tribology of self-assembled monolayersin this thesis, application of thin organic coatings to...

APPLICATION OF BIOCOMPATIBLE THIN ORGANIC COATINGS TO IMPROVE TRIBOLOGY

OF TI6AL4V ALLOY

BHARAT PANJWANI

NATIONAL UNIVERSITY OF SINGAPORE

2011

APPLICATION OF BIOCOMPATIBLE THIN ORGANIC COATINGS TO IMPROVE TRIBOLOGY

OF TI6AL4V ALLOY

BHARAT PANJWANI (B.Tech, IIT Kanpur, India)

A THESIS SUBMITTED

FOR THE DEGREE OF MASTER OF ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2011

Preamble

Preamble

This thesis is submitted for the degree of Master of Engineering in the

Department of Mechanical Engineering, National University of Singapore under

the supervision of Associate Professor Sujeet Kumar Sinha. No part of this thesis

has been submitted for any degree or diploma at any other University or

Institution. As far as the author is aware, all work in this thesis is original unless

reference is made to other work. Parts of this thesis have been published and

under review for publication as listed below:

Journal

1. B. Panjwani, N. Satyanarayana and S. K. Sinha. "Tribological

characterization of a biocompatible thin film of UHMWPE on Ti6Al4V

and the effects of PFPE as top lubricating layer", Journal of the

Mechanical Behavior of Biomedical Materials 4 (2011) 953-960. (a part

of Chapter 4)

2. B. Panjwani and S. K. Sinha. “Evaluation of tribological properties of

PFPE over-coated 3-glycidoxypropyltrimethoxy silane self-assembled

monolayer on Ti6Al4V surface”, manuscript in preparation. (a part of

Chapter 5)

Conference

1. B. Panjwani, N. Satyanarayana and S. K. Sinha, “Improving the tribology

of Ti6Al4V through a biocompatible thin UHMWPE coating”, ICMAT

2011 - International Conference on Materials for Advanced Technologies,

Singapore from 26 Jun to 1 July, 2011.

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Acknowledgements

Acknowledgements

This is the great opportunity to acknowledge and express my thanks to

people for their support and encouragement in my postgraduate studies. First of

all, I would like to express my earnest gratitude and sincere thanks to my

supervisor Associate Professor Sujeet Kumar Sinha for providing me this

priceless opportunity to pursue my postgraduate studies. I am pleased to thank my

graduate advisor Assoc. Prof. Sujeet Kumar Sinha for his invaluable guidance,

supervision, encouragement, support and offering this great opportunity to work

with him.

I would like to express my special thanks to Dr. Nalam Satyanarayana for

his consistent help and support offered during my research work. I would also like

to thank Dr. R. Arvind Singh, Dr. Mohammed Abdul Samad and Dr. Myo Minn

for their support and valuable discussions.

I would like to say thanks to all my colleagues, Ehsan, Jonathan, Keldron,

Nam Beng, Prabakaran, Robin, Sandar, Sekar, Srinivas, Yaping, Yemei, for

stimulating research environment of mutual support and help in the team. I would

also like to thank all my friends, Amit, Archit, Chandra, Luv, Meisam, Sashi,

Srinivasa, Tapesh for their friendship and support.

I am grateful to the lab staff, Mr. Thomas Tan Bah Chee, Mr. Abdul

Khalim Bin Abdul, Mr. Ng Hong Wei, Mr. Maung Aye Thein, Mr. Juraimi Bin

Madon, Mr. Suhaimi Bin Daud, for their continuous support and assistance. Many

thanks to Mr. Juraimi Bin Madon for his technical expertise and support offered

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Acknowledgements

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in the fabrication of fixtures. I would also like to express my sincere thanks to the

ME dept office staff, Ms. Teo Lay Tin, Sharen and Ms. Thong Siew Fah, for their

support.

Finally, I want to express my gratitude and sincere thanks to my family for

their support, love and encouragement.

Table of Contents

TABLE OF CONTENTS

Page Number

Preamble

Acknowledgements

Table of Contents

Summary

List of Tables

List of Figures

List of Notations Chapter 1 Introduction

1.1 Importance of tribology

1.2 Brief history of tribology

1.3 Tribological applications

1.3.1 Industrial tribology

1.3.2 MEMS/NEMS tribology

1.3.3 Biomedical tribology

1.4 Importance of titanium and titanium alloys

1.4.1 Industrial applications

1.4.2 Consumer durables

1.4.3 Medical applications

1.4.4 MEMS applications

1.5 Titanium and titanium alloys tribology

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1 1 2 3 3 3 4 4 5 6 7 7 8

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Table of Contents

1.6 Objectives of the thesis

1.7 Methodology in the present thesis

1.8 Structure of the thesis

CHAPTER 2 Literature Review

2.1 Surface engineering and tribology

2.2 Existing tribology solutions for titanium alloys

2.2.1 Surface treatments

2. 2.1.1 Thermally sprayed coatings

2. 2.1.2 Electroplating and electroless plating systems

2. 2.1.3 Physical vapor-deposited coatings

2. 2.1.4 Surface modifications

2.2.2 Thermo-chemical processes

2.2.2.1 Nitriding

2.2.2.2 Oxidizing

2.2.3 Energy beam surface alloying

2.2.3.1 Laser gas nitriding

2.2.3.2 Electron beam alloying

2.2.4 Duplex treatments

2.3 Ti6Al4V alloy surface treatments for biomedical applications

2.3.1 Plasma nitriding

2.3.2 Bio-ceramic coatings

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Table of Contents

2.4 Thin film coatings in tribology

2.4.1 Polymer coatings in tribology

2.4.1.1 UHMWPE polymer coating tribology

2.4.2 Self-assembled monolayers coatings

2.4.2.1 Applications of SAMs coatings on titanium

2.4.2.2 Applications of SAMs coatings in MEMS tribology

2.5 Friction and wear mechanisms in polymer tribology

2.6 Solution-based coating methods for polymers

2.7 Use of PFPE as a top layer

2.8 Pretreatment methods

2.9 Biocompatibility testing

2.9.1 In-vitro testing

2.9.2 In-vivo animal testing

2.9.3 Clinical testing

CHAPTER 3 Materials and Experimental Procedures

3.1 Materials

3.2 Coatings preparation procedure

3.3 Polymer coating thickness measurement method

3.4 Contact angle measurement

3.5 Optical microscope

3.6 FE-SEM surface morphology observation

3.7 AFM surface topography measurement

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Table of Contents

3.8 FTIR-ATR analysis

3.9 XPS analysis

3.10 Cytotoxicity assessment

3.11 Tribological characterization

CHAPTER 4 Tribological Characterizations of Thin UHMWPE Film and PFPE Overcoat 4.1 Physical characterizations

4.1.1 Coating thickness measurement

4.1.2 Water contact angle results

4.1.3 SEM surface morphology

4.1.4 AFM surface morphology

4.2 Chemical characterizations

4.2.1 FTIR analysis results

4.2.2 XPS analysis results

4.3 Tribological characterization of UHMWPE coating

4.4 Investigation of underlying wear mechanism

4.5 Effect of PFPE overcoat on UHMWPE coating

4.6 Explanation of wear resistance increase by PFPE overcoat

4.7 Biocompatibility assessments

4.7.1 Cytotoxicity test results

4.8 Potential applications of coatings

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Table of Contents

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CHAPTER 5 Tribological Evaluations of Molecularly Thin GPTMS SAMs Coating with PFPE Top Layer 5.1 Physical characteristics of the coatings


5.1.2 AFM morphology results

5.2 Chemical characteristics of UHMWPE coating


5.3 Tribological characterizations

5.4 Optical microscopy of wear track and counterface surface

5.5 Biocompatibility test


5.6 Potential applications of GPTMS/PFPE coating

CHAPTER 6 Conclusions CHAPTER 7 Future Recommendations References Appendix A Cytotoxicity Test Procedures

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Summary

Summary

Titanium and its alloys have been extensively used in many biomedical

and industrial applications due to their high specific strength with acceptable

elastic modulus, corrosion resistance and biocompatibility. However, high

coefficient of friction and low wear resistance of titanium and its alloys limit their

usage in some applications.

To improve the tribological properties of titanium and its alloys, various

surface modifications, coatings and treatments have been explored. In spite of

these developments, there is still a need to further investigate effective solutions

to improve tribological properties of titanium and its alloys.

In this thesis, application of thin organic coatings to improve tribology of

titanium and its alloys has been explored with emphasis on biomedical

applications. Ti6Al4V alloy, a commonly used titanium alloy, has been chosen as

substrate material in the studies of this thesis.

In the first study, ultra-high molecular weight polyethylene (UHMWPE)

polymer thin film (thickness of 19.6±2.0 µm) was coated onto substrate using dip-

coating method. Physical characterizations (contact angle, thickness

measurement, Field emission-scanning electron microscopy (FE-SEM)

morphology and atomic force microscopy (AFM) imaging), biocompatibility test

(cytotoxicity) and chemical characterizations (Fourier transform infrared

spectroscopy-attenuated total reflectance (FTIR-ATR) and X-ray photoelectron

spectroscopy (XPS)) were carried out for the obtained UHMWPE coating.

Tribological characterization of this coating was carried out using 4 mm diameter

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Summary

Si3N4 ball counterface in a ball-on-disk tribometer for different normal loads (0.5,

1.0, 2.0 and 4.0 N) and rotational speeds (200 and 400 rpm). This coating

exhibited low friction coefficient (0.15) and high wear life (> 96,000 cycles) for

the tested conditions. Perfluoropolyether (PFPE) overcoat on UHMWPE coating

further increased the wear resistance of coating as tested at even higher rotational

speed (1000 rpm). UHMWPE coatings (with and without PFPE overcoat) meet

the requirements of cytotoxicity test using the ISO 10993-5 elution method. Due

to their low surface energy, wear resistance and noncytotoxic nature, the thin

coatings of UHMWPE and UHMWPE/PFPE can find various applications in

biomedical implants and devices.

Despite having suitable properties for biomedical applications, higher

thickness of UHMWPE and UHMWPE/PFPE coatings may prevent their usage in

micro-electro-mechanical systems (MEMS) biomedical applications. In the

second study of this thesis, 3-glycidoxypropyltrimethoxy silane (GPTMS) self-

assembled monolayers (SAMs) with PFPE overcoat has been deposited onto

substrate. For comparison, PFPE coating has also been formed onto same

substrate. Ti6Al4V alloy specimens with PFPE overcoat and GPTMS/PFPE

composite coating showed low coefficient of friction and high wear durability as

tested at 0.2 N normal load and rotational speed of 200 rpm. The wear durability

of the obtained GPTMS/PFPE coating is much higher than that for only PFPE

coating. Obtained coatings were also characterized by contact angle measurement,

AFM imaging and XPS analysis. Formed PFPE and GPTMS/PFPE coatings are

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biocompatible in nature. Due to the combination of hydrophobicity, low friction

coefficient, high wear resistance and noncytotoxicity, these coatings can find

usage in biomedical applications where low coating thickness may be crucial.

Molecular thickness (< 4 nm) of these coatings is particularly advantageous for

their applications in biomedical MEMS devices.

List of Tables

List of Tables

Table 4.1 Table 4.2 Table 5.1 Table 5.2 Table 5.3

Measured water contact angle values for different specimens. Summary of tribological tests on Ti6Al4V/UHMWPE specimens. Measured water contact angle values for different specimens. Measured surface roughness for different Specimens in AFM imaging. Coefficient of friction for specimens tested in the study.

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List of Figures

List of Figures

Figure 1.1 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 4.1 Figure 4.2 Figure 4.3

Research methodology followed in the research studies. Schematic of typical SAM molecule structure and attachment with substrate. General classification of the wear of polymers [Briscoe and Sinha 2002]. Schematic representations of wear mechanisms (N: normal load; V: sliding velocity). (a) Adhesive wear. (b) Abrasive wear. A schematic diagram of contact angle measurement. Experimental apparatus. (a) Dip-coating machine. (b) Clean air furnace. Optima contact angle measurement set-up. Experimental instruments. (a) Optical microscope set-up. (b) Ball-on-disk tribometer. (c) Ball-on-disk tribometer stage. Ball-on-disk tribometer schematic (R: Track radius; r: Ball radius; F: Normal load; ω: Rotational speed of the disk). Step-height measurement method. Measured water contact angle values for different specimens. (a) Ti6Al4V. (b) Ti6Al4V/O2 plasma treated. (c) Ti6Al4V/O2 plasma treated/UHMWPE. (d) Ti6Al4V/O2 plasma treated/UHMWPE/PFPE. Surface morphology of Ti6Al4V/UHMWPE surface using FESEM. (a) At lower magnification, 100x. (b) At higher magnification, 500x.

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List of Figures

Figure 4.4 Figure 4.5 Figure 4.6 Figure 4.7 Figure 4.8 Figure 4.9

AFM morphology of surfaces. (a) Polished Ti6Al4V alloy surface (scan area: 40µm×40µm, vertical scale: 1µm). (b) Ti6Al4V/UHMWPE surface (scan area: 40µm×40µm, vertical scale: 5µm). FTIR-ATR spectrum of the UHMWPE coating on Ti6Al4V substrate. XPS spectra of specimens. (a) UHMWPE coating on Ti6Al4V. (b) UHMWPE/PFPE coating on Ti6Al4V. Variation of friction coefficient as a function of the sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 3 mm, normal load: 0.5 N, spindle speed: 200 rpm). Variation of friction coefficient vs. sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 4 N, spindle speed: 400 rpm). Wear track morphology. (a) FESEM morphology of wear track for bare Ti6Al4V alloy for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after 1,000 cycles, magnification 60X. (b) FESEM morphology of wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 sliding cycles, magnification 80X. (c) AFM surface morphology inside the wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 cycles (scan area: 40µm×40µm, vertical scale: 500 nm).

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List of Figures

Figure 4.10 Figure 4.11 Figure 5.1 Figure 5.2 Figure 5.3 Figure 5.4 Figure 5.5 Figure 5.6 Figure 5.7

Wear track and counterface analysis. (a) EDX analysis of wear track for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after completion of 175,000 cycles. (b) EDX analysis of Ti6Al4V surface without polymer coating. (c) Optical image of Si3N4 ball for high normal load tribology test after completion of 175,000 cycles, 100X. (d) Optical image of Si3N4 ball after cleaning with acetone for high normal load sliding tribology test after completion of 175,000 sliding cycles, 100X. Effect of PFPE overcoat on wear life (track radius = 2 mm, normal load = 4 N, spindle speed = 1000 rpm). Water contact angle measurement from representative samples. (a) Ti6Al4V. (b) Ti6Al4V (after O2 plasma treatment). (c) Ti6Al4V/PFPE. (d) Ti6Al4V/PFPE (heat treated). Water contact angle measurement from representative samples. (a) Ti6Al4V/GPTMS. (b) Ti6Al4V/GPTMS/PFPE. (c) Ti6Al4V/GPTMS/PFPE (heat treated). AFM imaging (scan area: 5µm×5µm, vertical scale: 100 nm). (a) Ti6Al4V. (b) Ti6Al4V/PFPE. (c) Ti6Al4V/PFPE (heat treated). (d) Ti6Al4V/GPTMS. (e) Ti6Al4V/GPTMS/PFPE. (f) Ti6Al4V/GPTMS/PFPE (heat treated). Wide scan XPS spectra of Ti6Al4V/GPTMS specimens. Comparison of C1s peaks for bare Ti6Al4V/GPTMS and Ti6Al4V in C1s scan. Wear durability (number of sliding cycles before failure) of tested specimens in the study. Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/PFPE, (b) Ti6AL4V/PFPE (heat treated) and (c) bare Ti6Al4V) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm).

