Enhanced Wear Performance of Hybrid Epoxy-ceramic Coatings on

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Applications of Polymer, Composite, and Coating Materials

Enhanced Wear Performance of Hybrid Epoxyceramic Coatings on Magnesium Substrates Junjie Yang, Shichun Di, Carsten Blawert, Sviatlana Lamaka, Linqian Wang, Banglong Fu, Pingli Jiang, Li Wang, and Mikhail Larionovich Zheludkevich ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10612 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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ACS Applied Materials & Interfaces

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Enhanced Wear Performance of Hybrid Epoxy-ceramic Coatings on Magnesium Substrates

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Junjie Yanga,b*, Shichun Dia, Carsten Blawertb, Sviatlana V. Lamakab, Linqian Wangb, Banglong Fuc,

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Pingli Jiangb, Li Wangc，Mikhail L. Zheludkevichb,d*

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a) School of Mechatronics Engineering, Harbin Institute of Technology, West Dazhi Road 92, 150001

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Harbin, P.R. China

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b) Magnesium Innovation Centre (MagIC), Helmholtz-Zentrum Geesthacht, 21502 Geesthacht,

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Germany

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c) Institute of Materials Research, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany

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d) Faculty of Engineering, University of Kiel, Kaiserstrasse 2, 24143 Kiel, Germany

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*Corresponding authors:

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Junjie Yang,

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Tel: +86 186-4655-0485, Email: [email protected]

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Prof. Dr. Mikhail L. Zheludkevich,

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Tel: +49 415287-1988, Fax: +49 415287-1960, Email: [email protected]

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Abstract

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Epoxy-based polymer was deposited as sealing agent on porous anodized coatings prepared by

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plasma electrolytic oxidation (PEO) to construct multilayered “soft-hard” coatings on Mg substrates.

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Different thicknesses and microstructures of the top epoxy layer were achieved by employing

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different dip-coating strategies. Atomic force microscopye (AFM), pull-off tests and nanoindentation

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tests were conducted to study the surface roughness, the adhesion strength of the epoxy layer and the

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mechanical properties of each component in the hybrid coating system. The micropores and other

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defects on the anodized layers were sealed by the epoxy polymer, which decreased the surface

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roughness. The dominant abrasive wear behavior of blank PEO coatings was significantly reduced

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by the epoxy layers, and the wear mechanism of the hybrid coatings was proposed considering both

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the microstructure of the hybrid coatings and the mechanical properties of the different components

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in the hybrid system.

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Key words: Magnesium; PEO; Epoxy polymer; Hybrid coating; Wear properties. 1

ACS Paragon Plus Environment

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Funding: This work was supported by the China Scholarship Council [Grant number:

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201506120140];

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1. Introduction

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Mg and its alloys are regarded as promising alternatives to iron, aluminum and copper alloys,

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especially in the transportation industry, due to their high strength-to-weight ratio, damping capacity

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and excellent vibration and shock adsorption ability.1-3 However, the poor corrosion and wear

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resistance of Mg and its alloys have restricted their application as structural materials.4-5 Therefore,

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improving these basic but practical properties of Mg-based materials through suitable surface

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modification methods has become an imperative issue for industrial applications.

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Plasma electrolytic oxidation (PEO) is considered one of the most promising technologies for the in

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situ production of ceramic coatings on the surface of Mg and its alloys. Because of the involvement

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of complex electro-, thermal and plasma-chemical reactions during the PEO treatment, the resultant

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ceramic coatings are characterized by superior adhesion strength, high hardness and electrical

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resistivity as well as higher thermal and chemical stabilities, which in turn, enhance the corrosion

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and wear performance of the Mg substrate.6-7 However, the long-term performance of PEO-coated

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components under aggressive conditions remains a concern due to the porous microstructure.8-9 Thus,

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corrosion and wear attacks are preferentially initialized from the weak points on the surface, e.g.,

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open pores and cracks, where the concentration of corrosive species and stress usually occurs.10-12

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Modifying the parameters of the power supply13-14 and the electrolyte composition15-16 or adding

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micro/nano particles9, 17-19 can increase the compactness and hardness of PEO coatings; alternatively

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introducing lubricants (e.g., PTFE, LDH and MoS2) can reduce the wear attack of both the coatings

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and counterparts. Nevertheless, the porous microstructure of PEO coatings is difficult to change

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considering the dielectric breakdown growth mechanism. Benefiting from the mentioned intrinsic

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properties of PEO coatings, PEO is an ideal pretreatment process, and several duplex surface

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modification strategies, e.g., chemical conversion,20-21 chemical vapor deposition (CVD),22

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electrophoretic deposition,23 hydrothermal treatment24 and sol-gel,25-26 have been developed with the

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aim of changing the surface conditions to overcome the disadvantages associated with PEO coatings.

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Although most of these reported methodologies increased the corrosion resistance of PEO coatings

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by sufficiently reducing the number of pores and cracks by depositing a protective layer on the

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porous surface, few investigations have been performed regarding the adhesion strength between the

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duplex layer and the PEO coatings as well as their tribological properties.

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Epoxy resins can react with either themselves through catalytic homopolymerization or with a

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number of coreactants, including multifunctional amines, acids, phenols, alcohols and thiols. The 2


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reaction of polyepoxides with themselves or with hardeners forms a polymer with favorable

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chemical resistance and mechanical properties.27 Although a significant improvement in the

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corrosion resistance by the application of epoxy polymer coatings has been reported in the

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literature28-29 and in our recent work30, the tribological properties of these coatings have been rarely

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discussed. In the present work, a durable, soft epoxy polymer was used to seal the porous

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micro-features of PEO coatings. A simple dip-coating strategy was employed to conduct the

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deposition process, and the influence of the dipping repetition on the surface morphology and the

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thickness of the epoxy layer was investigated using a series of surface analysis techniques (i.e.,

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scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray

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diffraction (XRD) and atomic force microscopy (AFM)). The adhesion strength between the epoxy

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layers and the different pretreated surfaces was studied by pull-off tests, and the wear properties and

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wear degradation mechanism were evaluated by ball-on-disc dry sliding wear tests and

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nanoindentation tests.

