Electrophoretic Co-Deposition of Polyetheretherketone And

materials

Article Electrophoretic Co-deposition of Polyetheretherketone and Graphite Particles: Microstructure, Electrochemical Corrosion Resistance, and Coating Adhesion to a Titanium Alloy

Aleksandra Fiołek 1,*, Sławomir Zimowski 2, Agnieszka Kopia 1, Alicja Łukaszczyk 3 and Tomasz Moskalewicz 1,*

1 Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Czarnowiejska 66, 30-054 Kraków, Poland; [email protected] 2 Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology, Mickiewicza Av. 30, 30-059 Kraków, Poland; [email protected] 3 Faculty of Foundry Engineering, AGH University of Science and Technology, Reymonta 23, 30-059 Kraków, Poland; [email protected] * Correspondence: [email protected] (A.F.); [email protected] (T.M.); Tel.: +48-12-617-4527 (T.M.)

Received: 11 June 2020; Accepted: 20 July 2020; Published: 22 July 2020

Abstract: The present study explores the possibilities of fabricating a graphite/polyetheretherketone (PEEK) composite coating on a Ti-6Al-4V titanium alloy through duplex treatment consisting of electrophoretic deposition (EPD) and heat treatment. It has been found that the electrophoretic co-deposition of graphite and PEEK microparticles can be performed from environmentally-friendly pure ethanolic suspensions. Zeta potential measurements and a study of the interaction between both particle types with the use of transmission electron microscopy allowed potential mechanisms of particle co-deposition to be indicated. Microstructure characterization was performed on macro-, micro- and nanoscale using visible light microscopy, X-ray diﬀractometry and electron microscopy. This allowed the coating homogeneity and distribution of graphite particles in the polymer matrix to be described. Graphite particles in the form of graphene nanosheet packages were relatively evenly distributed in the coating matrix and oriented parallel to the coating surface. The heat-treated coatings showed high scratch resistance and no adhesive type destruction was observed, but they were highly susceptible to deformation. The corrosion measurements were performed with use of electrochemical techniques like open circuit potential and linear sweep voltamperometry. The coated alloy indicated better electrochemical corrosion resistance compared with the uncoated alloy. This work showed the high versatility of the electrophoretic co-deposition of graphite and PEEK particles, which combined with post-EPD heat treatment allows composite coatings to be fabricated with controlled distribution of graphite particles.

Keywords: graphite; polyetheretherketone; composite coating; electrophoretic deposition; microstructure

1. Introduction Polyetheretherketone (PEEK) is a high-performance thermoplastic polymer, with an amorphous or semi-crystalline structure depending on the applied processing and heat treatment route [1,2]. This material exhibits high thermal stability, easy processing and high chemical resistance. In addition, it combines low density with good mechanical and tribological properties, such as high mechanical strength and elastic modulus, as well as wear resistance [3–5]. The introduction of a reinforcement phase

Materials 2020, 13, 3251; doi:10.3390/ma13153251 www.mdpi.com/journal/materials Materials 2020, 13, 3251 2 of 15 into the PEEK matrix may result in unique properties being given to the final material, therefore PEEK is often used as the matrix for various types of composites. For example, Ba¸stanet al. [6] introduced hydroxyapatite (HA) microparticles into the PEEK matrix to increase bioactivity. Vail et al.[7] used the polytetrafluoroethylene (PTFE) filaments in the PEEK matrix to reduce the coefficient of friction (COF) and wear rate. PEEK may be used as a matrix for bulk composites, but also as a matrix of composite coatings improving various properties of metallic materials. In recent years, PEEK and PEEK-based coatings have been produced by various methods, such as thermal spraying [8] and printing technology [9]. Another method that allows the production of this type of coating is electrophoretic deposition (EPD). This is a method in which there is fast-growing interest due to the short time of coating production, uncomplicated and inexpensive equipment as well as great flexibility in the selection of materials for deposition [10]. Due to the unique properties of PEEK, the EPD of both PEEK [11–14] and PEEK-based [6,15–19] composite coatings has been widely studied, also with respect to improving the tribological properties of metallic materials. In our previous works, we have shown that it is possible to fabricate repeatable and homogeneous, semi-crystalline or amorphous PEEK [20] and composite PEEK-based coatings [21–25] using EPD and post-EPD heat treatment. However, despite the fact that these coatings improved the wear resistance of titanium alloys, the reduction of the coefficient of friction was not at a satisfactory level, usually about 0.3 in cooperation with an alumina ball. Therefore, the challenge is to develop wear resistant coatings with low a COF, at the level of 0.1 or below this value. It should be emphasized that the development of low-friction PEEK-based coatings, with the exception of the MoS2/PEEK708 coating [25], is missing in the available literature. In those works, also the kinetics and mechanism of the electrophoretic co-deposition of PEEK particles with oxides [21], nitrides [22–24] and sulfides [25] were investigated. It was found that only Al2O3 [21] and PEEK [20] particles were successfully deposited by anaphoresis from a suspension containing only ethanol as a dispersion phase. In the case of the co-deposition of PEEK with Si3N4 [22,23] or TiN [24] nanoparticles or MoS2 [25] nanosheets, it was necessary to additionally stabilize the suspension by using cationic chitosan polyelectrolyte or polyethylenimine (PEI). Although this addition allowed the cathodic co-deposition of polymer microparticles with nitride or sulfide nanoparticles, the degradation of chitosan during heat treatment caused the formation of porosity in the coatings, which resulted in a decrease in the subsequent scratch resistance and tribological properties. In particular, in terms of tribological properties, it is important that the coating is as uniform and devoid of surface irregularity as possible. No less important is the selection of the appropriate reinforcement phase, which, in combination with the matrix, gives unique properties to the composite. For example, it is possible to significantly reduce the coefficient of friction by using a material with a self-lubricating capacity [26]. Graphite is one of the allotropic forms of carbon and, next to molybdenum disulfide, is the most commonly used solid lubricant. Due to its hexagonal structure with the axial ratio c/a in the elementary cell of about 2.7 (where a and c are parameters of the elementary cell), it exhibits strong anisotropy of thermal and electrical properties. Graphite consists of layers of carbon atoms connected by strong covalent bonds, but between the layers there are weak van der Waals forces. Individual layers can be easily moved relative to each other, therefore, graphite has excellent lubrication properties [27,28]. The individual two-dimensional one-atom-thick layers of graphite are called graphene. It is characterized by low density, high mechanical strength, high surface area and high chemical stability [29–32]. Due to its outstanding properties, graphene is extremely attractive from a mechanical and tribological point of view [33]. Graphene has already been used as a filler in the composites of various polymers, for example, polyimide [34], polytetrafluorethylene [35] or polyacrylonitrile [36]. In recent years, several papers have also been published describing the production and properties of graphene/PEEK composites [37–43]. The EPD of pure graphene has also been reported in the literature and the authors of those works proved that it is possible to obtain graphene coatings on various substrates, e.g., silicon [44] or titanium alloy [32]. However, as far as we know, the present paper is the first study regarding the electrophoretic deposition of Materials 2020, 13, 3251 3 of 15 composite PEEK-based coatings incorporating graphite particles consisting of many layers of graphene stacked together. The aim of the present work is to develop deposition conditions and investigate the EPD mechanism, as well as the heat treatment process, for the fabrication of novel nanocomposite graphite/PEEK 708 coatings on Ti-6Al-4V alloy substrates. It is also particularly interesting to compare the EPD process and mechanism of the co-deposition of PEEK and graphite particles with respect to the co-deposition of PEEK with oxides, nitrides and sulfides.

