Enhanced Lubricity of SnO2

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Enhanced lubricity of SnO2 nanoparticles dispersed polyolester nanofluid Venkataramana Bonu, Niranjan Kumar, Arindam Das, Sitaram Dash, and Ashok Kumar Tyagi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03506 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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Industrial & Engineering Chemistry Research

Enhanced lubricity of SnO2 nanoparticles dispersed polyolester nanofluid Venkataramana Bonu, Niranjan Kumar,* Arindam Das,* Sitaram Dash, Ashok Kumar Tyagi

Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India.

Corresponding Author *Corresponding author. Tel./fax: +91 4427480081. E-mail address: [email protected] (N. Kumar). [email protected] (A. Das)

ACS Paragon Plus Environment

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Abstract Nanofluid lubrication is a novel approach for enhancing energy efficiency of the sliding interfaces which is useful for reducing friction and wear of the machine elements. The SnO2 nanoparticles (NPs) of 25 nm size and concentration 0.03 mg mL–1 dispersed in polyolester (POE) oil is found to exhibit significant reduction in friction coefficient and wear up to 38 and 42%, respectively, in comparison to neat POE oil. It is also found that the lubrication efficiency depends on size of the NPs, dispersion stability, and concentration. Fourier transform infrared red spectroscopy confirmed that the chemical stability of the POE was preserved after the tribology test and there was no product due to oxidation reaction. Formation of low shear strength tribofilm containing organic compounds and SnO2 nanoparticles was key factor in reduction of the friction and protection against wear and deformation. Keywords: Nanofluids, Interface, Nanofluid friction, Lubrication.


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1. Introduction Controlled

size, shape, and composition of metal-oxides nanoparticles (NPs) play an important

role in a wide range of advanced applications including heterogeneous catalysis, microelectronic and photo-electronics devices.1–4 Moreover, study of inorganic micrometer-scaled particles dispersed in oil have become a demanding research subject in the field of tribology for improved energy efficiency and prolonging the lifetime of the sliding components.5–7 Such materials are directly used in the engine oil additives. Considering environmental impact on the automobile industry, it has become imperative to improve the fuel efficiency by reducing the frictional resistance of the components. Therefore, the study of friction reducing additives and effective lubrication has become one of most important subject in nanotechnology, colloidal and interface where interdisciplinary approach is highly appreciated for tailoring the energy efficient materials. Designing of such nano-structured materials and nano-additives with desired morphologies for energy conservation in tribology application is gaining importance, because one-third of the total mechanical energy becomes wasted in friction.8,9 The energy efficiency of mechanical devices with moving components like bearings or gears is mitigated through friction. Additionally, friction causes wear, eventually leading to mechanical failure of moving parts. In this aspect, use of inorganic small particles as lubricating additives are useful which is reported to enhance the lubrication mechanism via formation of tribofilm on the sliding surfaces and/or providing a novel ball- bearing effect.8 This aspect minimizes frictional energy and degradation of the base oil, therefore, increases the load-carrying capacity.5–7 Enhanced tribological performance in micrometer-scaled calcite calcium carbonate NPs additives was attributed to the improved toughness of the protective film, due to the reinforcing effect of the bimodal grain size distribution.10 Whereas high dispersibility of In–Sn alloys NPs exhibited better anti wear


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properties than that of monometallic indium or tin NPs in oil medium.6 It was predicted that NPs rolled between the sliding bodies and, therefore for the ball bearing effect, the spherical particles with high hardness is a primary requisite for converting the sliding effect into rolling mechanism effectively.11 In this respect, Sn and In–Sn alloy NPs are constituted as soft materials that undergo plastic deformation under the localized stress. Novel example related to spherical shape of sub-micrometer sized particles such as ZnO, TiO2, CuO 9 and Fe3O4

