Electrodeposition of Zn/TiO2 coatings on Ti6Al4V produced by selective laser melting, the characterization and corrosion resistance

Recently, additive manufacturing techniques have begun to be implemented extensively in the production of implants. Ti6Al4V alloy is a material of choice for implants due to its low density and high biocompatibility. Recent research, however, has demonstrated that Ti6Al4V alloy emits long-term ions (such as Al and V) that are hazardous to health. Surface modifications, including coating, are therefore required for implants. The electrodeposition method was utilized to deposit Zn-doped TiO2 onto the surfaces of Ti6Al4V samples, which were manufactured via the selective laser melting method. The effects of processing time, amount of TiO2 addition, microstructure of anode materials, and resistance to wear and corrosion were investigated. The coating hardness and thickness increased with increasing processing time and TiO2 concentration. It has been observed that the addition of TiO2 to zinc anode coatings results in an increase in wear and a decrease in corrosion rate. It was noted that the specimens exhibiting the most significant wear also possessed the highest hardness value. The specimens were generated utilizing a graphite anode, underwent a 30-min processing time, and comprised 10 g l−1 of TiO2.


Introduction
Implant construction is the primary application of biomedical materials in numerous fields, including orthopaedics, dentistry, drug delivery, and tissue engineering.In order to be deemed suitable for implantation, a material must demonstrate that it is devoid of any detrimental biological effects and is compatible with the functioning of the human body [1,2].
Musculoskeletal system implants frequently incorporate titanium and its alloys due to their superior biomechanical and biochemical compatibility compared to alternative metallic materials like stainless steel and Co-based alloys [2,3].Although Ti alloys possess the most desirable properties, they can prolong the manufacturing process of complex geometry components like bone implants.Additive manufacturing (AM) methodologies have recently been implemented to fabricate metallic components featuring elaborate geometric intricacies.Additive manufacturing (AM), or rapid prototyping (RP), encompasses all technologies that utilize layer-by-layer material fabrication to create end-use components with the necessary industrially acceptable properties.Unlike conventional manufacturing processes, the AM method enables the fabrication of parts that closely resemble the net shape without rough machining [4,5].
Selective laser melting (SLM) is an additive manufacturing (AM) technique that utilizes computercontrolled focused laser scanning of a metallic powder bed to generate three-dimensional, nearly net-shaped components [5].SLM is one of the techniques that has gained widespread medical application in recent years, particularly in the prosthetic knee and hip implant industries, which manufacture bone implants.Research on the microstructure and mechanical properties of SLM-produced titanium alloys reveals that these exhibit enhanced hardness and tensile strength compared to conventionally machined and cast products.As mentioned earlier, the circumstance is linked to developing the (α′) HCP martensite phase and epitaxial growth [6].
It has been noted that prolonged corrosion of the Ti6Al4V alloy, utilized in fabricating a diverse range of implants owing to its exceptional mechanical properties, can result in adverse biological effects or even allergic reactions due to the liberation of metal ions.Vanadium (V) in the Ti6Al4V alloy has been identified as potentially cytotoxic and capable of inducing long-term adverse reactions in body tissues; aluminum (Al) is associated with neurological disorders, including Alzheimer's [7,8].
To improve the properties of implant submaterials, various surface modifications are applied with physical modifications such as laser texturing [9] and different coating methods such as pulsed laser coating [10], PVD [11] and pulsed electrochemical coating [12].Of these modifications, the coating process is a preferred method for developing properties such as desired biocompatibility, fatigue resistance, corrosion resistance, prevention of bone and soft tissue adhesion (osteoconduction) and surface hardness, as it can be applied with many different techniques [13].One of the most important features of electrophoretic coating, which is one of the coating techniques, is that particles suspended in the liquid medium can be coated.Electrophoretic deposition (EPD) is a highly preferred method for producing bioactive coatings on complex form substrates due to its ability to deposit at ambient temperature.The EPD method offers several advantages, including its versatile application, simple device and equipment requirement, short processing time, cost-effectiveness, ease of modification, and the ability to accumulate particles at the desired density in the final product [14,15].
Although numerous studies have used other antibacterial elements with antimicrobial properties, such as Ag and Cu, their potential cytotoxicity is still a concern.Zinc is a vital trace element that serves essential functions in the human body, such as DNA synthesis, enzyme activity and cell division.Various studies have reported that Zn can increase cytocompatibility [16] and exhibit antibacterial activity [17].In recent times, there have been reports indicating that Zn ions can effectively impede bacterial growth and reduce bacterial adhesion and growth.A primary advantage of safeguarding metal surfaces with Zn/Zn alloy coating is providing a cathodic corrosion protection layer.This layer effectively prevents and impedes the corrosive environment from compromising the substrate material for an extended period [18].The Ti6Al4V surface was coated with Zndoped ZrO 2 /TiO 2 porous coatings (Zn-ZrO 2 /TiO 2 ) by Wang et al utilizing a hybrid technique consisting of magnetron sputtering and micro-arc oxidation.It was discovered to possess remarkable antibacterial properties against S. aureus and an acceptable level of cytotoxicity.Additionally, they underscored the extent of compatibility [19,20].Using the electrodeposition method, He et al produced a superhydrophobic surface with a Zn/ZnO/TiO 2 coating on Ti6Al4V alloy in their study.Mirak et al deposited biocompatible zinchydroxyapatite-titania and zinc-hydroxyapatite nanocomposite coatings onto NiTi shape memory alloy using electrodeposition.In comparison to Zn-HA and bare NiTi, Zn-HA/TiO 2 composite coatings exhibited the highest corrosion resistance, according to the authors [21].
This research aimed to safeguard the primary metal of a custom-made implant against corrosion caused by bodily fluids.In light of this rationale, Ti6Al4V components manufactured by SLM were coated with Zn-doped TiO 2 via the EPD method.Various counter electrodes were employed at different concentrations and times.

