Voltage-Driven Growth of Phosphorus Tribofilms

Voltage-Driven Growth of Phosphorus Tribofilms | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Voltage-Driven Growth of Phosphorus Tribofilms Yun Zhao, Jie Zhang, Hugh A. Spikes, JANET S. S. WONG This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8888807/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 10 You are reading this latest preprint version Abstract Ashless phosphorus-based lubricant additives, which are increasingly deployed in next-generation formulations, often suffer from slow tribofilm formation and poor film stability, limiting their effectiveness under demanding operating conditions. As mechanical systems become increasingly electrified, understanding how lubricants respond to electrical stimuli, and developing strategies that exploit such stimuli, have become critical for ensuring reliable operation. Here, we investigate the tribological performance of bis(2-ethylhexyl) phosphite (BEPite) in polyalphaolefin (PAO2) lubricated steel/steel contacts under controlled voltage and current. BEPite produces thicker and denser tribofilms on anodic rubbing surfaces when a voltage is applied. Increasing the current between lubricated contacts from < 0.01 mA to 0.5 mA has little effect on tribofilm formation, whereas increasing voltage enhances it. This enhancement is only observed in rubbing contacts. Chemical analysis reveals the presence of oxidized PO 3 2− -related species and abundant iron in the tribofilm. This suggests voltage-driven triboelectrochemical oxidation, as well as enhanced additive adsorption and reaction, promote tribofilm formation. Either iron oxides or released Fe ions may alter the tribofilm structure. These phenomena appear general for phosphorus-based ashless additives, as both phosphites and phosphates with different structures have seen increased tribofilm growth at anodic rubbing surface. This work demonstrates that voltage alone can intensify triboelectrochemical reactions, providing new insights for the design of next-generation lubricants. Electrified tribology phosphite phosphate tribofilm iron oxidation and voltage Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction The rapid development of electrification has posed challenges to lubricated systems. For instance, battery systems in electric vehicles operate at 200-900 V, with voltages on motors reaching up to ~10% of the supply voltage and currents of tens of amperes. Current discharge can potentially lead to premature damage in EV drivetrain [1, 2]. The detrimental effect of electric potential has also been seen in wind turbine gearboxes and industrials motors [3]. Negative effects on performance of lubricant has also been reported [4-6]. Thus, a good understanding of lubricant behaviour under electric conditions, and potentially how it may be used to improve machine reliability is crucial. Antiwear additives are important additives in fully-formulated lubricants. Zinc dibutyldithiophosphate (ZDDP), the most common antiwear additive, has been a target of replacement due to component reliability and environmental issues [7-9]. Ashless phosphorus-based additives, such as phosphates and phosphites, are potential candidates. These additives often suffer from limitations, including slow tribofilm growth and insufficient surface protection, making the enhancement of their performance a key research direction [10, 11]. In this work, we examine how an electrified contact may promote tribofilm formation of phosphite additives in the boundary lubrication regime. 2. Background Although lubricants with hydrocarbon oil as base fluid exhibit very low ionic conductivity, application of current and electric field can still impact the performance of lubricants. Numerous studies have shown that performance of fully formulated oils is influenced in electrified contact [ 12 – 17 ]. For example, both wear volume in formulated engine oil and electric vehicle oil under 1 A and 2 A currents increased by 20–50 times, but only on the anodic surface, while the cathodic surface remained unaffected [ 13 ]. A plausible explanation is that electric current is concentrated at the asperity contacts, generating significant localized heat that can potentially induce lubricant decomposition [ 5 ]. An interesting phenomenon has been demonstrated where the tribofilm formed on the anode becomes electrically polarized and subsequently adhered to the cathode [ 13 ]. Kadiric et al. [ 5 ] found that even a small current (< 10 mA) could affect tribofilm formation in an automatic transmission fluid, with tribofilm promoted on the anode but being suppressed on the cathode. Both studies highlight that electrifying a rubbing contact can alter the behaviour of additives and hence tribofilm formation, consequently affect wear of tribo-pairs. Additive performance may be impacted by electrochemical reactions even under mild voltage conditions. Under boundary lubrication, electrochemical reactions may be intensified at sliding interfaces due to shear stress. Most existing work to date about specific additive performance under electric field has focussed on film-forming additives such as zinc dibutyldithiophosphate (ZDDPs), molybdenum dithiocarbamate (MoDTC), and related chemistries [ 5 , 12 , 16 , 18 – 22 ]. In 1981, Yamamoto and Hirano found that applying voltage and current simultaneously to rubbing pairs enhanced the tribofilm formation of tricresyl phosphate (TCP), whereas excessive voltage and current reduced it [ 23 ]. Ozimina later reported that 1 wt.% ZDDP in acetonitrile could decompose at potentials within ± 1 V [ 20 ]. Applied voltages between 0–3 V have also been shown to influence additive adsorption and electrochemical reaction in ZDDP-containing propylene carbonate/diethyl succinate systems [ 24 ] and an organic molybdenum additive in PAO2 [ 25 ]. Wear reduction was observed as a result. For ashless additives, tricresyl phosphate has been reported to enhance the scuffing resistance of surface films under microampere‑current and millivolt‑voltage conditions [ 26 ]. By contrast, other studies that apply electrical current have shown detrimental effects on wear performance [ 27 , 28 ]. An applied voltage may also impact tribofilm formation through altering the properties of rubbing surfaces. It has been shown that electrifying a contact could modify metal surface energy and thereby influence frictional performance [ 29 ]. Applied voltage can affect metal surface oxidation [ 30 ], lubricant bubble generation [ 31 ], and base oil wettability [ 32 ]. Applying current can also induce similar phenomena described above, including surface interactions [ 23 , 33 ] and bubble generation [ 34 ]. Prior work has shown that additive binding and tribochemical pathways differ between metallic iron and iron oxide surfaces [ 35 ], and that iron cations generated due to metal-surface oxidation can promote the polymerisation of iron polyphosphates near the substrate-tribofilm interface [ 36 ]. Additional studies have demonstrated that adding metal ions into lubricants enhances tribofilm formation [ 37 ], confirming the role of metal ions on tribofilm formation. From a triboelectrochemical perspective, promoting oxidation of the metal rubbing surface may facilitate the tribofilm-formation of phosphorus-based additives and thereby improve their overall tribological performance. In this study, we control the voltage and current between lubricated contacts and investigate their influence on tribofilm formation of phosphite and phosphate additives to examine if voltage alone, without significant current, can modify their tribofilm formation. The chemical characteristics of the resulting tribofilms under different voltage biases were compared to evaluate the extent to which electrical stimuli modify interfacial chemistry. Complementary static electrochemical measurements were performed to examine the importance of rubbing in observed voltage-dependent tribofilm formation. 2. Materials and Methods 2.1. Materials Cleaning solvents include heptane (≥ 99%) and toluene (≥ 99.5%). The lubricant additives used are bis(2-ethylhexyl) phosphite (96%), bis(2-ethylhexyl) phosphate (97%), dibutyl phosphite (96%), tris(2-ethylhexyl) phosphite, and tris(2-ethylhexyl) phosphate (97%), all purchased from Sigma-Aldrich without further purification. PAO2 (polyalphaolefin SpectraSyn 2) is chosen as the base oil. Lubricants are prepared by dissolving phosphorus-based additives into PAO2 to achieve a phosphorus concentration of 800 ppm. The mixture is stirred with a magnetic stir bar at room temperature for 2 h. The transparent solution is then allowed to stand for 10 min and is used immediately thereafter. Most results in this study uses bis(2-ethylhexyl) phosphite in PAO2 as the model lubricant because it forms a visible tribofilm under the test conditions, enabling clear examination of effect of applied voltage of tribofilm formation. 2.2. Tribometer and test conditions Friction tests are conducted using a mini-traction machine with an electrochemistry module (MTM-EC, PCS Instruments), as shown schematically in Fig. 1 a. The MTM-EC employs a ball-on-disc configuration, with voltage applied across the ball and disc in a two-electrode setup. A balance resistor, with a maximum value of 1 MΩ, is connected to the contact in series and is used to control the electric circuit current. Voltage and current between the ball and disc are continuously monitored using an oscilloscope. The balls and discs are made of AISI 52100 steel and are from PCS Instruments. They are cleaned before testing using the following procedure: (1) rinse with toluene and wipe with toluene-soaked tissue; (2) ultrasonicate in toluene for 15 min, then soak in fresh toluene for 12 h; (3) rinse again with toluene, wipe with toluene-soaked tissue, and ultrasonicate for another 15 min in fresh toluene; (4) before testing, wipe and rinse with toluene, dry with compressed air, and treat with oxygen plasma for 1 min. Test conditions are summarized in Fig. 1 b and 1 c. The normal load is 31 N (maximum Hertzian pressure of 0.95 GPa), entrainment speed 50 mm s − 1 , and slide-roll ratio (SRR) 20%, with the ball and disc rotating at 45 and 55 mm s − 1 , respectively. Tests are performed at 80°C. In the MTM-EC software, a + 5 V setting connects the positive terminal to the ball (anodic) and the negative terminal to the disc (cathodic). A -5 V setting reverses this polarity. Unless otherwise stated, the balance resistor is set to 1 MΩ to minimize current effects. Depending on the contact conditions during sliding, the measured potential is typically lower than the set value and may fluctuate. The measured value is determined by the amount of solid-to-solid contact and electrical properties of any tribofilm formed on the surface. For simplicity, the set voltage is used to label each condition. The test duration is 2 h, unless specified otherwise. After testing, specimens are rinsed with heptane and dried with compressed air. They are stored for further analysis. Each experiment is repeated at least twice to ensure reliability. 2.3. Characterizations After each test, wear scars may form on rubbing surfaces. Rubbed surfaces are first examined using an optical microscope to assess dimensions and colour of these scars (RH-2000 digital microscope, Hirox, Tokyo, Japan). The thickness of tribofilm on discs is measured using scanning white light interferometry (SWLI, Bruker Contour GT-K). The topographic profile is an average of a band of 5-pixel width. Before examining with SWLI, selected areas of the tribofilm on the disc are removed by etching with 0.1 M oxalic acid for 20 s to expose the buried steel surface. Residual oxalic acid is then wiped away using a DI water-wetted lint-free tissue. This is followed by gold coating on both etched and unetched regions (Fig. 4 a). This facilitates the evaluation of wear depth and the actual tribofilm thickness [ 38 ]. Chemical analyses are conducted using Energy dispersive X-ray spectroscopy (EDX, Tescan Mira) and time-of-flight secondary ion mass spectrometry (TOF-SIMS, DektakXT, Bruker). EDX is used to map the element distribution in the tribofilm, while TOF-SIMS is employed to characterize the chemical states of elements at different depths. 3. Results and discussion Under open-circuit potential (OCP) conditions, no external voltage is applied between the ball and disc. So, both voltage and current are considered negligible. In this state, the ball exhibits a wear track with no obvious tribofilm. On the disc, a patchy tribofilm, with low surface coverage, is formed (see Fig. 2 c). 3.1. Comparative importance of current and voltage on BEPite tribolfilm formation Under an applied voltage, the reaction is primarily attributed to the electric current, which induces localized heating through its concentration at asperity contacts [ 5 ]. To compare the importance of current and voltage on tribofilm formation, the balance resistor is adjusted to control current. The power supply provides a fixed voltage, while the impedance in the circuit varies. In addition to the fixed balance resistor, the resistance between the sliding contact drops to 0 Ω when asperities are in direct contact, but becomes very high when the contacts are separated by a tribofilm. As a result, the voltage and current across a sliding contact exhibit large fluctuation. We record data continuously and then average the readings over each 0.6 s. Changing the resistance of the balance resistor from 1 MΩ to 10 kΩ corresponds to changes in the real average voltage and current from 1.16 V and < 0.01 mA to 0.26 V and 0.5 mA, respectively (Fig. 2 a and 2 b). A higher current combined with lower voltage (10 kΩ resistance) produces tribofilms similar to those formed under OCP conditions, whereas a lower current (1 MΩ resistance) combined with higher voltage results in a pronounced increase in tribofilm formation (Fig. 2 c). These results indicate that voltage plays a more decisive role than current in driving tribofilm growth in our rubbing conditions. Thus, the following discussion focuses on the effects of applied voltage. 