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Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy
Journal Pre-proof Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy N.N. Gosvami, I. Lahouij, J. Ma, R.W. Carpick PII:
S0301-679X(19)30589-4
DOI:
https://doi.org/10.1016/j.triboint.2019.106075
Reference:
JTRI 106075
To appear in:
Tribology International
Received Date: 19 August 2019 Revised Date:
9 November 2019
Accepted Date: 14 November 2019
Please cite this article as: Gosvami NN, Lahouij I, Ma J, Carpick RW, Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy, Tribology International (2019), doi: https://doi.org/10.1016/j.triboint.2019.106075. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy N. N. Gosvami1,‡,*, I. Lahouij†, J. Ma2,#, and R. W. Carpick1,* 1
Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA.
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Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA ‡
†
Present Address: Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Present Address: MINES ParisTech, PSL – Research University, Centre de mise en forme des matériaux (CEMEF), CNRS UMR 7635, CS 10207 rue Claude Daunesse, 06904, Sophia Antipolis Cedex, France #
Present Address: W. L. Gore and Associates, 1202 Sheldon Drive, Newark, DE, 19711 *[email protected], *[email protected]
Abstract Zinc dialkyldithiophosphate (ZDDP)-based antiwear additives are crucial in automotive lubricants, where its effectiveness in reducing wear of ferrous alloys is well established. However, prior studies of light-weight aluminum-based alloys reveal that ZDDP is not as effective an anti-wear agent on Al-based surfaces for reasons that remain under debate. Here we use in situ atomic force microscopy (AFM) to study nanoscale ZDDP-derived tribofilms at the sliding interface between an alumina microcolloid probe and substrates comprised of either aluminum (with native oxide) or aluminum oxide (single crystal sapphire). The experiments reveal that ZDDP tribofilms form on both substrates, supporting the model of tribofilm formation as a thermally-activated, stress-assisted process that does not require cation exchange from wear debris originating from the substrate.
Keywords: zinc dialkyldithiophosphates (ZDDP), boundary lubrication, antiwear tribofilms, atomic force microscopy 1
1. Introduction Zinc dialkyldithiophosphates (ZDDPs), one of the most successful and widely used antiwear additives, are used in a wide range of automotive engine lubricants and also for antiwear protection of gears, bearings, and other engineering components [1]. ZDDP reduces the wear of surfaces in relative motion in the boundary regime by transforming into a surface-bound tribochemical film. Owing to the complexity of the buried, multi-asperity contacting interfaces intrinsic to macroscale contacts, the mechanism(s) by which ZDDP reacts at the sliding interface has remained poorly understood for decades, although many advances have been made [1]. A recent in situ single-asperity approach using an atomic force microscope (AFM) showed that the growth of ZDDP antiwear tribofilms is a tribochemical reaction which can be described by Arrhenius-type reaction kinetics where the growth rate increases exponentially with temperature and contact pressure [2]. This work provided experimental confirmation of stress-assisted growth of zinc phosphate-derived films predicted by atomistic simulations [3, 4]. Subsequent in situ work confirmed these observations with various types of ZDDPs at the nano [5] and microscales [6], and recent macroscale work provided compelling support for the idea that shear stresses, in particular, are critical for the tribofilm formation [7]. Several macroscale studies have explored the growth of ZDDP tribofilms on a variety of material surfaces [1, 8-15]. One motivation behind such studies is to enable the replacement of steel engine components with lightweight, non-ferrous alloys, e.g., aluminum or magnesium alloys, which can improve energy efficiency substantially. However, due to the complex nature of the interface in macroscale tests, questions remain regarding the nature of ZDDP interactions with such materials. Particularly for aluminum-silicon (Al-Si) alloys, which have been studied the most, several researchers have reported that robust tribofilms do not form on the softer Al
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matrix, but robust antiwear pad-like structures do form on the surfaces of the harder silicon inclusions, similar to the tribofilms observed on steel surfaces [16-19]. It is not clear why robust tribofilm formation is apparently inhibited on Al surfaces, which warrants performing further investigation of ZDDP tribofilm growth on Al-based substrates. There are multiple theoretical mechanisms proposed to explain the growth of ZDDP antiwear films preferentially on ferrous substrates, e.g., steels. One mechanism proposed by Mosey et al. is based on molecular dynamics (MD) simulations, which revealed that antiwear film grows via contact-pressure induced cross-linking of thermal decomposition products of zinc phosphates [3, 4]. Al, being a soft metal, is unable to withstand the high contact pressures typically required for the cross-linking. Instead, it plastically yields and wear (e.g., by plowing) occurs during sliding, preventing tribofilm growth. Another model is based on the Hard and Soft Acid-Base (HSAB) theory, which predicts favorable cationic exchange occurs between Fe+3 (from Fe-containing substrates) and zinc phosphates, which are initially weakly adsorbed in the form of a thermal film [20, 21]. The cationic exchange involves chemical digestion of Fe-oxide wear debris (e.g., in the form of nanoparticles) in the zinc phosphate matrix, which results in a robust interconnected phosphate glass. The HSAB model was recently combined with MD simulations, which showed that tribofilm growth is favorable on certain non-ferrous materials including silicon due to favorable chemical digestion of silica particles, but alumina particles cannot be easily digested [17]. Both mechanisms are consistent with the lack of observation of ZDDP-derived tribofilms on aluminum, and thus it is not clear which mechanism, or both, are at play. To address these open questions and to validate the theoretical predictions, we use the recently established in situ tribofilm growth method to explore ZDDP-derived tribofilm growth
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[2, 6], and in particular, how the substrate material plays a role. Studies were performed on mirror polished ultrapure Al (99.99% purity) substrates, and on atomically smooth sapphire substrates, in both-cases using wear-resistant alumina microspherical probes.
2. Materials and Methods The 99.99% pure Al substrates (Laurand Associates Inc., Great Neck, NY, USA) were electro-polished to mirror finish using the method described in ASTM E1558-09 (2014) [22] prior to all the AFM experiments. Sapphire single crystal c-plane wafer substrates (MTI Corp., Richmond, CA, USA) with RMS surface roughness of ~0.35 nm were used after solvent cleaning with isopropyl alcohol and ethanol. The procedures for AFM probe preparation and AFM tribofilm experiments are described in detail in [6]. We briefly summarize these procedures below and point out the few specific differences in this study. Commercial silicon cantilevers (PPP-NCH, Nanosensors, Neuchatel, Switzerland) used in these measurements were first calibrated for their normal and lateral spring constants using the Sader method [23]. The cantilever deflection sensitivities along the normal direction were obtained by taking force curves on the Al substrate surface, whereas lateral force sensitivities were obtained from the static friction vs. distance curves [6]. The probes were prepared in a similar manner as those in [6], but instead of steel microspheres, we glued alphaalumina microspheres (Inframat Corp., Manchester, CT, USA) on calibrated cantilevers using an inverted optical microscope (Olympus BX51) by first applying a small quantity of a two-part epoxy (J-B Weld, Sulphur Springs, TX, USA) using a sharp tungsten wire (TGW0325, World Precision Instruments, Sarasota, FL, USA) and then placing the microcolloid using a strand of hair. The cantilevers were stored overnight in a dry, N2-purged box to allow the epoxy to cure 4
fully. The diameters of the microspheres attached to the cantilevers were estimated either from optical imaging (Olympus BX51) or using field emission scanning electron microscopy (SEM, JOEL 7500F). The microspheres vary in diameter; in this study, the microsphere used for measurements on the Al sample had a diameter of 39 µm, and the one used for measurements on the sapphire sample had a diameter of 35 µm. The microspheres have a RMS roughness of ca. 85 nm (measured by AFM over an area of 15x15 µm2 after removing the background curvature of the sphere). The normal and lateral spring constants were corrected after gluing the microcolloid, which results in a variation in the effective length of the cantilever (distance between the base of the cantilever to the center of the glued microcolloid) as well as the tip height. All the experiments were performed using a commercial atomic force microscope (Keysight 5500, Keysight Technologies, Santa Clara CA, USA), equipped with a sample heating plate, a liquid cell, and a temperature controller (Model 321, Lake Shore Cryotronics Inc., Westerville, OH, USA), schematic shown in Figure 1. For the experiments in this study, a mixture of 99 wt% base oil (SpectraSyn polyalphaolefin (PAO) 4 cSt, ExxonMobil, Houston, TX, USA) with 1 wt% zinc dialkyldithiophosphate (ZDDP) antiwear additive (HiTEC 1656, a mixture of primary and secondary ZDDP, Afton Chemical Corp., Richmond, VA, USA) was used for all the experiments. This is consistent with the typical amount of ZDDP used in commercial engine oils which corresponds to approximately 800 ppm P as limited by current regulations [1], and is the same or close to levels used in multiple previous studies[5, 7, 18, 19] including our own [2, 6]. Data analysis was performed using WSxM 5.0 software [24] and using custom MATLAB routines. To characterize the Al contact interface, a transmission electron microscopy (TEM) lamella was fabricated by the focused ion beam (FIB) lift-out technique [25] using a dual-beam
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FIB (FEI Strata DB235). To limit the extent of ion beam damage, the surface was first protected with electron beam induced Pt deposition and subsequently coated with a 2 µm ion beam induced Pt protective layer. For final polishing of the lamella, a 5 kV ion beam was used at 2° incidence to the lamella surfaces. The sample was then imaged with a TEM/STEM (JEOL 2010F, JEOL Ltd., Tokyo, Japan) operating with 200 kV accelerating voltage and equipped with Energy Dispersive Spectroscopy (EDS).
