**4. Ion implantation techniques**

Subsequently, diamond-like carbon (DLC) coatings were introduced, following the strategy of favoring sliding with very low coefficients of friction (0.1), maintaining relatively high hardness (H > 15 GPa). Although these specific coatings showed high efficacy in laboratory tests, the clinical tests performed to date for hip and knee prostheses showed undesirable results, being the decohesion of the layers and the subsequent localized corrosion the main

On the other hand, TiN and DLC coatings showed very good biocompatibility so they are being marketed in other types of prostheses such as dental implants and heart valves. For these specific purposes, the absence of high loads which can delaminate the coating as it happened in the case of hip and knee prosthesis enables its introduction into the market. In **Figure 6**, two representative examples of Ti dental implants obtained which are coated by TiN

However, one of the main problems concerned to this type of coatings is the adhesion and corrosion behavior, being one of the hot topics in the community scientific their implementation and introduction of them in a massive way in the market of hip and knee prostheses. One of the most promising approaches is the use of the aforementioned HIPIMS. This PVD technology allows obtaining layers of much greater adhesion and density, giving excellent results in terms of corrosion and wear resistance. A representative example is shown in **Figures 7** and **8**, respectively. In **Figure 7** is shown a cross section of a sample coated with TaN DC magnetron sputtering (left), where a columnar growth with micron-sized grains can be appreciated. On the other hand, the photograph on the right shows a layer of TaN coating by HIPIMS where a greater density and compactness can be observed. The corrosion resistance in terms of polarization resistance as a function of time (4, 24, and 168 h, respectively) is shown in **Figure 8**.

Another research line which is showing promoting results is ceramic coatings doped with bactericidal elements (mostly Ag or Cu) in the form of nanoparticles embedded in the TiN or CrN. The controlled release of Cu or Ag ions provides a bactericidal effect which makes possible the prevention of infections due to bacterial proliferation on the surfaces of the prostheses. Doped metal nitrides and carbonitrides deposited by pulsed magnetron sputtering are widely tested [60, 61]. In addition, Cu and Ag concentrations between 5 and 25% have been shown to be highly efficient, avoiding the proliferation of different contagious types of bacteria such as *S. aureus*, *P. aeruginosa*, and *S. epidermis*, increasing bactericidal efficacy with increased concentration of Ag and Cu, respectively. Finally, another extended solution is the coatings of diamond-like carbon(DLC) doped with Ag because the experimental results of depositing these layers by means of magnetron sputtering point to the fact that silver segregates in the form of nanoparticles (order of 3 nm), and in high concentrations appear nanofibers of Ag on

**Figure 6.** TiN magnetron sputtering coating (left) and diamond-like carbon (DLC) magnetron sputtering coating (right)

(left) or DLC (right) magnetron sputtering can be clearly appreciated.

the surface, showing very good antibacterial behavior [62].

on Ti dental implant. Courtesy of the commercial company Flubetech.

cause of the failure.

206 Advanced Surface Engineering Research

Ion implantation techniques consist in the superficial modification of materials by ion bombardment. By means of these techniques is possible to improve surface properties of different materials [63, 64]. Although these techniques have their origin in the nuclear industry [65, 66], the first works on semiconductor applications appeared in Bell Laboratories in 1948 when Kingsbury and Ohl carried out studies of implantation of light ions on wafers of Si [67, 68]. Metallurgical application of ion implantation was firstly reported simultaneity in the 1970s by Harwell laboratory (UK), and Naval Research Laboratory (USA). These early works were mainly focused on nitrogen implantation of steels. From these first results up to today ion implantation techniques have been introduced in many different industrial applications (mostly in aeronautical or biomedical sector).

Ion implanters equipment consists of a series of characteristic elements (see **Figure 9**) such as a source of ions (capable of producing sufficient quantities of certain types of ions), one or two acceleration stages (potential differences of the order of 100,000 V), a mass separator magnet, and a thermal chamber (high-vacuum chamber where the samples whose surface is to be treated) are placed. The whole process of generation, acceleration, and implantation is carried out in high vacuum, of the order of 10−5–10−6 mbar to ensure that the average travels of the ions far surpass the distance that separates the source of the whites to implant. In addition, ion implantation process is performed in an approximate range of energies of 25–300 KeV, although most of the studies are carried out to energies between 50 and 200 keV. The relevant parameters of these treatments are the type of ion, the implanted dose, the implantation energy, and the temperature of the process, which in the most cases is kept deliberately below a certain level (being able to talk about temperature environment), as it can be observed in **Figure 9**. By a properly selection of all these parameters, physical-chemical properties of the surface of the implanted samples can be perfectly controlled [69].

