Characterization and lubrication performance of diesel soot nanoparticles as oil lubricant additives

Abstract

Diesel soot, a complex product of incomplete combustion, enters lubricant oils and acts as an additive. Thus, diesel soot significantly affects the performance of lubricants. In particular, the dispersion and concentration of diesel soot in lubricant media performs a key function. In this study, diesel soot was dispersed in PAO 4 oil with 1 wt% sorbitan monooleate (Span 80) as a dispersing agent. The chemical and structural features of three kinds of diesel soot (from loader soot, cement tanker soot, and bulldozer soot) were monitored by AFM, TEM, XRD, FT-IR, and Raman spectroscopy. The tribological behavior of different concentrations of loader soot provided with the optimal physicochemical properties and dispersion properties in comparison with other soot types was investigated using a UTM-2 tribometer. Tribology test results showed that diesel soot, as an additive to PAO 4 oil, significantly reduced both friction and wear of steel balls and plates. The lubricant with a diesel soot concentration of 0.01 wt% exhibited the optimal anti-wear performance, with a wear rate reduction of 75.2%. Bearing effects, as well as chemical and electrochemical actions were the main anti-wear mechanisms of diesel soot as an oil additive.

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1 Introduction

Diesel engine combustion produces the highly carbonaceous material known as soot.¹ The part of soot retained in engine lubricant is known as “engine” soot, which can significantly affect the performance of lubricants. The portion expelled to the environment via the exhaust is known as “exhaust” soot, which is one of the most important sources of atmospheric particulate matter with diameters of 2.5 micro metres or less (PM 2.5). Diesel soot has been considered as a harmful substance to human health and the environment.² Moreover, diesel soot impairs engine performance by damaging cylinders and reducing the efficiency of power generation.^3,4 Liu et al.⁵ found that increased soot affects not only the viscosity of oil but also the abrasive friction surface, causing mechanical damage. Therefore, the properties of diesel soot and tribological characteristics when particles are suspended in engine lubricant should be understood. The present work indicates that a comprehensive understanding of diesel soot engine tribology may be achieved by including the effects of physical structure and chemistry of soot.^6–8 Uy⁹ has characterized diesel soot by using a variety of analytical techniques. Cai et al.¹⁰ collected onion-like carbon nano-spheres from candle soot that were found to exert an efficient lubricant effect. George¹¹ indicated that the anti-friction properties of SAE 15W-40 formulated oil were strengthened by the addition of 2 wt% carbon black. Zhang et al.¹² studied the effects of different concentrations of tribological properties of graphene as lubricant additives and found that a graphene concentration of 0.02% exhibits the optimal anti-wear properties.

Several researchers believe that diesel soot perform a harmful action in the lubrication system.^13,14 The present study aims to explore the advantages of diesel soot by characterizing the morphology, structure, and composition of lubricant oils and finding the optimum concentration, thereby reducing wear mechanism for the anti-friction property.

2 Materials and methods

2.1 Soot collection

Three samples of diesel exhaust soot were collected from pipes leading to after-treatment systems of diesel vehicles for this study. All samples were collected at the location away from the exhaust port by 10 cm to 15 cm and then stored in a thermostatic oven. Diesel vehicles include a loader, a concrete mixer truck, and a bulldozer (Fig. 1). Among these vehicles, the loader engine is the most problematic because it often works under overloaded conditions. The amount of soot scraped from the pipe was considerably larger than those of the other diesel vehicles. The concrete mixer truck frequently travels on flat roads comprising about 75% highway and 25% city driving. The service life of the bulldozer engine exceeds a decade, during which the engine has undergone several repairs. At present, this bulldozer works for 2 months to 3 months in a year. The soot samples and their associated detailed parameters are listed in Table 1.

Fig. 1 Images of diesel engine vehicles.

