WOODHEAD PUBLISHING IN MATERIALS

WOODHEAD PUBLISIIING IN MATERIALS

Surface engineering of

light alloysAluminium, magnesium and

titanium alloys

Edited by Hanshan Dong

CRC PressBoca Raton Boston New York Washington, DC

W o o d h e a d p u b l i s h i n g l i m i t e dOxford Cambridge New Delhi

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10 Plasma assisted surface treatment of aluminium alloys to combat wear 323

F. ashraFizadeh, Isfahan University of Technology, Iran

10.1 Introduction 32310.2 Effect of plasma on surface oxide film 32810.3 Plasma nitriding of Al alloys 33110.4 Physical vapour deposition (PVD) coatings of Al alloys 33510.5 Duplex surface treatment 34810.6 Load bearing capacity and interface design 35110.7 Summary 35810.8 References 359

11 Plasma immersion ion implantation (PIII) of light alloys 362 Y. Xin and p. k. Chu, City University of Hong Kong, China

11.1 Introduction 36211.2 Processes and advantages of plasma immersion ion

implantation (PIII) 36311.3 PIII surface modification of (titanium) Ti Alloys 36911.4 PIII surface modification of magnesium (Mg) alloys 37511.5 PIII surface modification of Al alloys 38711.6 Future trends 39211.7 Sources of further information and advice 39311.8 References 393

12 Laser surface modification of titanium alloys 398 t. n. baker, University of Strathclyde, UK

12.1 Introduction 39812.2 Lasers used in surface engineering 39912.3 Laser surface modification methods 40012.4 Laser processing conditions for surface engineering 40512.5 Laser surface melting and cladding 41012.6 Laser surface alloying 41312.7 Effect of laser surface modification on properties 41912.8 Summary 43312.9 Acknowledgements 43312.10 Sources of further information and advice 43412.11 References 434

13 Laser surface modification of aluminium and magnesium alloys 444

J. C. betts, University of Malta, Malta

13.1 Introduction 444

H. Liao*University of Technology of

Belfort-Montbeliard90010 Belfort cedex France

E-mail: hanlin.liao@utbm.fr

H. Wang School of Mechanical and

Materials EngineeringJiujiang UniversityJiujiang 332005P. R. China

E-mail: wanght7610@163.com

Chapter 9

S. Abela Department of Metallurgy and

Materials Engineering University of Malta, Msida MSD06Malta

E-mail: ssabel@eng.um.edu.mt

Chapter 10

F. Ashrafizadeh Department of Material

EngineeringIsfahan University of Technology Isfahan 8415683111Iran

E-mail: ashrafif@cc.iut.ac.ir

Chapter 11

Y. Xin and P. K. Chu*Department of Physics and

Materials ScienceCity University of Hong KongTat Chee AvenueKowloonHong KongChina

E-mail: paul.chu@cityu.edu.hk

Chapter 12

T. N. BakerDepartment of Mechanical

EngineeringUniversity of StrathclydeGlasgow G1 1XJUK

E-mail: neville.baker@strath.ac.uk

Chapter 13

J. C. BettsDept. of Metallurgy and Materials

Engineering Faculty of Engineering University of MaltaMsidaMalta, MSD2080

E-mail: jbetts@eng.um.edu.mt

xiiiContributor Contact Details

11Plasma immersion ion implantation (PIII)

of light alloys

Y. Xin, City University of Hong Kong, China and Chongqing University, China, and P. K. CHU,

City University of Hong Kong, China

Abstract: Plasma immersion ion implantation (Piii) or plasma source ion implantation (PSii), which overcomes the line-of-sight limitation inherent to beam-line ion implantation is an important surface modification technique for light alloys. in this chapter, the processes and advantages of the Piii technique are introduced, which is followed by an up-to-date overview of the progress and current status of PIII surface modification of light alloys including Mg, Al and Ti alloys. The structural changes in light alloys after Piii and the resulting improvements in the corrosion resistance, surface mechanical properties, as well as biological performance of the treated alloys are described. new trends in Piii surface treatments are also discussed.

Key words: light alloys, plasma immersion ion implantation, corrosion resistance, surface mechanical properties, biological performance.

11.1 Introduction

Plasma immersion ion implantation (Piii) or plasma source ion implantation (PSii) is a well established technique based on plasma science. The unique advantage of Piii is processing of samples with a complex shape, because the technique circumvents the line-of-sight restriction encountered in conventional beam-line ion implantation (Conrad, 1987; Tendys, 1988). As the typical implantation depth is quite shallow, generally about several nanometers to micrometers from the surface, Piii does not usually alter the bulk properties of the materials. Hence, this technique has become an important and effective method for the surface modification of various electrically conducting (Conrad, 1987; Liu, 2007a; Kostov, 2004; Poon, 2005) and insulating materials (Zhang, 2006, 2007a; Powles, 2007). One of its important applications is surface modification of light alloys such as Mg-based, Al-based, and Ti-based alloys. in this chapter, the principle and advantages of Piii are introduced. A summary of recent progress in surface modification of light alloys, namely Mg, Al, and Ti alloys by PIII is presented. The structural changes in these light alloys after undergoing Piii and the resulting corrosion resistance, surface mechanical properties, as well as biological performance of the treated alloys are described. new

iii surface treatment of light alloys are discussed at the end of the chapter.

11.2 Processes and advantages of plasma immersion ion implantation (PIII)

PIII was first introduced back in the mid-1980s by Conrad et al. (1987) and Tendys et al. (1988). initially called plasma source ion implantation (PSii), this technique is now frequently referred to as plasma immersion ion implantation (Piii) in order to distinguish it from conventional beam-line ion implantation. in Piii, the specimens are surrounded by a low temperature plasma and are pulse-biased to a high negative potential relative to the chamber wall, as illustrated in Fig 11.1. The ions generated in the plasma shroud are accelerated across the sheath formed around the specimens and are implanted into the surface of targets according to plasma physics. The ion energy and angle of incidence depend on the scattering mean free path, which is function of the plasma gas pressure, accelerating voltage, and sample topography. Owing to the absence of transport optics and mass selection, PIII can provide a high ion flux. The principle governing the PIII process depends on plasma physics and such details can be found readily in many books and references, such as those listed in the Section 11.6, Bibliography and so they will not be discussed here. in short, the advantages of Piii are described in the following (Chu, 1996):

11.1 Schematic diagram of the PIII process.

Plasma sources chamber

Positive ions

High voltage pulser

+

–

(i) it is not subject to normal thermodynamic constraints such as impurity solubility.

(ii) The instrumentation is simple and cheaper than traditional ion implanters.

(iii) The implantation flux can be as high as 1016cm–2s–1 and the implantation energy can vary from 10 to 105 eV.

(iv) non-planar samples can be implanted with good conformality and uniformity without special target manipulation.

(v) Multiple processes, such as simultaneous and consecutive implantation, deposition, and etching can be carried out.

(vi) insulators can be treated.

These unique characteristics make PIII very attractive for the modification of the surface properties of a myriad of materials.

