The Role of Diesel Soot in the Tribological Behavior of 150SN Base Oil

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1 Lubrication Research China Petroleum Processing and Petrochemical Technology 2017, Vol. 19, No. 2, pp June 30, 2017 The Role of Diesel Soot in the Tribological Behavior of 150SN Base Oil Su Peng 1 ; Xiong Yun 1 ; Liu Xiao 1 ; Yang He 2 ; Fan Linjun 1 ; Liu Ping 1 (1. Department of Oil Application & Management Engineering, Logistic Engineering University, Chongqing ; 2. Sinopec Research Institute of Petroleum Processing, Beijing ) Abstract: The diesel soot was collected from diesel engine exhaust pipe. The morphology and structure of the collected diesel soot was characterized by HRTEM, XRD and XPS and its tribological behavior was investigated by a SRV IV oscillating reciprocating friction and wear tester. Test results showed that the tribological behavior of diesel soot was largely influenced by the test load. Under a low load, the diesel soot could reduce the wear volume of the disc. While under a high load, the diesel soot could reduce the friction coefficient of base oil. Based on the characterization of the worn scars by the SEM technique, the 3D surface profiler and the Raman spectroscopy, it was assumed that the core-shell structure of diesel soot with several graphitic layers played important roles. On one hand, its spherical and special structure could make it roll between friction pairs to reduce wear under a low load. On the other hand, its outer-shell graphite layers could be peeled off to form lubrication film to reduce friction under a high load and shear force. Key words: diesel soot; tribological behavior; core-shell structure; 150SN base oil 1 Introduction Soot is one kind of microscopic carbonaceous particles and is also the common by-product formed during the combustion process taking place in the engine [1]. Owing to its high combustion temperature and high air-fuel ratio, diesel engines tend to produce more soot than gasoline engines [2]. In recent years, soot contamination in diesel engines become serious as the exhaust gas recirculation (EGR) technology is widely employed [3-4]. Soot contamination of the lubricating oil can lead to intense wear in critical components and increase frequency of oil changes [5]. Hence, the importance of understanding the tribological behavior of diesel soot is highlighted. The tribological behavior of soot has already been attracting great attention of researchers. For example, Wei, et al. [6] investigated the candle soot serving as the particular lubricant additive in water and discovered that these candle soot additives could effectively reduce both friction and wear in water. Hu, et al. [7] discovered that the uniformly dispersed carbon black could improve the antifriction properties of a formulated engine lubricant CD 15W-40 and its tribological effect can be attributed to absorption and agglomerate effects. Liu, et al. [8] showed that bio-fuel soot caused abrasive wear and corrosive wear and it could influence the formation of boundary lubrication film. The main objective of this paper was to explore the tribological behavior of diesel soot and try to figure out its anti-wear and friction-reducing mechanism in the 150SN base oil. 2 Experimental 2.1 Diesel soot collection and characterization Diesel soot was directly scraped away from the tail pipe of a six-cylinder diesel engine (F6L913, Beijing Beinei Diesel Engine Co., Ltd.). The enriched exhaust soot was then dried in a vacuum oven at 120 o C for 5 hours to remove water. A high resolution transmission electron microscope (HRTEM) (JEM-2100, JEOL) was used to analyze the morphology and nanostructure of the diesel soot. The soot was spread onto a copper grid and its measurement was done at an acceleration voltage Received date: ; Accepted date: Corresponding Author: Professor Xiong Yun, xy @sina.cn, Telephone:

$of 200 kv. An X-ray diffractometer (XRD-6100, Shimadzu) was utilized to characterize the phase structure of the soot.$ 2 Friction and wear testing The basestock lubricant used in this study was the 150SN base oil without any additives, with its main physicochemical properties shown in Table 1.

2 Friction and wear testing The basestock lubricant used in this study was the 150SN base oil without any additives, with its main physicochemical properties shown in Table 1.

The friction and wear tests were conducted on a SRV IV oscillating reciprocating friction and wear tester (Optimol Instruments Prüftechnik GmbH).

The oscillating upper ball was a standard 10-mm diameter ball, which was made of AISI 52100 steel with a hardness of 59 61 HRC.

