Summary of Chemistry doctoral thesis: Electrodeposition of hydroxyapatite/modify carbon nanotubes coating on alloys to apply for bone implants

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Objectives of the thesis: Selecting of suitable conditions to synthesize HAp-CNTsbt nanocomposite coating on 316LSS and Ti6Al4V; HAp-CNTbt coating has biocompatibility and protection ability for the substrate in comparison with HAp coating.

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Nội dung Text: Summary of Chemistry doctoral thesis: Electrodeposition of hydroxyapatite/modify carbon nanotubes coating on alloys to apply for bone implants

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY INSITUTE FOR TROPICAL TECHNOLOGY SUMMARY OF PhD THESIS IN CHEMISTRY ELECTRODEPOSITION OF HYDROXYAPATITE/MODIFY CARBON NAOTUBES ON ALLOYS TO APPLY FOR BONE IMPLANTS Specialization: Theoretical and Physical chemistry Code: 9 44 01 19 Hanoi 2019 1
The dissertation completed at: Academic supervisors: 1. 2. Reviewer 1: Reviewer 2: Reviewer 3: 2
INTRODUCTION  Reason to choose the topic Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is the main inorganic component in human bones and teeth, has high biocompatibility. HAp is applied in medicine with different forms: powder, ceramic, composite and coating. Synthetic HAp has the same composition in natural bone and has good biocompatibility. However, pure HAp coating has a relatively high solubility in physiological environment and poor mechanical properties leading to faster degradation of the material and reducing the fixed ability between the implant material and the host tissue. Some reports show that the doping of carbon nanotubes to create HAp- CNTcomposite significantly improves the mechanical properties of materials such as corrosion resistance and mechanical strength. The thesis: "Electrodeposition of hydroxyapatite/modify carbon nanotubes coating on alloys to apply for bone implants" shows investigation to synthesize HAp-CNTsbt coating on 316LSS and Ti6Al4V.  Objectives of the thesis: - Selecting of suitable conditions to synthesize HAp-CNTsbt nanocomposite coating on 316LSS and Ti6Al4V. - HAp-CNTbt coating has biocompatibility and protection ability for the substrate in comparison with HAp coating. • Main content of the thesis: 1. Study on effect of the scanning potential range, scanning rate, number of scans, CNTbt amount, and synthesis temperature on the characteristics of HAp-CNTbt coating. Selecting of suitable conditions for synthesie HAp-CNTbt/316LSS and HAp-CNTbt/ Ti6Al4V materials. 2. Determination of roughness, elastic modulus and hardness of 316LSS, Ti6Al4V, HAp/316LSS, HAp/Ti6Al4V, HAp-CNTbt/316LSS, and HAp-CNTbt/Ti6Al4V. Determination of dissolutione of HAp and HAp-CNTbt coating in 0.9% NaCl solution. 3. Research on biocompatibility and electrochemical behavior of 316LSS, Ti6Al4V, HAp/316LSS, HAp/Ti6Al4V, HAp-CNTbt/316LSS, and HAp-CNTbt/ Ti6Al4V in SBF solution. CHAPTER 1: OVERVIEW 1.1. Overview for Hydroxyapatite 1.1.1. Properties of Hydroxyapatite 1.1.1.1. Structural properties Hydroxyapatite (HAp) exists in two structural forms: hexagonal (hexagonal) and monoclinic (monoclinic). Hexagonal HAp is usually formed during synthesis at temperatures between 25 and 100 °C. The monoclinic form is mainly created by heating the hexagonal HAp at 850 °C in air, then cooling to room temperature. 1.1.1.2. Physical properties HAp exists in crystal with some parameters: molar mass of 1004.60 g, density of 3.08 g/cm3, hardness in the Mohs scale by 5, melting temperature of 1760oC, and boiling temperature 2850 oC. The dissolution of HAp in the water is 0.7 g/L. HAp crystals usually have rod- shape, needle-shape, scale-shape, fibrous-shape, spherical-shape, and cylindrical-shape. 3
1.1.1.3. Chemical properties • HAp reacts with acids to form calcium salts and water. • HAp is relatively thermally stable, which is decomposed slowly at temperature range of 800°C ÷ 1200°C, to form oxy-hydroxyapatite. • At bigger temperatures (> 1200°C), HAp is broken down to β - Ca3(PO4)2 (β - TCP) and Ca4P2O9 or CaO. 1.1.1.4. Biological properties HAp has a high biological compatibility, non-toxic, non-allergenic to the human body and has high antiseptic properties. 1.1.2. Methods of synthesis of hydroxyapatite coating a. Physical method The physical method is the method of creating HAp coating from ions or phase transition. These methods have the advantage of being able to easily fabricate HAp coating with a thickness of µm. Several physical methods are used: plasma, vacuum evaporation and magnetron sputtering [2, 37]. b. Electrochemical method Electrochemical method has many advantages in making thin coating on metal or alloys for biomedical applications. Electrochemical technique is a simple technique that allows the synthesis of HAp coating at low temperatures. The obtained HAp coating is of high purity, good adhesion to the substrate and we can control the coating thickness. HAp coating with thickness of nm size are synthesized on different substrates by electrochemical method such as: Electrophoresis method, Anode method, Cathode deposition method. 1.1.3. Application of HAp 1.1.3.1. Application of HAp powder HAp powder with nano size is mainly used for medicine and calcium supplements. In addition, HAp is used as a slow-release nitrogen fertilizer for plants. 1.1.3.2. Application of HAp porous ceramic Porous ceramic of HAp is used in making dentures and repairing dental defects, making artificial eyes, making bone graft details and repairing bone defects. 1.2.3.3. Application of HAp composite HAp is combined with biodegradable polymers such as polylactic acid, polyacrylic acid, chitosan ... to create replacement materials for bone. 1.2.3.4. Application of HAp coating HAp coating on the suface of biomedical materials is applied in dentistry, orthopedic bone. 1.2. Overview of carbon nano tubes materials 1.2.1. Properties 1.2.1.1. Structure of CNT: CNT are graphene sheets which are rolled up to form a hollow cylinder. Depending on rolling direction, CNT materials are divided into armchair, zigzag and chiral types. 1.2.1.2. Physical properties 1.2.1.2.1. Mechanical properties CNT have a good mechanical property, durable and low density. So, they are used as reinforcement for rubber, polymer, and metals to improve durability and abrasion resistance for materials. 1.2.1.2.2. Electrical properties 4
