effect of silver addition on corrosion resistance and … · 2015-11-21 · in this study table 3....
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Effect of Silver Addition on Corrosion
Resistance and Biocompatibility of
Nickel-Titanium alloy
Uk-Hyon Joo
Department of Dentistry
The Graduate School, Yonsei University
Effect of Silver Addition on Corrosion
Resistance and Biocompatibility of
Nickel-Titanium alloy
Directed by Professor Kyoung-Nam Kim
The Doctoral Dissertation
Submitted to the Department of Dentistry,
the Graduate School of Yonsei University
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
Uk-Hyon Joo
July 2007
This certifies that the Doctoral Dissertation
of Uk-Hyon Joo is approved
-----------------------------------------------
Thesis Supervisor : Kyoung-Nam Kim
-----------------------------------------------
Thesis Committee Member #1 : Kwang-Mahn Kim
-----------------------------------------------
Thesis Committee Member #2 : Yong-Keun Lee
-----------------------------------------------
Thesis Committee Member #3 : Chung-Ju Hwang
-----------------------------------------------
Thesis Committee Member #4 : Keun-Taek Oh
The Graduate School
Yonsei University
July 2007
감감감사사사의의의 글글글
6년전 새로운 세계로의 설레임과 두려움을 가지면서 학교로 돌아온 후
많은 시간이 흘러갔습니다. 작은 결실을 맺고 앞으로 나아가게
되었습니다. 항상 따뜻한 말씀과 더불어 정열적인 추진력으로 저를
지도해주신 김경남 교수님께 먼저 감사드립니다. 부족한 논문임에도
많은 조언과 격려를 해주신 김광만 교수님, 이용근 교수님, 황충주
교수님께 이 자리를 빌어 감사드립니다. 항상 정열적인 탐구 자세를
보여주시고 새로운 세계로의 길을 보여주신 오근택 박사님께
감사드립니다.
낯선 환경에서 적응할 수 있도록 관심과 조언을 해주신
치과생체재료공학교실의 김남이 선생님, 이상배 선생님께도 감사의 뜻을
안 남길수 없습니다. 많은 시간을 함께 보냈던 김연웅 박사님, 이영주
선생님 또한 빠질 수 없는 동료였습니다. 그리고 바쁜 의국 생활을 잘
이끌었고, 이끌고 있는 정해경 선생님, 김우현 선생의 관심도 잊을 수
없습니다. 많은 시간을 함께 하지 못했던 후배들, 오영일, 김동현,
김민철, 이세호, 이병현, 임기형. 대학원생활을 통해 많은 대화를
나누지 못해 아쉽습니다.
연구 활동에 있어 다른 관점을 보여주시고 조언을 해주신 지질자원
연구원 최국선 박사님께 늦게나마 감사의 뜻을 전할 수 있게 되어서
기쁩니다. 금속공학 선후배 관계를 떠나 회사 조직 내에서 많은 조언과
용기를 주신 덕창이 형. 언제 조용히 좋은 시간을 보냈으면 합니다.
초기 대학원 생활을 함께하면서 잘 챙겨주지 못했던 심형민, 박지호,
강동국. 앞으로 각자의 분야에서 좋은 결과를 낼 수 있을 것입니다.
우리 팀의 막내 문승균, 탁창우. 따뜻한 말을 많이 해주지 못해
미안하네여.
회사 생활과 연구 활동을 병행에 있어서 관심과 격려를 해주었던 우리
BMK 식구 들. 감사합니다. 여러분들의 도움이 없었다면 좀 더 험난한
시간을 보내게 되었을 것입니다.
나의 오랜 벗. 이왕준, 김준철, 정현수, 박성언. 각자의 위치에서
열심히 살고 뛰어다니는 든든한 동반자들입니다. 황상현, 한상윤,
오홍석, 조인제 자주연락을 나누진 못해도 항상 그대들을 생각하면
가슴이 탁 트입니다.
마지막으로 음으로 양으로 많은 격려를 해주신 어머니, 누님과 매형,
장인어른과 장모님. 드디어 긴 시간이 지나고 부족하나마 결실을 보게
되었습니다. 더 열심히 뛰겠습니다. 저에게 너무나 소중한 저의 아내
류선임과 우리 연재. 항상 가족은 저에게 버팀목이 되어 주고 있습니다.
사랑합니다.
항상 열정적인 자세로 더 나은 모습을 보여드리겠습니다.
2007년 7월
주 욱 현 올림
i
TABLE OF CONTENTS
LIST OF FIGURES .………………………………………………………… ⅲⅲⅲⅲ
LIST OF TABLES . ….……………………………………………………… ⅳⅳⅳⅳ
ABSTRACT ..………………………………………………………………… ⅴⅴⅴⅴ
I. Introduction ……………………………………………………………… 1
1. Shape memory effects ..………………………………………………… 2
2. Superelasticity ..………………………………………………………… 3
3. Biocompatibility of the nickel-titanium alloys ……………………… 5
II. Materials and Methods ..………………………………………………… 9
1. Materials ...……………………………………………………………… 9
2. Constituent analysis of the alloys ……………………………………… 10
3. Phase identification and microstructure observation .……………… 10
4. Transformation temperature measurement ………………………… 11
5. Electrochemical test …………………………………………………… 11
6. Microhardness test ……………………………………………………… 12
7. Surface characterization of the alloys .………………………………… 13
8. Ion release test .………………………………………………………… 13
9. Cytotoxicity ..…………………………………………………………… 14
10. Statistical analysis ……………………………………………………… 17
ii
III. Results …………………………………………………………………… 18
1. Chemical compositions ………………………………………………… 18
2. Phase identification …………………………………………………… 20
3. Transformation temperature ………………………………………… 22
4. Electrochemical property : Corrosion resistance .…………………… 24
5. Microhardness test .…………………………………………………… 28
6. Surface characteristics ..……………………………………………… 29
7. Ion release test ………………………………………………………… 36
8. Cytotoxicity ...…………………………………………………………… 37
IV. Discussion .……………………………………………………………… 39
V. Conclusions ……………………………………………………………… 50
References …………………………………………………………………… 52
Abstract (in Korean) ………………………………………………………… 59
iii
LIST OF FIGURES
Figure 1. The schematic diagram of the shape memory effect and .……… 4
superelasticity of nickel-titanium alloy
Figure 2. The schematic diagram for the transformation temperature .… 12
determination
Figure 3. X-ray diffraction patterns of experimental nickel-titanium …… 20
alloys
Figure 4. Photomicrographs of (a) NT and (b) NTS3 ..…………………… 21
Figure 5. Transformation temperatures of experimental nickel ………… 22
-titanium alloys measured using a differential scanning calorimeter
Figure 6. Anodic polarization curves of experimental nickel-titanium .… 24
alloys in artificial saliva
Figure 7. Open circuit potentials behaviors with time (a) and open ...…… 25
circuit potentials value (b) of experimental nickel-titanium
alloys in artificial saliva
Figure 8. Potentiostatic behaviors of experimental nickel-titanium ……… 26
alloys at (a) 250 mV , (b) 0 mV , (c) -250 mV (SCE)
in artificial saliva
Figure 9. Microhardness (Hv) of experimental nickel-titanium alloys ...… 28
Figure 10. XPS survey spectrum for nickel-titanium alloy .……………… 29
after argon ion sputtering
iv
Figure 11. The chemical species on the surface of NT alloy ……………… 31
Figure 12. The chemical species on the surface of NTS2 alloy …………… 32
Figure 13. Nickel ion release according to the period of immersion …… 36
in artificial saliva
Figure 14. The cell viability of experimental nickel-titanium alloys .…… 38
after MTT test
v
LIST OF TABLES
Table 1. Typical properties of nickel-titanium alloys ..…………………… 3
Table 2. Experimental nickel-titanium alloys design used .....……………. 9
in this study
Table 3. Constituents of artificial saliva …………………………………… 12
Table 4. The binding energy of the chemical species ……………………… 14
Table 5. Description of decolorization index, lysis index and ..…………… 16
interpretation of response index used in the agar diffusion test
Table 6. Chemical compositions (wt%) of experimental ..………………… 19
nickel-titanium alloys
Table 7. Transformation temperatures of experimental nickel-titanium … 23
alloys (n=2)
Table 8. Fraction of each element in survey spectrum with change of ..… 30
take off angle
Table 9. The result of chemical species on the surface of experimental … 34
nickel-titanium alloys
Table 10. Cytotoxicity of experimental nickel-titanium alloys tested .…… 37
in this study
vi
ABSTRACT
Effect of Silver Addition on Corrosion Resistance and
Biocompatibility of Nickel-Titanium alloy
Uk-Hyon Joo
Department of Dentistry
The Graduate School, Yonsei University
(Directed by Professor Kyoung-Nam Kim, D.D.S., Ph.D.)
The equiatomic or near-equiatomic nickel-titanium alloys are unique materials
which possess a shape memory effect and superelasticity and they have come to be
widely used not only in aerospace engineering but also in industrial and medical
fields. However the properties of such alloys are extremely sensitive to the precise
nickel-titanium ratio, the addition of alloying elements and processing etc.. For the
purpose of improving fatigue property, a third element such as Cu and Fe is added to
nickel-titanium alloy. But because nickel-titanium alloys form unstable passive film
or reveal lowered corrosion resistance depending on third elements, a cautious
approach is needed in medical applications which require high corrosion resistance.
The purpose of this research was to investigate the effect of silver addition to nickel-
titanium alloys on corrosion resistance and biocompatibility for biomedical
application. Arc melting, homogenization, hot rolling and solution heat treatment
were performed to prepare silver added nickel-titanium alloys. First, the physical
properties of the experimental nickel-titanium alloys were investigated by phase
identification, phase transformation temperature and microhardness. The corrosion
resistance was evaluated by electrochemical test. The effect of silver addition on the
biocompatibility of the alloys was studied by surface characterization, ion release test
and cytotoxicity.
