50

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

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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.