3.2. Effect of applied voltage on BEPite tribofilm formation When a voltage is applied, the potential difference between the ball and disc increases gradually with rubbing time (Fig. 3 a). At the start of sliding, substantial asperity contacts cause a short circuit, with measured voltage close to 0 V. As the tribofilm develops, the resistance across the contact increases, leading to a rise in the measured voltage. The morphology of the tribofilm leads to occasional micro-short-circuits, causing large voltage fluctuations. With the balance resistor in series, the current remains stable at ~ 0.01 mA (Fig. 3 b). The disc tribofilm forms under anodic condition is more uniform than that observed at OCP, whereas the ball wear track shows no obvious tribofilm, see Fig. 3 c. For a cathodic disc, tribofilm is observed mostly at the centre of its wear track. Tribofilms are also formed at the centre of the wear track of the anodic ball. These observations indicate that tribofilm formation depends on the applied voltage and, critically, on its polarity. Notably, the friction coefficients under electrified and non-electrified conditions are similar ( Figure S1 ). The tribofilm formed on the ball surface is not clearly distinguishable under either biased or unbiased conditions. The reason is unclear and may be due to the different physical properties and contact conditions between the ball and the disc, which promote preferential transfer of the tribofilm to the disc surface. To enable a more reliable assessment of the effect of applied voltage on tribofilm formation, the tribofilm on the disc is selected as the primary focus for subsequent analysis. The effect of voltage on tribofilm formation on an anodic disc is investigated over a range of applied negative voltage while keeping the balance resistor at its maximum value to minimize current effects, see Fig. 4 b. For programmed voltages of -2 V, -5 V, and − 8 V, the actual average voltages across the lubricated contacts are − 0.56 V, -1.16 V, and − 0.97 V, respectively ( Figure S2 ). At -2 V, blue deposits (labelled as ‘thick’ in Fig. 4 b) appear on the disc surface, with localized increase in thickness but poor uniformity. At -5 V, tribofilm growth is enhanced, with more blue regions and an average thickness of 80–90 nm (Fig. 4 c, and for repeat tests see Figure S3 and S4 ). Further increasing the set voltage to -8 V does not increase thickness because the actual voltage across the contact does not change significantly. In contrast, applying a positive voltage does not lead to increase in tribofilm thickness but can impact the morphology of the tribofilm ( Figure S5 ). The increased thickness of tribofilms on anodic discs shows that an anodic bias results in higher tribofilm growth rate or stronger tribofilm (resisting film removal) or both. Note that the profiles of the cleaned area show no measurable wear, although occasional surface scratching can occur during the process ( Figure S6 ). This suggests that tribofilms forms in anodic discs under the applied negative voltage offer good antiwear protection. Figure 5 shows how the tribofilm thickness on anodic discs at -5 V changes with rubbing time (see also Figure S7-10 ). In the first 30 min, the tribofilm islands reach only 15–20 nm, scattered across the wear track (see also Figure S7 and S8 ). By 60 min, the thickness increases to ~ 40 nm (see also Figure S9 ), and the surface becomes densely covered with granular and elongated deposits. Further rubbing produces thicker tribofilms, reaching ~ 60 nm at 90 min (see also Figure S10 ) and ~ 80 nm at 120 min. Tribofilm growth under unelectrified (OCP) conditions is slower (Fig. 6 and Figure S11-14 ). After 30 min, the film thickness is ~ 15 nm ( Figure S11 ), with fewer deposits compared to that on an anodic disc under − 5 V. This indicates that voltage can influence tribofilm formation even at the earliest stages. Prolonged rubbing increases tribofilm thickness marginally. After 120 min it reaches only ~ 40 nm, with partial surface coverage. Extending the rubbing time to 240 min does not lead to further growth of the tribofilm thickness ( Figure S14 ). These results show that applied voltage increases tribofilm growth rate or produce more robust films that remain stable at higher thicknesses, or both. 3.3. Chemical analysis of disc tribofilm formed at OCP and − 5V Chemistry of tribofilms formed under electrified and non-electrified conditions is investigated using EDS, see Fig. 7 . For direct comparison, the selected regions include both the wear tracks and the immediate unrubbed areas. In both cases, oxygen (O, red) and phosphorus (P, green) are detected. In non-electrified condition (Fig. 8 a), the distributions of O and P in the wear track are slightly elevated. In the electrified condition (Fig. 8 b), the distributions of O and P form stripe-like features which match the tribofilm morphology, showing that this tribofilm consists of O- and P-containing compounds. The wear track on the anodic disc contains 2.52 wt.% P and 4.17 wt.% O, nearly twice the proportions observed on an unelectrified disc (Fig. 7 c). On the other hand, the unelectrified disc contains more Fe. This result is consistent with tribofilms on anodic discs being thicker, and having higher surface coverage (see Fig. 4 ). 3D TOF-SIMS (Fig. 8 ) is performed in a ~ 100 µm × 100 µm area at the centre of each wear track (wear track width ~ 250 µm). As phosphite-derived tribofilms typically contain C, O, and P, the negative ion spectra are analysed for C − , CH − , P − , PO − , PO 2 − , and PO 3 − fragments (Fig. 9 ). Phosphorus in the tribofilms on both discs is primarily present as PO 2 − and PO 3 − , with small amount of other P-containing fragments (P − and PO − ) detected (Fig. 9 c-f). Overall, PO 3 − and PO 2 − accounts for 57% and 41% of total P-containing fragments in the anodic tribofilm, whereas in the OCP tribofilm, the proportions are 52% and 44%, respectively (Fig. 8 d). No PO 4 − (the oxidation product of phosphite) forms under OCP conditions, while it is detectable in anodic tribofilm, confirming that anodic polarization can drive this oxidation reaction ( Figure S15 ). The distribution of iron oxides in the tribofilm is also examined, as surface oxidation may be an important contributing factor to tribofilm formation. The key marker for iron oxides in TOF-SIMS include Fe + , FeO + , FeOH + , Fe 2 O + , O − , O 2 − , FeO 2 − , and FeO 2 H − . Among these, only O − , O 2 − , Fe + , and FeO + exhibit obvious signals (Fig. 9 ). Depth profiling is conducted in 10 nm increments for a total of five cycles and confirms that the tribofilms formed under OCP is much thinner than that formed in -5 V (Fig. 8 a-c). The depth distributions and relative intensities of various fragments are shown in Figs. 8 , 9 and 10 . C − and CH − , representatives of organic and carbonaceous species, are concentrated within the top 1–2 nm, consistent with previous reports [ 39 ], and are more abundant in the OCP tribofilm than in the anodic tribofilm (Fig. 8 a and 8 b, Fig. 10 a and 10 b). The tribofilms on both the OCP and the anodic discs show the highest P-base fragment densities on their top surfaces. For the anodic tribofilm, the densities of P-base fragments drop within the few nanometers before stabilizing or reducing in slower rates. The high fragment concentration detected at the tribofilm surface probably originates from recent deposition and surface reactions during sliding. With increasing depth, these newly formed species may undergo further changes, such as organic species removal and inorganic species aging, leading to the formation of more stable tribofilm which are more resistant to rubbing. Depth profiles of iron oxide fragments show high intensities across the entire sputtering range. They co-exist with P-base fragments throughout the tribofilm, indicating a stable, iron-phosphorus-rich structure. These results demonstrate that iron actively participates in tribofilm formation, leading to the development of a dense, iron-phosphorus-enriched tribofilm under anodic conditions. These depth profiles show that the through-thickness chemistry of the anodic tribofilm is relatively constant with the amount of phosphorus and iron oxide-related species decay slowly below the top surface (Figs. 9 and 10 ). The intensities of P-base fragments are much lower in OCP tribofilms and are only detected in the first 10 nm (Fig. 10 ). Iron oxide-related fragments concentrate in the top ~ 10 nm and gradually decrease in abundance at depths of 20–30 nm, consistent with a previous report about iron oxide distribution within the tribofilm [ 40 ]. This exceeds the depth range of phosphorus-containing species, suggesting a tribofilm structure comprising an organic/carbonaceous top layer, a middle layer containing phosphorus and iron, and an underlying oxide layer. 3.4. How anodic potential promotes tribofilm formation Under open-circuit potential (OCP), the tribofilm contains a high proportion of partially decomposed phosphite species, resulting in an organic-rich film. In contrast, anodic polarisation favours more complete phosphite decomposition, producing a tribofilm enriched in inorganic phosphate species \(\:\left({\text{PO}}_{3}^{-}\right.\) , \(\:{\text{PO}}_{2}^{-}\) , \(\:\left.{\text{PO}}_{4}^{3-}\right)\) that better stabilises the film under shear. A potential mechanism for the formation of the tribofilm under applied anodic potential compared to OCP conditions is illustrated in Fig. 11 . Phosphite additives adsorb onto the steel surface through chemisorption via the phosphorus group [ 41 ], assisted by physical interactions​ (van der Waals forces) between the organic carbon chains and the surface. When an anodic potential is applied, the resulting electrostatic field strengthens chemisorption​ while concurrently weakening the physical adsorption​ of the organic chains because of electronegativity difference [ 42 ]. This potential-dependent interfacial reorganization influences the initial adsorption stage and later reactions. Cleavage of the C-O bond, as generally seen with ZDDPs [ 43 ], will favour the formation of PO 3 − , whereas scission of the O-P bond should lead predominantly to PO 2 − . The anodic film also contains PO 4 − . The weakened interaction between carbon chains and the surface will lead to reduced carbon species on rubbing surfaces. Anodic polarization also promotes oxidation of the steel surface, generating iron oxides and releasing iron ions. These species enhance both the chemisorption and complexation​ of phosphite derivatives, thereby accelerating tribofilm growth. The incorporated iron ions may play a vital structural role, acting as cross-linking centres, connecting phosphate groups to form a more coherent and robust three-dimensional networks within the tribofilm [ 44 ]. This iron-stabilized architecture should significantly improve the mechanical integrity and shear resistance of the film under tribological stress. The overall chemical nature of the OCP and anodic tribofilms observed here is consistent with tribofilms and thermally formed films reported in the literature [ 39 , 44 ]. The films are dominated by a mixture of carbon-containing species, phosphate-based compounds, and iron-phosphate/iron-oxide complexes, with an oxide-rich interfacial layer adjacent to the steel substrate. The key distinction due to the applied anodic potential therefore lies not in the identity of the chemical species, but in their relative abundance, distribution, and degree of inorganic cross-linking. Under anodic polarization, the tribofilms exhibit a higher fraction of inorganic phosphate species and iron-coordinated structures, whereas films formed under OCP or purely thermal conditions tend to retain a larger proportion of organic fragments from incomplete phosphite decomposition. This suggests that anodic polarization does not introduce new chemistry, but rather shifts the balance of existing tribochemical pathways, promoting oxidation, enhanced iron incorporation, and the formation of a more inorganic, mechanically robust network compared with conventional phosphite tribofilms reported in the literature. 