Simple and scalable synthesis of super-repellent multilayer nanocomposite coating on Mg alloy with mechanochemical robustness,high-temperature endurance and electric protection

Shuqi Wang ,Yaming Wang,* ,Junchen Chen ,Yongchun Zou,c ,Jiahu Ouyang ,Dechang Jia ,Yu Zhou

a Institute for Advanced Ceramics,Harbin Institute of Technology,Harbin 150080,China

b Key Laboratory of Advanced Structure-Function Integrated Materials and Green Manufacturing Technology,Harbin Institute of Technology,Harbin 150001,China

c Center of Analysis and Measurement,Harbin Institute of Technology,Harbin 150001,China

d School of New Energy and Materials,Southwest Petroleum University,Chengdu 610500,China

Abstract Multi-functionalization is the future development direction for protective coatings on metal surface,but has not yet been explored a lot.The effective integration of multiple functions into one material remains a huge challenge.Herein,a superhydrophobic multilayer coating integrated with multidimensional organic-inorganic components is designed on magnesium alloy via one-step plasma-induced thermal field assisted crosslinking deposition (PTCD) processing followed by after-thermal modification.Hard porous MgO ceramic layer and polytetrafluoroethylene (PTFE) nano-particles work as the bottom layer skeleton and filler components separately,forming an organic-inorganic multilayer structure,in which organic nano-particles can be crosslinked and cured to form a compact polymer-like outer layer with hierarchical surface textures.Remarkably,the chemical robustness after prolonged exposure to aqua regia,strong base and simulated seawater solution profits from polymer-like nanocomposite layer uniformly and compactly across the film bulk.Moreover,the self-similar multilayer structure coating endows it attractive functions of strong mechanical robustness (>100th cyclic rotary abrasion),stable and ultra-low friction coefficient (about 0.084),high-temperature endurance,and robust self-cleaning.The organic-inorganic multilayer coating also exhibits high insulating property with breakdown voltage of 1351.8±42.4V,dielectric strength of 21.4±0.7V/μm and resistivity of 3.2×1010 Ω·cm.The excellent multifunction benefits from ceramic bottom skeleton,the assembly and deposition of multidimensional nano-particles,and the synergistic effect of organic inorganic components.This study paves the way for designing next generation protective coating on magnesium alloy with great potential for multifunctional applications.

Keywords: Magnesium alloy;Multifunctional multilayer coating;Mechanochemical robustness;Robust wettability;High-temperature endurance.

1.Introduction

With increasing demands for lightweight to reduce fuel consumption and enhance the running power force,magnesium alloys,with a density two-thirds that of aluminum,are attractive for the aerospace,automobile,electronic equipment and biomedical engineering fields [1–3].Generally,except for high strength-to-weight ratio and damage tolerance,magnesium alloys have the advantages of excellent machinability,excellent dimensional stability,high thermal conductivity,conductivity and high damping characteristic [4–7].However,magnesium alloy itself has poor creep resistance,high chemical activity and dissolves fast in corrosive aqueous solution and/or humid atmosphere containing aggressive ions,which seriously impedes its wide applications [8–11].Another major issue that prevents its using in airspace is their low stability at high temperatures,which can be related to flammability [12].Among them,poor corrosion resistance of magnesium and its alloys is the critical problem to be solved in recent years [13–15].In the context of corrosion protection,the superhydrophobic coating is an emerging technology holding a unique fascination,which is the ability to isolate the bulk metal materials from the external corrosion environment [16,17].As such,artificial superhydrophobic coatings on magnesium alloys have been designed and prepared by methods including self-assembly [18],sol-gel [19],etching technique [20,21] and electrodeposition [2],etc.For instance,Tan et al.[22] fabricated a superhydrophobic composite coating with active corrosion resistance for AZ31B magnesium alloy protection.A hydrophobic surface was prepared by the microarc oxidation and subsequent stearic acid surface modification of Mg alloy,achieving corrosion protection [23].Singh et al.[24] explored an environment friendly nickel electrodeposition on AZ91 magnesium alloy and investigated its corrosion behavior.

Despite these progresses,unfortunately,with the fast advancement of modern techniques,more fickle requirements emerged,and the monotonous superhydrophobicity has been unable to handle new and strict challenges.Meanwhile,the future application of magnesium alloys will be extended to more complex environment and field,which requires them to possess more functions to fulfill the ever-growing demands.To our knowledge,the superhydrophobic coating materials with multifunction were realized via integrating biological activity,embedding various antibacterial agents as well as electrically and thermally conductive filler,etc.[25–28],which can be used under different environments,resulting in opening a new avenue for research into functional applications.Regrettably,only limited material is able to deal with these emerging requirements and challenges.Moreover,unlike the extensive development of self-cleaning,anti-icing and oil/water separation,as one natural function of superhydrophobic composite coating,electrical protection and long-term corrosion protection have been underutilized,which may widen the application fields of super-repellent materials in precision electronic devices protection.It can be seen that multi-functionalization is the future development direction of advanced protect coatings.The effective integration of multiple functions into one material remains a huge challenge,relying on the creative design for composite component,structure,and/or surface state.

