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Effect of TiO2 nanoparticles on wettability and tribological performance of aqueous suspension
Wear 376-377 (2017) 786–791
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Effect of TiO2 nanoparticles on wettability and tribological performance of aqueous suspension Linghui Kong, Jianlin Sun n, Yueyue Bao, Yanan Meng School of Materials Science and Engineering, University of Science and Technology Beijing, 100083 Beijing, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 August 2016 Received in revised form 13 January 2017 Accepted 18 January 2017
Aqueous suspensions containing different concentration of nano-TiO2 (50 nm) are prepared, in which sodium polyacrylate (PAAS) as the dispersant is used. Their wettability and tribological properties are discussed with Dataphysics OCA50 contact angle measurement device and the MRS-10A four-ball tester, respectively. The rubbed surfaces after friction tests are also analyzed by scanning electron microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS). Wetting results show that the wettability of aqueous suspension is sharply deteriorated with the addition of PAAS, possibly caused by adsorption of PAAS on the substrate alters the characterization of the substrate, which promote the substrate change from homogeneity to heterogeneity, and eventually the wetting state transits from Wenzel to Cassie. And when the nano-TiO2 is further added to the aqueous suspension, the wettability is slightly better, which is mostly due to adsorbed sodium polyacrylate desorb from substrate surface and return to the body phase. In another aspect, four-ball experimental results reveal that the nano-TiO2 as the additive exhibits good anti-wear and friction reduction properties as well as load-carrying capacity. SEM analysis indicates that nano-TiO2 is deposit on the rubbed surface and so prevents direct contact of tribo-pairs. & 2017 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Wettability Tribological performance Aqueous suspension
1. Introduction Water is a low cost lubricant with a high cool capacity, but its low viscosity and corrosive property makes it unacceptable for most tribological application. However with the development of nanotechnology, more and more scientists begin to attempt to add certain additives to the aqueous solution, so as to obtain double effects in both lubricating and cooling [1]. One of the most important applications is on the metalworking process, where the water acts a certain role on lubricating, cooling and washing [2]. In the 21st century, some researchers try to add the nanoparticles to the base oil, and satisfied results are gained on the lubricating aspects. Selected nanoparticles vary from metals such as Ag [3], Cu [4–6], Pd [7], Ni [8]; oxides such as CuO [9], Fe3O4 [10], ZnO [11], Al2O3 [12]; and to rare earth compounds such as LaF3 [13,14], CeBO3 [15] CeO [16]. Besides, the researches of nanoparticles in the aqueous solutions were also emphasized. Zhang [17] studied the tribological property of water-soluble copper/silica in the aqueous system and found that a tribofilm composed of FeS, FeSO4 and SiO2 was formed and so reduced the friction. Zheng [18] reported that Fe3O4/MoS2 nanocomposites in the aqueous media showed excellent lubricating properties. Cho [19] investigated the hexagonal boron nitride nanon
Corresponding author. E-mail address: [email protected] (J. Sun).
http://dx.doi.org/10.1016/j.wear.2017.01.064 0043-1648/& 2017 Elsevier B.V. All rights reserved.
sheets as a lubricant additive in water and found that repeated exfoliation and deposit of h-BN occurred on the sliding surfaces, forming tribo-films which can reduce friction and wear. Currently, the dispersion of nanoparticles in lubricating oils is still a challenge for application of nano-additives. The main factor which affects the stability of nano suspension is the tendency of nanoparticles towards aggregation due to the presence of strong vander Waals attractive forces. To overcome this obstacle, a series of solutions are put into practice, the most popular way is the addition of dispersant. Low molecular weight sodium polyacrylate is the most commonly used dispersant [20,21], the dispersing mechanism is shown in Fig. 1. Since sodium polyacrylate contains hydrophobic carbon chain and the hydrophilic carboxyl, the carboxyl is adsorbed on the surface of the nanoparticles, and the carbon chain is free in water and acts as steric stabilized effect. The nano-TiO2 possess particularly physical chemical and electrical performance, its tribological behavior as the additive in lubricant oils is studied in several papers [22–24], and all results show that the tribological properties of lubricants are improved by the addition of nano-TiO2. However, present works have been weighted heavily on the anti-wear and tribological properties of nanofluid, and seldom study the physicochemical properties such as viscosity and wettability. Therefore this paper chooses the nano-TiO2 as the additive, add to the deionized water, where the sodium polyacrylate is used as the dispersant [25]. Wettability and tribological properties of these asprepared aqueous suspensions are analyzed.
L. Kong et al. / Wear 376-377 (2017) 786–791
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Fig. 2. Sketch of sessile drop method.
Fig. 1. Sketch of bismuth ferrite dispersed with sodium polyacrylate in aqueous media [20].
Fig. 3. Sketch of the tribo-paris and wear scar morphology.
2. Experimental methods
example is measured 5 times, and then takes the average value.
2.1. Preparation of aqueous suspension
2.3. Tribological tests
Nano-TiO2 particles and sodium polyacrylate of this paper chosen are provided by Sinopharm Group Co. Ltd., China. Detailed parameters of TiO2 nanoparticles are shown in Table 1. Molecular weight of sodium polyacrylate is 2100. The preparation process of aqueous suspension is as follows: 1 g is added to the 200 mL deionized water, which aims to prevent the aggregation of nano-TiO2, and then stirs liquid until the hexametaphosphate is fully dissolved. pH value of the aqueous media is adjusted beyond 10, which is to increase the Zeta potential of aqueous media [25], so that better dispersing effect will be obtained. Subsequently, TiO2 nanoparticles with different concentrations are added. Finally place the mixture in ultrasonic dispersion for 30 min. From this way, aqueous suspensions with various concentration of nano-TiO2 (0%, 0.25%, 0.5%, 0.75%, 1.0%) that dispersed with 0.5% sodium polyacrylate are prepared. Likewise, aqueous suspensions with various concentration of nano-TiO2 that dispersed with 1.0% sodium polyacrylate are also prepared by the above mentioned method.
