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

1. Introduction

Gear oils used in industries and automotive engines are often subjected to heavy loads, due to which they experience high temperatures and pressures causing higher friction and surface damage leading to failure of the system. To prevent failure, conventional engine and gear oils are dispersed with extreme pressure (EP) and antiwear (AW) additives that react chemically with the metal surfaces, forming easily sheared layers and thereby preventing severe wear and seizure. Allotropes of carbon such as graphite, fullerenes, carbon nanotubes, and graphene have attracted the interest of the researchers due to their special properties. The hybridization of the atomic orbital of carbons in carbon nanotubes and fullerenes is of type sp2, similar to that of graphite, making a perfect hexagonal array of atoms. The sp2 C-C bond of CNTs is considered one of the strongest in solid materials; thereby CNTs are expected to yield exceptionally good mechanical properties. Lubricants dispersed with allotropes of carbon are being extensively studied for their lower friction coefficients, thereby improving antiwear properties. MWCNTs possess large surface area compared to many inorganic nanomaterials and can be very easily surface-modified. Several novel studies are made on the effect of dispersion of multiwalled carbon nanotubes on the wear and friction characteristics of lubricants. The length of the nanotube synthesized by existing methods is known to be thousands of times larger than their width and thus limits their functionality for many applications. Owing to their large length to diameter ratios, MWCNTs despite use of surfactant tend to form agglomerates faster, thereby leading to them settling in the liquid medium. This aspect is a main challenge in obtaining stable dispersion in liquid medium. The most common and frugal method to reduce the size of MWCNTs is ball milling which shortens the length of MWCNTs and obtaining open ends. However, ball milling for extended period produces defects and damages the graphite structure rendering it useless for dispersion in lubricants. Studies are made on the effect of ball milling on the structure and defect generation. Pierard et. al. [1] studied the effect of ball milling on the structure of single-walled carbon nanotubes. Raman spectroscopy was employed to study the defects produced in various hours of ball milling time. It is found that there exists an optimum time to keep the tubular structure intact without defects. In case of single-walled carbon nanotubes, ball milling times of over 50 hours completely destroy the structure producing amorphous carbon. Dresselhaus et. al.[2], Cancado et. al. [3], and Paton et. al. [4] suggested methods to detect defects and evaluate purity of carbon nanotubes and graphene. It was proposed that the intensity of G-band and the D/G ratio can be highly useful to determine both the purity and the defect density of carbon nanotubes and graphene. Chen et. al. [5] first studied the effect of dispersed of ball-milled and stearic acid modified multiwalled carbon nanotubes (MWCNTs) on the stability and thereby improvement in the lubricating properties of liquid paraffin base oil. The friction and wear tests were conducted on a pin-on-plate wear-testing machine. It is found that ball-milled MWCNTs could form stable suspensions and improve the antiwear and antifriction properties. Engine oils dispersed with nanomaterials are investigated for tribological property enhancement by several researchers. A detailed review of the prior art is listed in Table 1.

As given in Table 1, most of the studies were done with dispersion of larger amounts of MWCNTS in base lubricants or paraffinic oils without additives. Only few studies were made on commercial grade formulated lubricating oils. Further in the studies, due to dispersion of MWCNTs in higher concentrations, the viscosity of the lubricant is found to be significantly enhanced. This enhancement in viscosity is the major reason for the improved tribological properties. Furthermore higher concentrations of any nanomaterials may lead to faster agglomeration rates and the stability of the suspension will get compromised. Since MWCNTs have long length ranging from 1 to 25 microns and diameter in nanometers, there is a strong ability of entanglement of individual nanotubes leading to formation of clusters. These clusters tend to become hard and make the MWCNTs lose their special properties. Moreover, the stability of nanofluid has not been assessed quantitatively in the studies made so far. Further the defects produced in MWCNTs due to ball milling and its consequential effects on tribological properties also need to be assessed. The present study is aimed at investigating improvements in antiwear, antifriction, and extreme pressure properties of formulated EP 140 grade gear engine oil dispersed with surface-modified and ball-milled 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 to stabilize the MWCNTs in oil medium and the stability of the suspensions is investigated over a period of 2 months. The effect of decrease in length of MWCNTs due to ball milling on the tribological properties has been studied. The repeatability of results over a period of 60 days is investigated to observe consistent performance and the average values are reported. The effect of additives in the oil along with surface-modified MWCNTs has resulted in use of lesser amount of MWCNTs in the lubricant dispersion compared to studies reported in the literature which is one of the novel features of the study. The paper also compares chemical and physical routes for dispersion of MWCNTs in lubricant by comparing performance of surface-modified pristine MWCNTs and surface-modified ball-milled MWCNTs, respectively.

