Switching Brake Materials to Extremely Wear-Resistant Self

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Surfaces, Interfaces, and Applications

Switching Brake Materials to Extremely Wear-Resistant SelfLubrication Materials via Tuning Interface Nanostructures Qinglun Che, Ga Zhang, Ligang Zhang, Huimin Qi, Guitao Li, Chao Zhang, and Feng Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02166 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

Switching Brake Materials to Extremely Wear-Resistant Self-Lubrication Materials via Tuning Interface Nanostructures Qinglun Chea,b, Ga Zhang*b,c, Ligang Zhangb, Huimin Qib,d, Guitao Lib, Chao Zhange, Feng Guoa a

b

College of Mechanical Engineering, Qingdao University of Technology, Qingdao 266033, China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China c Qingdao Center of Resource Chemistry & New Materials, Qingdao 266071, China d University of Chinese Academy of Sciences, Beijing 100049, China e College of Mechanical Engineering, Yangzhou University, Yangzhou 225127, China *Corresponding author: Prof. Ga Zhang E-mail: [email protected] Tel: +86-931-4968041 Fax: +86-931-4968180

Keywords: Brake material, Self-lubrication, Nanostructure, Friction Interface, Nanoparticles

Abstract Tribological performance of motion components is one of the key aspects that must be considered in a wide range of applications such as vehicles, aircrafts and manufacturing equipment. This work demonstrates that further addition of only low-loading hard nanoparticles into a formulated non-asbestos organic brake material directly switches its functionality to a self-lubrication material. More importantly, the newly developed nanocomposites exhibit an extremely low wear rate. Comprehensive investigations on the friction interface reveal that the great friction and wear-reduction is due to formation of a nanostructured lubricious tribofilm. Tribofilm formation is continuously fed by complex molecular species released from the bulk nanocomposites, for which nanoparticles digested within the tribofilm greatly enhance its robustness and lubricity. This work gains insight into the crucial role of the interface nanostructure and paves a route for developing extremely wear-resistant self-lubrication composites for numerous applications.

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1 Introduction Non-asbestos organic (NAO) brake materials are being widely used in fields of transportation vehicles, machinery and windmill industries due to their relatively high friction coefficient and very high wear resistance, low cost and environmental friendliness.1,2,3 In general, NAO brake materials consist of phenolic resin as binder, various functional fillers, e.g. reinforcing fibers, coarse abrasives, solid lubricants, and ordinary fillers, e.g. barite, talcum and chalk. Contents of the fillers in the NAO brake material in total are rather high, i.e. usually higher than 85 wt.%.4,5,6

Based on vast numbers of trials, hundreds of brake products of different formulations were developed for various applications.7,8 The friction coefficients of formulated NAO brake materials usually lie in the range of 0.3-0.6.9,10,11 Depending on service conditions, the specific wear rate, defined as volume loss per unit meter per unit load, of optimally formulated NAO brake materials lies in the magnitude of 10-8 mm3/Nm.12,13 It was surmised that the inclusion of high fraction wear-resistant fillers accounted for the high wear resistance.14,15

There is no general consensus yet on underlying mechanisms determining the high friction of NAO brake materials.3,5 Nevertheless, it has been recognized that generation of a stable tribofilm on the steel counterface is important for getting a high and steady friction coefficient.16,17 The tribofilm is usually very thin and it is hard to characterize its nanostructure without tedious and expensive sample preparations. Research efforts have been dedicated to identifying structures of the tribofilms formed on steel surfaces rubbed with NAO brake materials and it has been demonstrated that the tribofilms mainly consist of iron oxide crystals.18,19

Polymer self-lubrication composites have been increasingly utilized as sliding bearings, bushings and seals, owing to their self-lubrication characteristics.20-23 Reinforcing fibers and solid lubricants ACS Paragon Plus Environment

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are often added into polymer matrix for formulating high-performance self-lubrication composites.24,25 Nevertheless, contents of the fillers in self-lubrication composites are much lower in comparison to those in NAO brake materials, i.e. usually lower than 30 wt.%.26,27,28 Similar to NAO brake materials, a high wear resistance of such self-lubrication composites is always required for extending the lifespan. Furthermore, in contrast to that of brake materials, a low friction coefficient of the self-lubrication composites is highly desired for minimizing energy consumption. In the last decades, great progress has been achieved in the field of polymer self-lubrication composites. Advanced self-lubrication composites exhibiting friction coefficients lower than 0.10 and specific wear rates in the magnitude of 10-7 mm3/Nm have been successfully developed.29,30

