Extraordinarily Low Friction and Wear of Epoxy-Metal Sliding Pairs

Subscriber access provided by University of Sunderland

Article

Extraordinarily Low Friction and Wear of Epoxy-Metal Sliding Pairs Lubricated with Ultra-Low Sulfur Diesel Lihe Guo, Guitao Li, Yuexia Guo, Fuyan Zhao, Ligang Zhang, Chao Wang, and Ga Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04352 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Extraordinarily Low Friction and Wear of Epoxy-Metal Sliding Pairs Lubricated with Ultra-Low Sulfur Diesel Lihe Guo†,‡, Guitao Li†, Yuexia Guo†,‡, Fuyan Zhao†,*, Ligang Zhang†, Chao Wang†, Ga Zhang†, §,* †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, No. 18, Tianshui Middle Road, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, No. 19, Yuquan Road, Beijing 100049, China § Qingdao Center of Resource Chemistry & New Materials, No. 36, Jinshui Road, Qingdao 266071, China *

Corresponding authors: Prof. Ga Zhang and Dr. Fuyan Zhao E-mail: [email protected]; [email protected] Tel: +86-931-4968041 Fax: +86 931 4968180

Abstract Utilization of ultra-low sulfur diesel (ULSD) having intrinsically low lubricity has been mandated in more and more countries for meeting increasingly stringent emission regulations. Our work demonstrated that epoxy hybrid nanocomposites, reinforced with carbon fibers and Al2O3 or AlN nanoparticles, offered extraordinarily low friction and wear when lubricated with ULSD. More interestingly, when sliding took place under lubricant starvation conditions, almost no wear took place thanks to the formation of a nanostructured tribofilm. We investigated comprehensively heat and stress activated physical and chemical actions occurring at friction interfaces. It was revealed that the lubricious and robust tribofilm grew while complex molecular species released from the nanocomposites were fed constantly onto the contact zone. The present work paved a route for tuning interface nanostructures of large numbers of motion components subjected to harsh lubrication conditions towards extremely high energy efficiency and long lifespan.

Keywords: Ultra-low sulfur diesel; Lubrication; Tribofilm; Epoxy; Nanocomposites

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction Friction and wear exert a major influence on efficiency and lifespan of machinery, and thus play an important role in global economy. Introduction of improved tribological designs can bring about marked economic results.1-3 Nonetheless, considerable energy is still being dissipated to overcome friction, and huge economic losses are caused by wear of vast numbers of motion components and their replacement.4 In transportation vehicle sector, around one third of fuel is consumed to overcome friction in the engine system, transmission system, tire-road contact, brake contact and so on.5 Considerable efforts are being dedicated to developing higher energy-efficient vehicles by reducing frictional energy loss, not only for saving economic costs but also for minimizing environmental impact caused by engine emission.

In order to meet the increasingly stringent emission regulation of fuel combustion, it is necessary to reduce the sulfur content in fuel.6,

7

However, adoption of the

hydrodesulfurization process makes the diesel fuel loss its lubricity due to removal of intrinsically available polar substances in diesel, e.g. oxygen- and nitrogen-based trace compounds, which is helpful to form protective tribofilms.8, 9 Fuel lubrication is, however, required for the motion components of fuel delivery and injection systems to protecting their mating surfaces from wear and scuffing.10,

11

Lubrication additives of chemical

ingredients are usually blended into diesel for reducing friction and wear of the components. Nevertheless, these chemical ingredients can themselves have unfriendly impact on the environment.8, 12 A novel strategy to enhance the lifespan and reliability of the components is to design tribo-materials having high tribological performance even when lubricated with ultra-low sulfur diesel (ULSD).

It has been recognized that formation of an easy-shear tribofilm at sliding interfaces is of great significance for reducing friction and wear of the sliding pairs subjected to mixed and boundary lubrication regimes.13-16 The tribofilm separates effectively direct rubbing


Page 2 of 35

Page 3 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the tribo-pairs. In particular, the tribofilm usually exhibits a low modulus so that the contact stress is lowered with tribofilm thickening. Growth of the tribofilm is a rather complex process governed by various tribo-physical and chemical actions, which are most probably thermally activated and stress assisted.3, 17, 18 Zinc dialkyldithiophosphate (ZDDP) is the most commonly used antiwear additives in lubricant oil. Gosvami et al.19 observed in-situ nucleation, growth and thickness saturation of ZDDP-derived tribofilms via well-defined single-asperity sliding nanocontacts. The authors inferred that tribofilm growth was continuously fed by ZDDP molecules from the oil into the contact zone, where tribo-chemical reactions occurred.

It should be noted that growth and sacrifice of a tribofilm reach equilibrium in the steady friction process. The durability and shear characteristic of the tribofilm determine its functionality. It is of great interest if one can tune the nanostructures of the tribofilm via regulating tribo-physical and chemical actions at the interface. We believe that there is a design target of tribo-materials for motion components with ULSD lubrication. For instance, one can design to promote tribofilm formation by compounding functional fillers into the tribo-materials. Thus, when the functional fillers are released from the bulk materials onto the friction interface, they can trigger beneficial chemical reactions or be digested into the tribofilm.

Owing to the self-lubrication characteristic and chemical resistance, polymeric materials are being increasingly utilized as sliding materials for applications under dry friction and harsh lubrication conditions. Depending on the molecule structures, friction-induced heat and stress can break the molecular chains and the free radicals can finally chelate with the metallic counterpart, yielding a surface-bonded tribofilm.20-23 In particular, the tribological performance can be significantly improved by filling with appropriate fillers or their combinations. Reinforcing fibers with a high modulus usually benefit the



Page 4 of 35

abrasion resistance of the matrix materials.24, 25 Layer-structured solid lubricants, e.g. graphite and MoS2, readily transfer onto the dry sliding counterface, and thus help to form a lubricating layer.26, 27 When oxide nanoparticles in polymer nanocomposites are released onto the dry rubbing interface, they can be “tribo-sintered” into a compact tribofilm and thus enhance its durability.28-30 However, the presence of liquid lubricant at the interface usually hinders materials transfer, and suppresses the physical and chemical actions which are thermally activated. The mechanisms governing tribofilm growth of polymer composites subjected to boundary lubrication have not been well understood yet, which limits the development of tribo-materials that offer better performance.

In the present work, tribological behaviors of epoxy (EP) reinforced with short carbon fibers (SCF) and ceramic nanoparticles (Al2O3 or AlN) were explored under ULSD lubrication conditions. Evolution of friction and wear as functions of sliding time and ULSD quantity was investigated. In particular, structures and properties of the tribofilms formed on the steel counterface were characterized in-depth. Moreover, tribo-chemical reactions were analyzed in-depth for elucidating formation mechanisms of the tribofilms. It is expected that the output of this work can pave a route for tuning tribofilm structures of motion components subjected to harsh lubrication conditions.