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dispersed in lubricant

reduced the friction and wear significantly via the rolling mechanism. Besides favorable tribological properties of above mentioned micrometer/sub-micrometer particles, quantification of friction and wear behavior are unknown for several cheaply produced covalently bonded NPs which provide large surface to volume ratio. Effective utility of metal-oxide nano-sized particles are yet to be understood for lubrication characteristic. Although, synthesis and other aspects of SnO2 NPs have been well documented 12, the tribology of oil based SnO2 nano-dispersoids is yet to be investigated. It is noteworthy to mention that in above work the laser was used to re-shape the micron and sub micron particles in spherical shape for friction and wear reducing additives. The reported reduction in friction by Hu et al 9 is around 50%. Song et al

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reported 40% and

20% reduction in the friction and wear, respectively. Here, friction and wear behavior of SnO2 NPs dispersed in POE is investigated. The effect of NPs size, dispersion time, and concentration of the NPs on the tribological properties of SnO2 NPs is also investigated. Micro-Raman technique and energy-dispersive X-ray spectroscopy (EDS) was used to characterize the tribofilm formation in the wear track. Wear and friction of steel contacting bodies in the presence of nanofluid is explained by the formation of a tribofilm. After the tribology test, a degradational property of nanofluid is also characterized.


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2. Experimental 2.1. Materials synthesis and charcterizations SnO2 NPs were synthesized by a soft chemical method. Ammonia solution (NH4OH, Merck) of 0.05 mol/L was added dropwisely to the mixture of stannic chloride (SnCl4, Spectro Chemicals) and distilled water (MilliQ, 18 MOhm cm) under a continuous magnetic stirring. The resulting white colored gel was kept in a hot bath at a temperature of 80 oC. Further, this gel was washed with distilled water several times to remove Cl- ions. Washed gel was dried at a temperature of 100 oC. As-prepared NPs were annealed in air for 1 hour at 300, 500 and 800 oC to obtain different sizes of NPs. Morphology of SnO2 particles, wear tracks and wear morphology separated from lubricant were analyzed by a field emission scanning electron microscope (FESEM, carl zeiss supra 55). High resolution transmission electron microscopy (HRTEM, Libra 200 Zeiss) was employed for the structural characterization of SnO2 NPs. Surface chemical analysis of SnO2 NPs was investigated by the X-ray photo electron spectroscopy (XPS; M/s SPECS GmbH, Germany). Micro-Raman spectroscopy (InVia, Reinshaw) was carried out for chracterizing SnO2 NPs and wear tracks using 514.5 nm excitation of an Ar+ laser with 1800 gr/mm grating, and thermo electric cooled CCD detector in a back scattering mode. EDS technique was used for chemical mapping of wear track. Optical absorption properties of the NPs dispersed in POE were studied with the aid of UV-Vis spectroscopy (Avantes). Dynamic light scattering (DLS) measurement of SnO2 NPs dispersed in POE were carried out by Malvern UK (4700) equipped with multi tau correlator and He-Ne laser (633 nm). DLS data was collected for 15 minutes for each sample. Fourier transform infrared spectroscopy (FTIR, Bruker MB-3000) was carried out to probe infrared active vibrational modes of SnO2 NPs dispersed in POE before and after the triblogy test.


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2.2. Tribo-characterization of SnO2 NPs-POE system Synthesized SnO2 NPs were dispersed in POE oil at the concentration of 0.5 mg/ml. It was further diluted with POE-oil for low concentration samples. Friction behavior of SnO2 NPs dispersed in POE was measured by a ball-on-disc micro- tribometer (CSM Instrument, Switzerland) operating in a linear reciprocating mode. The POE is synthetic oil which is widely used as lubricant in various industrial applications. One drop of NPs dispersed POE was applied between the steel-steel contact interfaces ensuring the presence of lubricant between the sliding interfaces. Size, dispersion time and concentrations of SnO2 NPs dispersed in POE were also varied for the detailed tribo-test investigation. For understanding the role of dispersion of NPs, the 30 kHz ultrasonicator was used for different time scale. Dispersion was carried out keeping the sample (NPs-POE) in eppendorf tube in water poured sonicator bath. The calibrated K-type thermocouple was directly connected through the tube for the measurement of NPs-POE temperature during the sonication. The water was changed for every 15 minutes to avoid any overheating. 100Cr6 steel ball of diameter of 6 mm was used as sliding member against the lubricated 316LN steel disc. Chemical composition and physical properties of the 100Cr6 steel ball and 316LN steel disc is given below in the table 1 and 2, respectively.

