Oil-Soluble Silver–Organic Molecule for in Situ ... - ACS Publications

May 10, 2016 - Northwestern University, 2145 Sheridan Road, Evanston, Illinois ... Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23...
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Oil-Soluble Silver−Organic Molecule for in Situ Deposition of Lubricious Metallic Silver at High Temperatures Michael Desanker,†,∥ Blake Johnson,‡,∥ Afif M. Seyam,⊥ Yip-Wah Chung,*,§ Hassan S. Bazzi,⊥ Massimiliano Delferro,*,†,¶ Tobin J. Marks,*,†,§ and Q. Jane Wang*,‡ †

Department of Chemistry, ‡Department of Mechanical Engineering, and §Department of Materials Science and Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States ⊥ Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar ¶ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States S Supporting Information *

ABSTRACT: A major challenge in lubrication technology is to enhance lubricant performance at extreme temperatures that exceed conventional engine oil thermal degradation limits. Soft noble metals such as silver have low reactivity and shear strength, which make them ideal solid lubricants for wear protection and friction reduction between contacting surfaces at high temperatures. However, achieving adequate dispersion in engine lubricants and metallic silver deposition over predetermined temperatures ranges presents a significant chemical challenge. Here we report the synthesis, characterization, and tribological implementation of the trimeric silver pyrazolate complex, [Ag(3,5-dimethyl-4-n-hexyl-pyrazolate)]3 (1). This complex is oil-soluble and undergoes clean thermolysis at ∼310 °C to deposit lubricious, protective metallic silver particles on metal/metal oxide surfaces. Temperature-controlled tribometer tests show that greater than 1 wt % loading of 1 reduces wear by 60% in PAO4, a poly-α-olefin lubricant base fluid, and by 70% in a commercial fully formulated 15W40 motor oil (FF oil). This silver−organic complex also imparts sufficient friction reduction so that the tribological transition from oil as the primary lubricant through its thermal degradation, to 1 as the primary lubricant, is experimentally undetectable. KEYWORDS: silver complex, lubrication additive, friction reduction, oil degradation, lubrication comprise typical lubricants.8,9 These degradation pathways lead to irreversible reductions in viscosity and the generation of oilinsoluble acids and salts that corrode surfaces and form performance-damaging sludges.7 To address these issues, the additive packages used in modern lubricants contain compounds designed to preserve the longevity of the lubricant and include friction modifiers, viscosity modifiers, dispersants, corrosion inhibitors, and antioxidants.10 Solid lubricants, applied either as a surface coating or as a lubricant additive, are well-suited for high-temperature operations. Most solid lubricants, such as graphite and molybdenum disulfide, have robust 2D lamellar structures and weak intracrystalline interactions, enabling low-friction sliding of basal planes when sheared.10 Note that the ductility of soft metals can also be utilized in lubrication. The low shearstrength of metallic films can form a smooth “glaze layer” on tribosurfaces that lubricates sliding contacts, and the low reactivity of noble metals enables this mechanism to function at

1. INTRODUCTION The ever-present demand for improved engine performance and reduced emissions drives the design of increasingly sophisticated lubrication technologies. Lubricating oils and greases are engineered to function over a broad range of temperatures and loading conditions. Modern engines operate at higher temperatures, speeds, and pressures than previous engine generations and therefore require lubricants capable of handling these harsher conditions.1,2 Reliable performance under extreme conditions is also critical in emergency and combat situations. In automotive engines, the temperature at surfaces of critical tribological components can easily reach 200 °C, while asperity contacts can generate “flash temperatures” up to 1000 °C for μs durations.2 Furthermore, extreme pressures and temperature in the contact zones can lead to plastic deformation, wear away mating surfaces, and catalyze undesirable chemical reactions, which degrade the surfaces and lubricant.3−6 Conventional lubricants and oils undergo degradation via three main pathways: bond scission, thermolysis, and oxidation.7 Furthermore, the high temperatures and pressures of typical engines create an environment that is hostile to the long-chain hydrocarbon molecules, which © 2016 American Chemical Society