It has been well recognized that tribological performance is not an intrinsic property of materials but a system behavior. Formation of protective surface-bonded tribofilms mitigates direct rubbing of polymer composites and their metallic counterparts and therefore is regarded essential for obtaining low friction and wear.27,28,30 Despite arguments on underlying mechanisms, it has been demonstrated that compounding SiC,27,31 TiO2,32,33 SiO218,23,28,30 and Al2O334 nanoparticles into polymer matrices or their conventional self-lubrication composites significantly lowers the friction and wear. It was surmised that possible rolling of nanoparticles32,33,35 and improved tribofilm uniformity27,30,33,36 accounted for the improved tribological performance.

In this work, low-loading hard oxide nanoparticles were further added into a formulated NAO brake material. The tribological characteristics of the composites were comparatively investigated. In particular, nanostructures of the friction interface were investigated in depth. It was the objective of this work to explore the possibility to transform a NAO brake material to a self-lubrication material by tuning the interface structure. It is expected that the outcome of the present work will gain



insight into the crucial role of interface structures of the wear resistant materials.

2 Experimental 2.1 Materials and preparation Cashew nut oil modified phenolic resin (Sumitomo Bakelite Co. Ltd, Japan) was used as binder resin. This kind of phenolic resin has been widely utilized for manufacturing automotive brake pads. The primary purpose of the modification is to improve the toughness and heat resistance of the binder resin.37,38 Barite powders and talcum powders serving as ordinary fillers were purchased from Shandong Zhenhua Industrial Co. Ltd (China) and Shandong Anquan Chemical Technology Co. Ltd (China), respectively. The main compositions of the barite and talcum powders were BaSO4 (purity: 95%) and Mg3Si4O10(OH)2 (purity: 94%), respectively. Aramid pulp and carbon fibers employed as reinforcing fillers were supplied by Shanghai Teijin Aramid Co. Ltd (China) and Nantong Senyou Carbon Fiber Co. Ltd (China), respectively. Graphite flakes (RGB390TS, Superior graphite, US) were used as solid lubricants.

As a reference composite, a NAO brake material was prepared by adapting the formulations of Lu39 and Ma et al.40 The volume fractions of the ingredients were determined following the Golden Section approach as often utilized for formulating brake materials.12,39,41 The term 0.618n (n=0, 1, 2, 3 and so on) was applied for determining the volume fractions. That is, volume fractions of the resin binder (Vb=0.236, 0.6183), reinforcing fillers (VF=0.382, 0.6182), and ordinary fillers (Vf=0.382, 0.6182) were respectively 23.6%, 38.2% and 38.2%. Our experiments confirmed the high performance of the reference brake material designated below as BraM-C (abbreviation of conventional brake material).

Based on the reference formulation, 1.0-5.0 vol.% ceramic nanoparticles, i.e. ZrO2, SiO2 and Al2O3, ACS Paragon Plus Environment

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were further added into BraM-C. The nanoparticles were supplied by Beijing DK Nano Technology Co. Ltd (China). The average diameters of the ZrO2, SiO2, and Al2O3 nanoparticles were 30, 20 and 70 nm, respectively. Scanning Electron Microscope (SEM) morphologies of the nanoparticles are shown in Figure S1 (Supporting information). The nanocomposites are denominated below according to the type and fraction of the nanoparticles. For example, BraM-1.5Zr refers to the composite containing 1.5 vol.% nano-ZrO2. Detailed compositions of the conventional brake material and the nanocomposites are listed in Table 1. Densities of the composite materials studied were measured based on the Archimedes principle and are illustrated in Table 1.