3. Results and Discussions To investigate ZDDP-derived tribofilm growth on non-ferrous substrates (pure Al or sapphire), sliding against a non-ferrous counter surface (alumina microspherical probe), we used similar sliding conditions which had been used for steel on steel contacts in our previous study using the recently reported AFM based approach (temperature 115±5 ˚C, mean Hertzian contact pressure of ~0.2-0.5 GPa, sliding distance of 10-15 µm per stroke, and sliding speed of ~200 µm/s) [6]. The temperature chosen is one at which ZDDP tribofilm growth is readily observed both in macroscopic studies [1] and our own previous nanoscale studies [2, 6]. Note that the roughness of the probe will create local asperity contacts within the nominal contact zone with contact pressures well in excess of the nominal pressure. Similar to the observations for steel-steel tribopairs at the microscale [2], no tribofilms form at room temperature under similar experimental conditions. This is consistent with the fact that, in general, the growth of tribofilms from ZDDP is not observed at room temperature [1]. However, in contrast to the observations on steel substrates, the pure Al substrate undergoes severe wear during sliding for extended sliding periods (Movie S1). A zoomed out topographic AFM image of the worn surface after one such experiments is shown in Figure 2(a), which
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reveals a large depression within the sliding zone. This is due to some degree of wear, which we suggest further below is potentially attributable to plastic deformation in the form of an indented sliding track. The simultaneously recorded friction coefficient values were high (above 0.9) and showed large fluctuations due to continuous wear (Figure 2(b)). Consistent results were seen in extended tests out to 1000 cycles, and the behavior was reproducible. This behavior was reproduced in a replicate experiment. Similar experiments were then repeated, but at elevated temperatures (115±5 ºC) (Figure 3). During the initial stage of sliding, the Al substrate again starts to exhibit wear in the form of plastic compression, and significant fluctuations in the friction force are observed. No clear signature of tribofilm growth is seen during this phase. This behavior was reproduced in a replicate experiment. The causes of the rapid and the slower fluctuations seen in friction during this initial sliding phase are unknown, but could be due to multiple factors including material transfer to the tip; evolution of the contact area during sliding as the substrate plastically deforms; random and sudden plastic yield events; and nascent tribofilm nucleation events. Notably, large fluctuations in friction have been observed in the macroscopic sliding of high purity Al and associated with plastic modification of the Al surface [26]. This wear behavior continues to occur with increasing sliding time, as evident from the continuous change in surface topographic features (Movie S2). However, the deformation process slows down and the coefficient of friction is significantly reduced after 500-600 sliding cycles, at which point nucleation and subsequent growth of the tribofilm is observed. We note that some of the image frames (e.g. apparent in Movie S2) show vertically aligned bands. Such features are well-known to be due to the mechanical instabilities of the AFM piezo scanner
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and/or the cantilever which are excited at the turnaround points at the left and right sides of the image when scanning at high rates, as we do here [27]. These cause fluctuations in the applied normal force across the scan, and thus can influence subsequent surface wear and/or tribofilm growth. In particular, before tribofilm growth starts, the bands are most pronounced, potentially due to the strong adhesive and frictional interactions between the tip and the bare substrate. Once the tribofilm starts to grow, these bands are greatly diminished. Figure 3(a) shows the zoomed out topography image of the sliding zone, revealing the characteristic morphology of the tribofilm, which is very similar to the rough, pad-like morphology of films produced via a steel-steel tribopair involving a microscale colloidal AFM probe [6], and similar to the pad-like morphology seen in tribofilms grown with nanoscale AFM probes [2, 5] and in macroscale tribometry experiments [1, 7]. This topography is clearly distinct from the morphology changes seen just due to wear of Al without tribofilm growth. As discussed further below, chemical analysis further establishes that a ZDDP-derived tribofilm has formed. Figure 3(b) shows the simultaneously recorded friction coefficient data, where fluctuations are observed up to ~600 cycles. However, once the tribofilm starts to grow after ~600 cycles (as evident in the distinctive topography seen in Movie S2), these fluctuations are significantly reduced. The fluctuations in friction may be attributed to significant wear occurring in the first ~600 cycles. Small-scale wear of ductile material like Al can consist of a large combination of plastic pop-in events due to random nucleation of dislocations and/or the encountering of random pre-existing dislocations in the sliding zone [28, 29], leading to significant fluctuations in the local topography, and thus the local friction force. The reduction in wear once the tribofilm starts to grow is thus correlated with reduced fluctuations in friction.
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The mechanism for the delayed onset of tribofilm formation is not certain, but one possibility is that the plastic deformation leads to work-hardening, which is intrinsic for pure Al [30] including during indentation [29] and sliding contact [26]. It is indeed likely that plastic deformation is occurring in the first few hundred cycles. A simple estimate for the experiment in Fig. 2 using the Greenwood-Williamson model of rough elastic contact of flat surfaces with one having the same roughness as our probe predicts a nearly fully plastic contact (we assumed, to consider a limiting case, a yield stress of pure Al of 570 MPa as this is at the upper end of reported values for pure Al after heat treatment [31]). This is by the application of compressive normal stress alone; the additional application of shear during sliding will also contribute to plastic deformation. As it plastically deforms, the expected work hardening then is hypothesized to occur until the constant applied contact pressure is no longer sufficient to cause further plastic deformation. Thus, the plastic deformation-induced wear diminishes or ceases, and increased compressive contact stress can be stably supported by the substrate. This increased compressive contact stress will also increase the shear stresses in the contact. Since ZDDP tribofilm growth is accelerated exponentially by increased contact stresses [2, 6, 7], these higher stresses should induce the nucleation and subsequent growth of a robust ZDDP tribochemical film. In other words, we are hypothesizing that contact stresses that are sufficiently high to drive rapid tribofilm growth could not be initially achieved on the soft Al substrate; once the Al is workhardened due to repeated compressive plastic deformation, higher contact stresses could be achieved. This idea is directly based on the observation of compressive stress thresholds for nucleating crosslinking of polyphosphate chains in atomistic simulations of zinc phosphates [3, 4], the observation of a clear shear stress-dependence for the formation of ZDDP tribofilms in macroscopic tribometry experiments [7], and our prior observation of an exponential dependence
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of ZDDP-derived tribofilm growth on compressive contact stress [2]. Notably, the atomistic studies predicted that, without work hardening, the yield stress of pure Al is too low to support tribofilm formation [3]. Our observations are consistent with these simulation results.