Among various multifunctional composite coatings,polytetrafluoroethylene represents one of the most promising candidates due to its chemical stability,high dry lubricity,wear-resistance,hydrophobicity,remarkable corrosion resistance and insulation properties [29].Simultaneously,incorporation of PTFE with other ceramic materials has been used successfully in many industries,such as wear resistant components,electrical insulator,corrosion protection,heat exchanger,radiative cooling and so on.Additionally,the overall performance of composite coatings could be adjusted by their nanocomposite,multilayered structure design and the properties of building blocks.Hence,the integration of PTFE nanoparticles and ceramic porous skeleton into an organicinorganic multilayer structure with proper composition and hierarchical surface textures can be an effective strategy to develop multifunctional composite coatings.

Herein,we report anin-situplasma-induced thermal-field assisted crosslinking deposition (PTCD) technique to fabricate an organic layer-loading ceramic multilayer membrane on the AZ31 alloy surface consisting of ceramic porous layer as bottom layer skeleton and organic nanoparticles fillers as sealing and protection components.Meanwhile,compared with other coating techniques (electrochemical deposition,dipping,spin-coating,multi-step,etc.),the simple and environmentally friendly PTCD method can prepare multilayer structure coating with hierarchical surface textures in one step.The ceramic layer and organic layer grow simultaneously,which can further improve the sealing effect of the PTFE outer layer and overall performance of coating.Furthermore,benefitting from the integration of multidimensional nano-particles and the synergistic effect among organic and inorganic components,the multilayer coating exhibits attractive multifunction.It achieves outstanding performances of high enticing mechanical robustness (>100 cycles),stability to high-temperature exposure and robust self-cleaning ability.Remarkably,the chemical robustness after prolonged exposure to aqua regia,strong base and simulated seawater solution benefits from polymer-like nanocomposite layer uniformly and compactly across the film bulk.Moreover,the coating possesses high electrical insulating property that can be applied in the field of electrical equipment for insulation protection,as well as abrasive resistance protection.

2.Materials and methods

2.1.Preparation of the super-repellent multilayer coatings

One-step plasma-induced thermal-field assisted crosslinking deposition (PTCD) technique was implemented toin-situgrow the organic-inorganic multilayer coating on commercial AZ31 alloys.The specimens were cut from an as-extruded AZ31 alloy plate with a chemical composition (in wt.%) of 3.1% Al,0.9% Zn,0.2% Mn and balance Mg,provided by Yin He Company,China.The special PTCD treatment of AZ31 alloy samples was performed using a bipolar pulse AC plasma generator,and the experiment parameters were set as constant voltage of 500V,treatment time of 20min,frequency 600Hz,duty cycle 10.0% and the electrolyte temperature of 50–60°C.For the electrolytes,10g/L sodium silicate,4g/L sodium tungstate,4g/L potassium fluoride,2g/L sodium hydroxide and 12vol.% nano-PTFE emulsion with size of 100–150nm (Dupont,USA) were dispersed in DI water at various concentrations with vigorous stirring.For convenience,the coatings with and without PTFE adding were denoted as MP coating and MgO coating.The as-obtained MP coating has a hydrophobic contact angle of 135.9±1.8°The as-prepared MP multilayer coatings were annealed in air at～260 °C for～1h to form superhydrophobic multilayer coatings,called as MPT coating.

2.2.Characterization

The coating thickness was measured with a coating thickness gauge (TT230,Time Inc US).Surface morphologies and cross-sectional morphologies were characterized using a scanning electron microscope (SEM,MERLIN Compact,Oxford company,Germany),and elementary composition was analyzed by energy dispersive spectroscopy (EDS,MERLIN Compact,Oxford company,Germany).Before testing,the samples were inlaid with conventional metallographic method and sprayed with gold.Functionalities were investigated by Fourier transform infrared (FT-IR) spectroscopy using a Perkin Elmer Frontier spectrometer.The phase composition of the samples was characterized and analyzed by X-ray diffraction (XRD,Philips X′Pert) with a Cu-Kαsource and the ICSD Patterns were selected as the database of XRD patterns.The contact angle measurements were characterized by a contact angle meter (NAVITAR),where the static contact angle values were the average of at least five measurements of a droplet (～5 μL) at different positions on each sample.

2.3.Robust superhydrophobic tests

The cyclic rotary abrasion test was conducted by the sample against a counter-rotating surface of PRESI sandpaper (standard glasspaper,grit no.240,made in France)under an applied 100g load.The distance from center of rotary platform to center of the sample was 5cm.Around with distance of about 19cm was defined as one cycle,and the water-repellency of the sample was assessed every 20 cycles,then the coating thickness reduction was measured by thickness gauge (TT230,Time Inc US).The high-temperature endurance of the superhydrophobic coating was tested by the heat treatment experiment: samples were subjected to extreme high-temperature environment (500°C for 100min and 400°C for 24h in air).The water droplets bouncing experiments were observed using a high-speed camera (Phantom Camera Control 1.3) equipped with the Nikon camera lens(AF Nikkor 50mm f/1.8D).The self-cleaning property was measured on the coatings by ink,dust and Cr2O3powder.