Table 1. Compositions and densities of composite materials investigated Composite Materials Ingredients (vol.%) BraM-C

BraM-1Zr

BraM-1.5Zr

BraM-3Zr

BraM-5Zr

BraM-3Si

BraM-3Al

Nano-ZrO2

0

1

1.5

3

5

0

0

Nano-SiO2

0

0

0

0

0

3

0

Nano-Al2O3

0

0

0

0

0

0

3

Graphite

9

9

9

9

9

9

9

25.8

26.8

26.3

26.4

24.6

26.4

26.4

Barite Talcum

9

9

9

9

9

9

9

Aramid pulp

23.6

21.6

21.6

20

20

20

20

Carbon fiber

9

9

9

9

9

9

9

23.6

23.6

23.6

23.6

23.6

23.6

23.6

2.40

2.38

2.40

2.45

2.45

2.32

2.35

Phenolic resin 3

Densities of composite (g/cm )

In order to get uniform distribution of the fillers, all ingredient materials were mixed in a high-speed blender (DFY-250C, Linda machinery Co. Ltd, China) at a rotation speed of 3000 rpm. In order to avoid overheating of the phenolic resin, the mixing process was conducted intermittently, i.e. the mixing was stopped after each 15 seconds. Evenly mixed ingredients were put into a die and hot pressed at 5 MPa in the die with stepwise increasing the temperature to 165 °C. In order to release sufficiently the gas generated during the process, the low pressure (5 MPa) was maintained for 200 min. The pressure was afterwards enhanced to 30 MPa and then maintained for 30 min. The ACS Paragon Plus Environment


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specimen (approximately 4 mm thick) was naturally cooled to ambient temperature before demolding. Finally, the samples were annealed at 180 °C for 5 h for relieving residual stress. Diagrams in Figure S2 (Supporting information) illustrates detailed sintering and annealing parameters.

2.2 Tribological tests Tribological tests were conducted at room temperature in air ambience on a Pin-On-Disc (POD) tribometer (TRM 1000, Wazau, Germany). Photograph of the test rig and scheme of the contact configuration of the friction pair are given in Figure S3 (Supporting information). GCr15 bearing steel (GB/T18254-2016) discs (LS2542) were used as counterpart. Before the test, the steel disc was ground with a 1000 grit silicon carbide paper so that a mean roughness Ra=0.25 µm was obtained. The specimens had dimensions of 4×4×12 mm3. As seen from Figure S3 (Supporting information), during POD tribo-tests, the steel disc (upper) rotated with the stationary composite pin (below). The radius of rotation was 16.5 mm. In order to explore the tribological behaviors under harsh friction conditions, the normal loads varied from 6 to 30 MPa while the sliding speed was fixed at 1 m/s. Each test was repeated for at least three times. The friction coefficient was measured online by a torque sensor and the specific wear rate Ws of the composite pin was calculated using the following equation: Ws=∆m/(ρ L F)

(1)

where ∆m was the mass loss of the composite pin, ρ was the density of the specimen, L was the sliding distance in total, and F was the applied load.

2.3 Analyses of worn surfaces and tribofilms The worn surfaces of composite pins and steel discs were inspected using an optic microscope


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(Olympus BX41, Japan) and a Field Emission Scanning Electron Microscopy (FE-SEM, Merlin Compact, Carl Zeiss, Germany) after depositing a thin gold layer for enhancing electrical conduction. Elemental compositions of composites’ surfaces and tribofilms generated on the steel surfaces were analyzed using an Energy Dispersive X-ray Spectroscopy (EDS, Energy 350, Oxford, UK) attached onto the FE-SEM.

In order to gain insight into the tribofilms generated on the steel counterface, their nanostructures were characterized in-depth using a Transmission Electron Microscope (TEM, Tecnai G2 TF20S-TWIN, FEI, US) in combination with Selected Area Electron Diffraction (SAED) and Scanning Transmission Electron Microscopy combined with Energy Dispersive X-ray Spectroscopy (STEM/EDS). A slice of tribofilm’s cross-section was prepared by Focused Ion Beam (FIB) machining in a Dual Beam SEM/FIB instrument (Quanta 3D FEG, FEI, US). In order to elucidate possible tribochemical reactions, functional groups of tribofilms were analyzed by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR, TENSOR 27, Bruker, Germany) using a zinc selenide ATR unit. The spectra were recorded while scanning in selected regions in 4000-500 cm-1 wavenumber range at a resolution of 4 cm-1.

