Oshida Y. Bioscience and Bioengineering of Titanium Materials

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Chapter 4

Oxidation and Oxides

4.1. Formation of Titanium Oxides 81

4.1.1 Air-Formed Oxide 82

4.1.2 Passivation 84

4.1.3 Oxidation at Elevated Temperatures 85

4.2. Influences on Biological Process 86

4.3. Crystal Structures of Ti Oxides 87

4.4. Characterization of Oxides 90

4.5. Unique Applications of Titanium Oxide 91

4.6. Oxide Growth, Stability, and Breakdown 91

4.7. Reaction with Hydrogen Peroxide 94

References 97

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Chapter 4

Oxidation and Oxides

4.1 FORMATION OF TITANIUM OXIDES

Titanium is a highly reactive metal and will react within microseconds to form

an oxide layer when exposed to the atmosphere [4-1]. Although the standard elec-

trode potential was reported in a range from -1.2 to -2.0 V for the Ti ↔ Ti

3⫹

electrode reaction [4-2, 4-3], due to strong chemical affinity to oxygen, it easily

produces a compact oxide film, ensuring high corrosion resistance of the metal.

This oxide, which is primarily TiO

, forms readily because it has one of the high-

est heats of reaction known (H⫽⫺915 kJ/mol) (for 298.16⫺2000° K) [4-4–4-6].

It is also quite impenetrable by oxygen (since the atomic diameter of Ti is 0.29 nm,

the primary protecting layer is only about 5–20 atoms thick) [4-7].

The formed oxide layer adheres strongly to the titanium substrate surface. The

average single-bond strength of the TiO

to Ti substrate was reported to be about 300

kcal/mol, while it is 180 kcal/mol for Cr

/Cr, 320 kcal/mol for Al

/Al and 420

kcal/mol for both Ta

/Ta and Nb

/Nb [4-8]. Adhesion and adhesive strength of

Ti oxide to substrates are controlled by oxidation temperature and thickness of the

oxide layer as well as the significant influence of nitrogen on oxidation in air. In

addition, adhesion is greater for oxidation in air than in pure oxygen [4-9], suggest-

ing that the influence of nitrogen on the oxidation process is significant.

In addition to these oxidation conditions, it is well known that several parameters

can modify the accommodation of the stresses developed during the oxidation of a

metal and consequently play an important role in maintaining the protective proper-

ties of oxide layers. The results show that the adhesion of the oxide layers to the

metal substrate decreases as the layer thickness increases. It is shown that the adhe-

sion of the oxide layers decreases when the oxidation temperature increases, despite

the increase in oxide plasticity [4-10]. Adhesive strength between Ti substrate and

TiO

is also related to the thermal mismatching when the Ti/TiO

couple was sub-

jected to the elevated temperatures. Although TiO

does not exhibit any phase trans-

formation up to its melting point (i.e., 1885°C), its substrate Ti metal has an

allotropic phase transformation at 885°C (below which it is HCP – hexagonal

closed-packed crystalline structure – and above which it becomes BCC – body-cen-

tered cubic crystalline structure). Accordingly, when Ti substrate is exposed beyond

this phase transformation temperature, the bond strength between Ti and TiO

will

be weakened due to the significant differences in coefficients of thermal expansion,

particularly during the cooling stage passing through the 885°C transus temperature.

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The performance of titanium and its alloys in surgical implant applications can

be evaluated with respect to their biocompatibility and capability to withstand the

corrosive species involved in fluids within the human body [4-11]. This may be

considered as an electrolyte in an electrochemical reaction. It is well documented

that the excellent corrosion resistance of titanium materials is due to the formation

of a dense, protective, and strongly adhered film – which is called a passive film.

Such a surface situation is referred to a passivity or a passivation state. The exact

composition and structure of the passive film covering titanium and its alloys is

controversial. This is the case not only for the “natural” air oxide, but also for films

formed during exposure to various solutions, as well as those formed anodically.

