Nanoscale Generation of Robust Solid Films from Liquid-Dispersed

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Surfaces, Interfaces, and Applications

Nanoscale Generation of Robust Solid Films from Liquid Dispersed Nanoparticles via In-situ Atomic Force Microscopy: Growth Kinetics and Nanomechanical Properties Harmandeep S Khare, Imène Lahouij, Andrew Jackson, Gang Feng, Zhiyun Chen, Gregory Cooper, and Robert W Carpick ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16680 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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ACS Applied Materials & Interfaces

Nanoscale Generation of Robust Solid Films from Liquid Dispersed Nanoparticles via In-situ Atomic Force Microscopy: Growth Kinetics and Nanomechanical Properties Harmandeep S. Khare†§, Imene Lahouij†‡, Andrew Jackson†, Gang Feng§, Zhiyun Chen§§, Gregory D. Cooper§§, Robert W. Carpick†* † Department

of Mechanical Engineering & Applied Mechanics University of Pennsylvania Philadelphia, PA 19104 Department of Mechanical Engineering Villanova University Villanova, PA 19085 §

Pixelligent Technologies Inc. Baltimore, MD 21224 §§

Abstract Inorganic metal-oxide nanoparticles have been identified as candidate materials for replacing zinc dialkyldithiophosphate (ZDDP) anti-wear additives in automotive lubricants due to their ability to form protective anti-wear tribofilms. Yet, the nanoscale mechanisms which control the growth and properties of these tribofilms remain unknown. Here we report on an in-situ study of the kinetics of nanoparticle tribofilm growth using colloidal-probe atomic force microscopy (AFM). We report on the nucleation and growth of anti-wear tribofilms formed with a dispersion of a novel zirconia (ZrO2) nanoparticle additive in lowviscosity oil. Tribofilms in this study are generated in-situ at the lubricated nanoscale contact of the colloidal-probe AFM, which helps elucidate the effects of localized contact parameters on tribofilm nucleation and growth. The results strongly support that ZrO2 nanoparticle tribofilms grow through a process of stress-activated tribosintering. In contrast to ZDDP-derived tribofilms, ZrO2 tribofilm growth occurs across temperatures from -25°C to 100°C. Nanomechanical properties of the resulting tribofilms are found to approach values for single crystal and conventionally-sintered macroscale structures, suggesting the potential for robust wear protection across a broader range of conditions than those where ZDDP is effective. Keywords: Nanoparticle, AFM, Additive, Anti-Wear, Lubricant * corresponding author Robert W. Carpick, Ph.D. Department of Mechanical Engineering and Applied Mechanics University of Pennsylvania Philadelphia, PA 19104 [email protected] (215) 898-4608

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current address Department of Mechanical Engineering Gonzaga University 502 E. Boone Ave., SEAS AD Box 26 Spokane, WA 99258 §

current address MINES ParisTech, PSL – Research University, CEMEF – Centre de mise en forme des matériaux, CNRS UMR 7635, CS 10207 rue Claude Daunesse, 06904, Sophia Antipolis Cedex, France ‡


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INTRODUCTION Anti-wear additives are crucial for reducing wear and preventing failure of lubricated interfaces which experience direct contact due to inadequate lubricant load support. This is a particular concern in low viscosity oils which have potential to substantially reduce viscous energy losses in vehicles and industrial machinery.1 Anti-wear additives typically undergo mechanochemical dissociation at the sliding interface to form surface-bound tribofilms, which protect the contacting surfaces from subsequent damage. The efficacy of tribofilms in providing wear resistance is generally determined by a combination of their intrinsic mechanical properties and their adhesion to the substrate. Zinc dialkyldithiophosphates (ZnDTP or ZDDP) are a class of anti-wear additives for automotive lubricants universally used due to their ability to form effective tribofilms at sliding interfaces, which act as protective layers for the underlying metal.2 This is in addition to ZDDP’s low cost and good solubility in oil. ZDDP-derived tribofilms form as a result of stress-assisted, thermally activated chemical reactions3-4 at sheared contacts.5 At room temperature, only thin and weakly-bound ZDDP-derived tribofilms are able to form, while in many instances, no tribofilms are observed at all.6-7 They bond robustly to steel surfaces, and exhibit a lower elastic modulus and hardness than typical engineering materials (e.g., steel), which allows them to sacrificially cushion applied stresses at the interface, and thereby prevent wear.3 Furthermore, ZDDP-derived tribofilms exhibit graded mechanical properties through their thickness, with an intrinsically patchy morphology.2 However, despite their importance for wear protection, thiophosphate byproducts from ZDDP combustion lead to poisoning of after-exhaust treatment systems, which results in an increase in automotive tailpipe emissions.8 More crucially, existing ZDDP technology provides inadequate wear protection in ultra-low viscosity lubricants (often formulated as polyalphaolefin and Group V blends), where the lower oil viscosity dramatically diminishes fluid film thickness, thereby increasing the likelihood of contact severity and accelerated wear.9 Increasing the ZDDP concentration for these low-viscosity lubricants is prohibited by already stringent emissions regulations which ultimately aim to eliminate sulfur and phosphorous-containing additives in oil. Moreover, despite reducing wear, ZDDP additives also tend to increase friction.2 Ultra-low viscosity lubricants are necessary for meeting increasingly aggressive automotive fuel efficiency and emissions standards.10-11 As a result, it is crucial to identify new anti-wear additives which can provide the necessary wear protection in these novel base stock blends without relying on sulfur and phosphorous compounds. Nanoparticles with diameters in the range 1-100 nm have been identified as a promising sulfur and phosphorous-free anti-wear alternative to ZDDP.12 However, several challenges remain. Metal and metaloxide nanoparticles are often unable to solubilize in non-polar lubricant base stocks, resulting in poor dispersibility and sedimentation. Additionally, suspended particles in lubricants are more susceptible to cause optical haziness and opacity in formulated lubricants. This makes fresh formulations visually 3 ACS Paragon Plus Environment


indistinguishable from aged oil, precluding facile lubricant condition monitoring which relies primarily on visual inspection of optical clarity. Nano-sized additives are preferred over their micron-sized counterparts due to their ability to entrain within microscale asperities of engineered surfaces without causing abrasive wear. However, an increase in specific surface energy for nanoscale particles make them significantly more susceptible to agglomeration and precipitation. When adequately dispersed, nanoparticles can improve wear resistance by generating a protective tribofilm, which is thought to be composed of discrete nanoparticles, nanoparticle agglomerates, or nanoparticle-reinforced tribopolymer networks.13-25 For example, Kato and Komai studied the anti-wear performance of various metal oxide nanoparticles at a dry sliding interface, and noted that oxides with relatively higher oxygen diffusion coefficients and smaller particle diameters had a greater tendency to generate surface tribofilms.26 It was suggested that these tribofilms form under shear and at room temperature due to a surface diffusion-mediated process of tribosintering. Although scaling laws predict a reduction in energetic barriers to the melting and sintering of nanoscale particles,27-28 it is unclear if tribofilms observed in the work of Kato and Komai consist of sintered nanoparticles (which would be marked by grain coarsening mediated by grain boundary migration), coalesced nanoparticles with mismatched lattice orientations, or mere clusters of irregularly-shaped nanoparticles which have undergone plastic deformation. Nevertheless, tribosintering has attracted considerable attention as a possible mechanism for describing the growth of nanoparticle tribofilms. Hernández Battez et al. showed an improvement in the extreme pressure performance of CuO, ZnO and ZrO2 nanoparticles dispersed in a polyalphaolefin (PAO6) base oil, attributing this to their ability to deposit tribofilms on the wear track. An improvement in extreme pressure performance was seen to correlate with higher bulk material hardness. It was hypothesized that tribofilms developed through a process of tribosintering and once formed, these may reduce adhesive wear and provide greater load support.15 However, neither tribosintering growth kinetics nor the reduction of adhesion were experimentally verified. A number of other macroscale experimental studies similarly report the generation of interfacial tribofilms from nanoparticle additives, and correlate this to improvements in anti-wear and extreme pressure performance. Furthermore, wear performance is often also seen to benefit from a corresponding reduction in nanoparticle size or improvements in effective material properties such as hardness, which offer some indirect and empirical insights into mechanisms of nanoparticle-derived tribofilm growth and wear protection. However, direct measurements of tribofilm growth kinetics for nanoparticles, which can provide a more definitive understanding of these mechanisms, are non-existent.


