tribological properties of a tightly woven · tribological properties of a tightly woven...

TRIBOLOGICAL PROPERTIES OF A TIGHTLY WOVEN

CARBON/CARBON COMPOSITE

A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES

OF MIDDLE EAST TECHNICAL UNIVERSITY

BY

KERİMAN KARAVELİ

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR

THE DEGREE OF MASTER OF SCIENCE IN

METALLURGICAL AND MATERIALS ENGINEERING

JUNE 2005

Approval of the Graduate School of Natural and Applied Sciences.

Prof. Dr. Canan ÖZGEN

Director

I certify that this thesis satisfies all the requirements as a thesis for the degree of Master of Science.

Prof. Dr. Tayfur ÖZTÜRK Head of Department

This is to certify that we have read this thesis and that in our opinion it is fully adequate, in scope and quality, as a thesis for the degree of Master of Science.

Prof. Dr. Abdullah ÖZTÜRK Supervisor

Examining Committee Members

Prof. Dr. Muharrem TİMUÇİN (METU,METE) Prof. Dr. Abdullah ÖZTÜRK (METU,METE) Inst. Dr. Caner DURUCAN (METU,METE) Prof. Dr. Hasan MANDAL (Anadolu Unv.) Prof. Dr. Servet TURAN (Anadolu Unv.)

I hereby declare that all information in this document has been obtained and

presented in accordance with academic rules and ethical conduct. I also declare that,

as required by these rules and conduct, I have fully cited and referenced all material

and results that are not original to this work.

Name, Last Name : Keriman Karaveli

Signature :

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ABSTRACT

TRIBOLOGICAL PROPERTIES OF A TIGHTLY WOVEN

CARBON/CARBON COMPOSITE

Karaveli, Keriman

M.Sc., Department of Metallurgical and Materials Engineering

Supervisor: Prof. Dr. Abdullah Öztürk

June 2005, 77 pages

Tribological properties of a tightly woven Carbon/Carbon (C/C) composite were

assessed experimentally in accord with the ASTM pin on disk technique. The C/C

composite used in this study was a commercial material (K-Karb) obtained in a

panel form. The composite consists of graphite fiber reinforced graphite matrix

developed for aerospace applications. The fiber reinforcement was in a plain weave

woven fabric form.

The tests were conducted by sliding zirconia ball against the C/C composite. The

friction coefficient and wear rate were determined as functions of applied load,

sliding speed, sliding distance and lubrication in ambient laboratory conditions.

Mean friction coefficient of the composite was 0.135 µ when tested at ambient

atmosphere and 0.113 µ in lubricated environment at a load of 5 N, sliding speed of

0.5 cm/s, and sliding distance of 100 m. The wear volumes determined from surface

profile traces obtained on the wear tracks after completion of the tests were used for

calculations of the specific wear rates. The specific wear rates of the composite were

0.754 x 10-4 mm3/N.m at ambient atmosphere and 0.437 x 10-4 mm3/N.m in

lubricated environment at the load of 5 N, sliding speed of 0.5 cm/s, and sliding

iv

distance of 100 m. The specific wear rate of the composite decreased with

increasing sliding distance, sliding speed, applied load and also, decreased in

lubricated environment.

Keywords: C/C composite, tribology, friction, wear, lubricant.

v

ÖZ

SIKI ÖRGÜLÜ BİR KARBON/KARBON KOMPOZİTİN

TRİBOLOJİK ÖZELLİKLERİ

Karaveli, Keriman

Yüksek Lisans, Metalurji ve Malzeme Mühendisliği Bölümü

Tez Danışmanı: Prof. Dr. Abdullah Öztürk

Haziran 2005, 77 sayfa

Sıkıca örülü karbon fiberlerle takviye edilmiş bir Karbon/Karbon (K/K) kompozitin

tribolojik özellikleri ASTM pin on disk tekniğine göre deneysel olarak

değerlendirildi. Bu çalışmada kullanılan K/K kompozit plaka şeklinde ticari bir

malzeme olan K-Karb idi. Kompozit, grafit fiber takviyeli grafit matristen oluşmuş

ve havacılık uygulamaları için geliştirilmiştir. Fiber takviyesi düz örgü şeklindedir.

