ANALYSIS OF CORROSION FATIGUE FOR
COMMERCIALLY PURE TITANIUM USING
NITROGEN ION IMPLANTATION
NURDIN ALI
A thesis submitted in
fulfillment of the requirement for the award of the
Doctor of Philosophy
Faculty of Mechanical and Manufacturing Engineering
Universiti Tun Hussein Onn Malaysia
APRIL 2014
v
ABSTRACT
The objective of this research is to determine the corrosion fatigue behaviours for
commercially pure titanium (CpTi) using nitrogen ion implantation. A series of
studies was conducted to obtain the mechanical properties, corrosion resistance and
fatigue and corrosion fatigue behaviours and to develop model prediction of
corrosion fatigue life of nitrogen ion implanted CpTi (Nii-Ti). Initially, nine
specimens of CpTi were implanted nitrogen ion with the energy of 80, 100 and 115
keV and dose of 0.5x1017, 1.0x1017 and 2.0x1017ions/cm2 to characterize its surface
properties and to obtain corrosion resistance. The result shows that energy of 100
keV and dose of 2.0x1017ions/cm2 was the optimal implanted parameter. In the
second study, fatigue and corrosion fatigue test were performed to investigate the
fatigue and corrosion fatigue behaviours. The fatigue specimens were implanted with
the energy of 100 keV and dose of 2.0x1017ions/cm2. The fatigue tests were carried
out for Nii-Ti specimens in saline solution and for CpTi and Nii-Ti specimens in
laboratory air by means of axial loading condition at stress level between 240 and
320 MPa. The results were nitrogen ion implantation improved slightly the fatigue
life of CpTi and Nii-Ti with the fatigue strength of 250 MPa and 260 MPa,
respectively. Finally, the prediction of corrosion fatigue life was developed based on
corrosion pit growth law. The stress amplitudes of 250, 260 and 280 MPa were
selected to measure penetration rate of specimens at various elapsed times using
electrochemical method in saline solution, then established the empirical model. The
result shows that the expression fits the experimental data well. In conclusion, the
effects of nitrogen ion implantation on surface properties and adhesion strength of
nitride layers improved the fatigue and corrosion fatigue life of Nii-Ti.
vi
ABSTRAK
Objektif kajian ini ialah untuk menentukan tingkah laku lesu kakisan untuk titanium
komersil murni (CpTi) menggunakan nitrogen ion implantasi. Satu siri kajian telah
dijalankan untuk menentukan sifat-sifat mekanikal, rintangan kakisan, kelesuan dan
tingkah laku lesu kakisan dan membangunkan pengiraan daripada model hayat lesu
kakisan CpTi setelah ditanamkan ion nitrogen (Nii-Ti) Dalam kajian awal, sembilan
spesimen CpTi yang telah ditanamkan ion nitrogen dengan tenaga 80, 100 dan 115
keV dan dose 0.5x1017, 1.0x1017 dan 2.0x1017ions/cm2 untuk ciri-ciri sifat
permukaan dan rintangan kakisan. Hasil kajian menunjukkan bahawa tenaga 100
keV dan dose 2.0x1017ions/cm2 adalah parameter optimum penanaman itu. Dalam
kajian kedua , ujian lesu dan kakisan lesu telah dijalankan untuk melihat lesu dan
hayat lesu kakisan. Spesimen lesu telah pun ditanam ion nitrogen dengan tenaga 100
keV dan dose 2.0x1017ions/cm2. Ujian lesu dengan dijalankan bagi spesimen CpTi
dan Nii-Ti dalam persekitaran makmal dan ujian lesu kikisan untuk spesimen Nii-Ti
dalam larutan masin dengan keadaan beban paksi dalam julat tegasan antara 240
MPa. dan 320 MPa. Berdasarkan kajian ini didapati bahawa penanaman ion nitrogen
meningkat sedikit hayat lesu CpTi dan Nii-Ti dengan kekuatan lesu masing-masing
iaitu pada 250 MPa dan 260 MPa. Dalam kajian akhir terhadap angaran hayat
kakisan lesu telah dibangunkan berdasarkan hukum pertumbuhan kakisan lubang.
Amplitud tegasan 250 MPa, 260 MPa dan 280 MPa telah dipilih untuk melihat kadar
penusukan yang terjadi terhadap specimen Nii-Ti pada pelbagai masa berlaku
mengunakan kaedah elektrkoimia dalam larutan masin; kemudian menubuhkan
model empirik bagi anggaran hayat kakisan lesu. Keputusan kajian menunjukkan,
pengiraan dan ujikaji boleh dibandingkan dan bersesuian juga. Sebagai kesimpulan,
kesan penanaman ion nitrogen pada sifat-sifat permukaan dan ketegasan lekatan
lapisan nitrida boleh meningkatkan hayat lesu dan hayat lesu kakisan Nii-Ti.
vii
TABLE OF CONTENTS
TITLE
DECLARATION
DEDICATION
CHAPTER 1
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF FIGURES
xii
LIST OF TABLES
xvi
LIST OF SYMBOLS AND ABBREVIATIONS
xvii
LIST OF APPENDICES
xx
INTRODUCTION
1
1.1. Background of research
1
1.2. Problem statements
2
1.3. Objective
3
1.4. Scope of research
3
viii
CHAPTER 2
1.5. Contributions of research
4
1.6. Thesis organization
4
LITERATURE REVIEW
6
2.1
Titanium in biomedical applications
6
2.2
Mechanical Loading imposed on an implant device
2.3
in human body
9
Ion implantation technique for surface modification
11
2.3.1
Review of ion implantation technique
12
2.3.2
Theoretical prediction of penetration depth
15
2.3.3
Surface modification of CpTi by nitrogen
ion implantation
2.4
2.5
2.6
15
Corrosion effects associated with biological
environment
17
2.4.1
Corrosion forms of pure titanium
17
2.4.2
Effect of nitrogen ion implantation on
corrosion resistance of titanium
19
Fatigue and corrosion fatigue of metals
20
2.5.1
Mechanism of corrosion-fatigue
23
2.5.2
Stress -life approach in corrosion fatigue
23
2.5.3
Crack initiation in corrosion fatigue
25
2.5.4
Crack propagation in corrosion fatigue
28
Trends in fatigue of titanium in various
environment
31
2.6.1
32
Fatigue of titanium in laboratory air
ix
2.6.2
Corrosion fatigue of titanium in SBF
environment
2.7
Trends in corrosion fatigue modeling
34
2.8
Fatigue life prediction under various environment
35
2.8.1
Prediction of fatigue life under laboratory air
environment
2.8.2
2.9
36
Prediction of crack initiation life under
corrosion fatigue
CHAPTER 3
33
Summary of literature review
37
38
METHODOLOGY OF RESEARCH
39
3.1 Materials
39
3.2 Experimental procedures
41
3.2.1 Preparation of specimen for ion implantation
41
3.2.2 Nitrogen ion implantation process
43
3.2.3 Nitride phase observation
44
3.2.4 Surface hardness evaluation
45
3.2.5
47
Electrochemical studies
3.3 Tensile properties test and wear resistance evaluation
51