3.5. Universality of applied potential effect on P-based tribofilm The above effect of applied potential on promoting tribofilm growth is not unique to BEPite. Voltage-induced effects on tribofilm formation are also seen for other phosphorus-containing additives that show limited tribofilm growth under OCP conditions (Fig. 12 ) . For example, diisopropyl phosphite forms tribofilm rapidly at OCP, and so applying voltage produces minimal effect ( Figure S16 ). By contrast, dibutyl phosphite, tris(2-ethylhexyl) phosphite, bis(2-ethylhexyl) phosphate, and tris(2-ethylhexyl) phosphate, which all show limited tribofilm formation at OCP, show obvious changes in their tribofilms under − 5 V. For dibutyl phosphite, friction coefficient remains unchanged, while tribofilm thickness slightly increases under − 5 V (Fig. 12 a and 12 b). Tris(2-ethylhexyl) phosphite (Fig. 12 d) and bis(2-ethylhexyl) phosphate (Fig. 12 f) produce no obvious tribofilm at OCP, but tribofilms are formed at -5 V, accompanied by an increase in friction coefficient (Fig. 12 c and 12 e). Tris(2-ethylhexyl) phosphate (Fig. 12 h and 12 g) shows large differences in tribofilm morphology and friction coefficient between OCP and − 5 V. At OCP, the wear track is wider and tribofilm highly heterogeneous, while under − 5 V the wear track becomes narrower and the tribofilm more uniform, corresponding to a reduction in friction coefficient from 0.25 to 0.10. These results indicate that the application of negative potential can be effective in promoting tribofilm formation/tribofilm growth in a variety of P-base additives. 3.6. Effect of applied potential alone on steel surface chemistry One question that arises from the above tests is whether the observed effect on BEPite tribofilm on anodic discs in a rubbing contact occurs without rubbing? To explore this, the growth of the voltage-induced surface film is evaluated with a static electrochemical cell. A steel ball and a steel disc are brought into contact in lubricant. The separation between the two steel specimens is controlled at approximately 1 µm using a digital micrometer (Fig. 13 a). In the absence of mechanical motion, electrochemical effects are isolated from tribochemical effects. Applying a 5 V bias to steel discs in BEPite in PAO2 for 4 h leads to no obvious surface film under optical microscopy (Fig. 13 b). Raman spectroscopy likewise reveals no discernible signatures of iron oxides, suggesting that any oxidation, if present, occurs at very low levels ( Figure S17 ). More sensitive TOF-SIMS depth profiling shows only a slight increase in FeO + fragments on the anodic surface after holding at 5 V for 4 h, both in neat PAO2 and in PAO2 containing 800 ppm BEPite (Fig. 13 c and d ). Carbon‑rich residues are also observed, likely due to the adsorption of tenacious carbon ( Figure S18 ). The disc immersed in a phosphite‑containing PAO2 solution has a thin PO 2 − - and PO 3 − -containing layer, which adheres strongly and cannot be removed by rinsing with heptane ( Figure S18 and S19 ). This enhancement is confined to the top ~ 1–2 nm of the interface, indicating extremely limited electrochemical oxidation and surface film formation under static conditions. These results support the proposition that the substantial tribofilm growth observed during sliding under applied voltage arises from triboelectrochemical processes that occur only when mechanical shearing is present. This comparison highlights the mechanical force plays a decisive role in activating voltage-driven interfacial reactions, which subsequently contribute to tribofilm formation. 4. Conclusion Ashless P-based antiwear additives are more environmentally-acceptable alternatives to conventional antiwear additives ZDDP. However, they suffer from relatively low tribofilm growth compared to metal-containing P additives [ZDP] or some P-S based ones. In this work, the tribofilms of P-based ashless additives formed under electrified conditions have been examined. Under our experimental conditions, tribofilm formation is primarily governed by applied voltage rather than current, with an anodic surface exhibiting faster tribofilm growth and greater film thickness. Chemical analysis suggests that iron participation plays an important role in tribofilm formation. The tribofilm structure consists of an outer layer of organic species and carbon, and an inner layer rich in phosphorus and iron. Compared with the tribofilm formed under OCP, applying − 5 V produces a denser and thicker film, resulting in improved stability. The discovery that anodic oxidation and additive decomposition act cooperatively to build dense and stable tribofilms highlights a new electrochemical-tribochemical coupling mechanism. These insights provide a foundation for designing electro-responsive additives and smart lubricants capable of dynamically adapting to next-generation electrified systems. Declarations Competing interests The authors declare no conflict of interest. Author Contribution Y.Z., H.A.S. and J.S.S.W.: conceptualization and design of the study. Y.Z.: experimental investigation, data acquisition, analysis and interpretation and writing of the original draft. J.Z. : support with experimental investigation, data acquisition, and interpretation, J.S.S.W.: supervision, funding acquisition, project administration, and revision of the original draft. All authors reviewed the manuscript. Acknowledgement This research is financially supported by the Shell University Technology Centre (UTC) for Mobility and Lubricants and the EPSRC InFUSE Prosperity Partnership (EP/V038044/1). Y.Z. is funded by Shell and UKRI via IDLA studentship. Data Availability The data that support the findings of this study are available from the corresponding author upon reasonable request. References Kliman, G., Stein, J. Induction motor fault detection via passive current monitoring—a brief survey. In: Proc. 44th Meeting of the Mechanical Failures Prevention Group, pp. 49–65 (1990). Hadden, T., Jiang, J.W., Bilgin, B., Yang, Y., Sathyan, A., Dadkhah, H., Emadi, A. A review of shaft voltages and bearing currents in EV and HEV motors. In: IECON 2016—42nd Annual Conference of the IEEE Industrial Electronics Society, pp. 1578-1583 (2016). https://doi.org/10.1109/IECON.2016.7793357 Sheng, S. Report on wind turbine subsystem reliability-a survey of various databases (presentation). Technical report, National Renewable Energy Lab. (NREL), Golden, CO (USA) (2013). https://docs.nrel.gov/docs/fy13osti/59111.pdf Yamamoto, Y., and F. Hirano. Scuffing resistance of phosphate esters II: Effect of applied voltage. Wear 66 (1981): 77-86. https://doi.org/10.1016/0043-1648(81)90034-X Yousuf, A., Spikes, H., Guo, L. and Kadiric, A. Influence of Electric Potentials on Surface Damage in Rolling-Sliding Contacts Under Mixed Lubrication. Tribology Letters 73 (2025), 45. https://doi.org/10.1007/s11249-025-01977-2 Cao-Romero-Gallegos, J.A., Farfan-Cabrera, L.I., Erdemir, A. and Pascual-Francisco, J.B. Lubricated sliding wear of gear material under electrification-A new approach to understanding of the influence of shaft currents in the wear of EV transmissions. Wear, 523 (2023), p.204782. Spikes H.A. Beyond ZDDP, Lubrication Science, 20 (2008), p. 77-78. https://doi.org/10.1002/ls.60 Gangopadhyay, A.K., Jensen, R.K., Carter III, R.O., Uy, D., O'Neill, A.E., Simko, S.J., Gao, H., Stockwell, R.T., Phillips, C.B. and Graham, M.E. Development of zero‐phosphorus engine oils. Lubrication Science, 20(2008), pp.163-180. https://doi.org/10.1002/ls.54 Devlin, M.T., Guevremont, J., Sheets, R., Loper, J., Guinther, G., Thompson, K. and Jao, T.C. Effect of metal‐free phosphorus anti‐wear compounds on passenger car emissions and fuel economy. Lubrication Science, 20(2008), pp.151-161. https://doi.org/10.1002/ls.44 Spikes, Hugh. Low‐and zero‐sulphated ash, phosphorus and sulphur anti‐wear additives for engine oils. Lubrication Science 20 (2008), p. 103-136. Kim, B. Tribological performance of ashless antiwear additives under extreme pressure conditions. Thesis, 2010. https://mavmatrix.uta.edu/context/materialscieng_dissertations/article/1041/type/native/viewcontent Gangopadhyay, A., Paputa Peck, M.C. and Simko, S.J. Wear control in a lubricated contact through externally applied electric current. Tribology transactions, 45(2002), pp.302-309. https://doi.org/10.1080/10402000208982553 Peng, Z., Nassif, A., Georgi, F., Montmitonnet, P. and Lahouij, I. Electric polarity: A key factor in lubricated wear of bearing steel. Tribology International, 209 (2025), p.110748. https://doi.org/10.1016/j.triboint.2025.110748 Cao-Romero-Gallegos, J.A., Farfan-Cabrera, L.I., Erdemir, A., Pascual-Francisco, J.B. Lubricated sliding wear of gear material under electrification - a new approach to understanding of the influence of shaft currents in the wear of EV transmissions. Wear 523 (2023), 204782. https://doi.org/10.1016/j.wear.2023.204782 Aguilar-Rosas, O.A., Farfan-Cabrera, L.I., Erdemir, A., CaoRomero-Gallegos, J.A. Electrified four-ball testing-a potential alternative for assessing lubricants (e-fluids) for electric vehicles. Wear 522 (2023), 204676. https://doi.org/10.1016/j.wear.2023.204676 Deshpande, P., Yelkarasi, C., Lee, S., Farfan-Cabrera, L.I., Erdemir, A. Electrotribochemical formation of abrasive nanocarbon particles under electrified conditions on lubricated sliding contacts. Carbon 228 (2024), 119425. https://doi.org/10.1016/j.carbon.2024.119425 Farfan-Cabrera, L.I., Hernández-Peña, A., Resendiz-Calderon, C.D., Lee, P., Sanchez, C., Lee, S., Erdemir, A. Electrified four-ball testing of ZDDP and MoDTC as additives in low-viscosity synthetic oil. Wear 571 (2025), 205835. https://doi.org/10.1016/j.wear.2025.205835 Farfan-Cabrera, L.I., Lee, S., Skowron, S., Erdemir, A. Enhancing lubrication of electrified interfaces by inert gas atmosphere. Journal of Tribology 147 (2024), 051104. https://doi.org/10.1115/1.4066649 Blankespoor, R.L. Electrochemical oxidation of zinc bis(O,O-dialkyl phosphorodithioates-S, S’). Mediation by 1,1’-Bis(methoxycarbonyl)ferrocene. Inorg. Chem. 24 (1985), 1126–1128. https://doi.org/10.1021/ic00202a003 Stezeryanskii, E.A., Litovchenko, K.I., Kublanovsky, V.S. Electro-oxidation of zinc diisooctyldithiophosphate. J. Electroanal. Chem. 390 (1995), 143–145. https://doi.org/10.1016/0022-0728(95)03893-L Ozimina, D. The assessment of ZnDTP tribochemical reactivity by electrochemical simulation. Lubr. Sci. 13 (2000), 45–57. https://doi.org/10.1016/S0167-8922(02)80019-0 Xu, X., Brandon, N., Spikes, H. Study of zinc dialkyldithiophosphate using electrochemical techniques. In: Proc. Leeds-Lyon Symp. Boundary and Mixed Lubrication, Vienna Sept. 2001, pp. 175–181. https://doi.org/10.1002/ls.3010130105 Ali, M.K.A., Zhang, C., Yu, Q., Sun, Y., Zhou, F., Liu, W. Do electrification-temperature effects deteriorate ZDDP tribofilms in electric vehicles transmission? Insights into antiwear mechanisms using low-saps oils. Wear 564–565 (2025), 205746. https://doi.org/10.1016/j.wear.2025.205746 Yamamoto, Y., Yagi, J. Higaki, H. Effect of externally applied electric field on friction and wear characteristics. Vibration, Control Engineering, Engineering for Industry, 35 (1992), pp.641-646. https://doi.org/10.1299/jsmec1988.35.641 He, S., Meng, Y., Tian, Y.: Correlation between adsorption/desorption of surfactant and change in friction of stainless steel in aqueous solutions under different electrode potentials. Tribol. Lett. 41 (2011), 485–494. https://doi.org/10.1007/s11249-010-9604-6 Liu, C., Li, W., Ouyang, C., Tian, Y. and Meng, Y. Voltage-assisted tribofilm formation of sulfur-and phosphorus-free organic molybdenum additive on bearing steel surfaces in industrial base oils. Tribology Letters, 70(2022), p.19. https://doi.org/10.1007/s11249-022-01562-x Yamamoto, Y. and Hirano, F., 1981. Scuffing resistance of phosphate esters II: Effect of applied voltage. Wear, 66(1), pp.77-86. Peng, Z., Nassif, A., Georgi, F., Montmitonnet, P. and Lahouij, I., 2025. Electric polarity: A key factor in lubricated wear of bearing steel. Tribology International, 209, p.110748. Ali, M.K.A., Sun, Y., Zhang, C., Yu, Q., Zhao, C., Zhou, F. and Liu, W., 2025. Improving tribological performance of electrified steel interfaces in e-mobility systems using ash-sulfur-less oil additives based on amine salts-phosphoric esters. Tribology International, 205, p.110561. Endo, K., Fukuda, Y., Takamiya, O.: Wear behaviours of metals under lubricated conditions and the effects of small electric potential. Trans. Jpn. Soc. Mech. Eng. 14, 1281–1288 (1971) He, Y., Luo, J. and Xie, G., 2007. Characteristics of thin liquid film under an external electric field. Tribology international, 40(10-12), pp.1718-1723. Głogowski, M., Smykowski, D. and Pietrowicz, S., 2023. The Effect of an Electric Field on the Sliding Friction of the Silicone Rubber against Selected Metals in Motor Base Oils. Energies, 16(9), p.3954. Shor, G.I., Blagovidov, I., Eustigneev, E., Lapin, V.P.: Antiwear and antiscuff properties of lubricating oils in relation to electric potential at metal-oil phase boundary. Chem. Tech. Fuels Oil 8, 770–772 (1972) Lu, R., Kawada, S., Tani, H. and Koganezawa, S., 2023. The influence of electric current on the friction behavior of lubricant molecules. Tribology Online, 18(3), pp.83-90. Xie G, Luo J, Guo D, Liu S, Li G. Damages on the lubricated surfaces in bearings under the influence of weak electrical currents. Science China Technological Sciences. 2013 Dec;56(12):2979-87. Yang, Q. and Duan, F., 2024. Tribological properties of phosphate ester confined between iron-based surfaces. Langmuir, 40(7), pp.3738-3747. Sharma, V., Gabler, C., Doerr, N. and Aswath, P.B., 2015. Mechanism of tribofilm formation with P and S containing ionic liquids. Tribology international, 92, pp.353-364. Luiz, J.F. and Spikes, H.A., 2020. Tribofilm formation, friction and wear-reducing properties of some phosphorus-containing antiwear additives. Tribology Letters, 68(3), p.75. Benedet, J., Green, J.H., Lamb, G.D., Spikes, H.A., 2009. Spurious mild wear measurement using white light interference microscopy in the presence of antiwear films. Trib Trans. 