Methionine as a Friction Modifier for Tungsten Carbide-Functionalized

Research Article pubs.acs.org/journal/ascecg

Methionine as a Friction Modifier for Tungsten CarbideFunctionalized Surfaces via in Situ Tribo-Chemical Reactions Manel Rodríguez Ripoll,*,† Vladimir Totolin,*,† Pedro O. Bedolla,† and Ichiro Minami‡ †

AC2T research GmbH, Viktor-Kaplan-Straße 2/C, 2700 Wiener Neustadt, Austria Division of Machine Elements, Luleå University of Technology, 97187 Luleå, Sweden

‡

W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: This work presents a novel method for generating in situ low friction tribofilms in lubricated contacts using αamino acid L-methionine as an additive. Methionine is an environmentally acceptable natural organosulphur compound that is commonly used in the food industry. Our approach relies in the use of steel surfaces functionalized with tungsten carbide particles that are tailored to interact with methionine via a tribo-chemical reaction. The results show that after an induction period, the friction drops dramatically by 60% down to values of 0.06 when methionine was used as an additive in lubricated tungsten carbide-functionalized surfaces. The low friction could only be achieved by the coexistence of tungsten from the functionalized surfaces and sulfur from methionine, which led to the presence of tribo-chemically generated tribofilms. Ab initio simulations indicate that the tribo-chemical reaction for forming tungsten disulfide is energetically favorable, thus attributing the observed friction reduction mechanism to the in situ formation of this compound during the sliding process. The concept of functionalizing surfaces to react with specific additives opens up a wide range of possibilities, which allows tuning surfaces to target specific additive interactions. This synergy can be exploited for using novel green additive technology, thus allowing more environmentally friendly formulations with outstanding tribological performance. KEYWORDS: Amino acid, Methionine, Friction modifier, Tribo-chemical reaction, Boundary lubrication, Ab initio simulation

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INTRODUCTION

treatment, which results in air pollution, is driving the need for developing alternative environmental acceptable additives which will eventually substitute compounds containing heavy metals such as Zn and Mo.4 A recent alternative proposed to reduce friction in sliding contacts relies in the in situ generation of low friction tribofilms using functionalized surfaces.5 The method is based on the use of functionalized surfaces containing submicrometer tungsten carbide (WC) particles which react with sulfur-containing extreme pressure (EP) additives. By these means, a tribochemical reaction between the S from the additive and the W from the surface leads to the formation of low friction tribofilms. The formed tribofilms have a complex chemical composition of iron and tungsten oxides and sulfides. The authors attributed the low friction properties to the formation of WS2 at the sliding interface. Tungsten disulfide is a transition metal dichalcogenide (TMDs) which has a lamellar structure

Modern lubricants contain various additives in order to enhance the properties of base oils and fulfill multiple functions in lubricated contacts such as friction and wear reduction as well as corrosion protection, among others.1 Adsorption of lubricant additives on sliding contacts is essential for friction control, reduction of wear, and lubrication. Unveiling the adsorption mechanism at solid−lubricant interfaces requires often the combination of experiments and simulation tools. The structural information on hexadecylamine surfactants adsorbed on iron oxides surfaces was simulated using molecular dynamics (MD), which showed the formation of a surface film of 15 to 20 Å at the highest surface coverage.2 In a recent work, MD showed the friction reduction mechanism of organic friction modifiers in boundary lubrication. The coefficient of friction was lower for higher surface coverages by the formation of ordered solid-like structures.3 Most of the commonly used friction modifiers and antiwear additives pose serious environmental concerns, in particular, those composed of organic phosphorus, zinc, and sulfur. A growing awareness of the lubricant ash levels that poison catalysts used for exhaust © 2017 American Chemical Society

