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SMBS, VIT University, Vellore , India 30

Abstract Titanium and its alloys are having very attractiveproperties enabling them to be used in the fields of aero space,

biomedical, marine and many other corrosive environments. The

application of these alloys is more attractive today in the field of

biomedical implant materials due to their superior biocompatibility

and strength. These alloys have high coefficient of friction and poor

abrasive wear resistance which results in wear of the implant during

its fixation in the body. Implant wear is a common phenomenon

which is resulted due to high friction between artificial materialswhich is much higher than healthy and natural joints. The

corresponding wear of the implant results in the accumulation of

wear debris in the body tissues which results in inflammation, pain

and loosening of implant resulting in shorter life period of the

implant. Solution treatment of the alloy is one of the important

techniques to improve the sliding wear properties of the alloy. These

properties can be obtained by changing the microstructure of the

alloy where the formation martensitic structure (acicular or retained

) is resulted. The formation of martensitic structure in titanium alloy

results in improved hardness value with a subsequent improvement in

its sliding wear behavior. In this work the both the Ti-6al-4V and Ti-

6Al-4V implant alloy materials are subjected to heat treatment

above their transformation temperature followed by slow cooling in

furnace, air and water. These specimens were further aged and testedfor dry sliding wear properties against hardened steel disc using a

pin- on-disc apparatus. Weight loss method with a optimal load of

50N and a sliding distance of 500metres is considered for conducting

the wear test.An improvement in wear rate is observed under

different heat treatment condition and also high wear resistance of Ti-

6Al-7Nb is reported. The conditions which favor the above results

are properly discussed with the help of Energy dispersive X-ray

spectroscopy (EDS) and an analysis of the wear tracks of thesame is

done using Scanning Electron Micrograph (SEM) is reported.

KeywordsBiomaterials, Transformation temperature, Wearresistance, Vickers micro hardness Value.

F. B.K.C.Ganesh is working asa research scholar in Mechanical

Engineering Department, College of Engineering (A), Andhra University,

Visakhapatnam. Email: bkcganesh @ yahoo.com. Mobile +91 98854 25863

S. N.Ramanaih .Ph.D is working as Associate Professor in the department

of Mechanical Engineering ,College of Engineering, Andhra university,

Visakhapatnam.

T. P.V.Chandrasekhar Rao,Ph.D is working as a Associate professor in

Mechanical Engineering Department, Lakireddy Balireddy college of

Engineering, Mylavaram , Vijayawada..

T.S.V.N.Pammi Ph.D is working as Co-ordinator ,Advaced Analytical

Laboratory,DST-PURSE programme, Andhra University, Visakhapatnam.

I. INTRODUCTIONi-6Al-4V and Ti-6Al-7Nb are binary alloys of titanium

which have gained wide popularity as Implant

materials.Ti-6Al-4V which has been used extensively for

aerospace and marine applications is also being used as

biomaterial due to its excellent biocompatibility and corrosion

resistance. Though these metals have excellent strength to

weight ratio and low density, these alloys have a high

coefficient of friction which results in implant wear. This

factor is very important while considering the friction between

the artificial implant material coming in contact with the

healthy and natural bone joint. Two main properties have been

suggested with respect to the poor tribological properties of

titanium alloys. [1].

1. Low resistance to plastic shearing and low work hardening

2. Low protection exerted by the surface oxide which may

form as a consequence of frictional heating

The impact of the above factors such as low sliding wear

resistance and protection exerted by the surface oxides has

promoted many surface modification techniques which are

under continuous development to improve the wear resistance

of these alloys while working in a specific tribological

environment.

Titanium exists in various allotropic forms. At low

temperatures, it has a closed packed hexagonal crystal

structure known as , whereas above 883oC, it has a body

centered cubic structure known as . The to

transformation temperature of pure titanium either increases or

decreases based on nature of the alloying elements. The

alloying element such as aluminium, oxygen, nitrogen, etc that

tend to stabilize are called alpha stabilizers and the addition of

these elements increase beta transus temperature, while

elements that stabilize phase are known as stabilizers such

as vanadium, molybdenum, niobium, iron ,etc and addition of

these elements depress the transus temperature. Some of the

elements such as zirconium and stantium which do not have

marked effect on the stability of either of the phase but form

solid solutions with titanium are termed as neutral elements.