We then conducted similar experiments on a flat sapphire substrate, which is significantly harder (more than 3 times harder than pure Al), and therefore more wear-resistant than Al, while allowing the same experiments without significantly affecting the surface chemistry (i.e., the sapphire surface chemistry is similar to that of the Al sample, whose surface consists of a thin aluminum oxide layer). Figure 4 shows the representative results of experiments conducted using the sapphire substrate but conducted in otherwise the same manner as the experiments whose results are shown in Figure 3. Figure 4(a) shows the zoomed-out AFM topography image of the sliding zone after ~800 cycles. Unlike the case of the Al substrates, no wear was observed on the sapphire (i.e., the AFM images of the sliding zone revealed flat, uniform, unchanging topography). Correspondingly, we did not observe large fluctuations in the measured friction coefficient (Figure 4(b)), unlike the fluctuations initially observed on the pure Al sample. This supports our earlier suggestion that, when the substrate is undergoing plastic deformation, the irregularity and randomness of the process lead to large fluctuations in friction. Continued sliding on the sapphire substrate resulted in clear nucleation and growth of a tribofilm (Movie S3). For the first several hundred cycles (~300) no tribofilm growth was detected; after this period, several tribofilm nucleation sites (localized raised regions) were observed at multiple points within the sliding zone. These continued to grow with further sliding, leading to the formation of a stable tribofilm across the entire sliding zone. The nucleation and growth process of the tribofilm, as well as its morphology, is similar to the nanoscale
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experiments conducted on steel [6]. It is important to note that in all the experiments, no clear evidence of probe wear was observed (Figure S1 & S2). This indicates that the alumina probes are rather durable for such tests. In contrast, steel microcolloid probes showed some degree of wear, as reported in our previous study [6]. In addition, we do observe build up of material on the alumina probe used for the experiments on Al (Figure S1). We attribute this to transferred Al from the substrate which accompanies the plastic wear we observe. This may account for the different friction coefficient observed for the experiments on the Al substrate. We note that friction will also be affected by the overall probe size, the evolving roughness of the surface of the sample, and the fine scale roughness of the probe. We thus focus on the overall trends in tribofilm growth; a study of specific factors controlling friction will be the subject of a future effort. An ideal way to validate our hypothesis that strain hardening of the Al occurred to allow tribofilm growth would be through nanoindentation experiments. Unfortunately, the need to isolate the plastically-indented regions from AFM experiments and then conduct localized nanoindentation experiments proved challenging, particularly due to the fact that the surfaces were roughened appreciably after the wear and the tribofilm growth. While this will be pursued in future studies, we did further explore the idea that work-hardening assisted the tribofilm growth on pure Al by conducting SEM and cross-sectional TEM imaging of the sample and tribofilm [32]. Figure 5 shows SEM and TEM images of the same region of the Al substrate that was probed and the results reported in Figure 3. Figure 5(a) shows an oblique-angle SEM image of the sliding zone which shows a clear depression with an oval perimeter due to the surface wear. A lamella was sectioned from this sample region using focused ion beam (FIB) to conduct cross-sectional imaging of the surface, as shown in Figure 5(b). The magnified image of the
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lamella, acquired using TEM, is shown in Figure 5(c). This image was acquired within the central region of the sliding zone where significant wear had been observed. As clearly seen, the Al surface shows significant roughening near the surface due to wear. In addition, subsurface regions with darker contrast are clearly observed. The darkened contrast is typical of plastically deformed metals with a high density of dislocations within the grains located under the sliding interface [33] and extends several hundred nanometers deep. On top of these, a tribofilm is seen with thickness in this region of approximately 60 nm. The interface between the tribofilm and the Al undulates somewhat, but is observed to exhibit an abrupt change in contrast, within the image resolution of ca. 5 nm. While there may well be Al wear debris in the film, the tribofilm itself can be clearly delineated from the substrate. Figure 5(d) shows a similar magnified image acquired using TEM, but away from the central region, i.e., still within the contact region but closer to the peripheral region where the contact stresses experienced by the Al sample would have been lower. The tribofilm thickness in this region is significantly smaller (approximately 35 nm). In addition, the regions with darker contrast underneath the surface are smaller than those in Figure 5(c), extending only ~100 nm deep. Although qualitative, the cross-sectional TEM images indicate a correlation between tribofilm thickness and the extent of work hardening attributable to the contrast change expected from subsurface plastic deformation. The contention that work hardening occurs is also consistent with the well-established work-hardening behavior of Al subject to indentation and sliding [26, 28, 29]. Scanning mode TEM (STEM) was then used to conduct energy dispersive spectroscopy (EDS) to perform chemical analysis of the TEM lamella. Twenty EDS spectra were recorded across the tribofilm at different locations of the lamella to obtain direct evidence of the formation of a ZDDP-derived tribofilm. Figure 6 illustrates representative EDS spectra recorded from the
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Al substrate, the tribofilm-substrate interface, and the tribofilm itself. As expected, the substrate is composed overwhelmingly of aluminum (Al), while the interface is rich in Al and O, as expected for the oxide that will be present near the original free surface; it could also indicate that additional surface oxidation occurred prior to the tribofilm formation as a result of the wear process observed initially. The EDS spectrum of the tribofilm shows the presence of Zn, P, and S, which are attributed to the ZDDP-derived tribofilm formed by sliding. This supports our claim that the observed structures in Figures 3(a) and, correspondingly, 4(a), contain the expected elemental constituents of ZDDP-derived tribofilms. These regions’ morphologies are very similar to those of ZDDP-derived tribofilms reported elsewhere [2, 5-7, 34], further supporting our contention that the observed regions are ZDDP-derived tribofilms. As seen in Figure 6, Cu and Ga peaks appear in the three spectra recorded across the TEM lamella. Cu element is due to the Cu TEM grid on which the sample was mounted. The Ga is contamination that occurred during Ga+ focused ion beam milling (FIB) process. In addition, peaks corresponding to Pt were observed from the spectra recorded within the upper layer of the tribofilm (not shown here), close to the Pt layer (an area roughly marked with dashed blue lines in Figure 5c). This is a common effect due to Ga+ ion etching of the protective Pt film that was deposited on top of the tribofilm, causing changes in the structure and composition of the upper 20 nm of the tribofilm. We note that substantial Al and O peaks appear in the tribofilm spectrum, suggesting the embedding of wear debris generated during the sliding experiment into the tribofilm. This is reminiscent of iron oxide particles observed embedded within the tribofilms grown on steel [35], attributed to substrate wear processes during sliding. Notably, the content of P and S in the tribofilm, based on comparing peak heights in EDS spectra, is lower than that observed for
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ZDDP-derived tribofilms grown on ferrous substrates [2, 35]. Further study is needed to reliably quantify this difference and explore its origins. Growth of robust tribofilms of an alumina probe sliding on the Al and on the sapphire substrates, with similar morphology and growth properties as the films observed on steel substrates, clearly indicates that presence of iron is not necessary for the growth of ZDDPderived antiwear tribochemical films. Previous studies based on the hard and soft acid-based (HSAB) model [20], and supported by reactive MD simulations [17, 21], predict that such tribofilms would not grow at Al/alumina or alumina/alumina interfaces because the exchange of Zn with Al is not favored. The HSAB model is based on cationic exchange between the wear debris of substrates containing Fe or other elements whose exchange with Zn is favored in the formation of zinc polyphosphates. This exchange then results in digestion of the wear debris in the zinc phosphate matrix, which is favored thermodynamically due to the increase in configurational entropy, leading to the growth of the protective antiwear tribofilm. Such cation exchange may well occur in tribofilm growth processes where such cations are present, such as Fe3+, Si4+, and others. Such processes will certainly affect the growth, morphology, and tribological properties of the tribofilms. However, our results demonstrate that such cation exchange is not needed to produce tribofilms in the first place.
4. Conclusions In summary, we report the first results where ZDDP antiwear tribochemical film growth has been monitored in situ between two non-ferrous sliding surfaces, namely, an alumina probe sliding against pure Al and against sapphire. Our results reveal that tribofilm growth occurs on pure Al as well as on sapphire substrates for an alumina probe sliding in ZDDP-containing base
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oil at 115±5°C. In addition, the pure Al substrate undergoes substantial wear in the form of compressive plastic deformation as soon as sliding begins, whereas no such damage is observed on the sapphire substrate. Stable sliding on Al is subsequently established, potentially due to the eventual strain hardening of the Al, and the growth of ZDDP-derived tribofilms with the typical pad-like morphology occurs. On sapphire, no wear is observed prior to the onset of tribofilm growth. For both Al and sapphire, the tribofilm morphology is similar to that observed on steel substrates. The results are consistent with previous results on steel-steel tribopairs whereby stress, as well as elevated temperatures are required for the tribofilms to grow. These results are consistent with the original predictions of Mosey et al. [3, 4] whose modeling results showed that zinc phosphate molecules will crosslink with sufficient applied pressure. While cation exchange may occur within tribofilms formed on surfaces containing species with favorable interactions according to the HSAB principle, this work shows that such processes are not required to form tribofilms; rather, this work suggests that sufficiently hard substrates that can enable sufficiently high contact stresses at asperity contacts are the prerequisite for tribofilm formation. Further work is required to determine how the composition and properties of these tribofilms differ from those formed on steel or other substrates.
Acknowledgements This work was supported by the National Science Foundation under grant CMMI1728360, by the University of Pennsylvania through the School of Engineering and Applied Sciences, and by the Vagelos Integrated Program in Energy Research (VIPER). This work was
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carried out in part at the Singh Center for Nanotechnology, which is supported by the NSF National Nanotechnology Coordinated Infrastructure Program under grant NNCI-1542153. NNG would also like to acknowledge SERB (ECR/2016/001014) for financial support.
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Figures
Figure 1: Schematic of the colloid probe AFM used for the sliding tests.
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(a)
(b) Figure 2: (a) Zoomed out contact mode AFM topography of the sliding zone on a pure Al substrate after 500 cycles. Consistent results were seen when testing up to 1000 cycles under the same conditions. Experiments were performed at room temperature in a ZDDP-containing lubricant. No tribofilm is seen; only a plastically deformed region of the Al substrate is observed. (b) Coefficient of friction measured during the same sliding test using an applied load of 8.7 µN (corresponding to mean Hertzian stress of 0.17 GPa; stresses at the individual asperities will be much higher as discussed in the text).