Testler zirkonyadan oluşturulmuş bir kürenin K/K kompozit numune üzerinde

kaydırılması ile gerçekleştirildi. Sürtünme katsayısı ve aşınma hızı; uygulanan

yükün, kayma hızının, kayma mesafesinin ve yağlayıcının fonksiyonu olarak normal

laboratuar koşullarında belirlendi. Kompozitin ortalama sürtünme katsayısı 5 N yük

altında, 0,5 cm/s kayma hızında, 100 m kayma mesafesinde laboratuar atmosferinde

0,135 µ; yağlayıcının bulunduğu bir ortamda 0,113 µ olarak bulundu.

Testlerin tamamlanmasından sonra elde edilen yüzey profil izlerinden faydalanılarak

belirlenen aşınma hacimleri, spesifik aşınma oranlarının hesaplanılmasında

kullanıldı. Kompozitin spesifik aşınma oranı 5 N yük altında, 0,5 cm/s kayma

hızında, 100 m kayma mesafesinde laboratuar atmosferinde 0,754 x 10-4 mm3/N.m

ve yağlayıcının bulunduğu bir ortamda 0,437 x 10-4 mm3/N.m olarak bulundu.

vi

Spesifik aşınma oranları, artan kayma mesafesi, kayma hızı, uygulanan yük ile ve

yağlayıcının bulunduğu bir ortamda azalmaktadır.

Anahtar Kelimeler: K/K kompozit, triboloji, sürtünme, aşınma, yağlayıcı.

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To my dear parents;

Halise and Timur KARAVELİ

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ACKNOWLEDGEMENTS

First, I wish to express my deepest gratitude to my supervisor Prof. Dr. Abdullah

ÖZTÜRK, for his helpful guidance, advice, criticism, encouragement and insight

throughout the every stage of this study.

I would like to thank my parents for their complimentary love, devotion and

unshakable faith in me during my life.

I must also thank Nilüfer Sevindi Önersoy and Esin Mungan Özdemir for their

endless guidance, motivation and support over the years.

I would like to express my frank thanks to Ahmad Changizi, providing invaluable

friendship and motivation from the time I came to METU through the final writing

of this thesis. The research assistants Selen Gürbüz and Gül Çevik are also

acknowledged for their help, motivation and moral support.

I would like to thank my homemate Aslı Tayçu for her understanding and

motivation for the last one year, during the writing of this thesis.

Thanks are also extended to my lecturers in Anadolu University, Department of

Materials Science and Engineering for their understanding and support.

Finally, I would like to express my special thanks to Dt. Gürel Pekkan for his

neverending patience, understanding, encouragement and moral support in me.

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TABLE OF CONTENTS

PLAGIARISM………………………………………………………………………iii

ABSTRACT.........................................................................................................…..iv

ÖZ.........................................................................................................................…..vi

DEDICATION....................................................................................................….viii

