Organic-Modified Silver Nanoparticles as Lubricant Additives - ACS

Subscriber access provided by LAURENTIAN UNIV

Article

Organic-Modified Silver Nanoparticles as Lubricant Additives Chanaka Kumara, Huimin Luo, Donovan N Leonard, Harry M. Meyer, and Jun Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13683 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Organic-Modified Silver Nanoparticles as Lubricant Additives Chanaka Kumara,†₤ Huimin Luo, ‡ Donovan N. Leonard,† Harry M. Meyer,† Jun Qu†* †

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ₤Department of Chemistry, University of Tennessee, Knoxville, Tennessee, 37996, United States

ABSTRACT Advanced lubrication is essential in human life for improving mobility, durability, and efficiency. Here we report the synthesis, characterization, and evaluation of two groups of oil-suspendable silver nanoparticles (NPs) as candidate lubricant additives. Two types of thiolated ligands, 4-(tertbutyl)benzylthiol (TBBT) and dodecanethiol (C12), were used to modify Ag NPs in two size ranges, 1-3 and 3-6 nm. The organic surface layer successfully suspended the Ag NPs in a poly-alpha-olefin (PAO) base oil with concentrations up to 0.19-0.50 wt%, depending on the particle type. Using the Ag NPs in the base oil reduced friction by up to 35% and wear by up to 85% in boundary lubrication. The two TBBTmodified NPs produced a lower friction coefficient than C12-modified one, while the two larger NPs (3-6 nm) had better wear protection than the smaller one (1-3 nm). Results suggested that the molecular structure of the organic ligand might have a dominant effect on the friction behavior while the NP size could be more influential in the wear protection. No mini-ball-bearing or surface smoothening effects was observed in Stribeck scans. Instead, the wear protection in boundary lubrication was attributed to the formation of a silver-rich 50-100 nm thick tribofilm on the worn surface, as revealed by morphology examination and composition analysis from both the top surface and cross-section.

KEYWORDS: Silver nanoparticles, lubricant, additive, boundary lubrication, anti-wear, friction reduction, tribofilm

*

P.O. Box 2008, MS-6063, Oak Ridge, TN 37831-6063, (865) 576-9304, [email protected]

ACS Paragon Plus Environment

1


Page 2 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Lubricants have always been essential in human history and the focus has gradually shifted from mobility in ancient era to durability in modern life and then to efficiency currently and in the foreseeable future. Lubrication is an interfacial phenomenon with three regimes: hydrodynamic, mixed, and boundary lubrication.1 A lubricated contact interface is an extremely dynamic system with the lubricant experiencing transient mechanical and thermal stress. Particularly in boundary and mixed lubrication, surface asperity collisions inevitably occur, where friction modifiers and anti-wear additives play important roles. Nanoparticles (NPs) are widely used as catalysts, chemical sensors, and semiconductors in solar cells2-5. Recent studies suggested good potential for NPs as novel lubricant additives.6-10 Metallic, carbonbased, and ceramic NPs are believed to have distinct mechanisms for friction and wear reduction. Hard ceramic NPs, e.g., ZrO2, SiO2, and Al2O3, may function as micro ball-bearing rolling at the sliding interface under a mild pressure, as abrasive providing a polishing effect under a moderate pressure, or as reinforcement in the tribofilm under an extreme pressure.11-14 Hard carbon-based, such as diamond, NPs15 may act as ball-bearing or increase the surface hardness16 by incorporating into contact area. Soft carbonbased, e.g., graphite and fullerene, NPs may be exfoliated to form a protective film upon contact and rubbing.17-19 On the other hand, the softer metallic NPs, e.g., Ag, Cu, and Zn, have their own advantages: (1) (1) they may physically be pressed and/or smeared onto the contact area under pressure resulting in a NPs-containing softer and more compliant layer, which allows easier deformation for faster running-in with better conformal contact to reduce asperity contact pressure, decreases the resistance to shear for lower friction, and provides a cushion for asperity collisions with less noise,20 and (2) they may participate in the tribofilm formation either by tribochemical reactions as a metal cation supply.21-25 Silver NPs are of our particular interest because of their chemical stability11, robust synthetic protocol, and great potential in friction reduction and anti-wear properties10, 19, 26-30 Major challenges for using metal NPs as lubricant additives include particle oxidation, agglomeration, and precipitation in oils. Surface modification may be used to convert these metal NPs into organic miscible NPs. Previous studies


