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CVD-Graphene Coating for Improving Friction and Wear of Grey Cast Iron at Interfaces Khagendra Tripathi, Gobinda Gyawali, and Soo Wohn Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07922 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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CVD-Graphene Coating for Improving Friction and Wear of Grey Cast Iron at Interfaces Khagendra Tripathia*, Gobinda Gyawalia, Soo Wohn Leeb* a

Research Center for Eco Multi-Functional Nanomaterials, Sun Moon University, 31460, South

Korea b

Department of Environmental and Bio-chemical Engineering, Sun Moon University, Korea *

Email: [email protected] & [email protected]

Abstract This study reports the influence of CVD-graphene on the tribological performance of grey cast iron (GCI) from the internal combustion engine (ICE) cylinder liners by performing a ball-ondisk friction tests. The Graphene-coated specimen exhibited a significant reduction (~ 53%) of friction as compared to the uncoated specimen, whereas wear resistance increased by 2-folds and 5-folds regarding the wear of specimen and ball, respectively. Extremely low shear strength and highly lubricious nature of graphene attribute to the formation of a lubricious film between the sliding surfaces and decreases the interaction between surfaces in the dry environment. Under the applied load, uniform film of iron oxides such as Fe2O3, Fe3O4, FeOOH is found to be formed between the surfaces. It is proposed that the graphene encapsulation with the metal debris and oxides formed between the specimens increase the lubricity and decreases the shear force. The transformation of graphene/graphite into nanocrystalline graphites across the contact interfaces following the amorphization trajectory further increases the lubricity of the film that ultimately reduces friction and wear of the material. Keywords: graphene, CVD, friction, wear resistance, grey cast iron, solid lubricants 1. Introduction Grey cast iron (GCI) is widely used in diesel engine components such as cylinder heads and piston rings due to its unique combination of properties such as excellent thermal conductivity, vibration damping ability, and good machinability.1 The material also yields a surface with excellent wear characteristics. Relatively low friction and wear of the material is mainly due to the uniform distribution of the graphite flakes on the materials as discussed its effect in previous studies.2,3 The graphite flake behaves as a solid lubricating additive on the surface. However, there is much scope for the improvement in friction and wear of the material by employing the surface modification processes such as laser surface texturing,4,5 and ultrasonic nanocrystalline


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surface modification6 as explained earlier in our previous studies. In these days, many studies have focused on thin films coatings and achieved an excellent improvement in the friction and wear.7 Based on previous studies, the coating materials such as metal carbides (WC, TiC, CrC), diamond-like carbon (DLC), and molybdenum disulfide (MoS2) have found to exhibit low friction (< 0.2) when sliding against steel8–12 in boundary lubrication. High hardness, low friction, high wear resistance, low surface energy and chemical inertness of the coatings are supposed to be beneficial for the enhancement of tribological performance of materials. The purpose of the explanation here is to modify the cast iron specimen by the introduction of graphene thin films to improve the tribological properties of the material. Despite its extreme thinness, it can have a much efficient reduction of friction and wear by coating the surface with >3 layers of graphene.13–15 Therefore, more studies on the uses and advantages of the few-tomultilayer graphene coatings to different materials would help to develop graphene and graphene composite films as a useful coating material for the tribological applications in future. Since the discovery of graphene16 in 2004 as the thinnest, the strongest and highly conducting material, it has been considered as a wonder material which can have diverse fields of applications in the science and technology. Graphene has also created utmost attention to the tribological community regarding its implementation as a low friction and wear material due to its superior mechanical properties such as high modulus of elasticity (~1 TPa) and high fracture toughness (125 GPa).17,18 Besides this, it has high thermal conductivity19(~5000 Wm1

K-1) and super charge-carrier mobility.16 Studies in the past have presented graphene as very

much active coating material as a solid lubricant20 at different environment compared to graphite and other conventional solid lubricants such as metal oxides (CuO, ZnO) and metal sulfides (MoS2, WS2). It is some time that graphene and graphene nanocomposite coatings have triggered its use to protect the materials from corrosion.7,21 Some of the recent studies on the tribological behavior of graphene thin films coatings at nano scale and macro scale projected it as a highly efficient coating material for the tribological applications. One of the previous studies demonstrated an achievement of an extraordinary wear resistance by 1 to 4 graphene layers coating at the various environment.22 Similarly, the self-assembled graphene oxide thin film

on

the

silicon

dioxide

substrate

via

covalent

interaction

using

3-

aminiopropyltrimethooxysilane found to have exhibited a very low friction and remarkable high wear resistivity.23 Liang et al. have revealed graphene oxide film as good solid lubricants for silicon-based microelectromechanical systems (MEMS) due to the significant improvement in the friction and wear of silicon wafer.

24

Moreover, some studies showed the synergistic



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approach using nanofluids as efficient lubricants, prepared by mixing graphene onto the engine and paraffin oils reducing friction and wear for the tribological applications.

