Rheological and tribological behaviour of nanofluids: an experimental evaluation

Commercially available synthetic engine oils of SAE grades 5W40 were taken as base fluids for preparing nano fluids by blending 0.1% and 0.2% concentration by weight of the functionalized Cu and MoS2 nanoparticles in them. The base fluids and their blends were characterized using standard ASTM and IS testing methods for their physicochemical and tribo performance behaviours. The rheological studies were performed on a Rheometer while the tribological investigations were performed on tribo-tester. The study reveal that a small improvement in anti- friction behaviour of about 6% and 4% reported for the 0.2 wt% of Cu and MoS2 nanoparticles respectively in the tested lubricants indicate that finished products have little scope to improve anti-friction properties under the influence of already present additives, however anti-wear properties showed significant enhancement up to 20% for 0.2% MoS2 and up to 15% for 0.2% nano-Cu indicating better anti-wear properties of MoS2 and Cu nanoparticle blends. Rheological studies reveal that blending of nanoparticles has marginal influence on the rheological behaviour of the lubricants. The lubricants show shear thinning behaviour at low shear rates. However at higher shear rates, lubricants behave as Newtonian fluids.


Introduction
Rheological properties of lubricants largely determine the flow behaviour of lubricants that include shear stress, shear rate, yield stress, viscosity, compliance behaviour etc. Rheological properties in turn influence tribo-performance behaviour of the lubricants and ultimately influencing the machine life.Rheological parameters can be influenced by dispersing nano-particles in them.Modern Chemistry lays emphasis on the synthesis of nano additives that help enhance the flow behaviour of the lubricating oils and hence influencing tribological properties.Nanofluids have drawn the attention of researchers since 1995 when they were first time formulated [1].Nanoparticles of size 1 to 100 nm are blended with the base fluids to form nanofluids. Scientific community has paid due attention towards nanofluids due to their improved thermal properties and it is obvious from the comprehensive reviews [2,3,4,5].Mariano et al. reported enhancement of upto 27% in thermal conductivity of Co 3 O 4 -EG nanofluids [6].Heat transfer properties besides depending on thermal conductivity of nanofluids, do also depend on the flow behaviour which is determined by rheological studies as the rapidity of the fluid motion helps dispose of the heat from the production site and thus saving the machine component [7].Rheology of lubricants focuses on the study of flow and deformation behaviour of lubricants with high molecular weight in general as the flow behaviour of low molecular weight liquids are largely Newtonian [8].Detailed review of Newtonian and non-Newtonian flow behaviour was provided by H.A. Barnes using the concept of yield stress citing various equations on yield stress and viscosity models [9].Rheological study of nano-scale aluminium suspensions was carried out and reported that the paraffin oil / Al suspension exhibited non-Newtonian properties with respect to concentration of nano-particles, whereas the hydroxyl terminated polybutadiene / Aluminium suspensions exhibited linear shear stress shear rate curves up to a concentration of 50 vol.