Oshida Y. Bioscience and Bioengineering of Titanium Materials

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stress–strain loop with some stress hysteresis. This property applies to the parent

phase undergoing a stress-induced martensitic transformation. The other unique

property associated with NiTi is SE. When SME alloys are deformed, a superelas-

tic alloy deforms reversibly to very high strains up to 10% by the creation of stress-

induced phase. When the load is removed, the new phase becomes unstable and the

material regains its original shape. Unlike SME alloy, no change in temperature is

needed for the alloy to recover its initial shape. There is a term called pseudoelas-

ticity, which is a more generic term that encompasses both superelastic and rubber-

like behavior [2-27].

Because of the uniqueness of SME and the SE associated with NiTi, there

have been many applications in both the industrial and dental/medical fields,

including dental and orthopedic implants, artificial heart valve, stunt, endodon-

tic files, orthodontic archwires, etc. [2-28–2-33]. Superelastic devices take

advantages of their large, reversible deformation and their applications are not

limited to medical/dental fields, but can be found in parabola antenna used at the

space shuttle, frames for eyeglasses, and sporting goods such as fishing line.

2.2.8 Ti-Cu

Aiming for developing an alloy for dental casting with better mechanical properties

than unalloyed CpTi, Ti-Cu (Cu: 0.5–10 wt.%) was cast in an argon-arc melting

furnace. It was found that (i) the mean tensile strength was significantly higher than

for cast CpTi, and (ii) increases of 30% in the tensile strength and yield strengths

of 40% over CpTi were obtained for the Ti-5Cu alloy [2-34].

2.2.9 Ti-Mo

TiMo alloy is widely used as an orthodontic archwire for performing an ortho-

dontic mechanotherapy [2-35]. When NiTi wire, TMA (-phase Ti-Mo) wire, and

austenitic stainless steel wire were compared, it was found that (i) TMA showed

the greatest plastic strain and springback, followed by NiTi and stainless steel,

(ii) NiTi wire showed the highest stored energy value in bending-torsion, followed

by TMA and stainless steel, and (iii) the highest spring ratio (stiffness) in bending-

torsion was found in stainless steel, followed by TMA and NiTi [2-36]. Lin et al.

[2-37] studied alloying effect of Fe on the Ti–Mo alloy, and found that Ti–Mo with

iron contents in the range of 2–5 wt.% appeared to have a great potential for use

as an implant material.

2.2.10 Ti-5Al-2Mo-2Fe (SP700)

Ti-6Al-4V is the most widely used titanium alloy. However, Ti-6Al-4V still suffers

from limited applications in non-aerospace fields compared to CpTi, aluminum

18 Bioscience and Bioengineering of Titanium Materials

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alloys, and steels. The reasons for this include (1) poor hot-workability, with a

normal hot-working temperature range that is too high and too narrow, (2) poor cold-

workability that virtually prohibits cold-working, (3) poor hardenability that is fre-

quently inconsistent, and (4) a high superplastic forming temperature of around

900

C that results in rapid die wear. To overcome these disadvantages associated with

Ti-6Al-4V, a new beta-rich alpha-beta duplex phase titanium alloy has been devel-

oped (called SP700, since the newly developed Ti alloy remarkably exhibits high

superplastic capability at operating temperatures around 700

C). It was reported that

SP700 provides excellent hot- and cold-workability, as well as greatly improved

superplastic forming characteristics (i.e., the strain rate sensitivity exponent, m-value,

is about 1.0, indicating that flow is close to that of a Newtonian viscous flow), high

strength, and high toughness. Dental denture bases have been fabricated using this

SP700 alloy by superplastic forming technique [2-38].

2.2.11 Other Ti-based alloys

An interesting study was done by Brème et al. [2-39], using isoelastic porous sin-

tered Ti-30Ta alloy and titanium wire loop, to accomplish the mechanical compat-

ibility in endosseous dental implant systems. Their mechanical properties were

optimized by the production parameter such as sintering and diffusion bonding. The

functionality was tested after insertion into an artificial jaw, which had properties

corresponding to the natural mandibular. It was reported that the elastic properties

of both implants are similar to the properties of the bone, and the implant has a safe

anchorage bone ingrowth [2-39].

Qazi et al. [2-6] developed metastable -Ti alloy (mainly Ti-35Nb-7Zr-5Ta) and

solution-treated, followed by aging at 482

C. It was reported that (i) the heat-

treated Ti-35Nb-7Zr-5Ta alloys exhibited 0.2% off-set yield strength of 1300 MPa

with 8% elongation, due to ⫹ and ⫹⫹-phase precipitations, and (ii) this

enhanced strength makes the alloy candidates for bone plates and screws.