The tribological performances of as-prepared aqueous suspension are investigated using an MSR-10A four-ball apparatus, four bearing steel balls with a diameter 12.7 mm are made by GCr15 and their hardness is 64-66HRC. As shown in Fig. 3. In this technique, one steel ball under load is rotated against three stationary steel balls, these three stationary balls held in the form of a cradle. all balls are immersed in the lubricant. The PB value tests are conducted at a rotary speed of 1450 rpm. And the wear tests are conducted at a rotary speed of 1200 rpm for 30 min while the load is 200 N. The wear scar diameter (WSD) on the three lower balls is measured using an optical microscope with an accuracy of 0.01 mm. The morphology of worn steel surfaces was observed with SEM.
2.2. Wetting tests
Wettability is the potential of a surface to interact with liquids with specified characteristics. The wettability of aqueous suspension directly affects the spreading process on the metal surface. And then affects its tribological properties acts on this metal. So it's imperative to study the wettability of the nano-TiO2 aqueous suspension on the metal surface. Wettability as a very important physico-chemical property of materials is governed by both chemical composition and geometric structure of the substrate surface. The characteristics of substrate such as homogeneity, roughness and material have significant effects on wettability. So this paper chooses two stainless steels. And their roughness (Ra) are 0.30 μm (rough metal surface) and 0.05 μm (smooth metal surface) respectively. This experiment firstly investigates the wettability of aqueous suspension with different concentration of sodium polyacrylate (PAAS) and the results are shown in Fig. 4. From Fig. 4 it's demonstrated that the contact angle of deionized water is 36.4° (on the rough metal surface) and 37.4° (on the smooth metal surface), when 0.25% sodium polyacrylate is added to the deionized water, the contact angle of this aqueous media raises dramatically, but with the further increase of dispersant concentration, the growth sharply slows down. Meanwhile, Fig. 4
The contact angle measuring device is utilized to study the wettability of aqueous suspension, which is based on the sessile drop method. The droplet volume is precisely controlled at 4 mL, it's so small that the influence of gravity can be negligible, So the sessile drop method assumes that the sessile drop is a part of an ideal sphere. As a result, the side view of the drop is an ideal circle. Thus, the contact angle is calculated with the height and the base diameter of the drop. Fig. 2 shows the schematic drawing of drop with an acute contact angle, in which, θ is the contact angle, h and d are the height and base diameter of the drop, respectively. Each
Table 1 Detailed parameters of nano-TiO2. Name Crystal form TiO2
Anatase
Size
Purity Morphology Specific surface Area
50 nm 99.8%
Spherical
120 m2/g
Density
0.05 g/cm3
3. Results and discussion 3.1. Effects of PAAS and nanoparticles on wettability
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L. Kong et al. / Wear 376-377 (2017) 786–791
80
CH C O
Ra=0.30
70
Contact angle
H2C
Ra=0.05
n
O Na 60
Fig. 6. Sketch of PAAS structure.
50
H2C
40
CH C O
30 0.00
0.25
0.50
0.75
n
O Na
1.00
H2C
CH C O O-
n
+ nNa+
Fig. 7. Hydrolytic process of sodium polyacrylate in aqueous media.
Concentration of PAAS/% Fig. 4. Variation of wettability of aqueous suspension with the concentration of the dispersant.
is also shown that, Under given dispersant concentration, wettability of aqueous media on the rough metal surface is evidently better than that on the smooth metal surface, indicating that morphology of substrate will affect the wettability of aqueous media and the wettability of aqueous media on the metal is improved with the roughness of metal increases. Aqueous media containing 0.5% sodium polyacrylate is chosen as the base fluid, subsequently different concentration of TiO2 nanoparticles are added. Wettability of the suspension is analyzed and the results are listed in Fig. 5. From Fig. 5 it's known that when nano-particles are further added to the suspension, the wettability gets a little better compared with base fluid. For 0.25% nanoparticle adds to the base fluid, the contact angle of aqueous declines to 66.5° (on the rough metal surface) and 70.5° (on the smooth metal surface). And when the concentration of nano-TiO2 attains to 1.0%, the contact angle are 63.1° (rough metal) and 65° (smooth metal), separately. Likewise, under given dispersant concentration, the wettability of aqueous suspension on the rough metal is still better than that on the smooth metal, indicating that roughness exerts a great influence on the wettability of aqueous suspension, when the roughness increases, better wettability is achieved. This phenomenon is match up with Wenzel model.
The Sodium polyacrylate (PAAS) is the most common dispersant and its structural formula is shown in Fig. 6. Sodium polyacrylate (PAAS) belongs to organic compound, when dissolved in the aqueous media, as listed in Fig. 7, hydrolytic reaction is
Ra=0.05 Ra=0.30
Contact angle
72
θ=
aP 1 + aP
69
(3.1)
This result is that, the real surface is comprised by two parts, one is the original surface, and the other is the hydrophobic surface that adsorbed dispersant formed. As shown in Fig. 8a. Thus the whole wetting system is shifted from Wenzel wetting state to Cassie wetting state. The Cassie theory can be illustrated as formula (3.2).
cos θ = f1 cos θ1 + f2 cos θ 2
3.2. Analysis of wetting mechanism
75
occurred and the PAAS Polymer is dissociated with the form of polymer chains and the Na cation, polymer chains tend to gather in the interface due to weak interaction force between particles. And this phenomenon eventually results in its concentration of interface exceed that of body concentration, positive adsorption is formed in this way. In addition, polymer chains contain hydrophobic carbon chain and the hydrophilic carboxyl. When aqueous suspension dropped on the metal, hydrophobic carbon chains tend to adsorb on the solid surface thus the adsorbed surface is replaced by hydrophobic portion and forms the hydrophobic surface. In the other aspect, desorption is also occur between those adsorbed polymer chains, it makes them return to the body phase and change into free dispersant. And finally dynamic balance is gained. The balanced adsorption amount is decided by the Langmuir equation that is described as formula (3.1).
