Surface Modification by Ion Irradiation

Ion implantation (II) and plasma-based ion implantation (PBII) processes are also well established to offer wide possibilities for the surface modification of materials. The application potential of these modified surfaces is very high in the different fields of the modern technology, like the microelectronic, metallurgical, and biological industries. However, these processes are different with respect to coating deposition processes. Atomic species are introduced in the near-surface region by a nonequilibrium process which depends on the parameters used, like ion energy, ion current, chamber vacuum, and temperature conditions. The modified surface region can then be constituted of embedded phases, buried layers, and also, under some conditions, to form a coating layer. In the general case, a graded region is produced from the near-surface to deeper regions. The modified region is, in almost all cases, localized at depths on the order of ten nanometers to thousands of nanometers. Because of this, the nanoindentation technique is the most indicated to obtain mechanical properties such as hardness, elastic modulus, as well as elastic recovery and surface toughness.

Nitrogen is the most used atomic species that has been irradiated to improve surface hardness in metals by II or PBII processes because of its ability to form nitride compounds. However, other atomic species like C, B, Cr, Ti, and O can also be used to prevent wear and corrosion [110]. Notwithstanding, there exist a great number of commercially available metal alloys; the number of reported works in the literature about mechanical properties obtained by the nanoindentation technique for metal surfaces modified by II or PBII is small. Equally as performed for coatings, the influences of the substrate (matrix) and very near-surface region need to be excluded, or at least minimized, in order to obtain the actual hardness and elastic modulus values for the ionic modified surfaces. Surface modifier processes typically produce graded regions. These regions may be formed by a natural oxide layer, the presence of a solid solution of implanted ions, and some stoichiometric phases. The description of a unique hardness value for the material surface is then almost impossible. Consequently, it is recommended to analyze the ionic effect on surface hardening by using the hardness-to-depth profile to compare the different physical parameters of the irradiation process used to modify the surface.

A typical hardness profile for the pure iron surface submitted to N ion implantation is shown in Figure 9. The N peak position is around 70 nm in depth for both substrates. High hardness occurs in the near-surface region, and slowly decreases until it reaches the substrate value. The high surface hardness is attributed to the formation of iron nitrides. The distribution range of the N ions is not sharply defined (it is a Gaussian-like distribution). Then N atoms in solid solution are also present before and after the ion peak

Figure 9. Hardness values as a function of depth for N irradiated iron.

Contact I

Figure 9. Hardness values as a function of depth for N irradiated iron.

position. Because of the influence of high hardness in this graded region, substrate hardness values are only reached at very deep depths.

Table 1 summarizes the hardness values obtained using the nanoindentation technique (low applied loads <100 mN), in different commercial metals and metals alloys submitted to different ion irradiation processes (II and PBII).

Additional mechanical property information can also be extracted from the load/unloading curves in the surfaces modified by II or PBII processes. It is well known that II or PBII processes introduce surface damage. The damage profile is basically a function of the ion species and its energy. In this region, atomic displacements, vacancies, Frankel pairs, and extended defects are observed. These defects may alter the elastic field in the region around the tip contact, delaying the plastic deformation by increasing the tip ending pressure, and finally reaching the necessary energy to emit dislocations and form microcracks. This specific phenomenon can be observed in a load/unloading cycle, and it is characterized by a typical tip incursion (pop in) [111]. The extension of the tip incursion at an applied load and time can be then used to estimate the energy to create dislocations. The pattern of the tip incursion allied to electronic microscopy was then used to confirm these hypotheses. At this point, it is important to distinguish this phenomenon from pop in that occurs in dislocation-free materials.

Polymers are also widely used in most industrial applications due to their excellent chemical and physical properties, easy working processes, and low operational costs. On the other hand, when polymers are submitted to load-bearing and abrasive conditions, the mechanical and tribo-logical properties need to be enhanced. The use of II and PBII processes has shown to be a very efficient technique to modify polymer surfaces because there is a large energy transference in II and PBII processes when compared to other more traditional processes like X-ray, y-ray, ultraviolet rays (UV light), and electron beams. The polymers present a lower bond strength compared to metals, ceramics, or semiconductors; then the transferred energy to the electrons and atoms by the incoming ions stimulates chemical reactions and chain scissions that, at the final stage of the process, produce hardening and stiffening of the polymer surface by a cross-linking mechanism of the chains or

Table 1. Relative hardness (RH) for different metal alloys submitted to different ion implantation processes.

Material

Ion process

Ion type

RH

Ref.

Fe

II

N

3

[111]

Ar

3

AISI 1020

II

N

3

Ar

3

N + Ar

3

AISI D2

II

N

1.4

[112]

C

1.8

AISI M2

II

N

1.48

Cr

1.02

N + Cr

1.43

AISI P20

II

C

1.56

AISI M3:2

II

N

1.51

C

1.26

Cr + C

1.42

AISI 420

II

N

1.55

A286

II

N

1.87

Y

1.13

Y + N

1.09

N + Ar

8.0

AISI 316L

II

N

1.6

[113]

SS 316L

II

N

2.72

[114]

AISI 304

II

C

1.3

[115]

AISI 440C

II

C

1.15

TI

1.15

AISI 304

PBII

N

2.5

[116]

Al 7075

PBII

O

2.5

[117]

CoCrMo alloy

II

B

1.5

[118]

N

1.2

C

1.3

B+N

1.4

B+C

1.8

304L

II

N

1.23

[119]

AISI S7

II

N

1.53

Ni

II

Ti + C

2.0

[120]

Ni80Fe20

II

Ti + C

2.0

Zr

II

N

4.0

[121]

Al

II

N

5.0

[122]

Al-6061

II

N

5.0

Ti6Al4V

II

N

3.0-5.0

[123]

Ti6Al4V

PBII

N

10.0

[17]

Ti6Al4V

II

C

2.0

[124]

Si

PBII

C

2.5

[125]

N

1.1

Al

II

N

5

[126]

also by the formation of a three-dimensional microstructure which restrains the chain movements.

Literature data about mechanical and tribological properties, using the nanoindentation technique, of polymers, like polyisoquinoline (PIQ), poly-2-vinylpiridine (PVP), polyacrylonitrile (PAN), polyethylene (PE), polycarbonate (PC), polyetherimide (PEI), polystyrene (PS), Kapton polypropylene (PP), polypropylene (PP), polystyrene (PS), polyethersulfone (PES), polyethylene terephthalate (PET), polyhedral oligomeric silsesquioxane (POSS), poly(ether ether ketone) (PEEK), polystyrene (PS), fotoresist AZ 1350, C60 films, and plasma-formed polymers, in a bulk or coating form, submitted or not to surface-modifying processes, can be obtained by consulting the references [68, 127-141].

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