Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles

Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles

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Materials Today: Proceedings xxx (xxxx) xxx

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Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles Vineet Sharma, Rajat Joshi, Himanshu Pant, Vipin Kumar Sharma ⇑ Department of Mechanical and Automation Engineering, Maharaja Agrasen Institute of Technology, Rohini Sector-22, Delhi 110086, India

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Article history: Received 29 March 2019 Received in revised form 4 August 2019 Accepted 20 August 2019 Available online xxxx Keywords: Wear Friction Lubrication Bronze Graphite

a b s t r a c t The present paper discusses the use of graphite particles in the lubricant for enhancing the friction properties of lubricant. Graphite particles were produced using the ball milling methods and mixed with multi-grade SAE 15W-40 lubricating oil using a magnetic stirrer and ultrasonicator. The graphite mixed lubricant was tested for its frictional behaviour using a linear reciprocating tribometer. Experiments were performed at three load values with constant stroke length and frequency. It is observed that graphite mixed lubricant resulted in reduction in coefficient of friction (COF) values. For the considered operating parameters, there is 33–36% reduction in the COF values. The overall temperature generation during the testing was also low for the graphite mixed lubricant testing. Ó 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2nd International Conference on Computational and Experimental Methods in Mechanical Engineering.

1. Introduction Friction is an important phenomenon which runs most of our machines but is also a bane which leads to wear and energy losses due to heat, sound etc. Lubricants are used to reduce the resistance between two surfaces and improve the relative motion of different mating parts. Lubricants can be solid or liquid. Solid lubricants are used for very high temperatures and are relatively expensive. Hence, liquid and semi-solid lubricants are the favourable choices for common applications. Mineral oils, organic oils etc. are usually used as base compounds for lubricants. But the properties of the base compounds of the lubricants are usually non-flexible and limited and hence additives are added to the lubricants to enhance their properties. Even the lubricants used have certain additives added to them like Viscosity Index Improvers which impart properties that are universally recognized. Researchers have used nanoparticles to improve the tribological properties. A brief review is presented here for the better understanding of different nanoparticles. Babu et al. (2014) [1] used the Al2O3 and ZnO nanoparticles mixed lubricating oil for analyzing the performance of the journal bearing. Mathematical models were developed depicting the relation between the lubricant viscosity and temperature. The addition of nanoparticles increases the viscosity of the ⇑ Corresponding author. E-mail address: [email protected] (V.K. Sharma).

lubricant and hence affects the performance of the bearing. Zin et al. (2013) [2] studied the effect of copper nanoparticles on the tribological properties of the commercial lubricant used in IC engines. Copper particles in two sizes of 60 nm and 130 nm were prepared and mixed in the lubricant in different proportions. Tribological tests were conducted in boundary, mixed and elastohydrodynamic lubrication condition. It is concluded by authors that, copper particles with 130 nm mean diameter performed better in reducing the COF in all the tested lubrication regimes. Kole et al. (2010) [3] prepared the nanofluid using the alumina particles in vehicle coolant. Oleic acid was used as a surfactant to ensure the long-lasting mixing of the alumina particles. Different fractions of alumina particles were mixed and tested at a temperature range of 10–50 °C. It is concluded that, with higher percentage of alumina particles the viscosity of the lubricant increases. Omrani et al. (2016) [4] discussed the use of solid materials to improve the tribological properties. Carbonous materials, boron nitride and molybdenum disulfide (MoS2) are some of the materials which are used to produce the self-lubricating composites. Sharma et al. (2017) [5] suggested the use of fly-ash particles in the metal matrix to improve the tribological properties. Pin on disc experiments were performed to obtain the wear and COF values. Zhao et al. (2017) [6] studied the lubricating properties of graphene. Friction and wear experiments with different grapheme exfoliated in base oil were performed and compared with the properties of base oil. The higher degree of exfoliation resulted with lower friction values

https://doi.org/10.1016/j.matpr.2019.08.190 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2nd International Conference on Computational and Experimental Methods in Mechanical Engineering.

Please cite this article as: V. Sharma, R. Joshi, H. Pant et al., Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.190

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as compared to low exfoliated mixed lubricant. Sharma et al. (2018) [7] utilized the graphite powder and prepared an aluminium graphite metal composite. The graphite particles helped in enhancing the tribological properties of the aluminium. From the literature review it is found that graphite has been used as a possible additive to enhance the tribological properties of materials. On similar terms as that of Sharma et al. (2018), Fuentes et al. (2003) fabricated aluminium graphite composite using the powder metallurgy route. The graphite reinforcement was pretreated with copper before use. The use of graphite greatly enhances the rheology of the composite which made itself lubricating in nature [8]. Zhang et al. (2015) used the exhaust diesel soot as a possible lubricant additive in the commercial used lubricant. The wear and friction properties were evaluated with the help of ball on plate tribo-meter. It is reported that the diesel soot mixed lubricant performed better as compared to the graphite mixed lubricant at higher temperatures [9]. Zhang et al. (2014) presented a review about the usage of graphite and nano-materials as the possible additive in lubricant. The particle size, its aspect ratio and concentration largely affects the performance of the nano-particle mixed lubricant [10]. It is analysed from the literature that most of the studies has lacked to show the effect of low amounts of graphite in the commercial SAE 15W 40 lubricant oil. So, in the present paper 0.05 vol% graphite particles were mixed in the lubricant. Ball milling method was used to produce the graphite particles. Magnetic stirring and ultrasonicating techniques were utilized to mix the graphite particles. For the evaluation of tribological properties, a reciprocating pin on plate setup has been used and experiments were performed to find out the coefficient of friction. Moreover, the temperature rise between the tribopairs has also been determined.