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2. Results

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2.1 Microstructure and composition of the hybrid coatings

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Fig. 1 presents the secondary electron (SE) images of the surfaces pretreated by hydrofluoric acid

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(HF) etching and PEO, separately. Pickled in 12 wt.% HF (Fig. 1(a)), the Mg specimen has a

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brownish surface characterized by intensive parallel scratches that are introduced during grinding

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with SiC papers. This typical surface morphology suggests that the fluoride conversion (MgF2) layer

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is thin (Fig. 1(c)). In contrast, the anodized coatings are observed to be much thicker. The surface

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shows numerous irregular micron-scale pores and cracks, as indicated in Fig. 1(b). The formation of

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micropores is mainly ascribed to the dielectric breakdown caused by short-lived discharges and the

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entrapment of gas bubbles within the discharge channels during the solidification process.16 As

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shown in the Fig. 1(b) inset, an average of 340 pores is detected in the examined areas, and the group

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with the smallest pore area of 10-50 µm2 accounts for more than three fourths of the overall pores.

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With increasing pore size, the number of pores decreases significantly (only two pores were larger

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than 200 µm2). A closer observation of some of the pores, especially the large ones, reveals that

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sub-pores can be distinguished on the walls of the discharge channels. This phenomenon suggests a

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repetitive discharge behavior at the same position. Again, with consecutive increases in coating

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thickness (normally in the micro-arc oxidation stage),31 the increase in the breakdown energy results

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in enlarged and stable sparks on the coating surface, which cause larger pores on the sintered surface.

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These surface features contribute to the surface roughness, which is Ra = 1.1 ± 0.1 µm for the

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PEO-coated samples. The phase composition of the PEO-treated Mg specimens was examined by 3



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XRD, and the pattern with identified peaks is illustrated in Fig. 1(d). In the XRD spectrum,

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magnesium oxide (MgO) and magnesium phosphate (Mg3(PO4)2) are the main phases of the PEO

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coatings, and these phases are obtained by the electrochemical conversion reaction of the Mg

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substrate with anions of the electrolyte during the constant discharging process. In addition, weak

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Mg peaks are observed by XRD due to the penetration of the PEO coating. Accordingly, a much

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higher hardness can be expected for the PEO coating than for the Mg substrate.

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After dipping the samples in the epoxy polymer formulation and subsequently curing at 150 °C for

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90 min, homogeneous top layers are observed on all the epoxy-covered samples. In addition, a

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significant decrease in surface roughness was observed and the Ra values of ME3, PE1 and PE3

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were 52 ± 12, 190 ± 21 and 83 ± 13 nm, respectively, revealing a leveling effect due to the deposition

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of the epoxy polymer on the surface. To further investigate the differences among the hybrid coatings,

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the surface and cross-section views of ME3, PE1 and PE3 are presented in Fig. 2. From the top view,

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pre-treated surfaces are rarely observed for all specimens, which implies a relatively high thickness

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of the epoxy layer on all pre-treated Mg substrates. These results are consistent with the roughness

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values. The properties of the epoxy layer are synergistically dependent on the surface conditions (e.g.,

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porosity and composition), the epoxy formulation (e.g., viscosity and composition) and the post-cure

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treatment.27 Moreover, it should be noted that open pores are still detectable on the surface of PE1

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prepared with single dipping in the epoxy formulation, whereas a significant reduction in the number

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of open pores is observed when the dipping repetition in the epoxy formulation increases to three

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times (PE3). This distinct difference in the surface morphologies between PE1 and PE3 indicates that

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a multi-dipping strategy improves the sealing of pores, especially large ones, thus increasing the

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compactness of the hybrid coatings. This assertion is supported by the cross-section morphology

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images. As shown, epoxy layers with a uniform thickness are formed on the pre-treated surfaces, and

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the discharge channels and cavities within the PEO coatings are filled with the epoxy polymer due to

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the penetration of the epoxy formulation during the immersion period. Moreover, the top epoxy layer

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of ME3 has a thickness of 16.7 ± 0.3 µm, which is apparently thicker than the top epoxy layer of the

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PE samples (PE1: 13.5 ± 0.4 µm and PE3: 15 ± 1.6 µm). These differences confirm that the epoxy

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component is consumed by the porous anodized coatings due to penetration into the pores, and this

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process is favored by increasing the dipping repetition.

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2.2 Pull-off test and fracture analysis

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As one of the most critical parameters of composite coatings, the adhesion strength between the

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deposited layer and the substrate should be measured and guaranteed prior to further applications.

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Fig. 3 presents a schematic illustration of the components (dolly and specimen) in the pull-off tests 4


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and optical images of surface-detached specimens in groups with the corresponding dolly. A

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homogeneous morphology can be observed on the surface of both the dollies and detached

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specimens, which indicates the uniform detachment (more than a 90% contact area) of epoxy layers

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and the accuracy of the obtained adhesion values. In detail, the adhesion strengths for PE1 (Fig. 3(c))

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and PE3 (Fig. 3(d)) are 1.6 ± 0.3 MPa and 1.6 ± 0.2 MPa, respectively, which are only negligibly

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different from that of ME3 (1.7 ± 0.3 MPa, Fig. 3(b)). These results suggest that the porous anodized

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layers have a negligible influence on the adhesion of the epoxy layer on the treated sample surface

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compared to the fluoride conversion layer.

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To explain the obtained adhesion strength values, cross-section and surface views of the

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coating-detached area of ME3 and PE3 are prepared and examined by SEM/EDS (see Fig. 4).

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Consistent with the results given by the optical images, uniform removal of the epoxy layers can be

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observed for both ME3 and PE3 from the cross-section view (Fig. 4(a) and 4(b)). In addition, the

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presence of parallel scratches (Fig. 4(c)) and the elemental distribution of the cross-section (Fig. 4(a))

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suggest the complete removal of the fluoride conversion layer to expose the Mg substrate; a thin

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wavy residual layer (approximately 1 µm thick) is identified adjacent to the Mg substrate at a higher

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magnification, which can be assigned as the inner compact layer of the PEO coatings. Hence, it can

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be assumed that only the outer porous PEO layers, rather than the whole anodized coatings, are

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removed during the pull-off test, indicating that the derived values represent the adhesion strength

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between the outer and inner PEO layers, with much higher adhesion strength between epoxy and

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outer PEO layer (otherwise, damage to the hybrid coating should be observed there). The surface

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morphology of the inner PEO layer is shown in Fig. 4(d) to deduce the fracture mechanism of the

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hybrid PE coatings. In general, most of the detached area is characterized by coarse and rough

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microfeatures with a few exceptions of discontinuous smooth and glossy regions. Regular circular

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features suggest the locations of discharge channels where sparks occurred during the PEO treatment,

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and the smooth regions can be correlated with the so-called “pore band” reported in other works.32-33

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These results agree well with the formation and growth mechanism of PEO coatings proposed by

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Hussein et al,34 in which the pore band is most likely introduced into the coating as a result of

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entrapment and accumulation of gas bubbles within the coatings, thus separating the anodized

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coatings into two distinct layers. Hence, based on this theory, the fracture of the hybrid PE coatings

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may initialize from the weak point (i.e., the pore band) within the hybrid coating system.

