J Nanopart Res (2016) 18:377 DOI 10.1007/s11051-016-3679-4
RESEARCH PAPER
Minimizing of the boundary friction coefficient in automotive engines using Al2O3 and TiO2 nanoparticles Mohamed Kamal Ahmed Ali & Hou Xianjun & Ahmed Elagouz & F.A. Essa & Mohamed A. A. Abdelkareem
Received: 5 September 2016 / Accepted: 18 November 2016 # Springer Science+Business Media Dordrecht 2016
Abstract Minimizing of the boundary friction coefficient is critical for engine efficiency improvement. It is known that the tribological behavior has a major role in controlling the performance of automotive engines in terms of the fuel consumption. The purpose of this research is an experimental study to minimize the boundary friction coefficient via nano-lubricant
Electronic supplementary material The online version of this article (doi:10.1007/s11051-016-3679-4) contains supplementary material, which is available to authorized users. M. K. A. Ali (*) : H. Xianjun (*) : A. Elagouz : M. A. A. Abdelkareem Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China e-mail:
[email protected] e-mail:
[email protected] M. K. A. Ali : H. Xianjun : A. Elagouz : M. A. A. Abdelkareem Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan University of Technology, Wuhan 430070, China M. K. A. Ali : A. Elagouz : M. A. A. Abdelkareem Automotive and Tractors Engineering Department, Faculty of Engineering, Minia University, El-Minia 61111, Egypt F. Essa Mechanical Engineering Department, Faculty of Engineering, Kafrelsheikh University, Kafrelsheikh, Egypt F. Essa School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan 430070, China
additives. The tribological characteristics of Al2O3 and TiO2 nano-lubricants were evaluated under reciprocating test conditions to simulate a piston ring/cylinder liner interface in automotive engines. The nanoparticles were suspended in a commercially available lubricant in a concentration of 0.25 wt.% to formulate the nanolubricants. The Al2O3 and TiO2 nanoparticles had sizes of 8–12 and 10 nm, respectively. The experimental results have shown that the boundary friction coefficient reduced by 35–51% near the top and bottom dead center of the stroke (TDC and BDC) for the Al2O3 and TiO2 nano-lubricants, respectively. The anti-wear mechanism was generated via the formation of protective films on the worn surfaces of the ring and liner. These results will be a promising approach for improving fuel economy in automotive. Keywords Nanoparticles . Nano-lubricants . Boundary friction coefficient . Anti-wear . Engine oil . Surface science
Introduction Improving the tribological performance of internal combustion engines is a fundamental strategy for improving fuel economy in automotive (Ali and Xianjun 2015). The lubrication in an internal combustion engine is classified into three general regimes: boundary, mixed, and elastohydrodynamic/hydrodynamic (Ali et al. 2015). The total friction of the piston ring assembly consists of boundary friction at asperity contact
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locations and hydrodynamic friction due to shearing of lubricant (Styles et al. 2014). During one stroke, the different lubrication regimes can occur over the stroke depending on running conditions. During the stroke, the boundary and mixed lubrication regimes occur close to the top and bottom dead center of stroke (TDC and BDC), while the hydrodynamic lubrication regime occurs at the middle stroke (Priest and Taylor 2000). Moreover, boundary lubrication usually occurs during starting, stopping, and periods of severe operation. Hence, there is a great deal of asperity contact owing to lubricant film between the worn surfaces which is insufficient. In this case, it is the chemical and physical properties of thin tribo-films, which govern the tribological characteristics of the piston ring assembly (Wong and Tung 2016). Many failures of sliding systems are caused by inadequacies during boundary lubrication. Generally, the boundary lubrication regime occurs under heavy an applied load and low-speed condition (Priest 2000). Nevertheless, hydrodynamic friction increases under these conditions of high sliding speed and lower loading (Hsu and Gates 2005). The frictional power losses of piston ring/cylinder liner interfaces represent a nearly 35 to 45% in an engine (Lawrence and Ramamoorthy 2016; Shahmohamadi et al. 2015). Therefore, in order to enhance engine performance, it is imperative to have a minimization of the boundary friction coefficient of the piston ring assembly. The nanoparticles were studied as nanolubricant additives in order to improve the tribological performance of engines, oil properties, and fuel economy. The diameter of the nanoparticles (1–100 nm) showed a noticeable effect on the tribological properties of engine oils (Gulzar et al. 2016; Joly-Pottuz et al. 2008). Nanoparticles are different from traditional bulk materials because they possess high specific surface areas and extremely small sizes (Boshui et al. 2015). Nano-lubricant additives have received a particular attention owing to the use as friction modifiers, anti-wear additives, and solid lubricants on rubbing surfaces in tribological applications (Ali and Xianjun 2015; Xiaohong et al. 2006). The tribological performance depends on grain size, shape, and concentration of nanoparticles (Wu et al. 2007). The concentration is an essential matter because they can act negatively if there is a surplus of nanoparticles (Alves et al. 2016). Furthermore, the spherical morphology of nanoparticles used in nano-lubricant additives is an important factor that provides a rolling