11.2.1 Gaseous PIII

Both gaseous and solid ion implantation can be performed by means of Piii, which requires a low temperature and, typically, high-density plasma. in most applications, the plasma is produced by an electrical discharge. According to the nature of the implanted ions, they can be classified into gaseous plasma discharges such as radio frequency (RF) discharge and microwave discharge, glow discharge, and metal plasma discharge, as in the case of vacuum arc discharge. RF discharge can be classified into two groups according to the coupling of the RF power with the load: capacitive coupling and inductive coupling. The normal setup contains a RF generator, an antenna, and a matching network, as shown in Fig 11.2. Generally, the generator works at a frequency of 13.56 MHz, which is above the ion plasma frequency and below the electron frequency. Thus, electrons respond to the variation in the electric field, but ions experience the average field. The working pressure during discharge ranges from about 10–1 to 10 000 Pa (Chu, 1996). The plasma density in the RF glow discharge during low-pressure discharge varies from 109 to 1011 cm–3 and can reach 1012 cm–3 under medium pressures (1–100 Torr) (Miyake, 2000). in low-pressure discharge, the ion mean free path is typically longer than the sheath dimension at the substrate, giving rise to collisionless conditions. The plasma potential of the RF discharge typically falls into the range of 10–30 V above the wall potential (Lieberman, 1994; Ehlers, 1979). Glow discharge is triggered by two methods: thermionic filament or pulsed high voltage emission. Thermionic discharge relies on electron emission from a hot cathode to ionize the background gas. Plasma generation requires the suitable selection of a cathode heated by a floating power supply. Refractory

metals are usually used as the filament cathode as they are easy to fabricate and have sufficiently low evaporation rates. The electron currents range from 1 A to several kA, depending on the gas, plasma density, and plasma size. in addition, the applied voltage between the cathode and anode can be lower than the ionization energy and is usually in the range of 25–100 V, where the maximum of the ionization cross-section occurs for most gases (Lieberman, 1994). in pulsed high-voltage glow discharge, the pulsed voltages play dual roles, namely generation of the plasma, and acceleration of ions through the sheath to the substrate. Since it works for many electrode geometries and gases, the method is very versatile. The pulsed high voltage generates a sudden electric field between the substrate (cathode) and surrounding system (anode). The emitted free electrons from the cathode are accelerated and collide with gas molecules or the anode. ions produced during the collisions continuously strike the cathode to generate secondary ions, which are accelerated back into the discharge volume (Lieberman, 1994).

11.2.2 Metal PIII

Metal plasma immersion ion implantation needs a cathode arc discharge system, as shown in Fig 11.3. A typical cathodic arc plasma source comprises two parts: a plasma production unit and a macro-particle filter. The characteristics of arc discharge are relatively high current (tens or hundreds of Amperes) and low voltage (10–80 V) between the cathode and anode. The vacuum is sustained by materials originating from the cathode. As shown in Fig 11.4, a small area on the cathode is heated by the arc and many particles together with a high density metal plasma are ejected. During the cathodic arc process, an ensemble of luminous cathode spots moves in a rapid and

Matching netwrok

RF cathode

Plasma

Ground anode

RF powerC1 C2

P

11.2 Schematic diagram of a capacitively-coupled plasma sources with equivalent electrical circuit.

Trigger

Cathode

Magnetic filter duct

11.3 Schematic diagram of a cathodic arc plasma source with a magnetic filter duct.

Inactive spot

Active spot

+

Ion

Anode

Ions

Macroparticles

Photon

Electrons

+

+

+

+

+

+

Evaporation

++

++

++

++

Metal ion flux

11.4 Particle emission from the arc spots (Lafferty, 1980).

chaotic manner across the surface. The plasma expands in all directions from the cathode spots towards the anode and vacuum chamber walls. The cathode spot is usually small, while producing an intense plasma with current densities of 106–1012 A–2 (Lieberman, 1994). The spot velocity is determined by factors including the nature of the cathode, residual vacuum, and external magnetic field. The ejected macroparticle size ranges from 0.1 to 100 mm in diameter. The electron density in the cathodic plasma can reach 1020

cm–3 and the expanding plasma produces a highly ionized jet with mean ion charges state greater than 1+ for many elements. A curved duct filter with a magnetic field is usually utilized to mitigate particle contamination from the arc. Another aim of the filter is to increase the plasma transportation efficiency through the duct, and an optimized 90° curved duct can achieve plasma transport efficiency of about 35% (Schulke, 1997). In metal PIII, the metal ions condense on the material’s surface as a film in the time duration between the high voltage pulses. ions accelerated by the high voltage bias pulses bombard the deposited metal atoms, giving rise to recoil implantation. This affects the elemental depth profiles which may deviate from the usual Gaussian-like shape (Schulke, 1997).

11.2.3 Hybrid PIII processes

Many researchers, such as those in City University of Hong Kong, have developed several hybrid processes such as Piii/nitriding (Piii&n) and Piii/deposition (Piii&D). in order to perform these hybrid processes, different types of hardware have been designed. The gaseous plasma is usually produced inductively or by capacitively coupled radio-frequency plasma sources as well as hot filament glow discharge plasma sources, and sustained by direct high-voltage glow discharge between the chamber wall and the target (Tian, 2001a). In the City University of Hong Kong instrument, four sets of filtered cathodic arcs are used to generate various metal ions free of macroparticles. The sputtering system can be operated either by RF or DC, even when high-voltage pulses are applied to the samples. There is also an evaporation system for solid elements such as calcium, phosphorus, and sodium that are difficult to handle using conventional cathodic arcs (Li, 2003). A schematic of the evaporation system is displayed in Fig 11.5. Evaporation of the powders is accomplished by filament heating and the solid entering the chamber is further ionized by the RF. The high-voltage switch is hard-tube-based and the pulsed voltage, pulsed frequency and pulsing phase relative to the vacuum arc phase can be flexibly adjusted to optimize various processes. In addition, a high frequency/low frequency modulator has been developed for plasma nitriding or sample biasing during physical vapour deposition (Tian, 2004). Both Piii&n (Tian, 2000a, 2000b) and Piii&D (Tian, 1999, 2001b) process have been successfully conducted using this facility (Bilek, 2000).

PIII/nitriding (PIII&N)

In conventional nitriding processes, nitrogen must first absorb onto the sample surfaces and then diffuse into the substrate by thermal effects and the established concentration gradient. The surface conditions and materials’ species play a critical role on the effective particle flux arriving at and penetrating the materials’ surface. For example, it is quite difficult for nitrogen to penetrate the surface oxide barrier on aluminium alloys or stainless steels since n possesses quite small diffusion coefficients in the dense oxide layers. Consequently, a higher sample temperature is required but this may degrade certain properties. in the Piii/nitriding process, the mechanism of particles trapped on the top surface can altered. The absorption and reaction mechanisms in conventional plasma nitriding still play an important role, but the direct penetration of high energy particles by implantation is responsible for subsequent diffusion. in this way, the processing temperature can be lowered without compromising the resulting surface properties (Bilek, 2000).

PIII/Deposition (PIII&D)

Piii&D, which is low-temperature process, is a variation of the conventional ion beam assisted deposition technique. The major difference lies in the

Gas inlet RF system

Antenna

Plasma

Substrate

Vacuum chamber

Pump

Heating

Power

11.5 Schematic illustration of evaporation in the PIII and PIII&D processes.

acceleration mechanism of impinging ions and the non-line-of-sight process. The ions can be generated by sputtering, evaporation, cathodic arc, etc. During Piii&D, typical multiple plasmas are used. it is common to have gas plasmas composed of one or more species, and for metal plasmas to be generated simultaneously in the same vacuum chamber. The implantation process can be performed before deposition to form a gradient layer, and simultaneous deposition and implantation can also be carried out. By adjusting the durations of the high voltage (implantation process) and low voltage (deposition process), flexible processes can be carried out to optimize the surface properties of the modified layer (Bilek, 2000).