52, p. 841-846. Lorenz, M., Pawlicki, A.A., Hysmith, H.E., Cogen, K., Thaker, H. and Ovchinnikova, O.S., 2022. Direct multimodal nanoscale visualization of early phosphorus-based antiwear tribofilm formation. ACS Applied Materials & Interfaces, 14(30), pp.35157-35166. Xia, Y., Zhang, S., Wang, Z. and Zhang, C., 2024. New insight into tribofilm composition and structure of iron polyphosphate-rich tribofilm from formulated engine oil by ToF-SIMS. Journal of Materials Science, 59(21), pp.9533-9546. Restuccia, P., Pedretti, E., Benini, F., Loehlé, S. and Righi, M.C., 2025. Phosphorus-based lubricant additives on iron with Machine Learning Interatomic Potentials. arXiv preprint arXiv:2512.23583. Yang, C., Guo, Y., Zhang, H. and Guo, X., 2024. Utilization of electric fields to modulate molecular activities on the nanoscale: from physical properties to chemical reactions. Chemical Reviews, 125(1), pp.223-293. Spikes, H., 2025. Mechanisms of ZDDP—an update. Tribology Letters, 73(1), p.38. Saba, C.S. and Forster, N.H., 2002. Reactions of aromatic phosphate esters with metals and their oxides. Tribology Letters, 12(2), pp.135-146. Additional Declarations No competing interests reported. Supplementary Files MTMphosphiteSI260213.docx Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 30 Mar, 2026 Reviews received at journal 19 Mar, 2026 Reviews received at journal 27 Feb, 2026 Reviewers agreed at journal 22 Feb, 2026 Reviewers agreed at journal 21 Feb, 2026 Reviewers agreed at journal 19 Feb, 2026 Reviewers invited by journal 18 Feb, 2026 Editor assigned by journal 16 Feb, 2026 Submission checks completed at journal 16 Feb, 2026 First submitted to journal 15 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8888807","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":593674788,"identity":"9c0b1290-4afa-4def-b561-b11a1a71b59f","order_by":0,"name":"Yun Zhao","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Yun","middleName":"","lastName":"Zhao","suffix":""},{"id":593674792,"identity":"0d2b5384-be0f-4e6e-8787-94400dcd10e4","order_by":1,"name":"Jie Zhang","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Jie","middleName":"","lastName":"Zhang","suffix":""},{"id":593674795,"identity":"42c95ced-e3a0-4de2-b3a4-b6039b159acb","order_by":2,"name":"Hugh A. Spikes","email":"","orcid":"","institution":"Imperial College London","correspondingAuthor":false,"prefix":"","firstName":"Hugh","middleName":"A.","lastName":"Spikes","suffix":""},{"id":593674797,"identity":"fc11d846-0101-4a89-a89a-fdd1483fdb77","order_by":3,"name":"JANET S. S. WONG","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA40lEQVRIiWNgGAWjYBACxgYwZcMDphIYLJBF8WpJg2mRIKwFCg7DGERoYW7gMXxc8Ou8jO7s5mMfHtRIMPC3H2CTnIHXYTzGxjP7bvOY3TmWPCPhmASDxJkENskNeLXwbpPm7QFquZFjzJDABnTYDQY2yQeEtZwDasn/zJDwT4JBnigtPD8OgGxhZkhsk2AwAGnB67Bm/s/GvA3JIL8YMyT2SfAYnklstsTnfcP2tsTHPH/s7M1uNz9m/PHNRk7u+OGDN3vwaWkGWdXGAIsRBh6CESkPJv8gtIyCUTAKRsEowAAAlW5ICPsIrmMAAAAASUVORK5CYII=","orcid":"","institution":"Imperial College London","correspondingAuthor":true,"prefix":"","firstName":"JANET","middleName":"S. S.","lastName":"WONG","suffix":""}],"badges":[],"createdAt":"2026-02-15 23:53:31","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8888807/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8888807/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103173259,"identity":"24127fd0-b25a-491a-8a06-9459bfa29c70","added_by":"auto","created_at":"2026-02-22 15:09:55","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":293603,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExperimental setup and testing conditions.\u003c/strong\u003e \u003cstrong\u003ea.\u003c/strong\u003e MTM test rig equipped with voltage and current control system, which shows an application of -5V via the power supply. \u0026nbsp;\u003cstrong\u003eb.\u003c/strong\u003e Chemical structure of bis(2-ethylhexyl) phosphite. \u003cstrong\u003ec.\u003c/strong\u003e MTM testing conditions.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/40a1b0282ed68fce240b9e99.png"},{"id":103173243,"identity":"b5b0de00-11ad-46d5-a152-7831267f8f37","added_by":"auto","created_at":"2026-02-22 15:09:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":416616,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eWorn surfaces formed from BEPite solutions under different current conditions.\u003c/strong\u003e \u003cstrong\u003ea.\u003c/strong\u003e Real time voltage between rubbing contacts at different balance resistor values. \u003cstrong\u003eb.\u003c/strong\u003e Real time current between rubbing contacts at different balance resistor values. Data are averaged over intervals of 0.6 s. \u003cstrong\u003ec.\u003c/strong\u003e Optical microscope images of the ball and disc after rubbing. The change in balance resistor from 1 MΩ to 10 kΩ corresponds to average voltage and current changing from 1.16 V and 0.01 mA to 0.26 V and 0.5 mA. The differences in tribofilm formation on the ball and the disc may arise from differences in ball-disc contact conditions, specific test parameters, or possible transfer of tribofilm from the ball to the disc surface.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/ca9a8e3683edc17b5047b165.png"},{"id":103173251,"identity":"932c172e-03bb-45e9-a691-ca93579f3b40","added_by":"auto","created_at":"2026-02-22 15:09:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":392859,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of voltage on worn surfaces formed in BEPite solutions\u003c/strong\u003e \u003cstrong\u003ea.\u003c/strong\u003eReal time voltage between rubbing contacts. \u003cstrong\u003eb.\u003c/strong\u003e Real time current between rubbing contacts. Data are averaged over intervals of 0.6 s. \u003cstrong\u003ec.\u003c/strong\u003e Optical microscope images of the ball and disc after rubbing.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/c9c533b4c5774d99249f0c46.png"},{"id":103173230,"identity":"c96b457e-963e-461a-a09e-e16702b69beb","added_by":"auto","created_at":"2026-02-22 15:09:48","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":354263,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eBEPite tribofilms formed under different voltage conditions. a. \u003c/strong\u003eTribofilm etching process for film thickness measurement.\u003cstrong\u003e b.\u003c/strong\u003e Optical microscope images of discs after 2-hour rubbing under different applied voltages. Insert SWLI images show etched tribofilm for thickness measurement. White scale bars correspond to 100 mm. \u0026nbsp;\u003cstrong\u003ec.\u003c/strong\u003eThickness of tribofilm along the rubbing direction formed under different voltages. Repeat tests showing profiles parallel and perpendicular to the rubbing direction are in \u003cstrong\u003eFigure S3\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/2ce1289ce5bef034a29a19f9.png"},{"id":103173261,"identity":"017feb91-5cac-45b0-bcf4-a15f717d9294","added_by":"auto","created_at":"2026-02-22 15:09:55","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1094206,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eBEPite tribofilm on anodic discs over different rubbing durations under -5V. a. \u003c/strong\u003eOptical microscope images of the disc after different rubbing durations under -5V. White scale bars correspond to 100 mm. \u0026nbsp;\u0026nbsp;Insert images show etched tribofilm for thickness measurements. \u003cstrong\u003ec.\u003c/strong\u003e Thickness of tribofilms formed under -5V at different rubbing durations.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/c88b4afd5188efbc441239b6.png"},{"id":103173255,"identity":"76b7146b-6884-4367-bc86-c9b5c409e099","added_by":"auto","created_at":"2026-02-22 15:09:54","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":932654,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFrictional performance of BEPite in PAO2 over different rubbing durations under OCP. a. \u003c/strong\u003eOptical microscope images of the disc over different rubbing durations under OCP. White scale bars correspond to 100 mm. \u0026nbsp;\u0026nbsp;Insert images show etched tribofilm for thickness measurement. \u003cstrong\u003ec.\u003c/strong\u003e Thickness of tribofilm formed under OCP at different rubbing durations.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/709af538d60212407c1ba64d.png"},{"id":103173249,"identity":"5ff3d872-b9d4-4eed-8685-84bd3bd66882","added_by":"auto","created_at":"2026-02-22 15:09:53","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":874668,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEDS analysis of tribofilm formed on the disc at OCP and -5V. a.\u003c/strong\u003e Element mapping of the tribofilm formed at OCP. The map area is shown in the SEM image (left) \u003cstrong\u003eb.\u003c/strong\u003e Element mapping of the tribofilm formed at -5V. The map area is shown in the SEM image (left). \u003cstrong\u003ec.\u003c/strong\u003e Weight percentage of various elements in the tribofilm. \u003cstrong\u003ed.\u003c/strong\u003e EDS spectra of tribofilms formed at OCP and -5V. Note the colour map in (a) and (b) are for elemental distribution only.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/0c921fb6b3f1cad540c73051.png"},{"id":103173263,"identity":"3da63af7-cffb-489f-ba7b-452008f98dda","added_by":"auto","created_at":"2026-02-22 15:09:56","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":626686,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eNegative ion TOF-SIMS depth profiling of tribofilm formed on the disc at OCP and -5V. \u003c/strong\u003e3D distributions of negative ion fragments in the top 50 nm of tribofilms formed \u003cstrong\u003ea.\u003c/strong\u003e at OCP, and \u003cstrong\u003eb.\u003c/strong\u003e at -5V. \u003cstrong\u003ec.\u003c/strong\u003e TOF-SIMS depth profiles of total negative fragments. \u003cstrong\u003ed.\u003c/strong\u003e Percentage of P-based negative ion fragments.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/29efd0a814879541e9a9b8de.png"},{"id":103173221,"identity":"ec085985-976a-4f61-8657-38036b379ff7","added_by":"auto","created_at":"2026-02-22 15:09:46","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1615769,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTOF-SIMS depth profiling of iron oxide in the tribofilm formed on the disc at OCP and -5V. a.\u003c/strong\u003e 3D tomography of O\u003csup\u003e-\u003c/sup\u003e and O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. \u003cstrong\u003eb.\u003c/strong\u003e TOF-SIMS depth profiles of O\u003csup\u003e-\u003c/sup\u003e. \u003cstrong\u003ec.\u003c/strong\u003e TOF-SIMS depth profiles of O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. \u003cstrong\u003ed.\u003c/strong\u003e 3D tomography of Fe\u003csup\u003e+\u003c/sup\u003e and FeO\u003csup\u003e+\u003c/sup\u003e. \u003cstrong\u003ee.\u003c/strong\u003e TOF-SIMS depth profiles of Fe\u003csup\u003e+\u003c/sup\u003e. \u003cstrong\u003ef.\u003c/strong\u003e TOF-SIMS depth profiles of FeO\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/731af65f167d78359c58d178.png"},{"id":103173222,"identity":"961b9d52-be59-4c0d-8b6d-f05f531d484e","added_by":"auto","created_at":"2026-02-22 15:09:46","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1185871,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eNegative ion TOF-SIMS depth profiles of individual fragments in the tribofilm formed on the disc at OCP and -5V. a.\u003c/strong\u003e C\u003csup\u003e-\u003c/sup\u003e. \u003cstrong\u003eb.\u003c/strong\u003e CH\u003csup\u003e-\u003c/sup\u003e \u003cstrong\u003ec.\u003c/strong\u003e P\u003csup\u003e-\u003c/sup\u003e. \u003cstrong\u003ed.\u003c/strong\u003e PO\u003csup\u003e-\u003c/sup\u003e. \u003cstrong\u003ee.\u003c/strong\u003e PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. \u003cstrong\u003ef.\u003c/strong\u003e PO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image10.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/e7dfa6b83bd688ad979b45ef.png"},{"id":103173250,"identity":"ad05512c-1a24-4079-b684-a40d377f405a","added_by":"auto","created_at":"2026-02-22 15:09:53","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":314175,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic illustration of tribofilm formation on a steel surface under (a) OCP and (b) anodic potential conditions.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"image11.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/bea83f5c70219a0560535e55.png"},{"id":103173272,"identity":"ca8d891e-6a02-48e1-a468-33042449fb6e","added_by":"auto","created_at":"2026-02-22 15:09:59","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":546082,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFrictional performance of phosphite and phosphate of various molecular structures in PAO2. \u003c/strong\u003eCOF results and wear track of \u003cstrong\u003ea. \u003c/strong\u003eand\u003cstrong\u003e b.\u003c/strong\u003edibutyl phosphite; \u003cstrong\u003ec.\u003c/strong\u003e and \u003cstrong\u003ed.\u003c/strong\u003e tris(ethylhexyl) phosphite;\u003cstrong\u003e e.\u003c/strong\u003eand \u003cstrong\u003ef.\u003c/strong\u003e bis(2-ethylhexyl) phosphate (BEPate);\u003cstrong\u003e g.\u003c/strong\u003e and \u003cstrong\u003eh.\u003c/strong\u003etris(ethylhexyl) phosphate.\u003c/p\u003e","description":"","filename":"image12.