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2.3 Dry sliding wear test

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Dry sliding wear tests were carried out utilizing steel balls as the counterpart for characterizing the

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tribological properties of the produced coatings. Two different load levels (5 N and 10 N) were 5



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separately applied on the surface of the hybrid coatings, and the variations in the magnitude of the

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friction force and friction coefficient were recorded synchronously. Fig. 5 presents the friction

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coefficient (f, no units) curves obtained for 20 m of dry sliding for different hybrid coatings under

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both load conditions. With a 5 N load (Fig. 5(a)), the blank PEO coating shows the most spontaneous

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increase in the friction coefficient, reaching more than 0.5 after 3 m of sliding before decreasing

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suddenly to approximately 0.4 at 2 m. Then, the friction coefficient fluctuates severely between 0.4

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and 0.5 throughout the remaining distance. Protected by a single thick epoxy layer on the etched Mg

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substrate, ME3 exhibits a much milder increase in f from 0.05 (0 m) to 0.55 (7 m), which

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corresponds to the running-in process of the steel ball into the epoxy layer. The coefficient curve then

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decreases slightly and overlaps the curve PEO sample after 11 m of sliding. For the composite

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coatings (PE1 and PE3) with the introduced PEO intermediate layer, a significant difference mainly

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occurs at the initial part of the test. Sealed by a relatively thin and less compact epoxy layer, PE1

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reveals a similar pattern to the blank PEO sample but with a slower running-in process. After

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reaching 0.45 at 2 m, PE1 shows only a minor increase to 0.6 in a linear pattern until the end of the

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test. However, the initialization of the running-in period is greatly delayed to approximately 8 m

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when the triple dipping strategy is used to produce the PE3 coating. By the end of the wear test, PE3

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presents a friction coefficient of 0.68, which is the highest among all the samples in the same group.

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These results reveal that the coefficient curves become more stable after the deposition of the epoxy

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layers on the PEO layers, even for the sample (PE1) with a low thickness and imperfect surface

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morphology. This improvement in wear performance can be ascribed to the smooth surface

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morphology and physical properties of the epoxy component.

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For all coated specimens, under a load of 10 N (Fig. 5(b)), the friction coefficients exhibit a similar

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tendency to those obtained under a 5 N load but with a shortened running-in period. In brief, the

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friction coefficients of bare PEO and PE1 increase linearly up to more than 0.55 at approximately 2.5

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m, followed by transient drops to 0.35. However, after a period of stabilization, both coefficients are

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sustained at approximately 0.45. ME3 shows a spontaneous increase in f to 0.3 after 2 m of sliding,

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followed by a moderate growth to 0.45 at 11 m of sliding. Again, PE3 reveals the most durable wear

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protection among all the coated samples, as it sustains a low friction coefficient (0.1) until 3 m of

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sliding. After a short period of running-in and stabilization, the friction coefficient curve of PE3

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overlaps with those of the other samples.

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2.4 Wear track analysis

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Laser confocal microscopy was applied to analyze the wear tracks on all the coated specimens, and

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the reconstructed 3D models of the wear tracks in each group with their cross-section profiles

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obtained under 5 N and 10 N load are given in Fig. S2 and Fig. S3, respectively. The wear track 6


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depth (dwear) and wear rate (Rwear) tested under 5 N and 10 N load conditions are given in Fig. 6. The

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wear rate is defined in terms of mm3(Nm)-1 by dividing the lost volumes (Vlost) by the applied load (F)

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and the total sliding distance (L), as shown in Eq. 1.

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(1) × The wear depth of the wear track of the blank PEO samples (reference) tested under 5 N is 207.8 ±

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5.8 µm, which is unexpectedly higher than that (154.4 ± 7.0 µm) of the wear track produced under

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10 N. Both of the depth magnitudes of the wear tracks are at least seven times greater than the

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thickness of the PEO coating, which implies encroachment of the steel ball into the Mg substrate and

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complete failure of the coating. The shallower track obtained under a higher loading (10 N) may be

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explained by the hardening effect of Mg caused by the excessive plastic deformation and elevated

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temperature of the contact pair during dry sliding wear. Moreover, these wear depth results are

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consistent with the wear rates, with values of 2.28 × 10-3 mm3(Nm)-1 and 1.85 × 10-3 mm3(Nm)-1

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under 5 N and 10 N, respectively. With the introduction of top epoxy layers (PE1 and PE3), the

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depths of the wear tracks are reduced by a factor of six. In detail, the wear track depths on PE1 are

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8.2 ± 1.2 µm (5 N) and 26.5 ± 1.0 µm (10 N), and the wear track depths on PE3 are 9.7 ± 1.7 µm (5

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N) and 27.4 ± 1.4 µm (10 N). Correspondingly, the wear rates are reduced by one order of magnitude

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for the epoxy-covered samples. PE1 reveals the lowest wear rates of 9.6 × 10-5 mm3(Nm)-1 at 5 N

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and 7.87 × 10-5 mm3(Nm)-1 at 10 N, which are slightly lower than those of PE3 (5 N: 1.25 × 10-4

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mm3(Nm)-1 and 10 N: 1.51 × 10-4 mm3(Nm)-1). The slightly higher wear rates for PE3 are attributed

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to the thicker epoxy layer, which is primarily removed during the dry sliding test.

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For a better understanding of the wear mechanism of different types of coating systems, the

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morphologies of the wear tracks and the corresponding steel balls were observed by SEM and optical

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microscope respectively, as shown in Fig. 7. Note that the presence of wear defects can only be

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identified on the surface of the steel balls tested against blank PEO-coated samples under both loads;

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hence, the micrographs of the intact ball surface of the epoxy-covered samples are not shown in the

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present work. As shown in Fig. 7(a) and 7(b), the width of the wear track on the PEO-coated sample

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tested under 5 N is 1.9 mm, which is 0.5 mm narrower than that of the track produced under 10 N.