3.4 Fatigue testing in laboratory air and in saline solution
53
3.5 Prediction of corrosion fatigue life
58
3.6 Development of corrosion fatigue testing method
61
3.6.1
Corrosion chamber
3.6.2
Justification of fatigue test specimen
62
dimension
64
3.7 Summary of research methodology
66
x
CHAPTER 4
RESULTS AND DISCUSSION
67
4.1
67
4.2
Analysis of surface properties
4.1.1
Formation of nitride phase
67
4.1.2
Surface hardness assessment
74
4.1.3
Corrosion behaviour of Nii-Ti
77
4.1.4
Assessment of Nii-Ti surface properties
85
4.1.5
Tensile properties and wear resistance
87
Fatigue and corrosion fatigue of CpTi and Nii-Ti
4.2.1
Fatigue of CpTi and Nii-Ti
4.2.2
S-N curve of Nii-Ti in laboratory air and in
93
4.2.3
Formulation of stress-cycle relationship
95
4.2.4
Fracture surface analysis of failure specimen
96
4.2.5
Scheme of crack geometry at fracture
Corrosion fatigue behaviour of Nii-Ti
4.3.1
4.3.2
100
100
Corrosion behaviour of Nii-Ti during
corrosion fatigue
4.5
99
Characteristics of corrosion rate during
corrosion fatigue
4.4
92
saline solution
surface
4.3
92
Fatigue life prediction
105
108
4.4.1
Fatigue life prediction of Nii-Ti
108
4.4.2
Prediction of corrosion fatigue life of Nii-Ti
109
Summary of results and discussion
112
xi
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
114
5.1
Conclusions
114
5.2
Recommendations
116
REFERENCES
117
APPENDICES
129
VITA
156
xii
LIST OF FIGURES
FIGURE
2.1
TITLE
PAGE
Examples of titanium application: (a) artificial hip joint
(b) bone plate implant and (c) hip replacement
2.2
8
Schematic illustration of a cross-section of a deformed
metallic biomaterial surface showing the complex interactions
between the material’s surface and the physiologic
environment
11
2.3
Schematic of an ion implantation system
13
2.4
Schematic of the ion implantation process
13
2.5
Schematic view of the path of an individual ion in process of
ion implantation
2.6
Cross-sectional illustration of thin film and surface-modified
layer formation
2.7
14
17
Theoretical conditions of corrosion, immunity and passivation of
titanium
18
2.8
Schematic illustration of cyclic loading parameters
21
2.9
Schematic description of total fatigue life and its relevant
factors
22
2.10
Schematic description of total corrosion fatigue life
26
2.11
Corrosion pit formation mechanism and fatigue crack
initiation of CpTi in saline solution
27
2.12
Typical fatigue crack growth rate behaviour
30
2.13
S-N curve of CpTi of untreated and sandblasted samples
33
3.1
Flow chart of research methodology
40
3.2
Grinding and polishing machine for sample preparation
42
3.3
Form and shape of the specimen prior to preparation process
43
xiii
3.4
Cockcroft-Walton accelerator Type 200 keV/200 μA
44
3.5
X-Ray diffraction-BRUKER D8 ADVANCE
45
3.6
Vickers hardness test procedure
46
3.7
Hardness testing repetitions
47
3.8
Shimadzu Hardness Micro Vickers tester
47
3.9
Shape and form of specimen for electrochemical study
48
3.10
Electrochemical test set up
49
3.11
Apparatus of corrosion test WPG 100, WonATech
50
3.12
Dimension of wear test specimen
52
3.13
Pin-on Disc wear tester 20LE
53
3.14
Shape and dimension of the fatigue specimen according
to ASTM 1801-97
54
3.15
Machining procedure of fatigue specimen
55
3.16
Flow chart of the machining process of fatigue specimen.
56
3.17
Shimadzu SERVOPULSER 100 kN fatigue machine
57
3.18
Set up of corrosion fatigue testing
58
3.19
Apparatus set up for corrosion test of corrosion fatigue
specimen
59
3.20
Flowchart of corrosion fatigue behaviour modeling
60
3.21
Scanning Electron Microscope (SEM) JSM-61180LA
61
3.22
Drawing of the corrosion chamber
62
3.23
Corrosion chamber for current test (a) before installed
position and (b) in loading condition
3.24
Specimen with the diameter gage of 7 mm failed at the grid
section
3.25
63
64
Specimen failure due to buckling for stress amplitude of 280
MPa and 300 MPa.
65
3.26
An example of the specimen used for current study
65
3.27
Specimen of fatigue failure
65
xiv
4.1
Spectrograph for Nii-Ti with (a) 0.5x1017 ions/cm2 dose and
(b) 2.0x1017 ions/cm2 dose
4.2
69
XRD patterns for the original and implanted CpTi (a) at
different energies, (b) at different doses and (c) the
verification of unit cell formation
4.3
71
The projected range (Ri) of nitrogen ion implantation with
different energy: (a) energy of 80 keV (b) energy of 100 keV,
and (c) energy of 115 keV
73
4.4
Effect of beam energy on penetration depth
73
4.5
Surface hardness of CpTi and Nii-Ti, (a) effect of dose; and
(b) effect of energy on surface micro-hardness
4.6
Tafel polarization curves of CpTi and Nii-Ti in 3.5% NaCl at
(a) different doses and (b) different energies
4.7
80
Potentiodynamic polarization curves of CpTi and Nii-Ti in
saline solution at different energies and doses
4.8
77
84
Curve of hardness vs corrosion rate (a) in 3.5% NaCl
(b) in SBF solution
86
4.9
Microstructure of CpTi in original condition
88
4.10
Stress-strain relationships of CpTi and Nii-Ti
90
4.11
Wear resistance surface of CpTi and Nii-Ti with energy of
100 keV and dose 2.0x1017ions/cm2
4.12
S-N curves of CpTi and Nii-Ti in compare with the work of
Fleck & Eifler
4.13
94
Log-log plot of S-N curve for obtaining the Wöhler’s and
Basquin’s formula
4.15
93
S-N curve of CpTi and Nii-Ti in laboratory air and
in saline solution
4.14
91
96
SEM image of CpTi specimen tested at 260 MPa in laboratory
air showing (a) overview of the fracture surface, (b) specimen
surface crack initiation, (c) transgranular crack initiation and
propagation
97
xv
4.16
SEM image of Nii-Ti specimen tested at 280 MPa in
laboratory air showing (a) overview of the fracture surface (b)
fatigue-crack and propagation (c) transgranural crack
initiation and granular crack propagation
4.17
98
SEM image of Nii-Ti specimen tested at 280 MPa in saline
solution showing (a) overview of the fracture surface (b)
corrosion pit formation, fatigue-crack and propagation (c)
specimen surface corrosion
4.18
Scheme of the crack geometry at surface of Nii-Ti
4.19
Polarization curves of corrosion fatigue specimen tested at the
99
100
stress, (a) 250 MPa (b) 260 MPa and (c) 280 MPa.
102
4.20
Penetration rate as a function of time.