(3.2)
Where ƒ1 is the fractional area of the original surface with contact angle θ1, ƒ2 is the fractional area of the surface that adsorbed dispersant formed with contact angle θ2, θ is the Cassie contact angle. When nano-TiO2 particles are added, as shown in Fig. 8b, the concentration of free sodium polyacrylate of body phase is decreased due to this dispersant is liable to adsorb on the nanoparticles as well. This phenomenon ultimately results in the adsorbed sodium polyacrylate desorb from substrate surface and return to the body phase, the hydrophobic area reduces and the apparent angle decreases to a certain degree. 3.3. Extreme, anti-wear and friction reduction properties of aqueous suspension
66 63 60 0.00
0.25 0.50 0.75 1.00 Concentration of nano-TiO 2/%
Fig. 5. Variation of wettability of aqueous suspension with the concentration of the nanoparticles.
Aqueous media containing 0.5% and 1.0% sodium polyacrylate are separately chosen as the base fluids, subsequently different concentration of TiO2 nanoparticles are added. PB values of the suspensions are analyzed and the results are listed Fig. 9. It is known that aqueous media containing 0.5% Sodium polyacrylate (PAAS) is merely 216 N. The addition of nano-TiO2 can greatly increase the PB value of suspension. The load-carrying capacity of aqueous suspension improves dramatically with the nanoparticles
L. Kong et al. / Wear 376-377 (2017) 786–791
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Fig. 8. Sketch of the dispersant and nanoparticle altered contact angle. (a) Only the dispersant added (b) nanoparticles further added.
Dispersed using 1.0% PAAS Dispersed using 0.5% PAAS
385
PB/N
350 315 280 245 210 0.00
0.25
0.50
0.75
1.00
Concentration of nano-TiO2/% Fig. 9. The variation of PB with additive concentration of aqueous suspension.
concentration increases. And when the concentration of nano-TiO2 reaches 1.0%, the PB is almost attained to 373 N. Further observation reveals that when TiO2 nanoparticles are dispersed using different concentration of Sodium polyacryalte, respectively, the PB values will differ from each other even the aqueous suspension are added with the same amount nanoparticles. As shown in Fig. 9, when the nanoparticles concentration is 1.0%, The PB values are 353 N and 373 N while 1.0% and 0.5% sodium polyacrylate are using separately. The most likely reason to explain this phenomenon is that, sodium polyacryalte can dramatically increase the viscosity of the deionized water, which is bound to affect the liquidity of nanoparticles in the aqueous suspension, in this way there are not adequate nanoparticles to enter into the tribo-pairs
3.4. Analysis of tribological behavior From the study on the wettability above, it's known that the wettability of aqueous suspension containing nano-TiO2 is comparatively weak, thus the improvement of tribological properties is not decided by the influence of nanoparticles on the wettability in this situation. So as to investigate the tribological behavior of tribopairs, SEM is used to analyze the worn surface of steel ball. Fig. 11a shows the SEM micrographs of the worn steel surfaces lubricated by aqueous media containing 0.5% Sodium polyacrylate (PAAS) (load: 200 N; speed: 1200 rpm; time: 30 min; room temperature). The worn surfaces of the steel balls in this case show signs of severe scuffing and adhesion, accompanied by deep grooves and rough
0.12
Dispersed using 1.0% PAAS Dispersed using 0.5% PAAS
0.57
0.51 0.48 0.45
Dispersed using 1.0% PAAS Dispersed using 0.5% PAAS
0.11
Fiction coefficient
0.54 W SD /m m
when superfluous sodium polyacryalte is added, and eventually PB value is lower in comparison with that dispersed with less sodium polyacryalte. Fig. 10 shows the change of friction coefficient and wear scar diameter as a function of the concentration of nano-TiO2 (load: 200 N; speed: 1200 rpm; time: 30 min; room temperature), it is seen that nano-TiO2 as additive in the base fluid (aqueous media containing 1.0% sodium polyacrylte) are effective in reducing friction coefficient and wear scar diameter of the tribo-pairs. The wear scar diameter and friction coefficient evidently decline with increasing concentration of nano-TiO2. However, for the aqueous suspension containing 0.5% sodium polycrylate, the wear scar diameter and friction coefficient experience a declining process and then slightly rise when nanoparticle concentration exceeds 0.5%. This may be caused by excess nanoparticles deposited on the metal surface and reunion phenomenon is occurred.
0.10 0.09 0.08 0.07
0.42
0.06
0.39 0.00
0.25
0.50
0.75
1.00
0.00
0.25
0.50
0.75
Concentration of nano-TiO /%
Concentration of nano-TiO2/%
(a)
(b)
1.00
Fig. 10. Variation of WSD and friction coefficient with additive concentration of aqueous suspension containing 0.5% sodium polyacrylate (a) Variation of WSD, (b) variation of friction coefficient.
790
L. Kong et al. / Wear 376-377 (2017) 786–791
Fig. 11. Wear scar lubricated by aqueous suspension that containing 0.5% sodium polyacrylate without nano-TiO2. (a) SEM micrographs (b) EDS spectra.