2. Materials and Methods Graphite particles in the form of graphene nanosheet stacks and PEEK 708 (Victrex Europa GmbH, Hofheim am Taunus, Germany) microparticles were used as composite coating components. Graphite was delivered by Nanostructured and Amorphous Materials, Inc. (Houston, TX, USA). According to the manufacturer, the particle diameter was in the range of 5–10 µm, while the thickness 3 was from 4 nm to 20 nm. PEEK 708 has a melting point of 374 ◦C, a density of 1.32 g/cm and a particle size of up to 10 µm. As a substrate for coating deposition, a two-phase (α + β) Ti-6Al-4V titanium alloy (BÖHLER Edelstahl GmbH, Düsseldorf, Germany) was used. The alloy in the form of a bar with a 22 mm diameter was cut into discs with a thickness of 3 mm. The surface of discs was ground using sandpaper with a gradually increasing gradation from 200 to 3000 and then polished using a Struers (Ballerup, Denmark) standard colloidal silicon suspension (OP-S, 0.04 µm). Directly before the deposition, the surface was washed with distilled water and degreased with technical ethanol. The following suspensions were prepared for coating deposition: 1 g/L, 2 g/L or 4 g/L of graphite powder and 30 g/L of PEEK powder were mixed with pure ethanol (with a purity of 99.8 %), which was the dispersion medium. The prepared suspension was then magnetically stirred for 5 min and ultrasonically dispersed for 15 min to eliminate the particle agglomerates. The zeta potential of graphite and PEEK particles as a function of the suspension’s pH was measured using the Laser Doppler Velocimetry (LDV) technique with a Zetasizer Nano ZS 90 of Malvern Instruments Ltd., (Malvern, UK). The pH value was increased with sodium hydroxide and lowered with citric acid, and its value was controlled with an ELMETRON CPC-505 pH-meter (Zabrze, Poland). Direct current electrophoretic deposition was carried out in a standard two-electrode system using an EX752M PSU Multi-mode power supply (Huntingdon, UK). Austenitic stainless steel was used as a counter electrode (cathode), while the titanium alloy, on which the coating was deposited, was an anode. Constant voltage in the range of 10–100 V with 10 V changes and a constant time of 40 s were applied. The distance between electrodes was constant at 10 mm and the suspension was magnetically stirred (~ 50 rpm) throughout the deposition process to avoid particle sedimentation. As-deposited coatings were left for 1 h until they dried in atmospheric air. The coated alloy was heat treated in a Carbolite-Gero LHT 4/30 (Derbyshire, UK) laboratory oven. The treatment consisted of heating samples with the furnace to a temperature of 390 ◦C at a constant rate of 4.5 ◦C/min, holding this temperature for 40 min, and then cooling with the furnace to room temperature (RT) with a cooling rate of 2 ◦C/min. Investigations of the coating components and coating microstructure were carried out using an FEI Nova NanoSEM 450 (Eindhoven, the Netherlands) scanning electron microscope (SEM), JEOL JEM-2010 ARP microscope (Tokyo, Japan) and Tecnai TF 20 X-TWIN (FEI) (Eindhoven, the Netherlands) transmission electron microscopes (TEM). Thin foils for TEM investigation of graphite particles and the suspension used for EPD were obtained by placing a drop of the ethanol containing graphite particles or EPD suspension, respectively, on a copper grid and air-drying. The coating microstructure was investigated also on cross-section lamellae, that were cut by a focused ion beam (FIB) using an FEI QUANTA 3D 200i device (Eindhoven, the Netherlands). Phase analysis was conducted with the use of X-ray diffraction (XRD) in Bragg–Brentano geometry using a Panalytical Empyrean DY1061 diffractometer (Almelo, the Netherlands) and in TEM by selected area electron diffraction (SAED). The electron diffraction patterns and fast Fourier transformation (FFT) patterns were Materials 2020, 13, 3251 4 of 15 interpreted with the use of Java Electron Microscopy Software (JEMS, Pierre Stadelmann, Switzerland). Image analysis was performed using Gatan’s ‘Digital Micrograph’ (Pleasanton, CA, USA) software. In order to determine a scratch resistance of the coatings the scratch tests were carried out using a Rockwell C diamond stylus with a radius of 0.2 mm with the use of Micro-Combi Tester (MCT) from CSM Instruments (Peuseux, Switzerland). During the tests the load increased linearly from 0.01 to 30 N on a scratch length of 5 mm with a constant sample velocity of 5 mm/min. In the scratch tests, the critical load (Lc) corresponding to the load at which some failure of the coating occurs, involving the load LC1 at which the first cohesive cracks appear or the load LC2 causing adhesive damage, was investigated. Additionally, the scratch tests with a constant load equal to 0.25 N at the same parameters (5 mm of the scratch length and 5 mm/min of the velocity) were performed to determine the COF of the coatings vs. a Rockwell C diamond stylus. The measurements were repeated three times for each sample at defined test parameters. The paper presents representative results of the performed tests. A PGSTAT302 AUTOLAB potentiostat/galvanostat (Utrecht, the Netherlands) was used in corrosion resistance experiments. All potentials were measured vs. SCE (3M KCl solution) and the counter electrode was made of platinum plate. The measurements were performed using a classical three-electrode cell. NaCl (3.5 wt. %) aqueous solution was used as the electrolyte. Experiments were performed in aerated solutions at the temperature of 25 ◦C. The polarization curves were obtained with a scan rate of 1 mV/s.

3. Results and Discussion Graphite particles used as a component of composite coatings were in the form of graphene packages with several layers (Figure1a). The electron di ffraction pattern deriving from the package showed a polycrystalline character (Figure1b). The XRD pattern contained a strong crystalline diffraction peak from graphite (hexagonal primitive, hp) at the 2Θ angle of 26.4◦ (Figure1c). The HRTEM image of a graphene sheet with a few layers clearly shows distinct lattice fringes (Figure2a). The interplanar distance between the fringes measured in a fast Fourier transform (FFT) pattern (Figure2b) and in a line profile pattern performed along the line marked in an inverse fast Fourier transform image (IFFT) (Figure2c) of 0.339 nm, corresponded to the (002) plane of graphite with a hexagonal structure. Materials 2020, 13, x 5 of 15

Figure 1. FigureTransmission 1. Transmission electron electron microscopy microscopy (TEM) (TEM micrograph) micrograph (a ) (a and) and selected selected area area electron electron diffraction (SAED) patterndiffraction of (SAED the area) pattern marked of the area as marked A in Figure as A in 1Figurea with 1a with identification identification ( (b)),, as as well well as the as the X-ray X-ray diffraction (XRD) pattern (c) of graphite particles consisting of graphene layers stacked diffraction (XRD) pattern (c) of graphite particles consisting of graphene layers stacked together. together.