ACKNOWLEDGEMENTS………………………………………………………...ix

TABLE OF CONTENTS……………………………………………………………x

LIST OF TABLES…………………………………………………………………xii

LIST OF FIGURES…………………………………………………………….…..xv

CHAPTER

1. INTRODUCTION………………………………………………………………..1

2. THEORY…………………………………………………………………………5

2.1 CARBON/CARBON CONCEPT……………………………………….5

2.2 PROCESSING OF CARBON/CARBON COMPOSITES……………...6

2.3 PROPERTIES OF CARBON/CARBON COMPOSITES………………7

2.4 APPLICATIONS OF CARBON/CARBON COMPOSITES…………...8

2.5 TRIBOLOGICAL PROPERTIES…………………………………….....9

2.6 STANDARDIZATION OF THE TESTING METHOD……………….13

2.7 PIN ON DISK TRIBOLOGICAL TESTING METHOD……...………15

3. EXPERIMENTAL PROCEDURE………………………………………………18

3.1 SPECIMEN PREPERATION………...………………………………..18

3.2 TESTING……………………………………………………………….19

x

3.2.1 Tribological Testing…………………………………………..19

3.2.2 Surface Profile Measurement……...…………………………21

3.3 WORN VOLUME AND SPECIFIC WEAR RATE

CALCULATIONS……………………………………………………...22

3.4 MICROSCOPIC OBSERVATIONS…………………………………...23

3.4.1 Optical Microscopy (OM)…………………………………...…….....23

3.4.2 Scanning Electron Microscopy (SEM)…………………………….....23

3.5 EXPERIMENTAL FLOWCHART…………………………………….23

4. RESULTS AND DISCUSSION………………………………………………....25

4.1 GENERAL……………………………………………………………...25

4.2 FRICTION COEFFICIENT……………………………………………26

4.3 WORN VOLUME……………………………………………………...45

4.4 SPECIFIC WEAR RATE………………………………………………55

4.5 SURFACE CHARACTERIZATION………………………………..…62

4.5.1 Optical Microscopy (OM)…...…………………………….…62

4.5.2 Scanning Electron Microscopy (SEM)……………………….65

5. CONCLUSIONS………………………………………………………………...67

FUTURE WORKS…………………………………………………………………68

REFERENCES……………………………………………………………………..69

xi

LIST OF TABLES

TABLE

3.1 Properties of the C/C composite used in the present study………………...19

3.2 Tribological test conditions………………………………………………...20

4.1 Mean friction coefficient of the C/C composite measured without lubricant at

loads of 2.5 N, 5 N and 10 N, at sliding speeds of 0.5 cm/s and 1 cm/s for

different sliding distances…………………………………………………..27

4.2 Center line average surface roughness value of the C/C composite measured

without lubricant at loads of 2.5 N, 5 N and 10 N, at the sliding speeds of 0.5

cm/s and 1 cm/s for different sliding distances prior to the

wear……………………………...………………………………………….28

4.3 Center line average surface roughness value of the C/C composite measured

without lubricant at loads of 2.5 N, 5 N and 10 N, at the sliding speeds of 0.5

cm/s and 1 cm/s for different sliding distances after the

wear………………………………………………………………………....28

4.4 Mean friction coefficient of the C/C composite measured with lubricant at a

load of 5 N and a sliding speed of 0.5 cm/s for different sliding distances...29

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4.5 Center line average surface roughness values of the C/C composite measured

with lubricant at a load of 5 N and a sliding speed of 0.5 cm/s for different

sliding distances prior to the wear………………………………………….30

4.6 Center line average surface roughness values of the C/C composite measured

with lubricant at a load of 5 N and a sliding speed of 0.5 cm/s for different

sliding distances after the wear....…………………………………………..30

4.7 Worn volume of the C/C composite measured at loads of 2.5 N, 5 N and 10

N and at sliding speeds of 0.5 cm/s and 1 cm/s for different sliding distances

without lubricant……………………………………………………………46

4.8 Worn volume of the C/C composite measured at a load of 10 N and a sliding

speed of 0.5 cm/s with lubricant for different sliding distances……………46

4.9 Mean wear track area of the C/C composite measured without lubricant at

loads of 2.5 N, 5 N and 10 N, at sliding speeds of 0.5 cm/s and 1 cm/s for

different sliding distances. …………………………………………………51

4.10 Mean wear track area of the C/C composite measured with lubricant at a load

of 5 N and a sliding speed of 0.5 cm/s for different sliding distances…...…51

4.11 Specific wear rate of the C/C composite measured without lubricant at loads

of 2.5 N, 5 N and 10 N at sliding speeds of 0.5 cm/s and 1 cm/s for different

sliding distances…….…………….………………………………………...55

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4.12 Specific wear rate of the C/C composite measured with lubricant at a load of

5 N and a sliding speed of 0.5 cm/s for different sliding distances

.……………………………………………………………………………...55

4.13 A comparison of friction coefficient and specific wear rate values obtained

for C/C composite in this study with those reported in the literature for

selected materials………...…………………………………………………62

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LIST OF FIGURES

FIGURE

2.1 Variation of specific strength of several classes of high temperature

engineering materials with temperature……………………………………...8

2.2 The representative types of wear volume curves…………………………...11

2.3 Schematic representations of the wear modes……………………………...12

2.4 Photograph of a tribometer…………………………………………………16

2.5 Geometry of wear track, radius, and forces on disc………………………...17

3.1 Schematic illustration of fiber fabric wave pattern of the C/C composite

investigated…………………………………………………………………18

3.2 Schematic representation of the experimental procedure for determining the

tribological properties of the C/C composite studied………………………24

4.1 Optical micrograph of a specimen showing the texture of the composite (X

20).………………………………………………………………………….25

4.2 Variation of the friction coefficient of the C/C composite studied as a

function of sliding distance, number of rotational laps, and sliding time. Data

xv

were obtained at the sliding speed of 0.5 cm/s and at the applied load of 10 N

in ambient condition up to;

