lubricant and the antiwear, antifriction, and extreme pressure properties have improved significantly with dispersion shortened MWCNTs. Ball milling for longer periods produces defects on the surface of MWCNTs reducing their advantage as oil additives. 1. Introduction being extensively studied for their lower friction coefficients, thereby improving antiwear properties. MWCNTs possess Gear oils used in industries and automotive engines are large surface area compared to many inorganic nanomate- often subjected to heavy loads, due to which they experience rials and can be very easily surface-modified. Several novel high temperatures and pressures causing higher friction and studies are made on the eeff ct of dispersion of multiwalled surface damage leading to failure of the system. To prevent carbon nanotubes on the wear and friction characteristics failure, conventional engine and gear oils are dispersed with of lubricants. The length of the nanotube synthesized by existing methods is known to be thousands of times larger extreme pressure (EP) and antiwear (AW) additives that react chemically with the metal surfaces, forming easily sheared than their width and thus limits their functionality for many layers and thereby preventing severe wear and seizure. applications. Owing to their large length to diameter ratios, MWCNTs despite use of surfactant tend to form agglom- Allotropes of carbon such as graphite, fullerenes, carbon nanotubes, and graphene have attracted the interest of the erates faster, thereby leading to them settling in the liquid researchers due to their special properties. The hybridization medium. This aspect is a main challenge in obtaining stable of the atomic orbital of carbons in carbon nanotubes and dispersion in liquid medium. The most common and frugal fullerenes is of type sp2, similar to that of graphite, making a method to reduce the size of MWCNTs is ball milling which perfect hexagonal array of atoms. The sp2C–C bond ofCNTs shortens the length of MWCNTs and obtaining open ends. is considered one of the strongest in solid materials; thereby However, ball milling for extended period produces defects CNTs are expected to yield exceptionally good mechanical and damages the graphite structure rendering it useless for dispersion in lubricants. Studies are made on the effect of properties. Lubricants dispersed with allotropes of carbon are 2 Advances in Tribology ball milling on the structure and defect generation. Pierard reported in the literature which is one of the novel features et. al. [1] studied the eeff ct of ball milling on the structure of the study. The paper also compares chemical and physical of single-walled carbon nanotubes. Raman spectroscopy was routes for dispersion of MWCNTs in lubricant by comparing performance of surface-modified pristine MWCNTs and employed to study the defects produced in various hours of ball milling time. It is found that there exists an optimum time surface-modified ball-milled MWCNTs, respectively. to keep the tubular structure intact without defects. In case of single-walled carbon nanotubes, ball milling times of over 50 2. Experimental hours completely destroy the structure producing amorphous carbon.Dresselhaus et.al.[2],Cancado et.al.[3], and Paton .. Materials. In the present study, multiwalled carbon et. al. [4] suggested methods to detect defects and evaluate nanotubes produced by CVD method have been procured purity of carbon nanotubes and graphene. It was proposed from M/sCheap TubesInc., USA. The size of MWCNTsis that the intensity of G-band and the D/G ratio can be highly 20-40 nm in diameter, 25 microns in length, and 95% of useful to determine both the purity and the defect density of purity. All other chemicals purchased are of GR grade. The carbon nanotubes and graphene. Chen et. al. [5] first studied surfactant is AR grade procured from M/s Sigma Aldrich the effect of dispersed of ball-milled and stearic acid modified India Pvt limited. GL4 (EP 140 grade) gear oil is selected as multiwalled carbon nanotubes (MWCNTs) on the stability base lubricant. and thereby improvement in the lubricating properties of liquid paraffin base oil. The friction and wear tests were .. Ball Milling of Multiwalled Carbon Nanotubes. As the conducted on a pin-on-plate wear-testing machine. It is found length of the carbon nanotubes is