2 of 200 kv. An X-ray diffractometer (XRD-6100, Shimadzu) was utilized to characterize the phase structure of the soot. The X-ray photoelectron spectroscopy (XPS) (ESCALab-250, Thermo Fisher Scientific) was employed to probe the functional groups on the soot particle surface. 2.2 Friction and wear testing The basestock lubricant used in this study was the 150SN base oil without any additives, with its main physicochemical properties shown in Table 1. Diesel soot was added into the base oil at a mass percentage of 0%, 1%, 3%, and 5%, respectively, and the stock solution was dispersed ultrasonically for 24 hours before each tribological test [9]. The friction and wear tests were conducted on a SRV IV oscillating reciprocating friction and wear tester (Optimol Instruments Prüftechnik GmbH). The contact between the frictional pairs was obtained by pressing the upper running ball against the lower stationary disc. The oscillating upper ball was a standard 10-mm diameter ball, which was made of AISI steel with a hardness of HRC. Meanwhile, the stationary lower disc had the dimensions of 24 mm (diameter) 7.9 mm (thickness), which was also made of AISI steel with a hardness of HRC. The friction and wear tests were assessed in air (at a RH of 46%). The test conditions employed to investigate the effect of diesel soot on wear and friction properties of 150SN base oil included: an applied load of 300 N, a temperature of 50 o C, an oscillating frequency of 50 Hz, a sliding amplitude of 1 mm, and a test duration of 30 min. The testing load to probe the effect of load on the tribological behavior of diesel soot ranged from 50 N to 500 N and the testing temperature was also specified at 100 o C. Wear volumes of lower discs were measured by a three-dimensional (3D) surface optical profiler (NPFLEX, Bruker). Each friction and wear test was repeated three times and the average values of friction coefficient and the wear volume were calculated. After tribological tests, the wear scars on the lower disc were analyzed by scanning electron microscopy (SEM) (S-3700N, HITACHI) and Raman spectroscopy (invia Reflex, RENISHAW). Table 1 Physico-chemical properties of base oil Items 150SN Test method Kinematic viscosity at 40 o C, m 2 /s GB/T 265 Kinematic viscosity at 100 o C, m 2 /s 5.52 GB/T 265 Viscosity index 114 GB/T 2541 Sulfur content, % 1.4 SH/T 0689 Water content, % Trace GB/T 260 Pour point, o C -12 GB/T 3535 Density at 20 o C, kg/m GB/T Results and Discussion 3.1 Materials HRTEM images, the XRD spectra and XPS spectra of diesel soot are presented in Figures 1 to 3 Figure 1 HRTEM images of diesel soot Figure 2 XRD spectra of diesel soot Figure 1(a) shows that diesel soot particles were spherical or nearly spherical with their diameter ranging from 20 nm to 50 nm and the agglomerated soot particles were arranged in a chain-like conformation. Figure 1(b) shows that the diesel soot shared a core-shell structure. The outer shell was comprised of graphite taking the shape of a turbostratic structure with a thickness of about

nm. The inner core is comprised of amorphous carbon with a diameter of about 3 5 nm [10-11]. In Figure 2, the diesel soot shows two broad reflection with the maximum intensity detected at 26.0 and 44.

3 nm. The inner core is comprised of amorphous carbon with a diameter of about 3 5 nm [10-11]. In Figure 2, the diesel soot shows two broad reflection with the maximum intensity detected at 26.0 and The reflection at 2θ = 26.0 and 44.2 was assigned to the 002 and 101 lattice planes of graphite [10]. This fact indicates that the diesel soot contained graphite carbon. Figure 3 shows the C 1s spectra of diesel soot. The binding energy E B at ev could be attributed to C-C and C-H groups, and E B at ev could possibly be attributed to C-OH and C-O-C groups, and E B at ev could possibly be attributed to C=O and COO- groups [11]. These results demonstrated that functional groups on the surface of the diesel soot were mainly composed of C-C and C-H groups, while some oxygen functional groups were also identified on the diesel soot surface, such as C-OH, C-O-C, C=O and COO- groups. important role in improving the friction and wear performance of the 150SN base oil. Figure 4 Frictional traces and disc wear volume upon using the 150SN base oil with different contents of diesel soot (under a load of 300 N and at a temperature of 50 o C) Figure 3 XPS spectra of diesel soot (C1s) 3.2 Friction and wear properties of diesel soot in 150SN base oil Figure 4 shows the effects of diesel soot on friction and wear performance of the 150SN base oil. It can be seen from Figure 4 (a) that diesel soot at different concentration added to 150SN base oil could reduce the friction coefficient as compared to the neat base oil. When the diesel soot content reached 5% in base oil, the sample exhibited a lowest friction coefficient as compared with other cases. It can be seen from Figure 4(b) that when the diesel soot was added to the 150SN base oil, the anti-wear property of 150SN base oil became better. It was evident that the 3% of diesel soot added to 150SN base oil exhibited the best anti-wear property. These results indicated that diesel soot played an 3.3 Effects of load on wear and friction In order to illustrate the effect of load on the tribological behavior of diesel soot, the average friction coefficients and wear volumes of samples lubricated by 150SN base oil containing 1%, 3%, and 5% of diesel soot, respectively, were measured under 4 different loads. The results are shown in Figure 5. Figure 5 (a) shows that, at lower load (50 N) diesel soot could not reduce the friction coefficient, but at high load (100 N, 300 N, and 500 N) the diesel soot could reduce the friction coefficient of base oil. For example, under a load of 100N and 300 N, the 150SN base oil containing 3% of diesel soot exhibited a lowest friction coefficient. While, under a load of 500 N, the 150SN base oil containing 5% of diesel soot exhibited a lowest friction coefficient. The friction-reducing properties of diesel soot could possibly be attributed to its core-shell structure with 91