2.4.Electrochemical measurement

The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were tested using a Gamry600(Reference 600,Gamry,USA) electrochemistry workstation.Experiments were carried out using a 3.5wt.% NaCl electrolyte in a classical three-electrode cell,in which the samples were used as the working electrode,and a platinum plate and a saturated calomel electrode (SCE) as counter electrode and reference electrode,respectively.The exposed area of the samples with size of 20mm×15mm×1.5mm was 0.3 cm2.The samples were allowed to stabilize at their open circuit potential (OCP) for 30min before the measurements started.The electrochemical impedance spectroscopy (EIS)measurements used a sine signal with an amplitude of 5mV.The range of measured frequencies extended from 105Hz to 10-2Hz,then,the experimental impedance plots were fitted to equivalent circuits using the Zview software.Meanwhile,dynamic measurement of polarization curves was recorded from -2.0 VSCEto 0.5 VSCEat a sweep rate of 1mV/s.

2.5.Chemical robustness tests

To further estimate chemical robustness,we used harsh chemical corrosion environments including the aqua regia (a mixture of concentrated hydrochloric acid and nitric acid in a 3:1 vol ratio)-a strongly acidic and very potent oxidizing agent,1M basic,sodium hydroxide solution.The samples were immersed in 3.5wt.% NaCl simulated seawater solution for 260h.Besides,weight change tests of samples were conducted by precision electronic balance (LIANG PING,precision 0.0001g).Firstly,the samples were completely immersed in a 3.5wt.% NaCl solution,and were taken out at regular intervals with repeatedly washed in DI water,water then were dried naturally,weighed and recorded.Meanwhile,the digital photographs of the various specimens after immersed in 3.5wt.% NaCl solution were recorded.

Relative weight change per unit area can be calculated according to following equation:

wherewtrepresents the quality of the coatings after a certain period of immersion,w0shows the original quality of the coated samples without immersion,tandsare immersing time and sample area,respectively.

2.6.Electrical insulation and tribological tests

The dielectric strength was determined by withstanding voltage tester (Nanjing Instrument Factory,China) with AC 5KV Ferranti step up transformer operating at 50Hz.The applied voltage was increased at a constant rate of 100V/s until electrical breakdown occurred.The volume resistance was determined by CHT3530 high resistance meter.The electrical resistivity of samples can be calculated according to following equation:

WhereRv(Ω) is the volume resistance of the specimen,ρ(Ω·cm) represents volume resistivity,d(cm) andl(cm) are the diameter of the upper electrode and the thickness of the coating separately.

The tribological behavior of the coating was evaluated using a pin-on-disk tester under the dry sliding conditions.The test was carried out in the laboratory (25±2 °C and 30% r.H.) under 3N load with a sliding speed of 0.15m/s.

Fig.1.Growth and characterization of the multilayer coatings: a),b) and c) Surface morphologies of MgO,MP and MPT;d) Water contact angles of different coatings;e) Cross-sectional morphology of MgO coating and corresponding element distribution (f);g) Cross-sectional morphology of MPT coating and corresponding element distribution (h);i) XRD patterns of MgO,MP and MPT;j) FT-IR analysis of MPT.

2.7.Adhesion strength tests

The samples were scribed by a crosshatch cut made of tungsten carbide with a guide and blade interval.Then,the 3M898 tape was pressed down firmly onto the surface with a sufficient pressure to remove air bubbles and ensure good contact between the tape and the coating surface.After that,the tape was torn off quickly along the directions that vertical to one of the cross scratch lines.Finally,the macro morphologies of crosshatch cut area were taken before and after tape peeling test.

3.Results and discussion

3.1.Growth and characterization of multilayer coating

The water-repellency is commonly believed to be due to the presence of synergistic binary geometric structures,which reduce the energy of the surface [30].The hydrophilic MgO coating exhibits typical porous morphology with microporous(ranging from 0.1μm to 4.5μm) and rough pleated-structure(Fig.1a).After adding PTFE nanoparticles,the nanoparticles are interwoven and accumulated on the surface irregularly,thus forming rough surface texture at microscale (Fig.1b).This irregular accumulation of organic nano-particles may be vital for the superhydrophobic surface to obtain the hierarchical roughness.After thermal modification,the microand nanoscale surface textures have already trapped enough amount of air to prevent the penetration of the water droplet,which makes the superhydrophobicity with contact angle reaching 161.7±2.1° on the surfaces (Fig.1c).

Besides,the cross-sectional views and respective line scanning of the coatings are depicted in Fig.1e-h.There are different sized pores,cracks and local defects throughout the MgO coating,and the corresponding distribution of elements mainly consists of Mg,Si and O elements.After the treatment in the PTFE containing solution,the thick and compact polymer-like nanocomposite layer covers the entire surface of the MgO coating,accompanied by disappearance of defects and micro-cracks,which exhibits a strong sealing effect on the MgO coating,marked in Fig.1g.Thus,a layered architecture is designed on Mg alloy to provide multiple corrosion barriers and functional characteristics,which includes an inner barrier layer,an intermediate layer,and a compact polymer-like nanocomposite layer.

