An Investigation on the Tribological Performances of the SiO2/MoS2 Hybrid Nanofluids for Magnesium Alloy-Steel Contacts
© The Author(s). 2016
Received: 27 February 2016
Accepted: 6 July 2016
Published: 15 July 2016
Hybrid nano-materials offer potential scope for an increasing numerous novel applications when engineered to deliver availably functional properties. In the present study, the SiO2/MoS2 hybrid nanoparticles with different mass ratios were employed as lubricant additives in the base oil, and their tribological properties were evaluated using a reciprocating ball-on-plate tribometer for magnesium alloy-steel contacts. The results demonstrate that the SiO2/MoS2 hybrid nanoparticles exhibit superior lubrication performances than individual nano-SiO2 or nano-MoS2 even in high load and diverse velocity cases. The optimal SiO2/MoS2 mixing ratio and the concentration of SiO2/MoS2 hybrid nanoparticles in the base oil are 0.25:0.75 and 1.00–1.25 wt%, respectively. The excellent lubrication properties of the SiO2/MoS2 hybrid nanoparticles are attributed to the physical synergistic lubricating actions of nano-SiO2 and nano-MoS2 during the rubbing process.
Magnesium and its alloys are promising materials in transportation, electronics industries, or aerospace for their excellent properties, such as low density, high specific strength, and electromagnetic compatibility [1, 2]. Nowadays, magnesium alloy products are mainly fabricated by casting and die-casting . However, the poor mechanical properties of the cast alloys are not sufficient to meet the demand of most load-bearing structural components. Compared with casting products, wrought magnesium alloys fabricated by plastic deformation processes, such as rolling, extrusion, and forging, seem to be more attractive owing to their competitive productivity and performance . It should be noted that the tribological interaction always takes place during forming process as two contact surfaces (tool steel and the metal) move relative to each other. These create a great challenge to achieve high-quality product and extended tool life .
The use of liquid lubricants to decrease friction is an efficient way to improve energy efficiency and mechanical durability, especially in the case of magnesium and its alloys. Unfortunately, so far, there are no suitable forming fluids for the forming process of Mg alloy, even at some conditions, the forming fluid used for Al alloy forming is casually used and the result is not satisfied. The application of conventional oil-based lubricant in Al alloy forming processes relies heavily on sulfur-, chlorine-, and phosphorous-containing additives. These additives form easily sheared tribo-layers on metal surfaces, thus controlling friction and reducing wear . However, the fast chemical degradation of these additives during application is accompanied by a loss of their lubrication performances. Even worse, the abovementioned additives cause negative effects on the environment even at low concentrations during the disposal of waste fluids . Therefore, the exploration of the harmless lubricant with the excellent tribological performances still continues. Some efforts have already been made in order to find novel lubricant additives for magnesium alloy, such as N-containing compounds , borates , and ionic liquids . All of them as lubricant additives are capable of forming tribo-layers with low shear strength to protect the contacting surfaces from damage, thereby improving the lubrication performances. Nonetheless, some problems are associated with their applications. For instance, many amide-based friction reducing agents for lubricating oils, including straight-chain amides, are not liquid at low temperature . The borate without active elements, i.e., nitrogen, sulfur and chlorine, cannot be used as good lubricant additive for magnesium alloy. Additionally, boron is an electron-deficient element and has a great affinity with oxygen borate esters susceptible to hydrolysis in the presence of moisture . The high cost and tedious procedure for the preparation of ionic liquids are main problems to put them into industrial application after 10 years of extensive development . These disadvantages are key problems to replace the traditional phosphors- and chlorine-containing additives with the aforementioned additives.