Received: February 5, 2016 Accepted: May 10, 2016 Published: May 10, 2016 13637

DOI: 10.1021/acsami.6b01597 ACS Appl. Mater. Interfaces 2016, 8, 13637−13645

Research Article

ACS Applied Materials & Interfaces extreme temperatures.11 Silver coatings on contact surfaces have demonstrated friction and wear reduction over temperatures ranging from 25−750 °C.12−17 Silver nanoparticles have also been shown to greatly increase surface fatigue life, decrease friction and wear, and work synergistically with other lubricant additives.18−22 However, silver nanoparticles are costly to produce, difficult to suspend in oil, and often require a surfactant to prevent the particles from agglomerating.10 An alternative strategy for the delivery of lubricious silver is to use a silver-containing molecular precursor. Such molecules are designed to undergo thermolysis at elevated temperatures, depositing a layer of metallic silver on mechanical surfaces. In previous research, we synthesized and characterized three generations of silver precursor molecules and evaluated their performance as extreme temperature additives in motor oil.23−26 The Gen-I additive (Figure 1a) was used to grow

2. EXPERIMENTAL SECTION 2.1. Materials. Silver(I) oxide, 99+% (99.99%-Ag) PURATREM was obtained from Strem Chemicals and used as received. Acetyl acetone, potassium carbonate, 1-bromohexane, hydrazine monohydrate, and organic solvents were purchased from Sigma-Aldrich and used as received without further purification. Acetone was dried over CuSO4 and vacuum distilled. PAO4, a poly-α-olefin oil, supplied by Ashland Chemicals, and a commercially available 15W40 motor oil were used as model base fluids. All deuterated solvents (99+ atom % D) were purchased from Cambridge Isotope Laboratories and used as received. The 52100 steel bar stock was cut into 1 cm × 1 cm squares and polished to ∼10 nm roughness as measured by atomic force microscopy (AFM). Elemental analyses were performed by Galbraith Laboratory, Knoxville, Tennessee (USA). NMR spectra were recorded on a Varian UNITY Inova 500 (FT, 500 MHz, 1H; 125 MHz, 13C) and an Agilent DD2 F500 (VT, 500 MHz, 1H; 125 MHz, 13C) spectrometers. Chemical shifts (δ) for 1H and 13C spectra are referenced using internal solvent resonances. For 2D-DOSY NMR studies to measure the nuclearity of 1 in solution, a 10 wt % solution of additive 1 in toluene-d8 was used in an Agilent DD2 F500 spectrometer. The gradient pulse strength was varied in 16 linear steps from 2−60 Gcm−1 to obtain complete signal attenuation. The calibration was confirmed by measuring the diffusion coefficients of benzene, octadecyltriethoxysilane, and tristearin in a mixture in chloroform-d1 at 22 °C. The gradient pulse duration (δ) and the diffusion delay (Δ) were kept constant at 2 ms for δ and 50 ms for Δ. Spectra were measured at 22 °C with a 90° pulse duration of 7.9 μs and a relaxation delay of 2 s. Solution stability of additive 1 was investigated by heating a sealed NMR sample in toluene-d8 for 1 h at 275 °C. Toluene-d8 was used because of its high boiling point. Fourier transform infrared (FTIR) spectra (4000−700 cm−1) were recorded on a Bruker Tensor 37 FTIR spectrometer equipped with a Mid-IR detector for use between 4000 and 700 cm−1. For liquid injection field desorption ionization (LIFDI), a Linden ChroMasSpec (Linden GmbH, Auf dem Berge 25, D-28844 Weyhe, Germany) source was interfaced to a Waters Inc. (34 Maple Street, Milford, MA 01757) gas chromatography mass spectrometry (GCMS) system. The emitter is a 10 μm tungsten wire with graphite dendrites (whiskers). The liquid is introduced onto the graphite dendrites via capillary without breaking vacuum. A 1 mg/mL sample was prepared in toluene. Approximately 10 μL of this sample was pulled through the capillary by dipping the atmospheric end into the solution for about 5 s. A camera is used to monitor the wetting of the graphite whiskers by the solution. Once the solvent evaporates, a current ramp of 0−90 mA in 3 min is passed through the tungsten wire. The sample evaporates frp, the emitter in ∼1.6 min. Instrumental tuning is performed by direct infusion of a freshly prepared CH2Cl2 solution (1 nM) of 1 into a continuous flow of methanol from a solvent delivery system (200 μL min−1). Working parameters were set as follows: spray voltage, 3.5 kV; capillary voltage, 15 V; capillary temperature, 200 °C; tube lens, 65 V. Samples were analyzed in flow injection mode using a six-port valve equipped with a 2 μL sample loop. Mass spectra were recorded in full scan analysis mode in the range 0−1500 m/z. Thermogravimetric analysis (TGA) was performed on a TA Q50 ultramicro balance instrument (ramp rate = 5 °C min−1 and under an N2 flow rate of 90 mL min−1) at atmospheric pressure. Films of silver additive 1 were thermolyzed on 52100 stainless steel substrates at 350 °C for 1 min. Film chemical compositions were assessed with an Omicron ESCA Al Kα probe Xray photoelectron spectrometer (XPS) under high vacuum (20%) for effective silver wear and friction reduction and requires added dimethyl sulfoxide (DMSO) for adequate solubility in base oils. Lower precursor loadings are clearly necessary to reduce the required silver, ensure better oil solubility, and accommodate other additives in the additive package. Note that additive packages typically constitute 10−15 wt % of entire lubricant formulations.27 A silver additive that can achieve equal or superior functionality at lower loadings would be highly desirable to meet the requirements of modern automotive systems. Here, we report a P- and S-free trinuclear silver−pyrazolate complex bearing long alkyl chains (1) to enhance oil solubility, which achieves noteworthy wear and friction reduction at high temperatures and at low additive loadings (>1 wt %). This complex has been synthesized, fully characterized, and tribologically evaluated in PAO4 and fully formulated engine oils. 13638