thousands of times larger that ball-milled MWCNTs could form stable suspensions and than their width, ball milling of MWCNTs is a common improve the antiwear and antifriction properties. Engine oils procedure to generate short and open-ended nanotubes. dispersed with nanomaterials are investigated for tribological Ball milling apparatus consists of tungsten carbide lined property enhancement by several researchers. A detailed bowls containing tungsten carbide lined ball. Arrangement review of the prior art is listed in Table 1. is provided to perform ball milling under argon atmosphere As given in Table 1, most of the studies were done with to prevent oxidation of material during ball milling. The dispersion of larger amounts of MWCNTS in base lubri- speed of rotation of bowls is set at 400 RPM and the ratio of cants or paran ffi ic oils without additives. Only few studies balls to MWCNTs is taken as 10:1 to ensure minimal damage were made on commercial grade formulated lubricating oils. to the tubular structure of the MWCNTs. Higher speeds Further in the studies, due to dispersion of MWCNTs in and ratios will increase the impact and thereby attrition higher concentrations, the viscosity of the lubricant is found of MWCNTs. Ball milling was performed for 4 and 20 to be significantly enhanced. This enhancement in viscosity hours at 400 RPM to avoid damage to the structure. After is the major reason for the improved tribological properties. ∘ ball milling, the MWCNTs are heated in air at 600 Cto Furthermore higher concentrations of any nanomaterials remove amorphous carbon generated during ball milling may lead to faster agglomeration rates and the stability of process. These ball-milled MWCNTs are characterized using the suspension will get compromised. Since MWCNTs have HRSEM and transmission electron microscopy to determine long length ranging from 1 to 25 microns and diameter the average length of the MWCNTs. in nanometers, there is a strong ability of entanglement of individual nanotubes leading to formation of clusters. These .. Electron Microscopy. Figure 1(a) shows HRSEM image of clusters tend to become hard and make the MWCNTs lose pristine long length entangled MWCNTs. Figure 1(b) shows their special properties. Moreover, the stability of nanou fl id MWCNTs ball-milled for 5 hours with a small change in has not been assessed quantitatively in the studies made length of MWCNTs as compared to Figure 1(a). Figures so far. Further the defects produced in MWCNTs due to 2(a) and 2(b) show images of 10- and 20-hour ball-milled ball milling and its consequential effects on tribological MWCNTs. properties also need to be assessed. The present study is aimed It can be seen that the average length of 10-hour ball- at investigating improvements in antiwear, antifriction, and milled MWCNTs is under 4 microns. From Figure 2(b) (TEM extreme pressure properties of formulated EP 140 grade gear image) it can be observed that the length of 20-hour ball- engine oil dispersed with surface-modified and ball-milled milled CNTs has come down to around 150 nm size. MWCNTs. Ball milling is performed in inert gas medium and at a low intensity to prevent damage to the structure of MWCNTs. A simpler surface modification technique is used .. Raman Spectroscopy. Raman spectroscopy is employed to assess the formation of defects during ball millings and to stabilize the MWCNTs in oil medium and the stability of shown in Figure 3 for pristine MWCNTS, 5-, 10-, and 20- the suspensions is investigated over a period of 2 months. The effect of decrease in length of MWCNTs due to ball hour ball-milled MWCNTs. The sp2 structure of MWCNTs causes first order peaks D and G bands that are approximately milling on the tribological properties has been studied. The −1 −1 repeatability of results over a period of 60 days is investigated located at 1350 cm and 1580 cm , respectively. Defect free to observe consistent performance and the average values are MWCNTs due to intact hexagonal graphite structure make reported. The eeff ct of additives in the oil along with surface- the G-band sharper. Defects in MWCNTs make the G-band modified MWCNTs has resulted in use of lesser amount of peaks wider and shorter. On the other hand, D band peak MWCNTs in the lubricant dispersion compared to studies represents lattice defects and finite crystal