Received: April 23, 2017 Revised: June 15, 2017 Published: June 20, 2017 7030

DOI: 10.1021/acssuschemeng.7b01258 ACS Sustainable Chem. Eng. 2017, 5, 7030−7039

Research Article

ACS Sustainable Chemistry & Engineering with a strong bonding between the metal and chalcogenide atoms which contrasts with the weak chalcogenide−chalcogenide interaction between the layers, allowing them to easily slide over each other. This makes WS2 suitable to be used as a solid lubricant.6−8 This method proved to be effective in terms of friction and wear reduction since the low friction tribofilm is constantly formed on-demand during the sliding process for protecting the surface. While this approach is promising and can be readily implemented in various engineering applications, the EP additives used as a sulfur carrier in this study were sulfurized olefins. It has been reported that these type of EP additives may pose environmental concerns, in particular, during their manufacturing process that requires the use of sulfur monochloride which leads to the presence of chlorine residues in the final product.9 For this reason, the current study aims to provide new fundamental insights regarding the in situ generation of low tribofilms using environmentally acceptable lubricants. Among the available organosulphur compounds, amino acids are particularly interesting since they are essential components in living tissues, and therefore, they are inherently safe chemicals.10 Amino acids are readily available from food and pharmacy industries. Of all proteinogenic amino acids, only cysteine and methionine contain sulfur. The consideration of sulfurcontaining natural amino acids for tribological applications was proposed in the previous works.10,11 It was shown that antiwear additives derived from cysteine are able to provide comparable wear protection as those achieved by zinc dialkyldithiophosphate (ZDDP). The main inconvenience of sulfur-containing proteinogenic amino acids for potential tribological applications is their polar nature, which limits the number of potential solvents to be used. As a consequence, most of the effort was devoted to synthesize polar derivatives concurrent against ZDDP in synthetic lubricants. Other works12,13 focused on the synthesis of ionic liquids from amino acids. Opposite to cysteine, methionine is the remaining sulfurcontaining proteinogenic amino acid that has not been used in tribological applications so far. The only found tribological application of methionine is as one of the components in a lubricating glycoprotein derived from bovine synovial fluids in the context of cartilage lubrication.14 Methionine is an essential amino acid that is nontoxic, readily available, and inexpensive. Methionine has been proposed as a green corrosion inhibitor for steel in sulfuric acid solution due to its antioxidant properties and as potential replacement for inorganic chromates or nitrites.15,16 The aim of this work is the in situ generation of low friction tribofilms using methionine. The use of methionine as a friction modifier offers a novel potential application for this environmentally safe compound, which has been mostly used in the food industry so far. Methionine is able to fulfill the requirements and specifications recently defined for developing greener formulations.17

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Figure 1. Molecular structure of L-methionine. applications.19 In tribological applications, glycerol has been proposed as a potential substitute of mineral oils20 due to its good performance under boundary and elastohydrodynamic lubrication.21 This mixture is thus an example of a fully green formulation since both the solvent and the additive have a good biocompatibility and are harmless to humans. The lubricant mixtures were tested against stainless steel AISI 304functionalized surfaces. The AISI 304 substrate had a hardness of 230 HV1. The surfaces contained hard WC particles, which were embedded in the substrate using a machine hammer peening technique.22 The particles had a diameter of 0.8 μm. The average surface roughness of the sample after embedding the particles was 0.6 μm. A thorough description of the embedding process is found elsewhere.23 Tribological Tests. The friction and wear performance of the lubricant mixture was evaluated using reciprocating sliding conditions with an SRV tribometer (Optimol, Germany). The tests were performed at an oscillating frequency of 25 Hz using an amplitude of 2 mm. A 10 mm diameter 100Cr6 steel ball was used as a counterbody. The normal load was set to 10 N, based on a previous work.5 Prior to the tests, the functionalized surfaces and the balls were cleaned in an ultrasonic bath for 10 min using toluene and petroleum ether. Afterward, they were fixed in the sample holder, lubricated with 0.2 mL of lubricant mixture to ensure fully immersed contact conditions, and tested for 4 h while continuously monitoring the coefficient of friction. At least three repetitions were performed for every test condition to ensure the statistical repeatability of the results. The tribological tests were performed at a constant temperature of 100 °C. This temperature is representative for lubricants operating in combustion engines. Additionally, this testing temperature helps in reducing the viscosity of glycerol since glycerol has a high room temperature viscosity of about 945 mPa s.24 This high viscosity means that under moderate contact loads, the thickness of glycerol inhibits tribolfim formation due to lower shear stresses at the contact interface.20,25 It has been recently shown that, at least for ZDDP additives, the rate of tribofilm formation depends on shear stress.26 Morphological and Chemical Analyses of Tribofilms. The morphology of the tribofilms was analyzed using a JEOL JSM 6500 F scanning electron microscope (Jeol, Japan) at an acceleration voltage of 20 kV. Energy dispersive X-ray analyses (EDX) were performed to reveal the elemental composition of the tribofilms. Cross-section lamellas perpendicular to the generated tribofilms were milled using a focused ion beam for transmission electron microscopy (TEM) investigations. Prior to milling, the tribofilms were protected by depositing an organometallic Pt precursor. The TEM analyses were performed with a TECNAI F20 field emission TEM (FEI, Hilsboro, OR, USA) at 200 kV. The device was operated for imaging in bright field imaging mode. Scanning transmission images (STEM) were taken using a high angle annular dark field detector operated in dark field mode. For EDX analysis, an EDAX Apollo XLTW silicon drift detector was used. The chemical composition of the tribofilms was investigated using X-ray photoelectron spectroscopy (XPS). XPS analyses were performed using a Thermo Fisher Scientific theta probe (East Grinstead, UK) equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) and Ar+ ion gun. During the measurements, the base pressure inside the XPS chamber was kept constant at values in the range of 10−7 Pa. Prior to the XPS analysis, the tested samples were ultrasonically cleaned in toluene followed by petroleum ether (both HPLC grade), with each cleaning sequence lasting for 10 min. After being transferred inside the XPS chamber, the samples were sputtered