25, 26

Therefore,

graphene-based engine nanofluids and dispersed paraffin oil mixed with graphene oxide nanoparticles were found to be much more efficient liquid lubricants as compared to that engine oil or paraffin oil for the tribological applications. Therefore, studies in the past and ongoing studies at present put forward graphene as a much talked about thin film solid-lubricant in the tribological community. Graphene, being proportionately new coating material, remains relatively unexplored regarding its application to tribological purposes. This study presents the coating of few-tomultilayer graphene on the GCI by using the chemical vapor deposition (CVD) process and is investigated its significance to reduce the friction and wear. The role of graphene layers in the improvement of the tribological behavior of the GCI has been demonstrated well enough with the description of the function of the graphene in the process. Effect of graphene on the friction and wear behavior of GCI has not been reported yet among the scientific community to the best of our knowledge. In this respect, the study has its novelty as a step to develop graphene as an effective coating material in future. Indeed, this article will contribute to the tribological community and also to the people working in the field of surface, interfaces, and in the development of coatings technology. Furthermore, it will help in the development of thin films coating towards the application in tribological components. 2. Experimental details 2.1 GCI Sample preparation Specimens with dimensions of 15 ×15 × 4 mm3 were prepared from the ICE cylinder. The specimens were first ground to flatness and then mirror polished using an alumina suspension down to the particle size of 1 µm. The chemical composition of the cast iron specimen employed in this study is listed in Table 1. The polished specimen (average surface roughness ~ 0.0778 µm) was used for graphene coatings by CVD. The Vickers’s surface hardness of sample before coating was measured to be 225 HV, approximately. 2.2 Graphene growth on GCI by CVD Graphene has been synthesized on the GCI specimen by thermal CVD process. Figure 1 presents a schematic diagram of the CVD apparatus and main steps for the deposition of graphene. The deposition of the graphene can be described mainly in three steps: (i) Heating of the substrate, (ii) Reaction to the substrate, and (iii) Cooling of the substrate. At first, the


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substrate was kept inside the CVD furnace and heated to the growth temperature (at the rate of 10 oC/min) in the presence of Argon (Ar) gas. In the second step, the methane (CH4) gas was introduced into the furnace in the presence of hydrogen gas (H2) and Ar gas and allowed to deposit graphene at the surface within a specified period of growth time by decomposition of CH4 at high temperature. After the completion of the growth time, the flow of CH4 and H2 was stopped, and the sample was allowed to cool naturally to room temperature (RT) in the presence of Ar. During the process, the CH4 (g) decomposed into its constituents in the presence of H2 (g), and the diffusion of carbon occurs. Due to the high solubility of carbon (C) to iron (Fe), the diffusion takes place up to the depth of several micrometers from the surface. The diffused C- atoms segregate on the surface during the cooling process. The process continues until the carbon covers the surface. Due to the natural cooling of the substrate, the graphene layers of different thickness was observed to be deposited over the surface. In the process of deposition of graphene, 500, 100 and 50-200 sccm of Ar, H2 and CH4 were used, respectively and specimens so prepared are referred to PC50, PC100, PC150, and PC200 under different CVD conditions. Raman spectroscopy, AFM and SEM extensively used to optimize the coating conditions. 2.3 Friction and wear tests A ball-on-disk tribometer (CSM Instruments, Switzerland) was used to perform the friction and wear tests under a normal load of 5 N (Hertzian contact pressure = 0.395 GPa) and a speed of 5 cm/s in dry condition. A unidirectional sliding of a bearing steel ball (SAE52100; diameter = 12.7 mm and Vickers hardness ~ 848 HV) is configured in the wear tests with a radius of rotation of 3 mm for an hour. Sample specification employed for the wear tests is listed in Table 2. The wear volume of the specimens was calculated as, V = 𝐴 × 2𝜋𝑅

(1)

where, A is the area of wear profile across the width of the wear track, and R is the wear track radius of the specimens. The specific wear volume the balls in ball-on-disk tests configuration was calculated as, 𝜋ℎ 3𝑑 2 V = ( )( + ℎ2 ) 6 4

(2)

where, d is the mean wear scar diameter, r is the radius and h is the worn height of the ball which is given by the relation,



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(3)

𝑑2 2 √ h=r− 𝑟 − 4 The specific wear rate of both the specimens and the balls was determined as, W=

𝑉 𝐹. 𝑙

(4)

where, V is the wear volume, F is the normal load applied, and l is the total sliding distance during the wear tests. Also, a set of tribotests were performed (stroke length = 1 mm) in the reciprocating sliding condition to explore the lifetime of the graphene coating by varying the applied load of 1, 2, 3, 5, 7.5, and 10N, respectively in dry condition. Besides, tribotests were carried out to explain the role of graphene films and third body particles generated in the contacts for 15 consecutive tests by washing the tribo-interfaces intermittently using ethanol after every 5 min of sliding cycles under 5N (0.395 GPa) load. 2.4 Surface analyses Four specimens coated at four different experimental conditions Surface morphologies of the GCI before and after synthesizing graphene films have been observed by SEM (Nanoeye, SNE3000M, South Korea) and AFM (Brucker, Innova tapping-mode AFM). Confocal Raman measurements were done with a LabRAM HR800 instrument having a 532 nm laser as the excitation source for the identification of the graphene films and thicknesses.

X-ray

photoelectron spectroscopy (XPS) was used to analyze the graphene/cast iron surface before and after wear using a ESCA 2000, XPS (sigma probe) using MXR1 gun within a spot size of 200 µm. FET TitanTM 80-300 is used to observe the high resolution transmission electron microscopy (HRTEM) images and for the energy dispersive X-ray spectroscopy (EDS) measurements of the graphene-coated surface before and after the friction and wear test. The average surface roughness and wear track profiles were measured by using a two-dimensional (2D) surface profilometer (Surftest SV-600, Japan). The worn surfaces, as well as the wear mechanism associated with the specimens, are demonstrated by the SEM micrographs and the EDX results obtained inside the worn region. It has also been observed the wear scar on the balls by the SEM and elemental EDX spectra inside the worn facets to enlighten the wear phenomena. Moreover, Raman spectroscopy is used to demonstrate the wear phenomena involving graphene for the coated specimen. For this, the wear debris/particles/layer in the