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List of Figures

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Figure 5.8 Figure 5.9 Figure 5.10 Figure 5.11 Figure 5.12 Figure 5.13

Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/GPTMS/PFPE, (b) Ti6AL4V/GPTMS/PFPE (heat treated) and (c) Ti6Al4V/GPTMS) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm). Optical micrographs of Ti6Al4V specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/GPTMS specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/PFPE specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/PFPE (heat treated) specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/GPTMS/PFPE specimen’s wear track and counterface surface after 100,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.

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List of Notations

List of Notations

ADLC: Amorphous diamond-like carbon

AFM: Atomic force microscopy

ASTM: American society for testing and materials

CVD: Chemical vapor deposition

DLC: Diamond-like carbon

EDX: Energy-dispersive x-ray spectroscopy

Epoxy SAM: 3-Glycidoxypropyltrimethoxysilane

FE-SEM: Field emission- scanning electron spectroscopy

FTIR-ATR: Fourier transform infrared spectroscopy-attenuated total reflectance

GPTMS: Glycidoxypropyltrimethoxysilane

HSS: High speed steel

ISO: International standards organization

L-B: Langmuir-Blodgett method

MEM: Minimum essential medium

MEMS: Micro-electro-mechanical systems

MPa: Mega Pascal

NEMS: Nano-electro-mechanical systems

PDMS: Polydimethylsiloxane

PE: Polyethylene

PEEK: Poly ether ether ketone

PFPE: Perfluoropolyether

PI: Polyimide

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List of Notations

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PMMA: Polymethylmethacrylate

PS: Polystyrene

PTFE: Polytetrafluoroethylene

PVD: Physical vapor deposition

RMS: Root mean square roughness

SAMs: Self-assembled monolayers

SEM/EDS: Scanning electron microscope equipped with x-ray energy dispersion spectroscopy Si3N4: Silicon nitride

TO: Thermal oxidation

UHMWPE: Ultra-high-molecular-weight polyethylene

XPS: X-ray photoelectron spectroscopy

Chapter 1: Introduction

Chapter 1

Introduction

1.1 Importance of tribology

Tribology is defined as the discipline to study the science and technology

of interacting surfaces in relative motion and of associated subjects and practices

[Jost 1966]. The word “Tribology” was originated from the Greek word “Tribos”

which means rubbing [Dowson 1979]. Tribology investigates the principles and

related practices of friction, lubrication and wear phenomena to understand the

interaction of contact surfaces in a given environment.

Friction is defined as the resistance between interacting surfaces under

relative motion. Wear can be described as the material removal phenomenon due

to interaction of surfaces in relative motion. Friction and wear are often

unavoidable phenomena in sliding and rolling surface contacts. Lubrication is the

method employed to reduce friction and wear of contact surfaces in relative

motion by interposing a material called lubricant. Lubricant can be of any

material state such as solid, liquid and gas or a combination of them.

Friction and wear play important roles in many places in natural

phenomena as well as man-made devices such as automobile, manufacturing etc.

Friction and wear are often undesirable factors in many applications and

adversely affect the performance and efficiency of systems thus scientists and

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engineers strive to come up with means to minimize friction and wear to increase

life and durability of such systems.

Tribology has grown into an important discipline for studying friction,

lubrication and wear principles in order to improve the efficiency of mechanical

systems.

1.2 Brief history of tribology

Although full appreciation of significance of tribology as an independent

discipline has been recognized only recently, human civilization had realized the

importance of friction and wear phenomena since ages. Wheel is the most

important mechanical invention of human civilizations and has been important

milestone in the journey to come up with solutions to address friction and wear

phenomenon in transportation. Known oldest wheel, discovered in Mesopotamia,

dates back to 3500 BC although archaeologists believe that it was invented around

8,000 BC. In 1880 BC, the Egyptians used sledges to transport large statues and

made use of water to lubricate sledges. Leonardo Da Vinci (1452-1519) is known

as the first person to study friction systematically as indicated by the sketches

discovered several hundred years later. Amonton (1699) stated through

experiments that friction force is directly proportional to the applied normal load

and is independent of the apparent area of contact. These two laws are known as

Amonton’s laws of friction. Charles Augustine Coulomb (1785) discovered the

third law that kinetic friction force is independent of sliding velocity. These three

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laws of friction were discovered on the basis of experimental observations and

were related to dry friction.

1.3 Tribological applications

Tribology as a discipline has grown tremendously in sync with the

scientific and technical developments in the world. Being a multidisciplinary

discipline, tribology keeps reinventing itself with developments in science and

technology. Today, tribology has found place in every aspects of everyday life.

Due to the growth of knowledge and interest in tribology for different

applications, this discipline has been further divided into different areas.

1.3.1 Industrial tribology

One of the important factors affecting the performance of the machines is

the nature of interacting surfaces. Thus friction and wear become important

considerations in the functioning of machines. Industrial tribology has found

important place in production, manufacturing, fabrication, aviation, aerospace and

marine sectors.

1.3.2 MEMS/NEMS tribology

With the development of MEMS/NEMS applications, it has been observed

that friction and wear at small length scales become limitations for efficiency and

durability of devices. At small length scales, surface forces become predominant

compared to inertial forces. Thus MEMS/NEMS devices require specialized

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solutions to address tribological limitations to reduce friction and increase wear

durability.

1.3.3 Biomedical tribology

With the development of biomedical engineering, researchers are

investigating the application of tribological principles for the improvement of

functioning of medical implants and patients’ comfort.

With continued research by tribologists, this area has grown tremendously

and has been able to make useful contributions to biomedical engineering.

Tribology has found importance in improving implants life and in reducing

patient trauma in biomedical applications.

1.4 Importance of titanium and titanium alloys

British mineralogist and chemist, William Gregor, discovered titanium

metal in 1791. A Berlin chemist, Martin Klaporth, independently isolated titanium

oxide in 1795. He named it titanium after Greek mythological name “Titans”. The

most popular titanium alloy Ti6Al4V was developed in the late 1940s in the

United States [Leyens and Peters 2003].

Titanium and its alloys are widely used in biomedical, aerospace, aviation,

marine, chemical industry, sports and leisure applications due to its high specific

strength and excellent corrosion resistance.

The ASTM defines a number of alloy standards from Grade 1 to 38

(ASTM B861 - 10 Standard Specifications for Titanium and Titanium Alloy

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Seamless Pipe). Ti6Al4V is the most commonly used titanium alloy for

biomedical and industrial applications. Its chemical composition consists of 6%

aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen and

remaining of titanium. It is significantly stronger than pure titanium while

stiffness and thermal properties are same as that of pure titanium (although

thermal conductivity is about 60% lower in Grade 5 Ti compared to that of pure

Ti). Grade 5 is heat treatable and is an excellent combination of strength,

corrosion resistance, weldability and fabricability. Grade 5 can be used upto 400

degrees Celsius temperature [Leyens and Peters 2003]. Ti6Al4V is most widely

used among titanium alloys [Donachie 2000] and is considered as the

"workhorse" of the titanium industry.

1.4.1 Industrial applications

Titanium and its alloys are used in industrial applications due to its high

specific strength, fatigue strength and creep resistance at high temperature

[Leyens and Peters 2003].

The much higher payoff for weight reduction in aircraft and spacecraft is

the driving factor for the usage of titanium and titanium alloys. In jet engine,

titanium is the second most common material after Ni-based super-alloys. It is

widely used in airframe and gas turbine engine due to the weight saving

considerations.

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Highly stressed components of helicopters such as rotor mast and head are

made from titanium alloys. In space applications, titanium alloys are used

extensively due to small payload requirement of space vehicles.

Titanium is reactive metal but is extremely corrosion resistant due to its

stable oxide layer at surfaces. Due to its corrosion resistant behavior, titanium

alloys are popular in chemical, process and power generation industries. Heat

exchangers, condensers, containers, apparatus and steam turbine blades are made

from titanium alloys. It is also used in photochemical refineries and flue gas

desulphurization plants.

Titanium alloys show excellent corrosion resistance in seawater and sour

hydrocarbons, thus they are widely used in marine and offshore applications.

Titanium alloys are used in automobile industry to improve performance at

reduced weight although their use has been limited to racing and high

performance sports car due to higher cost.

1.4.2 Consumer durables

In sports and leisure, titanium alloys are used in making golf clubs, tennis

racquets, baseball bats, pool cues, high speed cycling, scuba diving equipment,

expedition and trekking equipment [Leyens and Peters 2003]. Titanium alloys

have also found usage in architecture due to its excellent immunity to

environmental corrosion and a low coefficient of thermal expansion.

In jewellery and fashion industry, titanium alloys are gaining popularity

due to its lightweight, corrosion resistant, hypo-allergic nature and possibility of

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creating a large range of surface finishes by utilizing anodizing and heat

treatment. Besides titanium alloys are finding place in musical instruments,

optical instruments, information technology and security applications due to its

versatile properties.

1.4.3 Medical applications

Excellent compatibility with the human body makes titanium a key

material for biomedical implant materials. It is resistant to corrosion from body

fluids. Their excellent fatigue property, high specific strength and low modulus of

elasticity make it a preferred material for orthopedic devices. Bone fracture plates,

screws, nails and plates for cranial surgery are made from titanium alloys [Leyens

and Peters 2003].

Shape memory property of Nitinol (a titanium alloy) makes it suitable for

some specialized applications such as stent. Titanium has widespread usage in

dental implants due to its biocompatibility and low thermal conductivity.

1.4.4 MEMS applications

Titanium is also been proposed as a potential MEMS (Micro-electro-

mechanical systems) material for its physical and mechanical properties. Titanium

and titanium alloy MEMS can be preferably used in biomedical applications due

to its excellent biocompatibility.

As a potential MEMS material for its physical and mechanical properties

as well as biocompatibility, titanium alloy can be used in many MEMS

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applications [Aimi et al. 2004]. In MEMS applications, lubrication is required to

reduce adhesion, friction and wear to ensure the reliability and durability of

devices.

The durability and reliability of MEMS/NEMS devices are affected by

surface properties such as adhesion, friction, and wear [Bhushan 2003, 2004,

2005]. This requires the application of ultra-thin lubricant films having low

friction and low adhesion as well as high wear durability to protect the contact

surfaces in MEMS/NEMS devices.

1.5 Titanium and titanium alloys tribology

Titanium and titanium alloys have found many applications due to its high

strength-to-weight ratio, excellent corrosion resistance and biocompatibility.

Unalloyed titanium is as strong as steel but has 45% less weight. Titanium can be

alloyed with aluminium, vanadium, molybdenum and iron to produce lightweight

strong alloys to produce alloys of importance in biomedical, industrial, marine,

automotive and aerospace applications.

Application of titanium alloys in many areas is limited by its tribological

properties such as high friction coefficient, poor wear durability and low surface

hardness. Its poor tribological properties are caused by severe adhesive wear with

a strong tendency to seizure, low abrasion resistance and the lack of mechanical

stability of the oxide layer [Budinski 1991; Yildiz et al. 2009]. Titanium tribology

has found great interest among researchers due to possible application of titanium

alloys with improved tribological properties in many potential areas.

8


In industrial applications, various surface treatments such as thermo-

chemical processes, energy beam surface alloying and duplex treatments have

been proposed by researchers to address the tribological limitations of titanium

alloys [Bloyce 1998].

For biomedical applications, plasma nitriding and bio-ceramic coatings

are widely investigated solutions to improve the tribological properties of alloys

in orthopedic implants [Molinari et al. 1997; Yildiz et al. 2008; Fei et al. 2009].

1.6 Objectives of the thesis

The objective of this thesis is to evaluate some of the potential solutions

for surface modifications of titanium and titanium alloys to improve its

tribological properties.

In the first study, UHMWPE and UHMWPE/PFPE thin film coatings were

evaluated to address Ti6Al4V alloy tribological limitations. Experimental

characterizations of the physical, chemical and tribological properties of coatings

were carried out. This study also investigates the underlying mechanism for

excellent UHMWPE thin film tribological properties. In this study, following

approaches have been used:

Use of UHMWPE polymer coating to improve the tribological properties

of Ti6Al4V alloy.

Use of PFPE as a top layer to further improve the wear resistance of

UHMWPE film.

9


In the second study, GPTMS self-assembled monolayers (SAMs) coating

with PFPE overcoat was evaluated to address tribological limitations of Ti6Al4V

alloy. In this study, following approaches have been used:

Use of PFPE to improve the tribological properties of Ti6Al4V alloy.

Use of PFPE overcoat to improve the tribological properties of GPTMS

SAMs coated Ti6Al4V alloy.

1.7 Methodology in the present thesis

To achieve above objectives, UHMWPE polymer and GPTMS SAMs

coatings were deposited onto Ti6Al4V alloy substrate. Oxygen plasma treatment

was used to clean and improve adhesion properties of Ti6Al4V alloy substrate

with coatings. PFPE top layer was used to enhance the wear durability of

coatings.

Following process diagram (Fig. 1.1) represents the summary of the steps

followed in this thesis for different studies.

Figure 1.1: Research methodology followed in the research studies.

10


11

1.8 Structure of the thesis

Present thesis consists of a total of seven chapters. Literature review in the

field of titanium and titanium alloys tribology as well as thin film coatings is

presented in Chapter 2. Chapter 3 provides the detailed information of materials

and experimental characterization techniques (physical, chemical, biological,

tribological) used in this thesis. Chapter 4 presents the results and discussions for

the tribological evaluation of UHMWPE thin film coating on Ti6Al4V alloy

substrate and the effect of PFPE overcoat. Chapter 5 consists of results and

discussions for the tribological characterization of PFPE overcoated bare

Ti6Al4V alloy and GPTMS SAMs deposited Ti6Al4V alloy. Chapter 6

summarizes the conclusions drawn from studies and Chapter 7 presents the

recommendations for future studies from the work presented in this thesis.

Chapter 2: Literature Review

Chapter 2

Literature Review

2.1 Surface engineering and tribology

All solids are bound by surfaces. Surfaces act as an interface between

solid and its environment. Many of the properties of the solids are governed by

the solid’s interaction with the environment through the surfaces. Surface is the

most important part in many engineering applications since most failures such as

corrosion, fatigue and wear initiate at surfaces. Even though many solids have

desired bulk properties, they lack suitable surface properties. Surface engineering

involves modifying surface properties of the components to suit different

applications. Surface engineering techniques are used in the automotive,

aerospace, missile, power, electronic, biomedical, textile, petroleum,

petrochemical, chemical, power, steel, cement, machine tools and construction

industries. Tribological properties such friction coefficient and wear are mainly

affected by surface properties. For tribological applications, surface engineering

consists of surface modifications, surface coatings and treatment techniques.

Surface modification also consists of changing the texture of a component.

Objective of surface engineering in tribology is to reduce friction coefficient and

increase wear durability at contact interfaces in many applications.

Many materials possess required bulk properties such as strength and

toughness but do not possess suitable surface properties for tribological

applications. In these cases, modifying surface properties to reduce coefficient of

12


friction and wear resistance by surface engineering serves the purpose [Bhushan

and Gupta 1991].

2.2 Existing tribology solutions for titanium alloys

Higher friction coefficient and low wear durability of titanium and

titanium alloys are attributed to the presence of mechanically unstable titanium

oxide film and the high surface energy leading to adhesive wear [Budinski 1991;

Yildiz et al. 2009]. In titanium and titanium alloys, surface wear occurs by

adhesive and abrasive mechanisms as well as by subsurface damage via plastic

deformation. The different processes used for the tribological improvements of

titanium alloys are surface treatments (thermally sprayed coatings, electroplating

and electroless plating systems, physical vapour-deposited coatings and surface

modifications), thermo-chemical processes (nitriding and oxidising), energy beam

surface alloying (laser gas nitriding and electron beam alloying) and duplex

treatments [Bloyce 1998].

