Coatings In Simulated Body Fluid Research Articles

Ti3C2Tx MXene-functionalized Hydroxyapatite/Halloysite nanotube filled poly– (lactic acid) coatings on magnesium: In vitro and antibacterial applications

Magnesium (Mg) stands out in temporary biomaterial applications due to its biocompatibility, biodegradability, and low Young's modulus. However, controlling its corrosion through next-generation polymer-based functional coatings is crucial due to the rapid degradation behavior of Mg. In this study, the function of 2D lamellar Ti3C2Tx (MXene) in Hydroxyapatite (HA) and Halloysite nanotube (HNT) hybrid coatings in biodegradable poly– (lactic acid) (PLA) was investigated. The morphological and structural characterizations of the coatings on Mg were revealed through HRTEM, XPS, SEM-EDX, XRD, FTIR, and contact angle analyses/tests. Electrochemical in vitro corrosion tests (OCP, PDS, and EIS-Nyquist) were conducted for evaluate corrosion resistance under simulated body fluid (SBF) conditions. The bioactivity of the coatings in SBF have been revealed in accordance with the ISO 23,317 standard. Finally, antibacterial disk diffusion tests were conducted to investigate the functional effect of MXene in coatings. It has been determined that the presence of MXene in the coating increased not only surface wettability (131°, 85°, 77°, and 74° for uncoated, pH, PHH, and PHH/MXene coatings, respectively) but also increased corrosion resistance (1857.850, 42.357, 1.593, and 0.085 × 10–6, A/cm2 for uncoated, pH, PHH, and PHH/MXene coatings, respectively). It has been proven that the in vitro bioactivity of PLA-HA coatings is further enhanced by adding HNT and MXene, along with SEM morphologies after SBF. Finally, 2D lamellar MXene-filled coating exhibits antibacterial behavior against both E. coli and S. aureus bacteria.

Development of dual-effect AW/Ag/CHI bioactive and antibacterial coating for orthopedic implant surfaces

Titanium implants, which are widely used in orthopedic implant applications, require surface treatments due to their low osteoinduction properties and the risk of postoperative infection. In this study, apatite wollastonite (AW)-based coatings with chitosan (CHI) and silver (Ag) added were applied to the titanium surface using dip coating and drop casting techniques. The obtained coating surfaces were characterized by FESEM, EDS, XRD, FTIR, and TGA–DSC. Biomineralization behaviors were examined by immersing the resulting coatings in simulated body fluid with pH 7.2–7.4. The antibacterial activities of the coatings obtained using S. aureus and E. coli bacterial colonies were examined. Following the successful coating of AW nanoparticles on the Ag and CHI implant substrate, the Ca/P ratio of the coated surface was obtained as 1.4–1.92% after the biomineralization process performed at 37° for 7 days. Ag particles were successfully distributed homogeneously with a 40–50-nm diameter in Ag-doped coatings. It was concluded that the successfully produced composite coating with bioactive and antibacterial properties is a suitable candidate material for metallic implant surfaces.Graphical abstract

In Vitro Wear of Titanium Reinforced Hydroxyapatite Coatings in Simulated Body Fluid

Abstract Titanium-reinforced hydroxyapatite (Ti-HAP) coatings have been deposited over 254 SMO stainless steel using a low-velocity oxygen fuel technique. FESEM, XRD, Vickers microhardness, and shear strength tests have been performed to characterize the developed coatings. Further, in vitro wear behaviors of the coatings were evaluated using a pin-on-disc wear tester in simulated body fluid. The results reveal that 40Ti-HAP coating possesses superior wear resistance compared to 60Ti-HAP, Ti, and HAP coatings, attributed to higher surface hardness and higher shear strength values. Amongst the tested samples, the friction coefficient was lower for Ti coating, followed by 60Ti-HAP composite coating. The surface roughness was a prominent factor in reducing the friction coefficient of coating samples. Local detachment and brittle fracture were the dominant wear modes in Ti-HAP coating, whereas an additional micro-plowing wear mode was observed in pure hydroxyapatite and titanium coating.