104
4.21
Variation of experimental constant as a function of stress
amplitude
105
4.22
Corrosion rate versus elapsed time in saline solution
106
4.23
Variation of Ecorr as a function of elapsed time
107
4.24
Variation of current density (icorr) as a function of elapsed
time
107
4.25
S-N curves of experimental and analytical data for Nii-Ti
108
4.26
S-N curves of corrosion fatigue initiation life of model and
corrosion fatigue life obtained by experimental work
4.27
111
S-N curves of predicted corrosion fatigue life (Nf) of model
and corrosion fatigue life (Nf) obtained by experimental work
112
xvi
LIST OF TABLES
TABLE
TITLE
PAGE
2.1
Physical properties of unalloyed titanium
9
3.1
Chemical composition of CpTi
41
3.2
Mechanical properties of CpTi
41
3.3
Experimental design of nitrogen ion implantation process
42
3.4
Simulated body fluid solution reagent
48
3.5
Comparison of mpy unit with equivalent metric-rate
expressions
51
3.6
Experimental design for fatigue and corrosion fatigue test
54
4.1
Width and peak of intensity XRD pattern for CpTi, TiN and
Ti2N
4.2
72
The projected range (Ri) and longitudinal straggle Ri at
different energies
74
4.3
Comparison of different method of hardness test
74
4.4
Micro-hardness distribution at different energies and doses
75
4.5
Corrosion behaviour of Nii-Ti in 3.5% NaCl
81
4.6
Corrosion behaviour of Nii-Ti in SBF solution
85
4.7
Material properties of CpTi and Nii-Ti
90
4.8
Fatigue life and corrosion fatigue life of CpTi and Nii-Ti in
the averaged number of cycles
95
4.9
The electrochemical properties of corrosion fatigue
103
4.10
Critical size of initiation crack
109
xvii
LIST OF SYMBOLS AND ABBREVIATIONS
A
-
Area
ASTM
-
American Society for Testing and Materials
Al
-
Aluminium
Al2O3
-
Aluminium Oxide/ Alumina
At%
-
Atomic Percentage
a.u.
-
Auxiliary unit
BATAN
-
Badan Tenaga Nuklir Nasional (National Nuclear
Energy Agency of Indonesia)
bcc
-
Body centred cubic
cm
-
Centimetre
C
-
Paris’s material constant
CE
-
Counter Electrode
CpTi
-
Commercially pure titanium
,
-
Fatigue crack growth rate
EW
-
Equivalent weight
Ecorr
-
Corrosion potential
f
-
Frequency
F
-
Corrosion degradation factor
fcc
-
Face-centred cubic
h
-
Hour
Hz
-
Hertz
HF
-
Hydrofluoric acid
HNO3
-
Nitric acid
H2O
-
Water
Icorr
-
Total anodic current
xviii
icorr
-
Corrosion current density
K
-
Stress Intensity Factor
k
-
Strength coefficient
Kc
-
Critical stress intensity factor
KIC
-
Fracture toughness
Kpc
-
Stress intensity factor at instance of crack initiation
Kt
-
Stress concentration factor
keV
-
Kilo electro Volt
K
-
Stress intensity factor range
ΔΚth
-
Fatigue threshold stress intensity factor range
l
-
Liter
m
-
Paris’s material constant
mV/s
-
Milivolt per second
mg
-
Miligram
mm
-
Milimetre
MPa
-
Mega Pascal
mpy
-
Mil per year
mV
-
Milivolt
n
-
Number of element
N
-
Number of cycle
2Nf
-
Reversals to failure
nA
-
nanoampere
NaCl
-
Sodium chloride
N2
-
Nitrogen
Nii-Ti
-
Nitrogen ion implanted Cp Ti
O2
-
Oxygen / Air
R
-
Stress ratio, (
RE
-
Reference Electrode
Sa
-
Stress amplitude
Scf
-
Fatigue strength in corrosive medium
Sf
-
Fatigue strength in laboratory air
Sm
-
Mean stress
S’f
-
Fatigue strength coefficient
xix
SBF
-
SEM/EDS
Simulated Body Fluid
Scanning Electron Microscopy/Energy Dispersion
Spectroscopic
-
Stress range (
Ti
-
Titanium
TiO2
-
Titanium Dioxide
TixN
-
Titanium Nitride
WE
-
Working Electrode
-
Plastic strain
-
Elastic strain
-
Epsilon
-
Fatigue strength
κ
-
Kappa
ϴ
-
Theta
α
-
Alpha
ρ
-
Rho (density)
γ
-
Gamma
-
Chi
μA
-
Microampere
μm
-
Micrometer
W
-
Atomic weight of the element
XRD
-
X Ray Diffraction
-
Geometry correction factor
ɛ
)
xx
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
Quality Certificate Sample
129
B
Calculation of Wear Resistance
130
C
Phase Diagram of Titanium (Ti) – Nitrogen (N)
133
D
X-ray Diffraction Patterns of CpTi and TiN
134
E
Calculation of Material Behaviour
140
F
Calculation of Material Exponents
149
G
Prediction of Corrosion Fatigue Life
151
H
Certificate of Nitrogen Ion Implantation
152
I
Nitrogen Ion Implantation of Fatigue Specimens
153
J
List of Publications
154
CHAPTER 1
INTRODUCTION
This chapter begins with the background of the research and important elaboration,
followed by problem statements, objective, scope, contributions of the research and
organization of the thesis.
1.1
Background of research
Metallic biomaterials are the most appropriate implant materials to replace failed
hard tissue at present. Stainless steels, cobalt based alloys, titanium and its alloys are
the three most used metals for biomaterials in fabrication of medical devices.
Recently, titanium and titanium alloys are getting much more attention as
biomaterials because they have high specific strength, low density, good resistance
to corrosion, moderate elastic module of 100-110 GPa (Leyens & Peters, 2003;
Majumda et al., 2008), no allergic problems and good biocompatibility (non-toxic and
not rejected by the human body) among other metallic biomaterials. They exhibit a
high corrosion resistance due to the formation of a stable passive layer (TiO2) on its
surface.
Therefore, they have been extensively used in the last several decades as
materials for dental implants, and medical devices. Nowadays, they are also
considered by medical engineering as the material of choice for medical application
(Elias et al., 2008; Niinomi, 1998; Van Noort, 1987), i.e. the prosthetics, internal
fixation, inner body devices and instrumentation.
Since significant benefit of titanium for patients, surgeons and engineers, the
use of the titanium would steadily increase in the near future. Besides the increasing
2
of titanium used as implant materials are due to the increase in the olders generation
or aging population worldwide, the trend toward more active lifestyles, and the
ability to control health care cost.
The population ratio of the aged people has grown rapidly, so the number of
the elder demanding and replacing failed tissue with artificial devices from
biomaterials is also increasing, particularly, the amount of usage of instruments for
replacing failed hard tissues such as artificial hip joints, and dental implants.
Pure titanium and Ti–6Al–4V are still the most widely used among the
titanium alloys where they meet a demand almost the market of titanium
biomaterials. Basically, they are developed as structural materials particularly for
aerospace structures (Niinomi, 2003; Luetjering & Williams, 2003).
Therefore, the development of titanium targeted for biomedical applications
are highly required. Accordingly, the research and development on titanium
composed of non-toxic elements were started (Silvaa et al., 2004), and are under
development which increase continuously. Although commercially pure titanium
(CpTi) exhibit several advantages as biomaterial, but its resistance to wear is lower
than Ti alloy. It is therefore necessary of surface treatment of commercially pure
Titanium (CpTi) to enhance the resistance to wear by ensuring no decline in
corrosion resistance.