2.2.1 Surface treatments

2.2.1.1 Thermally sprayed coatings

In thermal spraying process, solid rod or powder of metal and/or ceramic

is partially or fully melted and sprayed onto the substrate on which it re-solidifies.

No special pre-treatment is required for titanium and titanium alloys before

thermal spraying. Plasma spraying, detonation gun, high-velocity oxy-fuel and

vacuum plasma spraying are used to deposit these coatings [Bloyce 1998].

13


Hard materials such as WC-Co, Mo and Cr-Ni are sprayed onto Ti6Al4V

to provide wear resistant coatings. 75 µm of molybdenum is sprayed onto the

stems of titanium automotive valves to prevent galling. Molybdenum coating

exhibits low coefficient of Friction, lubricant retention ability, hardness and wear

resistant properties. In aero engines and other gas turbine applications, sprayed

coatings on titanium are used to improve wear durability. Tungsten carbide-cobalt

(WC-Co) material is sprayed onto titanium alloy mid-span support faces to

provide protection against fretting wear in the low-pressure compressor. Thermal

spraying methods are more suitable for localized areas than for complete surface

of components.

2.2.1.2 Electroplating and electroless plating systems

Since passivating film of TiO2 acts as a weak interface between the

coating and titanium alloy substrates, pretreatments methods such as abrasive

blasting and copper strike are required prior to plating. Hard chrome plating,

coatings based on electroless nickel and soft metallic coatings including silver and

copper, are used on titanium alloy substrates to protect against wear. Oil seal

collars, pistons, racing car fly-wheels and bearing housings are plated with hard

chrome [Bloyce 1998].

2.2.1.3 Physical vapour-deposited coatings

Metals, alloys, compounds or metastable materials are deposited on

different substrates using physical vapour deposition (PVD) methods. TiN coating

14


using PVD method is coated onto titanium alloy parts for racing cars and

aerospace components where strength-to-weight ratio is an important

consideration. TiN is coated onto pump parts and valve components in oil,

chemical and food industries due to its corrosion resistance property [Bloyce

1998].

Diamond-like carbon (DLC), amorphous diamond-like carbon (ADLC),

hydrogenated carbon films (a-C:H) [Kustas et al 1993] and MoS2 [Buchholtz and

Kustas 1996] coatings by PVD method are finding applications due to their low

coefficients of friction and wear durability. Applications of the most of the

developed PVD coatings is limited to low contact stress areas to avoid plastic

deformation of the substrate.

2.2.1.4 Surface modifications

Ion implantation is one of the widely used surface modification techniques

for titanium alloys [Perry 1987]. Commonly implanted species are nitrogen and

carbon for Ti6Al4V alloy. Improvement in the wear durability is caused by the

increase in surface hardness. An increase in hardness results in resistance to the

plastic deformation and thus, oxide layer can support higher stresses. Various

titanium prosthetics in biomedical applications are ion-implanted.

By anodizing titanium and titanium alloys, layers of TiO2 less than 100

nm thickness are produced on the surface to increase the wear resistance of the

alloys. Anodizing improves the tribological properties of titanium and titanium

alloys compared to untreated material but it is not able to support higher loads.

15


Anodizing is frequently used on titanium fasteners and considered as minimum

base treatment for titanium and titanium alloys to improve tribological properties

[Bloyce 1998].

2.2.2 Thermo-chemical processes

Titanium can be thermo-chemically alloyed with interstitial elements such

as boron, carbon, nitrogen and oxygen. Phase equilibrium exists for boron and

carbon with titanium element so only a thin compound layer can be produced with

boron and carbon which means that solid solution hardening does not exist below

the thin compound layer. Thus this condition is similar to surface modification

produced by PVD coatings [Bloyce 1998]. Solid-solution-hardened diffusion

zones exist below the surface compound layers in the case of nitriding and

oxidizing so these methods are more useful considering depth-hardening criteria.

2.2.2.1 Nitriding

Nitriding of titanium and titanium alloys has been widely used effectively

as a surface treatment for protection against wear [Molinari et al. 1997].

Components treated by nitriding include racing engine components, surgical

instruments, racing car components, watchcases, precision mechanical parts and

golf club heads. Plasma ion or glow discharge nitriding have long been applied in

the surface treatment of titanium-based materials. Treatment gases such as

nitrogen, nitrogen-hydrogen mixtures, nitrogen-argon mixtures or cracked

ammonia and temperatures in the range 700-900 ºC are used in plasma nitriding.

16


Racing car steering racks, gears and ball valves are treated using plasma nitriding

[Bloyce 1998].

2.2.2.2 Oxidising

Tribological properties of titanium and its alloys can be improved by

oxidizing. Oxygen in solution in αTi results in the significant strengthening of the

material although it affects adversely the properties such as tensile ductility,

fracture toughness and fatigue crack growth. Considering balance of the change in

the properties, oxidising is used in limited cases. Controlled oxidizing in lithium

carbonate salt baths is used for the production of titanium pistons. A treatment

based on oxidizing in air has been used to surface treat Ti22V4Al alloy valve

spring retainers. Diffusion hardening has been successfully applied in the surface

treatment of the biomedical alloy Ti-13Nb-13Zr alloy to improve tribological

properties [Mishra et al. 1994]. Ti6Al4V alloy treated by a thermal oxidation

(TO) process exhibits low coefficient of friction and low wear rates. Improvement

in properties can be attributed to the formation of oxide layer and a hardened

diffusion zone. TO is used for surface treatment of auto-engine components.

To address the tribological limitations of titanium alloys by oxidising,

different solutions such as plasma nitriding, plasma immersion ion implantation,

plasma spraying, PVD, CVD and laser surface treatments have been explored

[Bloyce 1998].

17


2.2.3 Energy beam surface alloying

Energy beam surface alloying method changes the chemical composition

of the material in the liquid state during surface melting. Greater depth of

hardening at surface is obtained through this method compared to other surface

engineering methods.

2.2.3.1 Laser gas nitriding

Nitrogen is a widely researched alloying element for titanium and titanium

alloys. Laser gas nitriding is carried out using CO2 laser, a gas jet of blowing

nitrogen or nitrogen and argon at the melt pool. Different structures are obtained

by controlling the amount of nitrogen in the gas jet [Bloyce 1998].

2.2.3.2 Electron beam alloying

Different alloying elements can be used to increase surface hardness and

wear durability by using electron beam alloying method [Bloyce 1998]. This

process can produce different microstructures with defined properties by

controlling the amount of alloying elements. Silicon and carbon alloyed surfaces

exhibit significant improvements in tribological performance. Titanium alloyed

with silicon and carbon results in a tough hard layer, whereas titanium alloyed

with silicon and nitrogen results in a hard wear-resistant layer. Electron beam

alloying method is a line-of-sight method and is not suitable for complex

geometries.

18


2.2.4 Duplex treatments

Combination of two processes can be used to obtain the best improvement

in tribological properties as observed in different studies. Relatively deep cases in

material surfaces can be achieved by combining surface alloying processes and

thermo-chemical treatments. Low-friction hard surfaces can be obtained using a

combination of thermo-chemical processes and PVD coating processes [Dong et

al. 1996].

2.3 Titanium alloys surface modifications for biomedical

applications

Plasma nitriding, CVD, PVD, plasma immersion ion implantation, plasma

spraying and laser surface treatments are used to improve wear and corrosion

properties of titanium and its alloys in medical implants applications [Yildiz et al.

2009].

2.3.1 Plasma nitriding

Thermo-chemical diffusion plasma nitriding process is one of the common

methods to produce hard and wear resistant nitrides on the surface of titanium

alloys. Compound and diffusion layers formed on the surface as a result of the

nitriding process increase the surface hardness, wear and corrosion resistance of

the titanium alloy medical implants.

19


2.3.2 Bio-ceramic coatings

Different bio-ceramic coatings made of inorganic material such as TiN,

TiAlN and Al2O3 on the surface of titanium alloys exhibit anti-allergic and non-

cancerous nature as well as good corrosion resistance and excellent tribological

properties. Al2O3 coating shows lower friction coefficient (0.4), high strength,

wear resistance, chemical inertness and excellent corrosion resistance in hip–

prosthesis applications [Yildiz et al. 2009]. TiAlN deposited by PVD techniques

is used in many implant applications. TiAlN coating has good mechanical

properties, high wear resistance and biocompatibility.

2.4 Thin film coatings in tribology

Thin film surface coating of functional material is commonly used in

various fields of technology such as optical devices, electrical equipment, tools

for cutting, forming etc [Hedenquist et al. 1992; Sproul 1996; Zweibel 2000].

Depending upon the application, pure metals, compounds and ceramics are used

as coating materials. Thin surface coating technique has many advantages.

Surface properties can be tailored by coating while bulk properties of the

materials are retained. This provides capability to provide optimized properties

for the desired application. Materials that are difficult to synthesize utilizing other

methods can be used as a coating material. Since coating requires usage of a small

amount, expensive material can also be used as thin film coatings.

Thin film coatings have also become popular in tribology due to their

effectiveness and potential applications [Holmberg 1994]. Thin film coating

20


method can be applied to improve tribological performance of materials by

applying a thin layer of a low friction coefficient and wear resistant material.

Various methods such as gaseous state and solution state as well as molten and

semi-molten state processes can be applied to deposit coatings.

2.4.1 Polymer coatings in tribology

A polymer can be defined as a molecule composed of many (poly) parts

(mer) joined together by chemical covalent bonds. If all monomer segments of

polymer are the same, it is called a homopolymer. If the monomer segments of

polymer are different, it is called a copolymer. Polyethylene is a polymer formed

from monomer ethylene (C2H4). Ethylene is a gas having a molecular weight of

28. The chemical formula for polyethylene (PE) can be written as -(C2H4)-,

where n is the degree of polymerization. Polymer thin films, when coated onto

various metallic substrates such as steel and aluminium, show excellent

tribological properties [NASA report 1980; Fusaro 1987].

Hard coatings such as metal, ceramic, nitride, carbide and oxide are used

widely to provide wear resistant layers onto components in different engineering

applications. Recently, researchers are showing interest to explore applications of

polymer coatings in different applications due to their good wear and corrosion

resistance, self lubricating characteristics, low noise emission, cost effectiveness

and impact loading performance [Demas and Polycarpou 2008; Zhang et al 2010].

Due to the higher cost and operational difficulties of PVD and CVD coating

21


techniques, polymer coatings can provide a good alternative protecting

technology [Klein 1988].

Commonly used polymers for coatings are polytetrafluoroethylene

(PTFE), polyetheretherketone (PEEK), ultra high molecular weight polyethylene

(UHMWPE), epoxy, polydimethylsiloxane (PDMS) and polymethylmethacrylate

(PMMA). PTFE coating is used in tribological applications due to its excellent

properties such as low friction coefficient, high chemical resistance and high

temperature stability. However, PTFE coating has poor wear and abrasion

resistance which lead to failure in the machine parts [Unal et al. 2004]. PEEK is a

suitable polymer for coating due to its excellent tribological properties, chemical

resistance and high strength. Many researchers [Lu and Friedrich 1995; Friedrich

and Schlarb 2008] have investigated friction and wear behavior of PEEK based

composites.

2.4.1.1 UHMWPE polymer coating tribology

UHMWPE is a linear homopolymer. In UHMWPE, the degree of

polymerization can be upto 200,000. UHMWPE can have average molecular

weight of upto 6 million g/mol. UHMWPE molecular weight can not be measured

by conventional means and is measured using intrinsic viscosity. UHMWPE

polymer has outstanding physical and mechanical properties. It is a unique

polymer with excellent impact resistance, abrasion resistance, chemical inertness

and lubricity. UHMWPE exhibits high wear resistance compared to PMMA,

polystyrene (PS), PEEK, and PE. It has been used in many industrial applications.

22


Bulk UHMWPE is commonly used for orthopedic applications due to its excellent

wear resistance.

Besides usage of bulk UHMWPE, there are different studies to explore the

application of UHMWPE coating for different tribological applications.

UHMWPE, as a thin film coating, has shown low coefficient of friction and wear

resistance in some studies [Satyanarayana et al. 2006; Minn and Sinha 2008;

Abdul Samad et al. 2010]. Due to its excellent wear resistance, UHMWPE

coating has the potential to provide a solution for tribological limitations of

titanium alloys in biomedical and industrial applications.

2.4.2 Self-assembled monolayers coatings

Molecularly thin films have received widespread interest due to their

ability to tailor the functionality of the constituent molecules. Molecular thin films

facilitate the investigation of intermolecular forces, molecule-substrate and

molecule-solvent interactions. These interactions affect interfacial properties such

as wettability, biocompatibility and corrosion resistance of the surfaces for

different types of materials [Mrksich 2000; Yan et al. 2000]. Chemical

transformations on molecular films can be investigated in details which can

provide new mechanistic insights as well as methods to tailor surface properties.

Such transformations allow functionalities such as the tethering of biologically

important molecules to surfaces at precisely controlled positions. These

functionalities are of the significant importance in different studies of chemical

biology and micro-array technology [Petty 2002; Pirrung 2002]. Development of

23


molecular films by the chemisorptions between the substrate and head group of

organic molecules provides a method for making ultra-thin organic films having

controlled thickness [Azzam et al. 2002].

Ultra-thin organic molecular layers have also found application as the

lubricants for MEMS systems [Bhushan et al. 1995 (a); Komvopoulos 1996;

Rymuza 1999]. There are primarily two methods to form lubricant layers;

Langmuir–Blodgett method and Self-assembly method [Ulman 1991].

Langmuir–Blodgett (L-B) method cannot be applied for three dimensional

surfaces and is used mainly for flat surfaces such as magnetic recording media

[Ando et al. 1989; Bhushan et al. 1995 (b)] as well as L–B films have only

physically bonding with the substrate by vander Waals forces [Koinkar and

Bhushan 1996]. Compared to this, self-assembled monolayers (SAMs) have easy

preparation methods and possess excellent properties such as low thickness, stable

chemical and physical properties as well as good covalent bonding with the

substrate. Moreover, the properties of the SAMs can be tailored by changing the

type and length of the molecules, terminal group and the degree of cross-linking

within the layer. These properties make SAMs more attractive than the L–B films

[Ulman 1991].

SAMs are ordered molecular assemblies formed spontaneously by the

immersion of a substrate into a solution of the active surfactant due to adsorption

of the surfactant with affinity of its head group to substrate [Ulman 1991]. SAMs

consist of three building blocks (Fig. 2.1). A head group bonds covalently

strongly to a substrate. A tail group constitutes outer surface of the film. A body

24


chain connects head and tail group. For strong binding of SAM molecules to the

substrate, head should contain a group that bonds chemically with the substrate.

Molecular structure and cross-linking of SAMs also affect the friction and wear

properties of coatings.

Figure 2.1: Schematic of typical SAM molecule structure and attachment with substrate.

SAMs are usually formed by solution-based method (immersing a

substrate in a SAMs solution that is reactive to the substrate surface) though vapor

based deposition methods are also used [Ulman 1991]. In general, SAM

molecules are not aligned perpendicularly to the substrate and have tilt angle with

respect to surface. Alkane-thiolates adsorbed on Au exhibit a tilt angle of 30-35o

with the surface normal [Ulman 1996].

By having SAMs with different terminal groups, the film surface can be

made hydrophilic or hydrophobic. A non-polar methyl (-CH3) or trifluoromethyl

(-CF3) terminal group are used for a hydrophobic surface film with low surface

energy. To achieve a hydrophilic film, surface terminal groups such as alcohol (-

OH) or carboxylic acid (-COOH) groups are used. The commonly used surface

25


head groups are thiol (-SH), silane (e.g. trichlorosilane -SiCl3), and carboxyl

groups (-COOH). The commonly used substrates are gold, silver, silicon and

steel. There are many research studies of various SAMs having interaction with

different substrate. The alkyl-silane SAMs on Si [DePalma and Tillman 1989;

Ruhe et al. 1993; Srinivasan et al. 1998; Cha and Kim 2001; Ren et al. 2002;

Sung et al. 2003] and alkyl-thiol SAMs on Au [Lio et al. 1997; Bhushan and Liu

2001; Liu et al. 2001] have been widely studied.

