ANTI-WEAR EFFECT AND THE ROLE OF DISPERSION TEMPERATURE OF NANOFLUIDS CONTAINING AL₂O₃ NANO-ADDITIVES

Nabhan A. and Rashed A.

Production Engineering and Mechanical Design Department, Faculty of Engineering, Minia University, El-Minia 61111, Egypt.

ABSTRACT

The present investigation is focusing on the enhancement of the tribological performance of 20W-50 engine oil by the addition of Al₂O₃ nanoparticles. The anti-wear properties of engine oil have been tested under different sliding speeds 0.84, 1.26, 1.68, 2.1, and 2.52  m/s and operating temperature of 50, 75, and 100˚C using a tribometer with stainless steel disc and plate to simulate the working conditions in the engine. The experiments have been performed on the samples lubricated by different nanofluids prepared by dispersing the nanoparticles at three different mixed temperatures of 50ºC, 100ºC, and 150ºC. The study concluded that the lowest wear rate is achieved when the sliding surfaces were lubricated by the nanofluids mixed at temperature of 50°C. It can be found that the wear rate reduces by 34.4% during the lubrication by nanofluid containing 0.4 wt.% of Al₂O₃ nanoparticles content. It is also concluded that the increase in the mixing temperature could impede the dispersion of nanoparticles in the oil, in which the dispersion stability decreases.  

KEYWORDS

Wear rate, nano-lubricant additives, engine lubricant oil, Al₂O₃.

INTRODUCTION

Nowadays, the use of nanomaterials as additives to engine oils is one of the areas that attracts great interest from researchers around the world. Nanoparticles are characterized by playing an important role by reducing the friction coefficient between the sliding parts, [1, 2]. This is due to the mechanics of the tribological system for nanoparticles such as protective film, mending effect, rolling effect, and polishing effect, [3–5]. In situ many nanomaterials have been used as additives to lubricant oil, [2] and grease, [6] to improve frictional properties and increase the wear resistance of sliding surfaces. Through Stribeck curve it displays the friction conditions for the internal engine parts during worked under one of the various lubrication regimes, as illustrated in Fig. 1, [7]. The boundary lubrication is introduced as ultra-thin film of lubricant that in which the sliding surface asperities [8]. The boundary lubrication occurs oftentimes when the sliding parts are so close together that the surface interactions under overload and low speed, [9–11]. Wherefore the chemical and physical properties of the lubricant are of major importance, [12, 13].

Based on the characteristic chemical compounds, the various types of nanomaterials have been classified into metals, metal oxides, carbon-based, ceramics and polymers, [2]. The PTFE nanoparticles are used as lubricant additive to enhance the tribological performance of oils, generally more effective in the extreme pressure then the other nanoparticles, [14–17]. Copper oxide (CuO) used as nano-additives engine oil. Further it was found that CuO nanoparticles play a vital role for improving the tribological performance. The results display that nano-lubricant exhibits clearly reducing of coefficient of friction and anti-wear behavior by 53% and 24%, respectively, [18–20]. Carbon-based, such as carbon nanotubes, carbon nanofibers, and graphene are distinguished to have an effective role in improving the thermal conductivity of the fluids [21]. In contrast, increasing amount of the carbon leads to reduce stability and do not useful in the viscosity of nano-lubricants, [22, 23].In an attempt to reach the better results it was using hybrid nano-additives that may combine carbon and other types of nanoparticles. Hybrid carbon nanotubes with TiO₂, Al₂O₃, and other nanoparticles play a desirable role in improve the pressure characteristics and increase their thermal conductivity, [24–27].

Fig. 1 The Stribeck curve showing the transition from boundary to mixed and finally to hydrodynamic lubrication regimes with clarified an impact role of the nano-additives, [7].

