Tribological Properties of MoS2 Particles as Lubricant Additive on EN31 Alloy Steel and AISI 52100 Steel Ball

Tribological Properties of MoS2 Particles as Lubricant Additive on EN31 Alloy Steel and AISI 52100 Steel Ball

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 4 (2017) 9967–9971 www.materialstoday.com/proceedings ICEMS 20...

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 4 (2017) 9967–9971

www.materialstoday.com/proceedings

ICEMS 2016

Tribological Properties of MoS2 Particles as Lubricant Additive on EN31 Alloy Steel and AISI 52100 Steel Ball M.S.Charooa*, M.F.Wanib, M. Haniefc, M.A.Ratherd a,b,c

Centre for Tribology, Mechanical Engineering Department, National Institute of Technology, Srinagar-190006, J&K India d

Department of Chemical Engineering, National Institute of Technology, Srinagar-190006, J&K India

Abstract Main objective of this research study is to find out tribological behaviour of rolling bearing material EN31 alloy steel under lubricated conditions with various sizes of MoS2 particles as additive by weight (0.5% and 1%) in conventional lubricant SAE 20W40. Four sizes of MoS2 particles viz., 35 μm, 7 µm, 3 μm and 90 nm were used as additives in the lubricant SAE 20W40 on a tribo-pair consisting of EN31 steel disc and AISI 52100 steel ball. Friction and wear tests were conducted on universal tribometer equipped with high resolution piezo load sensor at three different loads (75 N, 100 N and 125 N) and sliding speed of 0.03 m/s for 10 minutes. It was observed in this study that as the size of the particles decreased, the friction coefficient and wear volume decreased which was attributed to the rolling of the small sized particles in the contact zone. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Recent Trends in Engineering and Material Sciences (ICEMS-2016). Keywords: Friction; Wear; MoS2; EN31

1. Introduction Lubricant performs a number of critical functions; it includes lubrication, reduction in friction and wear, cooling and protecting metal surfaces against corrosive damage. Most of the lubricants that are produced from mineral oil don’t have all the desired tribological properties. It has already been established that addition of solid lubricant particles, such as, MoS2, h-BN and WS2 to mineral oil improve the tribological properties of lubricants to a * Corresponding author : M.S.Charoo, Department of Mechanical Engineering, NIT Srinagar, J&K, India Tel.:+91-9797793483. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Recent Trends in Engineering and Material Sciences (ICEMS2016).

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large extent [1-4]. In these research studies, it has been observed that MoS2 micro/nano particles mixed with lubricating oil possess better friction and wear reducing properties, as compared, to the base oil. Extensive research in open literature reported that addition of solid lubricant particles in particular concentration and size show excellent friction and wear reduction characteristics [5-7]. Particles with layers in structure under strong intra-planar and weak inter-planar bonds have been accepted as offering a very good lubrication potential. It is evident from the above literature review that addition of particular concentration of micro particles to the conventional lubricant reduces the friction coefficient and increases the wear resistance. Researchers used different sizes of MoS2 as lubricant additives, i.e. micro particles, nanoparticles and nanotubes. However, very little research has been carried out to study the effect of MoS2 in conventional lubricants in practical application, such as, EN31 disc and AISI 52100 ball tribo-pair. In this article, two prong approach was adopted to ascertain the effect of various concentrations of MoS2 as lubricant additive on EN31 disc and AISI 52100 ball tribo-pair at various loads and constant sliding speed. In the first set of experiments the effect of MoS2 as additive was studied using a four ball wear test machine. In the second set of experiments, the effect of the MoS2 particles as additive was studied on actual tribo-pair using a universal tribotester. Nomenclature ASTM COF MoS2 SAE WSD

American society for testing materials coefficient of friction molybdenum disulphide society of automotive engineers wear scar diameter

2. Experimental details/procedure 2.1 Preparation of lubricant The commercially available engine oil of grade SAE 20W40 and molybdenum disulphide (MoS2) were procured from the commercial source namely Intelligent Materials Pvt. Ltd. Nanoshel LLC (in Collaboration). The MoS2 particles with a diameter of 35 μm, 7 µm, 3 μm and 90 nm which were black in colour with density of 4.2 g/cm3 and melting point of 1180 oC were added in base engine oil of grade SAE 20W40 in two concentrations (weight % 0.5 and 1.0). The MoS2 mixed engine oil was kept in a test tube and stirred for 2 hours using WENSAR ultrasonicator at a temperature of 50 oC and frequency of 40 kHz. 2.2 Sample preparation A reciprocating friction and wear test on EN31 steel disc of diameter 30 mm and 7 mm thick, against 10 mm diameter AISI 52100 steel balls were conducted using a ball-on-disc configuration. Disc was polished using different grit size of sand papers made of SiC (silicate carbide) viz. 80, 220, 400, 600, 1000, 1200, 1500 and 2000 with a unidirectional surface texture. This was followed by rubbing on the velvet cloth using diamond paste of 3 micron, 1 micron and 0.25 micron to obtain a mirror polished surface. 2.3 Friction and wear tests Wear tests were carried out on a four ball wear test machine in order to support the argument that tribological properties of liquid lubricant are improved by addition of MoS2 particles as additives. Friction and wear experiments were conducted with a tribometer (Magnum India), using a pin-on-disc arrangement. All the experiments were conducted at room temperature. The tests for friction and wear were performed at constant sliding velocity of 0.03 m/s and different loading conditions (75 N, 100 N and 125 N) on the contact surfaces. The length of stroke was kept 2 mm in all the tests and test duration was fixed as 10 minutes for each experiment. All experiments were repeated three times in order to reproduce the results.