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(a)
(b) Figure 3: (a) Zoomed out AFM topography of the sliding zone on the pure Al substrate after 2000 cycles., experiments were performed in a ZDDP-containing oil at 115±5 ˚C. (b) Coefficient of friction measured during the same sliding test using an applied load of 9.5 µN (corresponding to mean Hertzian stress of 0.18 GPa; stresses at the individual asperities will be much higher as discussed in the text). Initially, high friction which reduces, then rises again with large fluctuations eventually starts to follow a progressively stabilizing trend after 500-600 cycles, which corresponds with the onset of observable tribofilm growth (see Movie S2). The cause of the rapid and the slower fluctuations in friction during the initial sliding phase are unknown but may be due multiple factors including material transfer to the tip, evolution of the contact area during sliding as the substrate plastically deforms; random and sudden plastic yield events; and nascent tribofilm nucleation events.
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.
(a)
(b) Figure 4: (a) Zoomed out contact mode AFM topography of the sliding zone on the sapphire substrate after 794 cycles. Experiments were performed in a ZDDP-containing oil at 115±5 ˚C. (b) Coefficient of friction measured during the same sliding test using an applied load of 15.7 µN (corresponding to mean Hertzian stress of 0.48 GPa; stresses at the individual asperities will be much higher as discussed in the text). Overall, the rapid fluctuations in friction are not as large as those seen on the Al substrate. The cause of the significant changes around 300-350 cycles is likely due to the nucleation and growth of the tribofilms. They could also be due to changes in the tip shape and/or material transfer or other factors that are difficult to control.
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Figure 5: (a) SEM image of the Al substrate, clearly revealing the worn region where sliding tests were performed. (b) SEM image of a lamella prepared using FIB sectioning across the sliding zone to examine the cross-section of the surface and subsurface regions, (c) TEM image of the central region of the lamella showing the cross-section where tribofilm, as well as subsurface region can be identified. Darker contrast within the grains underneath the sliding zone indicates significant increase in dislocation density (work hardening) and correspondingly thicker ZDDP tribofilm formed within the sliding zone. The blue dashed lines roughly mark the Pt contaminated layer of the tribofilm. (d) TEM imaging revealing thinner tribofilm away from the severely worn region and grains do not exhibit darker contrast indicating absence of work hardening.
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Figure 6: EDS spectra recorded as shown in the STEM micrograph (inset) within (1) the substrate, (2) the interface and (3) the tribofilm. Spectrum 3 reveals the presence of Zn, P, and S derived from the ZDDP tribofilm. Al and O are also seen, as expected from the Al substrate; some of the O may be from the tribofilm. C is from contamination and hydrocarbons in the residual oil as well as hydrocarbons in the tribofilm. The Cu peak is due to the Cu TEM grid on which the sample was mounted, and Ga is from the FIB milling. 22
References
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[15] Dawczyk J, Morgan N, Russo J, Spikes H. Film Thickness and Friction of ZDDP Tribofilms. Tribol Lett. 2019;67:34. https://doi.org/10.1007/s11249-019-1148-9. [16] Grun F, Summer F, Pondicherry KS, Godor I, Offenbecher M, Laine E. Tribological functionality of aluminium sliding materials with hard phases under lubricated conditions. Wear. 2013;298:127-34. https://doi.org/10.1016/j.wear.2012.11.048. [17] Onodera T, Martin JM, Minfray C, Dassenoy F, Miyamoto A. Antiwear Chemistry of ZDDP: Coupling Classical MD and Tight-Binding Quantum Chemical MD Methods (TBQCMD). Tribol Lett. 2013;50:31-9. https://doi.org/10.1007/s11249-012-0063-0. [18] Pereira G, Lachenwitzer A, Kasrai M, Norton PR, Capehart TW, Perry TA, et al. A multitechnique characterization of ZDDP antiwear films formed on Al (Si) alloy (A383) under various conditions. Tribol Lett. 2007;26:103-17. https://doi.org/10.1007/s11249-006-9125-5. [19] Shimizu Y, Spikes HA. The influence of aluminium–silicon alloy on ZDDP tribofilm formation on the counter-surface. Tribol Lett. 2017;65:137. https://doi.org/10.1007/s11249-0170915-8. [20] Martin JM. Antiwear mechanisms of zinc dithiophosphate: a chemical hardness approach. Tribol Lett. 1999;6:1-8. https://doi.org/10.1023/A:1019191019134. [21] Martin JM, Onodera T, Minfray C, Dassenoy F, Miyamoto A. The origin of anti-wear chemistry of ZDDP. Faraday Discuss. 2012;156:311-23. https://doi.org/10.1039/C2FD00126H. [22] ASTM E1558-09, Standard Guide for Electrolytic Polishing of Metallographic Specimens. 2014. https://doi.org/10.1520/E1558-09R14. [23] Green CP, Lioe H, Cleveland JP, Proksch R, Mulvaney P, Sader JE. Normal and torsional spring constants of atomic force microscope cantilevers. Rev Sci Instrum. 2004;75:1988-96. https://doi.org/10.1063/1.1753100. [24] Horcas I, Fernández R, Gomez-Rodriguez J, Colchero J, Gómez-Herrero J, Baro A. WSXM: a software for scanning probe microscopy and a tool for nanotechnology. Rev Sci Instrum. 2007;78:013705. https://doi.org/10.1063/1.2432410. [25] Giannuzzi LA, Stevie FA. A review of focused ion beam milling techniques for TEM specimen preparation. Micron. 1999;30:197-204. https://doi.org/10.1016/S0968-4328(99)000050. [26] Kuo S, Rigney D. Sliding behavior of aluminum. Materials Science and Engineering: A. 1992;157:131-43. https://doi.org/10.1016/0921-5093(92)90020-2. [27] Schitter G, Stemmer A. Identification and open-loop tracking control of a piezoelectric tube scanner for high-speed scanning-probe microscopy. IEEE Transactions on Control Systems Technology. 2004;12:449-54. https://doi.org/10.1109/TCST.2004.824290. [28] Bolduc M, Terreault B, Reguer A, Shaffer E, St-Jacques R. Selective modification of the tribological properties of aluminum through temperature and dose control in oxygen plasma source ion implantation. J Mater Res. 2003;18:2779-92. https://doi.org/10.1557/JMR.2003.0388. [29] Barnoush A, Welsch MT, Vehoff H. Correlation between dislocation density and pop-in phenomena in aluminum studied by nanoindentation and electron channeling contrast imaging. Scripta Materialia. 2010;63:465-8. https://doi.org/10.1016/j.scriptamat.2010.04.048. 24
[30] Varma S, LeFevre B. Large wire drawing plastic deformation in aluminum and its dilute alloys. Metallurgical Transactions A. 1980;11:935. https://doi.org/10.1007/BF02654706. [31] Aluminium - Specifications, Properties, Classifications and Classes, https://www.azom.com/article.aspx?ArticleID=2863. [accessed 14 October 2019]. [32] Dawczyk J, Ware E, Ardakani M, Russo J, Spikes H. Use of FIB to study ZDDP tribofilms. Tribol Lett. 2018;66:155. https://doi.org/10.1007/s11249-018-1114-y. [33] Williams DB, Carter CB. Transmission electron microscopy: a textbook for materials science. 2nd ed. New York: Springer; 2009. . https://doi.org/10.1007/978-0-387-76501-3. [34] Taylor LJ, Spikes H. Friction-enhancing properties of ZDDP antiwear additive: part I— friction and morphology of ZDDP reaction films. Tribol Trans. 2003;46:303-9. https://doi.org/10.1080/10402000308982630. [35] Njiwa P, Minfray C, Le Mogne T, Vacher B, Martin J-M, Matsui S, et al. Zinc dialkyl phosphate (ZP) as an anti-wear additive: comparison with ZDDP. Tribol Lett. 2011;44:19. https://doi.org/10.1007/s11249-011-9822-6.
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Supplementary Materials Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy
N. N. Gosvami1,‡,*, I. Lahouij†, J. Ma2,#, and R. W. Carpick1,* 1
Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA.