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Despite the harsh interfacial conditions of macroscopic, multi-asperity contacts (e.g., interfacial wear, plastic deformation, generation and entrainment of third bodies, tribochemical reactions.), nanoparticle additives are still able to nucleate and generate tribofilms at the sliding contact. Although favorable intrinsic material properties are clearly important for generating functional tribofilms, they provide little insight into the nature of these interactions at the contact interface. To obtain a more fundamental understanding of tribofilm growth mechanisms, direct and in-situ measurements of nanoparticle interactions with both the interface, as well as with each other are warranted. In-situ measurements of any contact phenomena are inherently challenging given the inaccessibility of the buried contact interface. A direct investigation of tribofilm growth from nanoparticle additives, in particular, is especially difficult since that nanoparticles of interest are orders of magnitude smaller than contact length-scales commonly probed with macroscale tribological instrumentation (10-1000 µm). Insitu electron microscopy provides the necessary length-scale resolution for visualizing nanoparticle interactions.29 However, this technique is not easily amenable to liquid environments native to nanoparticle additives. Liquid cell colloidal probe atomic force microscopy (AFM), on the other hand, offers a unique platform for conducting growth kinetics measurements at the 5-1000 nm contact length-scales relevant to nanoparticle additives, within their native liquid lubricant environments. Furthermore, AFM allows in-situ visualization and quantification of tribofilm topography, friction, adhesion, and mechanical properties. The ability to accurately modulate contact force, and therefore stress (derived from suitable contact mechanics models) at single- and few-asperity contacts enables direct measurements of the tribofilm growth process without introducing the confounding effects of large-scale plastic deformation and multi-asperity contact and wear. In this paper, we use colloidal probe AFM to investigate the fundamental interactions of nanoparticle additives within a buried sliding nanoscale interface, through in-situ measurements of tribofilm topography and growth kinetics. Through these measurements, we propose mechanisms for describing the nucleation and growth of anti-wear tribofilms generated from nanoparticle lubricant additives. Commercially available zirconia (ZrO2) nanoparticles that are highly monodisperse with an average diameter of 5 nm are used in this study as the nanoparticle additive system (Pixclear PC14-10-L01, Pixelligent Technologies Inc.). The nanoparticles, synthesized using an established solvothermal technique,30 are treated and capped with an organic ligand to improve their dispersion stability in oil.31 Dispersions of the capped ZrO2 nanoparticles in oil were optically clear with no observable settling of nanoparticles. Dispersions were found to maintain their stability for extended periods of time (> 4 years), which eliminated the need for mixing or stirring of these solutions at any point prior to testing. This is



unlike many other nanoparticle additives, which require frequent mechanical stirring to prevent settling. Similarly prepared ZrO2 nanoparticle dispersions in oil were shown to improve wear protection in macroscale sliding and rolling experiments, under a wide range of contact conditions.32 RESULTS AND DISCUSSION AFM sliding experiments were performed with steel microspheres (colloidal probes) mounted to conventional silicon AFM microcantilevers. The size of the colloidal steel microspheres in this study varied between 10 µm and 70 µm. Regions in AFM sliding experiments typically consisted of a nominally square, 5 × 5 µm2 scan window, where the probe was raster-scanned repeatedly at a prescribed normal load, as illustrated by the red square in Fig. 1 (a). A single raster scan of the 5 × 5 µm2 square window is referred to as a single scan cycle; typical experiments comprised of several hundred scan cycles. Tribofilm topography and friction were measured in-situ during the process of tribofilm growth. To accurately quantify tribofilm volume, periodically a larger area was imaged to include the unaffected steel surrounding the tribofilm; this provided an accurate reference for tribofilm height. Each of these ‘imaging scans’ consisted of a 10 × 10 µm2 scan window and were performed using the same colloidal probe at a slower scan speed and higher pixel resolution to improve accuracy of subsequent volume measurement. This is illustrated by the green square in Fig. 1 (a). Furthermore, to minimize damage during imaging to the already generated ZrO2 tribofilm, imaging contact stress was reduced to approximately half the contact stress applied during tribofilm growth scan cycles. 10 × 10 µm2 imaging scans were collected at the beginning of the test (i.e. at 0 cycles), and periodically during the experiment (typically at 40, 100 or 500-cycle intervals), and finally at the end of the experiment. Fig. 1 (a) shows the topographic images of the substrate at 0, 3600 and 7000 cycles, the latter two clearly showing growth of a ZrO2 tribofilm at different stages. Scanning was performed with 10 wt.% ZrO2 in PAO4, at 25°C under a nominal initial Hertzian contact stress of 183 ± 10 MPa. Evolution of the tribofilm volume on the steel surface as a function of scan cycles is shown in Fig. 1 (b) and can broadly be divided into three regimes: induction, growth, and saturation. During the induction period, no observable tribofilm nucleation or growth occurs despite the applied initial contact stress of 183 ± 10 MPa in this case. This induction period corresponds to a sliding duration of approximately 2.5 h at a speed of 195 µm/s in the AFM. As a reference, macroscale experiments conducted to generate tribofilms with similar ZrO2 nanoparticles were run at contact stresses ≤ 1 GPa, with sliding speeds of 40 mm/s and a ball diameter of 12.7 mm. With a reduction in ZrO2 concentration or an increase in base oil viscosity, a significant increase in induction period is typically observed (see Supporting Information Figs. S1 and S2). No other changes in topography (such as wear of substrate) could be seen during this period, although a decrease in friction