2.4.2.1 Applications of SAMs coatings on titanium

Increasingly, titanium based substrates are gaining importance. SAMs

have been widely investigated as a versatile tool for surface modification in

various applications such as microelectronics, corrosion resistance and biosensors.

They also facilitate studies of wetting, adhesion, friction and related phenomena

[Ulman 1991].

For decades, mineral or metallic surface properties have been tuned by

SAMs [Van Alsten et al. 1999; Barriet et al. 2003; Noel et al. 2006;

Satyanarayana and Sinha 2005; Raman et al. 2006]. Material degradation can be

controlled without altering the visual aspect of the material by forming SAMs of

alkyl perfluorinated chains which are known for their chemical inertness.

The formation of SAMs on metals [Folkers et al. 1995] or metal oxide

surfaces [Gao et al. 1997] is widely employed for the fabrication of model

surfaces with highly controlled chemical properties. SAMs can be used in the

modification of metal oxide surfaces for the investigation of protein adsorption

26


[Shumaker-Parry et al. 2000], for the study of cell behavior [Noel et al. 2006] as

well as for the fabrication of tailored sensor surfaces [Nyquist et al. 2000].

Passivation of metal surfaces, adhesion promotion, and interface corrosion

protection in metal/lacquer systems are other examples of industrial applications

of SAMs [Van Alsten et al. 1999].

Surface modification of titanium and its alloys is of great importance for

their practical applications in biomedical implants. A recent interest has emerged

for organic functionalization of the native oxide surfaces of tantalum, titanium

and related alloys due to their wide use as biocompatible materials in implants

[Viornery et al. 2002; Gawalt et al. 2003]. Covalent surface modification of

titanium dioxide is of great interest in view of its applications in medical implants,

catalysis and polymer fillers.

2.4.2.2 Applications of SAMs coatings in MEMS tribology

Self-assembled monolayers (SAMs) can reduce adhesion and stiction as

well as control friction and wear at the contact interfaces. Thus a lot of attention

has been paid to study them in tribological applications [Choo et al. 2007; Singh

and Yoon 2007].

Even though some SAMs exhibit low coefficient of friction, the wear

resistance achieved by these monomolecular layers is not sufficient to provide

high wear life to the high velocity moving MEMS components. Therefore,

researchers have realized the importance of the mobile portion as the top layer

which can provide self-repairability due to the migration of mobile molecules into

27


the sliding contact resulting into high wear resistance [Katano et al. 2003]. This

concept has been well studied for hard disk lubrication.

2.5 Friction and wear mechanisms in polymer tribology

This section of the thesis has a brief review of polymer tribology with

emphasis on the associated friction and wear mechanisms.

In polymer tribology, friction between two sliding surfaces is primarily

affected by two terms, ploughing term and adhesion term [Briscoe and Sinha

2002].

(a) Ploughing term: This term of the friction is the result of plowing of the

asperities of the counterface into polymer besides the sub-surface

deformation involving plastic flow and fracture depending upon contact

conditions.

(b) Adhesion term: This term of the friction is caused by the adhesive

interaction of very low thickness layer of the polymer in contact with

counterface.

If hardness of one of the sliding surface is high relative to another surface,

plowing term becomes important. Adhesion term depends upon the interfacial

shear stress at the contact surfaces in the absence of hard asperities. Shear stress

(τ) depends upon the contact pressure P as given in Equation (2.1) [Bowden and

Tabor 1986].

τ = τ0 + αP (2.1)

28


where τo is defined as the initial shear stress; α is the pressure coefficient; τ is

defined as the friction force (F) divided by real contact area (A); P is calculated as

the ratio of applied normal load (L) to real contact area (A) [Briscoe et al. 1973].

Thus Equation (2.1) can be rewritten as Equation (2.2) as given below.

F/A = τ0 + α (L/A) (2.2)

Initial shear stress (τ0) can be neglected in most conditions (especially high load

conditions) [Briscoe and Tabor 1975]. Therefore, Equation (2.2) modifies as

Equation (2.3) as follows.

F = α L (2.3)

Where α = μ (coefficient of friction) is the ratio of friction force to the applied

normal load.

Due to visco-elastic nature of polymers, their tribological behavior is more

complex than that for metals or ceramics. For polymers, friction coefficient

depends upon load, contact geometry and loading time [Bowden and Tabor 1986].

General classification of the wear of polymers is shown in Fig. 2.2 [Briscoe and

Sinha 2002].

29


Figure 2.2: General classification of the wear of polymers [Briscoe and Sinha 2002].

In the generic scaling approach, friction is defined as interfacial and bulk

types which result into interfacial and cohesive nature of wear. In the second

approach, phenomenological methods are used to define wear by the different

prevailing mechanisms at the interface. Third approach defines wear using

material characteristics of different polymer types (Fig. 2.2).

There are different mechanisms that contribute to wear such as adhesion,

abrasion, fatigue, erosion and corrosion in sliding conditions (see Fig. 2.3 for

schematic representations of adhesive and abrasive wear mechanism). In most

polymers, wear occurs primarily by adhesive, abrasive and fatigue mechanisms

[Opondo and Bessell 1982]. In adhesive wear, isolated spots on sliding interfaces

adhere and afterwards, shear takes place in sliding at some point other than

original adhering interface. Adhesive wear is the purest and most important form

30


of wear and it can not be eliminated fully in sliding conditions. This form leads to

material transfer from the worn part to the counterface. Load and geometry of

contact as well as surface energy of sliding surfaces affect the characteristics of

the transfered layer. The nature of formed transfer layer greatly affects friction

and wear rate of polymers. In some sliding conditions, polymer has linear

molecules and thin transfer layer protects the counterface. This condition

generates very less friction and wear rate by resulting sliding between polymer

and polymer due to easy shearing in the sliding conditions. Molecular orientation

with respect to sliding direction also affects the friction coefficient. If molecular

orientation is aligned with sliding direction, friction force reduces [Briscoe and

Sinha 2002]. Thick transfer layers in sliding conditions lead to high wear rate.

Molecule types and glass transition temperature of polymer also affect nature of

formed transfer layer.

Figure 2.3: Schematic representations of wear mechanisms (N: normal load; V: sliding velocity). (a) Adhesive wear. (b) Abrasive wear.

Abrasive wear is affected by two-body and three-body mechanisms. In

two body abrasive wear, polymer surface and counterface are involved. If

31


generated debris or foreign particles are trapped between sliding surfaces, it

results into three-body abrasive wear. Abrasive wear depends upon bulk material

properties of polymer as investigated by many researchers [Ratner et al. 1964;

Lancaster 1969 and 1973]. Equation (2.4) describes the relationship between wear

rate and bulk material properties.

Se

LV

(2.4)

Where V = Wear Volume; = Proportionality constant; = Normal load; L =

Sliding velocity; = Hardness of the polymer; = Ultimate tensile stress; =

% Elongation to break.

S e

Equation (2.4) has been validated experimentally in different studies

[Ratner et al. 1964]. Fatigue wear occurs due to the repetitive cyclic loading in

sliding conditions and wear particles are generated in the form of delaminated

flakes by the phenomenon of the fatigue crack growth. Fatigue wear is generally

smaller than adhesive and abrasive wear but can result into severe damage for

mechanical components such as bearings.

2.6 Solution-based coating methods for polymers

Many solution-based coating techniques have been developed by making

use of the physical interaction between the deposited molecules and the substrate

[Advincula et al. 2004]. Following are some of the widely used solution-based

coating techniques:

Spin coating

32


Dip-coating

Printing/droplet evaporation

Doctor blading

Spray coating

In the above mentioned deposition techniques, molecules are adsorbed

from solution onto substrate and solvent evaporates during the coating process.

Layers with desired thickness and homogeneity can be deposited without

significant effort by controlling deposition conditions.

In this thesis, dip-coating method was used to coat thin films of

UHMWPE on Ti6Al4V substrate as well as to overcoat PFPE. In dip-coating

process, a substrate is dipped into a liquid coating solution for the specified time

duration and is withdrawn afterwards from the solution at a controlled speed. Dip-

coating method is an excellent method for producing high-quality and uniform

coatings. Being a cost effective method, it can be easily used in laboratory set-up.

2.7 Use of PFPE as a top layer

Perfluoropolyether (PFPE) lubricants were developed in the early 1960’s

and have been used as lubricants in different applications. PFPE lubricants, a

unique class of lubricants and functional fluids, have found usage in different

applications because of their versatile nature. PFPE lubricants have following

advantageous properties:

Good lubrication properties

33


Low volatility

Non-flammable

Chemical inertness

Low surface tension

Excellent oxidative and thermal stability

Effective in wide temperature range

Non-cytotoxic and biologically inert

Good radiation resistance

PFPE has been applied as a top layer to improve the wear durability of

high speed steel (HSS) tools [Fox-Rabinovich et al. 2002]. It was observed that

after applying PFPE as a top layer, coefficient of friction decreased and wear

resistance of tool improved. Improved properties were attributed to the top layer

of PFPE. Use of a mobile hydrocarbon-based lubricant as top layer also improved

the wear life of monolayers deposited onto Si based MEMS in a study [Eapen et

al. 2005]. PFPE overcoat also resulted in the improved tribological properties of

UHMWPE films deposited on Si substrate [Satyanarayana et al. 2006; Minn et al.

2008]. The method of using PFPE as a top layer to improve the tribological

properties of polymer and SAMs coatings has also been investigated in this thesis.

2.8 Pretreatment methods

Pretreatment methods for surface cleaning and surface energy

modification play a very important role in the adhesion of thin films coating onto

34


a substrate. The wetting property of the substrate affects significantly the adhesion

between the film and the substrate. High surface energy of the substrate is one of

the most important factors that strongly influence the adhesion strength of the

coating [O’Brien et al. 2006]. Hydrophobic substrate surface results into low

adhesion strength of the applied coating.

Surface energy of the surface can be estimated using water contact angle

(θ), which is defined by the Young’s Equation [Wu. 1982]. The contact angle is

defined as the angle at which a liquid/vapor interface meets a solid surface. The

contact angle is a system specific property and is determined by equilibrium of

the drop under the action of interfacial energies (Fig. 2.4). Surface energy of the

interface can be calculated by water contact angle of the surface. The relation

between the contact angle (θ) and interfacial energies is defined as given below

(Equation 2.5).

γLV cosθ = γSV – γSL

(2.5)

γSV = Surface free energy of solid S.

γLV = Surface free energy of liquid L.

γSL = Interfacial free energy between solid and liquid.

θ = Contact angle.

35


Figure 2.4: A schematic diagram of contact angle measurement.

Depending upon the surface energy, surfaces can be divided into high

energy surfaces and low energy surfaces. High energy surface exhibits

hydrophilic nature while low energy surface exhibits hydrophobic nature [Wu

1982]. Different physical and chemical treatments can be used for modifying

surface energy. Higher surface energy (hydrophilic surface) of the substrate

promotes adhesion of the deposited coating.

After going through various surface treatments for titanium alloys, oxygen

plasma was chosen as pretreatment method for Ti6Al4V alloy substrate for

coatings of UHMWPE polymer and SAMs. Oxygen plasma is an environmental-

friendly surface pretreatment technique with cost effectiveness and easier

processing method. Plasma cleaning is one of the effective methods for pre-

treatment of metals to increase surface energy to improve the adhesion between

the film and the substrates. Pre-treatment generates functional groups which cause

an increase in the hydrophilic nature of the surface.

In normal conditions, surfaces exposed to the atmosphere are covered by

organic or inorganic contaminants, CO2 and hydrocarbon. These contaminants

36


prevent good bonding of the coating. Oxygen plasma discharge subjects the

surface to very high energy bombarding electrons which break the molecular

bonds on the surface. Thus, contaminants covering the surface are removed in the

form of CO2 due to the reaction of carbon with the free oxygen radicals in the

plasma. Oxygen radicals generated in the oxygen plasma oxidize the surface

resulting in functionalized groups. “Functionalized groups making the surface

hydrophilic” is called the oxidative effect. Higher wettability assists in improved

adhesion property of the surface [Loh 1999].

Oxygen plasma treatment has been used to create pure and stoichiometric

surface oxide layer on Ti. Oxide layers generated by oxygen plasma method

exhibit reduced stiction for hydrocarbon contaminations [Aronsson et al. 1997].

Oxygen plasma treatment also hydroxylizes Ti surface [Yoshinari et al. 2006] to

facilitate covalent attachment of SAMs on surface. Gas plasma has been used in

SAMs deposition studies as pretreatment for metal oxides to attach SAMs

[Mahapatro et al. 2006]. Stability of octadecyltriethoxysilane and

octadecyltrichlorosilane SAMs deposited on mica was enhanced greatly by the

application of gas plasma pretreatment process [Kim et al. 2002, 2008].

2.9 Biocompatibility testing

Biocompatibility refers to the evaluation of the suitability of materials for

use in implantable medical devices. Biocompatibility examines the interaction

between a medical device and tissues as well as physiological systems of the

patient treated with the device. An evaluation of biocompatibility is an important

37


and early part of the overall testing of a device for intended medical application.

The biocompatibility of a device depends on different factors. Some of the

important factors are the following:

• Physical and chemical nature of device materials.

• Types of patient tissue that will come in contact with the device.

• Duration of device exposure.

ISO 10993 “Biological Evaluation of Medical Devices” documents the

testing strategies that are acceptable in Europe and Asia. Part 1 of the ISO 10993

standard has the general guidance on selection of tests. Part 2 covers requirements

of animal welfare. Parts 3 through 19 are guidelines for specific test procedures

and guidelines on testing-related issues.

Different biocompatibility tests such as cytotoxicity, hemocompatibility,

sensitization, irritation, systemic toxicity, genotoxicity and implantation have

been advised in ISO 10993 as per the requirements of device such as nature and

duration of contact. Any medical implant needs to go through following

categories of tests before getting approval to be used in medical practice.

2.9.1 In-vitro testing

In this method, testing is carried out using cells and tissues outside the

body in an artificial environment.

38


39

2.9.2 In-vivo animal testing

Studies are carried out for implant material in an animal model.

2.9.3 Clinical testing

Tests are carried out in human subjects to verify the safety and

effectiveness of a medical device for intended applications.

Chapter 3: Materials and Experimental Procedures

Chapter 3

Materials and Experimental Procedures

This chapter provides detailed information on the materials used in sample

preparation, coating preparation procedures and descriptions of experimental

methodologies used in physical, chemical, biological and tribological

characterizations of the formed coatings in this thesis.

3.1 Materials

Ti6Al4V-ELI Grade 5 (as per ASTM B265) specimens were used as the

substrate to form coatings. Size of the Ti6Al4V alloy specimen used for coating

deposition was 25mm×25mm (5mm thickness). Ti6Al4V alloy specimens were

procured from Titan Engineering, Singapore.

In the chapter 4 of this thesis, grinding of titanium alloy samples was done

using SiC abrasive papers using 800, 1000 and 1200 grit sizes successively. After

grinding, samples were polished using 5 μm alumina powder paste and 1 µm

diamond paste subsequently. Afterwards, samples were sonicated using acetone,

ethanol and distilled water for the duration of 10 min in each solution and dried

with N2 gas. After grinding, polishing and cleaning, surface roughness of

obtained Ti6Al4V alloy specimens was measured as 44 nm. Roughness of the

Ti6Al4V alloy specimen was measured using AFM in a scan area of

40μm×40μm. AFM roughness measurements were carried out at three different

40


locations onto the sample surface and it provided same roughness value within the

deviation of 5 nm.