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

have shown the feasibility of using surface modification to enhance the oil solubility of metallic NPs, such as Au,21 Ag,19, 28 Pd,23 Cu22 and Ni.24 In this study, we used thiol ligands to modify the silver NPs due to the stable metal-thiol interface and potential for improved oil suspendability.31-3217 Ag NPs modified by 4-(tert-butyl)benzylthiol were reported with excellent stability in ambient environments.33-34 Recent advances in the total structural analysis of thiolated silver NPs revealed unique structures of silver thiolated protecting layers at the interface.31, 35-3620,21 However, there is no report in the literature investigating oil solubility and tribological behavior of thiolated silver NPs as lubricant additives. In this study, three samples of thiol-modified Ag NPs were synthesized: two 4-(tert-butyl)benzylthiol functionalized, with metallic core particle sizes of 13 nm and 4-5 nm (denoted as NP-TBBT-1 and NP-TBBT-2, respectively), and one dodecanethiol functionalized with core particle size of 3-6 nm (abbreviated as NP-C12). They were characterized by UV-visible spectroscopy, transmission electron microscopy, and thermogravimetric analysis. These thiolmodified Ag NPs are stable in both dry and liquid forms and can be stably dispersed in lubricating oils. Two tribological bench tests were carried out on the NP-containing oils: one test to investigate the friction behavior in elastohydrodynamic and mixed lubrication and the other to evaluate the friction and wear performance in boundary lubrication. Focus ion beam (FIB) aided cross-sectional STEM/EDS examination and XPS surface chemical analysis of the worn surfaces were performed for fundamental understanding.

2. EXPERIMENTAL AND MATERIALS 2.1. Synthesis of silver nanoparticles Synthesis of 4-(tert-butyl)benzylthiol functionalized 1-3 nm silver NPs (Ag NP-TBBT-1). Ultrasmall silver nanoparticle were synthesized using the method similar to

Murray et al with minor

modification33. Briefly, AgNO3 (1mmol /170 mg) was dissolved in acetonitrile ( 2 mL) and mixed with a toluene solution (30 mL) of tetraoctylammonium bromide (1 mmol/ 550 mg) and stirred for 30 min. This solution was cooled in an ice bath. Then, 4-(tert-butyl)benzyl thiol (3.1 mmol) was added and stirred for