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Moreover, a typical worn surface morphology is revealed for the worn PEO coatings under both

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loads, and it is characterized by the presence of deep parallel grooves along the sliding direction of

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the ball movement. Large pieces of debris are also shown on the worn surface, which may result

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from the triboxidation of the Mg substrate and the transfer of Mg onto the steel ball after the

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complete removal of the PEO coating. As the counterparts slid against the blank PEO coatings, the

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surface of the steel ball acquired scoring marks and a flattened surface when tested under 5 N; the

=

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1

phenomenon related to the adhesive transfer of Mg was observed on the steel ball surface tested

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under 10 N. These results confirm the three-body abrasive wear mechanism of PEO-coated Mg due

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to the presence of the hard oxide on the surface, which serves as an abradant when gradual damage

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to the coating occurs.35-36

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Covered with a single epoxy layer, ME3 shows much narrower and smoother wear tracks (Fig. 7(c)

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and 7(d)) compared to the PEO-coated samples under both loads. The wear track on the ME3 surface

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has a general wear morphology under the 5 N load, featuring a uniform contact surface surrounded

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by fine scattered epoxy powders along the track. For the 10 N load, slightly narrower wear tracks

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characterized by glazed surfaces and deformed edges are found on the worn surface, which implies

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that more severe plastic deformation of the epoxy layer occurs during the dry sliding period.

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Moreover, transverse cracking and detachment of the epoxy layer at the interface are also manifested

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in the wear track, suggesting failure of the single epoxy layer (even if with a higher thickness) to

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protect Mg from wear attack.

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Sealing porous anodized coatings with the epoxy polymer layer effectively strengthens the wear

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resistance of the coating (see Fig. 7 (e) to 7(h)). Compared to the wear tracks of blank PEO-coated

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samples (Fig. 7(a) and 7(b)), significantly reduced wear attack can be identified in the wear track of

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PE1 under a 5 N load, and the wear track is characterized by smearing marks and shallow grooves.

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These characteristics indicate the restricted expansion of the wear track by the epoxy component,

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even though the thin top epoxy layer is gradually consumed during the wear process. However, it is

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interesting to note that a less defective surface featuring uniform scale-like cracks is found on the

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wear track of PE1 surface when the load increases to 10 N (Fig. 7(f)). This phenomenon can be

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explained by cracking of the underlying anodized coating caused by the excess load (10 N), which

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correlates with the sharp decrease in the friction coefficient after 2.5 m of sliding (Fig. 5(b)). In

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addition, this situation is further improved by increasing the dipping repetition in the epoxy

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formulation to three times. Under 5 N and 10 N load conditions, PE3 has wear tracks with minor

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surface morphology damage due to the compact surface microstructure and probably due to the

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slightly higher thickness of the top epoxy layer.

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3. Discussion

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The properties and composition of the oxide layer produced by the PEO treatment are mainly

30

determined by the electric parameters of the power supply, the composition of the electrolyte and the

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processing duration.37-39 In typical situations, the thickness and porosity of the PEO coatings increase

32

with increasing energy input (i.e., current density/voltage, duty ratio and treatment period) in the

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same electrolyte system. As shown in the surface and cross-section SEM images (Fig. 1 and Fig. 2),

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the blank PEO coatings are characterized by a porous surface morphology featuring numerous 8


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micropores, extrusions, and scattered particles (see Fig. 1). As a result, gradual removal of the PEO

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coatings tends to initialize at the surface due to the stress concentration caused by the limited contact

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area between these surface features and the smooth steel ball, which corresponds to the drastic

4

reduction in the friction coefficient during the dry sliding tests (Fig. 5). The anodized coating

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composition (e.g., MgO) offers a higher hardness than that of the Mg substrate, which can easily

6

induce abrasive wear attack when the debris of the anodized coating is generated and accumulates

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within the contact pair. Therefore, the presence of these fine, hard particles can not only accelerate

8

the wear of PEO coatings but also destroy the steel counterpart simultaneously during the dry sliding

9

process. In conclusion, the abrasive wear mechanism, in combination with the adhesive wear

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mechanism, is highlighted for PEO-coated Mg under various load conditions.

11

Sealing the porous PEO coatings with an epoxy polymer through a dip-coating process effectively

12

enhances the wear resistance of the PEO coatings. In addition to the reduction in the surface

13

roughness, the enhancement in wear performance is preferentially correlated with the properties of

14

the epoxy layer and the reinforced structure of the hybrid coating system. Nanoindentation was

15

carried out to determine the mechanical properties of different layers. Fig. 8(a) shows

16

load-displacement cycle curves, where both the loading and unloading curves reveal non-linear

17

behaviors, indicating plastic deformation of these materials. The maximum load of the epoxy

18

polymer at the target displacement (1 µm) is 4.4 mN, which is clearly less than that of the PEO (34.1

19

mN) and Mg substrate (13.5 mN). These curves illustrate the nature of the different materials in the

20

hybrid coating, and the softer epoxy polymer requires a lower load to induce a comparable

21

penetration depth. Fig. 8(b) shows the indentation hardness data as a function of the indentation

22

displacement for the three different materials. The hardness values, especially for PEO, become

23

stable when the penetration of the indenter exceeds 200 nm. The fluctuation of the PEO curve

24

presented at the beginning of the measurement may result from the less compact microstructure of

25

the PEO coating. Hence, the hardness (H) is calculated by averaging the data between 400 nm and

26

600 nm, and the determined hardness values for the epoxy polymer, PEO and Mg are 0.24 ± 0.03

27

GPa, 3.12 ± 0.88 GPa and 0.71 ± 0.01 GPa, respectively. Unsurprisingly, a similar tendency for the

28

elastic modulus (E) can be found for the three materials, and the values for the epoxy polymer, PEO

29

and Mg are 4.9 ± 0.5 GPa, 58.4 ± 9.8 GPa and 39.6 ± 1.4 GPa, respectively. Although the epoxy

30

polymer has the lowest hardness and elastic modulus among the three materials, revealing a kind of

31

“soft” nature, it shows better wear resistance when considering the plastic index factor (H/E) shown

32

in Fig. 8(b). The plastic index, revealing the fracture toughness and resistance to plastic deformation,

33

is usually adopted to demonstrate the wear resistance of materials, and a better wear resistance is 9



1

indicated by a higher plastic index value.40 Therefore, the epoxy layer also provides durable

2

resistance against wear attack probably resulting from its highly strain-dependent and strain

3

rate-dependent properties.