In the case of silver added nickel-titanium alloys, the actual silver contents were
less than the silver contents added in this study. The recovery rate for silver range was
vii
28.48 ~ 41.50%. The silver added nickel-titanium alloys had mixed austenitic and
martensitic phase and exhibited high martensitic fraction. Silver added nickel-
titanium alloys increased the transition temperature range and did not show an
increase in hardness value, compared with nickel-titanium alloy (p>0.05). From the
results of the electrochemical test, the silver addition was considered to improve
corrosion resistance and form a stable passive film. In potentiodynamic test, silver
added nickel-titanium alloys had low passive current densities below 10 ㎂/cm2. Also,
the current densities of alloys rapidly decreased with immersion time and
subsequently showed a stable potentiostatic behavior below 1 ㎂/㎠. According to
high resolution spectral analyses for spectra, the surface film formed on experimental
nickel-titanium alloys was mainly titanium oxide, such as TiO2. For the silver added
nickel-titanium alloys, nickel element was not observed in the outer layer of the
passive film and the silver existed as metallic state in the inner layer of the passive
film. From the results of ion release test, there was no significant difference in nickel
ion release depending on silver content, compared to nickel-titanium alloy (p>0.05).
The silver added nickel-titanium alloys showed almost no toxicity and the cell
viability above 80%, compared to nickel-titanium alloy.
From the above results, we concluded that silver added nickel-titanium alloys
increased their transformation temperatures, strengthened their corrosion resistance
and formed a stable oxide film. Further study on the biocompatibility, the chemical
stability of the passive film, transformation temperature change, and the working
process for these alloys is necessary to ensure that these materials can be safely used
in the dental and medical fields.
Key words: Biocompatibility, Corrosion resistance, Electrochemical property,
Ni ion release, Passive film, Surface characterization, Nickel-titanium alloy,
Silver (Ag) addition
1
Effect of Silver Addition on Corrosion Resistance and
Biocompatibility of Nickel-Titanium alloy
Uk-Hyon Joo
Department of Dentistry
The Graduate School, Yonsei University
(Directed by Professor Kyoung-Nam Kim, D.D.S., Ph.D.)
I. Introduction
The nickel-titanium alloys were first developed in the early 1960s by the Naval
Ordnance Laboratory in the U.S., which was an accidental discovery obtained during
the research on aerospace engineering. The new alloy was named Nitinol. The "Ni"
and "Ti" are the atomic symbols for nickel and titanium. The "NOL" stands for the
Naval Ordnance Laboratory where it was discovered. At that time Nitinol was
reported to exhibit the shape memory effect. The shape memory effect of
nickel-titanium alloys attracted great attention of scientists and their in-depth study on
the property resulted in nickel-titanium alloys with excellent elastic recovery and
superelasticity. The equiatomic or near-equiatomic nickel-titanium alloys are unique
materials with a shape memory effect and superelasticity. As a result, nickel-titanium
alloys with the shape memory effect and superelasticity came to be widely used not
only in aerospace engineering but also in industrial and medical fields. Shape memory
alloys are rapidly becoming a promising class of functional materials due to their vast
potential for applications in various heat-sensitive devices and even heat engines.
2
1. Shape memory effects
The nickel-titanium alloys contain 55 wt% nickel and 45 wt % titanium. The
nickel-titanium alloys have an inherent ability to alter their type of atomic bonding
which results in unique changes in the mechanical properties. Such changes in
mechanical properties are related to a function of temperature and stress.
The nickel-titanium alloys at high temperature show the BCC structure which is
referred to as the austenite phase or parent phase. So the structure of its alloys at low
temperature is the HCP structure which is referred to as the martensite phase or
daughter phase. When it is cooled through a critical transformation temperature range
(TTR), the nickel-titanium alloy shows dramatic changes. The crystal structure of the
alloy changes and it is known as the martensitic transformation. The martensitic
transformation is closely linked to a solid-solid phase transformation and occurs
without diffusion. This phenomenon causes a change in the physical properties of the
alloy and gives rise to the shape memory effect (Buehler WJ et al., 1968). This shape
memory effect of the nickel-titanium alloy is the result of a phase transformation from
martensite phase to austenite phase. When nickel-titanium alloy is exposed at low
temperature(below Ms temperature), the alloy is the twinned-martensite state and its
state can easily be deformed to a single orientation by the de-twinning
process(re-orientation of variant of the martensitic phase) to deformed martensite.
Shape memory effect is observed when nickel-titanium alloys are formed into a
certain shape at a high temperature and memorized it. The alloy does not go back to
its original shape when transformed under the martensite transformation temperature,
but when it is heated over higher temperature than austenite transformation
temperature, it goes back to its original shape. The total atomic movement between
adjacent planes of atoms is less than full interatomic distances. In this way,
deformations even up to 8 % strain can be recovered.
The schematic diagram of the shape memory effect of nickel-titanium alloy is
shown in Figure 1. The nickel-titanium alloy in martensite phase is more ductile than
in austenite phase. Table 1 shows the properties of the nickel-titanium alloy in each
phase.
3
Table 1. Typical properties of nickel-titanium alloys (Thompson SA, 2000)
Austenite Phase Martensite Phase
Density (g/ cm3)
Melting Point ( )℃
Ultimate Tensile Strength (MPa)
6.45
1310
690-1380
Young’s Modulus (GPa)
Yield Strength (MPa)
120
379
50
138
2. Superelasticity
The transformation from austenite to martensite can be occurred by the stress. For
most metals, in general, an external force applied to the materials result in a
permanent deformation. For nickel-titanium alloys, however, it is not the case. When
a stress is applied to nickel-titanium alloys, stress-induced martensitic transformation
occurs, resulting in the so-called superelasticity. Superelasticity refers to the ability of
nickel-titanium alloys to form stress-induced martensite on loading and to return to
the original shape on unloading.
At a temperature above the Af temperature, the martensitic phase can be induced
by straining the sample. After reaching a critical stress, the sample starts to transform
into martensite, the value of stress increases linearly with temperature. During further
straining, the stress at which the transformation occurs is almost constant until the
material is fully transformed. A continuous straining will lead to elastic loading of the
martensite, followed by plastic deformation. When the strain is limited to the start of
elastic loading of the martensite and the applied stress is again released, the reverse
transformation will occur at a lower stress level than during loading, leading to the
reverse movement of the first induced strain (Humbeeck VJ et al., 1998).
When an external force is applied on the alloys, the changes associated with the
transition and orientation between the austenite and martensite are generated on its
alloy. If the force or stress decreases, then nickel-titanium alloys reverse to original
shape without permanent deformation. Figure 1 shows the schematic diagram of
4
superelasticity of nickel-titanium alloy.
(a) (a) (a) (a) shapeshapeshapeshape memory effect memory effect memory effect memory effect
(b) superelasticity(b) superelasticity(b) superelasticity(b) superelasticity
Figure 1. The schematic diagram of the shape memory effect and
superelasticity of nickel-titanium alloy.
1111
2222
3333
Heating
1111 2222 Loading
Unloading
5
3. Biocompatibility of the nickel-titanium alloys
Nickel-titanium alloys generally have high resistance to corrosion, good working
ability, and excellent stability in cyclic applications. Most of the earlier developments
of shape memory alloys were used as the thermally activated parts of actuation and
coupling. In recent years, a large number of commercial applications have arisen for
biomedical alloys (Melton KN, 1994; Pelton AR et al., 1995; Besselink PA, 1995). In
particular, the greatest commercial successes for shape memory alloys are linked with
the use of superelasticity (Pelton AR et al., 1995).
Nickel-titanium shape memory alloys have attracted considerable attention as
materials for medical implants and spine fixation devices (Duerig T et al., 1999). It is
known that the biocompatibility of the implants fabricated from nickel-titanium
depends on a corrosion resistant titanium oxide layer avoiding the allergic and toxic
effects of nickel (Humbeeck VJ et al., 1998). However, nickel either in metallic or in
oxidized state is also detected on the surface of nickel-titanium and its amount is
depending on the surface treatment. Nickel-titanium alloys are widely used for dental
orthodontic applications because of its high-quality mechanical (Moorleghem WV et
al., 1998; Torrisi L, 1999; Brantley WA et al., 2001) and anticorrosion (Moorleghem
WV et al., 1998; Rondelli G et al., 2000; Thierry B et al., 2000; Shabalovskaya SA,
2001; Huang HH et al., 2003) properties. Nickel-titanium alloy wires have been used
extensively for alignment and leveling of teeth in the initial orthodontic treatment step
even though they have a few disadvantages. It is difficult to attach orthodontic
accessories to the wires and the wires are hard to permanently deform. However,
although there is a protective passive film on the nickel-titanium wire, Ni and/or Ti
ions may still be released from the alloy surface into the oral environment through
corrosion processes (Huang HH et al., 2003). The corrosion resistance of orthodontic
wire is an important factor in determining its biocompatibility because the corrosion
process has negative consequences on biocompatibility. It is clear that the risk
potential associated with the corrosion of Ni-containing alloys is the adverse effect of
Ni (Koster R et al., 2000; Ryhanen J et al., 1997; Guyuron B et al., 1992; Veldhuizen
AG et al., 1998). Many orthodontists desire nickel-titanium wires with an elastic
6
modulus which is inherently variable, or which can be varied by manipulation in
order to optimize the tooth movement. Also many engineers have researched the
application for the root canal instruments.
The extensive use of nickel-titanium alloys in the production of stents, especially
delicate ones like vascular stents, imposes severe requirements on surface
homogeneity and stability. The nickel-titanium alloys which have a self-expanding
property and are able to get a good MRI image are attractive materials for the stents.
In spite of the interesting properties of nickel-titanium alloy and its possible
interesting biomedical applications, the implantation of nickel containing materials in
human body needs great caution. Although nickel is nutritionally essential, it is well
known that nickel is capable of eliciting toxic and allergic responses (Zaprianov ZK,
1983). Therefore, in the biocompatibility estimation of nickel-titanium implant which
attention has been drawn to the potential nickel release, especially when such
nickel-titanium devices are implanted into young patients which results in a
potentially longer host exposure. If nickel-containing alloys are used in human body,
the ions released from them over the long term can cause adverse effects. The
predominant systemic effects in humans of long term exposure to nickel include
allergies, dermatitis, and asthma. Nickel is one of the most common causes of allergic
contact dermatitis, especially in women (Fisher AA, 1986; Fisher JR et al., 1982; Bass
JK et al., 1993). There is some clinical evidence of carcinogenicity resulting from
implant alloys (Smith GK, 1981; Williams DF, 1982). Maijer and Smith have reported
that the factors affecting metallic ion release from orthodontic appliances are
corrosion resistance of the orthodontic materials, galvanic corrosion between
dissimilar metals, or interaction with microbacterial metabolisms in dental plaque,
surface area and cleanness of the appliances etc. (Maijer R et al., 1982, 1986). The
studies have measured the rate/concentration/level of metallic ions released from
metallic appliances in various environments (Park HY et al., 1983; Berge M et al.,
1982; Shin JS et al, 2003), however, most of these studies have reported that
corrosion or ion release from metallic orthodontic appliances does not present any
problem clinically (Huang HH et al., 2003; Barrett RD et al., 1993; Bishara SE et al.,
1993; Kerosuo H et al., 1995). On the other hand, arguments that hypersensitivity to
7
alloys could occur in certain patients with metallic prostheses have generated reports
to the effect that nickel ions from stainless steel released into the oral cavity, or in
contact with soft tissue, can induce hypersensitivity (Romaguera C et al., 1989;
Temesvari E et al., 1988; Dunlap CL et al., 1989).