METHODOLOGY

Lubricant Mixture and Functionalized Surfaces. The lubricant mixture was prepared using 1 wt % L-methionine reagent grade (Figure 1) of >99.5% purity (Sigma-Aldrich, USA). Glycerol was selected as an environmentally acceptable carrier fluid since methionine is soluble in glycerol. Glycerol is readily available nowadays as a byproduct of biodiesel18 and is commonly used for the production of food additives, pharmaceuticals, and personal care products and in industrial 7031


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ACS Sustainable Chemistry & Engineering

Figure 2. Friction performance of 1 wt % methionine in glycerol against steel and WC-functionalized surfaces (a) and 1 wt % methionine in glycerol and pure glycerol against WC-functionalized surfaces (b). using soft Ar+ for 20 s, with 3 kV and 1 μA sputter current in order to remove the remaining contaminants. The sputtered area was approximately 3 mm × 3 mm. The survey scans were acquired at a spot resolution of 400 μm and a pass energy of 200 eV. The identified elements were then acquired using high resolution scans (at a pass energy of 50 eV), and the resulting binding energies were referenced to the adventitious carbon at a binding energy of 284.6 eV. Ab Initio Modeling of Reaction Mechanism. First-principles calculations within the framework of density functional theory (DFT) were carried out.27 The Kohn−Sham equations were solved iteratively with the Vienna Ab-Initio Simulation Package (VASP)28−33 using the formulation proposed by Perdew, Burke, and Erzenholf (PBE)34,34 for the exchange-correlation energy functional, a plane-wave basis, and employing periodic boundary conditions. The projector augmented wave (PAW)35 method was applied to describe the interaction between the core and the valence electrons. van der Waals forces were taken into account in our calculations since, as shown by previous studies, they play key roles in interactions at metal/organic interfaces such as the one considered in the present work. The empirical van der Waals correction DFT+D3 proposed by Grimme36 was applied in all systems here considered. In this method, the following expression is added to the total DFT energy Edisp = −

1 2

Nat Nat

⎛

C6ij

⎝

rij6, L

∑ ∑ ∑ ′⎜⎜fd ,6 (rij , L) i=1 j=1 L

+ fd ,8 (rij , L)