Table 1 Lists of samples and their associated parameters

Sample	Soot type	Engine type	Engine rated power (kW)	Engine rated speed (r min^−1)	Vehicles rated load (kg)	Engine service life (years)	Engine oil
Soot A	Exhaust	Loader	162	2800	5000	4	0#
Soot B	Exhaust	Concrete mixer truck	96	2300	4000	2	0#
Soot C	Exhaust	Bulldozer	382	2000	22 000	10	0#

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2.2 Characterization

Contact mode atomic force microscopy (AFM, SPI3800N, Japan) and transmission electron microscopy (TEM, JEM-2100F) were used to characterize the primary particle and aggregate size, as well as the bulk and surface microstructures of diesel soot. Diesel soot X-ray diffraction (XRD) measurements were performed using PANalytical X'Pert Pro X-ray diffractometer with nickel-filtered Cu Kα (1.54 Å) radiation as the X-ray source. The pattern was recorded in the 2θ range of 10° to 50° with a step size of 0.016. Fourier transform infrared spectroscopy (FTIR, Nicolet 5700) was conducted using a Perkin-Elmer spectrum one spectrometer in the range of 4000 cm⁻¹ to 500 cm⁻¹ with a KBr pellet. Raman spectrum (Lab Ram HR) characterization utilized a 532 nm laser as excitation source. The zeta potential of particles in alcohol with a dispersant was measured by Zetasizer Nano-ZS90. To study the dispersion of diesel soot, three kinds of diesel soot were prepared and left to disperse for 360 h.

2.3 Experimental methods

Tribology tests were conducted with a ball–plate contact wear model by using a UMT-2 testing machine. The upper ball sample was GCr15 steel (wt%: 1.0 C, 1.49 Cr, 0.31 Mn, 0.26 Si, 0.009 P, 0.004 S) with a diameter of 9.525 mm and hardness of 766 HV fixed in a special clamp. The lower plate sample was an alloy cast iron RTCr2 (wt%, 3.11 C, 2.21 Si, 0.50 Mn, 1.63 Cr) with a size of 25 mm × 12 mm × 6 mm fixed in a square tank equipped with lubricating oil. The bottom of the tank was fitted with a heating device that can control the experimental temperature. In this study, different concentrations of diesel soot (Table 2) were added into PAO 4 oil as lubricant for comparison. Given the high surface activity of nanoparticles, diesel soot is easy to aggregate.^14–16 To improve dispensability, 1 wt% Span-80 (C₂₄H₄₄O₆, Chengdu, Changzheng Glass Co., Ltd) was utilized as a dispersing agent. The mixed oil was stirred for over 10 min and then subjected to 15 min of ultrasonic vibration to create a uniform suspension in oil. The entire wear process was performed under the following conditions: normal load of 10 N, sliding speed of 5 mm s⁻¹, reciprocating sliding distance of 8 mm, test time of 6000 s, and temperature of 100 °C.

Table 2 Concentrations of diesel soot of Western test

Test plan	PAO 4 + 1 wt% SP 80 + (x) wt% diesel soot
Step 1	0	—	—	—	—	—	—	—	—	—	—	—	—
Step 2	—	0.001	0.005	—	—	0.01	—	—	0.05	0.1	0.5	1	5
Step 3	—	—	—	0.006	0.008	—	0.02	0.04	—	—	—	—	—

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After the wear test, specimens were cleansed with acetone and ethanol and then dried. The morphologies and tribo-chemistry of worn surfaces were analyzed through optical microscopy (OM, OLYMPUS BX60M), scanning electron microscopy (SEM, JSM-7001F, JEOL, Tokyo, Japan), 2D profiler (NanoMap-D), energy dispersive X-ray spectroscopy (EDX, EDAX-7760/68M), and Raman spectrum (Lab Ram HR) characterization.