4

The critical parameters (i.e., dipping repetition) involved in the dip-coating process can also play an

5

important role in determining the wear performance of hybrid coatings, because they directly affect

6

the surface morphology of the specimen. Obvious differences can be distinguished by comparing the

7

wear results of the PE1 and PE3 samples (Fig. 5 and Fig. 7), and these differences are reasonably

8

correlated with the surface conditions. As shown in Fig. 2, a considerable decrease in the number of

9

open pores can be observed when the dipping repetition increases from one time (PE1) to three times

10

(PE3), which means that single dipping in the epoxy formulation is not sufficient for sealing all the

11

pores within the anodized coating. This result is because volatile solvents (i.e., ethanol and DMSO)

12

account for more than 70% of the volume of the epoxy formulation, and these solvents evaporate

13

during the solidification process at 150 °C, resulting in exposure of the incompletely sealed pores.

14

Accordingly, increasing the dipping repetition favors penetration, and probably saturation, of the

15

epoxy formulation into the pores in the PEO coatings, which increases the surface compactness.

16

Considering the factors affecting the mechanical properties of the materials and the surface

17

microstructure, the wear mechanisms of the hybrid coatings are proposed, as schematically

18

demonstrated in Fig. 9. As previously discussed, a small amount of coating debris may be easily

19

generated on the porous PEO coating under both loads due to its unstable microstructure, which

20

features micro-pores, cracks and other defects (as shown in Fig. 1(b)). The hard, tiny ceramic debris

21

present within the contact pair induces abrasive wear attack on the anodized coatings and steel ball,

22

especially when the anodized layer is completely removed from the Mg surface (Fig. 9(a)). In the

23

case of PE1 (Fig. 9(b)), damage to the coating may initialize from the open pores, where the stress

24

concentration is usually present along the edges. Under a 5 N load, a general wear out of the hybrid

25

coating is revealed from the smooth friction coefficient curve (Fig. 5(a)) and the less defective

26

surface morphology of the wear track (Fig. 7(e)) compared to the blank PEO specimen tested under

27

the same conditions. In addition to the protective segregation provided by the top epoxy layer, the

28

production of epoxy debris in mixed with PEO debris can also contribute to reducing the abrasive

29

wear attack when the steel ball slides against the anodized coating. However, PE1 follows a similar

30

pattern in the friction curve to that of PEO, showing an abrupt drop in the friction coefficient after

31

2.5 m of sliding for PE1 (Fig. 5(b)), which indicates a similar failure mechanism to that of PEO. This

32

behavior can be explained by the unfilled voids within the hybrid coating induced by the insufficient

33

penetration of the epoxy solution into the pores of the anodized layer (Fig. 2(b)). As a result, failure

34

of the hybrid coating occurs when the contact stress exceeds its elastic deformation limit (mainly the 10


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1

limit of the anodized layer). Despite this fact, a less defective surface morphology and the lowest

2

observed volume loss are revealed for PE1 throughout the dry sliding test, demonstrating the benefit

3

of the soft top epoxy layer. Furthermore, the collapse behavior and damage observed for PEO and

4

PE1 are effectively avoided by PE3 (Fig. 9(c)), which is fabricated with triple dipping in the epoxy

5

formulation. This significant enhancement in wear performance can mainly be ascribed to the

6

reinforced microstructure and properly sealed surface of the PEO coatings due to the increased

7

penetration of the epoxy formulation into the pores in the PEO coatings.

8

In summary, treating porous PEO coatings with durable epoxy polymer using a dip-coating strategy

9

was proven to enhance the wear performance of PEO coatings under dry sliding wear conditions. In

10

the optimized hybrid coating system, sealed PEO layers mainly provide steady support against the

11

applied loads, while the top epoxy layer reduces the wear attack through plastic deformation. Both

12

the mechanical properties and the saturation of the sealing agent within the porous coatings are

13

responsible for this increase in performance.

14

4. Conclusion

15

In the present work, an epoxy polymer was deposited on PEO surfaces via a dip-coating process

16

followed by a high-temperature curing treatment. Different dipping strategies were employed to

17

study the influence of the dipping repetition on the coating morphology and the tribological

18

performance of the fabricated coatings. The surface morphology, composition, adhesion strength and

19

dry sliding behavior under 5 N and 10 N load conditions were investigated. Based on the obtained

20

results, the main conclusions can be drawn as follows:

21

(1) Uniform epoxy layers can be formed on the surface of PEO layers through a dip-coating

22

procedure. Increasing the dipping repetition can effectively decrease the number of open pores in the

23

epoxy layer but has a slight influence on the thickness of the epoxy layer.

24

(2) Similar adhesion strength values (approximately 1.7 MPa) are found for the ME3 and PE (PE1

25

and PE3) coatings. However, detachment occurs between the outer and inner layers of the PE

26

coatings, which indicates a higher adhesion strength between the epoxy layers and the porous PEO

27

surface. The morphology of the inner PEO layers is first shown, and it consists of crater-like

28

residuals (discharge channels) and smooth areas (pore band). This finding provides physical support

29

for the layered microstructure of PEO coatings and the existence of the pore band.

30

(3) Abrasive and triboxidation wear mechanisms are revealed for bulk PEO-coated Mg under 5 N

31

and 10 N loads, respectively, because of the rough surface topology and plastic deformation of the

32

Mg substrate. Covering the PEO layers with the epoxy component not only reduces the surface

33

roughness but also increases the compactness of the PEO layers, which enhance the resistance

34

against wear attack. PE3, produced with three dipping repetitions in the epoxy solution, demonstrates 11



1

the best wear performance among all the tested samples as a result of the sufficient infiltration of the

2

epoxy component into the pores of the PEO layers.

