Tribological performance of halloysite clay nanotubes as green lubricant additives

Wear 376-377 (2017) 885–892

Contents lists available at ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

Tribological performance of halloysite clay nanotubes as green lubricant additives Laura Peña-Parás n, Demófilo Maldonado-Cortés, Patricio García, Mariana Irigoyen, Jaime Taha-Tijerina, Julia Guerra Universidad de Monterrey, Departamento de Ingeniería, Av. Morones Prieto 4500 Pte. San Pedro Garza García, N.L. 66238, Mexico

art ic l e i nf o

a b s t r a c t

Article history: Received 30 August 2016 Received in revised form 30 December 2016 Accepted 11 January 2017

Nanoparticles have been recently explored as lubricant additives for improving tribological performance of metal-forming tools. Halloysite clay nanotubes (HNTs) are naturally-occurring, low-cost, and non-toxic nanoparticles, making them attractive as green lubricant additives. In this study, HNTs were dispersed at varying concentrations (0.01, 0.05, and 0.10 wt%) within a polymeric lubricant for metal-forming applications. The ITeE-PIB Polish method for testing lubricants under scuffing conditions was conducted with a four-ball tribotester in order to obtain the scuffing load and load-carrying capacity (poz) under extreme pressures (EP), as this method has demonstrated to be sensitive to extreme-pressure additives; a blockon-ring test was used to obtain wear volume loss and coefficient of friction (COF). Surface roughness of worn materials was characterized with an optical 3D-surface measurement system. Results showed that 0.05 wt% HNTs delayed scuffing initiation, and increased the scuffing load and load-carrying capacity by 50% and
72%, respectively. Wear volume loss and COF were both lowered by
70%, and surface roughness was reduced due to the formation of a tribofilm. The improved lubrication provided by environmentally-friendly HNT lubricant additives shown in this study may result in considerable cost savings through lower energy consumption and longer tool life. & 2017 Elsevier B.V. All rights reserved.

Keywords: Scuffing Extreme pressure Lubricant additive Roughness

1. Introduction Lubrication is key for metal-forming operations as a mean to reduce contact between tooling surfaces and reduce friction and wear. Recent studies have shown improvements in the tribological properties of lubricants for these applications with nanoparticle additives, resulting in conservation of materials and energy [1–9]. For example, Ghaednia, et al. [10] found a decrease in the coefficient of friction (COF) of 23% with CuO nanoparticles at a concentration of 2.0 wt%, attributed to a reduction in the real area of contact of surfaces by nanoparticles. Alves et al. [1] studied the tribological behavior of CuO and ZnO nanoparticles in a vegetableoil under extreme pressures (EP); results showed that nanoparticles may form a tribofilm, and fill surface valleys creating a smoother surface, reducing friction and wear. Inorganic fullerenelike (IF) nanoparticles such as MoS2 and WS2 have shown to be highly effective at EP conditions through the formation of a thin tribofilm on the sliding surfaces by gradual exfoliation of the tubes ´external sheets [5,11–17]. Experiments using an in situ HRTEM nanoindenter under compression of hollow WS2 nanotubes by n

Corresponding author. E-mail address: [email protected] (L. Peña-Parás).

http://dx.doi.org/10.1016/j.wear.2017.01.044 0043-1648/& 2017 Elsevier B.V. All rights reserved.

Lahouij et al. [18] demonstrated that exfoliation occurs at a contact pressure of 0.35 GPa, where the particle began to tear. Similarly, another study by Lahouij et al. [19] showed exfoliation of MoS2 at a higher contact pressure of 0.5 GPa, due to the nanoparticle´s larger Young´s modulus of 238 GPa, compared to WS2 (150 GPa). Furthermore, they found that under sliding conditions these fullerenes may also act as nano-bearings, further enhancing their lubricating properties. Kalin et al. [14] obtained a 50% reduction in COF under severe boundary lubrication conditions by exfoliated sheets of MoS2 nanotubes. Likewise, Rapoport et al. [11] showed that WS2 improved tribological behavior of paraffin oil by a smoothing effect with nanoparticles filling valleys, and a protective film by exfoliated nanosheets. Some nanoparticles based on metals [20], metal oxides [21], and carbon (like CNTs) [22] have shown to induce toxicological effects, limiting their application in conventional products. Therefore, there is an increasing demand for environmentally friendly nanoparticle additives [23]. Clay mineral nanomaterials are naturally occurring materials that are not hazardous to the environment [24,25]. In this study, we propose Halloysite clay nanotubes (HNTs) as green additives for improving tribological properties of lubricants since biocompatibility experiments have shown them to be non-toxic, and environmentally friendly [26,27]. These hollowed multilayered tubular natural materials consist of

886

L. Peña-Parás et al. / Wear 376-377 (2017) 885–892

Fig. 1. SEM micrograph of halloysite clay nanotubes (HNTs).