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ACS Applied Materials & Interfaces funnel. The reaction was refluxed overnight, then cooled and concentrated to dryness. The residue was then dissolved in Et2O (200 mL) and filtered to remove solids, and the filtrate was concentrated to dryness to give 37.10 g of dark yellow liquid. The impurities n-hexyl bromide and acetylacetone were distilled off at 35 °C and 30 mTorr, and the product was purified by silica gel column chromatography and eluted with ethyl acetate/hexane (1:8) to give 14.28 g of 3-hexyl-2,4-pentanedione as a light-yellow liquid (42% yield). A solution of N2H4·H2O (3.26 mL, 66.7 mmol) in MeOH (20 mL) was slowly added (over 20 min) via cannula transfer to a 0 °C solution of 3-hexyl-2,4-pentanedione (12.30 g, 66.7 mmol) in MeOH (50 mL). The mixture was stirred for 12 h at room temperature and then refluxed for 4 h. The solvent was then removed in vacuo, and the light yellow liquid was dried to give 11.00 g (93% yield) of 3,5dimethyl-4-n-hexyl-pyrazole (LHPz). 1H NMR (CDCl3): δ = 2.32 (t, 2H, −CCH 2 CH 2 −), 2.19 (s, 6H, −CH 3 ), 1.42 (q, 2H, −CCH2CH2CH2−), 1.28 (m, 6H, alkyl chain), 0.88 (t, 3H, −CH2CH3) ppm. 13C NMR (CDCl3): δ = 141.89, 115.78, 31.87, 30.76, 29.16, 23.14, 22.79, 14.20, 10.97 ppm. ESI-MS (p.i., CH2Cl2/ MeOH 95:5, m/z, I%) = 181.03 m/z [LH]+. 2.3. Synthesis of [Ag(3,5-dimethyl-4-n-hexyl-pyrazolate)]3 (1). Solid Ag2O (7.16 g, 30.9 mmol) was added to a solution of LHPz (11.03 g, 61.2 mmol) in anhydrous MeOH (100 mL) during stirring. Then MeOH (35 mL) was added, and the mixture stirred at room temperature for 24 h, then refluxed for 6 h, followed by further room temperature stirring for 18 h. Solvent was then evaporated under reduced pressure, and the crude product was dissolved in hexanes and filtered to remove insoluble particles. The filtrate was then concentrated, and the product recrystallized from hexanes to give [Ag(3,5-dimethyl-4-n-hexyl-pyrazolate)]3 (1) as a white powder (77% yield). 1H NMR (CDCl3): δ = 2.34 (t, 6H, −CCH2CH2−), 2.04 (s, 18H, −CH3), 1.44 (qu, 6H, −CCH2CH2CH2−), 1.31 (m, 18H, alkyl chain), 0.90 (t, 9H, −CH2CH3) ppm. 13C NMR (CDCl3): δ = 147.14, 113.49, 32.01, 31.30, 29.38, 24.13, 22.94, 14.31, 12.65 ppm. Anal. Calcd: Ag, 37.56%; C, 46.01%; H, 6.67%; N, 9.76%; found: Ag, 38.02%; C, 45.52%; H, 6.59%; N, 9.87%. LIFDI-MS (p.i., CH2Cl2, m/ z) = 181.03 m/z [LH]+; 861.10 m/z [AgLH]3+. 2.4. Single Crystal X-ray Structure. Single-crystal data were collected with a Bruker Smart APEXII area detector diffractometer (Mo Kα; λ = 0.71073 Å). Cell parameters were refined from the observed setting angles and detector positions of selected strong reflections. Intensities were integrated from several series of exposure frames that covered the sphere of reciprocal space.28 A multiscan absorption correction was applied to the data using the program SADABS.29 The structures were solved by direct methods30 and refined with full-matrix least-squares (SHELXL-97)31 using the Wingx32 software package. Graphical material was generated with the OLEX2 program.33 2.5. Tribology Experiments. Silver additive 1 was combined with commercial polyalphaolefin (PAO4) and FF oils. PAO4 is a commonly used base fluid for commercial engine oils, and the FF oil used in these tests is a military grade 15W-40 oil with a standard additive package. Additive 1 was dissolved in the minimal amount of hexane (1.0 mL/ 1.0 g silver additive) and then added to the above oils to achieve additive concentrations of 1.0, 2.5, and 5.0 wt %. Before tribology evaluation, the oil−silver additive mixtures were stirred with a magnetic stir bar for 30 min to ensure a homogeneity. 2.5.1. Lubricated Sliding Interaction of Stationary Pin-on-Disk. Temperature-ramped pin-on-disk tribological tests were performed to simulate boundary lubrication in an engine valve train. The tests were designed to start under normal operation conditions and then slowly ramps up to an extreme temperature. The increasing temperature simulates the failure of the oil above its thermal limit and tests Ag additive 1’s continued performance at high temperatures. A schematic of the pin-on-disk tribometer (CETR UMT-2 tribometer) used in this study is given in Figure 2, and Table 1 outlines the experimental conditions. A ball made of M50 bearing steel with 62 HRC hardness was used to apply a vertical load to a 52100 steel disk with 50 HRC hardness. The surface of the disk was coated with a 2 mL solution of 1 in the desired base oil. All tribometer tests were performed three times