Sample preparation and electrodeposition coating
The Ti6Al4V Grade 5 ELI samples, produced by selective laser melting (SLM), were used as the substrate.The chemical composition of Ti6Al4V Grade 5 ELI is given in table 1. SLM process was performed with Ermaksan ENA Vision 250 SLE device using 0.085 laser diameter, 1000 mm s −1 laser speed, 280 W Laser power, and 0.120 Hatch distance parameters.Samples with dimensions of 20 × 20 × 3.5 cm were cut from the sheets produced by SLM with a precision cutting device.After the cut samples were sanded with 220 and 400-grit mesh sandpaper, the surface was washed with alcohol.
The electrochemical solution given in table 2 was used for the electrodeposition process.According to the process groups shown in table 3, the EPD process was carried out at two different times, 30 and 60 min, with the addition of two different TiO 2 (anatase,∼ 325 mesh, 99%) 5 g l −1 and 10 g l −1 .Additionally, to compare the TiO 2 accumulation change, coating processes were carried out with graphite and pure (99.9%)Zn counter electrode.

Characterization of implant material surfaces
The coating thickness of the electrophoretic deposition (EPD) applied samples was measured using a Leica Microsystems optical microscope.Pre-and post-in vitro corrosion process examinations of the EPD-applied samples were conducted through SEM and EDS analyses.The analyses were performed with a field emission scanning electron microscope, specifically the HITACHI SU5000 (FE-SEM).
Micro-hardness Analysis: Vickers microhardness measurements were conducted using a Highwood HWMMT-X3 hardness device.In the test, a 50-gf load was applied for 10 s.
In vitro corrosion tests: The corrosion behaviour was assessed in commercial Hank's Balanced Salt Solution (Biowest X0510 HBSS 10X without Calcium, without Magnesium, with Sodium Bicarbonate, without Phenol Red) at room temperature.The working cell was equipped with three standard electrodes: Pt as the counter electrode, a common calomel electrode as the reference electrode, and the working electrode.Subsequently, the samples were immersed in the solution, and an open circuit potential (OCP) was applied for 30 min.The data was obtained by covering the potential from − 0.25 V versus OCP to 0.25 V.The scan rate was 0.17 mV s −1 .The graphs calculated corrosion current density (i corr ) and corrosion potential from the extrapolated points where the anodic and cathodic overpotentials were about ±50 mV.
Wear resistance test: The wear test was conducted using a pin-on-plate test system, applying a load of 15 kg at a speed of 80 mm s −1 for 900 s.The counter abrasive was a 5 mm diameter bearing ball made of 100Cr6 material.Wear amounts were compared using the weight difference method.Weight difference measurements were made with a precision scale.A discernible alteration in surface morphology ensues from the augmentation of TiO 2 quantity, which possesses a ceramic and exceedingly rigid phase, coupled with the extension of the processing time from 30 min to 60 min.Surface roughness increases when examining SEM images, primarily due to doubling the processing time.The SEM images indicate that surface morphology varies between processes utilizing the same amount and duration of TiO 2 .Graphite and zinc anodes are nonetheless utilized independently.