Fig.2.Schematic of the organic-inorganic multilayer coating formation mechanism and the structure.a) Surface charged and movement of nanoparticles in the electrolyte.b) Schematic illustration of the formation process and structure of nanocomposite coating,subjected to the synergy of thermal field,gradient pressure field and microarc discharge effects.

The MPT coating is primarily composed of Mg,Si,O and F,indicating a uniform organic composite layer with hierarchical surface textures on the MgO bottom layer (Fig.1h).The surface phase composition and chemical composition of coatings are characterized by X-ray diffraction and Fourier transform infrared spectroscopy (Fig.1i,j).With addition of PTFE nanoparticles,a new diffraction peak at 18° appears,which indicates the existence of an integrated PTFE phase(Fig.1i).Furthermore,the peaks at 500–650 cm-1and 1150–1250 cm-1confirm the presence of -CF2and -CF3functional groups (Fig.1j),which also proves the successful ‘fluorination’ of the coatings via addition,mixing and deposition.

Moreover,to further understand the process of organicinorganic multilayer coating formation,the schematic growth mechanism of coating is illustrated in Fig.2.The coating formation process was mainly attributed to the combined dynamic equilibrium of bottom MgO layer formation and top polymer-like layer deposition by directional migration,chemical bonding and crosslinking curing of nanoparticles.When the voltage was applied on the electrode,the reactions of dissolution and oxygen evolution for the anodic process occur between the Mg anode and oxide interface.The oxide formation reaction was attributed to the transformation of Mg2+into MgO between the Mg anode and electrolyte interface(Fig.2a).Simultaneously,with the incorporation of nanoparticles,the negatively charged PTFE particles move towards the Mg anode when an electric field is generated between the anode and cathode potential by applied voltage (Fig.2a).In this process,the nanoparticles are subjected to the synergy of thermal field,gradient pressure field and plasma discharge effects,based on the combination of chemical,electrochemical and thermal foundations (Fig.2b).As a result,a top PTFE layer is formed on the bottom MgO layer by directional migration,deposition and crosslinking curing of nanoparticles [31].

3.2.Mechanical robustness and high-temperature endurance

Mechanical robustness is the major challenge for superhydrophobic coatings,which is attributed to the fact that the weak mechanical performance of micro-nano structural surface is prone to wear [32].The mechanical robustness of the wettability inside the MPT coating was investigated by the employment of an abrasive approach [33] (Fig.3a),which were community-widely used.Due to continuous PTFE nanoparticles characteristics and hierarchical topography inside the coating,the wettability remains above 152° after 100 abrasion cycles in 240# sandpaper (Fig.3b)confirming the durable superhydrophobicity of the MPT coating.Furthermore,owing to the solid mechanical strength endowed by the self-similarity design of multilayer structure,only tiny mass loss and thickness change occurs on the MPT coating after 100 abrasion cycles (Fig.3c).To further understand the durable special wettability,the SEM analysis is employed to observe the exposed morphology after sandpaper abrasion (Fig.3d).The original MPT coating surface is changed obviously by abrasive process with signs of abrasion and local PTFE nanoparticles.Although severe physical insults resulting from the performance of abrasive test appear on the MPT,the organic-inorganic multilayer coating is capable of preserving dual-scale architectures for non-wetting performance on the surface.

Fig.3.Mechanical robustness and thermal stability of multilayer coatings.a) A schematic of the mechanical abrasion tests performed using the sandpaper(grit no.240) abrasion tests;b) Photograph of water droplet and contact angle,after the 100th cycle rotary abrasion;c) The contact angles change and coating thickness reduction with abrasion cycles with 100g load,and contact angles of the multilayer coating remained above 152° after 100 abrasion cycles;d) A SEM image of the multilayer coating after 100th cycle abrasion;The changes of contact angle at high temperatures: the multilayer coating remains superhydrophobic after heating for 18h at 400°C and contact angle still maintains above 145° after heating for 24h at 400°C (corresponding the SEM images(Inset)),and contact angles change for the coating of 100 abrasion cycles reheated treatment with 400°C (e);Contact angle still maintains above 150° after high temperature exposure for 100min at 500°C (f);g) and h) surface and cross-sectional morphologies of the coating after high temperature exposure (500°C)for 100min.

