Hard wearproof PEO-coatings formed on Mg alloy using TiN nanoparticles

Journal Pre-proofs Full Length Article Hard wearproof PEO-coatings formed on Mg alloy using TiN nanoparticles Dmitry V. Mashtalyar, Sergey L. Sinebryukhov, Igor M. Imshinetskiy, Andrey S. Gnedenkov, Konstantine V. Nadaraia, Alexander Yu. Ustinov, Sergey V. Gnedenkov PII: DOI: Reference:

S0169-4332(19)32878-8 https://doi.org/10.1016/j.apsusc.2019.144062 APSUSC 144062

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

13 June 2019 8 September 2019 16 September 2019

Please cite this article as: D.V. Mashtalyar, S.L. Sinebryukhov, I.M. Imshinetskiy, A.S. Gnedenkov, K.V. Nadaraia, A. Yu. Ustinov, S.V. Gnedenkov, Hard wearproof PEO-coatings formed on Mg alloy using TiN nanoparticles, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144062

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Hard wearproof PEO-coatings formed on Mg alloy using TiN nanoparticles Dmitry V. Mashtalyara,b, Sergey L. Sinebryukhova, Igor M. Imshinetskiya *, Andrey S. Gnedenkova, Konstantine V. Nadaraiaa,b, Alexander Yu. Ustinova,b and Sergey V. Gnedenkova

a

Institute of Chemistry Far Eastern Branch, Russian Academy of Sciences, 159, Prosp. 100-letya Vladivostoka, Vladivostok 690022, Russia b

Far Eastern Federal University, 8 Sukhanova St., Vladivostok, 690950, Russia

* е-mail: [email protected] Keywords: magnesium alloys; protective coatings; plasma electrolytic oxidation; titanium nitride; nanoparticles Abstract In order to increase the hardness and wearproof of magnesium alloy parts it is necessary to use protective coatings. These surface layers were formed by plasma electrolytic oxidation (PEO) in the electrolyte containing nanosized particles of titanium nitride. X-ray photoelectron spectroscopy and Raman spectroscopy showed the presence of TiN, TiOxNy, and TiO2 compounds in the composition of the PEO-layer. Oxidation of TiN in TiOxNy and TiO2 was realized during the PEO treatment. It has been shown that the best protective mechanical properties are attributed to the coatings formed in electrolytes with TiN concentrations of 2–3 g/l. Mechanical properties of the obtained layers have been studied. It has been established that the hardness of nanoparticlescontaining coatings is twofold higher in comparison with the PEO-coating formed in the base electrolyte; its wear resistance increases in 2.2 times. Thus, the incorporation of TiN nanoparticles into the composition of the PEO-coating improves significantly mechanical properties of the surface of magnesium alloys. This, in turn, makes it possible to expand the field of application of these materials in such industries as aviation, automotive, development of high-tech products and devices.

1. Introduction Application field of magnesium alloys in the modern world is constantly growing. However, it is significantly limited by their high corrosion rates and low wearproof. The main method of protecting magnesium alloys involves applying protective paints or oxide coatings that enhances resistance to corrosion and wear.

Note that major disadvantages of magnesium alloys can be eliminated by plasma electrolytic oxidation (PEO) [1–6]. This method allows obtaining protective coatings on different metals and alloys without thorough surface preparation before forming corrosion protective, wearproof PEOlayers with high adhesion to the substrate [7–12]. The PEO method allows modifying the surface by varying the processing polarization mode and composition of electrolyte. One of the ways to improve the quality of the PEO-coatings is using of nanoparticles (NPs) in the composition of the electrolyte. Nanosized particles either substantially modify or give additional properties to surface layers of the treated materials. Thus, anticorrosive, magnetoactive, catalytic or other coatings can be obtained, that will greatly expand the application area of treated alloys [13–22]. Addition of nanosized aluminum oxide into electrolytes for plasma electrolytic oxidation results in enhancement of the mechanical and corrosion behavior of coatings formed on magnesium alloy [23–25]. Microhardness of nanoparticles-containing coatings is almost in twofold higher in comparison with the coatings obtained without particles addition, while the corrosion current density decreases in 7 times. Biomedicine is an area that can widely use the metallic implants with PEO-coatings modified with nanomaterials [26–28]. Incorporating silver nanoparticles into surface layers of coatings imparts them antibacterial properties [29–31]. Hydroxyapatite particles embedded in coatings accelerate a regeneration of bone tissues [32–34]. In this work, the features of the formation and properties of coatings obtained by PEO method in silicate-fluoride electrolytes containing titanium nitride (TiN) nanoparticles were studied. Advantages of titanium nitride have been proved in various industrial areas due to its high hardness, durability and chemical resistivity. It is actively used for formation protective films on cutting surfaces of tools, in friction assemblies, medical implants and other objects [35–40]. The use of titanium nitride nanoparticles has significantly expanded the scope of application of this compound, on its basis are created new composite material