Table 1. Chemical composition of 100Cr6 steel ball counterbody and 316 LN disc used for friction measurement: C%

Si%

N%

Ni%

Mn%

P%

S%

Cr%

Mo%

Al%

Cu%

Fe%

100Cr6

0.98

0.2

----

----

0.32

0.025

0.015

1.43

0.1

0.05

0.3

(Balance)

316LN

0.03

1.0

0.2

12.6

2.0

0.045

0.03

17.2

2.8

----

----

(Balance)


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Table 1. Mechanical properties of 100Cr6 steel ball counterbody and 316 LN disc used for friction measurement: Tensile strength (GPa)

Yield strength (GPa)

Modulus of elasticity

Poisson's

(GPa)

ratio

100Cr6

0.76

0.45

200

0.3

316LN

0.51

0.2

210

0.28

Surface roughness of the steel disc and ball were ~18 nm and 28 nm, respectively. All the tests were conducted at 4 cm/s sliding speed with a 2 N load. The stroke length of wear track and sliding distance were maintained at 3 mm and 100 m, respectively. Surface roughness of the wear track was measured by Dektak 6M–stylus profiler fixing 5 mg contact load at scanning speed of 1.5 µm/s. A sharp diamond stylus tip with 12.5 µm radius of curvature was scanned across the wear track to generate 2D roughness profile. The stylus is mechanically coupled to the core of linear variable differential transformer (LVDT) sensor. The measurement was repeated several times. This technique was also utilized to measure the dimension of the wear track.

3. Results and discussion 3.1. Characterization of NPs Morphological and structural features of NPs are shown in Figure 1. High resolution TEM image of the as-prepared material clealry shows the average size of the NPs is 2.5 nm (Figure 1a) which is less than the excitaton bohr radius of 2.7 nm for SnO213 which conforms to quantum dots (QDs). FESEM image of the SnO2 NPs annealed at 800 oC displays an average size of ~25 nm (Figure 1b). Zoomed HRTEM image of a single NP (inset of Figure 1a and 1b) shows the (110) crystalline plane belonging to the rutile tetragonal SnO2 phase (JCPDS #41-1445). Selective area electron diffraction (SAED) pattern of the as-prepared and the annealed NPs contain (110), (101)


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and (211) planes of the rutile tetragonal SnO2 phase (Figure 1). Ring like SAED pattern of the as-prepared NPs indicates the existence of planes in all orientations. For the particle size confirmation detailed structural characterizations of all the different size NPs can be found in our earlier work.13

Figure 1. HRTEM image of (a) 2.5 and (b) 25 nm SnO2 NPs along with SEM image. Corresponding SAED patterns of both the NPs are given

XPS study of the 2.5 and 25 nm NPs is carried out to investigate the surface chemistry. Figures 2a,&b and 2c,&d show Sn3d and O1s spectra of 2.5 and 25 nm NPs. The signature of SnO2 appears as a spin-orbit doublet at 487.3 eV (3d5/2) and 495.7 eV (3d3/2) with an area ratio of 1.5 implying the presence of Sn4+ chemical state in both the NPs.14 Atomic weight percentage ratios between ‘O’ and ‘Sn’ (O:Sn) are approximately 2 and 1.55 for the 2.5 and 25 nm NPs respectively. High amount of –OH groups on the surfaces of 2.5 nm NPs are observed as shown in Figure 2 which is found to be responsible for the large amount of ‘O’ in the 2.5 nm NPs.


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Relatively low ratio of 1.55 for the 25 nm NPs indicates the high oxygen deficiency due to substoichiometry of the NPs surface. This increased defects density is a manifestation of strong desorption of ‘O’s from the surface of NPs due to annealing of the as-prepared QDs at a high temperatures of 800 oC.