a) 1 m………………………………………………………………31

b) 10 m………………………………………………………………31

c) 100 m………………………………………………………………31



were obtained at the sliding speed of 0.5 cm/s and at the sliding distance of

100 m in ambient condition at;

a) 2.5 N.................................................................................................34

b) 5 N....................................................................................................34

c) 10 N....................................................................................................34



were obtained for the sliding speed of 1 cm/s and for the applied load of 10

N in ambient condition up to;

a) 1 m………………………………………………………………35

b) 10 m………………………………………………………………35

c) 100 m………………………………………………………………35



were obtained for the sliding speed of 1 cm/s and for the sliding distance of

100 m in ambient condition at;

a) 2.5 N……...……………………………………………………….36

b) 5 N…………………………………………………………………36

c) 10 N……………………………………...…………………………36



xvi

were obtained for the sliding speed of 0.5 cm/s and for the applied load of 5

N with lubricant up to;

a) 1 m……………………………………………………………….38

b) 10 m……………………………………………………………….38

c) 100 m……………………………………………………………….38



was obtained for the sliding speed of 0.5 cm/s and for the applied load of 10

N in ambient condition up to the sliding distance of 1000 m…………........39

4.8 Variation of the mean friction coefficient of the C/C composite studied as a

function of sliding distance for the applied loads of 2.5 N, 5 N and 10 N at

ambient condition. Data was obtained at the sliding speeds of;

a) 0.5 cm/s……………………………………………………………..41

b) 1 cm/s……………………………………………………………….41

4.9 Variation of the mean friction coefficient as a function of sliding distance for

lubricated and unlubricated conditions. The sliding speed was 0.5 cm/s and

the applied load was 5 N……………………………………………………42

4.10 Variation of the mean friction coefficient as a function of sliding distance for

the sliding speeds of 0.5 cm/s and 1 cm/s at ambient condition. The applied

load was 5 N………...……………………………………………………...44

4.11 Variation of worn volume of the C/C composite studied as a function of

sliding distance for the applied loads of 2.5 N, 5 N and 10 N. Condition was

ambient. Data was obtained at the sliding speeds of,

a) 0.5 cm/s……………………………………………………………..49

b) 1 cm/s……………………………………………………………….49

xvii

4.12 Variation of worn volume of C/C composite studied as a function of sliding

distance with and without lubricant. The applied load was 10 N and the

sliding speed was 0.5 cm/s. Data was obtained for 1 m, 10 m and 100 m…50

4.13 Schematic representation of the wear track of the C/C composite. Data were

obtained after tribological testing at a load of 10 N and at a sliding speed of

0.5 cm/s for the sliding distances of;

a) 1 m……………………………………………………………….53

b) 10 m……………………………………………………………….53

c) 100 m……………………………………………………………….53

4.14 Schematic representation of the wear track of the C/C composite. Data were

obtained after tribological testing for a sliding distance of 100 m at sliding

speed of 0.5 cm/s at the loads of;

a) 2.5 N..................................................................................................54

b) 5 N.....................................................................................................54

c) 10 N....................................................................................................54

4.15 Variation of specific wear rate studied at loads of 2.5 N, 5 N and 10 N at

ambient atmosphere. The data were obtained at the sliding speeds of;

a) 0.5 cm/s……………………………………………………………..58

b) 1 cm/s……………………………………………………………….58

4.16 Variation of specific wear rate of C/C composite studied as a function of

sliding distance with and without lubricant. The applied load was 5 N and the

sliding speed was 0.5 cm/s. Data was obtained for 1 m, 10 m and 100 m…59

4.17 OM image taken after the tribological test performed for 100 m sliding

distance at a load of 10 N in unlubricated condition (x 10). Data was

obtained at sliding speeds of;

a) 0.5 cm/s…………………………………………………………….63

b) 1 cm/s………………………………………………………………63

xviii

4.18 OM image taken after the tribological test performed for 100 m sliding

distance at a load of 5 N at a sliding speed of 0.5 cm/s (x 10). Data was

obtained;

a) with lubricant……………………………………………………….64

b) without lubricant……………………………………………………64

4.19 OM images taken after the tribological test performed for 1000 m sliding

distance at a load of 10 N, at a sliding speed of 0.5 cm/s without lubricant (x

10)…………………………………………………………………………..64

4.20 SEM images taken after the tribological test performed for 100 m sliding

distance at a load of 10 N and at a sliding speed of 0.5 cm/s without lubricant.