Figure 2. High resolution transmission electron microscopy (HRTEM) micrograph of graphene sheet with magnified details given at the top (a), fast Fourier transformation (FFT) patterns from the area marked as A in Figure 2a and its identification (b), inverse fast Fourier transformation (IFFT) image of the spot pairs marked by red circles in Figure 2b (c), and intensity profile for the line marked as red in its IFFT image showing its d-spacing as 0.339 nm (d).

Materials 2020, 13, x 5 of 15

Figure 1. Transmission electron microscopy (TEM) micrograph (a) and selected area electron diffraction (SAED) pattern of the area marked as A in Figure 1a with identification (b), as well as the Materials 2020, 13,X 3251-ray diffraction (XRD) pattern (c) of graphite particles consisting of graphene layers stacked 5 of 15 together.

Figure 2. HighFigure resolution2. High resolution transmission transmission electron electron microscopymicroscopy (HRTEM (HRTEM)) micrograph micrograph of graphene of graphene sheet sheet with magnifiedwith magnifie detailsd details given given at the at the top top (a ),(a), fast fast FourierFourier transformation transformation (FFT) (FFT)patterns patterns from the fromarea the area marked as A in Figure 2a and its identification (b), inverse fast Fourier transformation (IFFT) image marked asof A the in spot Figure pairs2 amarked and its by identificationred circles in Figure ( b 2b), inverse(c), and intensity fast Fourier profile transformationfor the line marked (IFFT)as red image of the spot pairsin its markedIFFT image by showing red circles its d-spacing in Figure as 0.3392b ( nmc), ( andd). intensity profile for the line marked as red in its IFFT image showing its d-spacing as 0.339 nm (d).

PEEK particles were thoroughly investigated in our previous work [20]. XRD studies showed an almost completely amorphous structure of the polymer powder with a trace amount of crystalline phase. The equivalent circle diameter of the particles was in the range of 2–15 µm. Both in our previous works [20,45] and in the works of other authors [12,13,46], it has been proven many times that pure PEEK coatings can be successfully deposited on an anode from an ethanol-based suspension without any other additives. However, our experimental studies showed that the deposition of graphite from pure ethanol is impossible. According to the literature data [32,44], pure graphene coatings were successfully electrophoretically deposited from an isopropanol-based suspension. In those works, they firstly added graphene powder to isopropyl alcohol and then, after 1 h of ultrasonic dispersion, used magnesium nitrate (Mg(NO3)2 6H2O) additive to charge the suspension. In the case of composite coatings, interactions between the co-deposited particles in the dispersion phase greatly influence the final success of EPD. In our earlier work [21], we deposited a homogeneous PEEK-based coating containing Al2O3 from pure ethanol. However, there is much more frequently a situation in which it is necessary to use additional substances that will facilitate or even allow deposition. For example, Boccaccini et al. [16] and Seuss et al. [47] used citric acid monohydrate for the deposition of PEEK/bioglass coatings to stabilize the ethanol-based suspension. Ba¸stanet al. [6] used HCl and NH4OH to obtain the specific pH of the suspension that prevented sedimentation during the deposition of polyetheretherketone and hydroxyapatite particles. In our previous works, we also used the addition of polyethylenimine or chitosan polyelectrolytes to ethanol for the co-deposition of PEEK with nitrides (Si3N4, TiN) [22–24] and sulfides (MoS2)[25]. Although the use of the polyelectrolytes made it possible to deposit macroscopically homogeneous coatings, unfortunately, the thermal degradation of PEI or chitosan during heat treatment after deposition resulted in the formation of open porosity in the coatings. Because of the similarity in some respects, e.g., the form of thin platelets combined in packages or similar surface potential, it seemed that graphite particles may behave similarly to MoS2 nanosheets. Interestingly, in the present work, it turned out that, in contrast to MoS2/PEEK coatings, the graphite/PEEK coatings can be deposited from pure ethanol containing 1 g/L of graphite without the need for any additives. However, at higher graphite concentrations of 2 g/L and 4 g/L in ethanol, Materials 2020, 13, x 6 of 15

Because of the similarity in some respects, e.g., the form of thin platelets combined in packages or similar surface potential, it seemed that graphite particles may behave similarly to MoS2 nanosheets. Interestingly, in the present work, it turned out that, in contrast to MoS2/PEEK coatings, theMaterials graphite/PEEK2020, 13, 3251 coatings can be deposited from pure ethanol containing 1 g/L of graphite without6 of 15 the need for any additives. However, at higher graphite concentrations of 2 g/L and 4 g/L in ethanol, no deposition processprocess waswas observed,observed, regardlessregardless of of the the suspension’s suspension’ pH,s pH, due due to to faster faster sedimentation sedimentation of ofthe the suspension suspension in in comparison comparison with with the the suspension suspension with with the the graphite graphite concentration concentration of 1of g 1/L. g/L. To understand understand and and explain explain the the phenomenon phenomenon of deposition of deposition of PEEK of PEEK and graphite and graphite from pure from ethanol, pure ethanolzeta potential, zeta potential measurements measurements and TEM and observations TEM observations of the suspensionof the suspension were performed.were performed. The zetaThe zetapotential potential of both of both graphite graph andite and PEEK PEEK particles particles in ethanol in ethanol was was negative negative over over the entirethe entire measured measured pH pHrange, range, reaching reaching the maximum the maximum values values of –38.6 of at –38.6 pH = at11 pH and = –43.911 and at pH–43.9= 10.5,at pH respectively = 10.5, respectively (Figure3). (Figure 3).

