1Influence of graphene oxide additive on the tribological and electrochemical corrosion properties of a PEO coating prepared on AZ31 magnesium alloy

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Influence of graphene oxide additive on the tribological and electrochemical corrosion properties of a PEO coating prepared on AZ31 magnesium alloy Yulin Zhang, Fei Chen, You Zhang, Cuiwei Du PII:

S0301-679X(19)30649-8

DOI:

https://doi.org/10.1016/j.triboint.2019.106135

Reference:

JTRI 106135

To appear in:

Tribology International

Received Date: 25 September 2019 Revised Date:

4 December 2019

Accepted Date: 22 December 2019

1 Please cite this article as: Zhang Y, Chen F, Zhang Y, Du C, Influence of graphene oxide additive on the tribological and electrochemical corrosion properties of a PEO coating prepared on AZ31 magnesium alloy, Tribology International (2020), doi: https://doi.org/10.1016/j.triboint.2019.106135. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

1

Influence of graphene oxide additive on the tribological and

electrochemical corrosion properties of a PEO coating prepared on AZ31 magnesium alloy Yulin Zhang 1,3, Fei Chen 3,4, You Zhang 3,4, Cuiwei Du Du 1,2 * 1

Corrosion and Protection Center, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China. 2 Beijing Advanced Innovation Center for Materials Genome Engineering, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China. 3 College of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China. 4 Beijing Key Lab of Special Elastomeric Composite Materials, Beijing 102617, China.

Abstract: Graphene oxide (GO) was incorporated into a plasma electrolytic oxidation (PEO)-based coating to further promote the tribological properties and corrosion resistance of AZ31 Mg alloys. The results demonstrated that the embedded GO additives reduced the surface roughness and porosity, promoted the formation of blocked pores and minor-sized structures, and enhanced the microhardness of the GO-containing coatings. The composite coating prepared with 20 mL·L-1 GO additive was sufficient for sustaining a low and stable friction coefficient and high wear endurance. In addition, the inclusion of a GO additive in the composite coating could effectively decrease the corrosion rate and enhance the polarization resistance. Key words: Plasma electrolytic oxidation; Graphene oxide; Tribology properties; Corrosion resistance 1. Introduction The low density, high specific strength and superior machinability of magnesium and its alloys have expanded their applications in many fields, such as electronic products, aerospace and biomaterials[1-2]. Unfortunately, the low wear resistance of magnesium alloys caused by severe plow and adhesive wear has restricted their widespread utilization[3-4]. Moreover, their inferior corrosion resistance cannot meet the requirements of industrial applications[5-7]. Therefore, suitable surface modification should be applied to magnesium alloys to overcome these drawbacks. Some conventional surface strengthening technologies, such as chemical vapor deposition[8], physical vapor deposition[9], plasma thermal spray[10] and chemical conversion[11], have been applied to enhance the surface properties of magnesium alloys. However, these surface modification methods fail to provide a protective Corresponding author. Tel./fax: +86 010-62333975 E-mail address: [email protected] (C.W. Du) or [email protected]

coating with a high wear resistance due to the mechanical bonding between the coating and the Mg substrate[12]. Recently, plasma electrolytic oxidation (PEO) has been extensively applied to provide firm metallurgically combined coatings possessing considerably high hardness and superior adhesion strength on a metal surface[13-15]. Unfortunately, conventional PEO-based coatings constantly display a relatively high friction coefficient when sliding against steel counterparts under dry and nonlubricated conditions[16-17]. Moreover, the inevitable micropores and microcracks that form in the PEO coating accelerate the deterioration of the corrosion resistance[18-19]. Therefore, it seems essential that new methods should be extensively explored to further improve the tribological and corrosive performances of conventional PEO-based coatings. In recent years, the majority of researchers trying to improve the wear and electrochemical corrosion resistances of conventional PEO-based coatings have focused on preparing composite coatings incorporating various additives, such as Al2O3, SiC, SiO2, alumina sol, and hydroxyapatite[20-24]. The results have confirmed that the composite coatings exhibit obvious efficient protection. Typically, entrapping hard particles (WC, ZrO2, Si3N4, etc.) in the coating results in an enhancement in the hardness and has a positive effect on the wear resistance[25-27]. However, the impact of these hard particles in enhancing the anticorrosion and antifriction properties of PEO coatings have yet to be completely investigated. To promote the wear and corrosion resistances, Gnedenkov et al.[28] carried out PEO of AZ31 Mg alloy by adding superdispersed polytetrafluoroethylene (SPTFE) particles to a silicate-based electrolyte and discovered that the MgO/SPTFE composite coating exhibited better wear and corrosion resistances. Similarly, Ma et al.[29] carried out PEO of a Mg alloy with the addition of hydroxyapatite (HA) particles to prepare a biocomposite coating that displayed superior corrosion resistance behavior in simulated body fluid conditions. Furthermore, self-lubricating materials, for example, graphite, graphene, and MoS2[30-32], have also been applied to improve the antifriction properties of PEO coatings on Mg alloys. Recently, graphene oxide (GO) has increasingly displayed potential for utilization and has proven to be an optimal self-lubricating and corrosion-inhibiting material for metal substrates[33-36]. Aidin et al.[37] demonstrated an approach to efficiently improve the anticorrosion properties by adding GO to an alkaline phosphate-based electrolyte to prepare a MgO/GO composite coating. Moreover, a two-part coating of PEO combined with a GO film prepared using a layer-by-layer self-assembly technique on ZK60 magnesium alloy was accomplished by Qiu et al.[38]. The results demonstrated that the GO film with a decreased corrosion rate served as a corrosion-inhibiting barrier and provided effective physical separation between the sheltered substrate and aggressive medium. In another study, Zhao et al.[39] prepared a GO/micro-arc oxidation (MAO) composite coating on a Mg alloy in a phosphate solution containing (0~3 g·L-1) GO by a two-step MAO coating process. However, the effect of the GO concentration on the tribological performances of these composite PEO coatings have not been studied, and their approaches are inconvenient for two-step PEO processing. In the present work, the aim is to promote the tribological properties and

