Xue-Jun Cui ,Chung-Ming Ning ,Gung-An Zhng ,Lun-Lin Shng ,Li-Ping Zhong ,Ying-Jun Zhng

a School of Materials Science and Engineering,Sichuan University of Science and Engineering,Zigong 643000,China

bState Key Laboratory of Solid Lubrication,Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences,Lanzhou 730000,China

c Key Laboratories of Fine Chemicals and Surfactants in Sichuan Provincial Universities,Zigong 643000,China

Abstract A reliable,high-performance coating procedure was developed using PDMS to modify a duplex MAO/DLC coating on an AZ31B Mg alloy.First,the duplex MAO/DLC coating was fabricated via a combined MAO and unbalanced magnetron sputter process.Subsequently,a PDMS solution was used to modify the MAO/DLC coating via a conventional dip-coating method.The surface characteristics,bond strength,hardness,tribological behaviour,and corrosion resistance of the coated samples were evaluated via SEM,CA,Raman spectroscopy,friction and wear behaviour,polarisation curve,and NSS tests.The PDMS modificatio reduced the HIT of MAO/DLC coating from 15.96 to 8.34GPa;this is ascribed to the penetration of PDMS,which has good rheological properties to form a viscoelastic Si-based organic polymer layer on the MAO/DLC coating.However,the PDMS-modifie MAO/DLC coating was denser,hydrophobic,and had higher bond strength compared with MAO-and MAO/DLC-coated samples.Moreover,the PDMS modificatio reduced the COF and wear rate of the duplex MAO/DLC coating.This indicates that the PDMS improved the tribological behaviour owing to the transferred Si oxide that originated from the Si-O network of the PDMS,as well as the low graphitisation of the DLC layer during sliding.Furthermore,the corrosion current density of the MAO/DLC-coated sample modifie by PDMS for 10min decreased by two order of magnitude compared with that of the MAO/DLC-coated sample but by fi e orders of magnitude compared with that of the bare substrate.The NSS tests proved that the PDMS layer slowed the corrosion of the Mg alloy under long-term service,enhancing the corrosion protection efficien y.The results are attributed to the high bond strength and lubricant MAO/DLC layer,and the dual role of sealing and hydrophobicity of PDMS.Therefore,PDMS modificatio is promising for the fabrication of protective materials for Mg alloys that require corrosion and wear resistance.

Keywords: Magnesium alloy;Microarc oxidation;Diamond-like carbon;Polydimethylsiloxane;Tribological behaviour;Corrosion resistance.

1.Introduction

The low corrosion and wear resistances of Mg alloys have greatly restricted their engineering applications in the automotive,aerospace,and mechanical engineering field [1,2],for example,the pistons,rings,and connection rods of Engine should be of good corrosion and wear resistance when they are made of Mg alloys.Therefore,in the last several decades,different surface treatments and coating processes have been employed to prevent corrosion and wear,such as cold spraying [3,4],chemical conversion coating [5,6],electrodeposition[7],electroless plating [8],and anodic oxidation [9-11].Most of these methods produce surfaces that provide good corrosion and wear protection [5,11] but are not hard enough,are not lubricating,and cannot satisfy the requirements of high corrosion and wear resistance.

Diamond-like carbon (DLC),owing to its high hardness,good adhesion,low coefficien of friction (COF),and chemical inertness,is promising for improving the resistance of Mg alloys to corrosion and wear[12-14].However,the interfacial bond strength of DLC coatings on Mg alloys is significantl reduced owing to differences in material properties between the hard DLC coating and soft Mg alloy,such as the elastic modulus,the melting points,and the high internal stress of the DLC coating.Therefore,metallic buffer layers,such as Al[15],Ti [16],Cr [17,18],W [19],and Si [20],have been deposited on the interface between DLC coatings and Mg alloys to reduce the internal stress of the DLC coating and improve its adhesion.Nonetheless,the degree of the improvement in the interfacial bond strength is limited,and there is a distinct separation at the Mg alloy substrate/DLC coating interface[20].Moreover,metallic buffer layers have negative effects on the corrosion protection offered by DLC coatings owing to the large potential difference with Mg.This accelerates the corrosion of Mg alloys through the formation of a corrosion microcell when the coated samples are in corrosive environments [14,18].Hence,an inert transition layer should exist between the hard DLC coating and soft Mg alloy to fully utilise the protective properties of the DLC coating [14].