Although nickel-titanium alloys have good biocompatibility (Ryhanen J et al.,
1999; Kapanen A et al., 2001) and cytotoxicity (Kapanen A et al., 2002; Ryhanen J et
al., 1999), corrosion resistance (Ryhanen J et al., 1997), strength and ductility
(Shabalovskaya SA, 2001) and present no problems for bone formation (Ryhanen J et
al., 1999; Kapanen A et al., 2001) along with specific functional properties such as the
shape memory effect and superelasticity, they have a restricted application when used
in human body due to their high nickel content.
Nickel-titanium alloys are known to be susceptible to thermal cycling fatigue,
which is a limiting factor in the long term usage. Many researchers have studied to
improve the fatigue properties and to decrease the thermal hysteresis range with
ternary NiTi-X alloys. Particularly, third elements such as Cu, Fe, Co and Cr were
added to nickel-titanium. Mohammed ES et al. reported that Cu addition to
nickel-titanium alloys affected their biocompatibility in vitro because of Cu ion
release (Mohammed ES et al, 2001). Cu addition to nickel-titanium alloys resulted in
increased susceptibility to pitting corrosion. Also Ni and Cu dispersed in oxide film in
the metallic state and oxide layer was not stable (Iijima M et al, 1998).
Therefore, it is necessary to develop new alloys that can minimize the adverse
effects of nickel release or search for other elements that can substitute for the nickel
element in nickel-titanium alloys for biomedical applications. The elimination of
surface nickel and preparation of completely passive surface layers that are
structurally and chemically uniform is a desirable target to pursue in the development
of stable nickel-titanium alloys surfaces for long-term implantation.
The purpose of this study was to investigate the effect silver addition on corrosion
resistance and biocompatibility of nickel-titanium alloy for biomedical applications.
To investigate the physical properties of silver added-nickel-titanium alloys,
compositional analysis, phase transformation temperature measurement, phase
identifications, microhardness were performed. To evaluate the corrosion resistance
8
and biocompatibility of the alloys, the potentiodynamic tests, open circuit potential
measurements, potentiostatic tests, surface characterization, ion release test, and
cytotoxicity evaluations were performed.
9
II. Materials and Methods
1. Materials
(1) Alloy design and melting
Arc melter was used to make experimental alloys. The experimental
nickel-titanium alloys used in this study were presented in Table 2. The raw materials
were sponge type titanium (purity 99.99%), granular nickel (99.99%) and granular
silver (99.99%). These alloy samples were made by melting each one five times in an
arc melter.
Table 2. Experimental nickel-titanium alloys design used in this study
at% wt% Element
Alloy Ni Ti Ag Ni Ti Ag
NT 50.00 50.00 0.00 55.00 45.00 0.00
NTS1 49.75 50.00 0.25 54.67 44.83 0.50
NTS2 49.50 50.00 0.50 54.27 44.72 1.01
NTS3 49.25 50.00 0.75 53.87 44.62 1.51
NTS4 49.00 50.00 1.00 53.48 44.52 2.00
(2) Heat treatment and specimen preparation
The experimental nickel-titanium alloys were then heat-treated in a vacuum
furnace at 950 for ℃ 48 hours to homogenize them. Subsequently, they were
hot-rolled to 2 mm thickness at 950 ℃, and then solution-annealed in a vacuum
furnace at 950 for 1 hour℃ and quenched in a water bath. These specimens were step
by step polished with silicon carbide (SiC) papers from 200 to 2000 grit, then
ultrasonically cleaned in acetone and in ethyl alcohol solution for five minutes each.
10
Finally, they were rinsed in distilled water and dried.
2. Constituent analysis of the alloys
Chemical compositions of the experimental nickel-titanium alloys were obtained
using energy-dispersive spectroscopy (Kevex Superdry, Kevex instruments, USA).
Silver in particular, which was added in small quantities to the nickel-titanium alloy,
was analyzed by atomic absorption spectroscopy (6601 model, Shimadzu Co., Japan)
after the silver added nickel-titanium alloys had been dissolved in the mixed solution
of nitric and hydrofluoric acid at about 70 ℃. The elements such as carbon, sulfur,
oxygen and nitrogen were analyzed to investigate the degree of contamination of the
alloys during the arc melting, vacuum heat treatment and hot rolling. Elements such
as carbon, sulfur were detected with a CS analyzer (CS-200 model, LECO, St. Joseph,
MI, US) and oxygen and nitrogen were detected with a NO analyzer (TC-300 model,
LECO, St. Joseph, MI, US).
3. Phase identification and microstructure observation
Phase identification by X-ray diffraction (XRD, D-Max Rint 2400 model, Rigaku,
Japan) was performed using the Kα rays of a Cu target in the 20° 65∼ ° scan range, at
a scan rate of 4°/min. The temperature of measurements was room temperature. The
numbers for the ICDD (International Centre for Diffraction Data) standards used to
index the x-ray diffraction peaks were 18-0899 for austenitic phase and 27-0344 for
martensitic phase.
In order to facilitate microstructure investigation, the alloys were polished using
200 to 2000 grit SiC paper and 0.05 ㎛ diamond paste. Subsequently, they were
ultrasonically cleaned in ethyl alcohol solution, dried, and etched in Kroll’s solution
(95 ml H2O + 3.5 ml HNO3 + 1.5 ml HF) for about 3 min. Optical microscopy was
used to observe the microstructure of the alloys.
11
4. Transformation temperature measurement
Thermal analysis was performed using a differential scanning calorimetry (DSC,
Differential scanning calorimetry 2920, TA Instrument, US). 10.0 ㎎ of each sample
was measured by a electronic balance (A200S, Sartorius, Germany), placed in the
sample pan and covered with an aluminum cover. Each sample was cooled to the
temperature of -50 ℃ and heated to 150 ℃, and cooled again to -50 ℃. The scan rate
was 10 ℃/min. Draw a tangent line from where the DSC heating curve deviates from
the base line. Austenite starting temperature (As) is the temperature at the intersection
point of the tangent line and the base line. Draw a tangent line when the curve reaches
again the base line during the continued heating. Austenite finishing temperature (Af)
is the temperature at the intersection of the tangent line and the base line. After that,
draw a tangent line from where the DSC cooling curve deviates from the base line.
Martensite starting temperature (Ms) is the temperature at the intersection of the
tangent line and the base line. Draw a tangent line again when the cooling curve
reaches again the base line during the continued cooling. Martensite finishing
temperature (Mf) is the temperature at the intersection of the tangent line and the base
line. Figure 2 shows the schematic diagram for the method of transformation
temperature determination.
5. Electrochemical test
To investigate the corrosion properties of each alloy, potentiodynamic testing with
a potentiostat (Electrochemical Interface, SI 1287, Solartron Instrument, Hampshire,
UK) was performed in artificial saliva at a temperature of (37 ± 1) (Gjerdet NR et ℃
al., 1987; Johansson BI et al., 1989). The area exposed to the solution was 1 cm2, the
potential scanning rate 1 mV/sec, and the scanning range used -600~1600 mV (SCE).
Open circuit potentials were measured for two hours and potentiostatic testing was
performed at 250 mV, 0 mV, and –250 mV (SCE) for two hours in artificial saliva.
Table 3 showed the constituent of artificial saliva used in this study.
12
Figure 2. The schematic diagram for the transformation temperature
determination. As, Af, Ms, Mf indicate the austenite starting temperature,
austenite finishing temperature, martensite starting temperature, martensite
finishing temperature, respectively.
Table 3. Constituents of artificial saliva
Constituent Concentration(g/ℓ)
NaCl 0.40
KCl 0.40
CaCl2·2H2O 0.795
NaH2PO4·2H2O 0.780
Na2S·9H2O 0.005
CO(NH2)2(Urea) 1.0
Distilled water 1000 mℓ
6. Microhardness test
A Vickers microhardness tester (MXT-α7E model, Matsuzawa Seiki Co., Japan)
was used to measure the microhardness of each alloy using a 1000 g load. Ten
measurements were obtained on each specimen.
13
7. Surface characterization of the alloys
Chemical elements and chemical state of the surface of the alloys were analyzed
using X-ray photoelectron spectroscopy (XPS). Test specimens were used after
finished potentiostatic test at 250 mV (SCE) and were analyzed immediately after
finishing potentiostatic test by the XPS. After potentiostatic test, test specimens were
removed from the corrosion cell, and all specimens were kept in a vacuum desiccator
prior to analysis. Polished specimens were also analyzed for the purposes of
comparison. XPS spectra were taken with a pass energy of 23.5 eV using
monochromatic Al Kα X-rays produced by the Perkin-Elmer Φ 5800 ESCA system
(Perkin-Elmer, Boston, MA, USA). The base pressure in the chamber was maintained
at 2×10-10 torr during spectra acquisition, and binding energy shifts were referenced
by setting the hydrocarbon peak in the C 1s spectra to 284.8 eV. The spot size was
400 um x 400 um. The accuracy of the measured binding energy was ± 0.2 eV. XPS
data was smoothened by Savitsky-Golay method and visualized using a Shirley
background, and the line shape of the XPS spectra was compiled with a mixed
Gaussian-Lorentzian sum function. In order to obtain an in-depth compositional
profile difference of the oxide films, the take-off angle of the analyzed photoelectrons
with respect to the specimen surface was varied, taken at 15° and 75°. Table 4 shows
the binding energy of chemical species used in this study.