The electrical properties of CNT depend strongly on its structure. The electrical conductivity of CNTs is corresponding to semiconductor or metal. 1.2.1.2.3. Thermal properties CNTs are good thermal conductors, at room temperature the thermal conductivity of CNTs is about 3,103 W / m.K. 1.2.1.2.4. Emisivity properties The electronic emissivity ability of CNT is very high. 1.2.1.2. Chemical properties About chemical property, CNT are relatively inert. To improve the chemical activity of CNT, CNT usually are modified to create surface defects. 1.2.2. Application of CNT materials CNT are used in energy storage, electronic, reinforcing materials and medical applications (CNT are used in biosensors, drug delivery, and nanotechnology application for bone implants). 1.2.3. Modification of CNT - CNT is modified by oxidizing agents, a combined reaction, and substitution reaction 1.3. Composite of hydroxyapatite/carbon nano tubes (HAp-CNTbt) HAp-CNTbt composite is synthesized by many different methods. The research results show that the presence of CNT improved the mechanical properties for HAp by the increase of elastic modulus and the hardness. 1.4. In vitro and In vivo tests The results of biocompatibility of HAp-CNT in Hanks solution or simulated body fluid solution (SBF) show that the material has good biocompatibility with the development of new apatite crystals. The results of in vitro test by cells (osteoblast) showed that there is a good growth. 1.5. Investigation in Vietnam In Vietnam, there are some reports about HAp powder, coating, ceramic and composite. Since 2011, Dinh Thi Mai Thanh et al. (Institute of Tropical Technology) investigated on HAp powder, PLA/HAp composite and HAp coating on the surface of 304LSS, 316LSS, TiN/316LSS, Ti6Al4V and CoNiCrMo. We realize that, investigation of HAp-CNT coating is quite new in Vietnam. This study aims are selection of suitable conditions to synthesize HAp-CNTcoating on the surface of 316LSS, Ti6Al4V substrates by scanning potential method. Chapter 2. CONDITION AND EXPERIMENTAL METHOD 2.1. Chemicals and experimental conditions 2.1.1. Chemicals - Ca(NO3)2.4H2O, NH4H 2PO4, NaNO3, NaCl, NaHCO3, KCl, Na2HPO4.2H2O, MgCl2.6H2O, CaCl2, KH2PO4, MgSO4.7H2O, C6H 12O 6, NH4OH, HCl, HNO3 67 % and H2SO4 98 %. CNTs: 90 % of pure, d = 20 – 100 nm, L = 1 - 10 µm is produced at Insitute of Material Science. - The materials of 316LSS (100×10×2 mm) and Ti6Al4V (12×10×2 mm) were purchased from Gloria Technology Material Company (Taipei, Taiwan) with element components are listed in Table 2.1 and 2.2. Table 2.1. The element component of 316LSS Element Al Mn Si Cr Ni Mo P Fe Component (%) 0.3 0.22 0.56 17.98 9.34 2.15 0.045 69.405 5
Table 2.2. The element component of Ti6Al4V Element Ti Al V C Fe Component (%) 89,63 6,04 4,11 0,05 0,17 2.1.2. Electrodeposition of HAp-CNTon 316LSS or Ti6Al4V * Preparation of substrate: 316LSS and Ti6Al4V were polished by SiC paper of 600, 800 and 1200 (Japan). After that, they were clearned and dried. The working area was limited of 1 cm2 by epoxy. * Modification of CNTs: 4 g CNTs were put in a container containing 200 ml of H2SO4 and HNO3 (3:1) acids with 1 h of ultrasonic. Then, the mixtre was heated at 110oC for 1 h using a concender. CNTs were obtained using centrifuge to neutral pH and dried at 80oC for 48 h. Then, 0.05 g CNT or CNTbt was dispersed into two tubes containing 50 mL of Ca(NO3)2 3x10-2 M, NH4H2PO4 1.8x10-2 M, NaNO 3 0.15 M solution (the solution is used to synthesize HAp coating) with pH o = 4.3 and ultrasound for 20 minutes. These two tubes were left on the shelf for 7 days to observe the dispersion of CNT or CNTbt. The pH of the two solutions containing CNT and CNTbt were also measured. * The different conditions to synthesize HAp and HAp-CNTsbt coating. HAp and HAp-CNTsbt coating were synthesized by scanning potential method. The electrolyte solution contains 3x10-2 M Ca(NO3)2, 1.8x10-2 M, NH4H 2PO4, and 0.15 M NaNO3. The coatings were synthesized in a cell of three electrodes: the working electrode is 316LSS or Ti6Al4V, The counter electrode of Platinium; and reference electrode of Ag/AgCl (SCE). The factors investigated: Table 2.3. The conditions to synthesize HAp-CNTbt/316LSS and HAp-CNTbt/Ti6Al4V Survey factor Fixed installation factor 1 - Scanning potential: 0 ÷ -1.4 V; Scanning rate of 5 mV/s, 5 scans, 45 oC 0 ÷ -1.5 V; 0 ÷ -1.6 V; 0 ÷ -1.65 V; 0 ÷ - and CNTbt 0.5 g/L. 1.7 V; 0 ÷ -1.8 V; 0 ÷ -1.9 V; 0 ÷ -2.0 V and 0 ÷ -2.1 V 2 - Concentration of CNTbt: 0.25; 0.5; 0.75 0 ÷ -1.65 V (for 316LSS); and 1 g/L 0 ÷ -2.0 V (for Ti6Al4V); 5 mV/s; 5 scans, 45 oC. 3 - Synthesis temperature: 30, 45, 60 oC 0 ÷ -1.65 V (for 316LSS); 0 ÷ -2.0 V (for Ti6Al4V); 5 mV/s; 5 scans, CNTbt 0.5 g/L 4 - Scanning rate: 2, 3, 4, 5, 6 and 7 mV/s. 0 ÷ -1.65 V (for 316LSS); 0 ÷ -2.0 V (for Ti6Al4V); 5 scans, 45 oC, CNTbt 0.5 g/L 5 - Scanning times: 3, 4, 5 and 6 scans 0 ÷ -1.65 V (for 316LSS); 0 ÷ -2.0 V (for Ti6Al4V); 5 mV/s; 45 oC, CNTbt 0.5 g/L 2.1.3. In vitro test in SBF solution 1 L of SBF solution containing: NaCl (8 g/L); NaHCO3 (0.35 g/L); KCl (0.4 g/L); Na2HPO4.2H2O (0.48 g/L); MgCl2.6H 2O (0.1 g/L); CaCl2 (0.18 g/L); KH 2PO4 (0.06 g/L); MgSO4.7H2O (0.1 g/L) and glucozo (1 g/L). The initial pH is 7.3. Electrochemical behavior 6