The effect of dispersing diamond and SiO₂ nanoparticles to paraffin oil on the tribological performance was investigated, [28]. It was found that paraffin oil using both nanoparticles as additives have friction coefficient and wear less compared with the pure paraffin oil. The nano-lubricant paraffin with SiO₂ nanoparticles of 0.05 to 0.5 wt. % contents have the best tribological performance, [29].  The results can show that paraffin oil dispersing by tiny size of SiO₂ nanoparticles anti-wear and anti-friction characteristics under high loads. MoS₂ and SiO₂ nano-lubricants used to study the frictional properties of contacts between magnesium alloy and steel using a reciprocating sliding ball-on-flat tribometer. The results showed that MoS₂ nano-lubricants has more clearly effect as compared with the SiO₂ nano-lubricants in terms of the load carrying capacity and the lubrication film stability, [30]. The rheological stability and friction coefficient of SAE 40 Lubricating oil using SiO₂ nanoparticles as additives with contents up to 1% by weight were studied, [31]. The fully flooded condition testing results can show that the friction coefficient decreased with the increase of the concentration of nano-lubricant up to 0.6 % while the coefficient of friction starts increasing with nano-lubricant with content above 0.6%. Titanium and aluminum oxides nanoparticles are considered the most used as additives for motor oils due to their excellent tribological, chemical and thermal properties. The nano-lubricant oil contents of TiO₂ nanoparticles reduce friction coefficient clearly, it may be due to form of a thin film on contact surfaces [32, 33]. The results of engine oil dispersing by TiO₂ nanoparticles showed the ability to withstand higher loads by of 35%, [34]. The effect of spherical titanium oxide nanoparticles, the average diameter was 50 nm, in reducing friction between two pieces of cast iron were evaluated. The rapeseed oil with 5wt% of TiO₂ nanoparticles concentration reduces the mean surface roughness by 80.84%, [35]. While, the friction coefficient and the wear scars decreased approximately by 15.2% and 11%, respectively [36]. Hybrid TiO₂/SiO₂ nano-lubricants is considered a desirable way on the control the friction coefficient and wear resistance of rubbing surfaces, [37]. The 1.0% wt. of TiO₂ nano-lubricant shows that the friction coefficient and wear rate reduced in the ranges 39–47% and 50%, respectively, as compared to the additives free engine oil, [38].

The nanoparticles were stirred in base oil with contents of with 0.5% and 1.0% by weight. The experiments evaluated under five temperatures starts from 50ºC up to 150ºC in the sequence increment of 25ºC. The 1.0% wt. of TiO₂ nano-lubricant shows that the friction coefficient and wear rate reduced in the ranges 39–47% and 50%, respectively, as compared to the additives free engine oil. The tribological properties improved as the lubricant composition of 0.5% wt. of SiO₂ nanoparticles while with increasing the composition of nanoparticles to 1.0% wt. the friction coefficient and wear rate again increased. The thin-film reformation effect and the rolling mechanism of the nano-lubricants of the frictional contact interfaces reduce and prevent the asperity contact between rubbing surfaces. The tribological properties of the modified Al₂O₃/SiO₂ composite nanoparticles as lubricating oil additives were investigated by four-ball and thrust-ring tests in terms of wear scar diameter, friction coefficient, and the morphology of thrust-ring, [39].   Titanium and aluminum oxides nanoparticles are considered the most used as additives for motor oils due to their excellent tribological, chemical and thermal properties, [40]. The influence of using the hybrid of Al₂O₃/TiO₂ as a nano-additives in engine oil was investigated, [41, 42]. The results illustrated that the friction coefficient and wear rate of the ring decreased in the ranges 39–53% and 25–33%, respectively. The lubricant oil, SAE 20W40, with using Al₂O₃ nanoparticles with 20 nm of grain size as nano-additives were studied. The results showed that 0.5% wt. of Al₂O₃ nano-lubricant reduce the friction by 49.1% and 21.6% under flooded and starved conditions, as compared to SAE 20W40 lubricant oil, [43]. The wear rate was reduced by using 0.5% wt. of Al₂O₃ nano-lubricant with 40–80 nm of grain size, [44]. Adding of 0.1wt% concentration and 78 nm grain size of Al_­2O₃ nanoparticle to engine oil leads the friction coefficient to decrease by ranges of 17.61% and 23.92% for the four-ball and the thrust-ring test, respectively, [45]. The thermal conductivity improved by 37.49% using 3.0% wt. of Al₂O₃ nano-lubricant, [46]. Nanoparticles can easily enter into small gaps between sliding surfaces because of their ultrafine sizes, whereas the micron-scale traditional additives cannot, [47]. A study of influence of mixed temperatures on the tribological performance of Al_­2O₃ nano-lubricants under different operating conditions by using friction and wear tribometer. It was found that Al₂O₃ added to 20W-50 engine oil enhanced frictional property significantly and reduced vibrations. The study concluded that the lowest coefficient of friction was obtained at 0.4 wt. % concentration of Al₂O₃ nanoparticles and working temperature of 50°C, [48].

The current work aims to complete the previous study in order to measure the wear rate and monitor the worn surfaces topography.

