Ion implantation of titanium based biomaterials

Progress in Materials Science 56 (2011) 1137–1177

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Ion implantation of titanium based biomaterials Tapash R. Rautray ⇑, R. Narayanan, Kyo-Han Kim Department of Dental Biomaterials, School of Dentistry, Kyungpook National University, 2-188-1 Samduk-dong, Jung-gu, Daegu, Republic of Korea Institute for Biomaterials Research & Development, Kyungpook National University, 2-188-1 Samduk-dong, Jung-gu, Daegu, Republic of Korea Brain Korea 21 Project, Kyungpook National University, 2-188-1 Samduk-dong, Jung-gu, Daegu, Republic of Korea

a r t i c l e

i n f o

Article history: Received 9 December 2009 Received in revised form 23 February 2011 Accepted 9 March 2011 Available online 23 March 2011

a b s t r a c t Titanium and its alloys are widely used as implant materials. Their integration in the bone is in general very good without fibrous interface layer. However, titanium and its alloys have certain limitations. Metal ions are released from the implant alloy and have been detected in tissues close to titanium implants. The release of these elements, even in small amounts, may cause local irritation of the tissues surrounding the implant. Cell and tissue responses are affected not only by the chemical properties of the implant surface, but also by the surface topography or roughness of the implants. To overcome the problem of ion release and to improve the biological, chemical, and mechanical properties, many surface treatment techniques are used. Any surface treatment that would elicit favorable response from tissues can be applied to enhance the usefulness of the implants. In view of this, the current review describes surface modification of titanium and titanium alloys by ion beam implantation. Ó 2011 Elsevier Ltd. All rights reserved.

Abbreviations: ALP, alkaline phosphatase; AMS, accelerator mass spectrometry; BCP, biphasic calcium phosphate; Cp-Ti, commercially pure titanium; DCPD, dicalcium phosphate dihydrate; DIBAD, dual ion-beam assisted deposition; EIS, electrochemical impedance spectroscopy; EQT, electrostatic quadruple triplet; ESA, electrostatic analyzer; FFR, fretting fatigue resistance; FT-IR, Fourier transform infra-red spectroscopy; HA, hydroxyapatite; IBAD, ion-beam assisted deposition; IBED, ion beam enhanced deposition; IBSD, ion beam sputtering deposition; MC-SCNICS, multi cathode source of negative ions by cesium sputtering; m-SBF, modified SBF; PIXE, proton-induced X-ray emission; PVD, physical vapour deposition; RBS, Rutherford backscattering spectrometry; SBF, simulated body fluid; SEM, scanning electron microscope; SRIM, Stopping and Range of Ions in Matter; TCP, tricalcium phosphate; TRIM, Transport of Ions in Matter; UHMWPE, ultra high molecular weight polyethylene; XRD, X-ray diffraction. ⇑ Corresponding author at: Department of Dental Biomaterials, School of Dentistry, Kyungpook National University, 2-188-1 Samduk-dong, Jung-gu, Daegu, Republic of Korea. Tel.: +82 53 660 6897, fax: +82 53 422 9631. E-mail address: [email protected] (T.R. Rautray). 0079-6425/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmatsci.2011.03.002

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Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Titanium and titanium alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Osseointegration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calcium phosphate ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Dicalcium phosphate dihydrate [CaHPO42H2O] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Tricalcium phosphate [Ca3(PO4)2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Hydroxyapatite [Ca10(PO4)6(OH)2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Ion implantation principles and parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Ion–solid interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Energy loss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. High dose implantation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Atomic mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7. Radiation damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8. Radiation enhanced diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Ion implantation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Typical accelerator system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Ion source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Injection system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Post-acceleration system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Implantation beam line and chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Dose and SRIM/TRIM calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Ion-beam assisted deposition (IBAD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Principles and methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. IBAD coating features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Advantages of IBAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. Dual-ion-beam-assisted deposition (DIBAD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. HA coating using Ca and P ion implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Implantation of some specific ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. Carbon and carbon monoxide ion implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Oxygen ion implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3. Nitrogen ion implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4. Sodium ion implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5. Magnesium ion implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6. Silver ion implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.7. Argon ion implantation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Properties of ion-implanted HA coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1. Stress shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3. Wear resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4. Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5. Fatigue resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6. Fretting fatigue resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7. Fracture toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.8. Adhesion of the coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.9. Corrosion properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Biological activities of ion-implanted HA coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. Bioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. Testing the in vivo bioactivity using SBF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3. In vitro cell activity of ion-implanted titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4. In vivo results of ion-implanted titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5. Anti-bacterial action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1139 1140 1141 1141 1141 1141 1142 1142 1143 1144 1146 1147 1148 1148 1148 1148 1149 1149 1149 1149 1150 1150 1151 1152 1152 1153 1154 1155 1155 1159 1160 1160 1160 1160 1162 1162 1163 1163 1163 1163 1164 1165 1165 1165 1166 1167 1167 1168 1168 1169 1170 1170 1171 1171

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Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1172 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1172 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1172

1. Introduction Materials used for implant applications should have good mechanical strength, high chemical stability, excellent corrosion resistance and biocompatibility [1,2]. Titanium is extensively used in bone anchoring systems, such as dental and orthopedic implants as well as osteosynthesis applications. It displays mechanical properties similar to bone. Table 1 gives the mechanical properties of various materials used for implant applications. Titanium forms a protective and stable oxide film. This film can provide excellent corrosion resistance [3] and can adsorb proteins and induce differentiation of bone cells [4]. Titanium integration in the bone is in general very good without fibrous interface layer. However there are some areas of concern regarding the use of titanium. Some issues to be addressed in this regard include the long-term stability of hip joint prostheses [5] and healing response as well as osseointegration of dental implants. Altering the titanium surface can help in tackling these issues as surface parameters play an important role to obtain effective implant–tissue interaction and osseointegration. A common way to improve the osseointegration is the coating of titanium surface with hydroxyapatite (HA; the mineral component of the bone). Many processes like plasma-spraying, biomimetic, electrolytic, sol–gel, sputtering, ion implantation, laser cladding, etc. are used to produce HA coatings on implant metal surfaces [6,7]. Although they are commercially popular, plasma processes produce thick coatings containing various phases of mixed crystallinity. These thick coatings however, give rise to delamination and differential dissolution of the phases. Longer time-scale, poor adhesion and low crystallinity are the problems usually associated with biomimetic process. The electrolytic processes produce coatings with poor adhesion strengths. Sol–gel processes involve longer times and require a post-processing annealing to obtain more desirable properties. On the other hand, ion implantation methods are better than these techniques and they have many applications for surface treatment of biomaterials. One major advantage of these methods is tremendous increase of wear resistance of orthopedic joints. In addition ion implantation modifies orthodontic appliances, surgical tools and other sensitive medical components, such as heart valves. This technique can provide modification of titanium surface for HA synthesis or property improvement. HA is synthesized by implantation of Ca and P [8]. Improvements in wear resistance and biocompatibility are achieved by implantation of other ions [9]. Ion implantation technique [10] enables to inject any element into the near-surface region of any substrate. It is a non-equilibrium process and therefore, is capable of producing materials with compositions and structures unattainable by other conventional equilibrium methods (such as thermal diffusion or alloying). Ion beam systems are characterized by having a preferred direction, i.e., the direction of ion beam propagation. In the ion beam method, a beam of high energy (>10 keV) ions is allowed to fall on a target, kept in a vacuum chamber. The incident ions lose their energy due to collisions with the target atoms and come to rest in the near-surface region.

Table 1 Mechanical properties of several materials. Material

Elastic modulus (GPa)

Yield strength (GPa)

UTS (GPa)

Pure Ti Ti–6Al–4V Ti–13Nb–13Zr Co–Cr–Mo Stainless steel Bone

105 110 79 200–230 200 1–20

692 850–900 900 275–1585 170–750 –

785 960–970 1030 600–1795 465–950 150–400

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Table 2 Advantages of ion implantation. Possibility of accurate dose and depth control Most ions can be applied Possibility of addition of many dopants Process is possible at ambient temperature Minimal lateral spread of dopants beneath a mask

Ion implantation is carried out in vacuum and hence it is an ultra-clean process and thus, high purity layers can be synthesized. Concentration and depth distribution of impurities are easily determined and controlled. Excellent adhesion between the implanted layer and substrate is found as there is no demarked interface between the implanted layer and substrate. There is no strict limit on the solubility as this is a non-equilibrium process. Therefore solid solubility limit can be exceeded. Bulk properties of the substrate are not affected largely because the process is performed at low substrate temperatures. For the same reason there is no significant dimensional change of the substrate. The process is also extremely controllable and reproducible and can be tailored to affect different surfaces in desired ways [9]. Advantages of this technique are succinctly described in Table 2. Table 3 gives an idea about the various properties improved by ion implantation method. In spite of all these advantages, it should be mentioned that, ion implantation is a violent process and produces implantation damages on the surface. These damages can improve tribological [11] or corrosion resistance of the substrate. If the implantation damages are not required they can be removed by post-implantation annealing [12]. Ion beam implantation methods are based on accelerator or plasma type. This article deals with accelerator-based ion beam implantation of different ions, formation of HA coating on titanium system and properties of such coatings. Following sections will deal briefly about titanium and its alloys, osseointegration and calcium phosphate ceramics before discussing in detail about the ion beam principles and parameters, as applied to titanium system. 2. Titanium and titanium alloys An ideal metallic biomaterial used in dental or orthopaedic fields should have (a) reasonably low density, (b) little or no cytotoxic metals in its composition, (c) high strength and long fatigue life, (d) low elastic modulus (comparable with that of cortical bone) and (e) large room-temperature plasticity so that it can be easily formed and (f) good casting properties so that it can be cast into defectfree components [13]. Ti alloys are light and have very high strength to weight ratio. They possess good biocompatibility, outstanding mechanical properties, excellent corrosion resistance and good formability. For all these reasons they are used in dental and orthopaedic applications. Commercially pure titanium (cp-Ti) is used widely in dental implants. Alloys such as Ti–6Al–4V and Ti–6Al–7Nb have superior mechanical properties and are used in various orthopedic and osteosynthesis systems such as parts of hip and knee implants, bone screws or plates [14]. Ti system is useful for implant materials because of their high corrosion resistance as compared to stainless steel and Co–Cr–Mo alloys. A passive oxide (primarily TiO2) film protects the surface of titanium and its alloys. This stable and adherent passive oxide film protects Ti alloys from pitting corrosion, intergranular corrosion, and crevice corrosion attack and in large part is responsible for the excellent biocompatibility of Ti alloys.