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worn region is analyzed through Raman spectra observed at various positions correlating the image from the Raman microscope. 3. Results and discussion 3.1 Raman analysis of graphene on GCI Raman spectroscopy is the most accessible and convenient tool in identifying atomic and electronic properties of carbon allotropes such as fullerenes, carbon nanotubes,27 nanodiamonds,28 diamond-like- carbon,28 amorphous carbon,28 graphite, and graphene.27,29,30 Especially, it identifies the defects, disorder, the orientation and the number of layers of graphene and graphite without any damage to the material.31,32 The thickness and quality of graphene can be identified from the characteristics Raman peaks of graphene/graphite.30 Raman spectra of the graphene-coated GCI at various regions (marked in the Raman image) is displayed in Figure 2. The spectra show three most prominent features at ~ 1347, ~1586 and ~ 2687 cm-1 namely D, G and 2D peaks, respectively identifying the result as the Raman fingerprints of graphene. The G band is associated with the doubly degenerate in-plane longitudinal optic (iLO) and in-plane transverse optic (iTO) phonon modes with E2g symmetry at the zone center (Γ-point), and it results from a normal first-order Raman scattering process in graphene.29 On the other hand, the D and 2D bands result from the second-order processes. The 2D band involves with two iTO phonons near the K point, whereas the D band involves with one iTO phonon and one defect at the K point.29 The 2D band can be termed as the overtone of the D-band as the frequency of the band is almost double to the D-band frequency. Besides, the D band is silent for infinite layers but becomes Raman active for a few layers with the substantial number of defects.29 The appearance of D peak (~1347 cm-1) suggests the formation of disordered graphene with inherent defects on it that may include vacancies and strained hexagonal/non-hexagonal distortions that lead to the non-uniformity, corrugation, and twisting of the layers.33 The low-intensity peak, namely G*, at ~ 2450 cm-1 results due to the intervalley double resonant Raman process similar to that of the 2D band, but involving one iLO and iTO phonons.29 Another weak peak that appeared at ~ 2940 cm-1 is attributed to the combination of the D and G peaks.30 Therefore, few-to-multilayer graphene can be confirmed by the characteristics spectra of graphene on GCI specimen. The 2D band positions are found to be shifted right of 2700 cm-1 on increasing the thickness of the graphene as seen in Figure 2. The position of the 2D band is found to be at ~ 2688 cm-1 for single-to-bilayer graphene whereas the multilayer graphene (> 4-5 layers) exhibited the 2D band at ~ 2712 cm-1. Graphene



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prepared by the flow of 50 sccm of CH4 has been characterized thoroughly in each characterization. 3.2 XPS and TEM analysis X-ray photoelectron spectroscopy (XPS), being a surface-sensitive quantitative spectroscopic technique, is used to measure the elemental composition identifying the chemical states of the graphene deposited on the GCI specimen. Figure 3 shows the survey spectra of the graphene on the grey cast iron. A broad and asymmetric tail of the C1s spectrum towards higher binding energy indicates the sample with a high concentration of sp2 carbon. A major peak at ~ 283.4 eV in the deconvoluted C1s spectrum corresponds to the C=C bonding of graphene/graphite. Other peaks at ~ 284.4 eV, ~285.7 eV, and 286.8 eV correspond to C-C/C-H, O-C-O, and OC=O bonding and attributed to the different forms of hydrocarbons and oxidized carbon and iron, respectively. Figure 4 (a) shows the HRTEM image of the graphene grown directly on the grey cast iron. It confirmed the morphology and the structures of the deposited material being a few layers (611 layers) graphene on the cast iron substrate. It can be seen that the graphene is well adhered to the substrate with negligible gap between the graphene layers and the substrate indicating a very good adhesion of the films to the substrate. Moreover, EDS elemental spectrum, STEM/HAADF intensity profiles and elemental weight percentage along a line across the substrate-graphene interface has been measured (See Figure 4a-b & Figure S1, Supplementary Information). The results show the presence of the iron reach region (deep beneath the substrate), iron oxides/oxidized carbon rich region (the substrate-graphene interface) and carbon rich regions (graphene). 3.3 Surface morphology and roughness analysis The average surface roughness of the uncoated and graphene-coated specimen are measured to be (0.0778 ± 0.00789) µm and (0.3258 ± 0.065) µm, respectively. Increment in the surface roughness of the coated specimen compared to the uncoated specimen is attributed mainly to the reorientation and recrystallization process during high-temperature graphene synthesis and little to the non-uniform coating of the graphene layers on the surface. The roughness of the annealed sample without graphene coating under the same conditions employed in the CVDgraphene deposition is measured to be (0.2786 ± 0.0326). It supports surface roughness increase for the graphene-coated specimen. The surface topography of the uncoated and coated specimen can be easily distinguished as shown in Figure 5a-b. A uniform distribution of


