Surface Evaluation of Tricalcium Phosphate Bioceramic Coating on SS-316L by Electrophoretic Deposition

Abstract


INTRODUCTION
The global demand for healthcare devices, especially bone tissue implants, is increasing each year due to the growing aging population and other health factors (Drevet & Benhayoune, 2022;Gheno et al., 2012;Li et al., 2017).Therefore, current developments in orthopedic or dental surgeries are limited to inert metals such as titanium alloys, stainless steel, and CoCr alloys (AlMangour et al., 2020;Drevet et al., 2018;Farrakhov et al., 2021;Koumya et al., 2021;Nkonta et al., 2021;Sheremetyev et al. 2022;Trincă et al., 2021).This alloy is used because it has mechanical properties suitable for bone tissue replacement and has good biocompatibility with the body environment.According to the International Union of Pure and Applied Chemistry (IUPAC), biocompatibility is the ability of a material to come into contact with biological systems without causing deleterious effects (Drevet & Benhayoune, 2022;Ghasemi-Mobarakeh et al., 2019).
Stainless steel 316 L (SS-316L) has earned a wide reputation as an implant in reconstructive surgery due to its excellent mechanical, biomedical and chemical properties.Coated SS-316L is of particular interest in metal implants to stabilize biological structures (e.g.building bone tissue and creating joint prostheses) and accelerate the healing process or replace damaged biological tissue (Koumya et al., 2021;Kurgan, 2013) However, the main problem of SS-136L metal is related to its low bioactivity and biocompatibility.To avoid this problem, coating SS-136L metal with biocompatible and bioactive materials using tricalcium phosphate was proposed (Hosseinalipour et al., 2010).
The material used for surface modification of SS-316L is a biomaterial based on tricalcium phosphate (TCP, [Ca3(PO4)2]), which exhibits excellent biocompatibility when used inside the human body (Anselme, 2000;Dorozhkin, 2015;Fiume et al., 2021).Additionally, TCP biomaterials possess osteoconductivity, non-immunogenicity, and the ability to stimulate strong bonding with bone tissue.TCP coating on SS-316L as a surface modification aims to overcome the poor mechanical properties of bioceramics.Therefore, TCP/ SS-316L composite material is proposed to overcome the problems of less active bioactivity of SS-316L and poor TCP mechanical properties (Tulinski & Jurczyk, 2012).The bioactive TCP coating over SS-316L, which is similar to the main mineral component of human bone, not only acts as a barrier layer but also promotes the natural growth of bone cells (Ma et al., 2022;Shao et al., 2016;Yuan et al., 2023).
There are different methods to create a calcium phosphate coating on the implant surface, such as plasma spraying (Chambard et al., 2019;Heimann, 2016), magnetron sputtering (Safavi et al., 2021;Surmenev et al., 2021;Surmeneva et al., 2019), pulsed laser deposition (Popescu-Pelin et al., 2017), deposition electrophoresis (Bartmanski et al., 2017;Kollath et al., 2013) or electrodeposition (Olivier et al., 2020a;Olivier et al., 2020b;Vidal et al., 2019).Electrophoretic deposition and electrodeposition are alternative methods to synthesize calcium phosphate films at low temperatures (Gao et al., 2018;Shirkhanzadeh, 1998).Electrophoretic deposition (EPD)-based coating processes have been carried out, including ceramic thick film deposition, lamination and body forming (Boccaccini et al., 2010;Caicedo et al., 2020).Electrophoretic deposition for TCP biomaterials on stainless steel metal is the most practical technique because it is inexpensive and relatively simple technique.This allows a more homogeneous TCP biomaterial layer to be achieved on the surface (Ananth et al., 2015).The deposition rate can be controlled by changing certain parameters such as electrode voltage and deposition time (Bai et al., 2010).
This study focuses on observing the microstructural evolution of a TCP biomaterial deposited on a metal substrate.The metal substrate chosen for this study is stainless steel type SS-316L with very low carbon content.Surface modification on SS-316L will be carried out using TCP biomaterial through the electrophoretic deposition technique at room temperature.

EXPERIMENTAL PROCEDURES Materials And Preparation
A commercial SS-316L with dimensions of 20 mm × 10 mm × 3 mm was used as the substrate.Before the coating process, the substrate surface was mechanically ground using silicon carbide (SiC) abrasive paper ranging from P120 to P1200 grit to achieve a uniform surface roughness.The samples were then cleaned using methanol and distilled water and air-dried.Subsequently, the substrate was immersed in a 4% NaOH solution for 10 minutes, followed by pickling in a 1% HCl acid solution for an additional 10 minutes.Afterward, the substrate was cleaned with distilled water and methanol and air-dried.