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/860905cd91f6969ed0b6c2b7.png"},{"id":103173268,"identity":"b283584a-f97c-49ab-a94f-0e6a7f9def9d","added_by":"auto","created_at":"2026-02-22 15:09:56","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":354242,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemically driven interfacial oxidation under static conditions. a.\u003c/strong\u003e Schematic of the setup used to probe static electrochemical reactions in which a steel ball and a steel disc are held in close proximity in a lubricant under an applied voltage. \u003cstrong\u003eb.\u003c/strong\u003e Optical images of the steel disc before and after applying 5 V between ball and disc with 800 ppm BEPite in PAO2 for 4 h. Scale bars correspond to 500 μm. \u003cstrong\u003ec.\u003c/strong\u003e TOF-SIMS depth profiles of FeO\u003csup\u003e+\u003c/sup\u003e fragments of a fresh disc and an anodic disc. \u003cstrong\u003ed.\u003c/strong\u003e 3D TOF-SIMS tomographic reconstruction of FeO\u003csup\u003e+\u003c/sup\u003e distribution of discs in \u003cstrong\u003ec\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"image13.png","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/1af987190dcf71b1d0207225.png"},{"id":103505135,"identity":"794f3278-2fc3-4f49-a28a-6097201ec617","added_by":"auto","created_at":"2026-02-26 13:24:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":10250852,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/143de652-ab4a-42b1-a97a-72f574f94fa4.pdf"},{"id":103173245,"identity":"bb4df1cc-091b-4f8c-9c39-3dff9892e4b3","added_by":"auto","created_at":"2026-02-22 15:09:53","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":14530890,"visible":true,"origin":"","legend":"","description":"","filename":"MTMphosphiteSI260213.docx","url":"https://assets-eu.researchsquare.com/files/rs-8888807/v1/485cb2c4a9bcabce02fa6b8d.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Voltage-Driven Growth of Phosphorus Tribofilms","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe rapid development of electrification has posed challenges to lubricated systems. For instance, battery systems in electric vehicles operate at 200-900 V, with voltages on motors reaching up to ~10% of the supply voltage and currents of tens of amperes. Current discharge can potentially lead to premature damage in EV drivetrain [1, 2]. The detrimental effect of electric potential has also been seen in wind turbine gearboxes and industrials motors [3]. Negative effects on performance of lubricant has also been reported [4-6]. Thus, a good understanding of lubricant behaviour under electric conditions, and potentially how it may be used to improve machine reliability is crucial.\u003c/p\u003e\n\u003cp\u003eAntiwear additives are important additives in fully-formulated lubricants. Zinc dibutyldithiophosphate (ZDDP), the most common antiwear additive, has been a target of replacement due to component reliability and environmental issues [7-9]. Ashless phosphorus-based additives, such as phosphates and phosphites, are potential candidates. These additives often suffer from limitations, including slow tribofilm growth and insufficient surface protection, making the enhancement of their performance a key research direction [10, 11]. In this work, we examine how an electrified contact may promote tribofilm formation of phosphite additives in the boundary lubrication regime.\u0026nbsp;\u003c/p\u003e\n\u003ch3\u003e2. Background\u003c/h3\u003e\n\u003cp\u003eAlthough lubricants with hydrocarbon oil as base fluid exhibit very low ionic conductivity, application of current and electric field can still impact the performance of lubricants. Numerous studies have shown that performance of fully formulated oils is influenced in electrified contact [\u003cspan additionalcitationids=\"CR13 CR14 CR15 CR16\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. For example, both wear volume in formulated engine oil and electric vehicle oil under 1 A and 2 A currents increased by 20\u0026ndash;50 times, but only on the anodic surface, while the cathodic surface remained unaffected [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. A plausible explanation is that electric current is concentrated at the asperity contacts, generating significant localized heat that can potentially induce lubricant decomposition [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. An interesting phenomenon has been demonstrated where the tribofilm formed on the anode becomes electrically polarized and subsequently adhered to the cathode [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Kadiric et al. [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] found that even a small current (\u0026lt;\u0026thinsp;10 mA) could affect tribofilm formation in an automatic transmission fluid, with tribofilm promoted on the anode but being suppressed on the cathode. Both studies highlight that electrifying a rubbing contact can alter the behaviour of additives and hence tribofilm formation, consequently affect wear of tribo-pairs.\u003c/p\u003e \u003cp\u003eAdditive performance may be impacted by electrochemical reactions even under mild voltage conditions. Under boundary lubrication, electrochemical reactions may be intensified at sliding interfaces due to shear stress. Most existing work to date about specific additive performance under electric field has focussed on film-forming additives such as zinc dibutyldithiophosphate (ZDDPs), molybdenum dithiocarbamate (MoDTC), and related chemistries [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan additionalcitationids=\"CR19 CR20 CR21\" citationid=\"CR19\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. In 1981, Yamamoto and Hirano found that applying voltage and current simultaneously to rubbing pairs enhanced the tribofilm formation of tricresyl phosphate (TCP), whereas excessive voltage and current reduced it [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Ozimina later reported that 1 wt.% ZDDP in acetonitrile could decompose at potentials within \u0026plusmn;\u0026thinsp;1 V [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Applied voltages between 0\u0026ndash;3 V have also been shown to influence additive adsorption and electrochemical reaction in ZDDP-containing propylene carbonate/diethyl succinate systems [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e24\u003c/span\u003e] and an organic molybdenum additive in PAO2 [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Wear reduction was observed as a result. For ashless additives, tricresyl phosphate has been reported to enhance the scuffing resistance of surface films under microampere‑current and millivolt‑voltage conditions [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. By contrast, other studies that apply electrical current have shown detrimental effects on wear performance [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAn applied voltage may also impact tribofilm formation through altering the properties of rubbing surfaces. It has been shown that electrifying a contact could modify metal surface energy and thereby influence frictional performance [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Applied voltage can affect metal surface oxidation [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e30\u003c/span\u003e], lubricant bubble generation [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e31\u003c/span\u003e], and base oil wettability [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Applying current can also induce similar phenomena described above, including surface interactions [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e33\u003c/span\u003e] and bubble generation [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Prior work has shown that additive binding and tribochemical pathways differ between metallic iron and iron oxide surfaces [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e35\u003c/span\u003e], and that iron cations generated due to metal-surface oxidation can promote the polymerisation of iron polyphosphates near the substrate-tribofilm interface [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Additional studies have demonstrated that adding metal ions into lubricants enhances tribofilm formation [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e37\u003c/span\u003e], confirming the role of metal ions on tribofilm formation. From a triboelectrochemical perspective, promoting oxidation of the metal rubbing surface may facilitate the tribofilm-formation of phosphorus-based additives and thereby improve their overall tribological performance. In this study, we control the voltage and current between lubricated contacts and investigate their influence on tribofilm formation of phosphite and phosphate additives to examine if voltage alone, without significant current, can modify their tribofilm formation. The chemical characteristics of the resulting tribofilms under different voltage biases were compared to evaluate the extent to which electrical stimuli modify interfacial chemistry. Complementary static electrochemical measurements were performed to examine the importance of rubbing in observed voltage-dependent tribofilm formation.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eCleaning solvents include heptane (\u0026ge;\u0026thinsp;99%) and toluene (\u0026ge;\u0026thinsp;99.5%). The lubricant additives used are bis(2-ethylhexyl) phosphite (96%), bis(2-ethylhexyl) phosphate (97%), dibutyl phosphite (96%), tris(2-ethylhexyl) phosphite, and tris(2-ethylhexyl) phosphate (97%), all purchased from Sigma-Aldrich without further purification. PAO2 (polyalphaolefin SpectraSyn 2) is chosen as the base oil. Lubricants are prepared by dissolving phosphorus-based additives into PAO2 to achieve a phosphorus concentration of 800 ppm. The mixture is stirred with a magnetic stir bar at room temperature for 2 h. The transparent solution is then allowed to stand for 10 min and is used immediately thereafter.\u003c/p\u003e \u003cp\u003eMost results in this study uses bis(2-ethylhexyl) phosphite in PAO2 as the model lubricant because it forms a visible tribofilm under the test conditions, enabling clear examination of effect of applied voltage of tribofilm formation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Tribometer and test conditions\u003c/h2\u003e \u003cp\u003eFriction tests are conducted using a mini-traction machine with an electrochemistry module (MTM-EC, PCS Instruments), as shown schematically in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The MTM-EC employs a ball-on-disc configuration, with voltage applied across the ball and disc in a two-electrode setup. A balance resistor, with a maximum value of 1 MΩ, is connected to the contact in series and is used to control the electric circuit current. Voltage and current between the ball and disc are continuously monitored using an oscilloscope. The balls and discs are made of AISI 52100 steel and are from PCS Instruments. They are cleaned before testing using the following procedure: (1) rinse with toluene and wipe with toluene-soaked tissue; (2) ultrasonicate in toluene for 15 min, then soak in fresh toluene for 12 h; (3) rinse again with toluene, wipe with toluene-soaked tissue, and ultrasonicate for another 15 min in fresh toluene; (4) before testing, wipe and rinse with toluene, dry with compressed air, and treat with oxygen plasma for 1 min.\u003c/p\u003e \u003cp\u003eTest conditions are summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. The normal load is 31 N (maximum Hertzian pressure of 0.95 GPa), entrainment speed 50 mm s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and slide-roll ratio (SRR) 20%, with the ball and disc rotating at 45 and 55 mm s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. Tests are performed at 80\u0026deg;C. In the MTM-EC software, a\u0026thinsp;+\u0026thinsp;5 V setting connects the positive terminal to the ball (anodic) and the negative terminal to the disc (cathodic). A -5 V setting reverses this polarity. Unless otherwise stated, the balance resistor is set to 1 MΩ to minimize current effects. Depending on the contact conditions during sliding, the measured potential is typically lower than the set value and may fluctuate. The measured value is determined by the amount of solid-to-solid contact and electrical properties of any tribofilm formed on the surface. For simplicity, the set voltage is used to label each condition. The test duration is 2 h, unless specified otherwise. After testing, specimens are rinsed with heptane and dried with compressed air. They are stored for further analysis. Each experiment is repeated at least twice to ensure reliability.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Characterizations\u003c/h2\u003e \u003cp\u003eAfter each test, wear scars may form on rubbing surfaces. Rubbed surfaces are first examined using an optical microscope to assess dimensions and colour of these scars (RH-2000 digital microscope, Hirox, Tokyo, Japan). The thickness of tribofilm on discs is measured using scanning white light interferometry (SWLI, Bruker Contour GT-K). The topographic profile is an average of a band of 5-pixel width. Before examining with SWLI, selected areas of the tribofilm on the disc are removed by etching with 0.1 M oxalic acid for 20 s to expose the buried steel surface. Residual oxalic acid is then wiped away using a DI water-wetted lint-free tissue. This is followed by gold coating on both etched and unetched regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). This facilitates the evaluation of wear depth and the actual tribofilm thickness [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Chemical analyses are conducted using Energy dispersive X-ray spectroscopy (EDX, Tescan Mira) and time-of-flight secondary ion mass spectrometry (TOF-SIMS, DektakXT, Bruker). EDX is used to map the element distribution in the tribofilm, while TOF-SIMS is employed to characterize the chemical states of elements at different depths.