Table 3 Properties influenced by ion implantation (given under different categories). Mechanical Chemical Electrical Optical

Microhardness, friction and wear Corrosion, passivation and reactivity Resistivity and semiconductivity Colour, reflectivity and transmission

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Many of the alloys contain a mixture of a and b phases, whose morphologies and distributions can be altered by processing or heat treatment. In addition, depending upon alloy composition, the metastable b phase can decompose on cooling to mixtures of a and b, martensite or a metastable transition phase. Alloying additions also can influence the microstructures. Table 4 gives the details of alloying elements added to titanium and their effect in stabilizing the various phases. 3. Osseointegration Osseointegration [15,16] is described as direct contact (at the light microscope level) between living bone and implant [16]. Based on histology, osseointegration is defined as the direct anchorage of an implant by the formation of bone tissue around it without the growth of fibrous tissue at the bone– implant interface. A 100% bone-to-implant contact does not occur. No consensus opinion could be arrived as to the extent of bone-to-implant contact required for acceptance of the connection as osseointegration or on the criteria for definition of the term. Irrespective of all the confusions involved in the definition, effective osseointegration requires a bioinert or bioactive material and a surface configuration that are osteophilic i.e. attractive for bone deposition. Surface properties of implants influence the elaboration of bone-to-implant contact and HA coating on a suitable bioactive implant surface will enhance this contact. Thus an effective technique for effectively coating HA on such a substrate is mandatory. 4. Calcium phosphate ceramics Of the many phases of calcium phosphate ceramics HA has been studied extensively from the biomedical point of view. Other phases such as dicalcium phosphate dihydrate (DCPD) and tricalcium phosphate (TCP) have also been investigated for biomedical applications. 4.1. Dicalcium phosphate dihydrate [CaHPO42H2O] DCPD occurs as the mineral brushite and crystallizes in the monoclinic space group Ia. Unit cell parameters are: a = 5.812(2) Å, b = 15.180(3) Å, c = 6.239(2) Å and b = 116.42°. DCPD is normally obtained as the primary crystalline product when calcium phosphate is precipitated at low pH and temperatures. For example, DCPD is the product obtained on electrolysis at room temperature, from solutions containing dissolved calcium and phosphorus compounds at acidic pH [17]. 4.2. Tricalcium phosphate [Ca3(PO4)2] TCP is a polymorph ceramic and has tendency to resorb fast. It exhibits two phases known as a-TCP and b-TCP. Variation in sintering temperature and humidity determine which of these phases is being formed. Once implanted inside the body TCP undergoes partial conversion to HA. In some applications fast degradation of b-TCP is detrimental. To slow down the rate of biodegradation, biphasic calcium phosphate (BCP) ceramics (i.e. composite ceramics that consist of mixtures of both the HA and bTCP phase) have been proposed [18,19].

Table 4 Alloying elements of titanium and their effects.

1 2 3 4 5 6

Alloying Element

Effect

Aluminum Tin Vanadium Molybdenum Chromium Zirconium

a stabilizer a stabilizer b stabilizer b stabilizer b stabilizer a and b strengthener

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4.3. Hydroxyapatite [Ca10(PO4)6(OH)2] HA is the main mineral component of bone and teeth. It can form a real bone with the surrounding bone tissue upon implantation [20]. HA deposition is common in osteoarthritic cartilage. Large number of HA crystals are present in synovial fluid and synovial fluid leukocytes [21]. HA is hexagonal with a = 9.432 Å and c = 6.881 Å. Among the various calcium phosphates, stoichiometric HA has very slow rates of dissolution and precipitation. As already indicated, presence of HA is the key to promote osseointegration at the bone-implant interface. HA is brittle and therefore it is applied as coating on the metallic implant systems. Ion implantation is an advanced surface modification technique using which HA can be formed on the metals. The next sections deal with basic principles and operational process parameters of ion implantation. 5. Ion implantation principles and parameters When an energetic ion passes through a substrate, a variety of processes occur due to ion- solid interactions. If the ions get incorporated into the substrate after loosing all their energies, the process is called ion implantation [11]. In ion implantation process, ions are accelerated and directed towards a substrate (titanium in the present case). The energy of the ions is typically in the range of several keV to few MeV depending on the desired depth of implantation. With this level of energy, the ions penetrate the surface and create significant changes (Fig. 1). However the energy of ions is so selected that they do not cause a deep penetration inside the substrate. So the modifications are therefore confined to the near-surface region [9] and a depth of 1 lm from the surface can be achieved (Fig. 2). As a first step electrons are stripped off from atoms to form the ions, which are then extracted by attracting them to an oppositely charged region. By creating a high graded potential difference in the accelerator tank, the ions are accelerated. Then they pass through a magnet that can select ions of a desired species and charge. A series of electrostatic and magnetic lens elements shapes the ion beam and scans it over an area in an end-station that contains the parts to be treated. The entire process is conducted in high vacuum. Important ion implantation parameters are ion species, beam energy, fluence (or total number of ions that bombard a surface) and beam current density or flux. An important

Fig. 1. Illustration of various processes in ion–solid interactions.

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Fig. 2. Depth distribution of ions inside the solid.

feature is, the process provides a great deal of control over these parameters and can greatly influence the ultimate effects on the substrate [9]. Table 5 gives various stages of development in ion implantation technique used for surface modification of biomaterials. Various atomic processes accompanying ion implantation are described in the ensuing sections. 5.1. Ion–solid interaction When an energetic ion penetrates into a substrate its energy is lost because of (i) elastic and (ii) inelastic collisions. Elastic (nuclear) collision takes place with the nuclei of the target atoms in the solid. In this process a part of the ion energy is transferred to the nuclei (atoms) of the target such that the total kinetic energy remains constant. Such collisions cause displacements of atoms from their equilibrium positions in the solid. This is the main mechanism for slow ions (low ion energy regime). Inelastic or electronic collision of ions with atoms causes excitations of electrons in the solid medium. This is the predominant mechanism of energy loss for fast ions (high energy regime). Inside the substrate, ions lose energy by both these collision processes. Or in other words, the energy of an ion decreases through its path of penetration into the substrate. For a high energy ion, inelastic loss is dominant during the initial stages of its path and elastic energy loss is dominant towards the end of its path. Atoms in the solid experience a temporary or permanent relocation from their original lattice sites, causing an accumulation of defects (vacancies and interstitials) producing a structural transformation Table 5 Chronological developments of ion implantation processes. Year

Development

1906 1930 1940 1960 1970 1980

Physics and principles of ion implantation were postulated Development of low energy ion sources Originating from ion source, light ions were accelerated Major efforts to study effects of implanting dopants into semiconductors Surface properties of metals studied by ion implantation Proven and established in micro electronics industry and explorations were made in biomedical industry Application to biomedical implants in dental and orthopaedic devices

1990s and 2000s

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from crystalline to an amorphous state [11,22]. Atoms at or near the surface may receive sufficient energy to surmount the surface potential barrier and leave the substrate. This process is called sputtering. This results in a flux of ejected atomic and/or molecular species, which can either be neutral or have various charge states [23–25]. In the high ion energy regime (a few MeV onwards) the electronic stopping is predominant over nuclear stopping. Through the initial ion–electron interaction and the later electron–lattice interaction, a part of the ion energy is transferred to the lattice. Both of these can give rise to bond breaking and defect formation. Thus, both the low and the high energy regimes can be used for the modification of the surface of substrates. Due to its collision with an atom in the substrate an ion can be scattered in all directions. As a result of these collisions atoms inside the substrate get recoiled. The recoiled atoms again transfer energy to neighbouring atoms and form a collision cascade. In this way ion-irradiation produces structural modification of substrates which gives rise to phenomena like sputtering and implantation. Ion irradiation also induces mixing in bilayers or multilayers and produces alloys. 5.2. Energy loss The incident ions lose their energy due to collisions with substrate atoms. The major processes [11] by which the ions lose their energy in the target material are: (1) Direct collision with the nucleus due to the screened electrostatic interaction, (2) interaction with electrons in the substrate and (3) charge exchange processes between the ion and the atoms of the substrate. All these processes are energy dependent and make different contributions to the energy loss along the ion trajectory. Treating the above mentioned processes independently, differential energy loss equation can be written as

      dE dE dE dE ¼ þ þ dx dx nucl: dx elect: dx exch:

ð5:1Þ

Variation of the first two components with energy is shown in Fig. 3. Both these components increase with energy, reach a maximum and then decrease. Nuclear stopping predominates at lower energies whereas electronic collisions dominate at higher energies. 2 An ion moving with velocity v > Zh1 2e , where Z1 is the atomic number of the ion, e is electron charge  2 ) = v is Bohr velocity, has a high probability of and h is h/2p where h is Plank’s constant and (e2/h 0 being fully stripped of its electrons. In such a case, the energy loss can be obtained from classical calculations of Bohr [26] as



  dE 4Z 2 e2 N ¼ 1 2 B dx elect: mv

ð5:2Þ

Fig. 3. Variation of the nuclear and electronic contributions to the specific energy loss dE/dx as a function of ion velocity v.

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where m is the electronic mass, N is the atomic density of the target and B is the dimensionless stopping number, given by

B ¼ Z 2 lnðPmax =Pmin Þ Z2 being the atomic number of the target and Pmax and Pmin are the upper and lower limits of the ion– electron separation at closest approach. B is a measure of penetration through electron shells. In 1930, Bethe [27] revised the formula taking into consideration quantum mechanical aspects of the situation. The new value of B is given by

B ¼ Z 2 lnðmv2 =IÞ where I is the average excitation energy of the electrons in the target. The calculations of Bloch [28] based on Thomas–Fermi model of the atom give value of I as

I ¼ KZ2 where K = 1 eV. Considering the shell corrections, Walske [29] and Bichsel [30] corrected the above formula further, as

B ¼ Z 2 lnð2mv2 =IÞ  C K  C L where CK and CL are contributions arising from the stopping processes due to the K and L shell electrons. The third term in Eq. (5.1) comes from charge exchange processes between ion and the target atom. The energy loss due to this process is given as

    X dE mv 2  ¼N /z1 rz1 ;z1 1 J þ dx exch: 2

ð5:3Þ

where rz1 ;z1 1 is the charge exchange cross-section for electron capture, /z1 is the probability of such a process, J is the electron binding energy in target atom for electron capture process or electron binding energy in projectile ion for electron loss process and v is the ion velocity. Energy loss due to collisions between ion and nucleus is given by Bohr [31] as

  Z Tm dE  ¼N TdrðE; TÞ dx nucl: T min

ð5:4Þ

where dr(E, T) is the differential scattering cross-section at energy E, involving an energy transfer between the transfer limits T and T + dT. Tmin is the minimum energy transfer whereas Tm represents the upper limit for energy transfer. T and dr(E, T) depend upon the interaction potential V (r ) between the two particles. Linhard et al. [32,33] proposed a theory (Linhard, Scharff and Shiutt theory abbreviated as LSS theory) that developed a comprehensive concept of atomic stopping. By using a differential cross-section based on Thomas–Fermi potential between the atoms, LSS theory gives a relationship for nuclear stopping in terms of dimensionless length q, and energy e. These two quantities are defined as:

M1 M2

q ¼ Rpa2 N

! ð5:5Þ

ðM1 þ M2 Þ2

and

e¼E

 a  e2

M2 Z 1 Z 2 ðM 1 þ M 2 Þ

where

0:8853a0 a¼ 1=2 2=3 Z 1 þ Z 22=3

 ð5:6Þ

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where a0 is the Bohr’s radius whose value is  0.0529 nm. R is the range of ions, N is the number of atoms per unit volume, Z1 and Z2 are atomic numbers of the incident ion and target atom respectively. M1 and M2 are the corresponding masses. The nuclear stopping Sn as a function of e is defined [32,33] as



de dq

Sn ðeÞ ¼



¼r

n

M1 þ M2 4pe2 Z 1 Z 2 M 1

ð5:7Þ

r is the nuclear stopping cross-section. Nuclear stopping Sn(e) varies with e as e1/2. At low energies,   nuclear stopping ddqe increases, reaches a maximum value around e  0.35 and then falls off. Similar to nuclear stopping, electronic stopping also can be expressed in terms of e and q. The electronic stopping Se(e) is given [32,33] by 

 de ¼ ke1=2 dq e

Se ðeÞ ¼

ð5:8Þ

where

k¼

" # 1=2 1=2 ne 0:0793Z 1 Z 2 ðM 1 þ M 2 Þ3=2 2=3

2=3

3=2

3=2

ðZ 1 þ Z 2 Þ3=4 M1 M 2

1=6

and ne  Z 1 :

