Surface characterization of EPD coating on AZ91 Mg alloy produced by powder metallurgy

İbrahim Aydin^a, Ali İhsan Bahçepinar^a,*, Canser Gül^b

^aManisa Celal Bayar University, Vocational School of Technical Sciences, Machinery and Metal Technologies Department, Manisa

^bManisa Celal Bayar University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Manisa

*(Corresponding author: ali.bahcepinar@cbu.edu.tr)

1. INTRODUCTIONTOP

Mg alloys are using in many fields such as automobile or space industries. AZ series Mg alloys such as AZ31, AZ80, and AZ91 are studied frequently for their biocompatibility, lightness and their relative corrosion resistance comparing to the other mg alloys (Agarwal et al., 2016). These properties of Mg alloys raise the question of whether it can be used as a permanent implant in recent years due to its mechanical properties close to the natural bone (1.8–2.1 g·cm⁻³) (Brandes and Brook, 1998; Bandyopadhyay et al., 2010; Ivanova et al., 2014). Mg alloys are lightweight metals with density ranging from 1.74 to 2.0 g·cm⁻³, that is much lighter than titanium alloys (4.4–4.5 g·cm⁻³), Co-Cr alloys (4.5–8.3 g·cm⁻³) (Ivanova et al., 2014; Zhang et al., 2015).

AZ series alloys mostly consist of α-grains and secondary β-phases. These secondary phases such as Mg17Al12 precipitate at the grain boundaries and cause the presence of localized galvanic couples between α-grains and secondary phases. But this mechanism not the only reason for the fast degradation of magnesium alloys (Sahoo and Panigrahi, 2016; Wang et al., 2017; Chowdary et al., 2018). Pitting corrosion is also affecting the degradation especially in the presence of chloride ions (Tahmasebifar et al., 2016). The human body is a very harsh environment that contains many ions such as chlorine and so on (Salman et al., 2013). Therefore, various studies have been carried out to prevent this degradation in mg alloys.

Various studies were carried out to increase the bioactivity, corrosion or wear resistance of Mg and its alloys (Chen et al., 2008; Salman et al., 2013; Esmaily et al., 2016). Some of them were designed to provide various properties using reinforcing elements (Esmaily et al., 2016), while others aimed at forming an oxide layer on the surface using heat treatments (Niu et al., 2016; Chen et al., 2017). Coatings were made to ensure that similar properties are also available (Brusciotti et al., 2013; Rojaee et al., 2013).

Coatings were made to ensure that Hydroxyapatite (HA) is a calcium phosphate product, which is the essential component of the human bone, having a high bio-activity and bio-compatibility (Bogdanoviciene et al., 2006; Javidi et al., 2008; Iqbal et al., 2012; Morteza, 2018a). However, due to its low mechanical properties, it is not appropriate to be directly used in human body (Miranda et al., 2019). Its application of being coated on the surfaces of the metallic biomaterials having high mechanical properties supports its usage in human body more (Javidi et al., 2008; Iqbal et al., 2012; Luo et al., 2018). Also HA has a significant place in biomaterials due to the fact of supporting the bone growth (Javidi et al., 2008; Miranda et al., 2019).

There are various methods like sol-gel, biomimetic method, electrochemical precipitation, plasma spraying and electrophoretic deposition (EPD) for the HA coating of metallic implants (Abdeltawab et al., 2011; Iqbal et al., 2012; Bartmanski et al., 2018). Among these the one which has the highest potential of usage in industry is EPD method. With this method, the coating process occurs with the deposition of the charged particles which are spread in an electrolytic suspension on the counter electrode moving by the effect of electrical force (Pang et al., 2009; Sreekanth and Rameshbabu, 2012; Drevet et al., 2016; Morteza, 2018b). EPD method, which is preferred in industry, is a simple method due to its advantages like coating thickness control, creating homogeneous coatings, low equipment cost, coating materials with complex structures (Iqbal et al., 2012; Morteza and Shahrabi, 2013; Khalili et al., 2016; Morteza, 2018b; Sabzi et al., 2018). The biggest disadvantage of EPD method is that it needs sintering at high temperatures (Khalili et al., 2016). Alcohols are used as solvents in the preparation of EPD suspensions. The reason is that alcohols act as proton donors. Ethanol in the alcohols group is an easy-to-find solvent commonly used in the EPD method (Abdeltawab et al., 2011).

2. MATERIALS AND METHODSTOP

2.1. Implant material selectionTOP

AZ91 Mg alloy powders containing the components given in Table 1 were hot compacted by hot isostatic pressing (HIP) at 275 MPa and 325 °C for 1 h for substrate production. Powders, which have particle size distribution given in Table 2, were selected because of their commercial availability and frequently used in biomaterial applications. The density values of the samples reached up to 98.6% of the theoretical density, according to the Archimedes’ Principle. Produced AZ91 Mg alloy was cut in to 30×10×2 mm³ dimensions.