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


another 30 min. NaBH4 (10 mmol/0.380 g) dissolved in ice-cooled water (4 mL) was added to the mixture and the reaction was continued for another 3 hour at 0 oC. The organic phase was isolated and solvent removed by rotary evaporation. The product washed with methanol several times to remove excess thiol and other byproducts. Finally, the product was extracted with toluene to isolate ultra-small tert-butylbenzyl thiol functionalized silver nanoparticles, NP-TBBT-1. Synthesis of 4-(tert-butyl)benzylthiol functionalize 4-5 nm silver NPs (Ag NP-TBBT-2). 4-(tertbutyl)benzylthiol functionalized silver nanoparticles were synthesized according to the following procedure. AgNO3 (1 mmol/ 170 mg) was dissolved in acetonitrile (2 mL), mixed with toluene (30 mL) and stirred for 30 min. To this solution, 4-(tert-butyl)benzylthiol (2 mmol) was added and stirred for another 30 min. Next, tert-butylamine Boran (10 mmol/0.435 g) powder was added to the mixture and the reaction was continued for another 2 hours at room temperature. The solvent was removed by rotary evaporation and washed with methanol several times to remove excess thiol and other byproducts. Finally, the product was extracted with toluene to isolate 4-(tert-butyl)benzylthiol functionalized silver nanoparticles, NP-TBBT-2. Synthesis of dodecanethiol functionalized 3-6 nm silver NPs (Ag NP-C12). Dodecanethiol functionalized silver nanoparticles were synthesized in similar to a published report37 with minor modification. Briefly, AgNO3 (1 mmol/ 170 mg) was dissolved in acetonitrile (2 mL), mixed with toluene (30 mL), and stirred for 30 min at 70 oC. Dodecanethiol (2 mmol) was added and stirred for another 30 min. Next, tert-butylamine Boran (10 mmol/0.435 g) powder was added to the mixture and the reaction was continued for another 2 hours at 70 oC. After cooling to room temperature, the solvent was removed by rotary evaporation and washed with ethanol several times to remove excess thiol and other byproducts. Finally, the product was extracted with toluene to isolate the dodecanethiol functionalized silver nanoparticles, NP-C12. While one could concern the sulfur content, the amount of sulfur introduced by the organic ligands is negligible. For example, 0.5 wt% of the NP-C12 would only add 0.007 wt% sulfur to the lubricant, which is orders of magnitude below the upper limit of sulfur (0.5 wt%) in a commercial GF-5/6


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

automotive engine oil, regulated by ILSAC, International Lubricants Standardization and Approval Committee.

2.2. Characterization Thermogravimetric analysis (TGA) of silver nanoparticles was performed with an AutoTGA2950 (TA Instruments) under a N2 environment at a 10 °C/ min heating rate. Transmission electron microscopy (TEM) images were taken with an Hitachi HF-3300 at 300 kV. Samples suitable for TEM analysis were prepared by drop-casting a toluene solution of nanoparticles onto lacey carbon film supported on Cu grids. Images were analyzed using ImageJ software (version 1.46r). UV-Visible spectra were recorded in toluene with a Varian Cary 5000 spectrophotometer in a quartz cell at 300–1100 nm. FTIR spectra of the NPs were measured using a Perkin Elmer FTIR spectrometer with a Universal ATR accessory. A Hitachi S4800 scanning electron microscope (SEM) equipped with an energy-dispersive spectroscopy (EDS) was used for the worn surface analysis and qualitative elemental mapping. The nanostructure of the tribofilm was also analyzed with the Hitachi HF-3300 TEM coupled with a Bruker solid state EDS detector. Initially, an area on the wear scar was selected and a protective evaporated carbon layer followed by an ion beam deposited tungsten layer were used to protect the tribofilm. Thin cross sections of the wear scar were lifted out using a Hitachi NB5000 FIB equipped with a gallium ion source. X-ray photoelectron spectroscope (XPS) was used to analyze the chemical composition of the tribofilm ( ThermoScientific K-Alpha XPS). X-rays were generated using monochromatic Al-Kα source and the emitted photoelectron were analyzed with a hemispherical energy analyzer. Ion sputtering was performed using Ar+ (under 2 kV ) for 15 S to remove any surface adsorption species and contamination. The relative elemental composition was calculated by measuring the peak areas of the primary core levels of the elements present and normalized with tabulated sensitivity factors. Composition depth profiles were acquired using Ar-ion sputtering.


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3. Lubrication regime determination A lubrication regime usually is determined by the ratio (λ) between the lubricant film thickness (h) and the composite roughness (σ) at the contact area: λ=h/σ. There are four commonly-defined regimes1: boundary (BL, λ < 1), mixed (ML, 1 ≤ λ < 3), elastohydrodynamic (EHL, 3 ≤ λ < 10), and hydrodynamic lubrication (HL, λ ≥ 10). BL involves solid-solid surface asperity collisions resulting in a relatively high friction coefficient (usually 0.05–0.15) and surface deformation and/or wear. In contrast, an EHL or HL contact is fully separated by a liquid film and thus has a much lower friction coefficient (usually 0.001–0.03) with no wear. ML is the transition stage between BL and EHL. In this study, the lubricant film thickness (h) was calculated using the Hamrock and Dowson formula1. The composite 2 2 roughness is defined as σ = σ 1 + σ 2 , where σ1 and σ2 are the root-mean-square (RMS) surface

roughness of the two surfaces in contact.