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graphite flakes is visible on the uncoated specimen. Similarly, the few-to-multilayer graphene is found to be formed on the coated specimen as confirmed earlier by Raman spectroscopic analysis. Graphene was deposited except at the flakes regions that have not covered the flakes. Thus, both the deposited graphene and the graphite flakes are supposed to be involved in reducing the wear and friction of sliding components. AFM surface topography further investigates the surface morphology in smaller scale (see Figure 5c-d). It provides the substantial proof of graphene films on the grey cast iron surface along with the graphite flakes and shows the significant increase in the nanoscale surface roughness of the coated specimen, is mainly due to the deposition of few-to-multilayer graphene and also due to the recrystallization at high-temperature CVD process. 3.4 Coefficient of friction of the uncoated and coated specimens Figure 6 shows the COF of the uncoated and coated specimens under a linear tribotest using a load of 5 N and a sliding speed of 5 cm/s in dry condition. Initially, the COF of the coated specimen started comparatively from a higher value. For the first 20 sec of sliding, the friction of the coated specimens is quite high (Inset diagram (I)). It may be due to the increase in the surface roughness. As the rough surface becomes smooth during sliding, the actual friction between the specimens can be recorded. The period at which the COF was high, is considered as the running-in period of the friction test. During this running-in period friction would be in decreasing order. As soon as the contact surfaces have worn out to fit the irregularities perfectly, the friction reaches to the steady state value. At this stage, the contact pressure is much lower than the start of the experiment that results in very low friction and wear for an extended run of the experiment. After 300 sec of sliding, the specimen attained a stable COF (Inset diagram (II)). At one stage, the friction of both uncoated and coated specimens is similar. The lower friction for the uncoated specimen post the few minutes of sliding is due to the lubricious nature of uniformly distributed graphite flakes to some extent.2-4,6 Then, the friction of the uncoated specimen became unstable and higher. As we already reported in our study that the effect of the graphite flakes remains until and unless the flakes chip-out completely.6 Holes and cracks may appear due to the chipping out of the flakes from their respective positions. These holes and cracks appeared in the contact region make the surface rougher and resulted in an unstable and higher friction. So, the uncoated specimen exhibited higher COF of about 0.35 at the end of the cycles, and also the friction was not found to be stable. The effect of the graphene films can be explained on this basis that more stable and much lower COF is obtained for the graphene-coated specimen. The graphene-coated specimen exhibited stable friction value



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much earlier and till the end of the experiment can be attributed to the existence of graphene film throughout the experiment. The tiny fluctuations of on the COF values of the coated specimen is attributed to the slight interaction of the third body particles, called wear debris generated at the contact region of the surfaces. A much lower friction of the coated specimen compared to the uncoated specimen is attributed to extremely low shear and highly lubricious nature of graphene.13,20,34 Although four coated specimens were prepared under different CVD conditions, there was no significant difference in the friction among the specimens. Therefore, it is focused mainly on the reduction of friction and wear of the one of the coated specimen with most uniform graphene under least flow of methane gas (PC50). Afterwards, PC50 has been considered as the graphene coated specimen for other characterizations such as SEM. AFM, Raman spectroscopy, XPS, and TEM, etc. 3.5 Effect of load on the coefficient of friction and wear Reciprocating friction tests as a function of the applied load has been accessed. It explored the lifetime of graphene films for different loads and showed that the life time of the graphene films found to be shortened by few cycles under high loads as illustrated in Figure 7 and also presents significant friction difference between uncoated and coated specimens. In addition, it shows a clear transition of friction regime dependent on the applied load depicting two different friction regimes with the increase of sliding cycles for the graphene-coated specimen. The first region belongs to the very low coefficient of friction (< 0.10) under the existence of lubricious and easy shear behavior of well-adhered graphene layers to the substrate. The second region (COF≥ 0.15) corresponds to the sliding cycles where the friction mechanism is governed by the formation of a lubricious layer of the iron oxides and detached graphene indicating the possibility of the formation of the combined layer of a-C/graphene and various metal oxides (mainly iron oxides). It controlled the friction and wear for this part of sliding cycles. The COF values are found to be varied subtly with the increase/decrease of loads disclosing the effect of load to the shear of the contacting surfaces. Worn surfaces of the sliding specimens under different applied loads during the reciprocating friction tests can be observed (see Figure S1 & S2, supporting information) for both uncoated and coated specimens. The wear tracks and ball wear scars for the coated specimen are found to be much narrower and smaller than for the uncoated specimens for each set of the applied load. It projects the efficient contribution of graphene films at the sliding interfaces.


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Additionally, the tribo-interfaces were cleaned with ethanol intermittently after each successive sliding cycles for 5 min under the load of 5N load and speed of 160 rpm for the identification of the role of graphene. It showed the friction profiles at each tests disclosing the transition of the friction dependent on the graphene’s role governing the friction mechanism (see Figure 8). Figure 8 showed that the coated specimen exhibited very low friction (µ < 0.08) for first 3 consecutive sliding cycles in the tests and soon after this a clear transition of from the low friction to higher friction is seen corresponding to the 4th and 5th tests. Post this set of sliding cycles; the friction increased gradually and exhibited comparatively higher friction after the end of 15th set of sliding test. The coated specimen exhibited the COF of (0.08 ~ 0.15) after 12th consecutive tests, whereas the friction resulted even higher for the remaining set of tests. The findings in our experiment have been categorized into three different stages. The first stage corresponds to ultra-low friction solely dependent on the easy shear behavior of the highly lubricious graphene layers at the contact interfaces. The second stage corresponds to moderate friction values. The friction in this stage can be attributed to the formation of the lubricious layer of iron oxides as well as the graphene debris developed during the process. At last, the comparatively higher friction in the third stage is the result of the involvement of limited flakes of graphene and mostly the formation of the oxides layer. Both types of friction tests with and without washing the tribo-surfaces with ethanol resulted that the graphene layers are completely responsible to low friction and wear for the first 15-20 min of sliding cycles. It projected a lifetime of graphene layers at the tribo-interfaces and also the role of graphene with the combination of other lubricious oxides layers on the friction and wear of the substrate. 3.6 Wear analysis by SEM and EDX observation Figure 9 shows the SEM micrographs of the wear tracks and the EDX spectra of the elements inside the worn surfaces. It provides the worn surface features as well as the wear mechanism associated with both the uncoated and graphene-coated specimen based on the EDX elemental analysis. The elements detected inside the worn surfaces mainly involves in governing the wear mechanism of the surfaces in contact. Many holes and cracks noticed inside the worn surfaces of the uncoated specimen and had originated from the edges or chipped-out graphite flakes. The chipped out flakes behave as the lubricant to some extent for the uncoated GCI specimen, controlling the wear and friction. It can be observed that the formation of the films by mixed graphite and wear debris is not uniform over the region of wear that attributes to the large