2
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA ‡
†
Present Address: Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Present Address: MINES ParisTech, PSL – Research University, Centre de mise en forme des matériaux (CEMEF), CNRS UMR 7635, CS 10207 rue Claude Daunesse, 06904, Sophia Antipolis Cedex, France #
Present Address: W. L. Gore and Associates, 1202 Sheldon Drive, Newark, DE, 19711 *[email protected], *[email protected]
Figure S1: AFM topography of the alumina probe (acquired using a sharp probe in contact mode) used for experiments performed on pure Al at 110 ˚C at 9.5 µN load. The image shows build-up of material on the surface. However, the curvature of the probe is intact, i.e., no flattening of the probe due to wear, in contrast to our observations for steel microspheres sliding on steel [1]. We attribute this to wear of the much softer Al (which accompanies the plastic deformation we observe), which resulted in transfer of material to the probe surface. As shown in the Fig. S2, no such deposit is seen on an alumina probe when slid against a sapphire sample. As well, in our previous experiments (steel microsphere on steel substrate), we did not observe any tribofilm build up on probe surface in absence of the substrate wear [1]. We proposed that the lack of tribofilm build up could be due to continuous sliding of the probe, which inhibits the adsorption of reactive species on the surface of the probe as needed to form a stable tribofilm, and promotes shear-induced removal of any nascent deposited tribofilm. 26
Figure S2: AFM topography of the alumina probe (acquired using a sharp probe in contact mode) used for experiments performed on sapphire at 110 ˚C at 15.7 µN load. The image does not show any build up of material on the probe surface, and the curvature of the probe is intact, i.e., no flattening of the probe surface due to wear. This is in contrast to our experiments using a steel microcolloid sliding against a steel substrate in a ZDDP-containing oil, where wear of the probe is evident by a clear flattening of the end of the probe [1].
Movie S1: In situ imaging for an alumina probe sliding against pure Al in a lubricant containing ZDDP at room temperature. Movie S2: In situ imaging for an alumina probe sliding against pure Al in a lubricant containing ZDDP at 115±5 ˚C Movie S3: In situ imaging for an alumina probe sliding against sapphire in a lubricant containing ZDDP at 115±5 ˚C
References for the Supplementary Materials:
[1] Gosvami NN, Ma J, Carpick RW. An In Situ Method for Simultaneous Friction Measurements and Imaging of Interfacial Tribochemical Film Growth in Lubricated Contacts. Tribol Lett 2018;66:154. https://doi.org/10.1007/s11249-018-1112-0
27
Highlights • • • • •
Tribology of lightweight metals is critical for reducing wear and friction. ZDDP additives form protective antiwear tribofilms at sliding interfaces. However, ZDDP tribofilm growth on non-ferrous metals is not well established. We study ZDDP-derived tribofilms in situ at the nanoscale. We show tribofilm growth on Al-based materials via stress and thermal activation.
Nitya Nand Gosvami: Conceptualization, AFM Experiments, WritingOriginal draft preparation. Imène Lahouij: FIB/TEM Experiments and Data Analysis, Reviewing and Editing Manuscript: Jaron Ma: AFM Experiment, Software, Data Analysis. Robert Carpick: Supervision, Writing- Reviewing and Editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0301-679X(19)30589-4
DOI:
https://doi.org/10.1016/j.triboint.2019.106075
Reference:
JTRI 106075
To appear in:
Tribology International
Received Date: 19 August 2019 Revised Date:
9 November 2019
Accepted Date: 14 November 2019
Please cite this article as: Gosvami NN, Lahouij I, Ma J, Carpick RW, Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy, Tribology International (2019), doi: https://doi.org/10.1016/j.triboint.2019.106075. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy N. N. Gosvami1,‡,*, I. Lahouij†, J. Ma2,#, and R. W. Carpick1,* 1
Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA.
2
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA ‡
†
Present Address: Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Present Address: MINES ParisTech, PSL – Research University, Centre de mise en forme des matériaux (CEMEF), CNRS UMR 7635, CS 10207 rue Claude Daunesse, 06904, Sophia Antipolis Cedex, France #
Present Address: W. L. Gore and Associates, 1202 Sheldon Drive, Newark, DE, 19711 *[email protected], *[email protected]
Abstract Zinc dialkyldithiophosphate (ZDDP)-based antiwear additives are crucial in automotive lubricants, where its effectiveness in reducing wear of ferrous alloys is well established. However, prior studies of light-weight aluminum-based alloys reveal that ZDDP is not as effective an anti-wear agent on Al-based surfaces for reasons that remain under debate. Here we use in situ atomic force microscopy (AFM) to study nanoscale ZDDP-derived tribofilms at the sliding interface between an alumina microcolloid probe and substrates comprised of either aluminum (with native oxide) or aluminum oxide (single crystal sapphire). The experiments reveal that ZDDP tribofilms form on both substrates, supporting the model of tribofilm formation as a thermally-activated, stress-assisted process that does not require cation exchange from wear debris originating from the substrate.
Keywords: zinc dialkyldithiophosphates (ZDDP), boundary lubrication, antiwear tribofilms, atomic force microscopy 1
1. Introduction Zinc dialkyldithiophosphates (ZDDPs), one of the most successful and widely used antiwear additives, are used in a wide range of automotive engine lubricants and also for antiwear protection of gears, bearings, and other engineering components [1]. ZDDP reduces the wear of surfaces in relative motion in the boundary regime by transforming into a surface-bound tribochemical film. Owing to the complexity of the buried, multi-asperity contacting interfaces intrinsic to macroscale contacts, the mechanism(s) by which ZDDP reacts at the sliding interface has remained poorly understood for decades, although many advances have been made [1]. A recent in situ single-asperity approach using an atomic force microscope (AFM) showed that the growth of ZDDP antiwear tribofilms is a tribochemical reaction which can be described by Arrhenius-type reaction kinetics where the growth rate increases exponentially with temperature and contact pressure [2]. This work provided experimental confirmation of stress-assisted growth of zinc phosphate-derived films predicted by atomistic simulations [3, 4]. Subsequent in situ work confirmed these observations with various types of ZDDPs at the nano [5] and microscales [6], and recent macroscale work provided compelling support for the idea that shear stresses, in particular, are critical for the tribofilm formation [7]. Several macroscale studies have explored the growth of ZDDP tribofilms on a variety of material surfaces [1, 8-15]. One motivation behind such studies is to enable the replacement of steel engine components with lightweight, non-ferrous alloys, e.g., aluminum or magnesium alloys, which can improve energy efficiency substantially. However, due to the complex nature of the interface in macroscale tests, questions remain regarding the nature of ZDDP interactions with such materials. Particularly for aluminum-silicon (Al-Si) alloys, which have been studied the most, several researchers have reported that robust tribofilms do not form on the softer Al
2
matrix, but robust antiwear pad-like structures do form on the surfaces of the harder silicon inclusions, similar to the tribofilms observed on steel surfaces [16-19]. It is not clear why robust tribofilm formation is apparently inhibited on Al surfaces, which warrants performing further investigation of ZDDP tribofilm growth on Al-based substrates. There are multiple theoretical mechanisms proposed to explain the growth of ZDDP antiwear films preferentially on ferrous substrates, e.g., steels. One mechanism proposed by Mosey et al. is based on molecular dynamics (MD) simulations, which revealed that antiwear film grows via contact-pressure induced cross-linking of thermal decomposition products of zinc phosphates [3, 4]. Al, being a soft metal, is unable to withstand the high contact pressures typically required for the cross-linking. Instead, it plastically yields and wear (e.g., by plowing) occurs during sliding, preventing tribofilm growth. Another model is based on the Hard and Soft Acid-Base (HSAB) theory, which predicts favorable cationic exchange occurs between Fe+3 (from Fe-containing substrates) and zinc phosphates, which are initially weakly adsorbed in the form of a thermal film [20, 21]. The cationic exchange involves chemical digestion of Fe-oxide wear debris (e.g., in the form of nanoparticles) in the zinc phosphate matrix, which results in a robust interconnected phosphate glass. The HSAB model was recently combined with MD simulations, which showed that tribofilm growth is favorable on certain non-ferrous materials including silicon due to favorable chemical digestion of silica particles, but alumina particles cannot be easily digested [17]. Both mechanisms are consistent with the lack of observation of ZDDP-derived tribofilms on aluminum, and thus it is not clear which mechanism, or both, are at play. To address these open questions and to validate the theoretical predictions, we use the recently established in situ tribofilm growth method to explore ZDDP-derived tribofilm growth
3
[2, 6], and in particular, how the substrate material plays a role. Studies were performed on mirror polished ultrapure Al (99.99% purity) substrates, and on atomically smooth sapphire substrates, in both-cases using wear-resistant alumina microspherical probes.