Figure 3. Zeta potential as a function of pH for graphite in ethanol and polyetheretherketone (PEEK) Figure 3. Zeta potential as a function of pH for graphite in ethanol and polyetheretherketone (PEEK) 708 in ethanol. 708 in ethanol. A sharp drop in the zeta potential value of the graphite particles to –17.2 mV was observed at pH A sharp drop in the zeta potential value of the graphite particles to –17.2 mV was observed at = 8. It is supposed that this behavior may be due to the addition of citric acid to the ethanol in order to pH = 8. It is supposed that this behavior may be due to the addition of citric acid to the ethanol in decrease the pH of the suspension from 8.7 (original pH) to 8, which provided citrate anions in the order to decrease the pH of the suspension from 8.7 (original pH) to 8, which provided citrate anions suspension and affected the electrophoretic mobility of graphite particles. Since the citrate anions were in the suspension and affected the electrophoretic mobility of graphite particles. Since the citrate already present in the suspension, further acid addition did not cause such a significant change in zeta anions were already present in the suspension, further acid addition did not cause such a significant potential, which was in the range from –25.9 mV to –27.5 mV at pH = 4.5–7.5. However, at pH below 4.5, change in zeta potential, which was in the range from –25.9 mV to –27.5 mV at pH = 4.5–7.5. However, the zeta potential began to decrease gradually. The original, not adjusted pH of the suspension used for at pH below 4.5, the zeta potential began to decrease gradually. The original, not adjusted pH of the deposition was 8.7. Although the potential was not the highest at this point, it was quite sufficient for suspension used for deposition was 8.7. Although the potential was not the highest at this point, it deposition because it oscillated around 30 mV for both particle types. According to the literature [48], was quite sufficient for deposition because it oscillated around 30 mV for both particle types. this is the value used to assume that the suspension is relatively stable. TEM investigation of the According to the literature [48], this is the value used to assume that the suspension is relatively suspension (containing 1 g/L of graphite) used for EPD revealed that the graphite particles were most stable. TEM investigation of the suspension (containing 1 g/L of graphite) used for EPD revealed that often adsorbed on the surface of large PEEK particles and a representative image is shown in Figure4a. the graphite particles were most often adsorbed on the surface of large PEEK particles and a It should be noted that, in this case, the PEEK particles were not completely covered with graphite. representative image is shown in Figure 4a. It should be noted that, in this case, the PEEK particles In contrast, it was observed that the PEEK particles in the suspension containing 2 g/L of graphite were not completely covered with graphite. In contrast, it was observed that the PEEK particles in were completely or nearly completely covered with graphite particles (Figure4b). Such behavior of the suspension containing 2 g/L of graphite were completely or nearly completely covered with particles indicates the electrostatic interactions between them and may explain the differences in the graphite particles (Figure 4b). Such behavior of particles indicates the electrostatic interactions behavior of the particles during the EPD process, deposition and its absence, respectively, in relation between them and may explain the differences in the behavior of the particles during the EPD to graphite concentration in the suspension. However, on the other hand, graphite particles and process, deposition and its absence, respectively, in relation to graphite concentration in the separate PEEK particles were also observed in the suspension. It should be mentioned that there are suspension. However, on the other hand, graphite particles and separate PEEK particles were also very large differences in both particle types, e.g., in nature, in size: micro and nano, in morphology: observed in the suspension. It should be mentioned that there are very large differences in both 3D and 2D, and in properties: hard and soft. However, regardless of these differences, the charge of particle types, e.g., in nature, in size: micro and nano, in morphology: 3D and 2D, and in properties: both was interestingly the same. The electrostatic interaction occurs much more frequently between hard and soft. However, regardless of these differences, the charge of both was interestingly the same. particles with opposite charges. For example, in the study of Yousefpour et al. [49], where negatively The electrostatic interaction occurs much more frequently between particles with opposite charges. charged PTFE combined into composite particles with positively charged hydroxyapatite, or in the For example, in the study of Yousefpour et al. [49], where negatively charged PTFE combined into work of Castro et al. [50], where such a situation occurred between ZrO and MgO particles. However, composite particles with positively charged hydroxyapatite, or in the work2 of Castro et al. [50], where a similar interaction between co-deposited particles to that observed in our study was described by such a situation occurred between ZrO2 and MgO particles. However, a similar interaction between Ba¸stanet al. [6] based on the co-deposition of PEEK microparticles and hydroxyapatite nanoparticles. According to them, HA nanoparticles have higher specific surface energy, which causes interaction between HA and PEEK particles in the suspension. HA particles cover the surface of PEEK and form the PEEK/HA complex despite the fact that both particle types have a positive zeta potential. Materials 2020, 13, x 7 of 15 Materials 2020, 13, x 7 of 15 co-deposited particles to that observed in our study was described by Baştan et al. [6] based on the Materialsco-2020depositiondeposited, 13, 3251 particles of PEEK to microparticles that observed andin our hydroxyapatite study was described nanoparticles. by Baştan According et al. [6] tobased them, on HAthe7 of 15 nanoparticlesco-deposition have of PEEK higher microparticles specific surface and energy, hydroxyapatite which causes nanoparticles. interaction Accordingbetween HA to and them, PEEK HA particlesnanoparticles in the have suspension. higher specific HA particles surface cover energy, the surfacewhich causes of PEEK interaction and form between the PEEK/HA HA and complex PEEK Based on the investigation results provided by zeta potential and TEM described above, it is possible despiteparticles the in thefact suspension. that both particle HA particles types coverhave athe positive surface zeta of PEEK potential. and form Based the on PEEK/HA the investigation complex to proposeresultsdespite provided twothe fact deposition thatby zeta both potential mechanisms, particle and types TEM whichhave described a maypositive above, occur zeta separatelyit potential.is possible Based orto propose together on the two atinvestigation deposition the same time (Figuremechanisms,results5). They provided consist which by zetamay of (i)potential occur graphite separately and adsorption TEM or described together on theabove,at the PEEK same it is possible surfacetime (Figure to and propose 5). then They complextwo consist deposition of particles (i) (partiallygraphitemechanisms, covered adsorption which with graphite)onmay the occur PEEK depositseparately surface together and or thentogether oncomplex the at the positively particles same time (partially charged (Figure covered electrode 5). They with consist (anode) graphite) of and(i) (ii) independentdepositgraphite together adsorption deposition on theon of thepositively both PEEK particle surface charged types and electrode then on thecomplex (anode) anode. particles and However, (ii) (partially independent as confirmed covered deposition with experimentally, graphite) of both the depositionparticledeposit togethertypes of on graphite onthe the anode. positively from However, pure charged ethanol as confirmed electrode is impossible, experimentally,(anode) and therefore (ii) independentthe deposition the first deposition mechanismof graphite of fromboth is much morepureparticle plausible. ethanol types is on impossible, the anode. therefore However, the as first confirmed mechanism experimentally, is much more the plausible. deposition of graphite from pure ethanol is impossible, therefore the first mechanism is much more plausible.

Figure 4. TEM images of the graphite particles adsorbed to the PEEK 708 microparticle in the FiguresuspensionFigure 4. TEM 4. TEM images used imagesfor of electrophoretic the of graphite the graphite particles deposition particles adsorbed (EPD adsorbed) containing to the to PEEK the 1 g/708 PEEKL (a microparticle) and 708 2 microparticle g/L (b in) of the graphite. suspension in the usedInsuspension for the electrophoretic case used of the for suspension electrophoretic deposition containing (EPD) deposition containing 1 g/ L(EPD of graphite) 1 containing g/L(a ) the and 1 PEEK g/ 2L g /(L( a part) band)icles of 2 graphite.g/ areL ( b covered) of graphite. In the only case of the suspensionpartially.In the case containing of the suspension 1 g/L of containing graphite the 1 g/ PEEKL of graphite particles the are PEEK covered part onlyicles arepartially. covered only partially.