corrosion resistance of conventional PEO coatings. The PEO coating incorporating GO additive was well-examined by Raman and X-ray photoelectron spectroscopy (XPS) methods. Additionally, the effects of GO additive on the wear and corrosion resistance were investigated by a ball-on-disk tribometer and electrochemical workstation, respectively. 2. Experimental 2.1 Substrate preparation The nominal composition of the specimens (with a size of 25 × 25 × 4 mm) is displayed in Table 1. Before PEO processing, the surfaces of the samples were successively abraded with emery papers and then polished to a surface roughness of Ra=0.1 µm. 2.2 PEO process The aqueous electrolyte used in the PEO process consisted of sodium silicate (15.0 g·L-1), sodium hydroxide (6 g·L-1), sodium hexametaphosphate (1 g·L-1) and a GO dispersion in distilled water. The mass fraction of GO additive in the GO-dispersion solution was 1.0 wt.%. Fig. 1 presents the SEM morphologies of the GO sheets which produced by Beijing Carbon Century Technology Co., Ltd. Moreover, to keep the GO additive fully dispersed in the silicate-based electrolyte, the electrolytic bath was equipped with an ultrasonic system. The pH value for the silicate-based electrolyte without the GO-dispersion solution was approximately 13. The PEO process was performed with a 60 kW homemade unipolar electrical source using a steel electrolytic tank, which was utilized as the cathode as well. The temperature was kept below 30 ℃ by a stirring and water-cooling system. All of the PEO treatments were carried out at a constant voltage of 460 V, and the fabricated coatings possessed superior properties. In addition, other pulse parameters, such as the frequency, duty cycle and working time, were fixed at 1000 Hz, 40% and 15 min, respectively. The samples prepared with different concentrations of the GO-dispersion solution were denoted as GO-0, GO-5, GO-10, GO-20 and GO-40, as shown in Table 2. Fig. 1. SEM Morphologies of GO sheets.

2.3 Characterization of the PEO coatings The surface and cross-sectional morphologies of the PEO coatings were observed by scanning electron microscopy (SEM, SSX-550) assisted by SEDX-500 (Window scanning model). The percentage of porosity in the coating top surface was analyzed using ImageJ software. The crystallographic structures of the specimens were investigated using X-ray diffraction (XRD, Bruker D8) with a step size of 0.04°. Raman spectroscopy (RM2000) and XPS (ESCALAB-220IXL) were carried out to characterize the elemental composition. The microtopography and roughness of the specimens were further investigated using laser scanning confocal microscopy (LSCM, VK-X250). The Vickers hardness of the specimens was evaluated by an HMV-IT hardness tester with an applied load of 0.3 kg and a loading duration of 20 s. The tribological properties of the specimens were determined on a ball-on-disc tester (MS-T4000) with a rotational speed of 200 rpm under dry sliding conditions. All specimens were tested with 3 mm diameter balls (GCr15, HRC66) and a normal

load of 4 N at 25 ℃. The entire testing time was 10 min. The radius (r) of the wear scars was 5 mm. SEM and energy dispersive X-ray spectroscopy (EDS) were applied to observe and analyze the wear scars of the worn specimens after the wear testing. Furthermore, the cross-sectional area (S) of the wear scar was measured by LSCM. The wear volume (V) and wear rates (W) were obtained using equation 1 and 2, respectively: V = S × 2 ×π× r (1) V W= (2) N×L Where N is the applied load (N), L is sliding distance (m). Electrochemical tests were investigated using a typical three-electrode workstation (CS360) in a 3.5 wt.% NaCl solution. Electrochemical impedance spectroscopy (EIS) measurements were performed at the open circuit potential (OCP) with an applied amplitude of the sinusoidal signal of 10 mV in the frequency range between 105 Hz and 10-2 Hz before the electrochemical measurement system became stable (approximately 20 min). Afterwards, the potentiodynamic polarization tests were carried out with a scanning rate of 2 mV/s and a scanning range from -0.5 to 0.5 V vs the OCP. Table 1 Nominal composition of the Mg alloy substrate. Element