a) X 300..………………………………………………………………66

b) X 1000…………………………………………………………..…..66

xix

CHAPTER 1

INTRODUCTION

Carbon fiber-reinforced carbon-matrix composites, the so-called carbon/carbon

(C/C) composites are of great importance since they possess a variety of unique

engineering properties. These composites have prominent structural properties of

high specific strength and specific modulus as well as excellent functional

characteristics such as high thermal conductivity and thermal capacity, outstanding

thermal shock resistance, low density, good wear resistance, self-lubricating

capability. Moreover, they retain their high thermal and chemical stability in inert

environments (1,2). The variety of properties is tied to constituents, matrix and

reinforcement, the processing conditions and the development of the fiber/matrix

interface bond strength.

The combination of the desirable engineering properties make C/C composites

useful for special applications such as exit nozzles for rockets, nose caps and leading

edges for missiles and the space shuttle (3), sporting goods, racing car components,

disk brakes for racing cars, military and civilian aircrafts (4,5). Also, due to their

biocompatibility with the human body, applications of C/C composites are gradually

extending to biomaterials such as hip joint replacement, heart valves and skeletal

parts (6). Their thermal properties play a more significant role for space applications

while their mechanical properties are the most desirable for biomedical and

metallurgical applications. For brake pad applications their tribological properties

are the key parameters. Currently, ~81 % of C/C composites are used in aircraft

brake disks, ~18 % are used in space rocket technology, and only 1 % is used in the

rest of the applications (3).

1

The extreme tribological requirements for brake pads have been the impetus for

low-density C/C composites, which exhibit a high and stable coefficient of friction

at high sliding speeds. In addition, C/C composites are lighter compared with the

conventional brake pads contribute to the weight saving the aircraft. Thus they have

replaced the conventional metallic brake pads in both military aircrafts such as the

US F-16 and F-18, the French Mirage 2000, and civilian aircrafts such as Boing 747

Airbus and Concorde (4,5). Recently attempts have been made to use C/C

composites as the brake pad material in railway locomotives (5).

The only drawback of C/C composites is their sensitivity to high temperature

oxidation that may be reduced by oxidation resistant coatings (7).

Materials with good tribological properties have been the focus of increasing

research activities for brake pad applications. Advanced ceramic materials have

excellent prospects for tribological applications. Consequently, clarification of wear

processes of ceramic materials has received much attention over the last two

decades (8). Studies (9-13) conducted on the sliding wear behavior of advanced

ceramics have revealed that thin layers, so-called tribofilms, consisting of fine wear

particles, or debris, are observed on the wear surfaces. Tribofilm might play an

important role in the sliding wear behavior of ceramics. Actual wear often occurs at

the contact interfaces where a tribofilm is present. The characterization of the

tribofilms and their role on the wear behavior is still a subject of considerable

scientific and engineering interest.

The wear processes, which reduce service life of C/C composites, are very complex,

involving the interaction of multiple damage modes that may combine in a variety

of ways to produce various failure modes (10). The tribological properties of C/C

composites depend upon not only material properties but also the initial surface

finish and experimental test conditions. Although both constituents of C/C

composites are based on the same element, this does not simplify the composite

behavior because the morphology of each constituent may range from carbon to

2

graphite. Given the wide selection of suitable fibers, reinforcement geometries,

matrix precursors and processing conditions, C/C composites can be produced from

one-directional to n-directional forms using unidirectional and woven cloth fibers

(8). Typical factors that can affect tribological behavior are the properties of the

materials, the nature of the relative motion, the nature of the loading, the shape of

the surface(s), the surface roughness, the ambient temperature, and the composition

of the environment in which the wear occurs (10). It has been observed (8) that the

environment plays a significant role in determining the tribological behavior of C/C

composites as in the case of carbons and graphite. Studies (8-12) on the combined

influence of environment and temperature on the tribological behavior of C/C

composites revealed that the dusting wear occurs whenever there is a lack of

lubricating gases/vapors in the environment.