3 Results and discussion 3.1 Significantly modified tribological behavior by adding nanoparticles Figure 1 shows the effect of ZrO2 nanoparticles on the friction coefficient and wear rate obtained when sliding takes place at 6 MPa. It is demonstrated that further adding nano-ZrO2 into the brake material BraM-C dramatically lowers the friction coefficient and wear rate. Moreover, the friction coefficient and wear rate decrease continuously with enhancing the volume fraction of nano-ZrO2 from 1% to 3%. The addition of 3% nano-ZrO2 into BraM-C decreases the friction coefficient from



0.35 to 0.16 (see Figure 1a). In comparison to the original BraM-C, the wear rate is reduced by more than 90% by adding 3% nano-ZrO2 (see Figure 1b). The lowest wear rate, i.e. 3.1×10-8 mm3/Nm, is obtained when BraM-3Zr is sliding against the steel disc. According to our best knowledge, such a wear resistance is higher than those of polymer self-lubrication composites ever reported in literatures. Further enhancement of the volume fraction of nano-ZrO2 from 3% to 5% does not lead to significant change of the friction coefficient and wear rate.

Figure 1. Effect of volume fraction of nano-ZrO2 on (a) mean friction coefficient and (b) specific wear rate of the original brake material BraM-C; (c) evolutions of friction coefficients of BraM-C and BraM-3Zr as a function of sliding time. Pressure: 6 MPa. Figure 1c compares evolutions of the friction coefficients of BraM-C and BraM-3Zr as a function of time recorded from the sliding at 6 MPa. It is manifested that the friction coefficients of the both composites increase in the starting stage most probably owing to tribo-oxidation of the steel surface.5,7,41 Nonetheless, the friction coefficient of BraM-C decreases slightly from 0.5 h to 1 h and ACS Paragon Plus Environment

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finally becomes steady at about 0.35. The slight decrease of the friction coefficient is caused by material transfer from the composite pin onto the steel surface, as demonstrated below. With respect to the sliding of BraM-3Zr, the friction coefficient decreases continuously from 0.2 h till 2.5 h and finally it becomes steady at about 0.16. Moreover, after the running-in stage, fluctuation of the friction coefficient of BraM-3Zr is much smoother than that of BraM-C, indicating generation of a more stable friction interface.

Hence, our work demonstrates the feasibility of transforming a NAO brake material to a self-lubrication material by adding only low-loading ZrO2 nanoparticles. The nanocomposites containing nano-ZrO2 have a great potential for applications as extremely wear-resistant self-lubrication materials. As revealed below, the dramatically decreased friction and wear after adding nano-ZrO2 is mainly in association with generation of a nanostructured lubricous interface. When more and more ZrO2 nanoparticles are released from the bulk composites, they play an important role in the structure of the rubbing interface.

In order to explore roles of various oxide nanoparticles, tribological performances of BraM-3Zr, BraM-3Al and BraM-3Si were comparatively studied at pressures ranging from 6 to 30 MPa. Similar to the role of nano-ZrO2, further adding SiO2 or Al2O3 nanoparticles into BraM-C leads to dramatically decreased friction coefficient and wear rate (see Figure 2a and b). The friction coefficients of the three nanocomposites are in a similar level, although the friction coefficient of BraM-3Al is slightly higher than those of BraM-3Si and BraM-3Zr. The friction coefficient of BraM-C does not vary significantly with increasing the pressure from 6 to 30 MP, which remains in the range of 0.30-0.35. The high reliability of BraM-C as a brake material is thus verified, for which a stable and relatively high friction coefficient are necessary. Similarly, the friction coefficients of



BraM-3Si and BraM-3Al are not sensitively dependent on the pressure. Nevertheless, BraM-3Zr exhibits a friction coefficient as low as 0.10 when sliding against the steel disc at 30 MPa. To our best knowledge, such a friction coefficient is lower than those of most self-lubrication composites reported in the literature.