11.3 PIII surface modification of titanium (Ti) alloys

Titanium and its alloys are widely used as structural materials in aerospace and aeronautic crafts due to their high strength-to-weight ratio and high temperature resistance (Jackson, 2006; naka, 1996). Usually, the corrosion resistance of Ti alloys is good, due to the naturally-formed dense and corrosion resistant surface titanium oxide layer. Another important application of Ti-based alloys is for orthopaedic implants, such as bone screws, bone plates and artificial hip joints, due to their excellent biocompatibility, mechanical strength, and chemical stability (niinomi, 2008). Figure 11.6 shows one set of bone fixation implants – bone screws, and a bone plate for fixation of a fractured bone (Liu, 2004a). According to their applications, special surface treatments are usually required. in industrial applications, treatments to enhance the corrosion resistance and tribological properties are usually carried out. Titanium-

Bone screw

Bone plate

11.6 Bone plate and bone screws made of titanium (Liu et al., 2004a).

base implants are generally considered to be bio-inert in a physiological environment, as the naturally-formed titanium oxide can hardly induce growth of hydroxyapatite (HAP) from body fluids, leading to formation of fibre capsule around the implant (Long, 1998). Thus, a suitable surface treatment needs to be performed in order to improve the ability to induce precipitation HAP (named bioactivity or bone conductivity) and enhanced osteoblast adhesion (biocompatibility).

11.3.1 Influence of PIII process on corrosion and surface mechanical properties

nitriding is found to be very effective for enhancing the surface hardness and wear resistance of titanium alloys (Zhecheva, 2006; Sha, 2008; Pohrelyuk, 2007). Thus, nitrogen Piii can improve the tribological properties of titanium alloys. The formation of a hard and dense Tin layer is responsible for enhancement of surface hardness and wear resistance. Without substrate heating, the modified layer is usually several tens of nanometers thick, depending on the applied voltage. Heating the substrate will lead to diffusion of the implanted n, and n can penetrate more than one hundred nanometers (Mandl, 2007). As shown in Fig 11.7, pure nitrogen implantation usually causes recoil implantation of C-H contamination from the surface, but the utilization of a gas mixture of n2/H2 can exclude C implantation (Lacoste, 2002). Hence, n2/H2 mixtures with various volume ratios are usually adopted in practice. implantation also induces changes in the surface morphology. As demonstrated in Fig. 11.8, the untreated sample typically exhibits a two phase structure, the matrix and b phase. After pure nitrogen implantation, the two phases are more evident and the bright structure (b phase) becomes rougher. This is natural in Piii treatment as Piii induces slight sputtering of the surface. implantation using n2/H2 results in complete modification of the surface morphology suggesting more severe sputtering-induced damage. The influence of nitrogen PIII on the surface mechanical properties of Ti-6Al-4V has been systematically investigated (Ueda, 2003). Figure 11.9 shows that the near surface possesses the highest hardness, which is several times higher than that of the substrate. In deeper sites, increased hardness (~70%) is observed, even at depths eight times that of the maximum penetration of nitrogen (30 nm). The increased dose results in higher hardness, but we believe that a treatment time longer than 2h (corresponding to the highest dose in Fig. 11.9) poses little influence on the surface hardness. Although the modified layer is very thin (about 30 nm), a reduction of the coefficient of friction of about one third can be obtained compared with the untreated samples. The influence of nitrogen PIII on the corrosion resistance of Ti-6Al-4V has also been studied (da Silva, 2007). Different kinds of nitrogen Piii processes,

N2

O

Ti

N

C

0 500 1000 1500 2000 2500 3000Depth (Å)

Ato

mic

%

100

80

60

40

20

0

N2+H2

OTi

N

C

0 500 1000 1500 2000 2500 3000Depth (Å)

Ato

mic

%

100

80

60

40

20

0

11.7 Depth profiles of Ti, O, N and C measured by XPS in titanium after PIII using N2 and N2 + H2 (PN2:PH2 = 5:4), respectively (Lacoste et al., 2002).

17 µm

(a) (b) (c)

17 µm 17 µm

11.8 SEM images of the Ti–6Al–4V alloys: (a) untreated sample; (b) after PIII processing using N2; (c) after PIII processing using N2/H2 mixture (da Silva et al., 2007).

such as pure n2 implantation and n2/H2 co-implantation (with different gas ratios) have been performed. Electrochemical tests reveal that all these Piii processes do not improve the corrosion resistance of the treated alloys. On the contrary, they all pose slightly deleterious effects. More negative corrosion potentials are observed on all the treated samples and the passive current densities are all about ten times higher than that of the untreated sample, as shown in Fig 11.10. Oxygen and carbon Piii have also been performed on Ti-6Al-4V. The structure evolution after implantation has been investigated (Han, 1996). Carbon implantation can also improve the surface mechanical properties of the treated sample, whereas oxygen implantation seems to be more effective in enhancing the corrosion resistance of Ti alloys (Loinaz, 1998).

11.3.2 Modification of Ti alloy by PIII for biological applications

As aforementioned, Ti-based implants are typically bio-inert in a physiological environment. Piii has been extensively employed to engineer a functional surface on Ti-base implants. it is well accepted that Ti-OH groups play an important role in the bioactivity of Ti-based implants. The mechanism of the Ti-OH groups inducing precipitation of HAP is illustrated in Fig. 11.11. The surface with Ti-OH groups is negatively charged in a physiological environment, with a pH of around 7.4. The negatively charged surface attracts positively charged Ca2+ ions, forming positively charged calcium

TAV st

TAV-30 min

TAV-60 min

TAV-90 min

TAV-120 min

0 50 100 150 200 250 300 350 400Depth (nm)

Har

dn

ess

(GP

a)

7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

11.9 Surface hardness of Ti-6Al-4V subjected to nitrogen PIII at a bias voltage of 20 kV as a function of penetration depths. (Ueda et al., 2003).

titanate on the surface. With accumulation of positive charges on the surface, the surface becomes positively charged, consequently attracting negatively charged phosphate ions. The absorbed Ca2+ and phosphate ions combine to form apatite (Kokubo, 2003). The HAP then grows by consuming more Ca2+ and phosphate ions from the surrounding body fluids. Thus, fabrication of surface Ti-OH groups is an effective way to enhance the bioactivity of titanium-based implants. Hence, H2 and H2O Piii have been conducted into

Untreated 1:30h(N)

1:30h(N:H)

(N:H)3:00h

1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01Current density (A.cm–2)

Po

ten

tial

(V A

g/A

gC

l)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

–0.5

–1.0

11.11 Illustration of HAP precipitation process on the surface with Ti-OH groups (Kokubo et al., 2003).

SBF SBFHCO3– HCO3–SO42– SO42–

PO42– PO42–

PO42–

K+ K+

Na+ Na+

Cl– Cl–

OH– OH–

OH OHOH OHOH OHOH OHOH OHOH OHOH OH

Mg2+ Mg2+

Ca2+Calcium titanate

Calcium phosphate

Ti Ti

Ti Ti

Ti Ti

Ti Ti

Ti Ti

Ti Ti

Ti Ti

Ti Ti

Ti Ti

Ti Ti

O OO O

O OO OO OO O

11.10 Potentiodynamic polarization curves of Ti-6Al-4V subjected to PIII at a bias voltage 11 V and substrate 400°C: N, pure N2 PIII; N-H; PIII with N2 and H2 mixture gas (da Silva et al., 2007).

titanium (Xie, 2005). Ball-like HAP clusters are observed on the surface of sample after H2 + H2O PIII and soaking in synthetic body fluid (SBF) for only 14 days (see Fig. 11.12). The dramatically increased Ti-OH groups after H2 + H2O implantation can be clearly discerned by XPS analysis, as shown in Fig. 11.13. The increased number of Ti-OH groups account for the improved bioactivity.