% [10].Viscoelastic study of perfluoropolyether (PFPEs) lubricants revealed that at high shear rate region they exhibited Newtonian flow behaviour that decreased which took a non-Newtonian bend with increasing temperatures [11].Similar other studies on various combinations of nanoparticles -fluid blends exhibited different sets of behaviour at different conditions that depended on size of the nano particles/ concentrations or other physical parameters.In one such study CuO-DI nanofluid with 29nm size CuO nanoparticles exhibited non Newtonian flow behaviour [12].Water-Al 2 O 3 and water-CNT nanofluids at low concentrations were reported to exhibit hysteresis behaviour with gradual loading and unloading of the stress [13].A comprehensive review on viscosity of nanofluids was produced citing various viscosity models developed so far and how they correlated with experimental observations [14].PEG 200-TiO 2 nanofluid exhibited shear thinning behaviour with particle mass fraction 1% for a temperature variation from -10 °C to 40 °C [15].Heat transfer oil blended with CuO, Al 2 O 3 and TiO 2 exhibited shear thinning behaviour as against the base fluids that exhibited shear thickening behaviour [16].ZnO and ZrO 2 nanoparticles blended with PAO at 0.5, 1.0 and 2.0 wt% concentrations were investigated with respect to shear rates from 10 6 to 10 7 s -1 and temperatures from 40 to 100 °C respectively and reported to exhibit shear thinning behaviour [17].Engine oil-TiO 2 blends at concentrations 0.05 to 2.5 wt% were prepared and their viscosity measured at different temperatures and concentrations.Empirical models of viscosity were then developed using the obtained data and compared with other existing models [18].Rheological behaviour of the lubricants influences their tribological behaviour.Additive Chemistry largely influences the tribo-performance behaviour of the lubricant as it affects the film forming capability and hence affecting the anti-wear and anti-friction properties.Nanoparticles as additives influence the tribo-performance behaviour by varying proportions.Cu nanoparticles as oil additives were reported to have better anti-wear properties than zinc dialkyl dithiophosphate (ZDDP) [19].Cu nanoparticles blended with motor oil were reported to change the topology of the worn surfaces and reduced friction effectively at higher loads [20].Surface modification of Cu nanoparticles by DTC8, tetradecyl hydroxamic acid and oleic acid have been reported to improve tribo-performance of lubricants as an additive [21,22,23] TiO 2 , CuO, SiO 2 nanoparticles as additives have also been studied and found to possess significant anti-wear and anti-friction properties [24,25,26].ZnO, Al 2 O 3 and grapheme oxide as oil additives have also been studied and reported to have significant results [27,28].Blends of single-walled carbon nano-horns (SWCNHs) in engine oil were reported to reduce friction at all tested concentrations and temperatures [29].A review work throws sufficient light on the lubrication capabilities of various nanoparticles like Cu, Ni, Al, Pb, CuO, ZnO, TiO 2 , MoS 2 , WS 2 , diamond, graphite, CNTs, graphene, BN, Al 2 O 3 , SiO 2 , composites Al 2 O 3 / SiO 2 , ZrO 2 / SiO 2 and polymer PTFE [30].Viscosity and friction factor of Al 2 O 3 nanoparticle dispersed in water at 6 vol.% exhibited Newtonian behaviour and a correlation between the viscosity and friction factor established [31].Platelet MoS 2 nanoparticles of average size 50 nm blended with lubricants were studied and reported to enhance tribo-performance behaviour and stability significantly [32].From the literature survey it is evident than sufficient amount of studies were undertaken to investigate rheological, thermal and tribological studies of nanofluids.It is observed from the survey that the combined studies of rheological and tribological of nanofluids are scarce and there is no reliable dependency between the two parameters yet.Authors decided to carry out rheological and tribological investigation of synthetic commercial engine oils by blending them with Cu and MoS 2 nanoparticles.Nanoparticles chosen have shown promising results as oil additives for enhancing tribo-performances.

Cu nanofluid
The Copper Nano-particles (Avg.Size: 45 nm) were purchased from from M/s. Reinste Nano ventures India Pvt. Ltd.The surface morphology showed their spherical shape and elemental analysis done by EDX confirmed the presence of nanoparticles.The oleic acid was used to functionalize copper nanoparticles to reduce the settling rate and enhance the dispersion stability of nano-fluids [23].