FigureFigure 5. Schematic 5. Schematic drawings drawings ofof the possible possible co co-deposition-deposition mechanism mechanism of the ofgraphite the graphite and PEEK and 708 PEEK 708 particlesparticlesFigure 5. from fromSchematic ethanol drawings showing showing of the the possibleadsorption adsorption co- depositionof ofgraphite graphite mechanism on onthe thePEEK PEEKof thesurface graphite surface and and deposition PEEK deposition 708 of of complexescomplexesparticles on from the on theanodeethanol anode ( ashowing), (a and), and independent theindependent adsorption depositiondeposition of graphite of of bothon both the types types PEEK of ofparticles surface particles andon the on deposition theanode anode (b ).of (b). complexes on the anode (a), and independent deposition of both types of particles on the anode (b). TheThe selection selection of theof the basic basic deposition deposition parameters, parameters, voltage voltage and and time, time, was was performed performed in an in an experimental way, and the preliminary evaluation of the coating homogeneity was carried out by experimentalThe selection way, and of the the preliminary basic deposition evaluation parameters, of the voltage coating and homogeneitytime, was performed was carried in an out macroscopicexperimental observations.way, and the Thepreliminary coatings evaluationwere macroscopically of the coating homogeneous homogeneity and was repeatable carried out after by by macroscopic observations. The coatings were macroscopically homogeneous and repeatable after macroscopic observations. The coatings were macroscopically homogeneous and repeatable after deposition from the suspension containing 1 g/L of graphite at pH = 8.7 in the voltage range from 50 V to 70 V. After deposition at voltages in the range of 20–40 V, coatings were too thin and did not cover the substrate completely, while the deposition at the highest voltages in the range of 80–100 V led to thick coatings being obtained, but they started to lose uniformity. It should be pointed out that the EPD process was also carried out from suspensions at lower pH after adding citric acid and at higher pH after adding sodium hydroxide. Speciﬁc suspensions were selected in which the zeta Materials 2020, 13, x 8 of 15

MaterialsV to2020 70, 13V., After 3251 deposition at voltages in the range of 20–40 V, coatings were too thin and did not8 of 15 cover the substrate completely, while the deposition at the highest voltages in the range of 80–100 V led to thick coatings being obtained, but they started to lose uniformity. It should be pointed out that potential of graphite in ethanol was relatively high, i.e., at pH = 4.5, 6, 7, 9.5 and 11 (Figure3). However, the EPD process was also carried out from suspensions at lower pH after adding citric acid and at depositionhigher waspH after not observed adding sodium at any hydroxide. of the following Specific suspensionspH values. were Moreover, selected faster in which sedimentation the zeta of the suspensionpotential of was graphite detected, in ethanol compared was relatively to the virgin high, i.e. suspension, at pH = 4.5 without, 6, 7, 9.5 any and additives. 11 (Figure The 3). zeta potentialHowever, at pH deposition= 8.7 was was not not the observed highest possible, at any of but the at following the same pH time values. it was Moreover, suﬃcient faster to allow depositionsedimentation and good of quality the suspension coatings was to bedetected obtained., compared In addition, to the the virgin suspension suspension used without for deposition any was environment-friendlyadditives. The zeta potential pure ethanolat pH = and8.7 wa dids not not the require highest any possible, additional but substances,at the same time such it as was acids or bases,sufficient which is to an allow important deposition advantage. and good Finally, quality thecoatings coating to be deposited obtained at. In 70 addition, V and 40 the s wassuspension selected for furtherused microstructural for deposition studies. was environment-friendly pure ethanol and did not require any additional substances, such as acids or bases, which is an important advantage. Finally, the coating deposited at Coatings after deposition and drying were submitted to heat treatment in order to densify them 70 V and 40 s was selected for further microstructural studies. and to increase the adhesion to the substrate. Figure6 shows SEM images of coatings directly after Coatings after deposition and drying were submitted to heat treatment in order to densify them depositionand to andincrease heat the treatment. adhesion Itto can the besubstrate. seen from Figure Figure 6 shows6a that SEM the images PEEK particlesof coatings uniformly directly after covered the substratedeposition and and between heat treatment. them the It graphite can be seen particles from wereFigure sporadically 6a that the PEEK distributed. particles Figure uniformly6b shows that thecovered coatings the substrate after heat and treatmentbetween them were the densegraphite and particles no cracks were sporadically or porosity distributed. occurred. Figure Moreover, the PEEK6b shows changed that its the morphology coatings after to heat a continuous treatment coating were dense matrix, and in no which cracks relatively or porosity evenly occurred. embedded graphiteMoreover, particles the arePEEK visible changed on theits morphology coating surface. to a continuous However, coating it can alsomatrix, be in seen which that relatively the graphite contentevenly was embedded rather low. graphite A characteristic particles are spherulitic visible on microstructurethe coating surface. can However, also be observed it can also in be the seen coating, whichthat suggests the graph theite formation content was of a rathercrystalline low. structure A characteristic in the polymerspherulitic during microstructure heat treatment. can also It be should observed in the coating, which suggests the formation of a crystalline structure in the polymer during be noted that, according to the literature [20], a PEEK microstructure composed of spherulites is heat treatment. It should be noted that, according to the literature [20], a PEEK microstructure preferred in terms of tribological properties, because spherulites cause strengthening of the coating composed of spherulites is preferred in terms of tribological properties, because spherulites cause and reducestrengthening its destruction. of the coating It can and be reduce seen in its Figure destruction.6b that Itthe can graphite be seen in particles Figure 6b were that orientedthe graphite parallel to theparticles coating were surface, oriented which parallel is also to thepreferable coating in surface, terms which of the is subsequent also preferable tribological in terms of properties. the The coatingsubsequent thickness tribological was properties. measured The as 110 coatingµm. thickness was measured as 110 μm.

FigureFigure 6. SEM 6. SEM images images of of the the graphite graphite/PEEK/PEEK 708 as as-deposited-deposited (a () aand) and heat heat-treated-treated (b) coatings. (b) coatings.

TheThe XRD XRD studies studies conﬁrmed confirmed that that PEEK PEEK after after heating heating and and cooling cooling with with a furnace a furnace changed changed its its structurestructure from from amorphous amorphous to semi-crystalline to semi-crystalline (Figure (Figure7). One7). One wide wide amorphous amorphous peak peak from from the polymer the can bepolymer seen incan the be XRD seen patternin the XRD of thepattern as-deposited of the as-deposited coating, whilecoating, four while strong four strong crystalline crystalline peaks are peaks are clearly visible in the pattern after the heat treatment of the sample. In both XRD patterns, clearly visible in the pattern after the heat treatment of the sample. In both XRD patterns, before and before and after heat treatment, crystalline peaks from graphite at the 2Θ angles of 26.4° and 54.2° after heat treatment, crystalline peaks from graphite at the 2Θ angles of 26.4 and 54.2 are also present. are also present. ◦ ◦ To investigate the coating microstructure on a cross-section by TEM, a lamella was cut out with the use of FIB. Similarly to the SEM study performed on plan-view samples, it was observed that the

Materials 2020, 13, x 9 of 15

To investigate the coating microstructure on a cross-section by TEM, a lamella was cut out with the use of FIB. Similarly to the SEM study performed on plan-view samples, it was observed that the graphite particles consisted of graphene packages of 200 nm thick and oriented parallel to the coating surfaceMaterials 2020occurred, 13, 3251 in the dense polymer matrix (Figure 8a). Only individual small graphite particles9 of 15 were visible in the deeper areas of the coating (Figure 8b).