It has been reported (8) that the average coefficient of friction depend on heat

treatment temperature at which the composite was processed prior to testing and on

the bulk density and Young’s modulus of the composite. The coefficient of friction

and wear rate varied in a cyclic fashion as a function of the orientation of the carbon

fiber with respect to the sliding surface within the composite. It has been shown (14)

that different fiber orientations at the wear face do not change the qualitative

features of the wear mechanism of the composite.

The literature suggests that the frictional transitions in C/C composites can be

usefully studied only when sliding wear tests are carried out under controlled

conditions of constant applied load and sliding velocity. Although information on

the tribological behavior and properties of C/C composites is gathered in the open

literature, the data are sparse. There is not a single document covering the effect of

different test parameters and environmental conditions upon which the tribological

properties of these important engineering materials depend. Consequently, a clear

understanding of the effects of different test parameters on the tribological

properties is essential if they are to be used in applications requiring high resistance

to wear and friction. Any contribution to this particular research area will be an asset

3

to brake pad technology. Hence, studies on the tribological properties of C/C

composites have both scientific and practical significance.

The purpose of this study was to determine the tribological behavior of a tightly

woven C/C composite under various experimental conditions. Tribological

properties were assessed experimentally in accord with the ASTM pin on disk

technique. The friction coefficient and specific wear rate were determined as

functions of the applied load, sliding speed, sliding distance, sliding time, and

lubricant in ambient laboratory conditions in order to understand the effects of

different test parameters on the service life of C/C composites. Tribological testing

was supplemented with microstructural characterization to provide information

required to explain observed behavior. Fractographic analysis of wear surfaces was

conducted to examine the location of the damage and wear mechanisms occurred

during the tests. The results of this study were correlated with the results of the

studies reported in the literature.

4

CHAPTER 2

THEORY

2.1 CARBON/CARBON CONCEPT

Carbon has four allotropes: diamond, graphite, carbines and fullerenes, each having

significant scientific and technological importance (15). Its most abundant allotrope,

graphite, can take many forms with respect to microstructure, amorphous to highly

crystalline structure, highly dense with density of 2.2 g/cm3 to highly porous with

density of 0.5 g/cm3, and different shapes. These types of graphites are called

synthetic carbons and in technical terms, engineered carbons. Examples are cokes,

graphite electrodes, mechanical carbons, glassy carbons, carbon black, porous

carbons, activated carbons, carbon fibers and composites. Solid carbons are

preferred for structural applications under extreme environmental conditions of

temperature or corrosion. This is mainly because, theoretically, carbon materials

with covalently bonded atoms possess very high specific strength (40–50 GPa) and

retain this strength at elevated temperatures in the temperature range over 1500 ºC

(16). However, the normal bulk synthetic graphite exhibits less than 2 % of the

theoretical strength. Therefore, for long there has been a quest by scientists to

explore and achieve the maximum possible strength in carbon materials. This has

led to the development of C/C composites in 1958 (17) and promoted the attainment

of mature structural material in the 1980’s. These composites have densities in the

range 1.6–2.0 g/cm3, much lower than those of metals and ceramics and hence make

lower component weight an important consideration for aerovehicals (18).

5

2.2 PROCESSING OF CARBON/CARBON COMPOSITES

Carbon fiber reinforced carbon matrix composites require different processing

techniques. Carbon fibers are prepared from pitch or polyacrylonitrile (PAN), and

matrices are prepared from organic binders such as resin and pitch or chemical

vapor infiltration (CVI) (19). The most popularly used C/C formulae include CVI

carbon matrix reinforced with PAN-based carbon fabric laminates (designated as

‘PAN-CVI’) and phenolic resin char-CVI hybrid matrix reinforced with chopped

mesophase pitch-based carbon fiber yarns (designated as ‘pitch-resin-CVI’) (20).