Fabrication and characterization of hydroxyapatite coatings on anodized magnesium alloys by electrochemical and chemical methods intended for biodegradable implants

The fabrication of hydroxyapatite (HAP)/MgO composite coatings on Mg alloy is crucial for enhancing the corrosion resistance and biocompatibility of biomedical implants. In this study, we aimed to investigate the effects of two different surface modification methods, i.e., electrochemical (electrodeposition, ED) and chemical (solution treatment, ST), on the phase structure, degradation properties, and biocompatibility of the composite coatings in comparison to the anodized coating. The surface morphologies and crystalline structures of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Subsequently, the degradation rate of the coatings in simulated body fluid were comprehensively evaluated by using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and scanning electrochemical microscopy (SECM) tests. Additionally, in-vitro cell proliferation assays were employed to quantitatively assess the biocompatibilities of the coatings. The results showed that both ED and ST methods were effective in depositing HAP on anodized Mg alloy, resulting in different surface morphologies with hydroxyapatite layer thicknesses of 2.71 μm and 3.56 μm, respectively. In this way, the HAP-deposited coatings exhibited improved corrosion resistances and biocompatibilities compared with those of the anodized coating. Specifically, the ST-deposited composite film displayed superior degradability and biocompatibility which was attributed to its filiform surface morphology and thicker HAP layer. Overall, the present study demonstrates the potential of HAP/MgO composite coatings for biomedical applications, with implications for the development of advanced implant materials with improved performance and biocompatibility.

(TiZrNbTaMo)N nanocomposite coatings embedded with silver nanoparticles: imparting mechanical, osteogenic and antibacterial traits to dental implants

Designing highly osteogenic and effective antibacterial biomaterials with a prolonged service life for oral implants remains an ongoing challenge. In this work, the multi-element (TiZrTaNbMo)N high entropy nitride (HEN), with different concentrations of Ag, was co-deposited onto surgical grade commercially pure titanium by a reactive sputter-deposition technique using a double cathode glow discharge plasma system. The effects of Ag concentration on the microstructure and mechanical properties of the HEN-Ag nanocomposite coatings were investigated by various testing methods. Potentiodynamic polarization measurements were used to evaluate the corrosion resistance of the coatings in simulated body fluid. The antibacterial activity of the HEN-Ag nanocomposite coatings against Escherichia coli (E. coli), Staphylococcus aureus (S. aureus) and Candida albicans (C. albicans) strains was examined based on the bacterial inhibitory zones and bacterial killing efficiency. The cytocompatibility and osteogenesis activity of the HEN nanocomposite coatings using MC3T3-E1 cells were evaluated by a CCK-8 assay and an alkaline phosphatase quantitative assay. These studies demonstrate that the Ag-free HEN coating was composed of a face-centered-cubic (FCC) nitride phase, which displays good mechanical properties, anti-corrosion performance and osteogenic function, but no antibacterial properties were observed. In contrast, the HEN0.9Ag0.1 coating, composed of amorphous/nanocrystalline high-entropy nitride embedded with silver nanoparticles ∼10 nm in diameter, displays good mechanical and anti-corrosion performance, together with the excellent cell compatibility and antibacterial performance, which offer potential for providing outstanding protective efficiency for dental implants. Although the HEN0.8Ag0.2 nanocomposite coating exhibited superior antibacterial efficacy, potential for in vitro cytotoxicity was identified.