Microarc oxidation (MAO),which is also called plasma electrolytic oxidation,can grow ceramic-like coatings in situ and has been widely applied to fabricate anti-corrosion and anti-wear coatings on Mg alloys [10,11,21].Notably,MAO coatings can firml bond to the Mg alloy substrate,primarily via mechanical interlocking [22].Generally,the structure of the MAO coating comprises a dense inner layer and a porous outer layer [23,24].The inner layer acts as a physical barrier to the corrosive medium,while the outer layer can greatly enhance the bond strength for a surface coating [25,26].Hence,replacement of a metallic buffer layer with an MAO coating enhances the interfacial bond strength.Composite coatings have been developed using the MAO coating as the interlayer on Mg alloys,including electroless plating [27],hydrophobic coatings [28,29],and metal nitride coatings [30-32],as well as DLC coatings [33-35].MAO coatings,when applied as an interlayer,have improved the adhesion of DLC coatings on AZ80 Mg alloys.Additionally,the composite coating has enhanced the corrosion and wear protection of the Mg substrate [33].However,many types of defects,such as cracks,pinholes,and pores,as well as structural changes,may be introduced during the preparation of the coatings [36-38],even in high-quality graphene film [39,40].These defects can provide channels or paths for corrosive ions and induce galvanic corrosion or form a corrosion microcell to accelerate coating failure when the coated samples are in corrosive environments[36,41].

Hydrophobic coatings prepared using low-surface-energy polymers have proven effective for protecting Mg alloys from corrosion [28,29,42].Modern technology is focusing on the development of reliable,high-performance,low-cost,and easy-application coating materials that are suitable for use in harsh environmental conditions.Polydimethylsiloxane(PDMS)—a silicone elastomer—is attractive for the development of hydrophobic coatings owing to its desirable properties,such as its low surface energy,excellent permeability,chemical inertness,thermal stability,ease of handling,and conformance to submicron features [43-48].However,little research has been performed on the protection offered by PDMS-modifie DLC and MAO/DLC coatings to Mg alloys.In this study,PDMS was used to modify a duplex MAO/DLC coating on an AZ31B Mg alloy.The effects of the PDMS and dipping time on the microstructure,adhesion strength,hardness,tribological,and corrosion behaviour of the duplex MAO/DLC coating were investigated.

2.Material and methods

2.1.Preparation of PDMS-modifie duplex MAO/DLC coatings

The fabrication routes for the PDMS-modifie duplex MAO/DLC coatings are shown in Fig.1.Before MAO,sheets of the semi-continuous casting AZ31B Mg alloy with a size of 30mm×30mm×2mm (mass fraction:2.94% Al,0.9%Zn,0.23% Mn,0.01% Si,0.01% Cu,0.00053% Ni,0.003%Fe,and remainder Mg) were ground and polished with a series of SiC papers (from 240 grit to 1200 grit),degreased ultrasonically in acetone for 10min,and dried in a cold air fl w.MAO was conducted using equipment and a process that were previously reported [28,32].The MAO-treated samples were then rinsed successively for 10min with deionised water and 15min with absolute ethanol alcohol in an ultrasonic bath and subsequently dried in cold air.

The DLC coating was deposited on the MAO-coated samples and a Si wafer using pulsed direct-current equipment,i.e.,a patented closed-fiel unbalanced magnetron sputter ion plating system (UDP-650,Teer Coating Ltd.) at the State Key Laboratory of Solid Lubrication,Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences.A graphite target with a diameter of 60 mm was used.First,the coated samples and Si wafer were placed in a vacuum chamber.After the base pressure was evacuated to vacuum conditions(3.0×10-3Pa),high-purity Ar gas (99.99%,16 cm3/min) was introduced to sputter the substrates (bias voltage:-500 V;frequency:250 Hz;duty cycle:80%;treatment time:30 min) to remove surface contaminants.Next,the chamber pressure was maintained at 0.1 Pa by controlling the fl w of C4H10gas(16 cm3/min).Subsequently,the H-doped DLC coating (in this study,all DLC coatings were doped with H) was deposited with a bias voltage of -70 V,frequency of 250 Hz,and duty cycle of 80% for 5 h.Thus,an MAO/DLC coating with a thickness of approximately 2 μm (determined using a surface profilometer was prepared on the AZ31B Mg alloy.

