**1.3.6 Effect of crystallization treatment on surface morphology and corrosion resistance**

Figure 1-23 shows the optical microscope morphologies of the deposited samples at the current density of 20mA/cm2 for 7min before and after crystallization. It can be seen from figure 1-21a and figure 1-21b that before crystallization the surface Ti-O film, presenting bigger lump in shape, covered on the surface of TiNi SMA; it can be seen from figure 1-21c and figure 1-21d that the crystallized Ti-O film in smaller lump in shape distributed on the substrate surface. This is because of that the crystallization treatment made the hydrate (mainly Ti(OH)4) of amorphous Ti dehydrate water molecules and the crystal lattice recombinate to form the anatase type of TiO2. Due to that the current density in cathodic-electrodeposition was higher, the Ti-O film obtained was thicker too, so, in the process of crystallization, there might be uneven dehydration and stress distribution existing, thus bringing about serious crystal defects to affect the properties of Ti-O film to a large extent.

Fig. 1-23. Optical microscope result by current 20mA/cm2 for 7 min (a, b untreated; c, d heat treated)

Ti-O Film Cathodically-Electrodeposited on


unannealed samples in Hank's solution (PH7.45)

presenting a better cathodic-polarization property.

1. untreated 2. heat treated


E/V(SCE)

solution (PH7.45)




E/V(SCE)


0.00

0.05

0.10

the Surface of TiNi SMA and Its Bioactivity and Blood Compatibility 23

 annealed unannealed

0 1 0 2 0 3 0 4 0 5 0 6 0

t / m i n

012345

1

2

l g i / n A c m - <sup>2</sup>

Fig. 1-26. The anodic-polarization curves of annealed and unannealed samples in Hank's

Fig. 1-25. The Corrosion potential change plotted as a function of time of annealed and

Figure 1-26 shows the polarization curves of the deposited samples at current density of 5mA/cm2 before and after crystallization at 450C. It can be seen from the polarization curves that the breakdown potential of the uncrystallized sample was 1.3V, the current density in the process of polarization always was lower than that of the crystallized sample, and a longer passivation region emerging illustrates that the deposited sample at current density of 5mA/cm2 before crystallization presented a better corrosion resistance in Hank's solution of PH7.45. Due to the single structure of unannealed Ti-O film, it is easy for the crystallized sample to arise more crystal defects, and bring about the crystal structure not unitary, thus presenting a poor corrosion resistance. From the cathodic-polarization curves of view, the slope of the sample 's cathodic-polarization curves before crystallization was larger, but the current density was lower than that of the crystallized sample, thus

Figure 1-24 shows the optical microscope morphologies of the deposited samples at current density of 5mA/cm2 for 4min before and after crystallization. It can be seen from figure 1- 22a and 1-22b that Ti-O film was even and smooth, and from 1-22c and 1-22d that the film distributed in type of tiny particles. Similarly, this is because of that the crystallization treatment made the hydrate (mainly Ti(OH)4) of amorphous Ti dehydrate water molecules and the crystal lattice recombinate to form the anatase type of TiO2. Because the film was thinner, the crystal defects caused in the process of crystallization was less, so that the effect on the film's properties would not be serious.

Figure 1-25 shows the corrosionpotential-time curves of the deposited samples at current density of 5mA/cm2 before and after crystallization in Hank's solution of PH7.45. it can be seen from the figure that the corrosion-potential of the sample before crystallization basically reached a stability at 5min, about -0.1V; and that of the crystallized one was about - 0.19V, presenting a more negative potential. This illustrates that in the Hank's solution of PH7.45, the deposited TiNi SMA sample at current density of 5mA/cm2 before crystallization displayed a better thermodynamic stability.

