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Chapter 3
Corrosion and wear behavior of the laser nitrided
biomedical titanium and its alloys
3.1 INTRODUCTION
Inspite of the fact that titanium and its alloys have several attractive properties,
which enables one to use these materials for various biomedical applications, poor
wear resistance of these materials prevent their usage for the load bearing applications
such as ball of the hip implants and femoral and tibial component of the knee implants
as has been reported by several workers (Dong et al; 1999, Marc Long et al; 1998,
Geetha et al; 2009). Currently, implants with high wear and corrosion resistance and
enhanced biocompatibility are required to serve for a longer period (more than 15
years) when implanted especially in younger patients.
In order to improve the wear resistance of titanium and its alloys, several
surface modification techniques such as solid state nitriding chemical vapor
deposition (CVD), plasma nitriding and physical vapor deposition (PVD), ion
implantation have been used (Thair et al; 2002, Garcia et al; 1998, Sathish et al;2010,
Nolan et al;2006, Leitao et al; 2000). In CVD and plasma nitriding technique, one of
the serious disadvantages is that the whole work piece is to be heated in order to
perform nitriding, while PVD is a line of sight technique and it is difficult to coat
internal parts. The disadvantage in ion implantation and CVD is that the depth of the
coatings or the hardening layer is restricted by the diffusivity of the nitrogen into the
substrate resulting in a thin layer of surface modified zone. The failure of the nitrided
implants in clinical use due to the formation of a thin layer of modified zone is well
documented by Melinda et al, (Melinda et al; 1997, Harman et al, 1999).These
drawbacks can be overcome by nitriding the titanium surface in the molten state using
laser technology, which is termed as laser surface nitriding which has been performed
by many groups (Mridha et al; 1994, Weerasinghe et al; 1996, Fengjiu Sun et al;
2005, Roy et al; 2001, Geetha et al; 2004).The advantages of this method are low
processing time, coating on complex shapes and the ease with which thicker modified
layer can be obtained. Nitrogen concentration in the surface region can be varied
51
during laser gas assisted processing (Yilbas et al; 2006) and increasing the nitrogen
concentration in the melt layer also improves the surface hardness of the substrate
material by TiN formation (Ettaqui et al; 1997). The amount of TiN formed on the
surface is determined by the factors such as the type of metal and alloy composition,
environment, scanning speed, substrate surface morphology and preheat treatment
conditions. Considerable studies have been carried out to optimize the laser gas
assisted nitriding process to obtain surfaces with high hardness and without cracks
and porosity. Nitriding is carried out at either in argon or helium environment to
minimize the oxidation reactions in the melt pool during laser irradiation of the
surface and dilute the concentration of the nitrogen to avoid crack formations
(Abboud et al; 2008, Peter Schaaf, 2002, Ignatiev et al, 1993, Xin et al, 1996). The
formation of TiN, martensite and alpha needles with nitrogen were found to improve
significantly the wear properties of the titanium surface (Gerders et al; 1995). Several
studies on wear behavior of the laser nitrided surfaces have revealed that this process
enhances the wear resistance of Cp titanium, TiAl and Ti-6Al-4V alloys several fold
(Ettaqui et al; 1997, Perez et al; 2006, Jiang et al; 2000).
Ti-13Nb-13Zr alloy is considered to be a better alternative to Ti-6Al-4V
biomedical grade, as the former exhibits lower modulus of elasticity which is close to
bone and consists of non toxic alloying elements. Several studies have compared the
corrosion behavior of this alloy with the standard materials such as Ti-6Al-4V, 316
stainless steel etc and it has been found that Ti-13Nb-13Zr alloy exhibits enhanced
corrosion resistance in simulated body conditions when compared to other alloys
(Khan et al; 1999). Further, Ti-13Nb-13Zr alloy possesses low wear resistance and
also it exhibits very high corrosion under wearing conditions and hence it is necessary
to enhance the wear resistance of this alloy by suitable methods so that they can be
used as bio implants (Khan et al;1999, Majumdar et al; 2008). Geetha et al have tried
laser nitriding on Ti-13Nb-13Zr alloy at low scanning speed and reported that the
nitrided alloy exhibited better corrosion resistance in simulated body conditions when
compared to the bare substrate (Geetha et al; 2004). However, laser nitriding of Ti-
13Nb-13Zr alloy at low scanning speed resulted in dense dendrites consisting of large
amounts of TiN and some cracks. There are only meager reports on the surface
modification and wear characteristics of this alloy (Kovacs et al; 1993, Mishra et al;
1993, Johansson et al; 2004) and this motivated us to undertake the present studies.