Figure 2. Schematic of temperature-controlled CETR UMT-2 ball-ondisk tribometer with heating chamber.

Table 1. Tribometer Evaluations Conditions and Sample Formulations test parameter

value

applied load (N) Hertzian contact pressure (GPa) disk rotation speed (rpm) radial location of contact (mm) sliding contact speed (m/s) total sliding distance (m) base oil Ag additive concentration (wt %) temperature range (°C) rate of temperature change (°C/s) test duration (min)

25 2.15 200 20 0.42 754 PAO4 15W-40 0.0, 1.0, 2.5, 5.0 180−350 0.1 30

for each oil sample. The composite roughness (Rq) of the contacting surfaces was about 10 nm ±1.6 nm. Roughness was measured using a Zygo NewView 7100 white-light interferometer (WLI) and SPIP surface analysis software. The surface of the disk was coated with a 2 mL solution of additive 1 in either PAO4 or the FF lubricant. All tribometer tests were performed at least twice for each oil sample. The lubricant film thickness was estimated based on the sample characterization and the tribological test conditions, which is smaller than the composite surface roughness of the two mating surfaces in most of the contact zone, confirming that the tests took place in the mixed and boundary lubrication regimes. Under these conditions, lubricant viscosity changes due to heating would not notably influence the friction and wear results on a comparative basis. In Table 1, the testing temperature range (180−350 °C) refers to the surface temperature of the lower rotating disk in the tribometer, where the machine’s heating element and thermal sensor are located. The temperature range was chosen to simulate a temperature transition for a typical tribo-interface in an engine. The modeling of engine tribology34 reveals that the temperature of valve train components may reach about 200 °C under normal conditions. The tests were designed to start below this temperature value, at 180 °C, and then ramp up to 350 °C. Even though the overall testing temperature starts below the decomposition temperature of the additive complex (shown to be 313 °C in the TGA data, vide infra), asperity contacts are expected to induce flash temperatures sufficient to cause Ag deposition throughout the test. To confirm this, a tribological model35 was used to calculate the possible flash temperature based on the theory of a moving heat source over a semi-infinite solid36 with considerations of asperity contact, heat conduction through the surfaces, and convection through fluid flow (although the convection is negligible in the current cases). This analysis revealed a flash temperature increase of 246 °C above the ambient temperature, which is sufficient for Ag deposition. 2.5.2. Wear Analysis by White Light Interferometry (WLI). After friction tests were completed, the disks were sonicated in a hexane bath to remove oil residues. The volume of the wear scar was then measured using the WLI. The wear volume and material buildup 13639

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ACS Applied Materials & Interfaces around the wear scar were used to calculate the wear rate, which is defined as the volume removed per unit load per sliding distance. Energy-dispersive X-ray spectroscopy (EDS) on Hitachi S4800−II and Hitachi SU8030 scanning electron microscopes (SEMs) was used to examine the morphology of the disk surface and deposited silver content.