In **Figure 10**, a schematic diagram of the used plasma immersion ion implantation-enhanced deposition (PIIIeD) processing system is shown. For this figure, the process chamber is similar to a conventional plasma immersion ion implantation (PIII) combined with an RF magnetron sputtering and a glow discharge (GD) plasma source. In this system, thin films are deposited simultaneously with 3-D implantation of argon ions, improving film adhesion and relaxing film stress. As metal ions are rare in a magnetron discharge, the auxiliary electrode for glow discharge plasma helps to ionize some metal neutrals which are also implanted into the substrate during the high-voltage pulses.

The basic difference between ion beam implantation (II) and plasma immersion ion implantation (PIII) consist in PIII the target is an active part of the system, and it is biased at pulsed high voltage. On the other hand, in II, the target is isolated from the ion beam generation (it is not active part of electric circuit), and both treatments have relevant differences as it can be shown in **Table 1** [71].

Conventional ion implantation is a ballistic process where kinetic energy from ions promotes ion implantation in the target. Plasma immersion ion implantation (PIII) is a combined process where temperature and voltage were defined to obtain an implantation profile. According to this, the temperature is an important parameter in PIII, dominating in many cases the final implantation characteristics. In addition, ion energy is the other key parameter in ion implantation, much more important in II where is the most relevant parameter affecting implantation profile. However, the temperature is not considered a key factor in plasma immersion ion implantation, being the typical implantation energies from a few KV up to no more of 30 kV. The most extended applications of ion implantation technologies have been in biomedical devices. There are different functionalities that ion implantation can achieve in hip, knee, and other prosthesis. One of the first applications was the implantation of Ca and P to obtain a surface with characteristics similar to hydroxyapatite [72]. As in the case of PVD, this solution has not been incorporated into the market due to the good performance of the hydroxyapatite grown by plasma spraying. In this way, ion implantation has been more relevant in the strategies aimed at increasing the hardness and wear resistance of the prostheses. Conventional ion implantation was the first attempt to apply on different alloys of titanium and CoCr in order to reduce the wear of the prosthesis. The results obtained were not satisfactory due mainly to the low thickness of the modified layer (typically around 0.1 microns) [71]. In order to overcome the problem of the low thickness of the conventional ionic implantation, low

**Figure 10.** Schematic diagram of the used plasma immersion ion implantation-enhanced deposition (PIIIeD) processing system. The process chamber is similar to a conventional PIII combined with an RF magnetron sputtering and a glow

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discharge (GD) plasma source. Reprinted with permission of [70].

**Figure 9.** Scheme of a mass separation ion implanter. Ion implantation, the invisible shield. Courtesy of R. Rodríguez, T. Tate & N. Mikkelsen, SPRINT RA372 project.

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or two acceleration stages (potential differences of the order of 100,000 V), a mass separator magnet, and a thermal chamber (high-vacuum chamber where the samples whose surface is to be treated) are placed. The whole process of generation, acceleration, and implantation is carried out in high vacuum, of the order of 10−5–10−6 mbar to ensure that the average travels of the ions far surpass the distance that separates the source of the whites to implant. In addition, ion implantation process is performed in an approximate range of energies of 25–300 KeV, although most of the studies are carried out to energies between 50 and 200 keV. The relevant parameters of these treatments are the type of ion, the implanted dose, the implantation energy, and the temperature of the process, which in the most cases is kept deliberately below a certain level (being able to talk about temperature environment), as it can be observed in **Figure 9**. By a properly selection of all these parameters, physical-chemical properties of

In **Figure 10**, a schematic diagram of the used plasma immersion ion implantation-enhanced deposition (PIIIeD) processing system is shown. For this figure, the process chamber is similar to a conventional plasma immersion ion implantation (PIII) combined with an RF magnetron sputtering and a glow discharge (GD) plasma source. In this system, thin films are deposited simultaneously with 3-D implantation of argon ions, improving film adhesion and relaxing film stress. As metal ions are rare in a magnetron discharge, the auxiliary electrode for glow discharge plasma helps to ionize some metal neutrals which are also implanted into the sub-

The basic difference between ion beam implantation (II) and plasma immersion ion implantation (PIII) consist in PIII the target is an active part of the system, and it is biased at pulsed high voltage. On the other hand, in II, the target is isolated from the ion beam generation (it is not active part of electric circuit), and both treatments have relevant differences as it can be

**Figure 9.** Scheme of a mass separation ion implanter. Ion implantation, the invisible shield. Courtesy of R. Rodríguez,

the surface of the implanted samples can be perfectly controlled [69].

strate during the high-voltage pulses.