3 Results and discussion

3.1 Characterization of additive particles

The image in Fig. 2 shows the HR-TEM and AFM images of primary particles and agglomerates of different diesel soot types. The particles are nearly circular in plane and elliptical in cross-section. The smallest unit of diesel soot is the primary particle¹⁷ with sizes ranging from 20 nm to 50 nm (Fig. 2a), 50 nm to 70 nm (Fig. 2b) and 0 nm to 70 nm (Fig. 2c). Soot A presents an evident layered structure. The agglomerates present a modest branching structure and fall in the size range of 30 nm to 150 nm (Fig. 3). The AFM images did not only prove the TEM results but also showed a high degree of diesel soot agglomerates with a size of 2 (Fig. 3a), 10 (Fig. 3b), and 5 nm (Fig. 3c). All three kinds of diesel soot are nanoparticles with relatively uniform particle size, and the primary particle size of soot A is evenly distributed and relatively small.

Fig. 2 HR-TEM images of different diesel soot nanoparticles.

Fig. 3 AFM images of different diesel soot nanoparticles.

The XRD pattern of graphite 2H phase is known to exhibit three distinctive peak values in the 2θ range of 10° to 60°. These values are 26.8° (002), 42.3° (101), and 54.9° (102) according to the standard card PDF41-1487 database.¹⁸ The XRD spectrum of diesel soot is shown in Fig. 4a. The XRD results showed that strong diffraction peaks at 15° to 32° appeared in soot A and soot B, and weak diffraction peaks can be observed in soot C. Why the peaks of soot C were weak? Although the diffraction measurement of a material was always accompanied by noise or background. But for soot C, the reason maybe the test sample was not dry enough. These peaks suggest that the soot powder is amorphous in structure. The three kinds of diesel soot exhibited a relatively sharp diffraction peak in the vicinity of 2θ = 26.8° near the graphite [002] crystal plane. The peak shape of soot C is sharper, and the width at half-maximum of soot C is narrower, indicating a higher graphitization degree of soot C. In addition, soot A presented significant diffraction peaks in the vicinity of 2θ = 44.6°, which is close to graphite [101] crystal plane. Thus, the degree of crystallinity of soot A and soot C is relatively better than that of soot B.^19–21

Fig. 4 XRD, FT-IR, and Raman spectra of different diesel soot types.

As shown in the FT-IR spectrum (Fig. 4b), the two medium absorption bands observed at 2970 cm⁻¹ to 2853 cm⁻¹ should be attributed to the stretching vibration of the CH bond of –CH₃ and –CH₂ groups. The band at 1739 cm⁻¹ corresponds to the functional group C=O. However, the peak of C=O is not detected in soot C. The C=C absorption peak at 1635 cm⁻¹ is the standard peak of graphite, and the band at 1240 cm⁻¹ can be attributed to the absorption peak of C–O–C with an sp² structure.

The Raman spectrum (Fig. 4c, excited by a 532 nm laser) showed the typical D (1250 cm⁻¹ to 1450 cm⁻¹) and G peaks (1500 cm⁻¹ to 1700 cm⁻¹) of carbon crystal. Soot B exhibited the strongest D peak among the three kinds of diesel soot, indicating a high defect density in diesel soot. The ratio of D peak and G peak (I_D/I_G) follows a trend of soot B > soot A > soot C, indicating that the extent of graphitization is soot B < soot A < soot C. These findings were also verified by the XRD²² test results.

The dispersion stability of diesel soot is shown in Fig. 5. A 0.05 wt% solution was selected for comparison because the concentration of 0.01 wt% was very low for observation. The figure shows that the colour of the (2), (4), and (6) dispersion bottles added with dispersant is darker, indicating the improved effect of dispersion after dispersant addition. After being allowed to stand for 48 h, the dispersion began to precipitate partially, except for the (2) bottle dispersion. However, the dispersion added with the dispersant remained darker. After 120 h, the precipitation of dispersion was more evident and substantially reached a stable state. After 360 h, the (2) bottle remained unchanged. The above phenomenon shows that Span 80 effectively allows diesel soot and graphite to disperse well in the base oil. The soot A dispersion with the added dispersant showed the optimum effect.