Over the past few years, the addition of nanoparticles as lubricant additives into base fluids is a rapidly progressing field of research because nanoparticles are different from traditional bulk materials due to their extremely small size, high specific surface area, and variety of particle chemistries [14, 15]. Meanwhile, due to the tiny usage of nanoparticle additive, the negative influence on the environment is greatly suppressed [16–18]. Among of the nanoparticles, the layered nanomaterials with different shapes and morphologies, such as MoS2 nanoparticles, are widely used as additives to liquid lubricants . Previous studies have shown that MoS2 nanoparticles can effectively reduce the friction in the boundary lubrication for steel/steel contacts and titanium alloy/steel pairs based on the formation of diverse types of tribo-film [20, 21]. Moreover, it was also reported that MoS2 nanoparticles can decrease the friction and wear even on relatively inert surfaces, such as diamond-like carbon (DLC) film [22, 23]. Despite many attempts, the combination of nano-MoS2 with other nanoparticles or compounds is of particular significance in terms of their application as lubricant additives because the combinations usually exhibit more prominent lubrication performances in contrast with individual nanoparticles attributed to the synergistic effect among two or more components. In this respect, Kunhong Hu et al.  synthesized TiO2/MoS2 nano-clusters and the tribological properties of the as-prepared TiO2/MoS2 nano-clusters as lubricant additive were investigated using a four-ball tribometer. It was found that the MoS2/TiO2 (2:1) nano-clusters achieve the lowest friction coefficient (μ = 0.045), which is 30.8 and 40 % lower than pure MoS2 and pure TiO2, respectively. Yanbin Zhang et al.  reported the use of carbon nanotubes (CNT)/MoS2 hybrid nanoparticles in minimum quantity lubrication for Ni-based alloy grinding. The results demonstrated that lower grinding forces and better ground surface were achieved by CNT/MoS2 hybrid nanofluids. Moreover, the optimal MoS2/CNT mixing ratio and nanofluid concentration are 2:1 and 6 wt%, respectively. Yufu Xu et al.  investigated the tribological behaviors of esterified bio-oil (EBO) and EBO containing graphene or/and MoS2 for steel/steel pairs by a point contact undirectional sliding tribometer. A synergistic effect on friction reduction and wear protection was observed with the graphene/MoS2 hybrids when added into EBO. These researches primarily demonstrate that MoS2-based hybrid nanoparticles have great potential as lubricant additives and are worth carrying out further study. As compared with the aforementioned nanoparticles, the SiO2 nanoparticles have attracted a great deal of research attention as a lubricant additive owing to excellent tribological performances, low cost, and facile preparation. Previous investigations about the lubrication properties of SiO2 nanoparticles mainly focused on Al alloy forming process, such as Al alloy machining and drilling [27, 28]. Experimental results showed that SiO2 nanofluids cause a decrease in friction coefficient and increase in surface quality of workpiece during machining process. This effect has been attributed to the rolling action of nanoparticles between the contact surfaces. In our early work, the tribological behaviors of nano-SiO2 and nano-MoS2 as lubricant additives for magnesium alloy/steel pairs have been studied and found that nano-MoS2 achieves better anti-wear behavior than nano-SiO2, while nano-SiO2 possesses better friction reducing behavior than nano-MoS2 . In view of their respective special tribological characteristics, it is worth to study how the two particles behave together as lubricant additive and try to further optimize the comprehensive tribological performances.
The aim of the current study is to provide an effective SiO2/MoS2 hybrid nanoparticles additive for the development of magnesium alloys forming fluid. The tribological behaviors of the SiO2/MoS2 hybrid nanoparticles were investigated by considering the friction coefficient and wear volume. Further, the lubrication mechanism was discussed in detail.
Friction and Wear Test
Mechanical properties of extruded AZ31 magnesium alloy used in this study
0.2 % YS/MPa
The as-extruded sheet was cut into samples with dimensions of 10 mm (TD) × 20 mm (ED) × 3 mm (ND). Prior to examination, the samples were polished with 1000 grit silicon carbide paper to a mean surface roughness of Ra ~0.08 μm. The ball slides at a stroke of 6 mm back and forth along extrusion direction. In the first group of experiments, the influence of the SiO2/MoS2 mixing ratio on the lubrication behaviors of nanofluids was discussed. The normal load during the tribological tests was 8 N corresponding to maximum Hertzian contact stress 446 MPa of at least 50 % higher than the yield strength of magnesium alloy sheets. The sliding speed was 30 mm/s, and the sliding test duration was 1.5 h. Lubricant oil was supplied to the top of the plate before testing, covering the entire surface. Friction coefficient and wear volume were used as characterization parameters. Five different SiO2/MoS2 mixing ratios were designed: pure MoS2, SiO2/MoS2 (0.25:0.75), SiO2/MoS2 (0.5:0.5), SiO2/MoS2 (0.75:0.25), and pure SiO2. The concentration of these additives in the base oil was 1 wt%. The optimal SiO2/MoS2 mixing ratio was obtained from the first group of experiments. However, the concentration of nanoparticles in the base fluids is another fundamental issue because excessive concentration will make nanoparticles agglomeration and thereby destroy lubrication properties. In the second group of experiments, the influence of the concentration of SiO2/MoS2 hybrid nanoparticles on lubrication properties was discussed by changing the mass fraction of the SiO2/MoS2 hybrid nanoparticles in the base oil. The nanofluids were prepared as 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, and 2.00 % by weight of SiO2/MoS2 hybrid nanoparticles into the base oil. The optimal concentration was determined. In the third group of experiments, the influence of normal load and sliding speed on the lubrication properties of the fluids was performed with a normal load varying from 1 to 10 N (corresponding maximum Hertzian contact stress from 223 to 480 MPa) and a sliding velocity ranging from 10 to 80 mm/s for 30 min. The friction coefficient was recorded by the tribometer in real time. Three sets of tests at the same normal pressure and sliding velocity were conducted to obtain each datum point to verify the repeatability and accuracy of the test results. Using nanofluids, the wear of the AISI 52100 steel ball was not measurable and there was almost no variation in the roughness measurements. For this reason, the wear volume of AZ31 magnesium alloy plate was calculated to evaluate the lubricant effectiveness in wear reduction in the overall lubricating system.