Some surface processing, such as sandblasting, induces rough and
contaminated surfaces and it might be an increasing in risk to failure due to this
surface condition results in higher corrosion susceptibility. Electrochemical
investigations of the corrosion behaviour of CpTi and Ti alloys have always
demonstrated very good passivity condition of the surface. However, the study about
ensuring reliability of medical implant is still insufficient. Therefore, the prediction
of the corrosion fatigue life of pure titanium with nitrogen ion implanted surface as
the implant material would be a valuable contribution on ensuring the sustainability
of the implant devices.
1.2
Problem statements
Nitrogen ion implantation was introduced onto CpTi to modify the surface condition
for more reliable performance and to increase its surface resistance to wear and
corrosion. However, the problem with ion implantation is crystallographic damage,
3
produces implantation damage on the surface (Rautray et al., 2011) and point defect
in target crystal on impact resulting imperfection in lattice crystal such as vacancies
and interstitials known as point defects. Ion implantation introduces both a chemical
change in the target surface and a structure change in the crystal structure that could
be surface defect in the form of a void or micro crack those can cause premature
failure of the implanted device.
Several research works studied on corrosion behaviour of CpTi and Titanium
alloys (Fukumoto et al., 1999; Sundararajan, & Praunseis, 2004; Raman et al., 2005),
however more work are necessary in the area of fatigue as well as corrosion fatigue
for the nitrogen ion implanted CpTi (Vardiman & Kant, 1982).
This study focuses more comprehensive on the analysis of corrosion fatigue for
commercially pure titanium using nitrogen ion implantation. The nitrogen ion
implantation might create surface damage and lattice disorder in the near surface of
the material. Therefore, the nitrogen ion implanted CpTi, with surface damage, when
it is applied a cyclic stress in body fluid or saline solution environment could be the
potential factors caused the corrosion fatigue failure for the proposed biomedical
material. Intensive research is needed to prove the fatigue behaviour of CpTi after
implantation with nitrogen ion.
1.3 Objective
The objective of this research is to analyze corrosion fatigue behaviours for
commercially pure Titanium using nitrogen ion implantation. A series of studies was
conducted to achieve specific objectives as follows:
1. To obtain the optimum parameter of CpTi after nitrogen ion implantation.
2. To investigate the influence of the nitrogen ions implantation on fatigue and
corrosion fatigue properties.
3. To develop an empirical model based on experimental data for corrosion
fatigue life prediction.
1.4
Scope of research
The scopes of this research are as follow:
4
1. Introducing the nitrogen ion onto surface of CpTi by ion implantation method
to modify the surface hardness that can improve the wear and corrosion
resistance. Beam energy of 80, 100 and 115 keV and dose of 0.5x1017,
1.0x1017 and 2.0x1017 ions/cm2 are used as the variables proposed for
nitrogen ion implantation process.
2. Specimens that had been implanted with nitrogen ion were analyzed for the
formation of nitride phase. Surface hardness and corrosion rate of the Nii-Ti
specimens were tested to verify the optimal parameter of implantation
process. In addition, the tensile and wear resistance tests were also to be
carried out on the nitrogen ion implantation specimens.
3. The nitrogen ion implanted specimens were tested the fatigue in laboratory
air and in saline solution with the frequency of 10 or 20 Hertz and stress ratio,
R, of -1.
4. Based on the experimental results, the empirical model was developed for
estimation of corrosion fatigue life for CpTi using nitrogen ion implantation.
1.5
Contributions of research
Corresponding to the above objective, some important points could be expressed as
contribution to the knowledge and professional usage as well as industrial
application. There are three prominent contributions that can be provided from the
result of this research.
1. Improvements in corrosion resistance and mechanical properties for Ti used
biomedical applications have practical importance. Different energies at
different doses for implanting nitrogen ion are a good approach.
2. The use of nitrogen ion implantation is a good technique for the improvement
of fatigue strength of Ti base materials.
3. The penetration growth law for Nii-Ti was established for contribution to the
service life estimation of Ti base materials in acidic environments.
1.6
Thesis organization
The present thesis comprised of five chapters that were organized in order to address
the objectives referred to in section 1.3 which are:
5
•
Chapter 1: The description of research overview was discussed and the
investigations performed in this area was briefly reviewed. The knowledge gap
for significant corrosion behaviour and mechanical properties of CpTi with
surface modification is extracted from the state-of-art to define the research
objectives. The problem statements, research objective, scope of the research and
the research contributions are described. The overall contents of the thesis are
also summarized in this chapter.
•
Chapter 2: The basic theory to support the implementation of the whole research
is discussed in this chapter.
•
Chapter 3: The details of the experimental investigations are presented. The
properties of the CpTi, the fabrication process and equipment used in the research
activities are described. The loading set-up, experimental conditions and
measuring systems employed to collect the experimental data are explained.
•
Chapter 4: The achieved results of the research are presented and discussed
following the objective of the research. The most important findings are also
described.
•
Chapter 5: The conclusions derived from experimental and theoretical
investigations are presented. The future works as recomendations are also stated
in this chapter.
6
CHAPTER 2
LITERATURE REVIEW
The current research on the effect associated with nitrogen ion implanted
commercially pure titanium (Nii-Ti) in body fluid environment are important to
understand and keep up to date as it changes with the latest technology and materials.
Establishing a framework for the present study, the basic concept involved in surface
modification, corrosion and corrosion-fatigue are reviewed. Later, specific examples
are outlined to show basic trends found in the literature. Finally, empirical model of
fatigue-life time prediction was briefly described.
2.1
Titanium in biomedical applications
Titanium is found in the earth‟s crust at the level of about 0.63% by mass and it is the
seventh-most abundant element metal (73.8%) (Barksdale, 1968; Barbalace, 2006). It
is recovered from TiO2 which is rich with deposits of rutile and ilmenite, FeTiO3,
that are found on every continent (Luetjering & Williams, 2003). Since its discovery
in 1791, and up until Kroll‟s innovative process development in 1932, there had been
no practical methods to recover titanium metal from these ores because of its
prominent affinity for oxygen. Since the modern ore extraction, beneficiation and
chemical processes are discovered, then it is enabled the large-volume manufacturing
of high-grade TiO2. This compound became an important pigment for paints and
commercial products, and of titanium metal for the production of the CpTi grades,
titanium-based alloys and other alloys systems. The Dupont Company was the first to
produce titanium commercially in 1948. Today, the primary consumer of titanium is
7
the aerospace field, whereas the other market such as medicine, automotive are
gaining increased acceptance (Leyens & Peters, 2003).
Commercially pure titanium is unalloyed titanium. At service temperatures it
consists of 100% -hcp phase. As a single-phase material, its properties are controlled
by chemical elements (iron, oxygen and interstitial impurity elements) and grain size.
It is classified into Grades 1 through 4 depending on strength and allowable levels of
the chemical elements i.e. iron, carbon, nitrogen, and oxygen. CpTi ASTM Grade 2
has the yield strength of 275 MPa. (Luetjering & Williams, 2003 and Niinomi,
1998).
In the galvanic series of metals, titanium has a standard reduction potential of
-1.63 Volts which is close to aluminum of -1.662 Volts. Therefore, titanium is very
active in Electro-motif force (Emf) series about 1.2 V more active than iron with
standard electro potential of -0.44 (Winston & Uhlig, 2008). The excellent resistance
of titanium to general corrosion in most environments is well-known. This is the
result of stable protective surface film, which basically consists of TiO2. This thin
oxide film makes the titanium passive as long as the integrity of film is continuously
formed and maintained, generally caused by which most oxidizing environment. On
the other hand, titanium is not corrosion resistant under reducing condition, where
the protective nature of oxide film breaks down such as in sulfuric, hydrochloric and
phosphoric acid is not good (Luetjering & Williams, 2003).