2.2.12 Intermetallic Alloys

Although not ordinary alloys, titanium aluminide-based alloys, including TiAl

(-phase) and Ti

Al (2 phase), are used for high temperature applications. The

titanium aluminide-based alloys provide an inherently low-density material and

exhibit excellent creep-rupture properties [2-13, 2-40]. TiAl amorphous alloy pro-

vides high strength, linear elastic behavior, and the infinite fatigue life necessary

for high device reliability. Tregilgas [2-41] developed amorphatized TiAl alloys

for a material for the digital micromirror device chip. In order to enhance the wear

resistance of -TiAl (Ti-47Al-2Nb) alloy, the surface of this alloy was modified in

a nitrogen-ion plasma atmosphere [2-42].

Materials Classification 19

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REFERENCES

[2-1] Collings EW. The physical metallurgy of titanium alloys. Columbus, OH:

The American Society for Metals, 1984. pp. 3–5.

[2-2] Ivansishin OM, Markovsky PE. Enhancing the mechanical properties of

titanium alloys with rapid heat treatment. J Metals 1996;48:48–52.

[2-3] Eylon D, Vassel A, Combres Y, Boyer RR, Bania PJ, Schutz RW. Issues in

the development of beta titanium alloys. J Metals 1994;46:14–15.

[2-4] Niiomi M. Recent metallic materials for biomedical applications. TMS

Metallurgical and Materials Transactions 2002;33A;477–486.

[2-5] Jha SK, Ravichandran KS. High-cycle fatigue resistance in beta-titanium

alloys. J Metals 2000;52:30–35.

[2-6] Qazi JI, Marquardt B, Rack HJ. High-strength metastable beta-titanium

alloys for biomedical applications. J Metals 2004;56:49–51.

[2-7] Niiomi M. Fatigue performance and cyto-toxicity of low rigidity titanium

alloy, Ti-29Nb-13Ta-4.6Zr. Biomaterials 2003;24:2673–2683.

[2-8] Niiomi M, Kuroda D, Fukunaga K, Morinaga M, Kato Y, Yashiro T,

Suzuki A. Corrosion wear fracture of new  type biomedical titanium

alloys. Mat Sci Eng A 1999;A263:193–199.

[2-9] ASTM designation F2066-01. Standard specification for wrought titanium-

15 molybdenum alloy for surgical implant applications. Philadelphia PA:

American Society for Testing and Materials. 2001. pp. 1605–1608.

[2-10] ASTM designation draft #3. Standard specification for wrought titanium-

35Niobium-7Zirconium-5Tantalum alloy for surgical implant applications

(UNS R58350). Philadelphia, PA: American Society for Testing and

Materials. 2000.

[2-11] American Society for Testing and Materials designation draft #6. Standard

specification for wrought titanium-3Aluminum-2.5Vanadium alloy for

surgical implant applications (UNS R58350). Philadelphia PA, ASTM,

2000.

[2-12] Paton NE, Williams JC. Effect of hydrogen on titanium and its alloys. In:

American society for metals. Hydrogen in metals. 1974. pp. 409–432.

[2-13] Titanium and Titanium Alloys – Source Book. Donachie MJ, editor. Metals

Park, OH: American Society for Metals. 1982.

[2-14] Mountford JA Jr. Titanium alloys for the CPI (Chemical Process Industry).

Adv Mater Processes 2001;159:55–58.

[2-15] Hernandez FE. Corrosion behavior for commercially pure titanium with

different grades and from different material suppliers. Indiana University

Master Degree Thesis, 2003.

[2-16] Lim WC. Comparison of electrochemical corrosion behavior among four

grades of commercially pure titanium. Indiana University Master Degree

Thesis, 2001.

[2-17] Binon PP. Implants and components: entering the new millennium. Int

J Oral Maxillofac Implants 2000;15:76–94.

20 Bioscience and Bioengineering of Titanium Materials

Else_BBTM-OSHIDA_ch002.qxd 9/14/2006 9:41 PM Page 20

[2-18] McCraken M. Dental implant materials: commercially pure titanium and

titanium alloys. J Prosthodont 1999;8:40–43.

[2-19] Moore BK, Oshida Y. Materials science and technology in dentistry. In:

Encyclopedic handbook of biomaterials and bioengineering. Wise DL,

Trantolo DJ, Altobelli DE, Yaszemski MJ, Gresser JD, Schwartz ER, edi-

tors. New York: Marcel Dekker. 1995. pp. 1325–1430, Chapter 48.