10 µm

11.12 SEM views of H2+H2O PIII of pure titanium after soaking in SBF for 14 days (Xie et al., 2005).

O1s

(b)

Ti-O

Ti-OH

H2O

528 530 532 534 536Binding energy (eV)

(a)

11.13 High resolution XPS O1s spectrum acquired from pure titanium before (a) and after (b) H2 + H2O implantation (Xie, 2005).

Ca is a biological active element in bone tissue growth. Much work has been carried out to investigate the effect of Ca Piii on the biological properties of titanium alloys. However, due to the low melting point and high activity of Ca, PIII of Ca via the traditional approach is very difficult. Ca, P and na implantation has been successfully performed by Liu and co-workers (2004b) using the evaporation system stated above (see Fig. 11.5) as confirmed by the XPS results in Fig. 11.14. However, the XPS results also imply that deposition is inevitably present during implantation. Thus, this treatment should be described as Piii iii&D to fabricate a thick modified layer with a thick deposition layer (about 0.5 mm in thickness) and implantation layer (about 0.6 mm), as shown in Fig. 11.15. The modified layer has a bi-layered structure with the top layer being calcium hydroxide and the inner layer being calcium titanate, as schematically shown in Fig. 11.16. in the vacuum chamber, the top layer after Piii&D is mainly composed of CaO. When exposed to air, CaO turns into calcium hydroxide and/or carbonates. The SEM views (Fig. 11.17) demonstrate that the original smooth surface becomes very rough, with many ball-like protuberances. After soaking in SBF for 14 days, needle-like HAP covers the entire surface of the treated sample (Fig 11.18). The good bioactivity of the Piii&D titanium alloys are considered to be associated with the dissolution of the top calcium hydroxide layer. The increased Ca ion concentration in the vicinity and local pH both benefit nucleation of HAP. The biological response of the Ca implanted titanium alloys has also been studied by in vivo and in vitro experiments. The Ti surface implanted with Ca enhances the expression of certain bone-associated components and promotes MG-63 cell adhesion and spreading. In vivo experiments show that bone formation on the Ca implanted surface is superior to that of the untreated one (Liu, 2005). Using the same equipment, na and P Piii has also been carried out. no obvious cytotoxicity effects are discovered (Liu, 2005; Maitz, 2005) whereas good bioactivity is observed from the na implanted samples. in comparison, only a small improvement in bioactivity is observed from the P implanted samples (Liu, 2004b). Modification of the biological properties of titanium using PIII and PIII&D is effective and promising. The advantage is that the modified layer bonds strongly to the substrate as there is no distinctive interface between the modified layer and substrate. However, more effective processes and better understanding of the corresponding mechanism are still required.

11.4 PIII surface modification of magnesium (Mg) alloys

Magnesium and its alloys possess many unique properties, such as low density, high specific strength and good electromagnetic shielding effects. They are

Atomic concentration (%)

Atomic concentration (%)

Atomic concentration (%)

100 80 60 40 20 0

100 80 60 40 20 0

90 80 70 60 50 40 30 20 10 0

P Ti O

Na

Ti O

Ca

O Ti

0 10

20

30

40

50

60

Dep

th (

nm

)

0 20

40

60

80

10

0 12

0 14

0D

epth

(n

m)

0 2

4 6

8 10

Sp

utt

er t

ime

(min

)

11.1

4 X

PS

dep

th

pro

file

of

elem

ents

o

f p

ure

tit

aniu

m

sub

ject

ed t

o P

, C

a an

d

Na

PIII

(Li

u,

2004

b).

in addition, the surface mechanical properties of magnesium alloys are usually unsatisfactory. in practical applications, suitable surface treatments are necessary. Surface modification of magnesium alloys falls mainly in two families, namely (i) coating the base materials by anodising, plasma electrolytic oxidation (PEO),

(c/s

)

1E8

1E7

1 000 000

100 000

10 000

Deposition layer

Implantation layer Ti substrate

16O40Ca48Ti

0.0 0.5 1.0 1.5 2.0Depth (µm)

11.15 XPS depth profile of pure titanium after Ca PIII&D (Liu, 2005).

Substrate

Calcium titanate

Calcium hydroxide

11.16 Schematic illustration of the structure of titanium subjected to Ca PIII&D.

chemical conversion coating and various ceramic coating (Xin, 2008) and (ii) ion implantation (Zhang, 2007b). Fabrication of a coating may provide better corrosion protection, but the adhesion strength is a crucial issue. Usually, ion implantation generates a gradient layer which does not easily delaminate from the substrate. However, ion implantation can usually provide

NONE SEI 10.0kV X2.000 10µm WD 8.0mm

11.17 SEM views of Ca PIII&D treated titanium (Liu, 2005).

NONE SEI 10.0kV X1.000 10µm WD 8.0mm

11.18 SEM views of Ca-PIII&D treated titanium after soaking in SBF for 14 days (Liu, 2005).

only moderate effects on the corrosion resistance and surface mechanical properties. Thus, the choice of surface modification techniques should take into account various needs. As such, surface treatments of Mg alloys by Piii can be classified into two groups: gaseous PIII and metal PIII.

11.4.1 Gaseous PIII of magnesium alloys

implantation of gaseous ions aims at fabricating a barrier layer on the surface to block attack from aggressive ions in the environment. Up to now, oxygen, hydrogen, and nitrogen have been implanted into magnesium alloys. MgH2 formed by electrochemical ion reduction (EIR) can significantly improve the corrosion resistance of magnesium alloy (Bakkar, 2005). Researchers have tried to generate a denser MgH2 barrier layer via hydrogen Piii (Bakkar, 2005). Hydrogen implantation conducted at a bias voltage 30 kV and substrate temperature of 200°C leads to a penetration depth of about 200 nm for H in AZ91 magnesium alloys, as shown in Fig 11.19. The potentiodynamic polarization results in Fig. 11.20 show that the corrosion potential of the treated pure magnesium is positively enhanced by more than 200 mV. The improvement in the corrosion resistance is more obvious in the Piii AZ91 magnesium alloy, with an enhanced corrosion potential of nearly 300 mV. Another significant change is the dramatically enhanced pitting corrosion potentials. The electrochemical test also implies that both the hydrogen Piii pure magnesium and AZ91 magnesium alloys

27–Al

25–Mg

16–O

1–H

19–F

0 100 200 300 400 500 600 700 800 900Depth from the surface (nm)