Fig.1.Schematic diagram of the PDMS modifie duplex MAO/DLC coatings on Mg alloy.

The MAO/DLC-coated samples were then immersed in a PDMS solution consisting of 3 g of PDMS,0.3 g of a curing agent,and 100 mL of ethyl acetate for 5 or 10 min.Subsequently,the samples were removed from the PDMS solution and cured at 80°C for 8h.For analysis,the PDMSmodifie coatings were designated as MAO/DLC/PDMS-5 and MAO/DLC/PDMS-10,respectively.

2.2.Structure characterisations

The surface microstructures of the coatings and wear tracks on the coatings,as well as the elemental distribution of the wear tracks,were investigated via field-emissio scanning electron microscopy (SEM,HITACHI SU8020,Japan) in conjunction with energy-dispersive X-ray spectroscopy (Oxford,England).Prior to the characterisation,Au film were sputtered on the test samples for conductivity.An optical microscope was used to observe the scratch surface morphologies.Raman spectroscopy (Horiba Jobin Yvon LabRAM HR800,France) with an Ar+beam at a wavelength of 532 nm was used to measure the atomic bonds of the coatings and the wear tracks on the coatings.

2.3.Properties evaluation

The bond strength between the coatings and the Mg alloy substrate was measured using a multifunctional material surface performance tester (MFT-4000).The coated samples were fi ed horizontally on the loading platform;the loading speed was 11.4 N/min,the critical load was 1 N,the termination load was 20 N,and the scratch length was 5 mm.The loading force and friction force ranged from -0.02 to 0.01 N and -0.02 to 0 N,respectively.

The indentation hardness (HIT) and modulus (EIT) values of the coating were investigated using a nanoindentation tester (TTX-NHT2) under linear loading with a maximum load of 10 mN and a maximum depth of 0.4 μm.The values were calculated using the Oliver-Pharr method.At least fi e measurements were performed at different locations of each coated sample,and the average values were regarded as the fina HITand EITof the sample.

The water contact angles (CAs) of the samples were measured using an optical CA metre with a 3-μL water droplet at ambient temperature.The CAs were measured at fi e different positions on the same sample,and the average value was calculated.

The tribological behaviours of the coated samples were assessed via a friction test using a ball-on-disc tribometer(CSM)at room temperature(22±2°C)with a relative humidity(RH)of 27%±2%in air.During the reciprocating friction tests,a GCr15 steel ball (diameter:6 mm) was used as the friction counter body.The normal load was 2 N,the reciprocating length was 5 mm,the sliding time was 4000 s,and the sliding frequency was 5 Hz.The total sliding distance was 200 m.Each sample was tested thrice at different positions,and the average COFs were calculated.After the friction test,the worn surfaces were observed as described in Section 2.2,and their wear volumes were calculated using a twodimensional noncontact optical profilomete (KLA-Tencor D-100,USA).The specifi wear rates(W)of the coated samples were calculated using Eq.(1).

Here,Arepresents the area of the wear track,Srepresents the length of the wear track,Nrepresents the normal load,andLrepresents the sliding distance.

The corrosion resistance of the bare and coated samples was investigated via a polarisation curve test in a 3.5wt%NaCl solution at room temperature using an electrochemical test system(VersaStat3F-400,Princeton Applied Research,USA).In this test system,the sample with an exposed area of 1 cm2was used as the working electrode,a Pt sheet with an area of 4 cm2was used as the counter electrode,and a saturated calomel electrode was used as the reference electrode.The open-circuit potential (OCP) was measured to ensure that the sample was stabilised in the NaCl solution.Polarisation tests could not be conducted until the OCP changes were±<10 mV during the 5-min period of measuring the potentials.The scanning rate of the polarisation curve test was 1 mV/ S,and the scanning region was set as ±500 mV with respect to the OCP.Each sample was tested at least thrice at different positions to ensure reliability and reproducibility.The corrosion potential (Ecorr) and corrosion current density(icorr) were calculated using the VersaStudio software,according to the mostly linear polarisation behaviour in the Tafel region.