[2-20] Iijima D, Yoneyama T, Doi H, Hamanaka N, Kurosaki N. Wear properties

of Ti and Ti-6Al-7Nb castings for dental prostheses. Biomaterials 2003; 24:

1519–1524.

[2-21] Soma H. Clinical application of Ti-6Al-7Nb alloy “Ti-Alloy Tough”.

Quintessence Dent Technol 1998;23:98–104.

[2-22] Semlitsch MF, Weber H, Streicher RM, Schoen R. Joint replacement com-

ponents made of hot-forged and surface-treated Ti-6Al-7Nb alloy.

Biomaterials 1992;13:781–788.

[2-23] Tuominen S, Wojcik C. Alloys for aerospace. Adv Mater Processes 1993;

147:23–26.

[2-24] Higo Y, Ouchi C, Tomita Y, Murota K, Sugiyama H. Properties evaluation

and its application to artificial hip joint of newly developed titanium alloy

for biomaterials. Proc. 3rd Japan International SAMPE Symposium 1993.

pp. 1621–1625.

[2-25] Zwicker U, Brème J. Investigations of the friction behavior of oxidized

Ti-5%Al-2.5%Fe surface layers on implant material. J Less-Common

Metals 1984;100:371–375.

[2-26] Buehler WJ, Gilfrich JV, Wiley KC. Superelasticity in TiNi Alloy. J Appl

Phys 1963;34:1467–1469.

[2-27] Duerig TW, Melton KN, Stöckel D, Wayman CM, editors. Engi-

neering aspects of shape memory alloys.London: Butterworth-

Heinemann. 1990.

[2-28] Civjan S, Huget EF, DeSimon LB. Potential applications of certain nickel-

titanium (Nitinol) alloys. J Dent Res 1975;54:89–96.

[2-29] Bensmannn G, Baumgart F, Haasters J. Osteosyntheseklammern aus NiTi.

Herstellung, Vorversuche und klinischer Ensatz. Tech Mitt Krupp, Forsch-

Ber 1982;40:123–134.

[2-30] Miura F, Mogi M, Ohura Y, Hamanaka H. The superelastic property of the

Japanese NiTi alloy wire for use in orthodontics. Am J Orthod Dentofacial

Orthop 1986;90:1–10.

[2-31] Yoneyama T, Doi H, Hamanaka H, Okamoto Y, Mogi M, Miura F. Super-

elasticity and thermal behavior of Ni-Ti alloy orthodontic arch wires. Dent

Mater J 1992;11:1–10.

[2-32] Yoneyama T, Doi H, Hamanaka H. Bending properties and transformation

temperatures of heat treated Ni-Ti alloy wire for orthodontic appliances.

J Biomed Mater Res 1993;27:399–402.

[2-33] Iijima M, Endo K, Ohno H, Mizoguchi I. Effect of Cr and Cu addition on

corrosion behavior of Ni-Ti alloys. Dent Mater J 1998;17:31–40.

Materials Classification 21

Else_BBTM-OSHIDA_ch002.qxd 9/14/2006 9:41 PM Page 21

[2-34] Kikuchi M, Takada Y, Kiyosue S, Yoda M, Woldu M, Cai Z, Okuno O,

Okabe T. Mechanical properties and microstructures of cast Ti-Cu alloy.

Biomaterials 2003;19:174–181.

[2-35] Ho WF, Ju CP, Chern Lin, JH. Structure and properties of cast binary

Ti-Mo alloys. Biomaterials 1999;20:2115–2122.

[2-36] Drake SR, Wayne DM, Powers JM, Asgar K. Mechanical properties of

orthodontic wires in tension, bending, and torsion. Am J Orthod 1982;82:

206–210.

[2-37] Lin DJ, Chern Lin JH, Ju CP. Structure and properties of Ti-7.5Mo-xFe

alloys. Biomaterials 2002;23:1723–1730.

[2-38] Ouchi C, Minakawa K, Takahashi K, Ogawa A, Ishikawa M. Development

of -rich - titanium alloy: SP-700. NKK Technical Review

1992;65:61–67.

[2-39] Brème J, Biehl V, Schulte W, D’Hoedt B, Donath K. Development and

functionality of isoelastic dental implants of titanium alloys. Biomaterials

1993;14:887–892.

[2-40] Jha SK, Larsen JM, Rosenberger AH. The role of competing mechanisms

in the fatigue-life variability of a titanium and gamma-TiAl alloy.

J Metals 2005;57:50–54.

[2-41] Tregilgas J. Amorphous hinge material. Adv Mater Processes 2005;163:

46–49.