Electronic stopping increases linearly with velocity over a very wide range and hence becomes the dominant process for energies greater than e  3. At much energies ddqe passes through a max higher  imum, then falls off as e1. The overall rate of energy loss ddqe is obtained by adding the appropritotal ate losses due to electronic stopping and nuclear stopping. 5.3. Range After losing all their energies by the processes (described until now) along their path, the incident ions come to rest. When a beam of energetic ions is incident on a target, the distance travelled between successive collisions and the energy transfer per collision varies from ion to ion. As a result, all ions of the given type and energy do not travel the same distance inside the host material. If E0 is the initial energy of the incident ion, the total path covered by the ion as it is brought to rest is given by

Rtot ¼

Z 0

E0



dE ðdE=dxÞtotal

ð5:9Þ

where R is the range of ions. Eq. (5.9) is helpful in calculating the depth of penetration which is an important parameter for bioactivity. For low doses, the depth distribution of these ions is approximately Gaussian in nature. The peak corresponds to the most probable range called projected range, Rp (Fig. 4). It is the average depth to which an ion will penetrate before coming to rest, in the direction of incident beam. The LSS theory gives relation between R and Rp [34] as

R q M2 ¼ ¼1þb Rp qp M1

ð5:10Þ

where b is a slowly varying function of E and R. However, b ffi 13 is a good approximation. The straggling around this mean value, Rp is denoted by DRp (Fig. 5). It represents spread in the ion path lengths. Actual implanted distribution can be predicted with the help of DRp. If the mean square fluctuation about the average total path length is denoted by DR2 and the square of the mean range is R2 , the ratio DR2 =ðR2 Þ can be taken as a measure of straggling. LSS theory can be used to calculate the 2

2

1 þM 2 Þ . DRR2 against reduced ion straggling if the depth distribution is assumed to be Gaussian. A plot of ðM4M 1 M2

energy e, for different values of k (where k is the electronic stopping parameter) shows that, straggling drops off with increase in e and k.

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Fig. 4. Basic processes during ion implantation. (a) Implantation of dopant atoms (implantation profile); (b) contribution of energy to nuclear subsystem (nuclear energy losses); (c) contribution of energy to electronic subsystem (electron energy losses).

Fig. 5. Projected range Rp and the distribution of the implanted atoms DRp and DRt. Also shown is the range R of the ions.

5.4. High dose implantation High dose implantation may be defined as the total implanted ion concentration exceeding 1012 ions/cm2. Under this condition, effects like atomic mixing, sputtering, ion beam induced migration and chemical effects play an important role in determining the state of the implanted material. These effects also influence the depth profiles of the trajectory ions.

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5.5. Atomic mixing When a beam of energetic ions impinges on a substrate containing an existing (impurity) layer, mixing of the impurity and host atoms takes place. A large number of atomic collisions are initiated along the trajectory of the incident ion. Due to elastic collisions between incident ions and impurity atoms, redistribution of impurity atoms takes place. Redistribution of impurity atoms also takes place due to elastic collisions between impurity atoms and secondary knock ions. Similarly, in case of heavy ions incident on light substrate, relocation of matrix atoms also results from these collisions. As a result of all these collisional processes, substantial mixing of the impurity atoms and substrate atoms takes place. This is known as atomic mixing or cascade mixing. Atomic mixing has a very little effect on projected range, RP, but affects the straggling DRp substantially. As a result the profile (Fig. 5) broadens [35]. Another possible result of this phenomenon is formation of metastable phases. 5.6. Sputtering Studies on sputtering of elemental materials [36–38] indicate that, (i) heavier ions or heavier targets give high sputtering yield and (ii) sputtering yield is inversely proportional to the surface binding energy. Elemental sputtering phenomena [22] are explained based on a collision cascade principle. Sputtering yield is directly proportional to nuclear stopping power of the incident ion in the nearsurface region. High dose implantation increases the chances of sputtering. Some of the ions of the implanted species are also removed because of sputtering. In the case of multi elemental targets, different atoms may have different energies. Their binding energies and ejection probabilities will also be different. As a result, sputtering yield will be different for two different kinds of atoms. Hence one element may be preferentially sputtered over others. This is called preferential sputtering and the surface composition may be altered because of this [39,40]. Surface layer several hundred angstroms thick are enriched with heavy species. The depth up to which these compositional changes occur is dependent on specific ion and its energy. Steady composition is reached when an amount of material comparable to the thickness of the altered surface has been sputter-removed [41]. 5.7. Radiation damage An energetic ion incident on the target initiates a collision cascade inside it. At low doses, these regions of disorder are isolated; with increasing dose they start overlapping resulting in a completely disordered layer. This damage caused by the incident radiation is mainly due to energy loss by nuclear collision. The displacement energy (energy required to displace the atom from the lattice site), Ed, determines the number of defects but does not influence their spatial distribution [42]. Depth distribution of the damage is determined by various parameters like implanted ion species, dose rate, total dose energy of the ion, temperature during implantation, etc. To remove the disorder regions caused by low dose ion implantation, annealing is done. Typically this would involve low temperatures (<600 °C). On the other hand the amorphous layer resulting from high dose implantation requires higher temperatures (>600 °C) for annealing and the reordering is epitaxial. 5.8. Radiation enhanced diffusion Radiation increases vacancy concentration and promotes vacancy-assisted diffusion. Substitutional atoms are also ejected into interstitial sites, from where they diffuse very rapidly. As a result, diffusion of substitutional atoms is promoted even at lower temperatures. This phenomenon is known as radiation enhanced diffusion. Enhanced diffusion takes place in the region where radiation damage is produced. Even in those regions which did not suffer radiation damage, diffusion may result from migrated vacancies and interstitial atoms. The enhanced diffusion continues even after implantation is over, due to the

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migrated defects until they are lost to the sinks. The decay time depends on the thermally activated jump rate for migration. If the temperature during implantation is sufficiently high to allow for damage annealing, the enhanced diffusion is suppressed or even nullified. After this discussion on the principles of ion implantation, the next section deals with a typical accelerator-based ion implantation process giving details about ion source, injection systems, implantation beam line, etc. 6. Ion implantation processes 6.1. Typical accelerator system The ion implanter may be a low energy or medium energy accelerator. The accelerator consists of a steel cylinder (called as main tank of accelerator) pressurized with SF6 insulating gas to prevent sparking by trapping electrons. Internal support column is provided in this tank to support the high voltage terminal and other accelerator components. A charging system is equipped inside it for controlling and maintaining constant terminal voltage. Gas-stripping system is used to convert negative ions into positive with different charge states. High terminal voltage is achieved by the Van de Graaff principle by successive charging of the terminal by the induction of charge from the pelletron chain [43]. The vacuum inside the accelerating tube is maintained by using combination of cryogenic pumps, turbo-molecular pumps, and titanium ion pumps at regular intervals over the length of the entire beam line. It is capable of accelerating a variety of ion species over a broad range of energy for use in Rutherford back scattering spectrometry (RBS), proton induced X-ray emission (PIXE), ion implantation, accelerator mass spectrometry (AMS), and nuclear physics experiments. Fig. 6 shows a typical electrostatic accelerator. 6.2. Ion source Ion sources with multiple characteristics are strongly recommended for accelerators. Multi cathode source of negative ions by cesium sputtering (MC-SCNICS) has long been used as a reliable sputter cathode ion source with rapid and precise cathode change characteristics. Cs+ ions are produced by tantalum ionizer immersed in cesium vapour. The Cs+ ions are accelerated to the negatively biased cold cathode, which then sputter out negative ions from cathode materials. Multiple cathodes of same or different materials can be mounted at the same time in the cathode disk in a pelletron accelerator [44–48]. In general, ion beam currents are functions of cathode composition, cathode potential, cesium ion flux and cathode temperature [43]. 6.3. Injection system Extractor–einzel lens assembly is arranged to accelerate the negative ion beam emerging from the ion source. The pre-accelerating tube focuses the beam through the 45° Electrostatic Analyzer (ESA) into the 90° double focusing injection magnet. The injection magnet focuses the beam into the X–Y slits. Faraday cup and X–Y slits are provided immediately after the injection magnet to stop the unwanted ions and also measure the ion beam current injected into the accelerator. A beam profile monitor is provided before the accelerator to see the beam shape and optimize the beam transmission. An X–Y beam steerer is provided to correctly position the negative ion beam for injection into the accelerator. Einzel lens is placed before the accelerator entrance to focus the ion beam at the terminal-stripper. Terminal-stripper is a gas-stripper used to convert the negative ions to positively charged species. The lens achieves the necessary focal length using high voltage for negative ion beam energy in the range of tens of keV. The beam enters through the low energy end of the accelerator tank where negative ions are attracted to the positively charged high voltage terminal and thus accelerated. At the terminal, these ions are charge-stripped by passing them through gas-stripper system and converted into positive

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Fig. 6. Schematic diagram of a typical ion implanter.

ions. As the positive ions exit the stripper and drift into the second stage of the accelerator, they are repelled by the high voltage terminal and thus accelerated once again. This dual acceleration gives the single charged ions twice the energy [49]. If q is the charge (in coulomb) and VT is the terminal voltage (in MV), then total energy E (in MeV) can be calculated as

E ¼ ET þ E0 ¼ V T ð1 þ qÞ

ð6:1Þ

where ET is terminal energy (in MeV) and E0 is ion source energy (in keV). 6.4. Post-acceleration system At the exit end of high energy accelerating column, an electrostatic quadruple triplet (EQT) lens is provided to focus the beam into the 90° double focusing analyzing magnet. Before the analyzing magnet, there is a beam profile monitor to see the beam shape. The analyzing magnet bends the beam of selected charge state of positive ions by 90° and focuses into the switching magnet. In the beam line there are Faraday cups to measure the beam current. The magnetic quadruple doublet focuses the beam into the switching magnet. The switching magnet has a stainless steel chamber with seven exit ports at 0°, ±15°, ±30°, ±45° which can switch the ion beam to different targets placed at the vacuum chambers in the beamlines. 6.5. Implantation beam line and chamber Implantations are usually performed at 15° beam line. The beam line is equipped with raster scanner (Fig. 7) operating at specific frequencies, both in the horizontal and vertical directions. The voltage applied to raster scanner can be large enough to obtain a uniform implanted region. The vacuum in implantation chamber is approximately 106 mbar. To maintain this level of vacuum the system is equipped with rotary and turbo-molecular pumps. The implantation chamber

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Fig. 7. Schematic diagram of beam scanner.

Fig. 8. Schematic diagram of an ion implantation chamber.

