Meng-fei Guo,
Zhen-bing Cai*,
Zu-chuan Zhang and
Min-hao Zhu
Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China. E-mail: caizb@swjtu.cn
First published on 23rd November 2015
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.
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.
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![]() |
10 | 0# |
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 | — | — | — | — | — |
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.
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
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 CO. 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 (ID/IG) 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.
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.
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) |
Total sliding distance (S) can be calculated as:
S = v × t | (3) |
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%.
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.
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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
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.
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
Sample | Soot A | Soot B | Soot C |
---|---|---|---|
Zeta potential (mV) | −0.516 | −0.994 | −0.831 |
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 effect35–37 (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.
In conclusion, the particle size of diesel soot, dispersion stability, surface charge, electrochemical action, and concentrations of diesel soot are important factors influencing lubrication.
(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.
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