8. Ion release test
Metal ion release tests were performed by immersing the specimens in artificial
saliva kept in a 37 , 5% CO℃ 2 humidified atmosphere incubator. All specimens were
ultrasonically cleaned in ethanol for 1 minute and dried. After cleaning of surface, all
specimens were placed respectively in a sterilized bottle with same surface area to
solution volume ratio. Each specimen immersed in artificial saliva was withdrawn at
different times, ranging from 3 days to 4 weeks, A Graphite Furnace Atomic
Absorption Spectrophotometer (GFAAS, SpectrAA 220FS, Varian, Palo Alto, CA,
USA) was used to measure released metal ions concentration in the withdrawn
14
solution, focusing on nickel and silver content.
Table 4. The binding energy of the chemical species (Unit : eV)
Chemical Species Binding Energy
1. Ti 2p 3/2 (m) 453.9
2. Ti2+ (TiO) 455.1
3. Ti3+ (Ti2O3) 457.3 Ti 2p
4. Ti4+ (TiO2) 458.7
1. Ni 2p 3/2 (m) 852.6
2. Ni2+ (NiO) 853.6 Ni 2p
3. Ni2+ (Ni(OH)2) 855.4
1. O2- 530.2
2. OH- 531.4 O 1s
3. H2O 532.9
1. Ag2+ (AgO) 367.7 Ag 3d
2. Ag 3d 5/2 (m) 368.2
9. Cytotoxicity
(1) Agar diffusion test
The agar overlay method was used to evaluate the cytotoxicity of these materials.
Four specimens were prepared for each material to perform the cytotoxicity testing,
and processed to ensure that a surface area of 1 ㎠ came into contact with agar;
surfaces were sterilized with ethylene oxide gas, and cleansed with distilled water and
dried. Gutta Percha was used as a positive control, and glass as a negative control. A
solution was produced by cultivating L-929 cells (NCTC clone 929), a mouse
fibroblast cell-line, in α-MEM medium. A solution (10 mL) in α-MEM medium was
15
added to a Petri dish and cultivated for 24 hours. Then the α-MEM medium was
removed and 10 mL of Eagle's agar medium at 45 50∼ ℃ was added to each Petri
dish and left to stand at room temperature for 30 minutes. After the Eagle's agar
medium had solidified, neutral red vital stain solution (10 mL) was added slowly to
the center of the dish and then spread over the surface and left for 30 minutes.
Immediately the dyeing solution was removed, the specimens were placed in contact
with the agar and incubated for 24 hours in a 37 , 5% CO incubator. First, the ℃ ℃
Petri dish was placed on top of a white paper, then the zone index was measured after
observing the size of the discolored area, and the lysis index was measured by
calculating the lysed ratio of the cells in the discolored area using an inverted phase
contrast microscope (CK2, Olympus, Japan). Decolorization distance was measured
with a ruler. Decolorization index and lysis index were determined by standard of ISO
7405:1997(E). Table 5. shows the decolorization index, lysis index, response index.
Finally, the response index was measured by the zone and lysis indices.
16
Table 5. Description of decolorization index, lysis index and interpretation of
response index used in the agar diffusion test
Decolorization
(Zone)
index
Description of decolorization
0 No detectable zone around or under sample
1 Zone limited to area under sample
2 Zone not greater 5 mm in extension from sample
3 Zone not greater 10 mm in extension from sample
4 Zone greater than 10 mm in extension from sample,
but not involving entire plate
5 Zone involving entire plate
Lysis index Description of zone
0 No observable lysis
1 Up to 20% of the zone lysed
2 Over 20 % to 40 % of the zone lysed
3 Over 40 % to 60 % of the zone lysed
4 Over 60 % to 80 % of the zone lysed
5 Over 80 % lysed within the zone
Cytotoxicity Response index (zone index/lysis index)
None 0/0
Mild 1/1 ~ 1/5, 2/1
Moderate 2/1 ~ 2/3, 3/1 ~ 3/5, 4/1 ~ 4/3
Severe 4/4 ~ 4/5, 5/1 ~ 5/5
17
(2) Cell viability : MTT assay
The cell viability was determined with the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5
-diphenyltetrazolium bromide) assay (Page M et al., 1988; Mosmann T, 1983).
Extracts of each alloy were prepared in accordance with international standard (ISO
10993-12, 1996). Extraction was performed by autoclaving 121 for 1 hour; the ℃
ratio between surface of test specimen and the volume of extraction media was 3
cm2/mℓ. Each 20 µℓ of extracts were inserted in the cell cultured wells. As a control,
20 µℓ of medium was used, then incubated in a 37 , 5% CO℃ 2 humidified
atmosphere incubator for 24 hours. For the MTT assay, 50 µℓ of MTT solution (5
mg/mℓ) was added to each well of a 96-well plate (BD Falcon, Bedford, MA, USA),
and incubated for 4 hours. The supernatant was removed, and the formazan crystals
produced were dissolved in 200 µℓ of dimethylsulfoxide, and quantified by measuring
their optical density at 570 nm using an ELISA reader (Precision Microplate Reader,
Sunnyvale, CA, USA). Viability rates of control group were set to represent 100%
viability. The results of the other test materials were expressed as a percentage of the
control to yield comparable data.
10. Statistical analysis
SPSS version 12.0 was used for the statistic analysis and significant differences
were analyzed by a one-way ANOVA, followed by the Tukey’s post hoc test. The
significance was accepted at the 95% confidence interval.
18
III. Results
1. Chemical compositions
The chemical compositions of the alloys in this study are presented in Table 6. It
was found that all the alloys contained more than 50 wt% concentration of nickel. The
concentration of nickel was in the range of 52.92% ~ 55.30% and as for Ti, it was in
the range of 44.54% ~ 46.33%. The concentration of silver in the NTS series was in
the range of 0.20 ~ 0.83%. In the nickel-titanium alloys, the concentration of titanium
remained about 45 wt %. When silver was added to nickel-titanium, the silver content
was actually less than the added content based on alloy design. NTS1 ~ NTS4
exhibited relatively low recovery rates for silver as in the range of 28.48 ~ 41.50%.
The concentrations of oxygen, nitrogen, carbon, and sulfur were in the range of
0.02 ~ 0.24%, 0.00 ~ 0.08 %, 0.03 ~0.04, and 0.01% respectively.
19
Table 6. Chemical compositions (wt%) of experimental nickel-titanium alloys
(n=2)
Ni Ti Ag
NT 55.12 ± 1.66 45.01 ± 1.46 -
NTS1 55.30 ± 2.04 44.54 ± 2.02 0.20 ± 0.07
NTS2 54.02 ± 0.46 45.70 ± 1.64 0.33 ± 0.12
NTS3 54.33 ± 1.39 45.28 ± 1.99 0.43 ± 0.15
NTS4 52.92 ± 1.81 46.33 ± 1.98 0.83 ± 0.37
O N C S
NT 0.02 ± 0.01 0.01 ± 0.00 0.03 ± 0.01 0.01 ± 0.01
NTS1 0.02 ± 0.00 0.00 ± 0.00 0.03 ± 0.02 0.01 ± 0.01
NTS2 0.03 ± 0.01 0.00 ± 0.00 0.03 ± 0.02 0.01 ± 0.01
NTS3 0.10 ± 0.04 0.01 ± 0.00 0.03 ± 0.01 0.01 ± 0.01
NTS4 0.24 ± 0.10 0.08 ± 0.00 0.04 ± 0.02 0.01 ± 0.01
20
2. Phase identification
Figure 3 shows the results of the phase identification of the experimental
nickel-titanium alloys. All alloys had mixed phases of austenite with cubic structure
and of martensite with monoclinic structure. The main peak was (110) for austenitic
phase and (002), (-111), (020), (012) for martensitic phase. NT showed a strong
austenitic phase peak. The silver added nickel-titanium alloys, NTS series, showed
the mixed phase with austenitic peak and martensitic peak, and high fraction of
martensite. Furthermore, austenitic and martensitic phases were identified in the
microphotographs of the nickel-titanium alloys presented in Figure 4. They showed
surface distortions which are characteristics of martensitic phase transformation.
20 30 40 50 60 700
200
400
600
800
1000
1200 Austen itic Phase
M artensitic Phase
NTS3
NTS4
NTS2
NTS1
NT
Inte
nsity
(A
rbita
ry U
nit)
D iffrac tion A ng le (2 the ta)
Figure 3. X-ray diffraction patterns of experimental nickel-titanium alloys.
21
(a) NT
(b) NTS3
Figure 4. Photomicrographs of (a) NT and (b) NTS3.
0.01mm mm
Surface distortion
0.01mm
Martensitic Phase with twin structure
22
3. Transformation temperature
The results of the transformation temperature showed that when the alloys were
heated, the As and Af temperature was 28.6～100.9 and ℃ 39.8～132.7 , ℃
respectively, and when the samples were cooled, the Ms and Mf temperature was
25.8～78.8 and ℃ 10.1～47.4 , respectively. ℃ Transformation temperatures of the
alloys were presented in Figure 5 and Table 7. The silver added nickel-titanium alloys,
NTS series, showed significantly higher transformation temperatures than NT.
0 50 100-40
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
Cooling
NTS4
Heating
Exothermic
NTS4
NTS2
NTS2
NT
NT
NTS1
NTS1
NTS3
NTS3
Hea
t fl
ow(m
W)
Temperature(oC)
Figure 5. Transformation temperatures of experimental nickel-titanium alloys measured using a differential scanning calorimeter.
23
Table 7. Transformation temperatures of experimental nickel-titanium
alloys (n=2)
Sample
As ( )℃ ( Austenite
starting temperature )
A f ( )℃ ( Austenite finishing
temperature )
Ms ( )℃ ( Martensite
starting temperature )
M f ( )℃ ( Martensite
finishing temperature )
NT 28.6 ± 6.8 39.8 ± 7.5 25.8 ± 7.3 10.1 ± 5.1
NTS1 90.6 ± 6.9 132.7 ± 2.9 73.8 ± 6.1 32.5 ± 3.2
NTS2 100.9 ± 10.3 123.1 ± 6.6 74.4 ± 3.5 47.4 ± 1.9
NTS3 98.7 ± 6.7 125.8 ± 8.5 78.8 ± 4.1 42.4 ± 5.8
NTS4 72.6 ± 13.2 103.6 ± 2.6 60.1 ± 3.6 30.8 ± 4.9
.