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is observed within the high stress sliding region (see Supporting Information Fig. S3). It is reasonable to assume this decrease in friction corresponds to the removal of surface oxides or adsorbed contaminants, which helps condition the surface prior to nucleation of ZrO2 tribofilms. Tribofilm nucleation finally occurs at approximately 3000 cycles, after which the tribofilm grows rapidly and reaches saturation volume at approximately 4500 cycles. The tribofilm growth regime between 3000 and 4500 cycles is seen to comprise of an initial growth phase which is highly non-linear, Fig. 1 (b, inset), followed by linear growth until 4500 cycles. A movie showing all images in a similar series of tribofilm growth measurements is available in the Supporting Information Movie S1. We hypothesize that the difference in growth rates between these two phases reflect the nature of interactions experienced by ZrO2 nanoparticles. The initial phase of growth is dominated by the interactions between the steel surface and the nanoparticles which first arrive at the interface. After nucleation, growth is driven predominantly by the interactions of newly arriving nanoparticles with the preexisting layer of deposited nanoparticles. The final, saturation region beyond which there is an absence of significant and measurable growth, is observed to begin at 4500 cycles. Despite the absence of significant growth, a large variation in measured tribofilm volume is observed, seen in the scatter in the data in Fig. 1 (b). This variation occurs due to a continuous competition between tribofilm growth and subsequent removal of the upper fewnm, which despite the observable instability, results in no net increase or decrease of tribofilm volume and height over longer sliding durations. Removal of the superficial layers ZrO2 tribofilm in this saturation regime was confirmed with the optical ex-situ observation of numerous ZrO2 debris situated in the periphery of the tribofilm. The typical ZrO2 tribofilm growth process therefore consists of three distinct regions: an induction period (culminating in film nucleation), a two-phase growth period, and a self-limiting saturation stage. In order to quantitatively deduce parametric variations in growth kinetics, we define the following terms: (1) the tribofilm volumetric growth rate is defined as the slope of a linear regression fit to measured volume within the linear phase of tribofilm growth (2) tribofilm nucleation is defined to occur when the measured tribofilm volume exceeds 0.02 µm3, which would represent an approximately 1 nm increase in average height across the sliding region, and (3) the saturation tribofilm volume corresponds to the average measured volume during saturation. These three regimes are annotated in Fig. 1 (b). It is worth noting that concurrent with tribofilm growth on the substrate, later-stage ZrO2 tribofilm growth is also observed on the AFM probe. It is anticipated that tribofilms on the probe will contribute to the overall uncertainty in later-stage contact stress, but do not affect reported values of initial contact stresses.



Figure 1: (a) A sequence of three topographic images of the steel substrate at 0, 3600 and 7000 cycles, showing progression of ZrO2 tribofilm growth. Scanning was performed in 10 wt.% ZrO2 in PAO4 at 25°C, at a nominal contact pressure of 183 ± 10 MPa. A movie showing all images from a similar growth experiment are available in Supporting Information Movie S1; (b) Volumetric growth and corresponding mean tribofilm height as a function of scan cycles for the tribofilm shown in (a). Tribofilm growth consists of three distinct regions: induction, growth (comprised of a non-linear and linear phases), and saturation; (c) SEM micrograph of a central region of a ZrO2 tribofilm, showing a distinct pad-like morphology; (d) EDS spectrum of a ZrO2 tribofilm overlaid on a reference 52100 steel spectrum (measured away from the tribofilm, on the same substrate). EDS spectrum of the 52100 steel shows Fe-K/L, Cr-K peaks and C- K peaks, whereas the EDS spectrum of the zirconia tribofilm shows Zr-L and O-K x-ray peaks, as well as the Fe-K/L, Cr-K, and C- K peaks.

A high-resolution scanning electron microscopy (SEM) micrograph of the central region of tribofilm from Fig. 1 (a) is shown in Fig. 1 (c). The tribofilm morphology in Fig. 1 (c) shows a series of distinct pad-like features, with a characteristic lateral length scale of 260 ± 50 nm. The Hertzian contact diameter estimated for the probe diameter and normal load applied during this test is 184 ± 25 nm. AFM topography images collected during the evolution of the ZrO2 tribofilm reveal no changes in surface morphology until 8 ACS Paragon Plus Environment

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nucleation. These images reveal that nucleation occurs with the formation of a small number of discrete tribofilm sites or pads. Over time, each pad grows significantly in thickness, and to some extent in lateral dimension, until it reaches a steady state value. The typical mean terminal thickness depends on sliding conditions, ranging between 20 nm and 70 nm for normal stress varying between 170 MPa and 900 MPa, respectively, with a typical RMS roughness roughly 10% of tribofilm thickness (measured over a 5  5 µm2 area). Concurrently, more pads appear in between preexisting ones, and undergo a similar process of vertical and lateral growth. From the SEM micrograph, some evidence of directionality within these pads is also observed – pads appear elongated more strongly along the AFM fast-scan direction (left to right in the image), than along the AFM slow scan direction (top-bottom direction). In an AFM, the fast scan direction represents the direction along which shear stresses are applied within the contact, in many ways similar to linear wear tracks generated on macroscale tribometers. The transverse ‘slow scan’ direction of the raster-scanned AFM tribofilm can then be thought to consist of multiple fast scan wear tracks, offset a certain distance from each other (determined by the prescribed AFM line-step resolution). Elongated pads as well as the correlation between its characteristic size to the estimated contact diameter suggest that contact stresses directly influence this pad-like morphology. Although the exact origins of the pad-like morphology are unclear, we hypothesize these occur as part of sliding -induced instabilities, which depend on applied load (and contact diameter), interfacial friction, and sliding speed. AFM tip-induced rippling instabilities, which have some similarities to the features seen here (most notably, the existence of laterallyperiodic topographic features), have been observed for a number of other material systems.33-37 It is plausible that initial nucleation of any one pad occurs due to entrainment and entrapment of ZrO2 particles, with its progressive growth being self-propagated, in part, since the protruding (pre-existing) pad will experience greater contact stress than the neighboring pad-free zones. Sliding instabilities, if assumed to be periodic, will impose periodic variations in contact stress and perhaps in contact residence times. It is plausible that preferential nucleation and growth at pinning sites of these instabilities, compounded with the self-propagating growth of existing pads, cascade into the morphology seen in Fig. 1 (c). In such a scenario, pad connectivity in the fast scan direction would arise from tip-assisted material transport between pads in the slip direction (since stresses are applied in normal and shear directions). Furthermore, for similar lubricant-additive dispersions, the overall morphology should exhibit a dependence on sliding speed and contact size. The effects of these parameters in elucidating the origins of this pad-like morphology remain the subject of a future study. The energy-dispersive x-ray spectroscopy (EDS) spectrum of the tribofilm in Fig. 1 (a) and (c) is shown in Fig. 1 (d). The EDS spectrum of 52100 steel at an unworn location offset from the tribofilm is also overlaid in Fig. 1 (d). EDS spectra shown in Fig. 1 (d) were both collected at an accelerating voltage of 15 keV. 9 ACS Paragon Plus Environment