The “natural” oxide film on titanium ranges in thickness from 2 to 7 nm, depend-

ing on such parameters as the composition of the metal and surrounding medium,

the maximum temperature reached during the working of the metal, the surface

finish, etc.

Passivity is a property of a metal commonly defined in two ways. One of these

is based on a change in the electrochemical behavior of the metal, and the other on

its corrosion behavior [4-12]. Passivity is an unusual phenomenon observed dur-

ing the corrosion of certain metals, indicating a loss of chemical or electrochemi-

cal reactivity under certain environmental conditions [4-13]. Such passivity

appears on certain the so-called passivating metals, many of which are “transition

metals”, characterized by an unfilled group of electrons in an inner electron shell.

Passivating metals are, for example, Fe (covered with Fe

and/or Fe

), Cr

(with Cr

), Zr (with ZrO

), and Ti (with TiO

) [4-14]. This type of passivity is

ascribed to an invisibly thin but dense and semiconducting oxide film on the metal

substrate surface, displacing the electrode potential of the metal strongly in the

positive (or more noble) direction (e.g., Ti passive film exhibits its stable passiv-

ity up to 1.5–2.0 V). Hence, comparing the electrode potential (⫺1.2 to ⫺1.5 V)

to this stable passivity potential, surface nobility was remarkably improved.

Using the electron diffraction techniques (TED: transmission electron diffrac-

tion method on stripped films, and RED: reflection electron diffraction method on

bulk specimens with formed oxide film) and other surface microanalytical tech-

niques, composition and structure of the oxide film that forms on titanium under

various conditions were investigated. The findings are summarized as follows.

4.1.1 Air-formed oxide

When fresh titanium is exposed to the atmosphere by such cutting acts as lathing,

milling, or sawing, an oxide layer begins to form within nanoseconds [4-7]. After

only 1 s, a surface oxide (with 2–7 nm in thickness) will be formed. Air oxidation at

room temperature produced titanium monoxide (TiO) with small quantities of tita-

nium oxide, Ti

. Titanium has proved to be a highly successful material for

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implants inserted into human bone. Under certain conditions, titanium will establish

and maintain a direct contact with the bone tissue in a process called osseointegra-

tion. The mechanisms underlying this behavior are not yet understood. However, it

is clear that the properties of the implant surface are of vital importance for a suc-

cessful osseointegration. The properties of titanium implant surfaces are determined

by the thin (20–70 Å) oxide film that covers the metal. Thus, the biocompatibility of

titanium implants is associated with the surface titanium oxide and not with the bulk

titanium metal [4-1, 4-15–4-17]. The surface oxide is also formed during the implant

preparation procedures. During the machining of the implants, pure titanium metal

is exposed to air and is rapidly oxidized. In the following cleaning and autoclaving

procedures, oxide film is then modified and grows. In order to obtain an oxide film

that is reproducible with respect to the chemical composition, structure and thick-

ness, it is important that the different steps of the implant preparation are performed

under carefully controlled conditions. Even small changes in the preparation proce-

dures might lead to considerable changes of the implant surface [4-18].

As mentioned previously, because the surface air-formed oxide is nearly impen-

etrable, once this thin passivating film has formed, oxygen is prevented from reach-

ing the metal beneath and further oxide layer thickening is quickly halted because

the oxide film is dense and semiconductive (not like an electron-conductive metal

substrate). The compositional structure and exact thickness of the passivating oxide

layer is dependent on many factors associated with its formation. These include

such factors as type of machining, surface roughness, coolants used during machin-

ing, and sterilization procedures. It should be no surprise that reported biocompa-

tibility of devices is prepared by different investigators, and therefore may vary to

a significant degree [4-7].