(Fig. 8) has two view ports; one port for electrical connections to the suppressor assembly (Fig. 9) and another port for viewing the beam display on the target. 6.6. Dose and SRIM/TRIM calculation Each ion is typically one atom and the actual amount of material implanted in the target is the integral over time of the ion current. This amount is called dose. The current (ion current and secondary

Fig. 9. Suppressor with target holder.

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electron induced current emitted due to ion bombardment on the surface of target) is measured by a current integrator. Dose can be calculated from the number of counts (N) measured by the current integrator, given as

D ¼ NS=qe

ð6:2Þ

where D = dose or ion fluence (ion/cm2), q = charge state of the ion, e = electronic charge, S = scale at which the digital current integrator is set. For the calculation of penetration depth, projectile range (dE/dx)elec and (dE/dx)nucl of the particle in the material, the Stopping and Range of Ions in Matter (SRIM)/Transport of Ions in Matter (TRIM) code is used [50]. SRIM/TRIM uses the details of ion species, energy of ions and the target composition, to determine the depth of penetration and straggle of ions in the substrate (Fig. 10).

7. Ion-beam assisted deposition (IBAD) 7.1. Principles and methodology While ion implantation modifies the surface properties by penetration of the ions into the surface of a substrate, IBAD (Fig. 11) is a thin film deposition process that adds a layer on the surface of the substrate to form a coating. Due to the low operating pressure of the various broad-beam ion sources, it has been possible to combine ion bombardment and sputtering with evaporative deposition. Depending on the configuration, these two techniques can operate independently in the same chamber. The ion beams are generated from an ion source in high vacuum conditions as in the case of conventional ion beam techniques [51]. In the first of the two actions, an inert-gas ion beam is used to controllably sputter atoms from a target (HA in the present case) for getting the supply of target atoms. These are impinged on the substrate (titanium in the present context) and form a film. In the second action, inert (e.g. Ar+) or reactive gas ions (e.g. Oþ 2 ) are directly focused at the substrate (titanium in the present context) for modifying the properties of the growing film by some combination of bombardment, reaction, or burial within it.

Fig. 10. Depth of penetration an straggle of ions inside the substrate.

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Fig. 11. Schematic diagram of IBAD system and process. The technology combines evaporation with concurrent ion beam bombardment to produce adherent dense coatings with virtually any substrate/coating material combination.

7.2. IBAD coating features Fig. 12 shows the XRD patterns of ‘IBAD formed and subsequently heat-treated’ Ca and P layers on titanium [51]. When the ion beam is not used to assist deposition, TCP is formed after heat treatment, (Fig. 12a). When an ion beam with the current of 0.6 A is applied to the substrate during deposition, the composition of the layer is unchanged (Fig. 12b). But, when the current of the ion beam is

Fig. 12. XRD pattern of coating layer after heat treatment in vacuum at 630 °C. Ion beam currents are (A) 0 A, (B) 0.6 A, (C) 0.8 A, (D) 1.0 A, (d) TCP, (h) HA and (e) the Ti peak [51].

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increased to 0.8 A, weak HA peaks are detected instead of TCP peaks (Fig. 12c). When the beam current is increased to 1.0 A the HA peaks became stronger (Fig. 12d). Thus the composition of the coating layer is strongly influenced by the ion beam used or indirectly by the ion beam current. SEM micrographs of the coating layer before and after the heat treatment are shown in Fig. 13 [51]. Morphologies were similar for all the four beam current conditions of 0, 0.6, 0.8 or 1 A. Before the heat treatment, the layer was rather featureless as shown in Fig. 13a. After heat treatment, however, the layer became severely cracked (Fig. 13b) apparently due to a thermal expansion mismatch between the substrate and the layer. The effect of these cracks is reduction in bond strength [52]. Similar features were obtained irrespective of the current conditions. HA obtained on titanium substrate by IBAD has a tensile bond strength of around 70 MPa. This exceeds the tensile bond strength of 51 MPa typically associated with the plasma spray deposition process. The higher bond strengths associated with the IBAD prepared films are seen as a consequence of an atomic intermixing interfacial layer formed by ion dynamic intermixing. The interfacial layer is 25 lm thick and consists of a transitional structure that includes amorphous HA, amorphous calcium phosphates, amorphous Ti phosphate compounds and a two stage gradient of interrupted titanium. A chemical bond is suggested to form at the HA/Ti interface as a consequence of the energetic ion bombardment process. The coating thickness associated with the IBAD process is typically in the range of 2–4 lm and can be made thicker with extended exposure times. 7.3. Advantages of IBAD Ion bombardment of films during sputter deposition is particularly effective in modifying film properties. IBAD provides independent control of the deposition parameters and more specifically,

Fig. 13. SEM of the coating layer (A) before and (B) after the heat treatment. Similar morphology was obtained for all the four beam currents of 0, 0.6, 0.8 and 1 A [51].

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the characteristics of the ions bombarding the substrate. Besides providing independent control of parameters such as ion energy, temperature and arrival rate of atomic species during deposition, this technique is especially useful to create a gradual transition between the substrate material and the deposited film, and for depositing films with less built-in strain than is possible by other techniques. These two properties can result in films with a much more durable bond to the substrate. To summarize, IBAD can produce coatings with excellent substrate-bonding properties but with no known long-term clinical experience [53]. 7.4. Dual-ion-beam-assisted deposition (DIBAD) Another hybrid, ion-beam-assisted deposition called dual-ion-beam-assisted deposition (DIBAD) produces surface layers similar to those achieved by sputtering and evaluation, characterized by superior adhesion and composition gradients at the interface (Fig. 14). These qualities are obtained by virtue of high impact energy achieved by simultaneous ion beam bombardment from an ion source. A typical system in commercial use employs a multiple pocket electron beam evaporator which can generate a choice of metallic or metalloid vapours. Two ion sources, one of high energy for enhancing the deposition and the other of lower energy for cleaning the substrate prior to coating, are arranged to process substrates on a controlled temperature rotating holder. The resulting DIBAD system combines desirable features of evaporation and ion implantation. Coatings on titanium can be grown from within the substrate, for best adhesion. IBAD and DIBAD can produce highly dense and pin-hole-free films with controlled porosity. Compared with ion implantation, high through-put and low-cost productions are possible in these techniques. Further sections will deal with HA formation and its properties.

8. HA coating using Ca and P ion implantation Objective of surface modification is to improve the macroscopic surface properties such as hardness, wear resistance and corrosion resistance for enhanced applications. As a surface modification technique, ion implantation was first used in the semiconductor industry to introduce controlled dopants into the semiconductor materials [54,55]. Its application to metals and alloys began in the

Fig. 14. Schematic diagram of a DIBAD system.

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mid seventies. Since then it has been used widely to modify surface micro/nano structures of these materials [56]. Persistent efforts have been devoted to improve the integration of metallic implants with the surrounding bone tissue. In finding out factors capable of being actively involved in the osseointegration process, researchers have studied how calcium and/or phosphorous ion implantations modify surface chemistry and bioactivity both in vitro and in vivo [57]. These two types of ions are very important for HA nucleation or formation. Therefore these implantations are discussed in detail. The implanted calcium ions improve the adhesion of the apatite to the titanium substrate. Calcium ions are implanted by dynamic ion mixing to induce a strong bond between the apatite film and the titanium substrate [58]. In this instance the implanted calcium ions serve as a binder and accelerate calcium phosphate precipitation on titanium. Hanawa et al. [59] studied the properties of Ca2+ ion implanted titanium. With an ion beam current density of 50 lA/cm2 and acceleration energy of 18 keV, titanium surfaces were modified to have CaTiO3 of thickness 10 nm. Upon subsequent immersion of this modified surface in biofluid, bone conduction is accelerated. They suggested three mechanisms for which calcium phosphate is precipitated and bone conduction is accelerated and these are: (1) dissolution of calcium from the implanted Ca2+ ions (2) retardation of micro-dissolution of titanium by the surface-modified layer (3) enhanced adsorption of phosphate ions to the modified surface due to presence of Ca2+ ions. In Fig. 15, the effect of Ca2+ ion concentration on the depth distribution is indicated [60]. The surface of calcium-ion-implanted titanium consists of calcium titanate when ions are implanted at the rate of 1016 and 1017 ions/cm2. An additional phase of CaO is seen at 1018 ions/cm2 [61]. The outermost surface in all these three cases is calcium hydroxide. Modified surface layers are very thin; about 6, 8 and 13 nm on specimens implanted at 1016, 1017 and 1018 ions/cm2, respectively. This modified layer is believed to improve the hard-tissue compatibility. In another study treatment of the Ca2+ ion-implanted titanium surfaces in physiological solution produced needle-like HA whereas no such feature was observed on the control non-implanted titanium surface [62]. In vitro culturing of bone cells on the Ca ion implanted Ti samples showed that the cell-material interaction increased. But bioactivity was dependent on the ion implantation dosage [63–65] as high dose of Ca ions (1017 ions/cm2) significantly enhanced cell spreading, formation of focal adhesion plaques, and expression of integrins. Another benefit of this high dose is increase of corrosion resistance under stationary conditions [66]. Fig. 16 shows the SEM of Ca-ion-implanted titanium surface after (a) implantation and (b) 3000 h of exposure to simulated body fluid (SBF) [66]. There is a network of calcium on the implanted surface and this network is widened and spread after long-term exposure. Fig. 17 shows the SEM of the cells cultured on the surface of Ca ion implanted Ti after 4 days exposure to SBF [66]. Cell extension and attachment are clearly visible in this figure indicating cell acceptance on the surface. Figs. 16 and 17 have been obtained under the Ca2+ dose rate of 1017 ions/cm2 with ion energy of 25 keV. The implanted sample was exposed for 3000 h in SBF before the surface was inspected (Fig. 16).

Fig. 15. Schematic illustration of cross-sections of surface-modified layers of titanium specimens with and without calcium-ion implantation [59].

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Fig. 16. SEM of titanium surface (a) Ca ion implanted Ti (b) 3000 h of exposure in SBF. Dose rate was 1  1017 ions/cm2 with ion energy of 25 keV [66].

Fig. 17. SEM of the cells cultured on the surface of Ca ion implanted Ti after 4 days exposure. Dose rate was 1  1017 ions/cm2 with ion energy of 25 keV [66].