Experimental Material preparation All materials were prepared on the basis of a Bisphenol-A diglycidyl ether epoxy resin (EP, WSR618) with an epoxide equivalent weight of 182-192 g/equiv, supplied by Nantong

Xingchen

Synthetic

Material

Co.,

Ltd.

(China).

Amine

hardner

triethylenetetramine (TETA) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Polyacrylonitile-based SCF (Nantong Senyou, China) having a mean diameter of 7 µm and length of 30-100 µm was used as the reinforcing filler. The SCF was supplied


Page 5 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


without sizing treatment and used as-received. Al2O3 nanoparticles (average size: 30 nm) were commercially supplied by XuanCheng JingRui New Material Co., Ltd. (China). AlN nanoparticles (average size: 50 nm) were supplied by Shanghai Chaowei Nanotechnology Co., Ltd. (China). In order to disperse evenly the nanoparticles and to enhance the adhesion with the polymer matrix, they were treated with silane coupling agent (KH-550, Sinopharm Chemical Reagent Co., Ltd, China) before being incorporated into the matrix.

Detailed compositions and designations of the EP composites were listed in Table 1. Volume fractions of SCF and the nanoparticles were 10% and 5%, respectively. Neat EP was prepared in the present work as a reference material. Respective fillers were dispersed into EP resin by stirring in a high speed vacuum dissolver (Dispermat CN-10, VMA-Getzmann, Germany). In order to prepare the hybrid nanocomposites, required fraction of the nanoparticles were firstly dispersed into EP resin and then SCF was blended into the mixture using the same dissolver. The compounds were then blended with the curing agent for 15 min. Finally, the mixture was poured into release agent-coated metallic molds and kept at room temperature for 2 h gel time, followed by curing at 100 oC for 1 h.

Table 1. Designations and compositions of EP composites (vol.%). Designations

EP

SCF

EP

100

SCF/EP

90

10

Al2O3/SCF/EP

85

10

AlN/SCF/EP

85

10

Al2O3

AlN

5 5

Tribological tests Tribological tests were conducted using a Plate-On-Ring (POR) test rig (MRH-1A, Jinan Yihua, China). The contact configuration of the sliding pair was illustrated schematically



Page 6 of 35

in Fig. S1. The polymer plates have dimensions of 50×10×5 mm3. Standard bearing steel rings (GCr15, GB/T 18254-2002) were utilized as counterparts and the diameter of the steel ring was 60 mm. All tests were conducted at room temperature. The steel ring and the polymer plate were grounded with SiC metallographic abrasive papers and the mean roughness Ra was controlled at around 0.20 µm and 0.12 µm, respectively. Prior to the tribological tests, the counterpart ring and polymer plate were cleaned thoroughly with petroleum ether in an ultrasonic bath. ULSD (sulfur content 10 ppm according to Chinese National V Standard) purchased from Golden Shield Petrochemical Group Co., Ltd (China) was utilized as lubricant. The viscosity of the diesel was ~5.4 MPa·s at room temperature. The applied load was fixed at 100 N and the sliding speed was 1 m/s. The volume loss of the specimen was measured and the specific wear rate was calculated according to the following formula:

Ws = ′ × (2 arcsin −

√42 − 2) (3⁄ ∙ )

(1)

Where L' is the width of polymer testing specimens (10 mm), R is the radius of steel ring (30 mm), W is the width of the wear scar (mm), F is the normal load applied on polymer specimens, and L is the total sliding distance (m). Each test was repeated for at least 3 times for calculating the average friction coefficient and wear rate.

During the test, the diesel was pushed continuously by a high-precision micro-injection pump (Baoding Lange Constant Flow Pump Co., Ltd, LSP02-1B, China) and transported onto the sliding interface through an elastomer tube. Before each test, the wear track on the steel ring was pre-wetted by pushing about 300 µL diesel on the surface. Each test lasted 3 h so that the friction process reached a stable stage. Influence of diesel quantity on the tribological performance of the EP materials was evaluated. In order to simulate the situation of lubricant starvation, the polymer plates rubbed with the pre-wetted ring


Page 7 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and no further diesel was transported onto the sliding interface throughout the 3 h test. Under this condition, a continuous diesel film was not noticeable from the ring surface especially in less than 1 h due to diesel evaporation, confirming the lubricant starvation situation. Moreover, the sliding with adequate lubricant supply was also simulated by continuously feeding the diesel fuel onto the sliding interface. Two flow rates of the diesel, i.e. 200 µL/h and 1000 µL/h were concerned in the present work. It was observed that with continuous diesel injection, adequate diesel gathered in the contact wedge area during the whole testing process. A continuous lubricant film was noticeable on the ring surface and hence supply of adequate diesel was ascertained.

It is surmised that under the lubricant starvation condition, only diesel molecules but not a continuous liquid film is present on the ring surface.31 With respect to the sliding with the supply of adequate diesel, a continuous diesel film was present on the ring surface. The minimal film thickness (hmin) of the diesel film was 157 nm as calculated using the Hamrock-Dowson equation3, 32 and accordingly the lambda ratio (λ) was 0.77. It was thus corroborated that the sliding system lied in the boundary lubrication regime. That is, the hydrodynamic role of the lubricant film was negligible and majority of the load was born by solid-solid contact.

Characterization of the tribofilms The wear track of the steel ring rubbed with the EP-based materials was inspected using an optical microscope (Axio Imager A2m, Zeiss). Worn surface morphologies of the steel rings were analyzed using a Field Emission Scanning Electron Microscope (FE-SEM, Merlin Compact, Zeiss). The samples were coated with a thin layer of platinum to enhance the electrical conductivity. Elemental compositions of the tribofilms formed on the steel counterface were analyzed using an Energy Dispersive X-ray Spectroscopy (EDX, Energy 350, Oxford) instrumented onto the FE-SEM.



In order to gain an insight into the mechanisms governing the formation and function of the tribofilms, their nanostructures and nanomechanical properties were characterized. The tribofilms’ nanostructures were analyzed using a High Resolution Transmission Electron Microscopy (HR-TEM, Tecnai G2 TF20 S-TWIN, FEI) combined with EDX and Selected Area Electron Diffraction (SAED). Cross-sectional lamellas of tribofilm on the steel counterface were prepared by Focused Ion Beam (FIB) machining in a Dual-Beam SEM/FIB instrument (Quanta 3D FEG, FEI, Netherlands). Prior to FIB machining, the area of interest was coated with a platinum cap layer by ion beam assisted deposition.

Functional groups of the tribofilm formed on the steel surface and the original surface of the corresponding EP composite were analyzed by using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy (TENSOR 27 IFS120HR, Bruker) with a zinc selenide ATR unit. Then, a laser Raman spectrometer (DXR, Thermo Scientific) equipped with in an excitation laser source at 532 nm was conducted for the tribofilm rubbed with all four materials.