UHMWPE polymer powder was procured from Ticona Engineering

Polymers, Germany through a local Singapore supplier. UHMWPE polymer

powder was of GUR X143 grade. Physical properties of the UHMWPE powder

used are as follows:

Melt flow index 190/15 = 1.8±0.5 G/10 min

Density = 0.33±0.03 g/cm3

Average particle size = 20±5 μm

Decahydronapthalin (decalin) was used as the solvent to dissolve

UHMWPE powder. PFPE (Z-dol 4000) with chemical formula (HOCH2CF2O-

(CF2CF2O)p-(CF2O)q-CF2CH2OH; ratio p/q is 2/3) was procured from Solvay

Solexis, Singapore. H-Galden (ZV60) was used as solvent for PFPE and procured

from Ausimont INC. Chemical formula of H-Galden is (HCF2O-(CF2O)p-

(CF2CF2O)q-CF2H, ratio p/q is 2/3). Ethanol, acetone, and distilled water were

used for cleaning of titanium alloy sample surfaces before any surface treatment

and coating.

In the chapter 5 of this thesis, grinding of titanium alloy samples was done

using SiC abrasive papers using 160, 320, 800 and 1000 grit sizes successively.

After grinding, samples were polished using 1 µm and 0.25 µm diamond pastes

subsequently. Afterwards, samples were sonicated using acetone, ethanol and

distilled water for the duration of 15 min in each solution and dried with N2 gas.

Resulting surface roughness (rms) of Ti6Al4V specimens (after grinding,

41


polishing and cleaning) was 12 nm (measured by AFM in a scan area of

5μm×5μm). 3-Glycidoxypropyltrimethoxysilane (CH2–O–CH–CH2–O–(CH2)3–

Si–(OCH3)3) (purchased from Aldrich) was used for the preparation of SAMs

solution. Toluene was used as the solvent to form SAMs solution. A commercial

PFPE Zdol 4000 (molecular weight 4000 g/mol, with OH terminal groups at both

ends, monodispersed) was used to form an overcoating layer.

Hydrofluoropolyether solvent (H-Galden ZV) purchased from Ausimont Inc. was

used for the preparation of PFPE solution without further purification. Toluene

(99.5% anhydrous), ethanol (99.8%), acetone and distilled water were also used

for cleaning the samples.

3.2 Coatings preparation procedure

In the chapter 4 of this thesis, oxygen plasma treatment was carried out on

dried Ti6Al4V alloy specimens (after grinding, polishing and cleaning) in Harrick

Plasma Cleaner. RF power supply of 18 W was used for oxygen plasma

treatment. Titanium alloy surfaces were treated with oxygen plasma under

vacuum for the time duration of 10 min. Oxygen plasma treatment on titanium

alloy surfaces removes contaminations and helps in better adhesion of the

UHMWPE coatings. After oxygen plasma treatment, specimens were used for

dip-coating in UHMWPE solution. Dip-coating was carried out within 30 min of

oxygen plasma treatment to avoid any contamination of the surface after plasma

cleaning.

42


UHMWPE powder dissolution was carried out in decalin first by heating

the polymer solution to 80 °C for 20 min followed by heating to 160 °C for 40

min. UHMWPE polymer powder solution in decalin was prepared at 3 wt%

concentration. To assist dissolution process, magnetic stirrers were used. After the

heating process, solution turns transparent from white which indicates uniform

and complete dissolution.

Dip-coating process was carried out immediately using dipping and

withdrawal speeds of 1.9 mm/s and an intermediate soaking time of 35 sec in

solution (See Fig. 3.1 (a) for dip-coating machine). After completion of dip-

coating, samples were kept in air for 1 min. Afterwards, samples were kept in

clean air furnace at 100 ◦C for a time duration of 20 hrs (See Fig. 3.1 (b) for clean

air furnace). Samples were cooled slowly to room temperature after heat

treatment. Thus obtained UHMWPE coated samples were kept in desiccator

before using it for Ti6Al4V/UHMWPE specimen characterizations.

To achieve overcoat of PFPE on UHMWPE, UHMWPE coated samples

were dip coated in PFPE solution. PFPE dip coating solution was prepared using

0.5 wt% PFPE in H-Galden solvent. Dip-coating was carried out in PFPE solution

for 1 min soaking time and using dipping as well as withdrawal speeds of 2.1

mm/s. For PFPE coating, no heat treatment was carried. These samples were

stored in desiccator till further characterizations for Ti6Al4V/UHMWPE/PFPE

specimens.

43


Figure 3.1: Experimental apparatus. (a) Dip-coating machine. (b) Clean air furnace.

In the chapter 5 of this thesis, Ti6Al4V alloy specimens (after grinding,

polishing and cleaning) were oxygen plasma treated for 10 min. Plasma treated

Ti6Al4V alloy specimens were immersed into the epoxy SAM solution (using

toluene as the solvent) at a concentration of 1 vol% and left for 18 h. Afterwards,

the samples were washed with toluene and ethanol to remove any physisorbed

SAM molecules and dried with N2 gas. Samples were kept in desiccator overnight

before any further characterization for GPTMS coating. This procedure is similar

to that reported in an earlier study [Luzinov et al. 2000] except that the process

was carried out in ambient atmosphere of 25°C and a relative humidity of ~70%

in this study.

PFPE was dip-coated onto the GPTMS SAMs modified samples. Dipping

and withdrawal speeds of 2.1 mm/s were used for the dip-coating process with an

intermediate soaking time of 60 sec in solution. After dip coating, heat treatment

was carried out for samples at 150 ◦C for approximately 1 hr in clean air furnace.

44


3.3 Polymer coating thickness measurement method

In the literature, polymer film thickness has been measured by using

different methods such as non-contact laser sectioning/SEM [Minn and Sinha

2008], profilometer [Satyanarayana et al. 2006], AFM [Lobo et al. 1999] and

focused ion beam/SEM [Abdul Samad et al 2010].

In this study, stylus profilometer in 2D scan mode was used to measure

thickness. Contact force of 25.5 mg and scan distance of 500 µm was used for

scanning. Step-height method was used to measure the thickness from the scanned

profile. Scratches were created in UHMWPE coating using a sharp corner of

another bare Ti6Al4V alloy sample to expose Ti6Al4V substrate. This method

made scratch only on the polymer coating as the scratching tip and the substrate

were of same materials. For measurements of average and standard deviation,

data from three samples were used. In each sample, three scratches were made

and data from three different locations on each scratch were used to measure

thickness. Method used in this study was based on the assumption that bare

Ti6Al4V sample corner will not penetrate another Ti6Al4V substrate significantly

to affect validity of measurement. To validate this assumption, scratches were

created using similar method on bare Ti6Al4V alloy samples. It was observed that

the scratch-depth created in this way on bare Ti6Al4V alloy substrate did not

exceed 0.7 µm in any of the case. This value has been taken into account in

standard deviation measurement of the coating thickness.

45


3.4 Contact angle measurement

Contact angle measurement is one of the methods to evaluate surface free

energy of surfaces. Contact angle measurement is simple, inexpensive and widely

used technique for characterizing the wettability of surfaces. Contact angle is

sensitive to the conditions such as chemistry and topography of top layer. Water

(polar) is the popular liquid to measure the contact angle. Other liquids such as

formamide (polar), hexadecane (non-polar) and diiodomethane (non-polar) are

also used in contact angle measurements.

In this study, static contact angle is used to measure hydrophilic or

hydrophobic nature of the different surfaces (See Fig. 3.2 for contact angle

measurement set-up). VCA Optima Contact Angle System (AST Products, Inc.

USA) was used to measure static contact angle. Distilled water droplets with a

volume of 0.5 µl were used for the measurements. To arrive at average contact

angle and standard deviation of different surfaces, data were collected for three

different samples for five different locations for each sample.

Figure 3.2: Optima contact angle measurement set-up.

46


3.5 Optical microscope

Olympus microscope was used to study counterface and wear track

conditions before and after completion of tribological tests (See Fig. 3.3 (a) for

optical microscope set-up). This microscope uses monochromatic light source and

allows observations at 50, 100, 200 and 500 magnifications. Each silicon nitride

ball, used in the tribological tests, was observed under optical microscope before

test at various magnifications to ensure cleanliness of the counterface. After

completion of each tribological test, wear track and counterface were observed to

investigate underlying wear and friction mechanisms.

Figure 3.3: Experimental instruments. (a) Optical microscope set-up. (b) Ball-on-disk tribometer. (c) Ball-on-disk tribometer stage.

3.6 FE-SEM surface morphology observation

Surface morphology of the Ti6Al4V and UHMWPE coated Ti6Al4V

surfaces was observed using Hitachi S-4300 Field Emission Scanning Electron

47


Microscopy (FESEM). Thin gold coating was carried out on polymer deposited

Ti6Al4V surfaces to make the surface conductive. JEOL, JFC-1200 Fine Coater

was used to perform gold coating. A current of 10 mA and the coating duration of

30 sec were used to deposit gold film on polymer coated surfaces.

3.7 AFM surface topography measurement

Atomic force microscopy (AFM) is a very high resolution microscopy

having resolution on the order of the fractions of a nanometer. In AFM imaging,

cantilever with a sharp tip at its end is used to scan the specimen surface. Tip

interaction with the surface causes tip to deflect due to operating forces between

the tip and the sample. Deflection of the tip is measured by a laser reflected from

the top surface of the cantilever into photodiodes. AFM operates in primarily two

modes. Contact Mode and Tapping mode [Digital Instruments MultiModeTM

Instruction Manual 1997]. In contact mode, tip scans the surface and tip deflection

is used as feedback signal. During scanning, force between the tip and the surface

is kept constant by maintaining a constant tip deflection.

In the tapping mode imaging, the tip is oscillated up and down at near its

resonance frequency while scanning the sample surface. The reflected laser beam

from the tip generates a sinusoidal signal in the photodiode array whose amplitude

provides the image of the scanned surface. As the oscillating tip is scanned over

the surface, tip interactions with the surface cause the change in the amplitude of

oscillating tip. In contact mode, scanning over the surface can cause damage or

48


modify surface features. Thus tapping mode is preferred over contact mode to

measure topography of polymer coatings.

Tip geometry also affects the accuracy of measurements. Sharp tip

generates accurate representation of film while blunt tip provides erroneous

imaging results. Besides tip geometry, the spring constant and the resonance

frequency of tip are important considerations for the accuracy of imaging. A small

spring constant is effective in measuring small forces and a high resonance

frequency is recommended for making tip insensitive to vibrations and

mechanical noise.

Topographic (roughness) measurement of specimens’ surfaces was carried

out using Atomic force microscope (Dimension 3000 AFM, Digital Instruments,

USA). Images were scanned in air using a monolithic silicon tip using the tapping

mode. In measurements, set point voltage used was 1-2 V and scan rate of 0.5 Hz

was used.

3.8 FTIR-ATR analysis

FTIR-ATR (Fourier transform infrared spectroscopy-attenuated total

reflectance) is an analytical technique to identify organic and inorganic materials

by measuring the absorption of infrared radiation by the sample material versus

wavelength. Molecular components and structure of the material can be identified

by infrared absorption. When an infrared radiation is irradiated over the material,

molecules are excited by absorbed IR radiation into a higher vibrational state. The

energy difference between excited vibrational state and ground state determines

49


the wavelength of absorbed light by a particular molecule. Thus absorbed

wavelengths by the sample are characteristic of its molecular structure.

In FTIR spectroscopy, an interferometer is used to modulate the

wavelength generated from a broadband infra-red source. A detector is used to

measure the intensity of transmitted or reflected light as a function of its

wavelength. Thus detector is able to provide the interferogram (intensity of light

as a function of its wavelength). Interferogram is analyzed by computer using

Fourier transforms to obtain a single-beam infrared spectrum. The FTIR spectra

are plotted as intensity versus wave number (in cm-1). The wavenumber is the

reciprocal of the wavelength. The intensity is plotted as the percentage of light

transmittance or absorbance vs wavenumber.

Bio-Rad FTIR model 400 spectrophotometer was used to collect FTIR-

ATR spectrum in air for UHMWPE film. This spectrum was collected from 32

scans at a resolution of 4 cm−1 at four different locations. The spectra were

collected at 5 replicate points. Plasma treated bare Ti6Al4V was used for

background scan for ATR-FTIR spectrum.

3.9 XPS analysis

X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic

technique to measure chemical state, elemental composition, empirical formula

and electronic state of the elements of the material. XPS is operated in ultra high

vacuum (UHV) conditions.

50


In XPS spectra, material is irradiated with x-rays and kinetic energy and

numbers of electrons that escape from the top 1 to 10 nm of the material surface

are analyzed. X-rays are usually generated either by Mg or Al source. X-rays

cause a core electron to be emitted from the sample. The kinetic energy of thus

emitted electron is detected using an electron multiplier and is equal to the

difference between the energy of the X-ray (1253 eV for Mg, 1486 eV for Al) and

the binding energy of the electron.

In this study, XPS is used to study the chemical state of the sample

material. XPS (Thermo Fisher Scientific Theta Probe) was used to study chemical

states of the coatings. A monochromatized Al Kα X-ray source (1486.6 eV

photons) was used at a constant dwell time of 100 ms and pass energy of 40 eV. A

photoelectron take-off angle of 90o was used to obtain the core level signals. All

binding energies (BE) were referenced to the C1s hydrocarbon peak at 284.6 eV.

3.10 Cytotoxicity assessment

The cytotoxicity test was carried out by NAMSA (Northwood, OH, USA).

This test was done according to the guidelines of “International Organization for

Standardization 10993-5: Biological Evaluation of Medical Devices, Part 5: Tests

for In Vitro Cytotoxicity” (see Appendix A for details on cytotoxicity testing

methods). Before cytotoxicity testing, Ti6Al4V/UHMWPE specimens were

sterilized using ethylene oxide and degassed. Fifteen specimens were used for

each test for the cytotoxicity testing using ISO elution method-1×MEM extract

method.

51


In a single preparation, test specimens were extracted in single strength

minimum essential medium (1×MEM) at a temperature of 37 ◦C for the time

duration of 24 hours. The negative control, reagent control and positive control

were also prepared as per the requirements mentioned in testing standards.

Triplicate monolayers of L-929 mouse fibroblast were dosed with prepared

extracts. Incubation was carried out at a temperature of 37 ◦C in the presence of

5% CO2 for a time duration of 48 hours. Following the incubation, the triplicate

monolayers were microscopically (100 magnification) examined for any abnormal

cell morphology and cellular degeneration.

3.11 Tribological characterization

Tribological tests were carried out in this study using UMT-2 (Universal

Micro Tribometer, CETR, USA) (see Fig. 3.3 (b) and (c) for tribometer set-up). It

can be operated in both ball-on-disk and ball-on-plate modes. This tribometer can

apply normal loads upto 500 gm (5 N) and rotational speeds of upto 5000 rpm

(1.05 m/s at a track radius of 2 mm). In this study, tests were carried out in ball-

on-disk mode under dry conditions. Ball-on-disk mode measures dynamic friction

coefficient. Many contact geometries can be simulated using ball-on-disk mode

thus this mode was chosen to evaluate tribological properties of coatings in this

thesis. Ball-on-disk mode schematic has been shown in Fig. 3.4. A Si3N4 ball of 4

mm diameter was used as the counterface for tribological tests. Si3N4 ball had the

surface roughness of 5 nm as provided by the supplier. The ball was thoroughly

cleaned using acetone before each test. Si3N4 ball is used as the counterface since

52


53

Si3N4 material has much higher hardness when compared to Ti6Al4V alloy and is

inert towards organic species.