ls and coatings with increased hardness and

corrosion properties [41–45]. Therefore, application of the titanium nitride nanoparticles in electrolyte for PEO can substantially improve the properties of surface layers of treated material and allows using the magnesium alloys in implant surgery, automobile industry, aircraft engineering, etc. Presented work continues the previous study [34], which was aimed on the formation of coatings and the research of their electrochemical properties. The main attention in this paper is devoted to the study of the process of incorporation of TiN NPs into PEO-coating, the behavior of nanoparticles at the plasma microdischarges, as well as the effect of this process on the morphological features, composition, and mechanical properties of the obtained surface layers. Note

that studies of the mechanism of formation of PEO-layers in electrolytes of a similar composition with the use of TiN nanoparticles previously have not been carried out.

2. Experimental Rectangular plates of a size of 15 mm × 20 mm × 2 mm made of the МА8 magnesium alloy (1.5 wt.% Mn; 0.15 wt.% Ce; Mg – balance) were used as samples. Before oxidation for the surface standardization, the samples were mechanically grind by sandpaper with different grain size (600, 800, and 1200), washed with distilled water, and degreased by alcohol. Based on the positive results of earlier studies [46] in present work the electrolyte containing sodium fluoride (5 g/l) and silicate (15 g/l) was selected to treat samples using the PEO. The titanium nitride nanopowder (CAS 25583-20-4) with the particle size of 20 nm produced by the ABCR GmbH company (Germany) was used to produce the composite coating. Concentration of titanium nitride particles in the working electrolyte was equal to 1, 2, 3, and 4 g/l. To reduce the aggregation of nanoparticles in a aqueous medium the ultrasonic treatment (UST) was used. UST was carried out using a Bandelin HD 3200 ultrasonic homogenizer (Bandelin Electronics, Germany) equipped with a titanium probe at operating frequency of 20 kHz and power 140 W. A VS70T titanium probe with a diameter of 12.7 mm was placed in a 100 ml RZ3 rosette cell filled with suspension. For intensive circulation of the suspension was carried out in a pulsating mode: 1 second of ultrasonic treatment and 2 seconds of pause. The pulsating mode of ultrasonic dispersion allows changing the portion of the suspension under the probe. After each stage lasting 2 min a 30 min pause was made. Since during the plasma electrolytic oxidation the coating growths, as a rule, realized at anodic polarization, to ensure the maximal incorporation of particles into the coating an anionic surfactant (sodium dodecyl sulfate) was used as a stabilizer of the dispersed system. Its concentration in the electrolyte was equal to 0.5 g/l. The electrolyte conductivity measured by a HI 255 Hanna Instruments Conductivity Meter was around 15–16 mS over the whole concentration range (0–4 g/l). The size and –potential of particles in suspensions were measured on a Zetasizer Nano ZS (Malvern Instruments, UK). All chemicals were used as received. The coating process was carried out using a plasma electrolytic oxidation device equipped with a conventional reversible thyristor rectifier as power supply. The frequency of polarizing pulses was 300 Hz and the duty cycle was 50 %. All samples were treated in a two-stage bipolar PEO-mode. At the first stage, the anodic component was stabilized galvanostatically at a current density of 0.5 А/cm2; and the cathodic phase was stabilized potentiostatically at −30 V. The duration of the first PEO-stage was equal to 200 s. The anodic voltage attained 300 V at this stage. During the second stage for 600 s the anodic component was reduced potentiodynamically from 300