Fig. 5 Optical images of dispersion property of different diesel soot types in base oil: (1) PAO 4, (2) PAO 4 + 0.05 wt% soot A + 1 wt% SP, (3) PAO 4 + 0.05 wt% soot A, (4) PAO 4 + 0.05 wt% soot B + 1 wt% SP, (5) PAO 4 + 0.05 wt% soot B (6) PAO 4 + 0.05 wt% soot C + 1 wt% SP, and (7) PAO 4 + 0.05 wt% soot C.

In summary, the characterization results of the three kinds of diesel soot show that soot A (the exhaust soot of loader) exhibits the optimal physical, chemical, and dispersion properties among the diesel soot types. Therefore, soot A was selected as the concentration for testing the lubricating oil additives.

3.2 Friction and wear

The average friction coefficients (COF) of different soot A concentrations are compared in Fig. 6. Each set of data represents an average of two repeat tests. The figure shows that dispersant addition (1 wt% SP 80) and different additive concentrations of soot A in PAO 4 oil significantly reduce the friction coefficient in comparison with that of PAO 4 base oil. For example, the COF of PAO 4 was 0.57, and the value decreased to 0.12 after adding 0.01 wt% concentration of soot A. However, after adding the dispersant and additive, COF initially increased (0 wt% to 0.001 wt%), subsequently decreased (0.001 wt% to 0.01 wt%), and then remained virtually unchanged (0.01 wt% to 5 wt%). In addition, COF decreased by different degrees with the addition of different concentrations of soot A. This finding showed that a competition of dispersants and additives occurred in the lubrication process, and the concentration of soot A exerted a major effect on friction.

Fig. 6 Average friction coefficient with different soot A concentrations.

Considering that the wear amount of balls is markedly lower than that in plates, worn plate scars were analysed in this study. The wear rate can be defined as the volume loss, V (mm³) on per loading force, F (N) and total sliding distance, S (mm).

Wear rate = V/F × S (1)

Volume loss (V) can be calculated as:

V = A × d (2)

where A is the average cross section area of the wear track and d is single sliding distance.

Total sliding distance (S) can be calculated as:

S = v × t (3)

where v is sliding speed and t is test time.

The image in Fig. 7 shows the maximum width and depth of worn plates under the conditions of adding different soot A concentrations in PAO 4 base oil. The highest value of maximum width and depth appears at the condition of PAO 4 lubrication, and the lowest value appears with the addition of 0.01 wt% soot A in PAO 4 base oil. These results are consistent with the results of wear rate in Fig. 8, and a similar trend is observed in Fig. 7 and 8. The wear rate and the maximum width and depth are significantly reduced after the addition of soot A. For example, the maximum reduction of wear rate is 76% at the optimal concentration of 0.01 wt%. This result may be ascribed to the formation of a protective film on the surface of the friction pair, which separates the friction pair of direct contact to reduce the wear rate. The lubricant achieving the optimum effect at the concentration of 0.01 wt% may be ascribed to competition of additives and dispersant in the friction process, thereby achieving a mutually beneficial situation at a concentration of 0.01 wt% soot A. Notably, the wear rate at the concentration of 0 wt% (1 wt% SP 80) is less than that at 0.001 and 0.005 wt%. This finding is attributed to the low soot concentration, which is insufficient for forming stable protective films. Extremely high soot concentration is known to increase wear and tear.^4,23–25 However, in this study, the wear rate at a high concentration remains low, which may be ascribed to the dispersion producing considerable precipitation in the course of friction when the concentration exceeds 0.05%.

Fig. 7 Maximum width and depth of worn plates.

Fig. 8 Wear rate of worn plates.