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3. Results and discussion 3.1 Wear preventive characteristics of lubricating fluid (Four ball test) Wear tests were conducted separately on lubricant SAE20W40 alone and lubricant SAE 20W40 with 1.0 wt% MoS2 on wear testing machine as per ASTM D 4172 standard. Wear scar diameter (WSD) of the three stationery balls were measured using microscope equipped with the four ball wear test machine. It was observed that WSD of the balls in the lubricant SAE 20W40 alone were greater than WSD of the balls in the lubricant SAE 20W40 + 1.0 wt% MoS2. There was an average reduction of almost 20% in the WSD of the ball with additive in the lubricant. 3.2 Friction coefficient (COF) For measuring the coefficient of friction under dry and lubricated conditions, tests were performed at constant sliding speed of 0.03 m/s and different loads at room temperature. The base lubricant SAE 20W40 added with different concentrations of MoS2 particles (0.5 wt%, 1.0 wt%) were also tested at the same operating conditions for comparison. Figure 1-3 shows the variation of coefficient of friction of different concentration of MoS2 additive lubricant at different loading conditions for 10 minute test duration. The lubricant with additives shows better coefficient of friction as compared to the base oil. From the results it is concluded that the reduction in coefficient of friction may be the ball bearing effect of solid particles dispersed in the lubricant, on the contacting surfaces. It is also observed that as the load increased the value of coefficient of friction increased. The reason for increase in friction coefficient may be the reduction in lubricating film thickness between the contacting surfaces as the contacts are in boundary lubrication regime. At all the loading condition the value of coefficient of friction of MoS2 nanoparticles in lubricant is lower as compared to the base oil, and it is assumed that the value of friction coefficient decreased because of the hexagonal structure of the MoS2, the layers of MoS2 shear off by application of load and these layer formed protective layer on the surface. 0.35

Normal Load = 75 N

SAE 20W40 (Avg. COF= 0.120)

0.3

SAE 20W40 + 0.5 wt% MoS2 90 nm (Avg. COF= 0.0313)

0.25 COF

dry (Avg. COF= 0.314)

SAE 20W40 + 1 wt% MoS2 90 nm (Avg. COF= 0.0363)

0.2

SAE 20W40 + 0.5 wt% MoS2 35 µm (Avg. COF= 0.0573)

0.15

SAE 20W40 + 1 wt% MoS2 35 µm (Avg. COF= 0.0613)

0.1

SAE 20W40 + 0.5 wt% MoS2 7 µm (Avg. COF= 0.0416)

0.05

SAE 20W40 + 1 wt% MoS2 7µm (Avg. COF= 0.0496)

0 100

200

300

400

Time (s)

500

600

SAE 20W40 + 0.5 wt% MoS2 3 µm (Avg. COF= 0.044) SAE 20W40 + 1 wt% MoS2 3 µm (Avg. COF= 0.0451)

Fig. 1. Coefficient of friction under dry and lubricated conditions with MoS2 particles dispersed as additives at normal load of 75 N.

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Normal Load = 100 N

0.45

SAE 20W40 (Avg. COF= 0.194)

0.4 0.35 0.3 COF

dry (Avg. COF= 0.379)

0.25 0.2 0.15 0.1 0.05 0 100

200

300 400 Time (s)

500

600

SAE 20W40+0.5 wt% MoS2 90 nm (Avg. COF= 0.0119) SAE 20W40+1 wt% MoS2 90 nm (Avg. COF= 0.0275) SAE 20W40+0.5 wt% MoS2 35 µm (Avg. COF= 0.0143) SAE 20W40+ 1 wt% MoS2 35 µm(Avg. COF= 0.0183) SAE 20W40+0.5 wt% MoS2 7 µm (Avg. COF= 0.0816) SAE 20W40+ 1 wt% MoS2 7 µm (Avg. COF= 0.112) SAE 20W40+0.5 wt% MoS2 3 µm (Avg. COF= 0.0416) SAE 20W40+ 1 wt% MoS2 3 µm (Avg. COF= 0.0616)

Fig. 2. Coefficient of friction under dry and lubricated conditions with MoS2 particles dispersed as additives at normal load of 100 N.