3

5. Experimental

4

5.1. Materials and reagents

5

Commercial pure magnesium (Mg) strips (35 mm × 35 mm × 4 mm) were used as the substrate. The

6

chemical composition analyzed by Spark OES (Spark analyser M9, Spectro Ametek, Germany) is (in

7

wt.% ) 0.0129 % Al, 0.00325 % Ca, 0.00538 % Fe, 0.0201 % Mn, 0.00178 % Zn, 0.0207 % Si and

8

Mg balance. All specimens were ground using emery papers up to 1200 grit, rinsed with deionized

9

water and ethanol, and eventually dried under a compressed air flow at room temperature. The

10

chemicals and solvents used in the present work are of analytical grade from Sigma-Aldrich and used

11

without any further purification.

12

5.2. Pretreatments for bulk Mg

13

Prior to the deposition of the epoxy polymer, the Mg specimens were pretreated by acid pickling and

14

a PEO treatment. An HF protocol was employed to treat Mg as a reference.41 In detail, Mg specimens

15

were immersed in 12 wt.% HF for 15 min until a homogeneous light-brownish conversion MgF2

16

layer formed (images are shown in Fig. 1S) on the Mg surface. In addition, PEO treatments were

17

carried out in constant current mode supplied by a pulsed DC source. The current density, frequency

18

and duty cycle were 50 mA/cm2, 200 Hz and 10 %, respectively. During the PEO treatment, the Mg

19

samples and stainless steel tube served as anode and cathode, respectively. A typical alkaline solution

20

composed of KOH (2 g/L) and Na3PO4 (10 g/L) was used as the electrolyte for the PEO treatments.

21

The temperature of the electrolyte was regulated within 20 ± 1 °C by a water cooling system and

22

constant mechanical stirring. All the PEO pretreatments lasted for 10 min and were followed by

23

rinsing in distilled water and drying in compressed air.

24

5.3. Synthesis and deposition of the sealing agent

25

The formulation of epoxy polymer primarily consists of three components: poly(bisphenol

26

A-co-epichlorohydrin) glycidyl end-capped (PBA, CAS: 25036-25-3), 3-aminopropyltriethoxysilane

27

(APTES, CAS: 919-30-2) and diethylenetriamine (DETA, CAS: 111-40-0). The addition of APTES

28

mainly increases the adhesion strength between the epoxy component and the pretreated Mg, and

29

DETA serves as a hardener to shorten the synthesis period and modify the mechanical properties of

30

the epoxy component. The three components were prepared separately. PBA was dissolved in a

31

mixture of dimethyl sulfoxide (DMSO) and ethanol (volume ratio of 1:1), and the APTES and DETA

32

were dissolved in ethanol. After the solution was stirred for 1 h, the silane and amine components

33

were successively added into the epoxy solution under constant stirring. The concentration of the

34

solute components in the final formulation was 3 wt. % APTES, 35 wt. % epoxy and 4 wt. % DETA. 12


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1

The obtained final mixture was stirred for another 6 h at room temperature.

2

A PC-controlled single vessel dip coater (KSV NIMA) was used to conduct the dip-coating for the

3

pretreated specimens (i.e., HF-etched and PEO-coated Mg) in ambient environment. Single and

4

triple-dipping regimes were used for the dip-coating process, and the names of the samples and

5

parameter details are given in Table 1. Shortly after dip-coating, all the coated samples were cured at

6

150 °C for 90 min in an oven with a fully opened fan to maintain a flowing air conditions.

7

5.4. Characterization

8

The adhesion of the epoxy component between the etched and PEO-coated Mg substrates was

9

investigated with pull-off tests. A manually operated hydraulic pull-off adhesion tester (PosiTest

10

AT-M, DeFelsko) was used to conduct the experiments. The diameter of the dollies was 20 mm, and

11

the experiments were conducted according to the instructions in ASTM D4541/D7234. Each

12

measurement was repeated at least four times to guarantee the reproducibility of the results.

13

Cross-sections of distinct layers were prepared by cold embedding the coated samples in resin,

14

polishing successively using 800, 1200, and 2500 grit emery papers, and disc polishing with

15

colloidal silica (1 µm) to a mirror finish. A thin Au film was sputtered on the surface to avoid

16

charging effects on electrically nonconductive materials during the SEM study.

17

Surface roughness measurements were carried out in the AFM (Dimension Icon, Bruker) contact

18

mode. The length, width and thickness of the cantilever were 450, 50 and 2 µm, respectively. The

19

force constant was typically 0.2 N/m. The tip height was 15 to 19 µm, and the typical curvature

20

radius was 8 nm.

21

Nanoindentation experiments were carried out to characterize the mechanical properties of the

22

different components (i.e., epoxy, PEO and Mg substrate) in the hybrid coating system. To avoid the

23

influence of the sublayer(s), all the indentation measurements were conducted on the cross-section of

24

the sample, which was embedded in resin and polished to a mirror finish. Each layer in the hybrid

25

coating was determined at least at three different sites to ensure the reproducibility of the results. A

26

commercial apparatus (Nano Indenter G200, Agilent) equipped with a Berkovich indenter tip

27

(three-sided pyramid with a normal angle of 65.3° to the base) was used for the measurement. A

28

compliance indentation system was employed during the indentation measurement, and the

29

indentation tip was calibrated against fused silica. The target indentation depth, the hold period at the

30

maximum load and the indentation strain-rate target were 1 µm, 10 s and 0.05 s-1, respectively, for all

31

measurements to reduce the impact of factors such as tip blunting, thermal drifting and creep of

32

materials on the indentation results.

33

Dry sliding wear tests were performed using a ball-on-disc oscillating tribometer (Tribotec) under 13



1

ambient conditions (temperature: 25 ± 1 °C, humidity: 30 % RH). Stainless steel balls (AISI 52100)

2

with a diameter of 6 mm were used as the counterparts during the wear test. The applied normal

3

loads in the tests were 5 N and 10 N with a linear sliding speed of 0.05 m/s. The sliding amplitude

4

was 10 mm, and the sliding distance was 75 m (15000 s). All wear tracks on the worn surfaces were

5

reconstructed using a color laser confocal microscope (LCM, VK-9700, Keyence). A combination of

6

a 0.5 µm pitch (vertical direction) and a 30% laser reflection intensity was employed to guarantee the

7

accuracy of the obtained results and the efficiency of the measuring process. SEM (Vega3, Tescan)

8

equipped with EDS detector was used to examine the surface and cross-section morphologies and

9

chemical composition of the PEO coating. The image analysis software ImageJ was employed to

10

quantify the distribution of the pores according to their size. Note that the observation was conducted

11

at a magnification of ×500 (approximately 0.23 mm2) using the SEM images at three random

12

locations on the surface and pores smaller than 10 µm2 were not included in the current study to

13

reduce the error involved during the transformation of SEM images into binary documents. GIXRD

14

(D8 Advance AXS, Bruker) with Cu Kα (λ=1.54056 Å) radiation at 40 kV and 40 mA was carried

15

out to determine the phase composition of the pretreated specimens. The incidence angle was 5°, and

16

each scanning step was 0.05° for 0.5 s in the 2θ range from 10° to 80°.