Figure 2. Effect of applied pressure on (a) mean friction coefficients and (b) specific wear rates of BraM-C, BraM-3Zr, BraM-3Si and BraM-3Al; evolution of friction coefficients of BraM-C, BraM-3Zr, BraM-3Si and BraM-3Al versus sliding time recorded during the siding at 18 MPa (c) and 30 MPa (d). As seen from Figure 2b, further addition of the nanoparticles reduces the wear rate by up to one order of magnitude. In comparison to BraM-3Zr and BraM-3Si, BraM-3Al exhibits slightly higher wear rates when sliding takes place at 6, 18 and 30 MPa. With increasing the pressure from 6 to 30 MPa, the wear rate of the original brake material BraM-C increases continuously. The lower wear resistance of BraM-C at high pressures can be attributed to decreased mechanical properties of the ACS Paragon Plus Environment

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binder resin when more friction heat is produced. Nevertheless, the nanocomposites demonstrated always a very high wear resistance in the pressure range studied. In particular, the wear rates of BraM-3Zr and BraM-3Si are in the magnitude of 10-8 mm3/Nm even when being subjected to the sliding at 30 MPa.

Figure 2c and d illustrate evolutions of friction coefficients of BraM-C, BraM-3Zr, BraM-3Si and BraM-3Al recorded during the sliding at 18 MPa and 30 MPa, respectively. As consistent with above observations, the friction coefficient of BraM-C increases in the starting stage due to tribo-oxidation of the steel and then it slightly decreases. Moreover, the friction coefficient of BraM-C fluctuates significantly even after the running-in stage. With respect to the sliding of BraM-3Zr at 18 MPa, the friction coefficient first increases and then decreases significantly following a similar tendency as observed at 6 MPa (cf. Figure 1c). Nevertheless, the increase of the pressure shortens the running-in duration. This is true especially when increasing the pressure to 30 MPa (Figure 2d). We believe that the tribofilm forms quickly under a high pressure since nanoparticles are released rapidly from the nanocomposite onto the sliding interface. With respect to the sliding of BraM-3Si and BraM-3Al, obvious increment of friction coefficient does not occur in the starting stage. This gives a hint that tribo-oxidation of the steel is inhibited from the beginning of the test. After the running-in stage, the friction coefficients of the three nanocomposites become similar. Nevertheless, when sliding takes place at 30 MPa, the friction coefficient of BraM-3Al increases further from about 0.1 to 0.2 after around 1 h (Figure 2d). Further efforts are required to elucidate whether this increment is caused by possible structural modification of the tribofilm as abundant hard Al2O3 nanoparticles are released onto the interface.

3.2 Structures of tribofilms and worn surfaces



Figure 3a and b show respectively SEM micrographs of the worn surfaces of BraM-C and BraM-3Zr generated after the sliding at 6 MPa. Two typical regions, i.e. primary plateaus and secondary plateaus, are clearly identified from the friction layers. The primary plateaus correspond to carbon fibers and aramid pulp, which exhibit a significantly higher abrasion resistance than phenolic resin and other fillers, i.e. BaSO4, Mg3Si4O10(OH)2 and graphite.42 Secondary plateaus are generated owing to retention and compaction of wear particles in vicinity of protruding fibers and pulp, i.e. primary plateaus. Thus, the primary plateaus play an important role in formation of the secondary plateaus.16,43,44

Figure 3. SEM micrographs of worn surfaces of BraM-C (a) and BraM-3Zr (b); (c)-(g) EDS elemental maps of the worn surface of BraM-3Zr (same area as b). Applied pressure: 6 MPa. Closer inspections of the worn surfaces demonstrate that the secondary plateaus on the worn surface of BraM-3Zr cover nearly the entire surface and are more compact, when compared to that of BraM-C (cf. Figure 3a and b). EDS elemental maps of the worn surface of BraM-3Zr indicate that the secondary plateaus comprise evenly mixed ZrO2, BaSO4 and Mg3Si4O10(OH)2 (Figure 3c-g). In particular, Zr-element covers majority of the primary plateaus as well (see Zr map in Figure 3c), demonstrating that a thin ZrO2 layer forms on the ends of the reinforcing fillers. It is believed that


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the thin ZrO2 layer sustains a very high rubbing stress and protects effectively the friction surface. Moreover, it is surprising that little C-element is present in the secondary plateaus (see C map in Figure 3e).