Titanium (Ti) and its alloy have been widely used for medical implants or
fixtures, owing to their superior specific strength, corrosion resistance, and
biocompatibility. The use of such materials can be proposed in response to a need for
artificial hard tissue, because they have characteristics that are advantageous to
biomedical engineering purposes (Balazic et al., 2007; Jagielski et al., 2006; Liu et
al., 2004; Fukumoto et al., 2000; Niinomi, 1998). Figure 2.1(a)-(c) show examples
on usage of Titanium for artificial hip joint bone plate implant and hip replacement,
respectively.
8
(a)
(b)
Figure 2.1 Examples of titanium application: (a) artificial hip joint (Liu et.al. 2004), (b)
bone plate implant and (c) hip replacement
(source: http://www.supraalloys.com/medical-titanium.php)
The material properties of Ti and its alloy have been proven to be well accepted by
human tissues, compared to other metallic biomaterials (Liu et al., 2004; Jagielski et
al., 2006). Table 2.1 shows the physical properties of unalloyed titanium (Liu et al.,
2004). The unalloyed titanium presented in Table 2.1 is classified as high grade
titanium. Its ultimate strength and yield strength are almost equal to the titanium
alloys with ultimate strength of 860-965 MPa (Sun et al., 2001; Geetha et al., 2009).
The performance of biomedical implants relies on the biocompatibility,
corrosion behaviour, mechanical properties, formability, and availability of the
materials (Liu et al., 2004; Balazic et al., 2007). CpTi is now widely used for hard
9
tissue replacement due to the fact that its properties are suited for the needs of
medical applications, except wear resistance when used for artificial hip joints.
Table 2.1 Physical properties of unalloyed titanium
(Liu et al., 2004)
Properties
Atomic number
Number of electrons
Atomic weight (g/mol)
Equivalent Weight (EW)
Crystal structure
Alpha, hexagonal closely packed (hcp)
c(Ǻ)
a(Ǻ)
Beta, body centered cubic (bcc) a(Ǻ)
Density (g/cm3)
Coefficient of thermal expansion, α, at 20°C (K-1)
Thermal conductivity (W/(mK))
Melting temperature (°C)
Boiling temperature (estimated) (°C)
Transformation temperature (°C)
Electrical resistivity
High purity (μΩCm)
Commercial purity (μΩCm)
Values
22
2
47.90
23.95
4.6832±0.0004
2.9504±0.0004
3.28±0.003
4.54
8.4×10-6
19.2
1668
3260
882.5
42
55
The nature of CpTi is its inherent corrosion resistance, which is attributed to the
spontaneous formation of a strong passivation oxide layer. The extent of this
corrosion resistance dictates its biocompatibility and is suitable to be used in a
physiological saline solution (Pompe et al., 2004). The nature of CpTi is also
sufficient elasticity, which has become necessary for apt design of hip joint
replacements; elasticity is a decisive factor for hard tissue replacement. The
characteristics of Ti and its alloy are characterized by the inert nature within the
human body: immune from the attack of bodily fluids, compatible with bone growth,
strong, and flexible.
2.2
Mechanical loading imposed on an implant device in human body
An implant is often subjected to cyclic loading during daily activity of the human
body. It can also be chemically attacked by the body fluid medium, under certain
conditions. Among the mechanical and chemical parameters that can influence the
10
corrosion-fatigue behaviour of such material, cyclic stress's parameter in body fluid
(saline solution) is need to be highlighted and verified its effect on stress cycles
before the final failure of implant devices. Another factor that also assists the fatigue
failure is crack length of material implant. Therefore, fatigue and other mechanical
properties such as toughness and wear resistant of biomaterial structure in a living
environment need to be evaluated and improved in order to confidently use the
implants for a long period of time.
The complexity of the service conditions and loads encountered on devices
implanted in the human body is generally quite high. Depending on the activities of
the patient, implanted device experiences of both static loading in the form of body
weight and also dynamic in the form of cyclic stress during walking or running.
These stresses are actually far under fatigue limit of the material implanted, however,
the implanted device can fail before its life time due to the mechanical and chemical
loading of implanted device. Figure 2.2 shows a schematic picture of the crosssection of a deformed metallic biomaterial surface, surrounded in a physiological
environment (Teoh, 2000). The Figure 2.2 illustrates three distinguishable layers,
namely (1) the molecular absorbed layer, (2) the passive oxide film, and (3) the
deformed layer. The molecular absorbed layer is an aqueous sandwich layer of biological
components to establish a good bond between the host tissue and the biomaterial. The
passive oxide film is protective passive film consist of either metal oxides or
hydroxides and act as a burrier protecting the metal surface from the corrosive
environment (Kim, J. J. & Young, Y. M., 2013), and deformed layer is surface and
subsurface damage arising from a spherical indentor.
The implant device is introduced into a patient must have sufficient strength to
sustain and transmit the load actions resulting from joint and muscular forces. The
actual stress working on the material implant in human body depends on the body
weight where a value of 2.5 BW (Body Weight) seems to be reasonable in hip
implants design. Peak forces are considered to be equal to 2.5 BW. While for loading
frequency is taken 0.5 Hz with an assumption that we could walk 2 h per day with a
pace of one step per second. (Cicero et al., 2007).
11
Physiological environment
Cells
Damage zone
Cyclic stress
Contact body
Molecular absorbed
Layers
Passive oxide layer
Deformed layer
Intrinsic microstruture
Figure 2.2 Schematic illustration of a cross-section of a deformed metallic
biomaterial surface showing the complex interactions between the material‟s surface
and the physiologic environment (Teoh, 2000)
Another researcher (Mudali et.al. 2003) argued that mechanical forces imposed on
the implant as follows: (1) the load varies with the position in the walking cycle and
reaches a peak of about four times of body weight at hip and three times the body
weight at the knee. The frequency of loading and load cycles encountered over a
specific time period are also important. A past rate of walking corresponds to one to
two million steps per year. For an active person, the number of steps taken may be
two or three times more taken by a normal person (2) the human body is a harsh
environment for metals and alloys having to be in an oxygenated saline solution with
salt content of about 0.9%, at pH 7.4 and temperature of 37+/-1 (Mudali et al., 2003)
2.3
Ion implantation technique for surface modification
Surface modification is a technique used to change the physical, chemical, electrical
or mechanical properties of metallic material surface such as wear resistant,
corrosion behaviour, fatigue properties and of enhancement of biocompatibility
particularly subjected to medical application. Several established methods of surface
modification are employed: shot peening, plasma nitriding, plasma immersion ion
implantation (PIII), magnetoelectopolishing, chemical etching, anodizing, ion
implantation etc. Ion implantation technique will be discussed in detail in this
section.
12
2.3.1 Review of ion implantation technique
Ion Implantation is a potential enhancement method for modifying the surface
properties of materials by insertion of accelerated atoms, within the first atomic
layers of the component using a high technology approach. It is similar to a coating
process, but it does not involve the addition of a layer on the surface. Originally, this
technique was developed to produce controlled doping of semiconductors, and still
used extensively in that capacity today and now it had being used to modify or
change the material‟s near-surface chemical composition or defect state.