The morphologies of the worn surfaces were determined by Zeiss AURIGA FESEM. The wear depth and wear track profiles after the friction tests were obtained by a noncontact 3D surface profiler (Olympus OLS4000), and the wear volume of the plates was calculated from the wear depth. The final values quoted for the wear volume of the specimen were averages of three tests results. The chemical compositions of the worn surfaces were characterized by a VG model Escalab 250 X-ray photoelectron spectroscopy (XPS) with Al-Kα radiation as the excitation source. And the binding energy of C1s at 284.6 eV was utilized as the reference. Prior to the analysis, the specimens were cleaned ultrasonically for 5 min with acetone, in order to eliminate the residual lubricant.
Results and Discussion
Effect of SiO2/MoS2 Mixing Ratio on Lubrication Performance
Figure 3 shows average wear volume and 3D optical microscopic images of the wear tracks lubricated with different samples. The variation of wear volume is similar to the change in friction coefficient in Fig. 2. In this respect, the SiO2/MoS2 (0.25:0.75) hybrid nanofluids also outperform the rest samples. In comparison with the base oil, the SiO2/MoS2 (0.25:0.75) hybrid nanofluids lead to a significant reduction of wear volume by 50.5 % (from 6.08 to 3.01 mm3 in mean value). In contrast, the pure MoS2 nanofluids and SiO2 nanofluids are less effective by offering 17 and 9.4 % wear volume reduction in contrast with the base oil (from 6.08 to 5.05 and 5.51 mm3 in mean value), respectively. Comparative results of friction coefficient and wear volume under different SiO2/MoS2 mass ratio further prove the lubrication advantages of the SiO2/MoS2 (0.25:0.75) hybrid nanofluids.
Effect of the Concentration of Hybrid Nanoparticles on the Lubrication performance
Effect of Normal Load and Sliding Speed on Lubrication Performance
Rubbing Surface Analyses
Lubrication Mechanism of the SiO2/MoS2 Hybrid Nanofluids
In contrast, the lubricity mechanism of spherical nano-SiO2 is quite different from that of nano-MoS2 platelets. The higher-magnification image of the worn surface in Fig. 9a and corresponding EDS reveal the presence of nano-SiO2 on the worn surface. Unlike nano-MoS2, nano-SiO2 possesses nano-scale size and excellent dispersion in the base lubricant. These characteristics allow the nano-SiO2 to easily enter the contact area, thereby resulting in faster running-in conditions (Fig. 2). The rolling effect was often proposed as a lubrication mechanism for spherical nanoparticles, although little research has been conducted on the direct visualization of the rolling behavior of the nano-SiO2 into the contact interfaces [33, 43]. In this way, nanoparticles decrease the shear stress and lead to the reduction in friction coefficient. Besides, even though the surface of the magnesium alloy plate was looking smooth, when we observed its micro meter and nanoscale image, the surface was complicated with ridges and valleys. When nanoparticles were added into the base oil, the filling of valleys of the contacting asperities will occur. This is also supported by the high magnification FESEM spectra (Fig. 9), which shows the distribution of nanoparticles on the worn surface. As shown in Fig. 9, it is clearly seen that nano-SiO2 fills up the grooves of the rubbing surfaces, while nano-MoS2 deposits on the flat of the worn surfaces. It demonstrates that the difference in the distribution of nanoparticles on the worn surface is substantial. This can be explained by the model proposed by Mosleh Mohsen . The authors suggested that the nanoparticles can fill up valleys and smooth the surface if the characteristic length l of flaky nanoparticles is smaller than the peak-to-valley roughness of the harder surface which is equal to 4 Ra, i.e., the condition of l < 4 Ra applies. With respect to spherical nanoparticles, the diameter d of the particle should satisfy d < 0.67 Ra. Comparatively, nano-SiO2 with an average size of about 30 nm acts as a third body material filling in the valley of surface with 0.08-μm surface roughness, increasing real contact area. As a result, the load can be distributed over a larger contact area, so that the effective contact pressure decreases and consequently the wear also reduce. However, nano-MoS2 platelets may be difficult to fill in the cavities because its length (more than 300 nm) is larger than 4 Ra in our study. In comparison with pure nano-SiO2 additive (Fig. 10b), the micro-cooperation for SiO2/MoS2 hybrid nanoparticles additive occurred in succession between the shearing-sliding of the nano-MoS2 platelets structure and the filling