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fluctuations of COF for the uncoated specimen. Also, the involvement of carbon inside the worn surface is found to be very low as observed by the EDX results inside the worn face of the uncoated GCI (see Figure 9e). The completely different design of worn surface and wear mechanism can be witnessed from the SEM images and EDX spectra from the worn face of the coated specimen. Unlike uncoated specimen, the worn region of the coated specimen showed the formation of uniform films between the sliding specimens. It is assumed to be due to the existence of graphene films between the sliding surfaces. The role of graphene in the formation of the lubricious carbon film is confirmed by the EDX analysis of the elements inside the worn surfaces of the specimens (see Figure 9f). A larger percentage of carbon is found to be associated with the coated specimen as compared to the uncoated specimen in the formation of this lubricious film. The worn surface EDX analysis revealed that the carbon inside the worn region of the coated specimen is found to be increased by ~500 % as compared to the uncoated specimen. The formation of the mixed mechanical layer associated with the elements shown in EDX results is a substantial proof for the formation of a uniform lubricious film between the specimen and the ball that attributed to low friction and wear. Moreover, the mechanism of the loss of material of the specimens is found to be inhibited by abrasive, adhesive, and oxidative wear, respectively. In the running-in period, the specimen erodes due to the abrasion. Then, the adhesive wear enters into the process of wear and oxidation of the metal debris causes the oxidative wear of the material. Wear profiles of the specimens (see Figure S4) and the quantified list of loss of materials (see Table 3) show a substantial reduction in wear post-coating of the specimen by graphene. The resistance to wear of the graphene-coated specimen increased by ~ 23% as compared to the uncoated specimen. The wear profiles indicate that the graphene-coated specimen born much narrower but deeper wear as compared to the uncoated specimen. The deeper wear of the coated specimen is attributed to the possible decrease in mechanical properties of the GCI at the hightemperature as reported earlier.1,35 However, the reduction in the quantity of wear is attributed to the efficient formation of the uniform lubricious film composed of iron oxides such as Fe2O3 and Fe3O4 and graphene-encapsulated Fe3O4 (will be discussed later in another section). The wear of the coated specimen decreased approximately by 2-folds in magnitude as compared to the uncoated specimen which explains graphene as the potential coating material for the reduction of the friction and wear of the GCI specimen.


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It is imperative to quantify and analyze the wear of the mating partner (steel ball) in the study to supplement the wear of the specimen. Figure 10a-b shows the SEM images of the wear scars developed on the balls slid against the uncoated and the coated specimens. The wear scar diameter is found to be reduced by two-folds in magnitude. It indicates the incommensurate contact area reduction during the rubbing of the contact surfaces. From the ball wear calculations, as listed in Table 3, it can be observed that the ball wear is reduced by five-folds (in magnitude) when slid against the graphene-coated specimen during the wear test. Similar elemental shares are seen inside the worn facets of the balls by the EDX analysis (see Figure 10c-d). The lesser ball-wear slid against the coated specimen as compared to the uncoated specimen is strictly due to the graphene films and its consequence in the formation of the thin lubricious film between the sliding interfaces due to extremely low-shear and lubricious nature of graphene films. Therefore, the reduction of wear can be attributed to the uniform lubricious film formed of iron oxides and graphene films at the interfaces of both ball and specimen. 3.7 Analysis of wear by Raman spectroscopic, XPS, and TEM analysis Raman spectra observed inside the worn surfaces of the specimens provide a meaningful explanation for the significance of graphene in reducing the wear and friction of both uncoated and coated specimens (see Figure 11 and Figure 12). It describes an active involvement of the graphene governing the wear mechanism of the coated specimen. The worn surface of the uncoated specimen indicates that the wear is mainly dominant of abrasive and adhesive wear. However, the oxidative wear exists relatively to a small degree which can also be witnessed from the detection of oxygen with the wear debris as earlier revealed by EDX. Raman spectra of the worn region display the partial formation of iron oxides films only in few areas (see black spots in Figure 11a-d). The graphitic wear debris was also noticed rarely inside the wear track resulted from the graphite flakes. Unlikely, the coated specimen exhibited abrasive wear dominance with both adhesive and oxidative wear. The formation of graphene-encapsulated iron oxides (Fe3O4) films is observed inside the wear track of the coated specimen, confirming its involvement in the reduction of wear. Graphitic peaks (only at partial areas) shows little effect on the wear and friction behavior of the uncoated specimen, whereas graphitic peaks overall region of the worn surface of the coated specimen predict its effectiveness in the wear and friction reduction. The increase in the D-peak intensity (~ 1350 cm-1) in the graphitic peaks indicates the growth in defect sites of the graphite crystals. It is evident to observe the defects after the formation of metal (Fe) oxides as well as graphitic debris post wear tests due to the generation of heat and pressure developed at the contact surfaces. In other words, the graphite