The SEM morphologies of worn surfaces and EDX spectroscopy of balls are shown in Fig. 9. As can be seen in Fig. 9a–c, the worn area with PAO 4 lubrication is relatively large with metallic luster, and the worn plate surface displays a number of clear grooves and considerably larger “peeling pits” (zones surrounded by white cracks). These results indicated that severe scuffing occurred in this case. With the addition of different concentrations of soot A in PAO 4 base oil (Fig. 9d–l), the worn ball areas were significantly reduced. Moreover, despite the abundant presence of “peeling pits” and grooves in the worn plate surface, the defects become relatively shallow and narrow. Thus, soot A additive provides anti-wear property to a certain degree. In particularly, at a concentration of 0.01 wt% (Fig. 9g–i), the ball shows the minimum wear area, and the plate only presents slight furrows. This finding demonstrates that the concentration of 0.01 wt% presents the optimum effect. Simultaneously, EDX spectroscopy (Fig. 9d, g and i) results showed that the content of carbon and oxygen in worn areas are higher, and the value of oxygen at the “accumulation” (area revolved by white line) zone increased significantly. This finding indicated that a chemical reaction occurred during friction process. In addition, “accumulation” is likely to include oxidation products and is more inclined to combine with soot. These results may explain the anti-wear of the base oil added in PAO 4 base oil.

Fig. 9 SEM micrographs of worn surfaces and EDX spectroscopy of balls. (a)–(c) PAO 4; (d)–(f) 0.005 wt% soot A; (g)–(i) 0.01 wt% soot A; (j)–(l) 0.05 wt% soot A.

The image in Fig. 10a shows that the wear scar of ball presents an oval shape instead of a circular shape and the edge of wear scar (I) was worn lightly, whereas the middle was worn severely (II). The explanation is shown in Fig. 10c: (i) the contact area of ball and plate in the X direction is larger than those in the Y direction, because the plate material is worn off in the Y direction. (ii) According to the calculation results of Hamilton and Goodman,²⁶ the stress distribution on the ball-on-plate contact surface under full slipping state is the outer part of the contact zone, which showed the minimum shear stress, whereas the middle zone showed the maximum shear stress. At the same time, EDX spectroscopy (Fig. 10a and b) indicates that the content of oxygen is significantly increased in the worn area and that the value of oxygen at the “accumulation” zone (Fig. 10a) is the highest. This finding indicates that an electrochemical reaction occurred in the worn area and that the production is easily attached to the worn surface. The Fig. 10b image also shows the hardness distribution of worn plates. The hardness values were in the order PAO 4 (worn area) > 0.01 wt% soot A additive oil (worn area) > substrate (unworn). Friction is known to result in the hardening of the tribo-pair and cause the plastic deformation layer to achieve increased hardness. Evidently, the additives reduced the hardness of the friction surface, because additives can promote the formation of an oil film. Another possibility is that a protective film with high carbon content was formed in oil-containing additives.^27,28

Fig. 10 SEM, EDX, hardness distribution of wear scars, and wear mechanism.

The Raman spectrum of the worn surface of balls and plates is shown in Fig. 11. The D and G characteristic peaks of graphite crystals were detected in the worn surface of balls and plates. In addition, the relative intensity of the D and G peaks of the worn ball surface was evidently stronger than those of plates. Notably, the relative intensity of the D peak was significantly stronger than that of the G peak under the condition of 0.01 wt% soot lubrication addition. Therefore, we can infer that a physical film was absorbed on the worn surface of balls and plates during lubrication. In addition, the relative intensity of the G peak is strengthened because of the enhanced contribution of graphite key in diesel soot, and the strengthening of D peaks may explain the increase in doping amount or the change in dipole moment.²⁹ At the same time, the Raman spectrum of the worn surface of balls and plates show a clear characteristic peak of Fe₃O₄ at 661 cm⁻¹, and the characteristic peak of α-Fe₂O₃ appears at 298 and 405 cm⁻¹.¹⁰ This finding may be ascribed to the occurrence of friction chemical reaction during the friction process, and forming iron oxides that can also protect the contact surface of ball–plate.

Fig. 11 Raman spectroscopy of worn balls and plates.