24
4. Electrochemical property : Corrosion resistance
Anodic polarization behaviors of the alloys in artificial saliva at 37 ℃ were
presented in Figure 6. The experimental nickel-titanium alloys showed relatively
good corrosion resistances in artificial saliva. In terms of passive current density of
anodic polarization curves, NT, NTS1 and NTS2 alloys had low passive current
density below 10 ㎂/cm2. NTS4 alloy showed high current density above 100
㎂/cm2. NTS3 had pitting potentials of about 600 mV (SCE).
1 E -3 0 .0 1 0 .1 1 1 0 1 0 0 1 0 0 0 1 0 0 0 0
-1 0 0 0
-5 0 0
0
5 0 0
1 0 0 0
1 5 0 0
2 0 0 0
N T S 1N T S 2
N T
N T S 3N T S 4
Pot
enti
al (
mV
vs.
SC
E)
C u r re n t D e n s ity (1 0 -6 A /c m 2 )
Figure 6. Anodic polarization curves of experimental nickel-titanium alloys in artificial saliva.
The results of the open circuit potentials (OCPs) of experimental nickel-titanium
alloys were presented in Figure 7. Low OCPs indicate relatively faster corrosion and
possibility of increased release of metal ions. Alloys’ OCPs increased initially and
showed stable OCP behaviors with immersion time. OCPs of the nickel-titanium
alloys were in the range of -279 ~ -217 mV (SCE) and did not show the significant
differences (p>0.05).
25
-500
-400
-300
-2000 1000 2000 3000 4000 5000 6000 7000 8000
Time(sec)
Po
ten
tial
(mV
vs.
SC
E) NT
NTS3NTS1
NTS2
NTS4
(a)
-400.00
-350.00
-300.00
-250.00
-200.00
-150.00
-100.00
-50.00
0.00NT NTS1 NTS2 NTS3 NTS4
Alloy
Po
ten
tial
(mV
vs.
SC
E)
(b)
Figure 7. Open circuit potentials behaviors with time (a) and open circuit
potentials value (b) of experimental nickel-titanium alloys in artificial saliva.
26
The results of the potentiostatic testing of nickel-titanium alloys, performed at 250
mV, 0 mV and -250 mV (SCE) in artificial saliva, are presented in Figure 8. At first,
the current density of alloys rapidly decreased with immersion time and subsequently
showed a stable potentiostatic behavior below 1 ㎂/㎠. In 250 mV (SCE), current
density of alloys were in the range of 0.08 ~ 0.5 ㎂/㎠ and those of most alloys
decreased with immersion time. In Figure 8(b), performed at 0 mV (SCE), current
density of alloys were in the range of 0.07 ~ 3.7 ㎂/㎠. The potentiostatic current
density of NTS1 was relatively high and other alloys showed stable passivity. Figure
8(c) is the result of potentiostatic testing performed at –250 mV (SCE). Most of the
alloys exhibited cathodic current density, except for NT, in the range of –0.33 ~ 0.5
㎂/㎠ and showed stable passivity with immersion time.
0.00E+00
1.00E-06
2.00E-06
3.00E-06
4.00E-06
5.00E-06
0 1000 2000 3000 4000 5000 6000 7000 8000
Time(sec)
Cu
rren
t D
ensi
ty(A
/㎠㎠ ㎠㎠)
NTS2
NT
NTS4
NTS1
NTS3
(a)
Figure 8. Potentiostatic behaviors of experimental nickel-titanium alloys at (a)
250 mV , (b) 0 mV , (c) -250 mV (SCE) in artificial saliva.
27
0.00E+00
1.00E-06
2.00E-06
3.00E-06
4.00E-06
5.00E-06
0 1000 2000 3000 4000 5000 6000 7000 8000Time(sec)
Cu
rren
t D
ensi
ty(A
/㎠㎠ ㎠㎠)
NT
NTS4
NTS1
NTS2
NTS3
(b)
-1.00E-06
0.00E+00
1.00E-06
2.00E-06
3.00E-06
4.00E-06
5.00E-06
0 1000 2000 3000 4000 5000 6000 7000 8000
Time(sec)
Cu
rren
t D
ensi
ty(A
/㎠㎠ ㎠㎠)
NTNTS1,2
NTS3NTS4
(c)
Figure 8. (Continued)
28
5. Microhardness test
Vickers microhardness values for experimental nickel-titanium alloys were
presented in Figure 9. Their hardnesses were distributed in the range of 216∼240 Hv,
but the silver added nickel-titanium alloys did not show a hardness increase with
silver contents (p>0.05).
0.0
50.0
100.0
150.0
200.0
250.0
300.0
NT NTS1 NTS2 NTS3 NTS4
Mic
ro V
icke
rs H
ard
nes
s (H
v)
Figure 9. Microhardness (Hv) of experimental nickel-titanium alloys.
29
6. Surface characteristics
02004006008001000120014000
1000
2000
3000
4000
5000
6000
Binding Energy (eV)
-O
1s
-T
i2p3
-C
1s
Figure 10. XPS survey spectrum for nickel-titanium alloy after argon ion sputtering.
The comparative surface characteristics of nickel-titanium alloys were evaluated by
means of X-ray photoelectron spectroscopy (XPS). The XPS results reveal
differences in the chemical compositions of the surface layers of the alloys,
suggesting concomitant differences in their protective characteristics.
After the potentiostatic test at 250 mV (SCE) in artificial saliva, the surface layers
of the nickel-titanium alloys were analyzed by XPS. The survey spectrum of
nickel-titanium alloys after 30 sec of ion sputtering is presented in Figure 10 and C 1s
peak and contamination layers were removed by argon ion sputtering. Table 8 shows
the area fractions of elements at the surface with take off angles set to 15° and 75°.
The silver added nickel-titanium alloy, NTS2, exhibited much higher Ti element
fractions than others in outer layer.
30
Table 8. Fraction of each element in survey spectrum with change of take off angle
O 1s Ti 2p Ni 2p Ag 3d Take off
angle 15deg 75deg 15deg 75deg 15deg 75deg 15deg 75deg
NT 69.64 62.27 27.88 34.55 2.47 3.19 - -
NTS2 65.03 69.22 34.57 26.73 0.40 3.87 0.00 0.19
From the result of the survey spectrum, O, Ti, and Ni elements were observed on
the surface layer of the NT. For NTS2, O and Ti element, and a small amount of Ni
was observed in outer layer and silver was respectively detected in inner layer. The
peak of the S and P was not shown in spectrum.
In order to obtain quantitative results from XPS analysis, curve fitting was
performed, employing the Gaussian-Lorentzian method. The results of curve fitting in
each alloy were presented in Table 9 and Figure 11 ~ 12.
31
Outer Layer
(15 Deg) Inner Layer
(75 Deg)
Ti 2p
4524544564584604620
100
200
300
400
500
600
Binding Energy (eV)
c/s
MAR19109.SPE
4
3
2
452454456458460462
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Binding Energy (eV)
MAR20109.SPE
4
3
2
Ni 2p
850851852853854855856857858300
350
400
450
500
550
600
650
700
Binding Energy (eV)
c/s
MAR19106.SPE
1
2
850851852853854855856857858
1000
1500
2000
2500
Binding Energy (eV)
MAR20109.SPE
2
1
O 1s
5285295305315325335345355360
500
1000
1500
Binding Energy (eV)
2
1
3
528529530531532533534535536
0
500
1000
1500
Binding Energy (eV)
MAR19109.SPE
2
1
3
Figure 11. The chemical species on the surface of NT alloy.
32
Outer Layer
(15 Deg) Inner Layer
(75 Deg)
Ti 2p
4524544564584604620
1000
2000
3000
4000
5000
6000
Binding Energy (eV)
MAR20106.SPE
2
3
4
452454456458460462
0
100
200
300
400
500
600
700
800
900
1000
Binding Energy (eV)
MAR19106.SPE
2
4
3
Ni 2p
850851852853854855856857858200
220
240
260
280
300
320
340
360
380
400
Binding Energy (eV)
c/s
MAR19109.SPE
850851852853854855856857858
2000
2500
3000
3500
4000
4500
5000
Binding Energy (eV)
MAR20106.SPE
2
1
Figure 12. The chemical species on the surface of NTS2 alloy.
33
Outer Layer (15 Deg)
Inner Layer (75 Deg)
O 1s
528529530531532533534535536200
400
600
800
1000
1200
1400
1600
1800
2000
Binding Energy (eV)
1
2
3
528529530531532533534535536
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Binding Energy (eV)
c/s
1
2
3
Ag 3d
36536636736836937037180
90
100
110
120
130
140
Binding Energy (eV)
c/s
MAR19106.SPE
365366367368369370371
500
550
600
650
700
750
Binding Energy (eV)
c/s
MAR20106.SPE
2
Figure 12. (Continued)
34
Table 9. The result of chemical species on the
surface of experimental nickel-titanium alloys
NT NTS2 Ti 2p Spectra
15 deg 75deg 15 deg 75deg
1. Ti 2p 3/2 (m) 0.00 0.00 0.00 0.00
2. Ti2+ (TiO) 1.93 16.15 3.31 11.98
3. Ti3+ (Ti2O3) 55.11 56.75 41.62 57.30
4. Ti4+ (TiO2) 42.96 27.10 55.07 30.71
NT NTS2 O 1s spectra
15 deg 75deg 15 deg 75deg
1. O2- 64.47 64.15 59.07 60.87
2. OH- 32.53 32.56 36.81 34.20
3. H2O 3.00 3.29 4.13 4.94
NT NTS2 Ni2p spectra
15 deg 75deg 15 deg 75deg
1. Ni 2p 3/2 (m) 60.69 74.73 - 69.05
2. Ni2+ (NiO) 31.35 25.24 - 29.20
3. Ni2+
(Ni(OH)2) 7.96 0.03 - 1.75
35
The Ti 2p spectrum was decomposed into four doublet spectra, originating from
Ti0, Ti2+, Ti3+, and Ti4+, according to the binding energy data (Asami K et al., 1993;
Gopel W et al., 1984). The binding energy value of the main peak is characteristic of
titanium in its normal oxidation state, Ti4+, in agreement with data previously reported
for TiO2 ( Asami K et al., 1993; Kaciulis S et al., 1999). From the result of the Ti2p
spectra, Ti oxide state, especially Ti4+ was observed in outer layer, excluding NT
alloy. In inner layer, the fraction of theTi3+ peak was high.