Another important element of bone mineral, namely, phosphorus was implanted on titanium surface at a dose of 1017 ions/cm2 [67]. This resulted in amorphous surface layers and produced TiP phase. Similar to the effect of Ca, P ions also increased the corrosion resistance under both short-term and long-term exposures to the test solution. A typical image of P-ion implanted samples exposed to SBF [67] is shown in Fig. 18. After an exposure of nearly 1700 h in SBF calcium phosphates were found on the P-ion implanted surface. These precipitates do not cover the surface completely but have an insular character. Phase identification or structural determination of both the Ca and P implantations is done using XRD. A typical XRD pattern of a Ti–6Al–4V sample implanted with calcium at 195 keV [68] (Fig. 19) shows peaks at 2h positions of 26°, 30°, 47° and 50° in addition to the Ti peaks. The significant feature of XRD of P ion implantation [68], shown in Fig. 20, is the broad distribution in the 2h range between 30° and 50° indicating partial amorphization of the implanted depth region. The metalloid phosphorus is well known for stabilizing amorphous phases. The amorphization due to P is more pronounced than in the case of Ca implantation and may contribute to increase in corrosion resistance. Dual Ca and P ion implantation is a gradual development in this area of HA research and several groups [8,68–70] have studied this aspect. In these methods, both Ca and P ions are incorporated into Ti surface. Further exposure of these surfaces to SBF forms crystalline calcium phosphates. Ca and P maintained at Ca/P ratio of 1.7 are incorporated into titanium surface, each with a total dose of 1017 ions/cm2. Subsequent annealing in air for 40 min at 500 °C followed by treatment in SBF at 37 °C forms HA [71,72]. Advantage of this dual implantation is that, crystalline calcium phosphates are rapidly nucleated on these surfaces during the SBF treatment [62]. Sites for nucleation of HA are produced enormously on the surfaces modified by ion implantation as the implanted surface contains amorphous CaO and P2O5 as the two major components [73] embedded in TiO2 phase. Exposure to SBF helps in creating conditions to alter the surface so as to (i) provide ionic components Ca2+ and HPO2 for HA formation, (ii) generate Ti–OH bonds and (iii) create a dynamic interaction process 4

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Fig. 18. SEM of the Ti surface (a) P-ion implanted (b) after 1700 h exposures in SBF. Dose rate was 1  1017 ions/cm2 with ion energy of 25 keV [67].

Fig. 19. XRD pattern of Ti6Al4V implanted with Ca, (a) without implantation, (b) Ca+ – 50 keV, 3  1017 ions/cm2 (c) Ca+ – 195 keV, 5  1017 ions/cm2, (d) and (c) annealed at 500 °C/1 h in vacuum [68].

between solution and surface. Similar to the earlier instance of Ca or P ion implantation, the dual ion implanted surface also shows partial amorphization [68] or the formation of nanocrystalline structures (Fig. 21). There is diffuse scattering around the position of (1 0 1) reflection in Fig. 21b, which can be interpreted as signal from the nanocrystalline precipitates. Additionally there is an improvement in biocompatibility over the single Ca or P implantation, a fact confirmed by the growth of bone marrow cells on the treated surfaces [57]. The cells cultured on titanium implanted with Ca + P ions expressed excellent spreading.

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Fig. 20. XRD pattern of Ti6Al4V implanted with P+: (a) without implantation, (b) P+ – 30 keV, 3  1017 ions/cm2, (c) P+ – 195 keV, 5  1017 ions/cm2, (d) as (c) annealed at 500 °C/1 h [68].

Fig. 21. XRD pattern of Ti6Al4V implanted with P+ and subsequently with Ca+: (a) without implantation, (b) P+ – 30 keV, 3  1017 ions/cm2 + Ca+ – 50 keV, 3  1017 ions/cm2 [68].

MeV implantation of Ca2+ and P2+ ions on titanium substrate to form HA was reported recently [8]. Calcium hydroxide was formed after heating the calcium implanted titanium for 3 h in air at 80 °C. Upon subsequent annealing for 5 min at 600 °C, HA was formed on the surface. Penetration depth of the HA layer in this method is much higher as compared to the keV ion implantation. The advantage of this technique is the avoidance of the time-consuming SBF treatment. 9. Implantation of some specific ions Besides Ca and P, other ions are also implanted to improve the overall stability, corrosion resistance, wear resistance or implant-tissue reactions. Sometimes these ions are also implanted to study more fundamental processes of ion beam interaction with solids. Ion species implanted onto titanium include C, CO, O, N, Na, Mg, Ag and inert ions (He, Ar).

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9.1. Carbon and carbon monoxide ion implantation Implantation of carbon (C+) and carbon monoxide (CO+) ions onto Ti significantly improves bone integration. This is made possible by covalent bonds on the implanted layer, normally not observed in the standard titanium oxide-bone interface. Carbon ion implantation is effective in reducing the wear associated with the titanium system [74]. Alkaline phosphatase (ALP) assay revealed a higher activity on CO+ ion implanted surfaces, with a remarkable difference in the early stage (3 days) as compared to control. Unlike the control samples CO+ ion implantation showed higher and faster levels of osseointegration [75] indicating the formation of new bone on the ion implanted surface. 9.2. Oxygen ion implantation Oxygen ion implantation enhances the corrosion resistance of the substrate material and this depends on the oxygen dose. Using different levels of oxygen implantation doses to modify Ni–Ti surfaces, Tan et al. showed that topographical changes were induced by oxygen ion implantation [76]. The original grooves on the Ni–Ti surface were filled up and smoothened by low and medium dose oxygen ion implantation. Medium dose oxygen-implanted samples showed the best corrosion resistance. Nanopores acting as pit initiators appeared on the Ni–Ti surface at the high dose and therefore the high oxygen dose did not offer protection against pitting corrosion [76]. 9.3. Nitrogen ion implantation Among the gaseous species, N has been studied most extensively due to its abundance in nature and its ability to improve surface properties. Nitrogen ion implantation has been applied successfully to metals and polymer biomaterials [77,78]. Most artificial joints consist of a metallic component articulating against polymer. The metallic component is mainly made from either titanium and its alloys (Ti–6Al–4V) or cobalt chromium (Co–Cr) alloys, while the polymer component is mainly ultra high molecular weight polyethylene (UHMWPE). The main problem existing in the artificial joints is the prosthetic wear debris which is believed to be the major reason for aseptic loosening (leading to the implant failure) and failure of artificial hip joint due to osteolysis [79–81]. By increasing the wear resistance or stability of the implants, this problem can be overcome. Nitrogen ion implantation is used exclusively for increasing the wear resistance. Increased wear resistance in the nitrided condition could be related to the formation of TiN and Ti2N [82,83]. Ion beam bombardment with nitrogen has been used to improve corrosion [84] and hardness in hip joints by which the wear of Ti–6Al–4V alloys is reduced by a high factor of 400 [85–87]. Oliver et al. studied the effect of nitrogen ion implantation on the wear behavior of a range of metals [88] and showed that a loose correlation exists between reduced wear and increased surface hardness. Great improvements in wear performance occur only when implantation produces a change in the dominant mode of wear. Formation of surface oxide layers during the wear process is often associated with these changes in wear behavior [88,89]. Besides increasing the wear resistance nitrogen implantation also enhances bone conductivity of titanium [90]. Nitrogen ion implantation also increased the corrosion resistance [84,91] especially at low doses [91]. Like the oxygen ion [76], nitrogen ion implantation at high doses and energy, was detrimental with regard to corrosion [91]. Maximum improvement of the corrosion resistance was observed with a dose of 1017 N+ ions/cm2 [92] out of the tested nitrogen dose rates of 1  1016, 1  1017, 6  1017 and 1  1018 N+ ions/cm2. Impregnation of Ti–6Al–4V with nitrogen ion is shown by Buchanan [93] to significantly improve the material’s resistance to wear-accelerated corrosion in both saline and serum solutions. Moreover, thin nitride film-formed titanium exhibiting high wear resistance [94] is commercially used in bone plates, dental implants, and artificial hip joints. 9.4. Sodium ion implantation Sodium ion implantation on titanium substrate with a subsequent heat treatment in air produces a Na2TiO3 layer [95] (Fig. 22). HA formed on this layer showed tremendous bioactivity relative to

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Fig. 22. XRD spectra of Ti after Na ion implantation (implanted and heat-treated) which shows the presence of Na2TiO3. Na ion dose rate ranged from 8  1016 to 4  1017 ions/cm2. Ion energies were in the range of 18–22 keV. Heat treatment was done for 1 h at 600 °C [95].

untreated surface. The thickness of this layer depends on the implantation energy. This treated surface is immersed in SBF to form HA. During the SBF treatment Na is leached out as NaOH. A TiO2  nH2O hydrogel remains at the surface with more number of Ti–OH groups. Ti–OH groups act as nucleation sites for the HA precipitation from the SBF [96]. FT-IR shows that the precipitates occur as carbonated 2 HA with its characteristic PO3 4 , P–OH and P–O bands along with as the CO3 band [95] as shown in Fig. 23. HA formed by the SBF treatment is either amorphous or microcrystalline [95]. On non-implanted Ti, the density of precipitates was much lower indicating that bioactivity of the sodium-implanted titanium surface was better than the unmodified surface [97,98]. Microscopic

Fig. 23. FTIR spectrum of a Ca–P precipitate on Na-implanted Ti. PO4, P–OH, P–O and CO2 3 bands are present in it. Na ion dose rate ranged from 8  1016 to 4  1017 ions/cm2. Ion energies were in the range of 18–22 keV [96].

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images of a Na-implanted and heat-treated surface exposed to SBF for 24 h (Fig. 24c and d) and a corresponding control sample without implantation (Fig. 24a and b) prove this effect. Heterogeneous precipitation occurred preferentially on the ion implanted surface, while little amount of precipitate was scattered on the control surface. With increased fluence of Na ions, more precipitate formed and more area was covered. Taking advantage of the fact that Na implantation helps in forming HA, Na was also implanted on titanium along with Ca and P [99,100] and was shown to enhance the osseointegration. 9.5. Magnesium ion implantation Howlett et al. studied the effect of Mg-ion implantation on the osseointegration of human bonederived cells. In this study, titanium discs were modified by ion beam implantation. Attachment and spreading of cultured cells were significantly enhanced on the implanted titanium compared to the unmodified control. The cells spread better and occupied more volume on the implanted surface [101]. 9.6. Silver ion implantation Fixation of implants with the surrounding bone depends on bone-implant contact, and the higher percentage of bone contact results in better stabilization of implants. When bone healings are associated with infection, the healing process becomes more complicated and the implant failure is expected since the implant becomes loosened due to the formation of bacteria colony on the implant surfaces. Ag incorporation into thee implants can solve these problems because it possesses antiinfective properties and can effectively reduce the deleterious effects of bacteria. Ag incorporated implants have been employed commercially in many approved devices. Metallic silver and silver salts have been used as bactericidal agents in dressings for burn injuries and in metallic silver coatings. The mechanism of the bactericidal action of silver is the action of released ions

Fig. 24. Light microscopy images after 24 h incubation in SBF of (a) and (b) titanium without implantation; (c) and (d) Na ionimplanted and heat-treated, titanium surface. Na ion dose rate ranged from 8  1016 to 4  1017 ions/cm2. Ion energies were in the range of 18–22 keV. Heat treatment was done for 1 h at 600 °C [96].