to 200 V, and the cathodic one was changed from −30 to −10 V. The electrolyte temperature was maintained in the range of 23–25 °С using a Smart H150-3000 chiller (LabTech Group, UK). X-ray photoelectron spectroscopy (XPS) hemispherical energy analyzer PHOIBOS-150 (SPECS, Germany) equipped with the ultra-high vacuum system was used for chemical analysis of the coating. The measurements were carried out at 5×10−7 Pa using non monochromatic Al Кα radiation with the energy of 1486.6 eV. The ion source was used to perform 5 min etching of samples by Ar+ beam with an energy of 5000 eV in the scanning mode. This procedure removed approximately 3 nm surface layer. In this work, Raman spectroscopy was used to investigate the structure of the TiN NPs and TiN-containing PEO-coating by means of Raman spectrometer alpha 500 (WITec, Germany). The obtained results were analyzed using WITec Control program. The laser with 532 nm wavelength and 20 mW irradiation power was used. Micro-Raman spectra were collected in the range from 100 up to 1200 cm−1 during 60 minutes (60 accumulated spectra). To obtain the 2D map of the compounds intensity distribution on the PEO-coating surface the Raman spectroscopy was also used. Spectra were acquired in the scanning mode from the area 54×36 μm, which contains 50×50 micro-Raman spectra with 1 s integration time. The distribution of the concentrations of Ti and N within the thickness of the PEO-layer was determined using the GDOES (glow discharge optical emission spectrometry) method, employing a GDS 850A (LECO, USA). The cathode voltage was equal to 1000 V and ion current was equal to 30 mA; a 4 mm diameter anode were used. Electron micrographs of sample surfaces were obtained using a Carl Zeiss EVO 40 electron microscope (Carl Zeiss, Germany). The apparent porosity of the coatings was estimated by analyzing SEM-images over the whole area of the top view (surface porosity) and through the whole coatings thickness (crosssection porosity) obtained at the same magnification using ImageJ 1.48v software (National Institutes of Health, USA). The fraction of the area occupied by the pores on the entire visible surface of the coating was estimated. The method of digital processing of SEM-images with the use of software algorithms is one of the common methods for determination of apparent porosity of the surface and cross-sections of the PEO-coatings, being sufficiently functional, accurate and efficient (OSP) tool [47,48]. The surface morphology was investigated using non-contact optical surface profiler technique. OSP370 device installed on the M370 electrochemical workstation (Princeton Applied Research, USA) was used. Data were analyzed using the Gwyddion 2.45 software. Surface topography analysis is presented in the form of the most common roughness parameters: Ra (the arithmetic average of the absolute values of the profile heights), Rt (the vertical distance between

the highest and lowest points of the profile), Rz (the average value of the absolute values of the heights of five highest profile peaks and the depths of five deepest valleys), Rv (distance between the deepest valley of the profile and the mean line), and Sreal/Spr (the ratio of the calculated real surface area value to the area of its orthogonal projection). Microhardness and elastic modulus of the coatings were determined using DUH-W201 dynamic ultramicrohardness tester (Shimadzu, Japan). The measurements of the universal microhardness (Hμ) were carried out on a cross-section using a Berkovich indenter at a load of 100 mN. Evaluation and comparative analysis of elastic-plastic properties of the coating was carried out using Shimadzu DUH Analysis Software Application v. 2.10. Universal microhardness was determind as the ratio of the applied load to the contact area at this load. For the Berkovich pyramid, the calculation was carried out according to the Eq. (1) [49]: 𝐻𝑈 = 𝐴

𝐹

𝑠

𝐹

≈ 26.43·ℎ2, (ℎ)

(1)

1

where 𝐴𝑠 (ℎ) =

3√3·tan⁡(𝑎) cos⁡(𝑎)

· ℎ12 ,

(2)

where F is an applied load (mN), h1 is a maximum penetration depth of the indenter (μm), and α is a constant depending on the shape of the indenter (for the Berkovich pyramid α = 65.03). The applied load in all measurements was equal to 100 mN, the loading rate was equal to 13.23 mN/s, and the holding time at the maximum load was equal to 5 s. The tests were carried out at room temperature. Tribological ball-on-disk test were carried out on CSM Tribometer (CSM Instruments, Switzerland). A corundum (α-Al2O3) ball with 10 mm diameter was used as a counterbody. All the studies were carried out under dry friction conditions in air at a temperature of 25 °C and a normal load of 10 N. The sliding speed was 50 mm/s; the track diameter was equal to 10 mm. Estimation of the area of cross-section of the wear track after tribological tests was carried out using a Metek Surtronic 25 precision contact profilometer (Taylor Hobson, UK). The sample material wear rate (W) was calculated according to the Eq. (3): 𝑊=

∆𝑉𝑠𝑎𝑚𝑝𝑙𝑒 𝑃𝑁

,

(3)

where ΔVsample is the samples volume loss during wear testg (mm3), N is the distance moved (mm), and F is the normal load (N). The sample volume loss was calculated according to the Eq. (4): ∆𝑉𝑠𝑎𝑚𝑝𝑙𝑒 = 𝑆 × 𝑙,

(4)

where l is the length of the of the wear track (mm), S is the area of the cross-section of the wear track (mm2). All experiments were carried out at least on 4 samples. The acquired data for each parameter were processed statistically and presented in text as average mean ± standard deviation.