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layers that comprise of edge defects, boundary defects, and oxidative defects degrade the quality of the layer and the defect peak increases. The increase in the D-band intensity after the wear tests of the coated specimen is due to the generation of more edges to graphite crystals breaking the symmetry of the micro-crystallites of graphite.36 Furthermore, the defect peak appeared ~1277 cm-1 (position 1 of wear track of the coated specimen seen in Figure 12d) is attributed to the smaller crystallites of the graphite that resulted due to the grinding of graphene/graphite36 during the sliding of the ball against the coated specimen. Therefore, it predicts the shift in the defect peak post the wear tests due to smaller graphite crystallites. Moreover, the oxidation of metals (especially Fe) can be witnessed from the Raman spectra observed of the iron oxides such as Fe2O3 and Fe3O4 (see Figure 12c). Raman peaks in the range of 60-900 cm-1 confirm the formation of thin layers of various iron oxides after the wear tests at RT along with the graphene-encapsulation. The spectra within the worn surfaces of the coated specimen are obtained according to as for the previous study of carbon-encapsulated Fe3O4 spectra.37 Different phases of iron oxides that appeared inside the worn region are listed in Table 4 corresponding to their vibrational frequencies. The coexistence of the mixtures of hematite, goethite, and magnetite along with some other oxides have been confirmed from the Raman spectra within the worn region.38 All the peaks of iron oxides observed in the studied range found to have coexisted in different phases. Various iron oxides at different phases and the encapsulation of graphene with those oxides altogether are supposed to behave solid lubricants film in dry condition. These oxides layers were resulted from metallic debris due to the abrasion of surface asperities at the contact under the applied shear force. These oxides or metallic debris agglomerates and compacts forming partial layers at the contact surface that now interfere in the contact between the ball and the specimen. Depending on the operating temperature in the test, the compaction of the oxide layers differs. In our study, no cracks on the compacted layers were noticed. It might be due to limited compacting of oxides layer. Previous studies explained that the debris only agglomerates and compacts in the limit at lowtemperature but badly compacts at high temperature.39,40 Thus, no bad compacting of the oxides and debris prohibited in the appearance of the cracks on the layers formed between the specimens. Despite the formation of the iron oxides layers on both the uncoated and coated specimens, graphene-encapsulation to the oxides to the layer formed between the contacting surfaces differentiates the wear and friction mechanism between the two specimens. The graphene-encapsulated Fe3O4 layer at the contact region as the thin lubricious film is responsible for the reduction of friction and wear of the coated specimen. It further sights into


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the necessity of uniform graphene film on the GCI surface to be more beneficial in the formation of a uniform film of graphene- encapsulated Fe3O4 between the contact surfaces that largely reduces the wear and friction. A study in the past revealed that the increase in the carbon content of the cast iron markedly reduced the friction and wear of the cast iron.41 It simply suggests that the graphene films on GCI increase the degree of involvement of carbon during sliding that ultimately reciprocates into the formation of uniform lubricating films of graphene-encapsulated iron oxides as predicted by the Raman analysis (see Figure 13). The Raman spectra supplement the results achieved through the EDX elemental spectra which showed the larger amount of carbon involved in the wear of the coated specimen in previous section. Fitted G and D peaks from the Raman spectra before and after wear of the graphene-coated specimen indicates the change in the positions as well as the broadening of the peaks (see Figure 13). The graphene coated specimen shows that graphite structure with relatively narrow FWHM (G & D), whereas the worn surface shows the graphitic structure with the shift in the G and D peaks, the sharp increase in FWHM, and 9-fold increment in the defect ratio (D/G) (see Table 5). The result is quite different with each other. The FWHM‘s of G and D peaks were nearly 3-times to that of the coated surface before wear. The shift of the G peak towards higher wavenumber (from 1580 to 1596 ), the increase in ID/IG from 0.17 to 1.5, and no dispersion in the G mode all describes the transformation of graphite/graphene to nanocrystalline graphite following a trajectory of amorphization during the wear test.42 The wear surface with the existence of amorphous carbon (a-C), iron oxides, carbon, silicon oxide, chromium along with the graphene/graphite predicts the transformation from nanocrystalline graphite/graphene to a-C and much more disorder and amorphization.43,44 Encapsulation of graphene/a-C with iron oxide (Fe3O4)37 further improve the wear properties from the Raman spectra across the worn surfaces. As in the previous study that revealed the increase in wear resistance of cast iron with Si-O additive,45 the lubricious layer formed with the composites of metal oxides, graphene/graphite, a-C, Si enhanced the wear resistance of the GCI. XPS has been used to identify the carbon/graphene encapsulated iron oxides in the wear surface of the coated specimen as indicated in Figure 14. It also confirms the oxidation state of the carbon/graphene encapsulated Fe3O4 nanocrystals and the formation of encapsulated layers at the tribo-interfaces. The calculated atomic content of different bonding from the XPS spectra is listed in Table 6 by fitting the C1s, O1s and Fe2p peaks deconvoluted to the Gaussian line shape. C1s deconvoluted into four peaks coinciding the graphene (C=C), adventitious carbon