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themselves. These positively charged metallic ions attach to the negatively charged bacterial cell wall and cause cell lysis and death. It can be said that, the amount of silver ions released from the surfaces of the as-implanted samples determines their anti-bacterial activity. Ag ion implantation provides an effective yet simple approach to minimize bacterial adhesion [102]. Silver ion was implanted into titanium and Ti–Al–Nb alloy to improve their anti-bacterial characteristics and wear performance [103]. Continuous release of required quantity of silver ions is needed and may not be achieved in these applications. Use of Ag nanoparticles can solve this problem as they have the advantage of a high specific surface area and a high fraction of surface atoms. Silver nanoparticles with the diameter of less than 10 nm created gradient of nanostructures [104,105] and these structures augment the anti-infective qualities of Ag. 9.7. Argon ion implantation Argon ion implantation improved the anti-bacterial performance of polymers used in biomedical industry [106,107]. For example, Ar+ ion implantation is used to reduce tissue inflammatory response of polyurethanes [108]. But, such a positive trend was not observed on the Ar+ implanted Ti [63]. Examination of morphology and adhesion of cells grown on the implanted titanium showed inferior quality of the attachment, as a greater proportion of cells appeared to remain rounded and poorly attached on the surface [63]. Hence Ar+ ion implantation is not a useful method for improving the surface properties of titanium. The next sections discuss the characteristics and performance of HA coatings. 10. Properties of ion-implanted HA coating Properties of HA coatings obtained by ion implantation methods, such as adhesion strength, mechanical properties, roughness, surface energy, corrosion, wear, in vitro and in vivo activities, are discussed next. 10.1. Stress shielding An important factor that can contribute to the failure of the implants is mismatch of the modulus of elasticity between bone and titanium in load bearing implants. The elastic modulus of bone and titanium are 10–30 GPa and 110 GPa, respectively. This difference causes large stress concentrations in the bone surrounding the implants [109]. In load bearing implant cases, in particular, biomechanical mismatch between the implant and the surrounding bone often leads to inhomogenous stress transfer. Called as ‘stress-shielding’ this leads to bone resorption, loosening of the implants and retarded bone healing. This is often corrected by a subsequent revision surgery [110]. In a way of minimising the ‘stress-shielding’, researchers adopt methods to decrease the strength and Young’s modulus of titanium by shaping it into an interconnected porous structure called as ‘titanium scaffolds’. Advantages of porous materials are twofold. They can provide biological anchorage for the surrounding bony tissue via ingrowth of mineralized tissue into the pore space [111]. And, their elastic moduli can be adjusted to match that of trabecular bone so as to limit stress shielding and prevent bone resorption at the implant interface. It is suggested that ion implantation of the porous titanium surface will give new dimension to solving the problem of mismatch of mechanical properties [111]. Besides, ion implantation can provide graded composition and such a gradation can be tailored to reduce the stress concentrations. 10.2. Hardness Ion-beam treatment can provide a hardened surface that can improve wear behavior in addition to increasing the resistance to dust erosion and salt corrosion, cyclic endurance, and material heat resistance. Ion implantation and ion mixing can produce surface-modified layer having a graded composition. The interface between the surface layer and substrate cannot be established clearly. This results

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in the formation of the fracture resistant interface. However tailoring the composition of this layer is difficult [84,112]. Hardening is attributed mainly to the formation of hard-phases of nitride, carbide and oxide precipitates. Ion implantation hardens the surface of materials by generating dislocations or introducing residual stresses at the vicinity of surface. An example is given to illustrate this. Ti–6Al–4V samples were implanted with carbon ions with energies of 120 keV and 60 keV and doses of 3  1017 ions/cm2 and 4  1017 ion/cm2, respectively. The implanted region exhibited a complicated graded microstructure, high density of TiC precipitates and dislocation networks. The precipitate density changed gradually with depth in accordance with the carbon content. In the depth region with the highest carbon content an almost continuous TiC film had formed. The maximum hardness occurs at that depth where the TiC content is maximal. In zones with a smaller TiC precipitate density, the hardness can be quantitatively explained as due to TiC dispersion hardening. The possibility to truly trace the hardness-depth profile of an ion implanted region and to relate this hardness-depth profile with the local variations in microstructure and composition augurs for the development of property-tailored ion implantation procedures [113]. Just to highlight the levels of hardness attainable, a threefold increase in microhardness at loads of 1–2 g is reported in titanium alloys [114–116]. 10.3. Wear resistance Ion-implanted titanium alloys showed very high resistance to wear in pin-on-disc experiments [9,88] and hip and knee joint simulation studies [9]. Oxygen and nitrogen implantations are used to render this benefit to titanium surface. Higher surface hardness alters the nature of wear from mild oxidative wear to severe adhesive wear. The native passive oxide layer formed on the surface of titanium or its alloys provides a useful low-friction surface; when this oxide is removed, rapid adhesive wear occurs against many counterface materials. To combat this oxide removal, Ti surface can be hardened. The increased hardness allows the material to resist plastic deformation and support the oxide at higher stresses. To illustrate the benefit of ion implantation surface nano structures provided by implantation significantly improve wear resistance of titanium nitride surface [117]. Jinyou et al. carried out the wear test for 3.5  106 cycles, and the weight losses of nitrogenimplanted CoCrMo, Ti6Al4V and 316L were reduced by 114%, 38% and 71% respectively. The wear depth of nitrogen implanted Ti6Al4V is similar to that of CoCrMo. But the wear resistance of Ti6Al4V was more satisfactory. This may be caused by abrasive wear. Ion implantation has many advantages compared with other surface treatments. One of the advantages is low temperature operation and therefore the absence of distortion or surface degradation of the parts. Another is no adhesion problems since there is no sharp interface. But ion implantation has a limitation of shallow penetration of the ions in the range of commercially used machines. But nitrogen ion implantation improves wear resistance of Ti6Al4V and it was suggested that the penetration of nitrogen ion was deep enough for prosthetic application [118]. Another report also demonstrated a definite potential for the use of N+ ion implantation total joint replacement applications [119]. N+ ion implantation was performed using 100 keV, 10 mA ion implantation system on Ti6Al4V substrates with ion doses of 2  1017 N+ ions/cm2 at an accelerating voltage of 90 keV. Knoop microhardness indentations were measured for the Ti6Al4V alloy after N+ ion implantation, at a load of 1 gf which shows increase in hardness and it was attributed to the formation of TiN precipitates. It was also found that N+ ion implantation improved the wear resistance of Ti6Al4V. Tests with all implanted couples showed the most improved tribological performance [119]. Nitrogen ion implantation forms a very thin TiN layer with a thickness no more than several hundred nanometers. Wear resistance of implanted titanium alloy would be weakened after the destruction of the thin layer. Nitrogen ion implantation could slightly harden the surface of titanium alloy and improve the wettability and wear resistance, but could not provide a long-term protection of titanium alloy due to the extraordinary low thickness of the TiN layer [120]. This is the disadvantage of the ion implantation process. In spite of this nitrogen ion implantation is popular as it improves the tribological performance of the titanium surface. b-Ti and shape-memory Ni–Ti wires are used in orthodontic treatments because of their desirable properties. Under the conditions operative in the oral environment these materials encounter high

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frictional forces which may even reduce their utility [9]. Ion implantation of these materials reduced their frictional characteristics against stainless steel, the predominant material used for brackets in orthodontic practice. Oxygen-implantation yielded better results [76] and further improvement in wear resistance can be obtained by post-implantation heat treatment. 10.4. Creep Ion plating provides surface nanostructures that can cause improvement in oxidation resistance [117]. Coating wise, platinum ion plating of high temperature titanium alloys can increase their creep and oxidation resistance. It is also found to improve the high cycle fatigue strength at room and elevated temperature. Ion plating of Pt is selected because it forms a sound and effective coating without risk of hydrogen contamination [121]. 10.5. Fatigue resistance Modification of surface chemistry and structure resulting from ion implantation can improve the substrate’s resistance to fatigue crack initiation. The damage induced by the ion beam is sufficient to develop dislocation tangles and residual stresses that contribute to the surface hardening. Also, since the stress amplitudes in high cycle fatigue are usually much lower than the yield stress, the surface residual compressive stresses created in an implanted layer increases the fatigue life. Carbon and nitrogen ion implantations are used for this purpose. The damage caused on the titanium surface due to the implantation is only beneficial in this case as it improves the surface hardness. However when the damage goes to the extent of affecting the base material severely and becomes irreparable then this technique becomes unsuitable for biomaterial applications. Carbon ion implantation was adopted to treat the surface of machined Ti–6Al–2Sn–2Zr–2Cr–2Mo specimens; stress-controlled fatigue tests were performed at room temperature and 400 °C. Ion implantation did not cause much change in the high cycle fatigue at room temperature but increased the high cycle fatigue strength at 400 °C [122]. Dual ion implantation is also used for improving the fatigue properties. Ti–6Al–4V has been implanted with carbon and nitrogen ions. Rotating beam fatigue tests showed improved fatigue life for both implants, with the superior carbon implantation giving a 20% increase in endurance limit and a factor of 4–5 lifetime increase at higher stresses over unimplanted material. A dose of 1  1017 ions/cm2 was required to obtain the maximum effect. Fatigue cracks have been observed to originate up to 150 lm below the surface, indicating a complex interaction between the implanted layer and the fatigue failure process [123]. To combat high temperature (or service) fatigue Pt ion plating of the high temperature titanium alloys is also used [121]. 10.6. Fretting fatigue resistance Bioimplants, such as hip joints and bone plates, are prone to fretting fatigue inside the body [124]. In fretting fatigue there is contact between two surfaces, where small, oscillatory sliding displacements occur between the surfaces while one or both of the contacting surfaces could be subjected to fluctuating stresses. Considerable oxidation can occur in the presence of aggressive environments to cause wear and material removal at the fretted surface [125,126]. The problem of fretting is it increases tensile and shear stresses at the contact surface and generates flaws, which lead to premature crack nucleation [127]. Hard nitride coatings are applied on the titanium substrates using ion implantation to reduce the chances of fretting. CrN and TiN hard coatings were applied on Ti6Al4V by ion beam enhanced deposition (IBED) and shot peening was combined with the hard coatings to study the duplex effect on FFR. However, an optimum composition of CrN showed better fretting fatigue resistance than TiN with the same processing parameters. Shot peening combined with IBED CrN hard coatings can improve greatly the fretting fatigue life of Ti6Al4V. The FFR of Ti6Al4V treated with CrN coating combined shot peening is much better than the shot peened one. TiN coating could not improve FFR as much as CrN coating. This