The EDS spectrum of the tribofilm shows C-Kα, O-Kα, Zr-Lα, Fe-Lα, and Fe-Kα peaks at 0.277 keV, 0.525 keV, 2.042 keV, 0.704 keV, and 6.403 keV, respectively. The EDS spectra of the unworn 52100 steel similarly shows C-Kα, Fe-Lα, and Fe-Kα peaks at 0.277 keV, 0.704 keV, and 6.403 keV, respectively. Both spectra also show a weak Cr-Kα peak at 5.414 keV. The absence of an O-Kα peak in the steel EDS spectrum suggests that no oxides are detected in the steel, and the O-Kα peak observed in the tribofilm spectrum is attributable to the ZrO2. Furthermore, C-Kα peaks appear in both spectra, but with a 10 % lower intensity for the tribofilm region. Some of this signal will arise from adventitious sources (likely hydrocarbon residue at the top surface of both regions), and some from the approximately 1% C in the steel. That the C signal is reduced for the ZrO2 suggests that dispersing ligands are absent in the tribofilms since their presence (nominally 10% - 20% of the weight of the nanoparticle comes from the capping ligand based on the proprietary capping chemistry of the nanoparticles) should increase the measured C intensity. This strongly indicates that the ligands are largely removed from the nanoparticle surface prior to nucleation. Iron peaks are observed in both spectra, attributed to the underlying substrate. The intensity of Fe-Kα, and especially Fe-Lα peaks in the tribofilm spectrum, are lower than corresponding peaks for the steel spectrum, since a large fraction of the interaction volume in the tribofilm EDS spectrum is devoid of iron (being comprised of ZrO2), thereby reducing the number of iron x-ray counts. It is worth noting that the tribofilm EDS spectrum is unable to distinguish iron in the underlying substrate from iron that is mechanically mixed with the ZrO2 tribofilm. The extent of mechanical mixing of steel with ZrO2, the microstructure of ZrO2 tribofilms and as well as the extent to which dispersing ligands comprise ZrO2 tribofilms can be determined from transmission electron micrographs of ion-milled tribofilm cross-sections – these results will be presented in a forthcoming manuscript. Mechanical properties (hardness and elastic modulus) of ZrO2 tribofilms and the steel substrate were characterized using the continuous stiffness measurement (CSM) nanoindentation method38 and depthcontrolled nanoindentation (also referred to as ‘single load’ indentation). To characterize the mechanical properties of a thin film, two possible artifacts must be taken into consideration: (1) a surface roughness effect at small indentation depths, resulting in an underestimation of the hardness and modulus due to the rough surface; and (2) a substrate effect at large indentation depths (beyond 20% of the film thickness), resulting in an overestimation of the hardness and modulus due to the stiffer and harder steel substrate.39 To attenuate the effect of these artifacts, a selected area of the tribofilm was imaged with the nanoindentor prior to each indentation in order to precisely select a low-roughness area to indent, as shown in Fig. 2 (a).


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Furthermore, the CSM method, which enables the measurement of the modulus and hardness as a function of indentation depth, was used to determine the appropriate range of indentation depth.

Figure 2: (a) and (b) 1 µm × 1 µm scan images of zirconia tribofilm carried out with a Berkovich tip showing a selected area (boxed region) before and after nanoindentation. (c) Hardness and (d) Young’s modulus as a function of indentation depth curves for the steel substrate and the zirconia tribofilm. Both hardness and modulus show a plateau at a depth range of 25-35 nm (grey region), indicating that this depth range is ideal for determining tribofilm properties.

Fig. 2 (c) and (d) show the CSM hardness and modulus as a function of indentation depth for the steel substrate and the ZrO2 tribofilm. For the steel substrate, both the hardness and the modulus are each nominally constant over the entire range of depths, indicating that the mechanical properties of the substrate are independent of depth, as expected. However, for the zirconia tribofilm, both the modulus and the hardness curves show a plateau in the depth range of 25-35 nm (corresponding to 14-20% of a 150 nm thick ZrO2 tribofilm), indicating that this depth range corresponds to minimized effects from surface roughness 11 ACS Paragon Plus Environment


and substrate. Hence, an increase in both hardness and modulus from 35 nm of indentation depth is attributed to the progressively increasing effect of the substrate on the measured values at increasing depths. Thus, measurements at the depth range of 25-35 nm are used to assign the mechanical properties of the zirconia tribofilm. Table 1 summarizes the hardness and modulus values obtained at this depth range after performing approximately 25 indents each on the tribofilm and steel. The 52100 steel substrate was measured to have a Young’s modulus of 217 ± 4 GPa and a hardness of 11.0 ± 0.6, which is consistent with values reported in the literature.40 The measured nanoindentation hardness value of 11.0 ± 0.6 GPa for 52100 steel used in this study is somewhat higher than reported values in the literature, which typically range from 6.6 - 9.4 GPa.40 This higher hardness is due to as-received 52100 steel substrates having been previously heat treated to harden these to a Rockwell C hardness greater than 60. As-received hardened substrates were subsequently polished and used in AFM testing. Young’s modulus of nanocrystalline monoclinic ZrO2, sintered at 1100°C with a theoretical density of 92%, has been reported to have a value of 199 ± 2 GPa in the literature.41 Single crystal monoclinic ZrO2 is known to have a measured hardness of 6.6 GPa,42 whereas sintered monoclinic zirconia has been reported to have hardness which varies between 4.1 GPa and 9.2 GPa.43-45 It should be noted that in bulk form, ZrO2 is known to exist in its monoclinic form at temperatures below 1170°C and pressures below 3 GPa.46 The ZrO2 tribofilm, generated in the AFM with ZrO2 nanoparticles known to be a combination of tetragonal and cubic phases, exhibited an average hardness of 7.3 ± 0.7 GPa and a Young’s modulus of 154 ± 5 GPa. The modulus and hardness values obtained from depth-controlled indentations carried out on tribofilms generated on silicon substrates at the macroscale are also reported Table 1. The results show that the modulus and hardness of ZrO2 tribofilms are not dependent on the nature of the substrate (steel or silicon) or the contact length-scale at which tribofilms were generated (AFM or macroscale). These results also show that ZrO2 tribofilms exhibit nominally similar hardness as the bulk, and modulus values which are nearly 77% of corresponding bulk values. Perhaps more significantly, both hardness and modulus of ZrO2 tribofilms are higher than corresponding values which have been reported in the literature for ZDDPderived tribofilms.40


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Samples

Source

Hardness (GPa)

Modulus (GPa)

52100 stainless steel

This work

11.0 ± 0.6

217 ± 4

52100 stainless steel

Literature40

8.0 ± 1.4

238.5 ± 21.5(*)

Nanoscale tribofilm on steel (generated in an AFM)

This work

7.3 ± 0.7

154 ± 5

Macroscale tribofilm on silicon (generated in a tribometer)

This work

7.9 ± 0.2

152 ± 1

6.7 ± 2.6

199 ± 2

Bulk monoclinic zirconia

Literature41-45

Table 1: Average nanoindentation Young’s modulus and hardness values obtained from approximately 25 indents, each for 52100 steel, and ZrO2 tribofilms on steel and silicon. Note that CSM method was only used for the steel and the nanoscale tribofilm while a single depth-controlled indentation mode was used to measure the mechanical properties of the macroscale tribofilm. Literature value of modulus reported for 52100 stainless steel indicated by (*) represents the reduced modulus of steel (E*), given by E* ~ E/(1-ν2), where E is the Young’s modulus and ν is Poisson’s ratio (0.3 for steel).