Fig. 12. Wear scar lubricated by aqueous suspension that containing 0.5% sodium polyacrylate with 0.5% nano-TiO2. (a) SEM micrographs, (b) EDS spectra.
wear scar, possibly due to ploughing effect of asperities during sliding process. Contrary to it, the worn steel surface lubricated by aqueous media (deionized water containing 0.5% sodium polyacrylate) containing 0.5% nanoparticles shows little sign of mild scuffing as shown in Fig. 12a, indicating that nano-TiO2 as additive in aqueous media is effective in anti-wear aspect. But as to the suspension containing 0.5% TiO2 nanoparticles that dispersed with 1.0% PAAS, as shown in Fig. 13a, the lubricating effect is inferior to that dispersed with 0.5% PAAS, possibly due to higher viscosity of aqueous suspension causes less nanoparticles enter into the surface of tribo-pairs. In order to further probe into the lubricating mechanism of nano-TiO2, EDS is utilized to reveal the compound distribution of worn surface. Area scanning is utilized to explore the distribution of Ti component on these worn surfaces. As illustrated in Figs. 11b and 12b, it is seen that there is higher Ti component distributed on the worn surface lubricated by aqueous suspension containing 0.5% nanoparticles in comparison with that of lubricated by aqueous media without nanoparticles, indicating that TiO2 nanoparticles are deposited on the worn surface so as to prevent direct contact of two metal surface. Besides, even under given nanoparticles concentration, as shown in Figs. 12b and 13b, the deposited nanoparticles are differ from each other when different concentration of sodium polyacrylate is contained in aqueous suspension. And the deposited nanoparticles amount in Fig. 12b is higher than that in Fig. 13b. This phenomenon further proves that when
dispersed using excessive sodium polyacrylate, nanoparticles can’t easily enter into the wear surfaces due to higher viscosity of aqueous suspension, which eventually lead to inferior lubricating effect. Anyway, according to the area scan results of Ti compound, it's easily concluded that nanoparticles are far away from completely covering the whole surface of tribo-paris. In this situation, as shown in Fig. 14, the surface of tribo-pairs is possibly comprised with three parts, they are liquid region, asperity contact region and nanoparticles deposited region. And the total friction could be expressed as formula (3.3).
T = τd Sd + τ b Sb + τ l Sl
(3.3)
Where the T is the total friction force, τd, τb, τl are successively the shearing stress of asperity contact region, liquid region and nanoparticles deposited region. And the Sd, Sb, Sl are successively the area of the three above region. From this way, a new synergistic lubrication model that related to asperity, nanoparticle and liquid film is put forwards.
4. Conclusions The wetting behavior of aqueous suspension is influenced by both concentration of nano-TiO2 and roughness of substrate. The addition of sodium polyacrylate makes the wetting state of
L. Kong et al. / Wear 376-377 (2017) 786–791
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Fig. 13. Wear scar lubricated by aqueous suspension that containing 1.0% sodium polyacrylate with 0.5% nano-TiO2. (a) SEM micrographs (b) EDS spectra.
Fig. 14. Sketch of synergistic lubrication model.
aqueous suspension change from Wenzel state to Cassie state. In the other aspect, the addition of nano-TiO2 may offset adverse effect of dispersant bring to the aqueous suspension on the wettability to a certain degree. The tribological performance of aqueous media is improved in a large degree by the addition of nano-TiO2. However this degree is decided by concentration of nano-TiO2 and dispersant. When 0.5% nano-TiO2 adds into the aqueous media (deionized water containing 0.5% sodium polyacrylate), both the PB and friction coefficient gain to the optimum value. As for aqueous media containing 1.0% sodium polyacrylate, the PB and friction coefficient reach optimal when 1.0% nano-TiO2 is added. When tribo-pairs is lubricated with aqueous suspension, the lubricated state is changed from classical mixed lubrication comprised with liquid and asperity, to the new mixed lubrication that comprised with liquid, asperity and nano deposited region. The coefficient is deceased due to nano deposited region partially instead of the asperity.
Acknowledgements The present study is financially supported by the National Natural Science Foundation of China (51274037).
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[4] X.L. Wang, Y.L. Ying, G.N. Zhang, Study on anti-wear and repairing performances about mass of nano-copper lubricating additives to 45 steel, Phys. Procedia (50) (2013) 466–472. [5] J. Padgurskas, R. Rukuiza, Tribological properties of lubricant additives of Fe, Cu and Co nanoparticles, Tribol. Int. 60 (7) (2013) 224–232. [6] V.S. Jatti, T.P. Singh, Copper oxide nano-particles as friction-reduction and anti-wear additives in lubricating oil, J. Mech. Sci. Technol. 29 (2) (2015) 793–798. [7] J.C. Sánchez-López, M.D. Abad, L. Kolodziejczyk, Surface-modified Pd and Au nanoparticles for anti-wear applications, Tribol. Int. 44 (6) (2011) 720–726. [8] R. Chou, A.H. Battez, J.J. Cabello, Tribological behavior of polyalphaolefin with the addition of nickel nanoparticles, Tribol. Int. 43 (12) (2010) 2327–2332. [9] L. Peña-Parás, T.J. aha-Tijerina, L. Garza, Effect of CuO and Al2O3, nanoparticle additives on the tribological behavior of fully formulated oils, Wear s332–333 (2015) 1256–1261. [10] G. Zhou, Y. Zhu, X. Wang, Sliding tribological properties of 0.45% carbon steel lubricated with Fe3O4, magnetic nano-particle additives in base oil, Wear 301 (1–2) (2013) 753–757. [11] A.H. Battez, J.E.F. Rico, A.N. Arias, The tribological behaviour of ZnO nanoparticles as an additive to PAO6, Wear 261 (3–4) (2006) 256–263. [12] T. Luo, X. Wei, X. Huang, Tribological properties of Al2O3 nanoparticles as lubricating