Al

Zn

Mn

Si

Fe

Cu

Mg

Content (wt%)

2.89

0.93

0.25

0.01

0.02

0.01

Balance

Table 2 The electrolytes used for preparing the PEO coating samples. Samples GO- 0 GO- 5 GO-10 GO-20 GO-40

Electrolyte Silicate-based electrolyte Silicate-based electrolyte+ 5 mL·L-1 GO-dispersion solution Silicate-based electrolyte+10 mL·L-1 GO-dispersion solution Silicate-based electrolyte+20 mL·L-1 GO-dispersion solution Silicate-based electrolyte+40 mL·L-1 GO-dispersion solution

3. Results and discussion 3.1 Morphology and porosity Fig. 2 shows the surface morphologies and elemental analyses acquired by SEM and EDS. As anticipated[10, 15], all of the as-obtained coatings displayed typical structures with numerous crater-like pores, pan-like structures and microcracks, which were randomly distributed throughout the surface. With increasing concentrations of GO additive in the silicate-based electrolyte, the number of crater-like pores increased distinctly, while the average diameters of the crater-like pores and pan-like structures were considerably reduced. As displayed in Fig. 2(a), the large-sized crater-like pores and pan-like structures were distinctly distributed on the coating surface. In comparison, several blocked pores appeared on the surface of sample GO-5, and the average size of the crater-like pores was reduced (Fig. 2(b)). In Fig. 2(c, d, e), the

majority of the crater-like pores on the surface of samples GO-(10~40) are filled by the molten oxide ejected from the discharge channels[27]. Furthermore, the surface morphologies of the GO-10 and GO-20 samples are more uniform and flat than those of the GO-0 and GO-5 samples. Nevertheless, as the concentration of GO additive was continuously increased to 40 mL·L-1, the sizes of the crater-like pores and pan-like structures of sample GO-40 unexpectedly increased, which may result in the aggregation of aggressive ions and fast channels for permeation of the aggressive ions[19]. The EDS area analyses were performed on the coating surface marked by the white rectangular block in Fig. 2, and the elemental contents are displayed in Fig. 2(f). The elemental surveys demonstrated that elements such as magnesium, oxygen and silicon were derived from the AZ31 alloy and the silicate-based electrolyte, respectively[30-31]. The composite coating prepared with the addition of GO additive had a different amount of carbon than the PEO-based coating. Moreover, the elements of carbon (wt.%) and oxygen (wt.%) detected in area ℃ were 7.61% and 49.87% more than those detected in area ℃, respectively. In fact, these enhancements in the amounts of carbon and oxygen could indicate the incorporation of GO additive into the composite coating. Fig. 2 SEM surface morphologies of the as-obtained coatings: (a) GO-0, (b) GO-5, (c) GO-10, (d) GO-20, and (e) GO-40; (f) the EDS area analysis at different positions.

Fig. 3 presents the SEM surface morphologies and their analyses using ImageJ software to calculate the porosity percentage. Obviously, the crater-like pores and microcracks on the top surface were well-filled by the red spots, and the porosity percentage results are depicted in Fig. 3(f). As the concentration of GO additive increased from 0 to 20 mL·L-1, the porosity percentage of the as-obtained coatings displayed an evident decrease. This result demonstrated that the reduction of the crater-like pore diameter and the formation of numerous blocked pores provided a significant contribution to the decrease in the porosity percentage[6]. However, with the continued enhancement of the GO additive concentration to 40 mL·L-1, the porosity percentage of sample GO-40 unexpectedly increased to 2.3%, which was two times more than that of sample GO-20 (1.2%). These results implied that there was a reasonable concentration of GO additive for the silicate-based electrolyte. Fig. 3 Surface porosity percentages of the as-obtained coatings: (a) GO-0, (c) GO-5, (e) GO-10, (g) GO-20, and (f) GO-40.

3.2 Cross-sectional morphologies and hardness values The cross-sectional morphologies are shown in Fig. 4. The morphologies exhibited the typical structural properties of the PEO coatings reported in previous research[25-27], where the PEO coatings were generally composed of a porous outer layer and a compact inner layer. The porous outer layer and micropores were distinctly observed, as displayed in Fig. 4(a-e); unfortunately, the compact inner layer was not evident due to its thin thickness. Furthermore, the total thickness of the as-obtained coatings was approximately 47 µm, which implied that the incorporation of the GO additive into the PEO coatings had no evident impact on the thickness.