There are three main routes to obtain C/C composites (21,22):

1. A woven carbon preform is impregnated under heat and pressure with pitch from

coal tar or petroleum sources. This is followed by pyrolysis. The cycle might be

repeated to obtain the desired amount of densification. The purpose of densification

is filling porosity with pyrolytic carbon, which increases the volume fraction of the

matrix continuously.

2. Carbon fiber/polymer composites are pyrolyzed to decompose the resin, generally

phenolics because they give high char strength, followed by reimpregnation and

repyrolysis to get a carbonaceous matrix bonded to carbon fibers.

3. Chemical vapor deposition from a gaseous phase onto and between the carbon

fibers in the preform

The gaseous infiltration, also known as carbon vapor deposition (CVD), is a

process, which is normally more expensive and technologically more sophisticated

than the liquid impregnation (6).

In the CVD process, the carbon matrix is formed from the decomposition of a

hydrocarbon, for example methane, which occurs at temperatures around 1100 ˚C

the deposition of the carbon matrix on the preform is very slow and can take many

days or weeks for practical purposes to be achieved. The liquid impregnation, on the

6

other hand, is faster than CVD (6). Using a CVD system, the surface is dominated

by large pores. The bond between fibers and matrix is much stronger than for a

resin-based composite. The fibers in the matrix are able to conduct more heat away

from the rubbing surface. This rougher surface topography accounts for the more

constant braking behaviour but also increases wear (23).

2.3 PROPERTIES OF CARBON/CARBON COMPOSITES

The constituents, both reinforcement and matrix, are likely to undergo a change in

properties during processing as influenced by heat treatment temperature,

differential dimensional changes, and thermal stresses (2).

In terms of the targeted properties, C/C composites cover a large range of materials.

The properties of interest are strength and stiffness, fracture toughness, frictional

properties, thermal conductivity and resistance to oxidation at high temperatures.

The operating mechanisms for these properties are quite different, especially in such

multiphase composite materials. Some of the most important and useful properties

of C/C composites are light weight, high strength at elevated temperatures in non-

oxidizing atmospheres, low coefficient of thermal expansion, high thermal

conductivity (higher than that of copper and silver), and high thermal shock

resistance (24-26). The mechanical properties of the constituents and their volume

fraction, bonding, and crack propagation mechanism control the mechanical

properties of the composites, whereas thermal properties are governed by thermal

transport phenomena (2).

The specific strength of C/C composites increases with temperature, in contrast to

that of metal and ceramics, whose specific strength decrease with increasing

temperature (27). The variations of specific strength of some engineering materials

with temperature are shown in Figure 2.1.

7

Figure 2.1 Variation of specific strength of several classes of high temperature

engineering materials with temperature (27).

2.4 APPLICATIONS OF CARBON/CARBON COMPOSITES

C/C composites have a wide variety of established uses dependent on their superior

mechanical properties that persist at high temperatures. C/C composites, developed

about three decades ago to meet the needs of the space programme, are nowadays

considered high performance engineering materials with potential application in

high temperature industries (2). Accordingly, steady growth also prevails in the civil

market segment. In terms of mass consumption, the main application of C/C

composites is still in high performance braking systems.

C/C composites were utilized in aerospace and defense applications such as rocket

nozzles for rockets, nose caps and leading edges for missiles and space shuttle,

nuclear reactors and especially for fusion devices (28). Newer applications such as

8

hot press dies, wind tunnel models, racing car components, commercial disk brakes

and sporting goods, etc, are being developed (29).

In engineering sectors, they are used in engine components, as refractory materials,

as hot-pressed dies and heating elements, as high temperature fasteners, liners and

protection tubes, as guides in glass industries. C/C composites have great potential

in energy sectors as polar plates for fuel cells, in storage batteries (2). As the

technology becomes more economical a viable, more and more applications get

evolved.

2.5 TRIBOLOGICAL PROPERTIES

Wear is damage to a solid surface as a result of relative motion between it and

another surface of substance (30). The damage usually results in the progressive loss

of material. Wear testing has been used to rank wear resistance of materials for the

purpose of optimizing material selection and development for a given application.

Standardization, repeatability, convenience, short testing time, and simple

measuring and ranking techniques are desirable in these tests. Wear is closely

related to friction, and lubrication; the study of these three subjects is known as

tribology (31). The word is derived from the Greek “tribos” meaning rubbing,

although the subject embraces a great deal more than just the study of rubbing

surfaces.