52
The objective of this work is to improve the hardness and wear resistance of
Ti-13Nb-13Zr alloy by subjecting it to laser nitriding process. The challenge which
lies with laser nitriding is to obtain crack free hard surface and it is found that it is
extremely difficult to optimize the processing conditions for laser nitriding as several
parameters have to be considered simultaneously. Amongst the various parameters, it
is well documented that scanning speed has a profound influence on the hardness and
crack formation. Mirdha et al found that the scanning speed has to be increased to
produce crack free hard surface when nitriding commercially pure titanium (Mirdha et
al; 1991). This chapter deals with the laser nitriding of commercially pure titanium
and Ti-13Nb-13Zr alloy performed at high scanning speed and the wear, corrosion as
well as the microstructural aspects. The corrosion and wear behavior of the specimens
were evaluated in Hank‟s solution in order to determine their performance in
simulated body conditions.
3.2 EXPERIMENTAL TECHNIQUES
3.2.1 ALLOY PREPARATION AND LASER NITRIDING
Commercially pure titanium and Ti-13Nb-13Zr alloy were melted using
nonconsumable vacuum arc technique and the Ti-13Nb-13Zr was hot rolled at 680°C
below the beta transus temperature (Tβ=735º C) at Defence Metallurgical Research
Laboratory, India. The composition of the Cp titanium and Ti1313 alloy are given in
Tables 3.1 and 3.2. The Commercially pure titanium and Ti-13Nb-13Zr alloy will
hereafter be referred as Cp Ti and Ti1313. The samples were shot blasted to enhance
the energy absorption. Nd: YAG pulsed laser, operating at a wavelength of 1.06 m
was used for nitriding the samples. As the studies carried out on laser nitriding of
Ti1313 alloy by Geetha et al. in dilute nitrogen environment (0.2 Pa N2/ Ar
atmosphere (partial pressure of N2 in the mixed gas)) at low scanning speed did not
reveal any prominent TiN peaks in XRD, in the current work, the Cp Ti and Ti1313
alloy were nitrided in pure nitrogen environment at high scanning speed. The details
of the processing parameters are given in Table 3.3.
53
Table 3.1 Composition of Cp Ti
Element Wt%
C 0.1
Fe 0.3
H 0.015
N 0.03
O 0.25
Ti Balance
Table 3.2 Composition of Ti-13Nb-13Zr alloy (Hot rolled)
Alloy
Designation Ti Wt % Nb Wt % Zr Wt%
Interstitials Wt%
O N C
Ti-13Nb-13Zr Balance 13.6 13.7 0.13 0.007 0.018
Table 3.3 Deposition Parameters of Nd: YAG Solid State Laser
Lasing Specifications Ti-13Nb-13Zr alloy* Cp titanium and
Ti-13Nb-13Zr alloy
Laser Wavelength (m) 1.06 1.06
Laser Spot Size (mm) 1.2 1.2
Laser pulse width (ms) 4 4
Laser Repetitive rate
(Hertz) 30 30
Laser Power (Watts) 100 100
Ambient pressure (Pa) (a) 0.2 Pa N2/Ar
(b) 1 Pa N2 1 Pa N2
Traverse speed ( mm/min) 600 720
* [Geetha et al, 2004]
3.2.2 MICROSTRUCTURAL ANALYSIS
Microstructural characterizations were carried out along X-Y and Y-Z sections
using optical microscope (Carl Zeiss) and scanning electron microscope (SEM). The
54
laser-melted samples were sectioned along its melt direction, polished using standard
metallographic procedure and etched using Kroll‟s reagent (5ml HF (40%) + 10ml
HNO3 (69-72%) + 85ml distilled water). The surface appearance, melt pool
configuration and microstructures were examined using Zeiss optical microscope and
JEOL JSM -6360 scanning electron microscope.
3.2.3 HARDNESS AND SURFACE ROUGHNESS
The micro hardness values of the laser nitrided specimens were measured
using micro Vickers hardness tester with a load of 200 g along the cross section as
this is the scale commonly used for hard coatings. The hardness was measured along
the nitrided cross section (X-axis) as well from the surface to the bulk (along Y-axis).