2.4. Tribological testing Stribeck curves were generated at room temperature using a variable load-speed cylinder-on-flat unidirectional sliding tester to study the impact of Ag NPs on the friction behavior in elastohydrodynamic and mixed lubrication regimes. A cylinder (1 inch diameter) of AISI 8620 alloy steel was set to rotate against a M2 bearing steel flat (13x12 mm2) under a constant load of 50 N with the sliding speed decreasing from 1.2 to 0.1 m/s at a 0.1 m/s interval. The actual contact length was about 8 mm because of the surface unevenness and edge tapering caused by polishing. The cylindrical and flat surfaces were polished using 1200 grit SiC abrasive paper prior to the test to reach average roughness of ~23 nm and ~13 nm (arithmetic average, Ra), respectively. Considering the RMS roughness (Rq) usually is 30% higher than Ra, the composite roughness σ of the cylinder-flat contact was estimated to be ~34 nm. Friction force was captured in situ by measuring the tangential force using a load cell. Nine repeat tests were performed for each lubricant.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

Boundary lubrication tests were carried out at room temperature using a Plint TE77 tribometer with an AISI 52100 steel ball (10 mm diameter) reciprocating sliding against an A2 tool steel flat (13x12 mm2). The ball was a standard grade 25 bearing steel ball and the surface roughness was measured to be ~15 nm (Ra). The steel flat surface was polished using 1200 grit SiC abrasive paper to reach a roughness of ~10 nm (Ra). The composite roughness σ of the ball-flat contact was estimated to be ~23 nm. All contact surfaces were cleaned with acetone followed by isopropyl alcohol before and after the test. Tests were conduct by applying a normal load of 100 N at 10 Hz oscillation with a 10 mm stroke for a sliding distance of 1000 m. The Hertzian contact stress at the beginning of the test is calculated to be 1.68 GPa. Friction force was captured in situ by measuring the tangential force with a piezoelectric load cell. Two repeat tests were performed for each lubricant. Wear volumes were quantified using a Wyko NT9100 white light interferometer.

3. RESULTS 3.1. Physicochemical properties of the Ag NPs Figure 1 shows the model structures of the Ag NPs synthesized in this work. Two types of ligand, 4-(tert-butyl)benzylthiol and dodecanethiol, were used to modify the NP surface. Bulky tert-butyl group substitution at the para-position of the benzyl group afforded excellent stability for silver NPs.33-34 Dodecanethiol was used to increase the NPs’ oil suspendability because of its longer alkyl chain. Figure 2a shows the UV-visible spectrum of the synthesized Ag NPs. NP-TBBT-1 displays a nearly featureless spectrum due to the ultra-small particle sizes (< 3 nm). The optical absorption features of NP-TBBT-2 shows emergence of a surface plasmon band around ~450 nm. In contrast to the TBBT-capped NPs, the spectrum of the NP-C12 Ag NPs has a dominant surface plasmon resonance (SPR) band centered at 480 nm. Surface plasmon resonance (SPR) is a collective oscillation of free electrons of the NPs in response to the incident light. The silver (4d10 5S1) NPs have sufficient free electrons that interact with incident radiation. It should be note that the NP shape and size as well as the organic ligand all effect the SPR band. This may be attributed to the ligand molecular structure and its bonding to the metallic core. The