oil additives, Ceram. Int. 40 (5) (2014) 7143–7149. [13] H. Xiao, J. He, L. Yu, Preparation and tribological properties of fluorosilane surface-modified lanthanum trifluoride nanoparticles as additive of fluoro silicone oil, Appl. Sci. Res. 316 (1) (2014) 515–523. [14] Z. Li, H. Xiao, L. Yu, Preparation of lanthanum trifluoride nanoparticles surfacecapped by tributyl phosphate and evaluation of their tribological properties as lubricant additive in liquid paraffin, Appl. Sci. Res. 292 (4) (2014) 971–977. [15] B. Chen, K. Gu, J. Fang, Tribological characteristics of monodispersed cerium borate nanospheres in biodegradable rapeseed oil lubricant, Appl. Sci. Res. (353) (2015) 326–332. [16] T. Shen, D. Wang, J. Yun, Tribological properties and tribochemical analysis of nano-cerium oxide and sulfurized isobutene in titanium complex grease, Tribol. Int. (93) (2015) 332–346. [17] C. Zhang, S. Zhang, L. Yu, Preparation and tribological properties of watersoluble copper/silica nanocomposite as a water-based lubricant additive, Appl. Sci. Res. 259 (42) (2012) 824–830. [18] X. Zheng, Y. Xu, J. Geng, Tribological behavior of Fe3O4/MoS2, nanocomposites additives in aqueous and oil phase media, Tribol. Int. (102) (2016) 79–87. [19] D.H. Cho, J.S. Kim, S.H. Kwon, Evaluation of hexagonal boron nitride nanosheets as a lubricant additive in water, Wear 302 (s1–2) (2013) 981–986. [20] C. Ponzoni, M. Cannio, R. Rosa, Stabilization of bismuth ferrite suspensions in aqueous medium with sodium polyacrylate characterized by different molecular weights, Mater. Chem. Phys. (149) (2014) 246–253. [21] T. Kefeng, S. Xingfu, S. Shuying, The rheological behavior and stability of Mg(OH)2 aqueous suspensions in the presence of sodium polyacrylate, Colloids Surf. A: Physicochem. Eng. Asp. (436) (2013) 1111–1120. [22] M.J. Kao, C.R. Lin, Evaluating the role of spherical titanium oxide nanoparticles in reducing friction between two pieces of cast iron, J. Alloy. Compd. 483 (1) (2009) 456–459. [23] Z. ZuoXin, S. Jianlin, et al., Research on lubrication behaviors of nano-TiO2 in water-based hot rolling liquid, Adv. Mater. Res. 643 (2013) 139–143. [24] G. Xiangyu, X. Yanqiu, C. Zhengfu, Tribological properties and insulation effect of nanometer TiO2 and nanometer SiO2 as additives in grease, Tribol. Int. (92) (2015) 454–461. [25] K. Ohenoja, J. Saari, M. Illikainen, et al., Effect of molecular weight of sodium polyacrylates on the particle size distribution and stability of a TiO2 suspension in aqueous stirred media milling, Powder Technol. 262 (262) (2014) 188–193.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Effect of TiO2 nanoparticles on wettability and tribological performance of aqueous suspension Linghui Kong, Jianlin Sun n, Yueyue Bao, Yanan Meng School of Materials Science and Engineering, University of Science and Technology Beijing, 100083 Beijing, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 August 2016 Received in revised form 13 January 2017 Accepted 18 January 2017
Aqueous suspensions containing different concentration of nano-TiO2 (50 nm) are prepared, in which sodium polyacrylate (PAAS) as the dispersant is used. Their wettability and tribological properties are discussed with Dataphysics OCA50 contact angle measurement device and the MRS-10A four-ball tester, respectively. The rubbed surfaces after friction tests are also analyzed by scanning electron microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS). Wetting results show that the wettability of aqueous suspension is sharply deteriorated with the addition of PAAS, possibly caused by adsorption of PAAS on the substrate alters the characterization of the substrate, which promote the substrate change from homogeneity to heterogeneity, and eventually the wetting state transits from Wenzel to Cassie. And when the nano-TiO2 is further added to the aqueous suspension, the wettability is slightly better, which is mostly due to adsorbed sodium polyacrylate desorb from substrate surface and return to the body phase. In another aspect, four-ball experimental results reveal that the nano-TiO2 as the additive exhibits good anti-wear and friction reduction properties as well as load-carrying capacity. SEM analysis indicates that nano-TiO2 is deposit on the rubbed surface and so prevents direct contact of tribo-pairs. & 2017 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Wettability Tribological performance Aqueous suspension
1. Introduction Water is a low cost lubricant with a high cool capacity, but its low viscosity and corrosive property makes it unacceptable for most tribological application. However with the development of nanotechnology, more and more scientists begin to attempt to add certain additives to the aqueous solution, so as to obtain double effects in both lubricating and cooling [1]. One of the most important applications is on the metalworking process, where the water acts a certain role on lubricating, cooling and washing [2]. In the 21st century, some researchers try to add the nanoparticles to the base oil, and satisfied results are gained on the lubricating aspects. Selected nanoparticles vary from metals such as Ag [3], Cu [4–6], Pd [7], Ni [8]; oxides such as CuO [9], Fe3O4 [10], ZnO [11], Al2O3 [12]; and to rare earth compounds such as LaF3 [13,14], CeBO3 [15] CeO [16]. Besides, the researches of nanoparticles in the aqueous solutions were also emphasized. Zhang [17] studied the tribological property of water-soluble copper/silica in the aqueous system and found that a tribofilm composed of FeS, FeSO4 and SiO2 was formed and so reduced the friction. Zheng [18] reported that Fe3O4/MoS2 nanocomposites in the aqueous media showed excellent lubricating properties. Cho [19] investigated the hexagonal boron nitride nanon
Corresponding author. E-mail address: [email protected] (J. Sun).
http://dx.doi.org/10.1016/j.wear.2017.01.064 0043-1648/& 2017 Elsevier B.V. All rights reserved.