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effect between rubbing surfaces. When the surface roughness is greater than the grain size, the valleys between asperities of the frictional surfaces can be filled for help to the formation of a tribo-boundary film via a physical mechanism on frictional surfaces that enhances the tribological behavior. The nanoparticles are of a unique significance in nano-lubrication because of excellent tribological properties and improve the load carrying ability (Li et al. 2011). The friction and wear tests were performed by Ingole et al. (2013) using a reciprocating pin-on-disk apparatus. The average grain size of TiO2 was 20–25 nm and 0.25– 2 wt.% particle concentration in base oil. The results showed that the friction coefficient reduction is under 0.25 wt.% of TiO2 nano-lubricant. Luo et al. (2014) investigated the effect of the addition of Al2O3/TiO2 nanoparticles to lubricating oil. The conclusions indicated that there was a reduction of friction about 18% for the four-ball test and 24% for the thrust-ring test using 75 nm of grain size for a 0.1 wt.% nanoparticle concentration. Peña-Parás et al. (2015) studied the influence of CuO and Al2O3 nano-lubricants with on tribological properties. The results exhibited a decrease by 18 and 14%, for the wear and friction coefficient, respectively. Zulkifli et al. (2013) investigated the influence of TiO2 nanoparticles with trimethylolpropane ester on tribological properties. The results revealed that the reduction coefficient of friction by 15% in comparison to the TMP ester without TiO2 nanoparticles. The surface roughness (Ra) decreased with the paraffin oil containing TiO2 nanoparticles compared to paraffin oil without nanoparticles (Kao and Lin 2009). There are some studies about nano-lubricant mechanisms to describe the tribological behavior, the rolling of nanoparticles between worn surfaces (Ali et al. 2016a, 2016b, 2016c), mending effect (Liu et al. 2004), polishing (Rapoport et al. 2003), reducing the real area of contact (Ghaednia et al. 2013), and the formation of a self-laminating protective film (Ali et al. 2016a, 2016b, 2016c). Nano-lubricants played a great role in the interaction between worn surfaces and lubricant to form a tribochemical film on frictional surfaces (sintering effect) under the heat generated by friction and high contact pressure (Wan et al. 2014). The formation of a lubricating coating on worn surfaces facilitated adsorption. These films could improve the surface properties and help separation between worn surfaces, yielding promising reduction of the friction coefficient (Bakunin et al. 2004).
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The purpose of this paper is minimizing of the boundary friction coefficient between the piston ring and cylinder liner using nano-lubricant additives. Minimizing boundary friction coefficient and wear continues to be a challenge. We have focused on enhancing the anti-wear and anti-friction by incorporating Al2O3 and TiO2 nanoparticles into lubricant oils. These findings lead to the conclusion that nanoparticles as nanolubricants can contribute to improving the efficiency and fuel economy of automotive engines. In addition, mechanisms of nano-lubricants were discussed using field emission scanning electron microscopy (FESEM, ZEISS ULTRA PLUS), non-contact 3D surface profilometer (Nanovea ST400 3D), and energy dispersive spectroscopy (EDS, Inca X-Act) to understand the major mechanisms leading to minimize the boundary friction coefficient.
Experimental methodology Tribotest rig The boundary friction reduction and anti-wear of nanolubricants were evaluated using test rig designed (Fig. 1) to mimic the sliding reciprocating motion of the piston ring/cylinder liner interface in an engine according to ASTM G181-11(Truhan et al. 2005). In this investigation, the piston ring and cylinder liner samples were used as the friction specimens and were cut from the actually fired engine components in an attempt to ensure that the materials tested are the same as in a real engine as shown in Fig. 2. A piezoelectric force transducer and charge amplifier were used to measure the normal contact and friction force generated on the piston ring during sliding. The signal output of friction during the reciprocating sliding motion was received using the DEWESoft 6.6.7 program for data acquisition connected to a PC to record the friction force value versus time of the experiment as shown in Fig. 1b. The friction coefficient was automatically recorded for many cycles of crankshaft rotation. Friction and wear testing The tribological experiments were conducted under different loads from 185 to 340 N and sliding speed from 0.25 to 0.66 m/s. The sliding speed is controlled by a frequency of AC motor. The temperature of the lubricant
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was kept at close to 60 °C (operation temperature in an engine) to reduce the viscosity effects of lubricant and to clarification the nanoparticles effect alone for minimizing of the boundary friction coefficient. In this setup, the compression ring (top ring) was used in tribological trials. To simulate the characteristics of the compression ring lubrication under oil starvation, so the amount of lubricants supplied to the interface between the ring and the liner should be a little amount (4 ml). The same quantity of the lubricants is used for the evaluation of the tribological characteristics of the piston ring/liner interface. The effects of lubricant starvation greatly reduce the film thickness; it is likely that the lubricant starvation conditions are helping to confirm the effect of nanolubricants in a current study. In these tests, each friction test was carried out at a constant contact load and average speed for duration of 20 min. Every test was repeated for three times with similar test conditions, and the friction coefficient presented in the results is the average of the tests. Table 1 illustrates the experimental parameters employed in the tribological tests. The wear results during this study have been offered in terms of specific wear rate which was calculated using the following formula: Specific wear rate ¼
V Fn S
ð1Þ
where V is worn volume (mm3), Fn is the applied load on ring (N), and S is the sliding distance (m). The volume worn of the ring was determined via the profiles of the worn scar cross-section measured using a surface profilometer.