Atomic Force Microscope (AFM) PeakForce QNM tapping mode (Dimension Icon, Bruker) was applied to mapping elastic modulus and topography of the tribofilm. The Young’s modulus was acquired using the Derjaguin-Muller-Toporov (DMT) model.33 In order to avoid possible impact of the steel substrate, PeakForce QNM measurements were conducted using ScanAsyst-Air probes with a nominal spring constant of 0.5 N/m. PeakForce QNM provided a new methodology approaching to mechanical properties of non-uniform thin films. Nevertheless, it should be noted that the DMT model is tip-dependent and the Young’s modulus is a relative estimation.34, 35

Results and Discussion


Page 8 of 35

Page 9 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Friction coefficient and wear rate Fig. 1a shows the friction coefficient evolutions of neat EP, SCF/EP, Al2O3/SCF/EP and AlN/SCF/EP rubbing with the steel ring pre-wetted with 300 µL ULSD (without further addition of diesel throughout the 3 h testing time). It is noticed that the friction coefficients of the EP-based materials started from 0.16-0.18. With respect to the sliding of neat EP, the friction coefficient gradually decreased in the first 1.5 h before reaching equilibrium at averagely 0.10. Moreover, significant fluctuation of the friction coefficient is noticed when neat EP rubbed with the pre-wetted steel ring. As identified below, free radicals of EP react with the steel surface and thus help to form a surface-bonded tribofilm. It is believed that the decrease of the friction coefficient is ascribed to tribofilm growth. Tribofilm formation is balanced by removal and replenishment processes.19, 36 It seems that the tribofilm of neat EP is not robust enough withstanding the rubbing stress. Hence, unsmooth evolution of the tribofilm is assumed to be the main reason for the fluctuation of friction coefficient.



Fig. 1. Friction coefficient evolutions of EP, SCF/EP, Al2O3/SCF/EP and AlN/SCF/EP with the presence of various quantities of diesel only pre-wetted (a); friction coefficient evolutions of SCF/EP sliding with dry and pre-wetted steel rings, and with lubrication of 200 µL/h and 1000 µL/h ULSD (b); friction coefficient evolutions of EP, SCF/EP, Al2O3/SCF/EP and AlN/SCF/EP with lubrication of 1000 µL/h ULSD (c); friction coefficient evolutions of (d) SCF/EP and (e) Al2O3/SCF/EP rubbing with the pre-wetted rings and when 200 µL diesel were suddenly dropped onto the wear track at 1 h; (f) specific wear rates of EP, SCF/EP, Al2O3/SCF/EP and AlN/SCF/EP as a function of ULSD quantity; (g) wear volumes of SCF/EP and Al2O3/SCF/EP rubbing with pre-wetted steel rings as a function of sliding time.


Page 10 of 35

Page 11 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Addition of SCF into EP matrix exerts an important role in the friction coefficient tendency. It is interesting to observe that the friction coefficient of SCF/EP decreased dramatically in the running-in stage from 0.18 to 0.016 (cf. Fig. 1a). In particular, the extremely low friction coefficient remained very stable under the lubricant starvation condition, giving a hint that a robust and easy-shear tribofilm was established.37 Nevertheless, the friction coefficient fluctuated significantly during the running-in period. The friction coefficients of Al2O3/SCF/EP and AlN/SCF/EP follow a similar tendency with that of SCF/EP, although the stable friction coefficient is slightly higher. However, the addition of Al2O3 or AlN nanoparticles suppresses greatly friction fluctuation in the running-in stage, indicating a smoother evolution process of tribofilms’ structures.

Fig. 1b compares the friction coefficients of SCF/EP obtained when sliding against dry steel, pre-wetted steel (no further diesel transportation), and when continuously lubricated with 200 µL/h and 1000 µL/h diesel. When sliding took place with the dry steel ring, the friction coefficient of SCF/EP was rather high, i.e. about 0.47, since no lubricating tribofilm formed.27,

28

In particular, the friction coefficient increases when a

tribo-oxidation layer forms on the steel surface.28 In comparison to the dry sliding, the presence of adequate diesel on the sliding interface reduced the friction coefficient of SCF/EP due to boundary lubrication of the diesel film.

Contrary to one’s expectation, when 1000 µL/h diesel is transported continuously onto the sliding interface, SCF/EP exhibits a much higher friction coefficient, i.e. 0.13 versus 0.016, when compared to that obtained under the diesel starvation condition (steel only pre-wetted with diesel) (cf. Fig. 1b). When 200 µL/h diesel was added onto the sliding interface, an extremely low friction coefficient was achieved as well. Nevertheless, the running-in duration was significantly longer in comparison to that obtained from the sliding against the pre-wetted steel. It was thus demonstrated that the presence of more



diesel retarded or even inhibited formation of the robust tribofilm. That is, SCF/EP exhibits the best tribological performance under the lubricant starvation condition when only some diesel molecules but not a continuous diesel film is present on the wear track.

When transporting 1000 µL/h diesel onto the sliding interface, Al2O3/SCF/EP and AlN/SCF/EP show lower friction coefficients than that of SCF/EP (cf. Fig. 1c). Moreover, addition of the nanoparticles mitigates the friction fluctuation. Nevertheless, similar to the tendency of SCF/EP, the hybrid nanocomposites show much higher friction coefficients with the presence of adequate diesel, in comparison to that acquired under the diesel starvation condition. It was thus manifested that formation of a robust tribofilm played a much more efficient lubrication role than a continuous diesel film did.

Figs. 1d and e show the similar friction coefficient tendencies of SCF/EP and Al2O3/SCF/EP, when they rubbed initially with the pre-wetted ring and then around 200 µL of diesel were suddenly dropped onto the wear track after 1 h sliding. It is observed that the addition of diesel dramatically increased the friction coefficients. This trial confirmed that the presence of adequate diesel at the interface increased the friction coefficient. It is suspected that the presence of more diesel dilutes the contents of the molecular species, which are released from the composites and fed constantly tribofilm growth at the interface. In this case, the balance of tribofilm growth was destroyed while adding diesel. Afterwards, a new running-in process took place and extremely low friction coefficients were obtained again later, when excessive diesel evaporated and “lubricant starvation” was emerged again.

Fig. 1f illustrates the specific wear rates of the EP-based materials when being lubricated with various quantities of diesel. In comparison to neat EP, the addition of SCF reduced the wear rate by 38.7% to 72.6%. What’s more, further addition of Al2O3 or AlN


Page 12 of 35

Page 13 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanoparticles into SCF-reinforced EP dramatically reduced the wear rate by up to one magnitude. The hybrid nanocomposites exhibit extraordinarily high wear resistance, which are more than 35 times higher than that of neat EP. Significant synergism of the SCF and nanoparticles in respect of wear-reduction was thus identified.38 In addition, the wear rates of both hybrid nanocomposites did not show a dependence on diesel quantity. However, SCF/EP showed the best wear resistance when rubbing with the pre-wetted steel ring.