Fig. 3. Load-carrying capacity of HNT nanolubricants obtained by the four-ball test at extreme pressures (EP).

Table 1 Material properties. Material Lubricant Polymeric lubricant fluid Nanoparticle Halloysite clay nanotubes (HNTs)

Specimens Balls Rings Blocks

Properties

Density (15 °C): 1.03 g/cm3 Viscosity (40 °C): 79.15 Cst Chemical formula: Al2Si2O5(OH)4  2H2O Morphology: tubular Size: 30–70 nm x 1–3 μm Hardness: 2 Mohs AISI 52100 steel, d: 12.7 mm, hardness: 60 HRC, Ra ¼ 0.25 mm AISI D2 steel, d: 35 mm, hardness: 62 HRC, Ra ¼ 0.65 mm AISI 1018 steel, dimensions: 15.75  10 x 6.35 mm, hardness: 78 HRB, Ra ¼ 0.65 mm

Fig. 4. Coefficient of friction curves for HNT nanolubricants obtained by the blockon-ring test.

Fig. 2. Friction torque curves over time at continuously increasing load for HNT nanolubricants with varying concentration.

Al2Si2O5(OH)4  2H2O [28], with external diameters of 30–190 nm, internal diameters of 10–100 nm, and lengths of 0.2 to
30 mm [24,28–30]. They have low hardness (
2 Mohs) and density (2.6 g/cm3) [28], and are low-cost [24]. Furthermore HNTs have a high modulus (140 GPa), comparable to that of IF-WS2, enabling them to withstand high loads under EP conditions. These nanoparticles have been studied for biomedical applications [27,31,32], corrosion protection [33,34], and mechanical reinforcing of

Fig. 5. Wear volume loss and average COF for lubricants with varying nanoparticle concentration obtained by the block-on-ring test.

polymer matrices [24,35]. HNTs have also shown to improve wear resistance in polymer matrices [35,36], however, their use as lubricant additive has not been yet explored. For this study, we propose HNTs as additives for a water-based polymeric lubricant used for metal-forming processes. Tribological properties of nanolubricants under EP conditions were determined

L. Peña-Parás et al. / Wear 376-377 (2017) 885–892

887

Fig. 6. Scanning electron micrographs of worn steel balls: (a) polymeric lubricant, (b) 0.01 wt% HNT nanolubricant, (c) 0.05 wt% HNT nanolubricant, (d) 0.10 wt% HNT nanolubricant.

by a four-ball test following the ITeE-PIB Polish test method for testing lubricants under scuffing conditions. Wear volume loss and COF under sliding conditions was also obtained by a block-on-ring test. Worn areas were characterized by Scanning Electron Microscopy (SEM), Optical microscopy and Energy Dispersive x-ray Spectroscopy (EDS) in order to provide an understanding on the HNTs tribological mechanisms.

2. Experimental details 2.1. Materials In this study, HNTs supplied by Sigma-Aldrich were dispersed in a water-based polymeric lubricant for drawing and forming applications. Fig. 1 shows an SEM micrograph with the morphology of the nanoparticles used for this study. The main properties of the lubricant and selected nanoparticle are also shown in Table 1.