Generally,superhydrophobic materials are difficult to survive in high temperature condition.The MPT coating was continuously exposed to high-temperature environment(400°C for 24h and 500°C for 100min in air),as shown in Fig.3e,f.The MPT coating remains superhydrophobic after high temperature exposure for 18h at 400°C and contact angle still maintains above 145° after 24h,which indicates excellent high-temperature endurance for a long time.The corresponding surface morphology consists of banded crosslinked PTFE with irregularly cross-distribution and exposed ceramic particles,which explains the highly durable superhydrophobicity on the MPT after high temperature exposure due to the self-similarity of multilayer micro-nano rough structures.Moreover,a more rigorous environmental test is implemented,that is,the coating was abraded 100 times before being heat treated at 400°C.The change trend of contact angle is in consistent with that of the original MPT coating with high temperature exposure of 400°C(Fig.3e),demonstrating the durable superhydrophobic ability.Undoubtedly,the MPT coating is seemed as impeccable and possessed opportunity to act as a candidate for promising applications such as anticorrosion and antifouling of ships as well as self-cleaning of aeronautics.Furthermore,the multilayer coating after exposing extreme high-temperature of 500°C for 100min still sustains superhydrophobicity with a contact angle of 151.3±0.8° Unfortunately,after high temperature exposure for 60min,cracks appeared at the margin of the coating,due to the organic-inorganic mismatch caused by thick polymer-like layer [34].To further understand the high-temperature durable wettability,the surface and cross-sectional morphologies of the multilayer coating after high temperature exposure are shown in Fig.3g and h.The network structures of surface are formed by organic and inorganic hybridization after high temperature treatment,which again endows the coating with superhydrophobicity.Such robustly superhydrophobic multilayer coating will help realize the technological potential of hydrophobic surfaces,by addressing some critical aspects of robustness.

Moreover,as shown in Fig.4,the MPT coating exhibits a stable water repellency performance,such as water flow impact,pH range,self-cleaning function and water droplets bouncing.In a word,the hierarchical surface texture and selfsimilar structure are still discerned in the multilayer coating after different cycles of destruction,which sufficiently proves the enticing interfacial bonding between the coating and substrate,providing highly stable avenues for the function applications.As shown in Fig.5,the results of scratching tests indicate that the MgO and MPT coatings both have an adhesion strength of zero grade with no exfoliation according to the ISO 2409 standard,which further confirms the superior interfacial bonding strength between the coating and substrate.

3.3.Corrosion resistance

We evaluated the corrosion resistance of the MgO,MP and MPT coatings in a 3.5wt.% NaCl aqueous solution from the electrochemical point of view by polarization curves(Fig.6).Generally,anodic polarization may sometimes produce concentration effects,as well as roughening of the surface which can lead to deviations from Tafel behavior[35].Therefore,extrapolation of the cathodic Tafel region back to zero overvoltage gives the net rate of the cathodic reaction at the corrosion potential,and this is also the net rate of the anodic reaction at the corrosion potential.

Fig.4.Self-cleaning performances of the MPT coating surface.a) Images of the superhydrophobic surface with various solutions (Water,Ink,Methyl orange and Methylene blue);b) Water flow impact of MPT coating;c) Statistics of the contact angles for the MPT coating surface with different pH value droplets;d) The ink removal test on the MPT coating surface.Self-cleaning tests process of superhydrophobic surface by e) dust and f) Cr2O3 green powder.g)Time-lapse photographs of water droplets bouncing on the MPT coating surface (Droplet sizes～10 μL.Drop height,40mm.).

As shown in Fig.6,the curves shown in the anodic branches reveals that the anodic curve cannot be fitted by the Tafel equation,since not well-defined peaks can be noted.However,the cathodic polarization curve displays a limiting diffusion currentidue to the reduction of oxygen.Thus,the cathodic reaction of Mg alloy surface coating in 3.5% NaCl obeys Tafel’s law in a small range of polarization curve,as well as the corresponding fitting results of corrosion potential(Ecorr) and corrosion current density (Icorr) were obtained and summarized in Table 1.As expected,the corrosion potential of the MP and MPT coating is significantly higher than that of the MgO coating,which indicates that the organic-inorganic multilayer coatings can more effectively cut off the circuit of the corrosion primary cell.Meanwhile,the corrosion current of MP and MPT is significantly lower than that of the MgO coating.Thus,it confirms that the blocking layer formed by superhydrophobic outer layer can significantly improve the corrosion resistance.

Table 2 Equivalent circuit parameters for the EIS data of Mg alloy matrix,MgO,MP and MPT samples in 3.5wt.% NaCl solution after 0.5h immersion.

Fig.5.The scratching methods to evaluate the adhesion strength of the layers: a) MgO sample;b) MgO sample after scratching test;c) MPT sample;d) MPT sample after scratching test.

Fig.6.Potentiodynamic polarization curves of the various coatings after 0.5h of immersion in 3.5wt.% NaCl solution.

Table 1 Polarization corrosion parameters of MgO,MP and MPT samples in 3.5wt.%NaCl solution after 0.5h immersion.

Moreover,in the electrochemical impedance spectroscopy(EIS) curves (Fig.7),the impedance modulus (|Z|f=0.01Hz)of MPT coating is four orders of magnitude higher than that of AZ31 alloy,demonstrating better corrosion protection performance.The Nyquist plot of the uncoated magnesium alloy is characterized by an inductive loop in the low frequency range as well as a capacitive loop in the high and medium frequency ranges (Fig.7b).The capacitive loop is related to the charge transfer process,while the inductive loop can be attributed to the dissolution and pitting of magnesium substrate [23].Obviously,the corrosion resistance of the coating is clearly enhanced due to the larger dimensions of the capacitive loop.Especially for the MPT coating,the diameters of the two capacitive loops are the largest among all specimens.Additionally,the capacitive arc of the MPT coating has not been observed at low-frequency part,indicating that super-repellent multilayer coating can effectively protect magnesium alloy from corrosion.From Fig.7c,the phase angles at an intermediate frequency become loftier and wider,indicating that the MPT coating is uniform and denser.Thus,the evolution of the EIS data demonstrates that superhydrophobic organic-inorganic multilayer coating has excellent corrosion resistance.