In order to evaluate the volume loss rates at different stages, additional tribological tests were conducted and the wear volumes of SCF/EP and Al2O3/SCF/EP rubbing with the pre-wetted steel ring were measured as a function of sliding time. As seen from Fig. 1g, the volume loss rate of SCF/EP gradually decreased versus sliding time, indicating that the wear was remarkably reduced due to formation of the tribofilm. In particular, it is rather impressive that almost no more wear of Al2O3/SCF/EP was noticed during the sliding from 1.5 h to 8 h. It is thus verified that tribofilm formation of the hybrid nanocomposite plays an extraordinary wear-reduction role. The above results give clear evidence that regardless of the poor lubrication of ULSD, the lifespan of motion pairs can be greatly extended if a high-performance tribofilm is established at the interface.

Tribofilm structures and tribo-chemistry Fig. 2a displays the optical micrograph of the steel ring rubbed with neat EP under the pre-wetting condition. Certain regions of the steel surface are covered with a tribofilm, as indicated by the arrow in Fig. 2a. As demonstrated below, free radicals deriving from broken epoxy molecules encountered with the steel counterface and thus helped to form a surface-bonded tribofilm. This is believed to be the main reason for the friction reduction of neat EP versus sliding time (cf. Fig. 1a). Nevertheless, it seems that the load-bearing capability of such a tribofilm is not enough to withstand the rubbing stress. In this case,



the tribofilm separates only partially contact of the sliding pair.

Fig. 2. Optical micrographs of the steel surfaces rubbed with EP (a), SCF/EP (b), Al2O3/SCF/EP (c) and AlN/SCF/EP (d) under the lubricant starvation condition; Optical micrographs of the steel surfaces rubbed with SCF/EP while 200 µL/h (e) and 1000 µL/h ULSD (f) was continuously transported onto the sliding interface.

When sliding took place with SCF/EP, a nearly continuous tribofilm formed on the pre-wetted steel surface, whereas some “smooth plateaus” are noticed, as indicated by arrows in Fig. 2b. Generation of these “smooth plateaus”, corresponding to a


Page 14 of 35

Page 15 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tribo-oxidation layer, is an indication of direct rubbing and mild abrasion of the hard reinforcing fibers with the steel.28, 29 Under boundary lubrication conditions, carbon fibers in the composite bear majority of the load, and thus high flash temperature and rubbing stress develop on fiber tips.15, 39 From the Raman spectra of the steel surfaces rubbed with neat EP and SCF/EP (cf. Fig. 3), fingerprints of iron oxide are identified, verifying tribo-oxidation of the steel surface. Moreover, D band (1353 cm-1) and G band (1606 cm-1) are observed from the spectra of the steel surfaces rubbed with neat EP and SCF/EP. The intensities of D and G bands for the steel surface rubbed with SCF/EP are higher than those of the surface rubbed with EP. This gives a hint that more carbon materials transferred onto the steel surface rubbed with SCF/EP. The intensity ratio of D band to G band (ID/IG) reflects defects degree of carbon materials.40,

41

ID/IG ratios of the steel surfaces rubbed with EP and SCF/EP are

respectively 1.59 and 1.03, indicating carbon materials in the tribofilm of SCF/EP has a more ordering structure than those in the tribofilm of EP. The above results allow us to infer that pulverized carbon fibers transfer onto the steel surface, which accounts for the dramatically reduced friction and wear (cf. Fig. 1).

In addition, the broad bands in the Raman spectra at approximately 2900 cm-1 are assigned to the D+G band, which is an indication of disordered structures and defects.42 In comparison to the tribofilm of neat EP, the intensity of the D+G peak of SCF-reinforced composites is much lower. This is in fact an additional proof that the carbon materials in the tribofilms of the composites have a more ordered structure when compared to that in the tribofilm of neat EP.



Fig. 3. Raman spectra of the tribofilms formed on steel surfaces rubbed with neat EP, SCF/EP, Al2O3/SCF/EP and AlN/SCF/EP under the lubricant starvation condition.

When Al2O3/SCF/EP and AlN/SCF/EP slid against the pre-wetted steel ring, a uniform tribofilm formed on the steel counterface as well (cf. Figs. 2c and d). More interestingly, unlike the observation from the steel surface rubbed with SCF/EP, almost no “smooth plateaus” are present on the steel surfaces rubbed with Al2O3/SCF/EP or AlN/SCF/EP. In comparison to the sliding of EP and SCF/EP, tribo-oxidation of the steel was mitigated when rubbed with the hybrid nanocomposites. The fingerprint of iron oxides at ~700 cm-1 is clearly noticeable from the spectrum of steel surfaces rubbed with EP and EP/SCF,43 whereas it becomes less obvious from the surfaces rubbed with Al2O3/SCF/EP or AlN/SCF/EP. Moreover, ID/IG ratios of the steel surfaces rubbed with Al2O3/SCF/EP or AlN/SCF/EP are respectively 0.82 and 0.69, indicating structures of carbon species in the tribofilms tuning more ordered (in comparison to tribofilms of EP and SCF/EP).

When transporting continuously 200 µL/h ULSD onto the sliding interface, more “smooth plateaus” were observed on the steel surface slid against SCF/EP (indicated by arrows in Fig. 2e). In particular, the tribofilm on the steel surface was destroyed with


Page 16 of 35

Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lubrication of 1000 µL/h ULSD and large “smooth plateaus” were generated (cf. Fig. 2f). In comparison to the sliding under the lubricant starvation condition, tribo-oxidation became severer when more ULSD was added onto the wear trace. Presence of adequate diesel lowered the boundary lubrication performance and hence aggravated direct rubbing of the friction pair. Friction and wear properties of SCF/EP, Al2O3/SCF/EP and AlN/SCF/EP show the similar tendency as a function of diesel quantity. It was disclosed that the presence of adequate diesel diluted the molecular species released from the composites that were necessarily required for tribofilm growth. It is thus inferred that the tribofilm formed under the diesel starvation condition plays a better boundary lubrication role than a continuous diesel film does.

Fig. 4. FTIR spectrum of AlN/SCF/EP bulk composite and ATR-FTIR reflectance results of the tribofilms formed on steel surfaces rubbed with AlN/SCF/EP under the diesel starvation condition. The inset shows a photograph of the steel ring rubbed with AlN/SCF/EP plate samples (two wear traces) and “Licp” were written on the two wear traces with a cotton swab dipped with acetone.

Fig. 4 compares the FTIR spectrum of the AlN/SCF/EP bulk composite and ATR-FTIR spectrum of the tribofilm formed on steel surfaces rubbed with AlN/SCF/EP under the



diesel starvation condition. Fingerprints of the chemical bonds of EP are identified from the spectrum of the bulk composite. The absorption peaks at 1612 and 1511 cm-1 are due to C-C stretching vibration of the aromatic ring.44, 45 The peaks at 1241 and 1183 cm-1 are assigned to the C-O asymmetrical aromatic stretching vibration, while the peak at 1036 cm-1 is due to the symmetrically stretching vibration of C-O. When respect to the spectrum of the tribofilm, the absorption peaks at 1511, 1241 and 1036 cm−1 are attributed to respective chemical bonds of EP. In addition, the absorption peak of Al-N bond is observed at 672 cm−1.46 This implies that AlN nanoparticles were released from AlN/SCF/EP bulk composite and compacted into the tribofilm.