Measurements were repeated five times for every specimen in order to obtain the
average micro hardness value and the hardness profile along melt direction is
presented in the later section. The average surface roughness (Ra) was measured using
Mitutoyo surf test-211 profilometer.
3.2.4 X-RAY DIFFRACTION ANALYSIS
The Philips 3121 X-ray diffractometer using Cu K radiation was set at 40 kV
and 20 mA for the XRD analysis and the data were collected for the 2 ranging from
20 to 90 in steps of 1/min.
3.2.5 CORROSION TESTING
Potentiodynamic polarization measurements were carried out on both un-
nitrided and laser nitrided Cp Ti and Ti1313 alloy in Hank‟s solution. The solution
was made up to one litre by adding distilled water and the solution was filled in
corrosion cell. The samples were mechanically polished up to 1000 grit SiC paper and
then rinsed with distilled water before subjecting to the corrosion studies. The sample
was then placed in teflon holder which consisted of a 6 mm diameter window and the
sample was exposed to test solution through this window.
For performing the electrochemical measurements, a conventional three-
electrode system was utilized. The sample to be tested was considered as the working
electrode (10mmx5mmx0.5mm), platinum foil as the counter electrode and saturated
55
calomel electrode (SCE) as the reference electrode. The corrosion testing was
performed using Potentiostat (Gill A.C, ACM make). Just before conducting the
polarization studies, Open Circuit Potential (OCP)–time measurements were carried
out for one hour to achieve a steady open-circuit potential, which was measured as the
corrosion potential Ecorr. When the specimen attained a constant potential after one
hour, potentiodynamic polarization was started from an initial potential of 250 mV
below the corrosion potential, Ecorr. The scan rate used was 0.166 mV/s as per ASTM
F2129 Standards. In order to verify the reproducibility of the data, the experiments
were repeated three times by exposing the various regions of the polished samples.
3.2.6 WEAR TESTING
The wear behavior of the laser nitrided and unnitrided CpTi and Ti1313
samples was studied in Hank‟s solution, at 37 1 C for 105 cycles. The wear rate was
calculated by measuring the weight loss after the wear testing using an electronic
weighing balance of 0.0001 g of accuracy. Three dimensional wear plots (sliding
distance vs stoke length vs friction coefficient) were also constructed using the data
collected during the wear test. In order to understand the wear mechanisms underlying
during various stages of wear testing, one sample was subjected to intermittent wear
studies. Among the above two materials studied, laser nitrided Ti1313 alloy was taken
as a test case and the weight loss of this sample was measured after every 20,000
cycles to examine the various stages of wear in a greater detail. As after 80,000
cycles, the nitrided zone was completely removed and predominant wear started to
occur in the base substrate, the experiment was terminated at this point.
3.3 RESULTS AND DISCUSSION
The observations made on both the unnitrided and laser nitrided samples using
OM, SEM and XRD are discussed in detail in the following sections.
3.3.1 SURFACE APPEARANCE, ROUGHNESS AND CRACKS
The surface morphologies of the Cp Ti and Ti1313 alloy laser nitrided at pure
nitrogen environment are shown in Figure 3.1 (a-c) and the surface roughness values
for the treated and untreated samples are reported in Table 3.4. The nitrided surfaces
56
of all the samples appeared to be shiny gold in color and were very rough with ridges
and troughs. The roughness values of the samples processed at high scanning speed
(720 mm/min) were found to be lower when compared with that of the samples
processed at low scanning speed (600 mm/min) (Geetha et al; 2004). The variation in
the roughness values is attributed to the changes with respect to the scanning speed
and nitrogen content (Mridha et al; 1998).
(a) (b)
(c)
Figure 3.1 Micrographs of the surface of the laser nitrided (a) & (b) Cp Ti and
(c) Ti1313 alloy
Ripples formed during laser processing Enlarged view of ripples
57
(a) (b)
Figure 3.2 Surface morphologies of (a) laser nitrided Cp Ti and (b) laser nitrided
Ti1313 alloy at high scanning speed
Minute cracks were observed in laser nitrided Cp Ti processed at high
scanning speed. In spite of the fact that Cp Ti and Ti1313 alloy were nitrided using
the same scanning speed, the formation of minute cracks found in Cp Ti (Figure 3.2
(a)) is attributed to the formation of considerably high amount of dendrites.
Table 3.4 Surface roughness of the laser nitrided alloys
Material
Pure nitrogen
environment.