However, the PEO coatings of samples GO-0 and GO-5 exhibited numerous large-sized pores in the cross-sectional morphologies, as shown in Fig. 4(a and b). In contrast, the coatings of samples GO (10~40) possessed a relatively dense structure and smooth cross-sectional surface, implying that the incorporation of the GO additive induced an enhancement in the density of the composite coating. Fig. 4(f) shows the microhardness variation along the depth of the as-obtained coatings. Obviously, the microhardness values exhibited an upward tendency throughout the depth to a maximum hardness (572.9 HV0.3Kg) and then sharply decreased to the same level as that of the AZ31 substrate. Moreover, the coatings of both samples GO-20 and GO-40 presented the highest microhardness values, which may be attributed to their dense microstructure and the formation of hard phases[30]. Fig. 4 The cross-sectional topographies and microhardness values of the as-obtained coatings: (a) GO-0, (c) GO-5, (e) GO-10, (g) GO-20, and (f) GO-40.

3.3 Surface roughness Fig. 5 shows the 3D topography images and roughness of the as-obtained coatings. The rapid accumulation and solidification of molten oxide ejected from the channels resulted in the formation of numerous independent triangular islands on the sample surface. Evidently, the values of Sa and Sq were significantly reduced as the concentration of GO additive increased to 20 mL·L-1. Thus, the results indicated that the composite coating possessed a lower surface roughness than the PEO-based coating, which was ascribed to the pores blocked by the molten oxides ejected from the discharge channels[23]. However, the number of large-sized triangular islands and the surface roughness of sample GO-40 were significantly higher than those of sample GO-20, which could be caused by the excessive amount of GO additive absorbed on the coating surface. Therefore, 20 mL·L-1 might be a suitable threshold for GO additive to be added into the silicate-based electrolyte. Fig. 5 The 3D topography images and surface roughness of the as-obtained coatings: (a) GO-0, (c) GO-5, (e) GO-10, (g) GO-20, and (f) GO-40.

3.4 Coating compositions The X-ray diffraction patterns and Raman spectra are presented in Fig. 6. As illustrated in Fig. 6(a), all coatings were mainly composed of MgO and Mg2SiO4 phases, apart from some weak peaks corresponding to the Mg substrate, which indicated that the X-rays easily penetrated the porous layer[29]. For the fundamental composition of the silicate-based electrolyte, no diffraction peaks associated with the GO additive were displayed in the XRD pattern, which was attributed to the content of the GO additive in the coatings being too low to be observed[35-36]. According to previous studies[38-40], although the incorporation of the GO additive into the coating did not result in the formation of a new phase, the intensity of the major diffraction peak increased with increasing concentrations of GO additive. Consequently, the GO additive added into the silicate-based electrolyte promoted the formation of MgO and Mg2SiO4 phases. To identify the existence of the GO additive in the composite coating, Raman measurements of the as-obtained coatings were carried out, as illustrated in Fig. 6(b).

For comparison, the spectra of the GO additive and GO-0 sample are also displayed. Two characteristic peaks appeared at approximately 1345 cm-1 and 1578 cm-1, which were designated as the peaks of D and G, respectively[38]. The characteristic peaks of D and G were not detected for the GO-0 coating without the incorporation of the GO additive. Furthermore, the intensities of the D and G peaks in the composite coatings increased with increasing concentrations of GO additive added into the silicate-based electrolyte, indicating that the GO additive was successfully incorporated into the coatings[35]. In comparison with the strong band representing the GO additive, the intensities of the D and G peaks for the composite coatings were relatively weak, which demonstrated that the content of the GO additive in the composite coatings was low. Fig. 6 XRD patterns and Raman spectra of the as-obtained specimens.

3.5 XPS survey spectra Fig. 7 illustrates the detailed chemical structures of the GO additive and as-obtained coatings. The survey spectrum (as shown in Fig. 7(a)) of the GO additive demonstrated that the major peaks were assigned to O 1s and C 1s, while the high-resolution C 1s spectral peaks at 284.7, 286.9, 288.6 and 289.2 eV (Fig. 7(b)) were assigned to C-C, C-O, C=O and O-C=O bonding, respectively[39]. The dominant peaks presented in the survey spectra (as displayed in Fig. 7(c, e, g, i, k)) were assigned as Mg 1s, O 1s and C 1s, implying that the as-obtained coatings were primarily composed of Mg, O, and C. Furthermore, the intensity of C 1s in the survey spectra was enhanced with increasing concentrations of GO additive. Therefore, the Mg originated from the substrate, while the other elements were mainly from the silicate-based electrolyte and GO additive. Compared to the high-resolution C 1s spectrum of the GO additive, the C 1s peaks at 284.7 and 286.0 eV of the GO-0 sample separately correspond to C-C and C-O bonding, suggesting that the C in the PEO-based coating came from impurities or carbon dioxide adsorbed on the coating surface. Furthermore, the intensity of the C 1s peak at approximately 291.3 eV was increasingly weakly associated with the increasing GO additive concentration, which indicated the elimination of carbonyl functional groups during the intense spark discharge process[37, 39]. In conclusion, these results confirmed that the GO additive was successfully entrapped in the composite coatings. Fig. 7 XPS survey scans of (a) GO additives and the as-obtained coatings: (c) GO-0, (e) GO-5, (g) GO-10, (i) GO-20, and (k) GO-40.