Figure 2. XPS spectra of NPs (a) Sn 3d (2.5 nm) (b) O 1s of SnO2 (2.5 nm) and (c) Sn 3d (25 nm) (d) O 1s of SnO2 (25 nm)

3.2. Tribology of SnO2 NPs in-POE Three stages systematic optimizations of the tribological experimental protocol are followed to achieve low friction and wear of SnO2- POE lubricated steel-steel sliding system. The systematic optimizations are size of the particles, dispersion time and concentration of SnO2 NPs dispersed in POE medium. In the first step, experiment is carried out with different sizes of NPs (Figure SI-1). In each experiment, 0.03 mg mL–1 of SnO2 NPs is dispersed in POE after


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ultrasonication for 15 minutes. Lowest friction coefficient of average value 0.12 is obtained for the 25 nm SnO2 NPs. This value is found to increase in similar tribology condition when the particle size is further increased to 40 nm. However, the value consistently increases with decreasing particle size. From the results, 25 nm SnO2 NPs is selected for further study of the tribo- experiment to understand the influence of dispersion time by ultrasonication. Results show that friction coefficient decreases with increasing dispersion time and the average value becomes 0.057 at 90 minutes dispersion (Figure SI-2). However, at higher ultrasonication time of 120 minutes the friction coefficient increased to 0.08. In order to probe the dispersion, the absorption spectra of the NPs dispersed in POE was recorded with respect to ultrasonication time (Figure SI-3). It is clear from the Figure SI-3 that the absorption edge of SnO2 at lower wavelengths is increasing with ultrasonication time. This indicates improvement in the dispersion with increasing ultrasonication time. However the absorption spectrum recorded after 120 minutes of ultrasonication time found to have higher absorption at tail region that is near high wavelengths. In order to investigate the size distribution of agglomerated particles in POE, the dynamic light scattering measurement was carried out (Figure SI-4). Result shows decrease in mean size of agglomerated SnO2 particles in POE with increasing the ultrasonication time (Figure SI-4a). At 90 minutes ultrasonication time, the mean size of the particle is the lowest and the corresponding value is approximately 0.51 µm where smaller particles have size of approximately 100 to 150 nm as shown in size distribution curve (Figure SI-4b). The stability of the dispersion is strongly influenced by gravity as the particle size becomes large. Gravitational length lg and Peclet number Pe characterize the influence of gravity on a particle as defined below15,16,

lg=6kBT/πd3∆ρg,

(1)


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Pe=d/2lg,

(2)

where, kB is the Boltzmann constant, T is the temperature, d is the diameter of the particle (mean value 0.51 µm is considered in calculation), ∆ρ and g is density difference between solvent and SnO2, and acceleration due to gravity, respectively. Calculation showed lg value is 1.076×10–4 cm and Pe number 0.23. The later indicates a longer sedimentation time. Here, the role of temperature during the ultrasonication could be ignored due to marginal increase in temperature (Figure SI-5). In final stage, the concentration of SnO2 NPs is optimized. Lowest friction coefficient and wear rate are measured for 0.03 mg mL–1 of SnO2 NPs with a dispersion time of 90 minutes (Figure 3). Wear dimension was measured by contact profilometer as proposed by Torres Pérez et al 17 and wear volume V of the line shaped track on flat specimen was calculated by method proposed by Qu et al 18.

1 2  = V πh (3Ro − h)  3   2 h = R R 2 − d scar o o  4

(3)

Here, h represents the worn depth of wear track, Ro is the diameter of spherical ball, dscar is the wear scar diameter. Once the wear volume is known, the wear rate, k is easily obtained from the following equation: k= V/(F×S)

(4)

Here, F and S are normal load and sliding distance, respectively. Systematic trend of friction coefficient and wear rate with concentration is not observed but this value is found to increase usually for high concentration of SnO2 NPs dispersed in POE. Value of friction coefficient is found to be 0.14 in POE lubricated steel-steel sliding system and it decreases significantly to a value of 0.057 for SnO2- POE lubricated steel-steel system with 0.03


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mg mL–1 concentration (Figure 3c). At this concentration, wear rate decreases to 1.2×10–7 mm3/Nm (Figure 3b). Tribology test was repeated and friction values were found to be reproducible. Any deviation existed within error margin has been mentioned in Figure 3a (inset). It is worth to mention that friction coefficient of steel-steel contact in dry and unlubricated condition is high ~0.9 (inset (b)) in Figure SI-1. Wear of steel disc has clearly shown wider dimension, while sliding system works in neat POE lubricated condition (left side Figure 4). In this condition, steel wear track deforms largely with high penetrating depth. Calculated wear rate remains also high value of 3.5×10–5 mm3/Nm.