Nanolubricants were prepared by adding of 0.01, 0.05, and 0.10 wt% HNTs into the polymeric lubricant, followed by homogenization for 5 min with a T10 Ultra Turraz homogenizer at 20,000 rpms; and ultrasonication for 5 min with a Cole-Parmer 500-Watt ultrasonic homogenizer with a frequency of 20 kHz. 2.2. Four-ball test The tribological properties of nanolubricants under EP conditions were determined with a T-02U four-ball tribotester, following the ITeE-PIB Polish test method for testing lubricants under scuffing conditions [37–39]. In this test, three lower balls covered with ∼8 mL of the selected nanolubricant are under a rotating ball at a given speed and increasing load, generating friction and wear on the contact points. The balls' material properties are shown in Table 1. Tests were run at a temperature of 25 °C, a rotational speed of 500 rpms, and a linearly increasing load ranging from 0 N to 7200 N, over a time of 18 s. This test shows the frictional torque

888

L. Peña-Parás et al. / Wear 376-377 (2017) 885–892

Fig. 7. SEM micrographs of worn blocks: (a) polymeric lubricant, (b) 0.01 wt% HNT nanolubricant, (c) 0.05 wt% HNT nanolubricant, (d) 0.10 wt% HNT nanolubricant.

until it reaches a value of 10 N.m, when seizure occurs and the oil film is destroyed. This torque value was adopted based on the life of the ball chuck [38]. The load at this point is called the seizure load (Poz). If a frictional torque of 10 N.m is not reached at the end of the run, then Poz is considered to be 7200 N, the maximum load of the test. The load for scuffing initiation (Pt) may also be obtained; it occurs when a sudden increase in frictional torque is observed indicating lubricant film breakdown. The load-carrying capacity or limiting pressure of seizure poz is calculated according to the following relationship:

poz = 0. 52

Poz WSD2

(1)

Here, WSD represents the average wear scar diameter in mm of the three lower steel balls obtained by an optical microscope. The coefficient 0.52 results from the force distribution in the four-ball tribological system [38]. In this test, a large value of poz (in N/mm2) is indicative of a more efficient lubricant under scuffing conditions.

2.3. Block-on-ring test A sliding wear test under high pressures was performed with a T-05 tribotester with a block-on-ring configuration, a conformal contact, and an oil bath chamber fixture, based on ASTM G-077-05 [40]. Basic characteristics of the block and ring materials are shown in Table 1. Nanolubricants were placed in the oil chamber allowing constant lubrication while the test ring rotated, covering it in lubricant due to centrifugal forces. All tests were run at a temperature of 25 °C, 200 rpms, 1080 s, and a load of 3000 N (corresponding to a contact pressure of ∼0.8 GPa). Wear volume loss in mm3 was obtained for each block by the following equation, per ASTM G-077-05 [40]:

volume loss =

⎛ D2 t ⎡ b b ⎞⎤ ⎢ 2 sin−1 −sin ⎜ 2 sin−1 ⎟ ⎥ ⎝ D 8 ⎣ D ⎠⎦

(2)

where t is the block with, D is the diameter of the ring, and b is the average wear scar width.

L. Peña-Parás et al. / Wear 376-377 (2017) 885–892

889

Fig. 8. Surface roughness profiles for blocks lubricated with: (a) polymeric lubricant, (b) 0.01 wt% HNTs, (c) 0.05 wt% HNTs, (d) 0.10 wt% HNTs.

2.4. Worn surface characterization Morphology of worn materials and their surface roughness characteristics were analyzed with an Alicona IF-EdgeMaster optical 3D surface measurement system and a Tescan Vega3 SB Scanning Electron Microscope (SEM) equipped with EDS for elemental analysis.

3. Results and discussions 3.1. Tribological behavior of nanolubricants Fig. 2 depicts the frictional torque at increasing load of each nanolubricant obtained by the ITeE-PIB Polish test method (fourball test). Scuffing initiation, where a sudden increase in the frictional torque appears, was observed at ∼6 s for the unreinforced polymeric lubricant with a Pt of 2400 N. As a reference, Michalczewski et al. [41] reported Pt values of
1000 N and
1800 for a mineral base oil and a mineral oil with EP additives (S-P compounds) using the same testing method. It can be noted that the addition of HNTs into the lubricant delayed scuffing initiation at all concentrations, appearing at 7.2, 9, and 7.6 s for 0.01, 0.05, and 0.10 wt% HNTs, respectively. At 0.05 wt% a Pt of 3600 N was obtained, indicating that at this concentration the nanoparticles were able to withstand higher loads before the lubricant film presented any breakdown. The lower improvement found at 0.10 wt% (Pt of ∼3050 N) can be attributed to agglomeration due to higher nanoparticle concentration. As proposed by Ghaednia et al. [10] smaller sized nanoparticles are more likely to infiltrate gaps between surfaces reducing contact and withstanding higher loads, compared to large agglomerates.