3.3 Lubrication mechanism

The tribological results of this research show that diesel soot displays excellent friction-reducing and anti-wear properties as additives in PAO 4, with the optimum effect observed at the concentration of 0.01 wt%. This finding compelled us to investigate the lubrication mechanism.

The zeta potential of different diesel soot particles in alcohol is shown in Table 3. The surface of the three kinds of diesel soot particles carry a small amount of negative charge, this charge may be ascribed to a small amount of oxygen-containing polar group on the surface.³⁰

Table 3 Zeta potential of different diesel soot particles

Sample	Soot A	Soot B	Soot C
Zeta potential (mV)	−0.516	−0.994	−0.831

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As seen from HR-TEM (Fig. 2) and AFM (Fig. 3) images, diesel soot is composed of nanoparticles with spherical and approximate ellipsoid structure. During the sliding process, the metal surface carries a positive charge because of the low-energy electrons emitted from contact convex points on the metal surface.³¹ Diesel soot with a negative charge (Table 3) can be adsorbed onto the surface of a sliding pair to form a protective layer (Fig. 12a). In addition, part of diesel soot is exfoliated by shear force to generate isolated graphite sheets.³² This formation was proven by the enhanced contribution of the graphite key (Fig. 11). As a result, graphite and magnetite can form a composite film (Fig. 12a) that effectively reduces friction and wear,^33,34 as can be obtained from the results of the EDX spectrum (Fig. 9d, g and j, and 11). At low concentrations of diesel soot (<0.01 wt%), the main mechanism of weak antifriction wear involves the increase in surface defects (Fig. 11), which in turn results in the ability of forming a graphite–magnetite film. At the same time, spherical and ellipsoidal structures easily roll and slide under an external force. Under shear force, diesel soot moves between contact surfaces to prevent the direct contact of friction pair. The motion is similar to that of bearing balls to achieve anti-friction anti-wear effect^35–37 (Fig. 12b). At a diesel soot concentration of 0.01 wt%, the optimal antifriction wear is attributed to the stronger ability of formation of graphite–magnetite film (Fig. 9g and 11), and the motions reach the optimum state. At high concentrations (>0.05 wt%), given dispensability reasons, a considerable amount of precipitate adhering to the surface changes the contact manner to achieve the wear reduction effect. However, this mechanism does not belong to the above-described anti-friction conditions.

Fig. 12 Lubrication mechanism diagram of diesel soot in PAO 4 base oil.

In conclusion, the particle size of diesel soot, dispersion stability, surface charge, electrochemical action, and concentrations of diesel soot are important factors influencing lubrication.

4 Conclusions

Three kinds of diesel soot were easily prepared from pipes leading to after-treatment systems of diesel vehicles.

(a) Diesel soot particles show almost circular in plane and elliptical in cross-section with a size of 20 nm to 70 nm, which demonstrates a degree of internal chaos turbostratic graphitization. The dispersion of diesel soot indicated that soot A exhibits better dispensability than other diesel soot types when SP 80 was utilized as a dispersant.

(b) Diesel soot as additive in PAO 4 oil displays excellent friction-reducing and anti-wear properties, with the optimum effect observed at the concentration of 0.01 wt%.

(c) Magnetite–graphite composite tribofilm occurred in the wear scars. Through a mechanism of rolling, sliding, and the composite motion of rolling and sliding between the friction pair, the direct contact of friction pair is blocked to achieve excellent friction reduction and anti-wear effect.

(d) The particle size, dispersion stability, surface charge, electrochemical action, and concentrations of diesel soot are important factors influencing lubrication properties.

Acknowledgements

This study was supported by National Science Foundation of China (51375407 and U1530136), the authors thank Dr Wen Yue from China University of Geosciences in Beijing for providing the PAO4 oil. And thank Dr Jin-fang Peng from Southwest Jiaotong University for SEM sample preparation and partial EDX analysis.

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