AZ91 Mg alloy, which we used in the study as a base material, was created with powder metallurgy method as an alternative to present biomedical materials. Besides it’s a candidate of new generation implant materials due to the fact that it’s lightweight and it has a structure closer to bone’s.

2.2. The preparation of the coatingTOP

In the study, AZ91 Mg alloyed implant materials were rubbed by using SiC emery paper and they were washed with detergent water. Then they were cleansed in ultrasonic bath with wash of pure water for 30 min, with wash of ethyl alcohol 30 min and with wash of pure water 30 min respectively. Finally, they were made ready for the coating process by having been dried in the oven at 25 °C.

In electrophoretic deposition process ethanol was used at a purity of 99.8%. By having added 1 g of HA and 1 g of PVA and 10 ml of N,N-Dimethylformamide (Sigma-Aldrich) in 100 ml of ethanol they were mixed in a magnetic mixer for 30 min and the homogeneous diffusion of HA powders was provided in a teflon beaker. In order to provide suspension stability, the Ph value was set as 4 by using HNO₃ and NaOH.

For the coating process AZ91 Mg alloys were placed as anode and cathode by making the surfaces face to each other to be coated. The distance between them was set as 10 mm. They were dipped in stable suspension prepared in Teflon beaker and were connected to DC power supply (Bio RAD Powerpac Basic). In order to examine the effect of the voltage which was applied at a constant period to the coating properties, the voltage effect period was kept at a constant value of 2 min, the voltage values were set as 100, 150, 200 and 250 V. The coating operations were performed at these parameters.

In order to increase the bonding strength of the coatings which were gained, the coatings were kept at room temperature for 24 hours, then they were heated up to 400 °C at a heating speed of 2 °C·min⁻¹. They were left to be cooled by having been kept at 400 °C for 1 hour.

3. RESULTSTOP

3.1. SEM-EDX resultsTOP

The microstructure examination analyzes of the coated samples were performed by using QUANTA 250 FEG model scanning electron microscope. 5000x images of the results gained on the microstructural examinations which were performed on SEM are given on Fig. 1. When SEM images are examined it is seen that the HA structure successfully occurred on the surface. At 250 V it was seen that there occurred deep holes and fractures in the structure.

The elemental component of the structures and the coated samples were determined by using EDX detector on QUANTA 250 FEG model scanning electron microscope. The EDX results of the coatings created at different voltage periods are given on Fig. 2. When the results were examined, it was detected that a calcium phosphate structure had occurred on all coatings. Besides the Ca/P rates calculated through these results for each parameter are given on Table 3.

3.2. XRD resultsTOP

In order to determine the phases which the coated samples contained and the concentration of these phases, PHILIPS X’PERT PRO X radiation diffraction device (XRD) was used. Figure 3 shows the results of the XRD tests. According to the test results, the HA crystals occurred at peak points of 25.7182°, 28.7945°, 31.6402°, 32.0520°, 32.6648°, 32.7802°, 33.8369°, 33.926°, 39.6934°, 46.2985°, 49.3822° and 70.7743° respectively.

3.3. Coating thickness resultsTOP

The thickness of HA coatings which were performed by using electrophoretic deposition method, was evaluated with ElektroPhysik MiniTest 730/Sensor FN 1.5 HD trademark device in terms of micrometer. The averages of the results were taken by performing evaluations on each coating surface 5 times. For each coating gained at 100, 150, 200 and 250 V values at a deposition period of 2 min five evaluations were performed and averages of the results were taken. The average values of the coating thicknesses gained from the evaluation results are given on Table 4.

3.4. Surface roughness resultsTOP

For the determination of the surface roughness values of the samples gained by the result of the coatings, Roughness Tester PCE-RT 1200 model device was used. On the coating the evaluation range was determined as 12.5 mm and the evaluation speed was determined as 0.5 mm·s⁻¹, the results were gained in terms of micrometer. Surface roughness was measured 5 times in each sample coated with 100, 150, 200 and 250 V for 2 min and averages of the results were taken. The average roughness values gained by the evaluation results are given on Table 5.

3.5. Corrosion test resultsTOP

The corrosion tests of the samples were carried out in simulated body fluid (SBF) prepared as an electrolyte. The chemicals used in the SBF preparation were given in Table 6. The first five chemicals were dissolved in 700 ml deionized water respectively. After the fifth chemical added, 15 ml of 1M HCl solution was added. Subsequently, the other chemicals were added sequentially. After dissolving the final chemical, the temperature was increased to 37 °C, body temperature, adjusted to pH 7.4 using 1M HCl solution. Then, deionized water was added to complete the solution volume to 1 L.

Corrosion tests were carried out by using Metrohm potantiotat-galvanostat corrosion device with three electrode system. A saturated calomel electrode (SCE) was used as a reference electrode, graphite electrode was used as a counter electrode and insulated sample outside the specific surface area was used as a working electrode. The open circuit potential of 120 s was measured in the prepared SBF solution and then corrosion tests were performed. Potential in the range of -250 mV and 250 mV was applied to the samples with 1 mV·s⁻¹. Electrochemical potential curves of the samples obtained in the measurement results was given in Fig. 4 and corrosion rates was given in Fig. 5.