2. Materials and Methods The 99.99% pure Al substrates (Laurand Associates Inc., Great Neck, NY, USA) were electro-polished to mirror finish using the method described in ASTM E1558-09 (2014) [22] prior to all the AFM experiments. Sapphire single crystal c-plane wafer substrates (MTI Corp., Richmond, CA, USA) with RMS surface roughness of ~0.35 nm were used after solvent cleaning with isopropyl alcohol and ethanol. The procedures for AFM probe preparation and AFM tribofilm experiments are described in detail in [6]. We briefly summarize these procedures below and point out the few specific differences in this study. Commercial silicon cantilevers (PPP-NCH, Nanosensors, Neuchatel, Switzerland) used in these measurements were first calibrated for their normal and lateral spring constants using the Sader method [23]. The cantilever deflection sensitivities along the normal direction were obtained by taking force curves on the Al substrate surface, whereas lateral force sensitivities were obtained from the static friction vs. distance curves [6]. The probes were prepared in a similar manner as those in [6], but instead of steel microspheres, we glued alphaalumina microspheres (Inframat Corp., Manchester, CT, USA) on calibrated cantilevers using an inverted optical microscope (Olympus BX51) by first applying a small quantity of a two-part epoxy (J-B Weld, Sulphur Springs, TX, USA) using a sharp tungsten wire (TGW0325, World Precision Instruments, Sarasota, FL, USA) and then placing the microcolloid using a strand of hair. The cantilevers were stored overnight in a dry, N2-purged box to allow the epoxy to cure 4
fully. The diameters of the microspheres attached to the cantilevers were estimated either from optical imaging (Olympus BX51) or using field emission scanning electron microscopy (SEM, JOEL 7500F). The microspheres vary in diameter; in this study, the microsphere used for measurements on the Al sample had a diameter of 39 µm, and the one used for measurements on the sapphire sample had a diameter of 35 µm. The microspheres have a RMS roughness of ca. 85 nm (measured by AFM over an area of 15x15 µm2 after removing the background curvature of the sphere). The normal and lateral spring constants were corrected after gluing the microcolloid, which results in a variation in the effective length of the cantilever (distance between the base of the cantilever to the center of the glued microcolloid) as well as the tip height. All the experiments were performed using a commercial atomic force microscope (Keysight 5500, Keysight Technologies, Santa Clara CA, USA), equipped with a sample heating plate, a liquid cell, and a temperature controller (Model 321, Lake Shore Cryotronics Inc., Westerville, OH, USA), schematic shown in Figure 1. For the experiments in this study, a mixture of 99 wt% base oil (SpectraSyn polyalphaolefin (PAO) 4 cSt, ExxonMobil, Houston, TX, USA) with 1 wt% zinc dialkyldithiophosphate (ZDDP) antiwear additive (HiTEC 1656, a mixture of primary and secondary ZDDP, Afton Chemical Corp., Richmond, VA, USA) was used for all the experiments. This is consistent with the typical amount of ZDDP used in commercial engine oils which corresponds to approximately 800 ppm P as limited by current regulations [1], and is the same or close to levels used in multiple previous studies[5, 7, 18, 19] including our own [2, 6]. Data analysis was performed using WSxM 5.0 software [24] and using custom MATLAB routines. To characterize the Al contact interface, a transmission electron microscopy (TEM) lamella was fabricated by the focused ion beam (FIB) lift-out technique [25] using a dual-beam
5
FIB (FEI Strata DB235). To limit the extent of ion beam damage, the surface was first protected with electron beam induced Pt deposition and subsequently coated with a 2 µm ion beam induced Pt protective layer. For final polishing of the lamella, a 5 kV ion beam was used at 2° incidence to the lamella surfaces. The sample was then imaged with a TEM/STEM (JEOL 2010F, JEOL Ltd., Tokyo, Japan) operating with 200 kV accelerating voltage and equipped with Energy Dispersive Spectroscopy (EDS).
3. Results and Discussions To investigate ZDDP-derived tribofilm growth on non-ferrous substrates (pure Al or sapphire), sliding against a non-ferrous counter surface (alumina microspherical probe), we used similar sliding conditions which had been used for steel on steel contacts in our previous study using the recently reported AFM based approach (temperature 115±5 ˚C, mean Hertzian contact pressure of ~0.2-0.5 GPa, sliding distance of 10-15 µm per stroke, and sliding speed of ~200 µm/s) [6]. The temperature chosen is one at which ZDDP tribofilm growth is readily observed both in macroscopic studies [1] and our own previous nanoscale studies [2, 6]. Note that the roughness of the probe will create local asperity contacts within the nominal contact zone with contact pressures well in excess of the nominal pressure. Similar to the observations for steel-steel tribopairs at the microscale [2], no tribofilms form at room temperature under similar experimental conditions. This is consistent with the fact that, in general, the growth of tribofilms from ZDDP is not observed at room temperature [1]. However, in contrast to the observations on steel substrates, the pure Al substrate undergoes severe wear during sliding for extended sliding periods (Movie S1). A zoomed out topographic AFM image of the worn surface after one such experiments is shown in Figure 2(a), which
6
reveals a large depression within the sliding zone. This is due to some degree of wear, which we suggest further below is potentially attributable to plastic deformation in the form of an indented sliding track. The simultaneously recorded friction coefficient values were high (above 0.9) and showed large fluctuations due to continuous wear (Figure 2(b)). Consistent results were seen in extended tests out to 1000 cycles, and the behavior was reproducible. This behavior was reproduced in a replicate experiment. Similar experiments were then repeated, but at elevated temperatures (115±5 ºC) (Figure 3). During the initial stage of sliding, the Al substrate again starts to exhibit wear in the form of plastic compression, and significant fluctuations in the friction force are observed. No clear signature of tribofilm growth is seen during this phase. This behavior was reproduced in a replicate experiment. The causes of the rapid and the slower fluctuations seen in friction during this initial sliding phase are unknown, but could be due to multiple factors including material transfer to the tip; evolution of the contact area during sliding as the substrate plastically deforms; random and sudden plastic yield events; and nascent tribofilm nucleation events. Notably, large fluctuations in friction have been observed in the macroscopic sliding of high purity Al and associated with plastic modification of the Al surface [26]. This wear behavior continues to occur with increasing sliding time, as evident from the continuous change in surface topographic features (Movie S2). However, the deformation process slows down and the coefficient of friction is significantly reduced after 500-600 sliding cycles, at which point nucleation and subsequent growth of the tribofilm is observed. We note that some of the image frames (e.g. apparent in Movie S2) show vertically aligned bands. Such features are well-known to be due to the mechanical instabilities of the AFM piezo scanner
7
and/or the cantilever which are excited at the turnaround points at the left and right sides of the image when scanning at high rates, as we do here [27]. These cause fluctuations in the applied normal force across the scan, and thus can influence subsequent surface wear and/or tribofilm growth. In particular, before tribofilm growth starts, the bands are most pronounced, potentially due to the strong adhesive and frictional interactions between the tip and the bare substrate. Once the tribofilm starts to grow, these bands are greatly diminished. Figure 3(a) shows the zoomed out topography image of the sliding zone, revealing the characteristic morphology of the tribofilm, which is very similar to the rough, pad-like morphology of films produced via a steel-steel tribopair involving a microscale colloidal AFM probe [6], and similar to the pad-like morphology seen in tribofilms grown with nanoscale AFM probes [2, 5] and in macroscale tribometry experiments [1, 7]. This topography is clearly distinct from the morphology changes seen just due to wear of Al without tribofilm growth. As discussed further below, chemical analysis further establishes that a ZDDP-derived tribofilm has formed. Figure 3(b) shows the simultaneously recorded friction coefficient data, where fluctuations are observed up to ~600 cycles. However, once the tribofilm starts to grow after ~600 cycles (as evident in the distinctive topography seen in Movie S2), these fluctuations are significantly reduced. The fluctuations in friction may be attributed to significant wear occurring in the first ~600 cycles. Small-scale wear of ductile material like Al can consist of a large combination of plastic pop-in events due to random nucleation of dislocations and/or the encountering of random pre-existing dislocations in the sliding zone [28, 29], leading to significant fluctuations in the local topography, and thus the local friction force. The reduction in wear once the tribofilm starts to grow is thus correlated with reduced fluctuations in friction.
8
The mechanism for the delayed onset of tribofilm formation is not certain, but one possibility is that the plastic deformation leads to work-hardening, which is intrinsic for pure Al [30] including during indentation [29] and sliding contact [26]. It is indeed likely that plastic deformation is occurring in the first few hundred cycles. A simple estimate for the experiment in Fig. 2 using the Greenwood-Williamson model of rough elastic contact of flat surfaces with one having the same roughness as our probe predicts a nearly fully plastic contact (we assumed, to consider a limiting case, a yield stress of pure Al of 570 MPa as this is at the upper end of reported values for pure Al after heat treatment [31]). This is by the application of compressive normal stress alone; the additional application of shear during sliding will also contribute to plastic deformation. As it plastically deforms, the expected work hardening then is hypothesized to occur until the constant applied contact pressure is no longer sufficient to cause further plastic deformation. Thus, the plastic deformation-induced wear diminishes or ceases, and increased compressive contact stress can be stably supported by the substrate. This increased compressive contact stress will also increase the shear stresses in the contact. Since ZDDP tribofilm growth is accelerated exponentially by increased contact stresses [2, 6, 7], these higher stresses should induce the nucleation and subsequent growth of a robust ZDDP tribochemical film. In other words, we are hypothesizing that contact stresses that are sufficiently high to drive rapid tribofilm growth could not be initially achieved on the soft Al substrate; once the Al is workhardened due to repeated compressive plastic deformation, higher contact stresses could be achieved. This idea is directly based on the observation of compressive stress thresholds for nucleating crosslinking of polyphosphate chains in atomistic simulations of zinc phosphates [3, 4], the observation of a clear shear stress-dependence for the formation of ZDDP tribofilms in macroscopic tribometry experiments [7], and our prior observation of an exponential dependence
9
of ZDDP-derived tribofilm growth on compressive contact stress [2]. Notably, the atomistic studies predicted that, without work hardening, the yield stress of pure Al is too low to support tribofilm formation [3]. Our observations are consistent with these simulation results.