Consequently, there can be a distinct modifications to the near-surface microstructure
and chemical, physical and mechanical properties which for example can appear as
changes in corrosion behaviour, electronic properties, stiffness, hardness, wear
resistance, friction response (Al Jabbari et al., 2012), or other surface-regionsensitive mechanical properties such as fatigue and contact fracture toughness
(Tanaka et al., 1996; Nastasi & Mayer, 2006).
Using highly energetic beams of ions (positively charged atoms), this
technique is used to modify surface structure and chemistry of materials at low
temperature. The moderate heating associated with the process virtually eliminates
any risks of distortion or oxidation effects and mostly the operating process is
between 150oC and 200oC which depend on the level of the ion beam flux (Woolley,
1997). The process does not adversely affect component dimensions or bulk material
properties. It has been used extensively and successfully for studying the mechanism
under-laying the so called reactive element effect. In general it is found that
influence of the implanted element is applied as a coating or present as an alloy or
oxide dispersed addition.
on implantation is also a powerful method for modifying the near surface
properties of material. To cater for diverse research and application, the implanting
facilities must be flexible. For example, ion beams of elements are desirable, dose
can cover the range from 109 ions cm-2 to 1018 ions cm-2, In ion implantation, high
energy ions are generated in an ion accelerator and implanted into the alloy surface.
The penetration depths are typically in the order of 1-100 nm while the concentration
distribution has a maximum value of up to several tens of percent (Stroosnijder,
1998; Nastasi & Mayer, 2006). In addition, more recent investigations have reported
that ion implantation had improved fatigue and corrosion resistance in metallic
13
alloys, polymers and ceramics.
Currently, the technique is most commonly
employed to treat the surface of cutting and machining tools, moulds, casting dies,
alloys for nuclear reactors containers, food packaging materials, medical implants,
biocompatible materials, etc (Agarwal & Sahoo, 2000). Figure 2.3 and Figure 2.4
show schematic of an ion implantation system and ion implantation process,
respectively.
Figure 2.3 Schematic of an ion implantation system (Nastasi & Mayer, 2006)
Figure 2.4 Schematic of the ion implantation process (Denison et al., 2004)
Improvement of the surface hardness as well as corrosion resistance that yielded by
ion implantation technique is influenced by the implantation parameters involve
14
energy, dose or time. The two key parameters defining the final-implantation profile
are dose D (in ions/cm2) and energy, E, (in kilo electro Volt, keV). The dose is
related to the beam current, I, expressed by the following formula (Equation 2.1):
(Spitzlsperger, 2003; Nastasi & Mayer, 2006; Wen & Lo, 2007).
D(dose) 
I (nA)t ( s)
1.6 x10 19 qA(cm 2 )
(2.1)
where D denotes implantation dose, I represent the beam current (nA), t is beam time
(s), q is charge state of the ion, and A is defined as the striking area (cm2).
A significant advantage of ion implantation is that the treated surface is an
integral part of the work piece and does not suffer from possible adhesion problems
associated with coatings. The moderate heating associated with the process virtually
eliminates any risks of distortion or oxidation effects. The process does not adversely
affect component dimensions or bulk material properties. Ion implantation produces
no dimensional changes in the work piece (Woolley, 1997). Figure 2.5 illustrates the
path of an individual ion in ion implantation process.
Modified
region
Figure 2.5 Schematic view of the path of an individual ion in process
of ion implantation (Hirvonen & Sartwell, 1994).
15
2.3.2 Theoretical prediction of penetration depth
The penetration depth of the implanted ions depends on the ion weight or mass,
energy and on material substrate (Hunsperger, 2009). As the ions penetrate into
substrate of the specimen, they lose their energy due to the interaction with the
electrons and the atoms of the specimen. Their energy continuously decreases when
the ion is traveling deeper into the substrate.
The energies of ion implantation range from several hundred to several
million kilo electrons volt (keV), yielding in ion distributions with average depths
from < 10 nm to 10 µm. Doses range from 1011 ions/cm2 for threshold adjustment to
1018 ions/cm2 for buried dielectric formation. Theoretical calculation was made using
the simulation program Transport of ions in matter (TRIM)-Stopping and Range of
Ions in Matter (SRIM) simulation software providing an approach of the penetration
depth penetration. SRIM is a computer program used to calculate interaction of ions
with matter and the core of SRIM is a program Transport of ions in matter (TRIM).
This open source computer programs were developed by Ziegler et al., 1985 and are
being continuously upgraded with the major changes occurring approximately every
five years. SRIM is based on a Monte Carlo simulation method, namely the binary
collision approximation with a random selection of the impact parameter of the next
colliding ion. It needs the ion type and energy (in the range 10 keV - 2 GeV) and the
material of one or several target layers as the input parameters.
The predictions of the implantation depth in the near surface of specimens
were provided from that software, and calculated the depth of implanted ions using
following equation:
𝑥𝑖 = 𝑅𝑖 + 𝛿𝑅𝑖
(2.2)
where, 𝑥𝑖 is penetration depth of implanted ions, Ri is ion range and 𝜹Ri is the
longitudinal straggling (Nastasi & Mayer, 2006; Saryanto et al., 2009; Suzuki, 2010).
2.3.3 Surface modification of CpTi by nitrogen ion implantation
Surface modification methods such as anodic oxidation treatment, (Song et al., 2007)
sandblasting, (Jiang et al., 2006) carbide coating, (Velten et al., 2002) plasma
16
nitriding, (Kapczinski et al., 2003) electrochemical treatment (Guilherme, 2005) and
nitrogen ion implantation (Jagielski et al., 2006; Fukumoto et al., 2000; Shikha et al.,
2008; Arenas et al., 2000) have been proposed to increase corrosion-resistant and
wear-resistant of material. CpTi with modified surface has good mechanical and
chemical properties as well as biocompatibility in human body environment.
The most common ion used for application in metallurgical surface treatment
is nitrogen. When the nitrogen ions penetrate and diffuse the surface of the specimen,
some of them wedge micro cracks, some occupy lattice spaces in crystalline
structures, and some form compounds through the chemical reaction (i.e.; titanium
nitride: TiN; Ti2N), resulting a new lattice properties as inherent of face center cubic
(fcc) and tretragonal space lattice. It has been established that nitrogen implantation
into metals can alter their surface properties such as hardness, friction, wear and
corrosion resistant, etc.
Among the treatment options, as mentioned previously, the nitrogen ion
implantation technique is a good method to enhance passivity and to reduce the
corrosion rate. This is due to the fact that the formation of TiN and Ti2N phases elude
the migration of ions and stabilize the TiO2 film growth on the surface of titanium
(Arenas et al., 2000; Mudali et al., 2003). Nitrogen ion implantation can modify the
Ti surface to produce wear-resistant species such as nitrides (TiN; Ti2N) other than
TiO2 from the surface. The surface of nitrogen ions implanted Cp Ti appeared to
consist of a mixture of TiO2 and TiN/Ti2N or a Ti oxynitride as shown in Figure 2.6
(Liu et al., 2004). Figure 2.6 illustrates the cross-sectional view of thin film and
surface-modified layer formation for surface modification of titanium by ion
implantation technique. Surface modification had been performed at specific energies
to assess the formation of the nitride phase, surface hardness, wear resistance, and
corrosion behaviour in various doses (Sundararajan, & Praunseis, 2004; Arenas et
al., 2000; Fukumoto et al., 1999). Still, the mechanical properties, chemical
composition, and corrosion resistance for CpTi surface implanted nitrogen ions in
various doses and energy need to be verified for achieving good result from
combining parameter between energy and dose as optimal parameter of the ion
implantation process.