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(C-C/C-H), C-O and O-C=O type moieties, respectively (see Figure 14b). The C=O and OC=O bonding accords with the carbon amount from the adventitious carbon and graphene. Similarly, O1s spectra is deconvoluted to four peaks (Figure 14c) that indicates a major peak located at 529.2 eV corresponds to oxide (O2-). Other three minor peaks correspond to OH-, CO or O-C=O, and moisture peak at about 530.4, 531.8, and 533.1 eV, respectively.46 The atomic content of carbon involved in the single and double bonding between C and O in the O1s spectra well matches with the carbon content involved in the C1s spectra. In the Fe2p spectra (see Figure 14d), both Fe3+ and Fe2+ bonding states found to be existed that depends on the C=O and O-C=O bonding. The atomic contents in the Fe2p spectra reveal the formation of FeO or Fe2O3 or Fe3O4 (in mixed form) at the tribo-interfaces. Carbon and graphene at the surfaces ultimately encapsulate with these form of iron oxides forming the highly lubricious layer at the contact surfaces that eventually decreased the friction and wear of the specimen. As a matter of fact, it is interesting to note that even after large cycles of triboevents (60 min) graphene layers can be seen (however distorted/disordered) on the worn surface which has been indicated in Figure 15a. Besides the existence of graphene layers, it has been revealed the formation of the layers of iron nanoparticles, amorphous phases of carbon (might have resulted due to the amorphization of graphene/graphite under tribostress) and some other oxides. The iron oxide nanoparticles have been confirmed on the worn areas by lattice spacing in reference to the JCPDS (pdf) card number 25-1402, 39-1346, and 88-315 for the iron oxides. Also, EDS elemental spectrum, STEM/HAADF intensity profiles and elemental weight percentage along a line across the worn surface of the coated specimen have been measured (See Figure 15b-c and Figure S5a-b, Supporting Information). The results showed the presence of various layers depending on the atomic contents of Fe, C and O involved across the line of observation. The involvement of carbon (some parts graphene and remaining amorphized carbon) with Fe and O across the worn surface of the coated specimen supported the mechanism of friction and wear by the encapsulation of a-C/graphene with the oxides for the long run of friction tests. Based on the characterization of worn surfaces through SEM, EDX and Raman spectroscopy, XPS and TEM, a schematic diagram is proposed illustrating the function of graphene in lubricating the sliding contacts as shown in Figure 16. It describes that the formation of iron oxides and graphitic crystallites exists at the init ial period of the experiment due to the grinding of graphite/graphene crystals due to the pressure applied on the specimen. Later, graphene encapsulation with the iron oxides takes places arising incommensurate reduction in the contact area and enhances in the lubricity of the mixed layer formed between the sliding specimens.


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The effective combination of graphene with the metal oxides and debris is explained to describe the significant reduction of the friction and wear. It is universally accepted that lubrication is paramount in the area of tribology as the lubricants have been used to reduce the wear and friction in bearings, gears, engines, etc. with components made of uncoated metal for many years. A great challenge for engineers is to optimize the lubricants and additive used for coated components. It is also crucial to achieving smooth surfaces of coated component to replace the oil or grease with a wear resistant coating that provides low friction. Vapor deposition processes are preferred to obtain the kind of wear resistant smooth coatings. Despite the increase in roughness of the coated GCI specimen, the graphene is proved to be the potential candidate, as a highly lubricious and extremely low shear material, to improving friction and wear of the material. Hence, graphene-coated by CVD and iron oxides or metal debris formed a uniform lubricious layer in the contact region due to the continuous sliding of the specimen with the applied contact pressure, which seems to synergistically contribute to the reduction of friction and the enhancement of the wear resistance of the GCI. In future, a study on the controlled roughness and more uniform graphene coating on the substrate is required and can be achieved further improvement in the friction and wear. 4. Conclusions Graphene coating on the grey cast iron is successfully employed to reduce the friction and wear of the material. Some of the key findings in this study are summarized as follows: a) Few-to-multilayer graphene is successfully synthesized on the GCI specimen by the CVD. b) Uniformly distributed graphite flakes on GCI plays as solid lubricant additive on the surface to reduce the friction and wear. c) The graphene-coated specimen exhibited a reduction of friction and wear by two-folds and five-folds in magnitudes, respectively as compared to the uncoated GCI specimen. d) The wear associated with the surfaces is found to be mainly adhesive and oxidative wear followed by abrasive wear at initial. e) The formation of lubricious thin films of iron oxides (Fe2O3, Fe2O4, FeOOH) at the worn surfaces were partly responsible for low friction and wear of both uncoated and graphenecoated specimen. f) Graphene-encapsulated iron oxides especially Fe3O4, nanocrystalline graphite crystals obtained from grinding of graphene/graphite, and other metal oxides across the worn



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surfaces of the graphene-coated specimen was mainly responsible for the significant decrease in the friction and wear. Acknowledgements This research was financially supported by Global Research Laboratory (GRL) program (grant: 2010-00339) of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea. References (1)

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Tables: Table 1 The chemical composition of the specimens obtained by EDX observation. Elements Wt%

C

Si

P

S

2.47 ± 0.16

0.43 ± 0.05

≤ 0.06

Cr

≤ 0.05 0.45 ± 0.08

Mn 1.03 ± 0.11

Ni

≤ 0.06 ≤ 0.1

Table 2 Specification of the grey cast iron samples employed to friction tests. Specimen

Name

Polished GCI

Uncoated

Graphene-coated (50 sccm of CH4)

PC50


PC100


PC150


PC200

Table 3 Disk and ball wear calculations and COF measurements. Quantities COF (observed)

Uncoated specimen

Graphene-coated specimen

0.34

0.16

Disk specimen Wear volume (mm3)

5.4 × 10-3

4.1 × 10-3

Wear rate (mm3.N-1.m-1)

6.0 × 10-6

4.6 × 10-6

Ball specimen Wear volume (mm3)

9.6 × 10-3

1.8 × 10-3

Wear rate (mm3.N-1.m-1)

10.7 × 10-6

2.06 × 10-6


Cu

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Table 4 Raman wavenumbers for different phases of iron oxides observed in the worn surfaces in this study Oxides

Peak positions (cm-1)

References

α-FeOOH (goethite)

386

[47]

β-FeOOH (alkaganetite)

386

[48]

γ-FeOOH (lepidocrocite)

655

[48]

δ-FeOOH (feroxyhite)

400

[48]

α-Fe2O3 (hematite)