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(a)

(b)

Figure 3. Friction coefficients vs sliding distance (a) at various concentrations (a) 0.01 (b) 0.02 (c) 0.03 (d) 0.05 (e) 0.1 (f) 0.15 (g) 0.2 mg mL–1 of 25 nm SnO2 NPs dispersed in POE for 90 minutes ultrasonication time. Average value of friction coefficient with standard deviation is plotted in inset. Figure 3(b) shows wear rate against above concentration.

Magnified SEM images show rougher wear track with larger cavities and grooves (left side Figure 4). This wear is due to the collision of SnO2 NPs with the sliding metallic contact. Load is carried away by the hard and small NPs which can pin the soft steel contacts and lead to a deformation and damage.7 Directional ripples along the wear tracks are attributed to sliding


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direction as shown in red arrows. After the tribo-test, the color of the POE becomes very dark (right side Figure 4) and this is possible due to tribo-chemical reaction. This change of color is a direct evidence for the formation of metal oxide scale.19 More wetting characteristic of darker lubricant on steel test specimen is observed. A large amount of metal oxide debris formation in POE-steel- steel sliding system is also noticed (Figure SI-6). In contrast, color of the sample remains unchanged when the test is performed for the SnO2 NPs-POE lubricated steel-steel contact, indicating anti-oxidative nature of SnO2 NPs. In this condition, with low concentration of NPs, the amount of separated wear particles is negligible and practically it was impossible to observe even in FESEM image. This observation demonstrates utility of significantly low amount of NPs with large surfaces in preventing wear.


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Figure 4. Left side of the figure denotes wear of steel disc (a) only with POE steel-steel contact and (b) SnO2-POE steel-steel lubricated contact (particle size 25 nm, concentration 0.03 mg mL– 1

, dispersion time 90 minutes). Red arrows represent sliding direction of ball against the steel

disc. Right side of the figure represents penetration depth in steel disc (a) only with POE steelsteel contact and (b) SnO2-POE steel-steel lubricated contact condition (particle size 25 nm, concentration 0.03 mg mL–1, dispersion time 90 minutes). Photograph shows pure POE and change in color of POE with and without SnO2 NPs after the tribology test. In order to investigate the chemical composition of wear track, the EDS mapping of important representative elements was carried out in the wear track of 0.03 mg mL–1 concentration (Figure 5). This belongs to wear track of lowest friction coefficient and this is shown in Figure 3c. The evidence of Sn is observed in the wear track indicating participation of SnO2 nanoparticles as a


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tribofilm in lubrication process. Sn element is not present in virgin 100Cr6 steel ball and 316 LN steel as shown from the table 1 and 2.

Figure 5. EDS mapping of wear track formed at concentration (a) 0.03 mg mL–1 of 25 nm SnO2 NPs dispersed in POE for 90 minutes ultrasonication time. The wear track refers to Figure 3c.

3.3. Vibrational studies 3.3.1. Raman spectra Raman analysis of the wear tracks is performed to investigate the chemical changes under different sliding conditions. Samples of the metallic steel surface is Raman inactive and does not show any peak, confirming absence of metal oxide (spot in black circle) (Figure 6a). However,