The O 1s spectrum of all specimens can be deconvoluted by three components,
which can be assigned to oxide species (O2-), hydroxide groups (OH-), and adsorbed
water (H2O) (Moulder JF et al., 1995; Lausmaa J et al., 1990; Vogler EA, 1996). From
the results of the fractions of O1s spectra, over 60% of components consisted of O2-
as oxide was observed. According to the high resolution spectral analyses for Ti and
O spectra, the surface film on nickel-titanium alloy was mainly titanium oxide, such
as Ti2O3, TiO2.
From the result of the spectral analysis for the Ni 2p spectra, mostly metallic
nickel and oxidized nickel were observed in inner layer. Especially, nickel element
was not observed in outer layer on NTS2. The silver existed as metallic state in oxide
inner layer.
36
7. Ion release test
The result of the Ni ion release in artificial saliva was shown in Figure 13. The
amount of the Ni ion release increased with immersion time (p<0.05). The amount of
the Ni ion release with silver addition showed no significant difference (p>0.05).
0.00
40.00
80.00
120.00
NT NTS1 NTS2 NTS3 NTS4
Ni i
on
(p
pb
)
3d 1w 2w 4w
Figure 13. Nickel ion release according to the period of immersion in artificial
saliva.
37
8. Cytotoxicity
(1) Agar diffusion test
The cytotoxicity of the nickel-titanium alloys is presented in Table 10. There was
almost no toxicity in any of the silver added alloys as indicated by the response index.
Table 10. Cytotoxicity of experimental nickel-titanium alloys tested in this study.
Zone Index
Lysis Index
Response Index
Cytotoxicity
NTS1 0 0∼ 0 0∼ 0/0 none(-)
NTS2 0 0∼ 0 0∼ 0/0 none(-)
NTS3 0 0∼ 0∼0 0/0 none(-)
NTS4 0 1∼ 1 1∼ 0/1 mild
NT 0 0∼ 0 0∼ 0/0 none(-)
Positive (Gutta Percha)
3 3∼ 4 4∼ 3/4 Moderate(++)
Negative(glass) 0 0∼ 0 0∼ 0/0 none(-)
38
(2) MTT Test
All experimental nickel-titanium alloys showed the cell viability above 80% as
compared control group. The result of the cell viability was shown in Figure 16.
87.990.8
85.1
93.388.6
0.0
20.0
40.0
60.0
80.0
100.0
NT NTS1 NTS2 NTS3 NTS4
Cel
l Via
bili
ty (
%)
Figure 14. The cell viability of experimental nickel-titanium alloys after MTT
test.
39
IV. Discussion
Many alloy systems with the shape memory effect or superelastic effects have
been developed such as Ti-Ni, Ni-Al, Ag-Cd, Au-Cd, Cu-Al-Zn, Cu-Au-Zn, Cu-Sn,
Cu-Zn, In-Tl, In-Cd, Ti-Ni-Cu, Ti-Ni-Fe, and Cu-Zn-Al. But only a few of them have
been developed as a commercial product, namely, binary NiTi, ternary NiTi -Cu, and
Cu-Zn-Al. Most of new shape memory alloy applications are based on NiTi, or NiTi
-Cu and NiTi-Nb alloys (Humbeeck JV et al., 1998). It is well known that the phase
transformation between high temperature austenite structure and low temperature
martensite structure is a reversible reaction (Khier SE et al., 1991). At high
temperature, the nickel-titanium alloy with a austenite phase which provides the cubic
structure of body centered cubic (BCC) or face centered cubic (FCC), cannot be
transformed easily. But at low temperature, martensite phase exhibits high ductility,
so it can be easily transformed.
Nickel-titanium alloys with these good properties are available, mainly for dental
applications, in the shape of thin wires and thin films. Also they are used for actuators
and sensors in electrical applications. Many researchers and clinicians, however, have
worried about nickel allergies of nickel-titanium alloys. The nickel-titanium alloys
have been used carefully in orthopedic and cardiac surgery in spite of the possibility
that the nickel will cause adverse effect such as allergies and carcinogenicity (Fisher
AA, 1986; Fisher JR et al., 1982; Bass JK et al., 1993; Smith GK, 1981; Williams DF,
1982).
In this study, the nickel-titanium alloys were made by the addition of silver. NT
alloy showed the high recovery rates of the alloying elements, but the silver added
nickel-titanium alloy, NTS alloys, showed low recovery rate for silver. These results
indicats that silver evaporated during the melting process because of significant
melting and boiling point differences among silver, titanium, and nickel (Ag; 961 , ℃
2155 , Ti; 1667 , 3285 , Ni; 1455 , 2920 ). T℃ ℃ ℃ ℃ ℃ hat is, the vapor pressure of
silver is higher than that of Ti or Ni. For the analysis of alloy constituents, the levels
of nickel, titanium, copper and cobalt were determined by energy dispersive
spectroscopy (EDS), but because the silver contents of the NTS alloys were below the
40
detection limit of EDS, the silver contents were analyzed by atomic absorption
spectroscopy (AAS) after the NTS alloys were dissolved in mixed acid solution. This
anomaly was considered to be due to the evaporation of silver compound by the
reaction with the acid solution during the dissolution of the NTS alloys, and the
evaporation of silver in arc melting
When the alloys are deformed to martensitic transformation, some distortion
occurs in each parent grain. This phenomenon causes a discrimination of martensitic
and austenitic phases. Phase fractions of the alloys depended on the contents and
types of added elements. The NTS alloys showed high martensitic phase fractions in
XRD patterns. These results were consistent with the results of phase transformation
temperature measurement. After heating, the austenitic transformation starting
temperatures (As) of the NTS alloys were higher than those of the other
nickel-titanium alloys. These results indicats that when the NTS alloys were solution
heat treated, the austenitic phase was transformed by high fraction to martensitic
phase during the cooling process. The NT alloy showed significantly lower As
temperature than the NTS alloys , which indicates that they had high austenitic phase
fraction because of the low driving force on the martensitic phase transformation
during cooling.
Transformation temperatures of the nickel-titanium alloys were dependent on the
chemical composition, working process, and heat treatment, etc. Especially
nickel-titanium alloys are extremely sensitive to the precise Ni to Ti ratio and alloying
elements. It was reported that at equiatomic composition, if the nickel contents are
increased by 1%, the transformation temperature is decreased by 100 ℃ (Hahn KH et
al., 1991). Binary alloys are commonly available with Ms temperature between
–50 100∼ ℃ and ternary alloys are available with Ms temperatures down to –200 . ℃
The deformation behavior and superelasticity can be improved by the addition of a
third element to equiatomic or near-equiatomic nickel-titanium alloys. A small
amount of Cr is effective in lowering the transformation temperature and in
improving rigidity. The addition of Cu and Nb is also effective in reducing stress
hysteresis and in enhancing the stability of the superelasticity against cyclic loading
(Boyer R et al., 1990). Oxygen forms inclusions such as a Ti4Ni2Ox, lower Ms, retard
41
grain growth, and increase strength. Nitrogen has similar effects as oxygen (Boyer R
et al., 1990). Fe, Al, Cr, Co and V tend to be substituted for nickel, but sharply
depress Ms (Boyer R et al., 1990). Pt and Pd tend to decrease Ms in small quantities
(5 10%), then tend to increase M∼ s temperature e at larger quantities, eventually
achieving Ms temperatures as high as 350℃ (Boyer R et al., 1990; Lindquist PG et al,
1990). Zr and Hf have been reported to increase Ms temperature and especially Hf has
been attractive as high-temperature shape memory effect alloy because the Ms
temperatures of this alloy was 270℃ (Boyer R et al., 1990).
To date, the effects on nickel-titanium of various elements other than Ag have
been studied. In this study, a small amount of silver addition to nickel-titanium was
found to increase the As temperature of nickel-titanium-Ag alloys. Silver causes a
large increase in Ms temperature.
The adverse effects of nickel in human body can be reduced by improving
corrosion resistance. It is important to improve the corrosion resistance of
nickel-titanium alloys in order to minimize the adverse effect of nickel. In this study,
silver was added to improve the corrosion resistance.
For medical devices of metallic biomaterials, corrosion resistance and chemical
stability of metals in human body is a very important factor. To evaluate the corrosion
resistance and electrochemical property of nickel-titanium alloys, potentiodynamic
and potentiostatic test were performed. These tests are general corrosion tests and
make it possible to predict corrosion property based on electrochemical data. From
anodic polarization curves of the nickel-titanium alloys evaluated in artificial saliva,
passive current density and pitting (breakdown) potentials were observed. The
nickel-titanium alloys with a low passive current density in artificial saliva were
considered to have high corrosion resistance due to the presence of a stable passive
film. Because artificial saliva contains a lot of chlorine ions which are aggressive and
able to break down the passive film, pitting corrosion can occur at a certain potential.
The pitting potential is defined as the potential at which the passive film is broken
down. Among the experimental nickel-titanium alloys, NTS3 showed pitting
potentials above 300 mV (SCE). The pitting potentials of nickel-titanium alloys were
higher than those of Ni-Cr alloys, compared with the pitting potentials of other
42
prosthetic alloys in oral environment (Senay C et al., 1992; Gil FJ et al., 1999).
Among the experimental nickel-titanium alloys, NTS4 showed the highest passive
current density, indicating the instability of its passive film. Because silver solubility
in nickel-titanium is very low, excessive amount of silver addition will lead to
precipitation causing compositional inhomogeneity and passive film instability.
The results of the potentiostatic test indicate the stability of the passive films
formed on metal surface. Potentiostatic current density of experimental
nickel-titanium alloys was maintained consistently without drastic increase during test
periods. This means that the passive film of nickel-titanium alloys is
electrochemically stable for a long period in saliva solution or human body fluid.
Heavy alloying of nickel-titanium is considered to initially decrease the stability of
the passive film and finally to destroy it.