Measured mechanical properties of tribofilms generated in the AFM suggest that compressive and shear stresses applied at the tribological contact are sufficient for generating nanostructures whose modulus approaches at least 70% of the values of samples generated through conventional sintering, and the hardness values measured are within the range of those reported in the literature for such sintered samples. Tribosintering of ZrO2 nanoparticles could be regarded as a plausible explanation for the observed mechanical properties of ZrO2 tribofilms, despite the fact that ZrO2 tribofilms were generated at 25°C whereas ZrO2 bulk sintering occurs at temperatures in excess of 1000°C.47 Mechanical properties of sintered bulk ceramics are typically a strong function of the degree of sintering and densification.48 The driving force for sintering and the rate of densification can be described as a function of temperature, particle size, particle surface energy, and applied external pressure.48 For tribosintering to adequately describe the process of tribofilm growth, it is reasonable to expect that ZrO2 tribofilm growth kinetics would show sensitivity to applied initial contact stress and temperature as well. To test this hypothesis, ZrO2 tribofilm growth kinetics were studied as a function of stress and temperature. In order to obtain growth kinetics across a wide range of initial contact stresses, three different cantilevers of varying effective flexural stiffness were used. On each of these cantilevers, a steel colloid with nominally identical probe diameter (41.7 ± 5.8 µm) was mounted at different positions along the length of the cantilever. With a combination of the three cantilevers, the initial Hertzian contact stress could be varied between 0.1 GPa and 1 GPa. Stress-dependent growth kinetics measurements were all performed at room temperature and in a 10 wt.% dispersion of ZrO2 in PAO4. 13 ACS Paragon Plus Environment


As shown in Fig. 3 (a) and (b), an increase in the nominal Hertzian contact pressure results in a roughly linear increase in tribofilm growth rate as well as the tribofilm volume, measured at saturation. Since all tribofilms were generated across a similar area (5  5 µm2), trends in tribofilm volume are directly correlated with average tribofilm height. In contrast, an increase in initial contact stress decreases the cycles needed for nucleation (i.e. induction period) and the total cycles to reach saturation, as shown in Fig. 3 (c) and (d), respectively. Together, these trends reveal that a higher initial contact stress results in faster nucleation, faster growth, and a thicker tribofilm (average height as well as volume) which reaches saturation sooner than an equivalent tribofilm generated at lower stress. A monotonic increase in tribofilm growth rate with increasing initial contact stress does not, however, uniquely identify a process of tribosintering. An increase in tribofilm growth rate with initial contact stress is observed for tribochemical films derived from ZDDP, which rely on Arrhenius rate law kinetics, where applied stress accelerates chemical reactions responsible for tribofilm growth. For these reactions, the decomposition of the free energy of activation into the internal activation energy and constituent stress activation terms yield an exponential dependence of reaction rate, or more specifically, the growth rate, on the applied contact stress.3, 5 Sintering rate laws, on the other hand, predict a linear relationship between densification rate and an applied external pressure.49 The increase in relative density of sintered, fine-grained ZrO2 nanoparticles with increasing applied stress has also been shown experimentally to follow a linear dependence.50 The reported linear dependence of sintering rate and densification on applied stress suggests that the linear increase in ZrO2 tribofilm growth rate with applied stress can be plausibly attributed to a mechanism of tribosintering. An extrapolation of the linear regression fit to the data in Fig. 3 (a) suggests that no measurable growth would occur below a certain threshold stress value. This threshold compressive stress, defined as the xintercept of the regression fit to the data, is estimated to be 74 MPa. Although the exact value of this threshold stress was not experimentally determined, sliding at lower initial contact stresses (< 50 MPa) did in fact show an absence of measurable ZrO2 tribofilm growth within experimental time scales (> 1000 cycles). Unlike sliding, where both compressive and shear stresses act within the contact, the predominant stress-state in bulk sintering in hydrostatic. Nonetheless, a similar threshold stress is seen in bulk sintering, below which densification is unaffected by externally applied stress.50-51 For sintering of fine-grained powders, the threshold stress marks the transition above which the external driving force for sintering from applied stress is much greater than the intrinsic driving force from curvature. Skandan et al. determined the value of threshold stress for ultra-fine 6 nm ZrO2 powder to be 35 MPa, whereas for 12 nm ZrO2 powder threshold stress was 15 MPa. It is worth noting that despite differences in the state of stress between the work of Skandan et al. and tribosintered ZrO2 (i.e. presence of shear stress), the existence of a threshold stress is qualitatively consistent with the results shown in Fig. 3 (a), and further supports tribosintering as 14 ACS Paragon Plus Environment

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a plausible mechanism for ZrO2 tribofilm growth. It should be emphasized that contact stress in the tribofilm growth process does not only affect the rate of densification, and conversely, densification is not affected only by applied contact stress. Unlike a close-packed arrangement of nanoparticles in a sintering preform, dispersed nanoparticles in the AFM fluid cell have significantly greater diffusivity. In order for dispersed nanoparticles to sinter, these need to first be entrained within the contact in sufficient numbers. It is also likely that dispersing ligands and their removal will manifest in additional energetic barriers to sintering of nanoparticles. Therefore, it is pertinent to note that in order for tribofilm nucleation and growth to occur, not only does the applied stress need to be greater than the sintering stress threshold, it also needs to be sufficiently high to overcome other energetic barriers, of which removal of dispersing ligands is one example. Given that ZrO2 tribofilms exhibit a lower modulus than the underlying steel, we further hypothesize that with an increase in ZrO2 tribofilm thickness, a progressive decrease will occur in the interfacial reduced modulus, and therefore also in the interfacial contact stress. The saturation of ZrO2 growth can therefore be viewed as the point where contact stress decreases to a value which precludes further tribofilm growth, resulting in the self-limiting behavior. This may also result in a gradient in the structure and properties of the film.

Figure 3: (a) Tribofilm growth rate as a function of initial contact stress shows a nearly linear relationship; (b) Saturation tribofilm volume plotted as a function of initial contact stress. The ultimate, average tribofilm thickness increases with increasing stress; (c) Cycles to nucleation as a function of initial contact stress shows that an increase in contact stress decreases induction period; (d) cycles to reach saturation volume also decreases with increasing initial contact stress.



Despite the dependence of the induction period and subsequent growth rates for ZrO2 tribofilms on stress depicted in Fig. 3, tribofilm nucleation and growth evolution is observed to involve a similar sequence of events for all experiments, independent of applied stress or substrate. These are highlighted in Fig. 4 (a-f), which show topographic images of an incipient ZrO2 tribofilm generated with a 10 wt.% dispersion of ZrO2 in PAO4, measured at 0, 500, 600, 700, 800 and 1000 scan cycles. The topography of the unworn steel, shown in Fig. 4 (a), reveals distinct criss-crossing scratches which are the result of prior surface finishing processes. At 500 cycles, a nucleation event is observed along one of these scratches, as shown in Fig. 4 (b). Progressive sliding results in an increase in tribofilm thickness near the vicinity of this nucleation site, shown in Fig. 4 (c). It also results in the appearance of a band of newly-formed tribofilm along the fastscan track corresponding to the position of the first nucleation site, as well as other regions of tribofilm nucleation below this location, as shown in Fig. 4 (d) and (e). Over time, the ZrO2 tribofilm is seen to encompass the entire raster-scanned region. Topographic images shown in Fig. 4 (a-f) strongly suggest that surface defects (pits or scratches) are key for tribofilm nucleation. Subsequent tribofilm growth, laterally and in thickness, likely occurs due to a combination of applied stresses, tip-assisted transport (both being directed along the fast scan direction) and progressive nucleation through anchoring of ZrO2 nanoparticles at other locations.

Figure 4: Plan-view topographic images (with high contrast) for tribofilm generated on a steel surface (1.3 nm Ra) at (a) 0 cycles, (b) 500 cycles, (c) 600 cycles, (d) 700 cycles, (e) 800 cycles, (f) 1000 cycles, showing gradual lateral and vertical growth of ZrO2 tribofilm; (g) Tribofilm induction for the four tribofilms grown on rough and smooth steel and silicon surfaces (with corresponding average roughness Ra shown on the top-right) show insensitivity to substrate chemistry, but a strong correlation with surface roughness. Measured growth rates for all four conditions (rough and smooth silicon and steel) were found to be nominally identical, with a value of approximately 0.3 µm3/100 cycles.