14


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Conflicts of interest The authors declare no conflict of interest.

ORCID Junjie Yang:

0000-0002-2609-7715

[email protected]

Shichun Di

0000-0001-7776-3549

[email protected]

Carsten Blawert

[email protected]

Lamaka V. Sviatlana

0000-0002-0349-0899

[email protected]

Linqian Wang

0000-0001-8407-2973

[email protected]

Banglong Fu

0000-0002-0258-8296

[email protected]

Pingli Jiang

0000-0001-6421-5068

[email protected]

Li Wang

0000-0001-6404-9856

[email protected]

Mikhail L. Zheludkevich

0000-0002-9658-9619

[email protected]

15



Acknowledgement The technical support of Mr. Volker Heitmann and Mr. Ulrich Burmester during this work is gratefully acknowledged. Junjie Yang, Banglong Fu and Pingli Jiang thank China Scholarship Council (Grand No. 201506120140, 201506220158, 201606310043) for the award of fellowship and funding. Dr. Li Wang acknowledges the Helmholtz Association for funding her Postdoc project at Helmholtz-Zentrum Geesthacht in the framework of the Helmholtz Postdoc program.

Supporting information Supporting Information Available: Optical surface image of reference bulk Mg (grounded using 1200# SiC paper) before and after 12 wt.% HF etching for 15 min, Figure S1; Reconstruction (by Laser confocal microscopy) of wear tracks in group with cross-section profile analysis on (a) the blank PEO, (b) ME3, (c) PE1 and (d) PE3 tested under 5 N loading for 75 m (15000 s) dry sliding wear, Figure S2; Reconstruction (by Laser confocal microscopy) of wear tracks in group with cross-section profile analysis on (a) the blank PEO, (b) ME3, (c) PE1 and (d) PE3 tested under 10 N loading for 75 m (15000 s) dry sliding wear, Figure S3.

16


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Chemical Conversion Routes on Magnesium-lithium Alloy J. Alloy. Compd. 2017, 706, 419-429. (21) Narayanan, T.; Lee, M. H. A Simple Strategy to Modify the Porous Structure of Plasma Electrolytic Oxidation Coatings on Magnesium Rsc Adv. 2016, 6 (19), 16100-16114. (22) Sun, M.; Yerokhin, A.; Matthews, A.; Thomas, M.; Laukart, A.; von Hausen, M.; Klages, C. P. Characterisation and Electrochemical Evaluation of Plasma Electrolytic Oxidation Coatings on Magnesium with Plasma Enhanced Chemical Vapour Deposition Post-Treatments Plasma Pro. Polym. 2016, 13 (2), 266-278. (23) Gnedenkov, S. V.; Sinebryukhov, S. L.; Mashtalyar, D. V.; Imshinetskiy, I. M. Composite Fluoropolymer Coatings on Mg Alloys Formed by Plasma Electrolytic Oxidation in Combination with Electrophoretic Deposition Surf. Coat. Tech. 2015, 283, 347-352. (24) Kim, Y. K.; Jang, Y. S.; Lee, Y. H.; Yi, H. K.; Bae, T. S.; Lee, M. H. Effect of Ca-P Compound Formed by Hydrothermal Treatment on Biodegradation and Biocompatibility of Mg-3Al-1Zn-1.5Ca Alloy; in Vitro and in Vivo Evaluation Sci. Rep. 2017, 7. (25) Wang, P.; Li, J. P.; Guo, Y. C.; Wang, J. L.; Yang, Z.; Liang, M. X. Effect of Zirconia Sol on the Microstructures and Thermal-protective Properties of PEO Coating on a Cast Al-12Si Piston Alloy J Alloy Compd. 2016, 657, 703-710. (26) Ivanou, D. K.; Yasakau, K. A.; Kallip, S.; Lisenkov, A. D.; Starykevich, M.; Lamaka, S. V.; Ferreira, M. G. S.; Zheludkevich, M. L. Active Corrosion Protection Coating for a ZE41 Magnesium Alloy Created by Combining PEO and Sol-gel Techniques Rsc Adv. 2016, 6 (15), 12553-12560. (27) May, C. Epoxy Resins: Chemistry and Technology, CRC press: 1987. (28) Lamaka, S. V.; Xue, H. S.; Meis, N.; Esteves, A. C. C.; Ferreira, M. G. S. Fault-tolerant Hybrid Epoxy-silane Coating for Corrosion Protection of Magnesium Alloy AZ31 Prog. Org. Coat. 2015, 80, 98-105. (29) Brusciotti, F.; Snihirova, D. V.; Xue, H.; Montemor, M. F.; Lamaka, S. V.; Ferreira, M. G. S. Hybrid Epoxy–silane Coatings for Improved Corrosion Protection of Mg Alloy Corrosion Sci. 2013, 67 (Supplement C), 82-90. (30) Yang, J. J.; Blawert, C.; Lamaka, S. V.; Snihirova, D.; Lu, X.; Di, S.; Zheludkevich, M. L. Corrosion Protection Properties of Inhibitor Containing Hybrid PEO-epoxy Coating on Magnesium Corrosion Sci. 2018, 140, 99-110. (31) Yang, J. J.; Lu, X. P.; Blawert, C.; Di, S. C.; Zheludkevich, M. L. Microstructure and Corrosion Behavior of Ca/P Coatings Prepared on Magnesium by Plasma Electrolytic Oxidation Surf. Coat. Tech. 2017, 319, 359-369. (32) Rapheal, G.; Kumar, S.; Scharnagl, N.; Blawert, C. Effect of Current Density on the Microstructure and Corrosion Properties of Plasma Electrolytic Oxidation (PEO) Coatings on AM50 Mg Alloy Produced in an Electrolyte Containing Clay Additives Surf. Coat. Tech. 2016, 289, 150-164. (33) Lu, X.; Blawert, C.; Scharnagl, N.; Kainer, K. U. Influence of Incorporating Si3N4 Particles into the Oxide Layer Produced by Plasma Electrolytic Oxidation on AM50 Mg Alloy on Coating Morphology and Corrosion Properties J. Magn. Alloy. 2013, 1 (4), 267-274. (34) Hussein, R. O.; Nie, X.; Northwood, D. O. An Investigation of Ceramic Coating Growth Mechanisms in Plasma Electrolytic Oxidation (PEO) Processing Electrochimica Acta 2013, 112, 111-119. (35) Rapheal, G.; Kumar, S.; Blawert, C.; Dahotre, N. B. Wear Behavior of Plasma Electrolytic Oxidation (PEO) and Hybrid Coatings of PEO and Laser on MRI 230D Magnesium Alloy Wear 2011, 271 (9), 1987-1997. (36) Bala Srinivasan, P.; Liang, J.; Blawert, C.; Dietzel, W. Dry Sliding Wear Behaviour of Magnesium Oxide and Zirconium Oxide Plasma Electrolytic Oxidation Coated Magnesium Alloy Appl. Surf. Sci. 2010, 256 (10), 3265-3273. (37) Hussein, R.; Northwood, D.; Nie, X. The Influence of Pulse Timing and Current Mode on the Microstructure and Corrosion Behaviour of a Plasma Electrolytic Oxidation (PEO) Coated AM60B Magnesium Alloy J. Alloy. Compd. 2012, 541, 41-48. (38) Srinivasan, P. B.; Liang, J.; Blawert, C.; Störmer, M.; Dietzel, W. Effect of Current Density on the Microstructure and Corrosion Behaviour of Plasma Electrolytic Oxidation Treated AM50 Magnesium Alloy. Appl. Surf. Sci. 2009, 255 (7), 4212-4218. 18