It is also evident that none of the lubricants presented seizure (Fig. 2), as a frictional torque higher than 10 N.m was not reached, likely due to the presence of additives in the polymeric lubricant; hence, Poz was taken as 7200 N for all materials. The load-carrying capacity or poz was then calculated according to Eq. (1) taking into account the seizure load (Poz) and the average of the WSDs of the three lower balls of the four-ball tribotest. Fig. 3 shows the loadcarrying capacity of HNT nanolubricants compared to the unfilled lubricants. Results had a similar behavior as the ones obtained for scuffing initiation, with poz showing an increasing trend up to 0.05 wt%, and a decrease in this improvement at higher HNT concentration, due to nanoparticle aggregation. The unfilled polymeric lubricant had a poz of ∼2300 N/mm2 In comparison, 8 cSt synthetic polyalfaolephin oil (PAO8) and RzR rapeseed refined oil without additives have shown poz values of ∼230 N/mm2 [2,42] and ∼260 N/mm2 [42], respectively, whereas fully formulated oils with EP additives API GL-4, API GL-5m (mineral base) and API GL-5s (synthetic) had load-carrying capacities of ∼1800 N/mm2 [2], ∼2400 N/mm2, and ∼2300 N/mm2, respectively [42]. The HNT nanolubricants prepared for this study showed a poz of up to ∼3900 N/mm2 (an increase of 72% compared to the polymeric lubricant) at a concentration of 0.05 wt%. Thus, the results shown in this study show significant improvements with respect to previously reported values found in the literature. Figs. 4 and 5 show the results obtained by the block-on-ring test. The effect of nanoparticle additive concentration on the COF with respect to time is displayed in Fig. 4. It was found that COF can be significantly reduced by adding HNTs into the polymeric lubricant. The best results were obtained with a concentration of 0.05 wt% HNTs; here, a very smooth curve with almost no variation was observed and the steady-state was achieved much earlier in the run, compared to 0.01 wt% and 0.10 wt%.

890

L. Peña-Parás et al. / Wear 376-377 (2017) 885–892

Fig. 9. EDS spectra and elemental analysis of worn steel balls lubricated with: (a) polymeric lubricant, (b) 0.01 wt% HNTs, (c) 0.05 wt% HNTs, (d) 0.10 wt% HNTs.

Wear volume loss results of blocks and their average COF obtained in the steady-state are depicted in Fig. 5. All HNTs nanolubricants exhibited lower wear volumes and COF valued compared to the unfilled lubricant. Also, a similar trend as with the EP four-ball test was observed in this test, with the best tribological results found at 0.05 wt%, and smaller improvements at higher concentration (0.10 wt%) likely due to nanoparticle agglomeration. Wear volume loss decreased by 41%, 70%, and 20% at HNT concentrations of 0.01, 0.05, and 0.10 wt%, respectively. Similarly, COF was reduced by 51%, 71%, and 49% at 0.01, 0.05, and 0.10 wt%, respectively. The possible tribological enhancing mechanisms for the

four-ball and block-on-ring test provided by HNTs are discussed in Section 3.2. 3.2. Worn surface analysis The wear scars of steels balls tested under EP-conditions lubricated with the unfilled and HNT-nanolubricants are shown in Fig. 6. As observed, the presence of HNT additives resulted in smaller WSDs (Fig. 6b–d) compared to those found in the polymeric lubricant (Fig. 6a). Because the scuffing wear appeared at least two seconds before for the polymeric lubricant without