8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nucleophilic TBBT ligand is expected to more strongly interact with the Ag atoms, act as shield and alter the electron density distribution of the metallic core. The strong covalent bonds suppress the electron density that is available for collectively response to the incident light, resulting a broad, less prominent SPR band. In contrast, the C12 ligand is less nucleophilic, compared to the TBBT, and thus the electron shielding effect is weaker in responding to the incident radiation. The TEM images of the NP-TBBT-1, NP-TBBT-2 and NP-C12 in Fig. 1 show nearly spherical shapes for all three samples of NPs. Note that the NPs’ distribution on the TEM grid depends on the type of solvent used, method of sample casting, and NP concentration. A toluene solution of NPa dropcast onto the TEM grid promoted the formation of a self-assembly monolayer. On the TEM grid, isolated NPs, self-assembly monolayers, and NP aggregated areas were all observed due to the fast solvent evaporation, and have no correlation to their dispersions in the lubricating oil. The smaller NP-TBBT-1 (1-3 nm) was produced by using a relatively strong reducing agent, NaBH4. A stronger reducing agent causes a faster reduction of metal ions, generating more nucleation events and resulting in smaller nanoparticles. In contrast, the NP-TBBT-2 was synthesized using a weaker reducing agent, tert-butylamine-borane. The slower reduction along with the lower metal to thiol molar ratio facilitated slower nucleation and allowed the silver particles to ripen over time to reach a final core particle size of 4-5 nm (see Figs. 1 and S1). A similar synthesis was also attempted at 70 oC, but the product decomposed prior to characterization. The NP-TBBT-1 and NP-TBBT-2 basically share the same organic layer but have different particle sizes. Both FTIR spectra of the NP-TBBT-1 and NP-TBBT-2 (see Fig. S2) show similar characteristic vibration bands of the tert-butylbenzyl group: the C-H stretching vibration of the benzene ring arises in the 3000-3100 cm-1 region and that of tert-butyl group appears around 2800-3000 cm-1, and the C=C bending vibration of the benzene ring carbon can be seen in the 1550-1650 cm-1 region. The FTIR analysis results confirmed the similarity of the ligand layers on the NP-TBBT-1 and NP-TBBT-2 NPs. The NP-C12 sample was synthesized using dodecanethiol with a protocol similar to the TBBT-2, but at 70 oC. The NP-C12 sample has a similar mean core particle size of 4.5 nm, albeit with a larger size variation from 3 to 6 nm indicating significant polydispersity.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

Figure 1. Model structures and morphology of thiol-modified Ag NPs. NP-TBBT-1: 4-(tert-butyl)benzyl thiol functionalized, core particle size of 1-3 nm, NP-TBBT-2: 4-(tert-butyl)benzyl thiol functionalized, core particle size of 4-5 nm, NP-C12: Dodecanethiol functionalized, core particle size of 3-6 nm.

Since these silver NPs are hybrid organic-inorganic materials, thermogravimetric analysis can be used to determine the organic content. Figure 2b presents the TGA curves of the thiol-modified NPs acquired by ramping the temperature from 20 to 700 oC at a heating rate of 10 oC/ min. Experiments were conducted in an inert atmosphere to avoid possible silver oxidation. The organic content of the thiolmodified NP was found to be NP-TBBT-1: 42 wt%, NP-TBBT-2: 35 wt %, and NP-C12: 30 wt%. The Ag:SR (R is the organic ligand attached to the NP) molar ratio was determined based on the TGA data to be 1:0.44, 1:0.32 and 1:0.23 for NP-TBBT-1, NP-TBBT2, and NP-C12, respectively. TGA results also indicated the thermo-sensitivity of the NPs as a function of temperature. Initial weight change of the both TBBT-modified NPs started at 100-150 oC as a result of removal of the TBBT protecting group (boiling point of TBBT: ~102 oC). Then, a significant weight change occurred in the 150-250 oC range upon complete loss of the organic layer. The weight remained constant above 250 oC. The boiling point of dodecanethiol is 266-283 °C, which is significantly higher than that of TBBT (102-