590,607,

[48]

γ-Fe2O3 (maghemite)

486, 670

[47]

Fe3O4 (magnetite)

158, 211.5,215.9, 284, 398, 667, 675

[47],[49]

Graphene/ Fe3O4

All Raman spectra and nature of peaks

[37]

Table 5 Analysis results of Raman spectroscopy of the graphitic crystals before and after wear of the coated specimen. Peak properties

Before wear

After wear

D position (cm-1)

1345

1333

G position (cm-1)

1580

1595

FWHMD (cm-1)

35

119

FWHMG (cm-1)

18

63.34

0.17

1.50

ID/IG

Table 6 Atomic content calculated by fitting XPS spectra using Gaussian line shape. C1s

O1s

Fe2p

Bond

Atomic content (%)

Bond

Atomic content (%)

Bond

Atomic content (%)

C=C

52.9

O-2

38.57

Fe2+tot

39.6

C-C/C-H

26.9

OH-

29.61

Fe3+tot

60.4

O-C-O

11.6

C-O, O-C=O

22.1

O-C=O

8.6

H2O

9.7



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Figures:

Figure 1 (a) Schematic diagram of CVD apparatus and (b) Graphical description of the process of deposition of graphene on GCI by CVD process.


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Figure 2 Raman spectra and images of CVD graphene on grey cast iron. (a) Raman image of graphene/GCI, (b) Raman image of graphene/GCI high magnification, (c) Raman spectra of graphene/GCI at various positions, and (d) & (e) deconvolution of the 2D peak of graphene in positions 4 and 2, respectively.



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Figure 3 XPS results of the graphene deposited on the grey cast iron. (a) Survey spectra, (b) C1s peak deconvolution with Gaussian peak fitting method.

Figure 4 (a) HRTEM image of graphene grown on Grey cast iron (PC50). EDX line profiles across a line on the graphene surface: (b) Drift corrected spectrum profile scanning. (c) Fe, O and C weight percentage (%) spectrum profile.


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Figure 5 SEM micrographs of the (a) uncoated specimens and (b) coated specimen. AFM images of the (c) uncoated and (d) coated specimens.



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Figure 6 Coefficient of friction of the uncoated and coated specimens under linear friction test under 5 N load. The inset diagrams shows the enlarged view of the nature of COF’s during running-in cycles and later cycles of the experiment, respectively.[ Linear friction tests; load= 5 N; sliding time = 60 min]


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Figure 7 (a) COF of the uncoated and the coated specimen from the reciprocating friction tests under various loads in dry condition. (b) Average COF of the specimens disintegrating the sliding cycle (at different stages of the friction tests) for the coated specimen. [Reciprocating friction tests; load = 1 ~ 10N; sliding time = 60 min]



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Figure 8 Friction profiles of the graphene coated specimen in reciprocating tribotests under 5N load after washing the tribo-interfaces intermittently after each sliding cycles of 5 min of sliding using ethanol. [Stage I: ultra-low friction regime, Stage II- medium friction regime, and Stage III- high friction regime.] [Reciprocating friction tests; load = 5 N]


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Figure 9 SEM images of wear tracks on (a) uncoated and (b) graphene-coated specimens. Enlarged view of worn surfaces of (c) uncoated and (d) graphene-coated specimens. Elemental composition of the elements on worn surfaces of (e) uncoated and (f) graphene-coated specimens by EDX observation. [Linear friction tests; load = 5N, sliding time = 60 min]



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Figure 10 Ball wear scar images and EDX results of worn surface of the balls slid against (a) & (c) uncoated and (b) & (d) graphene-coated specimens. [Linear friction test; load = 5 N and sliding time = 60 min]


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Figure 11 Raman images and spectra of the worn region of the uncoated specimen. (a) Raman image of the worn surface. (b) Raman spectra outside the wear track. (c) Raman spectra of iron oxides inside the wear track. (d) Raman spectra of graphene inside the wear track. [Linear friction mode; load = 5N and sliding time = 60 min]



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Figure 12 Raman images and spectra of the worn region of the graphene-coated specimen. (a) Raman image of the worn surface. (b) Raman spectra outside the wear track. (c) Raman spectra of iron oxides inside the wear track. (d) Raman spectra of graphene inside the wear track. [Linear friction test; load = 5N and sliding time = 60 min]


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Figure 13 Fitted D and G peaks of the spectra resulted of the graphene-coated specimen predicting the evolution of the peaks and properties before and after wear test. [Linear friction test; load = 5N and sliding time = 60 min]

Figure 14 XPS observation inside the worn surface of the graphene-coated specimen: (a) Survey spectra inside the worn surface. (b) C1s narrow scan spectra for the carbon/grapheneencapsulated iron oxide nanocrystals. (c) O1s narrow scan spectra for the carbon



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encapsulated iron oxides. (d) Fe2p narrow scan spectra for the carbon-encapsulated iron oxides.

Figure 15 (a) HRTEM image of worn part of the graphene-coated specimen after an hour tribotest (linear mode) under 5N load. EDX line profiles across a line on the worn surface: (b) Drift corrected spectrum profile scanning. (c) Fe, O and C weight percentage (%) spectrum profile.


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Figure 16 Schematic diagram explaining the formation of lubricious and low-shear film by the encapsulation of graphene with the metal (Fe) oxides at different steps: (a) CVD-graphene coated specimen, (b) ball-on-disc friction test with direction of motion, (c) iron oxides debris and graphite microcrystallites at the tribo-interfaces, & (d) encapsulation of graphene with metal oxides.


Graphene Coating via Chemical Vapor Deposition ... - ACS Publications

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