2. Experimental

2.1. Materials. In the present study, multiwalled carbon nanotubes produced by CVD method have been procured from M/s Cheap Tubes Inc., USA. The size of mWcNTs is 20-40 nm in diameter, 25 microns in length, and 95% of purity. All other chemicals purchased are of GR grade. The surfactant is AR grade procured from M/s Sigma Aldrich India Pvt limited. GL4 (EP 140 grade) gear oil is selected as base lubricant.

2.2. Ball Milling of Multiwalled Carbon Nanotubes. As the length of the carbon nanotubes is thousands of times larger than their width, ball milling of MWCNTs is a common procedure to generate short and open-ended nanotubes. Ball milling apparatus consists of tungsten carbide lined bowls containing tungsten carbide lined ball. Arrangement is provided to perform ball milling under argon atmosphere to prevent oxidation of material during ball milling. The speed of rotation of bowls is set at 400 RPM and the ratio of balls to MWCNTs is taken as 10:1 to ensure minimal damage to the tubular structure of the MWCNTs. Higher speeds and ratios will increase the impact and thereby attrition of MWCNTs. Ball milling was performed for 4 and 20 hours at 400 RPM to avoid damage to the structure. After ball milling, the MWCNTs are heated in air at 600[degrees]C to remove amorphous carbon generated during ball milling process. These ball-milled MWCNTs are characterized using HRSEM and transmission electron microscopy to determine the average length of the MWCNTs.

2.3. Electron Microscopy. Figure 1(a) shows HRSEM image of pristine long length entangled MWCNTs. Figure 1(b) shows MWCNTs ball-milled for 5 hours with a small change in length of MWCNTs as compared to Figure 1(a). Figures 2(a) and 2(b) show images of 10- and 20-hour ball-milled MWCNTs.

It can be seen that the average length of 10-hour ball-milled MWCNTs is under 4 microns. From Figure 2(b) (TEM image) it can be observed that the length of 20-hour ball-milled CNTs has come down to around 150 nm size.

2.4. Raman Spectroscopy. Raman spectroscopy is employed to assess the formation of defects during ball millings and shown in Figure 3 for pristine MWCNTS, 5-, 10-, and 20hour ball-milled MWCNTs. The [sp.sup.2] structure of MWCNTs causes first order peaks D and G bands that are approximately located at 1350 [cm.sup.-1] and 1580 [cm.sup.-1], respectively. Defect free MWCNTs due to intact hexagonal graphite structure make the G-band sharper. Defects in MWCNTs make the G-band peaks wider and shorter. On the other hand, D band peak represents lattice defects and finite crystal size. During defect formation, due to breaking of the 2D translational symmetry the D band peak will increase and become wider. Another peak G band which can be seen at 2700 [cm.sup.-1] Raman Shift represents amorphous defects in MWCNTs.

Table 2, provides the intensities of D, G, and G bands. In case of pristine MWCNTs, 5- and 10-hour ball-milled MWCNTs, there is no significant difference in the intensities of D, G, and G bands with ratio of intensities of D and G bands remaining marginally the same. In case of 20-hour ball-milled MWCNTs, a significant decrease in intensity of G and G bands is observed with increase of intensity of D band indicating mild destruction of graphite structure and formation of amorphous defects. In all cases, the peaks of all bands are sharp indicating either no defects or mild defect (in case of 20-hour ball-milled MWCNTs).