sheets as a lubricant additive in water and found that repeated exfoliation and deposit of h-BN occurred on the sliding surfaces, forming tribo-films which can reduce friction and wear. Currently, the dispersion of nanoparticles in lubricating oils is still a challenge for application of nano-additives. The main factor which affects the stability of nano suspension is the tendency of nanoparticles towards aggregation due to the presence of strong vander Waals attractive forces. To overcome this obstacle, a series of solutions are put into practice, the most popular way is the addition of dispersant. Low molecular weight sodium polyacrylate is the most commonly used dispersant [20,21], the dispersing mechanism is shown in Fig. 1. Since sodium polyacrylate contains hydrophobic carbon chain and the hydrophilic carboxyl, the carboxyl is adsorbed on the surface of the nanoparticles, and the carbon chain is free in water and acts as steric stabilized effect. The nano-TiO2 possess particularly physical chemical and electrical performance, its tribological behavior as the additive in lubricant oils is studied in several papers [22–24], and all results show that the tribological properties of lubricants are improved by the addition of nano-TiO2. However, present works have been weighted heavily on the anti-wear and tribological properties of nanofluid, and seldom study the physicochemical properties such as viscosity and wettability. Therefore this paper chooses the nano-TiO2 as the additive, add to the deionized water, where the sodium polyacrylate is used as the dispersant [25]. Wettability and tribological properties of these asprepared aqueous suspensions are analyzed.
L. Kong et al. / Wear 376-377 (2017) 786–791
787
Fig. 2. Sketch of sessile drop method.
Fig. 1. Sketch of bismuth ferrite dispersed with sodium polyacrylate in aqueous media [20].
Fig. 3. Sketch of the tribo-paris and wear scar morphology.
2. Experimental methods
example is measured 5 times, and then takes the average value.
2.1. Preparation of aqueous suspension
2.3. Tribological tests
Nano-TiO2 particles and sodium polyacrylate of this paper chosen are provided by Sinopharm Group Co. Ltd., China. Detailed parameters of TiO2 nanoparticles are shown in Table 1. Molecular weight of sodium polyacrylate is 2100. The preparation process of aqueous suspension is as follows: 1 g is added to the 200 mL deionized water, which aims to prevent the aggregation of nano-TiO2, and then stirs liquid until the hexametaphosphate is fully dissolved. pH value of the aqueous media is adjusted beyond 10, which is to increase the Zeta potential of aqueous media [25], so that better dispersing effect will be obtained. Subsequently, TiO2 nanoparticles with different concentrations are added. Finally place the mixture in ultrasonic dispersion for 30 min. From this way, aqueous suspensions with various concentration of nano-TiO2 (0%, 0.25%, 0.5%, 0.75%, 1.0%) that dispersed with 0.5% sodium polyacrylate are prepared. Likewise, aqueous suspensions with various concentration of nano-TiO2 that dispersed with 1.0% sodium polyacrylate are also prepared by the above mentioned method.
The tribological performances of as-prepared aqueous suspension are investigated using an MSR-10A four-ball apparatus, four bearing steel balls with a diameter 12.7 mm are made by GCr15 and their hardness is 64-66HRC. As shown in Fig. 3. In this technique, one steel ball under load is rotated against three stationary steel balls, these three stationary balls held in the form of a cradle. all balls are immersed in the lubricant. The PB value tests are conducted at a rotary speed of 1450 rpm. And the wear tests are conducted at a rotary speed of 1200 rpm for 30 min while the load is 200 N. The wear scar diameter (WSD) on the three lower balls is measured using an optical microscope with an accuracy of 0.01 mm. The morphology of worn steel surfaces was observed with SEM.
2.2. Wetting tests
Wettability is the potential of a surface to interact with liquids with specified characteristics. The wettability of aqueous suspension directly affects the spreading process on the metal surface. And then affects its tribological properties acts on this metal. So it's imperative to study the wettability of the nano-TiO2 aqueous suspension on the metal surface. Wettability as a very important physico-chemical property of materials is governed by both chemical composition and geometric structure of the substrate surface. The characteristics of substrate such as homogeneity, roughness and material have significant effects on wettability. So this paper chooses two stainless steels. And their roughness (Ra) are 0.30 μm (rough metal surface) and 0.05 μm (smooth metal surface) respectively. This experiment firstly investigates the wettability of aqueous suspension with different concentration of sodium polyacrylate (PAAS) and the results are shown in Fig. 4. From Fig. 4 it's demonstrated that the contact angle of deionized water is 36.4° (on the rough metal surface) and 37.4° (on the smooth metal surface), when 0.25% sodium polyacrylate is added to the deionized water, the contact angle of this aqueous media raises dramatically, but with the further increase of dispersant concentration, the growth sharply slows down. Meanwhile, Fig. 4
The contact angle measuring device is utilized to study the wettability of aqueous suspension, which is based on the sessile drop method. The droplet volume is precisely controlled at 4 mL, it's so small that the influence of gravity can be negligible, So the sessile drop method assumes that the sessile drop is a part of an ideal sphere. As a result, the side view of the drop is an ideal circle. Thus, the contact angle is calculated with the height and the base diameter of the drop. Fig. 2 shows the schematic drawing of drop with an acute contact angle, in which, θ is the contact angle, h and d are the height and base diameter of the drop, respectively. Each
Table 1 Detailed parameters of nano-TiO2. Name Crystal form TiO2
Anatase
Size
Purity Morphology Specific surface Area
50 nm 99.8%
Spherical
120 m2/g
Density
0.05 g/cm3
3. Results and discussion 3.1. Effects of PAAS and nanoparticles on wettability
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80
CH C O
Ra=0.30
70
Contact angle
H2C
Ra=0.05
n
O Na 60
Fig. 6. Sketch of PAAS structure.
50
H2C
40
CH C O
30 0.00
0.25
0.50
0.75
n
O Na
1.00
H2C
CH C O O-
n
+ nNa+
Fig. 7. Hydrolytic process of sodium polyacrylate in aqueous media.