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wear track formed in steel sample in dry and non-lubricated conditions shows strong peak at 673 cm–1 and a shoulder at 565 cm–1 (spot in red circle) related to the Raman mode of maghemite (Fe3O4) an inverse spinel with substantial oxygen disorder.20–22 This phase appears due to a tribochemical reaction between the metal and oxygen. The moist-humid condition presumably supplies oxygen and subsequent radicalization due to tribo-chemical reaction allows the formation of the oxide. Raman spectra inside of a wear track illustrate two distinct behaviors depending upon the locations in the POE-steel-steel lubricated system (Figure 6b). In the bright region (black circle), a broad feature around 610–850 cm–1 is due to the fluorescence from the organic compound of POE. In this location, weak spectral feature around 2860 and 2925 cm–1 are observed. However, strong features around 2860 and 2925 cm–1 observed in the region which is typically shown in the red circle correspond to symmetrical and asymmetrical stretching mode of –CH2 group of POE.23,24 In this spectrum, a strong peak at 711 cm–1 with relatively weak peaks at 575 cm–1 and 1330 cm–1 are also observed. These peaks are the signature of Raman modes of iron hydroxides (γ-FeOOH)

20,22

which can be formed due to a thermodynamically

activated reaction between maghemite (metal oxide) with hydroxyl functional group of POE. It is worth to mention that SnO2 is a good catalyst.12 In Figure 6 (c and d) two peaks around 1360 and 1608 cm–1 belong to stretching modes v(C=C) of polycyclic aromatic hydrocarbons.25 Additional peaks at 1120 and 1446 cm–1 belong to v(C–C) and v(C–O) stretching vibrations, respectively. However, weak spectra of Fe2O3 around 680–686 cm–1 are observed in wear track (Figure 6c and d). Peaks around 912, 1095–1100 cm–1 belong to stretching vibration of the v(C–C) cyclic alkanes26,27. Strong peaks of asymmetrical and symmetrical stretching modes of –CH2 components are accumulated in deeper location of these wear track indicating possible polymerization of organic compounds. This is evident from the development of peaks around


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1360 and 1608 cm–1 belonging to C=C vibrations which are strong while –CH2 component is enhanced in the wear track. This fact points to carburization of organic compounds like POE. In the region with a wave-number less than 1000 cm–1, the existing bands are associated with a distortion in the –CH2 groups (–CH2 rocking bands). However, those in the region with a wavenumber greater than 1000 cm–1 are associated with vibrations of C–O, C–C and –CH2 bonds as assigned above. From the Raman analysis of the wear track, it is clearly understood that carbon compounds and its carburization/polymerization process act as a protective layer against metal oxide component. Carburization/polymerization may occur due to direct channeling of mechanical energy into the chemical reaction or the involvement of electron/ions emitted during the wear process.28,29 These processes can be catalytically promoted due to availability of nascent surface and NPs.


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Figure 6. Raman spectra of (a) virgin steel surface and steel wear track in unlubricated contacting condition (b) POE steel-steel lubricated contact (c) 2.5 nm SnO2, dose 0.03 mg mL–1, dispersion time 90 minutes (d) 25 nm SnO2, dose 0.03 mg mL–1, dispersion time 90 minutes.

3.3.2. FTIR analysis FTIR measurements of SnO2 NPs dispersed in POE further decipher the change in functional group (Figure 7). A sharp stretching vibrational band of –C=O around 1744 cm−1 and 1650 cm−1 confirm ester linkages in the methyl group of the POE.30 In addition to the above bands, a sharp bands appearing at 1160 cm–1, 2924 cm−1 and 2854 cm−1 are indicative of –C–O (ester), –CH2 asymmetrical and symmetrical stretching, respectively. Peak at 1462 cm−1 belongs to –CH2 groups of alkyl chains that could not participate in cross-linking reactions.31 A band at 722 cm−1 is due to the presence of C–H vibrations.32 The C–O–C groups of POE around 1090 cm–1 is