C8ij ⎞ ⎟ rij8, L ⎟⎠

damping function to avoid singularities and near distances. Further details can be found elsewhere.36,37 To study the adsorption configuration at various stages of the reaction mechanism, we constructed a series of supercells containing a WC slab and a methionine molecule adsorbed on top of it. The adsorbed molecule and its orientation vary in each supercell. In constructing the adsorption surface of this slab, we considered the cleavage plane of WC (1010̅ )38 to be the most likely plane of rupture experienced by the embedded particles during tribological contact. Consequently, the resulting adsorption surface must be parallel to this plane. In WC, however, such a cut may produce two types of surfaces, exclusively carbon- or tungsten-terminated.39 For the purpose of simulating the adsorption state potentially leading to the formation of tungsten disulfide, this work focuses on the tungsten-terminated surface. Furthermore, the extent of this surface guarantees that the contribution to the total energy by spurious intermolecular interactions, which arise as a consequences of periodic boundary conditions, are less than 1 meV. The vacuum spacing in the z-direction (perpendicular to the surface) between repeated iron slabs was 16 Å, which suffices to avoid interaction with neighboring slabs that could affect the surface charge density and potential. The calculations setup was carefully selected for minimizing the total energy and forces, while ensuring their respective accuracy. In each supercell, the atoms constituting the adsorbed molecule, and the top four layers of the slab, were relaxed applying the conjugate gradient algorithm until all forces were smaller than 0.1 eV/Å, while the remaining atoms were kept fixed at the bulk positions. Total energies in the self-consistency cycle were converged to 10−6 eV, and a

(1)

The dispersion coefficients are geometry dependent as they are adjusted on the basis of local geometry around atoms i and j, and f is a 7032


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Figure 3. Morphology of the WC-functionalized steel surfaces lubricated with 1 wt % methionine in glycerol at low (a, left) and high magnification (a, right). High resolution TEM image of the low friction tribofilm using methionine as a friction modifier (b, top). TEM-EDX analysis of a section of the tribofilm (b, bottom). cutoff energy of 400 eV was applied to the plane-wave basis set. The kspace integrations were performed using a 2 × 2 × 1 Monkhorst−Pack mesh.40,41 The partial occupancies were set by applying a Gaussian smearing with a width of 0.2 eV. Test calculations with smaller widths were carried out to ensure that the selected value leads to the groundstate geometry. Dipole corrections were applied to compensate for the artificial electrostatic potential introduced by the periodic boundary conditions in the supercell approximation. The adsorption energies were calculated according to the following equation: mol + WC mol WC Eads = Etot − (Etot + Etot )

suggests that at the beginning of the test the contact operates under boundary lubrication since the viscosity of glycerol drops down to 14.8 mPa s at 100 °C.42 After this initial incubation time, friction starts to steadily decrease until reaching a final value of 0.06, at around 12,000 s after the start of the experiment. This result suggests that after an initial running-in period the observed steady drop in friction could be attributed to the progressive formation of a low friction tribofilm at the contact interface between the functionalized surface and the steel ball. The low friction values of 0.06 can only be achieved by the simultaneous combination of methionine and the tungsten carbidecontaining-functionalized surface. When sliding against conventional stainless steel samples, in absence of W at the contact interface, the results of the lubricant mixtures containing methionine lead to a clearly deficient friction performance. The initial coefficient of friction has a relatively high value of around 0.3, but soon after starting the experiment, friction spikes reach values higher than 0.7. These seizures in friction indicate the welding of asperities and subsequent pull out and breakage due to the incapability of the lubricant to prevent severe metal to metal contact (Figure S6a and b). This behavior becomes exacerbated as the experiment progresses, and toward the end of the test, peak values of friction over 1 could be observed (Figure 2a). These results show that in absence of the tungsten carbidefunctionalized surfaces, the mixture of methionine in glycerol is not effective in terms of friction reduction under the selected testing conditions.

(2)

is the total energy of the products adsorbed on the WC where Emol+WC tot mol slab, EWC tot is the total energy of the clean WC slab, and Etot is the total energy of the isolated gas-phase educts calculated using a cubic box with 15 Å side length.