of the materials in 50 ml of SBF solution was carried in the cell of three electrodes, at 37 ± 1 o C. 2.2. Methods 2.2.1. Electrochemical methods Dynamic scanning method, Method of measuring open-circuit potential and Electrochemical Impedance Spectroscopy. 2.2.2. Analysis methods Characteristics of these materials were determined by IR, SEM, EDX, TEM, XRD, AFM, TGA, measuring adhesion, determination of coating mass and thickness, determination of solubility of HAp and HAp-CNTsbt coating, and the methods to measure the mechanical properties of HAp and HAp-CNTsbt materials. CHƯƠNG 3: RESULTS AND DISCUSSTION 3.1. Modification of CNTs The IR spectrum of CNTs: C=C at 1630 cm-1, was overlap with the vibracation of –OH group in the water, the vibracation of –OH at 3400 cm-1. The IR spectrum of CNTsbt: the peak of –OH in water at 3400 cm-1. 2 peaks at 1720 cm-1 and 1385 cm-1 characteristic of C=O and C-OH. The results confirm that CNTs was modified successfully. Figure 3.2 shows that after 7 days soaked in water, CNTs was clumped by Van der Waals forces. CNTsbt dispersed well into water due to the presence of –COOH groups on the surface of CNTsbt, which reduces interaction of Van der Waals forces. SEM images show that CNTs and CNTsbt have tubular structures. CNTsbt §é truyÒn qua(%) 1720 1385 CNTs 1630 4000 3500 3000 2500 2000 1500 1000 500 -1 Sè sãng(cm ) Figure 3.1-3.3. IR spectra, dispersion and SEM images of CNTs and CNTsbt Bảng 3.3. Thành phần các nguyên tố của CNT và CNTbt Nguyên Nguyên tố Nguyên tố tố m% a% m% a% C 85.43 90.84 81.42 85.37 Figure 3.4. EDX spectra of CNTs and CNTsbt O 9.85 9.85 7.86 7.26 EDX spectrum of CNTs (Figure 3.4) shows Al 0.89 0.89 0.42 characteristic peaks of C, O, Fe, Al and Pt Fe 3.83 3.83 0.88 (Table 3.1). EDX spectrum of CNTsbt shows Total 100 100 100 100 characteristic peaks of C and O. The modification process of CNTs removed heavy metal catalysis. 3.2. Synthesis and characterization of HAp-CNTsbt composite 3.2.1. Effect of snanning potential range Cathode polarization curves of 316LSS and Ti6Al4V (Fig. 3.5): 0 ÷ -0,7 V/SCE, i≈ 0 because no reaction occurs. -0,7 ÷ -1,2 V/SCE, i increases slightly corresponding to reduction of H+, O2 in water. 7
5 0 Potential
hand, the diffusion of Ca2+ on the passive membrane of the substrate leads to the strong formation of surface interaction between 316LSS and HAp, improving the adhesion of HAp coating to the substrate [98-101]: FeOOH + Ca2+ → {FeOO -…Ca2+} + H+ (3.11) - 2+ 2- - 2+ 2- {FeOO …Ca } + HPO 4 → { FeOO …Ca …HPO4 } (3.12) {FeOO -…Ca2+} + PO43- + OH - → { FeOO-…Ca2+…PO43-…OH -} (3.13) For Ti6Al4V substrate, the mechanism of the adhesion between HAp coating and substrate was explained: There is oxide layer of TiO2 on the surface of Ti6Al4V. In the synthesis process, some reactions occured leading to the presence of corrosive products [98- 101]: {TiO 2} + 2H2O → Ti(OH)4 (3.14) + − {TiO2} + 2H2O → [Ti(OH)3] + OH (3.15) − + {TiO2} + 2H2O → [TiO2OH ] + H 3O (3.16) 2+ Ca ions into the solution diffused into the surface of titanium oxide. {Ti–OH} + Ca2+ → {TiO−···Ca2+} + H + (3.17) − 2+ 2− − 2+ 2- − {TiO ···Ca } + HPO4 → {TiO ···Ca ···HPO4 } + OH (3.18) − 2+ 3− − − 2+ 3- − {TiO ···Ca } + PO4 + OH → {TiO ···Ca ···PO4 ···OH } (3.19) FTIR spectra showed that potential range does not affect to the characteristic peaks of HAp and CNTs: PO 43-: 1040; 600 and 560 cm-1. The shilfting of C-OH(CNTsbt) from 1385 cm-1 to 1380 cm-1 was explained by reaction between Ca2+ of HAp and COO- of CNTsbt. Fig. 3.7-8. IR spectra of HAp-CNTsbt/316LSS Fig. 3.9-10. XRD of HAp-CNTsbt/316LSS and and HAp-CNTsbt/Ti6Al4V at different potential HAp-CNTsbt/Ti6Al4V at different potential XRD paterns presented that HAp-CNTsbt/316LSS materials had characteristic peaks of HAp and CNTs. Peak at 2θ ~ 32o of HAp. Peak at 25.88o of HAp was not observed because of overlap with peak at 26o of CNTs. XRD patern of the coating synthesized at 0 ÷ -1.6 V/SCE appeared characteristic peaks of DCPD (CaHPO4.2H2O. DCPD) at 2θ ~ 29.2o; 43o; 51o because, formed OH- was not enough to completely transfer HPO42- to PO43- at small potential range. For Ti6Al4V, XRD paterns of HAp-CNTsbt coating synthesized at 0 ÷ -1.6 và 0 ÷ -1.7 V/SCE was observed phase of DCPD. At larger potential range, the obtained coating composed phases of HAp and CNTs. SEM images showed that HAp-CNTsbt/316LSS had scales-shapes when they were synthesized at 0 ÷ -1.6 V/SCE; 0 ÷ - 1.65 V/SCE and has plate shapes with large size when they were synthesized at a wide range. SEM images of HAp-CNTsbt/Ti6Al4V had scales- shapes and uniform when they were synthesized in small potential ranges. At 0 ÷ -2.1 V / SCE, the coating was porous. TEM images were observed CNTsbt in the coating (Fig. 3.13). 9