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eUnder open-circuit potential (OCP) conditions, no external voltage is applied between the ball and disc. So, both voltage and current are considered negligible. In this state, the ball exhibits a wear track with no obvious tribofilm. On the disc, a patchy tribofilm, with low surface coverage, is formed (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec).\u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Comparative importance of current and voltage on BEPite tribolfilm formation\u003c/h2\u003e \u003cp\u003eUnder an applied voltage, the reaction is primarily attributed to the electric current, which induces localized heating through its concentration at asperity contacts [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. To compare the importance of current and voltage on tribofilm formation, the balance resistor is adjusted to control current. The power supply provides a fixed voltage, while the impedance in the circuit varies. In addition to the fixed balance resistor, the resistance between the sliding contact drops to 0 Ω when asperities are in direct contact, but becomes very high when the contacts are separated by a tribofilm. As a result, the voltage and current across a sliding contact exhibit large fluctuation. We record data continuously and then average the readings over each 0.6 s. Changing the resistance of the balance resistor from 1 MΩ to 10 kΩ corresponds to changes in the real average voltage and current from 1.16 V and \u0026lt;\u0026thinsp;0.01 mA to 0.26 V and 0.5 mA, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eA higher current combined with lower voltage (10 kΩ resistance) produces tribofilms similar to those formed under OCP conditions, whereas a lower current (1 MΩ resistance) combined with higher voltage results in a pronounced increase in tribofilm formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). These results indicate that voltage plays a more decisive role than current in driving tribofilm growth in our rubbing conditions. Thus, the following discussion focuses on the effects of applied voltage.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Effect of applied voltage on BEPite tribofilm formation\u003c/h2\u003e \u003cp\u003eWhen a voltage is applied, the potential difference between the ball and disc increases gradually with rubbing time (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). At the start of sliding, substantial asperity contacts cause a short circuit, with measured voltage close to 0 V. As the tribofilm develops, the resistance across the contact increases, leading to a rise in the measured voltage. The morphology of the tribofilm leads to occasional micro-short-circuits, causing large voltage fluctuations. With the balance resistor in series, the current remains stable at ~\u0026thinsp;0.01 mA (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe disc tribofilm forms under anodic condition is more uniform than that observed at OCP, whereas the ball wear track shows no obvious tribofilm, see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec. For a cathodic disc, tribofilm is observed mostly at the centre of its wear track. Tribofilms are also formed at the centre of the wear track of the anodic ball. These observations indicate that tribofilm formation depends on the applied voltage and, critically, on its polarity. Notably, the friction coefficients under electrified and non-electrified conditions are similar (\u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e).\u003c/p\u003e \u003cp\u003eThe tribofilm formed on the ball surface is not clearly distinguishable under either biased or unbiased conditions. The reason is unclear and may be due to the different physical properties and contact conditions between the ball and the disc, which promote preferential transfer of the tribofilm to the disc surface. To enable a more reliable assessment of the effect of applied voltage on tribofilm formation, the tribofilm on the disc is selected as the primary focus for subsequent analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe effect of voltage on tribofilm formation on an anodic disc is investigated over a range of applied negative voltage while keeping the balance resistor at its maximum value to minimize current effects, see Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. For programmed voltages of -2 V, -5 V, and \u0026minus;\u0026thinsp;8 V, the actual average voltages across the lubricated contacts are \u0026minus;\u0026thinsp;0.56 V, -1.16 V, and \u0026minus;\u0026thinsp;0.97 V, respectively (\u003cb\u003eFigure S2\u003c/b\u003e). At -2 V, blue deposits (labelled as \u0026lsquo;thick\u0026rsquo; in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) appear on the disc surface, with localized increase in thickness but poor uniformity. At -5 V, tribofilm growth is enhanced, with more blue regions and an average thickness of 80\u0026ndash;90 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, and for repeat tests see \u003cb\u003eFigure S3 and S4\u003c/b\u003e). Further increasing the set voltage to -8 V does not increase thickness because the actual voltage across the contact does not change significantly. In contrast, applying a positive voltage does not lead to increase in tribofilm thickness but can impact the morphology of the tribofilm (\u003cb\u003eFigure S5\u003c/b\u003e).\u003c/p\u003e \u003cp\u003eThe increased thickness of tribofilms on anodic discs shows that an anodic bias results in higher tribofilm growth rate or stronger tribofilm (resisting film removal) or both. Note that the profiles of the cleaned area show no measurable wear, although occasional surface scratching can occur during the process (\u003cb\u003eFigure S6\u003c/b\u003e). This suggests that tribofilms forms in anodic discs under the applied negative voltage offer good antiwear protection.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows how the tribofilm thickness on anodic discs at -5 V changes with rubbing time (see also \u003cb\u003eFigure S7-10\u003c/b\u003e). In the first 30 min, the tribofilm islands reach only 15\u0026ndash;20 nm, scattered across the wear track (see also \u003cb\u003eFigure S7\u003c/b\u003e and \u003cb\u003eS8\u003c/b\u003e). By 60 min, the thickness increases to ~\u0026thinsp;40 nm (see also \u003cb\u003eFigure S9\u003c/b\u003e), and the surface becomes densely covered with granular and elongated deposits. Further rubbing produces thicker tribofilms, reaching\u0026thinsp;~\u0026thinsp;60 nm at 90 min (see also \u003cb\u003eFigure S10\u003c/b\u003e) and ~\u0026thinsp;80 nm at 120 min.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTribofilm growth under unelectrified (OCP) conditions is slower (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cb\u003eFigure S11-14\u003c/b\u003e). After 30 min, the film thickness is ~\u0026thinsp;15 nm (\u003cb\u003eFigure S11\u003c/b\u003e), with fewer deposits compared to that on an anodic disc under \u0026minus;\u0026thinsp;5 V. This indicates that voltage can influence tribofilm formation even at the earliest stages. Prolonged rubbing increases tribofilm thickness marginally. After 120 min it reaches only\u0026thinsp;~\u0026thinsp;40 nm, with partial surface coverage. Extending the rubbing time to 240 min does not lead to further growth of the tribofilm thickness (\u003cb\u003eFigure S14\u003c/b\u003e). These results show that applied voltage increases tribofilm growth rate or produce more robust films that remain stable at higher thicknesses, or both.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Chemical analysis of disc tribofilm formed at OCP and \u0026minus;\u0026thinsp;5V\u003c/h2\u003e \u003cp\u003eChemistry of tribofilms formed under electrified and non-electrified conditions is investigated using EDS, see Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. For direct comparison, the selected regions include both the wear tracks and the immediate unrubbed areas. In both cases, oxygen (O, red) and phosphorus (P, green) are detected. In non-electrified condition (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea), the distributions of O and P in the wear track are slightly elevated. In the electrified condition (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb), the distributions of O and P form stripe-like features which match the tribofilm morphology, showing that this tribofilm consists of O- and P-containing compounds.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe wear track on the anodic disc contains 2.52 wt.% P and 4.17 wt.% O, nearly twice the proportions observed on an unelectrified disc (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). On the other hand, the unelectrified disc contains more Fe. This result is consistent with tribofilms on anodic discs being thicker, and having higher surface coverage (see Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e3D TOF-SIMS (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) is performed in a\u0026thinsp;~\u0026thinsp;100 \u0026micro;m \u0026times; 100 \u0026micro;m area at the centre of each wear track (wear track width\u0026thinsp;~\u0026thinsp;250 \u0026micro;m). As phosphite-derived tribofilms typically contain C, O, and P, the negative ion spectra are analysed for C\u003csup\u003e\u0026minus;\u003c/sup\u003e, CH\u003csup\u003e\u0026minus;\u003c/sup\u003e, P\u003csup\u003e\u0026minus;\u003c/sup\u003e, PO\u003csup\u003e\u0026minus;\u003c/sup\u003e, PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, and PO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e fragments (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). Phosphorus in the tribofilms on both discs is primarily present as PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and PO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, with small amount of other P-containing fragments (P\u003csup\u003e\u0026minus;\u003c/sup\u003e and PO\u003csup\u003e\u0026minus;\u003c/sup\u003e) detected (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec-f). Overall, PO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e accounts for 57% and 41% of total P-containing fragments in the anodic tribofilm, whereas in the OCP tribofilm, the proportions are 52% and 44%, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed). No PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (the oxidation product of phosphite) forms under OCP conditions, while it is detectable in anodic tribofilm, confirming that anodic polarization can drive this oxidation reaction (\u003cb\u003eFigure S15\u003c/b\u003e).\u003c/p\u003e \u003cp\u003eThe distribution of iron oxides in the tribofilm is also examined, as surface oxidation may be an important contributing factor to tribofilm formation. The key marker for iron oxides in TOF-SIMS include Fe\u003csup\u003e+\u003c/sup\u003e, FeO\u003csup\u003e+\u003c/sup\u003e, FeOH\u003csup\u003e+\u003c/sup\u003e, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csup\u003e+\u003c/sup\u003e, O\u003csup\u003e\u0026minus;\u003c/sup\u003e, O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, FeO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, and FeO\u003csub\u003e2\u003c/sub\u003eH\u003csup\u003e\u0026minus;\u003c/sup\u003e. Among these, only O\u003csup\u003e\u0026minus;\u003c/sup\u003e, O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, Fe\u003csup\u003e+\u003c/sup\u003e, and FeO\u003csup\u003e+\u003c/sup\u003e exhibit obvious signals (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDepth profiling is conducted in 10 nm increments for a total of five cycles and confirms that the tribofilms formed under OCP is much thinner than that formed in -5 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea-c). The depth distributions and relative intensities of various fragments are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. C\u003csup\u003e\u0026minus;\u003c/sup\u003e and CH\u003csup\u003e\u0026minus;\u003c/sup\u003e, representatives of organic and carbonaceous species, are concentrated within the top 1\u0026ndash;2 nm, consistent with previous reports [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e39\u003c/span\u003e], and are more abundant in the OCP tribofilm than in the anodic tribofilm (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea and \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb, Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eThe tribofilms on both the OCP and the anodic discs show the highest P-base fragment densities on their top surfaces. For the anodic tribofilm, the densities of P-base fragments drop within the few nanometers before stabilizing or reducing in slower rates. The high fragment concentration detected at the tribofilm surface probably originates from recent deposition and surface reactions during sliding. With increasing depth, these newly formed species may undergo further changes, such as organic species removal and inorganic species aging, leading to the formation of more stable tribofilm which are more resistant to rubbing. Depth profiles of iron oxide fragments show high intensities across the entire sputtering range. They co-exist with P-base fragments throughout the tribofilm, indicating a stable, iron-phosphorus-rich structure. These results demonstrate that iron actively participates in tribofilm formation, leading to the development of a dense, iron-phosphorus-enriched tribofilm under anodic conditions. These depth profiles show that the through-thickness chemistry of the anodic tribofilm is relatively constant with the amount of phosphorus and iron oxide-related species decay slowly below the top surface (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe intensities of P-base fragments are much lower in OCP tribofilms and are only detected in the first 10 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). Iron oxide-related fragments concentrate in the top\u0026thinsp;~\u0026thinsp;10 nm and gradually decrease in abundance at depths of 20\u0026ndash;30 nm, consistent with a previous report about iron oxide distribution within the tribofilm [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. This exceeds the depth range of phosphorus-containing species, suggesting a tribofilm structure comprising an organic/carbonaceous top layer, a middle layer containing phosphorus and iron, and an underlying oxide layer.