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EXPERIMENTAL RESULTS

Tribological Performance of Methionine as Lubricant Additive. The frictional performance of the lubricant mixture containing methionine in glycerol was investigated against WCfunctionalized surfaces under reciprocating sliding conditions (Figure 2). The friction results show that the mixture has an initial coefficient of friction of around 0.2. After initiating the rubbing process, friction remains at this value during the first hour of testing. This behavior 7033


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ACS Sustainable Chemistry & Engineering The friction performance of pure glycerol (Figure 2b) against hard WC-functionalized surfaces shows an initial COF value of 0.18, which slightly decreases toward 0.15 at the end of the test. The values of the coefficient of friction obtained are a factor 2.5 higher when compared to the values achieved using 1 wt % methionine in glycerol, and no tribofilm formation could be observed (Figures S6c and d). These findings clearly highlight the synergistic effect between methionine as a lubricant additive and WC-functionalized steel surfaces. Morphology and Tribo-Chemistry of Low Friction Tribofilm. The morphologies of the wear scars formed on the functionalized surfaces show the presence of a dark tribofilm under light microscopy (Figure S7). Scanning electron microscopy micrographs reveal a surface characterized by the presence of embedded tungsten carbide particles (Figure 3a). No visible signs of wear could be observed on any of the investigated functionalized surfaces. As previously reported, surfaces functionalized with hard WC particles provide a superior wear resistance when compared to conventional steel substrates.23 The functionalized surfaces tested with the methionine mixture reveal the presence of black spots evenly scattered throughout the wear scar. The black spots have a content of 6.8 wt % sulfur according to EDX analyses. A cross-sectional lamella was prepared from the tribofilm generated on a functionalized surface lubricated by the methionine mixture (Figure 3b). The bright field image shows a continuous tribofilm with a total thickness of about 1 μm. The presence of WC particles embedded on the steel surface can be observed at the interface between the steel surface and the formed tribofilm. EDX reveals the presence mostly of Fe, O, and S, likely indicating the dominance of iron sulfides and oxides. Tungsten can be detected sporadically and mostly in the vicinity of the tungsten carbide grains. The dark field images of the tribofilm reveal mostly an amorphous composition, with a sporadic presence of crystalline structures. In some localized regions close to the WC grains, lamellar structures could be identified, indicating a possible presence of WS2. XPS analyses reveal further insights into the chemical composition of the tribofilms. The spectra of the high resolution scans were processed using Gaussian/Lorentzian peak fitting. The high resolution W4f spectrum showed that the main component identified was WC outlined by the well-defined doublet W4f 7/2 and W4f5/2 at binding energies of 31.4 and 33.6 eV, respectively (Figure 4, top). The remaining binding energies could be ascribed to various types of oxides such as WO2 and WO3.5,43 The amount of WSx, if any, was too low to be detected inside the wear scar. The main component in the S2p high resolution spectrum corresponds to FeS with an S2p3/2 and S2p1/2 doublet at 161.6 and 162.8 eV, respectively (Figure 4, bottom). An additional doublet S2p3/2 and S2p1/2 could be detected at 162.0 and 163.4 eV, respectively, and was assigned to W−S bonds, according to prior findings.5,43,44 The remainder binding energies were ascribed to thiol bonds (163.6 and 164.8 eV) and SO2 species (167 and 168.2 eV). The high resolution peaks correlate well with the relative surface atomic concentrations from Table S1 showing that the tribofilms generated on the WC-functionalized surfaces are mainly composed of tungsten oxides (WO3 and WO2), FeS, and minor traces of WSx species. The high content of sulfur which is mostly in the form of sulfide (6.3 atomic %) as well as higher atomic % of Fe as compared to W allow us to suggest that the sulfur present in methionine reacted preferentially with the metal surface to generate a FeS-rich tribofilm as observed by the TEM analysis.