Due to the high flash temperature on the friction interface, the resin matrix of the wear particles decomposes and thus the nanoparticles dispersed in the resin are released.18,28 Owing to the high stress and flash temperature, released nanoparticles are finally tribo-sintered into a compact layer after mechanically mixing with pulverized BaSO4 and Mg3Si4O10(OH)2 powders. Tribo-sintering of ceramic nanoparticles preliminarily spreading onto a steel-steel friction interface was identified by Kato et al.45 As demonstrated below, the secondary plateaus of BraM-3Zr contain nearly identical ingredients as the tribofilm generated on the steel counterface. It is surmised that the secondary plateaus serve as an ingredient reservoir and thus continuously replenish the tribofilm by constantly feeding the species onto the friction interface. This is supposed essential for getting a stable friction coefficient when sacrifice and replenishment of the tribofilm reach a smooth equilibrium. Inclusion of ZrO2 nanoparticles as load-carrying components enhances the robustness of the secondary plateaus and the tribofilm sustaining harsh rubbing stress.

Note that BaSO4 and Mg3Si4O10(OH)2 have rather low hardness, i.e. Mohs hardness in the ranges of 3-4 and 1-1.5, respectively. When sliding takes place with BraM-C, secondary plateaus without the hard nanoparticles can hardly sustain the rubbing stress. Destruction of secondary plateaus was regarded as the origin of friction fluctuation.46,47,48 In fact, destruction of the secondary plateaus makes tribofilm replenishment difficult and therefore a stable interface is hardly established. With respect to the sliding of the nanocomposites, fluctuation of the friction coefficient is significantly suppressed, when compared to that of BraM-C.



Figure 4 displays micrographs of the steel surface rubbed at 6 MPa with BraM-C and BraM-3Zr. Discontinuous “smooth lands” take up majority of the surface area, as indicated by arrows in Figure 4a. Such “smooth lands” correspond to a tribo-oxidation layer of the steel surface and is an indication of direct rubbing of the sliding pair. When the reinforcing fillers rub with the steel surface, a high flash temperature develops on the convex points and thus leads to tribo-oxidation of the steel.28,30 Generation of such “smooth lands” causes high adhesion between the sliding pair.20,28 For the sliding of BraM-C, the increment of friction coefficient in the running-in stage observed above can be attributed to formation of the tribo-oxidation layer.

Figure 4. Optical micrographs of triboflims formed on steel surfaces rubbed at 6 MPa with BraM-C (a) and BraM-3Zr (b); SEM micrographs with a low (c) and high (d) magnifications of the tribofilm generated on the steel surface rubbed with BraM-3Zr; (e) EDS spectroscopy of the steel surface rubbed with BraM-3Zr at the location indicated by the white dot in c.


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Distinctly different morphologies are observed from the optical micrograph of the steel surface rubbed with BraM-3Zr (Figure 4b). In comparison to the steel surface rubbed with BraM-C, when sliding takes place with BraM-3Zr, much smaller “smooth lands” are generated, which takes up only minority of the surface area. This gives a hint that direct rubbing of BraM-3Zr with the steel is significantly mitigated, when compared to the sliding with BraM-C. Moreover, majority of the steel surface was covered by a continuous tribofilm, as seen from Figure 4c and d.

EDS analysis identifies clear signatures of Zr, Si, Mg, Ba and S elements from the tribofilm generated on the steel surface rubbed with BraM-3Zr (Figure 4e). Whereas, the small peak of Fe-element suggests that the fraction of iron oxide in the tribofilm is rather low. Moreover, the small peak of C-element corroborates that the binder matrix in the wear particles decompose owing to the high flash temperature.30 When the hard nanoparticles are released onto the interface, they abrade the tribo-oxidation layer on the steel surface. As evidenced below by FIB-TEM investigations, all wear products are finally compacted into a solid tribofilm as a result of stress and thermally activated tribo-sintering.

When the nanostructured tribofilm forms after the running-in process, it mitigates effectively direct rubbing of the sliding pair. It is believed that the fluctuation of friction coefficient in the running-in stage is related to structural evolutions of the tribofilm, which are governed in succession by tribo-oxidation of the steel surface, abrasion of tribo-oxidation layer by released nanoparticles and tribo-sintering of the wear products. Owing to the inclusion of the soft species, i.e. remnant polymer particles, graphite, BaSO4 and Mg3Si4O10(OH)2, the tribofilm can show an easy-to-shear characteristic. More importantly, digestion of the released hard nanoparticles within the tribofilm



enhances its load-carrying ability.18,28,30 After the running-in stage, loss and replenishment of the tribofilm reach equilibrium. That is, the secondary plateaus replenish continuously the tribofilm by constantly feeding the species onto the interface. On the other hand, worn events of the tribofilm back-transfer onto the composite surface and thus replenish the secondary plateaus. The authors believe that mutual ingredient feeding between the tribofilm and the secondary plateaus play an important role in maintaining a smooth friction process.