In order to elucidate the role of surface defects, and more broadly, surface roughness, volumetric growth of ZrO2 tribofilms was evaluated on steel surfaces finished to average surface roughness (Ra) values of 8.3 ± 2.3 nm and 1.3 ± 0.1 nm. This was achieved by varying the polishing time of the steel substrate against a 16 ACS Paragon Plus Environment

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rayon polishing pad in a diamond slurry. ZrO2 tribofilm growth measurements were also conducted on silicon wafers with surface roughness values of 0.33 ± 0.03 nm (as received) and 23.8 ± 4.8 nm, at nominally similar initial contact stresses. The rough silicon surfaces were obtained by lapping as-received wafers in a similar diamond slurry, which resulted in roughening of the surface. As shown in Fig. 4 (g), the reduction in induction period is more strongly correlated with an increase in surface roughness than substrate chemistry (silicon or steel). Induction periods are lowest on silicon with an average roughness Ra of 23.8 nm, whereas no nucleation at all is observed on silicon with an Ra of 0.33 nm during the first 1000 scan cycles. In fact, an additional 800 cycles of sliding on the smooth silicon substrate still did not result in nucleation of ZrO2 tribofilms. For steel, induction period is similarly reduced with an increase in substrate roughness. Similar trends in tribofilm nucleation were observed for sliding experiments conducted with a 3 mol.% yttria-stabilized zirconia (YSZ) substrate with varying surface roughness (see Supporting Information Fig. S4 and S5). Sliding within local regions of ultra-smooth YSZ (Ra < 1nm) did not result in tribofilm nucleation, whereas ZrO2 tribofilms readily grew on regions of the YSZ substrate with surface defects (such as scratches), and rapidly on regions with appreciable surface roughness. Similar to measurements on locally smooth YSZ substrates, sliding on a highly polished sapphire substrate did not result in tribofilm nucleation (Supporting Information Fig. S6). Although rough sapphire substrates were not evaluated, tribofilm nucleation is expected to be similar to rough silicon, steel and YSZ. Surprisingly, once tribofilms did nucleate, tribofilm growth rates were found to be similar for a given initial contact stress, independent of substrate type. For Fig. 4 (g), silicon and steel substrate both exhibit growth rates of 0.3 µm3/100 cycles, independent of the substrate’s surface roughness. A growth rate of 0.21 µm3/100 cycles was measured for ZrO2 tribofilms generated on a YSZ substrate at 397 ± 40 MPa (Supporting Information Fig. S4), which are nominally similar to interpolated values for a steel-on-steel contact from Fig. 3 (a). These results reveal two key characteristics of ZrO2 tribofilm growth. First, tribofilm nucleation necessarily requires the presence of surface defects such as scratches or surface pitting and is insensitive to substrate material chemistry. This is significant, since a large number of tribofilm-forming lubricant additives show preferential growth kinetics with different substrate material chemistries.52-53 We hypothesize that locallyrough sites act as locations for stochastic particle entrapment and points of interfacial stress concentration, which accelerate tribofilm nucleation. Secondly, once ZrO2 tribofilm nucleation does occur, subsequently rates of growth are driven primarily by applied contact stresses. In order to determine the extent to which the hypothesized mechanism of ZrO2 tribosintering could be thermally accelerated or retarded, ZrO2 tribofilm growth kinetics were studied as a function of temperature. Temperature-dependence of ZrO2 tribofilm nucleation and growth is also relevant in their use as commercial additives, where lubricants are exposed to a wide range of temperatures. The ZrO2 tribofilm



induction period and growth rate as a function of temperature are as shown in Fig. 5. Growth kinetics were measured within the range of -25°C (representative of colder climates) to 100°C (approaching the operating temperature of gasoline engines). Reported temperatures were measured and calibrated at the substrate surface and two or more replicate measurements were performed at each temperature. The contact was thermally equilibrated prior to testing and test temperature sequence was randomized in order to preclude the effects of thermal history at the contact. Since force transduction in the AFM relies on an optical lever which, in this case, crosses a liquid-air interface, the photodiode sensitivity, and therefore the applied contact stress is sensitive to thermally-induced changes in oil refractive index. Measurements shown in Fig. 5 were all conducted at a nominal contact pressure of 260 ± 33 MPa; growth rates and induction periods reported in Fig. 5 are therefore in general agreement with results reported in Fig. 3, which correspond to 25°C. Across all temperature-dependent measurements, the largest deviation in contact pressure as a consequence of changes in oil refractive index were found to be within 55 MPa, which is estimated (from Fig. 3) to result in a growth rate uncertainty of 0.05 µm3/100 cycles. This uncertainty is at least an order of magnitude lower than measured growth rate uncertainty across replicate measurements at a given temperature. As a result, vertical error bars in Fig. 5 represent standard deviation across different replicate measurements and the effect of variations in contact stress is considered to have a negligible influence on temperature-dependent growth rate differences reported in Fig. 5.

Figure 5: (a) Cycles to tribofilm nucleation as a function of sliding temperature shows that induction period is generally insensitive to temperatures above 0°C, but decreases sharply at -25°C resulting in much more rapid tribofilm nucleation; (b) Tribofilm growth rate as a function of temperature shows that growth rate peaks at 15°C and reduces dramatically with a further reduction in temperature to 5°C. In general, tribofilm growth is seen at all tested temperatures and growth rates at 100°C are only marginally lower than those at -25°C.

The tribofilm induction period and growth rate are plotted as a function of interfacial temperature in Fig. 5 (a) and (b). As shown in Fig. 5 (a), at temperatures above 0°C, the induction period for ZrO2 tribofilms shows little sensitivity to interfacial temperature. However, with a decrease in temperature from 25°C to 5°C, cycles to nucleation increase roughly from 1100 cycles to 1800 cycles. With a further reduction in 18 ACS Paragon Plus Environment