L. Peña-Parás et al. / Wear 376-377 (2017) 885–892

nanoparticles (Fig. 2) a lot of metal burr or wear debris is present on the perimeter of the wear scar in favor of the wear motion (Fig. 6a). However, nanolubricants (Fig. 6b-d) show complete absence of metal burr. As shown by Mosleh [6], nanoparticles may deagglomerate metallic transfer films under EP conditions. The smallest WSD with lower presence of pits was found for 0.05 wt% HNTs (Fig. 6c) thanks to the decrease in frictional torque compared to 0.01 wt% and 0.10 wt%, shown in Fig. 2. The increase in WSD for 0.10 wt% (Fig. 6d) is due to nanoparticle agglomeration. These agglomerates may form new asperities causing higher friction and wear, and rougher surfaces [43, 44]. The improvement in tribological properties provided by HNTs can be explained as follows: due to the pressures of the test HNT exfoliation occurs and a tribofilm is deposited onto the steel balls effectively lowering metal-metal contact. Because of the tubular shape of nanoparticles a rolling bearing effect is also proposed. This is in agreement with the findings obtained by Lahouij et al. [18,19], where fullerene-like hollow nanostructures nanoparticles with similar Young´s Modulus compared to HNTs exfoliate at a lower contact pressure than the one applied in this experiment. Similarly, Fig. 7 shows the SEM micrographs of worn blocks. The wear track for the unfilled polymer lubricant presents many grooves and furrows, as well as some metal burr due to adhesive wear in the absence of a tribofilm. In Fig. 7b-d a considerable decrease in furrow-depth and metal burr is evident. For the 0.01 wt% HNTs (Fig. 7b) due to the very low nanoparticle concentration there may be only some exfoliated sheet deposition. Fig. 7c with 0.05 wt% HNTs presents an evener surface with shallow furrows, explaining the smooth COF curve (Fig. 4), low average COF, and lower wear volume (Fig. 5), compared to the unfilled lubricant. This may be attributed the exfoliation of outer layers of HNTs forming a protecting tribofilm onto the surface. As previously mentioned, the nanoparticle clusters obtained at 0.10 wt% (Fig. 7d) have a detrimental effect on the nanoparticles´ tribological enhancing mechanisms, as it hinders their ability to infiltrate gaps between rubbing surfaces and provide a rolling-bearing effect under sliding conditions [19,45]. Surface roughness profiles of worn blocks are shown in Fig. 8. The surface roughness in Fig. 8a with the unfilled lubricant presents numerous asperities and an Ra of 0.65 mm. At 0.01 wt% HNTs (Fig. 8b) there was a slight decrease on roughness with an Ra of 0.61 mm The smoothest surface was obtained at 0.05 wt%, with an Ra of 0.41 mm. Lower roughness values delay the reduction of the lubricant layer film thickness, explaining the anti-wear and friction-reduction findings shown in Fig. 5. When nanoparticle concentration was increased to 0.10 wt% surface roughness increased (Ra ¼ 0.58 mm). Similar results were obtained by Sia, et al. [46], where nanoparticle clusters reduced the improvement on surface roughness. Fig. 9 presents the EDS spectra obtained on the worn surfaces of steel balls. Fig. 9a shows the spectrum and elemental analysis of the materials lubricated with the unfilled polymeric lubricants. Fig. 9b–d reveal some Al and Si elements present on the surface confirming exfoliation of HNT´s outer layers during the test. The deposition of HNT laminates formed a tribofilm that reduced friction and wear between rubbing surfaces, explaining the enhancements in load-carrying capacity and wear resistance. Based on findings by similar studies [14,17–19] and the testing conditions of our work a rolling bearing mechanism could also be present, however, further experiments should be done in order to confirm this mechanism. 4. Conclusions Nanoparticles of HNTs demonstrated to be good candidates as environmentally-friendly lubricant additives. The load-carrying

891

capacity obtained by the ITeE-PIB Polish Method increased by up to 72% at 0.05 wt% HNTs; a delay in scuffing initiation was found at all nanoparticle concentrations, as the load required for lubricant film breakdown also increased. Similarly, wear volume loss and COF were reduced by
70% at the same concentration, and smoother surfaces were obtained with lower groove density and metal debris deposition. EDS analysis confirmed the presence of a tribofilm, attributed to exfoliation of HNT´s outer sheets due to the high pressures of the tests. It was found that higher nanoparticle concentration (0.10 wt%) diminished the enhancing effect of HNTs due to agglomeration.

Acknowledgments Authors acknowledge the support from UDEM VIAC grants UIN14019 and UIN15013. The authors gratefully acknowledge ASF and CIIDIT for their contribution to this work.