Preparation of nano-lubricants Engine oil was used as base oil for the tribological experiments to demonstrate the effect of using nanoparticles as engine oil additives. The base engine oil used in this investigation was commercially available synthetic oil (Castrol EDGE professional A5 5W-30). The diameter of the Al2O3 and TiO2 nanoparticles in utilized nano-lubricants was about 8–12 and 10 nm, respectively. The Al2O3 and TiO2 nanoparticles were purchased from Nanjing XFNANO Materials Tech Co., Ltd. The compositions of nano-lubricants comprised Al2O3 or TiO2 nanoparticles in a concentration of 0.25 wt.% added to an oleic acid having a concentration of 1.75 wt.%. Therefore, nano-lubricants comprised
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Fig. 1 The designed bench tribometer of the piston ring/cylinder liner interface. a Photograph of tribometer. b The signal output of friction force between the piston ring and cylinder liner
2 wt.% additive solution (Al2O3 or TiO2 + oleic acid) and 98 wt.% engine oil as shown in Table 2. Oleic acid was added to the engine oil for the purpose of suspension and to minimize of nanoparticle agglomerates. The
mixing of nanoparticles with the engine oil is an important step towards the improvement in the nanolubricants quality to keep a beneficial effect of nanoparticles on tribological characteristics. To get a stable
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Fig. 2 The test specimens. Cylinder liner (a) and piston ring (b)
suspension of Al2O3 and TiO2 nanoparticles in the engine, a magnetic stirrer was used for blending the nanoparticles and the engine oil for 5 h. The particle size distributions of Al2O3 and TiO2 nanoparticles dispersed in engine oil were measured using dynamic light scattering (DLS, Zetasizer Nano ZS system) as shown in Fig. 3. The initial measurement of each tested sample was made after immediate preparation, and the second measurement was taken after 24 h. Further measurements were carried out at certain time periods. The initial measurement (time = 0) shows that the peak diameters for Al2O3 and TiO2 nanoparticles were 43 and 24 nm, respectively, which were larger than the primary sizes of each particle mentioned earlier (Al2O3 = 8–12 nm and TiO2 = 10 nm). This can be translated into a slight agglomeration of the nanoparticles in the engine oil due to strong intermolecular forces (van der Waals). DLS peak diameter of the nanoparticle sizes into engine oil was increased after 14 days, but did not form large clusters in engine oil as shown in Fig. 3a. This result suggests that the Brownian motion and the attractive forces (van der Waals) of the nanoparticles were greater than the repulsive forces. To reduce the aggregation and sedimentation of nanoparticles in engine oil, a magnetic stirrer was used for 20 min again just before the start of the tribological experiments.
Results and discussion Powder characterization The morphology of the Al2O3 and TiO2 nanoparticles was examined with FE-SEM and TEM images as shown in Fig. 4. The morphology of the Al2O3 and TiO2 nanoparticles was amorphous. This could lead to rolling effect between worn surfaces instead of sliding. The crystal structure of the nanoparticles was analyzed by X-ray diffraction (XRD). Figure 5 demonstrates the XRD patterns of the TiO2 and Al2O3 nanoparticles. It can be observed that the peaks are 2θ = 25.2°, 36.9°, 48°, 53°, 55°, and 62° with strong diffraction peaks at 25° and 48°, confirming that the TiO2 was in the anatase structure phase. Additionally, Al2O3 nanoparticles also exhibited typical diffraction peaks (311), (400), and (441) at 2θ = 37°, 45°, and 67°, respectively. Al2O3 diffraction peaks show a high degree of broadness due to a less degeneracy in the crystallites (Thamaphat et al. 2008), which suggests the crystal size of the Al2O3 being smaller than TiO2. The XRD data were analyzed with the assistance of MDI Jade 6 program.