Concentration of PAAS/% Fig. 4. Variation of wettability of aqueous suspension with the concentration of the dispersant.
is also shown that, Under given dispersant concentration, wettability of aqueous media on the rough metal surface is evidently better than that on the smooth metal surface, indicating that morphology of substrate will affect the wettability of aqueous media and the wettability of aqueous media on the metal is improved with the roughness of metal increases. Aqueous media containing 0.5% sodium polyacrylate is chosen as the base fluid, subsequently different concentration of TiO2 nanoparticles are added. Wettability of the suspension is analyzed and the results are listed in Fig. 5. From Fig. 5 it's known that when nano-particles are further added to the suspension, the wettability gets a little better compared with base fluid. For 0.25% nanoparticle adds to the base fluid, the contact angle of aqueous declines to 66.5° (on the rough metal surface) and 70.5° (on the smooth metal surface). And when the concentration of nano-TiO2 attains to 1.0%, the contact angle are 63.1° (rough metal) and 65° (smooth metal), separately. Likewise, under given dispersant concentration, the wettability of aqueous suspension on the rough metal is still better than that on the smooth metal, indicating that roughness exerts a great influence on the wettability of aqueous suspension, when the roughness increases, better wettability is achieved. This phenomenon is match up with Wenzel model.
The Sodium polyacrylate (PAAS) is the most common dispersant and its structural formula is shown in Fig. 6. Sodium polyacrylate (PAAS) belongs to organic compound, when dissolved in the aqueous media, as listed in Fig. 7, hydrolytic reaction is
Ra=0.05 Ra=0.30
Contact angle
72
θ=
aP 1 + aP
69
(3.1)
This result is that, the real surface is comprised by two parts, one is the original surface, and the other is the hydrophobic surface that adsorbed dispersant formed. As shown in Fig. 8a. Thus the whole wetting system is shifted from Wenzel wetting state to Cassie wetting state. The Cassie theory can be illustrated as formula (3.2).
cos θ = f1 cos θ1 + f2 cos θ 2
3.2. Analysis of wetting mechanism
75
occurred and the PAAS Polymer is dissociated with the form of polymer chains and the Na cation, polymer chains tend to gather in the interface due to weak interaction force between particles. And this phenomenon eventually results in its concentration of interface exceed that of body concentration, positive adsorption is formed in this way. In addition, polymer chains contain hydrophobic carbon chain and the hydrophilic carboxyl. When aqueous suspension dropped on the metal, hydrophobic carbon chains tend to adsorb on the solid surface thus the adsorbed surface is replaced by hydrophobic portion and forms the hydrophobic surface. In the other aspect, desorption is also occur between those adsorbed polymer chains, it makes them return to the body phase and change into free dispersant. And finally dynamic balance is gained. The balanced adsorption amount is decided by the Langmuir equation that is described as formula (3.1).
(3.2)
Where ƒ1 is the fractional area of the original surface with contact angle θ1, ƒ2 is the fractional area of the surface that adsorbed dispersant formed with contact angle θ2, θ is the Cassie contact angle. When nano-TiO2 particles are added, as shown in Fig. 8b, the concentration of free sodium polyacrylate of body phase is decreased due to this dispersant is liable to adsorb on the nanoparticles as well. This phenomenon ultimately results in the adsorbed sodium polyacrylate desorb from substrate surface and return to the body phase, the hydrophobic area reduces and the apparent angle decreases to a certain degree. 3.3. Extreme, anti-wear and friction reduction properties of aqueous suspension
66 63 60 0.00
0.25 0.50 0.75 1.00 Concentration of nano-TiO 2/%
Fig. 5. Variation of wettability of aqueous suspension with the concentration of the nanoparticles.
Aqueous media containing 0.5% and 1.0% sodium polyacrylate are separately chosen as the base fluids, subsequently different concentration of TiO2 nanoparticles are added. PB values of the suspensions are analyzed and the results are listed Fig. 9. It is known that aqueous media containing 0.5% Sodium polyacrylate (PAAS) is merely 216 N. The addition of nano-TiO2 can greatly increase the PB value of suspension. The load-carrying capacity of aqueous suspension improves dramatically with the nanoparticles
L. Kong et al. / Wear 376-377 (2017) 786–791
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Fig. 8. Sketch of the dispersant and nanoparticle altered contact angle. (a) Only the dispersant added (b) nanoparticles further added.
Dispersed using 1.0% PAAS Dispersed using 0.5% PAAS
385
PB/N
350 315 280 245 210 0.00
0.25
0.50
0.75
1.00
Concentration of nano-TiO2/% Fig. 9. The variation of PB with additive concentration of aqueous suspension.
concentration increases. And when the concentration of nano-TiO2 reaches 1.0%, the PB is almost attained to 373 N. Further observation reveals that when TiO2 nanoparticles are dispersed using different concentration of Sodium polyacryalte, respectively, the PB values will differ from each other even the aqueous suspension are added with the same amount nanoparticles. As shown in Fig. 9, when the nanoparticles concentration is 1.0%, The PB values are 353 N and 373 N while 1.0% and 0.5% sodium polyacrylate are using separately. The most likely reason to explain this phenomenon is that, sodium polyacryalte can dramatically increase the viscosity of the deionized water, which is bound to affect the liquidity of nanoparticles in the aqueous suspension, in this way there are not adequate nanoparticles to enter into the tribo-pairs
3.4. Analysis of tribological behavior From the study on the wettability above, it's known that the wettability of aqueous suspension containing nano-TiO2 is comparatively weak, thus the improvement of tribological properties is not decided by the influence of nanoparticles on the wettability in this situation. So as to investigate the tribological behavior of tribopairs, SEM is used to analyze the worn surface of steel ball. Fig. 11a shows the SEM micrographs of the worn steel surfaces lubricated by aqueous media containing 0.5% Sodium polyacrylate (PAAS) (load: 200 N; speed: 1200 rpm; time: 30 min; room temperature). The worn surfaces of the steel balls in this case show signs of severe scuffing and adhesion, accompanied by deep grooves and rough
0.12
Dispersed using 1.0% PAAS Dispersed using 0.5% PAAS
0.57
0.51 0.48 0.45
Dispersed using 1.0% PAAS Dispersed using 0.5% PAAS
0.11
Fiction coefficient
0.54 W SD /m m
when superfluous sodium polyacryalte is added, and eventually PB value is lower in comparison with that dispersed with less sodium polyacryalte. Fig. 10 shows the change of friction coefficient and wear scar diameter as a function of the concentration of nano-TiO2 (load: 200 N; speed: 1200 rpm; time: 30 min; room temperature), it is seen that nano-TiO2 as additive in the base fluid (aqueous media containing 1.0% sodium polyacrylte) are effective in reducing friction coefficient and wear scar diameter of the tribo-pairs. The wear scar diameter and friction coefficient evidently decline with increasing concentration of nano-TiO2. However, for the aqueous suspension containing 0.5% sodium polycrylate, the wear scar diameter and friction coefficient experience a declining process and then slightly rise when nanoparticle concentration exceeds 0.5%. This may be caused by excess nanoparticles deposited on the metal surface and reunion phenomenon is occurred.