Fig. 3.11. SEM images of HAp-CNTsbt/316LSS synthesized at different potential range Fig. 3.12. SEM images of HAp-CNTsbt/Ti6Al4V synthesized at different potential range 3.2.2. Effect of temperature Cathodic polarization curves of 316LSS and Ti6Al4V at different temperature were the same (Fig. 3.20 and 3.21). The temperature increased leading to the increase of reaction rate and current density. The temperature increased leading to the mass, thickness increased but the adhesion strength decreased (Table 3.5). Therefore, the temperature of 45 oC was chosen. 1 5 0 -1 0 -2 -5 -3 -4 -10 0.1 i (mA/cm2) -5 -15 0.0 i (mA/cm ) 2 -6 -0.1 -7 -20 -0.2 o -0.3 30 C -8 -25 -0.4 o 37 C -9 o 30 C -0.5 o 45 C -10 o 37 C -30 -0.6 o o -0.7 50 C -11 45 C o o -35 -0.8 60 C -12 50 C o -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -13 60 C -40 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 E (V/SCE) E (V/SCE) Fig. 3.20-21. Cathodic polarization curves of Fig. 3.22-23. XRD paterns of HAp-CNTsbt on 316LSS and Ti6Al4V at different temperature 316LSS and Ti6Al4V at different temperature Table 3.5. Mass, thickness and adhesion strength of HAp-CNTsbt followed temperature Temperature Mass (mg/cm2) Thickness (µm) Adhesion strength (MPa) (oC) 316LSS Ti6Al4V 316LSS Ti6Al4V 316LSS Ti6Al4V 30 1.16 1.18 3.80 3.40 14.03 12.00 37 1.61 1.54 5.30 5.10 13.08 11.10 45 2.10 2.08 6.90 6.30 13.2 10.40 50 3.28 3.13 11.98 11.86 8.45 7.22 60 3.73 3.81 12.20 11.40 6.05 6.00 XRD paterns showed that the temperature đo not affect to phase component of the coating (Fig. 3.22 and 3.23). HAp-CNTsbt coating composed phases of HAp and CNTs. SEM images of HAp-CNTsbt had scales shapes when they were synthesized at 30 oC and 45 o C. At 60 oC, the obtained coating had leaves shape with big size. 10
Fig. 3.24. SEM images of Fig. 3.25. SEM images HAp-CNTsbt/316LSS at of HAp-CNTsbt/Ti6Al4V different temperature at different temperature 3.2.3. Effect of CNTsbt concentration The amount of CNTsbt in the electrolyte increased, the cathode current density increased. The coating mass and thickness decreased with the presence of CNTsbt due to voluminous molecular structure of CNTs which prevented the formation of HAp in the substrate. However, the presence of CNTsbt in the coating improved the adhesion strength between the coating and substrate. From table 3.4. 0.5 g/L of CNTsbt was chosen for further investigation. 5 0 0 -1 -5 -2 -10 -3 -15 i (mA/cm ) 2 i (mA/cm ) 2 -4 -20 -5 -25 0g CNTs -6 -30 0 g/L CNTs 0.25g CNTs 0,25g/L CNTs 0.5g CNTs -35 -7 0,5 g/L CNTs 0.75g CNTs 0,75 g/L CNTs -8 1g CNTs -40 1 g/L CNTs -9 -45 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 E (V/SCE) E (V/SCE) Fig. 3.13. TEM images of HAp-CNTsbt on Fig. 3.14-15. Cathodic Polarization curves of 316LAA and 316LSS (A) and Ti6Al4V (B) Ti6Al4V in electrolyte with the different CNTsbt amount Table 3.4. The variation of mass, thickness and adhesion strength of HAp-CNTsbt synthesized at different amount of CNTsbt Thickness (µm) Amount of Mass (mg/cm2) Adhesion strength (MPa) ISO 4288-1998 CNTsbt (g/L) 316LSS Ti6Al4V 316LSS Ti6Al4V 316LSS Ti6Al4V 0,00 2.63 2.81 8.66 8.90 5.35 4.50 0.25 2.13 2.19 6.920 6.80 10.24 9.20 0.50 2.10 2.08 6.90 6.30 13.20 10.40 0.75 1.96 1.56 6.70 4.70 11.19 7.10 1,00 1.74 1.34 5.70 4.10 9.35 6.20 IR spectra showed characteristic peaks for vibracation of groups in HAp and CNTsbt (3.2.1 section). From TG/DTG diagram we can be calculated amount of CNTsbt in HAp-CNTsbt/316LSS and HAp-CNTsbt/Ti6Al4V was 5, 7, 7 and 6 % corresponding to CNTbt concentration of 0.25; 0.5; 0.75 and 1 g/L. Fig. 3.16-17. IR spectra of HAp-CNTsbt Fig. 3.24. TG/DTG diagram 0f HAp/316LSS (a) and with different amount of CNTsbt HAp/Ti6Al4V (b) 11
Fig. 3.25. TG/DTG diagram of Hap-CNTsbt /316LSS synthesized at 0 ÷ -1,65 V; 5 mV/s, 5 scans; 45 oC with CNTbt : 0,25 g/L (a); 0,5 g/L (b); 0,75 g/L (c) and 1 g/L (d) Fig. 3.26. TG/DTG diagram of HAp-CNTbt/Ti6Al4V synthesized at 0 ÷ -2 V; 5 mV/s, 5 scans; 45 oC with CNTbt : 0,25 g/L (a); 0,5 g/L (b); 0,75 g/L (c) and 1 g/L (d) 3.2.4. Effect of number scans The number of scans increased, mass and thickness increased but the adhesion strength decreased. Hap-CNTsbt coating synthesized with 3 scans had the adhesion of 14.5 MPa which was similar with adhesion of substrate and glue. When the number of scans increased (4 or 5 scans). Hap-CNTsbt coating was uniform. smooth. thick and completely covers for the substrate. Continue to increase the scans to 6 times. the adhesion between the coating and substrate was strongly reduced. Therefore, 5 scans were selected to synthesize HAp-CNTsbt/316LSS and HAp-CNTsbt/Ti6Al4V coatings. Table 3.6. Mass. thickness and adhesion strength of HAp-CNTsbt followed number scans Thickness (µm) Adhesion Number Mass (mg/cm2) ISO 4288-1998 (MPa) scans 316LSS Ti6Al4V 316LSS Ti6Al4V 316LSS Ti6Al4V 3 1.03 0.92 3.40 3.00 14.50 12.60 4 1.72 1.92 5.60 6.10 13.34 10.70 5 2.10 2.08 6.90 6.30 13.20 10.40 6 2.69 2.32 8.80 7.50 8.60 7.00 XRD paterns showed that number scans does not affect to phase component of the nanocomposite. HAp-CNTsbt coating had crystal structure and composed the phase of HAp and CNTs (Fig. 3.27 and 3.28). 1, 2 1 1 1.HAp; 2.CNTs 1, 2 1 1 1.HAp; 2.CNTs Fig. 3.27-28. XRD paterns of HAp-CNTsbt on 316LSS and Ti6Al4V at the different 1 Cêng ®é nhiÔu x¹ (%) 1 Cêng ®é nhiÔu x¹ (%) 1 6 lÇn quÐt 1 1 6 lÇn quÐt 1 5 lÇn quÐt 5 lÇn quÐt number scans 4 lÇn quÐt 4 lÇn quÐt 3 lÇn quÐt 3 lÇn quÐt 20 30 40 50 60 70 20 30 40 50 60 70 2 (®é) 2 (®é) 3.2.5. Effect of scanning rate Fig. 3.29 and 3.30 showed that scanning rate increased. i cathode decreased. Scanning rate increased from 2 to 7 V/s. the coating massdecreased but the adhesion increased. It can be explained as following: at low scanning rate icathode increased, the big amount of OH- and PO43- was formed leading to the increase of coating mass. However, the big value of icathode was advantaged for the reduction process of H+, H2PO4- and H 2O to form H2 gas on the surface of 12