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is due to CrN coating with better bonding strength and toughness than that of TiN using the same processing method [128]. Both hard CrN coatings with good toughness and soft CuNiIn coatings of low friction were applied on Ti6Al4V, and shot peening combined with coatings has also been studied in order to improve the FFR. As the contact stress concentration of fretting fatigue is high, coatings combined with shot peening achieved high levels. The fretting fatigue lifetime is largely dependent on the sliding contact conditions such as contact geometry, sliding distance and contacting materials. If cracks appear at an early test stage due to partial slip contact conditions, shot peening of coated and uncoated Ti6Al4V may improve the fretting fatigue resistance (FFR). Both shot-peened hard CrN and soft CuNiIn coatings were considered. When the contact stress is not severe and gross slip contact conditions were operative, both CrN and CuNiIn showed a better fretting fatigue resistance than that of shot-peened Ti6Al4V. So, it was found that hard CrN coatings are much more effective than soft coatings like CuNiIn. The addition of shot peening on these coatings may result in an additional improvement in fretting fatigue resistance as far as peeling off or cracking is not induced [129]. Xiaohua et al. studied extensively the composition distribution, bonding strength, hardness, and wear resistance of a OCr18Ni9 film deposited on a Ti–8Al–1Mo–1V (Ti811) titanium alloy surface by IBED. A OCr18Ni9 film was deposited on a Ti811 titanium alloy surface with high density, small grain size, low void radio, and high bonding strength. As a result, the hardness and wear resistance of the Ti811 alloy were increased to a remarkable extent. The FFR of the Ti811 titanium alloy was effectively improved at 350 °C because of the moderate hardness and good ductility of the OCr18Ni9 film. The FFR of the Ti811 alloy was improved by a factor of 6 as compared to the uncoated substrate at 350 °C, due to the synergistic role of the IBED film [130]. The physicochemical state of the surface layers of specimens of alloy VT18U was subjected to ionbeam (B+, N+, C+, Pd+, La+) and heat treatment and it was shown that the thickness of the ion alloyed layer does not exceed 1 lm. Tests were carried out for specimens in the original and irradiated conditions for cyclic endurance at 500 °C whose results indicate that it is possible to increase their life by several factors by means of ion-beam treatment. The physical and chemical hardening of the surface regions with ion implantation provided high resistance to fatigue failure for the material which causes a change in the mechanism for crack generation from the surface into the subsurface. The fact that the failure site is at greater depths compared with the projected travel is probably due to the high sensitivity of titanium alloys to stress concentrators. This was indicated by the quite random distribution of failure sites through the thickness of the surface layer in different ion alloyed specimens and the possibility of forming several sites in the surface region for a specific specimen [131]. IBAD coated platinum on the high temperature titanium alloys was originally developed to increase their creep and oxidation resistance [121]. This coating also improved the fretting fatigue life over the shot peened base material by 25%. . Although the wear resistance of titanium and its alloy is relatively low, ion implantation can harden the surface and reduce the friction coefficient, ultimately improving the wear and fatigue resistance. 10.7. Fracture toughness Ion implantation improves fracture toughness and fatigue strength. Lee et al. [132] modified titanium alloy with different techniques and studied hydrogen charging. Effect of hydrogen embrittlement on the fracture toughness of a titanium alloy under these surface modifications was investigated. Disk-shaped compact-tension specimens were first coated with different hard films and then hydrogen charged by an electrochemical method. Nitrogen ion implantation processing was carried out in a vacuum chamber at 555 °C to produce a TiNx layer of thickness 3.2 lm. Cathodic charging of hydrogen was carried out using 0.1 N HNO3 solution. The solution contained 1 g/l thiourea as a hydrogen recombination poison. Cathodic charging was done for 48, 96, or 144 h at 30 °C. Current density was maintained at a constant value of 10 mA/cm2. Fracture toughness testing was conducted immediately after the cathodic charging [132]. Surface studies revealed that fracture toughness of the as-received titanium alloy decreased with the increase of hydrogen charging time. Fracture toughness of the ion-implanted alloy (containing

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TiNx layer) decreased as well, but to a lesser extent after cathodic charging. The best result obtained was for the alloy coated with a CrN film (deposited by arc evaporation PVD) where fracture toughness was sustained even after hydrogen charging for 144 h. Obviously, the CrN film acted as a better barrier to retard hydrogen permeation, but it was at the sacrifice of the CrN film itself [132]. But the ion implanted TiNx remained intact although the fracture toughness reduced because of hydrogen charging thus indicating the efficacy of the ion implantation technique. 10.8. Adhesion of the coatings Bond strength of the substrate-HA coating on the implant material is very critical as it has to withstand crevice, bone growth stress and weight loss when abraded against bone. The strength of human bone is approximately 18 MPa [133]. Therefore the objective of an implant material (containing natural or modified surface) is that, it should have higher bond strength than natural human bone. Ion-beam coatings of HA on Ti have adhesion strengths higher than 30 MPa [134]. Due to the atomic mixing there is no definite boundary between the implanted layer and substrate. And hence adhesion strengths are high. Ion beam sputtering deposited (IBSD) and plasma-sprayed HA coatings have similar adhesion properties. IBAD coatings have higher adhesion strengths than those obtained by IBSD or plasma-spraying [135]. There is a wide atomic intermixed zone at the coating/substrate interface [136] in IBAD and it is this intermixed zone that contributes significantly to the improvement of the adhesive strength. The poor adhesion strengths of the plasma-coatings are thus, effectively avoided in IBAD [137]. 10.9. Corrosion properties Corrosion resistance is an important property of the material that determines its usefulness in implant applications. Krupa et al. [66] used polarization and electrochemical impedance spectroscopy (EIS) methods to show that, implantation improves the corrosion resistance of Ca ion implanted Ti. Calcium-ion implantation results in amorphization of the surface layer and increases the corrosion resistance of titanium under stationary conditions. Polarization tests indicate that, this implantation is advantageous only for a short exposure time of 13 h and not for long hours. As the exposure time increases, anodic current density increases to reach the values obtained for non-implanted titanium. This can be attributed to the alteration of the chemical composition of the oxide layer during the exposure. The migration of calcium ions from the implanted layer to the solution is facilitated during long hours of exposure, resulting in the thinning of the oxide layer. After sufficiently long time, the layer looses its real character and acquires electrochemical properties close to that of non-implanted titanium. At higher anodic potentials the implanted titanium undergoes pitting corrosion [66]. Impedance examination of the calcium implanted titanium indicates that the layer formed on the titanium surface is compact. However, with non-implanted titanium, an increase of the exposure time slightly increases the capacitance of the barrier layer and changes the structure of the layer [66]. Phosphorus ion implantation also enhanced the corrosion resistance of titanium [67]. Phosphorusion with a dose of 1017 ions/cm2 was implanted onto titanium. Surface layer formed after implantation was amorphous. From the distribution profiles of titanium and oxygen, it was inferred that a thin titanium oxide film is present on the sample surface. Phosphorus-ion implantation increases the corrosion resistance after short-term as well as longterm exposures. Krupa et al. [67] illustrated the benefits of P ion implantation. After an exposure of nearly 1700 h in physiological solution, calcium phosphate precipitates are seen on both implanted and non-implanted titanium. These precipitates form an insular layer. For the non-implanted titanium, anodic current density increased in the potential range of 1000–2500 mV. Above a potential of 2500 mV, the current density increased, its value being higher in longer exposed samples [67]. In the phosphorus-ion implanted samples, the anodic current densities are lower within the potential range of 1000 to 5000 mV. Unlike the non-implanted titanium, the current density stabilizes in this whole potential range. In the potential range of 1800–5000 mV, the current density is about 1 lA/cm2 and is lower by two orders of magnitude than that measured in the non-implanted titanium [67].

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Long-term exposure of the P-ion-implanted titanium in SBF has no significant effect on the shape of the impedance spectra, indicating that the implanted layer is stable and does not undergo any change. An increase of exposure time slightly increased the capacitance of the barrier layer in the nonimplanted titanium, because of the change in the structure of the surface layer [67]. Amorphous layers do not contain defects such as grain boundaries or contaminant segregation and this fact can be considered to be the reason for the increase in corrosion resistance. Increase of the corrosion resistance due to the phosphorus-ion implantation can also be attributed to the formation of TiP which itself has good corrosion resistance. Impedance measurements have specifically shown that long-term exposures do not affect essentially the properties of the surface layer [67] and this may be due to the presence of amorphous layers formed by P ion implantation. Aside from the individual Ca or P implantation, Ti surface was modified to contain both Ca and P ions [69]. Under both short-term and long-term exposure conditions polarization resistance and corrosion potential of implanted Ti are higher than those of non-implanted Ti, to suggest the benefits of this treatment. Potentiodynamic tests on oxide produced on titanium by IBAD method indicate that, the oxide layers enriched with Ca and P increase the corrosion resistance [73]. This increase in corrosion resistance is attributed to the presence of an intermediate layer, formed due to the implantation. A point defect occurs when the size of foreign cation (calcium in the present case) being incorporated in the crystal lattice does not match that of the host ion. According to the size factor, the presence of elements whose ionic radii are smaller than that of the host element hinders corrosion, whereas, the presence of elements whose ionic radii are larger than that of the host element increases corrosion. Calcium ions have larger ionic radii than the titanium ions and because of this, calcium cation vacancies are created. The higher the concentration of calcium in the coating the greater the number of calcium ion vacancies. There is a chance that these are not completely annihilated and so localized breakdown of the oxide may occur. This attack is further aggravated by the chloride ions of the SBF [138]. These concepts may explain the pitting of the coating containing Ca ions in the SBF medium. As already indicated nitrogen ion implantation also improves the corrosion resistance of titanium. Titanium implanted with nitrogen at a dosage of 3  1017 ions/cm2, improved corrosion resistance in Hank’s solution [139]. Corrosion improvement arrives from the formation of homogeneous distribution of small precipitates of TiN and Ti2N. Synergistic corrosion-wear is a highly degrading condition. Aside from the oxygen implantation [76], nitrogen implantation is also used to combat the corrosion-wear. Nitrogen ion implantation is reported to improve the corrosion-wear of Ti–6Al–4V alloy in saline and serum solutions [84]. The implantation did not give any difference in the static corrosion rates; but in the wear-corrosion regime, the nitrogen ion implantation offered protection to the alloy. 11. Biological activities of ion-implanted HA coating 11.1. Bioactivity Osseointegration of an implant containing HA coating is suggested to be the direct result of the reaction of HA coatings. These reactions include: (1) dissolution of HA, (2) precipitation of apatite, (3) ion exchange accompanied by absorption and incorporation of biological molecules, (4) cell attachment, proliferation and differentiation and (5) extra-cellular matrix formation and mineralization. In this cascade of events, dissolution of HA coating is a key step. This dissolution is an important requirement to induce the precipitation of bone-like apatite on the implant surface. Stoichiometric HA dissolves slowly and could show only a limited bioactivity in vivo. Besides the HA dissolution, various characteristics of these coatings (like surface roughness, porosity, chemical contents, etc.) influence the cell activity greatly. A rough surface provides initial stabilization until bone can grow and attach to the implant surface [140]. Rough coatings facilitate the adhesion of more number of osteoblasts [141]. Rough, textured and porous surfaces could stimulate the production of extra-cellular matrix [142]. Surfaces possessing high surface energy encourage cell activity, as cells attach and spread more easily on these surfaces [143]. Higher surface energy and more surface hydroxide groups (especially the basic type) attract the osteoblasts [141]. The OH

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groups on rough and higher surface energy titanium surfaces dissociate to give positive charges [141]. Proteins adsorb to positively charged surfaces easily and stimulate osteoblast attachment and spreading [144]. By this way, adhesion of osteoblasts is enhanced on the surfaces containing more hydroxide groups. Thus implantation of cations (Ca or Na) can provide the necessary positive charges for the osteoblast attachment. Fig. 25 shows the growth of bone cells [70] on pure titanium (a) and apatite surface layers produced on titanium implanted with Ca and P (b). Improved cell behavior is indicated on the implanted surface [70]. 11.2. Testing the in vivo bioactivity using SBF Bioactivity of the implanted surface is estimated by exposing it to SBF. The calcium-ion-implanted titanium surface is more positively charged than the titanium because of dissociation of hydroxyl radicals [145]. More phosphate ions of the body fluid are adsorbed to this surface because of electric charge attraction, with the consequent formation of a larger amount of calcium phosphate. Calcium ions are gradually released from the surface of implanted titanium [146,147] and this causes supersaturation of calcium ions in the body fluid near the surface, to eventually result in increased and accelerated precipitation of calcium phosphate [60]. Post-implantation treatments (heat- or hydrothermal-treatments) could augment the bioactivity. Calcium and phosphorus ions were implanted on a titanium surface and then this surface was heated at 500 °C in an oxygen atmosphere. The surface layer was enriched with CaO and P2O5. Exposed to a SBF solution, this surface showed a higher ability to nucleate HA than the control non-implanted titanium [100]. The two-step implantation of calcium and phosphorus ions onto titanium has also been combined with various post-implantation treatments with the aim to produce continuous HA layers. Pham et al. [100] subjected implanted titanium to hydrothermal heating, whereas Baumann et al. [70] heated the samples at 600 °C. The HA layers obtained by these investigators showed good adhesion to the substrate.