Investigation and optimization of In-Vitro behaviour of Perovskite barium titanate as a scaffold and protective coatings

The piezoelectric effect is widely known to have a significant physiological function in bone development, remodeling, and fracture repair. As a well-known piezoelectric material, barium titanate is particularly appealing as a scaffold layer to improve bone tissue engineering applications. Currently, the chemical bath deposition method is used to prepare green synthesized barium titanate coatings to improve mechanical and biological characteristics. Molarity of the solutions, an essential parameter in chemical synthesis, is changed at room temperature (0.1–1.2 Molar) to prepare coatings. The XRD spectra for as deposited coatings indicate amorphous behavior, while polycrystalline nature of coatings is observed after annealing (300 °C). Coatings prepared with solutions of relatively low molarities, i.e. from 0.1 to 0.8 M, exhibit mixed tetragonal – cubic phases. However, the tetragonal phase of Perovskite barium titanate is observed using solution molarities of 1.0 M and 1.2 M. Relatively high value of transmission, i.e. ∼80%, is observed for the coatings prepared with high molarities. Band gap of annealed coatings varies between 3.47 and 3.70 eV. For 1.2 M sample, the maximum spontaneous polarization (Ps) is 0.327x10−3 (μC/cm2) and the residual polarization (Pr) is 0.072x10−3 (μC/cm2). For 1.2M solution, a high hardness value (1510 HV) is recorded, with a fracture toughness of 28.80 MPam−1/2. Low values of weight loss, after dipping the coatings in simulated body fluid, is observed. The antibacterial activity of BaTiO3 is tested against E. coli and Bacillus subtilis. Drug encapsulation capability is also tested for different time intervals. As a result, CBD-based coatings are a promising nominee for use as scaffold and protective coatings.

Graphene oxide reinforced chitosan/polyvinyl alcohol antibacterial coatings on stainless steel surfaces exhibit superior bioactivity without human cell cytotoxicity

The study proposes an alternative therapeutics to diminish bacterial attachment in biomedical implants by modifying their surface with passive coatings. A uniform, thin-film of chitosan/polyvinyl alcohol/graphene oxide (CS/PVA/GO) was coated on 316 L stainless steel (SS) surface through spread casting followed by solvent evaporation. The abundant anchoring sites available at macromolecular interfaces of chitosan/PVA matrix facilitated a smooth, dense loading of GO. The effect of GO content on physicochemical features, antibacterial potential, and biocompatibility of coatings was thoroughly studied. The hybrid films displayed good adhesion behavior, and UV-protection ability with desired mechanical and thermal stability when coated on SS surface. Coatings manifested a 1.5–1.7 fold rise in antibacterial efficacy against Staphylococcus epidermidis and Staphylococcus aureus and exhibited a permanent biocidal response after 6 h of contact-active behaviour. We investigated a 3-fold generation of reactive oxygen species as the predominant antibacterial mechanism, which diminishes bacterial integrity by inducing protein leakage (8.5–9 fold higher) and suppressing respiratory chain activity as two secondary mechanisms. All coatings with varying GO content appeared non-haemolytic (<2%) with ultra-low cytotoxicity (<29.08%) against human hepatocellular carcinoma (HepG2) and peripheral blood mononuclear cells. The degradation rate of coatings in simulated body fluid exhibited a higher stability, indicated by a lower weight loss (69–78%) and a decrease in pH values as the GO content in coatings increased from 0.05 to 0.15 wt%. Such anti-infective coating is a step forward in inhibiting bacterial colonization on SS surfaces to extend its lifespan.

Role of TiO2 Nanoparticles in Wet Friction and Wear Properties of PEO Coatings Developed on Pure Titanium

The present study aims to explain how the incorporation of anatase TiO2 nanoparticles at three different concentrations, i.e., 1, 3, and 5 g/L, into the ceramic-like oxide plasma electrolytic oxidation (PEO) coatings on pure titanium substrate can affect the friction and wear behavior of the coatings in simulated body fluid (SBF) aqueous solution. For this purpose, a ball-on-disk friction and wear tester was utilized to characterize the wear performance of the PEO coatings. The morphology and dimensions (width and depth) of wear tracks were analyzed by scanning electron microscopy (SEM) and 2D depth profilometry, respectively. The results indicated that abrasive wear was identified in all PEO coatings; however, the coefficient of friction (COF), wear volume loss, and wear rate were strongly affected by the concentration of TiO2 nanoparticles. The coatings containing TiO2 nanoparticles presented a lower COF, less wear volume loss, reduced wear rate, and improved wear resistance due to having smoother surfaces and the presence of hard TiO2 nanoparticles on their surfaces and inside the pores. The coating with 3 g/L of TiO2 nanoparticles demonstrated the lowest wear rate value of 1.33 × 10−6 mm3/Nm (about a 32% reduction compared with that of coating without TiO2 nanoparticles) and the best wear protection properties among all coatings under investigation. The findings suggest TiO2 nanoparticles incorporated PEO coatings as a promising choice of surface treatment wherein the load-bearing capacity of titanium implants is critical.

Preparation and mechanical and biological performance of the Sr-containing microarc oxidation layer on titanium implants

A Strontium-doped titanium oxide coating was successfully prepared on a TA4 substrate with one-step microarc oxidation (MAO) technology. Then bioactivity of the coating was performed with the inducation test of bone-like apatite formation immersed in simulated body fluid (SBF). The biological activity of the coating was evaluated with cell culture experiments, and its corrosion resistance was characterized with electrochemical tests in the SBF. The composition and morphology of the coatings before and after SBF immersion were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The results showed that the coating had a rough porous structure, which was mainly composed of anatase, rutile and Ti phases. The SBF immersion test showed that abundant 3D micro nanopore structures were formed on the surface of the porous titanium oxide coatings, which enabled the coatings to exhibit good apatite nucleation ability. Faster precipitation of bone-like apatite by MAO-Sr coatings in SBF. Finally, in the biological compatibility test, compared with TA4, the contact angle between the MAO-Sr coating and SBF solution was lower, and the proliferation ability of bone marrow stromal cells (BMSCs) in the biological coating was stronger, indicating the good biocompatibility of the coating.

Effect of inserting the Zr layers on the tribo-corrosion behavior of Zr/ZrN multilayer coatings on titanium alloys

In this work, tribo-corrosion behavior of Zr/ZrN multilayer coatings in simulated body fluid (SBF) was investigated. The results indicated that Zr/ZrN-7 multilayer coatings possessed superior corrosion and tribo-corrosion resistance than the single ZrN coating. The effective inhibition of corrosive medium permeating by both forming a ZrO2 passive film and a multilayer structure contributed to the superior corrosion resistance. Furthermore, the improved corrosion, as well as suppressing crack propagation by the insertion of the Zr layer, enhanced the tribo-corrosion performance. Inserting Zr layers was found to be an effective method to improve the tribo-corrosion performance of Zr/ZrN multilayer coatings.

Corrosion, Wear, and Antibacterial Behaviors of Hydroxyapatite/MgO Composite PEO Coatings on AZ31 Mg Alloy by Incorporation of TiO2 Nanoparticles

Plasma electrolytic oxidation (PEO) is a promising surface treatment for generating a thick, adherent coating on valve metals using an environmentally friendly alkaline electrolyte. In this study, the PEO method was used to modify the surface of AZ31 Mg alloy. The composite coatings were formed in a phosphate-based electrolyte containing hydroxyapatite nanoparticles (NPs) and different concentrations (1, 2, 3, and 4 g/L) of TiO2 NPs. The results showed that the incorporation of TiO2 NPs in the composite coatings increased the porosity, coating thickness, surface roughness, and surface wettability of the coatings. The corrosion-resistance results of coatings in simulated body fluid (SBF) were tested for up to 72 h and all coatings showed superior corrosion resistance compared to the bare substrate. Among samples containing TiO2, the sample containing 1 g/L TiO2 had the highest inner layer resistance (0.51 kΩ·cm2) and outer resistance (285 kΩ·cm2) and the lowest average friction coefficient (395.5), so it had the best wear and corrosion resistance performance. The antibacterial tests showed that the higher the concentration of TiO2 NPs, the lower the adhesion of bacteria, resulting in enhanced antibacterial properties against S. aureus. The addition of 4 g/L of TiO2 NPs to the electrolyte provided an antibacterial rate of 97.65% for the coating.

Corrosion of Stirred Electrochemical Nano-Crystalline Hydroxyapatite (HA) Coatings on Ti6Al4V

Ti6Al4V substrates were electrochemically deposited with nano-crystalline hydroxyapatite (HA) from aqueous electrolytes. Cathodic HA coatings were obtained when the electrolyte was stirred using ultrasonic vibration. Two current densities of 20 mA/cm2 and 50 mA/cm2 were employed. Polarization and electrochemical impedance spectroscopy (EIS) were the techniques used to estimate the corrosion of coatings in simulated body fluid (SBF). The results indicate good corrosion resistance for the coating obtained at 50 mA/cm2 from ultrasonic stirring of the electrolyte.

The chitosan-based bioactive composite coating on titanium

This paper describes a promising electrophoretic deposition (EPD) approach for developing composite coatings based on chitosan with the additional components of hydroxyapatite, graphene, and gentamicin on titanium substrate. Bioactive properties were investigated in vitro by immersing the coatings in simulated body fluid (SBF) at 37 °C. The newly formed biomimetic layer on the top of the deposited chitosan-based coatings on Ti was confirmed by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and electrochemical measurements, while coatings' bioactivity was proved by alkaline phosphatase activity assay in MRC-5 and L929 tested cell lines. The biocompatibility towards MRC-5 and L929 cell lines was investigated by dye exclusion test (DET) implying the non-cytotoxic effect of coatings. Gentamicin release studies were monitored during 21-day by high-performance liquid chromatography coupled with mass spectrometry, indicating rapid release of gentamicin (≈40%) in the first 48 h and more than 60% after 14 days.

Effect of feedstock and plasma gun on the microstructure and bioactivity of plasma sprayed bioactive glass coatings

Plasma-sprayed 45S5 bioactive glass coatings were manufactured using two types of powder feedstocks (commercial and lab-made 45S5 glass) and two plasma torches (single and triple cathode) to analyse their influence on the microstructure and bioactivity of the coatings. Besides, the volatilisation of glass oxides such as Na2O and P2O5 during the deposition was studied. All coatings were microstructurally characterised by scanning electron microscopy, X-ray diffraction and X-ray dispersive energy analysis. Moreover, the bioactivity of the sprayed coatings was studied by immersing the coatings in Simulated Body Fluid until 14 days.The coatings obtained using the triple cathode torch showed similar thickness, less total porosity and greater microstructural homogeneity than the coating deposited with the single cathode torch. X-ray dispersive energy analysis revealed lower amounts of sodium and phosphorus at the surrounding of the lamellae, due to its volatilisation during the formation of the coatings. The volatilisation of these elements varied depending on the type of feedstock and plasma jet enthalpy. Concerning the bioactivity of the coatings, all of them have developed a hydroxycarbonate apatite layer on their surface.

Structure and tribocorrosion behavior of Ti/TiN multilayer coatings in simulated body fluid by arc ion plating

The TiN coating and Ti/TiN multilayer coatings were successfully fabricated on Ti6Al4V alloy by multi-arc ion plating. The Ti/TiN multilayer coatings with a total thickness of 4.6 μm and the TiN coating were prepared with a thickness of 5.6 μm. The XRD results indicate that the TiN phase of Ti/TiN multilayer coatings has a face-centered cubic crystal with the strong preferred orientation of (111) while the TiN coating has the face-centered cubic crystal structure with a preferred orientation of (200). For TiN coating, the hardness is (27.5 ± 1.9) GPa, the load-bearing capacity is (19.0 ± 3.6) N. For Ti/TiN multilayer coating, the hardness is (26.8 ± 1.7) GPa and the load-bearing capacity is (60.4 ± 4.0) N. The Ti/TiN multilayer coatings have excellent tribocorrosion resistance and lower coefficient of friction in simulated body fluid at 37 °C, compared with Ti6Al4V alloy and TiN coating, the tribocorrosion resistance of the multilayer coatings increased by approximately 58 and two times, respectively. The material loss which arises from tribocorrosion is not simply the summation of pure corrosion and pure wear loss, the synergism between corrosion and wear takes a large proportion of the total volume loss.

Zein/Bioactive Glass Coatings with Controlled Degradation of Magnesium under Physiological Conditions: Designed for Orthopedic Implants

Magnesium and its alloys are widely considered as temporary bio-implants owing to their mechanical properties and biocompatibility. However, the high corrosion rates and degradation in the physiological environment restrict the practical application of Mg as a biomedical device. Therefore, in this study, Zein/45S5 bioactive glass (BG) coatings were deposited via electrophoretic deposition (EPD) on pretreated pure magnesium (Mg) substrates, which controls the rapid degradation of magnesium. The set of EPD parameters was first optimized on stainless steel (SS) and then the optimum EPD parameters were applied to obtain zein/BG composite coatings on Mg substrates. The morphology of the obtained coatings was studied by scanning electron microscopy (SEM). SEM results showed that both zein and BG were successfully deposited on the surface of the Mg substrate. Electrochemical measurements consisting of open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization confirmed that the corrosion resistance of Mg improved after the deposition of zein/BG coatings. The in-vitro bioactivity study was carried out by immersing the zein/BG coatings in simulated body fluid for 3, 7, and 21 days. SEM, energy dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy results elucidated that the hydroxyapatite layer developed after 21 days of immersion in SBF, which confirmed the bone binding ability of the coatings.

Enhanced corrosion resistance and surface bioactivity of AZ31B Mg alloy by high pressure cold sprayed monolayer Ti and bilayer Ta/Ti coatings in simulated body fluid

Ti and Ta/Ti coatings were successfully applied on AZ31B Mg alloy using high pressure cold spray process. These coatings (particularly Ta/Ti coating) with high hardness lowered the wear rate of AZ31BMg alloy. In-vitro bioactivity test, infinite focus 3D measurement and EDS results revealed that the calcium-phosphate (Ca–P) compounds nucleation and growth capabilities on both the coatings surface are higher than AZ31B Mg alloy surface due to the highly passivating nature and rough surface (island-like) of the high pressure cold sprayed Ti and Ta/Ti coatings, which are further conducive to the biological fixation. Electrochemical corrosion and immersion tests disclosed that a dense layer of Ta on Ti coating was able to enhance the corrosion resistance of Ti/AZ31B Mg system in the Hank's balanced salt solution. Rct (charge transfer resistance) and │Z│f=0.01Hz values for Ta/Ti coated Mg alloy have remained slightly stable. This imparted that the rate of electrochemical processes at the electrode/electrolyte interface could remain very slow in the course of immersion time. Likewise, the OCP (open circuit potential) values of Ta/Ti coating remained constant during long term electrochemical corrosion test. This behavior could originate from fairly dense structure of the high pressure cold sprayed Ta layer which considerably impeded the penetration of corrosive solution into the coating interiors. Ta/Ti coating presented the most positive potential, highest Rct, and highest│Z│f=0.01Hz values, suggesting that it is the most thermodynamically stable/least to be corroded in Hank's balanced salt solution over time. In general, cold sprayed Ta/Ti coating might open a new way to broaden the utilization of Mg alloys in the field of biomaterials.

Pulsed electrodeposition of compact, corrosion resistant, and bioactive HAp coatings by application of optimized magnetic field