More interestingly, new chemical species are identified with the absorption peak at 1702 cm−1, corresponding to the stretching vibration of C=O of ketone.47 It is believed the C=O is a consequence of oxidation of C-O.46 During the friction process, rubbing stress and friction-induced heat can lead to breakage of the C-O band in EP molecular chains, as schematically illustrated in Fig. 5. When free radical transfer completes, oxidation of the carbon free radicals occurs and thus C=O is generated.47 In addition, the oxygen free radicals can react with the metallic ions of the steel substrate and the nanoparticles.21 Tribo-chemical reactions of the polymer with the steel can be essential for enhancing the adhesion of the tribofilm onto the steel surface,23, 40 while reactions of polymer with the nanoparticles can enhance affinity and robustness of the tribofilm.


Page 18 of 35

Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


OCH2CHCH2

O

OH

OCH2CHCH2

O

OH M

OCH2CCH3

MO

OH O2

OCH2CCH3

O

Fig. 5. Schematic illustrations of breakage of EP molecular chains, oxidation of the carbon free radicals, and reaction of oxygen free radicals with metal ions of steel substrate and nanoparticles.

It is interesting to observe that the tribofilms are removable using polar solvents such as acetone, DMP (dimethyl phthalate) and NMP (N-methyl pyrrolidone). As seen from the inset in Fig. 4, the tribofilms on the wear traces can be easily wiped off with a cotton swab dipped with acetone. On the contrary, the tribofilm shows a high resistance against non-polar solvents such as petroleum ether. According to the principle of similarity-compatibility, one can infer that the tribofilm consists mainly of polar materials. This supports the assumption that the tribofilm consists mainly of EP and its tribo-chemical products. Although no proof is provided, we believe chemical reactions of EP molecules, rather than physical transfer, play a dominant role in tribofilm formation.



Fig. 6. (a) TEM micrograph of the tribofilm cross-section on the steel surface rubbed with Al2O3/SCF/EP under the diesel starvation condition; (b) HR-TEM micrograph of the region I indicated in a; (c) EDS maps of C, N, Al and Fe elements of the region II indicated in a; (d) HR-TEM micrograph of the region III indicated in a; (e) TEM micrograph of tribofilm cross-section on the steel surface slid against AlN/SCF/EP under the diesel starvation condition; (f) HR-TEM micrograph of the region squared in e. The inset in d shows the SAED pattern of the tribofilm.

Fig. 6 illustrates FIB-TEM results of the tribofilms formed on the steel surfaces after rubbing with the Al2O3/SCF/EP and AlN/SCF/EP under the diesel starvation condition. The TEM micrographs provided direct evidence that after rubbing with the hybrid


Page 20 of 35

Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanocomposites, a continuous tribofilm covered the steel surface. It was corroborated that the protective tribofilms separated the direct rubbing and thus led to boundary lubrication. As has been recognized that a resilient tribofilm usually exhibits a higher load-carrying capability and a better anti-pressure property than an absorption film does.15, 39

Closer inspections disclosed that on the steel surface rubbed with Al2O3/SCF/EP a thin iron oxide layer with the thickness of 5-10 nm thick was established (lattice spacing of 0.25 and 0.29 nm) (cf. Figs. 2c and d). It is reasonably inferred that tribo-oxidation occurred owing to the direct rubbing at the interface prior to formation of the protective tribofilm. Afterwards, a compact tribofilm formed above this continuous oxide layer, in which only a small quantity of iron oxide crystals were observed. In fact, this provides evidence that the protective tribofilm inhibited effectively tribo-oxidation, and only abraded iron oxide debris from the oxide layer was finally mixed into the surface layer.

EDS mapping results demonstrate that the tribofilm of Al2O3/SCF/EP comprises abundant C- and N-elements from EP and significant fraction of Al-element from the nanoparticles (cf. Fig. 6c). As consistent with above observations, except for the oxide layer adjacent to the steel surface, only a little Fe-element was involved into the tribofilm. Moreover, HR-TEM graphs reveal that short-range ordered graphitic material with a grain size of a few nanometers is present in the tribofilm (squared areas in Fig. 6d). The inset SAED pattern in Fig. 6d shows that the dominating spacing is 0.34 nm. As described above, such graphitic material can be ascribed to the transfer of pulverized carbon fibers. Moreover, graphitization of the transferred C-materials can be activated by the temperature and stress at the interface, leading to generation of partially ordered structures.49,

50

These results allow us to infer that the tribofilm consists of Al2O3

nanoparticles and graphitic grains distributed in a polymeric matrix. It is assumed that the nanoparticles and nanosized graphitic grains endow the tribofilm with a high resilience



enduring rubbing stress.28, 49 When the rigid nanoparticles are released from the bulk composite, they can abrade the tribo-oxidation layer on the steel surface.29, 51-53 Otherwise, severe oxidation of the steel surface can reduce significantly the adhesion of the polymeric tribofilm.29

Similar to above observation with the sliding of Al2O3/SCF/EP, a continuous tribofilm formed on the steel surface rubbed with AlN/SCF/EP under the diesel starvation condition (cf. Fig. 6e). HR-TEM micrograph verifies that a compact tribofilm comprising nanosized graphitic grains, as indicated by rectangles, was established above the continuous oxide layer (cf. Fig. 6f). The nearly identical tribofilm structures of the two hybrid nanocomposites account for the similar triboglocal behaviors. Moreover, Al-element maps of the tribofilms of Al2O3/SCF/EP and AlN/SCF/EP verified that the nanoparticles distributed evenly in the tribofilms (Figs. S2 and S3).


Page 22 of 35

Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 7. Peakforce QNM mapping results of (a) height and (b) logarithm of DMT modulus of the tribofilm formed on steel surface rubbed with AlN/SCF/EP under the diesel starvation condition; (c) linear height profile along the dashed line in a and linear modulus profile along the dashed line in b.

Fig. 7 illustrates Peakforce QNM mapping results of the steel surface rubbed with AlN/SCF/EP under the diesel starvation condition. From the height map given in Fig. 7a, the surface topography of the tribofilm is not fully uniform, as consistent with the observations of TEM and optical micrographs. It is interesting that some nanosized protruding dots (indicated by arrows) are observed from the height map of the steel surface, as verified from the linear topographical profile (Fig. 7c) along the dashed line in Fig. 7a.