Figure 4. High resolution XPS spectra of W4f (top) and S2p (bottom) for 1 wt % methionine in glycerol.

reported in the literature for WS2 in boundary-lubricated contacts45 and for W−S-containing coatings in dry air contact conditions.7 As a consequence, it is suggested that during the sliding process the methionine molecule can interact with the WC surface. Various adsorption configurations were considered in order to evaluate the energetic viability of the proposed reaction as well as the favored reaction path (Figures S1, S2, and S3). Since the adsorption of methionine on the WC surfaces is a critical step that determines the progress of the reaction, our energetic analysis focuses on this process. In the calculated ground state geometry of L-methionine, the distance between the nitrogen atom and the oxygen atom bound to the hydrogen (2.71 Å) is close to the distance between tungsten atoms on the surface (2.85 Å). Accordingly, a reasonable first approach is to orient the molecule in a configuration where the nitrogen and the oxygen are located on top of tungsten atoms. Furthermore, because rubbing can promote dissociation of the acid hydrogen before the adsorption process occurs, the adsorption of the dissociated methionine molecule was also simulated in a configuration where the molecule is oriented as previously described. Moreover, the combination of rubbing and the experimental temperatures may stabilize a conformer of methionine, and therefore, two additional configurations were considered. These configurations were generated by rotating the atomic bonds in a way that places the acid oxygen, the nitrogen, and the sulfur atoms directly above the surface and approximately at the same distance from it. The main difference between them stems from the orientation of the carbon chain, as can be seen from the view parallel to the slab surface.

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MECHANISMS OF TRIBOFILM FORMATION Ab Initio Simulation of Interaction between Methionine and WC. As highlighted in the previous section, low friction tribofilms can be formed using methionine as an environmentally acceptable lubricant additive in conjunction with WC-functionalized surfaces. It was experimentally observed that low friction requires the simultaneous presence of W and S species at the contact interface. Furthermore, the low friction values of 0.06 achieved lie within the value range 7034


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ACS Sustainable Chemistry & Engineering Equilibrium Geometry and Energy of Adsorption Configurations. The initial step in the proposed reaction mechanism involving the adsorption of L-methionine on the (101̅0) WC surface is energetically favorable. The obtained adsorption energies of the order of several eV (Table 1) point to the formation of a chemical bond between the molecule and the metallic surface.

approximately half of the increase in adsorption energy. According to our calculations, the adsorption of such configuration is 2.24 eV more stable than the one previously considered. An analysis of the charge density surface (Figures S4 and S5) and LDOS (Figure 5b) as previously described shows a stabilization of this configuration by the formation of an additional tungsten−sulfur bond as well as van der Waals interactions between the carbon chain and the metallic slab. The existence of such an adsorbed configuration may have direct implications for the subsequent steps of the reaction mechanism leading to the formation of a tungsten sulfide tribolayer since the W−S bond may be already present during the initial steps of the reaction. The simulation of conformer B adsorption, where not only an increase in adsorption energy occurs but also a cleavage of an S−C bond takes place during atomic relaxation, additionally supports the existence of such a bond at early stages of the reaction. These results suggest the possibility of spontaneous formation of W−S−CH3 without any intermediates in the transition from dissociated Lmethionine, which increases the formation likelihood of the hypothesized tribolayer by reducing the number of reaction paths that could lead to different compounds. In Situ Conversion of Hard Coating (WC) to Low Friction Surface (WS2). On the basis of the ab initio simulations, the following in situ tribo-chemical reaction mechanism can be inferred. Methionine has two functional groups (NH2 and COOH) that can interact with the WCfunctionalized surfaces. Transition metals have a vacant datomic orbital. The electron configuration of W is [Xe]4f145d46s2. Therefore, W is ready to accept a maximum of six electrons on the 5d-atomic orbital. Methionine can donate a nonbonding electron pair (two electrons) on N and O. The W−O bond could be ionic depending on the environment. S in methionine can coordinate other W atoms (Figure 6a) according to the simulation (conformer B). The simulation suggests that the reaction starts with the dissociation of the C− S bond in methionine. It also indicates homolytic dissociation (radical reaction by one electron transfer) because other bonds in methionine are unreacted (Figure 6b). The intramolecular rearrangement of the radical species finally leads to formation of W−S−W bonds. Besides this reaction, due to the presence of Fe, the formation of WS2 is accompanied by formation of FeS. During the sliding process, both Fe and W partly oxidize, leading to the observed presence of FexOy and WOx. Once the W−S−W bonds are formed, WS2 has a lamellar structure with strong bonding between the metal and chalcogenide atoms which contrasts with the weak chalcogenide−chalcogenide interaction between the layers, allowing them to easily slide over each other. The lamellas have a good adhesion to metallic surfaces due to high polarization. In contrast to WS 2 nanoparticles, where the lubrication mechanism consists in exfoliation of the lamellas during sliding contact,46 in our case, according to the ab initio simulations, the W−S−W bonds are formed in situ via a tribo-chemical reaction. By relying on this mechanism, it is possible to achieve low friction tribofilms using sulfur coming from biomass, which meets the concept of green chemistry “use of renewable resources”. The requirement of tungsten may still pose a concern due to the potential presence of tungsten-containing degradation products. However, in our approach, W is the minor content of the substrate thanks to the peening technology applied, in contrast to coatings. In particular, our approach poses a substantial benefit in terms of sustainability in