MoS 2 nanofluid
The MoS 2 nanoparticles were synthesized hydrothermally by chemical treatment of ammonium heptamolybdate, citric acid and sodium sulphide.The TEM images along with elemental maps of synthesized MoS 2 nanoparticles show lamellar structure with 0.5636 nm separation between successive layers.The elemental maps show uniform distribution of Mo and S elements in the synthesized MoS 2 nanoparticles [33].
Standard testing procedures were adopted to measure physicochemical parameters such as density, viscosity, viscosity index, sulphated ash, TAN, TBN.Trace metal analysis for Zn, Mo and in lubricant samples was done by Inductively coupled Plasma Atomic Emission Spectrometer (ICP-AES); model: PS 3000 UV (DRE), Leeman Labs Inc. (USA).These metals form important constituents of EP additives.The tribo-performance investigations of Cu and MoS 2 nanofluids were investigated by using 4 Ball Tribo-tester (FBT) under ASTM D 4172 B standard test procedure [23,33].At the end of the test the contact wear is measured with the help of apochromotic microscope and reported as wear scar diameter in milimeter (mm).The worn out surface of used test specimens were analyzed by SEM and EDX analysis.The Rheological behaviour of nanofluids has been investigated using Anton Paar Austria make RHEOPLUS/32 MCR 301.The experiments have been performed using concentric cylinder geometry DG 26.7 [33].Different sets of experiments were performed to determine the variation of coefficient of viscosity and shear stress with shear rate for temperatures ranging from 30-50 °C.Similar tests were performed under viscoelastic environment by varying the shear rate from 1-20 s -1 .

Result and discussion
The physicochemical properties of prepared nano-fluids are tabulated in table1.Marginal changes in the characteristic parameters of nanofluids reported as compared to base fluids.It may be attributed to the instrumental error or to the blending of nano-particles in to the lubricants.Thus, the nano-fluids can be used for similar application as pre-defined for base lubricants.