the working electrode → obtained porous coating with low adhesion. So, scanning rate of 5 mV/s was chosen for further studies. 1 0 1 1: HAp; 2: CNTs 1 1: HAp; 2: CNTs 1,2 0 1,2 -5 1 1 1 1 1 1 7 mV/s -1 7 mV/s -10 Cêng ®é nhiÔu x¹ -2 6 mV/s 6 mV/s i (mA/cm ) Cêng ®é nhiÔu x¹ 2 -3 -15 i(mA/cm ) 2 5 mV/s -4 2mV/s -20 2mV/s 5 mV/s 3mV/s 3mV/s -5 4mV/s -25 4mV/s 4 mV/s 5mV/s 5mV/s 4 mV/s -6 6mV/s 6mV/s 7mV/s -30 -7 7mV/s 3 mV/s 3 mV/s -8 -35 -2.0 -1.5 -1.0 -0.5 0.0 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 20 30 40 50 60 70 E(V/SCE) E (V/SCE) 20 30 40 50 60 70 2 (®é) 2 (®é) Fig. 3.29-30. Cathodic polarization curves of Fig. 3.31-32 XRD of HAp-CNTsbt /316LSS and HAp- 316LSS. Ti6Al4V with different scanning rate CNTsbt /Ti6Al4V with different scanning rate Table 3.8. The variation of mass and adhesion strength of HAp-CNTsbt and 316LSS, Ti6Al4V with different scanning rate Scanning rate Mass (mg/cm2) Adhesion (MPa) (mV/s) 316LSS Ti6Al4V 316LSS Ti6Al4V 2 2.95 2.71 8.20 6.20 3 2.71 2.31 9.60 8.50 4 2.21 2.13 12.85 9.20 5 2.10 2.08 13.20 10.40 6 1.54 1.65 13.42 12.60 7 1.28 1.08 14.02 13.20 XRD paterns showed that the scanning rate doex not affect to phase component of the coating. HAp-CNTsbt coating had crystal structure and composed phase of HAp and CNTs. 3.2.6. Determination of mechanical and dissolution of materials  Surface roughness Ra values showed that the surface roughness of HAp and HAp-CNTsbt coatings is higer than that of the substrate. Fig. 3.33. AFM images of Fig 3.34. AFM images of 316LSS (a), HAp/316LSS Ti6Al4V (a), (b) and HAp- HAp/Ti6Al4V (b) and CNTsbt /316LSS (c) HAp-CNTsbt /Ti6Al4V (c)  Modulus The modulus of 316LSS, Ti6Al4V, HAp/316LSS, HAp-CNTsbt/316LSS, HAp/Ti6Al4V and HAp-CNTsbt/Ti6Al4V are 82 GPa,115 GPa, 86 GPa, 121 GPa, 93 GPa and 126 GPa, respectively which showed that CNTsbt increased modulus for the materials. 180 260 200 260 TKG316L 240 Ti6Al4V y = 115090 x + 3,0042 HAp/TKG316L 240 HAp/Ti6Al4V y= 120712 x + 3.1053 160 y= 82246x + 2.1493 2 180 2 2 220 R = 0,9996 y= 86247x + 2.2539 220 R = 0.9995 R = 0.9994 160 2 140 200 R = 0.9991 200 øng suÊt (MPa) 180 140 180 120 øng suÊt (MPa) 160 160 øng suÊt (MPa) øng suÊt (MPa) 120 100 140 140 100 80 120 120 100 80 100 60 80 80 60 40 60 60 40 40 40 20 20 20 20 0 0 0 0 0.0000 0.0005 0.0010 0.0015 0.0020 0.0000 0.0005 0.0010 0.0015 0.0020 0.0000 0.0005 0.0010 0.0015 0.0020 0.0000 0.0005 0.0010 0.0015 0.0020 §é biÕn d¹ng (%) §é biÕn d¹ng (%) §é biÕn d¹ng (%) §é biÕn d¹ng (%) 13
200 280 180 HAp-CNTs bt/TKG316L y= 92587x + 2.1495 260 HAp/CNTs bt/Ti6Al4V 240 y= 126219 x + 2,9425 Fig. 3.35. Modulus of 316LSS, HAp/316LSS, Hap- 160 2 220 2 CNTsbt /316LSS, Ti6Al4V, HAp/Ti6Al4V and Hap- R = 0.9995 R = 0,9994 140 200 øng suÊt (MPa) 180 øng suÊt (MPa) 120 160 100 80 140 120 CNTsbt/Ti6Al4V 100 60 80 40 60 40 20 20 0 0 0.0000 0.0005 0.0010 0.0015 0.0020 0.0000 0.0005 0.0010 0.0015 0.0020 §é biÕn d¹ng (%) §é biÕn d¹ng (%)  Hardness With the presence of 7.16 % CNTsbt in the nanocompostite of HAp-CNTsbt/316LSS. the hardness increased from 460 kgf/mm2 (4.5 GPa) to 573 kgf/mm2 (5.6 GPa), 7.25 % of CNTsbt into composite HAp-CNTsbt/Ti6Al4V. The hardness increased from 520 kgf/mm2 (5.1 GPa) to 612 kgf/mm2 (6.0 GPa). So, the hardness increased about 20-25 % with the presence of CNTsbt.  Dissolution of materials The dissolution of HAp and HAp-CNTsbt was determined Ca2+ concentration dissolved from the coating after these materials were immersed into 20 mL of 0.9 % NaCl with different time at 37 ± 1 oC. From Table 3.11, immersed times increased, the dissolution of the coating increased. The dissolution of HAp coating was large than that of HAp-CNTsbt coating. It means that the dissolution significantly reduced with the presence of CNTsbt. It can be explained by –COOH group on the surface of CNTsbt which created hydrogen bonding with –OH group in HAp. Thus, CNTsbt acts as a bridge connecting the HAp crystals together to make obtained tighter coating. Table 3.11. Ca2+ comcentration into the solution after immersion process into 0.9 % NaCl Material Ca2+ concentration (mg/L) 7 days 14 days 21 days HAp/316LSS 20.6 ± 0.3 25.3 ± 0.2 30 ± 0.2 HAp-CNTsbt/316LSS 13 ± 0.5 16.5 ± 0.2 19.4 ± 0.2 HAp/Ti6Al4V 21.3 ± 0.3 25 ± 0.4 29.5 ± 0.3 HAp-CNTsbt/Ti6Al4V 12.5 ± 0.4 16.3 ± 0.3 17.7 ± 0.3 3.3. Electrochmical behavior in SBF solution 3.3.1. The variation of pH solution pHo = 7.4, pH values of SBF solution increased after 1 immersed day. For SBF solution containing 316LSS and Ti6Al4V, pH slight change during immersion period and pH solution trend to decrease at long immersion time. After 21 imersed days, pH values of SBF solutions were 7.28 and 7.22 corresponding to the SBF containing 316LSS and Ti6Al4V. The increase of pH can be explained by translation between H2PO4- and OH- following the equations of (3.10) and (3.11). The decrease of pH solution was explained by the formation of new apatite crystals which consume OH- ions follows (3.7), (3.8) and (3.9) equations. For SBF solution containing HAp/316LSS and HAp-CNTsbt/316LSS, the variation of pH values is the same, pH value increased after 1 soaked day and strongly decreased after 5 soaked days. Aterthat, pH solution continue to increase and trend to strongly decrease after 14 and 21 soaked days. At 21 soaked days, pH solution containing HAp-CNTsbt/316LSS và HAp/316LSS were 6.5 and 6.9, respectively. For HAp/Ti6Al4V, pH solution increased from 7.4 to 7.75 when immersion time increased from 1 to 5 days. At longer immersion times, pH solution decreased. This value was 6.86 after 21 soaked days. The variation of SBF solution containing HAp-CNTsbt/Ti6Al4V fluctuate during immersion period. pH solution increased at the first times and strongly decreased after 21 14