Results and discussion
In order to determine how the elemental distribution reflected the various surface morphologies observed in SEM images, EDS analyses were conducted on every experimental sample.EDS mapping and exploring SEM images of G60-10 and Z60-10 samples with the greatest processing time and TiO 2 quantity are depicted in figure 2. The cumulative results of the EDS analyses performed on all test samples are presented in table 4.
Figure 2 further discerns that the morphology comprises plate-like regions, some of which extend outward, consistent with the findings of Guan et al [23].Sajjadnejad et al reported that particles within the Zn matrix close active defect areas (such as pores, voids, micro-holes, crevices, etc) against corrosion, forming a compact layer with a protective function [24].Similarly, SEM images reveal the accumulation of TiO 2 particles in the intermediate regions.
Table 4 indicates that sample Z60(10) exhibited the highest accumulation of TiO 2 , whereas sample G60(10) showed the lowest TiO 2 accumulation.Notably, for electrodeposition coatings produced using the graphite anode, a decreasing trend is observed in the amount of additional TiO 2 property units for the same processing time.Moreover, within coatings created with graphite anode, an augmentation in the accumulation of Zn on the surface is noted with increased processing time and the quantity of additional TiO 2 particles in the solution.Conversely, coatings manufactured with a Zn anode demonstrate a decrease in the amount of Zn accumulated on the surface as both processing time and the quantity of additional TiO 2 particles increase.Tuaweri et al investigated the impact of applied current density, deposition time, particle concentration, and agitation on the current efficiency and deposition characteristics of Zn-SiO 2 composite coatings.Their findings revealed that, over time, Zn dendrites encountered challenges in accumulating through the dense SiO 2 layer.In Zn anode coatings, the observed decrease in the accumulation of Zn on the surface, correlated with increased processing time and additional TiO 2 particle concentration, can be attributed to the electrochemical accumulation mechanism of zinc, as elucidated by Tuaweri et al [25].

Coating thickness
The images of the coating area taken with the optical metal microscope after all the test samples were coated are shown in figure 3, and the graph created according to the values of the approximate coating thickness measured during microscope imaging is shown in figure 4. Figure 5 shows an example of a cross-sectional EDS scan.
As shown in figures 3 and 4, a discernible pattern suggested that all samples' coating thickness increased as the amount of TiO 2 added increased.The specimens that underwent a 60-min treatment using a zinc anode and  5 g l −1 TiO 2 demonstrated a minimum thickness of 117 μm for the coating.The samples that underwent treatment with a zinc anode and 10 g l −1 TiO 2 for 30 min exhibited a maximum coating thickness of 240 μm as measured.Conversely, the thickness of the coating increased directly and proportionally with the duration of the processing process when a graphite anode was employed.It is worth mentioning that an extended processing time has been found to decrease the coating thickness of zinc anodes.