Friction and wear behaviour
The friction and wear behaviour of the nano-fluids generally depicts their tribo-performance.Figure 2 shows the variation in kinematic friction in terms of coefficient of friction (COF) over the entire experimental duration for the nanofluids formed using lubricants CE and SH.A higher COF due to static friction is observed in the beginning of the experiment which stabilizes over the time due to the formation of lubricant film.It is also observed that the COF decreases with increase in concentrations of the nanoparticles.Of the two base fluids selected, the SH base oil reported better anti-friction behaviour with the friction coefficient of 0.88 as compared to that of 0.90 for CE.Better performance of SH may be due to the higher dosage of Zn and P in the base fluid.When blended with nanoparticles the nanofluids of SH reported better anti-friction behaviour.Presence of nanoparticles augmented the performance of EP additives present in the base oil.The wear behaviour of nanofluids has been presented in terms of wear scar diameter (WSD), the lubricants with better anti-wear properties report smaller WSD and vice versa.Figure 4 shows the comparative assessment of the anti-wear behaviour of nanofluids.It is observed that blending of nanoparticles enhances the anti-wear property of the base fluids.It is further observed that the base fluids CE and SH have a wear scar diameter of 0.391mm and 0.446mm respectively, thus stating that CE has superior anti-wear properties over SH.As a result of this the nanofluids prepared using CE too has reported better anti-wear properties.The lowest values of wear scar diameter observed are 0.378mm and 0.336mm for the nanofluids of 0.2% Cu+CE and 0.2% MoS 2 +CE respectively.While for nanofluids of SH reported the values of 0.380mm and 0.357mm respectively for the nanofluids of (a) (b) 0.2% Cu+SH and 0.2%MoS 2 +SH respectively.When compared between the nanofluids of Cu and MoS 2 irrespective of the base fluid selected, the MoS 2 nanofluids reported superior anti-wear behaviour over the Cu nanofluids.This may be due to the fact that MoS 2 being lamellar solid forms thin sheets/layers that protects the surface from wear damage.On the contrary the spherical Cu nano particles are not be able to form sheets similar to MoS 2 as a result of which the surfaces are prone to wear damage.Thus, it is clear that, MoS 2 is a better anti-wear additive while Cu is better anti-friction additive.The percentage change in COF and WSD of the nanofluids with respect to the base fluids is given in Table 3. Marginal difference in the COF to the tune of 5.53% and 4.20% respectively for the 0.2% Cu and MoS 2 nanofluids of CE are visible in the table.Similarly, these values are of the order of 4.77% and 3.17% for the 0.2% Cu and MoS 2 nanofluids respectively of SH.Due to the existing additives present in the finished products, the anti-friction behaviour of nanoparticles are not that pronounced in the nanofluids.However, in case of anti-wear behaviour, substantial difference to the order of 3.28% and 14.06% is observed for the 0.2% Cu and MoS 2 nanofluids respectively of CE.While, 14.74% and 19.96% change in anti-wear behaviour has been observed for the 0.2% Cu and MoS 2 nanofluids respectively of SH. Figure 5 shows the wear scar diameter of the used test specimen balls along with the EDX analysis.Presence of Cu and MoS 2 nano-particles in varying concentrations has resulted in reduction in wear scar diameter for both the lubricants.However, it is also observed that for a given concentration of nanoparticles, MoS 2 nanoparticles result into more significant wear scar diameter reduction as compared to that of the Cu nanoparticles.The Cu nanoparticles being spherical in shape act as individual rolling elements in the contact vicinity thereby enhancing the anti-friction, anti-wear and load carrying capacity at higher loads and sliding speeds [19,20,21].On the contrary, the MoS 2 nanoparticles form lamellar sheets that can shear easily and also protect the surfaces from wear damage.As a result of this, the MoS 2 has shown superior behaviour over the Cu nanoparticles in terms of wear scar reduction.Figure 7 shows the viscosity shear rate variation at 30°C for base and nanofluids.It is reported that the viscosity of the lubricant reduces marginally with the nanoparticles addition.Of the two nanoparticles selected it is observed that MoS 2 nanofluids of both CE and SH have relatively lower viscosity as compared to the Cu nanofluids.This marginal difference in viscosity can be attributed to the lamellar  Figure 8 (a-c) shows the variation of lubricant viscosity with shear rate at different temperatures.It is observed that the lubricant viscosity decreases with increase in temperature irrespective of the lubricant and the nanoparticle selected.Further, it is also observed that the viscosity of lubricants decreases with increase in shear rate.However, this effect is more pronounced at lower temperatures.At higher temperatures the lubricant viscosity is almost independent of shear rates.This effect is also seen for the nanofluids of both MoS 2 and Cu.Thus, it can be ascertained that the temperature is more significant parameter over shear rate in influencing the lubricant viscosity.The shear stress/shear rate plots depicting the flow behaviour of base fluids and the nanofluids is shown in Figure 9 (a, b).The plots also represent the influence of temperature on the shear stress/shear rate behaviour.It is observed from the plots that at lower shear rates the lubricants exhibit shear thinning behaviour.This behaviour is observed for the shear rates of 10s -1 .However, beyond this the flow behaviour is almost Newtonian represented by a straight line.Blending of nanoparticles very marginally influence the flow behaviour as the curves marginally deviate from the base fluids with increase in the concentration of the nanoparticles.This phenomenon is irrespective to the base fluid and the nanoparticles selected.The temperature however has a pronounced effect, with shear stress decreasing with increase in temperature.A large deviation in shear stress/shear rate plot is observed around the shear rate 10s -1 .

Conclusions
In the present study Cu and MoS2 nanoparticles were blended at 0.1% and 0.2% (wt%) in commercially available synthetic engine oils of SAE grade 5W-40 post functionalization.Experimental investigation resulted into drawing following important conclusions:  Dispersion of nanoparticles in synthetic engine oils was stable for over 28 days. Physicochemical properties of synthetic engine oils and their corresponding nanofluids were similar.It means that small fraction of nanoparticles in the base oils do not significantly alter the characteristic behaviour. Marginal enhancement to the tune of 6% and 5% is being reported in the anti-fricition behaviour.It shows that additive package in the synthetic engine oils possesses good antifriction properties so there is a small scope its enhancement after dispersing nanoparticles. Anti-wear behaviour of lubricants reported up to 15% enhancement for Cu and 20% enhancement for MoS 2 nanofluids.The blemding of Cu and MoS 2 nano-particles aid in formation of thin lubricating films on the functional surfaces thereby resulting into reduced wear scar diameter. Blending of nanoparticles fluids marginally changes their rheological behaviour.With concentrations having very minimal role however, temperature has a very significant role in affecting the rheological behaviour of nanofluids.