17
Figure 2.6 Cross-sectional illustrations of thin film and surface-modified
layer formation (Liu et al., 2004)
2.4
Corrosion effects associated with biological environment
A multidisciplinary approach is necessary in studying corrosion-fatigue that involves
chemistry, electrochemistry, mechanics and metallurgy. In this section, the
chemistry, electrochemistry and mechanics that cause corrosion related failures are
described. First, it reviewed the various forms of corrosion that lead to failure in pure
titanium related with chemistry or electrochemistry that caused them. Then, the
mechanisms responsible for the effects of corrosion fatigue are discussed.
2.4.1 Corrosion forms of pure titanium
Pourbaix (1966; 1974) demonstrated that a metal could react in one of four ways
when exposed to a corrosive solution. He proposed the diagrams which are called
Pourbaix Diagram by varying the electrode potential and pH of the solution. The
diagrams demonstrate the corrosion activity that is thermodynamically favored in a
given system. A Pourbaix diagram for titanium in water at 25oC is shown in Figure
2.7. The diagram shows conditions of corrosion, immunity and passivation of
titanium in dependency of oxidizing potential and pH.
The diagram consists of four areas that represent the ways that a metal can
react to a corrosive solution. He demonstrated that a metal could be immune (passive
TiH2 region) from chemical reaction (passive TiH2 region), shown active corrosion
(Ti+2region) displayed passivity due to formation of a protective oxide film (Ti02
18
region), or suffered from pitting corrosion due to localize breakdown of a passive
film. If the protective oxide film breaks down locally, then the surface metal is exposed to
the solution and released its ion forming the pit.
Figure 2.7 Theoretical conditions of corrosion, immunity and passivation of titanium
(Pourbaix (1966; 1974)
The above diagram shows the significant differences in corrosion behaviour resulting
from different properties of TiO2 film. This film is very chemically resistant and is
attacked by very few substances. The following reactions are the basic corrosion of
Titanium in aqueous environment (Equation 2.3 – 2.4):
Ti  Ti 2   2e  E o  1.63V
(2.3)
2 H   2e   H 2 and
H 2O  O2  4e   4OH 
(2.4)
19
The basic reaction of Titanium in water is (Equation 2.5 – 2.6):
Ti  2 H  Ti 2   H 2
(2.5)
Ti  4 H 2 O  Ti (OH ) 4  2 H 2
(2.6)
Ti (OH ) 4 is a passive film of Titanium by a direct electrochemical reaction.
Balakrishnan et al., (2008) observed the formation of passive film on titanium
surface in SBF solution with the presence of Ca, P, Ti and O elements. The surface
passive TiO2 layer reacts with SBF solution which can be illustrated in Equations 2.7
– 2.11 as follows (Liu et al., 2004):
TiO 2  OH   HTiO 3
(2.7)
Ti  3OH   Ti (OH )   4e 
(2.8)
Ti (OH ) 3  e   TiO2 .H 2O  1 / 2 H 2 
(2.9)
Ti (OH )3  OH   Ti (OH ) 4
(2.10)
TiO2 .nH 2O  OH   HTiO 3 .nH 2O
(2.11)
The Ti–OH groups formed on the surface from the above reaction are negatively
charged and have a chemical affinity for Ca2+ and Na+ ions in the Simulated Body
Fluid (SBF) solution.
2.4.2
Effect of nitrogen ion implantation on corrosion resistance of titanium
Besides the design of the joint replacement, material selection plays an important
role. Materials for human body implants must be biocompatible, corrosion resistant,
and strong and have sufficient elasticity (Pompe et al., 2004).
Because it is absolutely inert in the human body, immune to attack from
bodily fluids, compatible with bone growth and strong, and flexible, titanium is most
biocompatible of all metallic implant, i.e. Stainless Steel 316L, cobalt alloys (Nasab
& Hassan, 2010; Niinomi, 2008) . Biocompatibility and corrosion resistance of the Ti
metal are the result of passive TiO2 film of 2 to 6 nm thickness formed on the surface
of Ti (Balakrisnan et.al., 2008; Tamilselvi & Rajendra, 2006; Raman et al., 2005;
Dearnley et al., 2004; Arenas et al., 2000). The corrosion behaviour of Ti and its
20
alloys has been studied using certain types of biological media (Tamilselvi &
Rajendra, 2006; Raman et al., 2005; Dearnley et al., 2004).
The implantation of nitrogen ion can enhance the passivability and reduce the
corrosion kinetics of the alloy surface with increasing tendency for repassivation and
with a significant decrease in ion release rates (Gordin et.al., 2013), and thus can
enhance corrosion resistance. This improvement arises from the formation of
precipitates of TiN and Ti2N, which screen underlying titanium atoms, avoiding their
migration and stabilizing the growth of the oxide film (Arenas et al., 2000).
Introducing to commercially pure titanium, Nitrogen-ion implantation
showed an improvement in the electrochemical behaviour of the passive film. Doses
between 4x1016 and 7x1016 ion/cm2 is recommended for orthopedic applications. A
detrimental effect is shown by implantation with higher doses due to the formation of
nitride phases (Sundararajan & Praunseis, 2004; Kapczinski et al., 2003) that can
accumulate surface damage, thus leading to increasing the corrosion resistance and
nanohardness (Shikha et al., 2008).
2.5 Fatigue and corrosion fatigue of metals
Fatigue is a process of progressive localized permanent structure change occurring in
a material subjected to condition that produce fluctuating stress and strain at point
that may culminate in crack or complete fracture after a sufficient number of
fluctuations.
The load histories for a real components, structures and vehicle are quite
diverse, at one extreme, they may be rather simple and repetitive, at the other
extreme, may be completely random. The constant amplitude loading is used to
obtain material fatigue behaviours/properties for used in fatigue design. Figure 2.8
illustrates schematically the basic fatigue loading for constant amplitude loading
pattern. The load ration (R-ratio) is defined as the ratio of minimum to maximum
stress amplitude as shown in Equation 2.12.
𝑅 = 𝑆𝑎 𝑚𝑖𝑛 /𝑆𝑎 𝑚𝑎𝑥
(2.12)
Where: 𝑆𝑎 , 𝑆𝑚 𝑎𝑛𝑑 ∆𝑆 (𝑆𝑚𝑎𝑥 − 𝑆𝑚𝑖𝑛 ) denote stress amplitude, mean load and stress
range respectively and f denote the frequency of the cycle stress. The R-ratio value of
fully reversals is -1 (R=-1) which means that the mean stress, 𝑆𝑚 = 0 and 𝑆𝑎 = -𝑆𝑎 .