intramolecular Ag···Ag distance of 3.45(9) Å indicates that a significant intramolecular argentophilic interaction is unlikely.25 However, a more significant intermolecular argentophilic interaction with a distance of 3.216 Å is observed, which leads to a staggered ladder stacking motif involving adjacent molecules in the crystal structure (Figure 3b). Silver additives with nitrogen donor ligands have been shown to form coordination polymers and oligomers in the solid state, but these structures can change in solution.25 Two-dimensional diffusion ordered NMR spectroscopy (DOSY NMR) was used to investigate the structure of additive 1 in solution (Figure S6). The diffusion coefficient of additive 1, compared to three reference compounds, was used to determine the molecular weight of additive 1 in solution (835.1 g mol−1). This indicates that it maintains its trimeric structure in solution. MS also provides useful information on molecular aggregation and fragmentation, which in turn are likely to influence thermolysis pathways.25 LIFDI MS was also used due to its gentle ionization characteristics, which preserve the structure of the trimeric complex. Ligand LHPz (180 m/z) and additive 1 (861 m/z) signals can be identified in the LIFDI MS spectrum (Figure S7). The occurrence of the LHPz fragment under gentle ionization conditions suggests the lability of the trimeric complex, which may represent the initial steps in silver film deposition. Atmospheric pressure TGA of Ag additive 1 was performed under N2 to evaluate the temperature at which thermolysis begins. The residue produced by thermolysis was then analyzed by X-ray photoelectron spectroscopy (XPS) and elemental analysis (EA). It was found that additive 1 begins to undergo thermolysis at ∼300 °C, which is a more desirable thermolysis temperature than the ∼200 °C of Gen-III additive (Figure 4a).24,26 Ramping temperature experiments (vide infra) have shown that the PAO4 and FF oil used as base oils in these experiments starts to decompose at 275 °C. At the completion of a TGA scan, 38.5% of the original mass remains (Figure 4b), indicating that the majority of the residue remaining after thermolysis is silver metal (38.02%). The thermal stability of additive 1 was also investigated in solution by NMR since this additive is designed for applications in motor oil. An NMR sample in toluene-d8 heated at 275 °C for 1 h shows no changes in structure (Figure 4b), indicating that additive 1 is stable under this conditions. 3.2. Solubility of Additive 1 in Base Oil. Solubility in base oil is an important property of tribologically promising high temperature additives and is something that challenged previous generations of silver−organic additives.24 The aforementioned Gen-III additive (Figure 1c) was highly promising in that it was S- and P-free and undergoes decomposition at high temperature to form metallic silver films. However, it requires very large loadings and forms a cloudy, opaque mixture in oil. In contrast, gentle heating of a 5 wt % loading 1 at 40 °C in PAO4 yields a homogeneous solution (Figure 5). 3.3. Silver Additive 1 Thermolysis. PXRD, XPS, and EA were performed to characterize the thermolysis residue. EA reveals that the composition of the residue is Ag, 93.6%; C, < 0.5%; H, < 0.5%; N < 0.5%. The remaining ∼4.5% is attributed to O, which presumably arises from Ag2O formation during thermolysis.37−39 The purity of the Ag film was also investigated by XPS (Figure 6a) and glancing angle/incidence X-ray diffraction (GIXRD, Figure 6b). The XPS spectrum exhibits characteristic metallic Ag 3d5/2 and 3d3/2 signatures,40