Tribological characterization Figure 6 clearly illustrates the effect of Al2O3 and TiO2 nano-lubricants on the average of the friction coefficient between the piston ring and cylinder liner with several
Table 1 Experimental parameters employed in the tribological tests
Table 2 The formulation of Al2O3 and TiO2 nano-lubricants
Parameter
Nanoparticles
Engine oil (5W-30)
Additive solution
Al2O3
98% oil
2% solution (0.25 wt.% nanoparticles + 1.75 wt.% oleic acid)
TiO2
98% oil
2% solution (0.25 wt.% nanoparticles + 1.75 wt.% oleic acid)
Value
Applied load
185 to 340 N
Average sliding speed
0.25 to 0.66 m/s
Temperature
60 °C
Lubricant amount
4 ml
Duration of test
20 min
Stroke length
65 mm
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Fig. 3 Particles size distribution of Al2O3 and TiO2 nanoparticles in engine oil measured by DLS as a function of time
Fig. 4 FE-SEM and TEM micrographs for a, b TiO2 nanoparticles and c, d Al2O3 nanoparticles
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J Nanopart Res (2016) 18:377 Fig. 5 XRD patterns of Al2O3 and TiO2 nanoparticles
Fig. 6 Mean friction coefficient between piston ring and cylinder liner under different contact loads and sliding speeds. a Under a contact load of 231.5 N. b Under an average sliding speed of 0.4 m/s
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of the speeds and the loads, as compared with engine oil without nanoparticle additives. The results showed that the coefficient of friction for all lubricants decreased with an increase in sliding speed and contact load due to frictional surface temperature and oxidation increment. This could be associated the formation of a thin molten layer at the asperity contacts causing a reduced of the shear stress and boundary friction coefficient. The use of only oleic acid as an additive slightly reduces the friction coefficient at a concentration of 1.75 wt.% and could be attributed to the chemical reactions on the frictional surfaces (Ali et al. 2016a, 2016b, 2016c). The nano-lubricants demonstrated lower average friction coefficient at all of the sliding speeds and applied loads. In particular, TiO2 nano-lubricant displays the lowest friction coefficient at all of the sliding speeds. The main reason of better tribological performance obtained by Al2O3 and TiO2 is the rolling effect of nanoparticles and forming a low friction tribo-film on the rubbing surfaces. The friction coefficient between the ring and the liner for all the lubricants with and without nanoparticles was measured with time as shown in Fig. 7. The test conditions of average sliding speed were 0.4 m/s and 278 N contact load. The results showed that the boundary friction coefficient reduced by 35–51% at TDC and BDC with the use of Al2O3 and TiO2 nano-lubricants, respectively, as compared to the commercial lubricant (5W-30). This enhanced decrement in boundary friction coefficient at ends of stroke with the use of Al2O3 and TiO2 nano-lubricants can be attributed to the formation tribo-film of Al2O3 and TiO2 nanoparticles and due to its
Fig. 7 Minimizing of the boundary friction coefficient between piston ring and cylinder liner using Al2O3 and TiO2 nanolubricants
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self-lubrication during boundary lubrication regime. Interestingly, nano-lubricant additives are most effective in the boundary lubrication regime. The most important observations of the behavior of friction curve during the stroke were as follows: friction coefficient with time shows the negative part. The reason is related to the change in sliding speed direction through the reciprocating sliding motion. Moreover, it was observed during one stroke that the lubrication regimes can occur at different points in the stroke depending on the speed, the contact geometry, and the surface topography. The boundary and mixed lubrication regimes occur near TDC and BDC, but the hydrodynamic lubrication occurs at the mid-stroke. It was also observed that the boundary friction coefficient (maximum value) is reached close to the TDC and BDC. The reason is related to the critically low sliding speed attained at the TDC and BDC, which prevents adequate access to the lube oil to these locations and the increase in the metal contact between worn surfaces (boundary lubrication). In contrast, the minimum friction coefficient was observed at a mid-stroke location because of the adequate access to the lube oil (hydrodynamic lubrication) as a result of the maximum sliding speed. The effect of Al2O3 and TiO2 nano-lubricants on the wear rate of the piston ring under different loads, average sliding speed of 0.53 m/s, and 15 km of sliding is illustrated in Fig. 8. It was evident that the wear rate of the ring decreased with an increase in the applied load. In high loads, the interface temperature and contact area become greater and lead to form an oxide film
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Fig. 8 Effect of Al2O3 and TiO2 nano-lubricants on wear rate of piston ring under different contact loads
(thermally activated process) on worn surfaces, assisted in lowering the wear rate in both nano-lubricants and engine oil without nanoparticles. The investigation was performed using engine oil with oleic acid to evaluate the real influence of the nanoparticles on the wear rate of the ring. The wear rate of the ring was slightly reduced, which demonstrates the effective role of nano-lubricants on the wear rate of the piston ring. The results showed a decrease in the wear rate for the use of Al2O3 and TiO2 nano-lubricants by 41 and 30%, respectively, as compared with the use of engine oil without nanoparticle additives under 185 N contact load. This could be attributed to the mechanism for nano-lubricants, which leads to the formation of a stable solid lubricant or an ultra-thin lubricating coating on worn surfaces, reducing the wear rate of the piston ring. Table 3 shows the summary of the tribological performance of the nanoparticles used as nano-lubricant additives.