Sec

on

dar

y io

n s

ign

al (

cou

nts

/s)

105

104

103

102

101

100

11.19 SIMS analysis of H distribution in pure magnesium subjected hydrogen PIII at 30 kV and substrate temperature of 300°C (Bakkar, 2005).

exhibit better corrosion resistance than samples treated by the EiR treatment. However, in our experiments, it was also found that the implantation voltages play an important role in the modification effects. Below 30 kV,

Ecorr.(mV) Icorr.(µA/cm2) Ipass.(µA/cm2)Untreated – 1370 1.633 4.7

H-EIR – 1249 0.296 1.3

H-PI3 – 1122 0.240 0.7

–1500 –1000 –500 0 500 1000 1500 2000 2500E, vs SCE (mV)

(a)

I (m

A/c

m2 )

101

100

10–1

10–2

10–3

10–4

Ecorr.(mV) Icorr.(µA/cm2)Ep.(mV)Untreated – 1430 0.474 1307

H-EIR – 1210 0.226 1680

H-PI3 – 1139 0.177 1907

–1500 –1000 –500 0 500 1000 1500 2000E, vs SCE (mV)

(b)

I (m

A/c

m2 )

101

100

10–1

10–2

10–3

10–4

11.20 Potentiodynamic polarization curves of pure magnesium (a) and AZ91 magnesium alloy (b): hydrogen PIII at 30 kV; H-EIR, electrochemical ion reduction. (Bakkar, 2005).

no improvement in the corrosion resistance could be obtained. This could be readily observed by in situ observation of the samples immersed in the test solution (simulated body fluids, SBF). As shown in Fig. 11.21, when exposed to the SBF, serious corrosion takes place immediately on the sample treated at 20 kV, but the magnesium alloy implanted at 30 kV is stable even after 1 day of immersion and only small corrosion sites can be seen. The potentiodynamic polarization test results shown in Fig. 11.22 are in good agreement with the results from in situ observation. The corrosion potential of the sample treated at 30 kV is enhanced by more than 200 mV, whereas no obvious enhancement in the corrosion potential can be seen from the sample after 20 kV Piii. Tian et al. have investigated the effects of nitrogen implantation on the corrosion behavior of magnesium alloys (Tian, 2005). The electrochemical test results in Fig. 11.23 show enhanced corrosion potential after nitrogen Piii. However, both the implantation voltage and dose affect the corrosion resistance. The corrosion resistance is not very good for both very low (M-22) and very high doses (M-44). it is believed that n implantation can result in the formation of Mg3n2 (Fukumoto, 2001). However, Mg3n2 is sensitive to atmospheric modification and a high amount of Mg3n2 will pose negative effects on the corrosion resistance. Thus, the enhanced corrosion resistance is ascribed to the compactness of the loose natural oxide layer and ion irradiation effects. it is also conjectured that the excess ions agglomerating and moving to the interface during high dose implantation make the surface more active. Researchers have also tried to fabricate dense oxide layers on magnesium by oxygen implantation and/or water Piii (Wan, 2007; Tian, 2006). As shown in Fig. 11.24, implantation of oxygen leads to enhancement in the

(a) (b)

11.21 In situ corrosion morphology observation of AZ91 magnesium alloys subjected hydrogen PIII: (a) implantation at 20 kV after exposure in SBF for 5 min; (b) implantation at 30 kV after exposure in SBF for 1 day.

Controlled E(corr) = 1731 mVH-PIII-20 E(corr) = 1713 mV

H-PIII-30 E(corr) = 1510 mV

–7 –6 –5 –4 –3 –2Log current density (mA/cm2)

Po

ten

tial

(m

V)

–1200

–1400

–1600

–1800

11.22 Potentiodynamic polarization curves of hydrogen PIII of AZ91 magnesium alloy: H-PIII-20, hydrogen PIII treatment at voltage 20 kV; H-PIII-30, hydrogen PIII treatment at voltage 30 kV.

–7 –6 –5 –4 –3 –2 –1 0Current log (I) (Log(A))

Po

ten

tial

(m

V)

–600

–800

–1000

–1200

–1400

–1600

–1800

Mg-00

Mg-22

Mg-42

Mg-44

11.23 Potentiodynamic polarization curves of AZ91 magnesium alloy after nitrogen PIII: Mg-00, untreated alloy; Mg-22, nitrogen PIII at 20 kV voltage for 4 h; Mg-42, nitrogen PIII at 40 kV for 2 h; Mg-44, nitrogen PIII at 40 kV for 4 h. (Tian, 2005).

binding energy of Mg-O and the binding energy increases with increased implantation dose. ion bombardment also induces a smoother surface. in fact, the higher the implantation dose, the smoother is the surface. The oxygen Piii magnesium alloys exhibit interesting changes of corrosion performance in different test solutions. Low dose implantation results in more negative corrosion potentials in a phosphate buffer solution (PBS). However, the higher implantation dose also degrades the corrosion potential, while a five orders of magnitude reduction in the corrosion current density is present. The chloride ion concentrations also affect the effects of oxygen Piii. in the solution containing a high concentration of chloride ions, dramatic degradation in the corrosion resistance takes place. This behavior is probably associated with the attack by chloride ions onto the Mg-O bonds. The chloride ions can transform the magnesium oxide ions into soluble magnesium chloride as follows (Song, 2003):

Mg + 2H2O Æ Mg(OH)2 + H2 [11.1]

Mg(OH)2 + Cl– Æ MgCl2 + 2 OH– [11.2]

Dissolution of magnesium oxide leads to more active regions on the surface and promotes further dissolution of the substrate. in a high concentration

Mg 2pMg-Mg

Mg-OO1s

O-MgO-O

526 528 530 532 536 538

PIII-4

PIII-2

PIII-1

Mg

46 48 50 52 54 56 58 60 62Binding energy (eV)

PIII-4PIII-2PIII-1

Mg

Inte

nsi

ty (

a.u

.)

11.24 Mg 2p and O 1s core level XPS spectra of untreated and oxygen PIII processed Mg: PIII-1, oxygen PIII at voltage 20 kV for 0.5 h; PIII-2, oxygen PIII at voltage 20 kV for 1 h; PIII-4, oxygen PIII at voltage 20 kV for 4 h. (Fukumoto, 2001).

of chloride ions, attack by aggressive chloride ions mitigates the protection efficiency of magnesium oxide. Gaseous Piii can only enhance the corrosion resistance to a certain extent. Among the gases, hydrogen implantation appears to be more effective. it can induce the formation of corrosion-resistant MgH2. nitrogen implantation and oxygen implantation result in the formation of non-corrosion-resistant Mg3n2 and MgO. The limited enhancement in the corrosion resistance is thought to arise from irradiation effects and surface compactness. The mechanical property changes after gas ion implantation are seldom covered in current publications