Graphene oxide, which possesses an excellent hydrophilicity and a large specific surface area, lightly attracted free electrons or negative ions generated by ionization to transform a negatively charged colloid when suspended in the silicate-based electrolyte. During the PEO process, the conductivity and viscosity of the silicate-based electrolyte were enhanced with the addition of GO additive[34]. Therefore, the micro-arc discharges generated on the anode surface were intensified due to the significant increase in the current intensity in a constant voltage mode. However, the concentration of negative ions in the silicate-based electrolyte increased

with the continued addition of GO additive, and then, the negative ions transferred to the substrate surface by electrophoresis, as displayed in Fig. 8 (a). Consequently, the electric potential difference between the anode and the electrolyte was distinctly enhanced, which resulted in an increase in the electron avalanche and a reduction in the breakdown voltage[35]. Accompanied with a decrease in the breakdown voltage, the number of micro-arc discharges per unit interval was evidently increased, while the energy of the individual spark was decreased, which led to the generation of numerous crater-like pores and a reduction in the average size of the pan-like structures. Therefore, the coating compactness was significantly improved due to the formation of minor-sized and blocked pores, as shown in the surface and cross-sectional morphologies. It is well known that the nanoparticles are incorporated in PEO-based coatings mainly by mechanical entrapment and electrophoretic deposition. According to previous studies[25-27], when the voltage applied on the anode reaches the breakdown voltage, electric sparks occur in the plasma discharging channel. The high temperature and pressure generated by plasma discharging melt the solid components and eject molten oxides from the discharging channel. Then, both the molten oxides in the inner layer and the electrolyte liquid are absorbed into the discharge channel for an extreme descent in pressure caused by the evacuation of the short-lived plasma. Consequently, the minor-sized GO additive was absorbed into the pores accompanying the electrolyte liquid, as shown in Fig. 8 (b). On the other hand, abundant GO additive is continually transferred to the anode surface by the cataphoretic effect, and most of the additive would not be absorbed into the discharge channel in a timely manner due to transitory discharge and the limited size of the micropores. Therefore, abundant GO additive absorbs on the coating surface or fills in the large-sized cracks and is wrapped by the molten oxide ejected from the discharge channel. Furthermore, because the addition of GO additive enhanced the electrolyte viscosity and decreased the energy of individual sparks, only a small amount of the molten oxide ejected from the discharge channel could travel farther[35-38]. Hence, the rapid solidification of the small amount of molten oxide formed minor-sized asperities, as demonstrated in Fig. 4, which had a great contribution to the lower roughness of the composite coatings. Moreover, the dispersion phase of GO additive in the coatings or asperities could act as nucleation sites to fabricate a finer microstructure with superior microhardness. Fig. 8 Schematic diagrams illustrating of the MAO coating growth

3.6 Tribological properties The typical friction coefficient evolution of the as-obtained specimens is displayed in Fig. 9. The entire wear process of different specimens was divided into three stages corresponding to the diverse primary wear mechanisms[34]. These include the following: stage ℃, plowing wear; stage ℃, abrasive wear; and stage ℃, adhesive wear. In the case of a Mg substrate, severe adhesive wear directly dominated the entire wear process, leading to the drastic fluctuation and higher value (≈ 0.58) of the friction coefficient, which was ascribed to the typical influence of “instantaneous welding” because of the high temperatures in the contact area between the frictional pairs[21]. Similarly, the adhesive wear for the as-obtained coatings at stage ℃ occurred