Evolution of surface topography The surface roughness of the frictional surfaces is one of the most significant parameters in controlling the tribological performance in an engine. The surface roughness of the piston ring and cylinder liner were investigated by the 3D optical profilometer after 15 km of sliding, as shown in Fig. 9. It can be seen in Fig. 9a, b the higher surface roughness of the ring and liner surface at using engine oil without nanoparticle additives. The results revealed a decrease in the surface roughness of the ring and the liner by 30–33 and 16–17.5%, respectively. The nanoparticles in the engine oil can fill scars and grooves of the rubbing surfaces (mending and polishing effects) to minimizing asperity contact and friction coefficient values. Figure 10 illustrates the surface profile of the worn surface of the cylinder liner lubricated by nano-
Table 3 Summary of the tribological performance of nano-lubricant additives Contact load (N)
Wear rate of the ring (mm3/N m)
Friction coefficient Engine Engine oil oil + oleic acid
Al2O3 nano-lubricant
TiO2 Engine Engine nano-lubricant oil oil + oleic acid
Al2O3 nano-lubricant
TiO2 nano-lubricant
185
0.0356
0.035
0.0313
0.03033
8.99E−8
5.28E−8
6.37E−8
232
0.0324
0.0327
0.02985
0.0302
6.39E−8
4.82E−8
5.22E−8
278
0.0319
0.0316
0.03024
0.03078
5.37E−8
3.33E−8
4.76E−8
340
0.0309
0.0304
0.02916
0.02858
4.54E−8
2.24E−8
2.74E−8
9.11E −8 6.66E −8 5.47E −8 4.87E −8
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Fig. 9 3D surface topography of the piston ring (a, c, e) and cylinder liner (b, d, f). a, b Use of 5W-30 lubricant. c, d Use of Al2O3 nanolubricant. e, f Use of TiO2 nano-lubricant
lubricants and engine oil without nanoparticles. The surface roughness values of the cylinder liner after experiment become diverse at different stroke positions due to change lubrication regimes in these regions. In boundary lubrication, the contacts between asperities become the dominating contact mechanism where the
average oil film thickness is less than the average surface roughness. The results showed the maximum of the surface profile of cylinder liner noted at TDC and BDC locations, which is clearly higher than that of the surface profiles in mid-stroke location. This observation could be attributed to the fact that the presence of boundary
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Fig. 10 2D roughness profiles of cylinder liner during the stroke at TDC, MID, and BDC locations for a engine oil (5W-30), b Al2O3 nanolubricant, c TiO2 nano-lubricant
lubrication regime is a major contributor to wear along with other factors, which include highest contact pressures and breakdown in the oil film due to very low speeds. On the other hand, this event is associated with the continuous existence of a lubricating oil film dynamically preserved at mid-stroke. Hence, the
enhanced asperity to asperity interaction prevails resulting in higher wear and friction at the extreme positions (TDC and BDC). Nevertheless, the friction coefficient of in the piston ring/liner interface increases with increasing surface roughness for liner especially at using engine oil without nanoparticles at TDC and BDC locations. It is
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immediately apparent from Fig. 10 that the maximum of the surface profile of cylinder liner for the use of Al2O3 and TiO2 nano-lubricant additives showed a lower asperity height. Nonetheless, the reduction in surface roughness for ring and liner can increase the scuffing resistance and allow the surface to bear a higher load. Also, the stick-slip friction depends on surface roughness. Characterization of the lubricating coating on the worn surfaces In this section, an explanation of the mechanisms for minimizing of the boundary friction coefficient and formation of lubricating coating on worn surfaces
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supported by FE-SEM and EDS evidence has been provided. Figure 11 shows the FE-SEM and EDS patterns with an elemental content of the piston ring surface to a comparison of the morphological evolution under a 308 N contact load and 0.258 m/s average sliding speed after a sliding distance of 15 km. The frictional surface of the piston ring using engine oil without nanoparticles showed deep scratches, grooves, and exfoliation as shown in Fig. 11a due to a breakdown of lubricant film thickness due to full starvation of lubrication between the ring and liner. This remark on piston ring surface confirms the result of high boundary friction coefficient and wear rate of the ring (Figs. 6 and 8). Furthermore, the tribo-boundary film (lubricating coating) formed on the worn surface of Al2O3 nano-
Fig. 11 FE-SEM and EDS patterns with elemental content of the piston ring surface for a engine oil (5W-30), b Al2O3 nano-lubricant, and c TiO2 nano-lubricant