der was inspected to check the fineness. For that the milled graphite powder was filtered by using an Ultra-filtration membrane (>0.001 nm). The scanning electron microscopic image (SEM) for the milled graphite particles is presented in Fig. 1(a,b). The Fig. 1 (a) was obtained at 300 magnification and Fig. 1(b) was obtained at 1000 magnification with 15 kV accelerating voltage. It is observed that, the average particle size was in the range of (20– 40 mm). During the capturing of the SEM image the graphite particles tend to agglomerate with each other, to solve this issue, very small amount of graphite particles were mixed with ethanol and later the mix was ultra-sonicated to break the agglomerates. After this a small drop of the mixture was taken on a white sheet and let the ethanol evaporate. This process provides pure graphite particles on the white sheet. This sample were later used for SEM analysis. The filtered graphite particles were later mixed with the lubricating oil. Multi-grade SAE 15W-40 was used as the base lubricant. The preparation of graphite mixed lubricant must confirm the adequate dispersion of particles in the lubricating oil and proper method is required to achieve the stability of the mixed particles against sedimentation. The proper mixing of the graphite powder

2. Materials and methods The graphite powder was prepared from a graphite block with the help of hand filing and ball milling technique [11]. The ball milling setup consists steel chamber which is connected with a motor. The motor rotates the chamber for a predefined time. In the steel chamber the graphite powder as obtained from the hand filing is poured into the steel chamber along with hardened steel balls of different diameters. Three balls of 50 mm diameter and 6 balls of 30 mm diameter were used for the milling of graphite powder. The mill was rotated at a speed which is not too fast or slow. During the fast milling operation, the steel balls strike with the wall of the cylindrical chamber and does not play much role to the powder production. The slow speed does create much energy for breaking the power to finer diameters. The ball milling setup was operated for 24 h in successive steps of 4 h at a rotational speed of 70 rpm. After the running of the ball mill, the sample pow-

Fig. 2. Graphite mixed SAE 15W-40 oil.

Fig. 1. SEM image of the milled graphite particles.

Please cite this article as: V. Sharma, R. Joshi, H. Pant et al., Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.190

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to the lubricating oil is obtained by using a magnetic stirrer and an ultrasonicator. Researcher’s have extensively used the magnetic stirrer [12] and ultrasonicator [13] for the proper mixing of powder with lubricant. Based on the previous studies, it was decided to add 0.05 vol% graphite powder to the SAE 15–40 lubricating oil (see Fig. 2). During the mixing, it was of prime importance that the graphite powder does not agglomerate and produces a stable dispersion. To ensure this, a dispersant Alkyl aryl sulfonate was used. The agglomeration of the powder particles behaves like a contaminant and may increase the possibility of indentation/scratch of the testing surfaces. The dispersant helps in suspending the graphite particles in the lubricating oil. There was no sedimentation reported even after keeping the mixed oil for 30 days.

2.1. Measurement of coefficient of friction Rotary tribometer and reciprocating tribometer are the two methods which are generally used to evaluate the tribological properties of tribopairs [14–17]. In this study a linear reciprocating tribometer was used to conduct the experiments. Other than reciprocating tribometer, rotary pin on disc tribometer has also been used by many researchers. For evaluating the effect of graphite particles in the lubricant, a Phosphorus-Bronze pin and a mild steel plate was used. The dimensions of the specimens and operating conditions are presented in Table 1. The operating conditions were decided by studying the previous literatures, and based on those input factors, pilot experiments were performed to finalize the input factors. The COF between tribopairs was measured with the help of a friction sensor installed on the tribometer. In this tribometer, the plate remains static and pin reciprocates with the help of a slider reciprocating element (Fig. 3(c)). Fig. 3(a) and (b) presents the photographic view of the tribometer used for the experiments. The continuous measurement of COF was obtained for analysing the effect of graphite particles on the frictional behaviour of the lubricant. For obtaining the accuracy in results, each set of experiments was performed three times and average was taken to draw the graphs. Fig. 3(a–c) presents the photographic view of the tribometer. The linear reciprocating tribometer consists the procedure to hold the specimens in the form of pin and flat disc shape. The pin sample was pressed against the flat disc sample under the effect of load. Dead weights are used to apply the required load on the pin. For obtaining the friction force the pin specimen moves in the reciprocating motion. Load on the pin, frequency, temperature, test duration and stroke length of the motion are the possible input parameters which may be varied to analyse the results. The flat plate holder consists a procedure to apply the lubricating oil. A temperature sensor and a friction force sensor were attached with the testing chamber for the measurement. The complete set is connected with a computer system for the continuous monitoring and recording of the data.