EXPERIMENTAL

Preparation of Al₂O₃ nano-Lubricant

Aluminum oxide nanoparticles (Al₂O₃, 80% alpha /20% gamma, purity 99.9%) has been purchased from US Research Nanoparticles, Inc. The average diameter of Al₂O₃ nanoparticles is 50 nm and the specific surface area 35 m²/g. Figure 2 illustrates the morphology of Al₂O₃ nanostructures investigated by SEM, which confirm the uniform shape of the nanoparticles with little imperfections. As a base material for lubrication, commercial mineral oil (Total 20W50 Quartz 5000SL) was chosen. The performance index of lubricant oil is shown in Table 1.  Different amounts of aluminum nanoparticles have been dispersed in the commercially available oil to form the required nanofluids. Moreover, the mineral oil-based nanofluid has been prepared by continuous stirring at 1500 rpm for 1.5 hours under three different temperatures of 50ºC, 150ºC, and 150ºC as shown in Fig.3. The mineral oil-based nanofluid has been left sure it is four weeks of preparation to monitor any agglomeration or sedimentation. The tribological and lubricous effects of five different amounts of nanoparticles of 0, 0.1, 0.2, 0.3, and 0.4 wt. % have been tested and evaluated.

Table 1. Performance index of the lubricating oil.

Fig. 2 SEM image of Al₂O₃ nanoparticles.

Fig. 3 Photograph of nano-lubricant oils carried out under mixing temperature (a)50ºC, (b) 100ºC and (c) 150ºC.

The Tribometer Test-Rig

The experimental stand of the test rig consists of aluminum friction plates slides against flat-panel pin at sliding speeds of 0.84, 1.26, 1.68, 2.1 and 2.52 m/s, Fig. 4. Experiments have been performed using the tested nanofluids specimens at operating temperatures of 50, 75 and 100˚C for 10 minutes to simulate the working conditions inside the engine.

Fig. 4 Schematic diagram of the wear test-rig.

The wear rates during the sliding have been expressed by weight loss measured using an electronic balance with an accuracy of 0.0001g by calculating the difference between the final and initial weights of specimens. Consequently, the wear scar resulted on the specimens is considered as indication to the worn surfaces and thus can be used as a vital indicator for wear resistance measurements. Finally, the worn surfaces on the steel plates have been examined using optical microscopy (OLYMPUS BX53M, USA).

RESULTS AND DISCUSSIONS

Influence of Nanoparticles on the Wear Rate

Figure 5 shows the influence of Al₂O₃ nanoparticles contents on the wear rate for samples mixed at temperature 50ºC under different working temperatures and sliding velocities. The results show that 0.4 wt. % of Al₂O₃ nano-lubricant sample has the lowest value of the wear rate. It can be seen that the wear rate of the aforementioned sample decreased by 26.6 % in the comparison with the base oil at operating temperature of 50 ºC. Meanwhile, the mineral oil-based nanofluid containing 0.4 wt.% of Al₂O₃ nanoparticles exhibited the best performance (34.4% reduction in the wear rate) at temperature of 75ºC for those nanofluids which have been prepared at temperature of 50 ºC. On the other hand, the wear rate has significantly increased at temperature above 75˚C, in which the wear rate lessened (19.7% in the comparison with the reference oil) at the operating temperature 100ºC. From the results reported, the wear rate is reduced for all nano-lubricant samples compared with lubricant oil with the reference oil. The enhanced tribological performance of the nanofluid is referred to the role of the rolling effect of the nanoparticles. Furthermore, the nanoparticles play a crucial role in the separating of the asperities contact, which leads to less deformation of the rubbing surfaces.Moreover, the formation of the Al₂O₃ nanoparticles enriched lubrication film helps to protect the sliding surfaces from the severe wear and reduces the contact time in the asperity region, [49, 50].

The performance of the nanofluids mixed at temperature of 100ºC have been evaluated at different working temperatures and sliding velocities sliding velocities as shown in Fig. 6. It is clear that the mineral oil-based nanofluid containing 0.3 wt. % of Al₂O₃ nanoparticles displayed the best wear rate at all operating temperatures as compared with the reference oil. This behavior is attributed to the rolling effect of the nanoparticles, which leads to the decrease of the metal-to-metal contact causing a decline in the wear rate. Fig. 7 shows that the wear rate at 0.3 wt.% of Al₂O₃ nano-fluid has been reduced by 25.6%, 24.7% and 19.81% at working temperatures of 50, 75 and 100ºC, respectively.  The wear rate of all nanofluids prepared at 150ºC of dispersion temperature, has been reduced. The results indicate that the general trend of the wear rate curve for all working temperatures has improved the tribological performance during the tests. Furthermore, the wear rate of the steel plate lubricated by mineral oil-based nanofluid containing 0.1 wt.% of Al₂O₃ nanoparticles has been reduced by 22.18 %, 21.53 %, and 23.37 %  in the comparison with the reference oil at working temperatures of 50, 75 and 100ºC, respectively. Moreover, increasing the content percentage of Al₂O₃ nano-additives above 0.1% is corresponding with the increase in the wear rate. This behavior could be attributed to the instable nanoparticle’s dispersion in the base oil at high mixing temperature. Table 2 compiles the enhancement percentages of wear rate for all samples lubricated by the proposed nanofluids. The increase of the mixing temperature impedes the dispersion and the suspending of nanoparticles in the base oil. Hence, dispersion of nanoparticles in the base oil is more stable at low mixing temperatures. Therefore, the Al₂O₃ nanoparticle content percentage of 0.4 wt. % exhibited promising performance at low working temperatures. As the mixing temperature rises, average agglomerate size goes down which leads to decrease the viscosity of nano-lubricant, [51].