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


103 oC). Therefore NP-C12 showed higher thermal stability compared to the NP-TBBT-1 and NP-TBBT2. The NP-C12 NPs remained stable up to ~250 oC, which is similar to the common hydrocarbon base oils and better than many commercial additives, such as zinc dialkyldithiophosphates (ZDDPs) that decompose at ~200 oC.

a

b

Figure 2. UV-visible spectra (a) and thermogravimetric data (b) for NP-TBBT-1, NP TBBT-2 and NPC12. UV-visible spectra were recorded in toluene solution and spectra were shifted for clarity.

The thiolated organic surface layer passivates the metallic core from oxidation, hinders the NP aggregation, and supports the solubility in hydrocarbon oils. The organic surface functional group and the core particle size together determine the oil-NPs colloidal stability. Stable suspensions of the three samples of thiol-modified Ag NPs were achieved in a poly-alpha-olefin (PAO) 4 cSt base oil at various treat rates. The synthetic PAOs are known to be more difficult to dissolve additives compared with mineral oil-based group I, II, and III oils. Due to its long-chain alkyl group, the NP-C12 can be directly dissolved in the PAO base oil at a concentration up to 0.50 wt% without aid of any dispersant (commonly used in commercial lubricants). The TBBT-modified NPs was first dissolved in toluene (~2 wt%) as a supporting media prior to being mixed with the PAO base oil. With the help of toluene, the maximum


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

concentrations of NP-TBBT-1 and NP-TBBT-2 that can be dissolved in PAO base oil are 0.19 and 0.38 wt%, respectively. Figure S3 shows the photographs of the PAO based oil containing the three different Ag NPs. Based on the TGA-determined silver content, the contents of silver in the oil are 0.11, 0.25, and 0.35 wt % for NP-TBBT-1, NP-TBBT-2 and NP-C12, respectively. Table 1 shows the viscosity of the testing oils with and without NPs. Insignificant changes in viscosity (0-2%) were observed upon addition of Ag NP-C12 into the PAO base oil. Mixing toluene into the PAO oil reduced the viscosity by 10-20% while the addition of NP-TBBT-1 and NP-TBBT-2 slightly increased the viscosity by 1-3%. The viscosity index (VI) is a measure of the variation in viscosity over an arbitrary temperature range (usually 40-100 oC). The addition of all three NPs modestly improved the VI compared to the PAO base oil.

Table 1. Viscosity of the base oil and the oils containing Ag NPs. Lubricant PAO base oil PAO+ 0.50 wt% Ag NP-C12 PAO+ 2 wt% toluene PAO+ 0.19 wt% Ag NP-TBBT-1 PAO+ 0.38 wt% Ag NP-TBBT-2

23 oC 28.33 28.33 23.38 23.86 23.88

Viscosity (cP) 40 oC 14.47 14.48 12.32 12.51 12.51

100 oC 3.33 3.39 3.01 3.07 3.10

Viscosity Index (VI) 98.1 106.7 96.1 103.1 107.7

The sulfur content in the organic ligand could be an environment concern. The upper limit of the sulfur content in a GF-5/6 automotive engine oil is 0.5 wt%. At the maximum concentration (0.5 wt%) of the NPs used in this study, the amount of sulfur is only 0.007 wt%, which is negligible.

3.2. Tribological performance of the Ag NPs as lubricant additives 3.2.1. Friction behavior in mixed and elastohydrodynamic lubrication The frictional behavior of the PAO base oil and PAO+0.38% NP-TBBT-2 were first investigated using a rotating cylinder sliding against a flat under a constant load of 50 N with the sliding speed