3. RESULTS AND DISCUSSION 3.1. Silver−Organic Additive Synthesis and Characterization. The synthesis of 1 is outlined in Scheme 1. The ligand Scheme 1. Synthesis of Lubrication Additive 1

was prepared by coupling an n-hexyl chain to acetylacetone, via an SN2 mechanism. Hydrazine was then used to close the nitrogen heterocycle (LHPz) under reflux conditions. LHPz was next reacted with silver oxide at 0 °C and then warmed to a room temperature to afford silver additive 1 in 77% yield as white powder. Compound 1 was fully characterized by standard spectroscopic (1H and 13C NMR) and analytical methods as well as by single-crystal X-ray diffraction. The crystallographic results reveal that the pyrazolate ligands and silver ions in additive 1 form a nine-membered ring structure, with each Ag+ ion coordinated to two nitrogen centers (Figure 3a). The

Figure 3. (a) Crystal structure of additive 1 (1). Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (o) are Ag1−N1 = 2.077, Ag1−Ag2 = 3.393, Ag1−N1−N2 = 118.56, N6− Ag1−N1 = 178.49. (b) Molecular packing in additive 1. Hydrocarbon chains are omitted for clarity. Intermolecular Ag···Ag distance = 3.216 Å. (c) Side view and (d) top view of crystal packing. 13640

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Figure 4. (a) TGA trace for Gen-III additive (red) and additive 1 (green). The weight loss data were recorded at the ramp rate of 5 °C min−1 and a 90 mL min−1 N2 flow rate. (b) 1H NMR spectra of additive 1 before and after 1 h of heating at 275 °C in toluene-d8 solution, showing no change in molecular structure.

1.0, 2.5, and 5 wt % loadings of 1 all reduce friction. A 2.5 wt % loading of 1 reduces friction by 20% in PAO4 and by 30% in FF oil. Loadings greater than 2.5 wt % do not decrease friction further. A similar friction−additive concentration relationship is also observed in the studies of previous generations of the silver additives.25,26 The pin-on-disk friction tests shown in Figure 8 were performed while the temperature was ramped from 180 to 350 °C over about 30 min. These plots show a steep increase in COF that begins around 275 °C for both base oils (PAO4 and FF oil). Initial COF measurements range between 0.15 and 0.20 for PAO4 and between 0.10 and 0.15 for FF oil, while the final measurements reach 0.30. Note that FF oil has lower initial COFs because the additive package includes friction modifiers. Both oils exhibit a similar spike in COF at ∼275 °C, which can be explained by oil degradation at high temperature. However, when a 2.5 wt % loading of 1 is used in either oil, remarkably, the COF does not increase even though the oil begins to degrade above ∼275 °C. The transition from oil as the primary lubricant to metallic silver as primary lubricant is clearly seamless. 3.5. Surface Analysis. The disks used in the temperature ramp tests (Figure 8) were next analyzed by WLI to determine the volume of the wear scars and the material buildup outside of the scar. The volume of material removed was normalized with load and the distance traveled during the test. It was found that a 2.5 wt % loading of additive 1 reduces wear by 60% in PAO4 and by 70% in FF oil (Figure 9). The trend in wear rate is similar to the trend in COF (Figure 7), with a 2.5 wt % loading producing the lowest amounts of wear. The Gen-III additive produces comparable friction and wear results at a 20 wt % loading.25 The increased efficiency of additive 1 is attributable to the increased number of silver atoms/molecule and improved solubility in oil, which enable better dispersion of the additive in base oil. SEM coupled with EDS was next used to analyze the elemental composition of the area in and around wear scars of the steel substrates used in the friction and wear tests. Figure 10 shows EDS spectra for areas outside and inside the wear scar after pin-on-disk tests were carried out with a 2.5 wt % loading of the additive in PAO4. Note that the EDS shows a much lower concentration of silver outside the wear scar, indicating that thermolysis and subsequent silver deposition largely occur

Figure 5. Vial of pure PAO4 (left) and a 5 wt % solution of 1 in PAO4 (right), which shows a light yellow tint and no insoluble material.