T. Tate & N. Mikkelsen, SPRINT RA372 project.

shown in **Table 1** [71].

208 Advanced Surface Engineering Research

**Figure 10.** Schematic diagram of the used plasma immersion ion implantation-enhanced deposition (PIIIeD) processing system. The process chamber is similar to a conventional PIII combined with an RF magnetron sputtering and a glow discharge (GD) plasma source. Reprinted with permission of [70].

Conventional ion implantation is a ballistic process where kinetic energy from ions promotes ion implantation in the target. Plasma immersion ion implantation (PIII) is a combined process where temperature and voltage were defined to obtain an implantation profile. According to this, the temperature is an important parameter in PIII, dominating in many cases the final implantation characteristics. In addition, ion energy is the other key parameter in ion implantation, much more important in II where is the most relevant parameter affecting implantation profile. However, the temperature is not considered a key factor in plasma immersion ion implantation, being the typical implantation energies from a few KV up to no more of 30 kV.

The most extended applications of ion implantation technologies have been in biomedical devices. There are different functionalities that ion implantation can achieve in hip, knee, and other prosthesis. One of the first applications was the implantation of Ca and P to obtain a surface with characteristics similar to hydroxyapatite [72]. As in the case of PVD, this solution has not been incorporated into the market due to the good performance of the hydroxyapatite grown by plasma spraying. In this way, ion implantation has been more relevant in the strategies aimed at increasing the hardness and wear resistance of the prostheses. Conventional ion implantation was the first attempt to apply on different alloys of titanium and CoCr in order to reduce the wear of the prosthesis. The results obtained were not satisfactory due mainly to the low thickness of the modified layer (typically around 0.1 microns) [71]. In order to overcome the problem of the low thickness of the conventional ionic implantation, low energy-high temperature implantation techniques [73], and plasma immersion ion implantation techniques have been performed [74]. By using these techniques, the resultant implantation profiles of more than 1 micron have been achieved (see figure GDOS profile of **Figure 11**), increasing the hardness and wear resistance.

in the polarization curves (Tafel plots) of **Figure 12**. However, this problem can be solved through the implantation of oxygen, obtaining for these specific cases an important increases

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Another application of interest for implantation by immersion in oxygen plasma is the reduction of the release of heavy ions to the blood flow observed in cardiovascular devices. The implantation of oxygen in stainless steels "pushes" the ions such as Ni and Cr into the surface, and as a promoting result, a significant decrease in the release of these ions have been obtained with an improvement in biocompatibility [71]. By means of oxygen implantation, heavy ions (Ni, Mo, and Cr) go deeper into the material as shown in **Figure 14**, where it is

**Figure 12.** Polarization curves for different samples in Ringer solution at 37.2°C. Reprinted with permission of [74].

**Figure 13.** Corrosion rates calculated from the polarization curves as function of nitrogen and oxygen implantation temperature. The samples labeled as "ref" are either only implanted with oxygen (with indicated temperature) or a

sample from untreated base material (circle). Reprinted with permission of [75].

of the corrosion resistance [75], as it can be observed in **Figure 13**.

Although the experimental results indicate an enhancement in the wear resistance, it has been also observed the precipitation of part of the chromium of the stainless steels, or of the alloys of CoCr, due to an increase in the temperature produced a decrease in the corrosion resistance, even at temperatures where carbides are not appreciated [74], as it can be appreciated


**Table 1.** A comparative scheme of the different parameters (geometry, temperature, thickness, batch time, ion energy, ion current, or industrial scaling) between plasma immersion ion implantation and ion beam implantation, respectively. Reprinted with permission of [71].