[2].Ti-6Al-4V is now replaced by Ti-6Al-7Nb due to its

excellent wear resistance and identical mechanical properties

consisting of + microstructure. It is also replaced due to

possibility of pain, inflammation and toxicity being

developed by elemental vanadium which is formed as a result

of abrasive wear of the implant when in contact with the

Comparison of dry sliding wear of Ti-6Al-4V

and Ti-6Al-7Nb implant alloys when subjected

to heat treatment

B.K.C.Ganesh, N.Ramanaih P.V.Chandrasekhar Rao and S .V.N.Pammi

T

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natural and healthy joint.

Mitsuo [3] reported that the application of stress by

rapid quenching results in the formation of martensitic

structure in steels which contain residual austenite in their

microstructures. This phenomenon is called stress or strain

induced martensitic transformation which enhances ductility or

fracture toughness of steel. Deformation induced martensitic

transformation also occurs in titanium where unstable beta

phase is retained at room temperature by rapid cooling suchas water quenching from a high temperature near the beta

transus temperature.

The surface modification and change in microstructure

can be obtained by heat treating the various samples at beta

transus temperature (transformation temperature) where

primary changes from hexagonally closely packed

crystallographic structure () to body centered cubic

crystallographic structure ().These specimens were

Subsequently cooled by rapid quenching ,furnace cooling and

air cooling followed by aging of these alloys at 500 0

centigrade .The objective of conducting the heat treatment

cycle is to improve the hardness of the alloy which is

beneficial in increasing the wear resistance of the alloy byretaining the transformed beta at the room temperature in

order to achieve strengthening of the alloy.

Wear rate calculation by pin and rotating disc machine

using weight loss method is one of the common techniques to

evaluate dry sliding wear behaviour of Titanium implant

materials. A. Molinari et.al,[4] investigated on dry sliding

wear mechanism of Ti-6Al-4V alloy. In their experimental

work it has been found that the wear volume of the rotating

specimens is reported as a function as sliding distance and the

load applied on the pin. An increase in wear volume is resulted

with an increase in applied load. O.Alam et.al, in their

experimental work of dry sliding wear have reported that

under a constant load of 45N applied on pin ,an increase ofwear rate is identified up to a sliding distance of 500 meters.

There after a steady state is attained which does not show any

appreciable change in the wear rate behaviour of the alloy.

B.D.Venkatesh.et.al [5] reported an improvement in various

mechanical properties when + alloys were heat treated

above the beta transus temperature and cooled by water

quenching, air cooling and furnace cooling. Sami Abualnoun

Ajel .et. al [6] have studied the influences of heat treatment on

Ti-6Al-7Nb implant alloy, where the improvement in the

strength of the alloy is reported to change with the

modification of the alloy microstructure.

It is evident that from the above literature that the effect

of various types of heat treatment influences the variouschanges in the respective microstructures which further results

in obtaining tailor made mechanical properties and

tribological properties of the implant alloy. In this work dry

sliding wear tests are conducted on Ti-6Al-4V and Ti-6Al-

7Nb implant alloys under various heat treating conditions.

These tests we conducted on a Ducom wear testing machine

considering an optimal load of 45N with sliding distance of

500metres under a constant velocity of 1m/sec.

II. MATERIALS AND METHODSBoth the materials are imported from Baoji Litai

Corporation, Shanxi,China.The chemical composition by

weight of the metals are as follows:

TABLE1

CHEMICALCOMPOSITIONOFIMPLANTALLOYS

%COMPOSITION OF

ELEMENT TI-6AL-4V TI-6AL-7N

TITANIUM 89.6 87.6

ALUMINIUM 6.29 5.80

VANIDIUM/NIOBIUM 3.95 6.50

IRON 0.09 0.037

CARBON 0.029 0.017

The implant alloys are heat treated at 950o centigrade for

one hour (beta transus temperature) in an Argon controlled

atmosphere for a period of one hour. It is then aged at 550o

centigrade for a period of three hours in an Air controlled

furnace. The pins were cut according to the standard

dimensions as shown in the figure 1 for the evaluation of wear

track and microstructure. All the cut specimens were

mechanically polished via a standard metallographic procedure

to a final level of 0.3 m alumina powder and etched with a

solution of water, nitric acid and hydrofluoric acid (80:15:5 in

volume).Scanning Electron Micrograph (SEM) analysis is also

conducted to study the wear track behaviour of various heat

treated specimens. Hardness values are measured by VickersMicro Hardness testing machine with a constant load of 0.5

kg. Average surface roughness (Ra) value of the wear track is

measured by surface roughness testing instrument.