[2-42] Thongtem S, McNallan M, Thongtem T. Surface modifications of -TiAl

alloys by nitrogen plasma deposition. Adv Technol Mater Mater Processing

2004;6:170–175.

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Chapter 3

Chemical and Electrochemical Reactions

3.1. Discoloration 26

3.2. Corrosion in Media Containing Fluorine Ion and Bleaching Agents 28

3.3. Corrosion Resistance, Under Influences of Environmental and 31

Mechanical-Assisted Actions

3.4. Metal Ion Release and Dissolution 42

3.5. Galvanic Corrosion 51

3.6. Microbiology-Induced Corrosion 53

3.7. Reaction with Hydrogen 61

References 64

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Chapter 3

Chemical and Electrochemical Reactions

The intraoral environment is hostile due to corrosive and mechanical actions. It is

continuously full of saliva, an aerated aqueous solution of chloride with varying

amounts of Na, K, Ca, PO

, CO

, sulfur compounds, and mucin. The pH value is

normally in the range of 6.5⫺7.5 but under plaque deposits it may be as low as

2.0. Temperatures can vary ⫾36.5

C, and a variety of food and drink concentra-

tions (with pH values ranging from 2.0 to 14.0) stay inside the mouth for short

periods of time. Loads can go up to 1000 N (normally 200 N as a masticatory

force), sometimes in an impact manner. Trapped food debris may decompose, and

sulfur compounds cause natural teeth and restorative materials to discolor. Under

these chemical and mechanochemical intraoral environments, materials in service

in the mouth are still expected to last for relatively long period of time [3-1].

It is well documented that Ti materials possess an excellent corrosion resistance

and form stable and protective oxides, such that Ti materials are one of the best

material choices for medical and dental applications. It is inevitable to note that

this chapter on corrosion, as well as the following chapter on oxidation occupy a

quite a large portion of this book. What it implies is that environments to which

titanium materials have been exposed are still diverse and vary widely in terms of

chemical, mechanical, physical, and any combinations of these parameters. In this

chapter, we will be reviewing the corrosion behavior of Ti materials when they are

exposed to various atmospheres.

It is known that all primitive living species from which Homo sapiens are orig-

inated were created from the ocean, containing about 32⫺35 mg/l of chlorine ion,

while the human body contains about 35 mg/l of chlorine ion and saliva has about

5 mg/l of it. Most of the information presented and reviewed in this chapter was

obtained from the in vitro tests, which were conducted in various corrosive media

mainly containing chlorine ions. Such media can be divided into four groups:

(1) containing less amount of chlorine ion (ca., 50 mg/dl Cl

⫺

, i.e. about 1% NaCl

concentration) to simulate saliva, and phosphate-buffered artificial saliva,

(2) since internal fluids have a concentration of chlorine ions seven times (35 mg/l

of chlorine ion) higher than that of oral fluids (5 mg/l of chlorine ion), simulated

body fluid with higher ion concentration (350 mg/dl Cl

⫺

), (3) lactic acid, simulat-

ing the chemistry of major constitutents of plaque, and (4) dental treatment agents,

including whitening agents and fluoride treatment agents. These corrosive media

cause dental materials discoloration, tarnishing, corrosion, or oxidation.

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3.1. DISCOLORATION

Currently, mouth-rinses, toothpaste, dental prophylactic agents containing fluo-

ride, and bleaching treatment agents are popular for esthetic purposes and preven-

tion of plaque and cavity formation. Normally, orthodontic patients are referred to

general dentists for fluoride treatments once in every 6 months during the course

of orthodontic mechanotherapy. Among various bleaching treatments for whiten-

ing stained teeth, an overnight (normally for 8 h) bleaching agent containing 10%

carbamide peroxide is popular. To achieve a satisfactory outcome, this treatment

must be repeated for two consecutive weeks. The corrosive effect of these agents

(normally containing fluoride and hydrogen peroxide) on dental metallic materials

has not been well documented, although it was reported to decrease the corrosion

resistance of titanium in solutions containing fluoride [3-2⫺3-6].

Titanium application in dentistry has steadily increased because of its numerous

biocompatibility benefits and mechanical advantages. In orthodontics, titanium is

valuable in the treatment of patients with an allergy to nickel and other specific sub-

stances [3-7]. Adell et al. [3-8] acknowledged the benefits of titanium and how the

body responds to it while working on permanent tooth replacements. Owing to their

biological advantages and excellent corrosion resistance, research and clinical evi-

dence supports the use of unalloyed titanium and its alloys in the body, which con-

sistently remain the first choice for implants in both medical and dental applications.