Figure 6. Ag film deposited on 52100 steel substrate after thermolysis of 1 at 350 °C for 10 min. (a) XPS spectrum, (b) GIXRD diffractogram (θ−2θ) in maroon. Peak positions and relative intensities for the powder pattern of cubic phase Ag (PDF 04− 0783) are presented in black.

with no detectable traces of contaminants, while the GIXRD reveals cubic metallic Ag (PDF 04−0783). 3.4. Evaluation of 1 as a Lubricant Additive: Friction and Wear Measurements. Silver additive 1 was added to oil at 1.0, 2.5, and 5 wt % loadings. A minimal amount of hexanes was added to aid in solubilizing the additive (1 mL hexanes/g of additive) before mixing. Addition of silver additive 1 to either oil creates a cloudy suspension at 25 °C that dissolves fully with heating to 40 °C and magnetic stirring. The effects of differing concentrations of 1 in PAO4 and FF oil were investigated by pin-on-disk tribometer measurements (Figure 7). Friction reduction is preferred but not absolutely necessary for high-temperature additives. The pin-on-disk tribometer was used to ensure that 1 did not have detrimental effects on the coefficient of friction (COF). Figure 7 shows that 13641

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Figure 7. Pin-on-disk measurements of friction with increasing loadings of 1 in (a) PAO and (b) in FF oil. Each data point is the average of a temperature ramp experiment from 180−350 °C over 30 min. The red line shows baseline friction for pure PAO and FF oil. The error bars represent the standard deviation between the two trials performed.

Figure 8. Pin-on-disk tribometer tests with lubricated contact. Oil mixture is a 2.5 wt % loading of 1 in (a) PAO4 and (b) FF oil. Temperature is ramped from 180 to 350 °C over 30 min. The red line shows baseline friction for pure PAO and FF oil. The friction curves are from one representative trial of the two trials performed.

Figure 9. Wear rate measurements for the temperature-ramped pin-on-disk tribometer substrates for 1 in (a) PAO4 and (b) FF oil. WLI is used to measure the volume of the wear scar at 12 positions, and the volumes for each wear scar are then averaged. The red line shows baseline friction for pure PAO and FF oil. The error bars represent the standard deviation between the three trials performed.

Figure 10. EDS of the (a) outside of the post-test wear scar and (b) the inside of the wear scar.

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by XPS and EDS analysis to deposit significant amounts of silver in the pure metallic form, indicating that the nanoparticles used as additives do not chemically react with steel surfaces but instead provide antiwear properties through formation of a boundary film with low shear stress.21 Nevertheless, Ag nanoparticles are costly to produce, difficult to suspend in oil, and often require a surfactant to prevent particle agglomeration.10 Additionally, MoS2 nanoparticles have been demonstrated by in situ TEM to provide surface protection by deforming, rolling, and shearing through the point of contact rather than by chemical reaction.43 The new metallic silver precursor additive reported here yields a product (silver metal) that functions similarly to MoS2, yet is even more inert and appears to undergo negligible chemical reaction with the surface, but rather is physisorbed via weak interactions. Plastic deformation and material removal are both observed in the wear scars (Figure 9), and small amounts of silver are found in the wear scar based on EDS analysis (Figure 11). The primary mechanism for reducing friction and wear is therefore attributed to the specific properties of silver, particularly its softness and low shear strength. In addition, silver provides a mechanical buffer between the interacting surfaces; the silver particles, as they roll or shear through the contacts, reasonably act as sacrificial surfaces.

