Tóm tắt Luận án Electrodeposition of hydroxyapatite/modify carbon naotubes on alloys to apply for bone implants

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 HApCNTsbt/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 HApCNTsbt/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 HApCNTsbt 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.

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) {FeOO-Ca2+} + HPO4 2- → { FeOO-Ca2+HPO4 2-} (3.12) {FeOO-Ca2+} + PO4 3- + OH- → { FeOO-Ca2+PO4 3-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]: {TiO2} + 2H2O → Ti(OH)4 (3.14) {TiO2} + 2H2O → [Ti(OH)3] + + OH− (3.15) {TiO2} + 2H2O → [TiO2OH −] + H3O + (3.16) Ca2+ ions into the solution diffused into the surface of titanium oxide. {Ti–OH} + Ca2+ → {TiO−···Ca2+} + H+ (3.17) {TiO−···Ca2+} + HPO4 2− → {TiO−···Ca2+···HPO4 2-} + OH− (3.18) {TiO−···Ca2+} + PO4 3− + OH− → {TiO−···Ca2+···PO4 3-···OH−} (3.19) FTIR spectra showed that potential range does not affect to the characteristic peaks of HAp and CNTs: PO4 3-: 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 and HAp-CNTsbt/Ti6Al4V at different potential Fig. 3.9-10. XRD of HAp-CNTsbt/316LSS and 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 ~ 29.2 o; 43o; 51o because, formed OH- was not enough to completely transfer HPO4 2- to PO4 3- 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). 10 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. -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 30 o C 37 o C 45 o C 50 o C 60 o C E (V/SCE) i (m A /c m 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 0.2 -40 -35 -30 -25 -20 -15 -10 -5 0 5 30 o C 37 o C 45 o C 50 o C 60 o C E (V/SCE) i (m A /c m 2 ) -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 Fig. 3.20-21. Cathodic polarization curves of 316LSS and Ti6Al4V at different temperature Fig. 3.22-23. XRD paterns of HAp-CNTsbt on 316LSS and Ti6Al4V at different temperature Table 3.5. Mass, thickness and adhesion strength of HAp-CNTsbt followed temperature Mass (mg/cm2) Thickness (µm) Adhesion strength (MPa) Temperature (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 oC. At 60 oC, the obtained coating had leaves shape with big size. 11 Fig. 3.24. SEM images of HAp-CNTsbt/316LSS at different temperature Fig. 3.25. SEM images of HAp-CNTsbt/Ti6Al4V 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. -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 E (V/SCE) i (m A /c m 2 ) 0g CNTs 0.25g CNTs 0.5g CNTs 0.75g CNTs 1g CNTs -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 E (V/SCE) i ( m A /c m 2 ) 0 g/L CNTs 0,25g/L CNTs 0,5 g/L CNTs 0,75 g/L CNTs 1 g/L CNTs Fig. 3.13. TEM images of HAp-CNTsbt on 316LSS (A) and Ti6Al4V (B) Fig. 3.14-15. Cathodic Polarization curves of 316LAA and 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 Mass (mg/cm2) Thickness (µm) ISO 4288-1998 Adhesion strength (MPa) Amount of 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 with different amount of CNTsbt Fig. 3.24. TG/DTG diagram 0f HAp/316LSS (a) and HAp/Ti6Al4V (b) 12 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 Mass (mg/cm2) Thickness (µm) ISO 4288-1998 Adhesion (MPa) Number 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). 20 30 40 50 60 70 2 (®é) C ­ ê n g ® é n h iÔ u x ¹ (% ) 4 lÇn quÐt 5 lÇn quÐt 1.HAp; 2.CNTs 6 lÇn quÐt 3 lÇn quÐt 1, 2 1 1 11 1 20 30 40 50 60 70 2 (®é) C ­ ên g ® é n h iÔ u x ¹ (% ) 4 lÇn quÐt 5 lÇn quÐt 1.HAp; 2.CNTs 6 lÇn quÐt 3 lÇn quÐt 1, 2 1 1 11 1 Fig. 3.27-28. XRD paterns of HAp-CNTsbt on 316LSS and Ti6Al4V at the different number scans 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 PO4 3- 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 H2O to form H2 gas on the surface of 13 the working electrode → obtained porous coating with low adhesion. So, scanning rate of 5 mV/s was chosen for further studies. -2.0 -1.5 -1.0 -0.5 0.0 -8 -7 -6 -5 -4 -3 -2 -1 0 1 E(V/SCE) i( m A /c m 2 ) 2mV/s 3mV/s 4mV/s 5mV/s 6mV/s 7mV/s -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -35 -30 -25 -20 -15 -10 -5 0 2mV/s 3mV/s 4mV/s 5mV/s 6mV/s 7mV/s i (m A /c m 2 ) E (V/SCE) 20 30 40 50 60 70 C ­ ê n g ® é n h iÔ u x ¹ 2 (®é) 3 mV/s 4 mV/s 5 mV/s 6 mV/s 1 1: HAp; 2: CNTs 1 1 1 1,2 7 mV/s 20 30 40 50 60 70 C ­ ên g ® é n h iÔ u x ¹ 2(®é) 3 mV/s 4 mV/s 5 mV/s 6 mV/s 1 1: HAp; 2: CNTs 1 1 1 1,2 7 mV/s Fig. 3.29-30. Cathodic polarization curves of 316LSS. Ti6Al4V with different scanning rate Fig. 3.31-32 XRD of HAp-CNTsbt/316LSS and HAp- 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 Mass (mg/cm2) Adhesion (MPa) Scanning rate (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 316LSS (a), HAp/316LSS (b) and HAp- CNTsbt/316LSS (c) Fig 3.34. AFM images of Ti6Al4V (a), HAp/Ti6Al4V (b) and 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. 0.0000 0.0005 0.0010 0.0015 0.0020 0 20 40 60 80 100 120 140 160 180 TKG316L y= 82246x + 2.1493 R 2 = 0.9994 §é biÕn d¹ng (%) øn g su Êt ( M P a) 0.0000 0.0005 0.0010 0.0015 0.0020 0 20 40 60 80 100 120 140 160 180 200 220 240 260 y = 115090 x + 3,0042 R 2 = 0,9996 Ti6Al4V §é biÕn d¹ng (%) ø n g su Êt ( M P a) 0.0000 0.0005 0.0010 0.0015 0.0020 0 20 40 60 80 100 120 140 160 180 200 HAp/TKG316L y= 86247x + 2.2539 R 2 = 0.9991 §é biÕn d¹ng (%) øn g s uÊ t (M P a) 0.0000 0.0005 0.0010 0.0015 0.0020 0 20 40 60 80 100 120 140 160 180 200 220 240 260 HAp/Ti6Al4V y= 120712 x + 3.1053 R 2 = 0.9995 §é biÕn d¹ng (%) ø n g su Êt ( M P a) 14 0.0000 0.0005 0.0010 0.0015 0.0020 0 20 40 60 80 100 120 140 160 180 200 HAp-CNTs bt /TKG316L y= 92587x + 2.1495 R 2 = 0.9995 §é biÕn d¹ng (%) ø n g s uÊ t (M P a) 0.0000 0.0005 0.0010 0.0015 0.0020 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 HAp/CNTs bt /Ti6Al4V y= 126219 x + 2,9425 R 2 = 0,9994 §é biÕn d¹ng (%) ø n g su Êt ( M P a) Fig. 3.35. Modulus of 316LSS, HAp/316LSS, Hap- CNTsbt/316LSS, Ti6Al4V, HAp/Ti6Al4V and Hap- CNTsbt/Ti6Al4V  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/mm 2 (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 Ca 2+ 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 Ca2+ concentration (mg/L) Material 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 15 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. Ca 2+ 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. -2 0 2 4 6 8 10 12 14 16 18 20 22 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 Thêi gian (ngµy) p H TKG316L HAp/TKG316L HAp-CNT bt /TKG316L Ti6Al4V HAp/Ti6Al4V HAp-CNT bt /Ti6Al4V -2 0 2 4 6 8 10 12 14 16 18 20 22 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Ti6Al4V HAp/Ti6Al4V HAp-CNT bt /Ti6Al4V TKG316L HAp/TKG316L HAp-CNTbt/TKG316L  m ( m g/ cm 2 ) Thêi gian (ngµy) Figure 3.36. The variation of pH of SBF solution follows immersion times Figure 3.37. The variation of mass follows 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). 16 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 17 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. 20 2 5 3 0 3 5 4 0 4 5 5 0 55 6 0 6 5 7 0 1 3 1 ,2 1 1  ® é  (c) C ­ ê n g ® é n h iÔ u x ¹ (a) (b ) 31 : H A p 2 : C N T s 3 : T K G 3 16 L 3 2 0 25 3 0 35 4 0 45 5 0 55 6 0 65 7 0 1 3 1,2 1 1  ® é  ( c ) C ­ ê n g ® é n h iÔ u x ¹ (a ) (b) 31: H A p 2: C N T s 3: T K G 316 L 3 Fig. 3.43. XRD paterns of 316LSS (a). HAp/316LSS (b) and HAp-CNTsbt/316LSS (c) after 21 immersed days Fig. 3.44. XRD paterns of Ti6Al4V (a). HAp/Ti6Al4V (b) and HAp-CNTsbt/Ti6Al4V (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 18 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. 0 2 4 6 8 10 12 14 16 18 20 22 -0.125 -0.100 -0.075 -0.050 -0.025 0.000 Ti6Al4V HAp/Ti6Al4V HAp/CNTs bt /Ti6Al4V TKG316L HAp/TKG316L HAp/CNT bt /TKG316L Thêi gian (ngay) E O C P ( V /S C E ) Fig. 3.45. The variation of EOCP of 316LSS, Ti6Al4V, HAp/316LSS, HAp/Ti6Al4V, HAp- CNTsbt/316LSS and HAp-CNTsbt/ Ti6Al4V following immersed times 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 -0.150 -0.125 -0.100 -0.075 -0.050 -0.025 0.000 0.025 0.050 1E-10 1E-9 1E-8 1E-7 1

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