Fig.7.Electrochemical impedance spectra,fitted results and equivalent circuits of Mg alloy matrix,MgO,MP and MPT coatings.a) Bode plots of |Z| vs;b)Nyquist plots;c) Bode plots of phase angle vs.frequency in 3.5wt.% NaCl solution;The equivalent circuits of the EIS plots for d1) uncoated matrix alloy;d2) MgO,MP and MPT coatings.

For better understanding the electrochemical behavior and microstructure in detail,the equivalent circuit models are shown in Fig.7d,and the fitting data are summarized in Table 2.Rsis the resistance of the solution,RpandQpare the resistance and capacitance of superhydrophobic outerlayer separately.Rbis the charge transfer resistance,andQbdenotes double layer capacitance.The impedance of constant phase element is described by the formula:

WhereQandjrepresentCPEconstant and imaginary number,ωis the angular frequency (ω=2πf/rads-1,fbeing the linear frequency),andαtheCPEexponential term representing the deviation from ideal capacitance [36].

The high-frequency capacitance loop of magnesium alloy is characterized byQpandRpto reflect the loose corrosion products layer (Fig.7d1).The low-frequency inductive loop represents the onset of pitting corrosion [37–39],which is characterized byRlandL.Moreover,the invisible inductive features of coating materials indicate the well inhibition effectiveness.TheRbvalues of the MP and MPT coatings are 8.47×105Ω·cm2and 7.82×106Ω·cm2,respectively,which are significantly higher than that of AZ31 alloy(182.3Ω·cm2).Such high inhibiting effect is ascribed to the polymer-like nanocomposite layer fully covering the whole surface and blocking the electron transfer.It also proves that multilayer coating with air-trapping effect,forming blocking layer with superior chemical stability,can inhibit the corrosive ions (such as oxygen ions,chloride ions) from participating in the electrochemical reaction.Additionally,from the fitting results shown in Table 2,it is evident that the MgO coating has both a low capacitance (QbandQp) and a high resistance,indicating that it effectively protects the substrate from corrosion.As is well known,the higher the porosity of a coating,the higher its surface area,and therefore capacitance,will be [40,41].Simultaneously,theQbandQpvalues of the MP and MPT coatings are significantly lower than MgO,having a lower capacitance response,which was implied that the compact PTFE outer layer plays an important role in improving overall compactness.Furthermore,the MPT coating exhibits higherRT(RT=Rb+Rp) values than the other coatings,thus confirming that the hydrophobic modification further improves the corrosion resistance of composite coatings.

To further evaluate long-term chemical robustness,the MPT coating is exposed to complicated and harsh aqueous phases,including NaCl simulated seawater solution for 260h,1M sodium hydroxide solution and the aqua regia for 72h(Fig.8).The MPT coating still sustains its hydrophobicity after long-term immersion in NaCl solution.It can be seen that the MPT coating can effectively inhibit the penetration of corrosive solutions into the coating.Although the corrosive medium can still adhere to the hydrophobic coating and cause damage to the superhydrophobic coating surface,due to the self-similar multilayer structure design of the coating,the upper coating is slowly destroyed,but the coating still has air-trapping effect to inhibit further penetration of the solution.Besides,the deterioration of superhydrophobicity is greatly governed by the desorption of hydrophobic molecules[42],and hydrophobic molecules tend to desorb from the substrate by some reaction,such as hydrolysis of chemical interface bonding.However,molecules with strong chemical interfaces are not prone to similar desorption,which also means that it is not easy to generate functional groups that can react with MPT coatings or its hydrolyzates.

Moreover,the relative weight of the MPT coating increases slowly with prolonging the immersion time (Fig.8a).Nevertheless,the relative weight of AZ31 alloy substrate increases rapidly and then drops quickly,reflecting the high chemical activity of magnesium alloy.It is attributed to the fact that magnesium alloy is prone to chemical reactions with seawater solutions and easily is adhered by salt solutions causing a continuous increase in relative weight.Subsequently,magnesium alloy is damaged and consumed during corrosion process,resulting in a continuous decrease in its relative weight,corresponding to Fig.8b.Additionally,the relative weight of MgO coating decreases slowly,which is due to the continuous corrosion consumption.The photographs of the samples after immersion in a 3.5wt.% NaCl aqueous solution are shown in Fig.8b.The uncoated AZ31 alloy faces severe corrosion,of which the surface is completely covered by corrosion products as well as visible corrosion defects on the surface of the MgO coating.Whereas,there is no obvious corrosion for the MPT coating,demonstrating better long-term corrosion protection.However,some defects and cracks appeared at the margin of the MPT coating after immersing for 216 h,which is unfavorable for corrosion protection.To further understand the corrosion behavior,SEM analysis was employed to observe the morphologies of coatings after immersion (Fig.8c-f).AZ31 alloy substrate is severely corroded,forming corrosion products such as nanosheets and nanoflowers (Fig.8c).There are two surface morphologies in the MgO coating(Fig.8d,e),including the formation of large cracks on the coating surface to lose the protection of the substrate,and corrosion products similar to substrate corrosion.Scanning electron micrograph of MPT coating (Fig.8f) shows no observable corrosion damage.Fig.8g depicts the XRD patterns of specimens after immersion in 3.5wt.% NaCl aqueous solution.For the AZ31 alloy and MgO coating,diffraction peaks corresponding to Mg(OH)2are significantly larger than MPT coating,which indicates that their corrosion products are mainly Mg(OH)2and consistent with considerable corrosion during the immersion.In contrast,the diffraction peaks of Mg(OH)2for the MPT coating are almost absent,indicating a better corrosion protection of the superhydrophobic multilayer coating.