Additionally,neutral salt spray (NSS) tests were performed to evaluate the corrosion protection of the coatings to the Mg alloy by in accordance with GB T 10125-2012/ISO 9227:2002.The borders of three parallel samples were masked by epoxy resin to conduct the NSS test.The tested samples were analysed every 8 or 12 h.Once corrosion phenomena on the surface of the samples were observed clearly,the samples were removed and photographed.When the last group of samples exhibited corrosion,all samples were removed to take photographs.Subsequently,the corrosion products were removed via mechanical methods,in accordance with GB T 16545-2015/ISO 8407:2009.

Fig.2.SEM images of the coated AZ31B Mg alloys:(a) MAO coating;(b) MAO/DLC coating;(c) MAO/DLC/PDMS-5 coating;(d) MAO/DLC/PDMS-10 coating.

3.Results and discussion

3.1.Surface and cross-sectional characterisation

Fig.2 shows the surface micro-morphologies of the MAO,MAO/DLC,MAO/DLC/PDMS-5,and MAO/DLC/PDMS-10 coatings on the AZ31B Mg alloy.Numerous crater-like features with round shrinkage pores are observed in Fig.2(a),which are typical features for MAO coatings [10,21].For the MAO/DLC coating in Fig.2(b),the deposited DLC layer was inhomogeneous on the MAO layer,and inherent micropores from the MAO layer remained on the duplex coatings.However,the PDMS-modifie duplex MAO/DLC coatings exhibited a low-porosity morphology,as shown in Fig.2(c)and (d),and became denser with the increasing dipping time.Compared with the MAO coating,the MAO/DLC,MAO/DLC/PDMS-5,and MAO/DLC/PDMS-10 coatings exhibited micropores with a smaller aperture,which should have facilitated the interstitial fillin of the DLC or PDMS layer.The DLC and PDMS modificatio improved the corrosion resistance of the MAO-coated AZ31B Mg alloy owing to the increased compactness.

Fig.3 shows the cross-sectional morphology and elements mapping of the MAO/DLC/PDMS-5 coated AZ31B Mg alloy.Clearly,the thickness of the MAO layer and the DLC layer is about 4μm and 2μm in Fig.3(a),respectively.The result agrees with that of the distribution of the element from the Fig.3 (b-f).It is observed that there are a few micro pores and crack in the MAO layer.Unfortunately,it is difficul to fin the PDMS layer on the surface of the DLC layer in Fig.1(a).However,the distribution of element shows some different results.Generally,the brighter the colour of the element appears,the higher the content of that element is.Thus,there are slight O and Si element in the DLC layer(below MAO layer in Fig.3d and e).Additionally,the distribution of C element is partially overlapping with MAO layer(Fig.3f).The results can conclude that the PDMS penetrated into the DLC and MAO layer.

The water CAs were measured to assess the hydrophobicity of the coated samples (Fig.4).Theoretically,materials with CAs of<90° are wet or hydrophilic,whereas those with CAs of>90° are hydrophobic or water-repellent [46].The data in Fig.4 clearly indicate that the CAs of the MAOand duplex MAO/DLC-coated Mg alloy were slightly less than 90°,indicating that these samples were hydrophilic.However,the PDMS-modifie duplex MAO/DLC coatings exhibited clear hydrophobicity,which is attributed to the intrinsic properties of the PDMS.PDMS is well known for its low surface tension and is capable of wetting most surfaces [43,49],allowing it to easily penetrate the pores and cracks on the surface of the duplex MAO/DLC coating.Thus,defects such as surface pores and cracks were sealed.The modifie duplex MAO/DLC coatings also exhibited a denser surface (Fig.2(c) and (d)).Moreover,PDMS is a low-surface-energy organic polymer;hence,the modifie duplex MAO/DLC coatings had good hydrophobicity (Fig.4).Therefore,the PDMS-modifie dual MAO/DLC coating can substantially improve the corrosion resistance of Mg alloys owing to its compact and hydrophobic surface.