(600mm/min)
(micron) *
Dilute nitrogen
environment.
(600mm/min)
(micron) *
Pure nitrogen
environment.
(720 mm/min)
(micron)
Ti-13Nb-13Zr 9.77 7.11 4.04
Cp titanium - - 3.92
* Geetha et al, 2004
Micro cracks Pores
58
3.3.2 MICRO HARDNESS AND PHASE ANALYSIS OF THE LASER
NITRIDED ALLOYS
There was a substantial increase in the hardness of the laser nitrided samples
when compared to the unnitrided samples. However, laser nitriding of Cp Ti led to
substantial increase in the hardness (784 ± 3 HV) (7.84±0.03 GPa) when compared to
Ti1313 alloy (390 ± 5HV)(390 ± 0.05 GPa ) (Figure 3.3). The high hardness of laser
nitrided Cp Ti is due to the formation of the columnar dendritic microstructure that
results from the unidirectional solidification whereas, the marginal improvement in
the hardness of laser nitrided Ti1313 alloy is due to the presence of scattered dendrites
present in the nitrided zone. This marginal increase in hardness clearly indicates that
less amount of TiN is formed under the present processing conditions. Inspite of the
fact that the hardness achieved in laser nitrided Ti1313 alloy was low, the hardness
values measured at various regions on the laser nitrided surface of Ti1313 alloy was
nearly the same and this is contradictory to the results of Geetha et al. where a large
variation in the hardness values were obtained for the nitrided surface that was
processed in pure nitrogen environment (Geetha et al; 2004). The low scanning speed
selected in their studies should have resulted in the formation of dense dendrites
consisting of large amount of TiN along with lesser amount of ZrN should have led
to high hardness. Hence, it is obvious that in the present study, high scanning speed
has resulted in less interaction time for nitrogen gas and titanium and subsequent
reduction in TiN formation and smaller melt pool depth. However, the marginal
increase in hardness as observed in figure 3.3 indicates that some nitrogen has
diffused into the titanium matrix and marginally improved the hardness and this is
further confirmed by the presence of the peak corresponding to TiN in the XRD
analysis. Thus it is clearly evident that the current scanning speed is not suitable for
enhancing the hardness of Ti1313 alloy, while it is highly suitable for Cp Ti as it has
resulted in considerable increase in the hardness
59
Figure 3.3 Microhardness profile of laser nitrided Cp Ti and Ti1313 alloy at high
scanning speed
The X-ray diffraction analysis of the laser nitrided Ti1313 alloy clearly
revealed the formation of TiN and α phases and with only one peak corresponding to
ZrN and TiN0.3. (Figure 3.4). Smaller interaction time and lesser quantity of nitrogen
in the melt favor the formation of substochiometric TiN0.3 phase rather than TiN. In
laser nitrided Cp Ti, the peaks corresponding to TiN are sharp and intense than those
of α Ti peaks. Oxide peaks were not observed in the X-ray diffraction analysis of
either of the samples.
Depth (µm)
Hard
nes
s (H
V)
60
Figure 3.4 XRD spectrum of un nitrided and laser nitrided Cp Ti and
Ti1313 alloy
3.3.3 MICROSTRUCTURE OF LASER NITRIDED ALLOYS
The surface and the cross section of the laser nitrided samples were analyzed
using Carl Zeiss microscope and SEM and the thickness of the laser nitrided zone was
measured using image analyzer (clemex Vision). The thickness of the laser nitrided
zone varied from 15 to 20 µm for Cp Ti and 5 µm to 10 µm for Ti1313 alloy
respectively. As pulsed laser was used for the surface nitriding, non overlapping
circular heat pulse has resulted in the variation in the thickness of the surface
modified zone. The SEM (Figure 3.5 (a)) clearly revealed the presence of two
different zones in the cross section of laser nitrided Cp Ti. The first zone consists of
densely populated large sized dendrites, while the second zone revealed the presence
of a mixture of small sized dendrites and large number of needle shaped acicular
martensite. Similar observations were made by several authors (Mohanad Soib
Selamat et al; 2001, Man et al; 2005). During rapid solidification (106 K/s), dendrites
(deg)
α - Ti
β - Ti
γ - TiN0.3
δ - TiN
ω - ZrN
61
grow along the direction in which the temperature difference is a maximum.The
orientation of dendrites within the melt pool is dependent on the stream direction and
below the dendrites a few acicular martensitic particles are found in Ti α‟ matrix
which leads to a decrease in the hardness in this zone compared to the top zone which
consists of mainly TiN. Formation of TiN and TiN0.3 particles spaced in ductile
martensitic matrix at the topmost region on laser nitriding of Ti and its alloys has been
reported by several authors. (Anizhecheva et al; 2005, Filip et al; 2008).On the other
hand, in the case of laser nitrided Ti1313 alloy only very less amount of TiN dendrites
were noticed (Figure 3.5(b). At higher magnification, in laser nitrided Cp Ti (Figure
3.5 (c)), it is evident that the amount of TiN is less as we move towards the substrate
as compared to the top most zone. This explains the reduction in the hardness as one
measures the hardness along the melt pool direction. The SEM of the nitrided Ti1313
alloy (Figure 3.5 (d)) did not reveal the presence of heat affected zone and also unlike
in the case of Cp Ti, the dendrite formation was not clearly observed in Ti1313.