between the ball and the remodified coating surface. In addition, the coefficient of friction (COF) values obtained for the composite coatings at stage ℃ of the wear processing reached a plateau, while the average COF values decreased significantly as the GO content incorporated into the composite coatings increased. In comparison with the Mg substrate, both coatings of samples GO-0 and GO-5 experienced a rapid increase in the COF in stage ℃ and thereafter arrived at relatively constant values of 0.36 and 0.32, respectively. Thus, the sudden increase in the COF values was primarily ascribed to the modification and spallation of the surface coatings. Furthermore, the fractured large-sized asperities (as shown in Fig. 5(a and b)) acted as hard abrasives entrapped in the large-sized pores, and cracks resulted in improving the density of the remodified coating surface. Therefore, the dominant wear mechanism in stage ℃ was abrasive wear, which was consistent with previous studies[21, 34]. It is well known that the tribological properties demonstrated by the evolution of the COF is mainly dependent on the surface roughness, hardness and self-lubricating property. As displayed in Fig. 3(f) and Fig. 5, the lower surface roughness and higher surface hardness of samples GO-(10~40) ensured that the COF has a momentary gentle increase in stage ℃. Additionally, the contact status of the friction pairs was mainly a “steel-on-protrusions” style, meaning that the steel balls wear the hard and minor-sized asperities first. Subsequently, the large-sized pores or cracks existing on the wear tracks could act as the reservoirs of the “third body” generated from the fragmentation of protrusions to alleviate plowing wear[22]. Therefore, there was no stage I observed in Fig. 9(b and c) because the soft and large asperities were rapidly worn out. In addition, the COF of sample GO-20 at stage ℃ maintained a lower and relatively constant value for a longer time than samples GO-10 and GO-40. According to the above analysis, the longer the time that stage ℃ and stage ℃ occupied, the higher the wear resistance of the coating. Hence, the coating of sample GO-20 had a higher tribological properties than the other coatings. Fig. 9 Friction coefficient evolution of the as-obtained samples at 25 ℃: (a) Mg substrate, (b) GO-0, (c) GO-5, (d) GO-10, (e) GO-20, and (f) GO-40.

Fig. 10 shows the wear scars and local enlarged images of the samples. Moreover, the corresponding results of the EDS elemental analysis are summarized in Table 4. Plow grooves, adhesive peelings and local detachments along the friction direction were distinctly observed on the worn surface of the Mg substrate, which indicated that severe adhesive wear dominated most of the wear processes apart from plow wear in the initial stage[21-22]. Therefore, only some Fe was explored in area ℃ (as displayed in Table 3), demonstrating minimal transfer of materials between the friction pairs because of the lower surface microhardness of the Mg alloy. For the as-obtained coatings (GO-0 and GO-5), the width and depth of the wear scars were decreased with increasing GO content incorporated into the composite coatings. As shown in Fig. 10 and the local enlarged images, the outer porous layers were almost damaged and removed, which was attributed to their lower surface microhardness and higher roughness. Additionally, abundant wear debris and fragments generated from the fragmentation of protrusions filled the large-sized pores and cracks because of the

pressure of the frictional balls. Consequently, abrasive wear was the primary wear mechanism for the entire wear process of samples GO-0 and GO-5. In contrast, the worn surface of sample GO-10 appeared smoother and milder, with limited areas observed in the local enlarged images of detachments and microcracks. Furthermore, both the GO-20 and GO-40 samples presented typical “fish scale” morphologies on the worn surface, which were caused by fatigue wear due to the cyclic stress of the stainless-steel balls and are emblematic of the higher surface hardness, lower roughness and higher wear resistance of these composite coating[34]. However, the “fish scale” morphology that formed on the worn surface of sample GO-20 appeared smoother and milder with no distinct local delamination. The excess addition of GO additive, for example, sample GO-40, brought about more severe abrasive and adhesive wear than observed for sample GO-20, as demonstrated by the local enlarged images of Fig. 10(f). This resulted primarily from the increasing surface roughness and decreasing hardness of sample GO-40. In addition, the amount of Fe detected on the worn surface decreased from 17.5% to 5.7% as the content of carbon in the composite coatings increased from 0 to 7.1%, indicating that less and less material transferred and less adhesive wear occurred on the coating surface. The unexpected increase in Fe for sample GO-40 was attributed to serious adhesive wear on the worn surface, which can be seen by the morphologies shown in Fig. 8(f). Fig. 10 Wear track morphologies of the specimens: (a) Mg substrate, (b) GO-0, (c) GO-5, (d) GO-10, (e) GO-20, and (f) GO-40. Table 3 Elemental content of the wear scars. Area

Mg (Wt%)

O (Wt%)

Si (Wt%)

C (Wt%)

Fe (Wt%)

85.7 41.8 44.2 44.3 45.7 42.4

12.2 35.5 36.8 37.1 38.1 37.3

0 5.2 4.4 3.8 3.4 3.2

0 0 2.3 4.2 7.1 8.2

2.1 17.5 12.3 10.6 5.7 8.9

The wear depth profiles of the specimens are displayed in Fig. 11, and the wear volumes and wear rates are summarized in Table 4. Compared with the Mg substrate, the wear depth of the samples GO-(0~10) decreased more or less regardless of the inconspicuous reduction of the width. However, both the wear depth and width of samples GO-20 and GO-40 decreased significantly, especially for sample GO-20, which exhibited higher antiwear and antifriction behaviors. Additionally, the applied composite coatings reduced the wear volumes of the Mg substrate, from 8.36×10-2 mm3 for the substrate to 7.06×10-2 mm3 for sample GO-0 and the lowest value of 1.69×10-2 mm3 for sample GO-20. The wear rates of the as-prepared samples decreased in the following order: Substrate > GO-0 > GO-5 > GO-10 > GO-40 > GO-20. Therefore, the antiwear and antifriction properties of the composite coatings were notably improved, which was ascribed to the alleviated abrasive and adhesive

wear due to the lower surface roughness, higher surface hardness and self-lubricating property with the GO additive incorporated into the composite coatings[21-22]. Fig. 11 Wear depth profiles of the specimens: (a) Mg substrate, (b) GO-0, (c) GO-5, (d) GO-10, (e) GO-20, and (f) GO-40. Table 4 Wear volumes and wear rates of the samples after the wear tests. Sample