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(39) Bala Srinivasan, P.; Liang, J.; Balajeee, R. G.; Blawert, C.; Störmer, M.; Dietzel, W. Effect of Pulse Frequency on the Microstructure, Phase Composition and Corrosion Performance of a Phosphate-based Plasma Electrolytic Oxidation Coated AM50 Magnesium Alloy. Appl. Surf. Sci. 2010, 256 (12), 3928-3935. (40) Leyland, A.; Matthews, A. On the Significance of the H/E Ratio in Wear Control: a Nanocomposite Coating Approach to Optimised Tribological Behaviour. Wear 2000, 246 (1), 1-11. (41) Su, H. Y.; Li, W. J.; Lin, C. S. Effect of Acid Pickling Pretreatment on the Properties of Cerium Conversion Coating on AZ31 Magnesium Alloy. J. Electrochem. Soc. 2012, 159 (5), C219-C225.

19



7. Figure captions Fig. 1 Surface morphology and phase composition of (a), (c) HF-etched Mg and (b), (d) PEO-coated Mg

Fig. 2 Cross-section and surface (inset) morphologies of (a) ME3, (b) PE1, (c) PE3, and (d) the thicknesses of pretreated and epoxy layers of different specimens

Fig. 3 Schematic illustration of the pull-off test and the detached sample surface in each group with the corresponding dolly (a) blank dolly surface (reference), (b) ME3, (c) PE1 and (d) PE3

Fig. 4 SEM image of the cross-section and surface morphologies of epoxy-detached samples (a), (c) ME3 and (b), (d) PE3

Fig. 5 Friction coefficient as a function of dry sliding distance and time for coated specimens (PEO, ME3, PE1 and PE3) tested under (a) 5 N and (b) 10 N loads

Fig. 6 Scratch depth and wear rate analysis of the wear tracks obtained after the dry sliding wear test under (a) 5 N and (b) 10 N loads Fig. 7 SEM images of wear tracks obtained after the dry sliding wear test under 5 N and 10 N loads for (a-b) PEO, (c-d) ME3, (e-f) PE1 and (g-h) PE3

Fig. 8 (a) Load-displacement curves and (b) derived results (i.e., hardness, elastic modulus and plastic index) of different components in hybrid PE coating systems obtained by nanoindentation Fig. 9 Schematic illustration of the wear mechanism of blank (a) PEO, (b) PE1 and (c) PE3 during

dry sliding wear

20


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8. Table Table 1 Nomination and parameters applied in dip-coating process Dipping parameters Substrate Dipping and withdraw speed Dipping Immersion period (mm/min) repetition (s) PEO (reference) Etched Mg (ME3) 108 3 5 PEO (PE1) 108 1 5 PEO (PE3) 108 3 5

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9. Figure list

Fig. 1 Surface morphology and phase composition of (a), (c) HF-etched Mg and (b), (d) PEO-coated Mg

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Fig. 2 Cross-section and surface (inset) morphologies of (a) ME3, (b) PE1, (c) PE3, and (d) the thicknesses of pretreated and epoxy layers of different specimens

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Fig. 3 Schematic illustration of the pull-off test and the detached sample surface in each group with the corresponding dolly (a) blank dolly surface (reference), (b) ME3, (c) PE1 and (d) PE3

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Fig. 4 SEM image of the cross-section and surface morphologies of epoxy-detached samples (a), (c) ME3 and (b), (d) PE3

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Fig. 5 Friction coefficient as a function of dry sliding distance and time for coated specimens (PEO, ME3, PE1 and PE3) tested under (a) 5 N and (b) 10 N loads

Fig. 6 Scratch depth and wear rate analysis of the wear tracks obtained after the dry sliding wear test under (a) 5 N and (b) 10 N loads

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5N

10N

PEO

ME3

PE1

PE3

Fig. 7 SEM images of wear tracks obtained after the dry sliding wear test under 5 N and 10 N loads for (a-b) PEO, (c-d) ME3, (e-f) PE1 and (g-h) PE3 27



Fig. 8 (a) Load-displacement curves and (b) derived results (i.e., hardness, elastic modulus and plastic index) of different components in hybrid PE coating systems obtained by nanoindentation

Fig. 9 Schematic illustration of the wear mechanism of blank (a) PEO, (b) PE1 and (c) PE3

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Graphical Abstract

Synthesis process of hybrid epoxy-ceramic coatings on Mg substrates

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Enhanced Wear Performance of Hybrid Epoxy-ceramic Coatings on

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