Table 1. Calculated Adsorption Energies for Various Adsorbed Molecules configuration L-methionine

dissociated L-methionine dissociated L-methionine conformer A dissociated L-methionine conformer B

adsorption energy (eV) 2.24 4.01 6.25 8.73

The resulting adsorption configuration suggests the formation of a bidentate chelate complex, where binding takes place via the amino and carboxyl groups. In the relaxed geometry, the hydrogen atoms bound to the nitrogen atom are displaced in the direction away from the metallic surface, and the separation between the tungsten atoms below the reacting oxygen and nitrogen atoms increases by 0.22 Å, while the remaining atoms of the surface are mostly unaffected. The distance between these reacting nitrogen and oxygen atoms, and their respective nearest tungsten atom is similar in the nondissociated molecule (2.28 and 2.34 Å, respectively), while in the dissociated one the oxygen−tungsten distance is smaller (1.95 Å), hinting a stronger bond in accordance to its higher adsorption energy. Further data supporting the formation of chemical bonds is obtained by analyzing the variation in the charge density induced by the adsorption process (Figures S4 and S5). This analysis shows charge transfer into the region where the chemical bonds are expected. To obtain a better understanding of the electronic properties of the chemical bonding arising during the adsorption process, we calculated the local densities of states (LDOS) of the oxygen, nitrogen, and tungsten atoms involved in the chemisorption. Interaction between the p-states of C and N atoms and the d-states of W atoms are observed in the resulting LDOS (Figure 5a). There is a clear broadening of the p-states, which is more pronounced for the binding oxygen than for the nitrogen atom. The most significant broadening occurs in the pz-states, as shown by a decomposition of the p-states in terms of the cell axes, which is expected from their closeness to the metallic surface. This broadening extends over the Fermi level, indicating a depopulation of the p-band and consequently an increase in the molecule−surface interaction via polarization of these atoms. Here, L-methionine can isomerize to produce an adsorbed configuration with lower energy due to the formation of an additional tungsten−sulfur bond. Several conformers of Lmethionine can be formed by rotation around the C−C, C−S, and C−N bonds. Two conformers capable of forming a polydentate configuration have been considered in this study, which will be referred as conformer A and conformer B (Figures S2 and S3). The energy barriers separating the transition between these conformers may be overcome by the temperature and pressures involved in the experiments presented in this work and stabilized by the adsorption process. For instance, the calculated energy difference between conformer A and the ground state is 1.18 eV, which is 7035


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Figure 5. Local density of states for the atoms forming the O−W (left) and N−W (right) chemical bonds during adsorption of dissociated Lmethionine on a WC (101̅0) surface (a). Local density of states for the atoms forming the O−W (top-left), N−W (top-right), and S−W (bottom) chemical bonds during adsorption of dissociated isomer of L-methionine (conformer A) on a WC (101̅0) surface (b). In all cases, the Fermi level is located at 0 eV.