For tribological characterization, the spindle rotational speeds of 200 and

400 rpm were used in different tests. Track diameter of 4 mm was used for all the

tests. The spindle rotational speed of 200 rpm gives a sliding speed of 41.9 mm/s

at the used track diameter. Applied normal loads from 0.2 N to 4.0 N were used

for various tests. At least three repetitions were carried out for every tested load

and speed combination. Tribological tests were carried out in a class 100 clean

booth environment. Temperature of 25±2 °C and a relative humidity of 55±5 %

were maintained in the clean booth for tests.

The wear life for the tribological tests was taken as the number of life

cycles after which the coefficient of friction exceeded 0.3 or a visible wear

appeared on the substrate whichever occurred earlier [Miyoshi 2001]. The FE-

SEM and optical microscope were used to observe the worn track surfaces after

appropriate number of sliding cycles to infer wear mechanism.

Figure 3.4: Ball-on-disk tribometer schematic (R: Track radius; r: Ball radius; F: Normal load; ω: Rotational speed of the disk).

Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat

Chapter 4

Tribological Characterizations of Thin UHMWPE Film

and PFPE Overcoat

In view of the stated objective of this thesis in Chapter 1, first study of this

thesis titled “Tribological characterizations of thin UHMWPE film and PFPE

overcoat” investigates the potential of thin UHMWPE polymer coating to address

tribological limitations of Ti6Al4V alloy. Rationale behind the selection of

UHMWPE polymer coating in this study has been explained in chapter 2.

UHMWPE polymer thin film was coated onto Ti6Al4V alloy substrate

using dip-coating method. Results of physical characterizations (contact angle,

thickness measurement, FE-SEM morphology and AFM imaging),

biocompatibility test (cytotoxicity) and chemical characterizations (FTIR-ATR

and XPS) of UHMWPE thin film have been reported in this chapter. Use of PFPE

overcoat to further improve the wear resistance of UHMWPE coating has been

evaluated at higher RPM. It was observed that the coating of PFPE onto a

Ti6Al4V substrate without any intermediate layer does not achieve high wear

durability.

Wear track and counterface conditions have been examined to infer underlying

friction and wear mechanism. Potential applications of this coating have been

suggested based upon prior literature studies and the results of this study.

54


4.1 Physical characterizations 4.1.1 Coating thickness measurement

Thickness of UHMWPE coating obtained in this study was measured

using stylus profilometer by step-height method. Fig. 4.1 shows the schematic of

thickness measurement in step-height method. Measured average thickness and

standard deviation of the characterized UHMWPE coatings are 19.6 µm and 2.0

µm respectively.

Figure 4.1: Step-height measurement method. 4.1.2 Water contact angle results

For different specimens, static contact angle measurements were carried

out using VCA Optima Contact Angle System. After grinding, polishing, cleaning

and drying processes, Ti6Al4V alloy surface exhibits water contact angle of

55.1±4.3◦. This measured value matches quite closely with value observed in an

earlier study 50.0±3.1◦ [Ponsonnet et al. 2003]. Bare Ti6Al4V (after polishing,

cleaning and drying processes), when exposed to O2 plasma treatment, showed

water contact angle of 10.4±1.1◦. This low water contact angle value indicates

increased surface free energy of the surface due to plasma treatment. Increased

surface energy of bare Ti6Al4V after oxygen plasma treatment affects the

adhesion of UHMWPE coating on bare Ti6Al4V substrate positively. Surface

energy is one of the significant factors which affect the adhesion of a coating on a

55


substrate [Silbernagl et al. 2009]. UHMWPE coating formed in this study

exhibited a water contact angle of 135.5±3.3◦ and UHMWPE/PFPE composite

layer showed a water contact angle of 128.5±2.9◦. High water contact values of

Ti6Al4V/UHMWPE and Ti6Al4V/UHMWPE/PFPE coatings indicate low

surface energy of the obtained coatings.

Bulk UHMWPE water contact angle values as documented in literature

are close to 90◦ [Chen et al. 2003]. Water contact angle is significantly influenced

by the surface chemistry and surface conditions. The differences in the water

contact angle of obtained UHMWPE coating in this study when compared to that

of bulk UHMWPE can arise due to different surface conditions such as roughness

[Torrisi et al. 2003].

Table 4.1 summarizes static water contact angle values for different

specimens prepared in this study. Fig. 4.2 shows the pictures of water contact

angle measurements of representative specimens from different surface

modifications.

Table 4.1: Measured water contact angle values for different specimens. Specimen Type Water contact angle (degree) Ti6Al4V 55.1±4.3 Ti6Al4V/O2 plasma treated 10.4±1.1 Ti6Al4V/O2 plasma treated/UHMWPE 135.5±3.3 Ti6Al4V/O2 plasma treated/UHMWPE/PFPE 128.5±2.9 Bulk UHMWPE [Chen et al. 2003] 90.0

56


Figure 4.2: Measured water contact angle values for different specimens. (a) Ti6Al4V. (b) Ti6Al4V/O2 plasma treated. (c) Ti6Al4V/O2 plasma treated/UHMWPE. (d) Ti6Al4V/O2 plasma treated/UHMWPE/PFPE. 4.1.3 SEM surface morphology

The surface morphology of the UHMWPE film on Ti6Al4V surface,

observed under FE-SEM, has been shown in Fig. 4.3. UHMWPE film exhibits

fibrous structure as observed in the surface morphology of the film. This coating

shows polymer chains protruding to the surface containing valleys between them.

Fig. 4.3 (a) and (b) show UHMWPE coating surface morphology at 100 and 500

magnification respectively. Images indicate the presence of uniformly distributed

polymer chain density in the UHMWPE coating.

Figure 4.3: Surface morphology of Ti6Al4V/UHMWPE surface using FESEM. (a) At lower magnification, 100x. (b) At higher magnification, 500x.

57


4.1.4 AFM surface morphology

AFM morphology imaging results are shown in Fig. 4.4. AFM imaging

was done for the scan area of 40µm×40µm. This scan area is much smaller than

used for SEM imaging. AFM morphology of Ti6Al4V alloy surface after

grinding, polishing and cleaning is shown in Fig. 4.4 (a). AFM measurement of

Ti6Al4V alloy surface shows an average roughness (Ra) of 44 nm. Fig. 4.4 (b)

shows sub-micron features of the UHMWPE polymer coating formed in this

study. Fig. 4.4 (b) shows the presence of islands and valleys in the formed

UHMWPE coating. The roughness (rms) of the UHMWPE coating measured

using scan area of 40µm×40µm is 0.795 µm. The roughness (rms) of the bulk

UHMWPE measured in an earlier study was 0.332 µm [Satyanarayana et al.

2006]. Different roughness value of the formed UHMWPE coating compared to

that of the bulk UHMWPE can cause differences in the observed water contact

angle results of the coating from the bulk polymer even though both have same

chemical structure.

Figure 4.4: AFM morphology of surfaces. (a) Polished Ti6Al4V alloy surface (scan area: 40µm×40µm, vertical scale: 1µm). (b) Ti6Al4V/UHMWPE surface (scan area: 40µm×40µm, vertical scale: 5µm).

58


4.2 Chemical characterizations

4.2.1 FTIR analysis results

Chemical nature of the coating was analyzed using FTIR-ATR analysis

spectrum. Spectrum was observed to be identical for all measurements at different

locations.

Fig. 4.5 shows the FTIR spectrum of UHMWPE coating on Ti6Al4V alloy

substrate. FTIR spectrum indicates CH stretching modes at 2866 and 2939 cm−1, a

band at 1475 cm−1 corresponding to polyethylene–methylene (CH2) bending and

CH2 rocking mode at 731 cm−1. This spectrum’s characteristics are similar to the

observed UHMWPE or PE polymers spectra characteristics in earlier studies

[Elliott 1969; Kang and Nho 2001]. This FTIR analysis validated that present

coating’s chemical nature is similar to that of bulk UHMWPE.

Figure 4.5: FTIR-ATR spectrum of the UHMWPE coating on Ti6Al4V substrate.


XPS spectra of UHMWPE coating on Ti6Al4V alloy substrate has been

shown in Fig. 4.6 (a). Observed XPS spectra of UHMWPE film coating indicates

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high carbon content due to the presence of polyethylene (CH2) group of

UHMWPE polymer. Absence of titanium peak in the observed XPS spectra

confirms the fact that the Ti6Al4V substrate is completely covered by the

UHMWPE polymer film and there are no pin-holes.

XPS spectra of UHMWPE/PFPE coating on Ti6Al4V substrate has been

shown in Fig. 4.6 (b). The presence of strong F1s peak in XPS spectra confirms

the presence of PFPE overcoat on UHMWPE.

Figure 4.6: XPS spectra of specimens. (a) UHMWPE coating on Ti6Al4V. (b) UHMWPE/PFPE coating on Ti6Al4V. 4.3 Tribological characterization of UHMWPE coating

Bare Ti6Al4V alloy tribological properties were characterized using Si3N4

as counterface in ball-on-disk tribometer and it exhibited a higher friction

coefficient of 0.6~0.7 as shown in Fig. 4.7. Higher friction coefficient of bare

Ti6Al4V alloy is attributed to high surface energy leading to adhesive wear and

the presence of mechanically unstable titanium oxide film [Budinski 1991; Yildiz

et al. 2009]. After coating of UHMWPE film onto the Ti6Al4V alloy surface, the

friction coefficient was reduced significantly from ~0.6 to ~0.15. Bulk UHMWPE

polymer has excellent lubrication properties and its tribological properties have

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been widely investigated and documented in earlier research studies [Wang et al.

1995].

In the initial study, tribological tests were carried out for the normal loads

of 0.5, 1.0 and 2.0 N. Five samples were characterized for each testing conditions.

Tests were carried out for the duration of 96,000 cycles (8 hours) at 200 rpm.

Sliding test beyond 96,000 cycles was not carried out due to long duration of

testing. The UHMWPE film did not fail in any of these tests for the tested

duration of 96,000 cycles. Table 4.2 shows the summary of the tribological tests

carried out on Ti6Al4V/UHMWPE film.

Table 4.2: Summary of tribological tests on Ti6Al4V/UHMWPE specimens.

Test Condition

No.

Test Parameters (Track Diameter,

Normal Load, Spindle Speed)

Maximum Friction Coefficient during

Test Duration Tested cycles

1 6 mm, 0.5 N, 200 RPM 0.15 96,000 (not failed) 2 4 mm, 1.0 N, 200 RPM 0.15 96,000 (not failed) 3 4 mm, 2.0 N, 400 RPM 0.11 96,000 (not failed)

Fig. 4.7 shows the variation of friction coefficient vs. number of sliding

cycles for the test condition No. 1 in Table 4.2. The test specimens exceeded

96,000 cycles without failure.

For comparison, bare Ti6Al4V substrate tribological properties were also

evaluated for the same testing parameters using Si3N4 as counterface in the ball-

on-disk tribometer. Bare Ti6Al4V substrate showed a higher coefficient of

friction of 0.6~0.7 as shown in Fig. 4.7. After coating of UHMWPE polymer film

onto the Ti6Al4V alloy surface, the coefficient of friction was reduced to ~0.15 as

shown in Table 4.2 and Fig. 4.7.

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Figure 4.7: Variation of friction coefficient as a function of the sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 3 mm, normal load: 0.5 N, spindle speed: 200 rpm).

Afterwards, four samples were tested at a high normal load of 4 N (track

radius = 2 mm, spindle speed = 400 rpm) till film failure was observed. Fig. 4.8

shows the variation of friction coefficient vs. number of cycles of revolutions for

a typical run at this high normal load test condition. Test result shows wear

durability of 187,000±8,000 cycles and a maximum coefficient of friction of 0.10

before failure of the film.

Figure 4.8: Variation of friction coefficient vs. sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 4 N, spindle speed: 400 rpm).

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4.4 Investigation of underlying wear mechanism

To investigate the underlying wear mechanism in the above tribological

studies, wear track and counterface conditions were investigated for one of the

high load test conditions (track radius = 2 mm, normal load = 4 N, spindle speed

= 400 rpm) where test was stopped after 175,000 sliding cycles to investigate

counterface and wear track conditions even though the film had not failed.

FESEM image of the wear track of Ti6Al4V alloy (Fig. 4.9 (a)) showed

that the specimen surface, without UHMWPE coating, is extensively damaged

only after 1,000 sliding cycles and there is significant accumulation of wear

debris around the wear track. Compared to this, wear track morphology of

Ti6Al4V/UHMWPE is smooth even after 175,000 sliding cycles as seen in Fig.

4.9 (b). AFM morphology (Fig. 4.9 (c)) of the surface inside the wear track

showed an average roughness (rms) of 35 nm which is very smooth compared to

the initial roughness (0.795 µm) of the UHMWPE polymer coating. Protrusions

and valleys of the polymer coating are flattened (ironed) in the initial running-in

period of the sliding cycles of the tribology test.

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Figure 4.9: Wear track morphology. (a) FESEM morphology of wear track for bare Ti6Al4V alloy for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after 1,000 cycles, magnification 60x. (b) FESEM morphology of wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 sliding cycles, magnification 80x. (c) AFM surface morphology inside the wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 cycles (scan area: 40µm×40µm, vertical scale: 500 nm).

EDX analysis at different locations inside the wear track confirms the

presence of polymer film proving that the coating was not removed even after

175,000 cycles of sliding (Fig. 4.10 (a)). Six locations were investigated in EDX

analysis and in all cases EDX analysis indicated high percentage of carbon (>

97%) and no Ti presence (only trace Ti peak was visible because of the detection

of the substrate). Compared to this, EDX analysis of bare Ti6Al4V alloy showed

high percentage of Ti (as seen in Fig. 4.10 (b)). Au peaks have also been observed

in the EDX analysis because of the gold coating deposited on the specimens

before FESEM/EDX analysis to make coating conductive as required in EDX

analysis. The optical image of Si3N4 ball (Fig. 4.10 (c)) at the end of tested

175,000 sliding cycles on the UHMWPE film showed that there was only a little

polymer transfer to the counterface. After cleaning the ball with acetone, no

permanent physical damage was observed on the Si3N4 ball surface (Fig. 4.10

(d)), indicating that the UHMWPE coating was able to protect the counterface as

well during tribological tests.

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Figure 4.10: Wear track and counterface analysis. (a) EDX analysis of wear track for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after completion of 175,000 cycles. (b) EDX analysis of Ti6Al4V surface without polymer coating. (c) Optical image of Si3N4 ball for high normal load tribology test after completion of 175,000 cycles, 100x. (d) Optical image of Si3N4 ball after cleaning with acetone for high normal load sliding tribology test after completion of 175,000 sliding cycles, 100x.

Since polymer coating presence is observed on the substrate and the

counterface surfaces which suggest that after certain number of sliding cycles,

contact is between polymer/polymer which can protect the substrate and the

counterface surfaces. The phenomenon of sliding between the polymer coating on

the substrate and the transferred polymer film on the counterface can result in low

friction coefficient and high wear resistance as observed in similar studies if the

polymer transfer layer formed on counterface is thin and material transfer is not

extensive [Briscoe and Sinha 2002].

4.5 Effect of PFPE overcoat on UHMWPE coating

Tribological tests were carried out at high rotational speed (1000 rpm) for

UHMWPE/PFPE coating to evaluate the effect of PFPE coating on UHMWPE

film wear durability. For the comparison, UHMWPE coating was also tested for

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similar conditions (track radius = 2 mm, normal load = 4 N, spindle speed = 1000

rpm). Five samples were tested for each surface coating to obtain the wear life.