3. Results and Discussion Preparation of the dispersive electrolyte containing nanoparticles for plasma electrolytic oxidation is an important stage of the composite coating formation. Nature of the dispersed solid phase influences on the behavior of NPs in different media. Size and shape of nanoparticles, method of their manufacture as well as the composition of the medium have to be taken into consideration for preparation of the stable dispersed electrolytic system. Preliminary assessment of the hydrodynamic diameter and –potential of the dispersed phase in dependence on UST and presence of surfactant in the dispersed media was carried out for aqueous suspensions containing titanium nitride nanoparticles in concentration of с = 0.1 g/l. This study shown that sequential reduction of the particles hydrodynamic diameter was registered after triple UST, while further processing resulted only in insignificant decrease of the hydrodynamic diameter (Table 1). The pH of the dispersed media is a very important parameter, which influences on the dispersed solid phase state. In Fig. 1 the evolution of the hydrodynamic diameter and –potential versus pH are presented. The studies were carried out in the electrolyte with a TiN particles concentration of 1 g/l. Increase of pH resulted in reduction of both particles hydrodynamic diameter and their electrokinetic potential. Lowest values were registered at pH = 10.7. At this pH values the hydrodynamic diameter is decreased down to 177 nm; –potential is decreased down to 51.6 mV (Fig. 1). Therefore, the decrease of the –potential in alkaline medium is favorable for stabilization of the dispersed electrolytic system. On the contrary, the lower pH in our case results in fast particles sedimentation. Therefore, alkaline electrolytes are preferable for plasma electrolytic oxidation. Influence of the electrolyte composition on the mode of plasma electrolytic oxidation is presented as a dependence of the current density and bath voltage on the PEO process time (Fig. 2). Current density at the initial stage of the process (200 s) was registered at 0.35 А/cm2 (Fig. 2 b). The voltage reached 280 V during coating formation in electrolyte without nanoparticles, whereas dispersed electrolytes with titanium nitride NPs showed reduction of the anode voltage growth rate after 100 s of the process, reaching the final anode voltage of Ufin = 250–270 V. Addition of titanium nitride nanoparticles interferes with anode reactions and delays sparking. Adding 1 g/l of TiN nanopowder reduces the final anode voltage at the initial process stage down to ~270 V (Fig. 2а). Subsequent increase in TiN concentration to 4 g/l results in a consistent decrease in voltage till ~250 V (Fig. 2 а) for this electrolyte composition. Addition of TiN nanoparticles into the electrolyte also led to an increase of the max value of cathodic current (Fig. 2b). Such behavior can be attributed to high conductivity of titanium nitride nanoparticles that were embedded into coatings, and it results the coatings resistivity decrease.

Changes in the phase composition of coatings formed in electrolytes with the different concentration of titanium nitride NPs were not registered by XRD [50]. This method probably fails to detect TiN phase in the coating due to its low concentration (less than 10 %) in studied samples. Analysis of XPS data of the sample obtained in electrolyte containing 3 g/l TiN nanoparticles (Fig. 3) shown that the coating contains Ti and N in the amount up to 0.1 and 1.4 at.%, respectively, (Table 2) in addition to the main elements (Mg, O, Si). With purpose to estimate the chemical state of elements in the coating composition the samples were Ar+ etched for 5 and 10 min. Etched surface was analyzed by XPS method (Table 2). Underlying layers contain substantially higher amounts of titanium (Table 2) as compared to the surface. Binding energy values Ti 2p and N 1s electrons led to the assumption that titanium is in the oxidized state: TiO2, TiOxNy, TiN (Fig. 3b). Presence of titanium oxynitride and oxide is caused by the chemical reaction between nanoparticles as a result of high-energy microdicharges realization during plasma electrolytic oxidation. Therefore, part of titanium nitride under these conditions is transformed to titanium dioxide and titanium oxynitride. Note that the TiOxNy possesses the extraordinary optical and electrical properties, as well as chemical stability. Its physical properties are between metallic TiN and insulated TiO2 [51]. Fig. 4 shows two micro-Raman spectra of TiN NPs obtained at different irradiation power. Our results are close to the work [52], where authors also detected and described the phase transformation of TiN (Fig. 4a) into TiO2 (Fig. 4b) (the mixture of rutile and anatase) with the irradiation power increasing. Therefore, for TiN NPs it is impossible to use the high irradiation power, which is higher than 30% from the maximum level. In the micro-Raman spectrum (Fig. 4a) of the TiN NPs phonon bands were detected in the acoustic range 150–400 cm−1 , which are related to heavy Ti4+ ions vibrations and in the optic range 550–700 cm−1, which are the result of lighter N3– ions vibrations [53–55]. The Raman peaks at 240, 320, 400–500 (maximum at 470), and 580 cm−1 are related to transverse acoustic (TA), longitudinal acoustic (LA), second-order acoustic (2A), and transverse optical (TO) modes of the cubic TiN nanocrystalline, respectively [52,54]. After formation of the PEO-coating with TiN NPs addition, Raman spectroscopy was also used to study the structure of the obtained surface layer and to confirm the presence of TiN in the composition of the PEO-coating. Two types of micro-Raman spectra were obtained from the different areas of TiN-containing PEO-layer (Fig. 5). Micro-Raman spectra presented in Fig 5a and Fig 5b were collected in points 1 and 2 indicated in Fig. 6, respectively. From Fig. 5a four bands can be seen at 210–280 (maximum at 265), 300–370 (maximum at 350), 380–470 (maximum at 420), and 530–650 (maximum at 610) cm−1. These bands are in agreement with the micro-Raman spectrum of TiN NPs and indicated that some part of TiN