Normal Load = 125 N

0.45

dry (Avg. COF=0.395) SAE 20W40 (Avg. COF= 0.191)

0.4 0.35

SAE 20W40+0.5 wt% MoS2 90 nm (Avg. COF= 0.0296)

COF

0.3

SAE 20W40+1 wt% MoS2 90 nm (Avg. COF= 0.0393)

0.25

SAE 20W40+0.5 wt% MoS2 35 µm (Avg. COF= 0.0065)

0.2

SAE 20W40+ 1 wt% MoS2 35 µm (Avg. COF= 0.0696)

0.15

SAE 20W40+0.5 wt% MoS2 7 µm (Avg. COF= 0.054)

0.1

SAE 20W40+ 1 wt% MoS2 7 µm (Avg. COF= 0.0605)

0.05

SAE 20W40+0.5 wt% MoS2 3 µm (Avg. COF= 0.0447)

0 100

200

300

400 Time (s)

500

600

SAE 20W40+ 1 wt% MoS2 3 µm (Avg. COF= 0.0564)

Fig. 3. Coefficient of friction under dry and lubricated conditions with MoS2 particles dispersed as additives at the normal load of 125 N.

3.3 Wear The quantitative wear loss was obtained by weighing EN31 sample using high precision digital balance before and after each test. Density of the material (7.2 X10-3 gm/mm3) was used to calculate the wear loss in mm3. Specific wear rate vs. load of the EN31 disc is shown in the Figure. 4 at various loads under lubrication using SAE 20W40 and SAE 20W40 + 0.5 wt% MoS2 with sliding velocity of 0.03 m/s. It is clear from the Figure 4 that wear loss is higher with lubricant SAE 20W40 and is also evident that in most of the cases the wear of EN31 disc decreased with the increase in concentration of MoS2 particles in SAE 20W40. It is inferred from these results that MoS2 particles as additive reduced the wear loss of about 30% to 60% in comparison to lubricant without additive. However, in all tests negligible wear was observed on AISI 52100 balls. This is attributed to higher wear resistance of steel ball due to its higher hardness value. Specific wear (K) was obtained from the Archards equation given below

M.S.Charoo et al. / Materials Today: Proceedings 4 (2017) 9967–9971

Specific wear rate x 10-6 (mm3/Nm)

Specific wear rate (k) =





(mm3/Nm)

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(1)

5.0 4.5 SAE 20W40

4.0 3.5

SAE 20W40+0.5 wt% 90 nm MoS2

3.0 2.5

SAE 20W40+0.5 wt% 3 µm MoS2

2.0 1.5

SAE 20W40+0.5 wt% 7 µm MoS2

1.0 0.5 0.0 75

100

125

SAE 20W40+0.5 wt% 35µm MoS2

Load (N)

Fig. 4. Specific wear coefficient vs. load for the tribo-pair with various sizes of MoS2 particles dispersed as additives in the lubricant.

4. Conclusion The solid particles of MoS2 in the base oil SAE 20W40 show excellent tribological properties under selected operating conditions. Four ball wear preventive test showed 20% decrease in the WSD of the balls with MoS2 particles as additive in the lubricant .Tribometer tests revealed that MoS2 particles as additive reduced the wear loss of about 30% to 60%, in comparison to lubricant without additive.. The value of coefficient of friction and wear loss is function of the size, load and concentration of the particles in the lubricant SAE 20W40. References [1] L. Rapoport, L. Feldman, Y. Homyonfer, M. Cohen, H. Sloan, J. Hutchison, Inorganic fullerene-like material as additives to lubricants: structure-function relationship, Wear. 225-229 (1999) 975-982. [2] X. Zhou. Study on the tribological properties of surfactant-modified IF-MoS2 Micrometer spheres as an additive in liquid paraffin, Tribology International. 40 (2007) 863–86. [3] Z. Pawlak, T. Pai, R. Bayraktar, E. Oloyede, A comparative study on the tribological behaviour of hexagonal boron nitride (h-BN) as lubricant micro particles-An additive in porous sliding bearings for a car clutch, Wear. 267 (2009) 1198-1202. [4] S. Bhaumik, S.D. Pathak, ' Analysis of anti-wear properties of CuO nanoparticles as friction modifiers in mineral oil (460cSt Viscosity) using pin-on-disk tribometer, Tribology in industry. 37 (2015) 196-203. [5] M. Kandeva, D. Karastoianov, B. Ivanova, V. Pojidaeva, Influence of nano-diamond particles on the tribological characteristics of nickel chemical coatings, Tribology in industry. 37 (2014) 181-187. [6] F. Deorsola, F. Russo, N. Blengini, G. Fino, Synthesis, characterization and environmental assessment of nanosized IF-MoS2 particles for lubricants applications, Chemical Engineering Journal, 195-196 (2012) 1-6. [7] L. Zhu, Microstructure and tribological properties of WS2/IF-IF-MoS2 multilayer films, Applied Surface Science, 258 (2012) 1944– 1948.