UHMWPE on Ti6Al4V showed a wear life of 28,000±8,000 cycles at these

sliding conditions. After PFPE coating, wear life increased to 60,000±14,000

cycles. Thus PFPE top layer improved the wear life of UHMWPE film as

evaluated at the high rotational speed testing condition. This comparison has also

been plotted in a bar chart shown in Fig. 4.11.

Figure 4.11: Effect of PFPE overcoat on wear life (track radius = 2 mm, normal load = 4 N, spindle speed = 1000 rpm).

4.6 Explanation of wear resistance increase by PFPE overcoat

The observed improvement in wear resistance by PFPE overcoat is similar

to the work done in earlier research studies [Satyanarayana et al. 2006; Abdul

Samad et al. 2010]. PFPE exhibits lower surface energy [Makkonen 2004],

excellent lubrication properties and thermal stability [Liu and Bhushan 2003].

These properties are beneficial for severe tribological condition for the tested

condition of 1000 rpm in this study.

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Since UHMWPE polymer surface does not have any reactive chemical

groups to react with PFPE molecules, possibility of any chemical interaction

between UHMWPE and PFPE can be eliminated. Therefore as hypothesized in

earlier studies, PFPE molecules can be entrapped during coating into rough

features of UHMEPE film such as valleys and can result into good lubrication

properties during sliding testing due to availability of lubricant [Satyanarayana et

al. 2006; Abdul Samad et al. 2010]. PFPE spreads well on polymer surfaces due

to low surface tension.

4.7 Biocompatibility assessments

Different biocompatibility tests have been suggested by ISO 10993

guidelines for biomedical devices based upon mode of contact, application area

and duration of contact in different applications. However cytotoxicity test is

common test applicable to all implant applications and used as a preliminary test

to evaluate the feasibility of using any coating in biomedical implant applications.

This test assesses the potential cytotoxic effects of leachable extracted from test

specimens’ surfaces.


In this study, UHMWPE and UHMWPE/PFPE coatings were tested for

cytotoxicity on Ti6Al4V alloy substrate using ISO elution method-1×MEM

(minimum essential medium) extract method. In cytotoxicity testing carried out in

this study, no cytotoxicity effects such as cell lysis were seen in any of the test

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wells during microscopic examination in the ISO elution method-1×MEM extract.

No change in the pH value was noticed after duration of 48 hours.

Grade should be less than the grade 2 (mild reactivity) for the tested

coatings to meet the requirements of the ISO elution method-1×MEM extract.

Based upon test analysis results, it was inferred that coatings exhibited

cytotoxicity level of grade 0 (reactivity: none) according to test guidelines and

thus meet the requirements of the ISO elution method-1×MEM extract.

4.8 Potential applications of coatings

As observed in this study, the UHMWPE and UHMWPE/PFPE thin film

coatings exhibit hydrophobicity, high wear durability and noncytotoxicity.

Hydrophobic coatings surfaces are water repellent and do not need a liquid

lubrication medium to reduce friction. Due to the low friction coefficient and self-

cleaning nature of surfaces, hydrophobic coatings can find many applications in

biomedical area. For example, in coronary guide-wire application, hydrophobic

coatings improve tactile feedback thus making it easier to grip by cardiologist

during operation. In endoscopic applications, hydrophobic coatings prevent the

sticking of organic material on the surface of the device. Low surface energy of

hydrophobic coatings has been observed to be beneficial in bio-film inhibition for

some in-vivo applications [Roosjen et al. 2006]. In a metallic stent, a hydrophobic

and noncytotoxic coating on stent surface can assist in improving the

compatibility of metal with body fluids by inhibiting release of potential cytotoxic

leachables from metal surface into body environment [Pendyala et al. 2009].

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69

Similarly, wear resistant property of a biocompatible coating is advantageous in

orthopedics and bio-devices applications. In considerations of the above-

mentioned potential applications, UHMWPE and UHMWPE/PFPE thin film

coatings studies can find usage in biomedical devices.

CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer

CHAPTER 5

Tribological Evaluations of Molecularly Thin GPTMS SAMs Coating with PFPE Top Layer

As investigated in the Chapter 4, a thin film of UHMWPE coating

exhibited low friction coefficient and high wear resistance. Use of PFPE as the

top layer further improved the wear resistance of the obtained thin UHMWPE

coating. Due to the combination of hydrophobicity, noncytotoxicity and excellent

tribological properties, potential applications of these coatings in biomedical

instruments were suggested.

In spite of having suitable properties for the biomedical applications,

higher thickness of UHMWPE (~19.6 µm) may prevent its usage in biomedical

MEMS applications. This chapter investigates the feasibility of using molecularly

thin GPTMS/PFPE coating (~4 nm) to address the tribological limitations of

titanium and its alloys in MEMS and in particular bio-MEMS applications. In this

study, GPTMS SAMs with PFPE overcoat has been deposited onto Ti6Al4V alloy

substrate. For comparison studies, PFPE has also been coated onto Ti6Al4V alloy

substrate. Obtained coatings have been characterized by contact angle

measurement, AFM imaging, XPS analysis and tribological tests. Wear track and

counterface surfaces have been examined to understand the underlying friction

and wear mechanism.

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5.1 Physical characteristics of the coatings


After polishing, cleaning and drying processes, bare Ti6Al4V alloy

showed a water contact angle value of 73±4◦. This measured value is close to the

value of 72◦ observed in an earlier study [Wang et al. 1997]. After 10 min of

oxygen plasma treatment of Ti6Al4V alloy, water contact angle was reduced to

6±1◦. This is due to the hydrophilic nature of Ti6Al4V alloy surface after oxygen

plasma exposure indicating increased surface energy. Surface energy is one of the

factors which significantly influence the adhesion of the coating onto the substrate

[Silbernagl et al. 2009]. PFPE coating on Ti6Al4V alloy exhibited a water contact

value of 52±3◦. PFPE coating on Ti6Al4V alloy after heat treatment showed

increased water contact angle value of 110±2◦.

GPTMS coating on Ti6Al4V alloy exhibited a water contact angle value

of 60±2◦ which is in a good agreement with the value of 62◦ reported in an earlier

study [Elender et al. 1996]. PFPE overcoat onto GPTMS coated Ti6Al4V alloy

showed a water contact angle of 90±5◦. After heat treatment, water contact angle

value of Ti6Al4V/GPTMS/PFPE was increased to 112±6◦.

Table 5.1 summarizes the water contact angle values (average and

standard deviation) for different specimens types prepared in this study. Fig. 5.1

and 5.2 show the water contact angle measurement pictures from the

representative specimens prepared in this study.

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Table 5.1: Measured water contact angle values for different specimens.

Specimen Type Water Contact Angle (°) Ti6Al4V 73±4

Ti6Al4V (after O2 plasma treatment) 6±1 Ti6Al4V/PFPE 52±3 Ti6Al4V/PFPE (heat treated) 110±2 Ti6Al4V/GPTMS 60±2 Ti6Al4V/GPTMS/PFPE 90±5 Ti6Al4V/GPTMS/PFPE (heat treated) 112±6

Figure 5.1: Water contact angle measurement from representative samples. (a) Ti6Al4V. (b) Ti6Al4V (after O2 plasma treatment). (c) Ti6Al4V/PFPE. (d) Ti6Al4V/PFPE (heat treated).

Figure 5.2: Water contact angle measurement from representative samples. (a) Ti6Al4V/GPTMS. (b) Ti6Al4V/GPTMS/PFPE. (c) Ti6Al4V/GPTMS/PFPE (heat treated).

Thus, GPTMS/PFPE coating is hydrophobic in nature indicating low

surface energy of the obtained coating. Observed increase in the water contact

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angle value after heat treatment of PFPE coatings is due to the reduction in the

available surface hydroxyl groups [Tao and Bhushan 2005]. Low surface energy

is one of the desirable properties for MEMS component coatings since high

surface energy leads to stiction resulting in the failure of components. Low

surface energy of the coatings suggests their potential applications in preventing

stiction arising due to the surface and capillary forces [Mastrangelo 1997].

5.1.2 AFM morphology results

Bare Ti6Al4V (after polishing, grinding and cleaning) showed the surface

roughness (rms) of 11.8 nm as measured from AFM imaging using the scan area

of 5µm×5µm. Scratches created in the polishing process can be observed in AFM

imaging (Fig. 5.3 (a)) results. Bare Ti6Al4V with PFPE overcoat exhibited the

surface roughness (rms) of 1.7 nm. PFPE coating on Ti6Al4V, after heat

treatment, showed the surface roughness (rms) of 1.2 nm.

GPTMS coating deposited on Ti6Al4V showed the surface roughness

(rms) of 6.8 nm. PFPE overcoat onto GPTMS coating exhibited the surface

roughness (rms) of 2.5 nm. After heat treatment, PFPE coating on GPTMS

showed the surface roughness (rms) of 5.0 nm.

Table 5.2 summarizes the measured surface roughness (RMS and Ra)

values of different specimens using 5µm×5µm scan area. Fig 5.3 shows the AFM

imaging results from different representative specimen types prepared in this

study.

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Table 5.2: Measured surface roughness for different Specimens in AFM imaging.

Specimen Type RMS (nm) Ra (nm) Ti6Al4V 11.8 9.6

Ti6Al4V/PFPE 1.7 1.3 Ti6Al4V/PFPE (heat treated) 1.2 0.7

Ti6Al4V/GPTMS 6.8 4.9 Ti6Al4V/GPTMS/PFPE 2.5 2.1

Ti6Al4V/GPTMS/PFPE (heat treated) 5.0 3.4

Figure 5.3: AFM imaging (scan area: 5µm×5µm, vertical scale: 100 nm). (a) Ti6Al4V. (b) Ti6Al4V/PFPE. (c) Ti6Al4V/PFPE (heat treated). (d) Ti6Al4V/GPTMS. (e) Ti6Al4V/GPTMS/PFPE. (f) Ti6Al4V/GPTMS/PFPE (heat treated).

Thickness of GPTMS coating obtained in an earlier study using similar

deposition method has been reported to be the order of (~1 nm) [Luzinov et al.

2000]. Thickness of PFPE coating deposited using similar procedure followed in

this study has been reported to be the order of (2~3 nm) in an earlier research

study [Eapen et al. 2002]. Since surface roughness of the used Ti6Al4V alloy

substrate is quite high (11.8 nm) in comparison with the deposited GPTMS SAMs

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and PFPE coatings in this study, it is difficult to infer data such as thickness and

topography of deposited coatings from AFM imaging results. As observed in the

AFM imaging results (Table 5.2), GPTMS coating has reduced the surface

roughness of the substrate surface. Observed significant change in the surface

roughness also suggests the possibility of forming GPTMS as multi-layer rather

than monolayer probably due to high humidity ambient conditions [Luzinov et al.

2000]. PFPE coatings onto different specimens also resulted into the reduction of

surface roughness value significantly which can be attributed to the possibility of

linear and flexible PFPE nano-particles filling up the surface textures features

such as valleys.

5.2 Chemical characteristics of UHMWPE coating

5.2.1 XPS analysis

Wide scan XPS spectra of Ti6Al4V/GPTMS specimen showed strong

C1s, O1s and Ti2p peaks as shown in Fig. 5.4. Due to nanometer thickness (< 1

nm) of GPTMS SAMs coating, substrate is also detected in XPS spectra. Fig. 5.5

shows the comparison of C1s peak of Ti6Al4V/GPTMS and Ti6Al4V specimens.

High resolution XPS spectrum of C1s scan for Ti6Al4V/GPTMS specimen

showed two strong peaks. Peak at 284.6 eV represents (C–C) bonds and 286.4 eV

corresponds to the (C–O) bonds in the epoxy SAM molecules. These two strong

peaks are indicative of epoxy SAMs presence as seen in earlier studies [Wong and

Krull 2005; Cloarec et al. 2002]. Compared to this, Ti6Al4V showed only one

C1s peak. Observed change in the XPS core-level spectra of C1s for

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Ti6Al4V/GPTMS specimen in comparison with the bare Ti6Al4V specimen

indicates the deposition of GPTMS SAMs coating.

Figure 5.4: Wide scan XPS spectra of Ti6Al4V/GPTMS specimens.

Figure 5.5: Comparison of C1s peaks for bare Ti6Al4V/GPTMS and Ti6Al4V in C1s scan.

5.3 Tribological characterizations

Bare Ti6Al4V alloy and surface modified Ti6Al4V alloy specimens’

tribological properties were evaluated using Si3N4 as counterface in a ball-on-

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disk tribometer. Table 5.3 tabulates initial friction coefficient (evaluated at the

end of 600 sliding cycles) values of specimens investigated in this study. Fig. 5.6

summarizes the wear life (average and standard deviation) of prepared specimens

in this study. Bare Ti6Al4V alloy and GPTMS deposited Ti6Al4V specimens

showed wear life of less than 100 sliding cycles for tested conditions (not shown

in Fig. 5.6). Wear durability data of samples from each specimen type were

collected from minimum six different samples using three different tracks on each

sample.

Table 5.3: Coefficient of friction for specimens tested in the study.

Specimen Type Initial Coefficient of Friction Ti6AL4V 0.5~0.6 Ti6AL4V/PFPE 0.12~0.13 Ti6AL4V/PFPE (heat Treated) 0.11~0.12 Ti6AL4V/GPTMS 0.5~0.6 Ti6AL4V/GPTMS/PFPE 0.11~0.12 Ti6AL4V/GPTMS/PFPE(heat Treated) 0.12~0.13

Figure 5.6: Wear durability (number of sliding cycles before failure) of tested specimens in the study.

Fig. 5.7 shows the variation in the coefficient of friction with sliding

cycles for representative PFPE coated Ti6Al4V alloy specimens. For comparison,

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result of the tribological characterization of bare Ti6Al4V alloy has also been

tabulated for the same testing conditions.

Bare Ti6Al4V alloy showed high coefficient of friction (0.5~0.6) as

shown in Fig. 5.7 and Table 5.3. High surface energy and mechanical instability

of oxide layer result into poor tribological properties of bare Ti6Al4V alloy

[Budinski 1991; Yildiz et al. 2009]. PFPE modified Ti6Al4V alloy (with heat

treatment as well as without heat treatment) specimens have showed lower

coefficient of friction although sample with heat treatment exhibited lower wear

durability (see Fig. 5.6 and Fig 5.7). Heat treatment increases the bonded portion

of PFPE on the substrate. Reduction in the wear durability can be attributed to the

reduction in the mobile portion of PFPE overcoat after heat treatment. Reduction

in the wear durability after heat treatment of PFPE coating has been observed in

an earlier study onto a different substrate [Miyake et al. 2006].

Figure 5.7: Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/PFPE, (b) Ti6AL4V/PFPE (heat treated) and (c) bare Ti6Al4V) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm).

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Fig. 5.8 shows the variation in the coefficient of friction with sliding

cycles from representative PFPE coated Ti6Al4V/GPTMS specimens. Similar

evaluation has also been obtained for the tribological test of Ti6Al4V/GPTMS

specimen.

Epoxy SAMs (GPTMS) modified Ti6Al4V alloy exhibited high

coefficient of friction (0.5~0.6) at the tested condition of 20 gm normal load and

200 rpm (see Table 5.3 and Fig. 5.8). Observed tribological properties of epoxy

SAMs modified substrate are similar to the observation in an earlier study

[Sidorenko et al. 2002]. PFPE overcoat onto epoxy-SAMs modified Ti6Al4V

alloy (with and without heat treatment) reduced coefficient of friction and

increased the wear durability (see Fig. 5.6 and 5.8). Epoxy-SAMs modified

Ti6Al4V alloy specimens with PFPE overcoat showed high wear durability

(90,700±13,900 sliding cycles) although heat treatment has resulted into some

reduction in the wear durability (74,700±24,000 sliding cycles).

Figure 5.8: Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/GPTMS/PFPE, (b) Ti6AL4V/GPTMS/PFPE (heat treated) and (c) Ti6Al4V/GPTMS) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm).