2.5. Surface Modification of MWCNTs. Pristine MWCNTs tend to agglomerate and form large particles clusters in liquid medium. Moreover, after the ball milling process, the MWCNTs tend to be compressed by the balls forming larger aggregates of entangled MWCNTs. To disentangle them and make them stable in the lubricant medium, it is required to modify the surface of MWCNTs with a surfactant to create stearic repulsions between individual nanotubes. To stabilize the nanoparticles in the liquid medium, a surfactant SPAN 80 (Sorbitan monooleate) is used to modify surface of MWCNTs during the preparation of oil. SPAN 80 is a nonionic surfactant with a hydrophilic-lipophilic balance of 4.6 which is ideally suitable for oils. It adsorbs on the surface of MWCNTs reducing their surface energy, thereby preventing agglomeration and settling of nanoparticles. To prepare surface-modified MWCNTs, SPAN 80 and multiwalled carbon nanotubes (both pristine and ball-milled) are taken in the ratio of 2:1 and ultrasonicated in a solvent for 30 minutes, which creates a mechanochemical reaction. This reaction coats the surfactant on to the surface of the MWCNTs.

2.6. FTIR Spectroscopy. The surface-modified MWCNTs are characterized for functional groups on the surface using Fourier transforms infrared spectroscopy as shown in Figures 4 and 5. Figure 4 shows pristine MWCNTs with no characteristic peak detected. Figure 4 shows the FTIR image of surface-modified MWCNTs with characteristics peaks between 1463 and 1486 [cm.sup.-1] wavelength indicating lipophilic groups attached to the surface.

2.7. Preparation and Evaluation of Stability of Lubricants with MWCNTs. The surface-modified long and ball-milled MWCNTs in 0.5 weight percent are dispersed in lubricating oil by processing it in a probe ultrasonicator (Hielscher UP400S) for about 30 minutes. The stability of the nanofluid for a period of 60 days is monitored by light scattering techniques. Ball-milled MWCNTs when dispersed in lubricating oils could form better stable suspension compared to long MWCNTs and gave better tribological properties.

The stability of the lubricants dispersed with MWCNTs is evaluated using light scattering techniques by means of zeta sizer (Horiba SZ 100). The zeta potential of the samples, an indicator of dispersion stability of MWCNTs in the lubricating oil medium, has been analyzed over a period of 60 days. A zeta potential value of [+ or -] 40 indicates good stability. As the viscosity of the gear oil is high, the oil samples are diluted using toluene before charging it into the zeta sizer to improve the transmittance of the oil so that accurate values can be obtained. Higher values of zeta potential values are found when the oil samples are dispersed with ball-milled MWCNTs. The variation of zeta potential immediately after preparation and 60 days after preparation is shown in Figures 6 and 7.

Figures 6(a) and 6(b) show the variation of zeta potential of lubricant dispersed with long MWCNTs and 5-hour ball-milled MWCNTs, respectively. As can be seen the zeta potential with long MWCNTs is much less indicating low stability. With five-hour ball-milled MWCNTs there is an improvement in the stability compared to stability of long MWCNTs. Figures 7(a) and 7(b) show the variation of zeta potential of lubricant dispersed with 10-hour and 20-hour ball-milled MWCNTs. As can be seen there is a significant improvement in the stability with 20-hour MWCNTs showing best stability. This can be attributed to lower agglomeration rates due to short length of MWCNTs.

2.8. Physicochemical Properties of Test Oils. The gear oils are manufactured by blending base stocks and additive components to meet the requirements of standards. Basic physicochemical properties should be in compliance with international standards for statutory purposes. The main physicochemical properties to be assessed for a gear oil are viscosity, viscosity index, pour point, flash point, total acid number, and copper strip corrosion resistance. All the properties for test oils are evaluated in 5 replicable experiments and the average values are reported.

As can be observed from Table 3, the effect of ball milling and surface modification has practically no effect on the physicochemical properties of the test oils. The viscosity and viscosity index are unchanged with dispersion of nanomaterials. Surface modification has no effect on pour point, flash point, and total acid number of the test lubricants.

2.9. Tests for Tribological Properties. The test oils are tested for improvement in tribological properties on a four-ball tester. The weight percentage of surface-modified and ball-milled MWCNTs is maintained as 0.5 wt %. The standard code of the tests, a rotating steel ball, is pressed against three steel balls firmly held together under a load and immersed in lubricant. The test parameters of load, duration, temperature, and rotational speed are set in accordance with standard test procedure.