size. During defect Advances in Tribology 3 Table 1: Summary of research carried out by authors. Base u fl id, type of nano Authors materials and Experimental apparatus Major findings concentration Mineral oil dispersed with It is foundthat MWCNTs outperformedgraphite with graphite and MWCNTs in Pin on disk tribometer and Bhaumik et al. [6] wear reduced by 70-75% and load bearing capacity concentration 0.1 to 0.6 Four-ball tester increasing by 20%. Wt%. Friction coefficient decreased by 10%, wear reduced by Paraffin oil dispersed with 30-40%. It was proposed that friction-reduction ability Chen et. al. [5] stearic acid modified Ring on plate of nano-lubricant depends on both nano materials and MWCNTs in 0.45% Wt % obtaining stable dispersion of nano-particle in lubricant. The friction coefficients and wear losses measured were lower for either oil or water dispersed with nanotubes Oil and water with Twin-disk machine for with friction coefficients reported as low as 0.063. It was Cornelio et. al. [7] SWCNTs & MWCNTs 0.01 measurements in proposed that decrease of friction and high wear to 0.05 Wt % rolling-sliding contact resistance are due to the formation of an amorphous carbon film transferred from the CNT’s on the surface. Friction coefficient decreased by 14 and 23% for the CuO nanoparticle concentrations of 1.0 and 2.0% wt, Mineral base oil dispersed disk-on-disk friction and respectively. It was suggested that the reduction in the Ghaednia et. al. [8] with CuO (9 nm) in 0.5 to 2 wear test real area of contact due to dispersion of nanomaterials wt% in lubricant is possible mechanism for reduction of friction eTh results indicate that the extreme pressure behavior poly-alpha-olefin(PAO6) of lubricant with nano particles is strongly dependent Hernandez Battez et. dispersed with CuO, ZnO Four-ball tester on the size and hardness of the nanoparticles. Particles al. [9] and ZrO2 in 0.5 to 2% wt with hardness less than surfaces in contact exhibited good EP behavior. Water dispersed with It is found that oxidation of MWCNTs produced defects sodium dodecyl sulfate on the surface leading to formation of better modified oxidized suspension. er Th e is a good improvement in Hu et. al. [10] Four-ball tester MWCNTs in 0.1% Wt (for tribological properties with surfactant modified application in metal MWCNTs compared to pristine MWCNTs indicating working u fl ids) strong influence of stability of suspension. Carbon nano-onions show better tribological poly-alpha-olefin dispersed properties than graphite powder. It is also found that with carbon nano onions Joly-Pottuz et. al.[11] pin-on-flat tribometer tribofilm formed by carbon onions converts wear and graphite powders in particles into ultrafine lubricious iron oxides, thus 0.1% wt preventing further abrasive wear process. A 50% reduction in friction and an increase in weld load by up to 100% are observed with dispersion of Mobil gear 627 and MWCNTs in lubricant. The Tribological performance is Khalil et. al. [12] paraffinic mineral oils in 0.1 Four-ball tribotester attributed to deposition of MWCNTs nanoparticles on to 2 wt% the worn surface resulting in decreasing the shearing stress, thus improving the tribological properties. Raw mineral oil (Sun Oil, Fullerenes have reduced friction by 30% by reducing the Japan) dispersed with Lee et. al. [13] disk-on-disk tribotester metal surface contacts. Further, it is found that volume fullerenes in 0.01 to 0.05% fraction is a key factor to control the friction and wear wt. Commercial gear oil The results indicate dispersion of nano graphite in dispersed with nano lubricant boosted the lubrication characteristics. The graphite (55 nm) in 0.1 to possible reason for improvement of properties is Lee et. al. [14] disk-on-disk tribotester 0.5% wt. Alkyl aryl suggested to be due to nanoparticles acting as ball sulfonate is used as bearing spacers between the friction surfaces reducing dispersant the contact between the plates. 