nanoparticles presented in the PEO-coating composition in the unchanged state. At the same time, Fig. 5b shows another type of the Raman spectrum. This spectrum also contains bands responsible for TiN, but as opposed to Fig. 5a it also contains bands at 160, 200, and 515 cm−1, which are responsible for the vibration modes of titanium-oxynitride [56–58]. These data indicate that as a result of PEO treatment some parts of TiN oxidized with the formation of TiOxNy and TiO2 [55]. It should be noted that peaks in the range from 750–900 cm−1 can be attributed to silicates in tetrahedral configuration [59,60] due to presence of Mg2SiO4 in the composition of PEO-coating [50]. Raman spectroscopy was also used in this work to show the intensity of TiN and TiO xNy distribution on the PEO-coating surface. The micro-Raman spectra were collected from the surface of PEO-layer and the definite range, which included the high intensity band, that responsible for specified substance, was chosen to present 2D intensity map. Fig. 6 shows the optical image (a) of the studied area from which the micro-Raman spectra were collected as well as the intensity distribution of TiN (b) and TiOxNy (c). The range at 360–440 cm−1 was used to show the TiN distribution, and 130–170 cm−1 was for TiOxNy presentation. Analysis of the results indicates that the maximum TiOxNy concentration presented in the top layers of the PEO-coating, whereas TiN compounds homogeneously distributed in down layers and pores of the PEO-coating. During the PEO treatment, the upper layers of the coating, that contain TiN particles, underwent more strong oxidation and transformed in TiOxNy and TiO2. At the same time, TiN in the pores and down layers of the PEO-layer stayed in unchanged state. These results confirmed by XPS data. Elemental composition of coatings was analyzed by atomic emission spectroscopy GDS850A (LECO®). Elemental layer analysis of coatings has shown that titanium and nitrogen content is registered even at minimum concentration (1 g/l) of the titanium nitride in the electrolyte (Fig. 7a). Titanium and nitrogen are unevenly distributed within the coating thickness. The amount of titanium in the coating increases proportionally to the concentration of titanium nitride in electrolyte. Data on titanium distribution in the coating (Fig. 7) show two maxima of Ti content in subsurface layers at the depth of 1 µm from surface and close to PEO-coating/substrate interface (nonporous layer). Such elemental distribution in the coating is determined by the mechanism of the particles incorporation into PEO-coatings during several stages: transport to the anode, embedding in situ of plasma discharges and participation of embedded particles in the process of coating growth. Two major stages can be defined. At the first stage, the negative charged TiN particles are absorbed due to the electrophoretic effect on the surface of oxidized alloy (anode) [61,62]. Deposited particles are not fully included into the composition of anodized layers and have no reliable adhesion to the