In wear test done as per ASTM D 4172, the average scar diameter formed on the bottom of three balls shows the ability of the lubricant to prevent wear. A larger diameter indicates poor antiwear behavior. The test is carried out for one hour at a load of 40 kgf and speed of 1200 RPM with temperature of oil maintained at 75[degrees]C.

The friction test is carried out to find the friction coefficient offered by the lubricant as per ASTM D 5183 code. 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[degrees]C. After "wear in", the used lubricating oil is discarded and balls are cleaned. Fresh lubricant sample is taken in the ball cup with the same worn test balls in place. The test is again started under the above conditions with load varying from an initial load of 10 kgf and increasing by 10 kgf at the end of each successive 10 min interval until there is a sharp rise in the Frictional Torque which indicates incipient seizure. This is called seizure load which is an important factor in determining the effectiveness of the lubricant.

ASTM D 2783 specifies the extreme pressure properties of lubricant in terms of weld load which is the ultimate load at which the lubricant evaporates due to high pressure and temperature resulting in all the four balls welded to each other. The standard also specifies another parameter called "load wear index" which indicates the behavior of lubricant in resisting the aforementioned weld conditions. The test is used to determine the load carrying properties of a lubricant 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:

temperature of oil: room temperature,

speed of rotation: 1760 RPM,

duration: 10 s,

load applied: 32 kgf to weld load.

A total of 10 readings are considered in the test and the corrected load is calculated for all ten readings. The load wear index is a single parameter that shows the overall EP behavior in a range between well below seizure and welding is calculated from the corrected load.

Corrected load = [LD.sub.H]/X (1)

where L is the applied load, kgf, X is the average scar diameter on the worn balls in mm and Hertz scar diameter, and [D.sub.H] = 8.73 x [10.sup.-2] [(L).sup.1/3] in mm. The load wear index is calculated from the expression LWI = (A/10) (kgf), where A is the sum of the corrected loads determined for the ten applied loads immediately preceding the weld load.

3. Results and Analysis

All the tests are conducted in ten repeatable trails over a period of 60 days to ascertain the influence of nanoparticles in terms of repeatable and reproducible results, and the average values are reported. The results of wear test conducted as per ASTM D4172 are as given below for different test oils. From Table 4, it can be observed that the average wear scar of lubricant with MWCNTs is much less than that of base lubricant. Further, with dispersion of long MWCNTs, the range of wear scar over 60 days of testing is higher compared to other oils due to poor lubricant suspension.

Table 5 shows the results of friction test in terms of average friction coefficient and seizure load. With dispersion of ball-milled MWCNTs, there is a good improvement in both seizure load and the friction coefficient.

The 10 sets of results of repeatable friction characteristics are plotted in graphs in Figure 8. In case of lubricant dispersed with long MWCNTs, although the performance improved during the initial days, there is steady decrease in the performance characterized by increase in friction coefficient due to poor stability over a period of time. Shortened MWCNTs due to their better stability gave a consistence performance over a period of 60 days with lubricant dispersed 10-hour ball-milled MWCNTs giving best performance. The variation of friction torque with time is plotted in Figure 9. From the graph it can be seen that the effect of nanomaterials is more significant at higher loads. In boundary lubrication regime there is significant contact between surfaces separated by a thin lubricant film. Increase of normal load would sweep the lubricant out of the contact region, reducing the lubricant film thickness between surfaces and increasing the chance of contact between surfaces in motion. Due to short length, the ball-milled MWNTs dispersed in lubricant could effortlessly glide and roll between the two contact surfaces in motion like spacers. This increases the pressure limits of the lubricant, thereby significantly reducing friction coefficient and improving the seizure load. Lubricant dispersed with ball-milled MWCNTs has shown consistent performance on the torque-time plot compared to base lubricant.

The extreme pressure properties, namely, last nonseizure load, weld load, and load wear index of test oils under consideration, are summarized in Table 6.

With dispersion of MWCNTs, there is an improvement in last nonseizure load, weld load, and load wear index. A 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 the last nonseizure load (LNSL) up to which the wear scar formed is uniform.