4 Advances in Tribology Table 1: Continued. Base u fl id, type of nano Authors materials and Experimental apparatus Major findings concentration All nano materials improved the tribological properties 4 types of Metal working with MWCNTs giving best results. Tribo-sintering of u fl ids dispersed with TiO , Afour-ball T-02U Pena-Paras et. al. [15] nano materials on the rubbing surfaces during Al O ,CuO and MWCNTs tribotester 2 3 machining is the reason proposed for the performance in 0.01 to 0.1% wt improvement. Nano diamond additives in oils improved the anti- wear Commercial oil dispersed properties and decreased the oil temperature compared Puzyr et. al. [16] with surface modified nano block-on-ring test setup to base oils. eTh anti-wear mechanism of ND additives diamonds in 0.01% wt was attributed to the formation of a hard and porous layer between the contact surfaces. SM gradeengineoil It was found that copper nano particle dispersed dispersed with Cu and Pinondisktester and Sarma et. al. [17] lubricant gave best results on engine tests. 3 to 5% TiO2 nano particles in engine test rig increase in thermal efficiency of the engine is observed. 0.025 to 0.1% wt Lubricant dispersed with WS nanoparticles gave EP 140 Transmission oil higher weld load and load wear index (LWI) than that dispersed with WS and of lubricant dispersed with MoS nanoparticles. The 2 2 Srinivas et. al. [18] Four-ball tester MoS nano materials in reason for better performance of WS nano particles is 2 2 0.5% wt. attributed to their lower hardness resulting in better deposition on the rubbing surfaces under load. The surface modification of multi walled carbon tubes CI 4 Engine oil dispersed plays prominent role in the improving stability and Srinivas et.al. [19] Four-ball tester with long MWCNTs thereby anti-wear and anti-friction properties of engine oils. Poly alpha olefin (PAO6) dispersed with carbon Block on-ring tribometer carbon-coated copper nanoparticles decreases wear and Viesca et. al. [20] coated nano particles in 0.5 and four-ball tester increases the load-carrying capacity of polyalphaolefin to 2% wt Table 2: Raman Spectra characteristics of MWCNTs. 󸀠 󸀠 MWCNTs Intensity of D band, ID Intensity of G band, IG Intensity of G band, IG Ratio of ID and IG Pristine 1536 2100 1594 0.731 5 hour ball milled 1261 1708 1082 0.738 10 hour ball milled 1410 1855 974 0.76 20 hour ball milled 1413 1266 774 1.11 formation, due to breaking of the 2D translational symmetry .. Surface Modification of MWCNTs. Pristine MWCNTs the D band peak will increase and become wider. Another tend to agglomerate and form large particles clusters in 󸀠 −1 peak G band which can be seen at 2700 cm Raman Shift liquid medium. Moreover, aer ft the ball milling process, represents amorphous defects in MWCNTs. the MWCNTs tend to be compressed by the balls forming Table 2,provides the intensities ofD,G,andG bands. larger aggregates of entangled MWCNTs. To disentangle In case of pristine MWCNTs, 5- and 10-hour ball-milled them and make them stable in the lubricant medium, MWCNTs, there is no significant difference in the intensities it is required to modify the surface of MWCNTs with of D, G, and G bands with ratio of intensities of D and G a surfactant to create stearic repulsions between individ- bands remaining marginally the same. In case of 20-hour ual nanotubes. To stabilize the nanoparticles in the liquid ball-milled MWCNTs, a significant decrease in intensity of medium, a surfactant SPAN 80 (Sorbitan monooleate) is Gand G bandsisobserved with increaseofintensity of D used to modify surface of MWCNTs during the prepa- band indicating mild destruction of graphite structure and ration of oil. SPAN 80 is a nonionic surfactant with a formation of amorphous defects. In all cases, the peaks of all hydrophilic-lipophilic balance of 4.6 which is ideally suitable bands are sharp indicating either no defects or mild defect (in for oils. It adsorbs on the surface of MWCNTs reduc- case of 20-hour ball-milled MWCNTs). ing their surface energy, thereby preventing agglomeration Advances in Tribology 5 (a) (b) Figure 1: HRSEM images of pristine and 5-hour ball-milled MWC NTS. (a) Pristine MWCNTS, (b) 5-hour ball-milled MWCNTS. (a) (b) Figure 2: HRSEM images of 10-hour and 20-hour ball-milled MWC NTs. (a) 10-Hour ball-milled MWCNTs, (b) 20-hour ball-milled MWCNTs. and settling of nanoparticles. To prepare surface-modiefi d coats the surfactant on to the surface of the MWC- MWCNTs, SPAN 80 and multiwalled carbon nanotubes NTs. (both pristine and ball-milled) are taken in the ratio of 2:1 and ultrasonicated in a solvent for 30 minutes, .. FTIR Spectroscopy. The surface-modified MWCNTs are which creates a mechanochemical reaction. This reaction characterized for functional groups on the surface using 6 Advances in Tribology 1000 1500 2000 2500 3000 -1 Raman Shift (cm ) 5 Hour BM MWCNTs Pristine MWCNTs 10 Hour BM MWCNTs 20 Hour BM MWCNTs Figure 3: Raman spectroscopy of pristine and ball-milled MWCNTs. 