Further, the some band like structures were seen to grow in random directions and this
may be attributed to non uniform cooling rates in this region due to the presence of
the alloying elements. Inspite of the fact that, peaks corresponding to TiN were
evident from XRD results, the low hardness measured in this region may be due to the
presence of scattered dendrites and other alloying elements in the surface. Thus the
hardness measured does not correspond directly to TiN phase as it is difficult to make
indentations on the scattered TiN phases present in lesser quantity.
Moulding
(a) (b)
Nitrided
zone
Figure 3.5 Cross-sectional (y-z cut section) morphologies of (a) laser nitrided Cp
Ti (b) Ti1313 alloy at lower magnification.
62
Zone I
Dendritric
structure
Zone II
Needle like
structure
(c) (d)
Figure 3.5 Cross-sectional (y-z cut section) morphologies of (c) laser nitrided Cp
Ti (d) Ti1313 alloy at higher magnification.
3.3.4 POTENTIODYNAMIC ANODIC POLARIZATION STUDIES
Anodic potentiodynamic polarization measurements were carried out for
unnitrided, laser nitrided Cp Ti and Ti1313 alloy in Hank‟s solution. The anodic
polarization behaviors of both the untreated and the laser nitrided Cp Ti and Ti1313
alloy are shown in figure 3.6. The common feature of the anodic polarization
behaviors of both the untreated and the laser nitrided specimens is the materials
ability to passivate in the Hank‟s solution even at the open circuit potential (ie) in the
absence of external potential. Ecorr values are noble for laser nitrided samples in
comparison with that of the unnitrided samples indicating that the nitrided samples are
less prone to corrosion compared with that of the counterparts.
From the anodic polarization curves, it is observed that the passive current
density of laser nitrided Cp Ti is 74% lesser when compared with that of the
unnitrided Cp Ti as shown in Table 3.5. This increase in the corrosion resistance of
Cp Ti is due to the presence of dense dendrites in the form of compact columnar
structure which has provided protection to the underlying substrate from environment.
These observations are similar to what has been reported earlier (Yilbas et al; 2006,
Kamachi Mudali et al; 2003).The enhancement in corrosion resistance on nitrided
layer is attributed to the rapid formation of a thin surface layer (200 A0) of TiO2 on the
TiN present in the nitrided zone (Starosvetsky et al; 2001). This thin oxide layer
inherits good corrosion resistance and adhere well to the TiN layer. Also, the nitrogen
(a) (a)
63
present in TiN dendrites may react with oxygen to form oxynitrides.These oxynitrides
along with oxides impede the dissolution at the surface, thus increasing the corrosion
resistance (BoTian et al; 2009).
The corrosion resistance of laser nitrided Ti1313 alloy with hardness 390±5
HV (3.90 ± 0.05 GPa) was found to be higher than the unnitrided sample, however
they exhibited slightly higher corrosion rate when compared with that of the laser
nitrided Cp Ti. This is due to the fact that the dendrites formed on Ti1313 alloy is less
compact and scattered and with less amount of TiN when compared to dendrites
formed on Cp Ti. Thus, high inhomogenity found in the microstructure on the surface
has provided a path for the corrosion agent to attack the surface leading to marginal
improvement in the corrosion resistance. This result is contradictory to the corrosion
behavior of the Ti1313 alloy which was laser nitrided at low scanning speed
(600mm/min) (Geetha et al; 2004).The enhancement in the corrosion resistance at low
scanning speed was attributed to densely packed dendrites consisting of high
concentration of TiN and minor amounts of Nb and Zr and high surface hardness
(1600± 20 HV)(16 ±0.02 GPa). Hence, it is clearly evident that an optimal amount of
TiN is required to improve the corrosion resistance and below which only marginal
changes in the corrosion behavior can be observed.