Table 1 Experimental conditions for friction tests. Property

Value

Lubricating oil Graphite particle size Pin material, Length and diameter Plate material, Length, breadth, thickness Load Frequency, Stroke Lubricating condition

SAE 15W-40, Graphite mixed SAE 15W-40 20–40 mm Phosphorous Bronze 15 mm, 8 mm Mild steel, 50 mm, 50 mm, 5 mm 50 N, 40 N, 30 N 5 Hz, Fully Flooded

Fig. 3. Photographs of linear reciprocating tribometer (a) location of pin and flat plate holder (b) Tribometer during an experimental run (c) Schematic presentation of working of tribometer.

3. Results and discussion The COF between the tribopairs of pin and flat plate in the considered lubricating conditions are presented in Fig. 4. From Fig. 4 (a–c), it is seen that for the initial 15–20 s of the experimental study the COF value increases very sharply for all the considered loading conditions. This increase could be because of the asperity contacts between the pin and flat counter face. After the initial time, the COF value starts decreasing for SAE 15W-40 and graphite particles mixed SAE 15W-40 lubricating oil and attains a near constant value. The whole COF curve can be divided into three parts, namely running in, mixed zone, and steady state. The region of the curve where the COF rises sharply for the initial few seconds is termed as running in. In this the surface asperities of the tribopairs may contact with each other and hence the COF values increases. After the removal of these surface asperities the COF values tend to reduce with time. The zone where the COF values are decreasing till a point is the mixed zone. After the mixed zone the COF value attains a near constant value in the steady state zone. Fig. 3

Please cite this article as: V. Sharma, R. Joshi, H. Pant et al., Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.190

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Fig. 5. Variation of temperature with time (a) at 50 N load, (b) at 40 N load, (c) at 30 N load.

Fig. 4. Variation of COF with time (a) at 50 N load, (b) at 40 N load, (c) at 30 N load.

presents the variation of COF with time for 50 N, 40 N and 30 N loads. For the 30 N load condition, the tribo-testing with pure SAE 15W-40 lubricant results in higher COF values for the initial sliding. COF value rises to 0.12 in the absence of graphite particles in the lubricant. The COF value for the graphite mixed lubricant is

0.08. The graphite particles in the lubricant may have formed a layer in-between the surface asperities and prevented the frictional contact between the tribopairs. After attaining the 0.12 COF value the friction coefficient starts decreasing for pure SAE 15W-40 oil till 180 s and attains near constant value thereafter. In the steady state region, the COF values for both the testing conditions are overlapping with each other and COF values for graphite mixed-lubricant starts decreasing for the last few seconds of the testing. There is no significant improvement in COF value for 30 N loading condition. As the load is further increase to 40 N

Please cite this article as: V. Sharma, R. Joshi, H. Pant et al., Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.190

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there is a significant improvement in COF value. The pattern of the variation of COF is same as that of 30 N load. The average COF value with pure SAE 15W-40 lubricant is 0.11 and for graphite mixed lubricant is 0.07. There is 36% improvement in the COF value. Similarly, for 50 N loading condition the overall improvement is 33% improvement in COF value. Apart from the COF, the temperature rise during the experimentation was also measured. Fig. 5(a–c) presents the variation of temperature rise at different loads. A temperature sensor was installed on the tribometer for this purpose. The average temperature rise in graphite mixed SAE 15W-40 lubricant is lower than the pure SAE 15W-40 lubricant. The percentage reduction with graphite mixed lubricant is 6–7% lower for all the considered loading conditions. The results obtained for the COF and temperature were in accordance with the results reported by Lee et al. [18].

4. Conclusions and future work The present paper addresses the effects of graphite particles on the friction behaviour of multi-grade SAE 15W-40 lubricating oil. Graphite powder was mixed with the lubricant and a linear reciprocating tribometer was used to evaluate the friction behaviour. Following conclusions may be drawn from this experimental study.  Mechanical ball milling operation was successfully used to obtain the fine graphite powder. The scanning electron microscope image indicates the average particle size as 20–40 mm.  The magnetic stirrer along with ultrasonication technique was successfully used to mix the graphite particles with lubricant. It is observed during the experimentation that graphite particles get completely mixed with the lubricant and there was no sign of any sedimentation for nearly 30 days.  The friction test concludes that, graphite particles helped in improving the friction behaviour of the lubricant. There was a 33–36% reduction in the average coefficient of friction value. It is also concluded that graphite particles mixed lubricant also help in less generation of heat during the experimentation. Nearly 6–7% reduced temperature generation was observed during the testing with the graphite mixed lubricant.  For the future work, mathematical relations for predicting the frictional behaviour and temperature rise during the reciprocating testing may be developed for theoretically explaining the effects.

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Please cite this article as: V. Sharma, R. Joshi, H. Pant et al., Improvement in frictional behaviour of SAE 15W-40 lubricant with the addition of graphite particles, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.190