anode material (Fig. 8a, b). At the second stage, plasma spark and arc discharges occur; chemical reactions are triggered by high-energy discharges. At that stage the part of the TiN particles are oxidized to titanium oxynitride or titanium dioxide. The depth of particle incorporation into PEOcoatings depends on their size. The nanoparticles that are smaller than the discharge channels can be deeper embedded into the coating in comparison with the microsized agglomerates (Fig. 8d) [63,64]. The relatively big particles or their agglomerates can be embedded deep into the heterooxide layer mainly through very big discharge channels realized at the end of PEO process. These particles are mostly embedded into the subsurface area. Analysis of scanning electron microscopy data revealed the differences in the morphology of surface layers (Fig. 9) formed on MA8 magnesium alloy in electrolytes with different contents of TiN NPs. Concentration of 2 g/l and over results in a “new growth” on the surface (Fig. 9c) caused by incorporation of particle agglomerates into coatings during plasma electrolytic oxidation [50]. Moreover, increasing of the concentration of nanoparticles in electrolyte leads to reduction of the average pore size. Analysis of SEM-images with use the ImageJ software has shown that the increase of the titanium nitride NPs concentration in the silicate-fluoride electrolyte reduces visible porosity of the formed coatings (Table 3) due to partial filling of pores with nanoparticles. 3D profiles of surface and relief profiles of the samples under study are presented in Fig. 10. Analysis of surface topography parameters (Table 4) enable one to conclude that addition of nanoparticles into the base electrolyte results in monotonic increase in the surface roughness. It also affects the quantity and size of cavities and protrusions. For all types of coatings, there is an excess of peak heights above the depths of the cavities, in contrast to the base coating obtained without the addition of nanoparticles. This is due to the incorporation of the NPs into the pores of the coating during the electrophoresis; therefore, the height of the peaks above the surface is greater than the depth of the cavities. Analysis of the results obtained by microhardness test (Table 5) demonstrated that nanoparticle-containing layers reveal the values of microhardness (Hµ) up to 4.5 GPa, which is higher than for the coatings formed in the base electrolyte (Hµ = (2.1 ± 0.3) GPa). Embedding titanium nitride nanoparticles in the amount of 1 g/l in silicate-fluoride electrolyte insignificantly increases the hardness of PEO-layer. Using electrolytes containing 2 g/l of titanium nitride or more results in substantial enhancement of the coating hardness. The tendency of decrease in microhardness and modulus of elasticity at particles concentration more than 2 g/l can be explained by lower intensity of ion transfer at coating/electrolyte interface and subsequent decrease of mechanical properties of the material. Results of tribological tests are presented in Fig. 11. The curves present dependence of the friction coefficient µ on the number of cycles during wear test. There is a general tendency for

positive effect of TiN nanoparticles in the coating composition on their mechanical properties (Table 6). Wearproof of nanoparticle-containing coatings is higher than one of coating formed in electrolyte without nanoparticles. This parameter improves for coatings obtained in electrolytes with TiN nanoparticles concentrations up to 3 g/l and become some worse for coating formed in electrolyte with TiN nanoparticles concentrations of 4 g/l (Table 6). Despite the partial oxidation of titanium nitride nanoparticles to oxynitride and titanium oxide, the incorporation of such nanostructured materials into the PEO-coatings allows forming the solid ceramic-like layers with high microhardness and wearproof performances. The incorporation of nanoparticles leads to a decrease in the porosity of the coatings, as evidenced by studies of the surface morphology and cross-sections of the coatings, as well as the results obtained by electrochemical impedance spectroscopy. During the process of plasma electrolytic oxidation, under the influence of the electrophoretic effect, particles are attracted in discharges zone and incorporated into the coatings at solidification. Therefore, the largest number of NPs is embedded into the coating, i.e. into the pores of the PEO-layer, that leads to the full or partial sealing the pores. From an analysis of other works (Table S2) devoted to the formation of PEO-coatings in electrolytes with nanoparticles, we can conclude that similar processes occur during coatings formation [63–70]. The mechanism of nanoparticles incorporation into the coating can be quite different as result of the differences in substrate material, PEO mode, base electrolyte composition, and particles properties (size, melting point, and chemical stability). The incorporation of particles can take place in two ways: reactive and inert [65]. In some cases, for example, when the silicon oxide [61, 72] or aluminum [73–77] particles are used, chemical reactions with the electrolyte components and the substrate material take a place. At that the new phases of silicates and aluminates formed. At the same time, when using particles of high-melting compounds, such as tungsten carbide [46], silicon carbide [78] or silicon nitride [79], the powders are inertly embedded. Particles are incorporated without formation of a new phase. The composites formed during this process as a rule do not change the corrosion properties of the obtained coating, but significantly increase the hardness and wear resistance PEO-layers (Table S3). In this work, the implementation of both formation mechanisms is observed. Despite the partial destruction of titanium nitride NPs in the upper layers of the coating, its compounds, apparently, do not enter into a chemical reaction with electrolyte components. The study of coatings by X-ray photoelectron (Fig. 3) and Raman (Fig. 4–6) spectroscopy revealed only compounds of titanium with nitrogen and oxygen. This made it possible to obtain a coating with mechanical characteristics exceeding the base PEO layers by more than 2 times (Tables 5, 6). Despite the decrease in porosity, the high conductivity of titanium nitride led to a slight

decrease in polarization resistance and an increase in the corrosion current density. Note that the protective corrosion characteristics of the coating remained at a high level, the value of the impedance modulus measured at lowest frequency exceeded by two orders of magnitude the values for the MA8 magnesium alloy without coating. Such coatings on magnesium alloys can be used in many industries where the protection from mechanical damage and aggressive media is reqiured.