150SN; 150SN+1% diesel soot; 150SN+3% diesel soot; 150SN+5% diesel soot

graphite outer shells, which might build up a continuous protective film

It can be seen from Figure 5(b) that all concentrations of diesel soot in

load (50 N, 100 N and 300 N), while under higher load (500 N) the diesel

These results demonstrated that the load played an important role in the

4.1 Morphology analysis of worn surface The morphology of the worn surfaces

concentration of diesel soot under a load of 300 N at a temperature of 50 o

Figure 6 The SEM micrographs and 3D surface profiler images of worn

4 150SN; 150SN+1% diesel soot; 150SN+3% diesel soot; 150SN+5% diesel soot 150SN; 150SN+1% diesel soot; 150SN+3% diesel soot; 150SN+5% diesel soot Figure 5 Effect of load on the tribological behavior of diesel soot graphite outer shells, which might build up a continuous protective film under high load [12-13]. It can be seen from Figure 5(b) that all concentrations of diesel soot in the 150SN base oil were confirmed to be able to decrease the wear under low load (50 N, 100 N and 300 N), while under higher load (500 N) the diesel soot could increase wear. These results demonstrated that the load played an important role in the tribological behavior of diesel soot. 3.4 Tribological mechanism analysis Morphology analysis of worn surface The morphology of the worn surfaces of disc which was lubricated by the 150SN base oil containing different concentration of diesel soot under a load of 300 N at a temperature of 50 o C as analyzed by the SEM and 3D surfaces profiler is shown in Figure 6. Figure 6 The SEM micrographs and 3D surface profiler images of worn surfaces lubricated by 150SN base oil with different contents of diesel soot (under a load of 300 N and at a temperature of 50 o C) 92

Figure 6(a) shows that, when there was no diesel soot in 150SN, the wear surface showed a lot of pits, while the signs of adhesive wear and large wear volume could be seen through 3D surface image

When 3% of diesel soot were added to the base oil, the wear surface displayed a lot of wide grooves and the number of pits decreased (Figure 6c).

These results indicate that the diesel soot mainly could lead to abrasive wear and with the increase of diesel soot, the pits on the wear scar decreased but the number of grooves increased.

Diesel soot particles are spherical or nearly spherical with their diameter varying from 20 nm to 50 nm, which possesses the typical core-shell structure in which graphitic layers are oriented