It is well recognized that the excellent tissue–bone compatibility of titanium is

mainly due to the properties of its stable surface oxide layer. Biocompatibility of

implant materials relies on the chemical and electrochemical stability of this surface

oxide layer, which interfaces with the soft and hard tissue and bone structure. Oxide

layers formed on titanium change from lower to higher oxides as oxidation progresses

and temperature increases. The following phases form in air: Ti⫹O→Ti(O)→

O→Ti

O→TiO→Ti

→Ti

→TiO

. Among these oxides of different

stoichiometry (e.g., TiO, Ti

, TiO

), TiO

is the most common and stable thermo-

dynamically. TiO

can have three different crystal structures – rutile, anatase, and

brookite – but also can be amorphous. TiO

is very resistant against chemical attack,

which makes titanium one of the most corrosion resistant metals.

Another physical property that is unique to TiO

is its high dielectric constant,

which ranges from 14 to 110 (in 10

cycles; dielectric strength: 350 V/mil, volume

resistivity: 10

–10

/cm) depending on crystal structure [4-19]. This high

dielectric constant would result in considerably stronger van der Waal’s bonds on

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TiO

than on other oxides. TiO

, like many other transition metal oxides, is cat-

alytically active for a number of inorganic and organic chemical reactions, which

also may influence the interface chemistry [4-20].

4.1.2 Passivation

Passive films can be formed by either chemically or electrochemically (or anodic)

treating titanium surfaces. Fraker and Ruff [4-21] found that more rigid oxidizing

conditions produce higher oxides of titanium alloy (thin films) in saline water

(3.5% NaCl), at a temperature range between 100° and 200°C, using a transmis-

sion electron microscopy. They found that oxides ranged from Ti

O to TiO

(anatase), with the higher oxides corresponding to the higher temperatures.

Concerning oxide films formed anodically on titanium, most investigators agree

that the film consists of TiO

. All three crystalline forms of TiO

(rutile, anatase,

and brookite) have been reported, as will be discussed later. It has also been

reported that the anodic film is either oxygen-excess or oxygen-deficient. The

controversy is widened by reports that the film is hydrated or contains mixed

oxides. In addition to variations in oxygen stoichiometry, the films may contain

various amounts of elements other than titanium and oxygen, or incorporate such

additional element(s) into TiO

structure. Thus, it is most probable that the anodic

film is not necessarily stoichiometric TiO

, and that the films studied by different

authors had different compositions due to differences in the conditions of oxida-

tion [4-21].

When titanium alloys (Ti-6Al-7Nb and Ti-6Al-4V) were chemically treated, sur-

face oxide films appear to be more complicated. Sittig et al. [4-22] treated these

alloys along with CpTi in nitric acid hydrogen fluoride and identified formed oxides.

Three types of oxides (TiO

, Ti

, and TiO) were identified on the CpTi substrate,

and it was found that the dissolution rate depends on grain orientations. On the other

hand, for Ti-6Al-7Nb and Ti-6Al-4V alloys, Al

, Nb

or V-oxides such as V

were formed in addition to TiO

oxide, and it was found that the selective -phase

dissolution and enrichment of the -phase appears to occur [4-22].

Studies on the nature and properties of thin films formed by the electrolytic oxi-

dation on titanium, which, in turn, determine the behavior of the electrode in the

open circuit, indicate the duplex nature of these films [4-23]. In both the acid and

alkaline media, the outer layer of the anodic oxide film was found to be more sus-

ceptible to dissolution than the inner layer, thereby indicating a more defective struc-

ture of the outer layer. The surface reactivity was found to be higher in

oxygen-saturated vs. nitrogen-saturated solutions. Galvanostatic anodization of tita-

nium has been studied in N

-deaerated, 1 N solution of NaOH, Na

, H

, HCl,

HClO

, HNO

, and H

at 25°C at current densities ranging from 4 to 50⫻10

A/cm

. From formation rate values, the following arrangement of passivation was

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obtained: (from the highest rate to the lowest rate) NaOH ⬎ Na

⬎ HClO

⬎

NHO

⬎ HCl ⬎ H

> H

[4-23].