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4. How anodic potential promotes tribofilm formation\u003c/h2\u003e \u003cp\u003eUnder open-circuit potential (OCP), the tribofilm contains a high proportion of partially decomposed phosphite species, resulting in an organic-rich film. In contrast, anodic polarisation favours more complete phosphite decomposition, producing a tribofilm enriched in inorganic phosphate species \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left({\\text{PO}}_{3}^{-}\\right.\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{PO}}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left.{\\text{PO}}_{4}^{3-}\\right)\\)\u003c/span\u003e\u003c/span\u003e that better stabilises the film under shear.\u003c/p\u003e \u003cp\u003eA potential mechanism for the formation of the tribofilm under applied anodic potential compared to OCP conditions is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e. Phosphite additives adsorb onto the steel surface through chemisorption via the phosphorus group [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e41\u003c/span\u003e], assisted by physical interactions​ (van der Waals forces) between the organic carbon chains and the surface. When an anodic potential is applied, the resulting electrostatic field strengthens chemisorption​ while concurrently weakening the physical adsorption​ of the organic chains because of electronegativity difference [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. This potential-dependent interfacial reorganization influences the initial adsorption stage and later reactions. Cleavage of the C-O bond, as generally seen with ZDDPs [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e43\u003c/span\u003e], will favour the formation of PO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, whereas scission of the O-P bond should lead predominantly to PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. The anodic film also contains PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. The weakened interaction between carbon chains and the surface will lead to reduced carbon species on rubbing surfaces.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAnodic polarization also promotes oxidation of the steel surface, generating iron oxides and releasing iron ions. These species enhance both the chemisorption and complexation​ of phosphite derivatives, thereby accelerating tribofilm growth. The incorporated iron ions may play a vital structural role, acting as cross-linking centres, connecting phosphate groups to form a more coherent and robust three-dimensional networks within the tribofilm [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. This iron-stabilized architecture should significantly improve the mechanical integrity and shear resistance of the film under tribological stress.\u003c/p\u003e \u003cp\u003eThe overall chemical nature of the OCP and anodic tribofilms observed here is consistent with tribofilms and thermally formed films reported in the literature [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The films are dominated by a mixture of carbon-containing species, phosphate-based compounds, and iron-phosphate/iron-oxide complexes, with an oxide-rich interfacial layer adjacent to the steel substrate. The key distinction due to the applied anodic potential therefore lies not in the identity of the chemical species, but in their relative abundance, distribution, and degree of inorganic cross-linking. Under anodic polarization, the tribofilms exhibit a higher fraction of inorganic phosphate species and iron-coordinated structures, whereas films formed under OCP or purely thermal conditions tend to retain a larger proportion of organic fragments from incomplete phosphite decomposition. This suggests that anodic polarization does not introduce new chemistry, but rather shifts the balance of existing tribochemical pathways, promoting oxidation, enhanced iron incorporation, and the formation of a more inorganic, mechanically robust network compared with conventional phosphite tribofilms reported in the literature.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Universality of applied potential effect on P-based tribofilm\u003c/h2\u003e \u003cp\u003eThe above effect of applied potential on promoting tribofilm growth is not unique to BEPite. Voltage-induced effects on tribofilm formation are also seen for other phosphorus-containing additives that show limited tribofilm growth under OCP conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. For example, diisopropyl phosphite forms tribofilm rapidly at OCP, and so applying voltage produces minimal effect (\u003cb\u003eFigure S16\u003c/b\u003e). By contrast, dibutyl phosphite, tris(2-ethylhexyl) phosphite, bis(2-ethylhexyl) phosphate, and tris(2-ethylhexyl) phosphate, which all show limited tribofilm formation at OCP, show obvious changes in their tribofilms under \u0026minus;\u0026thinsp;5 V. For dibutyl phosphite, friction coefficient remains unchanged, while tribofilm thickness slightly increases under \u0026minus;\u0026thinsp;5 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea and \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003eb). Tris(2-ethylhexyl) phosphite (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ed) and bis(2-ethylhexyl) phosphate (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ef) produce no obvious tribofilm at OCP, but tribofilms are formed at -5 V, accompanied by an increase in friction coefficient (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ec and \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ee). Tris(2-ethylhexyl) phosphate (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003eh and \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003eg) shows large differences in tribofilm morphology and friction coefficient between OCP and \u0026minus;\u0026thinsp;5 V. At OCP, the wear track is wider and tribofilm highly heterogeneous, while under \u0026minus;\u0026thinsp;5 V the wear track becomes narrower and the tribofilm more uniform, corresponding to a reduction in friction coefficient from 0.25 to 0.10. These results indicate that the application of negative potential can be effective in promoting tribofilm formation/tribofilm growth in a variety of P-base additives.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.6. Effect of applied potential alone on steel surface chemistry\u003c/h2\u003e \u003cp\u003eOne question that arises from the above tests is whether the observed effect on BEPite tribofilm on anodic discs in a rubbing contact occurs without rubbing? To explore this, the growth of the voltage-induced surface film is evaluated with a static electrochemical cell. A steel ball and a steel disc are brought into contact in lubricant. The separation between the two steel specimens is controlled at approximately 1 \u0026micro;m using a digital micrometer (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea). In the absence of mechanical motion, electrochemical effects are isolated from tribochemical effects.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eApplying a 5 V bias to steel discs in BEPite in PAO2 for 4 h leads to no obvious surface film under optical microscopy (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003eb). Raman spectroscopy likewise reveals no discernible signatures of iron oxides, suggesting that any oxidation, if present, occurs at very low levels (\u003cb\u003eFigure S17\u003c/b\u003e). More sensitive TOF-SIMS depth profiling shows only a slight increase in FeO\u003csup\u003e+\u003c/sup\u003e fragments on the anodic surface after holding at 5 V for 4 h, both in neat PAO2 and in PAO2 containing 800 ppm BEPite (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ec and \u003cb\u003ed\u003c/b\u003e). Carbon‑rich residues are also observed, likely due to the adsorption of tenacious carbon (\u003cb\u003eFigure S18\u003c/b\u003e). The disc immersed in a phosphite‑containing PAO2 solution has a thin PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e- and PO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e-containing layer, which adheres strongly and cannot be removed by rinsing with heptane (\u003cb\u003eFigure S18\u003c/b\u003e and \u003cb\u003eS19\u003c/b\u003e). This enhancement is confined to the top\u0026thinsp;~\u0026thinsp;1\u0026ndash;2 nm of the interface, indicating extremely limited electrochemical oxidation and surface film formation under static conditions.\u003c/p\u003e \u003cp\u003eThese results support the proposition that the substantial tribofilm growth observed during sliding under applied voltage arises from triboelectrochemical processes that occur only when mechanical shearing is present. This comparison highlights the mechanical force plays a decisive role in activating voltage-driven interfacial reactions, which subsequently contribute to tribofilm formation.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eAshless P-based antiwear additives are more environmentally-acceptable alternatives to conventional antiwear additives ZDDP. However, they suffer from relatively low tribofilm growth compared to metal-containing P additives [ZDP] or some P-S based ones. In this work, the tribofilms of P-based ashless additives formed under electrified conditions have been examined. Under our experimental conditions, tribofilm formation is primarily governed by applied voltage rather than current, with an anodic surface exhibiting faster tribofilm growth and greater film thickness. Chemical analysis suggests that iron participation plays an important role in tribofilm formation. The tribofilm structure consists of an outer layer of organic species and carbon, and an inner layer rich in phosphorus and iron. Compared with the tribofilm formed under OCP, applying\u0026thinsp;\u0026minus;\u0026thinsp;5 V produces a denser and thicker film, resulting in improved stability. The discovery that anodic oxidation and additive decomposition act cooperatively to build dense and stable tribofilms highlights a new electrochemical-tribochemical coupling mechanism. These insights provide a foundation for designing electro-responsive additives and smart lubricants capable of dynamically adapting to next-generation electrified systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eY.Z., H.A.S. and J.S.S.W.: conceptualization and design of the study. Y.Z.: experimental investigation, data acquisition, analysis and interpretation and writing of the original draft. J.Z. : support with experimental investigation, data acquisition, and interpretation, J.S.S.W.: supervision, funding acquisition, project administration, and revision of the original draft. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis research is financially supported by the Shell University Technology Centre (UTC) for Mobility and Lubricants and the EPSRC InFUSE Prosperity Partnership (EP/V038044/1). Y.Z. is funded by Shell and UKRI via IDLA studentship.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKliman, G., Stein, J. Induction motor fault detection via passive current monitoring\u0026mdash;a brief survey. In: Proc. 44th Meeting of the Mechanical Failures Prevention Group, pp. 49\u0026ndash;65 (1990).\u003c/li\u003e\n\u003cli\u003eHadden, T., Jiang, J.W., Bilgin, B., Yang, Y., Sathyan, A., Dadkhah, H., Emadi, A. A review of shaft voltages and bearing currents in EV and HEV motors. In: IECON 2016\u0026mdash;42nd Annual Conference of the IEEE Industrial Electronics Society, pp. 1578-1583 (2016). https://doi.org/10.1109/IECON.2016.7793357\u003c/li\u003e\n\u003cli\u003eSheng, S. Report on wind turbine subsystem reliability-a survey of various databases (presentation). Technical report, National Renewable Energy Lab. (NREL), Golden, CO (USA) (2013). https://docs.nrel.gov/docs/fy13osti/59111.pdf\u003c/li\u003e\n\u003cli\u003eYamamoto, Y., and F. Hirano. Scuffing resistance of phosphate esters II: Effect of applied voltage. Wear 66 (1981): 77-86. https://doi.org/10.1016/0043-1648(81)90034-X\u003c/li\u003e\n\u003cli\u003eYousuf, A., Spikes, H., Guo, L. and Kadiric, A. Influence of Electric Potentials on Surface Damage in Rolling-Sliding Contacts Under Mixed Lubrication. Tribology Letters 73 (2025), 45. https://doi.org/10.1007/s11249-025-01977-2 \u003c/li\u003e\n\u003cli\u003eCao-Romero-Gallegos, J.A., Farfan-Cabrera, L.I., Erdemir, A. and Pascual-Francisco, J.B. Lubricated sliding wear of gear material under electrification-A new approach to understanding of the influence of shaft currents in the wear of EV transmissions. Wear, 523 (2023), p.204782.