of nano-SiO2 particles during sliding process, resulting in excellent lubrication performances (Fig. 10c). However, the efficiency of the SiO2/MoS2 hybrid nanofluids was influenced by the mass ratio of SiO2/MoS2. For instance, the lubrication performances of SiO2/MoS2 (0.5:0.5) and SiO2/MoS2 (0.75:0.25) were a little inferior to that of the SiO2/MoS2 (0.25:0.75) at the test condition. Namely, there was an optimum dose of nano-SiO2 in the SiO2/MoS2 hybrid nanofluids. If excess nano-SiO2 was added into the SiO2/MoS2 hybrid nanofluids, the contact area was saturated with nano-SiO2. Any more nano-SiO2 took up the space of nano-MoS2 and disturbed its lubrication. The nano-SiO2 has outstanding mechanical properties especially in terms of hardness (Vickers hardness—1000 kgf/mm2); therefore, the excess hard nano-SiO2 as an abrasive plows the soft magnesium alloy surfaces (Vickers hardness—66.7 kgf/mm2) under the applied load. It will facilitate the abrasion of the magnesium alloy surface during rubbing process and thus results in the worse anti-wear property. This was confirmed by the poor anti-wear behaviors of the samples with high concentrations of nano-SiO2, such as SiO2/MoS2 (0.5:0.5), SiO2/MoS2 (0.75:0.25) and pure nano-SiO2 as shown in Fig. 3a. Therefore, the nano-SiO2 and nano-MoS2 mass ratio of 0.25:0.75 is the best proportion for synergetic lubrication effect under the selected testing conditions. In addition, from the XPS results of the worn magnesium alloy surface, nano-SiO2 is shown to prevent nano-MoS2 from oxidation, resulting in increased oxidation resistance of nano-MoS2 during rubbing process. The oxidation phenomenon may provide an important role in the sense. It could provide a “soft” oxide-like MoO3 which could serve to tether the nanoparticles to the magnesium alloy surface. However, if nano-MoS2 is overmuch oxidized into MoO3, the oxidation destroys the lubrication structure of layered MoS2 and thus results in the worse lubrication property . Therefore, the synergistic lubricating effect enables the SiO2/MoS2 hybrid nanofluids to integrate the advantages and eradicate the disadvantages of SiO2 and MoS2 nanoparticles at optimal ratio. So the SiO2/MoS2 (0.25:0.75) hybrid nanoparticles exhibit superior lubricating performances than individual nano-SiO2 or nano-MoS2 even in high load and diverse velocity cases. Previous work on the combination of MoS2 with other nanoparticles as lubricant additives has reported improvements in their lubricating performance, such as nano-TiO2 , carbon nanotubes (CNT) , and graphene . Since the testing method and analysis are completely different from the present study, comparable comparison between the results cannot be made. Even so, nano-SiO2 is less costly and facile preparation in contrast with the aforementioned nanoparticles. In conclusion, the SiO2/MoS2 hybrid nanoparticles are suggested as economic and environment friendly lubricant additives for the applications in forming process of magnesium alloy.
Compared with pure nanofluids, the SiO2/MoS2 hybrid nanofluids achieve lower friction coefficient and wear volume even in high load and diverse velocity cases for magnesium alloy/steel contacts under the test conditions.
The optimal SiO2/MoS2 mixing ratio and nanofluids concentration are 0.25:0.75 and 1.00–1.25 wt%, respectively. The 1.00 wt% SiO2/MoS2 (0.25:0.75) hybrid nanoparticles addition into the base oil shows reduction of friction coefficient by 46.6 % and reduction of wear volume by 50.5 % in contrast with the base oil.
The excellent lubrication properties of the SiO2/MoS2 hybrid nanoparticles are ascribed to the physical synergistic lubricating actions of nano-SiO2 and nano-MoS2 during the rubbing process. The synergistic lubricating effect enables the SiO2/MoS2 hybrid nanofluids to integrate the advantages and eliminate the disadvantages of SiO2 and MoS2 nanoparticles at optimal ratio.
The authors are grateful for the financial supports from Chongqing Science and Technology Commission (CSTC2013jcyjC60001, CSTC2014jcyjjq0041), National Natural Science Foundation of China (51531002, 51171212,51474043), and The National Science and Technology Program of China (2013DFA71070, 2013CB632200), and Education Commission of Chongqing Municipality (KJZH14101).
The authors declare that they have no competing interests.
HMX and BJ carried out the design and drafted the manuscript. BL and QHW prepared samples and carried out tribo-tests. JYX and FSP commented on the results and revised the manuscript. All authors read and approved the final version of the manuscript.
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