In this work three samples of each condition were heat

treated and subsequently cooled under different cooling

environments such as Cooling in a furnace, air and quench in

water followed by aging. These specimens were then tested by

wear testing machine and the results are reported

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Fig. 1.Dimensions of the (a) Friction pin (b) Rotating disc in

millimeters

III. RESULTS AND DISCUSSIONA. Wear track analysis

The following figure 2 shows the wear track micrographs of

both the alloys. It is evident from the microstructures that the

alloy Ti-6Al-4V hereafter referred as Ti64 has the presence of

a protective layer in its water quenched and air cooled

specimens. The presence of this layer is only seen when the

alloy is cooled at a faster rate which cannot been seen in a

slow cooling of the alloy in a furnace. Figue 3 shows the

scanning electron micrograph (SEM) of water quenchedspecimen. It consists of two layers,a protective and a non

protective layer. The energy dispersive spectrometry (EDS) of

the base and Water quenched specimen is shown in figure 4 (a)

,(b),(c).

From the EDS analysis of the base wear specimen it is clear

that the oxygen and carbon content are limited to 2.34 and

0.56 % by weight respectively. But when the specimen is heat

treated above the beta transus temperature and quenched with

water there is a formation of a protective and a non protective

layer as can be seen from the figure 3.The EDS analysis of the

non protective layer shows huge amount oxidation with its

weight % at 12.14 and carbon percentage of 1.11% as shown

in figure 4(b).whereas the EDS analysis of protective layershown in figure 4 (c) indicates an increase in the carbon at

1.71% by weight. This increase in the percentage of carbon

clearly suggest thatthere is a formation of carbide layer whichinhibits the amount of wear on this area of the specimen.

Further the EDS analysis of the non protective layer also

indicates a higher amount of metal to metal contact withincreasing percentage of iron content when in contact with the

steel disc.

1 (a) Base Material (b) Water Quenched

(C) Air cooled (d) Furnace cooled

2 (a) Base Material (b) Water Quenched

(C) Air cooled (d) Furnace cooled

Fig. 2. Wear Track of 1. Ti-6Al-4V and 2. Ti-6Al-7Nb subjected to

various heat treatment processes at 20 X magnification

It is evident from the literature that when titanium and its

alloys are exposed to oxygen containing atmosphere it results

in the formation of an oxide layer on its surface with an

oxygen diffusion zone beneath it. Hasen Gluryuz (2008).This

formation is more pronounced when the alloy is heat treated

above transformation temperature and cooled rapidly in air or

quenching by water. This formation further plays an important

role in developing a protective oxide layer which promotes

remarkable advantage of the alloy while working in a friction

and wear environment. Further the Scanning electron

micrograph of quenched specimen shows the presence of

acicular martensitic structure () in white globular primary

and matrix. The presence of needle like acicular alpha

greatly improves the hardness values of the quenched

specimen.. The microstructures have been consistent with

respect to thework done on this alloy by A.K.Jha .et.al. (2010)

and A.Molinari .et.al.(2010).

Fig 3 Scanning Electron Micrograph of Water Quenched wear

specimen.

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Fig 4 (a) EDS analysis of base wear specimen

Fig4 (b) EDS analysis of Non protective layer

Fig 4 (c) EDS analysis of Protective layer

Fig 5. Scanning Electron Micrograph of Quenched Specimen.

B. Weight loss and Wear rate analysisMaximum amount of weight loss has been reported from as

received material as shown in Figure 6, where as less amount

wear is reported from both water quenched and air cooledspecimens of Ti-6Al-4V alloy. The results commensurate with

the hardness values and microstructure behavior of all the heat

treated specimens. Various Wear properties including surface

roughness values of the heat treated specimens measured

perpendicular to the wear track are given in the Table1.The

value of as received material is high due low hardness of the

specimen. This is due to coarse wear track developed during

the testing procedure. The roughness value of water quenched

specimen is low due to formation martensitic structure by heat

treatment which resulted in an increase in the hardness value.