Materials 2020, 13, x 9 of 15

To investigate the coating microstructure on a cross-section by TEM, a lamella was cut out with the use of FIB. Similarly to the SEM study performed on plan-view samples, it was observed that the graphite particles consisted of graphene packages of 200 nm thick and oriented parallel to the coating surface occurred in the dense polymer matrix (Figure 8a). Only individual small graphite particles were visible in the deeper areas of the coating (Figure 8b).

Figure 7. Figure 7. XRDXRD patterns patterns of of the the graphite/PEEK graphite/PEEK 708 as as-deposited-deposited (1) and heat heat-treated-treated (2) coatings on the Ti-6Al-4V alloy. Ti-6Al-4V alloy. To investigate the coating microstructure on a cross-section by TEM, a lamella was cut out with the use of FIB. Similarly to the SEM study performed on plan-view samples, it was observed that the graphite particles consisted of graphene packages of 200 nm thick and oriented parallel to the coating surface occurred in the dense polymer matrix (Figure8a). Only individual small graphite particles Figure 7. XRD patterns of the graphite/PEEK 708 as-deposited (1) and heat-treated (2) coatings on the were visibleTi-6Al in the-4V alloy. deeper areas of the coating (Figure8b).

Figure 8. Microstructure of the outer area of the graphite/PEEK 708 coating heated at 390 °C and cooled with a furnace on a cross section. The graphite particle is located in the area directly close to the coating surface (a) and in the area about 3 μm away from the coating surface (b). Magnified details of the particle present in Figure 8a showing a layered struc ture are given at the top. TEM, focused ion beam Figure(FIB) lamellae. 8. Microstructure of the outer area of the graphite/PEEK 708 coating heated at 390 °C and Figure 8. Microstructure of the outer area of the graphite/PEEK 708 coating heated at 390 ◦C and cooled with acooled furnace with on a a furnace cross section. on a cross The section. graphite The particle graphite is particle located is in located the area in directlythe area directly close to close the coating to Scratchthe testscoating were surface carried (a) and out in the for area both about the 3 μmunfilled away from PEEK the coating surface and the (b). graphMagnifiedite/PEEK details coating surface (a) and in the area about 3 µm away from the coating surface (b). Magnified details of the under standardof the particle conditions present with in Figure an 8aincr showingeasing a loadlayered of struc 0–30ture N are (Figure given at 9a, theb) top. and TEM, at focuseda constant ion load of particle present in Figure8a showing a layered structure are given at the top. TEM, focused ion beam 0.25 N (Figurebeam 9c).(FIB )The lamellae. scratch tests results indicates very well adherence of the unfilled PEEK coating (FIB) lamellae. and graphite/PEEK composite coating to the substrate. Scratch with a Rockwell indenter did not Scratch tests were carried out for both the unfilled PEEK coating and the graphite/PEEK coating cause any adhesive failure or delamination in any of the tested coatings, even under a load of 30 N Scratchunder standard tests were conditions carried with out foran incr botheasing the unfilledload of 0 PEEK–30 N ( coatingFigure 9a, andb) and the at graphite a constant/PEEK load coating of (underFigure0.25 standard 10b, N (d).Figure H conditionsowever, 9c). The scratchthe with introduction tests an increasingresults indicatesof graph load veryite of 0–30fillers well adherence N reduced (Figure of 9thea,b) the scratch unfilled and at resistancePEEK a constant coating of load the compositeof 0.25and N graphite/PEEK (Figure coating9 c). compared The composite scratch to the coating tests unfilled results to the PEEK indicates substrate. coating. veryScratch The well with first adherence a singleRockwell cohesive of indenter the unfilled cracks did not PEEK in the graphcoatingcauseite and/PEEK any graphite adhesivecoating/PEEK werefailure composite located or delamination in coating the middle in to an they of substrate. the testedscratch Scratchcoatings, track with ateven the aunder load Rockwell aL C1load = indenter9 of N 30 ( FigureN did 10notc), cause while(Figure any for 10b, adhesivethed). pureHowever, failurePEEK the coating or introduction delamination they were of graph in observed anyite offillers the at reduced testedthe load coatings,the L C1scratch = 23 even Nresistance (Figure under of10 a the loada). The of number30 N (Figurecomposite of cracks 10 coatb,d). increaseding However, compared while the to the introductionincreasing unfilled the PEEK of load graphite coating. and fillerstheyThe firstals reduced o single occurred cohesive the scratchat the cracks edges resistance in the of the of scratchthe compositegraph tracks.ite/PEEK At coating a load coating compared of 27were N, locateda tosmall the infragment unfilled the middle PEEK of theof coating.the graph scratchite The/PEEK track first at coating singlethe load surface cohesive LC1 = 9 was N cracks (Figure detached, in the 10c), while for the pure PEEK coating they were observed at the load LC1 = 23 N (Figure 10a). The butgraphite the substrate/PEEK coating was not were exposed located ( inFigure the middle 10d). Meanwhile, of the scratch the track failure at the of load the pure LC1 = PEEK9 N (Figure coating 10 c),at number of cracks increased while increasing the load and they also occurred at the edges of the whilethis load for theconsisted pure PEEK only coatingin the formation they were of observed more extensive at the load and L deeper= 23 N cracks (Figure in 10thea). coating The number (Figure of scratch tracks. At a load of 27 N, a small fragment of the graphite/PEEKC1 coating surface was detached, 10b). The explicit destruction of the coatings at the loads of LC1 and 27 N was reflected in the change cracksbut increased the substrate while was increasing not exposed the load(Figure and 10d). they Meanwhile, also occurred the atfailure the edges of the ofpure the PEEK scratch coating tracks. at At a load ofthis 27 load N, aconsisted small fragment only in the of formation the graphite of more/PEEK extensive coating and surface deeper was cracks detached, in the coating but the (Figure substrate was not10b). exposed The explicit (Figure destruction 10d). Meanwhile, of the coating thes at failure the loads of theof L pureC1 and PEEK 27 N was coating reflected at this in loadthe change consisted only in the formation of more extensive and deeper cracks in the coating (Figure 10b). The explicit MaterialsMaterials2020, 201320, 3251, 13, x 10 of 1510 of 15

penetration depth of the indenter (Pd) at the maximum load was up to 75 μm and 65 μm for the destructioncomposite of and the pure coatings PEEK at coating the loadss, respectively, of LC1 and while 27, Nafter was unloading reﬂected, the in plastic the change deformation of the (R coursed) of frictionof both force coatings and in the the penetration scratch track depthwas approx. (Figure 40 9μm.a,b). It has been found that the graphite /PEEK coating isThe more beneficial highly effect susceptible of introducing to deformation the graphite than filler the into pure the PEEK PEEK coating. polymer Thematrix penetration manifested depth in the reduction of the COF of the graphite/PEEK coating during sliding contact with the diamond of the indenter (Pd) at the maximum load was up to 75 µm and 65 µm for the composite and pure stylus compared to the pure PEEK coating. This was confirmed by the scratch tests carried out at a PEEK coatings, respectively, while, after unloading, the plastic deformation (R ) of both coatings in the constant load of 0.25 N, which did not cause a large deformation of the polymerd coatings, and thus µ scratchthe track effect wasof the approx. mechanical 40 componentm. of friction force on the COF value was minimal (Figure 9c).