soaked days. The variation of pH solution can be explained as following: when HAp or HAp-CNTsbt coating imersed into SBF solution. there are two processes simultaneous occurs: the solubility of the coating and the formation of new apaptit crystals. When in SBF solution containing HAp or HAp-CNTsbt coatings. Ca2+ concentration increases in the area around of the material surface due to the dissolution of the coating and then OH- is accumulated by the ion exchange between Ca2+ and H+ lead to an increase in solution pH. The formation of apatite which consume OH- ions leading to the decrease of pH solution [16, 65]. 3.3.2. The variation of material mass Figure 3.37 shows the variation of mass of 316LSS and Ti6Al4V with and without HAp or HAp-CNTsbt coatings at differsent time into SBF solution. For the substrate, the variation of mass was almost not observed at the beginning of immersion and tended to increase slightly after 14 and 21 days of soaking. The mass of samples of 316LSS and Ti6Al4V increased 1.7 and 0.21 mg.cm-2 after 21 soaked days. For HAp or HAp-CNTsbt coatings, the mass slightly decreased after 1 soaked day and strongly increased at 3. 5 and 7 soaked days. After 21 soaked says. The mass variation was Δm = + 0.61 mg/cm2. For HAp-CNTsbt/316LSS, the mass slightly decreased after 1 soaked days (Δm = -0.05 mg/cm2) and strongly increased after 5 soaked days (0.68 mg/cm2). The mass trended to increase after 14 and 21 soaked days. The mass increased 0.82 mg/cm2 after 21 soaked days into SBF solution. For HAp/Ti6Al4V, at 3, 5 and 7 soaked days, the mass slightly decreased. The value strongly increased after 14 and 21 soaked days and reached of 0.65 mg/cm2 after 21 days. The variation of HAp-CNTsbt/Ti6Al4V slightly decreased at 1 and 3 soaked days and strongly increased at longer immersion days. After 21 soaked days, the mass increased of Δm = + 0.89 mg/cm2. The increase of material mass confirms the formation of new apatite crystals. The results showed that HAp-CNTsbt and HAp promoted the formation of new apatite crystals. 8.0 1.0 TKG316L 7.8 HAp/TKG316L 0.8 HAp-CNTbt/TKG316L 7.6 0.6 7.4 m (mg/cm ) 2 7.2 0.4 pH 7.0 0.2 6.8 TKG316L 6.6 HAp/TKG316L 0.0 HAp-CNTbt/TKG316L 6.4 Ti6Al4V Ti6Al4V -0.2 6.2 HAp/Ti6Al4V HAp/Ti6Al4V HAp-CNTbt/Ti6Al4V HAp-CNTbt/Ti6Al4V 6.0 -0.4 -2 0 2 4 6 8 10 12 14 16 18 20 22 -2 0 2 4 6 8 10 12 14 16 18 20 22 Thêi gian (ngµy) Thêi gian (ngµy) Figure 3.36. The variation of pH of SBF Figure 3.37. The variation of mass follows solution follows immersion times immersion times 3.3.3. Characterization of material  Surface morphology: SEM images of 316LSS, HAp/316LSS, HAp-CNTsbt/316LSS, Ti6Al4V, HAp/Ti6Al4V and HAp-CNTsbt/Ti6Al4V before and after immersed into SBF solution is shown in Figure 3.38-3.43. For 316LSS and Ti6Al4V, the formation of apatite crystals observed on the surface of materials after 21 soaked days. However, it is still possible to observe the positions of the substrate where apatite is not completely covered (Figure 3.38 and 3.41). 15
HAp/316LSS material had plate-like with larg size. After immersed days, the formation of apatite which had scale-like, to form coral-like on the surface of materials (Figure 3.39). HAp-CNTsbt/316LSS had scale-like. Apatite crystals formed with high density with coral- like after 14 and 21 soaked days (Figure 3.40). HAp/Ti6Al4V and HAp-CNTsbt/Ti6Al4V had scale-like. Apatite crystals formed with high density with coral-like after 14 and 21 soaked days (Figure 3.42 and 3.43). The results showed the biocompatibility of these materials in SBF solution. HAp- CNTsbt and HAp coatings promoted the formation of new apatite crystals. The results are suitable with the results of pH solution and mass variation. Figure 3.38. SEM images of 316LSS before and after 21 immersed days in SBF solution Figure 3.39. SEM images of HAp/316LSS before and after immersed in SBF solution Figure 3.40. SEM images of HAp-CNTsbt/316LSS before and after immersed in SBF solution Figure 3.41. SEM images of Ti6Al4V before and after immersed in SBF solution Figure 3.42. SEM images of HAp/Ti6Al4V before and after immersed in SBF solution Figure 3.43. SEM images of HAp-CNTsbt/Ti6Al4V before and after immersed in SBF solution  The phase component 16
XRD paterns of 316LSS and Ti6Al4V, after 21 days of soaking in SBF solution, there are two most characteristic peaks of HAp appeared at 2 of 25.8o and 32o. Besides, on the spectrum, there are peaks of 316LSS and Ti6Al4V substrates. This result confirmed the formation of apatite coating on the surface of the material after soaked in SBF on solution. XRD paterns of materials after 21 days of immersion in SBF solution did not observe any new peak appearance compared to XRD paterns before immersion. This result confirmed that after 21 days of immersion in SBF solution did not change the phase composition of the material. 1: H Ap 3 1: H A p 3 2: CNTs 2: C NT s 3 : T K G 3 16 L 3 3: T K G 316 L 3 3 3 1 1 1 ,2 1 ,2 Cêng ®é nhiÔu x¹ Cêng ®é nhiÔu x¹ (c ) (c) 1 1 1 (b) 1 (b) ( a) (a ) 20 25 30 35 40 45 50 55 60 65 70 20 25 30 35 40 45 50 55 60 65 70    ® é     ® é  Fig. 3.43. XRD paterns of 316LSS (a). Fig. 3.44. XRD paterns of Ti6Al4V (a). HAp/316LSS (b) and HAp-CNTsbt/316LSS (c) HAp/Ti6Al4V (b) and HAp-CNTsbt/Ti6Al4V after 21 immersed days (c) after 21 immersed days From the above results, it can be concluded that all of materials are biocompatible in SBF solution. After 21 days of soaking in SBF solution, the formation of new apatite crystals was observed. However, the formation of HAp crystals on HAp and HAp-CNTsbt coating are biger than that of the substrate. This result confirms good biocompatibility of HAp/316LSS materials, HAp/Ti6Al4V, HAp-CNTsbt/316LSS and HAp-CNTsbt/Ti6Al4V in SBF solution. The HAp-/CNTsbt and HAp coating are responsible for promoting the formation of apatite crystals. 