11.4.2 Metal PIII of magnesium alloys

it is well known that metal alloying elements affect both the mechanical properties and corrosion resistance of aluminium alloys. Many metal elements have therefore been implanted into magnesium alloys to achieve surface alloying. in conventional alloying processes, the contents of incorporated alloying elements depend on the solid solubility of elements in the substrate. However, in Piii, the amounts of alloying elements are not limited by solid solubility, leading to supersaturation of the alloying elements in the near surface. This process induces dramatic changes in the surface properties compared to conventional alloying processes. Metal Piii of Al, Ti, Zr, Ag and Ta have been studied. Aluminium is an important alloying element in magnesium alloys. The incorporation of Al not only enhances the mechanical properties, but poses great influence on the corrosion resistance. It is generally accepted that a higher content of Al will lead to better corrosion resistance. However, the sensitivity of stress corrosion increases with higher Al contents. Variations in the surface properties after Al implantation have been reported (Liu, 2007a; Lei, 2007). The Al content in the surface increases to about 30% after Al Piii, as shown in Fig. 11.25. The penetration depth of Al exceeds 100 nm. A high resolution Al 2p XPS spectrum (Fig. 11.25b) reveals that at depths below about 40 nm, Al is mainly in the form of Al2O3 and at depths between 40 and 60 nm, aluminium oxide and metallic Al co-exist. At depths over 60 nm, only metallic Al is present. Generally, Piii is not conducted at ultra high vacuum, and residual oxygen is implanted into the substrate at the same time. Formation of the aluminium oxide should be related to the incorporation of Al and implanted O. The corrosion resistance of Al-implanted AZ91 magnesium alloys has been evaluated by electrochemical impedance spectroscopy (EiS). The total impedance value of the Al-implanted sample, is about ten times higher than that of the untreated sample as shown in Fig. 11.26. The dramatically improved corrosion resistance is considered to result from the formation of a large amount of Al2O3. Al implantation can also

Co

nce

ntr

atio

n (

%)

80

60

40

20

0

Al

Mg

Zn

O

0 1 2 3 4 5Sputter time (min)

(a)

68 72 76 80 84Binding energy (eV)

(b)

12

7

6

4

3

0

Al2p

11.25 XPS depth profile and Al fine scanning spectra obtained from AZ91 magnesium alloys subjected Al PIII at bias voltage 10 kV for 4 h: sputtering rate ~ 20 nm/min referenced to SiO2 (Liu, 2007b).

AZ91Al-PIII&DTi-PIII&DZr-PIII&D

AZ91

100 150 200 250 Z¢/Ohm.cm2

Z≤/

Oh

m.c

m2

80604020

0–20–40

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Z¢ (Kohm.cm2)

Z≤

(Ko

hm

.cm

2 )

1.5

1.0

0.5

0.0

–0.5

–1.0

11.26 Electrochemical impedance spectra (EIS) of AZ91 magnesium alloy subjected Al, Ti and Zr PIII in SBF: Al-PIII&D, Al PIII at 10 kV for 4 h; Ti-PIII&D, Ti PIII at 10 kV for 4 h; Zr-PIII&D, Zr PIII at 10 kV for 4 h (Liu, 2007b).

enhance the wear resistance of magnesium alloy (Lei, 2007). Conventional ion beam implantation of Al via MEVVA (dose 2 ¥ 1016 ions/cm2) can reduce the wear rate by 30–40% compared to the untreated alloy. However, no such investigation upon Al Piii has been attempted, to our knowledge. Zr is also an important alloying element in magnesium alloys. Addition of Zr not only enhances the strength of magnesium alloys but also yields higher corrosion resistance. Liu et al. (2007a) have investigated the effects of Zr Piii on the corrosion resistance of AZ91 magnesium alloys. Similarly to Al PIII, Zr PIII produces a thick modified layer about 100 nm thick, as shown in Fig. 11.27. High resolution spectra also reveal the formation of ZrO at the near surface. Enhanced corrosion resistance is observed in the EiS measurement, as shown in Fig. 11.26. As Ti is a biologically friendly element, Liu has also tried to conduct Ti Piii on AZ31 magnesium alloy, to suppress the fast degradation rates in a physiological environment (Liu, 2007b). An electrochemical test in SBF also disclosed improved corrosion resistance to some extent, but it was not as effective as with Al Piii. in the depth profiles of a PIII-treated sample, it is noted that Al PIII and Zr PIII of AZ91 magnesium alloy both lead to much smaller contents of Mg in the modifier layers. Reduction in the active magnesium is also responsible for the improved corrosion resistance. it can be concluded that metal implantation into magnesium alloys is effective to improve both the corrosion resistance and surface mechanical properties. improvement in the corrosion resistance is often related to the formation of a corrosion resistant metal oxide and the dramatically reduced Mg content in the top surface layer.

Co

nce

ntr

atio

n (

%)

80

60

40

20

0

AlMgZnOZr

0 3 6 9 12 15 18Sputter time (min)

176 180 184 188 192Binding energy (eV)

Zr3d

19

17

14

11

10

8

6

3

0

19

14

11

10

8

3

0

(a) (b)

11.27 XPS depth profile and high resolution spectra of Zr 3d spectra of AZ91 magnesium alloy subject Zr PIII at a bias voltage 10 kV for 4 h: Sputtering rate 5.6 nm/min referenced to SiO2 (Liu, 2007b).

11.5 PIII surface modification of aluminium (Al) alloys

Al alloys have high strength-to-weight ratio and good formability, and have many applications in the aviation, space, as well as in automobile industries (Maddox, 2003; Wang, 1995). The corrosion resistance of Al-based alloys is generally considered to be good due to the native aluminium oxide surface layer. However, many of their applications are limited by the poor surface mechanical properties, such as low hardness and low wear resistance (see Chapter 1). Therefore, surface modification is conducted to enhance the surface mechanical properties. The most popular surface treatments are nitriding and PEO. Studies on Piii treatment of Al alloys are less common. Al2O3 has good tribological properties such as high hardness and wear resistance and a low friction coefficient. However, the naturally formed alumina layer on Al alloys is only several nanometer in thickness. Oxygen Piii has been carried out to fabricate a thicker aluminium oxide layer (Bolduc, 2003). The tribological properties of oxygen Piii-treated pure aluminium and alloyed aluminium (AA7075) are optimized by controlling the implantation dose. As shown in Fig. 11.28, the penetration depths of O at the bias voltage are around 90 nm, independent of implantation dose. The higher implantation dose results in increased O content across the modifier layer. It is noted that the depth profiles of O in the pure aluminium and AA7075 alloys are also similar for the same implantation dose. Evolution of the implantation dose may be observed up to near the saturation dose of 2.0 ¥ 1017 O/cm2 (triangles),

Implanted dose(1017O/cm2)

Pure Al AA70750.80 0.751.05 1.101.15 1.202.00 1.90

0 20 40 60 80 100Depth (nm)

O (

at.%

)

60

50

40

30

20

10

0

11.28 XPS depth profile of O pure Al and AA7075 subjected to oxygen PIII at low temperature with various dose (Bolduc, 2003).