Figure 3.6 Potentiodynamic polarization curves for Un nitrided, laser nitrided
Cp Ti and Ti1313 alloy
Laser nitrided Cp titanium
Laser nitrided Ti1313 alloy
Un nitrided Ti1313 alloy
Un nitrided Cp titanium
64
Table 3.5 Ecorr and Icorr values for un nitrided, laser nitrided Cp titanium and
Ti-13Nb-13Zr alloy in Hank’s solution.
Sample Ecorr (mV) I corr (µA/cm2)
Un nitrided Cp titanium -398 0.856
C.P Titanium-Laser Nitrided -220 0.634
Un nitrided Ti-13Nb-13Zr -384 1.040
Ti-13Nb-13Zr-Laser Nitrided -192 0.752
3.3.5 WEAR RESISTANCE AND WORN SURFACE MORPHOLOGY
The wear behavior and the wear rate of the laser nitrided samples tested using
reciprocatory wear testing equipment as per the ASTM G133 standard are reported in
Table 3.6 and Figure 3.7 (a-b) illustrates the worn surface morphologies of the laser
nitrided Ti1313 alloy and Cp Ti tested for 105 cycles. Amongst the two materials
tested, the wear rate of laser nitrided Cp Ti was lower when compared to the laser
nitrided Ti1313 alloy. The dendrites formed on the surface of the laser nitrided
Ti1313 alloy have been almost worn out and substrate was observed after 105 cycles.
On the other hand, in the case of laser nitrided Cp Ti, the worn surface morphology is
found to be smooth consisting of few dendrites (figure 3.7(a)) when compared to that
of the laser nitrided Ti1313 alloy. The difference in the worn surface morphologies of
the above two alloys are due to the variations in the amount of TiN in the dendrites.
As there is complete removal of the nitrided surface in Ti1313 alloy, it is evident that
initially the adhesive wear and then the abrasive wear have taken place. The adhesive
wear may cause the fragments of TiN dendrites to be pulled off and adhere to the
surface of the alumina ball. Then the presence of wear debris results in abrasive wear
that introduces the ploughing grooves which arise from the interaction of micro
cutting and plastic deformation (Cui et al; 2005).Wear rates of the unnitrided, laser
nitrided Cp Ti and laser nitrided Ti1313 alloy were all measured using mass loss
measurement method with an accuracy of 0.0001 g. The wear rate of the laser nitrided
Ti1313 alloy tested for 80,000 cycles was considerably lower than the unnitrided
Ti1313 alloy. Similarly, in the case of laser nitrided Cp Ti, the wear rate (3.76x10-6
mm3/N m) was found to be very less compared to the untreated Cp Ti (6x10
-6 mm
3/N
m) after 105 cycles. Further, the coefficient of friction (0.6) of laser nitrided Ti1313
alloy is found to be higher than that of the laser nitrided Cp Ti. The lower value of
65
friction coefficient for laser nitrided Cp Ti is due to the low surface roughness and
high hardness. The values obtained are in good agreement with the results that have
been reported earlier (Animesh Choubey et al; 2004, Xuekang Chen et al; 2007).
Even though the coefficient of friction for laser nitrided Cp Ti is found to be lesser
than the laser nitrided Ti1313 alloy, it exhibited marginally higher wear rate when
compared to the alloy.
The area under the three dimensional wear plot gives an estimate of the energy
dissipated during the wear processes (Fouvry et al; 2007). The increase in the loop
area indicates the increase in the wear of the tested specimen. From the three
dimensional wear plot of laser nitrided Ti1313 alloy, it is clearly evident that the wear
is very less at the initial stages (up to 40,000 cycles) and later on the there is
considerable increase in the wear (Figure 3.8). Further, increase in friction coefficient
value was observed above 40,000 cycles. In order to have better understanding of
wear mechanism, intermittent wear studies were carried out on laser nitrided Ti1313
alloy, the results of which are presented in Table 3.7. Worn surface morphologies of
the intermittent wear studies depict that, more amount of dendrites were noticed up to
40,000 cycles and later on the dendrites have worn off completely (Figure 3.9). Thus
the results obtained are well correlated with that of the three dimensional wear plot
obtained.