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temperature to -25°C, however, the induction period is dramatically reduced. Corresponding trends in tribofilm growth rate are shown in Fig. 5 (b). For a temperature decrease from 100°C to 15°C, an increase of nearly 2 is observed in tribofilm growth rate. Growth rates similarly increase when the temperature is reduced from 5°C to -25°C, but show a sharp discontinuity between 5°C and 15°C. Nonetheless, we find that ZrO2 tribofilm nucleation and growth occurs at all temperatures between -25°C to 100°C, albeit with some variation in growth rate and induction period. This is in contrast to ZDDP-derived tribofilms, which only show appreciable growth with increasing temperatures above 25°C,3, 6 with no observable tribofilm growth below 25°C. It remains unclear why ZrO2 tribofilms exhibit non-monotonic variations in temperature-dependent growth rate and induction. A decrease in temperature results in an increase in lubricant viscosity but can also decrease the Brownian velocity of dispersed nanoparticles and can increase the residence time of nanoparticles adsorbed on to the surface of the substrate or the tip. Taken together, changes in temperature can directly affect the diffusivity of nanoparticles in fluid, rates of adsorption and desorption to and from the substrate, as well as their propensity to be entrained within the sliding contact in sufficient numbers so as to induce nucleation and growth. Furthermore, the dispersing ligands on the nanoparticle surface will also participate in these interactions, while also exhibiting their intrinsic thermodynamic response to varying temperature. The thermodynamic interactions of individual nanoparticles with the substrate are highly relevant for a fundamental understanding of nanoparticle interactions during the nucleation phase but remain outside the scope of this work. However, the isolated effects of an increase in viscosity can be studied more tractably by dispersing ZrO2 nanoparticles in higher viscosity base stock. High viscosity, ambient temperature measurements with controlled viscosity were conducted with a 10 wt.% dispersion of ZrO2 in a high viscosity synthetic base oil (mPAO SYN 65). The kinematic viscosity of SYN65 at 25°C is evaluated to be 1390 cSt,54 whereas the kinematic viscosity of PAO4 at -25°C is estimated to be 1080 cSt. Tribofilm nucleation for SYN65 at 25°C (1390 cSt) occurred at 2000 cycles (Supporting Information Fig. S2). In contrast, nucleation with PAO4 at -25°C (1080 cSt) occurs only after 400 cycles, and with PAO4 at 25°C (34 cSt), nucleation occurs between 1000-1500 cycles. Therefore, at 25°C, despite a nearly 40 higher kinematic viscosity, nucleation in SYN65 is retarded by roughly 500 cycles. In contrast, at -25°C, where the viscosity of PAO4 (1080 cSt) is comparable to the viscosity of SYN65 at 25°C (1390 cSt), tribofilms nucleate nearly 1500 cycles sooner than SYN65 at 25°C. These results indicate that the temperaturedependent variation in tribofilm nucleation shown in Fig. 5 (a) is likely to be more strongly driven by the intrinsic tribosintering mechanism or by adsorption of nanoparticles on the substrate, than the effects of changing viscosity. As in the case of threshold sintering stress, it is plausible that the dispersing ligands participate in the temperature-dependent variation in tribofilm nucleation. A weaker interface between ZrO2 19 ACS Paragon Plus Environment


nanoparticles and the dispersing ligands, say, at reduced temperature, would favor easier cleavage of these ligands, which would favor nanoparticle precipitation and accelerate tribofilm nucleation. Electrostatically and electrosterically stabilized metal-oxide suspensions are known to exhibit a strong dependence of dispersion stability to changes in temperature as well as pH and ligand structure.55-56

Figure 6: (a) During the initial induction period, although no observable tribofilm nucleation occurs, capped ZrO2 nanoparticles undergo tip-assisted transport into the contact as well as adsorption to the surface, (b) interfacial shear stresses at discrete asperities result in the removal of capping ligands from surface-adsorbed ZrO2 nanoparticles, (c) shear stresses induce local nucleation of tribofilms, resulting in subsequent growth of thick ZrO2 tribofilms, which are ultimately self-limited due to gradually reducing contact stress.

Conclusion In summary, in-situ nanoscale measurements of ZrO2 tribofilm growth kinetics reported in this study provide completely new insights into mechanisms which drive growth of nanoparticle-derived anti-wear tribofilms. The results of this study are consistent with macroscale observations of tribofilm growth from various metal-oxide nanoparticle additives.15-16, 26, 57-58 The various steps associated with ZrO2 nanoparticle entrainment, removal of capping ligands, and subsequent nucleation and growth are illustrated in Fig. 6. We find evidence which strongly suggests the claim that tribofilms from nanoparticle anti-wear additives grow via a process of stress-induced tribosintering, likely also involving the removal of the dispersing ligands that are initially attached to the nanoparticles studied in this work. Nucleation of these tribofilms is strongly associated with topographic irregularities on the sliding surface such as scratches and pits. These features likely help in nanoparticle entrapment but may also act as stress concentration sites where growth is locally accelerated. Although surface roughness drives tribofilm nucleation, once nucleated, ZrO2 tribofilm growth rates are determined primarily by the applied initial contact stresses for a given temperature. The films grow until they reach a saturation state with a fluctuating thickness which varies within 20% of a mean thickness value. The attainment of a terminal thickness as opposed to unlimited 20 ACS Paragon Plus Environment

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growth is similar to that seen for ZDDP tribofilms.2-3 Since the growth rate during the growth phase depends on contact stress, we attribute the thickness saturation to the reduced contact stress that occurs due to the lower modulus of tribofilm at sufficient thickness. This reduces the growth rate and eventually leads to nanoparticles that are insufficiently attached to withstand the ongoing contact stresses, a mechanism similar to that invoked to explain the thickness saturation seen for ZDDP tribofilms.3 We show that ZrO2 tribofilms can be generated on a range of substrate materials, which further suggests that the mechanism of ZrO2 tribofilm formation is distinctly different from additives such as ZDDP, which can show stronger affinity for certain substrate chemistries than others. We also show that ZrO2 tribofilms are able to nucleate and grow across a wide temperature range from -25°C to 100°C. This is in contrast to conventional anti-wear additives, which are unable to deposit robust anti-wear tribofilms at room temperature, with even fewer which are able to provide wear protection between -25°C and 25°C.6-7 The combination of the demonstrated properties makes ZrO2 nanoparticles a highly promising candidate as an effective anti-wear additive for use in macroscale lubricants. Prior studies on ZDDP-derived tribofilm growth kinetics have shown that AFM-generated tribofilms are characteristically similar to ZDDP-derived tribofilms generated with macroscale, bench-top test instruments. We similarly expect that the fundamental mechanisms of ZrO2 tribofilm growth, as well as their novel properties outlined in this work, characteristically represent ZrO2 tribofilms at the macroscale. METHODS Synthesis, characterization and dispersion of ZrO2 nanoparticles ZrO2 nanoparticles were purchased from Pixelligent Technologies LLC, Baltimore, MD (Pixclear PC1410-L01, Pixelligent Technologies Inc.). These were synthesized using an established solvothermal technique which yields single-crystal ZrO2 nanoparticles30 and capped with an organic ligand to improve their dispersion stability in oil31. The size of ZrO2 nanoparticles was evaluated using transmission electron microscopy (TEM) as well as dynamic light scattering (DLS). In the TEM, ZrO2 nanoparticles were found to be crystalline and highly monodisperse, with an average diameter of 5 nm.31 The size as determined by DLS was 6.6 nm with a Dv(99.99) value of 17.8 nm (Fig. 7). A DLS Dv(99.99) value represents the diameter where 99.99% of the particles in the suspension are smaller than the given value. The value obtained using DLS is larger than the corresponding diameter from TEM since DLS captures the hydrodynamic diameter of a capped nanoparticle (i.e. a crystalline core with a shell of dispersing ligands), whereas TEM captures radius of only the crystalline core.



Figure 7: Dynamic light scattering (DLS) intensity plotted versus estimated diameter of capped nanoparticles. A large peak at approximately 7 nm shows that capped ZrO2 nanoparticles have a nearly uniform size.