The region above last nonseizure load is called extreme pressure zone in which the efficacy of additives in the gear oil comes into play. In this region, the pressures and temperature are very high and the additives are supposed to withstand these extremities. MWCNTs due to their good mechanical properties could form a barrier between the surfaces withstanding the extremities to improve weld load. There is a reduction in the wear scar with dispersion of pristine and ball-milled MWCNTs. The effect of dispersion of shortened MWCNTs is found to be quite visible in this region. Lubricant dispersed with 10-hour ball-milled MWCNTs improved the LNSL and could reduce the wear scars even in the extreme pressure regions. Lubricant dispersed with ball-milled MWCNTs due to decrease of the friction coefficient offered lower wear scars on the test balls. Further after last nonseizure load, it can be observed that the wear scar diameter with dispersion of nanomaterials is significantly low, resulting in improvement in load wear index as well as weld load. This is due to better dispersion owing to short length. Here the ball milling timing of MWCNTs plays an important role in defining the optimum length with minimum damage to the tube structure. The optimum length for attaining good stability and tribological properties can be assessed as 1 to 5 microns, which can be attained with 10 hours of ball milling at 400 RPM speed. 20-hour ball-milled MWCNTs could improve the load wear index by performing well in the antiwear region but, due to defect formation during ball milling, exhibited lower performance in EP region. This can be attributed to the formation of defects on the surface (Figure 3 and Table 2) leading to loss of special properties of MWCNTs.

4. Conclusions

(1) The ball milling of multiwalled carbon tubes prior to dispersion in lubricant plays an important role in improvement of stability, antiwear, antifriction, and extreme pressure properties of gear oil.

(2) Ball milling could shorten the MWCNTs making them stable in the lubricant for a period of more than 60 days.

(3) From Raman spectroscopy it can be observed that ball milling time of up to 10 hours did not produce any defects on the surface of MWCNTs but 20-hour ball milling produced mild defects on the surface.

(4) Long MWCNTs, although surface-modified, when dispersed in lubricants exhibited poor stability and could marginally improve the antifriction and extreme pressure properties.

(5) The physicochemical properties remain unaltered with dispersion of surface-modified and ball-milled MWCNTs.

(6) There is a good improvement in the wear scar diameters and friction coefficients with dispersion of shortened MWCNTs as short-length MWCNTs could slide between mating surfaces reducing contact.

(7) The load wear index and weld load of lubricants dispersed with shortened MWCNTs have improved significantly as the performance of short MWCNTs is good in both antiwear and extreme pressure region.

(8) At an optimum length of MWCNTs in the range of 1 to 3 microns, the lubricant could give best results while too short MWCNTs due to formation of defects despite forming good suspension could not give best performance.
Nomenclature

[D.sub.H]: Hertz scar diameter
EP:        Extreme pressure
AW:        Antiwear
L:         Load applied
L NSL:     Last nonseizure load
LWI:       Load wear index
MWCNTs:    Multiwalled carbon nanotubes
SL:        Seizure load
X:         Average scar diameter.


https://doi.org/10.1155/2018/2328108

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors gratefully acknowledge the support received from Hindustan Petroleum Corporation Ltd., India, for conducting the tests. The authors acknowledge the assistance from IIT Madras, IIT Kharagpur, and Osmania University, Hyderabad, in characterization. The authors sincerely thank the management of GITAM deemed University, India, for the support extended.

References

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Kodanda Rama Rao Chebattina (iD), (1) V. Srinivas (iD), (1) and N. Mohan Rao (iD) (2)

(1) Department of Mechanical Engineering, GITAM University, Visakhapatnam, India

(2) 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

Caption: Figure 1: HRSEM images of pristine and 5-hour ball-milled MWCNTS. (a) Pristine MWCNTS, (b) 5-hour ball-milled MWCNTS.

Caption: Figure 2: HRSEM images of 10-hour and 20-hour ball-milled MWCNTs. (a) 10-Hour ball-milled MWCNTs, (b) 20-hour ball-milled MWCNTs.

Caption: Figure 3: Raman spectroscopy of pristine and ball-milled MWCNTs.

Caption: Figure 4: FTIR spectroscopy of pristine MWCNTs.

Caption: Figure 5: FTIR spectroscopy of surface-modified MWCNTs.