\u003c/li\u003e\n\u003cli\u003eSpikes H.A. Beyond ZDDP, Lubrication Science, 20 (2008), p. 77-78. https://doi.org/10.1002/ls.60\u003c/li\u003e\n\u003cli\u003eGangopadhyay, A.K., Jensen, R.K., Carter III, R.O., Uy, D., O\u0026apos;Neill, A.E., Simko, S.J., Gao, H., Stockwell, R.T., Phillips, C.B. and Graham, M.E. Development of zero‐phosphorus engine oils. Lubrication Science, 20(2008), pp.163-180. https://doi.org/10.1002/ls.54\u003c/li\u003e\n\u003cli\u003eDevlin, M.T., Guevremont, J., Sheets, R., Loper, J., Guinther, G., Thompson, K. and Jao, T.C. Effect of metal‐free phosphorus anti‐wear compounds on passenger car emissions and fuel economy. Lubrication Science, 20(2008), pp.151-161. https://doi.org/10.1002/ls.44\u003c/li\u003e\n\u003cli\u003eSpikes, Hugh. Low‐and zero‐sulphated ash, phosphorus and sulphur anti‐wear additives for engine oils. Lubrication Science 20 (2008), p. 103-136. \u003c/li\u003e\n\u003cli\u003eKim, B. Tribological performance of ashless antiwear additives under extreme pressure conditions. Thesis, 2010. https://mavmatrix.uta.edu/context/materialscieng_dissertations/article/1041/type/native/viewcontent\u003c/li\u003e\n\u003cli\u003eGangopadhyay, A., Paputa Peck, M.C. and Simko, S.J. Wear control in a lubricated contact through externally applied electric current. Tribology transactions, 45(2002), pp.302-309. https://doi.org/10.1080/10402000208982553 \u003c/li\u003e\n\u003cli\u003ePeng, Z., Nassif, A., Georgi, F., Montmitonnet, P. and Lahouij, I. Electric polarity: A key factor in lubricated wear of bearing steel. Tribology International, 209 (2025), p.110748. https://doi.org/10.1016/j.triboint.2025.110748\u003c/li\u003e\n\u003cli\u003eCao-Romero-Gallegos, J.A., Farfan-Cabrera, L.I., Erdemir, A., Pascual-Francisco, J.B. Lubricated sliding wear of gear material under electrification - a new approach to understanding of the influence of shaft currents in the wear of EV transmissions. Wear 523 (2023), 204782. https://doi.org/10.1016/j.wear.2023.204782\u003c/li\u003e\n\u003cli\u003eAguilar-Rosas, O.A., Farfan-Cabrera, L.I., Erdemir, A., CaoRomero-Gallegos, J.A. Electrified four-ball testing-a potential alternative for assessing lubricants (e-fluids) for electric vehicles. Wear 522 (2023), 204676. https://doi.org/10.1016/j.wear.2023.204676\u003c/li\u003e\n\u003cli\u003eDeshpande, P., Yelkarasi, C., Lee, S., Farfan-Cabrera, L.I., Erdemir, A. Electrotribochemical formation of abrasive nanocarbon particles under electrified conditions on lubricated sliding contacts. Carbon 228 (2024), 119425. https://doi.org/10.1016/j.carbon.2024.119425\u003c/li\u003e\n\u003cli\u003eFarfan-Cabrera, L.I., Hern\u0026aacute;ndez-Pe\u0026ntilde;a, A., Resendiz-Calderon, C.D., Lee, P., Sanchez, C., Lee, S., Erdemir, A. Electrified four-ball testing of ZDDP and MoDTC as additives in low-viscosity synthetic oil. Wear 571 (2025), 205835. https://doi.org/10.1016/j.wear.2025.205835\u003c/li\u003e\n\u003cli\u003eFarfan-Cabrera, L.I., Lee, S., Skowron, S., Erdemir, A. Enhancing lubrication of electrified interfaces by inert gas atmosphere. Journal of Tribology 147 (2024), 051104. https://doi.org/10.1115/1.4066649\u003c/li\u003e\n\u003cli\u003eBlankespoor, R.L. Electrochemical oxidation of zinc bis(O,O-dialkyl phosphorodithioates-S, S\u0026rsquo;). Mediation by 1,1\u0026rsquo;-Bis(methoxycarbonyl)ferrocene. Inorg. Chem. 24 (1985), 1126\u0026ndash;1128. https://doi.org/10.1021/ic00202a003\u003c/li\u003e\n\u003cli\u003eStezeryanskii, E.A., Litovchenko, K.I., Kublanovsky, V.S. Electro-oxidation of zinc diisooctyldithiophosphate. J. Electroanal. Chem. 390 (1995), 143\u0026ndash;145. https://doi.org/10.1016/0022-0728(95)03893-L\u003c/li\u003e\n\u003cli\u003eOzimina, D. The assessment of ZnDTP tribochemical reactivity by electrochemical simulation. Lubr. Sci. 13 (2000), 45\u0026ndash;57. https://doi.org/10.1016/S0167-8922(02)80019-0\u003c/li\u003e\n\u003cli\u003eXu, X., Brandon, N., Spikes, H. Study of zinc dialkyldithiophosphate using electrochemical techniques. In: Proc. Leeds-Lyon Symp. Boundary and Mixed Lubrication, Vienna Sept. 2001, pp. 175\u0026ndash;181. https://doi.org/10.1002/ls.3010130105\u003c/li\u003e\n\u003cli\u003eAli, M.K.A., Zhang, C., Yu, Q., Sun, Y., Zhou, F., Liu, W. Do electrification-temperature effects deteriorate ZDDP tribofilms in electric vehicles transmission? Insights into antiwear mechanisms using low-saps oils. Wear 564\u0026ndash;565 (2025), 205746. https://doi.org/10.1016/j.wear.2025.205746\u003c/li\u003e\n\u003cli\u003eYamamoto, Y., Yagi, J. Higaki, H. Effect of externally applied electric field on friction and wear characteristics. Vibration, Control Engineering, Engineering for Industry, 35 (1992), pp.641-646. https://doi.org/10.1299/jsmec1988.35.641\u003c/li\u003e\n\u003cli\u003eHe, S., Meng, Y., Tian, Y.: Correlation between adsorption/desorption of surfactant and change in friction of stainless steel in aqueous solutions under different electrode potentials. Tribol. Lett. \u003cstrong\u003e41 \u003c/strong\u003e(2011), 485\u0026ndash;494. https://doi.org/10.1007/s11249-010-9604-6\u003c/li\u003e\n\u003cli\u003eLiu, C., Li, W., Ouyang, C., Tian, Y. and Meng, Y. Voltage-assisted tribofilm formation of sulfur-and phosphorus-free organic molybdenum additive on bearing steel surfaces in industrial base oils. Tribology Letters, 70(2022), p.19. https://doi.org/10.1007/s11249-022-01562-x\u003c/li\u003e\n\u003cli\u003eYamamoto, Y. and Hirano, F., 1981. Scuffing resistance of phosphate esters II: Effect of applied voltage. Wear, 66(1), pp.77-86.\u003c/li\u003e\n\u003cli\u003ePeng, Z., Nassif, A., Georgi, F., Montmitonnet, P. and Lahouij, I., 2025. Electric polarity: A key factor in lubricated wear of bearing steel. Tribology International, 209, p.110748.\u003c/li\u003e\n\u003cli\u003eAli, M.K.A., Sun, Y., Zhang, C., Yu, Q., Zhao, C., Zhou, F. and Liu, W., 2025. Improving tribological performance of electrified steel interfaces in e-mobility systems using ash-sulfur-less oil additives based on amine salts-phosphoric esters. Tribology International, 205, p.110561.\u003c/li\u003e\n\u003cli\u003eEndo, K., Fukuda, Y., Takamiya, O.: Wear behaviours of metals under lubricated conditions and the effects of small electric potential. Trans. Jpn. Soc. Mech. Eng. 14, 1281\u0026ndash;1288 (1971)\u003c/li\u003e\n\u003cli\u003eHe, Y., Luo, J. and Xie, G., 2007. Characteristics of thin liquid film under an external electric field. Tribology international, 40(10-12), pp.1718-1723. \u003c/li\u003e\n\u003cli\u003eGłogowski, M., Smykowski, D. and Pietrowicz, S., 2023. The Effect of an Electric Field on the Sliding Friction of the Silicone Rubber against Selected Metals in Motor Base Oils. Energies, 16(9), p.3954.\u003c/li\u003e\n\u003cli\u003eShor, G.I., Blagovidov, I., Eustigneev, E., Lapin, V.P.: Antiwear and antiscuff properties of lubricating oils in relation to electric potential at metal-oil phase boundary. Chem. Tech. Fuels Oil 8, 770\u0026ndash;772 (1972)\u003c/li\u003e\n\u003cli\u003eLu, R., Kawada, S., Tani, H. and Koganezawa, S., 2023. The influence of electric current on the friction behavior of lubricant molecules. Tribology Online, 18(3), pp.83-90.\u003c/li\u003e\n\u003cli\u003eXie G, Luo J, Guo D, Liu S, Li G. Damages on the lubricated surfaces in bearings under the influence of weak electrical currents. Science China Technological Sciences. 2013 Dec;56(12):2979-87.\u003c/li\u003e\n\u003cli\u003eYang, Q. and Duan, F., 2024. Tribological properties of phosphate ester confined between iron-based surfaces. Langmuir, 40(7), pp.3738-3747.\u003c/li\u003e\n\u003cli\u003eSharma, V., Gabler, C., Doerr, N. and Aswath, P.B., 2015. Mechanism of tribofilm formation with P and S containing ionic liquids. Tribology international, 92, pp.353-364.\u003c/li\u003e\n\u003cli\u003eLuiz, J.F. and Spikes, H.A., 2020. Tribofilm formation, friction and wear-reducing properties of some phosphorus-containing antiwear additives. Tribology Letters, 68(3), p.75.\u003c/li\u003e\n\u003cli\u003eBenedet, J., Green, J.H., Lamb, G.D., Spikes, H.A., 2009. Spurious mild wear measurement using white light interference microscopy in the presence of antiwear films. Trib Trans. 52, p. 841-846.\u003c/li\u003e\n\u003cli\u003eLorenz, M., Pawlicki, A.A., Hysmith, H.E., Cogen, K., Thaker, H. and Ovchinnikova, O.S., 2022. Direct multimodal nanoscale visualization of early phosphorus-based antiwear tribofilm formation. ACS Applied Materials \u0026amp; Interfaces, 14(30), pp.35157-35166.\u003c/li\u003e\n\u003cli\u003eXia, Y., Zhang, S., Wang, Z. and Zhang, C., 2024. New insight into tribofilm composition and structure of iron polyphosphate-rich tribofilm from formulated engine oil by ToF-SIMS. Journal of Materials Science, 59(21), pp.9533-9546.\u003c/li\u003e\n\u003cli\u003eRestuccia, P., Pedretti, E., Benini, F., Loehl\u0026eacute;, S. and Righi, M.C., 2025. Phosphorus-based lubricant additives on iron with Machine Learning Interatomic Potentials. arXiv preprint arXiv:2512.23583.\u003c/li\u003e\n\u003cli\u003eYang, C., Guo, Y., Zhang, H. and Guo, X., 2024. Utilization of electric fields to modulate molecular activities on the nanoscale: from physical properties to chemical reactions. Chemical Reviews, 125(1), pp.223-293.\u003c/li\u003e\n\u003cli\u003eSpikes, H., 2025. Mechanisms of ZDDP\u0026mdash;an update. Tribology Letters, 73(1), p.38.\u003c/li\u003e\n\u003cli\u003eSaba, C.S. and Forster, N.H., 2002. Reactions of aromatic phosphate esters with metals and their oxides. Tribology Letters, 12(2), pp.135-146.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Electrified tribology, phosphite, phosphate, tribofilm, iron oxidation, and voltage","lastPublishedDoi":"10.21203/rs.3.rs-8888807/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8888807/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAshless phosphorus-based lubricant additives, which are increasingly deployed in next-generation formulations, often suffer from slow tribofilm formation and poor film stability, limiting their effectiveness under demanding operating conditions. As mechanical systems become increasingly electrified, understanding how lubricants respond to electrical stimuli, and developing strategies that exploit such stimuli, have become critical for ensuring reliable operation. Here, we investigate the tribological performance of bis(2-ethylhexyl) phosphite (BEPite) in polyalphaolefin (PAO2) lubricated steel/steel contacts under controlled voltage and current. BEPite produces thicker and denser tribofilms on anodic rubbing surfaces when a voltage is applied. Increasing the current between lubricated contacts from \u0026lt;\u0026thinsp;0.01 mA to 0.5 mA has little effect on tribofilm formation, whereas increasing voltage enhances it. This enhancement is only observed in rubbing contacts. Chemical analysis reveals the presence of oxidized PO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e-related species and abundant iron in the tribofilm. This suggests voltage-driven triboelectrochemical oxidation, as well as enhanced additive adsorption and reaction, promote tribofilm formation. Either iron oxides or released Fe ions may alter the tribofilm structure. These phenomena appear general for phosphorus-based ashless additives, as both phosphites and phosphates with different structures have seen increased tribofilm growth at anodic rubbing surface. This work demonstrates that voltage alone can intensify triboelectrochemical reactions, providing new insights for the design of next-generation lubricants.\u003c/p\u003e","manuscriptTitle":"Voltage-Driven Growth of Phosphorus Tribofilms","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-22 15:09:18","doi":"10.21203/rs.3.rs-8888807/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-30T09:00:18+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-19T05:09:47+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-28T04:01:55+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"125715392754431282308337492878490573675","date":"2026-02-23T00:57:40+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"284195576742000858190837209810896635447","date":"2026-02-21T08:45:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"135148281088811592203583511597972223262","date":"2026-02-19T05:30:15+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-18T09:54:38+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-16T09:36:34+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-16T09:35:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Tribology Letters","date":"2026-02-15T23:45:23+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"13ed42b0-200d-486b-b19b-adb86c2a4443","owner":[],"postedDate":"February 22nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-03-30T09:15:15+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-22 15:09:18","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8888807","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8888807","identity":"rs-8888807","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}