Unless otherwise noted, dispersions consisted of a 10 wt.% loading of capped nanoparticles in a polyalphaolefin (PAO4) synthetic oil, with a reported kinematic viscosity of 4 cSt at 100°C, along with a 5 wt.% alkylated naphthalene (AN) co-solvent (ExxonMobil Chemical Co., Spring, TX). Although macroscale experiments for generating tribofilms with these ZrO2 nanoparticles were performed with 1 wt.% dispersions,32 which is a typical concentration for commercial anti-wear additives, a 10 wt.% dispersion of ZrO2 was selected in AFM experiments to account for the significantly smaller contact lengthscales and to allow generation of tribofilms within reasonably short experimental time-scales. A smaller number of experiments conducted with reduced concentrations of ZrO2 (down to 0.1 wt.%) in PAO4 also showed tribofilm growth with the AFM; a decrease in ZrO2 concentration resulted in significantly longer induction periods (see Supporting Information Fig. S1). Thus, to facilitate a higher throughput of experiments, most experiments were conducted using 10 wt.% ZrO2. In addition to PAO4, a small number of control experiments were conducted for a 10 wt.% ZrO2 dispersion in a higher viscosity metallocene PAO SYN 65 base oil (ExxonMobil Chemical Co.), with a reported kinematic viscosity of 65 cSt at 100°C (see Supporting Information Fig. S2). At 25°C, the addition of 10 wt.% ZrO2 and 5% AN to PAO4 was found to increase the viscosity of base PAO4 from 24.4 cP to 35 cP. All data reported in this study correspond to mixtures containing only base oil, the alkylated naphthalene co-solvent, and zirconia nanoparticles at a 10 wt. % concentration. No other co-additives were added to the tested oil dispersions. Colloidal probe AFM and substrate materials


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AFM measurements of tribofilm growth kinetics were performed on a Keysight Picoplus 5500 AFM, equipped with a fluid cell and a variable temperature substrate holder (Keysight Technologies, Santa Rosa, CA). All tribofilm growth studies were performed using steel colloidal microspheres of diameters ranging between 10 µm and 70 µm (NanoSteel Co., Providence, RI). Steel colloids were custom-mounted on tapping mode cantilevers (combination of TAP300-G, Budget Sensors, Sofia, Bulgaria and PPP-NCH, Nanosensors, Neuchatel, Switzerland) with a two-part epoxy (JB Weld, Sulphur Springs, TX), using a custom-built micromanipulator. Normal force calibration of AFM cantilevers was performed using the Sader Method59-60 prior to attachment of steel microspheres. The position of mounted steel colloid was varied along the longitudinal axis of several cantilevers in order to provide cantilevers with varying flexural stiffness. An off-end flexural length, determined from the actual location of mounted probe, was used to determine the true bending stiffness for the colloid-mounted cantilever.59 This corrected bending stiffness was used to determine the applied normal load, and subsequently to estimate the contact diameter and stress assuming Hertzian contact mechanics. The Hertz model was deemed appropriate since probe-sample adhesion was measured from force-distance curves to be negligible in the oil solution. All values of contact stress reported in this study refer strictly to the initial contact stress, measured for the steel-on-steel contact prior to ZrO2 tribofilm growth. It is assumed that as ZrO2 tribofilms grow at the interface, the actual contact stress will decrease if the elastic modulus of ZrO2 tribofilm is lower than steel. As a result, the actual contact stress at the interface will continuously decrease as ZrO2 tribofilm continues to grow, despite the applied normal load being nominally identical. ZrO2 tribofilm formation scans measuring 5 × 5 µm2 and the periodic 10 × 10 µm2 imaging were performed using the same colloidal probe. A custom MATLABTM script was used to process images and quantify parameters such as tribofilm volume and surface roughness. Unless otherwise noted, the sliding countersurface consisted of a disc-shaped 52100 steel coupon, hardened to Rockwell C hardness greater than 60 (Heckel Tool and Mfg. Corp., Eagle, WI). As-received steel coupons were polished to a nominal RMS roughness of 1.3 ± 0.1 nm using a polishing slurry. Surface roughness of the steel countersurface was measured using AFM topographic 10 × 10 µm2 images. This coupon was mounted on the AFM substrate holder and its test surface was submerged within a fluid cell. A smaller number of experiments were also conducted on silicon wafers (polished to different surface roughness values), yttria-stabilized zirconia (YSZ) and sapphire substrates. Tribofilm mechanical characterization The mechanical properties of tribofilms and the steel substrate were evaluated using an KeysightTM Nano Indenter G200 (Keysight Technologies, Santa Rosa, CA) in the continuous stiffness measurement (CSM) method38 and a Hysitron TI 950 TriboIndenter (Hysitron Corp., Minneapolis, MN) in depth-controlled indentation mode. 25 CSM indents and 25 depth-controlled indents per sample were performed. In both 23 ACS Paragon Plus Environment


cases, a Berkovich diamond tip was used. Surface imaging scans during indentation measurements were performed with a normal load of 2µN. In addition to ZrO2 tribofilms generated on steel substrates in the AFM, mechanical properties of a tribofilm generated on a silicon substrate were also evaluated. This additional film was generated using a macroscale ball-on-flat reciprocating tribometer, also with a 10 wt.% dispersion of ZrO2 in PAO4 at 25°C. A greater lateral coverage for the macroscale ZrO2 tribofilm helped minimize nanoindentation artifacts associated with edge effects, which could confound measured nanomechanical properties on AFM-generated tribofilms. ACKNOWLEDGEMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Small Business Innovation Research (SBIR) and Small Business Technology Transfer Program under an SBIRII Award Number DE-SC-0009222. I.L. acknowledges support from an Africk Fellowship. Use of University of Pennsylvania Nano/Bio Interface Center and Nanoscale Characterization Facility Instrumentation is acknowledged. The authors gratefully acknowledge K.T. Turner at the University of Pennsylvania and D.L. Burris at the University of Delaware for use of instrumentation, as well as D.P. Pope and I.W. Chen for useful discussions. CONFLICT OF INTEREST STATEMENT AJ, ZC, and GDC are affiliated with Pixelligent Technologies Inc., the commercial vendor for capped zirconia nanoparticles used in this work. ZC and GDC were employees of Pixelligent and both have equity in the company. AJ is a consultant to Pixelligent. No other co-authors declare a competing financial interest. SUPPORTING INFORMATION The Supporting Information is available free of charge via the Internet at http://pubs.acs.org The SI file contains further information on the effect of particle concentration, oil viscosity, substrate material and roughness on tribofilm growth, and effect of sliding on friction. REFERENCES 1. Wong, V. W.; Tung, S. C., Overview of Automotive Engine Friction and Reduction Trends– Effects of Surface, Material, and Lubricant-Additive Technologies. Friction 2016, 4 (1), 1-28. 2. Spikes, H., The History and Mechanisms of ZDDP. Tribol. Lett. 2004, 17 (3), 469-489. 3. Gosvami, N. N.; Bares, J. A.; Mangolini, F.; Konicek, A. R.; Yablon, D. G.; Carpick, R. W., Mechanisms of Antiwear Tribofilm Growth Revealed In-Situ by Single-Asperity Sliding Contacts. Science 2015, 348 (6230), 102-106. 4. Bec, S.; Tonck, A.; Georges, J. M.; Coy, R. C.; Bell, J. C.; Roper, G. W., Relationship Between Mechanical Properties and Structures of Zinc Dithiophosphate Anti-Wear Films. P. Roy. Soc. Lond. A Mat. 1999, 455 (1992), 4181-4203. 24 ACS Paragon Plus Environment

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Graphical Abstract:


Nanoscale Generation of Robust Solid Films from Liquid-Dispersed

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