Fig.3.Cross-sectional SEM image (a) and elements mapping (b-f) of the MAO/DLC/PDMS-5 coated AZ31B Mg alloy.

3.2.Bond strength and hardness

Fig.5 shows the scratch surfaces and adhesion of the coated AZ31B Mg alloys.The adhesion of the duplex MAO/DLC-coated Mg alloy was 13.845N (Fig.5(b)),which was higher than that of the MAO-coated sample (Fig.5(a))and that of a previously reported Cr/DLC-coated Mg alloy[14].This indicates that the MAO layer increased the bond strength at the interface between the hard DLC layer and the soft substrate.Additionally,the PDMS-modifie duplex MAO/DLC coatings,especially the MAO/DLC/PDMS-10 coating,exhibited the highest adhesion amongst the samples prepared in this study.The interaction between graphene and PDMS was relatively strong,and a substrate-induced prestrain was observed in the graphene layer [50].The PDMS permeated the pores or cracks of the surface on the duplex MAO/DLC coating,increasing the adhesion.

Fig.4.Contact angle (mean ± standard deviation) of the coated AZ31B Mg alloys.

Fig.6 shows theHITandEITvalues of the MAO-,MAO/DLC-,MAO/DLC/PDMS-5-,and MAO/DLC/PDMS-10-coated Mg alloys.The averageHITvalues were 12.36,15.96,11.28,and 8.34GPa,respectively.Apparently,the deposited DLC layer increased the hardness of the MAO-coated Mg alloy.However,the hardness significantl decreased with the increasing PDMS modificatio time.This is ascribed to the internal stress relaxation that occurred when the PDMS penetrated the pores.TheEITvalues exhibited a different trend from the hardness values.It is well known that the elastic modulus (EIT) is a reflectio of bond strength between atoms from microstructure.The higher the bond energy,the greater the bond strength,and the greater the elastic modulus value is.TheEITvalue of the duplex MAO/DLC coating was 58.31GPa,which was significantl lower than that of the MAO coating (153.81GPa).This indicates that the DLC layer had a relatively high internal stress but a low C-C bond strength,which is confirme by the scratch surfaces of the coatings in Fig.5.PDMS is a viscoelastic Si-based organic polymer and is known for its unusual rheological properties[43,49].The bond energy of Si-O from the PDMS layer is 460kJ/mol,higher than that of C-C (332kJ/mol) from the DLC layer.This results in increasing slightly on theEITvalues once the MAO/DLC coating was modifie by PDMS.Therefore,the PDMS penetrated the pores,increasing the plasticity of the duplex MAO/DLC coating and thereby reducing the hardness.It also improved the bond strength of the surface layer,thereby increasing theEITvalue.

3.3.Tribological behaviour

The tribological behaviour of the coated samples was evaluated via dry sliding friction tests,as shown in Figs.7-9.For the MAO coating,the COF fluctuate in the range of 0.83-0.88,accompanied by severe oscillation (curve in Fig.7).The oscillation implies that the MAO-coated Mg alloy had poor tribological characteristics.This is indicated by the severe wear and high wear rate shown in Figs.9(a) and 8,respectively.Scratches are observed along the sliding direction in Fig.9(a),indicating that the sliding was accompanied by abrasive wear [51].However,a closer examination of the wear track of the MAO-coated sample reveals that the MAO coating was intact.The MAO coating appeared to increase the wear resistance of the Mg alloy substrate.However,the COF of the duplex MAO/DLC coating remained below 0.25 and was very steady throughout the sliding test (Fig.7).The wear rate decreased from 16.4×10-6mm3/(N·m) for the MAO coating to 5.54×10-6mm3/(N·m) for the MAO/DLC coating(Fig.8).This indicates that the DLC layer significantl improved the tribological characteristics of the MAO-coated Mg alloy,which agrees with the literature [35].The PDMSmodifie duplex MAO/DLC coatings exhibited a lower COF and wear rate.The COF of the MAO/DLC/PDMS-5 coating was steady and remained below 0.21.The COF of the MAO/DLC/PDMS-10 coating was similar to that of the MAO/DLC/PDMS-5 coating in the early stages of sliding(before 400s).However,thereafter,a decreasing trend in the COF was observed.This indicates that the PDMS deeply penetrated more pores with increasing dipping time.The results confir the evolution trend of the bond strength and the hardness of the PDMS-modifie MAO/DLC coating with the increasing dipping time.