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decreasing from 1.2 to 0.1 m/s at a 0.1 m/s interval. The lubricant film thickness and λ-ratio for both lubricants with and without the NPs were calculated using the Hamrock and Dowson formula1, as shown in Table S1. Based on the lubrication regime definition1, the contact interfaces for both oils were predicted to be under elastohydrodynamic lubrication (3 ≤ λ < 10) at the sliding speed from 1.2 to 0.5 m/s and under mixed lubrication (1 ≤ λ < 3) from 0.4 to 0.1 m/s. The two experiment-generated Stribeck curves in the PAO base oil with and without NP-TBBT-2 are compared in Fig. 3. Each curve represents an average of nine replicates. Both curves showed a transition from elastohydrodynamic lubrication regime to mixed lubrication regime at around 0.5 m/s, which correlates well with the calculated λ-ratio in Table S1. The results show little change in friction behavior upon the addition of the Ag NPs, implying little microscopic ball-bearing effect. After testing, the contact area lubricated by the PAO + NP-TBBT-2 was examined using SEM and EDS, but no detectable Ag deposition was observed (see Fig. S4). Since the Stribeck curve scans were primarily in mixed and elasto-hydrodynamic lubrication regimes, the contact pressure was not high enough to produce a deposition layer of silver on the contact area.

Figure 3. Stribeck curves of the neat PAO base oil (Black) and PAO + NP-TBBT-2 (Red).

3.2.2. Friction and wear in boundary lubrication


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

Boundary lubrication tests were carried out on the NPs-containing oils using an AISI 52100 bearing steel ball reciprocating sliding against an A2 tool steel flat. Tests were carried out under a 100 N normal load at a 10 Hz oscillation with a 10 mm stroke for 1000 m sliding. The calculated minimum film thickness (hc) varied from 0 at the oscillation stroke ends to 18-20 nm at the stroke center. With the composite roughness of 23 nm, the λ is in the range of 0-0.87 for a given sliding stroke (0 at the stroke ends and 0.87 at the stroke center), which confirms the boundary lubrication regime at the beginning of the test. Figure 4 shows the friction coefficient traces of all tested fluids in the boundary lubrication tests. All three thiol-modified NPs evidently reduced the friction in boundary lubrication to various extents. With the help of toluene for suspension, the oils containing the two TBBT-modified NPs (Ag core sizes of 1-3 nm and 4-5 nm, respectively) showed similar friction behavior with ~35% reduction compared to the neat base oil. The NP-C12 (3-6 nm in core size) reduced the friction coefficient by ~15% when used alone and by ~20% when toluene was added. To further determine the impact of toluene, the base oil containing 2 wt% toluene alone was tested and showed ~15% friction reduction. It appeared some synergistic effect between the Ag NPs and toluene in reducing friction. Another interesting observation is that the two TBBT-NPs showed very similar friction behavior even though they have different particle sizes and, on the other hand, NP-TBBT-1 and NP-C12 having similar particle sizes but different organic ligands generated different friction behavior. This suggests that the friction behavior is primarily controlled by the organic ligand rather than the particle size.


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Effects of the three different Ag NPs on friction when added into the PAO base oil.

The wear results are summarized in Tables 2 and S2. All three NPs, when added to the base oil, effectively reduced the wear volumes of both the steel ball and flat. Interestingly, the impact of toluene, NP organic ligand, and particle size on wear is very different compared with that on friction. The best performance was achieved by using the NP-C12 alone with the total material loss reduced by >85%. Additional tests were conducted for the NP-C12 at lower concentrations of 0.10 and 0.25 wt% and results indicated proportionally weeker wear protection (see Table S3). The two larger NPs, NP-C12 and NPTBBT-2 (despite different organic ligands) are more wear protective than the smaller NP-TBBT-1. Basically opposite to the influence on the friction behavior, the particle size seemed to be more important than the organic ligand chemistry in wear protection. There was no synergy in wear protection for using NPs and toluene together. Toluene plus NP-C12 did not perform as well as NP-C12 alone and toluene plus NP-TBBT-1 showed poorer surface protection compared with the toluene alone. This was possibly caused by the competition between the two different wear protection mechanisms of the toluene (surface adsorption) and NPs (tribofilm formation).