4000 3500 3000 2500 2000 1500 1000 500 400 cm-1 Figure 4: FTIR spectroscopy of pristine MWCNTs. Fourier transforms infrared spectroscopy as shown in Fig- for a period of 60 days is monitored by light scattering tech- ures 4 and 5. Figure 4 shows pristine MWCNTs with no niques. Ball-milled MWCNTs when dispersed in lubricating characteristic peak detected. Figure 4 shows the FTIR image oils could form better stable suspension compared to long of surface-modiefi d MWCNTs with characteristics peaks MWCNTs and gave better tribological properties. −1 The stability of the lubricants dispersed with MWCNTs between 1463 and 1486 cm wavelength indicating lipophilic is evaluated using light scattering techniques by means of groups attached to the surface. zeta sizer (Horiba SZ 100). The zeta potential of the samples, an indicator of dispersion stability of MWCNTs in the . . Preparation and Evaluation of Stability of Lubricants lubricating oil medium, has been analyzed over a period of 60 with MWCNTs. The surface-modified long and ball-milled days. A zeta potential value of± 40 indicates good stability. MWCNTs in 0.5 weight percent are dispersed in lubricating As the viscosity of the gear oil is high, the oil samples are oil by processing it in a probe ultrasonicator (Hielscher diluted using toluene before charging it into the zeta sizer to UP400S) for about 30 minutes. The stability of the nanou fl id improve the transmittance of the oil so that accurate values %T Intensity (Arbitrary unit) Advances in Tribology 7 1471.78cm-1, 12.31%T 981.33cm-1, 910.80cm-1, 15.16%T 936.72cm-1, 15.78%T 1463.06cm-1, 12.30%T 618.26cm-1, 15.84%T 960.11cm-1, 14.64%T 1486.72cm-1, 12.12%T 665.99cm-1, 15.86%T 13 729.43cm-1, 15.23%T 12 718.65cm-1, 15.17%T 2000 1800 1600 1400 1200 1000 800 600 400 cm-1 Figure 5: FTIR spectroscopy of surface-modified MWCNTs. (a) (b) (c) (d) Figure 6: Zeta potential variation of nanou fl ids during 60 days. (a) Lubricants with long MWCNTs immediately aeft r preparation, (b) lubricant with long MWCNTs after 60 days, (c) lubricant with 5-hour ball-milled MWCNTs immediately aeft r preparation, (d) lubricant with 5-hour ball-milled MWCNTs aeft r 60 days. can be obtained. Higher values of zeta potential values are This can be attributed to lower agglomeration rates due to found when the oil samples are dispersed with ball-milled short length of MWCNTs. MWCNTs. The variation of zeta potential immediately after preparation and 60 days aer ft preparation is shown in Figures . Physicochemical Properties of Test Oils. The gear oils 6and 7. are manufactured by blending base stocks and additive Figures 6(a) and 6(b) show the variation of zeta potential components to meet the requirements of standards. Basic of lubricant dispersed with long MWCNTs and 5-hour ball- physicochemical properties should be in compliance with milled MWCNTs, respectively. As can be seen the zeta poten- international standards for statutory purposes. The main tial with long MWCNTs is much less indicating low stability. physicochemical properties to be assessed for a gear oil are With vfi e-hour ball-milled MWCNTs there is an improve- viscosity, viscosity index, pour point, ash fl point, total acid ment in the stability compared to stability of long MWCNTs. number, and copper strip corrosion resistance. All the prop- Figures 7(a) and 7(b) show the variation of zeta potential erties for test oils are evaluated in 5 replicable experiments of lubricant dispersed with 10-hour and 20-hour ball-milled and the average values are reported. MWCNTs. As can be seen there is a significant improvement As can be observed from Table 3, the eeff ct of ball milling in the stability with 20-hour MWCNTs showing best stability. and surface modification has practically no effect on the %T 8 Advances in Tribology (a) (b) (c) (d) Figure 7: Zeta potential variation of nanou fl ids. (a) Lubricants with 10-hour ball-milled MWCNTs immediately aeft r preparation, (b) lubricant with 10-hour ball-milled MWCNTs aeft r 60 days, (c) lubrica nt with 20-hour ball-milled MWCNTs immediately aeft r preparation, (d) lubricant with 20-hour