We then conducted similar experiments on a flat sapphire substrate, which is significantly harder (more than 3 times harder than pure Al), and therefore more wear-resistant than Al, while allowing the same experiments without significantly affecting the surface chemistry (i.e., the sapphire surface chemistry is similar to that of the Al sample, whose surface consists of a thin aluminum oxide layer). Figure 4 shows the representative results of experiments conducted using the sapphire substrate but conducted in otherwise the same manner as the experiments whose results are shown in Figure 3. Figure 4(a) shows the zoomed-out AFM topography image of the sliding zone after ~800 cycles. Unlike the case of the Al substrates, no wear was observed on the sapphire (i.e., the AFM images of the sliding zone revealed flat, uniform, unchanging topography). Correspondingly, we did not observe large fluctuations in the measured friction coefficient (Figure 4(b)), unlike the fluctuations initially observed on the pure Al sample. This supports our earlier suggestion that, when the substrate is undergoing plastic deformation, the irregularity and randomness of the process lead to large fluctuations in friction. Continued sliding on the sapphire substrate resulted in clear nucleation and growth of a tribofilm (Movie S3). For the first several hundred cycles (~300) no tribofilm growth was detected; after this period, several tribofilm nucleation sites (localized raised regions) were observed at multiple points within the sliding zone. These continued to grow with further sliding, leading to the formation of a stable tribofilm across the entire sliding zone. The nucleation and growth process of the tribofilm, as well as its morphology, is similar to the nanoscale
10
experiments conducted on steel [6]. It is important to note that in all the experiments, no clear evidence of probe wear was observed (Figure S1 & S2). This indicates that the alumina probes are rather durable for such tests. In contrast, steel microcolloid probes showed some degree of wear, as reported in our previous study [6]. In addition, we do observe build up of material on the alumina probe used for the experiments on Al (Figure S1). We attribute this to transferred Al from the substrate which accompanies the plastic wear we observe. This may account for the different friction coefficient observed for the experiments on the Al substrate. We note that friction will also be affected by the overall probe size, the evolving roughness of the surface of the sample, and the fine scale roughness of the probe. We thus focus on the overall trends in tribofilm growth; a study of specific factors controlling friction will be the subject of a future effort. An ideal way to validate our hypothesis that strain hardening of the Al occurred to allow tribofilm growth would be through nanoindentation experiments. Unfortunately, the need to isolate the plastically-indented regions from AFM experiments and then conduct localized nanoindentation experiments proved challenging, particularly due to the fact that the surfaces were roughened appreciably after the wear and the tribofilm growth. While this will be pursued in future studies, we did further explore the idea that work-hardening assisted the tribofilm growth on pure Al by conducting SEM and cross-sectional TEM imaging of the sample and tribofilm [32]. Figure 5 shows SEM and TEM images of the same region of the Al substrate that was probed and the results reported in Figure 3. Figure 5(a) shows an oblique-angle SEM image of the sliding zone which shows a clear depression with an oval perimeter due to the surface wear. A lamella was sectioned from this sample region using focused ion beam (FIB) to conduct cross-sectional imaging of the surface, as shown in Figure 5(b). The magnified image of the
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lamella, acquired using TEM, is shown in Figure 5(c). This image was acquired within the central region of the sliding zone where significant wear had been observed. As clearly seen, the Al surface shows significant roughening near the surface due to wear. In addition, subsurface regions with darker contrast are clearly observed. The darkened contrast is typical of plastically deformed metals with a high density of dislocations within the grains located under the sliding interface [33] and extends several hundred nanometers deep. On top of these, a tribofilm is seen with thickness in this region of approximately 60 nm. The interface between the tribofilm and the Al undulates somewhat, but is observed to exhibit an abrupt change in contrast, within the image resolution of ca. 5 nm. While there may well be Al wear debris in the film, the tribofilm itself can be clearly delineated from the substrate. Figure 5(d) shows a similar magnified image acquired using TEM, but away from the central region, i.e., still within the contact region but closer to the peripheral region where the contact stresses experienced by the Al sample would have been lower. The tribofilm thickness in this region is significantly smaller (approximately 35 nm). In addition, the regions with darker contrast underneath the surface are smaller than those in Figure 5(c), extending only ~100 nm deep. Although qualitative, the cross-sectional TEM images indicate a correlation between tribofilm thickness and the extent of work hardening attributable to the contrast change expected from subsurface plastic deformation. The contention that work hardening occurs is also consistent with the well-established work-hardening behavior of Al subject to indentation and sliding [26, 28, 29]. Scanning mode TEM (STEM) was then used to conduct energy dispersive spectroscopy (EDS) to perform chemical analysis of the TEM lamella. Twenty EDS spectra were recorded across the tribofilm at different locations of the lamella to obtain direct evidence of the formation of a ZDDP-derived tribofilm. Figure 6 illustrates representative EDS spectra recorded from the
12
Al substrate, the tribofilm-substrate interface, and the tribofilm itself. As expected, the substrate is composed overwhelmingly of aluminum (Al), while the interface is rich in Al and O, as expected for the oxide that will be present near the original free surface; it could also indicate that additional surface oxidation occurred prior to the tribofilm formation as a result of the wear process observed initially. The EDS spectrum of the tribofilm shows the presence of Zn, P, and S, which are attributed to the ZDDP-derived tribofilm formed by sliding. This supports our claim that the observed structures in Figures 3(a) and, correspondingly, 4(a), contain the expected elemental constituents of ZDDP-derived tribofilms. These regions’ morphologies are very similar to those of ZDDP-derived tribofilms reported elsewhere [2, 5-7, 34], further supporting our contention that the observed regions are ZDDP-derived tribofilms. As seen in Figure 6, Cu and Ga peaks appear in the three spectra recorded across the TEM lamella. Cu element is due to the Cu TEM grid on which the sample was mounted. The Ga is contamination that occurred during Ga+ focused ion beam milling (FIB) process. In addition, peaks corresponding to Pt were observed from the spectra recorded within the upper layer of the tribofilm (not shown here), close to the Pt layer (an area roughly marked with dashed blue lines in Figure 5c). This is a common effect due to Ga+ ion etching of the protective Pt film that was deposited on top of the tribofilm, causing changes in the structure and composition of the upper 20 nm of the tribofilm. We note that substantial Al and O peaks appear in the tribofilm spectrum, suggesting the embedding of wear debris generated during the sliding experiment into the tribofilm. This is reminiscent of iron oxide particles observed embedded within the tribofilms grown on steel [35], attributed to substrate wear processes during sliding. Notably, the content of P and S in the tribofilm, based on comparing peak heights in EDS spectra, is lower than that observed for
13
ZDDP-derived tribofilms grown on ferrous substrates [2, 35]. Further study is needed to reliably quantify this difference and explore its origins. Growth of robust tribofilms of an alumina probe sliding on the Al and on the sapphire substrates, with similar morphology and growth properties as the films observed on steel substrates, clearly indicates that presence of iron is not necessary for the growth of ZDDPderived antiwear tribochemical films. Previous studies based on the hard and soft acid-based (HSAB) model [20], and supported by reactive MD simulations [17, 21], predict that such tribofilms would not grow at Al/alumina or alumina/alumina interfaces because the exchange of Zn with Al is not favored. The HSAB model is based on cationic exchange between the wear debris of substrates containing Fe or other elements whose exchange with Zn is favored in the formation of zinc polyphosphates. This exchange then results in digestion of the wear debris in the zinc phosphate matrix, which is favored thermodynamically due to the increase in configurational entropy, leading to the growth of the protective antiwear tribofilm. Such cation exchange may well occur in tribofilm growth processes where such cations are present, such as Fe3+, Si4+, and others. Such processes will certainly affect the growth, morphology, and tribological properties of the tribofilms. However, our results demonstrate that such cation exchange is not needed to produce tribofilms in the first place.