Caption: Figure 6: Zeta potential variation of nanofluids during 60 days. (a) Lubricants with long MWCNTs immediately after preparation, (b) lubricant with long MWCNTs after 60 days, (c) lubricant with 5-hour ball-milled MWCNTs immediately after preparation, (d) lubricant with 5-hour ball-milled MWCNTs after 60 days.

Caption: Figure 7: Zeta potential variation of nanofluids. (a) Lubricants with 10-hour ball-milled MWCNTs immediately after preparation, (b) lubricant with 10-hour ball-milled MWCNTs after 60 days, (c) lubricant with 20-hour ball-milled MWCNTs immediately after preparation, (d) lubricant with 20-hour ball-milled MWCNTs after 60 days.

Caption: Figure 8: Variation of friction coefficient in 10 repeatable tests carried out during 2 months.

Caption: Figure 9: Variation of friction torque with time during friction test.

Caption: Figure 10: Variation of wear scar diameter with applied load during EP test.
Table 1: Summary of research carried out by authors.

                       Base fluid, type of
Authors                nano materials and     Experimental
                       concentration          apparatus

                       Mineral oil            Pin on disk
Bhaumik et al. [6]     dispersed with         tribometer and Four-
                       graphite and MWCNTs    ball tester
                       in concentration 0.1
                       to 0.6 Wt%.

Chen et. al. [5]       Paraffin oil           Ring on plate
                       dispersed with
                       stearic acid
                       modified MWCNTs in
                       0.45% Wt %

Cornelio et. al. [7]   Oil and water with     Twin-disk machine
                       SWCNTs & MWCNTs 0.01   for measurements in
                       to 0.05 Wt %           rolling-sliding
                                              contact

Ghaednia et. al. [8]   Mineral base oil       disk-on-disk
                       dispersed with CuO     friction and wear
                       (9 nm) in 0.5 to 2     test
                       wt%

Hernandez Battez et.   poly-alpha-            Four-ball tester
al. [9]                olefin(PAO6)
                       dispersed with CuO,
                       ZnO and ZrO2 in 0.5
                       to 2% wt

Hu et. al. [10]        Water dispersed with   Four-ball tester
                       sodium dodecyl
                       sulfate modified
                       oxidized MWCNTs in
                       0.1% Wt (for
                       application in metal
                       working fluids)

Joly-Pottuz et. al.    poly-alpha-olefin      pin-on-flat
[11]                   dispersed with         tribometer
                       carbon nano onions
                       and graphite powders
                       in 0.1% wt

Khalil et. al. [12]    Mobil gear 627 and     Four-ball tribotester
                       paraffinic mineral
                       oils in 0.1 to 2 wt%

Lee et. al. [13]       Raw mineral oil (Sun   disk-on-disk
                       Oil, Japan)            tribotester
                       dispersed with
                       fullerenes in 0.01
                       to 0.05% wt.

Lee et. al. [14]       Commercial gear oil    disk-on-disk
                       dispersed with nano      tribotester
                       graphite (55 nm) in
                       0.1 to 0.5% wt.
                       Alkyl aryl sulfonate
                       is used as
                       dispersant

Pena-Paras et. al.     4 types of Metal       Afour-ballT-02U
[15]                   working fuids          tribotester
                       dispersed with
                       Ti[O.sub.2],
                       [Al.sub.2][O.sub.3],
                       CuO and MWCNTs in
                       0.01 to 0.1% wt

Puzyr et. al. [16]     Commercial oil         block-on-ring test
                       dispersed with         setup
                       surface modifed nano
                       diamonds in 0.01% wt

Sarma et. al. [17]     SMgradeengineoil       Pin on disk tester
                       dispersed with Cu      and engine test rig
                       and TiO2 nano
                       particles in
                       0.025 to 0.1% wt

Srinivas et. al. [18]  EP 140 Transmission    Four-ball tester
                       oil dispersed with
                       WS2 and MoS2 nano
                       materials in 0.5%
                       wt.

Srinivas et.al. [19]   CI 4 Engine oil        Four-ball tester
                       dispersed with long
                       MWCNTs

Viesca et. al. [20]    Poly alpha olefn       Block on-ring
                       (PAO6) dispersed       tribometer and four-
                       with carbon coated     ball tester
                       nano particles in
                       0.5 to 2% wt

Authors                Major findings

                       It is found that
Bhaumik et al. [6]     MWCNTs outperformed
                       graphite with wear
                       reduced by 70-75%
                       and load bearing
                       capacity increasing
                       by 20%.