Figure S4 (Supporting information) compares morphologies of the steel surfaces slid at 30 MPa against BraM-C, BraM-3Zr, BraM-3Si and BraM-3Al, respectively. As consistent with above observations, after sliding with BraM-C, numerous “smooth lands” are generated owing to tribo-oxidation of the steel (Figure S4a (Supporting information)). With respect to the sliding of the three nanocomposites, tribofilms are generated covering large areas of the steel surface. It is apparent that tribofilms of the three nanocomposites show similar morphologies. It is assumed that the slight difference of nanocomposites’ tribological performances is ascribable to small discrepancies of the tribofilms in regards of modulus and shear strength.

Figure S5 (Supporting information) illustrates SEM micrographs and EDS elemental maps of the steel surface rubbed at 30 MPa with BraM-C. Only small peaks of Mg, Si, and Ba elements are identified from the steel surface. In particular, almost none of the above elements distribute on the plateau areas between the roughness grooves. The steel surface comprises mainly Fe and O elements deriving from tribo-oxidation products. Without inclusion of the hard nanoparticles, the soft tribofilm having a low load-carrying ability is easily destroyed by the rubbing stress.18,20 As a consequence, direct rubbing of the sliding pair and severe tribo-oxidation of the steel surface occur.


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Figure 5. SEM micrograph (a) and EDS maps of Zr (b), O (c), C (d), Fe (e), Mg (f) and Ba (g) elements of steel surfaces slid with BraM-3Zr at 30 MPa. On the contrary, significant fractions of Zr, Ba, and Mg elements distribute on nearly the entire surface of the steel counterpart rubbed with BraM-3Zr (see Figure 5a). In addition, the distribution of Zr, Mg and Ba elements coincide well with each other (Figure 5b, f and g), indicating that the wear products are evenly mixed on the interface. It is thus corroborated that the tribofilm consisting of both the hard and soft ingredients exhibits a high robustness sustaining harsh interfacial rubbing.

Figure 6a illustrates ATR-FTIR spectra of BraM-C bulk material and the tribofilm formed on the steel surface rubbed at 30 MPa with BraM-C. As seen from the spectrum of the tribofilm, the absorption peaks at 1186 and 610 cm-1 (S-O), 1615 and 1441 cm-1 (Mg-O), and 991 cm-1 (Si-O) indicate presence of wear products of BaSO4 and Mg3Si4O10(OH)2.49,50,51 Moreover, Fe3O4



generated due to tribo-oxidation of the steel is clearly identified with an absorption peak at 580 cm-1 (Fe-O).52 Figure 6b gives ATR-FTIR spectra of BraM-3Zr bulk material and the tribofilm formed on the steel surface rubbed with BraM-3Zr. Besides the ordinary fillers, signatures of ZrO2 are identified from the absorption peaks at 900 and 650 cm-1 (Zr-O) in the spectrum of the tribofilm (Figure 6b).

Figure 6. (a) ATR-FTIR spectra of unworn BraM-C and its tribofilm formed on steel surface after the sliding at 30 MPa; (b) ATR-FTIR spectra of unworn BraM-3Zr and its tribofilm formed on steel surface after the sliding at 30 MPa; (c) scheme of chain scission of phenolic resin and chelation of phenolic groups with the steel counterface. ACS Paragon Plus Environment

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The peaks at 3309.9, 1617, 1266, 1080 and 873 cm-1 in the spectra of BraM-C and BraM-3Zr bulk materials were attributable to chemical bonds of phenolic resin. However, most of these absorption peaks are not noticed from the spectra of the both tribofilms. More interestingly, new chemical species are identified from the tribofilms with absorption peaks at 826 cm-1 and 805 cm-1, as indicated by arrows in Figure 6a and b, corresponding to metal chelate salts of methyl phenolic alcohol Fe(OOC6H5) and Fe(OC21H30).53-57 This implies that tribo-chemical reactions between the polymer molecules and the steel occurred. As schematically depicted in Figure 6c, the molecular chains of phenolic resin are broken due to mechanical and thermal effects and thus phenolic hydroxyl free radicals and methyl phenol radicals are produced. The hydroxide radicals and methyl phenol radicals in an unstable state can lose their hydrogen atoms, methyl and phenolic groups to form phenolic hydroxyl free radicals and methyl phenol radicals, respectively. Finally, the phenolic hydroxyl free radicals and phenyl radicals chelate with the steel surface to generate metal-organic compounds. These results indicate that a chemically-bonded tribofilm is established while the polymer resin feeds molecular species onto the interface.