S (µm2)

L (mm)

V (mm )

3

W(mm3·N-1·m-1)

Substrate

5322.95

5

8.36×10-2

6.65×10-4

GO-0

4495.86

5

7.06×10-2

5.62×10-4

GO-5

3974.56

5

6.24×10-2

4.97×10-4

GO-10 GO-20

2750.08 1074.84

5 5

4.32×10-2 1.69×10-2

3.44×10-4

GO-40

2208.27

5

3.47×10-2

2.76×10-4

1.34×10-4

3.7 Electrochemical analysis The fitted results of the EIS spectra for the specimens are illustrated in Fig. 12(a and b). As shown in Fig. 12(a), two distinct capacitive loops at low and high frequencies were observed, which correspond to the responses of the compact inner layer and the loose outer layer, respectively[19, 26]. In addition, a straight line with an angle of 45° at low frequency was detected, which demonstrated that Warburg diffusion took place because the permeation process of the corrosive liquid into the compact inner layer was a moderate diffusion reaction[37-38]. Proverbially, the larger the radius of the capacitive loop is at low frequency, the higher the corrosion resistance of the coating[39]. With increasing concentrations of GO additive, the radius of the capacitive loop at low frequency was further enlarged until sample GO-20; however, a minor reduction in the capacitive loop radius was observed for sample GO-40. Similarly, the same tendency was observed in the Bode impedance plot as well, as illustrated in Fig. 12(b). Here, the values of the impedance at 0.01 Hz responding to the response of the compact inner layer could be clearly utilized to estimate the total corrosion resistance of the PEO coatings. Accordingly, the values of |Z| for varying specimens at the lowest frequency (0.01 Hz) were as follows: |Z|GO-20 (1539300 Ω·cm-2) ﹥ |Z|GO-40 (1378100 Ω·cm-2) ﹥ |Z|GO-10 (853600 Ω·cm-2) ﹥ |Z|GO-5 (268500 Ω·cm-2)﹥|Z|GO-0 (98244 Ω·cm-2), suggesting that sample E3 had the highest anticorrosion property. The electrical equivalent circuit models according to the fitted results of the EIS spectra are shown in Fig. 13 to quantitatively investigate the impedance values of the specimens. Additionally, the parameters corresponding to the fitted results are summarized in Table 5. For the equivalent circuits, the electrical elements were composed of resistance elements (Rs, Rp and Rb) and constant phase elements (Qp and Qb), where Rs is the resistance of the electrolyte, the resistance Rp and capacitance Qp represent the outer porous layer, Rb and Qb represent the inner dense layer, and the symbol W corresponds to the Warburg diffusion element. Rb for the substrate was only 3194 Ω·cm-2, which was lower than the Rb for the as-obtained coatings. With the

addition of GO additive, both the values of Rp and Rb were further enhanced, especially Rb, which increased from 106620 to 249320 Ω·cm-2 as the concentration of the GO-dispersion solution increased from 0 to 20 mL· L-1. Thus, the results demonstrated that the incorporation of GO additive into the composite coatings significantly improved the impedance values of the specimens. However, with the addition of excess GO additive, the Rp and Rb values of sample E4 were suddenly reduced to 4488 and 183640 Ω·cm-2, indicating that excessive agglomeration of GO additive would aggregate erosive ions on the coating surface and decrease the barrier effect of both the outer and inner layers[38]. Furthermore, the same regularity could be obtained from the varied values of W for the samples (GO-10, GO-20 and GO-40). The value of W for sample GO-20 increased by approximately 31.2% and 16.4% compared to that of the GO-10 and GO-40 samples, respectively, indicating that the erosive ions diffused slowly into the GO-20 composite coating. Fig. 12(c and d) illustrates the potentiodynamic polarization curves, the variation curves of the corrosion current density (Icorr) and polarization resistance (Rp) values of the specimens. Other electrochemical parameters, such as the corrosion potential (Ecorr) and anodic/cathodic Tafel slopes (βa and βc), are summarized in Table 6. The values of Rp were estimated using equation 1[19].