Peak Force QNM allows a relative estimation of the Young’s modulus based on the DMT model.54 Fig. 7b shows that the modulus of the tribofilm lies in the range of 100-1200 MPa. Linear modulus profile along the dashed line in b is illustrated in Fig. 7c. It is interesting to observe that the nanosized protruding dots correspond to the regions with a higher Young’s modulus than the matrix of tribofilm (average modulus around 450 MPa). It is interesting that the size of the protruding dots is in the range of 40-50 nm, which is similar to the size of the original AlN nanoparticles. Above height and DMT modulus results allow us to assume that such protruding dots are AlN nanoparticles entrapped into the tribofilm. Certainly, the presence of the nanosized nanoparticles enhances the load-carrying capability and durability of the polymer-based tribofilm. On the other hand, the soft polymer matrix of the tribofilm and graphitic crystals inside can endow the surface layer with an easy-shear characteristic.

Above results have two immediate consequences with respect to tuning structures and functionality of the interfaces lubricated with ULSD. First, more than tribo-physical



actions are leading to this layer; instead major tribo-chemical changes of the polymer matrix are occurring. It is supposed that tribofilm formation can be improved by designing optimally molecular structures of the polymer matrix. Second, tribofilm growth is fed constantly by molecular species released from the bulk composites. Tribo-physical and chemical products of such molecular species, e.g. rigid nanoparticles and graphitic nanocrystals, can exert an important role in the load-carrying capability and shearing property of the tribofilm. A robust tribofilm enduring severe rubbing stress can be tailored by optimizing the chemical structures and functional fillers of the composites. Hence, there is a design target of the materials for improving the boundary lubrication performance of numerous friction pairs operating in the media of low lubricity.

Conclusions Tribological behaviors of EP composites filled with hybrid SCF and Al2O3 or AlN nanoparticles were investigated under ULSD lubrication conditions. In particular, nanostructures and nanomechanical properties of tribofilms established on the steel counterface were comprehensively investigated. Following conclusions can be drawn: 1. SCF-reinforced EP exhibited extraordinarily low friction and wear when sliding took place under diesel starvation conditions. In particular, the friction coefficient was reduced by up to one magnitude after the running-in stage. Further addition of Al2O3 or AlN nanoparticles into SCF-reinforced EP led to greatly enhanced wear resistance. More interestingly, after the running-in stage, the hybrid nanocomposites showed nearly zero wear within a long-term friction process. Contrary to one’s expectation, the composites exhibited significantly higher tribological performance under lubricant starvation conditions, in comparison to the sliding lubricated with adequate diesel. Thus, the materials show a high potential for applications subjected to extremely harsh ULSD lubrication conditions.


Page 24 of 35

Page 25 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. Our results provided direct evidence that a nanostructured tribofilm, consisting mainly of tribo-chemical products of EP, nanosized graphitic crystals and nanoparticles, was established on steel surfaces rubbed with the nanocomposites. The unique structure comprising both the soft and hard species endows the tribofilm with a high load-bearing capability and easy-shear characteristic. It was corroborated that the nanostructured tribofilm separated almost entirely direct contact of the friction pair, and thus led to extraordinarily low friction and wear. 3. Utilization of ULSD with intrinsically low lubricity has been mandated for meeting increasingly stringent emission regulations. The present work proposed a design strategy of motions systems subjected to ULSD lubrication for overcoming the poor lubricity of the lubricant. The lubricious and robust tribofilm grew while complex molecular species released from the nanocomposites were fed constantly onto the contact zone. Based on further molecule level understanding of the tribo-physical and chemical actions at the friction interfaces, one can tailor the structure and functionality of the tribofilm via designing chemical structures of polymer matrices and by optimizing functional fillers.

Acknowledgments The authors acknowledge gratefully the financial support from the National Natural Science Foundation of China (Grant no. 51475446) and Chinese “Thousand Youth Talents Plan” program and “Innovation Leading Talents” program of Qingdao.

Supporting Information Schematic diagram of POR test with ULSD lubrication; SEM micrograph of the steel surface rubbed with Al2O3/SCF/EP; SEM micrograph of the steel surface rubbed with AlN/SCF/EP.

References



Page 26 of 35

(1) Shen, J. T.; Top, M.; Pei, Y. T.; De Hosson, J. T. M. Wear and Friction Performance of Ptfe Filled Epoxy Composites with a High Concentration of SiO2 Particles. Wear 2015, 322-323, 171-180, DOI: 10.1016/j.wear.2014.11.015. (2) Berman, D.; Erdemir, A.; Sumant, A. V. Graphene: A New Emerging Lubricant. Mater. Today 2014, 17 (1), 31-42, DOI: 10.1016/j.mattod.2013.12.003. (3) Desanker, M.; He, X.; Lu, J.; Liu, P.; Pickens, D. B.; Delferro, M.; Marks, T. J.; Chung, Y.; Wang, Q. J. Alkyl-Cyclens as Effective Sulfur- and Phosphorus-Free Friction Modifiers for Boundary Lubrication. ACS Appl. Mater. Interfaces 2017, 9 (10), 9118-9125, DOI: 10.1021/acsami.6b15608. (4) Holmberg, K.; Erdemir, A. Global Impact of Friction on Energy Consumption, Economy

and

Environment.

Fme

Trans.

2015,

43

(3),

181-185,

DOI:

10.5937/fmet1503181H. (5) Holmberg, K.; Andersson, P.; Erdemir, A. Global Energy Consumption Due to Friction

in

Passenger

Cars.

Tribol.

Int.

2012,

47,

221-234,

DOI:

10.1016/j.triboint.2011.11.022. (6) Schumacher, L. G.; Howell, S. A. Lubricating Qualities of Biodiesel and Biodiesel Blends. Sixth National Bioenergy Conference. Reno/Sparks, Nevada: Bioenergy 1994, pp 113-119. (7) DieselNet. Diesel Fuel Grades. USA2009. https://www.dieselnet.com/standards/us/ca_diesel.php (8) Agarwal, S.; Chhibber, V. K.; Bhatnagar, A. K. Tribological Behavior of Diesel Fuels


Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and the Effect of Anti-Wear Additives. Fuel 2013, 106 (2), 21-29, DOI: 10.1016/j.fuel.2012.10.060. (9) Nikanjam, M.; Henderson, P. T. In Lubricity of Low Aromatics Diesel Fuel, International Congress & Exposition, 1992, DOI: 10.4271/920825. (10)Lacey, P. I.; Lestz, S. J. Failure Analysis of Fuel Injection Pumps from Generator Sets Fueled with Jet A-1. Virginia. The Defense Technical Information Center of USA, AD-A234930, 1991, 1. (11) Lacey, P. I.; Lestz, S. J. Fuel lubricity requirements for diesel injection systems, Interim Report BILRF 270, Southwest Research Institute, San Antonio, TX, 1991. (12) Zhang, Q.; He, K.; Huo, H. Policy: Cleaning China's Air. Nature 2012, 484 (7393), 161-162, DOI: 10.1038/484161a. (13) Hammami, M.; Martins, R.; Abbes, M. S.; Haddar, M.; Seabra, J. Axle Gear Oils: Friction Behaviour under Mixed and Boundary Lubrication Regimes. Tribol. Int. 2017, 116, 47-57, DOI: 10.1016/j.triboint.2017.06.028. (14) Hsu, S. M.; Gates, R. S. Boundary Lubricating Films: Formation and Lubrication Mechanism. Tribol. Int. 2005, 38 (3), 305-312, DOI: 10.1016/j.triboint.2004.08.021. (15) Zhang, G.; Wetzel, B.; Wang, Q. Tribological Behavior of PEEK-Based Materials under Mixed and Boundary Lubrication Conditions. Tribol. Int. 2015, 88, 153-161, DOI: 10.1016/j.triboint.2015.03.021. (16) Lin, L.; Schlarb, A. K. The Roles of Rigid Particles on the Friction and Wear Behavior of Short Carbon Fiber Reinforced PBT Hybrid Materials in the Absence of