Fig.5.Scratch surfaces of the coated AZ31B Mg alloys:(a) MAO coating;(b) MAO/DLC coating;(c) MAO/DLC/PDMS-5 coating;(d) MAO/DLC/PDMS-10 coating.

Fig.6. HIT and EIT value (mean ± standard deviation) of the coated AZ31B Mg alloys.

Fig.7.COFs of the coated AZ31B Mg alloys as a function of the sliding time at 2N load in air with 27±2% RH.

Fig.8.Wear rate (mean ± standard deviation) of the coated AZ31B Mg alloys.

Fig.9 shows the SEM images and elemental mapping of the wear tracks of the coated AZ31B Mg alloys.The wear track of the MAO coating was broader than those of the duplex MAO/DLC and modifie coatings,indicating that the deposited DLC layer and the PDMS-modifie DLC layer significantl improved the wear resistance of the MAO coating.This improvement is ascribed to the compact microstructure (Fig.2) and high bond strength (Fig.5).As indicated by Fig.9 and Table 1,the wear tracks contained Fe and O.Thus,the Fe was transferred from the counter-body steel ball onto the wear track and formed oxides during sliding[52].Typically,a brighter colour of the elemental distribution corresponds to a higher content of the element [23].According to the energy spectra of Fe in Fig.9,the Fe oxide content of the wear tracks decreased in the following order:MAO>MAO/DLC>MAO/DLC/PDMS-5>MAO/DLC/PDMS-10.Fe was detected on the wear track of the PDMS-modifie duplex MAO/DLC coatings,indicating a lower COF for the modifie coatings.Si was also detected in the MAO/DLC/PDMS-5 and MAO/DLC/PDMS-10 wear tracks but not in the MAO/DLC wear track,confirmin that the PDMS penetrated the pores and cracks on the surface of the duplex MAO/DLC coating.This resulted in a high O content being detected in the MAO/DLC/PDMS coatings,as indicated by Table 1.This indicates that the Si-O network from the PDMS layer was broken,forming Si oxide owing to the high temperature caused by the sliding test (Fig.9(c)and (d) and Table 1).The wear mechanism is similar to that for an Si-doped DLC coating [53].The Si oxide layer can prevent contact between the MAO/DLC coating and the steel ball during sliding,resulting in a lower COF compared with the MAO/DLC coating.When the modificatio time was increased,a significantl larger amount of PDMS penetrated the pores,forming a thicker PDMS layer.Therefore,numerous Si oxides were formed with increasing sliding time.This resulted in a slow decline in the COF of the MAO/DLC/PDMS-10-coated sample,which finall reached a stable value (Fig.7).Hence,PDMS modificatio endowed the duplex MAO/DLC coating with excellent lubricating characteristics and effectively improved the wear protection of the AZ31B Mg alloy.

Table 1 Composition at wear tracks of the coatings in Fig.9 (Atom% ± abs.error%).