The base fluids and the nanofluids show shear thinning behaviour at lower shear rates and with increase in shear rates the behaviour changes to Newtonian.


At the critical shear rate of 10 s -1 the flow curves show troughs due to the transition from shear thinning to Newtonian behaviour.

2 . 1
Nanofluid preparation Two synthetic commercial engine oils of different make but similar grade SAE 5W-40 have been selected and coded as CE and SH for the present study.The nanofluids of Cu and MoS 2 have been prepared by blending 0.2% wt%. of functionalized Cu and MoS 2 nanoparticles in the CE and SH base fluids.The functionalized nanoparticles have been used to ascertain stable dispersion.The functionalized nanoparticles were blended through mechanical stirring followed by sonication to obtain the required nanofluids.The dispersion stability of as prepared nanofluids was assessed over a period of 28 days.The digital images of base fluids (CE & SH) and prepared nanofluids by using virgin and functionalized nanopartilces are shown in figure 1.It is obvious from the images that the use of functionalized nanoparticles results into long term dispersibility.

Figure 1 .
Figure 1.Lubricant blends of Cu and MoS 2 nanoparticles with CE and SH base oils.

Figure 3
shows the comparative chart of friction behaviour of nanofluids.The lowest values of friction coefficient observed for 0.2% concentration of nanoparticles are 0.085 and 0.084 for the lubricants CE and SH respectively blended with Cu nanoparticles while and corresponding values for MoS 2 blended CE and SH are 0.086 and 0.085 respectively.It is clearly observed that the Cu nanofluids reported lower friction when compared with that of MoS 2 nanofluids.The spherical Cu nanoparticles result in rolling contacts while the lamellar MoS 2 nano-sheets cause sliding contact.Thus, MoS 2 nanofluids report marginally higher friction as compared to Cu nanofluids.It is obvious from the figure that Cu is a better anti-friction additive while MoS 2 is better anti-wear additive on carrying out comparison of the two.

Figure 4 .
Figure 4. Comparative assessment of anti-wear behaviour of nanofluids.

Figure 6
Figure 6 depicts the viscosity temperature variation for the pure CE and SH lubricants and the nanofluids of 0.2% Cu and 0.2% MoS 2 in these lubricants.It is observed that for both the CE and SH lubricants there are negligible change in viscosity occurred with the blending of 0.2 wt % of nanoparticles.Minor deviations in viscosity curves is observed at low temperatures with Cu nanofluid showing slightly higher viscosity as compared to the MoS 2 nanofluid of both the CE and SH lubricants.
2 over the spherical shape of the Cu nanoparticles.The lamellar structure of MoS 2 offers least inter layer shearing resistance thus, resulting in lower viscosity.Viscosity of the lubricants reduces gradually with the rise in shear rate for lubricants and their nanoblends.Reduction in resistance with shear rate by the lubricant layers diminishes thus resulting in the reduction in viscosity.

Figure 7 .
Figure 7. Effect of shear rate on viscosity variation for (a) CE based lubricants and (b) SH based lubricants.

Figure 9 .
Figure 9. Shear stress/shear rate curves for lubricant samples of (a) SH and (b) CE.

Table 1 .
Physicochemical properties of nano-fluids.The concentration of trace metal in base lubricants are tabulated in table 2. It is observed that the concentration of Zn and P elements are approx.900 mg/l or 850 mg/l respectively while the concentration of Mo is almost negligible.The presence of these elements helps in enhancing the tribological performance of lubricants.