In the case of the alloys with the high OCP in test solution, oxidation can be
inhibited, then subsequently the alloys can decrease the possibility of corrosion. In
other words, high OCP means that test materials are in a relatively noble and stable
state in a certain environment. Potentiostatic results also demonstrate that the current
density decreased with immersion time and that the alloys in this study had very
stable passive films in artificial saliva. Nickel-titanium alloys, unlike Ti, were
reported to form Ni-Ti oxide on the surface. These oxides are unstable compared with
TiO2 and can release a tiny amount of Ni2+ ion initially. But in a short time stable
oxides can form on the surface, which shows excellent corrosion resistance (Gjerdet
NR et al., 1987). In this study, when NTS alloys were exposed to corrosion
environment, it is believed that silver-added nickel-titanium alloys have a good
corrosion resistance because of their stable passive films.
In general, there are two kinds of energy to be considered in corrosion process.
The first is a chemical driving force, as expressed by ∆G = - nF∆E that indicates what
corrosion will take place under certain conditions.
where n : the valence of the ion
F : the Faraday constant
∆E : the voltage or potential across the interface between the metal and
the solution.
43
If the free energy for oxidation is less than zero, then oxidation will occur
spontaneously, as in the metals in the alloys used for biomaterials. This potential is a
measure of the reactivity of the metals or the driving force for metal oxidation. The
more negative the potential of a metal in solution, the more reactive it will tend to be.
The second is the energy occurs when positive and negative charges (generally metal
ions and electrons, respectively) are separated from one another during corrosion. The
basic underlying reaction that occurs during corrosion is the increase of the valence
state - that is, the loss of electrons - of the metal atom to form an ion, as expressed by
the equation
M ↔ Mn+ + ne–
This oxidation event may result in free ions in solution, which then can migrate away
from the metal surface or can lead to the formation of metal oxides, metal chlorides,
organo-metallic compounds, or other chemical species. The ions are released into
human fluid or go on to form an oxide or another compound, and the electrons are left
behind in the metal or undergo other electrochemical reactions, such as reduction of
oxygen or hydrolysis of water. This separation between the charges is known as the
electrical double layer and creates an electrical potential across the metal-solution
interface. At equilibrium, the chemical energy balances with the electrical energy,
yielding the Nernst equation (Bard AJ et al, 1980) :
][][
lnM
M
nF
RTEE
no
+
+∆=∆
which states that there is an electrical potential across the interface between the
metal and the solution when metals are immersed in a solution. From this equation, a
scale of reactivity of the metal, known as the electrochemical series, can be
established. This scale ranks the equilibrium potential from most positive (noble) to
most negative (base).
Corrosion of metallic biomaterials lead to material degradation and produce the
release of ions with harmful effects on the organism. The presence or absence of a
protective passive film controls behavior of the metallic materials. The excellent
corrosion resistance of titanium alloys results from the formation of very stable,
44
continuous, highly adherent, and protective oxide films on metal surfaces (Shreir LL,
1979; Schutz RW et al., 1987; Escalas F et al., 1976; Craig RG, 1997). Titanium and
titanium alloys have an extremely high affinity for oxygen and the surface oxide films
on titanium surface form spontaneously and instantly when exposed to air or
environment. However, anhydrous conditions in the absence of a source of oxygen
may result in corrosion of titanium film because the protective film may not be
regenerated if damaged. Titanium alloys, like other metals, are subject to corrosion in
certain environments.
Generally, two essential features determine how and why a metal corrodes. One is
thermodynamic driving force and the other is kinetic barriers. The thermodynamic
driving forces that cause corrosion correspond to the energy required or released
during a reaction. The kinetic barriers to corrosion are related to factors that impede
or prevent corrosion reactions from taking place. Certain metals owe their resistance
to corrosion to the fact that their equilibrium potentials are very positive, indicating
that the chemical driving force for oxidation either is very small and negative or is
positive. Therefore, there is little or no driving force for oxidation unless the
potentials of these materials are raised well above their equilibrium potentials. Noble
metallic elements, such as Au and Pt are that have little or no driving force for
oxidation in aqueous solutions; hence, they tend to remain in metallic form
indefinitely in the human body. However, metallic biomaterials have more negative
potentials, they are much more likely to corrode. For titanium it has a very large
negative potential, this means a large chemical driving force for oxidation. If some
other process such as passivation does not intervene, titanium metal will react
violently with the surrounding oxygen, water, or other oxidizing species and will
revert to its ionic form. The kinetic barriers which govern the corrosion process of
metallic biomaterials prevent corrosion not only by energetic mechanisms but by
physical limitation of the rate at oxidation or reduction processes. The well known
process of passivation, or the formation of a metal-oxide passive film on a metal
surface, is an example of a kinetic limitation to corrosion. In general, kinetic barriers
to corrosion prevent the migration of metal ions from metal to human fluid, the
migration of anions from the solution to the metal, and the migration of electrons
45
across the metal-solution interface. Passive oxide films are the best known forms of
kinetic barriers to corrosion.
It is well known that the excellent corrosion resistance of titanium alloys results
from the formation of very stable, continuous, highly adherent and protective oxide
films on metal surfaces. Their stability and electrochemical property strongly depend
on the composition, structure and thickness of the oxide film. Therefore, the surface
characterization of titanium alloys is very important to understand the property of
titanium alloys. It was investigated the surface compositions and the chemical states
of formed passive oxide film on nickel-titanium alloys after electrochemical treatment
with X-ray photoelectron spectroscopy. Therefore, XPS and AES have been widely
applied for the analysis of surface layer of metals.
As mentioned in the literature(Wever DJ et al., 1998; Shabalovskaya SA et al.,
1995; Oshida Y et al., 1991; Nakayama Y et al., 1991), the biocompatibility of a
metal is related mainly to the surface passive film. This is, surface chemistry is
another aspect related to surface stability and biocompatibility, in particular. The XPS
surface analyses results for the nickel-titanium alloys are shown in Figure 11 ~ 14.
The oxides formed on the outermost surface of the nickel-titanium alloys included
TiO2 , metallic Ni and oxidized nickel. This oxide layer was composed mainly of
TiO2 but some Ti suboxides and Ni oxidized species were detected as well. Titanium
has a high affinity for oxygen, surface oxide films form spontaneously in an
oxygenated environment. From the XPS surface analyses results, the outermost
oxides, which provide the main corrosion resistance for the tested nickel-titanium
alloys, contained mainly TiO2, with small amount of NiO. There is no nickel element
on the NTS alloy, and mainly TiO2 and suboxides were observed. Therefore, the
outermost surface of the passive film on the tested nickel-titanium alloys was the
same, and mainly consisted of TiO2, which can provide the good biocompatibility of
nickel-titanium alloy (Koster R et al., 2000; Wever DJ et al., 1998; Shabalovskaya SA
et al., 1995). Because nickel-titanium alloy corrosion would lead to biologically
negative effects (Koster R et al., 2000; Ryhanen J, 1997; Guyuron B et al., 1992;
Veldhuizen AG et al., 1998), nickel-titanium orthodontic wire with long-term good
corrosion resistance, that is, a durable and protective TiO2 passive film, in an acidic
46
oral environment is clinically important.
There is a selective dissolution or selective oxidation of alloying explaining for
corrosion resistance enhancement. The difference in the electromotive force (EMF)
results from selective dissolution of alloying element with a lower electromotive force
among the alloying elements (Khan MA et al., 1999). Theory of selective oxidation is
that an alloy is selectively oxidized if one component, usually the most reactive one,
is preferentially oxidized. The alloy is formed from metal A and B where B is more
reactive than A. Therefore BO oxide is thermodynamically more stable than AO, and
this means that BO oxide preferentially formed on metal surface. When alloying with
a noble metal, it is an obvious example of selective oxidation. In alloy A-B, A is so
noble that AO oxide is not thermodynamically stable at the environmental pressure.
Then, only BO oxide can form if it is stable. From this theory, the addition of silver to
nickel-titanium is considered to improve the corrosion resistance because of easy
formation and strengthening of titanium oxide. This is, because the electromotive
force of the silver is much higher than that of titanium, a stable titanium oxide film
was formed on the surface of experimental nickel-titanium alloys by the selective
dissolution of titanium. Moreover, Stern and Wissenberg used the mixed-potential
theory to explain the electrochemical behavior of titanium alloyed with noble or
precious metal (Stern et al., 1959). The role of noble or precious metal is to enhance
the cathodic kinetics, shift the potential of the alloy surface to the passive region, and
promote the passivity for titanium. According to this theory (Stern et al., 1959), it is
thought that silver, classified precious metal, can promote the passivity for
nickel-titanium alloy.
The nature, composition, and thickness of the protective surface oxides that form
on titanium alloys depend on environmental conditions. Nickel-titanium alloy and
titanium alloy form several oxides such as TiO2, TiO, and Ti2O3. Actually, the
composition of the oxide layer has not been clearly defined, and it is unlikely that it
would correspond to the stoichiometric composition and, therefore, TiOx describes
more accurately the oxide form. Evidence shows that the ready formation of TiO2 in
the air might be due to the low free energy value of the reaction Ti + O2 → TiO2
which has a ∆G of 2856 kJ/mol, making the formation of TiO2 entropically favorable
47
(Beddoes J et al., 1999). Some reports suggest that the oxide thickness or
strengthening is increased as a log function of immersion time in electrolytes, others
have noted different directions of growth and steady-state levels of TiO2 (Eliades T,
1997). The Pourbaix diagram for titanium also reveals that the oxide film is stable
over a wide range of potentials (Pourbaix M, 1974).
The XPS results show differences in the chemical composition of the titanium
alloys surface layers, suggesting differences in their protective character. The
dominant Ti and O signals of the XPS spectra of Figure 11, 12, and Table 8 showed
that the surface film mainly consists of titanium oxide and small amount of Ni. These
results represented that immersion in artificial saliva and 250 mV load result in
passivation and formation of strengthened passive oxide film. From the high
resolution spectral analyses of Ti and O, the surface film on titanium was mainly TiO2
and suboxide containing small amount of hydroxyl groups and bound water. TiO and
Ti2O3 were located in the inner layer close to the surface. These results agreed with
oxide film layer model theory: TiO(in contact with the Ti substrate)/Ti2O3/TiO2(outer
part) (Blackwood DJ et al., 1989; Pouilleau J et al., 1997). This result has good
correlates and a thread of connection with many researchers’ reports and
explanations.