Micro-hardness and wear resistance analysis
Figure 6 depicts Vickers hardness values corresponding to varying amounts of TiO 2 addition.The samples treated for 30 min with a graphite anode and 10 g l −1 particle addition exhibited the highest hardness value.Conversely, the samples treated for 30 min with a zinc anode and 5 g l −1 particle addition displayed the lowest hardness value.In their investigation of the microstructure-corrosion resistance relationship in Zn-TiO 2 nanocomposite coatings, Sajjadnejad et al [26] the introduction of additional TiO 2 particles creates new nucleation sites, leading to smaller particles and, consequently, an increase in hardness.A comparison of figure 5 and table 4 reveals that, in coatings produced with a Zn anode, the hardness value rises proportionally with both processing time and the quantity of additional particles.The wear losses obtained from the wear tests are given in table 5. Figure 7 illustrates the variation in wear loss concerning the amount of TiO 2 addition.It was observed that with an increase in the quantity of additional particles in each process group, both hardness and wear loss exhibited an upward trend.The samples produced with the graphite anode, featuring the highest hardness value at 10 g l −1 addition and treated for 30 min (G30 (10)), displayed the most increased wear.It is understood that the coating material, which becomes brittle with increasing hardness, moves away from the main structure more easily under the load applied in the wear tests.In addition, it can be stated that with increasing coating hardness, the adhesion at the coating interface and the base metal weakens.In this case, it became easier to separate the coating from the surface due to the increasing surface temperature due to the applied load and friction.Studies have reported that the wear mechanism is not only related to the hardness of the material but also that different parameters affect wear [27,28].A similar situation was encountered in the wear tests in this study.It has been observed that the hardness of the material has a decreasing effect on wear resistance, not increasing it.
Another issue that affects the amount of wear is the amount of porosity in the microstructure of the coating.When the surface morphologies in the microscope images in figure 2 and figure 3 are examined, it can be said   that a very porous coating structure is formed.It is possible for the porous areas within the coating to be deformed much more easily and separated from the base metal during wear tests.The study conducted by Yılmaz and Buytoz reported that porosity affects hardness and wear resistance, and abrasive wear increases with increasing porosity [29].Figure 8 shows the SEM image of the microstructure of the coating area.When looking at the SEM image, intense porosity and roughness, especially in the upper parts of the coating, attract attention.In addition, TiO 2 particles in the structure were one of the factors affecting the wear behaviour.It is understood that TiO 2 particles in the very hard phase separate from the structure and emerge as a second corrosive agent.When the SEM images in figure 6 are scrutinized, it is evaluated that the TiO 2 particles separated   10), e)Z30(5), f)Z60(5), g)Z30( 10) ) and h)Z60 (10).
from the structure during the experiments got stuck between the steel test ball and the coating surface, acting like abrasive sandpaper grains, creating the abrasive wear mechanism.Singh et al reported that in three-body wear tests, the wear mechanism varies depending on the load, and material loss results from rolling the abrasive under smaller loads and micro-cutting under heavier loads.As a result, it is understood that factors other than hardness are effective on the wear behaviour of composites and that the subject needs to be examined from different aspects [30].SEM images taken from worn surfaces are shown collectively in figure 9 to explore the wear behaviour in more detail.
SEM analyses, as depicted in figure 8, offered insights into the morphology of the wear marks.Wear/film patterns and wear scar images were acquired for each group.In the investigated groups, the worn surface exhibited expansion with lines aligned in the direction of movement and baffle separation in the worn area, indicative of a compressive wear mechanism [31].Plastic deformation was observed along the wear scar, likely stemming from continuous contact pressure between the material surface and a harder body.Cracks beyond the worn region in the film were identified, aligning with findings reported by Alves et al [32].This phenomenon could be attributed to film degradation resulting from fatigue wear and film delamination.
Figure 10 presents the SEM and EDS examination of the wear path for sample G30 (10).Among the coatings fabricated with a zinc anode, minor wear was observed in sample Z30 (5).SEM and EDS data analysis in figure 10 reveals that zinc adheres to the surface, providing a protective layer for the underlying base material.
When figures 10 and 11 are examined, it is understood that the oxygen element is included in the structure, and some oxide layer is formed on the surfaces.It is natural for this situation to occur in wear tests carried out in open atmospheric conditions.Therefore, one of the alloying elements in the base metal is aluminum, which is sensitive to oxygen and can react quickly.In the EDS analyses in figures 10 and 11, all alloying elements in the coating material Zn and the base metal Ti6Al4V alloy were also detected.