Conclusion TiN nanoparticles were successfully incorporated into the PEO-layer formed on MA8 magnesium alloy. The relationship between the content of nanoparticles in the electrolyte and the presence of titanium nitride in the coatings composition was investigated. The TiN NPs are distributed within the thickness of PEO-coating and fill its pores. The partly transformation of the TiN nanoparticles to the titanium oxynitride and titanium dioxide during plasma chemical reaction take aplace. The optimal concentration of TiN nanoparticles in the electrolyte was found (2–3 g/l), which makes it possible to form coatings with the highest protective properties. Due to the high mechanical properties of the NPs the microhardness and wearproof of the formed nanoparticlecontaining coating were increased twofold in comparison with the PEO-coating formed in electrolyte without NPs.

Acknowledgements This work was supported by the Russian Science Foundation (№ 19-73-00078). The results of Raman spectrum were collected under the government assignments from Ministry of Science and Higher Education of the Russian Federation (project no. 265-2019-0001). Authors thank T. Mathenia and A. Isakov from the LECO Corporation for the elemental analyses.

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Figure captions Fig. 1. Dependences of hydrodynamic diameter (1) and ξ–potential (electrokinetic potential) (2) of titanium nitride nanoparticles on the pH of the electrolyte with a particles concentration of 1 g/l. Fig. 2. Evolution of the (a) voltage and (b) current density with time of the PEO process for samples in different electrolytes: (1) without nanoparticles and with TiN NPs concentration of (2) 1, (3) 2, (4) 3, (5) 4 g/l. Fig. 3. (a) XPS spectra of PEO-coating formed in electrolyte, containing 3 g/l TiN nanoparticles: Servey spectrum for (1) as-prepared sample and аfter (2) Ar+ etching for 5 min. (b) Ti 2p high resolution spectrum. (c) N 1s high resolution spectrum. Fig. 4. Micro-Raman spectra of the TiN NPs collected at (a) low 20% and high (b) 50% irradiation power of Raman laser. Fig. 5. Micro-Raman spectra of (a) TiN and (b) TiOxNy presented in the nanoparticles-containing PEO-layers composition. Spectra were collected in different points of the coating. Fig. 6. The optical image of the (a) studied area of the PEO-coating, and 2D map of (b) TiN and (c) TiOxNy intensity distribution on the surface of PEO-coating. Points 1 and 2 indicate places on the surface, where micro-Raman spectra presented in Fig 5a and Fig 5b were collected, respectively. Fig. 7. The distribution of elements whithin the thickness of coatings obtained in silicate-fluoride electrolyte with with TiN NPs concentration of (a) 1, (b) 2, (c) 4 g/l. Fig. 8. PEO-coatings formation scheme: (a) adsorption of nanoparticles by the positive charged substrate due to the applied voltage; (b) formation of anodized layer and incorporation of nanoparticles into the formed layer during the first stage of PEO process; (c) beginning of spark discharge and local melting of the substrate surface (d) growth of porous layer and nonporous sublayer, incorporation of TiN NPs into the coating. Fig. 9. SEM images of surfaces and cross-section (inserts) images of coatings obtained in different electrolytes: (a) without nanoparticles and with TiN NPs concentration of (b) 1, (c) 2, (d) 3, (e) 4 g/l. Fig. 10. 3D surface profiles of coatings obtained in different electrolytes: (a) without nanoparticles and with TiN NPs concentration of (b) 1, (c) 2, (d) 3, (e) 4 g/l. Fig. 11. Dependence of friction coefficient on number of cycles for samples with coatings obtained in different electrolytes: (1) without nanoparticles and with TiN NPs concentration of (2) 1, (3) 2, (4) 3, (5) 4 g/l.

Table 1. Performance of TiN nanopowder suspensions Hydrodynamic diameter, nm Surfactant

Zeta-potential, mV

UST cycles 1 2 3 4 1 2 3 4

–

+

Max*

FWHM*

Max*

FWHM*

399 286 259 247 356 218 202 197

126 102 67 75 97 62 56 59

–14.9 –25 –27.3 –28.1 –24.8 –25.9 –28.8 –29.2

10 9 7 8 8 7 6 6

*Max is the peak maximum at this value, FWHM is full width at half maximum.