Fig. 1-24. Optical microscope result by current 5mA/cm2 for 4 min (a, b unannealed; c, d annealed)

Figure 1-24 shows the optical microscope morphologies of the deposited samples at current density of 5mA/cm2 for 4min before and after crystallization. It can be seen from figure 1- 22a and 1-22b that Ti-O film was even and smooth, and from 1-22c and 1-22d that the film distributed in type of tiny particles. Similarly, this is because of that the crystallization treatment made the hydrate (mainly Ti(OH)4) of amorphous Ti dehydrate water molecules and the crystal lattice recombinate to form the anatase type of TiO2. Because the film was thinner, the crystal defects caused in the process of crystallization was less, so that the effect

Figure 1-25 shows the corrosionpotential-time curves of the deposited samples at current density of 5mA/cm2 before and after crystallization in Hank's solution of PH7.45. it can be seen from the figure that the corrosion-potential of the sample before crystallization basically reached a stability at 5min, about -0.1V; and that of the crystallized one was about - 0.19V, presenting a more negative potential. This illustrates that in the Hank's solution of PH7.45, the deposited TiNi SMA sample at current density of 5mA/cm2 before

a 200× b 1000×

c 200× d 1000× Fig. 1-24. Optical microscope result by current 5mA/cm2 for 4 min (a, b unannealed; c, d

on the film's properties would not be serious.

annealed)

crystallization displayed a better thermodynamic stability.

Fig. 1-25. The Corrosion potential change plotted as a function of time of annealed and unannealed samples in Hank's solution (PH7.45)

Figure 1-26 shows the polarization curves of the deposited samples at current density of 5mA/cm2 before and after crystallization at 450C. It can be seen from the polarization curves that the breakdown potential of the uncrystallized sample was 1.3V, the current density in the process of polarization always was lower than that of the crystallized sample, and a longer passivation region emerging illustrates that the deposited sample at current density of 5mA/cm2 before crystallization presented a better corrosion resistance in Hank's solution of PH7.45. Due to the single structure of unannealed Ti-O film, it is easy for the crystallized sample to arise more crystal defects, and bring about the crystal structure not unitary, thus presenting a poor corrosion resistance. From the cathodic-polarization curves of view, the slope of the sample 's cathodic-polarization curves before crystallization was larger, but the current density was lower than that of the crystallized sample, thus presenting a better cathodic-polarization property.

Fig. 1-26. The anodic-polarization curves of annealed and unannealed samples in Hank's solution (PH7.45)

Ti-O Film Cathodically-Electrodeposited on


cracking-lumping-peeling off-cracking again".

E/V(SCE)

solution (PH6.13)

the Surface of TiNi SMA and Its Bioactivity and Blood Compatibility 25

1. heat treated 2. untreated

012345

1

2

lgi/nAcm-2

Fig. 1-28. The anodic-polarization curves of annealed and unannealed samples in Fusayama

Figure 1-29 shows the optical microscope morphologies of the cathodically-electrodeposited samples at current density of 20mA/cm2 for different times. It can be seen from the figure that after deposited for 3min, obviously the Ti-O film began to emerge on the surface of TiNi SMA, fine, close and even, but there were some holes, which might be caused by the gas produced on the sample surface when cathodically-electrodeposited at the high current density. After deposited for 5min, the film thickness of the sample surface got increased apparently, but there were some big cracks emerging, adhered lumpily on the sample surface. After deposited for 7min, the Ti-O film fluffily adhered to the sample surface, bonding closely, with less cracks relatively, but uneven. The difference between the morphology of the sample deposited for 9min and that of the one for 7min was not great, fluffily adhered to the sample surface the same. Apparently, the surface of the sample deposited for 2h was different from that of the ones for short times, and its surface had no thicker fluffy Ti-O film, but with some holes. Similarly, the sample deposited for 12h had no fluffy Ti-O film emerging on the surface, but with some small cracks and porosities in different sizes and deepnesses. This is because of that at beginning of depositing, the Ti-O film nucleated and grew quickly on the sample surface, and its thickness increased quickly too. At the same time, with the increase of the thickness in the process of nucleation and growth, the Ti-O film on TiNi SMA surface continuously dissolved and peeled off, besides a lot of gas produced on the sample surface, that brought about uneven stress emerging internal Ti-O film, thus causing distributing lumpily. Because current density was higher, Ti-O film continuously formed and peeled off with time, thus gradually forming the fluffy distribution, and uneven very much. When to a certain time, because of reduction of Ti4+ in electrolyte, that made the level of Ti-O film peeling off far higher than that of forming, until the fluffy film disappearing completely, so some porosities in different sizes and deepnesses emerging. The cracks emerged in figure 1-27f might be caused by long time depositing, when drying, under the uneven stress, thus producing cracks. In short, at current density of 20 mA/cm2, Ti-O film was a growth process of "thickening-