                       Friction coefficient
Chen et. al. [5]       decreased by 10%,
                       wear reduced by 30-
                       40%. It was proposed
                       that friction-
                       reduction ability of
                       nano-lubricant
                       depends on both nano
                       materials and
                       obtaining stable
                       dispersion of nano-
                       particle in
                       lubricant.

Cornelio et. al. [7]   The friction
                       coefficients and
                       wear losses measured
                       were lower for
                       either oil or water
                       dispersed with
                       nanotubes with
                       friction
                       coefficients
                       reported as low as
                       0.063. It was
                       proposed that
                       decrease of friction
                       and high wear
                       resistance are due
                       to the formation of
                       an amorphous carbon
                       film transferred
                       from the CNT's on
                       the surface.

Ghaednia et. al. [8]   Friction coefficient
                       decreased by 14 and
                       23% for the CuO
                       nanoparticle
                       concentrations of
                       1.0 and 2.0% wt,
                       respectively. It was
                       suggested that the
                       reduction in the
                       real area of contact
                       due to dispersion of
                       nanomaterials in
                       lubricant is
                       possible mechanism
                       for reduction of
                       friction

Hernandez Battez et.   The results indicate
al. [9]                that the extreme
                       pressure behavior of
                       lubricant with nano
                       particles is
                       strongly dependent
                       on the size and
                       hardness of the
                       nanoparticles.
                       Particles with
                       hardness less than
                       surfaces in contact
                       exhibited good EP
                       behavior.

Hu et. al. [10]        It is found that
                       oxidation of MWCNTs
                       produced defects on
                       the surface leading
                       to formation of
                       better suspension.
                       There is a good
                       improvement in
                       tribological
                       properties with
                       surfactant modified
                       MWCNTs compared to
                       pristine MWCNTs
                       indicating strong
                       influence of
                       stability of
                       suspension.

Joly-Pottuz et. al.    Carbon nano-onions
[11]                   show better
                       tribological
                       properties than
                       graphite powder. It
                       is also found that
                       tribofilm formed by
                       carbon onions
                       converts wear
                       particles into
                       ultrafine lubricious
                       iron oxides, thus
                       preventing further
                       abrasive wear
                       process.

Khalil et. al. [12]    A 50% reduction in
                       friction and an
                       increase in weld
                       load by up to 100%
                       are observed with
                       dispersion of MWCNTs
                       in lubricant. The
                       Tribological
                       performance is
                       attributed to
                       deposition of MWCNTs
                       nanoparticles on the
                       worn surface
                       resulting in
                       decreasing the
                       shearing stress,
                       thus improving the
                       tribological
                       properties.

Lee et. al. [13]       Fullerenes have
                       reduced friction by
                       30% by reducing the
                       metal surface
                       contacts. Further,
                       it is found that
                       volume fraction is a
                       key factor to
                       control the friction
                       and wear

Lee et. al. [14]       The results indicate
                       dispersion of nano
                       graphite in
                       lubricant boosted
                       the lubrication
                       characteristics. The
                       possible reason for
                       improvement of
                       properties is
                       suggested to be due
                       to nanoparticles
                       acting as ball
                       bearing spacers
                       between the friction
                       surfaces reducing
                       the contact between
                       the plates.

Pena-Paras et. al.     All nano materials
[15]                   improved the
                       tribological
                       properties with
                       MWCNTs giving best
                       results. Tribo-
                       sintering of nano
                       materials on the
                       rubbing surfaces
                       during machining is
                       the reason proposed
                       for the performance
                       improvement.

Puzyr et. al. [16]     Nano diamond
                       additives in oils
                       improved the anti-
                       wear properties and
                       decreased the oil
                       temperature compared
                       to base oils.Te
                       anti-wear mechanism
                       of ND additives was
                       attributed to the
                       formation of a hard
                       and porous layer
                       between the contact
                       surfaces.

Sarma et. al. [17]     It was found that
                       copper nano particle
                       dispersed lubricant
                       gave best results on
                       engine tests. 3 to
                       5% increase in
                       thermal efciency of
                       the engine is
                       observed.