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assigned to the trans-conformation, while 1120 cm–1 is related to the gauche-conformation relative to C–C bond.33 Peak at 1380 cm–1 and 723 cm–1 belongs to –(CH2)n–, HC=CH– (cis) and –CH3 (methyl) groups, respectively.34 Absorption line at 1237 cm–1and 3006 cm–1 is –CH epoxy, =C–H (cis) and –C–O (epoxy), –CH2–, respectively.30,31 No visible difference is observed in FTIR spectra of samples confirming absence of strong bonding and hence interaction of functional groups with SnO2 NPs dispersed in POE. This fact also confirms that after the ultrasonication, the POE does not degrade because increase in temperature was kept controlled (Figure SI-4). After the tribology test, FTIR spectra are obtained from the buried lubricant collected from the wear tracks (Figure 8). It has been noticed that the functional and chemical nature of these spectra remain unchanged in comparison to same samples before the tribology test. This result indicates retention of chemical stability of the lubricant and also confirms that SnO2 NPs does not react covalently with the lubricant. It means SnO2 NPs are not oxidized and hence tribo-charging mechanism is avoided. In a tribology contact, a chemical bond is produced between the surfaces to some extent, and charges are transferred.35 In this process, intense electric fields are generated which induces chemical decomposition reaction of the lubricant and material transfer. Therefore, reducing tribo-charging is a great challenge for a chemist for developing advanced oil based lubricants. From this point of view, SnO2 NPs helps preventing tribocharging in POE medium.


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Figure 7. FTIR spectra of (a) pure POE (b) SnO2 NPs size 2.5 nm, dose 0.03 mg mL–1, dispersion time 90 minutes (c) SnO2 NPs size 25 nm, dose 0.03 mg mL–1, dispersion time 90 minutes.

Figure 8. FTIR spectra of buried lubricant collected from the wear tracks of (a) pure POE steelsteel lubricated contact and (b) SnO2 NPs size 25 nm, dose 0.03 mg mL–1, dispersion time 90 minutes


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3.4. Lubrication mechanism of SnO2 NPs-POE Based on above results, tribology mechanism can be explained in the following way. In unlubricated tribo- sliding condition, the friction coefficient of steel-steel surfaces remains very high ~0.9 (inset (b) in Figure SI-1). High friction coefficient may lead to increase the contact temperature of sliding interfaces. This is attributed to the formation of metal oxide phases as seen in the Raman spectra (Figure 6a). Friction coefficients of POE lubricated steel-steel sliding bodies show high values (Figure SI-1f). Consequently, the damage of the wear track ultimately results in significant increase in dimension of the track. Raman study of the wear track shows formation of metal-oxide scales and degraded organic compounds (Figure 6b). For the NPs of sizes 2.4 and 4 nm, the friction coefficient is higher than the neat POE (Figure SI-1a). The NPs of QDs size range agglomerates much stronger than the bigger size NPs due to high surface energies. So, this agglomeration might be the reason behind the higher values of friction coefficient. There can be another reason that the magnitude of roughness of the sliding surfaces is higher than that of these NPs. These QDs do not therefore, directly interact with the sliding bodies, leading to an increased contact area that allows garnishing wear and damaging.36 This typical mechanism was proposed for spherically shaped carbon onions/microsphere.37,38 Similar mechanism is therefore responsible to control the chemical changes and forming the oxide scales as shown in Raman results (Figure 6c). Friction coefficient of comparatively small particles dispersed in POE shows higher value than the same from the pure POE (Figure SI-1). Here, NPs with strong surface charges, defects and hydroxyl groups play a crucial role for enhancing friction. Similarly XPS results indicate a low number density of polar hydroxyl groups on the 25 nm SnO2 NPs and on dispersion, it shows the lowest value of friction coefficient. Here, the size of these dispersed spherical NPs can overcome the lubricant tribofilm barrier height and


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subsequently acts as a ball bearing. It is mentioned above that surface roughness of the steel substrate and ball is approximately 18 nm and 28 nm, respectively. However, increase in surface roughness across the wear track on 316 LN is significantly high (Figure SI-7). This is related to large deformation due to high contact pressure. The magnitude of roughness of wear track becomes larger than agglomerated SnO2 particle size as it is shown in Figure SI-7. In this condition, particle may interact within contact and provide lubrication through rolling effect. This is shown schematically in Figure 9. The larger particles may deform under the contact pressure and form deposit of SnO2 protective particles. The presence of this particle is evident from the EDS analysis (Figure 5). Moreover, large size distribution of the particles is more favorable to pack the various magnitudes of roughness and surface cavities under the sliding interfaces. The interface cavities act as a reservoir to protect the SnO2 particles which in turn provide rolling and lubrication. Under such condition, formation of oxide scales becomes thermodynamically unfavorable due to the generation of less thermal energy in sliding contact. This fact is clearly revealed in the Raman results which shows that oxide scale is negligible and the organic compounds are predominantly available in the wear track (Figure 6d). More importantly, the organic compounds act as a protective layer against wear and providing low shear strength tribofilm which easily shears under the tribo-stress. This phenomenon causes a decrease in friction coefficient and wear up to 38% and 25%, respectively, as compared to neat POE-steel-steel contact. It is worth to mention that improvement in lubrication is composite effect of organic compound and SnO2 nanoparticle.