The three dimensional wear plots shows the variations of coefficient of
friction with respect to the number of cycles and the displacement for the laser
nitrided Ti-13Nb-13Zr alloy tested in Hank‟s solution. The coefficient of friction
increased from 0.6-0.65 up to 40,000 cycles and later on there has been a sudden
increase in the coefficient of friction (0.8).The worn surface of the laser nitrided Ti-
13Nb-13Zr alloy is found to be smooth and no material transfer has taken place from
the alumina ball towards the coating. This is mainly due to the lower hardness of the
laser nitrided Ti-13Nb-13Zr alloy. Moreover the intermittent wear studies also clearly
revealed that after 40,000 cycles of wear testing, some deep abrasive scratches were
observed on the surface which gives an indication of the severity of the wear. The
presence of deep scratches on the worn surface is mainly due to the huge variations in
the hardness of the laser nitrided Ti-13Nb-13Zr alloy and the counterface alumina
ball. However no cracks were observed on the worn surface of the Ti-13Nb-13Zr
66
alloy. Also there has been an abrupt increase in the wear rate of the laser nitrided
samples tested after 40,000 cycles which very well corroborates with that of the 3-D
wear plot
(a) (b)
Wear
track
Worn
surface
Figure 3.7 Worn surface morphologies of (a) laser nitrided Ti1313 alloy and
(b) laser nitrided Cp titanium (after 105 cycles)
Figure 3.8 Three Dimensional Wear plot of laser nitrided Ti1313 alloy
Sliding direction Sliding direction
67
Table 3.6 Wear test Results
Coated Wear Friction
Coefficient
Un nitrided
Ti1313 alloy Wear rate = 4.443 x10
-6 mm
3/N m
0.7
Laser nitrided
Ti1313 alloy
Wear rate = 2.933x10-6
mm3/ N m
(after 80,000 cycles)
0.6
Un nitrided
Cp Ti Wear rate = 6 x 10
-6 mm
3/N m 0.5
Laser nitrided
Cp Ti Wear rate = 3.7x 10
-6 mm
3/N m
0.5
Table 3.7 Intermittent wear test results
No.of.cycles Mass loss
(gm)
Wear volume
(mm3)
Wear rate
(mm3/Nm)
Coefficient of
friction
20,000 0.0090 2.000 6.660X10-5
0.6
40,000 0.0134 2.978 9.927X10-5
0.5
60,000 0.0199 4.400 1.467X10-4
0.5
80,000 0.0311 6.910 2.303X10-4
0.6
68
(a) (b)
(c) (d)
Figure 3.9 Worn surface morphologies of laser nitrided Ti1313 alloy
(a) 20,000 cycles (b) 40,000 cycles (c) 60,000 cycles (d) 80,000 cycles
3.4 CONCLUSIONS
Laser nitriding of the biomedical materials such as Cp Ti and Ti1313 alloy
have led to increase in hardness, wear and corrosion resistance of the material. Our
studies make us to conclude that apart from other processing parameters discussed in
table 3.3, scanning speed and the concentration of TiN play a major role in the
enhancement of the above mentioned surface characteristics.
Sliding direction Sliding direction
Sliding direction Sliding direction
69
From our experimental work on Cp Ti and Ti1313 alloy, it is evident that
higher scanning speed (720 mm/min) results in less rougher surface than that
performed at the lower scanning speed (600 mm/min) due to the difference in the
concentration of the dendrites. Amongst the two materials viz; Cp Ti and Ti1313
alloy, a substantial decrease in the concentration of TiN dendrites was noticed for
Ti1313 alloy which is due to the high scanning speed and the presence of alloying
elements. The dendrites formed on laser nitrided Ti1313 alloy is less compact and
scattered when compared to dendrites formed on Cp Ti and this has indeed provided a
path for the corrosion agent to attack the nitrided layer leading to higher corrosion in
Ti1313 alloy. Though Ti1313 alloy exhibited high corrosion resistance, it exhibited
low wear rate when compared to the un nitrided samples. Hence, these studies bring
out the fact that several competing processing parameters used here with regard to
surface modification are to be properly optimized in order to achieve densely
populated TiN dendrites and a surface free from cracks.