References [1] S.M. Alves, B.S. Barros, M.F. Trajano, K.S.B. Ribeiro, E. Moura, Tribological behavior of vegetable oil-based lubricants with nanoparticles of oxides in boundary lubrication conditions, Tribol. Int. 65 (2013) 28–36, http://dx.doi. org/10.1016/j.triboint.2013.03.027. [2] L. Peña-Parás, J. Taha-Tijerina, L. Garza, D. Maldonado-Cortés, R. Michalczewski, C. Lapray, Effect of CuO and Al2O3 nanoparticle additives on the tribological behavior of fully formulated oils, Wear. 333 (2015) 1256–1261, http://dx.doi.org/10.1016/j.wear.2015.02.038. [3] L. Bogatu, C. Tanasescu, Optim. Balance Extrem. Press. Antiwear Addit. Gear Lubr. (2013) 904–908. [4] Z.J. Zhang, D. Simionesie, C. Schaschke, Graphite and hybrid nanomaterials as lubricant additives, Lubricants. 2 (2014) 44–65, http://dx.doi.org/10.3390/ lubricants2020044. [5] Z. Chen, X. Liu, Y. Liu, S. Gunsel, J. Luo, Ultrathin MoS2 nanosheets with superior extreme pressure property as boundary lubricants, Sci. Rep. 5 (2015) 12869, http://dx.doi.org/10.1038/srep12869. [6] M. Mosleh, M. Ghaderi, Deagglomeration of transfer film in metal contacts using nanolubricants, Tribol. Trans. 55 (2012) 52–58, http://dx.doi.org/ 10.1080/10402004.2011.626146. [7] J. Taha-Tijerina, L. Peña-Paras, T.N. Narayanan, L. Garza, C. Lapray, J. Gonzalez, E. Palacios, D. Molina, A. García, D. Maldonado, P.M. Ajayan, Multifunctional nanofluids with 2D nanosheets for thermal and tribological management, Wear. 302 (2013) 1241–1248, http://dx.doi.org/10.1016/j.wear.2012.12.010. [8] L. Peña-Parás, J. Taha-Tijerina, A. García, D. Maldonado, J.A. González, E. Palacios, P. Cantú, Antiwear and extreme pressure properties of nanofluids for industrial applications, Tribol. Trans. 57 (2014) 1072–1076, http://dx.doi. org/10.1080/10402004.2014.933937. [9] Z.Y. Xu, Y. Xu, K.H. Hu, Y.F. Xu, X.G. Hu, Formation and tribological properties of hollow sphere-like nano-MoS2 precipitated in TiO2 particles, Tribol. Int. 81 (2015) 139–148, http://dx.doi.org/10.1016/j.triboint.2014.08.012. [10] H. Ghaednia, R.L. Jackson, J.M. Khodadadi, Experimental analysis of stable CuO nanoparticle enhanced lubricants, J. Exp. Nanosci. (2013) 1–18, http://dx.doi. org/10.1080/17458080.2013.778424. [11] L. Rapoport, V. Leshchinsky, I. Lapsker, Y. Volovik, O. Nepomnyashchy, M. Lvovsky, R. Popovitz-Biro, Y. Feldman, R. Tenne, Tribological properties of WS2 nanoparticles under mixed lubrication, Wear 255 (2003) 785–793, http: //dx.doi.org/10.1016/S0043-1648(03)00044-9. [12] M. Gulzar, H. Masjuki, M. Varman, M. Kalam, R. Mufti, N. Zulkifli, R. Yunus, R. Zahid, Improving the AW/EP ability of chemically modified palm oil by adding CuO and MoS2 nanoparticles, Tribol. Int. 88 (2015) 271–279, http://dx. doi.org/10.1016/j.triboint.2015.03.035. [13] J. Tannous, F. Dassenoy, I. Lahouij, T. Le Mogne, B. Vacher, A. Bruhács, W. Tremel, Understanding the tribochemical mechanisms of IF-MoS 2 nanoparticles under boundary lubrication, Tribol. Lett. 41 (2011) 55–64, http://dx. doi.org/10.1007/s11249-010-9678-1. [14] M. Kalin, J. Kogovšek, M. Remškar, Nanoparticles as novel lubricating additives in a green, physically based lubrication technology for DLC coatings, Wear 303 (2013) 480–485, http://dx.doi.org/10.1016/j.wear.2013.03.009. [15] M. Mosleh, N.D. Atnafu, J.H. Belk, O.M. Nobles, Modification of sheet metal forming fluids with dispersed nanoparticles for improved lubrication, Wear 267 (2009) 1220–1225, http://dx.doi.org/10.1016/j.wear.2008.12.074. [16] B. Rahmati, A. a D. Sarhan, M. Sayuti, Morphology of surface generated by end milling AL6061-T6 using molybdenum disulfide (MoS2) nanolubrication in end milling machining, J. Clean. Prod. 66 (2014) 685–691, http://dx.doi.org/ 10.1016/j.jclepro.2013.10.048. [17] C.P. Koshy, P.K. Rajendrakumar, M.V. Thottackkad, Evaluation of the tribological and thermo-physical properties of coconut oil added with MoS2