Corrosion resistance
Figure 12 shows the potentiodynamic curves of TiO 2 -doped Zn-supported coatings.Corrosion current density (i corr ), corrosion potentials (E corr ) and corrosion rate (mpy) values obtained from polarization curves are summarized in table 6.The lowest current density value was seen in samples processed for 30 min with 5 g l −1 TiO 2 added using graphite anode.When the corrosion potential values obtained from the polarization diagram were examined, the lowest corrosion potential was seen as −1.04 V. Samples with 10 g l −1 addition for 60 min using graphite anode, samples with 5 g l −1 addition for 60 min and 10 g l −1 for 30 min using zinc anode have the lowest corrosion potential.A lower corrosion potential indicates that the coating has lower chemical stability.This shows that the coatings protect the substrate with a galvanic cell.Barranco et al reported that zinc is preferentially solvated when zinc matrix coatings nucleate.One of the cathodic reactions for zinc coatings in an oxygen-containing solution is the reduction of dissolved oxygen.This process causes the pH on the coating surface to increase locally, forming a thin and passive zinc-hydroxide layer.These hydroxide layers result in the formation of more corrosion products.However, due to the low solubility of corrosion products formed on the coating surface, the passive layer does not collapse, and therefore, the corrosion reaction occurs slowly [33].Figure 13 and tables 6 and 7 show the summary of the SEM images and EDS map results made after the test in the SBF.As a result of the EDS examinations, the harmony of the presence of zinc and oxygen in the coating in the area exposed to corrosion and the fact that the substrate material is not exposed show that galvanic protection has been achieved.Grain boundaries are high-energy regions sensitive to electrochemical activities, and the coating is expected to show lower corrosion current density due to the larger crystallite size [21].As seen in table 6, the lowest i corr value was observed in the coating made with a graphite anode, which has a spherical and large grain structure.Upon examination of the potentiodynamic curves, it was evident that the protection achieved by adding 10 g l −1 when using a graphite anode for a 30-min processing time was equivalent to the protection attained by adding 5 g l −1 when utilizing a zinc anode.Similarly, for a 60-min processing time, the values obtained by adding 10 g l −1 using a graphite anode mirrored those achieved by adding 5 g l −1 using a zinc anode.
Figures 14 and 15 show that the corrosion rate decreased as TiO 2 on the coating surface increased in the samples using zinc anode.

Conclusions
In the study, Ti6Al4V Grade 5 ELI samples were produced using a powder bed SLM method, and the TiO 2 ceramic material was coated homogeneously on these samples.• The coating thickness increased with the incremental addition of TiO 2 in all samples.For those using graphite anode, the coating thickness increased directly to the processing time.However, in the case of Zn anodes, it has been established that an increase in processing time results in a decrease in coating thickness.
• It has been observed that the amount of wear increases and the corrosion rate decreases with the addition of TiO 2 in coatings made using zinc anode.The samples, produced with a graphite anode and treated for 30 min, exhibited the highest wear, particularly those with the highest hardness value at 10 g l −1 TiO 2 addition.
• As additional TiO 2 particles increased in each process group, hardness and wear loss demonstrated an upward trend.
• The coating material becomes brittle with increasing hardness and moves away from the main structure more easily under the load applied in the wear tests.In addition, it can be stated that with increasing coating hardness, the adhesion at the coating interface and the base metal weakens.In this case, it became easier to separate the coating from the surface due to the increasing surface temperature due to the applied load and friction.TiO 2 particles in the very hard phase are separated from the structure and act as a second abrasive, creating the abrasive wear mechanism.
• Additionally, it has been concluded that wear increases as the amount of porosity at the microstructure level in the coating increases.The lowest current density value and corrosion rate were seen in samples processed for 30 min with 5 g l −1 TiO 2 added using graphite anode.The corrosion potential was noted to be at its lowest in samples with higher i corr values than others.Despite the seemingly rapid corrosion of zinc on the surface, the galvanic protection instigated by the formation of ZnOH contributes to increased corrosion resistance.

3. 1 .
Surface morphology SEM images of Zn-doped TiO 2 coatings on a Ti6Al4V alloy substrate manufactured using SLM are shown in figure1.The figures serve to emphasize the porous composition of the coating.It is apparent from the SEM images in the figure that the surface structure transforms the concentration of TiO 2 increases from 5 g l −1 to 10 g l −1 .

Figure 4 .
Figure 4. Average coating thickness values and cross-section image.

Figure 6 .
Figure 6.Hardness change according to TiO 2 addition amount.

Figure 7 .
Figure 7. Wear loss change according to TiO 2 addition amount.

Figure 8 .
Figure 8. SEM image showing the microstructure of the coating region.

Figure 13 .
Figure 13.The SEM images and EDS map results ma de after the corrosion test in the simulated body fluid (SBF), (a) G30-10 (b) G60-10.

Figure 14 .
Figure 14.The EDS analysis results following the coating process indicate the weight percentage of Ti in the deposited coating on the surface.

Figure 15 .
Figure 15.The variation in corrosion rate concerning the amount of TiO 2 addition.

Table 3 .
Treatment groups and sample numbering.

Table 7 .
The EDS results obtained after the corrosion test in the simulated body fluid (SBF).