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lubricant is smoother (without the exfoliation and scratches) and more compact than that of TiO2 nanolubricant (Fig. 11b, c). This observation may be confirmed by the 3D surface profiler using Al2O3 nanolubricant as shown in Fig. 9c. Al2O3 and TiO2 nanoparticle deposition on the rubbing surfaces of the ring was further confirmed by the EDS analysis with elemental content. The EDS analysis reveals that the Al2O3 and TiO2 nanoparticles were centered on the wear scratches, thereby enabling the nanoparticles to provide wear protection (tribo-film). Based on the SEM images, it was also observed that the surface morphology of the cylinder liner for the use of engine oil without nanoparticle additives showed a large area plastic deformation and an abrasion (Fig. 12a). When Al2O3 and TiO2 nanoparticles were added into engine oil, the exfoliation and scratches were eliminated and the worn surface of the liner was
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comparatively smooth and dominated by light abrasive wear as shown in Fig. 12b, c. This was due to Al2O3 and TiO2 nanoparticle deposition on the worn surfaces of the liner that ultimately led to decline surface roughness at TDC location as shown in Fig. 10c, d to minimize of boundary friction coefficient. It is interesting to observe the deposition of Al2O3 more than TiO2 nanoparticles on the worn surfaces of the ring and liner, notwithstanding its concentration (0.25 wt.%) was equal with TiO2 nano-lubricants as shown in Figs. 11 and 12. This suggested that the Al2O3 nano-lubricant was more effective in minimizing the wear (Fig. 8) via formation of a tribofilm on worn surface of the ring. In contrast, the TiO2 nano-lubricant was more effective in reducing the friction coefficient (Fig. 6) due to a majority of the TiO2 nanoparticles remained blended with the engine oil causing mode change of the friction from sliding to rolling friction.
Fig. 12 SEM and EDS patterns with elemental content of the cylinder liner surface at TDC location for a engine oil (5W-30), b Al2O3 nanolubricant. c TiO2 nano-lubricant
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Fig. 13 Mechanisms of nano-lubricants for minimizing of the boundary friction coefficient during boundary lubrication regime. a Rolling and mending effect, b tribo-boundary film, and c elements of a tribo-boundary film, corresponding to the blue dash line box
In order to ascertain the composition of a triboboundary film formed on the worn surface of the piston ring, we analyzed the cross-section of piston ring lubricated with Al2O3 nano-lubricant by FE-SEM and EDS spectrum. Figure 13 highlights how the formation of a lubricating coating on the worn surface of the ring. A tribo-boundary film that contains active elements is phosphorus (P), as shown in Fig. 13c. Based on the experimental evidence for Al2O3 nano-lubricant, a trace amount of P was detected (active element of ZDDP). This suggests the involvement of Al2O3 nanoparticles and oil additive package, specifically zinc dialkyldithiophosphate (ZDDP) anti-wear additive of the engine oil (5W-30) with the substrate surface to form the triboboundary film on the worn surface of the ring. This confirms the synergistic effects between the Al2O3 nanoparticles and oil additive package. The lubricating coating film formation can follow three different processes: melting of nanoparticles on the frictional surface, a reaction of them with
an active surface, and their tribo-sintering on the surface (Battez et al. 2008). The first processes are not feasible because the melting point of Al2O3 and TiO 2 is 2072 and 1843 °C, respectively. But, a chemical reaction and tribo-sintering process are the most suitable. When nanoparticles are employed under high compressive pressure (boundary lubrication conditions), the sintering process for nanolubricants starts as soon as the temperature increases above room temperature (Alves et al. 2016). This could be attributed the mechanism for Al2O3 nanolubricant, which leads to the formation of a stable lubricating coating as a solid lubricant (Fig. 13b), reducing the shear strength at the contact and hence reduce boundary friction coefficient. However at the same time, a physical film is formed by a deposition of Al2O3 and TiO2 nanoparticles. These nanoparticles in the engine oil can fill scars and grooves of the worn surfaces to facilitate the separation, reducing asperity contact. According to the previous
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evidence for nano-lubricants, a tribo-boundary (lubricating coating) film can be formed on the worn surfaces through a chemical reaction and a physical mechanism. This result suggests that all nanolubricant additives have positive functions for reducing boundary friction coefficient between the piston ring and cylinder liner via an ultra-thin lubricating coating and rolling and mending effect of nanoparticles on the worn surfaces.