4. DISCUSSIONTOP

Aydın et al. (2019) observed HA crystals at peak points of 2 Theta= 26.0078°, 28.1945°, 32.2494°, 34.1778°, 39.9130°, 48.1571° and 50.5511° on XRD results of the coatings which they gained in the HA coating process, which they had conducted on the surface of Tİ6Al7Nb alloy with EPD method. Kumar et al. (2016) observed HA crystals at peak points of 2 Theta= 26.06°, 31.62° on XRD results of the coatings which they gained in the HA coating process, which they had conducted on the surface of Mg-3Zn alloy at 20 V·cm⁻¹ for 10 min with EPD method. Iqbal et al. (2012) observed HA crystals at peak points of 25.91°, 28.94°, 32.2494°, 31.78°, 32.19°, 32.93°, 34.10°, 39.80°, 46.71°, 49.49° in the coating process they had conducted on 316L stainless steel.

By the results of conducted evaluations, when the coating thickness values are examined, it’s seen that surface roughness value increases due to the increasing voltage value. Aydın et al. (2019) determined the coating thicknesses as 4.38 μm, 5.43 μm, 7.60 μm, 9.42 μm respectively on the HA coatings, which they had performed on Ti6Al7Nb alloy by applying 50V,100V,150V,200V at a depositon period of 1 minute. Kwork et al. (2009) determined the coating thickness approximately as 10 μm in the HA coating process, which they had performed on Ti6Al4V bases at 200 V for 3 minutes. Bartmanski et al. (2018) conducted a hydroxyapatite coating process on Ti13Zr13Nb bases in the solution, which they had prepared by using 0.5 g of hydroxyapatite in 100 ml of ethanol, at 50 V for 1 min and they determined the coating thickness approximately as 29.35 μm. Dudek and Goryczka (2016) conducted a HA coating process on NiTi shape memory alloy at different voltages and periods, they determined the coating thicknesses as 2.6 μm at 80V in 1 min, 2.8 μm at 100 V in 1 min, 4.2 μm at 80V in 2 min.

By the results of conducted evaluations, when the roughness values are examined, it’s seen that surface roughness value increases due to the increasing voltage value. Aydın et al. (2019) evaluated the surface roughness values as 0.818 μm, 1.055 μm, 1.552 μm, 1.673 μm respectively on the HA coatings, which they had performed on Ti6Al7Nb alloy by applying 50 V,100 V,150 V and 200 V. Bartmanski et al. (2018) evaluated the surface roughness value as 1.26 μm by the result of the coating process on Ti13Zr13Nb bases in the solution, which they had prepared by using 0.5 g of hydroxyapatite in 100 ml of ethanol at 50 V for 1 min. Javidi et al. (2008) gained coatings at different voltages and periods in the HA coating process which they had performed on 316L stainless steel, they evaluated the surface roughness value as 1.8 μm on the coating they had gained at 90 V and at a deposition period of 180 s. Besides they determined that surface roughness increased within the increasing voltage.

Potential-current density graphs show that the potential values towards more positive values indicate higher corrosion resistance of these samples (Perez, 2004). In this case, it can be said roughly that the 100V/120s coated sample has the lowest corrosion resistance and the 250V/120s coated sample has the highest corrosion resistance. When the corrosion rates of the samples were examined, it was found that the results were consistent with their potential values. The coating thickness increased with increasing potential used in the coating process, and corrosion resistance of the samples tended to increase with increasing coating thickness.

When SEM images were examined in Fig. 1, it was observed that there were porosities on the sample surface coated with 250V/120s. However, the corrosion performances were examined; it was found that this sample was more resistant to corrosion than the others. In fact, it is expected that the corrosion resistance of such a porosity-containing material will be reduced. The higher corrosion resistance of this sample can be attributed to deep cavities seen do not reach the substrate material. When the Tafel curve of this sample was examined, it was found to be rougher than other curves. These sharp changes in the potential flow graph are associated with pitting corrosion (Perez, 2004). However, the findings support that the gaps were closed to the substrate surface. Furthermore, this is not only related to voids in the coating layer. The measured surface roughness values also contain information on corrosion behavior. In their study, Alvarez et al. (2010) emphasized that increased surface roughness prevents pitting corrosion and this may prevent the decrease in corrosion potentials (Alvarez et al., 2010).

The corrosion resistance of the uncoated sample was surprisingly not the lowest. The corrosion performance of the 100V/120s coated sample was lower than the uncoated AZ91D sample, which may be due to insufficient coating thickness. The presence of pits open to the bottom in small amounts of coated samples may decrease the corrosion resistance as it increases the ion concentration at these points (Song, 2011).

5. CONCLUSIONSTOP

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