which almost corresponds to stoichiometric Al2O3. SEM views of pure Al after oxygen Piii at the saturation dose are displayed in Fig. 11.29. Many nano-scale Al2O3 balls, about 20 nm in diameter, cover the whole surface. Oxygen Piii results in great changes in the surface mechanical properties, depending on the implantation dose. Changes in the surface mechanical properties of the two substrates as a function of implantation doses are demonstrated in Fig. 11.30. The highest hardness (about twice that of the untreated one) of treated AA7075 alloy occurs at a dose of 1.2 ¥ 1017 O/cm2, corresponding to 70–80% oxide. When the implantation dose increases to the saturation dose, the hardness dramatically drops, but it is still higher than that of the untreated. For pure Al, the hardness is independent of the implantation dose and is about twice that of the untreated one. The friction coefficients of both the pure Al and AA7075 alloy are about half of that of the untreated sample, and both are independent of implantation doses. The scratch depths observed from the oxygen Piii AA7075 alloy are reduced greatly at all loads, 30 mn, 20 mn, 10 mn. Diamond-like carbon films (DLC) possess many unique properties, such as ultra high hardness and low friction coefficient, and are desirable as coating materials (Hainsworth, 2007; Robertson, 2002). Piii&D is an attractive process to prepare adhesive DLC coatings on various substrates in order to impact their unique properties (Ensinger, 2006). Pre-implantation can generate a gradient layer that can release the stress resulting from the serious mismatch of mechanical properties between the DLC and substrate. High-voltage

11.29 SEM view of pure Al after oxygen implantation at a saturation dose 2 ¥ 1017 O/cm2 (Bolduc, 2003).

implantation between deposition cycles also benefits the growth of a compact and adhesive DLC coating. DLC coating has been successfully fabricated on pure aluminium via Piii&D (Xu, 2006) and the structural changes with processing parameters have been systematically investigated. The Raman spectra as a function of implantation voltage are shown in Fig. 11.31. The Raman spectrum of DLC is different from that of diamond C at 1332 cm–1

and that of single crystalline graphitic C at 1580 cm–1 (Liao, 2004a). The spectra can be deconvoluted into two bands identified as G and D, centered

Pure AlAA7075

Pure AlAA7075

AA707510 µN

20 µN

30 µN

0.0 0.8 1.0 1.2 1.4 1.6 1.8 2.0Dose (1017O/cm2)

(c)

(a)

(b)

Har

dn

ess

(GP

a)

Fric

tio

n

coef

fici

ent

Scr

atch

dep

th

(nm

)

5

4

3

2

1

01.0

0.8

0.6

0.4

0.2

0.0

120

100

80

60

40

20

0

11.30 Surface mechanical properties of pure Al and A7075 alloy subjected to implantation at a bias voltage 30 kV with various doses: Implantation is denoted as implantation dose (Bolduc, 2003).

at about 1545 and 1362 cm–1. The ratio of the G to D intensities (denoted as the integral area under the D and G bands) ID/IG is roughly proportional to the ratio of sp2/sp3 bonds. The ratio of sp2/sp3 (ID/IG) is one of the most important factors governing the quality of the DLC films. Generally, the lower the ratio, the closer the properties of the DLC films are to those of diamond. The spectrum of the sample produced at a bias voltage of 5 kV shows a broad and symmetrical band centered at 1560 cm–1, which differs from that of DLC films, and it corresponds to polymer-like carbon. The spectra at 10, 30 and 50 kV are all typical DLC films. It is obvious that in the range of 5–10 kV, there is an important voltage threshold value for the formation of the DLC film. The three DLC films are also different with each other according to the deconvoluted spectra. As the voltage increases from 10 to 50 kV, ID/IG shifts from 1.06 to 0.93 and then to 1.55. These changes correspond to the change of the ratio of sp2/sp3 resulting from the different ion bombardment energy. Below 30 kV, increased ion bombardment induces disorder in the C structure leading to increment of the sp3 bonds. However, when the ion energy is too high, the ions may damage the sp3 bonds. Therefore, there is an optimal energy favoring sp3 formation. The morphologies and roughness of the DLC fabricated at different

1200 1400 1600 1800Raman shift (cm–1)

(a)

1200 1400 1600 1800Raman shift (cm–1)

(c)

1200 1400 1600 1800Raman shift (cm–1)

(b)

1200 1400 1600 1800Raman shift (cm–1)

(b)

D

D D

G

G G

1.0

0.8

0.6

0.4

0.2

0.0

1.0

0.8

0.6

0.4

0.2

0.0

1.0

0.8

0.6

0.4

0.2

0.0

1.0

0.8

0.6

0.4

0.2

0.0

Inte

nsi

ty (

a.u

.)In

ten

sity

(a.

u.)

Inte

nsi

ty (

a.u

.)In

ten

sity

(a.

u.)

11.31 Raman spectra of the carbon films prepared for 3 h on Al as a function of implantation voltage: (a) 5 kV; (b) 10 kV; (c) 30 kV; (d) 50 kV (Liao, 2004b).

voltages also differ from each other (see Fig. 11.32). As the voltage increases, the morphologies become smoother and more compact. The changes in the morphology and roughness are related to the different ion bombardment effects at different energies. The hardness values of the fabricated DLC films are demonstrated in Fig. 11.33. These changes in the hardness is consistent with the change in ID/IG at different implantation voltages. The higher content of sp3 results in higher hardness. Piii has also been used as a pre-treatment process to generate a gradient layer for subsequent deposition of the coating (Knight, 1989; Liao, 2004b). C Piii and n Piii have been used to fabricate a gradient layer for subsequent deposition of DLC and Tin, respectively. This Piii pre-treatment process is found effective for enhancing the load-bearing capacity and bonding strength of the deposited coating. Studies on PIII surface modification of Al have not been extensive. This Piii process is more often used as a pre-treatment process or is combined with other techniques in deposition and nitriding. The development of effective hybrid treatments is promising for future studies and applications.

800.

00 n

M

800.

00 n

M

800.

00 n

M

800.

00 n

M

µM

µM

µM

µM

(a)

(c)

(b)

(d)

2 2

22

4 4

44

6 6

66

8 8

88

Ra: 26 nmRmax: 419 nm

Ra: 15 nmRmax: 249 nm

Ra: 19 nmRmax: 299 nm

Ra: 18 nmRmax: 204 nm

11.32 AFM morphologies and roughness of carbon films fabricated on Al substrate as a function of implantation voltage: (a) 5 kV; (b) 10 kV; (c) 30 kV; (d) 50 kV (Liao, 2004b).

11.6 Future trends

PIII is an important surface modification technique for the surface treatment of light alloys. The main advantage is its non-line-of-sight nature as compared to beamline ion beam implantation, allowing treatment of samples with complex geometries. In addition, the modified layers possess good bonding strength with the substrate, as there is no distinct interface between the modified layer and substrate. ion penetration is usually very shallow, typically ranging from tens of nm to more than 100 nm, depending on the nature of the substrate, implantation voltage, and substrate temperature. Processing parameters such as the implantation dose, implantation voltage, frequency, pulsing duration, and substrate temperature all affect the treatment. For Ti alloys, Piii can enhance the corrosion resistance and surface mechanical properties. However, the modified layer is usually too thin to satisfy the stringent requirements imposed by industry. Piii of biomedical Ti alloys to improve the biological performance, such as bioactivity and biocompatibility, is effective and promising. As the interactions between tissues and implants proceed at the interface, the functions of the generated functional layers are not limited by the thickness of the modified layers. Magnesium alloys are too active chemically, and the thin modified layer usually cannot provide satisfactory protection to the substrate. Therefore, in practical applications, PIII is not a good choice. In surface modification of Al alloys, hybrid treatments combining Piii and other techniques, such as deposition and nitriding, are promising applications. The choice of proper Piii techniques should consider the alloys and optimization of processing parameters such as the implantation dose, voltage, frequency, and pulsing duration, in order to achieve the best effects.