21
+𝑆𝑎
𝑁
−𝑆𝑎
Cycle (2 reversals)
𝑅=
𝑆𝑎
−𝑆𝑎
𝑆𝑚 = 0
=−1→Fully reversed
Figure 2.8 Schematic illustration of cyclic loading parameters
Most mechanical components and structures made of metal and alloy are subjected to
cyclic loading. Some those machine components such as super-heaters, propeller
shafts, turbines and pump elements, drilling equipment in petroleum industry
severely suffer from corrosion fatigue problem. Once cyclic loading occur in inert
environment, the structures or components suffer to fatigue failure. However, when
the components and structures subjected to cyclic loading and corrosive environment
even fresh water or atmospheric air, the Corrosion Fatigue “C-F” can occur (Ebara,
2007; Genel et al., 2000; Murtaza & Akid, 1996).
The first study of metal fatigue is believed to occur around 1829 by German
mining engineer Albert. The detailed research effort into metal fatigue was initiated
in 1842 following the railway accident in France. The cause of this accident was
traced to fatigue failure. A systematic investigation of fatigue failure was conducted
by Wöhler, during the period 1852-1869 in Berlin. His work led to characterization
of fatigue in term of Stress-life (S-N) curves and to the concept of fatigue „endurance
limit‟. Another well known fatigue researcher of this era was Fairbairn, (1864),
Geber, (1872), Goodman (1899), Ewing & Rosenhain (1900), Ewing and Humfrey
(1903). The development of metal fatigue research came in new era was begun by
Griffith (1921) and Paris et al., (1961). They applied the fracture mechanic concept
to solve fatigue problem of notch specimen. Later it will be described some detail
concepts of development in fatigue as well in corrosion fatigue.
In order to understand the fatigue mechanism, it is important to consider
various technical conditions that influence fatigue life and fatigue crack growth.
There are three technical conditions involved (a) material surface quality, (b) residual
22
stress and (c) environment effect. This understanding is necessary to analyze fatigue
properties of engineering structure in term of fatigue as a crack initiation process
(crack initiation period) followed by a crack growth period as shown in Figure 2.9.
Figure 2.9 Schematic description of total fatigue life and its relevant factors
(Schijve, 2009)
Corrosion fatigue is the metal cracking caused by combined action of a cyclic
loading and a corrosive environment. The severity of the action depends on the range
and frequency of the stress, the nature of the corroding condition and the time under
stress (Murtaza & Akid, 1996; Sivaprasad et al., 2006). So, corrosion fatigue is
influenced by various mechanical, chemical and structural parameters that interact
locally.
Corrosion fatigue is similar to stress corrosion cracking in many aspects. The
principal difference between these two types of environment enhanced cracking is in
the character of loading, which is static in stress corrosion cracking and repeated
loading in corrosion fatigue. Both fatigue life and fatigue limit are reduced in the
presence of corrosive environment as compared to the fatigue in neutral
environment. These are caused by interaction of electrochemical, metallurgical and
mechanical processes at the crack tip (Ramsamooj & Shugar, 2001a).
The aim of the above review is to describe some phenomenological
observation of corrosion fatigue failure of mechanical components and structures and
method of evaluation. In addition, some results proposed by previous researcher in
term of stress-life, strain-life and crack initiation and crack growth rate-stress
intensity range will be presented.
23
2.5.1 Mechanism of corrosion-fatigue
Under cyclic loading conditions, the embrittling environment can accelerate the
initiation of a surface flaw in an initially crack-free material and propagate the flaw
to certain critical size. Corrosion fatigue is a term which is commonly used to denote
the damage and failure of material under the combination action of cyclic stress and
any embrittling medium, although most wide spread adaptation is in the context of
aqueous environments. The corrosive environment produces corrosion products.
Corrosion fatigue is associated with two different mechanisms: Anodic dissolution
mechanism of corrosion fatigue and hydrogen assisted corrosion fatigue (Ramsamooj
& Shugar, 2001a; Marcus, 2002).
The mechanism of anodic dissolution cracks initiate at the surface sites of
localized concentration of tensile strength. A crack progresses along a specific path
which is composed of specific crystal planes within the grains. The mechanism of
anodic dissolution is mainly referred to as corrosion fatigue of carbon steels and
Stainless steels (Makhlouf et al., 2003) in water and also corrosion fatigue of
Aluminum alloys (Chlistovsky et al., 2007) and Titanium alloys in aqueous chloride
solutions. Genel et al., (2000) reported that cathodic polarization suppressed the
metal dissolution and pit formation, resulting in a noticeable increase in corrosion
fatigue strength up to 2.6 times that of free corrosion fatigue. In contrast to anodic
dissolution mechanism, hydrogen assisted corrosion fatigue is enhanced by cathodic
reaction: 2H+ + 2e- = H2 occurring on the crack tip surface. The atomic hydrogen
dissolves in the metal where its ions interact with the dislocations of the crystal
lattice causing decrease of the metal ductility (Suresh, 1998).
2.5.2 Stress -life approach in corrosion fatigue
Corrosion fatigue in aqueous media is an electrochemical behaviour. Cracks are
initiated either by pitting or at persistent slip bands “PSB” (Xie et al., 2002).
Corrosion fatigue can hence be reduced by alloy additions, inhibition, and cathodic
protection all of which reduce pitting (Congleton & Craig, 1982). Since corrosion
fatigue cracks initiate at the metal surface, surface treatments like plating, cladding,
nitriding (Genel et al., 2000) and shot-peening or sandblasting (Jiang et al., 2006)
were found to improve the materials' resistance to this phenomenon.
24
The important concept of stress-life was proposed by Wöhler. The method
characterizes the total fatigue life in terms of nominal stress amplitude and cyclic
number (S-N) curve (Stephens et al., 2001). The total fatigue is defined as
accumulation crack initiation, short crack, long crack and critical fracture or final
failure (Kaynak et al., 1996; De-Guang et al., 1998). The fatigue life equation is
written as follows (Equation 2.13):
S a     log N f
(2.13)
where Sa is stress amplitude, α and β are constants and Nf is number of cycle.
Basquin modified Wöhler‟s formula in term of correlation log-log scale, a linear and
power law relationship are commonly observed, as shown in Equation 2.14 and
Equation 2.15. The stress-life curve characterizes the contribution of crack initiation
and crack propagation processes to total fatigue life in nominally smooth specimen.
ln S a     ln N f
(2.14)
S a
 S a  S ' f (2 N f ) b
2
(2.15)
or
where 𝑆𝑎 is stress amplitude, 𝑁𝑓 is number of cycle and α and β are constants, 𝑆′𝑓 is
fatigue strength coefficient and 2𝑁𝑓 is reversals to failure and b is fatigue strength
exponent (Basquin‟s exponent).
A new approach on the study of early stages of corrosion fatigue cracking
was proposed by Acun‟a-Gonza‟lez et al., (2008). A visual recurrence analysis
applied to the electrochemical current oscillations registered during corrosion fatigue
tests allowed us to characterize the electrochemical dynamics on stainless steel
samples surface showing clearly the dynamics of localized corrosion, as well as the
formation and initial growth of short corrosion fatigue cracks.
Experimental study of corrosion fatigue behaviour welded steel structures has
found that the fatigue crack propagation in the corrosive medium is influenced
strongly by important weld-geometry parameter and accordingly, Paris‟s material
constant need to be determine experimentally to evaluate the corrosion fatigue life
(Wahab & Sakano, 2001). Ramsamooj & Shugar (2001b) proposed a new model for
117
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