FigureFigure 9. The 9. The scratch scratch test test results results (friction (friction force,force, penetration depth depth and and residual residual depth depth signals) signals) of PEEK of PEEK coating (a), and graphite/PEEK coating (b), and friction coeﬃcient of the coatings vs. diamond stylus Materials 2020coating, 13, x (a), and graphite/PEEK coating (b), and friction coefficient of the coatings vs. diamond stylus 11 of 15 in a scratchin a scratch test test at 0.25 at 0.25 N constantN constant load load ( c().c).

The open circuit potentials (OCP) were measured during a 20 h immersion test (Figure 11a). The potential value of the uncoated alloy at the beginning of the measurement equalled −0.05 V and changed in time, constantly increasing its value until it reached about 0.11 V after approx. 50,000 s. Similar plots, but with slightly lower values of potentials (−0.10–0.05 V), were observed for the graphite/PEEK coated alloy. Our previous study [20] of pure PEEK coatings indicated comparable OCP results for uncoated and semi-crystalline PEEK coated titanium alloys. In Figure 11a, the differences between OCP at the start and after approx. 50,000 s of measurement were significantly smaller. Because of the very small distance between OCP results for uncoated and graphite/PEEK coated alloys, it can be said that the corrosion resistance was similar for both the uncoated and coated titanium alloy. The anodic cyclic potentiodynamic polarization curves of both samples in sodium chloride solutions are presented in Figure 11b. A passive behaviour with large passive potential range resulted for both uncoated and coated alloy. The corrosion was defined by the limiting current density, which passes through the passivating film, thus becoming a measure of the film’s protective performance [51]. The passive current density (ip) was reduced from 5 μA/cm2 for the uncoated Figure 10. SEM images ofof thethe scratchscratch tracktrack show show the the failure failure of of PEEK PEEK coating coating at criticalat critical load load LC1 L=C1 23= 23 N sample to 0.33 μA/cm2 for the coated ones, while the corrosion potential (Ecorr) increased from ~ −0.50 N(a )(a and) and at 27at 27 N (Nb )(b as) as well well as as graphite graphite/PEEK/PEEK coating coating at LatC1 LC=1 9= N9 N (c )(c and) and at at 27 27 N N (d ().d). to −0.32 V, respectively. This shift in the polarization curve indicates improved corrosion resistance Theof the beneficial coated alloy eff asect the of coating introducing acts as the a protective graphite layer. filler According into the PEEK to our polymer previous matrix work [20 manifested], semi- in the reductioncrystalline of PEEK the COF coatings of the on graphitethe Ti-6Al/PEEK-4V substrate coating had during quite slidingsimilar contactresults of with polarization the diamond tests. stylus compared to the pure PEEK coating. This was confirmed by the scratch tests carried out at a constant load of 0.25 N, which did not cause a large deformation of the polymer coatings, and thus the effect of the mechanical component of friction force on the COF value was minimal (Figure9c). The open circuit potentials (OCP) were measured during a 20 h immersion test (Figure 11a). The potential value of the uncoated alloy at the beginning of the measurement equalled 0.05 V and −

Figure 11. Electrochemical measurements of uncoated Ti-6Al-4V and graphite/PEEK coated Ti-6Al- 4V alloy in 3.5 wt.% NaCl aqueous solution at 25 °C. Evolution of the corrosion potential vs. time (a) and polarization curves at 1 mV/s (b).

These studies showed that EPD is flexible enough to fabricate PEEK-based coatings incorporated with graphite particles, adheres well to the titanium alloy substrate and exhibits a tailored microstructure consisting of 2D graphite distributed in the coating areas close to the surface and oriented parallel to the surface. In our previous studies, we clearly demonstrated that the PEEK-based coatings containing hard Al2O3 [21] or Si3N4 [22] or TiN [24] nanoparticles significantly improved the wear resistance of the Ti-6Al-4V titanium alloy in dry sliding contact with an alumina ball. The wear index was 0.47 10−6, 1.41 10−6 and 1.10 10−6 [mm3/Nm], respectively, after the same ball-on-disc test conditions. In comparison with the pure PEEK708 coating, the wear rate was higher (2.61 × 10−6 [mm3/Nm]) during the same test conditions. However, the COF was still relatively high and equalled 0.27 for the pure PEEK708 coating [20], 0.25 for the Al2O3/PEEK708 coating [21], 0.26 for the Si3N4/PEEK 708 coating [22] and 0.30 for the TiN/PEEK 708 coating [24]. Similarly, in other works on PEEK and PEEK-based coatings, a relatively high coefficient of friction was demonstrated. For example, Li et al. [52] reported that the COF of PEEK coatings in cooperation with a 100C6 steel ball was around 0.3. Zhang et al. [53] also received a COF of PEEK and SiC/PEEK coatings of approximately 0.3 and 0.4, respectively, in cooperation with a 100C6 steel ball. Thus, in the search for new advanced PEEK-based coatings with a good balance between wear resistance and a low COF, the successful co-deposition of PEEK and graphite with a tailored microstructure is very promising. However, from the point of view of developing low-friction coatings, it seems advisable to increase the content of graphite, as a lubricant phase, in the coating. Therefore, future work will concentrate on the optimization of the PEEK to graphite volume fraction to obtain coatings with a good balance of wear resistance and COF as well as electrochemical corrosion resistance.

Materials 2020, 13, 3251 11 of 15 changed in time, constantly increasing its value until it reached about 0.11 V after approx. 50,000 s. Similar plots, but with slightly lower values of potentials ( 0.10–0.05 V), were observed for the Materials 2020, 13, x − 11 of 15 graphite/PEEK coated alloy. Our previous study [20] of pure PEEK coatings indicated comparable OCP results for uncoated and semi-crystalline PEEK coated titanium alloys. In Figure 11a, the differences between OCP at the start and after approx. 50,000 s of measurement were significantly smaller. Because of the very small distance between OCP results for uncoated and graphite/PEEK coated alloys, it can be said that the corrosion resistance was similar for both the uncoated and coated titanium alloy. The anodic cyclic potentiodynamic polarization curves of both samples in sodium chloride solutions are presented in Figure 11b. A passive behaviour with large passive potential range resulted for both uncoated and coated alloy. The corrosion was defined by the limiting current density, which passes through the passivating film, thus becoming a measure of the film’s protective performance [51]. 2 2 The passive current density (ip) was reduced from 5 µA/cm for the uncoated sample to 0.33 µA/cm for the coated ones, while the corrosion potential (Ecorr) increased from ~ 0.50 to 0.32 V, respectively. − − This shift in the polarization curve indicates improved corrosion resistance of the coated alloy as the coating acts as a protective layer. According to our previous work [20], semi-crystalline PEEK coatings Figure 10. SEM images of the scratch track show the failure of PEEK coating at critical load LC1 = 23 on the Ti-6Al-4V substrate had quite similar results of polarization tests. N (a) and at 27 N (b) as well as graphite/PEEK coating at LC1 = 9 N (c) and at 27 N (d).