ball-milled MWCNTs aeft r 60 days. physicochemical properties of the test oils. The viscosity and “load wear index” which indicates the behavior of lubricant viscosity index are unchanged with dispersion of nanomate- in resisting the aforementioned weld conditions. The test is rials. Surface modification has no effect on pour point, flash used to determine the load carrying properties of a lubricant point, and total acid number of the test lubricants. at high test loads usually encountered in gears. In this test on four-ball tester a series of 10 tests of 10-second duration are carried with varying load under the following conditions: . . Tests for Tribological Properties. The test oils are tested for temperature of oil: room temperature, improvement in tribological properties on a four-ball tester. speed of rotation: 1760 RPM, The weight percentage of surface-modified and ball-milled duration: 10 s, MWCNTs is maintained as 0.5 wt %. The standard code of load applied: 32 kgf to weld load. the tests, a rotating steel ball, is pressed against three steel A total of 10 readings are considered in the test and the balls firmly held together under a load and immersed in corrected load is calculated for all ten readings. The load lubricant. The test parameters of load, duration, temperature, wear index is a single parameter that shows the overall EP and rotational speed are set in accordance with standard test behavior in a range between well below seizure and welding procedure. is calculated from the corrected load. In wear test done as per ASTM D 4172, the average scar LD diameter formed on the bottom of three balls shows the Corrected load= (1) ability of the lubricant to prevent wear. A larger diameter where L is the applied load, kgf, X is the average scar indicates poor antiwear behavior. The test is carried out for diameter on the worn balls in mm and Hertz scar diameter, one hour at a load of 40 kgf and speed of 1200 RPM with −2 1/3 and𝐷 =8.73×10 (L) in mm. The load wear index is temperature of oil maintained at 75 C. calculated from the expression LWI = (A/10) (kgf), where A is The friction test is carried out to find the friction coef- the sum of the corrected loads determined for the ten applied ficient oer ff ed by the lubricant as per ASTM D 5183 code. loads immediately preceding the weld load. Initially the balls are subjected to “wear in” for one hour at a load of 40 kgf and speed of 600 RPM with temperature of oil maintained at 75 C. After “wear in”, the used lubricating 3. Results and Analysis oil is discarded and balls are cleaned. Fresh lubricant sample is taken in the ball cup with the same worn test balls in place. All the tests are conducted in ten repeatable trails over a The test is again started under the above conditions with load period of 60 days to ascertain the influence of nanoparticles in varying from an initial load of 10 kgf and increasing by 10 kgf terms of repeatable and reproducible results, and the average at the end of each successive 10 min interval until there is a values are reported. The results of wear test conducted as sharp rise in the Frictional Torque which indicates incipient per ASTM D4172 are as given below for dieff rent test oils. seizure. Thisiscalled seizure load whichisan important From Table 4, it can be observed that the average wear scar factor in determining the effectiveness of the lubricant. of lubricant with MWCNTsismuchlessthan that ofbase ASTM D 2783 specifies the extreme pressure properties lubricant. Further, with dispersion of long MWCNTs, the of lubricant in terms of weld load which is the ultimate load range of wear scar over 60 days of testing is higher compared at which the lubricant evaporates due to high pressure and to other oils due to poor lubricant suspension. temperature resulting in all the four balls welded to each Table 5 shows the results of friction test in terms of other. The standard also specifies another parameter called average friction coefficient and seizure load. With dispersion Advances in Tribology 9 Table 3: Physicochemical properties of test oils. ∘ ∘ Viscosity at 40 C Viscosity at 100 C Flash Point Pour Point Total Acid Number Copper strip Test oil Viscosity index ∘ ∘ (cSt) (cSt) C C (mg KOH/g) Corrosion test Base oil (EP140 gear oil) 408.4 27.9 94 194 -3 0.25 2a Base oil +0.5% pristine 409.1 27.75 93 192 -3 