3.4. Open circuit potential The change of open circuit potential (EOCP) of 6 materials in SBF solution by different immersion time is shown in Figure 3.45. At all times soaked, the EOCP of HAp-CNTsbt coating is always more positive than HAp coating and the two materials are always more positive than the substrates. The rule of changing the open circuit potential of 6 material samples when immersed in SBF solution is similar: EOCP moves to a negative potential at the beginning of the sample immersion time then more positive at long time of immersion. With 316LSS material, the EOCP moved more negatively at the beginning of the sample immersion. At longer immersion times, EOCP tended to move to a more positive direction and reached -88 mV after 21 days immersed in SBF solution. The EOCP value of HAp/316LSS is -73 mV at 1 day of immersion. It then tends to move towards the more positive during the remaining immersion process. After 21 days immersed, EOCP reaches -48 mV, much more positive than the time of one day immersion. The change of open circuit potential of HAp-CNTsbt/316LSS material is similar to that of HAp/316LSS material. EOCP values shifted to a more negative direction after 5 days of immersion. Then, it tended to move more positive during the remaining immersion period and reached -31 mV after 21 days. For Ti6Al4V, EOCP value plummeted after 7 days of immersion and it tended to move more positively at the next immersion time. After 21 days immersed in SBF solution, EOCP reached -79 mV. The change of open circuit potential of Ti6Al4V material is covered with 17
HAp and Hap-CNTsbt coating similarly during immersion process. At the time of 1 day soaking samples, EOCP values are -66 mV and -49 mV corresponding to HAp/Ti6Al4V and Hap-CNTsbt/Ti6Al4V materials. These two values plummeted after 7 days of immersion. Then, EOCP tended to move to a more positive direction and reached -38 mV and -21 mV after 21 days of immersion in SBF solution. The decrease of EOCP at the time of sample soaking for HAp or HAp-CNTsbt coating showed that coating infiltration phenomenon had occurred. EOCP variation is explained by membrane solubility or apatite formation during immersion. From this result it is possible to predict that HAp or HAp-CNTsbt coatings have a shielding effect on the substrate. At the same time, HAp and HAp-CNTsbt coatings also act as sprouts to promote the development of new apatite crystals on the surface of the material. This result will be further clarified in the section of total resistance measurement. Fig. 3.45. The variation of EOCP of 316LSS, 0.000 TKG316L HAp/TKG316L Ti6Al4V, HAp/316LSS, HAp/Ti6Al4V, HAp- -0.025 HAp/CNTbt/TKG316L Ti6Al4V HAp/Ti6Al4V HAp/CNTsbt/Ti6Al4V EOCP (V/SCE) CNTsbt/316LSS and HAp-CNTsbt/ Ti6Al4V -0.050 -0.075 following immersed times -0.100 -0.125 0 2 4 6 8 10 12 14 16 18 20 22 Thêi gian (ngay) 3.5. Polarizing resistance and density of corrosive current The Tafel polarization curves of the 6 materials in the potential range of Eo ± 150 mV is shown in Figure 3.46. From the slope of the Tafel polarization curve, the coefficient B (according to Equation 2.3) is calculated as 0.046; 0.040; 0.028; 0.026; 0.022 and 0.019 respectively corresponding to 316LSS, Ti6Al4V, HAp/316LSS, HAp/Ti6Al4V, HAp- CNTsbt/316LSS and HAp-CNTsbt/Ti6Al4V. 3.5. Điện trở phân cực và mật độ dòng ăn mòn Figure 3.46. Tafel polarization curves of 1E-5 316LSS (a), Ti6Al4V (b), HAp/316LSS (c), 1E-6 HAp/Ti6Al4V (d), HAp-CNTsbt/316LSS (e) i (A/cm ) 2 1E-7 and HAp-CNTsbt/Ti6Al4V (f) after 21 1E-8 a b c 1E-9 immersed days d f e 1E-10 -0.150 -0.125 -0.100 -0.075 -0.050 -0.025 0.000 0.025 0.050 E (V/SCE) Polarization resistance measurements were made in the potential range of Eo ± 10 mV in SBF solution with a scan rate of 1 mV/s (Figure 3.47). Polarized resistance value (Rp), corrosion current density (icorr) of the materials in SBF solution by immersion time is calculated according to Equations 2.1 and 2.2 with the coefficient B calculated above. 0.4 0.3 0.10 TKG316L HAp/TKG316L HAp/CNTsbt/316LSS 0.4 Ti6Al4V 5 ngµy 0.08 0.3 21 ngµy 0.2 0.2 0.06 3 ngµy 3 ngµy 0.2 7 ngµy 5 ngµy 0.04 0.0 i (A/cm ) 2 0.1 i (A/cm ) i (A/cm ) i (A/cm ) 2 2 2 0.02 -0.2 0.1 1 ngµy 0.0 0.00 -0.4 5 ngµy 3 ngµy -0.1 1 ngµy -0.02 1 ngµy 7 ngµy 5 ngµy 0.0 -0.6 14 ngµy -0.2 7 ngµy 14 ngµy -0.04 7 ngµy 21 ngµy 14 ngµy 14 ngµy 3 ngµy 21 ngµy -0.8 1 ngµy -0.06 21 ngµy -0.3 -0.1 -0.12 -0.11 -0.10 -0.09 -0.08 -0.10 -0.09 -0.08 -0.07 -0.06 -0.05 -0.09 -0.08 -0.07 -0.06 -0.05 -0.04 -0.03 -0.02 -0.11 -0.10 -0.09 -0.08 -0.07 -0.06 E (V/SCE) E (V/SCE) E (V/SCE) E (V/SCE) 18