Electrophoretic Deposition
EPD was carried out using a DC power supply.Two electrodes were prepared, with each SS316L acting as the cathode, and a graphite electrode was used as the anode in the EPD process, with a distance of 3 cm between the electrodes.The EPD process was conducted with a voltage of 10 for deposition times of 10, 20, and 30 minutes.The selection of this voltage is due to the better quality of the coating achieved at relatively low voltages to prevent layer peeling from the substrate (Assadian et al., 2015;Besra & Liu, 2007).The electrophoretic deposition scheme is illustrated in Figure 1.
The electrolyte solution was prepared by dissolving 0.064 M TCP or tricalcium phospahate [Ca3(PO4)2] in distilled water.TCP (Shuren Food Additive, ISO9001/ISO22000) is a food product sold in the food industry as a food grade anti-caking agent.Nitric acid (HNO3) was added to the electrolyte solution to lower the pH to 4.7.The electrolyte solution was continuously stirred at a speed of 100 rpm.The deposition process was carried out at room temperature.After the EPD process was completed, the substrate was air-dried at room temperature for 24 hours.The electrodeposition of TCP was carried out with different deposition time, which was 10 minutes for TCP10, 20 minutes for TCP20 and 30 minutes for TCP30.
Figure 1.Scheme of TCP coating on SS316L using electrophoretic deposition.

Characterization
Chemical composition of the substrate was tested using optical emission spectroscopy (OES, ARL 3460, Switzerland).The morphology of the raw material and coated results was characterized using a scanning electron microscope (SEM; Rigaku, SU 3500; Japan).Chemical element observations from the coating results were conducted using energy-dispersive spectrometry (EDS; Rigaku, SU 3500; Japan).Hardness testing was performed using Vickers microhardness (Future Tech, Japan) with indentations of 50, 100, and 300 gf for 30 seconds.The results of the Vickers microhardness testing were calculated using the Eq.(1).
where P is the indentation load, d1 and d2 are the diagonal distances of the indentation.Vickers microhardness testing is conducted to determine the hardness of both the substrate and its coating, with a specific focus on finding the optimal coating thickness value (Chicot et al., 1996).The calculation of the indentation depth is theoretically performed using Eq. ( 2).
where h is the indentation depth and d is the average diagonal of the indentation result (Iost et al., 2012).

Deposition Yield
In this study, each process parameter is analyzed to determine the optimal results.One of the indicators in the parameter analysis is the calculation of the deposition yield (Ahmed & Rehman, 2020).The deposition yield can be calculated through Eq. (3).
where ∆Weight is the difference in substrate weight between the initial condition and after EPD, and A is the coated surface.Table 1 shows the calculation results of the deposition yield and the hardness values of the substrate and TCP coatings.From Table 1, it can be concluded that a voltage of 10 V with a coating time of 30 minutes has the highest deposition yield.This deposition yield value indicates that the larger the value, the better the results of the deposition performed.

Morphology and Chemical Composition of Raw Materials
Table 2 shows the results of OES testing on the substrate and standard material for SS316L.The OES test results indicate that the substrate is SS316L stainless steel material with a composition of 16.24 wt.% Cr, 9.45 wt.% Ni, and 2.08 wt.% Mo, and a very low carbon (C) content of 0.03 wt.% C.These composition values are in accordance with the standard composition of SS316L.
Figure 2 shows the results of the initial characterization using SEM-EDS of the raw material TCP used in this research.Based on the results of the secondary electron image (SEI) from SEM shown in Figure 2a, the powder of the TCP raw material has an irregular shape with various sizes.The TCP particles have varying sizes ranging from 10-30 μm.On the other hand, the EDS test results are shown in Figure 2b.The results indicate that the raw material used consists of a composition of 25.11 at.% Ca, 14.13 at.%P, and 60.67 at.%O.

SEM-EDS Analysis of Biomimetic Coating
The SEM-EDS test results for SS316L material coated with TCP at different deposition times are shown in Figure 3.As seen in Figure 3(a), the coating deposited over a period of 10 minutes appears denser and more uniform, exhibiting a plate-like morphology of TCP on the substrate surface.However, a relatively short coating time  Moreover, an increase in deposition time results in a change in the morphology of the coating layer.
Additionally, the TCP layer formed on the SS316L substrate becomes thicker as the deposition time increases, as indicated by the weight gain after the deposition process in Table 2.The composition of the TCP layer is also identified in this study using EDS, as shown in Figures 3(b), (d), and (f).The EDS results reveal that there is no significant change in the composition of the TCP layer over different deposition time ranges.
Figure 5 shows a cross-sectional view between SS316L and the TCP layer with a deposition time of 30 minutes.The cross-sectional cut indicates two different morphologies formed from the deposition results.The TCP layer formed on the substrate surface has a dense structure and forms micro-sized plates.The deposited TCP layer on SS316L has a thickness ranging from 48.7 to 57.9 µm.Figure 5. Cross-section of TCP30.