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PFPE overcoat (with and without heat treatment) on bare Ti6Al4V alloy

and GPTMS SAMs modified alloy has reduced the coefficient of friction to

0.11~0.13 for different specimens investigated in this study as observed in Table

5.3.

5.4 Optical microscopy of wear track and counterface surface

Fig. 5.9 shows the optical micrographs of the wear track and counterface

surface after completion of 5,000 cycles of the sliding test for bare Ti6Al4V

specimen. There was extensive wear debris accumulation around the track (Fig

5.9 (a)) and counterface surface of Si3N4 ball showed severe damage (Fig 5.9

(b)). Similar wear track and counterface conditions were observed for GPTMS

deposited Ti6Al4V specimens after completion of 5,000 sliding cycles (as seen in

Fig. 5.10). Fig. 5.10 (a) and Fig. 5.10 (b) show the accumulation of wear particles

near wear track and damaged counterface surface respectively.

Figure 5.9: Optical micrographs of Ti6Al4V specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.

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Figure 5.10: Optical micrographs of Ti6Al4V/GPTMS specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.

Fig. 5.11 shows the optical micrograph images of the wear track and counterface

surface after completion of 10,000 sliding cycles for Ti6Al4V/PFPE specimen. As

observed in Fig. 5.11 (a), wear track shows mild impression of counterface with absence

of debris. There is a little material transfer to counterface surface. After cleaning with

acetone, counterface surface is smooth with no sign of any permanent damage. Compared

to this, wear track surface after completion of 10,000 sliding cycles using Ti6Al4V/PFPE

(heat treated) specimen shows the accumulation of debris and severe damage to

counterface surface attributed to the reduction in wear resistance after heat treatment (see

Fig. 5.12).

Figure 5.11: Optical micrographs of Ti6Al4V/PFPE specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. (c) Counterface after cleaning, magnification 100x.

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Figure 5.12: Optical micrographs of Ti6Al4V/PFPE (heat treated) specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.

Fig. 5.13 shows the wear track and counterface surface after completion of

100,000 sliding cycles using the coating of GPTMS/PFPE onto Ti6Al4V alloy in

one of the sliding test where coating had not failed. As seen in Fig 5.13 (a), no

wear debris was observed around wear track although mild scratch had formed by

the impression of the counterface. There was a little material transfer to the

counterface of Si3N4 ball (see Fig 5.13 (b)). After cleaning with acetone,

counterface showed smooth surface with no signs of damage as observed in Fig

5.13 (c).

Figure 5.13: Optical micrographs of Ti6Al4V/GPTMS/PFPE specimen’s wear track and counterface surface after 100,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. (b) Counterface after cleaning, magnification 100x.

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As seen in different optical micrograph images, before failure of the film,

mild scratching on the wear track without the presence of wear debris and smooth

counterface surface accompanied by little material transfer were observed. After

failure of the film, wear debris accumulation along the wear track and worn

counterface surface were noticed. Different optical microscope images support the

wear durability data shown in Fig. 5.6.

PFPE coating on bare Ti6Al4V alloy as well as GPTMS SAMs deposited

Ti6Al4V alloy has resulted into lowering of friction coefficient and the increase in

wear durability. This can be attributed to lower surface energy [Makkonen 2004]

as well as flexible and mobile nature of PFPE molecules [Mate 1992] resulting

into low resistance in the shearing at the sliding interface.

PFPE overcoat on bare Ti6Al4V alloy as well as GPTMS coated Ti6Al4V

may consist of three parts as speculated in earlier studies [Satyanarayana and

Sinha 2005; Satyanaryana et al. 2007]. PFPE top layer consists of molecules

trapped in surface texture, strongly adsorbed portion and the mobile portion.

Strongly adsorbed portion results due to strong physical adsorption and hydrogen

bonding interaction as well as covalent bonding formed due to the heat treatment.

During the initial sliding cycles, mobile as well as bonded portion of PFPE will

lubricate the contact region. After squeezing of the mobile portion of PFPE,

lubrication will be supported by the bonded portion of PFPE molecules. After

complete removal of the mobile and bonded portion of PFPE, high friction and

wear will be followed leading to the failure of the coating.

83


High wear resistance of Ti6Al4V/GPTMS/PFPE specimens compared

with Ti6Al4V/PFPE can be explained due to the possible interaction of PFPE

molecules with GPTMS intermediate layer. Chemical interaction of PFPE with

epoxy group has been reported in previous research studies [Ellis 1993; Elender et

al. 1996]. It has been observed in some studies that dual-lubricant layer

combination consisting of initially bonded layer having strong-polar top group

with top mobile layer having weaker polar group shows improved tribological

properties compared with the use of only any one of the monolayers [Katano et al.

2003; Satyanaryana et al. 2007].

5.5 Biocompatibility assessment

Biocompatibility of GPTMS has been evaluated in earlier research studies.

In one study, noncytotoxicity of GPTMS was validated by the assessment of

cytocompatibility of hybrids of gelatin–GPTMS and chitosan–GPTMS in an in-

vitro study [Ren et al. 2002]. In another study, biocompatibility of GPTMS SAM

was confirmed using both bacterial culture (E.coli DH5α, Gram negative bacteria)

and plant tissue culture (wheat seed) [Kumar et al. 2011].

Biocompatibility of PFPE polymer has been examined by researchers and

long-term biocompatibility of PFPE has been validated in different studies for

biomedical applications [Xie et al. 2006; Sweeney et al. 2008].

In this study, H-Gladen ZV60 was used as the solvent for PFPE. To

confirm that H-Galden has not affected the biocompatibility of PFPE, PFPE top

84


layer coated onto UHMWPE film was tested for cytotoxicity using ISO elution

method-1×MEM extract method.


In cytotoxicity testing carried out on PFPE over-coated UHMWPE film,

no cytotoxicity effects such as cell lysis were seen in any of the test wells during

the microscopic examinations of the ISO elution method- 1×MEM extract. No

change in the pH value was noticed after the duration of 48 hours.

Grade of the test results should be less than the grade 2 (mild reactivity) to

meet the requirements of the ISO elution method-1×MEM extract. Based upon

test analysis results, it was inferred that coatings exhibited cytotoxicity level of

grade 0 (reactivity: none) according to the test guidelines, and thus PFPE top layer

coating meets the requirements of the ISO elution method-1×MEM extract.

In view of the mentioned previous research studies and carried out

cytotoxicity test in this study, noncytotoxic nature of GPTMS and PFPE coatings

are indicated. It suggests the noncytotoxic nature of composite coating of

GPTMS/PFPE investigated in this study.

5.6 Potential applications of GPTMS/PFPE coating

GPTMS/PFPE coating exhibits hydrophobicity, noncytotoxicity and

excellent tribological properties (low friction and high wear durability).

Hydrophobic coatings exhibit low surface energy. Lower friction and water-

repellent nature of the hydrophobic coatings are useful in different biomedical

85


86

applications. Hydrophobic coatings on coronary guidewire assist in improving

tactile feedback to cardiologist. Low surface energy of biomedical coatings has

been found to be useful in bio-film inhibition for few in-vivo applications

[Roosjen et al. 2006]. Coatings, having low surface energy and noncytotoxic

nature, on metallic stent are beneficial in improving the biocompatibility of metal

surface with body fluids by inhibiting the release of cytotoxic extracts from the

metal surface into the body environment [Pendyala et al. 2009]. Thus, the

obtained composite coating of GPTMS/PFPE can find different applications in

biomedical devices. Due to the molecularly low thickness of this coating (< 4

nm), it can be particularly useful in biomedical MEMS applications where

thickness of the surface coating is a crucial consideration for its usage.

Chapter 6: Conclusions

Chapter 6

Conclusions

In the first study of this thesis titled: “Tribological characterizations of

thin UHMWPE film and PFPE overcoat”, a thin film of UHMWPE (thickness of

19.6±2.0 µm) was coated onto Ti6Al4V alloy substrate. Prior to polymer coating,

oxygen plasma pre-treatment of the substrate surface was carried out for cleaning

as well as better adhesion of the polymer coating to the substrate. Physical

characterizations (contact angle measurement, thickness measurement, SEM

morphology and AFM imaging), biocompatibility test (cytotoxicity assessment)

and chemical characterizations (FTIR-ATR and XPS) were also carried out for

the UHMWPE polymer thin coating. Tribological characterization of the coating

was carried out at different load conditions and rotational speeds using a ball-on-

disk tribometer against a counterface of 4 mm diameter Si3N4 ball.

From the observations and results of this research study, following

important conclusions can be drawn:

Obtained UHMWPE polymer coating has similar physical and chemical

structure as that of bulk UHMWPE polymer as validated by different

physical and chemical characterization of the coating.

Oxygen plasma treatment is an effective surface treatment method for

cleaning and good adhesion of polymer coating on Ti6Al4V alloy

substrate as indicated by increased surface energy of the substrate after

plasma treatment.

87


Formed UHMWPE polymer coating on Ti6Al4V alloy substrate exhibits

excellent tribological properties such as low coefficient of friction (0.15)

and high wear durability (> 96,000 cycles) under tested conditions. Good

mechanical and tribological properties of bulk UHMWPE polymer also

results into useful tribological properties of polymer coating.

UHMWPE and UHMWPE/PFPE coatings are non-cytotoxic in nature as

per the test requirements of the ISO elution method-1×MEM extract.

PFPE overcoat on UHMWPE polymer coating enhances wear resistance

for the tested high speed loading condition of 1000 rpm and the normal

load of 4 N by providing additional thermal stability and lubrication

properties.

Due to the combination of low surface energy, wear resistance and

noncytotoxicity of the coating, the thin film of UHMWPE (with and

without PFPE overcoat) can find usage in various applications of

biomedical devices.

In the second study of this thesis titled: “Tribological evaluations of

molecularly thin GPTMS SAMs coating with PFPE top layer”, the potential of

molecularly thin GPTMS/PFPE coating (~4 nm) to address the tribological

limitations of titanium and titanium alloy has been evaluated. Physical

characterizations (contact angle measurement, AFM imaging), chemical

characterization (XPS) and tribological tests were carried out for obtained

coatings specimens.

88


89

From the results and observations of this study, following important

conclusions can be drawn:

PFPE overcoat onto Ti6Al4V/GPTMS surface exhibits low surface energy

as indicated by high water contact angle of coatings.

PFPE overcoat on bare ti6Al4V alloy after oxygen plasma treatment

exhibits improved tribological properties such as low coefficient of

friction (0.11 ~ 0.13) and high wear durability compared to bare Ti6Al4V

alloy.

PFPE overcoat on GPTMS deposited Ti6Al4V alloy exhibits low friction

coefficient (0.11 ~ 0.13) and high wear resistance. Wear resistance of

PFPE overcoat on GPTMS deposited Ti6Al4V alloy is significantly higher

(90,700±13,900 sliding cycles) than PFPE overcoat without GPTMS

deposition (26,700±4,900) which can be attributed to increased bonding of

PFPE with substrate due to GPTMS intermediate layer.

PFPE overcoat also reduces surface roughness of the substrate due to

filling up of surface texture by nano-particles of PFPE.

Low coefficient of friction, high wear resistance, hydrophobicity and

biocompatible nature of the coatings are suitable for their proposed

applications in biomedical instruments. Due to molecular layer thickness

of the coatings (~4 nm), these coatings are particularly suitable for

biomedical MEMS applications.

Chapter 7: Future Recommendations

CHAPTER 7

Future Recommendations

Below are recommendations for future studies based upon the results and

understanding generated in this research work:

In this thesis, cytotoxicity tests for the UHMWPE and UHMWPE/PFPE

coatings were carried out. Cytotoxicity test is the preliminary test to assess

the feasibility of coatings for biomedical applications. To further assess

the suitability of obtained coatings in biomedical devices, additional

biocompatibility tests such as hemocompatibility, sensitization, irritation

studies, systemic toxicity, genotoxicity etc are recommended as per the

guidelines of ISO 10993 “Biological Evaluation of Medical Devices” to

address specific application requirements.

Besides in-vitro biocompatibility tests, in-vivo animal testing and clinical

testing are also recommended for future studies to assess the potential

usage of these coatings in biomedical applications.

Optimization studies of process parameters to obtain lower thickness

UHMWPE coating without deteriorating tribological properties are

recommended to broaden application areas.

Obtained composite coating of GPTMS/PFPE can be tested for different

biocompatibility tests such as hemocompatibility, sensitization, irritation,

systemic toxicity, genotoxicity etc as per the guidelines of ISO 10993

90

Chapter 7: Future Recommendations

91

“Biological Evaluation of Medical Devices” to satisfy specific application

requirements.

In-vivo animal testing and clinical testing are also recommended in future

studies for the obtained GPTMS/PFPE coating.

Optimization of PFPE (wt %) solution, dip-coating immersion time,

dipping and withdrawal speeds on Ti6Al4V alloy substrate can be carried

out to obtain PFPE coatings with improved tribological properties.

Heat treatment temperature as well as time duration optimization studies

can be carried out to investigate the potential to obtain coatings with

improved wear resistance at higher loads.

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Appendix A: Cytotoxicity Test Procedures

Appendix A

Cytotoxicity Test Procedures

In-vitro tests for cytotoxicity are carried out as per the guidelines of

“International Organization for Standardization 10993-5: Biological Evaluation of

Medical Devices, Part 5: Tests for In Vitro Cytotoxicity”. Cytotoxicity tests

evaluate the response of cells in contact with devices or their extracts. "Part 5:

Tests for In Vitro Cytotoxicity" contains details about the procedure of testing

using cell culture using direct or indirect contact with device or filter diffusion.

Extracts of devices or materials are exposed to a cell culture system such

as L929 mouse fibroblast cell line in the test. After exposure, loss of cell viability

indicates the presence of cytotoxic extracts. Extracts are samples of potentially

cytotoxic substances that can leach out from the device material into the tissue

from device during usage. Extraction medium is chosen according to the nature

and use of device as well as test method. Extracts are prepared by incubating the

device at a recommended surface area to extractant volume ratio. This ratio is

depended upon the thickness of testing device. Surface area to extractant volume

ratio is 60 cm2 per 20 ml extraction volume if the device thickness is greater than

or equal to 0.5 mm and 120 cm2 per 20 ml extraction volume if device thickness

is less than or equal to 0.5 mm.

In case surface area of device can not be determined, a weight to volume

ratio (4 g per 20 ml extraction volume) is used for extraction. Extractable

107

Appendix A: Cytotoxicity Test Procedures

108

substances should be maximized and device should be exposed to extreme

conditions without a significant degradation.

Other method of cytotoxicity testing by direct contact is the agar diffusion

or overlay assay. In this method, test device is placed directly on the mammalian

cell layer. Mammalian cell layer is protected from mechanical abrasion by using

an intermediate agar layer. Similar to other methods of cytotoxicity assessment,

the presence of cytotoxic leachables is indicated by the observed loss of cell

viability. The direct contact assay method requires the placement of device or

material onto the cell culture medium without the presence of an agar

intermediate layer. In a filter diffusion test, test device or material is placed on

one side of filter and exposed to cells grown on the opposite side of filter.

In general, cytotoxicty test involves a cell culture dish having one-half

million to one million cells. Device cytotoxicity is measured by the cell viability

after a period of exposure (approximately 24–72 hours). Cytotoxicity can be

measured by qualitative and quantitative analysis. To validate test results, positive

control materials (such as organo-tin-impregnated polyvinyl chloride material)

and negative control materials (such as USP-grade high-density polyethylene RS)

are also tested.

Following the test period, the cell layers are examined microscopically for

any abnormal cell morphology and cellular degeneration.

tribology of self-assembled monolayersin this thesis, application of thin organic coatings to...

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