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part of TiO2 remained dispersed in engine oil or became a third body to help facilitate the change from sliding to rolling friction (rolling effect), while Al2O3 nano-lubricant was more effective in improving the anti-wear performance owing to provide strong an ultra-thin lubricating coating forming on the worn surfaces of the ring and liner as a solid lubricant. 5. There should be more investigations on reducing fuel consumption in automotive via using nanoparticles as nano-lubricant additives.
Conclusions In short, reducing boundary friction coefficient means a decline of frictional power losses in engines and automotive fuel economy, while reducing wear means a longer life span for engine parts. Thanks to these nanolubricants, automotive engines operate safely and smoothly for a long time and lowering costs of maintenance. Based on the results presented above, it can be concluded that: 1. The boundary friction coefficient at the ends of the stroke (TDC and BDC) decreased by 35 and 51% for the Al2O3 and TiO2 nano-lubricants, respectively, as compared with engine oil without nanoparticles. This is due to the prevention of metal-to-metal contact by forming a boundary film on the sliding contact interfaces in automotive engines. 2. Al2O3 and TiO2 nanoparticles display better anti-wear performance compared with engine oil without nanoparticles. Hence, the wear rate of piston ring was also reduced by 41 and 30% for Al2O3 and TiO2 nano-lubricants, respectively. The main reason for the decrease in wear rate of the ring is the formation of an ultra-thin lubricating coating on the worn surfaces via a chemical reaction and a physical mechanism in boundary lubrication regime causing reducing metal-to-metal contact and a smoother surface. 3. The surface roughness was reduced for both of the piston ring and cylinder liner by 33–30% for the case of Al2O3 and 16–18% for TiO2 nano-lubricants, respectively, in comparison with the lubricant without nanoparticles. 4. TiO2 nano-lubricant played a critical role in boundary friction coefficient reduction, suggesting that
Acknowledgments The authors would like to express their deep appreciation to the Hubei Key Laboratory of Advanced Technology for Automotive Components (Wuhan University of Technology) for the continuous support. M.K.A. Ali acknowledges the Chinese Scholarship Council (CSC) for the financial support for their PhD studies in the form of CSC grant numbers 2014GF032. M.K.A. Ali also appreciates the financial support from the Egyptian Government. We also wish to thank the various anonymous reviewers for their helpful and valuable comments. Compliance with ethical standards Conflict of interest The authors declare that they have no conflicts of interest.
References Ali MKA, Xianjun H (2015) Improving the tribological behavior of internal combustion engines via the addition of nanoparticles to engine oils. Nanotechnol Rev 4:347–358 Ali MKA, Xianjun H, Mai L, Bicheng C, Turkson RF, Qingping C (2016a) Reducing frictional power losses and improving the scuffing resistance in automotive engines using hybrid nanomaterials as nano-lubricant additives. Wear 364:270– 281 Ali MKA, Xianjun H, Mai L, Qingping C, Turkson RF, Bicheng C (2016b) Improving the tribological characteristics of piston ring assembly in automotive engines using Al2O3 and TiO2 nanomaterials as nano-lubricant additives. Tribol Int 103: 540–554 Ali MKA, Xianjun H, Turkson RF, Ezzat M (2015) An analytical study of tribological parameters between piston ring and cylinder liner in internal combustion engines. P I Mech Eng K-J Mul. doi:10.1177/1464419315605922 Ali MKA, Xianjun H, Turkson RF, Peng Z, Chen X (2016c) Enhancing the thermophysical properties and tribological behaviour of engine oils using nano-lubricant additives. RSC Adv 6:77913–77924 Alves S, Mello V, Faria E, Camargo A (2016) Nanolubricants developed from tiny CuO nanoparticles. Tribolo Int 100: 263–271
377
Page 16 of 16