Rp =

βa × βc ………………………………equation 1 2.303 × ( β a + β c ) × I corr

As displayed in Fig. 12(c and d) and Table 6, the uncoated Mg alloy displayed an Ecorr value of -1.52 V and a corresponding Icorr value of 1.09 ×10-5 A·cm-2. Superficially, in comparison to the Mg substrate, minor enhancements in the Ecorr values of composite coatings were discovered, which demonstrated the limited reduction of the thermodynamic tendency of the corrosion emergence[37-39]. However, the Icorr values of the as-obtained coatings, especially for sample GO-20, were three orders of magnitude lower than that of the uncoated Mg substrate, implying that the chemical stability of the PEO-based coating was further improved by the incorporation of the appropriate content of GO additive. Therefore, for the as-obtained coatings, the corrosion tendency was GO-0 > GO-5 > GO-10 > GO-20 > GO-40, which was according to the results of the EIS analysis. In addition, the values of Rp evidently increased from 4.54×103 Ω·cm2 for the uncoated Mg alloy to a range from 6.45×104 to 1.88×106 Ω·cm2 for the as-obtained coatings, which was additional significant evidence for the increase of the chemical stability of the GO-containing coating. Due to the almost identical parameters of the PEO process, coating thickness and phase composition of the as-obtained coatings, the enhancement in the corrosion resistance can be mainly attributed to the incorporation of GO additive into the PEO-based coating. Therefore, as shown in Figs. 2 and 5, the embedded GO additive decreased the overall porosity and surface roughness of the composite coatings and, accordingly, provided efficient protection against corrosion. Fig. 12 EIS plots, polarization curves and variation curves of the corrosion current density and polarization resistance values.

Fig. 13 Equivalent circuit models for EIS in Fig. 11: a) for the substrate and samples E(0~1) and b) for samples E(2~4). Table 5 Fitting results of the EIS plots for the as-obtained specimens. Parameter

Substrate

GO-0

Rs (Ω·cm-2) 35.22 35.61 Rp (Ω·cm-2) 2663 1710 -2 -5 Qp (F·cm ) 1.86 ×10 8.00 ×10-8 0.8843 0.8745 n1 -2 Rb (Ω·cm ) 3194 106620 -2 -3 Qb (F·cm ) 1.8×10 1.70×10-5 0.7462 0.7366 n2 WR (Ω·cm-2)

-

-

GO-5

GO-10

GO-20

GO-40

35.53 2370 3.74×10-8 0.8832 129310 1.02×10-5 0.7794

36.21 2615 7.71×10-9 0.7946 178417 1.88×10-7 0.8582

35.76 4943 3.87×10-9 0.8031 249320 1.92×10-7 0.7713

35.63 4488 5.74×10-9 0.8143 183640 1.97×10-7 0.7911

-

190510

278790

233130

Table 6 Electrochemical data obtained from potentiodynamic polarization tests. Parameter

Substrate

GO-0

βa (mV) 0.39 0.73 βc (mV) 0.16 0.37 Icorr (A·cm-2) 1.09 ×10-5 1.66 ×10-6 Ia(mm·a-1) 1.53×10-1 8.30×10-3 -1.52 -1.47 Ecorr (V) Rp (Ω·cm2)

4.54×103

6.45×104

GO-5

GO-10

GO-20

GO-40

0.22 0.19 2.14×10-7 1.07×10-3 -1.46

0.18 0.14 6.07×10-8 3.04×10-4 -1.44

0.12 0.11 1.35×10-8 6.77×10-5 -1.40

0.18 0.15 1.98×10-8 9.93×10-5 -1.43

2.04×105

5.68×105

1.88×106

1.75×106

4. Conclusions 1. The morphology, roughness, hardness and porosity of the GO-containing PEO coatings varied with the GO additive concentration ranging from 0 to 40 mL·L-1. However, the GO additive concentration had no evident impact on the thickness of the coating. 2. The addition of GO additive increased the hardness and compactness and reduced the roughness, friction coefficient and wear volume of the composite coatings. The tribological properties of the PEO coatings increased in the following order: GO-20 > GO-40 > GO-10 > GO-5 > GO-0. 3. The electrochemical corrosion resistance of the PEO coatings was improved with the incorporation of GO additive due to the formation of blocked pores and compact microstructures. The corrosion tendency was as follows: GO-0 >GO-5 > GO-10 > GO-20 > GO-40. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51601015), the Beijing Natural Science Foundation (No. 2182017), the Science and Technology Project of Beijing Municipal Education Commission (KM201910017004), and the URT program of Beijing Institute of Petrochemical Technology (2018J00044 and 2018J00046).

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1, A GO-containing coating was fabricated on AZ31 magnesium alloy via plasma electrolytic oxidation. 2, The GO-containing coating displayed lower roughness and higher compactness and hardness than GO-free coating. 3, The GO-containing coating prepared with 20 mL·L-1 of GO additives was sufficient for sustaining a low, stable friction coefficient and high wear endurance. 4, The inclusion of GO additives in the composite coating could effectively decrease the corrosion rate and increase the polarization resistance of the as-obtained coatings.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Author Contributions Section： Du and Chen conceived and designed the study. Zhang performed the experiments and wrote the paper. Du, Chen, and Zhang reviewed and edited the manuscript. All authors read and approved the manuscript.