In order to gain insight into nanostructures of the tribofilm generated on the steel surface after rubbing with BraM-3Zr, FIB-TEM analyses were conducted. As seen from the TEM micrograph in Figure 7a, a continuous tribofilm having a thickness of about 100 nm covers the steel surface. Closer inspections of the tribofilm show that except for small amorphous particles of the binder resin (white phase in Figure 7a and b), the tribofilm has a uniform structure. From the inserts in Figure 7b, FFT analyses verify the presence of ZrO2, BaSO4, Mg3Si4O11 and graphite in the tribofilm, which correspond respectively to lattice fringes of 0.202 nm, 0.221 nm, 0.234 nm, 0.300 nm, 0.335 nm and 0.331 nm (JCPDS No. 24-1165, 24-1165, 24-1035, 84-1222, 80-0965, and



41-1487). SAED patterns demonstrate clear diffraction rings of ZrO2, BaSO4, Mg3Si4O11, and graphite (Figure 7c). It is surmised that Mg3Si4O11 was produced when Mg3Si4O10(OH)2 lost the crystal water during the friction process.58

Figure 7. (a) TEM micrograph of the tribofilm formed on steel surface slid against BraM-3Zr at 30MPa; (b) high-magnification TEM graph of the tribofilm’s cross-section with inserts showing FFT graphs of selected regions indicated in dashed squares; (c) SAED patterns of the tribofilm showing clear diffraction rings of ZrO2, BaSO4, Mg3Si4O11 and graphite; (d) STEM micrograph of the tribofilm and EDS maps of Zr (e), C (f), Fe (g), Mg (h) and Ba (i) elements. STEM graph of the tribofilm (left) and EDS maps of Zr, C, Fe, Mg and Ba elements are illustrated in Figure 7d, e, f, g, h and I, respectively. The EDS maps corroborate the uniform mixture of the wear products. In contrast to the tribofilm of BraM-C comprising mainly tribo-oxidation products, only a little iron oxide is present in the tribofilm of BraM-3Zr. This provides evidence that the nanostructured tribofilm inhibits effectively tribo-oxidation of the steel counterface by separating


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direct rubbing of the sliding pair.

4 Conclusions Our work demonstrates that the addition of low-loading hard nanoparticles into an original NAO brake material switches the material’s functionality on the opposite way to a self-lubrication material. More importantly, the newly developed nanocomposites exhibited an extremely high wear resistance. Nanostructures of tribofilms generated on the steel counterface were comprehensively investigated. It is revealed that when the nanocomposites slide against the steel counterface, lubricious and tightly-bonded tribofilms are generated minimizing direct rubbing of the friction pair. Tribofilm is formed owing to complex tribo-physical and chemical actions on the interface, and is replenished constantly as the nanocomposites feed hybrid molecular species onto the interface. When the nanoparticles are released onto the friction interface, they are “tribo-sinterred” into a compact tribofilm after being evenly mixed with other wear particles. The hard nanoparticles digested within the tribofilm enhance greatly its load-bearing ability. Our work proves the feasibility to tune the nanostructure and functionality of the friction interface by filling nanoparticles into NAO brake materials. It is expected that the outcome of this work can pave a route for developing extremely wear-resistant self-lubrication materials for a wide variety of applications.

Acknowledgements The authors acknowledge gratefully the financial support from National Key Research and Development Program of China (Grant no. 2017YFB0310703), National Natural Science Foundation of China (Grant no. 51475446), the “Innovation Leading Talents” program of Qingdao city and Natural Science Foundation of Shandong (Grant no. ZR2017QF010).



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Nanostructured tribofilm leads to dramatically reduced friction and wear


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Switching Brake Materials to Extremely Wear-Resistant Self

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