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Table 2. Effects of Ag NPs on wear when added into the PAO base oil. Lubricant PAO base oil PAO+ 0.50 wt% NP-C12 PAO+ 2 wt% tol PAO+ 2 wt% tol+0.19 wt% NP-TBBT-1 PAO+ 2 wt% tol+0.38 wt% NP-TBBT-2 PAO+ 2 wt% tol+0.50 wt% NP-C12

Wear volume (x106 µm3) Flat wear Ball wear Total wear 2.89±0.29 2.71±1.07 5.60±1.12 0.49±0.19 0.30±0.10 0.79±0.21 0.99± 0.24 1.60±0.54 2.06±0.59 1.27±0.13 2.52±0.03 3.79±0.13 0.58±0.03 0.40±0.10 0.98±0.10 0.55±0.12 1.15±0.16 1.66±0.21

In addition to the wear volume, surface roughness of the wear scars was measured along the sliding direction after each test. All the wear scars on the flats were slightly rougher than the unworn surface (10 nm), specifically, 14, 16, 31, and 20 nm (Ra) for the scars generated in the neat PAO, PAO+NP-TBBT-1, PAO+TBBT-2, and PAO+NP-C12, respectively. No clear trend of the worn surface roughness with the NP size or the type of the organic layer was observed.

3.2.3. Tribofilm characterization Silver-rich tribofilms have been detected on the worn surfaces on both the steel balls and the steel flats tested in the lubricants containing Ag NPs. Optical images of the wear scars on the steel balls tested in the oils without and with the NP-C12 are compared in Fig. S5. Evidently, the introduction of the silver NPs into the oil significantly reduced the wear scar size and altered the worn surface morphology. The worn surface lubricated by the NPs-containing oil seems to be largely covered by a layer of white (or light gray) deposit, which was further examined using SEM and EDS, as shown in Fig. 5. The EDS elemental mapping suggests that the tribofilm is silver rich (Ag Lα line at 2.98 keV). The high contact pressure and associated frictional heating in the boundary lubrication test are believed to facilitate the formation of such a silver-rich tribofilm. There was less oxygen in the area covered by the Ag-rich layer, which implies insignificant oxidation of the NPs. Although the NP surface was modified with thiol, the sulfur content in the tribofilm seems to be minimal.


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 µm

5 µm

20 µm

5 µm

Ag

Fe

S

O

Figure 5. SEM images and EDS elemental maps of the wear scars of the steel ball tested in PAO + NPC12. [Ag (Cyan), Fe (Blue), S (Pink), O (Green)]


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

For the steel flat specimen tested in the PAO oil containing NP-TBBT-2, a thin cross section was lifted out of the wear track using FIB for STEM/EDS examination. Figures 6 and S6 present the crosssectional STEM images and EDS elemental maps of the near surface zone, which clearly show a 50-100 nm protective tribofilm on the worn surface. Elemental mapping suggests that the matrix of the tribofilm is rich in Fe, O, and Cr, likely a mixture of FeOx and CrOx. There seem to be a number of NPs embedded in an amorphous matrix. These particles are spherical in shape with the diameter of a few nm. The Ag map overlaps very well with the particles, indicating that they are the Ag NPs from the lubricant. The Ag NPs have been confirmed to largely retain their metallic phase based on XPS analysis below (see Fig. 7c). Similar to the NP-C12 generated tribofilm, the NP-TBBT-2 produced tribofilm also contains little sulfur compounds. For comparison, we conducted similar cross sectional examination of the worn steel surface lubricated by the neat PAO base oil without NPs, as shown in Fig. S7. Unlike Fig. 6 where the NP-containing tribofilm providing a full coverage of the contact area with a relatively even thickness, the surface film in Fig. S6 appears to be thinner (

Organic-Modified Silver Nanoparticles as Lubricant Additives - ACS

Recommend Documents