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Figure 9. Rough contact interface with the formation of organic compound and sandwiched nanoparticles

The optimized concentration of NPs in the lubricant medium is a crucial parameter for providing rolling motion.11,39 Optimized concentration of nanohybrids can form a stable suspension in a aqueous medium that easily transferred onto the contact zone of the rubbing steel surfaces and, thereby forming a protective and lubricious layer to reduce the friction coefficient and wear.40 At a concentration of 0.03 mg mL–1 SnO2 NPs, lowest friction coefficient of 0.057 is achieved. In case of sufficiently lower concentration (less than 0.03 mg mL–1) of SnO2 NPs, the friction coefficient is dominated presumably by POE. Moreover, sufficiently low amount of NPs do not generate enough interaction points between the sliding bodies and therefore, conversion of sliding into rolling motion is not effective which subsequently restrict the triboflim formation. The rolling and load bearing mechanism of particles become ineffective due to the sufficiently large inter-particle distance.36 On the other hand, at high concentrations (more than 0.03 mg mL– 1

), the reduced inter-particle distance intercepts rolling motion. However, the exact mechanism

of particles as lubricant additives to enhance the tribological properties is not yet clearly


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understood. It is generally proposed that inorganic particles present in lubricating oil act as additives, behave as spacers and form a lubrication film.9,11,36 ,41

4. Conclusions The present work provides the insight to understand the lubrication properties of truly nanometer-scaled SnO2 NPs and demonstrates a facile and green approach to develop hybrid lubricants for tribological applications. The study proposed a novel and cheap method for the preparation of spherical SnO2 NPs which are chemically and mechanically stable in POE lube. A minute concentration of 0.03 mg mL–1 of SnO2 NPs in POE lubricated the steel-steel contact significantly, reducing the friction coefficient up to 38% and enhanced the anti-wear behavior up to 42%. Such an improved lubrication property could be explained by (a) effective suspension of SnO2 NPs in POE (b) formation of low shear strength organic tribofilm that shears easily under the contact stress and (c) deposition of SnO2 nanoparticle in the wear track, providing rolling motion. More importantly, POE nanofluid does not oxidize after the tribology test, indicating chemical stability. This work, therefore, opens up avenues for the development of novel nano/ultrananometer particles for saving the mechanical energy of the sliding devices. Future work may include studies of temperature dependent lubrication properties of SnO2 NPs dispersed in POE lube for the evaluation of real application of the product.

AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel./fax: +91 4427480081. E-mail address: [email protected] (N. Kumar). [email protected] (A. Das)

Notes


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The authors declare that there is no competing financial interest

ACKNOWLEDGMENT We acknowledge Mr. Shailesh, Safety group, IGCAR for his help in performing FTIR measurements. Dr. R.G. Joshi (MSG/IGCAR) is acknowledged for DLS measurement and for valuable technical discussions. The authors are thankful to Dr. B.V.R. Tata (MSG/IGCAR) for useful technical discussions on light scattering analysis. The help of Ms. N. Sreevidya (MMG/IGCAR) is acknowledged for EDS analysis. Supporting Information: Friction coefficients vs sliding distance of SnO2 NPs, Friction coefficients vs sliding distance of 25 nm SnO2 NPs, Absorption spectra of NPs, DLS measurement, Temperature vs. ultrasonication time of SnO2 NPs, Large area FESEM images of wear debris, Evolution of roughness of wear track with sliding distance.


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Enhanced Lubricity of SnO2

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