Fig. 5 Influence of Al₂O₃ nano-lubricant mixed at temperature 50°C on the wear rate at different working temperatures (a) 50ºC, (b) 75 ºC and (c) 100ºC.

Fig. 6 Influence of Al₂O₃ nano-lubricant mixed at temperature 100°C on the wear rate at different working temperatures (a) 50ºC, (b) 75 ºC and (c) 100ºC.

Fig. 7 Influence of Al₂O₃ nano-lubricant mixed at temperature 150°C on the wear rate at different working temperatures (a) 50ºC, (b) 75 ºC and (c) 100ºC.

Table 2 Reduction percentage of wear rate of best Al₂O₃ nano-lubricants.

Topography of the Worn Surfaces

Topography of worn surfaces nave been carried out to more clearly describe the contact mechanism of rubbing surfaces lubricated by the nanofluids prepared at different mixing temperatures. Figure 8 illustrates the wear tracks on the steel disc lubricated by the tested nanofluids at applied load of 20 N and average sliding speed of 1.68 m/s. Clear grooves parallel to the sliding direction can be clearly noticed on the wear track of those sample lubricated by the nanofluids mixed at 50ºC. Furthermore, some exfoliation could be seen as the results of the cracks propagation and contact fatigue, [52], as illustrated in Fig. 8 (a, b and c). On the other hand, the worn surfaces of the samples lubricated by nanofluids prepared at mixing temperatures of 75ºC were relatively smoother and free of surface cracks.

The formation of Al₂O₃ nanoparticles enriched tribo-film inhibits metal-to-metal contact and helps the mating surfaces to become smoother, [53]. Figure 8 (g, h and i) indicates that the relative higher spread of surface pits on the wear track. Furthermore, delamination wear mechanism is the dominant mechanism, which is directly corresponding to the higher wear rates. Moreover, the rise of the working temperature of frictional surfaces assisted to go down of the average agglomerate size which leads to decrease the viscosity of nano-lubricant. These results are attributed to the increase of the wear rate and plastic deformation, causing the exfoliation damage of the contact surfaces. 

Fig. 8 Microscopic images of the worn surfaces of Al₂O₃ nano-lubricant samples at different working temperatures (a, b and c) 50ºC, (d, e and f) 75 ºC and (g, h and i) 100ºC.

Considering Al₂O₃ nanoparticles as anti-wear additives that is referred to the reformation thin-film on the worn surfaces. This thin layer prevents the metal-to-metal contact between rubbing surfaces. However, the rolling effect ability of nanoparticles between the contact surfaces helps in delaying the damage of friction and wear. The mechanism of action of the nanoparticles in the contact can be summarized in their rolling on the sliding surfaces, filling the pits and fine cracks and separating the two contact surfaces.

CONCLUSIONS

In this work, different nanofluids prepared by dispersing different content percentages of Al₂O₃ nanoparticles in commercial mineral oil (Total 20W50 Quartz 5000SL) at different mixing temperatures have been prepared and evaluated tribologically. The experimental results proved that the mixing temperature plays a major role to determine the appropriate Al₂O₃ nano-additives percentage. For instance, the mineral oil-based nanofluids containing 0.4 wt.% of Al₂O₃ nanoparticles content and mixed at 50ºC exhibited the lowest wear rate. It can be seen that the wear rate reduces by 26.6%, 34.4%, and 19.7% under the working temperature of 50, 75, and 100ºC, respectively. Meanwhile, the higher mixing temperatures lead to a drastic increase in the wear rate as the result of the average agglomerate size, which reduces the viscosity of the nanofluids. The reduction observed in the wear rate can be attributed to the rolling of the nanoparticles. The tiny size of nanoparticles enables them to fill scratches and wear tracks and makes the friction surface smoother, where the sliding surfaces are protected by hard layer of nanomaterials and consequently, friction and wear decrease.

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