Metikoš-Hukovic´ et al. [4-24] employed structurally sensitive in situ methods,

such as photopolarization and impedance, to examine the passivation process and

the properties of the protective oxide layers on titanium. The thickness of anodic

oxidation and the non-stoichiometry of the surface oxide were correlated. It was

mentioned that the composition of the anodic film on titanium changes with the

relative potential from lower to higher oxidation stages according to the equation:

TiH

⫹ TiO → nTi

·nTiO

→ Ti

or Ti

→ Ti

→ TiO

. The character-

istic behavior of titanium can easily be seen at higher anodic potential (~ ⫹1.5 V

vs. SCE in 5 mol H

) when the electrode is covered with a nearly stoichio-

metric TiO

layer. The semiconducting properties of TiO

were investigated using

an anodic film stabilized at ⫹2 V (vs. SCE: saturated calomel electrode with

0.2444 V vs. NHE), and it was found that TiO

, like the lower titanium oxides, is

an n-type (metal-excess) semiconductor under anodic polarization [4-24].

Repassivation capability [4-25] associated with titanium substrate can be con-

sidered as another reason for the high corrosion resistance and biocompatibility of

titanium materials. Although titanium owes its high corrosion resistance to the

presence of an extremely thin oxide, when the thin film is disrupted mechanically,

underlying metal substrate will react quickly with the surroundings to reform the

oxide film. The mechanical and electrochemical parameters associated with the

fracture and repassivation of titanium oxide films were studied [4-26]. The scan-

ning electrochemical microscope has been modified to evaluate the high-speed

mechanical disruption of the oxide film of titanium and the associated repassiva-

tion process. A mechanically polished Ti-6Al-4V surface was immersed and

potentiostatically held in phosphate-buffered saline. An 18-m radius diamond

stylus was used to apply 30–50 m scratches with a controlled load and the result-

ant current transient was measured. It was found that (i) current transients were

detected at the lowest loads applied (0.008 N), and (ii) topographic images showed

that probe loads as low as 0.1 N caused permanent deformation in the surface of

about 0.1 m, and below 0.05 N no deformation could be seen [4-26]. Such capa-

bility of repassivation is sometimes called a self-healing ability.

4.1.3 Oxidation at elevated temperatures

Above 100°C, the surface is a lower oxide type, such as TiO. At elevated tempera-

tures (above 200°C), complex oxides are formed, such as Al

TiO

on Ti-6Al-4V and

NiTiO

on TiNi, in addition to rutile-type TiO

[4-15, 4-16]. Oxidation in air at

875–1050°C produced a flaky scale of TiO, Ti

, and rutile (TiO

). It was reported

that the high oxide TiO

appears on the surface of titanium in the presence of the

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most rigid conditions of oxidation; in less rigid conditions a lower oxide

(3Ti

·4TiO

) forms, while in still weaker oxidizing conditions, the formation of

even lower oxides (e.g., TiO) are possible [4-9].

The characteristics of high-temperature oxidation of Ti are the main obstacle to

strong Ti-ceramic bonding [4-27]. A thin oxide layer (~32 nm) with good adherence

to the substrate was formed when CpTi was oxidized at 750°C in 0.1 atm, and a

thick layer (~1 m) with poor adherence was formed when oxidized at 1000°C

[4-28]. In another study [4-29], the thickness of TiO

on the CpTi surface increased

with an increase in oxidation temperatures. Ti surface hardness also increases

substantially after oxidation at and above 900°C [4-28]. Therefore, the substrate is

now in the BCC structure range. Lower Ti-ceramic bond strength was attributed to

a thick TiO

zone on the metal surface when Ti was oxidized at higher temperatures

[4-30]. Enhanced bond strength between titanium and ceramic was reported when

porcelain was fired on cast Ti in a reduced argon atmosphere [4-31]. Modifying

Ti surfaces to control high-temperature oxidation has been examined. Published

studies show that Ti surface nitridation [4-32] or a thin Cr coating [4-33] is effec-

tive methods in limiting Ti oxidation at high temperatures. Improved Ti-ceramic

bonding was found when a thin layer of Si

was deposited on Ti surface [4-34].