Metal ions can be released from metallic biomaterials, such as artificial joints,
bone plates, screws and dental implants, etc., in the body. A number of researches
have been taken to study corrosion and metal ion release from biomaterials and have
demonstrated that metal ions are released and transported in vivo (Ferguson AB et al.,
1962; Brown SA et al., 1988). If a large amount of metal ions are released, it could be
generally harmful for human body causing various phenomena: transportation,
metabolism, accumulation in organs, allergy, and carcinoma (Hanawa T, 2004). Such
a metallic ion can attack the cells at the molecular level, affecting the structure of the
cells. For bone treatment, released metallic ions are able to interfere with the
osteoblast differentiation, and contributing to periprosthetic osteolysis by impairment
of normal osteogenesis (Sun ZL et al., 1997; Nichols KG et al., 1997). Therefore,
metal ion release from metallic biomaterials in vivo should be understood to discuss
the safety and biocompatibility of the materials.
48
The corrosion resistance can be determined by passive current density, corrosion
potential and pitting potential, which can be obtained by potentiodynamic testing.
Generally, a low passive current density indicates a stable passivity, which in turn
means a high corrosion resistance. In contrast, a high current density means that it is
highly likely that a large amount of metal ions released can be induced by corrosion
reaction. According to Faraday's law, the equation between the CPR (corrosion
penetration rate) and the current density [i, (A/cm2)] is as follows:
CPR (corrosion penetration rate) = Ka i / n ρρρρ
where K : constant,
a : atomic weight of the metal experiencing corrosion,
ρ : density of the metal,
n : the number of electrons associated with the ionization of each metal
atom
For example, nickel with a corrosion rate of 1 A/cm2 in an acidic solution is
estimated to have a CPR of 0.43 mili-inch per year.
CPR = [(0.129)(58.7)(1)] / [(2)(8.85)] = 0.43 mpy (mils per year)
The results of the ion release tests showed that for 4 weeks, the accumulated
amount of nickel ions released was relatively small; the average nickel ion intake for
a person through food was 5 ~ 10 ㎍ and through beverage was 0.43 ㎍/ℓ per day.
However, the amount of nickel ions released was far less than 600 ~ 2500 ㎍, which
is the allergy-inducing critical concentration (Bishara RD et al., 1993; Kaaber K et al.,
1978).
The clinical symptom of silver intoxication is argyrism giving rise to a grey-blue
color of the skin and mucosa(Landsdown ABG, 2002). It may be accompanied by
gastro-intestinal problems, anorexia, anemia, hepatic deficiency and respiratory
insufficiency. The most toxic reactions are found with silver chloride and sulfate.
Metallic silver alloyed with other metals, in particular with a certain amount of Au,
49
does not show significant toxic effects (Hildebrand HF et al., 1998).
According to the passivity, metallic biomaterials in aqueous solutions or human
body are systems in which active and passive surfaces exist simultaneously in contact
with electrolyte(Kelly EJ, 1982). Therefore, it is now thought that the surface oxide
film on the materials repeats a process of partial dissolution and reprecipitation in
aqueous solution. If the dissolution rate is larger than reprecipitation rate, more metal
ions gradually released. This process is ‘anodic dissolution’ in a narrow sense. If the
potential of a material anodically changes, anodic dissolution rate increases. Metal ion
release remaining on surface oxide film is relatively slow in vivo because the change
in potential of material in vivo is usually small. Passive oxide film on alloy plays an
important role as an inhibitor of ion release.
The agar diffusion test detects acute cytotoxicity of leachable components through
direct contact. The presence of toxic substance results in the decolorization in the
diffusion zone. Cell lysis occurs within this zone if the concentration of toxic
substance diffused is enough to cause cytolysis. However, no cytotoxicity was
observed due to minute amount of metal ion leaching and diffusion through the agar.
No abnormal morphology or cellular lysis was detected. There was no diffusion of
leachable components in alloys to cause cell lysis or decolorization. Metal ion release
was so limited that silver leaching effect did not occur.
The biocompatibility for biometallic materials is the most important property
because materials are used directly or indirectly in contact with human body. Then
the biocompatibility of materials essentially equates to corrosion resistance (Bergman
M et al., 1980; Widu F et al., 1999). That is, it is only by conversion to ions that the
alloying element can enter the surrounding organic system and thereby develop toxic
effects.
50
. ⅤⅤⅤⅤ Conclusions
We evaluated the effect of silver addition on corrosion resistance and
biocompatibility of nickel-titanium alloy as metallic biomaterials in this study.
1. Small quantities of silver were added to nickel-titanium alloy and the actual
silver contents were less than the added contents. The recovery rate for silver
range was 28.48 ~ 41.50%.
2. Silver added nickel-titanium alloys had the mixed phase with austenite and
martensite and high fraction of the martensite.
3. Silver added nickel-titanium alloys increased the transformation temperature.
4. Silver added nickel-titanium alloys did not show a hardness increase
(p>0.05).
5. Silver added nickel-titanium alloys increased the corrosion resistance of its
alloys, but at higher silver levels the corrosion resistance was decreased.
From the results of potentiostatic behavior, it was believed that these alloys
had stable passive films.
6. The passive films of silver added nickel-titanium alloys were composed of
mostly titanium oxide. Metallic nickel and silver existed in inner layer of the
passive films of the nickel-titanium alloys.
7. The amount of nickel ions released from the silver added nickel-titanium
alloys in the artificial saliva showed no significant difference, compared to
nickel-titanium alloy (p>0.05).
51
8. Silver added nickel-titanium alloys exhibited no cytotoxicity.
From the above results, it was shown that silver added nickel-titanium alloys
increased their transformation temperatures, strengthened their corrosion resistance
and formed a stable oxide film. Further study on the biocompatibility, the chemical
stability of passive film, transformation temperature change, and the working process
of these alloys is necessary to ensure that these materials can be used in the dental and
medical fields.
52
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ABSTRACT (IN KOREAN)
Nickel-Titanium 합금의합금의합금의합금의 부식저항성부식저항성부식저항성부식저항성 및및및및 생체적합성에생체적합성에생체적합성에생체적합성에 미치는미치는미치는미치는
은은은은 (Ag) 첨가의첨가의첨가의첨가의 영향영향영향영향
<지도교수지도교수지도교수지도교수 : 김김김김 경경경경 남남남남>
연세대학교연세대학교연세대학교연세대학교 대학원대학원대학원대학원 치의학과치의학과치의학과치의학과
주주주주 욱욱욱욱 현현현현
Nickel-titanium 합금은 형상기억효과와 초탄성 특성을 가진 재료로서,
항공산업 분야, 일반 산업 분야, 의료 분야 등에서 많이 사용되고 있다.
Nickel-titanium 합금은 nickel과 titanium의 정확한 원자수 비, 제 3 원소
첨가, 가공 공정 등에 매우 민감한 재료로서, 반복 하중에 대한 피로특성
개선 등의 목적으로 Cu, Fe 등 제 3원소를 첨가하기도 한다. 그러나 합금
원소 첨가에 따라 불안정한 피막을 형성, 내식성을 감소시키는 문제점이
있으며, 높은 내식성을 요구하는 의료 분야에서는 그 사용에 있어 주의가
요구되고 있다. 본 연구에서는 nickel-titanium 합금에 소량의 은 (Ag)
첨가에 다른 합금의 부식저항성, 생체적합성에 미치는 영향을 알아보고자
하였다. 합금은 아크 용해로에서 제조하여, 균질화 열처리, 열연공정,
용체화 열처리 등의 과정을 거쳐 판상 형태로 제작하였다. 원자수비 1:1인
nickel-titanium 합금과 은 (Ag)이 첨가된 nickel-titanium 합금에 대하여,
일반적인 특성을 평가하고자 상 분석, 미세조직관찰, 변태온도 측정, 경도
시험을 실시하였으며, 전기화학적 특성 분석을 통하여 실험합금의
내식성을, 표면 분석, Ni 이온 용출 실험, 세포독성 평가 등을 통해
생체적합성 평가를 시행하였다.
은(Ag)이 첨가된 nickel-titanium 합금은 Ni이나 Ti에 비해서 낮은 은(Ag)
회수율(28.5~41.5%)을 나타내었으며, 오스테나이트와 마르텐사이트 상이
60
혼합된 구조를 나타내었다. 은(Ag)이 첨가됨에 따라 마르텐사이트 상의
비율이 증가하였다. 은(Ag)이 첨가된 nickel-titanium 합금은 첨가되지 않은
nickel-titanium 합금과 비교 시 변태온도가 증가하였으며, 경도값은
증가하지 않았다(p>0.05). 전기화학적 특성 실험을 통해 합금의 내식성을
평가한 결과, 동전위 실험에서는 10 ㎂/cm2 이하의 낮은 부동태
전류밀도를, 정전위 실험 상에서는 시간이 지남에 따라 1 ㎂/㎠ 이하의
안정된 전류밀도 분포를 나타내었다.
XPS를 통해 얻어진 스펙트럼을 분석한 결과, 실험 합금 표면 피막 층은
주로 TiO2 등의 티타늄 산화물로 구성되어 있었다. 은(Ag)이 첨가된
nickel-titanium 합금의 피막 바깥층에서는 Ni이 관찰되지 않았으며, 피막
안쪽층에서 금속 상태의 Ni과 Ag이 존재하는 것으로 관찰되었다.
은(Ag)이 첨가된 nickel-titanium합금은 첨가되지 않은 nickel-titanium
합금과 비교 시 Ni 이온 용출량에 대한 유의차가 없었으며(p>0.05), 세포
독성을 나타내지 않았다.
이상의 결과로 은(Ag)이 첨가된 nickel-titanium 합금은 변태온도를
높이고, 내식성을 향상시키며, 안정된 산화막을 구성하여, 생체재료로
사용이 가능할 것으로 판단된다. 앞으로 다양한 분야에 생체재료로
사용하기 위해서, 은(Ag)이 첨가된 nickel-titanium 합금에 대한 화학적
안정성, 변태온도 조절, 가공 공정에 대한 연구가 더 필요할 것으로
생각된다.
핵심되는핵심되는핵심되는핵심되는 말말말말: 생체적합성, 내식성, 전기화학적 특성, Ni 이온 용출, 부동태 피막, 표면 특성 분석, 니켈-티타늄 합금, 은(Ag) 첨가