Hardness
Figure 6 presents a comparison of microhardness test results with indentation loads of 50 gf, 100 gf, and 300 gf for deposition times of 10, 20, and 30 minutes on the SS316L substrate.The hardness values of the SS316L substrate are influenced by the TCP layer.The highest hardness value is shown at the 50 gf load.This hardness value tends to decrease with increasing indentation load and coating thickness.The hardness test results are shown in Table 3.The decrease in hardness values is influenced by the presence of the TCP layer, which affects stress distribution during the indentation process.Therefore, in the TCP10 sample where the formed layer is thinner than TCP20 and TCP30, the hardness value approaches the hardness value of the substrate.Hardness testing on human bones with indentation loads between 10-100 gf has also been conducted by several previous studies.The hardness test results on human bones fall within the range of 33-45 VHN (Coats et al., 2003;Dall'Ara et al., 2007).Therefore, it can be assumed that the lowest hardness value of the coating is desirable for use in the human body.This is because if the hardness of the implant material is very high, it can lead to the bones being more easily worn away when in contact with the implant material (Louvier-Hernández et al., 2021).
The values obtained from Vickers hardness testing are influenced by the diagonal lengths and the depth of the indentation produced.This is also affected by the indentation load applied.Micro-Vickers test results indicate that the increase in the thickness of the layer formed on the surface of SS316L and the increase in the applied indentation load will lead to a decrease in hardness values.This is because, with smaller loads, the influence of elastic deformation on the material becomes a significant factor in reducing the values of both diagonal lengths and indentation depth, resulting in higher hardness values.As the applied indentation load increases, the influence of elastic deformation on the material can be minimized, leading to lower hardness values compared to lower indentation loads.
The indentations from hardness testing also exhibit impressions in the form of circular depressions due to the distribution of stress between the substrate and coating surfaces.Figure 7 shows the detachment halo's diameter results against penetration depth and the indentation loading zone.The results indicate a linear relationship between the impression diameter, indentation depth (h), and indentation load.This suggests that higher loads will cause plastic deformation, resulting in larger impression diameters and apparent indentation depths.

CONCLUSION
In this study, the topic revolves around the deposition of TCP [Ca3(PO4)2] on SS316L substrate with varying deposition times using the electrophoretic deposition technique.Samples were characterized to understand the changes in microstructure and mechanical properties resulting from the coating.The key findings from this study are (1) The highest deposition yield, with a value of 0.635, was observed in the deposition process with a time range of 30 minutes.(2) Microstructure testing results indicate that the TCP 10 sample has a more uniform Ca3(PO4)2 layer, but there is peeling and cracking in the coating.TCP20 and TCP30 exhibit irregular morphology in some areas, with no observable peeling of the layer.Additionally, the deposition process yields coatings with thickness in the range of 48.7-57.9µm.Hardness testing results show that an increase in deposition time and sample weight leads to a decrease in microhardness Vickers.The hardness test values also correlate with the applied indentation load, indicating that higher loads result in decreased hardness and increased indentation depth.
(a) SEM/EDS spectrum of TCP10, (b) EDS results of TCP 10, (c) SEM/EDS spectrum of TCP20, (d) EDS results of TCP 20, (e) SEM/EDS spectrum of TCP30, and (f) EDS results of TCP 30.(10 minutes) shows signs of peeling and cracks on the substrate (Figure 3(a)).On the other hand, Figure 3(c) and d indicate that the coating results are not uniformly distributed, evidenced by the accumulation of TCP particles in certain areas.However, as the coating time increases, the TCP particles adhering to the substrate show reduced signs of peeling and cracking.The TCP layers deposited for 20 and 30 minutes exhibit irregular morphologies.Figure 4(a) also shows the analysis results in areas where TCP particles do not accumulate.Analysis performed using point scanning EDS techniques indicates the formation of a thin film layer in the specified area (Figure 4(b)).These findings indicate that a longer deposition time enhances the adhesion capability of the TCP layer to the substrate, thereby reducing the occurrence of peeling in the coating layer.
(a) Morphology of SS316L-TCP10 at 1000x magnification and (b) chemical composition of the coating film on SS316L-TCP10.

Figure 7 .
Figure 7. Diameter of the deformation against the depth penetration of the SS316L/TCP coatings.

Table 2 .
Chemical composition of the SS316L.