Page 28 of 35

Solid Lubricants. Tribol. Int. 2017, 119, 404-410, DOI: 10.1016/j.triboint.2017.11.024. (17) Barry, P. R.; Chiu, P. Y.; Perry, S. S.; Sawyer, W. G.; Sinnott, S. B.; Phillpot, S. R. Effect of Temperature on the Friction and Wear of PTFE by Atomic-Level Simulation. Tribol. Lett. 2015, 58 (3), 50, DOI: 10.1007/s11249-015-0529-y. (18) Cosimbescu, L.; Demas, N. G.; Robinson, J. W.; Erck, R. A. Friction- and Wear-Reducing Properties of Multifunctional Small Molecules. ACS Appl. Mater. Interfaces 2017, 10 (1), 1317-1323, DOI: 10.1021/acsami.7b13271. (19) Gosvami, N. N.; Bares, J. A.; Mangolini, F.; Konicek, A. R.; Yablon, D. G.; Carpick, R. W. Tribology. Mechanisms of Antiwear Tribofilm Growth Revealed in Situ by Single-Asperity Sliding Contacts. Science 2015, 348 (6230), 102-106, DOI: 10.1126/science.1258788. (20) Gao, J.; Mao, S.; Liu, J.; Feng, D. Tribochemical Effects of Some Polymers/Stainless

Steel.

Wear

1997,

212

(2),

238-243,

DOI:

10.1016/S0043-1648(97)00140-3. (21) Gao, J. Tribochemical Effects in Formation of Polymer Transfer Film. Wear 2000, 245 (1), 100-106, DOI: 10.1016/S0043-1648(00)00470-1. (22) Urueña, J. M.; Pitenis, A. A.; Harris, K. L.; Sawyer, W. G. Evolution and Wear of Fluoropolymer

Transfer

Films.

Tribol.

Lett.

2015,

57

(2),

1-8,

DOI:

10.1007/s11249-014-0453-6. (23) Harris, K. L.; Pitenis, A. A.; Sawyer, W. G.; Krick, B. A.; Blackman, G. S.; Kasprzak, D. J.; Junk, C. P. PTFE Tribology and the Role of Mechanochemistry in the Development


Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of

Protective

Surface

Films.

Macromol.

2015,

48

(11),

3739-3745,

DOI:

10.1021/acs.macromol.5b00452. (24) Pei, X.; Bennewitz, R.; Schlarb, A. K. Mechanisms of Friction and Wear Reduction by Carbon Fiber Reinforcement of PEEK. Tribol. Lett. 2015, 58 (3), 1-10, DOI: 10.1007/s11249-015-0520-7. (25) Golchin, A.; Friedrich, K.; Noll, A.; Prakash, B. Tribological Behavior of Carbon-Filled PPS Composites in Water Lubricated Contacts. Wear 2015, 328-329, 456-463, DOI: 10.1016/j.wear.2015.03.012. (26) Zalaznik, M.; Kalin, M.; Novak, S.; Jakša, G. Effect of the Type, Size and Concentration of Solid Lubricants on the Tribological Properties of the Polymer PEEK. Wear 2016, 364-365, 31-39, DOI: 10.1016/j.wear.2016.06.013. (27) Zhang, G.; Sebastian, R.; Burkhart, T.; Friedrich, K. Role of Monodispersed Nanoparticles on the Tribological Behavior of Conventional Epoxy Composites Filled with Carbon Fibers and Graphite Lubricants. Wear 2012, 292-293 (15), 176-187, DOI: 10.1016/j.wear.2012.05.012. (28) Zhang, G.; Häusler, I.; Österle, W.; Wetzel, B.; Jim, B. Formation and Function Mechanisms of Nanostructured Tribofilms of Epoxy-Based Hybrid Nanocomposites. Wear 2015, 342-343, 181-188, DOI: 10.1016/j.wear.2015.08.025. (29) Qi, H.; Zhang, L.; Zhang, G.; Wang, T.; Wang, Q. Comparative Study of Tribochemistry of Ultrahigh Molecular Weight Polyethylene, Polyphenylene Sulfide and Polyetherimide in Tribo-Composites. J. Colloid Interface Sci. 2017, 514, 615-624, DOI:



Page 30 of 35

10.1016/j.jcis.2017.12.078. (30) Zhang, L.; Qi, H.; Li, G.; Zhang, G.; Wang, T.; Wang, Q. Impact of Reinforcing Fillers’ Properties on Transfer Film Structure and Tribological Performance of POM-Based

Materials.

Tribol.

Int.

2017,

109,

58-68,

DOI:

10.1016/j.triboint.2016.12.005. (31) Wo, H.; Dearn, K. D.; Song, R.; Hu, E.; Xu, Y.; Hu, X. Morphology, Composition, and Structure of Carbon Deposits from Diesel and Biomass Oil/Diesel Blends on a Pintle-Type

Fuel

Injector

Nozzle.

Tribol.

Int.

2015,

91,

189-196,

DOI:

10.1016/j.triboint.2015.07.003. (32) Hamrock, B. J.; Dowson, D. Isothermal Elastohydrodynamic Lubrication of Point Contacts: Part III-Fully Flooded Results. ASLE Trans. 1977, 27 (4), 275-287, DOI: 10.1115/1.3453074. (33) Derjaguin, B. V.; Muller, V. M.; Toporov, Y. P. Effect of Contact Deformations on the Adhesion of Particles. J. Colloid Interface Sci. 1975, 53 (2), 314-326, DOI: 10.1016/0021-9797(75)90018-1. (34) Haba, D.; Kaufmann, J.; Brunner, A. J.; Resch, K.; Teichert, C. Observation of Elastic Modulus Inhomogeneities in Thermosetting Epoxies Using AFM-Discerning Facts

and

Artifacts.