4. Conclusions In summary, we report the first results where ZDDP antiwear tribochemical film growth has been monitored in situ between two non-ferrous sliding surfaces, namely, an alumina probe sliding against pure Al and against sapphire. Our results reveal that tribofilm growth occurs on pure Al as well as on sapphire substrates for an alumina probe sliding in ZDDP-containing base
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oil at 115±5°C. In addition, the pure Al substrate undergoes substantial wear in the form of compressive plastic deformation as soon as sliding begins, whereas no such damage is observed on the sapphire substrate. Stable sliding on Al is subsequently established, potentially due to the eventual strain hardening of the Al, and the growth of ZDDP-derived tribofilms with the typical pad-like morphology occurs. On sapphire, no wear is observed prior to the onset of tribofilm growth. For both Al and sapphire, the tribofilm morphology is similar to that observed on steel substrates. The results are consistent with previous results on steel-steel tribopairs whereby stress, as well as elevated temperatures are required for the tribofilms to grow. These results are consistent with the original predictions of Mosey et al. [3, 4] whose modeling results showed that zinc phosphate molecules will crosslink with sufficient applied pressure. While cation exchange may occur within tribofilms formed on surfaces containing species with favorable interactions according to the HSAB principle, this work shows that such processes are not required to form tribofilms; rather, this work suggests that sufficiently hard substrates that can enable sufficiently high contact stresses at asperity contacts are the prerequisite for tribofilm formation. Further work is required to determine how the composition and properties of these tribofilms differ from those formed on steel or other substrates.
Acknowledgements This work was supported by the National Science Foundation under grant CMMI1728360, by the University of Pennsylvania through the School of Engineering and Applied Sciences, and by the Vagelos Integrated Program in Energy Research (VIPER). This work was
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carried out in part at the Singh Center for Nanotechnology, which is supported by the NSF National Nanotechnology Coordinated Infrastructure Program under grant NNCI-1542153. NNG would also like to acknowledge SERB (ECR/2016/001014) for financial support.
16
Figures
Figure 1: Schematic of the colloid probe AFM used for the sliding tests.
17
(a)
(b) Figure 2: (a) Zoomed out contact mode AFM topography of the sliding zone on a pure Al substrate after 500 cycles. Consistent results were seen when testing up to 1000 cycles under the same conditions. Experiments were performed at room temperature in a ZDDP-containing lubricant. No tribofilm is seen; only a plastically deformed region of the Al substrate is observed. (b) Coefficient of friction measured during the same sliding test using an applied load of 8.7 µN (corresponding to mean Hertzian stress of 0.17 GPa; stresses at the individual asperities will be much higher as discussed in the text).
18
(a)
(b) Figure 3: (a) Zoomed out AFM topography of the sliding zone on the pure Al substrate after 2000 cycles., experiments were performed in a ZDDP-containing oil at 115±5 ˚C. (b) Coefficient of friction measured during the same sliding test using an applied load of 9.5 µN (corresponding to mean Hertzian stress of 0.18 GPa; stresses at the individual asperities will be much higher as discussed in the text). Initially, high friction which reduces, then rises again with large fluctuations eventually starts to follow a progressively stabilizing trend after 500-600 cycles, which corresponds with the onset of observable tribofilm growth (see Movie S2). The cause of the rapid and the slower fluctuations in friction during the initial sliding phase are unknown but may be due multiple factors including material transfer to the tip, evolution of the contact area during sliding as the substrate plastically deforms; random and sudden plastic yield events; and nascent tribofilm nucleation events.
19
.
(a)
(b) Figure 4: (a) Zoomed out contact mode AFM topography of the sliding zone on the sapphire substrate after 794 cycles. Experiments were performed in a ZDDP-containing oil at 115±5 ˚C. (b) Coefficient of friction measured during the same sliding test using an applied load of 15.7 µN (corresponding to mean Hertzian stress of 0.48 GPa; stresses at the individual asperities will be much higher as discussed in the text). Overall, the rapid fluctuations in friction are not as large as those seen on the Al substrate. The cause of the significant changes around 300-350 cycles is likely due to the nucleation and growth of the tribofilms. They could also be due to changes in the tip shape and/or material transfer or other factors that are difficult to control.
20
Figure 5: (a) SEM image of the Al substrate, clearly revealing the worn region where sliding tests were performed. (b) SEM image of a lamella prepared using FIB sectioning across the sliding zone to examine the cross-section of the surface and subsurface regions, (c) TEM image of the central region of the lamella showing the cross-section where tribofilm, as well as subsurface region can be identified. Darker contrast within the grains underneath the sliding zone indicates significant increase in dislocation density (work hardening) and correspondingly thicker ZDDP tribofilm formed within the sliding zone. The blue dashed lines roughly mark the Pt contaminated layer of the tribofilm. (d) TEM imaging revealing thinner tribofilm away from the severely worn region and grains do not exhibit darker contrast indicating absence of work hardening.
21
Figure 6: EDS spectra recorded as shown in the STEM micrograph (inset) within (1) the substrate, (2) the interface and (3) the tribofilm. Spectrum 3 reveals the presence of Zn, P, and S derived from the ZDDP tribofilm. Al and O are also seen, as expected from the Al substrate; some of the O may be from the tribofilm. C is from contamination and hydrocarbons in the residual oil as well as hydrocarbons in the tribofilm. The Cu peak is due to the Cu TEM grid on which the sample was mounted, and Ga is from the FIB milling. 22
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Supplementary Materials Nanoscale in situ study of ZDDP tribofilm growth at aluminum-based interfaces using atomic force microscopy
N. N. Gosvami1,‡,*, I. Lahouij†, J. Ma2,#, and R. W. Carpick1,* 1
Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA.
2
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA ‡
†
Present Address: Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
Present Address: MINES ParisTech, PSL – Research University, Centre de mise en forme des matériaux (CEMEF), CNRS UMR 7635, CS 10207 rue Claude Daunesse, 06904, Sophia Antipolis Cedex, France #
Present Address: W. L. Gore and Associates, 1202 Sheldon Drive, Newark, DE, 19711 *[email protected], *[email protected]
Figure S1: AFM topography of the alumina probe (acquired using a sharp probe in contact mode) used for experiments performed on pure Al at 110 ˚C at 9.5 µN load. The image shows build-up of material on the surface. However, the curvature of the probe is intact, i.e., no flattening of the probe due to wear, in contrast to our observations for steel microspheres sliding on steel [1]. We attribute this to wear of the much softer Al (which accompanies the plastic deformation we observe), which resulted in transfer of material to the probe surface. As shown in the Fig. S2, no such deposit is seen on an alumina probe when slid against a sapphire sample. As well, in our previous experiments (steel microsphere on steel substrate), we did not observe any tribofilm build up on probe surface in absence of the substrate wear [1]. We proposed that the lack of tribofilm build up could be due to continuous sliding of the probe, which inhibits the adsorption of reactive species on the surface of the probe as needed to form a stable tribofilm, and promotes shear-induced removal of any nascent deposited tribofilm. 26
Figure S2: AFM topography of the alumina probe (acquired using a sharp probe in contact mode) used for experiments performed on sapphire at 110 ˚C at 15.7 µN load. The image does not show any build up of material on the probe surface, and the curvature of the probe is intact, i.e., no flattening of the probe surface due to wear. This is in contrast to our experiments using a steel microcolloid sliding against a steel substrate in a ZDDP-containing oil, where wear of the probe is evident by a clear flattening of the end of the probe [1].
Movie S1: In situ imaging for an alumina probe sliding against pure Al in a lubricant containing ZDDP at room temperature. Movie S2: In situ imaging for an alumina probe sliding against pure Al in a lubricant containing ZDDP at 115±5 ˚C Movie S3: In situ imaging for an alumina probe sliding against sapphire in a lubricant containing ZDDP at 115±5 ˚C
References for the Supplementary Materials:
[1] Gosvami NN, Ma J, Carpick RW. An In Situ Method for Simultaneous Friction Measurements and Imaging of Interfacial Tribochemical Film Growth in Lubricated Contacts. Tribol Lett 2018;66:154. https://doi.org/10.1007/s11249-018-1112-0
27
Highlights • • • • •
Tribology of lightweight metals is critical for reducing wear and friction. ZDDP additives form protective antiwear tribofilms at sliding interfaces. However, ZDDP tribofilm growth on non-ferrous metals is not well established. We study ZDDP-derived tribofilms in situ at the nanoscale. We show tribofilm growth on Al-based materials via stress and thermal activation.
Nitya Nand Gosvami: Conceptualization, AFM Experiments, WritingOriginal draft preparation. Imène Lahouij: FIB/TEM Experiments and Data Analysis, Reviewing and Editing Manuscript: Jaron Ma: AFM Experiment, Software, Data Analysis. Robert Carpick: Supervision, Writing- Reviewing and Editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: