Electrophoretic Deposition, Microstructure, and Corrosion Resistance of Porous Sol–Gel Glass/Polyetheretherketone Coatings on the Ti-13Nb-13Zr Alloy
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In this study, microporous composite sol–gel glass/polyetheretherketone (SGG/PEEK) coatings were produced on the Ti-13Nb-13Zr titanium alloy by electrophoretic deposition. Coatings with different levels of high open porosity were developed by introducing SGG particles of varying diameters into the PEEK matrix. The microstructure of the coatings was characterized by electron microscopy and X-ray diffractometry. The coatings with 40-50 µm thickness were composed of semicrystalline SGG particles consisting of hydroxyapatite, CaSiO3, some Ca2SiO4, and an amorphous phase containing Ca, Si, P, and O, homogeneously embedded in a semicrystalline PEEK matrix. The size of SGG particles present in the coatings strongly influenced the formation of microcracks and their adhesion to the underlying substrate. Microscratch tests showed that the coating containing SGG particles with a diameter smaller than 45 µm and open porosity of 45 pct exhibited good adhesion to the titanium alloy substrate, much better than the coating containing particles with a diameter smaller than 85 µm and total open porosity equal to 48 pct. The corrosion resistance was investigated in Ringer’s solution at a temperature of 310 K (37 °C) for a pH equal to 7.4 and in deaerated solutions with the use of open-circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy. The SGG/PEEK-coated alloy indicated better electrochemical corrosion resistance compared with the uncoated alloy.
KeywordsTitanium Alloy Bioactive Glass Open Porosity Electrophoretic Deposition Coating Adhesion
Titanium alloys are the most frequently used metallic materials for biomedical applications due to their high strength-to-weight ratio, high fatigue resistance, and good biocompatibility.[1,2] Today, the most important titanium alloys, which have found applications in medicine, are β alloys. They exhibit low elastic modulus and density, high strength, and ductility as well as good electrochemical corrosion resistance.[3, 4, 5] However, their use in medicine is limited not only by their relatively low hardness and poor tribological properties, but also by the very slow osseointegration between implants and surrounding bone tissues.[6,7] Titanium alloy osseointegration might be enhanced by surface treatment involving the deposition of porous coatings containing bioceramics, biopolymers, and their combinations. The porous coatings improve osseointegration by providing more space for bone growth. Moreover, the bond between the biomaterial and bone becomes stronger.[8,9] Porosity, pore diameter, distribution, and interconnectivity are important parameters, which determine the biomaterial bioactivity. In general, open porosity higher than 50 pct and interconnected pores with a mean diameter of 100 µm or higher are considered to be the minimum requirements to permit tissue ingrowth.[11,12] For example, Hadjicharalambous et al. showed that about 50 pct porosity and an average pore size of 150 μm are beneficial for cellular growth in zirconia ceramics. The important limitations to the use of materials, especially ceramics, with high porosity and pore size are their low mechanical properties. Thus, a good response to the improvement of the mechanical properties of highly porous coatings may be the introduction of a polymer and the deposition of composite polymer-based coatings incorporating ceramic particles.
Electrophoretic deposition (EPD) is a surface engineering method, which enables the codeposition of different ceramic or/and polymeric materials, producing dense or porous composite polymer-based coatings with high homogeneity and tailored thickness.[15,16] EPD consists of the movement of charged particles suspended in liquid and deposition onto a conducting substrate under the influence of an externally applied electrical field.[17,18] The advantages of this method are high purity, easy control of the coating thickness, high coating uniformity, the possibility of using complex shaped substrates, and the short deposition time of coatings.[19,20]
One of the popular materials with useful properties for a relatively strong porous coating matrix is polyetheretherketone (PEEK). PEEK is a nontoxic and bioinert material.[16,21,22] It is a crystallizable aromatic polymer with very good thermal and mechanical properties. This polymer is used to replace metal implant components, especially in long-term orthopedic applications. Furthermore, PEEK is noncytotoxic and can be repeatedly sterilized without evident deterioration of its mechanical properties.
The osseointegration process of titanium alloy might be enhanced by using a bioactive glass or glass-ceramic as a bioactive coating component. A typical feature common to all bioactive glasses, both melt or sol–gel derived, is the ability to interact with living tissue, in particular forming strong bonds to bone. Bioactive glasses are amorphous silicate-based materials exhibiting osteoconductive/osteoinductive properties.[10,25,26] They are promising materials for bone tissue engineering applications due to their excellent bioactivity, biocompatibility, and osteogenicity properties.[27,28] Bioactive glass stimulates new bone growth and, once implanted in the body, can react with physiological fluids and form a strong bond with bones. Its bioactivity is associated with the formation of a carbonated hydroxyapatite layer (HCA) on its surface, similar to the bone mineral.[29,30]
Glass-ceramics with crystalline or semicrystalline structures are produced by the transformation of the glass into a ceramic. The sol–gel glasses offer several advantages compared with the melt-delivered glasses. Due to their nonporous texture, the sol–gel-derived glasses exhibit a high specific surface area in comparison with melt-delivered glasses. The most significant advantage of sol–gel glasses is the presence of Si-OH groups in their structure, which are thought to play a role in HCA layer nucleation. Thus, their bioactivity is usually higher than that of bioactive glasses prepared by melting. Gel-derived bioactive glasses influence the faster formation of a strong bond with tissue, and also stimulate bone into regeneration.
According to our knowledge, there are very few papers available in the literature on the EPD of composite bioglass/PEEK coatings.[15,33] The main focus of the referred studies was the investigation of EPD parameters and the final quality of the composite coatings. However, there is no information about the EPD of sol–gel glass/PEEK coatings. The originality of this study is incorporation of the gel-derived glass-ceramic with a high surface area into the PEEK matrix and deposition of microporous coatings on near-β titanium alloy, as well as comparison of the sol–gel glass/PEEK coating microstructures and adhesion behaviors to the substrate with the bioglass/PEEK coatings deposited in our previous study.
The aim of this study was electrophoretic deposition of microporous SGG/PEEK coatings on the Ti-13Nb-13Zr alloy, as well as the characterization of the microstructure and selected properties, such as adhesion of the coatings to the titanium alloy substrate and electrochemical corrosion resistance in Ringer’s solution.
2 Methods and Materials
Composite sol–gel glass SGG/PEEK coatings were electrophoretically deposited on a near-β Ti-13Nb-13Zr alloy. The alloy, in the shape of a bar, was delivered by Xi’an Saite Metal Materials Development Co., Ltd., China. The microstructure of the alloy was described in detail in our previous paper. It was composed of α′ (hexagonal close-packed; hcp), β (body-centered cubic; bcc), and a small amount of α″ phase (orthorhombic, Cmcm space group). Disks of 27 mm diameter and 2 mm thickness were used as substrates for EPD. The samples were ground with sandpaper of 2000 grit and then cleaned with distilled water and ethanol.
The PEEK powder and SGG particles were used as coating components. The PEEK powder (VICOTE®704) was delivered by Victrex Europa GmbH, Germany. It has a melting point of about 616 K (343 °C), a crystallization peak at 433 K (160 °C), and a glass transition temperature of 416 K (143 °C). The bioactive sol–gel glass with nominal composition (mol pct) 54CaO-6P2O5-40SiO2 (A2) was produced by the sol–gel method described previously in the literature.[27,31] In the alcohol-based system, the following compounds were used as the starting materials: tetraethylorthosilicate (TEOS, Si(OC2H5)4), triethylphosphate (TEP, OP(OC2H5)3) (Sigma-Aldrich, USA), and calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) (POCH, Poland). HCl solution (1 M) (POCH, Poland) and distilled water were applied as the catalyst of the hydrolysis and polycondensation reactions. In brief, the first step was to mix and stir TEOS and ethanol in particular amounts. Subsequently, the appropriate volumes of distilled water and hydrochloric acid were added to the mixture, and next the TEP and calcium nitrate (dissolved in distilled water) were added gradually to the solution. After the addition of each reactant, the solution was stirred for 1 hour at room temperature (RT). The final solution was left at RT until the gel was formed. Subsequently, the gel was dried at 353 K (80 °C) for 72 hours and then heated up to 973 K (700 °C) for 20 hours. Afterward, it was sieved to obtain sol–gel glass powders with two different particle sizes: SGG1 with a diameter up to 45 µm and SGG2 with a diameter up to 85 μm. The SGG1 and SGG2 powders were used as a component materials for the EPD of composite coatings.
Different suspensions of SGG and PEEK powder in ethanol were used for coating deposition. They were prepared by adding 3.2-3.5 g of SGG1 or SGG2, 1.5 g of PEEK powder, and 1-5 g of citric acid to the 50 mL of ethanol and sonicating the mixture in an ultrasonic bath for 5 minutes in order to disperse the particles. Before deposition, the suspensions were mixed for 3 minutes using a magnetic stirrer. The pH values were measured using a Mettler Toledo EL20 pH-meter (China). The changes of pH in the suspensions were realized by adding different amounts of citric acid (C6H8O7). The EPD was carried out under constant voltage in the range of 30-70 V, and the deposition time ranged from 60 to 120 seconds. The distance between electrodes in the EPD cells was maintained at 10 mm. After EPD, the coated samples were dried at room temperature and subsequently heated at a temperature of 628 K (355 °C) for 20 minutes, heating rate 7.9 K/min (4.5 °C/min), then cooled using the furnace.
The microstructure of the coatings was investigated by scanning and transmission electron microscopy (SEM, TEM) as well as X-ray diffractometry (XRD). The SEM investigation was performed using a Nova NanoSEM 450 (FEI, the Netherlands). The TEM investigation was carried out using a JEOL JEM-2010 ARP microscope (Japan). The cross-sectional lamella for TEM investigation of coating microstructure was prepared by a focus ion beam (FIB) using an FEI QUANTA 3D 200i device (the Netherlands). Phase identification was performed by means of selected area electron diffraction (SAED) and XRD (Bragg-Brentano method). The XRD patterns were recorded using a Panalytical Empyrean DY 1061 diffractometer (the Netherlands). The diffractometer was operated with Cu Kα radiation at the 2θ range of 10 to 70 deg, using plan-view specimens. The total open porosity was determined by hydrostatic weighing based on the Archimedes principle. The test was performed for three porous specimens, and the average value was calculated. Tomographic datasets were obtained using the “slice and view” technique using a FEI Versa 3D dual-beam scanning electron microscope (the Netherlands) equipped with the AutoSlice and View™ G3 software. A regular cross section was milled using a Ga+ LMIS source at 30 kV and an ion current of 1 nA, and a voxel size of 36 × 36 × 120 nm. SEM imaging with an ETD detector at 5 kV was applied for 3D visualization of open porous SGG1/PEEK coating. The digital processing of data stacks and 3D visualization of reconstructed volume were performed using ImageJA 1.45b and Avizo Fire 6.3 software. The coating thickness was measured by contact profilometry using a CSM Instruments Micro-Combi Tester (MCT), Switzerland. The 5-mm trace length started in the uncoated area and finished on the coating surface. The difference in the recorded heights in these areas was equal to the coating thickness.
The adhesion of the coatings to the titanium alloy was investigated by the microscratch test method using the MCT. The microscratch tests were carried out using a Rockwell C indenter with a cone apex angle of 120 deg and tip radius of 200 μm. Tests were performed within the distance of 5 mm and with a load of 30 N. The critical loads Lc1 (cohesive cracks) and Lc2 (adhesive cracks) were determined from light microscopy (LM) observations, acoustic emission, and friction force signals. Average values of critical loads were calculated from three scratches.
The electrochemical studies of the samples were carried out using an Autolab PGSTAT302N potentiostat (the Netherlands). Ringer’s solution was used as the electrolyte for the corrosion study. The chemical composition of Ringer’s solution was as follows: 8.6 g NaCl, 0.3 g KCl, and 0.25 g CaCl2. The tests were carried out at a temperature of 310 K (37 °C). Electrochemical measurements were performed for a pH equal to 7.4 and in deaerated solutions. The linear sweep voltamperometry curves were recorded at a scan rate of 1 mV/s in the potential range from −1.3 to +1.5 V. The investigations were performed using a classical three-electrode cell, where the working electrode was a titanium alloy. Potentials were measured vs saturated calomel electrode (SCE), and the counter electrode was made of platinum wire. Electrochemical impedance spectra were acquired at the open-circuit potential. The amplitude of the perturbation signal was 10 mV, and EIS spectra were plotted in the frequency range from 105 to 10−3 Hz. The EIS data were fitted using the ZView software.
3 Results and Discussion
3.1 Electrophoretic Codeposition of SGG and PEEK
The electrophoretic deposition depends on (i) parameters related to the suspension including chemical composition, morphology, and particle size and the type of solvent, as well as (ii) the physical conditions of the deposition process such as potential difference and time. Particle size and morphology are important parameters of the EPD process. For stable suspensions, it is important that the particles remain well dispersed and stable to form homogeneous and uniform deposits. The problem with large particles, with a diameter greater than ~20 μm, is the tendency to undergo sedimentation. It is difficult to obtain uniform deposition from a sedimenting suspension of large particles. Therefore, continuous stirring of suspensions containing large particles is required during the coating deposition. To deposit coatings with different open porosity, two types of relatively large SGG particles with equivalent circle diameters of (ECD)—SGG1 < 45 μm and SGG2 < 85 μm—were used for the EPD of coatings in the present study.
The PEEK powder exhibited a nearly amorphous structure (Figure 2(b)); however, a low amount of crystalline phase was also present. Four weak diffraction peaks at 2θ angles of 18.68, 20.74, 22.92, and 29.15 deg occurred in the spectrum. The particles had a nearly spherical shape and ECD ranging from 2 to 15 μm (Figure 1(d)).
Electrophoretic codepositions of SGG and PEEK particles were carried out from the EtOH-based suspensions. During EPD, both the SGG and the PEEK particles were positively charged and moved toward the cathode (substrate material) under the electric field. Due to the large sizes of both the SGG and the PEEK particles, it was impossible to measure their zeta potential and electrophoretic mobility in EtOH using the laser Doppler microelectrophoresis method. Thus, the suspension’s chemical composition and EPD parameters were selected by trial and error. A similar chemical composition of the suspension, as well as similar deposition voltage and time to that used in our previous study for deposition of the bioglass/PEEK coatings on the Ti-6Al-7Nb alloy, was applied.
3.2 Microstructure of the SGG/PEEK Coatings
Both, the SGG1/PEEK and SGG2/PEEK coatings, were highly porous with pore diameters being up to 35 and 90 μm, respectively. The mean open porosities determined by hydrostatic weighing based on the Archimedes principle were equal to 45 ± 2 and 48 ± 2 pct for SGG1/PEEK and SGG2/PEEK, respectively.
3.3 Adhesion of the Coatings to the Ti-13Nb-13Zr Alloy Substrate
3.4 Corrosion Resistance
EIS Fitting Results Obtained from EIS Data
R 1 (Ω cm2)
CPE-T (Fs n−1 cm−2)
R 2 (Ω cm2)
3.8235 × 10−5
SGG1/PEEK on Ti-13Nb-13Zr alloy
6.5334 × 10−6
The SGG1/PEEK coating on the Ti-13Nb-13Zr alloy had a slightly higher CPE-P value which was closest to the capacitance (typical of passive coatings). The resistance (R2) of the SGG1/PEEK coating on the Ti-13Nb-13Zr alloy was higher than that of the uncoated Ti-13Nb-13Zr alloy, which suggested that the SGG1/PEEK coating on the Ti-13Nb-13Zr alloy had better corrosion resistance than the uncoated alloy.
Microporous SGG/PEEK coatings were successfully deposited on the Ti-13Nb-13Zr alloy by EPD. The coating homogeneity and uniformity depend strongly on the chemical composition of the suspension as well as on the electrophoretic voltage and time. The best-quality, homogeneous coatings were deposited from a suspension composed of 3.2 g sol–gel glass, 1 g PEEK, and 5 g of citric acid in EtOH at a potential difference of 50 V for 120 seconds.
The thicknesses of both types of coating were similar and in the range from 40 to 50 μm. Both the SGG1/PEEK and SGG2/PEEK coatings consist of HA (hp), CaSiO3 (tp), a small amount of Ca2SiO4 (op), and amorphous glass particles, with ECDs of up to 45 μm and up to 85 μm, respectively, embedded in a semicrystalline PEEK matrix. The PEEK crystallinity increased as a result of heating of the coated specimens above the PEEK melting point.
Both the SGG1/PEEK and SGG2/PEEK coatings were highly porous with pore diameters up to 35 μm and up to 90 μm, respectively. The mean volumetric open porosities, determined by hydrostatic weighing based on the Archimedes principle, were 45 and 48 pct, respectively.
The SGG1/PEEK coating exhibited very good adhesion to the titanium alloy substrate. The first adhesive cracks were observed after scratching under a load of 9 N. The SGG2/PEEK coating exhibited lower adhesion to the substrate due to the presence of large-sized SGG particles and deep microcracks bared to reveal the underlying substrate material.
The SGG1/PEEK coated alloy exhibited better corrosion resistance in Ringer’s solution compared with the uncoated alloy. This results from coating uniformity and homogeneity as well as its very good adhesion to the substrate material.
This study shows that electrophoretic deposition is very useful to deposit microporous composite sol–gel glass/PEEK coatings well adhered to titanium alloy substrates used in medical applications, and also improve the corrosion resistance of titanium alloy in Ringer’s solution.
This study was supported by the Polish National Science Centre (decision no. DEC-2013/09/B/ST8/00145). The authors appreciate the valuable contribution of Dr Ł. Cieniek (AGH University of Science and Technology) to SEM and DSc M. Kot (AGH-UST) during scratch tests.
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