**1.3.7 Effect of cathodic-electrodeposition time on surface morphology** 

Figure 1-27 shows the corrosionpotential-time curves of the cathodically-electrodeposited TiNi SMA samples before and after crystallization at 450C in Fusayama solution of PH6.13. It can be seen from the anodic-polarization curves that the stable corrosion potential of uncrystallized sample was more positive relatively, about -0.12V, but that of the crystallized one was more negative. That illustrates that the uncrystallized sample in Fusayama solution of PH6.13 possessed similarly a better thermodynamic stability.

Figure 1-28 shows the polarization curves of cathodically-electrodeposited TiNi SMA samples before and after crystallization at 450C in Fusayama solution of PH6.13. It can be seen from the anodic-polarization curves that at lower potential, the crystallized sample's in the process of anodic-polarization was lower than the uncrystallized one's, but the uncrystallized sample's passivation region was shorter, from 0.5V to 0.7V, with a 200mV span. On the other hand, the passivation region of uncrystallized one was very long, from 0.4V to 1.3V, with a 900mV span. It can also be seen that the passivated film of crystallized sample began to breakdown from 0.8V, its current density increased quickly and far away exceeded that of the uncrystallized one. The sample before crystallization was still in a passive state, until 1.3V, the current density began to increase apparently, and its breakdown potential was 500mV higher than that of crystallized sample, presenting a better property of anti-corrosion. Similarly, this is because of that the crystal defects caused by crystallization brought about the drop of the property of anti-corrosion. From the cathodicpolarization curves of view, the uncrystallized sample similarly presented a better cathodicpolarization property.

Fig. 1-27. The Corrosion potential change plotted as a function of time of annealed and unannealed samples in Fusyama solution (PH6.13)

Figure 1-27 shows the corrosionpotential-time curves of the cathodically-electrodeposited TiNi SMA samples before and after crystallization at 450C in Fusayama solution of PH6.13. It can be seen from the anodic-polarization curves that the stable corrosion potential of uncrystallized sample was more positive relatively, about -0.12V, but that of the crystallized one was more negative. That illustrates that the uncrystallized sample in Fusayama solution

Figure 1-28 shows the polarization curves of cathodically-electrodeposited TiNi SMA samples before and after crystallization at 450C in Fusayama solution of PH6.13. It can be seen from the anodic-polarization curves that at lower potential, the crystallized sample's in the process of anodic-polarization was lower than the uncrystallized one's, but the uncrystallized sample's passivation region was shorter, from 0.5V to 0.7V, with a 200mV span. On the other hand, the passivation region of uncrystallized one was very long, from 0.4V to 1.3V, with a 900mV span. It can also be seen that the passivated film of crystallized sample began to breakdown from 0.8V, its current density increased quickly and far away exceeded that of the uncrystallized one. The sample before crystallization was still in a passive state, until 1.3V, the current density began to increase apparently, and its breakdown potential was 500mV higher than that of crystallized sample, presenting a better property of anti-corrosion. Similarly, this is because of that the crystal defects caused by crystallization brought about the drop of the property of anti-corrosion. From the cathodicpolarization curves of view, the uncrystallized sample similarly presented a better cathodic-

0 10 20 30 40 50 60

 annealed unannealed

t / m i n

Fig. 1-27. The Corrosion potential change plotted as a function of time of annealed and

of PH6.13 possessed similarly a better thermodynamic stability.

polarization property.


unannealed samples in Fusyama solution (PH6.13)




E/V(SCE)



0.00

Fig. 1-28. The anodic-polarization curves of annealed and unannealed samples in Fusayama solution (PH6.13)