Fig. 25. Reflection electron micrograph of marrow bone cells on (a) Ti surface, (b) HA surface layer produced on dual ‘Calcium (fluence of 5  1017 ions/cm2 at 30 keV) and Phosphorus (fluence of 3  1017 ions/cm2 at 23 keV)’ ion-implanted titanium [70].

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Enhanced bond strength of a HA coating layer with a metal substrate and reduction in dissolution rate of such layer in a physiological saline solution, were achieved by applying an Ar ion beam during deposition [97]. The implanted layer was amorphous. Amorphous coatings usually exhibit higher dissolution in SBF. But composition and structure of the coating layer became similar to that of crystalline HA as a result of Ar ion beam assistance [51]. Induction of crystallinity and reduction in apatite dissolution rate are the benefits provided by the ion-beam. Tests are also done by exposing the titanium surface to modified SBF. Influence of calcium ion deposition on the apatite-inducing ability of porous titanium was investigated in a modified SBF (m-SBF). Porous Ti samples containing high content of calcium ions exhibited the best apatiteinducing ability, and produced a dense, carbonated apatite [148]. Calcium-ion implantation has proved to improve the bioactivity on flat as well as porous titanium. 11.3. In vitro cell activity of ion-implanted titanium In vitro cell culture studies have demonstrated improved bone cell adhesion, spreading, and growth on Ca ion implanted Ti surfaces compared with non-implanted Ti [63–65]. The implanted surface enhanced the expression of certain bone-associated components in vitro and increased the number of mitotic cells [149]. Ca ion implantation markedly enhanced the proportion of cells compared with those grown on the non-implanted Ti (control) surface. Enhanced hydrophilicity or increased surface energy by ion implantation are the reasons for the improved in vitro cell progression [150]. Calcium dose rates and culture time also influence the cell attachment significantly. Polished titanium discs were implanted with high, medium and low doses of calcium ions at 40 keV. MG-63 cells readily adhered to calcium-low and calcium-medium titanium surfaces; the initial binding of cells to calcium-high titanium surface was significantly reduced despite the high level of spreading on this particular surface [65]. However, this inhibition of cell adhesion on the calcium-high titanium surface was completely changed after 24 h, by which time cell attachment was significantly enhanced. a5ß1 integrin mediates cell adhesion to substrates via binding to fibronectin [151]. This process also generates intracellular signals that lead to the progression of cells through the cell cycle and increased cell proliferation. Since integrin function is critically dependent on the concentration of divalent cations [152] it is possible that the presence of more number of calcium cations might trigger enhanced ligand binding of the integrin receptors and stimulate integrin-mediated activation [65]. To put these ideas in a different perspective, positively charged surfaces are favorable for the adhesion of fibronectin. Calcium ions form the sites of positive charge and are helpful for the adhesion of fibronectin which promotes the attachment of osteoblasts [153]. This explains the enhanced cell attachment on the calcium-high titanium surface, especially after 24 h of culture. This point substantiates the discussion in Section 11.1. Low activity in the initial stages and high activity at 24 h of culture, on the calcium-high surface is explained by many factors [65]. Firstly, during ion implantation, an amorphous surface oxide layer differing chemically from the native titanium oxide is formed. Such amorphous layers could differently interact with biomolecules in solution. Oxide thickening might have occurred more on the calciumhigh titanium surface. Additionally, there could be time-dependent dissolution and leaching of ions from calcium implanted titanium on immersion in SBF [62,146,147]. These factors might affect the cell activity individually or in tandem with each other, especially on the calcium-high titanium, to play important roles after 24 h in culture medium compared with the initial periods [65]. 11.4. In vivo results of ion-implanted titanium As ion implantation technique improves biocompatibility and enhances in vitro cell activity, it is to be expected that in vivo results also show similar trend. Calcium ions were implanted onto titanium with 1017 ions/cm2 dose and this treated titanium was surgically implanted into rat tibia for 2, 8, and 18 days. Tetracycline and calcein labels indicated that Ca2+-implanted titanium is superior to bare titanium surface. A larger amount of new bone was formed on the Ca2+-treated surface, as early as 2 days after surgery. Part of the bone made contact with the Ca2+-treated surface. But, bone formation on the untreated sample was delayed [59].

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An improved in vivo response to Ca ion implantation has been attributed to the following factors: thickening of the oxide layer, formation of the Ca–P rich apatite layer, and/or release of the implanted ion from the modified Ca–Ti surface [59,146,147,149]. All these factors play major role in promoting bone cell adhesion, extra-cellular matrix communication and augmentation of tissue mineralization, leading to improved bone remodeling on the Ca-implanted titanium. Nitrogen implantation is mostly used to improve wear resistance of titanium surfaces. Additionally this ion improves in vivo response. Nitrogen-implanted titanium and Ti–6Al–4V screws were implanted in the tibia of rabbits. After 3 months of insertion bone filled a large portion of the area within the screws [154]. Around both control and nitrogen-implanted surfaces fibroblasts increased [90]. Macrophages close to the surface of control showed very low activity whereas macrophages close to the ion-implanted titanium had an active appearance as indicated by the presence of large golgi bodies in the cytoplasm. CO is reported to promote the differentiation and apposition of osteoblasts [155] that eventually leads to more complete osseointegration [156,157]. CO implanted titanium samples were placed in the jawbones of the dogs for 3–6 months. The CO ion implanted surface contains a variety of metastable C-rich compounds. Once implanted, these readily react with the surrounding biomolecules and promote the differentiation and apposition of osteoblasts to promote complete osseointegration. On the control, bone was absent after 3 months and appeared only after around 6 months. But lowdensity bone was present on the CO implanted surface just after 3 months indicating that modification of the titanium surface chemistry promoted early osteogenesis [158]. Working on Mg-ion implantation and testing the in vivo behavior Sul et al. proposed the mechanisms for osseointegration of titanium. Mechanical interlocking through bone growth in pores and biochemical bonding are suggested. To ascertain these they inserted Mg-ion incorporated and TiO2 stoichiometric implants into rabbit bone, respectively and used the removal torque values after 6 weeks of implantation as measure of osseointegration. Values for removal torque of the Mg-incorporated implants were significantly higher than those of the TiO2/Ti implants. On the Mg-implanted surface ion concentration gradient was detected between the implants and bone [159] indicating movement of ions along this path. In general it can be stated that the ions implanted onto titanium surface provide very good in vitro cell activity and in vivo bone-bonding. 11.5. Anti-bacterial action The problem of bacterial infection can be overcome by antibiotics, but antibiotics cannot always fight the proliferation of bacteria on biomaterial surfaces. One remedy to this is direct treatment of the surface with a long-lasting, safe and effective antimicrobial agent such as silver. Besides silver, other elemental implantations like copper [103,160], tin, platinum and zinc [161–163] have also been tried but they give some other problems. For instance, copper ion implantation is reported to lower the corrosion resistance of titanium [103,160] as well as zinc. For these reasons, silver is a very widely used element as far as anti-bacterial action is concerned. The anti-bacterial effect of various titanium surface modifications using Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans were studied by researchers. Many surface modifications were tried which includes (i) ion implantation (Ca+, N+, F+), (ii) oxidation (anodic oxidation, titania spraying), (iii) ion plating (TiN, alumina) and iv) Ag, Sn, Zn, Pt ion-beam mixing. The fluoride ion implanted specimens, when compared to the control untreated Ti, significantly inhibited the growth of P. gingivalis and A. actinomycetemcomitans. No release of fluoride ions from these samples under the dissolution tests were observed. Fluorine-ion implantation is helpful in curbing the activity of oral bacteria on Ti implants just like silver ion implantation [161].

12. Limitations Ca and P ions are used for improving the titanium surface to enhance osseointegration and Ag is used for anti-bacterial effect. Other ions like Na, F, Mg, etc. are also used for enhancement of other

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properties. There are many benefits in using ion implantation methods for implant applications and these are outlined in Section 1. Enhanced osseointegration is certainly an advantage. One big issue affecting the widespread use of ion beam implantation is, it is a capital-intensive technology. As a result, ion beam implantation is mostly used for high-value products and not for regular production line. Ion beam systems are characterized by having a preferred direction. Conventional ion beam implantation requires a normal incidence of ions to the substrate. As a consequence, ionbeam treatment is well-suited for planar substrates like wafers, but impractical for treatment of three-dimensional objects. Implantation of three-dimensional objects can be done but with lots of pre-implantation processing (application of masking and movable fixturing). This makes the method more complicated and expensive. Between the initial step of ion generation and final step of ion implantation, three intermediate costly steps, namely, beam extraction, beam focusing and beam scanning, are required. Requirement of high vacuum is another impediment. Moreover, during ion implantation, channeling may occur and it may distort the depth profile. Annealing is most often required because of the radiation damage due to high energy ion implantation. Some of these problems can be solved by using plasma-based technology where the equipment cost is low and masking/fixturing is not required [164]. SRIM/TRIM simulation gives excellent insight into the implantation process and the damage that accompanies ion implantation. However it ignores several features which are of importance in the real implantation process. This program assumes a statistically homogenous distribution of the host atoms based on the average density of the target materials. Effects occurring due to the periodicity of atoms in a single crystal (channeling) are neglected by the program. SRIM/TRIM also does not allow for any recombinative annihilation of defects so as to always overestimate the absolute magnitude of the damage, thereby giving erroneous results. The program also does not take into account, the rearrangement of atoms in the implantation-affected volume to form new phases. In spite of all these disadvantages, SRIM/TRIM computer simulation program is still popular as it gives excellent estimate of both the implant and the damage profiles and offers very valuable information about the target surface after ion implantation [165]. 13. Concluding remarks This review describes the ion implantation methods used to improve the mechanical, chemical and biological properties of biomedical titanium alloys. As there is no clear and identified interface between the implanted layer and substrate it gives excellent adhesion of the coating. Although it is a violent process substrate warpage is absent. Although the process is expensive and useful for high-value products, it is extremely controllable and can be tailored to implant different ions to form ultra-high purity coatings layers. Calcium and phosphorus implantation are useful in improvement of biocompatibility of titanium. While silver ion implantation is used for anti-bacterial applications, nitrogen ion implantation combats wear of the titanium surface. Future investigations should include extensive animal experiments and clinical trials, to continue with the understanding of bone responses to coated-implant surfaces. As other ions like strontium and silicon are also reported to improve the osteoconductive properties, these implantations can be experimented in the future. Acknowledgement The authors thank Brain Korea 21 (BK 21) project for supporting the work. References [1] Den Braber ET, de Ruijter JE, Smits HT, Ginsel LA, von Recum AF, Jansen JA. Effect of parallel surface microgrooves and surface energy on cell growth. J Biomed Mater Res 1995;29:511–8. [2] Singhvi R, Stepanopoulos G, Wang DIC. Effects of substratum morphology on cell physiology – review. Biotech Bioeng 1994;43:764–71. [3] Williams DF. Titanium and titanium alloys. In: Williams DF, editor. Biocompatibility of clinical implant materials. Boca Raton: CRC Press; 1981.

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