**Figure 11.** Glow discharge optical spectroscopy (GDOS) depth profiles of nitrogen after implantation at 400°C (ion energy 1.2 keV, current density 1 mA/cm<sup>2</sup> and fluence 3.5 × 1019 cm−2) for AISI 316. Reprinted with permission of [73].

in the polarization curves (Tafel plots) of **Figure 12**. However, this problem can be solved through the implantation of oxygen, obtaining for these specific cases an important increases of the corrosion resistance [75], as it can be observed in **Figure 13**.

energy-high temperature implantation techniques [73], and plasma immersion ion implantation techniques have been performed [74]. By using these techniques, the resultant implantation profiles of more than 1 micron have been achieved (see figure GDOS profile of **Figure 11**),

Although the experimental results indicate an enhancement in the wear resistance, it has been also observed the precipitation of part of the chromium of the stainless steels, or of the alloys of CoCr, due to an increase in the temperature produced a decrease in the corrosion resistance, even at temperatures where carbides are not appreciated [74], as it can be appreciated

**Table 1.** A comparative scheme of the different parameters (geometry, temperature, thickness, batch time, ion energy, ion current, or industrial scaling) between plasma immersion ion implantation and ion beam implantation, respectively.

Geometry Line of sight Conformal Temperature Room temperature 400°C

Batch time 10–100 h 0.1–2 h Ion energy 10–1000 kV 0.1–100 kV Ion current 1–100 mA 100–1000 mA Industrial scaling Low Medium

Thickness 0.1 micron 0.05–10 microns

**Ion implantation Plasma immersion**

**Figure 11.** Glow discharge optical spectroscopy (GDOS) depth profiles of nitrogen after implantation at 400°C (ion

and fluence 3.5 × 1019 cm−2) for AISI 316. Reprinted with permission of [73].

energy 1.2 keV, current density 1 mA/cm<sup>2</sup>

Reprinted with permission of [71].

increasing the hardness and wear resistance.

210 Advanced Surface Engineering Research

Another application of interest for implantation by immersion in oxygen plasma is the reduction of the release of heavy ions to the blood flow observed in cardiovascular devices. The implantation of oxygen in stainless steels "pushes" the ions such as Ni and Cr into the surface, and as a promoting result, a significant decrease in the release of these ions have been obtained with an improvement in biocompatibility [71]. By means of oxygen implantation, heavy ions (Ni, Mo, and Cr) go deeper into the material as shown in **Figure 14**, where it is

**Figure 12.** Polarization curves for different samples in Ringer solution at 37.2°C. Reprinted with permission of [74].

**Figure 13.** Corrosion rates calculated from the polarization curves as function of nitrogen and oxygen implantation temperature. The samples labeled as "ref" are either only implanted with oxygen (with indicated temperature) or a sample from untreated base material (circle). Reprinted with permission of [75].

to increase corrosion and wear resistance in heart valves and dental implants. In this line, the appearance of improved PVD techniques (HIPIMS) has achieved levels of adhesion and layer density that augur a wide use in other types of implants. Ion implantation and fundamentally the immersion in plasma are very effective tools to increase the wear resistance, maintaining

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Although all these techniques, and some more like plasma electrooxidation (PEO), have allowed significant advances already introduced in the market, there are still problems to be solved and challenges to be addressed within the surface treatments and design of novel materials. New manufacturing techniques such as additive manufacturing and social aspects such as the increase in life expectancy, determine that a continuous research is necessary to develop new materials and treatments compatible with this new framework. Probably one of the most promising ways of research is the use of combined treatment techniques within

the corrosion resistance of stainless steels and titanium and CoCr alloys.

PBLG-g-HA poly (γ-benzyl-L-glutamate) modified hydroxyapatite

materials and customized designs for each patient.

**Nomenclature**

HA hydroxyapatite

PVD physical vapor deposition

DLC diamond-like carbon CP calcium phosphates

PLLA poly (L-lactic acid)

IFM interferometry

CA cathodic arc

EB electron beam (EB)

MS magnetron sputtering

HIPIMS high power impulse magnetron sputtering

MPPMS modulated pulsed power magnetron sputtering

DCMS direct current magnetron sputtering

PIII plasma immersion ion implantation

FDA food and drug administration PSHA plasma sprayed hydroxyapatite SEM scanning electron microscopy

**Figure 14.** Compositional profiles for oxygen implanted into stainless steel at energies of 10, 20, and 30 keV. Reprinted with permission of [71].

possible to observe atomic concentration of Ni after oxygen implantation. As a consequence of this, migration tests show a reduction of the concentration of these ions in blood of more than 50%.

Finally, interesting approaches for the design and implementation of multifunctional layers (bactericides, wear resistance, and anticorrosion) have been tested by using combined treatment techniques. As a representative example, ionic implantation of Ag on ceramic layers previously deposited by PVD is a novel approach that allows to have a better control of the distribution of the Ag, solving potential problems of cytotoxicity by the excessive release of silver cations [76].
