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

Su Peng; Xiong Yun; Liu Xiao; Yang He; Fan Linjun; Liu Ping

(1. Department of Oil Application & Management Engineering, Logistic Engineering University, Chongqing 401311; 2. SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

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

Su Peng1; Xiong Yun1; Liu Xiao1; Yang He2; Fan Linjun1; Liu Ping1

(1. Department of Oil Application & Management Engineering, Logistic Engineering University, Chongqing 401311; 2. SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

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 infuenced 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 coeffcient of base oil. Based on the characterization of the worn scars by the SEM technique, the 3D surface profler 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 flm to reduce friction under a high load and shear force.

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 flm. The main objective of this paper was to explore the tribological behavior of diesel soot and try to fgure 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 120oC 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 voltageof 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 52100 steel with a hardness of 59—61 HRC. Meanwhile, the stationary lower disc had the dimensions of 24 mm (diameter)×7.9 mm (thickness), which was also made of AISI 52100 steel with a hardness of 58—60 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 50oC, 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 100oC. 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 Refex, RENISHAW).

Table 1 Physico-chemical properties of base oil

3 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 4—6nm. 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 refection with the maximum intensity detected at 26.0° and 44.2°. 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 C1sspectra of diesel soot. The binding energy EBat 284.6 eV could be attributed to C-C and C-H groups, and EBat 286.1 eV could possibly be attributed to C-OH and C-O-C groups, and EBat 289.1 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.

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 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 50oC)

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 coeffcient, but at high load (100 N, 300 N, and 500 N) the diesel soot could reduce the friction coeffcient 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 coeffcient. While, under a load of 500 N, the 150SN base oil containing 5% of diesel soot exhibited a lowest friction coeffcient. The friction-reducing properties of diesel soot could possibly be attributed to its core-shell structure withgraphite outer shells, which might build up a continuous protective flm 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.

Figure 5 Effect of load on the tribological behavior of diesel soot

These results demonstrated that the load played an important role in the tribological behavior of diesel soot.

3.4 Tribological mechanism analysis

3.4.1 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 50oC as analyzed by the SEM and 3D surfaces profler 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 50oC)

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 100oC.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 100oC.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 100oC).

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 100oC)

3.4.2 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.

Figure 9 Raman spectrum of diesel soot

It can be seen from Figure 9 that two major bands appeared at 1 350 cm-1and 1 590 cm-1. Previous studies revealed that the intensity peak at 1 350 cm-1was known as the D band and it was attributed to the disordered graphitic lattices[16]. Meanwhile, the intensity peak at 1 590 cm-1was 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

The ratio of the D band intensity (ID) to the G band intensity (IG) is often used as a measure to describe the degree of graphitization of a carbon material[18]. The smaller the value of this ratio (ID/IG) is, the higher the degree of graphitization would be. Tuinatra and Koenig developed the correlation Eq. (1) betweenID/IGand crystallite sizeLa, in whichCis 4.4 nm[19].

In order to fgure 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 ofLawas also calculated. These results are listed in Table 2.

Table 2 The degree of graphitization and crystallite size on worn scars and diesel soot

It can be seen from Table 2 that theID/IGratio 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 flm 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 ID/IGof 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 thefriction coeffcient 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).

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date: 2016-12-21; Accepted date: 2017-03-07.

Professor Xiong Yun, E-mail: xy0000001@sina.cn, Telephone: +86-23-86731412.