0 10 20 30 40 50 60Implanting voltage (kV)

HK

0.02

N (

GP

a)

7.5

7.0

6.5

6.0

11.33 Surface hardness of carbon films on Al as a function of implantation voltage (Liao, 2004b).

Development of PIII as an effective surface modification technique for light alloys in the future may fall in two categories, namely fabrication of a functional modified layer and development of hybrid PIII processes. The high energy of the implanted species benefits generation of various functional layers with unique properties, as well as good adhesion with the substrate. On the other hand, the development of Piii-based hybrid processes can generate much thicker modified layers with dramatically enhanced surface properties. Development of more effective Piii-based hybrid processes will lead to wider applications in surface modification of light alloys.

11.7 Sources of further information and advice

P. K. Chu, ‘improvement of Surface and Biological Properties of Biomaterials Using Plasma-based Technology’, in Trends in Biomaterials Research (iSBn 1-1-60021-361-8), P. J. Pannone (Ed.), nova Science Publishers, inc., new York, Chapter 2, pp. 81–108 (2007).

L. R. Shen and P. K. Chu, ‘improvement of Corrosion Resistance by Plasma Surface Modification’, in Trends in Corrosion Research, Vol. 3 (iSSn: 0972-4826), Marcel Dekker, india, pp. 41–57 (2004).

P. K. Chu and X. B. Tian, ‘Plasma-based Surface Modification’, in Surface Modification and Mechanisms: Friction, Stress, and Reaction Engineering (iSBn 0-8247-4872-7), G. E. Totten and H. Liang (Eds.), Marcel Dekker, new York, Chapter 15, pp. 543–601 (2004).

J. Matossian, G. Collins, P. K. Chu, C. Munson and J. Mantese, ‘Design of a Piii&D Processing Chamber’, in Handbook of Plasma Immersion Ion Implantation and Deposition (iSBn 0-471-24698-0), A. Anders (Ed.), John Wiley & Sons, new York, Chapter 6, pp. 343–379 (2000).

P. K. Chu, S. Qin, C. Chan, n. W. Chung and L. A. Larson, Plasma immersion ion implantation – a fledgling technique for semiconductor processing. Mater. Sci. Eng. R-Rep 1996; 17: 207–280.

M. A. Lieberman, A. G. Lichtenberg, Principle of Plasma Discharges and Materials Processing. Wiley, new York (1994).

11.8 ReferencesBakkar A and neubert V (2005), ‘improving corrosion resistance of magnesium-based

alloys by surface modification with hydrogen by electrochemical ion reduction (EIR) and by plasma immersion ion implantation (Piii)’, Corrosion Sci. 47, 1211–1225.

Bilek MMM, Evans P, McKenzie DR, McCulloch DG, Zreiqat H and Howlett CR (2000), ‘Metal ion implantation using a filtered cathodic vacuum arc’, J. Appl. Phys. 87, 4198–4204

Bolduc M, Terreault B, Reguer A, Shaffer E and St-Jacques RG (2003), ‘Selective modification of the tribological properties of aluminum through temperature and dose control in oxygen plasma source ion implantation’, J. Mater. Res. 18, 2779–2792

Chu PK, Qin S, Chan C, Chung nW and Larson LA (1996), ‘Plasma immersion ion implantation – a fledgling technique for semiconductor processing’, Mater. Sci. Eng. R 17, 207–280

Conrad JR, Radtke JL, Dodd RA, Worzata FJ and Tran nC (1987), ‘Plasma source ion-implantation technique for surface modification of metals’ J. Appl. Phys. 62, 4591–4596

da Silva LLG, Ueda M, Silva MM and Codaro En (2007), ‘Corrosion behavior of Ti-6Al-4V alloy treated by plasma immersion ion implantation process’ Surf. Coat. Technol. 201, 8136–8139

Ehlers KW and Leung KN (1979), ‘Some characteristics of tungsten filaments operated as cathodes in a gas-discharge’, Rev. Sci. Instrum. 50, 356–361

Ensinger W (2006), ‘Formation of diamond-like carbon films by plasma-based ion implantation and their characterization’, New Diam. Front. Carbon Technol. 16, 1–32

Fukumoto S, Yamamoto A, Terasawa M, Mitamura T and Tsubakino H (2001), ‘Microstructures and corrosion resistance of magnesium implanted with nitrogen ions’, Mater. Trans. 42, 1232–1236

Hainsworth SV and Uhure nJ (2007), ‘Diamond like carbon coatings for tribology: Production techniques, characterisation methods and applications’, Int. Mater. Rev. 52, 153–174

Han SH, Kim HD, Lee Y, Lee J and Kim SG (1996), ‘Plasma source ion implantation of nitrogen, carbon and oxygen into Ti-6Al-4V alloy’, Surf. Coat. Technol. 82, 270–276

Hort n, Huang YD and Kainer KU (2006), ‘intermetallics in magnesium alloys’, Adv. Eng. Mater. 8, 235–240

Jackson M and Dring K (2006), ‘Materials perspective – A review of advances in processing and metallurgy of titanium alloys’, Mater. Sci. Technol. 22, 881–887

Kaufmann H and Uggowitzer PJ (2001), ‘Fundamentals of the new rheocasting process for magnesium alloys’, Adv. Eng. Mater. 3, 963–967

Knight DS and White WB (1989), ‘Characterization of diamond films by Raman-spectroscopy’, J. Mater. Res. 4, 385–393

Kokubo T, Kim HM and Kawashita M (2003), ‘novel bioactive materials with different mechanical properties’, Biomaterials 24, 2161–2175

Kostov KG, Ueda M, Lepiensky A, Soares PC, Gomes GF, Silva MM and Reuther H (2004), ‘Surface modification of metal alloys by plasma immersion ion implantation and subsequent plasma nitriding’, Surf. Coat. Technol. 186, 204–208

Lacoste A, Bechu S, Arnal Y, Pelletier J, Vallee C, Gouttebaron R and Stoquert JP (2002), ‘Nitrogen profiles in materials implanted via plasma-based ion implantation’, Surf. Coat. Technol. 156, 125–130

Lafferty JM, ed. (1980), Vacuum Arcs – Theory and Application. Wiley, new YorkLei MK, Li P, Yang HG and Zhu XM (2007), ‘Wear and corrosion resistance of Al ion

implanted AZ31 magnesium alloy’, Surf. Coat. Technol. 201, 5182–5185Li LH, Poon RWY, Kwok SCH, Chu RK, Wu YQ and Zhang YH (2003), ‘Hybrid

evaporation: Glow discharge source for plasma immersion ion implantation’, Rev. Sci. Instrum. 74, 4301–4304

Liao JX, Xia LF, Sun MR, Liu WM, Xa T and Xue QJ (2004a), ‘The structure and tribological properties of gradient layers prepared by plasma-based ion implantation on 2024 Al alloy’, J. Phys. D – Appl. Phys. 37, 392–399

Liao J, Liu WM, Xu T and Xue QJ (2004b), ‘Characteristics of carbon films prepared by plasma-based ion implantation’, Carbon 42, 387–393

Lieberman MA and Lichtenberg AG (1994), Principle of Plasma Discharges and Materials Processing, new York, Wiley

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