5 Figure 6(a) shows that, when there was no diesel soot in 150SN, the wear surface showed a lot of pits, while the signs of adhesive wear and large wear volume could be seen through 3D surface image analysis. When 1% of diesel soot was added to the base oil, slight scratches appeared on the worn surface but there were still many pits on the wear scar (Figure 6b). When 3% of diesel soot were added to the base oil, the wear surface displayed a lot of wide grooves and the number of pits decreased (Figure 6c). When 5% of diesel soot were added into the base oil, the worn surface showed a considerably narrow and uniformly displayed grooves, indicating that the abrasive wear occurred (Figure 6d). These results indicate that the diesel soot mainly could lead to abrasive wear and with the increase of diesel soot, the pits on the wear scar decreased but the number of grooves increased. Diesel soot could provide some degree of anti-wear property which could be ascribed to its roller effect [7]. Diesel soot particles are spherical or nearly spherical with their diameter varying from 20 nm to 50 nm, which possesses the typical core-shell structure in which graphitic layers are oriented parallel to the external outer surface. This special structure and small size could made it roll easily between the friction pairs like a ball bearing [2, 14-15]. Figure 7 shows the 3D profile image of worn surface tested under a load of 50 N and at a temperature of 100 o C.And the roller effect of diesel soot can also be confirmed by Figure 7. When there was no diesel soot in the 150SN base oil, serious adhesive wear could be observed on the worn surface (Figure 7a). And when 3% of diesel soot was added to the base oil, a lot of narrow grooves appeared and wear volume decreased. It is assumed that in the case of low load the anti-wear performance of diesel soot could be achieved through rolling. But under high load, diesel soot could not decrease the wear, and instead it would increase the wear. This can be confirmed by Figure 8, which shows the 3D profile image of wear surface tested under a load of 500 N and at a temperature of 100 o C.This might occur due to the transition of diesel soot movement from rolling to sliding between friction pairs under high load. This transition mechanism was also proven by Bucholz, et al. [13] Figure 7 The 3D surface profiler images of worn surfaces lubricated by 150SN base oil and 150SN base oil with 3% diesel soot (under a load of 50 N and at a temperature of 100 o C). Figure 8 3D surface profiler images of worn surfaces lubricated by 150SN base oil and 150SN base oil with 3% of diesel soot (under a load of 500 N and at a temperature of 100 o C) Raman spectroscopic analysis In order to figure out the mechanism of the friction reducing property of diesel soot, the Raman spectroscopy was used to characterize the diesel soot and the worn scars after tribology test. Figure 9 shows the Raman spectrum of diesel soot. It can be seen from Figure 9 that two major bands appeared at cm -1 and cm -1. Previous studies 93

6 Figure 9 Raman spectrum of diesel soot revealed that the intensity peak at cm -1 was known as the D band and it was attributed to the disordered graphitic lattices [16]. Meanwhile, the intensity peak at cm -1 was known as the G band and it was analogous to ideal graphitic lattices [17]. These results proved the existence of graphitic carbon in diesel soot and were consistent with the HRTEM and XRD results. The Raman spectrums of three worn scars are shown in Figure 10. When there was no diesel soot in the 150SN base oil, the Raman spectrum of wear scar did not display the characteristic D (1 350 cm -1 ) band and G (1 590 cm -1 ) band. When 3 wt % of diesel soot were added to the 150SN base oil, the characteristic D (1 350 cm -1 ) band and G (1 590 cm -1 ) band appeared. However, the G band appearing under high load (300 N) can be clearly seen to have a higher intensity than the G band appearing under low load (50N). Figure 10 Raman spectra of diesel soot 150SN with 3% at 300 N; 150SN with 3% at 50N; 150SN at 300 N The ratio of the D band intensity (I D ) to the G band intensity (I G ) is often used as a measure to describe the degree of graphitization of a carbon material [18]. The smaller the value of this ratio (I D /I G ) is, the higher the degree of graphitization would be. Tuinatra and Koenig developed the correlation Eq. (1) between I D /I G and crystallite size L a, in which C is 4.4 nm [19]. L a = C / (I D / I G ) (1) In order to figure out the effect of load on the tribological behavior of diesel soot, the intensity of G band and D band in Figure 9 and Figure 10 was analyzed and the value of L a was also calculated. These results are listed in Table 2. Table 2 The degree of graphitization and crystallite size on worn scars and diesel soot Samples I D /I G L a, nm Note: Sample 1 was the diesel soot; sample 2 was the worn scar lubricated by 150SN base oil with 3% of diesel soot tested under a load of 50 N; and sample 3 was worn scar lubricated by 150SN with 3% of diesel soot tested under a load of 300 N. It can be seen from Table 2 that the I D /I G ratio of sample 3 was less than that of diesel soot. These results show that the high load strengthened the G band in worn surface, indicating that more graphite carbon was involved in forming the lubrication film under high load. It is assumed that, when the load was high, the outer-shell graphite in diesel soot could be exfoliated and the small graphitic fragments could slide between friction pairs to form a lubrication film under the action of shearing force and friction heat. This rearrangement of outer-shell graphite in forming lubrication film could explain the good lubrication ability of soot under a high load [20]. Thereby, under a low load (50 N), the ratio I D /I G of sample 2 was similar to that of diesel soot. This indicates that under a low load the outer-shell of diesel soot could not be peeled off and the diesel soot could just build up on the friction surface [21]. Hence, the diesel soot did not show frictionreducing property under a low load (50 N). 4 Conclusions Diesel soot could effectively improve the frictionreducing and anti-wear properties of the 150SN base oil, and its tribological behavior was also influenced by the testing load. For example, diesel soot could not reduce the 94