0.26 2a MWCNTs Base oil + 0.5% 10 hour 410.2 28.6 95 194 -3 0.25 2a ball milled MWCNTs Base oil + 0.5% 20 hour 411.5 28.4 95 194 -3 0.26 2a ball milled MWCNTs 10 Advances in Tribology Table 4: Results of wear test conducted as per ASTM D 4172 at 40 kgf load. Test Oils Average wear scar in microns Base lubricant (EP 140 transmission oil) 352.353 Base lubricant + long MWCNTs 347.071 Base lubricant + 5 hour Ball milled MWCNTs 335.085 Base lubricant + 10 hour Ball milled MWCNTs 317.954 Base lubricant + 20 hour Ball milled MWCNTs 323.547 Table 5: Results of friction test conducted as per ASTM D 5183. Test oil seizure load, kgf Average coefficient of friction Base lubricant (EP 140 transmission oil) 220 0.0901 Base lubricant + long MWCNTs 230 0.0839 Base lubricant +5 hour Ball milled MWCNTs 240 0.0784 Base lubricant + 10 hour Ball milled MWCNTs 260 0.0746 Base lubricant + 20 hour Ball milled MWCNTs 240 0.0783 0.1 of ball-milled MWCNTs, there is a good improvement in both seizure load and the friction coefficient. 0.095 The 10 sets of results of repeatable friction characteristics are plotted in graphs in Figure 8. In case of lubricant dispersed 0.09 with long MWCNTs, although the performance improved during the initial days, there is steady decrease in the per- 0.085 formance characterized by increase in friction coecffi ient due 0.08 to poor stability over a period of time. Shortened MWCNTs due to their better stability gave a consistence performance 0.075 over a period of 60 days with lubricant dispersed 10-hour ball-milled MWCNTs giving best performance. The variation 0.07 of friction torque with time is plotted in Figure 9. From the graph it can be seen that the effect of nanomaterials is 0.065 more significant at higher loads. In boundary lubrication 0.06 regime there is significant contact between surfaces separated by a thin lubricant film. Increase of normal load would 0.055 sweep the lubricant out of the contact region, reducing the lubricant lm fi thickness between surfaces and increasing the 0.05 chance of contact between surfaces in motion. Due to short 123456789 10 length, the ball-milled MWNTs dispersed in lubricant could Base lubricant effortlessly glide and roll between the two contact surfaces in motion like spacers. This increases the pressure limits of the Base Lubricant +pristine MWCNTs lubricant, thereby significantly reducing friction coefficient Base lubricant +0.5% 5 Hr and improving the seizure load. Lubricant dispersed with ball milled MWCNTs ball-milled MWCNTs has shown consistent performance on Base lubricant +0.5 %10 Hr the torque-time plot compared to base lubricant. Ball milled MWCNTs The extreme pressure properties, namely, last nonseizure Base lubricant+0.5 % 20 Hr load, weld load, and load wear index of test oils under Ball milled MWCNTs consideration, are summarized in Table 6. With dispersion of MWCNTs, there is an improvement Figure 8: Variation of friction coefficient in 10 repeatable tests in last nonseizure load, weld load, and load wear index. A carried out during 2 months. graph showing the variation of wear scar diameter with load is shown in Figure 10. The region between points 0 to 1 is normally designated as antiwear region in which all the lubricants exhibited similar behavior. Points 1 and 1 indicated gear oil comes into play. In this region, the pressures and the last nonseizure load (LNSL) up to which the wear scar temperature are very high and the additives are supposed formed is uniform. to withstand these extremities. MWCNTs due to their good The region above last nonseizure load is called extreme mechanical properties could form a barrier between the pressure zone in which the efficacy of additives in the surfaces withstanding the extremities to improve weld load. Friction coefficient Advances in Tribology 11 Table 6: Results of EP test conducted as per ASTM D 2783. Test oil Last non seizure load, kgf weld load, kgf load wear index Base lubricant (EP 140 transmission oil) 120 250 64.82 Base lubricant + long MWCNTs 120 315 69.54 Base lubricant +5 hour Ball milled MWCNTs 140 315 71.23 Base lubricant + 10 hour Ball milled MWCNTs 140 400 75.65 Base lubricant +20 hour Ball milled MWCNTs 120 250 72.81 2.5 1.5 0.5 0 5000 10000 15000 20000 Time Base Lubricant (EP 140 Transmission oil) Base Lubricant +0.5 % 10 Hr BM MWCNTs Base lubricant+0.5 % 5 hr BM MWCNTs