892

[18]

[19]

[20]

[21] [22]

[23]

[24]

[25]

[26]

[27]

[28]

[29] [30] [31]

[32]

L. Peña-Parás et al. / Wear 376-377 (2017) 885–892

nanoparticles at elevated temperatures, Wear (2015) 1–21, http://dx.doi.org/ 10.1016/j.wear.2014.12.044. I. Lahouij, E.W. Bucholz, B. Vacher, S.B. Sinnott, J.M. Martin, F. Dassenoy, Lubrication mechanisms of hollow-core inorganic fullerene-like nanoparticles: coupling experimental and computational works, Nanotechnology 23 (2012) 375701, http://dx.doi.org/10.1088/0957-4484/23/37/375701. I. Lahouij, F. Dassenoy, B. Vacher, J.-M. Martin, Real time TEM imaging of ompression and shear of single fullerene-Like MoS2 nanoparticle, Tribol. Lett. 45 (2012) 131–141, http://dx.doi.org/10.1007/s11249-011-9873-8. Z. Chen, H. Meng, G. Xing, C. Chen, Y. Zhao, G. Jia, T. Wang, H. Yuan, C. Ye, F. Zhao, Z. Chai, C. Zhu, X. Fang, B. Ma, L. Wan, Acute toxicological effects of copper nanoparticles in vivo, Toxicol. Lett. 163 (2006) 109–120, http://dx.doi. org/10.1016/j.toxlet.2005.10.003. C. Buzea, I.I. Pacheco, K. Robbie, Nanomaterials and nanoparticles: sources and toxicity., Biointerphases (2007), http://dx.doi.org/10.1116/1.2815690. S.Y. Madani, A. Mandel, A.M. Seifalian, A concise review of carbon nanotube's toxicology, Nano Rev. 4 (2013) 1–14, http://dx.doi.org/10.3402/nano. v4i0.21521. M. Zhang, X. Wang, X. Fu, Y. Xia, Performance and anti-wear mechanism of CaCO3 nanoparticles as a green additive in poly-alpha-olefin, Tribol. Int. 42 (2009) 1029–1039, http://dx.doi.org/10.1016/j.triboint.2009.02.012. Y. Lvov, E. Abdullayev, Functional polymer – clay nanotube composites with sustained release of chemical agents, Prog. Polym. Sci. 38 (2013) 1690–1719, http://dx.doi.org/10.1016/j.progpolymsci.2013.05.009. H.A. Duarte, M.P. Lourenço, T. Heine, L. Guimarães, Clay mineral nanotubes: stability, structure and properties, Stoichiom. Mater. Sci. - When Numbers Matter (2009) 3–25, http://dx.doi.org/10.5772/34459. A. Sánchez-Fernández, L. Peña-Parás, R. Vidaltamayo, R. Cué-Sampedro, A. Mendoza-Martínez, V. Zomosa-Signoret, A.M. Rivas-Estilla, P. Riojas, Synthesization, Characterization, and in Vitro Evaluation of Cytotoxicity of Biomaterials Based on Halloysite Nanotubes, Materials, 7, Basel 2014, pp. 7770–7780, http://dx.doi.org/10.3390/ma7127770. V. Vergaro, E. Abdullayev, Y.M. Lvov, A. Zeitoun, R. Cingolani, R. Rinaldi, S. Leporatti, Cytocompatibility and uptake of halloysite clay nanotubes, Biomacromolecules 11 (2010) 820–826, http://dx.doi.org/10.1021/bm9014446. M. Liu, Z. Jia, D. Jia, C. Zhou, Recent advance in research on halloysite nanotubes-polymer nanocomposite, Prog. Polym. Sci. 39 (2014) 1498–1525, http: //dx.doi.org/10.1016/j.progpolymsci.2014.04.004. D. Rawtani, Y.K. Agrawal, Multifarious applications of halloysite nanotubes: a review, Rev. Adv. Mater. Sci. 30 (2012) 282–295. R. Kamble, M. Ghag, S. Gaikawad, B.K. Panda, Review article halloysite nanotubes and applications: a review, J. Adv. Sci. Res. 3 (2012) 25–29. V. Vergaro, Y.M. Lvov, S. Leporatti, Halloysite clay nanotubes for resveratrol delivery to cancer cells, Macromol. Biosci. 12 (2012) 1265–1271, http://dx.doi. org/10.1002/mabi.201200121. M.J. Mitchell, C.A. Castellanos, M.R. King, Nanostructured surfaces to target