The Raman spectrum of a DLC coating is typically fitte using two Gaussian peaks [54,55]:the G peak around 1580 cm-1represents graphite,and the D peaks around 1360 cm-1represent a disordered graphite-like structure (not diamond).The G peak is due to the E2gsymmetric vibrational mode of graphite layers of sp2microdomains,and the D peak is assigned to the bond-angle disorder in the graphite structure,which is induced by linking with sp3C atoms and the lack of long-distance order in graphite-like microdomains [56,57].Typically,the G peaks of crystalline graphite or graphite-like C composed of only sp2C networks are sharp and appear at 1575-1580 cm-1,whereas the G peaks for amorphous C in which both graphite-like sp2and diamond-like sp3microdomains are randomly linked are broad and shift towards a low frequency [54,58].Regardless of the friction test results,the spectra of all the coatings exhibited the typical shape of a DLC spectrum (Fig.10).According to the G-peak posi tion and the intensity ratio of the D peak to G peak (ID/IG),the sp3/sp2ratios of the MAO/DLC coatings were determined[55,59,60].The D and G peaks of the duplex MAO/DLC coating before the wear test occurred at 1392 and 1556 cm-1,respectively (Fig.10a).However,the D and G peaks of the MAO/DLC/PDMS-5 and MAO/DLC/PDMS-10 coatings were shifted towards lower frequencies.The Si-O-Si bond was sp3-hybridised in the PDMS,which may have caused the shift in the D and G peaks.Moreover,the ID/IGratio decreased from 0.79 for the DLC coating to 0.72 for the MAO/DLC/PDMS-5 coating and 0.70 for the MAO/DLC/PDMS-10 coating.This suggests that the PDMS-modifie MAO/DLC coatings had an sp3-rich microstructure.However,the resulting sp3was derived from the Si-O-Si bond in PDMS,not the C-C bond in graphite.Therefore,the penetration of PDMS failed to increase the hardness of the duplex MAO/DLC coating but improved the adhesion of the DLC layer on the MAO coating.Therefore,compared with the MAO/DLC coating,the PDMS-modifie duplex MAO/DLC coatings exhibited lower hardness and COFs.

Fig.9.SEM images and elements mapping of the wear scar of the coated AZ31B Mg alloys:(a)MAO coating;(b)MAO/DLC coating;(c)MAO/DLC/PDMS-5 coating;(d) MAO/DLC/PDMS-10 coating.

Fig.10.Raman spectra of the coated AZ31B Mg alloys before and after wear test:(a) MAO/DLC coating;(b) MAO/DLC/PDMS-5 coating;(c)MAO/DLC/PDMS-10 coating.

Fig.11.Polarisation curves of the bare and coated AZ31B Mg alloys.

To investigate the wear mechanism of the MAO/DLC coatings,the Raman spectra of the coated samples after the friction test were examined,as shown in Fig.10.Before and after the friction test,the D and G peaks of the MAO/DLC coating shifted significantl towards higher frequencies,and the ID/IGratio increased from 0.79 to 0.96.This signifie an increase in graphite-like microdomains in the coating during the friction test,which endowed the MAO/DLC coatings with good tribological behaviour.For the PDMS-modifie MAO/DLC coatings,the D and G peaks were hardly shifted towards a higher frequency after the friction test;nevertheless,there was a slight increasing trend in the ID/IGratios.This indicates that graphitisation possibly occurred to a low degree during the friction test.It has been proven that the strong interaction between graphene and a PDMS substrate increases the barrier to shift the C atom out of the hexagonal structure,thereby hindering hydrogenation [50].It can be concluded that the interaction between the DLC layer and the PDMS layer restrains the graphitisation of the DLC layer during sliding.Thus,the PDMS not only enhanced the lubrication effect of the MAO/DLC coating but also slowed the graphitisation of the DLC layer.

3.4.Corrosion resistance

Fig.11 shows the polarisation curves of the bare and coated Mg alloy in the 3.5wt% NaCl solution.Compared with the bare AZ31B Mg alloy,the corrosion current densities of the coated samples were reduced,and the corrosion potentials were shifted slightly to the positive direction.This indicates that the coatings provided good corrosion protection to the Mg alloys.The fittin results of the polarisation curves from Fig.11 are presented in Table 2.Theicorrof the samples decreased in the following order:bare AZ31B>MAO>MAO/DLC>MAO/DLC/PDMS-5>MAO/DLC/PDMS-10.Theicorrof the MAO-coated Mg alloy was approximately three orders of magnitude smaller than that of the bare substrate.Theicorrof the MAO/DLC-coated Mg alloy was approximately one order of magnitude smaller than that of the MAO coating.According to Faraday’s law,a lower corrosion current density implies a higher corrosion resistance [61].Hence,the MAO and MAO/DLC coatings provided corrosion protection to the Mg alloy.Furthermore,the PDMS-modifie MAO/DLC-coated samples—particularly the MAO/DLC/PDMS-10-coated sample—exhibited anicorrwhich is two orders of magnitude smaller than that of the MAO-coated sample.Therefore,amongst all the samples tested,the MAO/DLC/PDMS-10 coating provided the best corrosion protection to the Mg alloy.