NiTi shape memory alloy possess spectacular properties, so it has a great potential to use as bio-implants. However, releasing toxic Ni ions from its surface have restricted its successful use in human's body. The present research attempts to modify the surface conditions of NiTi through deposition of calcium phosphate (CaP) coatings to overcome the abovementioned challenge, besides improving the other critical characteristics required for successful use of NiTi in bio-related applications. CaP deposits were deposited on NiTi by means of pulsed electrodeposition under magnetic field. Since the hydroxyapatite (HAp) has the closest Ca/P ratio to that of human's bone among the other CaP types, and considering its superior bioactivity, the processing variables including pulse current density, deposition time, as well as magnetic field direction and magnitude were adjusted in such a way that the Ca/P ratio of HAp obtains. The aim behind application of magnetic field was to attain favorable morphological and structural properties. Then, the characteristics of coatings electrodeposited under optimized conditions were investigated via Fourier transform Infrared (FTIR) and X-ray diffraction (XRD). Electrochemical potentiodynamic polarization studies test in Ringer's solution was performed to assess the corrosion behavior of the coatings. A noticeable increment in polarization resistance of NiTi, about 10 times, is obtained with deposition of the optimized coatings. Ni ion release was found to decrease by 89% after deposition of HAp coatings on NiTi. In vitro bioactivity of the coatings were studied through immersion of HAp coatings in simulated body fluid (SBF). Nucleation of apatite particles all over the coating microstructure demonstrate the desirable bioactivity of the deposits. In general, compact, corrosion resistive, and bioactive HAp coating with minimal Ni ion release were deposited on NiTi by controlling the operating conditions.

Preparation and Degradation Behavior of Composite Bio-Coating on ZK60 Magnesium Alloy Using Combined Micro-Arc Oxidation and Electrophoresis Deposition

In this work, ZK60 magnesium alloy was employed as the substrate for producing micro-arc oxidation (MAO) coating containing Ca and P. Electrophoretic deposition (ED) process was conducted on the micro-arc oxidized sample to prepare a hydroxyapatite (HA) layer on the original coating. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and Fourier transform infrared spectroscopy (FT-IR) were used to analyze the phase constituents and microstructures of both MAO and MAO-ED coatings. Corrosion resistance and degradation behavior of the coatings in SBF were investigated by electrochemical tests and simulated body fluid (SBF) immersion tests. The results indicate that a dense HA layer about 5μm in thickness has been successfully prepared on the MAO coating. The porosity of the MAO-ED coating decreases from 5.63% before ED process to 0.81%. The Ca/P ratio of the MAO-ED coating is 1.60, indicating a potential biocompatibility. The MAO-ED coating can induce the deposition of bioactive products from SBF. During 10-day SBF immersion test, the weight gain of MAO-ED sample keeps increasing, indicating that the deposition rate of the induced products is always higher than that of corrosion products before the 10 days of immersion. The deposition of induced products effectively protects the substrate from being corroded so that it ensures the good mechanical properties and biocompatibility during the initial stage of implantation. According to the changes in sample morphologies and the electrochemical measurements in SBF, a relevant degradation model has been suggested and the underlying mechanism for degradation behavior is discussed.

Corrosion behaviour of plasma sprayed graphene nanoplatelets reinforced hydroxyapatite composite coatings in simulated body fluid

Hydroxyapatite (HA) coatings, reinforced with varied concentration (0–2 wt%) of Graphene nanoplatelets (GNPs) have been deposited on titanium alloys (Ti–6Al–4V) substrate using atmospheric plasma spraying. Present work studies the effect of GNP concentration on the electrochemical behaviour of the HA coatings in simulated body fluid (SBF). The HA coating exhibited 15% porosity, whereas reinforcement of 1 wt% GNPs in HA (HA-1G) shows 13% porosity, further addition of 2 wt% GNPs in HA reduced the porosity to 10%. Reduction in porosity was achieved as GNPs easily accessed the inter-lamellae to fill the gaps at inter splat region and minimized the occurrence of post-plasma spray defects such as porosity, voids, microcracks etc. These consequences nextward resulted in the significant enhancement in corrosion resistance of the matrix. HA-1G displayed a significant reduction by 67% in the corrosion rate in SBF solution, while this reduction came to 87% for HA-2G coatings. Randomly oriented wrinkles in the GNPs after corrosion process and their hydrophobic nature effectively hindered the SBF infiltration into the coating and resisted their movement towards the underlying substrate. This in turn improved the overall corrosion resistance of the system.