Polymer

2014,

55

(16),

4032-4040,

DOI:

10.1016/j.polymer.2014.06.030. (35) Dokukin, M. E.; Sokolov, I. Quantitative Mapping of the Elastic Modulus of Soft Materials with Harmonix and Peakforce QNM AFM Modes. Langmuir 2012, 28 (46),


Page 31 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16060-16071, DOI: 10.1021/la302706b. (36) Ye, J.; Khare, H. S.; Burris, D. L. Transfer Film Evolution and Its Role in Promoting Ultra-Low Wear of a PTFE Nanocomposite. Wear 2013, 297 (1-2), 1095-1102, DOI: 10.1016/j.wear.2012.12.002. (37) Sahu, J.; Panda, K.; Gupta, B.; Kumar, N.; Manojkumar, P. A.; Kamruddin, M. Enhanced Tribo-Chemical Properties of Oxygen Functionalized Mechanically Exfoliated Hexagonal Boron Nitride Nanolubricant Additives. Materials Chemistry & Physics 2018, 207, 412-422, DOI: 10.1016/j.matchemphys.2017.12.050. (38) Vadivel, H. S.; Golchin, A.; Emami, N. Tribological Behaviour of Carbon Filled Hybrid UHMWPE Composites in Water. Tribol. Int. 2016, 124,169-177, DOI: 10.1016/j.triboint.2018.04.001. (39) Zhang, G.; Burkhart, T.; Wetzel, B. Tribological Behavior of Epoxy Composites under

Diesel-Lubricated

Conditions.

Wear

2013,

307

(1-2),

174-181,

DOI:

10.1016/j.wear.2013.08.014. (40) Liu, Q.; Duan, Y.; Zhao, Q.; Pan, F.; Zhang, B.; Zhang, J. Direct Synthesis of Nitrogen-Doped Carbon Nanosheets with High Surface Area and Excellent Oxygen Reduction Performance. Langmuir 2014, 30 (27), 8238-8245, DOI: 10.1021/la404995y. (41) Gao, L.; Wen, T.; Xu, J.; Zhai, X.; Zhao, M.; Hu, G.; Chen, P.; Wang, Q.; Zhang, H. Iron-Doped Carbon Nitride-Type Polymers as Homogeneous Organocatalysts for Visible Light-Driven Hydrogen Evolution. ACS Appl. Mater. Interfaces 2016, 8 (1), 617-624, DOI: 10.1021/acsami.5b09684.



Page 32 of 35

(42) Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the

Properties

of

Graphene.

Nat.

Nanotechnol.

2013,

8

(4),

235-246,

DOI:10.1038/NNANO.2013.46. (43) Liang, S.; Shen, Z.; Yi, M.; Liu, L.; Zhang, X.; Ma, S. In-Situ Exfoliated Graphene for High-Performance Water-Based Lubricants. Carbon 2016, 96, 1181-1190, DOI:10.1016/j.carbon.2015.10.077. (44) Pei, Y.; Wang, K.; Zhan, M.; Xu, W.; Ding, X. Thermal-Oxidative Aging of DGEBA/EPN/LMPA Epoxy System: Chemical Structure and Thermal-Mechanical Properties.

Polym.

Degrad.

Stab.

2011,

96

(7),

1179-1186,

DOI:

10.1016/j.polymdegradstab.2011.04.019. (45) Qian, L.; Ye, L.; Qiu, Y.; Qu, S. Thermal Degradation Behavior of the Compound Containing Phosphaphenanthrene and Phosphazene Groups and Its Flame Retardant Mechanism

on

Epoxy

Resin.

Polymer

2011,

52

(24),

5486-5493,

DOI:

10.1016/j.polymer.2011.09.053. (46) Sánchez, G.; Wu, A.; Tristant, P.; Tixier, C.; Soulestin, B.; Desmaison, J.; Alles, A. B. Polycrystalline AlN Films with Preferential Orientation by Plasma Enhanced Chemical Vapor

Deposition.

Thin

Solid

Films

2008,

516

(15),

4868-4875,

DOI:

10.1016/j.polymer.2011.09.053. (47) Bishnoi, S. W.; Rozell, C. J.; Levin, C. S.; Gheith, M. K.; Johnson, B. R.; Johnson, D. H.; Halas, N. J. All-Optical Nanoscale pH Meter. Nano Lett. 2006, 6 (8), 1687-1692, DOI: 10.1021/nl060865w.


Page 33 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48) Musto, P.; Ragosta, G.; Abbate, M.; Scarinzi, G. Photo-Oxidation of High Performance Epoxy Networks: Correlation between the Molecular Mechanisms of Degradation and the Viscoelastic and Mechanical Response. Macromol. 2008, 41 (15), 5729-5743, DOI: 10.1021/ma8005334. (49) Zhao, F.; Zhang, L.; Li, G.; Guo, Y.; Qi, H.; Zhang, G. Significantly Enhancing Tribological Performance of Epoxy by Filling with Ionic Liquid Functionalized Graphene Oxide. Carbon 2018, 136, 309-319, DOI: 10.1016/j.carbon.2018.05.002. (50) Wang, Y.; Gao, K.; Zhang, B.; Wang, Q.; Zhang, J. Structure Effects of sp2-Rich Carbon Films under Super-Low Friction Contact. Carbon 2018, 137, 49-56, DOI: 10.1016/j.carbon.2018.05.016. (51) Guo, L.; Qi, H.; Zhang, G.; Wang, T.; Wang, Q. Distinct Tribological Mechanisms of Various Oxide Nanoparticles Added in PEEK Composite Reinforced with Carbon Fibers. Composites. Part A 2017, 97,19-30, DOI: 10.1016/j.compositesa.2017.03.003. (52) Zhao, F.; Kasrai, M.; Sham, T. K.; Bai, Z. Characterization of Tribofilms Generated from Serpentine and Commercial Oil Using X-Ray Absorption Spectroscopy. Tribol. Lett. 2013, 50 (2), 287-297, DOI: 10.1007/s11249-013-0123-0. (53) Che, Q.; Zhang, G.; Zhang, L.; Qi, H.; Li, G.; Zhang, C.; Guo, F. Switching Brake Materials to Extremely Wear-Resistant Self-Lubrication Materials Via Tuning Interface Nanostructures.

ACS

Appl.

Mater.

Interfaces

2018,

10,

19173-19181,

DOI:

10.1021/acsami.8b02166. (54) Khelifa, F.; Druart, M. E.; Habibi, Y.; Rioboo, R.; Olivier, M.; Coninck, J. D.;



Dubois, P. Effect of Photo-Crosslinking on the Performance of Silica Nanoparticle-Filled Epoxidized Acrylic Copolymer Coatings. J. Mater. Chem. A 2013, 1 (35), 10334-10344, DOI: 10.1039/C3TA11668A.


Page 34 of 35

Page 35 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents (TOC) synopsis Formation of a nanostructured tribofilm reduces extraordinarily friction and wear of epoxy-steel sliding pairs lubricated with ultra-low sulfur diesel.


Extraordinarily Low Friction and Wear of Epoxy-Metal Sliding Pairs

Recommend Documents