Base Lubricant +0.5 % 20 Hr BM MWCNTs Base lubricant+0.5 % pristine MWCNTs Figure 9: Variation of friction torque with time during friction test. There is a reduction in the wear scar with dispersion of formation during ball milling, exhibited lower performance pristine and ball-milled MWCNTs. The effect of dispersion in EP region. This can be attributed to the formation of defects of shortened MWCNTs is found to be quite visible in this on the surface (Figure 3 and Table 2) leading to loss of special region. Lubricant dispersed with 10-hour ball-milled MWC- properties of MWCNTs. NTs improved the LNSL and could reduce the wear scars even in the extreme pressure regions. Lubricant dispersed 4. Conclusions with ball-milled MWCNTs due to decrease of the friction coefficient offered lower wear scars on the test balls. Further (1) The ball milling of multiwalled carbon tubes prior to after last nonseizure load, it can be observed that the wear scar dispersion in lubricant plays an important role in improve- diameter with dispersion of nanomaterials is significantly ment of stability, antiwear, antifriction, and extreme pressure low, resulting in improvement in load wear index as well properties of gear oil. as weld load. This is due to better dispersion owing to (2) Ball milling could shorten the MWCNTs making them short length. Here the ball milling timing of MWCNTs plays stable in the lubricant for a period of more than 60 days. an important role in defining the optimum length with (3) From Raman spectroscopy it can be observed that ball minimum damage to the tube structure. The optimum length milling time of up to 10 hours did not produce any defects on for attaining good stability and tribological properties can the surface of MWCNTs but 20-hour ball milling produced be assessed as 1 to 5 microns, which can be attained with mild defects on the surface. 10 hours of ball milling at 400 RPM speed. 20-hour ball- milled MWCNTs could improve the load wear index by (4) Long MWCNTs, although surface-modiefi d, when performing well in the antiwear region but, due to defect dispersed in lubricants exhibited poor stability and could Frictional Torque (N-m) 12 Advances in Tribology 10.00 1.00 0.10 50 500 Load applied in Kgf Base Lubricant Base lubricant + 0.5 % long MWCNTs Base lubricant+0.5 % 5 hour Ball milled MWCNTs Base lubricant+0.5 % 10 hour Ball milled MWCNTs Base lubricant+0.5 % 20 hour Ball milled MWCNTs hertz line Figure 10: Variation of wear scar diameter with applied load during EP test. marginally improve the antifriction and extreme pressure LWI: Load wear index properties. MWCNTs: Multiwalled carbon nanotubes SL: Seizure load (5) The physicochemical properties remain unaltered with dispersion of surface-modified and ball-milled MWC- X: Average scar diameter. NTs. (6) There is a good improvement in the wear scar diam- Conflicts of Interest eters and friction coefficients with dispersion of shortened MWCNTs as short-length MWCNTs could slide between The authors declare that they have no conflicts of interest. mating surfaces reducing contact. (7) The load wear index and weld load of lubricants dispersed with shortened MWCNTs have improved signifi- Acknowledgments cantly as the performance of short MWCNTs is good in both The authors gratefully acknowledge the support received antiwear and extreme pressure region. from Hindustan Petroleum Corporation Ltd., India, for (8) At an optimum length of MWCNTs in the range of 1 conducting the tests. The authors acknowledge the assistance to 3 microns, the lubricant could give best results while too from IIT Madras, IIT Kharagpur, and Osmania University, short MWCNTs due to formation of defects despite forming Hyderabad, in characterization. The authors sincerely thank good suspension could not give best performance. the management of GITAM deemed University, India, for the support extended. Nomenclature 𝐷 : Hertz scar diameter References EP: Extreme pressure AW: Antiwear [1] N. Pierard, A. Fonseca, J.-F. Colomer et al., “Ball milling effect L: Load applied on the structure of single-wall carbon nanotubes,” Carbon,vol. 42,no. 8-9, pp.1691–1697, 2004. LNSL: Last nonseizure load Mean scar diameter in mm Advances in Tribology 13 [2] M.S. Dresselhaus, A.Jorio,A.G. Souza Filho,and R.Saito, [18] V.Srinivas,R. 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