Srinivas et. al. [18]  Lubricant dispersed
                       withWS2
                       nanoparticles gave
                       higher weld load and
                       load wear index
                       (LWI) than that of
                       lubricant dispersed
                       with MoS2
                       nanoparticles.Te
                       reason for better
                       performance ofWS2
                       nano particles is
                       attributed to their
                       lower hardness
                       resulting in better
                       deposition on the
                       rubbing surfaces
                       under load.

Srinivas et.al. [19]   The surface
                       modifcation of multi
                       walled carbon tubes
                       plays prominent role
                       in the improving
                       stability and
                       thereby anti-wear
                       and anti-friction
                       properties of engine
                       oils.

Viesca et. al. [20]    carbon-coated copper
                       nanoparticles
                       decreases wear and
                       increases the load-
                       carrying capacity of
                       polyalphaolefn

Table 2: Raman Spectra characteristics of MWCNTs.

MWCNTs                 Intensity of   Intensity of   Intensity of
                        D band, ID     G band, IG    G' band, IG'

Pristine                   1536           2100           1594
5 hour ball milled         1261           1708           1082
10 hour ball milled        1410           1855           974
20 hour ball milled        1413           1266

MWCNTs                  Ratio of
                        ID and IG

Pristine                  0.731
5 hour ball milled        0.738
10 hour ball milled        0.76
20 hour ball milled        1.11

Table 3: Physicochemical properties of test oils.

Test oil                    Viscosity at    Viscosity at
                            40[degrees]C    100[degrees]C

                                (cSt)           (cSt)

Base oil (EP140 gear oil)       408.4           27.9

Base oil +0.5% pristine         409.1           27.75
MWCNTs

Base oil + 0.5% 10 hour         410.2           28.6
ball milled MWCNTs

Base oil + 0.5% 20 hour         411.5           28.4
ball milled MWCNTs

Test oil                     Viscosity      Flash Point   Pour Point
                                index       [degrees]C    [degrees]C

Base oil (EP140 gear oil)        94             194           -3

Base oil +0.5% pristine          93             192           -3
MWCNTs

Base oil + 0.5% 10 hour          95             194           -3
ball milled MWCNTs

Base oil + 0.5% 20 hour          95             194           -3
ball milled MWCNTs

Test oil                    Total Acid     Copper strip
                              Number

                            (mg KOH/g)    Corrosion test

Base oil (EP140 gear oil)      0.25             2a

Base oil +0.5% pristine        0.26             2a
MWCNTs

Base oil + 0.5% 10 hour        0.25             2a
ball milled MWCNTs

Base oil + 0.5% 20 hour        0.26             2a
ball milled MWCNTs

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

Table 5: Results of friction test conducted as per ASTM D 5183.

Test oil                                    seizure       Average
                                           load, kgf    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            240          0.0784
MWCNTs

Base lubricant + 10 hour Ball milled          260          0.0746
MWCNTs

Base lubricant + 20 hour Ball milled          240          0.0783
MWCNTs

Table 6: Results of EP test conducted as per ASTM D 2783.

Test oil                                   Last non     weld load,
                                            seizure         kgf
                                           load, kgf

Base lubricant (EP 140 transmission oil)      120           250

Base lubricant + long MWCNTs                  120           315

Base lubricant +5 hour Ball milled            140           315
MWCNTs

Base lubricant + 10 hour Ball milled          140           400
MWCNTs

Base lubricant +20 hour Ball milled
MWCNTs

Test oil                                   load wear
                                             index

Base lubricant (EP 140 transmission oil)     64.82

Base lubricant + long MWCNTs                 69.54

Base lubricant +5 hour Ball milled           71.23
MWCNTs

Base lubricant + 10 hour Ball milled         75.65
MWCNTs

Base lubricant +20 hour Ball milled          72.81
MWCNTs
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Article Details
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Title Annotation:Research Article
Author:Chebattina, Kodanda Rama Rao; Srinivas, V.; Rao, N. Mohan
Publication:Advances in Tribology
Article Type:Report
Geographic Code:9INDI
Date:Jan 1, 2018
Words:6260
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