0.7 0.6 0.5 HAp/Ti6Al4V 0.3 HAp/CNTsbt/Ti6Al4V Fig. 3.47. Polarization curves of materials in 0.2 0.4 0.3 5 ngµy 1 ngµy SBF solution at different immersion times i (A/cm2) i (A/cm ) 2 0.1 0.2 0.1 0.0 0.0 -0.1 21 ngµy 1 ngµy -0.2 -0.1 7 ngµy 5 ngµy 14 ngµy -0.3 21 ngµy 3 ngµy -0.4 -0.2 7 ngµy 3 ngµy 14 ngµy -0.09 -0.08 -0.07 -0.06 -0.05 -0.04 -0.03 -0.02 -0.5 -0.12 -0.10 -0.08 -0.06 -0.04 -0.02 E (V/SCE) E (V/CSE) The polarization resistance of Ti6Al4V is higher than that of 316LSS. Rp of HAp- CNTsbt or HAp coating is higher than that of Ti6Al4V, 316LSS and HAp-CNTsbt coating are higher than HAp coating. The polarization resistance of 316LSS is the lowest at all immersion times compared to Ti6Al4V, HAp/316LSS, HAp/Ti6Al4V, HAp-CNTsbt/316LSS and HAp-CNTsbt/Ti6Al4V. This value has fluctuations at different immersion times in SBF solution. The Rp decreases sharply after 3 days of immersion and continues to decrease slightly to 7 days of immersion. Then, it tends to increase at longer immersion points. At the time of 21 days immersion, Rp = 7.7 (KΩ.cm2) higher than one day of immersion (7.5 KΩ.cm2). The Rp variation is similar for Ti6Al4V but Rp of Ti6Al4V is always higher than 316LSS at all times of immersion. It shows that Ti6Al4V has better corrosion resistance than 316LSS. At the time of 1 day soaking samples, Rp = 10.5 (KΩ.cm2). This value tends to decrease at the beginning of the sample immersion time but tends to increase at longer sample immersion times. After 21 days of immersion, the polarization resistance Rp reaches 10.9 (KΩ.cm2). Rp value of HAp/316LSS materials, HAp-CNTsbt/316LSS, HAp/Ti6Al4V and HAp- CNTsbt/Ti6Al4V have fluctuations at the time of immersion. The cause of this variation is due to the formation of new apatite crystals and the dissolution of HAp or HAp-CNTsbt coatings during immersion process. The polarization resistance of Hap-CNTsbt coating is higher than that of HAp coating, which shows that the protection ability of Hap-CNTsbt coting is better than that of HAp coating. At the same time at long immersion times (14. 21 days), Rp of HAp-CNTsbt/316LSS, HAp-CNTsbt/Ti6Al4V increased stronger than HAp/316LSS and HAp/Ti6Al4V. This result shows that the formation of new apatite crystals of Hap-CNTsbt is better than that of HAp. Polarization resistance of HAp- CNTsbt/316LSS, HAp-CNTsbt/Ti6Al4V after 21 days of immersion in SBF solution were 20 KΩ.cm2 and 26.5 KΩ.cm2 respectively which is much higher than the one-day immersion (14.5 KΩ.cm2 and 16.9 KΩ.cm2). From the above results, it can be concluded that HAp- CNTsbt coating have better protection for 316LSS and Ti6Al4V substrates than HAp coatings. At the same time, it also promotes the formation of new apatite crystals. The corrosion current density (icorr) has fluctuations and fluctuations in the opposite direction compared to Rp (Figure 3.49). At all times soaked, the corrosion density of the 316LSS and Ti6Al4V substrates is always higher than that of HAp and HAp-CNTsbt coating. After 1 day of soaking, icorr is 2.3; 1.5; 2 and 1.1 µA/cm2 corresponding to HAp/316LSS, HAp-CNTsbt/316LSS, HAp/Ti6Al4V, HAp-CNTsbt/Ti6Al4V which is lower than that of 316LSS and Ti6Al4V materials (6 and 3.8 µA/cm2). This result shows the protective role of HAp-CNTsbt and HAp coatings for substrates. The corrosive current density of 316LSS increases sharply at 3 and 5 days soaking time then tends to decrease at the next soaking time. Increasing of the corrosion current density due to the attack of corrosive ions (Cl-, SO42-) in SBF solution to the material surface. After 21 days immersed in SBF solution, icorr reached at least of 5.9 µA/cm2. The results are 19
similar for Ti6Al4V material. The corrosion current density increases sharply at short immersion times and reaches the maximum value of 5.8 µA/cm2 after 7 days of immersion. At longer immersion times, icorr value plummeted and reached a minimum value of 3.7 µA/cm2 after 21 days of immersion. This is mainly due to the formation of new apatite crystals as a passive layer on the surface of material which can protect for the substrate. For HAp-CNTsbt or HAp coating, with long immersion periods (7.14 and 21 days). Corrosion current density tends to decrease. These results show the corrosion protection of HAp-CNTsbt and HAp coating for 316LSS, Ti6Al4V substrates. 30 10 MËt ®é dßng ¨n mßn (A/cm ) (f) 2 25 8 (e) Rp (k.cm ) 20 2 6 (a) (d) 15 (c) 4 (b) 10 (b) (a) (c) 2 (d) 5 (e) (f) 0 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 Thêi gian (ngµy) Thêi gian ng©m (ngµy) Fig. 3.48. The variation of Rp of 316LSS (a), Fig. 3.49. The variation of icorr of 316LSS (a), Ti6Al4V (b), HAp/316LSS (c), HAp/Ti6Al4V Ti6Al4V (b), HAp/316LSS (c), HAp/Ti6Al4V (d), HAp-CNTsbt/316LSS (e) and HAp- (d), HAp-CNTsbt/316LSS (e) and HAp- CNTsbt/Ti6Al4V (f) follows immersed times CNTsbt/Ti6Al4V (f) follows immersed times 3.6. Electrochemical impedance spectroscopy Bode impedance spectra of materials show the variations of log/Z/ follows logf at different immersion times in SBF solution (Figure 3.50). From the obtained results can be seen that for the substrates, the impedance value in the low frequency area decreases during immersion. The impedance resistance of Ti6Al4V is higher than that of 316LSS at all times of immersion. This shows that Ti6Al4V has better corrosion resistance than 316LSS. However, over time soak the sample, the impedance resistance is continuously decreasing. From the results of the decrease in pH and the mass increase of Ti6Al4V, it can be judged that: at different immersion time, there is the formation of apatite on the surface of the material but the formation is irregular, do not cover the substrate surface. This result will be confirmed by SEM image and X-ray diffraction of 316LSS and Ti6Al4V after 21 immersed days in SBF solution. 6.0 6.0 6.0 TKG316L 1 ngµy 1 ngµy 5.5 5.5 1 ngµy 3 ngµy 3 ngµy 5.5 5.0 5 ngµy 3 ngµy 5.0 5 ngµy 4.5 7 ngµy 7 ngµy 5.0 5 ngµy 14 ngµy 4.5 7 ngµy 4.0 14 ngµy 4.5 21 ngµy 4.0 14 ngµy 21 ngµy logIZI () 3.5 4.0 21 ngµy logIZI () logIZI () 3.5 3.0 3.0 3.5 2.5 2.5 3.0 2.0 1.5 2.0 2.5 1.0 1.5 2.0 0.5 1.0 HAp/TKG316L 1.5 -3 -2 -1 0 1 2 3 4 5 6 HAp/CNTsbt/TKG316L 0.5 logf (Hz) 1.0 -3 -2 -1 0 1 2 3 4 5 6 -3 -2 -1 0 1 2 3 4 5 6 logf (Hz) log (f) 20