only where needed. It is likely that heat and pressure from the contacting surfaces promotes thermolysis of the silver additive. Neither the PAO4 nor the fully formulated motor oil (FF oil) used has additives containing silver. Therefore, it can be concluded that the silver particles deposited on the metal surface result from thermolysis of additive 1. SEM was also used to investigate the size of the particles formed by thermolysis of additive 1 (Figure 11). The Ag is

4. CONCLUSIONS Silver−organic additives can be used to deposit metallic silver on mechanical surfaces at high thermolysis temperatures, thereby reducing friction and wear at temperatures where base oil degradation occurs. Indeed, additive 1 represents a new generation of silver−organic lubricant additives that exhibit useful solubility in nonpolar base oil. Additive 1 undergoes thermolysis between 313 and 332 °C to produce lubricous silver films. Temperature-ramped ball-on-disk experiments show that 1 significantly reduces friction at temperatures greater than ∼275 °C, the region where both PAO4 and FF oil fail to effectively reduce friction. The transition from oil as primary lubricant to metallic silver as primary lubricant is seamless. SEM and EDS analyses show that metallic silver is primarily deposited in wear scars, indicating that high temperature caused by asperity contacts increases the probability of thermolysis for the additive. The metallic silver is deposited acts as a protective buffer material between the contacting surfaces. Additive 1 is shown to be more effective at concentrations higher than 1 wt % loading. These results are a marked improvement over previous silver−organic additives that required higher loadings to achieve comparable antifriction and antiwear performance.

Figure 11. SEM images and 2D EDS maps of (a, b) a silver agglomeration next to a piece of wear debris, (c, d) area inside a wear scar with deep scars from asperity contact, and (e, f) agglomerations of silver particles inside a wear scar.

primarily deposited in the form of 10−100 μm in diameter particles in the wear scar. The box in the center of Figure 11, panel a highlights an agglomerate of silver next to a piece of wear debris. The 2D elemental mapping of Ag (Figure 11b) shows that the wear debris contains no Ag. Figure 11, panel c shows an image taken from the inside a wear scar, with deep tracks resulting from asperity contacts. The 2D mapping of this region (Figure 11d) shows a higher Ag concentration inside these deep scars, demonstrating that the silver is primarily deposited where it is needed as a result of high temperatures and pressures. Figure 11, panels e and f show higher magnification images and 2D mapping of agglomerated Ag particles inside the wear scar. The organo−silver additive 1 undergoes thermolysis at elevated temperatures and pressures to deposit Ag particles, the softness of which prevents the particles from acting as abrasives. This tribo-activation is similar to previously reported lubricant antiwear additives, such as zinc dialkyldithiophosphates (ZDDP, ZDTP), which are believed to supplement base oil by forming a thin film on the surface and protecting against wear in the boundary lubrication regime, where interfacial surface asperities come into contact.41,42 Silver nanoparticles studied as antifriction and antiwear additives have been shown

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01597. 1 H and 13C NMR spectra of LHPz and 1; DOSY NMR experiment; LIFDI-MS spectrum of 1; pin-on-disk tribometer parameters (PDF) Crystallographic data for compound 1 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 13643

DOI: 10.1021/acsami.6b01597 ACS Appl. Mater. Interfaces 2016, 8, 13637−13645

Research Article

ACS Applied Materials & Interfaces *E-mail: [email protected]. *E-mail: [email protected].

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Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare the following competing financial interest(s): A patent application partially based on this work has been filed (US Patent Application 15/013,878 2016).

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ACKNOWLEDGMENTS The authors gratefully acknowledge support by the National Priorities Research Program (NPRP Grant No. 5-192-1-046) of the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the author(s). M.D. was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. Purchases of the NMR and MS instrumentation at IMSERC was supported by the National Science Foundation (CHE-1048773 and CHE-0923236, respectively). Microscopy studies made use of the EPIC facility (NUANCE CenterNorthwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center, and the Nanoscale Science and Engineering Center (EEC-0118025/003), both programs of the National Science Foundation, the State of Illinois, and Northwestern University. The authors gratefully acknowledge Prof. H. Goudarzi for assistance with the LIFDI-MS analysis and M. M. Stalzer for assistance with the DOSY NMR experiments.

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DOI: 10.1021/acsami.6b01597 ACS Appl. Mater. Interfaces 2016, 8, 13637−13645