7 friction coefficient of base oil until the load was greater than 100 N, while the diesel soot could reduce the wear volume under a load which was less than 300 N. Diesel soot possesses the typical core-shell structure in which graphene layers are oriented in parallel to the external outer surface. This special structure make it roll easily between the friction pairs under a low load (below 300 N) to avoid direct contact of asperities and reduce adhesion wear. The Raman spectrum of the wear scar revealed the presence of graphite lubrication film under a high load. The outer-shell graphite of diesel soot could be exfoliated by shear force under high load and the graphite lubrication film was formed under the action of shear force and friction heat. Acknowledgement: We gratefully acknowledge the financial support of the Logistics Key Basic Research Program of PLA (BX214C006) and the Chongqing Science and Technology Achievement Transformation Fund (KJZH17139). References [1] Green D A, Lewis R. The effects of soot-contaminated engine oil on wear and friction: A review[j]. Journal of Automobile Engineering, 2008, 222: [2] Robert Bosch GmbH. Automotive Handbook[J]. 6th Edition. New York: John Wiley, 2004 [3] Aldajah S, Ajayi O O, Fenske G R, et al. Effect of exhaust gas recirculation (EGR) contamination of diesel engine oil on wear[j]. Wear, 2007, 263(1/6): [4] George S, Balla S, Gautam V, et al. Effect of diesel soot on lubricant oil viscosity [J]. Tribology International, 2007, 40(5): [5] Daido S, Kodama Y, Inohara T, et al. Analysis of soot accumulation inside diesel engines[j]. JSAE Rev, 2000, 21, [6] Wei J X, Cai M R, Zhou F, et al. Candle soot as particular lubricant additives[j]. Tribology Letters, 2014, 53(3): [7] Hu E Z, Hu X G, Liu T X, et al. The role of soot particles in the tribological behavior of engine lubricating oils [J]. Wear, 2013, 304: [8] Liu Tianxia, Hu Enzhu. Composition and structure as well as tribological performance of bio-fuel soot [J]. Tribology, 2014, 34(4): [9] Soejima M, Ejima Y, Uemori K, et al. Studies on friction and wear characteristics of cam and follower: influences of soot contamination in engine oil [J]. JSAE Review, 2002, 23: [10] Patel M, Aswath P B. Morphology, structure and chemistry of extracted diesel soot: Part I: Transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and synchrotron X-ray diffraction study[j]. Tribology International, 2012, 52: [11] Hu Enzhu. Hu Xianguo, Xu Yufu, et al. Investigation of morphology, structure and composition of biomass-oil soot particles [J]. Applied Surface Science, 2013, 271(1): [12] Vacher J P B, Ohmae N. Anti-wear and friction reducing mechanisms of carbon nano-onions as lubricant additives[j]. Tribology Letters, 2008, 30(1): [13] Bucholz E W, Phillpot S R, Sinnott S B. Molecular dynamics investigation of the lubrication mechanism of carbon nanoonions[j]. Comput Mater Sci, 2012, 54: [14] Matsumoto N, Joly-Pottuz L, Kinoshita H, et al. Application of onion-like carbon to micro and nanotribology[j]. Diam Relat Mater, 2007, 16: [15] Joly-Pottuz L, Matsumoto N, Kinoshita H, et al. Diamond derived carbon onions as lubricant additives[j]. Tribol Int, 2008, 41: [16] Sadezky A, Muckenhuber H, Grothe H, et al. Raman micro spectroscopy of soot and related carbonaceous materials: spectral analysis and structural information[j]. Carbon, 2005, 43(2): [17] Roy D, Chhowalla M, Wang H, et al. Characterization of carbon nano-onions using Raman spectroscopy[j]. Chemical Physics Letters, 2003, 373(1): [18] Esangbedo C, Boehman A L; Perez J M. Characteristics of diesel engine soot that leads to excessive oil thickening[j]. Tribology International, 2012, 47: [19] Tuinstra F, Koenig J L. Raman spectrum of graphite[j]. The Journal of Chemical Physics, 1970, 53(3): [20] Antusch S, Dienwiebel M, Nold E, et al. On the tribochemical action of engine soot[j]. Wear, 2010, 269: 1 12 [21] Hu Enzhu, Hu Kunhong, Xu Yong, et al. Friction-induced structure and composition change mechanism of bio-fuel soot particles[j]. Tribology, 2016, 36(2):

2016 International Conference on Engineering Tribology and Applied Technology

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