0.10 0.09 0.08 0.07
0.42
0.06
0.39 0.00
0.25
0.50
0.75
1.00
0.00
0.25
0.50
0.75
Concentration of nano-TiO /%
Concentration of nano-TiO2/%
(a)
(b)
1.00
Fig. 10. Variation of WSD and friction coefficient with additive concentration of aqueous suspension containing 0.5% sodium polyacrylate (a) Variation of WSD, (b) variation of friction coefficient.
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Fig. 11. Wear scar lubricated by aqueous suspension that containing 0.5% sodium polyacrylate without nano-TiO2. (a) SEM micrographs (b) EDS spectra.
Fig. 12. Wear scar lubricated by aqueous suspension that containing 0.5% sodium polyacrylate with 0.5% nano-TiO2. (a) SEM micrographs, (b) EDS spectra.
wear scar, possibly due to ploughing effect of asperities during sliding process. Contrary to it, the worn steel surface lubricated by aqueous media (deionized water containing 0.5% sodium polyacrylate) containing 0.5% nanoparticles shows little sign of mild scuffing as shown in Fig. 12a, indicating that nano-TiO2 as additive in aqueous media is effective in anti-wear aspect. But as to the suspension containing 0.5% TiO2 nanoparticles that dispersed with 1.0% PAAS, as shown in Fig. 13a, the lubricating effect is inferior to that dispersed with 0.5% PAAS, possibly due to higher viscosity of aqueous suspension causes less nanoparticles enter into the surface of tribo-pairs. In order to further probe into the lubricating mechanism of nano-TiO2, EDS is utilized to reveal the compound distribution of worn surface. Area scanning is utilized to explore the distribution of Ti component on these worn surfaces. As illustrated in Figs. 11b and 12b, it is seen that there is higher Ti component distributed on the worn surface lubricated by aqueous suspension containing 0.5% nanoparticles in comparison with that of lubricated by aqueous media without nanoparticles, indicating that TiO2 nanoparticles are deposited on the worn surface so as to prevent direct contact of two metal surface. Besides, even under given nanoparticles concentration, as shown in Figs. 12b and 13b, the deposited nanoparticles are differ from each other when different concentration of sodium polyacrylate is contained in aqueous suspension. And the deposited nanoparticles amount in Fig. 12b is higher than that in Fig. 13b. This phenomenon further proves that when
dispersed using excessive sodium polyacrylate, nanoparticles can’t easily enter into the wear surfaces due to higher viscosity of aqueous suspension, which eventually lead to inferior lubricating effect. Anyway, according to the area scan results of Ti compound, it's easily concluded that nanoparticles are far away from completely covering the whole surface of tribo-paris. In this situation, as shown in Fig. 14, the surface of tribo-pairs is possibly comprised with three parts, they are liquid region, asperity contact region and nanoparticles deposited region. And the total friction could be expressed as formula (3.3).
T = τd Sd + τ b Sb + τ l Sl
(3.3)
Where the T is the total friction force, τd, τb, τl are successively the shearing stress of asperity contact region, liquid region and nanoparticles deposited region. And the Sd, Sb, Sl are successively the area of the three above region. From this way, a new synergistic lubrication model that related to asperity, nanoparticle and liquid film is put forwards.
4. Conclusions The wetting behavior of aqueous suspension is influenced by both concentration of nano-TiO2 and roughness of substrate. The addition of sodium polyacrylate makes the wetting state of
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Fig. 13. Wear scar lubricated by aqueous suspension that containing 1.0% sodium polyacrylate with 0.5% nano-TiO2. (a) SEM micrographs (b) EDS spectra.
Fig. 14. Sketch of synergistic lubrication model.
aqueous suspension change from Wenzel state to Cassie state. In the other aspect, the addition of nano-TiO2 may offset adverse effect of dispersant bring to the aqueous suspension on the wettability to a certain degree. The tribological performance of aqueous media is improved in a large degree by the addition of nano-TiO2. However this degree is decided by concentration of nano-TiO2 and dispersant. When 0.5% nano-TiO2 adds into the aqueous media (deionized water containing 0.5% sodium polyacrylate), both the PB and friction coefficient gain to the optimum value. As for aqueous media containing 1.0% sodium polyacrylate, the PB and friction coefficient reach optimal when 1.0% nano-TiO2 is added. When tribo-pairs is lubricated with aqueous suspension, the lubricated state is changed from classical mixed lubrication comprised with liquid and asperity, to the new mixed lubrication that comprised with liquid, asperity and nano deposited region. The coefficient is deceased due to nano deposited region partially instead of the asperity.
Acknowledgements The present study is financially supported by the National Natural Science Foundation of China (51274037).
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