Element Weight% Atomic%

C K 1.71 5.33

O K 11.29 26.45

Al K 0.46 0.63

Ti K 83.26 65.17

V K 3.29 2.42

Total 100


C K 0.56 2.10

O K 2.34 6.53

Al K 1.79 2.96

Ti K 90.36 84.13

V K 4.41 3.86

Fe L 0.53 0.83

Total 100


C K 1.11 3.49

O K 12.14 28.57

Al K 1.60 2.24

Ti K 71.47 56.19

V K 4.30 3.18

Fe L 9.38 6.32

Total 100

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Presence of high amount of wear is reported from the wear

testing of as received material Ti64 alloy . This corresponds

to low hardness value which is Vickers hardness number

VHN 311 as compared to quenched specimen which is having

a hardness value of VHN 380.High amount of hardness

values in quenched specimen is due to the presence of

acicular (martensitic structure) which has formed due to

heat treatment above beta transus temperature followed by

water quenching and aging. The wear rate as shown in table 1is also high as in the case of as received material when

compared to air cooling or water quenched specimens. The

wear rate of furnace cooling specimen is greater than the

water quenched and air cooled specimens due to the

formation of lamellar plate like structure where there is no

presence acicular or retained beta .This is due to slow

cooling of the specimen where complete transformation of

body centered cubic crystallographic () structure to

hexagonally closely packed () structure has taken place,

which indicates more strength due to its high aspect (c/a) ratio

of hexagonally packed crystallographic structure.

The improvement of wear rate from base specimen to

quenched specimen correlates the Archard adhesive weartheory that if all junctions have the same diameter and if the

real area of contact is given by the normal load L divided by

the hardness H, the total volume of the material removed

during sliding through a distance S is given by

V=K LS/3H

Where K is the wear constant which gives the probability

that a wear particle will be formed from an asperity junction.

It is understood from the above equation that the amount of

the specimen is inversely proportional to the weight loss or

volume of the material removed. Therefore Ti64 specimens

show a proportional decrease in their wear rate with a

significant increase in their hardness values.

On the other hand the wear rate for the Ti7nb alloys is

almost consistent without any response to heat treatment.

There is also no presence of a protective layer as encountered

in the quenched specimen of Ti64 alloy. But the base wear

rate of Ti7nb is lesser that Ti64 alloy due to abrasive wear

resistance of adding niobium which is very much common in

manufacturing of resistant steel bolts and nuts as well cutting

tool industry for improving wear resistance. Similarly when

the wear rate of as received Ti-6Al-7Nb specimen is

considered the wear resistance is very high as compared to the

as received Ti-6Al-4V specimen though the hardness value of

Ti7Nb is lesser than Ti64 specimen .Therefore it is

understood that the improvement of the wear resistance

contradicts the Archard adhesive wear theory (1953), where

there is no correlation between the hardness and wear

resistance factors.

High wear resistance of Ti 7nb specimens is primarily due to

the presence of Nb2O5 in addition to TiO2oxide. Compared to

TiO2 oxide,Nb2O5 oxide is very good lubricating and hard

oxide. Due to the presence of dense hard niobium oxide a

higher surface roughness value is obtained as can be seen in

table 1 and due to its lubricating properties a higher amount of

wear resistance is resulted when compared toTi64 implant

alloy.

Fig 6.Weight loss of TI-6al-4V and Ti-6Al-7Nb under different

heat treating conditions

FCA-Furnace cooled and aged.

ACA-Air cooled and aged.

WQA-Water quenched and aged.

The wear resistane of the Ti-6Al-7Nb alloy is also due to

delamination wear as shown in the Scanning Electron

Micrograph of the alloy in figure 7.The sem image clearly

differentiates delaminated region with un delaminated region.TABLEII.

WEARPROPERTIESOFTI-6AL-4VANDTI-6AL-7NBUNDERVARIOUS

HEATTREATINGCONDITIONS

Heat treated

condition

Surface

roughness

(Ra)

Micro

Hardness

(HV0.5)

Wear Rate

X 10 -11

m3/m

Ti-6Al-4V

As Received

(Base Metal)2.11 311 1.954

Furnace cooled and

aged1.29 351 0.93

Air cooled and Aged 0.858 340 0.186

Water Quenched and

aged1.411 380 0.139

Ti-6Al-7NbAs received (Base

Metal)2.23 278 0.884

Furnace cooled and

Aged1.15 318 0.796

Air cooled and aged 1.24 283 0.766

Water Quenched and

Aged1.55 326 0.752

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The delamination of the alloy has taken place due to following

reseons[ 11].