Such excellent corrosion resistance of Ti materials might be jeopardized when

they are in contact with substances containing fluoride. An in vivo study by Harzer

et al. [3-7] showed that orthodontic brackets made of titanium show higher plaque

accumulation and more discoloration than stainless steel brackets. This was spec-

ulated to be due to the morpholical alteration of surface layers. When a surface

becomes rougher with higher unevenness, the outer surface appearance becomes

duller with discolored-like appearance. Nonetheless, studies have found that, in a

fluoridated medium (especially in acid-fluoridated solutions), titanium is degraded

[3-9, 3-10]. Watanabe and Watanabe [3-11] found that titanium orthodontic appli-

ances with high titanium content (such as unalloyed CpTi: commercially pure tita-

nium) have also shown lower tarnish resistance, especially when immersed in an

acidulated phosphate fluoride (APF) solution.

Teeth that have been exposed to a long-term coffee and/or cigarette usage are

normally stained. Viral bleaching of such teeth reflects patients’ increasing desire

to achieve an optimal esthetic appearance [3-12⫺3-15]. Internal discoloration

caused, for example, by tobacco, dentino-genesis imperfecta, or fluorosis, may be

removed. Oxygenating agents like carbamide peroxide or hydrogen peroxide,

, are used for effective bleaching purpose. Carbamide peroxide is used as a

vehicle for transporting H

. First, carbamide reacts with uric acid, ammonia,

26 Bioscience and Bioengineering of Titanium Materials

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and H

. Then, as a second step, H

reacts into H

O and O, which takes elec-

trons from the substance to bleach [3-13]. The bleaching efficacy is influenced by

the application time and the concentration of the effective agent. Such application

of the bleaching agents is performed in offices by clinicians (with relatively higher

concentration of H

) or at home by patients themselves (relatively lower con-

centration of H

) [3-14]. Commercially available bleaching products range from

3% to 9.5% for hydrogen peroxide (which can be converted to a range from 6% to

over 19% carbamide peroxide). Although the above discussion is regarding the

discolored natural teeth and whitening of such stained teeth, some patients under-

going the nightguard bleaching (e.g., for 8-h treatment) are likely to have some

metallic restorations (i.e., amalgam, crowns made of gold or porcelain fused to a

base metal, fixed or removable prosthodontic bridge or partial denture frameworks

made of base alloys, and/or titanium implant fixtures). Recently, it was found that

exposure of amalgam to common bleaching agents caused an increase in mercury

levels in the solutions [3-16].

The conscientious health-care provider needs to know the consequences of the

effects of fluoride agents and bleaching agents on various dental metallic materials

intraorally. While it is acknowledged that tarnishing of the metallic dental materi-

als is the visible consequence of these treatments, the implications on biological

effects and the structural integrity of the components are unclear. Clinicians need

to understand the potentially corrosive nature of the commercially available fluo-

ride and bleaching treatments on intraoral metals, and know when and where to use

them [3-17]. The discoloration efficacy of fluoride treatment agents (2.0% NaF

with pH 7.0, 0.4% SnF

with pH 7.0, and 1.23% APF with pH 3.5) was tested on

Ti-6Al-4V and 17Cr-4Ni PH (precipitation hardening) type stainless steel, and

bleaching agents (10% carbamide peroxide) were applied on CpTi (grade 2), 70Ni-

15Cr-5Mo, type IV gold alloy (70Au-10Ag-15Cu), and Disperalloy amalgam. The

degree of discoloration on these treated alloys was examined by a colorimeter and

naked eyes. After the baseline measurements of three L* (lightness), a* (position

on the red/green axis), and b* (position on yellow/blue axis), comparisons were

made with the Commission Internationale d’Eclairage (CIE-L*a*b) color system,

and the value of E* (defined as the Euclidean distance) can be calculated by

[(L

⫺L

)

⫹ (a

⫺a

)

⫹ (b

⫺b

)

]

1/2

, where subscripts “i” and “f” indicate the intial

value and final value, respectively. It was found that (i) all tested metallic materials

exhibit discoloration to various degrees, ranging from 10 to 18 in E*, (ii) the tooth

brashing between each treatment for both fluoride and bleaching treatments indi-

cate a remarkable reduction in the degree of discoloration (i.e., E* reduced to

2⫺8) of all tested materials, and (iii) results on naked eyes evaluation performed by

three clinicians did not agree well with those in the lower range of E*, whereas

when the discoloration advanced, both evaluations agreed well [3-17].

Chemical and Electrochemical Reactions 27

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