As is well known,superhydrophobic coatings are difficult to survive under fickle settings.Then,harsh chemical corrosion environments including 1M sodium hydroxide solution and the aqua regia (a mixture of concentrated hydrochloric acid and nitric acid in a 3:1 vol ratio)-a strongly acidic and very potent oxidizing agent were used,respectively,to further evaluate chemical robustness of the superhydrophobic multilayer coating.The MPT coating maintains a static contact angle of greater than 150° after 24h of NaOH exposure,as well as the coating after 100 abrasion cycle then was immersed in NaOH solution for a further 72h and still remains hydrophobicity (Fig.8h).The contact angle of the MPT coating is still higher than 150° after immersion in aqua regia for 12h.Notably,the coating of 100 abrasion cycle (as discussed in earlier sections) which was further immersed in aqua regia for 6h,also maintains hydrophobicity(Fig.8i).The reason for such excellent chemical resistance is the inherent chemical inertness of our rationally selected PTFE nanocomposite constituents.More importantly,the MPT samples are found to maintain highly efficient in repelling water droplets after the prolonged exposure in harsh chemical corrosion environments,indicating that the MPT is capable of withstanding corrosive liquids efficiently,and it was suitable for harsh application scenarios.

3.4.Proposed corrosion protection mechanism

Fig.8.Chemical robustness of the water-repellent multilayer coatings.a) The weight change of AZ31,MgO and MPT coating after immersed in 3.5 wt.%NaCl solution;b) Photographs of the various specimens immersed in 3.5 wt.% NaCl solution for 260 h;SEM morphology of samples after 260 h immersion in 3.5 wt.% NaCl solution: AZ31 (c);MgO coating (d),(e);MPT coating (f);g) XRD patterns of the various specimens after 260 h immersion in 3.5 wt.%NaCl solution;h) Effect of 1 M NaOH solution corrosion time: the superhydrophobicity of MPT coating is maintained above 150 ° after 24 h,and contact angle change of the coating after 100 abrasion cycles soaked in 1 M NaOH solution;i) Effect of aqua regia corrosion time on the water repellency of the MPT multilayer coating.Contact angle is maintained above 150 ° after 12 h,and contact angle change of the coating after 100 abrasion cycles immersed in aqua regia.

To better understand the corrosion behavior during longterm immersion,we proposed corrosion mechanism model of various samples(Fig.9).The corrosion mechanism of magnesium alloy surfaces is attributed to the fact that the corrosion ions are easily adsorbed on the native surface oxide (MgO),and then penetrate the coating/metal interface (Fig.9a).Meanwhile,the native surface oxide on Mg substrate is loose and not stable,unable to provide corrosion protection for the underlying substrate [43],resulting in that the existence of corrosion ions on metal surface causes corrosion reaction.Thus,adequate coatings are needed to prevent the diffusion of the corrosive ions to the metal surface.Furthermore,a corrosion mechanism model of the MgO coated AZ31 alloy was explored,shown in Fig.9b.When the MgO coated sample is immersed into the solution,at the initial stage,the MgO constituent on the outermost surface reacts with the corrosive solution and small amount of Mg(OH)2are formed.With the continuous reactions,the pores in the outer layer slowly enlarge.The corrosive solution goes through the outer layer and reaches the inner dense layer,as well as the interface between the dense layer and the AZ31 substrate (Fig.9b1).Once the corrosive solution reaches the interface layer,the substrate is easily corroded.Meantime,the corrosive medium penetrates all pores on the coating and reaches the substrate,leading to that more precipitates are accumulated on the sample surface.Once the AZ31 alloy corrodes away,the MgO coating and the precipitation layer formed after immersion will lose the support of the substrate and will detach from the substrate(Fig.8b).Accordingly,a newly synthesized superhydrophobic organic-inorganic multilayer coating decreases the corrosion ions adsorption on the AZ31 alloy surface,which indicated good corrosion protection efficiency.The excellent protection in the compact polymer-like outer layer is related to its ability to prevent the penetration of the aggressive ions by acting as a good physical barrier and increasing the tortuosity of the diffusion pathway for the corrosive ions(Fig.9c).Meanwhile,PTFE outer layer with robust superhydrophobicity adhered strongly to MgO bottom layer,forms a strong ‘pinning’and sealing effect,further blocking the corrosive diffusion pathways by making them longer and more wrapped,thus endowing the coating have good corrosion protection (Fig.9c1).