4.2. INFLUENCES ON BIOLOGICAL PROCESS

Sundgren et al. [4-35] and McQueen et al. [4-36], using Auger electron spec-

troscopy (AES) to study the change in the composition of the titanium surface dur-

ing implantation in human bone, observed that the oxide formed on titanium

implants grows and takes up minerals during the implantation. The growth and

uptake occur even though the adsorbed layer of protein is present on the oxide,

indicating that mineral ions pass through the adsorbed protein. Liedberg et al. [4-

37], using Fourier transform infraRed reflection absorption spectroscopy (FTIR-

RAS), showed that phosphate ions are adsorbed by the titanium surface after the

protein has been adsorbed. Using X-ray photoelectron spectroscopy (XPS), Hanawa

[4-38] demonstrated that oxides on titanium and titanium alloy (Ti-6Al-4V)

change into complex phosphates of titanium and calcium containing hydroxyl

groups which bind water on immersion in artificial saliva (pH 5.2). All these stud-

ies [4-35–4-38] indicate that the surface oxide on titanium reacts with mineral

ions, water, and other constituents of biofluids, and that these reactions in turn

cause a remodeling of the surface.

It was shown that titanium is in almost direct contact to bone tissue, separated

only by an extremely thin cell-free non-calcified tissue layer. Transmission electron

microscopy revealed an interfacial hierarchy that consisted of a 20–40 nm thick

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proteoglycan layer within 4 nm of the titanium oxide, followed by collagen bundles

as close as 100 nm and Ca deposits within 5 nm of the surface [4-39]. To reach the

steady-state interface described, both the oxide on titanium and the adjacent tissue

undergo various reactions. The physiochemical properties of titanium have been

associated with the unique tissue response to the materials: these include the bio-

chemistry of released corrosion products, kinetics of release and the oxide stoi-

chiometry, crystal defect density, thickness and surface chemistry [4-40].

While the conditions to produce an optimal oxide formation need to be better

understood, in general, the titanium passivating layer not only produces good cor-

rosion resistance, but it seems also to allow physiological fluids, proteins, and hard

and soft tissue to come very close and/or deposit on it directly. Reasons for this are

still largely unknown, but it may have something to do with things such as the high

dielectric constant for TiO

(50–170 vs. 4–10 for alumina and dental porcelain)

which should result in considerably stronger van der Waal’s bonds on TiO

than

other oxides; TiO

may be catalytically active for a number of organic and inor-

ganic chemical interactions influencing biological processes at the implant inter-

face [4-20]. The TiO

oxide film may permit a compatible layer of biomolecule to

attach [4-41, 4-42].

During implantation, titanium releases corrosion products into the surrounding

tissue and fluids even though it is covered by a thermodynamically stable oxide film

[4-43–4-45]. An increase in oxide thickness, as well as incorporation of elements

from the extracellular fluid (P, Ca, and S) into the oxide, has been observed as a func-

tion of implantation time [4-35]. Moreover, changes in the oxide stoichiometry,

composition, and thickness have been associated with the release of titanium corro-

sion products in vitro [4-46]. Properties of the oxide, such as stoichiometry, defect

density, crystal structure and orientation, surface defects, and impurities were sug-

gested as factors determining biological performance [4-17, 4-21, 4-47].

4.3. CRYSTAL STRUCTURES OF TI OXIDES

It should be clear that the proven high biocompatibility of titanium as an implant

materiasl is connected with the properties of its surface oxide. In air or water Ti

quickly forms an oxide thickness of 2–7 nm at room temperature. TiO

possesses

three crystalline structures: anatase, rutile, and brookite. Anatase-type TiO

is a

tetragonal crystalline system with a

= 3.78 Å and c

= 9.50 Å; the rutile-type TiO

is also a tetragonal structure, but the lattice constants are quite different from those

of anatase-type (i.e., a

=4.58 Å and c

=2.98 Å). The third type is brookite-type, and

has an orthorhombic crystalline structure with a

=9.17 Å, b

=5.43 Å, and c

=5.13 Å

[4-48]. Among these oxides, rutile is known to be the most stable phase [4-49].

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