better lubricating properties when compared to other green alternatives, such as rapeseed oil.20 The use of WS2 or MoS2 nanoparticles has shown a large potential for friction and wear reduction in sliding contacts using different contact conditions and mating materials.47−50 However, their use requires the need of dispersants in order to obtain stable lubricant mixtures. Recent results have raised concerns regarding this approach due to antagonistic effects between both.51,52 Our approach enables low friction without the need of adding dispersants, since the WC particles are already embedded and distributed throughout the substrate, so

comparison to commercially available lubricants relying on oil soluble metal-containing additives such as MoDTC and ZnDTP as friction modifiers. Oil soluble compounds are lyophilic and therefore affine to fats present in food. Therefore, there is a higher risk of heavy-metal intake. On the other hand, inorganic compounds have a much lower affinity to fats. Further, in our case, glycerol was used as the base lubricant for L-methionine, resulting in a green lubricant. Even though an extensive use of glycerol as a base lubricant cannot be immediately foreseen for being hygroscopic and having a viscosity higher than conventional base oils, it has been recently proposed that mixtures with up to 20 wt % water content have 7036


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Figure 6. Proposed reaction mechanism between methionine and tungsten carbide. Interaction between methionine and WC (a). Dissociation of CH2−SCH3 bond and formation of W−S−W bond (b).

that the low friction tribofilm is built in situ on demand during the test.

S1−S5). Morphology of the wear scars obtained under the lack of W or S at the contact interface (Figure S6). Morphology of the low-friction tribofilm and position of the TEM lamella (Figure S7). Chemical composition of the low friction tribofilm obtained by XPS (Table S1). (PDF)

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CONCLUSIONS The present work shows the feasibility of using methionine as a friction modifier via the in situ tribo-chemical formation of low friction tribofilms. The formation of low friction tribofilms requires the simultaneous presence of both, methionine as a sulfur carrier additive and a surface functionalized with tungsten carbide particles. The low friction tribofilms formed have a thickness of 1 μm and are mainly composed of iron sulfides. A plausible lubrication mechanism that relies on ab initio simulations suggests the in situ tribo-chemical formation of WS2. This mechanism may explain the observed coefficient of friction of about 0.06 seen in the experiments and the experimental observations that low friction only occurs under the simultaneous presence of S and W at the contact interface. With our approach, the synergy between the material surface and the lubricant additive can be exploited for using environmentally friendly compounds without known tribological applications, such as methionine. Our new approach deliberately exploits this synergy and opens the door for the design and construction of tribologically effective and environmentally acceptable systems. Conventionally, in many engineering applications where low friction is desired, this goal is achieved either by using suitable oil additives, many of them harmful for the environment and that often rely on the presence of ferrous surfaces for being functional.

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W Web-Enhanced Feature *

Video. Simulated ionic relaxation of an L-methionine conformer (conformer B) adsorbed on a WC (10−10) surface. The cleavage of the S−C bond and corresponding formation of S− C−CH3 occurs during the first steps of the simulation.

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AUTHOR INFORMATION

Corresponding Authors

*Manel Rodrı ́guez Ripoll. Telephone: +43 2622 816 00−309. E-mail: [email protected]. *Vladimir Totolin. Telephone: +43 2622 816 00−311. E-mail: [email protected]. ORCID

Manel Rodríguez Ripoll: 0000-0001-9024-9587 Ichiro Minami: 0000-0002-8972-2944 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Austrian COMET Programme (Project K2 XTribology. No. 849109) and carried out at the ‘‘Excellence Centre of Tribology’’. The authors thank Sara Spiller for performing a part of the XPS analyses, Johannes Bernardi for performing the TEM measurements, and Lukas Spiller for running the tribological tests. The functionalized surfaces were manufactured by Dr. Christoph Lechner (Technische Universität Wien).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01258. Ab initio simulation of L-methionine conformers. Four variants of L-methionine were simulated: equilibrium ground state, corresponding geometry with dissociation of the acid hydrogen, and two conformers of this dissociated state, named conformer A and B (Figures

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REFERENCES

(1) Minami, I. Molecular Science of Lubricant Additives. Appl. Sci. 2017, 7 (5), 445.

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Methionine as a Friction Modifier for Tungsten Carbide-Functionalized

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