Table 2 Fitting results of PDP curves related to Fig.11.

Table 3 Optical photographs of bare and coated samples before and after NSS test.

Table 3 presents optical photographs of the bare and coated Mg alloy after the NSS test.The bare AZ31B Mg alloy was severely destroyed within 24h of the NSS test (distinctly visible corrosion pits appeared at approximately 2h) owing to the inherent low corrosion resistance [28].Corrosion pits were observed on the MAO-coated Mg alloys,and one corrosion pit was observed on the MAO/DLC coating when the NSS test was performed for 144h.Large corrosion pits were visible for the MAO/DLC/PDMS-5 coating when the NSS test was performed for 456h;these appeared on the MAO/DLC/PDMS-10 coating at 720h.Thus,it can be concluded that the PDMS significantl improved the corrosion protection of the duplex MAO/DLC coating on the Mg alloy,with better effects in the case of prolonged dipping.Additionally,analysis of the surface appearance of the removed corrosion products after 720h of the NSS test indicated that the MAO/DLC/PDMS-10 coating slowed the corrosion of the Mg alloy.This confirm the conclusion derived from the polarisation curves.

The MAO/DLC coating provided better corrosion protection than the MAO coating to the Mg alloy.This is attributed to the deposited DLC layer,which covered the surface micropores of the MAO layer and reduced the number of surface pores and their size.Nevertheless,some pores remained in the MAO/DLC coating (Fig.2b).Moreover,pores with diameters of approximately 0.5nm are inevitable within such DLC films even for high-quality DLC film [39,40].The diameter of water molecules is approximately 0.32nm;therefore,water molecules with corrosive ions such as Cl-can penetrate the duplex MAO/DLC coating easily through these pores,resulting in corrosion.PDMS—a Si-based organic polymer—is known for its unusual fl w properties [43,49].It penetrates the surfaces pores and forms an organic polymer with a curing agent to seal the pores.PDMS is well known for its low surface energy and hence can be used to prepare hydrophobic layers [47,48].Thus,dense,hydrophobic MAO/DLC/PDMS coatings were fabricated.The PDMS layer played two crucial roles:1) the hydrophobic PDMS layer separated the corrosive solution or electrolyte from the coating on the Mg alloy and 2)the sealed PDMS layer blocked the diffusion path of corrosive ions (Cl-) from the corrosive solution,thereby inhibiting the corrosion of the Mg alloy.A longer modificatio time yielded a denser PDMS modificatio layer.This resulted in greater corrosion protection for the Mg alloy.Therefore,the MAO/DLC/PDMS-10 coated sample exhibited excellent corrosion resistance.

4.Conclusions

(1) PDMS reduced the indentation hardness but increased the compactness,hydrophobicity,bond strength,corrosion resistance,and wear resistance of the duplex MAO/DLC coating.Therefore,PDMS can be applied for the surface modificatio of a duplex MAO/DLC coating to improve the corrosion and wear protection of the Mg alloy.

(2) The excellent tribological behaviour of the PDMSmodifie MAO/DLC coating is attributed to the Si oxide layer transferred from the PDMS layer,as well as the mitigation effect of the PDMS on the graphitisation of the DLC layer during sliding.

(3) The enhanced corrosion resistance provided by the PDMS-modifie MAO/DLC coating was due to its perfect sealing barrier and the hydrophobicity of the PDMS layer.

Declaration of Competing Interest

We hereby confir that this manuscript is our original work.And it has not been published or been submitted simultaneously elsewhere.We declare that we have no conflic of interest.

Acknowledgements

This work was supported by Special Fund for Local Science and Technology Development from the Ministry of Science and Technology of China (2020ZYD053),Science and Technology Planning Project of Zigong (2019YYJC22),and Opening Project of Key Laboratories of Fine Chemicals and Surfactants in Sichuan Provincial Universities (2020JXY05).The authors thank Shi-Jie Song and Lin Wang for their assistance in the experiments.