Fig.9.Schematic representation of the corrosion protection of MgO and MPT coatings.

3.5.Electrical insulation and tribological performance

Fig.10 presents the electrical insulation and tribological performance of MgO,MP and MPT coating.Generally,band gap of PTFE phase is wider than ceramic phase at room temperature and has excellent withstand voltage.The MPT coating with compact polymer-like nanocomposite outer layer exhibits quite high electrical insulation with the breakdown voltage of 1351.8±42.4V and the dielectric strength of 21.4±0.7V/μm (Fig.10a).Furthermore,the MPT coating,with high polymer-like outer layer thickness up to 26μm,achieves a high volume resistance of 817.4±13.6 MΩand resistivity of 3.2×1010Ω·cm (Fig.10b),which is far beyond the MgO coating.Such superior electrical insulation is ascribed to the design of organic-inorganic multilayer structure hindering the electron transfer.Moreover,it is worth noting that the MPT coating shows the outstanding wear performance with stable and ultra-low friction coefficient (about 0.084) during the full duration of sliding test (Fig.10c),which is attributed to the fact that polymer-like nanocomposite layer has excellent self-lubrication and enticing interfacial bonding strength.

3.6.Comparative comprehensive performance

To further demonstrate the advantages of this method and the MPT coating,here we compared the performance of the MPT coating against existing other coatings systems.As shown in Table 3,ecumenically,anticorrosive coatings are formed by the methods of plasma electrolytic oxidation(micro-arc oxidation),spin-coating,electrodeposition,multistep,etc.[23,37,44–48].Obviously,the corrosion current of the composite coating is lower and the corrosion potential is higher,whose corrosion resistance is significantly better than the magnesium alloy substrate.Especially for the composite coatings prepared by spin-coating and multi-step method,compared with MPT,their corrosion current is lower,while the investigation of long-term chemical stability is often overlooked.Of them,Cui et.al[23]reported a hydrophobic surface fabricated through the micro-arc oxidation and subsequent stearic acid surface modification of AZ31 Mg alloy,achieving long-term chemical stability.Tsai,et.al[46]proposed a preparation method of anti-corrosion coating,and the coating surface shows long-term chemical durability reaching 150h.In this work,there is no obvious corrosion for MPT coating after 260h immersion,demonstrating good long-term corrosion protection.Meanwhile,a one-step PTCD method for preparing organic-inorganic multilayer coating is simple,fast and environmentally friendly.In this process,PTFE nano-particles participate in the formation and growth of the coating,and can further seal the ceramic bottom layer through directional migration,deposition and crosslinking curing of nanoparticles,which is beneficial to the overall compactness.In a word,the comparative results show that the synthesis method is simple and convenient,as well as the MPT coating has excellent comprehensive performances with robust superhydrophobicity,long-term chemical stability and high electric insulation.

Fig.10.Electrical insulating properties and tribological performance of coatings.a) Breakdown voltage and dielectric strength of MgO,MP and MPT coating.b) Volume resistance and electrical resistivity of MgO,MP and MPT coating.c) Friction coefficient of the MPT coating during dry sliding wear test.

Table 3 Comparative comprehensive performance of MPT coating with existing other coatings systems.

4.Conclusion

In summary,a novel superhydrophobic multilayer membrane on the AZ31 alloy surface,integrated with porous ceramic as bottom layer skeleton and PTFE nanoparticles fillers as sealed and protected components,was fabricated throughin-situplasma-induced thermal field assisted crosslinking deposition technique followed by after-thermal modification.Thanks to the multilayer structure design and the synergistic effect between organic and inorganic components,the coatings possess multiple functions,i.e.,robust superhydrophobicity with a water contact angle of 161.7±2.1°,mechanochemical robustness,thermal stability,strong self-cleaning function,as well as high electric insulating property.Remarkably,the chemical robustness after prolonged exposure to aqua regia,strong base and simulated seawater solution benefits from the uniform and compact distribution of polymer-like nanocomposite layer across the film bulk.Moreover,the MPT coating achieves enticing mechanical robustness (>100 cycles),stable and ultra-low friction coefficient (about 0.084),and stability to high-temperature exposure.Simultaneously,the coating has superior electric insulation with resistivity of 3.2×1010Ω·cm,breakdown voltage of 1351.8±42.4V,dielectric strength of 21.4±0.7V/μm that can be applied in the field of electrical equipment for insulation protection.This work provides a new inspiration and insight for preparing multifunctional protective coatings,which have great potential applications in harsh external environments in future.

Declaration of Competing Interest

The authors declare no competing financial interest.

Acknowledgments

The partial supports from the NSFC grant nos.51571077 and 51621091,National Basic Science Research Program(2012CB933900),Advanced Space Propulsion Laboratory of BICE and Beijing Engineering Research Center of Efficient and Green Aerospace Propulsion Technology (LabASP-2020-05),Aviation Science Foundation of China (NO.20163877014) and the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201202) are gratefully acknowledged.