During wear,dislocations are generated in the material due

to the plastic deformation of the surface by the slider.If an

oxide layer is present it is broken by the passage of the slider

thus exposing a fresh clean surface.This then allows those

dislocations nearly parelell to the surface to be eliminated due

to the action of stress at the free surface.With continuedsliding there will be pile up of dislocations at finite distance

from the surface which will lead to the formation of

voids.When these voids coalese crack formation takes place.

When the crack reaches a critical length the material between

the crack and the surface will shear producing a sheet like

particle.

This eventual presence of delaminated regions play an

important role in minimising the loss of material form the pin

there by improving the wear resistance.

Fig 7 Wear track of Ti-6Al-7Nb alloy

IV. CONCLUSIONS1. The Wear rate of Ti-6Al-4V quenched specimen is very low due to the

presence of protective oxide coating layer formed during heat treatment and

also due to the presence of acicular martensitic structure (retained beta) in its

microstructure.

2. Finer wear tracks were observed when the specimen hardness is increased

during rapid quenching and air cooling of the specimen below the

transformation temperature

3. Ti-6Al-7Nb resulted in higher wear resistance when compared to Ti-6Al-

4V alloy due to the presence delamination wear and Niobium oxide which is

hard and has lubricating properties.

4. Formation of protective oxide has taken place only in quenched and air

cooled specimens of Ti-6Al-4V, which indicates the presence of oxide layer

at times of faster rate of cooling the alloy below the transformation

temperature

5. Ti-6Al-4V implant alloy resulted in obtaining various dry sliding wearproperties when it is heat treated above its transformation temperature.

whereas Ti-6Al-7Nb alloy does not show much response to heat treatment.

REFERENCES

[1]. Md. Ohidul Alam, A.S.M.A.Haseeb. (2002)Response of Ti-6Al-4Vand Ti-24Al-11Nb alloys to dry sliding wear against hardened steel,

Journal of Tribology International, Vol 35, pp 357-362.

[2]. M.Geetha, A.K.Singh, R.Asokamani, A.K.Gogia.(2009) Ti basedbiomaterials, the ultimate choice for orthopedic implants - A Review

.Journal of Progress in Material Science, Vol 54, pp397-425.

[3]. Mitsuo Niinomi.(2008) Mechanical biocompatibilities of titaniumalloys for biomedical applications. Journal of Mechanical Behavior of

Biomedical Materials, Vol 1, pp 30-42.

[4]. A.Molinari,G Straffelini,B.Tesi,T.Bacci. (1997) Dry sliding wearmechanisms of the Ti6Al4V alloy, Journal of Wear, Vol 208, pp 105-

112.

[5]. B.D.Venkatesh, D.L.Chen, S.D.Bhole. (2009) Effect of heat treatmenton mechanical properties of Ti-6Al-4Valloy, Journal of Material

Science & Engineering (A), Vol 506, pp 117-124.

[6]. Sami Abualnoun Ajeel, Thair. L. Alzubaydi, Abdulsalam. K. Swadi.(2007) Influence of heat treatment conditions on microstructure of Ti-

6al-7Nb alloy as used surgical implant materials. Journal of

Engineering and Technology, Vol 25, pp 431 442.

[7]. A.K.Jha, S.K.Singh, M.S.Kiranmayee.(2010) Failure analysis oftitanium alloy (Ti6Al4V) fastener in aerospace application, Journal of

Engineering Failure Analysis, Vol 17 ,pp 1457-1465.

[8]. Hasen Gluyrez,Huseyin cimanglo (2008)Oxidation of Ti-6Al-4Valloy,Journalof alloy and compounds

.doi:10.1016/J.Jallcom.2008.04.024.

[9]. J,F.Archard, (1953) ,Journal of Applied Physics, Vol 24 ,pg 981-988.[10].N.P.Suh, (1973), Journal of Wear, Vol 25,pg 111-124.[11].N.P.Suh, (1974), The delamination theory of wear, MIT Report

submitted to NTIS U.S. Department of Commerce.

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