[33] [34]

[35]

[36]

[37]

[38] [39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

and kill circulating tumor cells while repelling leukocytes, J. Nanomater (2012), http://dx.doi.org/10.1155/2012/831263. E.Abdullayev, Y.Lvov, Halloysite Nanotubes for Sustained Release of Corrosion, Sci. International Coat. Symp. Technol, 2010. E. Abdullayev, V. Abbasov, A. Tursunbayeva, V. Portnov, H. Ibrahimov, G. Mukhtarova, Y. Lvov, Self-healing coatings based on halloysite clay polymer composites for protection of copper alloys, ACS Appl. Mater. Interfaces. 5 (2013) 4464–4471, http://dx.doi.org/10.1021/am400936m. D. Lingaraju, K. Ramji, M.P. Devi, U.R. Lakshmi, Mechanical and tribological studies of polymer hybrid nanocomposites with nano reinforcements, Bull. Mater. Sci. 34 (2011) 705–712, http://dx.doi.org/10.1007/s12034-011-0185-2. M.T. Albdiry, Morphological structures and tribological performance of unsaturated polyester based untreated/silane-treated halloysite nanotubes, Mater. Des. 48 (2013) 68–76, http://dx.doi.org/10.1016/j.matdes.2012.08.035. W. Piekoszewski, M. Szczerek, W. Tuszynski, The action of lubricants under extreme pressure conditions in a modified, Wear 249 (2001) 188–193, http: //dx.doi.org/10.1016/S0043-1648(01)00555-5. M. Szczerek, W. Tuszynski, A method for testing lubricants under conditions of scuffing. Part I. Presentation of the method, Tribotest 8 (2002) 2002. W. Piekoszewski, M. Szczerek, W. Tuszynski, A method for testing lubricants under conditions of scuffing. Part II. The anti-seizure action of lubricating oils, Tribotest 9 (2002) 35–48. ASTM, G77-05, Standard Test Method for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear Test, (2008). doi:http://dx.doi.org/10. 1520/G0077-05R10.2. R. Michalczewski, W. Piekoszewski, W. Tuszynski, M. Szczerek, J. Wulczyński, The new methods for scuffing and pitting investigation of coated materials for heavy loaded, lubricated elements, Tribology - Lubricants and Lubrication, Dr. Chang-Hung. Kuo (Ed. ), InTech (2011) 315–320, http://dx.doi.org/10.5772/ 24382. M. Kalbarczyk, R. Michalczewski, W. Piekoszewski, M. Szczerek, The influence of oils on the scuffing of concentrated friction joints with low-friction coated elements, Eksploat. I Niezawodn. – Maint. Reliab. 15 (2013) 319–324. Q. Wan, Y. Jin, P. Sun, Y. Ding, Tribological behaviour of a lubricant oil containing boron nitride nanoparticles, Procedia Eng. 102 (2015) 1038–1045, http: //dx.doi.org/10.1016/j.proeng.2015.01.226. C.J. Reeves, P.L. Menezes, M.R. Lovell, T.-C. Jen, The size effect of boron nitride particles on the tribological performance of biolubricants for energy conservation and sustainability, Tribol. Lett. 51 (2013) 437–452, http://dx.doi.org/ 10.1007/s11249-013-0182-2. H. Ghaednia, R.L. Jackson, The effect of nanoparticles on the real area of contact, friction, and wear, J. Tribol. 135 (2013) 041603, http://dx.doi.org/ 10.1115/1.4024297. S.Y. Sia, E.Z. Bassyony, A.A.D. Sarhan, Development of SiO2 nanolubrication system to be used in sliding bearings, Int. J. Adv. Manuf. Technol. 71 (2014) 1277–1284, http://dx.doi.org/10.1007/s00170-013-5566-9.