FigureFigure 11. Electrochemical 11. Electrochemical measurements measurements of of uncoated uncoated Ti-6Al-4VTi-6Al-4V and graphite/PEEK graphite/PEEK coated coated Ti- Ti-6Al-4V6Al- 4V alloy in 3.5 wt.% NaCl aqueous solution at 25 °C. Evolution of the corrosion potential vs. time (a) alloy in 3.5 wt. % NaCl aqueous solution at 25 ◦C. Evolution of the corrosion potential vs. time (a) and polarizationand polarization curves atcurves 1 mV at/s 1 ( bmV/s). (b).

TheseThese studies studies showed showed that that EPD EPD is is flexible flexible enough to to fabricate fabricate PEEK PEEK-based-based coatings coatings incorporated incorporated with graphite particles, adheres well to the titanium alloy substrate and exhibits a tailored with graphite particles, adheres well to the titanium alloy substrate and exhibits a tailored microstructure microstructure consisting of 2D graphite distributed in the coating areas close to the surface and consistingoriented of parallel 2D graphite to the surface. distributed In our in previous the coating studies, areas we closeclearly to demonstrated the surface andthat the oriented PEEK-based parallel to the surface.coatings In containing our previous hard Al studies,2O3 [21] we or clearlySi3N4 [22] demonstrated or TiN [24] nanoparticles that the PEEK-based significantly coatings improved containing the hardwear Al2O resistance3 [21] or of Si 3theN 4Ti[-226Al] or-4V TiN titanium [24] nanoparticlesalloy in dry sliding significantly contact with improved an alumina the ball. wear The resistancewear of theindex Ti-6Al-4V was 0.47 titanium 10−6, 1.41alloy 10−6 and in dry1.10 sliding10−6 [mm contact3/Nm], respe withctively, an alumina after the ball. same The ball wear-on-disc index test was 6 6 6 3 −6 0.47 10conditions.− , 1.41 10 In− comparisonand 1.10 10 with− [mm the pure/Nm], PEEK708 respectively, coating after, the the wear same rate ball-on-disc was higher (2.61 test conditions.× 10 In comparison[mm3/Nm]) withduring the the pure same PEEK708test condition coating,s. However, the wear the COF rate was was still higher relatively (2.61 high 10and6 equal[mmled3/Nm]) × − during0.27 the for same the pure test conditions. PEEK708 coating However, [20], 0.25 the COF for the was Al2 stillO3/PEEK708 relatively coating high and [21], equalled 0.26 for the 0.27 for Si3N4/PEEK 708 coating [22] and 0.30 for the TiN/PEEK 708 coating [24]. Similarly, in other works on the pure PEEK708 coating [20], 0.25 for the Al2O3/PEEK708 coating [21], 0.26 for the Si3N4/PEEK PEEK and PEEK-based coatings, a relatively high coefficient of friction was demonstrated. For 708 coating [22] and 0.30 for the TiN/PEEK 708 coating [24]. Similarly, in other works on PEEK example, Li et al. [52] reported that the COF of PEEK coatings in cooperation with a 100C6 steel ball and PEEK-basedwas around 0.3. coatings, Zhang a et relatively al. [53] alsohigh received coefficient a COF of friction of PEEK was and demonstrated. SiC/PEEK coatings For example, of Li etapproximately al.[52] reported 0.3 thatand 0.4, the respectively, COF of PEEK in coatingscooperation in cooperationwith a 100C6 withsteel ball. a 100C6 Thus, steel in the ball search was for around 0.3. Zhangnew advanced et al. [53 PEEK] also-based received coatings a COF with of PEEKa good and balance SiC/ PEEKbetween coatings wear resistance of approximately and a low 0.3COF, and 0.4, respectively,the successful in cooperation co-deposition with of aPEEK 100C6 and steel graphite ball. Thus,with a intailored the search microstructure for new advancedis very promising. PEEK-based coatingsHowever, with afrom good the balance point of betweenview of developing wear resistance low-friction and coatings, a low COF, it seems thesuccessful advisable to co-deposition increase of PEEKthe content and graphite of graphite, with as a a tailored lubricant microstructure phase, in the coating. is very Therefore, promising. future However, work will from concentrate the point of viewon of the developing optimization low-friction of the PEEK coatings, to graphite it seems volume advisable fraction to to obtain increase coatings the contentwith a good of graphite, balance as a of wear resistance and COF as well as electrochemical corrosion resistance.

Materials 2020, 13, 3251 12 of 15 lubricant phase, in the coating. Therefore, future work will concentrate on the optimization of the PEEK to graphite volume fraction to obtain coatings with a good balance of wear resistance and COF as well as electrochemical corrosion resistance.

4. Conclusions 1. Nanocomposite graphite/PEEK 708 coatings were electrophoretically deposited on titanium alloy substrates from an environmentally-friendly ethanol-based suspension containing 1 g/L of graphite. The optimal parameters to obtain homogeneous coatings were: voltage of 70 V and deposition time of 40 s. The EPD mechanism was investigated and discussed. The most probable mechanism of co-deposition of the particles consisted of the electrostatic interaction between them. As a result, graphite particles adsorbed on the surface of PEEK microparticles and graphite-PEEK complexes were deposited on the anode. 2. The post-EPD heat treatment densiﬁed the coatings and increased their adhesion to the titanium alloy substrates. The coating had high scratch resistance and there was no adhesive damage. The ﬁrst cohesive cracks appeared at the load LC1 = 9 N and their number grew with increasing load. The coating was characterized by high susceptibility to deformation, and plastic deformation of the coating after unloading was up to 40 µm. 3. As a result of duplex treatment, the graphite particles embedded in the polymer were oriented parallel to the coating surface, which is the most advantageous arrangement in the context of friction processes. However, their amount was rather low and the further optimization of their content in the coating to achieve a stable and low COF is necessary. 4. The coated alloy exhibited better corrosion resistance compared to the uncoated alloy in a sodium chloride solution at a temperature of 25 ◦C.

This study showed that a combination of electrophoretic deposition and post-EPD heat treatment is a convenient way of obtaining composite PEEK-based coatings incorporated with graphite particles consisting of graphene layer stacks oriented parallel to the coating surface. Such coatings are an important subject of research to improve the tribological properties of titanium alloys, while increasing frictional wear resistance and lowering the friction coeﬃcient.

Author Contributions: Conceptualization, A.F. and T.M.; formal analysis, A.F., S.Z., A.K., A.Ł. and T.M.; funding acquisition, A.F. and T.M.; investigation, A.F., S.Z., A.K., A.Ł. and T.M.; methodology, A.F. and T.M.; project administration, T.M.; resources, T.M.; supervision, T.M.; validation, A.F., S.Z., A.K., A.Ł. and T.M.; visualization, A.F. and T.M.; writing—original draft preparation, A.F.; writing—review and editing, T.M. All authors have read and agreed to the published version of the manuscript. Funding: The study was supported by AGH-UST (project no. 16.16.110.663). Acknowledgments: The authors appreciate the valuable contributions of Ł. Cieniek (AGH-UST) to SEM investigation and M. Gajewska (ACMiN AGH) for FIB lamella preparation. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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