Bakunin V, Suslov AY, Kuzmina G, Parenago O, Topchiev A (2004) Synthesis and application of inorganic nanoparticles as lubricant components—a review. J Nanopart Res 6:273– 284 Battez AH, González R, Viesca J, Fernández J, Fernández JD, Machado A, Riba J (2008) CuO, ZrO2 and ZnO nanoparticles as antiwear additive in oil lubricants. Wear 265:422–428 Boshui C, Kecheng G, Jianhua F, Jiang W, Jiu W, Nan Z (2015) Tribological characteristics of monodispersed cerium borate nanospheres in biodegradable rapeseed oil lubricant. Appl Surf Sci 353:326–332 Ghaednia H, Babaei H, Jackson RL, Bozack MJ, Khodadadi J (2013) The effect of nanoparticles on thin film elastohydrodynamic lubrication. Appl Phys Lett 103:263111 Gulzar M, Masjuki H, Kalam M, Varman M, Zulkifli N, Mufti R, Zahid R (2016) Tribological performance of nanoparticles as lubricating oil additives. J Nanopart Res 18:1–25 Hsu SM, Gates R (2005) Boundary lubricating films: formation and lubrication mechanism. Tribol Int 38:305–312 Ingole S, Charanpahari A, Kakade A, Umare S, Bhatt D, Menghani J (2013) Tribological behavior of nano TiO2 as an additive in base oil. Wear 301:776–785 Joly-Pottuz L, Vacher B, Le Mogne T, Martin J, Mieno T, He C, Zhao N (2008) The role of nickel in Ni-containing nanotubes and onions as lubricant additives. Tribol Lett 29:213–219 Kao MJ, Lin CR (2009) Evaluating the role of spherical titanium oxide nanoparticles in reducing friction between two pieces of cast iron. J Alloy Compd 483:456–459 Lawrence KD, Ramamoorthy B (2016) Multi-surface topography targeted plateau honing for the processing of cylinder liner surfaces of automotive engines. Appl Surf Sci 365:19–30 Li W, Zheng S, Cao B, Ma S (2011) Friction and wear properties of ZrO2/SiO2 composite nanoparticles. J Nanopart Res 13: 2129–2137 Li X, Cao Z, Zhang Z, Dang H (2006) Surface-modification in situ of nano-SiO2 and its structure and tribological properties. Appl Surf Sci 252:7856–7861 Liu G, Li X, Qin B, Xing D, Guo Y, Fan R (2004) Investigation of the mending effect and mechanism of copper nano-particles on a tribologically stressed surface. Tribol Lett 17:961–966 Luo T, Wei X, Zhao H, Cai G, Zheng X (2014) Tribology properties of Al2O3/TiO2 nanocomposites as lubricant additives. Ceram Int 40:10103–10109
J Nanopart Res (2016) 18:377 Peña-Parás L, Taha-Tijerina J, Garza L, Maldonado-Cortés D, Michalczewski R, Lapray C (2015) Effect of CuO and Al2O3 nanoparticle additives on the tribological behavior of fully formulated oils. Wear 332:1256–1261 Priest M (2000) Factors influencing boundary friction and wear of piston rings. Tribology series 38:409–416 Priest M, Taylor C (2000) Automobile engine tribology—approaching the surface. Wear 241:193–203 Rapoport L, Leshchinsky V, Lapsker I, Volovik Y, Nepomnyashchy O, Lvovsky M, Tenne R (2003) Tribological properties of WS2 nanoparticles under mixed lubrication. Wear 255:785–793 Shahmohamadi H, Mohammadpour M, Rahmani R, Rahnejat H, Garner CP, Howell-Smith S (2015) On the boundary conditions in multi-phase flow through the piston ring-cylinder liner conjunction. Tribol Int 90:164–174 Styles G, Rahmani R, Rahnejat H, Fitzsimons B (2014) In-cycle and life-time friction transience in piston ring–liner conjunction under mixed regime of lubrication. Int J Engine Res 15: 862–876 Thamaphat K, Limsuwan P, Ngotawornchai B (2008) Phase characterization of TiO2 powder by XRD and TEM. Kasetsart J(Nat Sci) 42:357–361 Truhan JJ, Qu J, Blau PJ (2005) The effect of lubricating oil condition on the friction and wear of piston ring and cylinder liner materials in a reciprocating bench test. Wear 259:1048– 1055 Wan Q, Jin Y, Sun P, Ding Y (2014) Rheological and tribological behaviour of lubricating oils containing platelet MoS2 nanoparticles. J Nanopart Res 16:1–9 Wong VW, Tung SC (2016) Overview of automotive engine friction and reduction trends—effects of surface, material, and lubricant-additive technologies. Friction 4:1–28 Wu Y, Tsui W, Liu T (2007) Experimental analysis of tribological properties of lubricating oils with nanoparticle additives. Wear 262:819–825 Xiaohong L, Zhi C, Zhijun Z, Hongxin D (2006) Surface-modification in situ of nano-SiO2 and its structure and tribological properties. Appl Surf Sci 252:7856–7861 Zulkifli N, Kalam M, Masjuki H, Yunus R (2013) Experimental analysis of tribological properties of biolubricant with nanoparticle additive. Procedia Eng 68:152–157. doi:10.1016/j. proeng.2013.12.161