Table 2. Binding energy (eV) and elemental composition (аt.%, in brackets) of the PEO-coating obtained in electrolyte containing 3 g/l TiN nanoparticles Analyzed area

Na 2s

Zn 2p

F 1s

O 1s

Ti 2p

N 1s

C 1s

Si 2p

Mg 2s

Surface

1073.1

1022.0

–

532.1

[0.1]

[1.4]

285.0

102.

89.

[0.5]

[0.8]

–

[18.2]

[67.9]

[4.5]

[6.6]

1072.2

1021.4

686.3

533.2

459.1

402.8

285.0

104.0

90.9

[1.3]

[0.5]

[1.1]

[9.5]

[0.6]

[0.3]

[48.4]

[9.8]

[10.5]

531.7

457.1

398.8

[10.8]

[0.2]

[1.4]

After Ar+ etching for 5 min

After Ar+ etching for 10 min

530.3

397.2

[4.7]

[0.9]

1073.1

1022.4

689.5

533.2

459.1

402.8

285.0

104.1

91.1

[1.3]

[0.3]

[2.1]

[9.3]

[0.9]

[0.5]

[46.6]

[9.5]

[9.9]

531.7

457.1

398.8

[10.5]

[0.3]

[2.5]

530.3

397.2

[4.6]

[1.6]

Table 3. Dependence of coating’s porosity and thickness on the concentration of TiN nanoparticles in the silicate-fluroide electrolyte used for PEO TiN particles concentration, g/l

Porosity, %

Thickness,

Surface

Cross-section

μm

Pore size (cross-section image), μm

0 (base PEOcoating)

5.0 ± 0.4

16.8 ± 2

20 ± 2

1.5 ± 0.7

1

4.7 ± 0.4

14.0 ± 1.7

19 ± 4

1.6 ± 0.6

2

4.5 ± 0.4

13.5 ± 1.9

21 ± 3

1.3 ± 0.7

3

4.1 ± 0.4

12.2 ± 1.8

20 ± 3

1.1 ± 0.4

4

3.9 ± 0.3

11.9 ± 1.6

21 ± 3

1.1 ± 0.5

Table. 4. Roughness parameters of the test samples with coatings obtained in a silicate-fluoride electrolyte without nanoparticles and with TiN nanoparticles in various concentrations TiN particles concentration, g/l

Ra, µm

Rz, µm

Rt, µm

Rv, µm

Sreal/Spr, %

0 (base PEO-coating)

0.75 ± 0.09

4.0 ± 0.6

5.0 ± 0.8

2.4 ± 0.5

1.78 ± 0.14

1

0.89 ± 0.07

4.9±0.4

6.2 ± 0.7

3.1 ± 0.5

2.7 ± 0.2

2

0.94 ± 0.08

5.1±0.6

6.7 ± 0.8

3.2 ± 0.6

2.83 ± 0.10

3

1.5 ± 0.3

8.6±1.5

10.8 ± 1.8

5.0 ± 0.7

7.9 ± 0.8

4

1.7 ± 0.2

9.2 ± 1.3

11.8 ± 1.4

5.5 ± 0.8

8.0 ± 0.6

Table 5. Microhardness and elastic modulus (Young modulus) of coatings formed onМА8 magnesium alloy TiN particles concentration, g/l

Hµ, GPa

Young modulus, GPa

0 (base electrolyte)

2.1 ± 0.3

60 ± 5

1

2.2 ± 0.3

62 ± 7

2

4.5 ± 0.5

102 ± 12

3

4.2 ± 0.5

98 ± 10

4

3.7 ± 0.3

97 ± 10

Table 6. Coatings tribological properties TiN particles concentration, g/l

Wear, mm3/(N·m)

Number of cycles to complete coating abrasion

0 (base electrolyte)

(4.3 ± 0.4)×10–5

(2.2 ± 0.2) × 103

1

(2.8 ± 0.2) × 10–5

(2.6 ± 0.2) × 103

2

(2.2 ± 0.2)·× 10–5

(3.8 ± 0.4) × 103

3

(1.9 ± 0.1)× 10–5

(4.9 ± 0.3) × 103

4

(2.5 ± 0.3)× 10–5

(3.3 ± 0.4) × 103



Protective coatings were formed on a magnesium alloy using TiN nanoparticles (NPs).



Formed coatings consist of TiN, TiOxNy, and TiO2 compounds.



Incorporation of TiN NPs increases surface layer hardness by twofold.



Obtained PEO coatings decrease wear of Mg alloy more than twofold.