The word tribology was introduced only just over thirty years ago and is defined as

the science and technology of interacting surfaces in relative motion and of related

subjects and practices (32,33). Collection of all the mechanical, chemical, and

environmental parameters that can affect wear and wear behavior is referred to as

the tribo-system. Typical factors that can affect wear behavior are the properties of

the materials, the nature of the relative motion, the nature of the loading, the shape

of the surface(s), the surface roughness, the ambient temperature, and the

9

composition of the environment in which the wear occurs (30). Coefficients of

friction and wear are parameters describing the state of contact bodies in a tribo-

system, and they are not material constants of the bodies in contact.

Depending on operating conditions and material selection, wear rate changes

drastically in the range of 10-15 to 10-1 mm3/N.m (34-37). Therefore, it is inherent

that designing of the operating conditions and selection for the materials are the

keys to controlling wear. As one way to meet these requirements, wear maps have

been proposed for prediction of wear modes and wear rates. A wear map is

considered one of the best descriptions of tribological condition and is useful in

selecting materials in a wide range of operating conditions.

In order to investigate the tribological behavior it is essential to have an

understanding of wear rate, varieties of wear modes, and wear mechanism (38,39).

Wear is the result of material removal by physical separation due to microfracture,

by chemical dissolution, or by melting at the contact interface. In addition, there are

several types of wear: adhesive, abrasive, fatigue, and corrosive. The dominant wear

mode may change from one to another for reasons that include changes in surface

material properties and dynamic surface responses caused by frictional heating,

chemical film formation, and wear. In general, wear does not occur through a single

wear mechanism (40).

Three representative types of wear volume curves are shown in Figure 2.2. Type I

shows a constant wear rate throughout the whole process. Type II shows the

transition from an initially high wear rate to steady wear at low rate. This type of

wear is quite often observed in metals (41). Type III shows catastrophic wear is the

period at which crack initiation takes place and depends on the initial surface finish,

material properties, and frictional conditions.

10

Figure 2.2 The representative types of wear volume curves (33).

Adhesive wear, Abrasive wear, Fatigue wear, are Corrosive wear are generally

recognized as fundamental and major wear modes (42). Schematic representation of

the wear modes is illustrated in Figure 2.3. Adhesive and abrasive wear are wear

modes generated under plastic contact. In the case of plastic contact between similar

materials, the contact interface has adhesive bonding strength. When fracture is

supposed to be essentially brought about as the result of strong adhesion at the

contact interface, the resultant wear is called adhesive wear, without particularizing

about the fracture mode.

In the case of plastic contact between hard and sharp material and relatively soft

material, the harder material penetrates to the softer one. When the fracture is

supposed to be brought about in the manner of micro-cutting by the intended

material, the resultant wear is called abrasive wear, recognizing the interlocking

contact configuration necessary for cutting, without particularizing about adhesive

forces and fracture mode.

11

Figure 2.3 Schematic representations of the wear modes (33).

In the case of contact in the running-in state, fatigue fracture is generated after

repeated friction cycles. When surface failure is generated by fatigue, the resultant

wear is called fatigue wear. In contact in corrosive media, the tribochemical reaction

at the contact interface is accelerated. When the tribochemical reaction in the

corrosive media is supposed to be brought about by material removal, the resultant

wear is called as corrosive wear.

Fatigue and corrosive wear can take place in both plastic and elastic contacts. The

material removal in adhesive, abrasive, or fatigue wear is governed by deformation

and fracture in the contact region, where fracture modes are fatigue, brittle or ductile

fracture. Such deformation and fracture are generated by mechanically induced

12

strains and stresses. Therefore, this type of wear is generally described as

mechanical wear. The material removal in corrosive wear is governed by the growth

of chemical reaction film on wear surface, where chemical reactions are highly

activated and accelerated by frictional deformation, frictional heating, microfracture,

and successive removal of reaction products. This type of wear is generally

described as chemical wear or tribochemical wear.

In some cases, material removal is governed by surface melting caused by frictional

heating or by surface cracking caused by thermal stress. These types of wear are

described as thermal wear, where frictional heating and partial high temperature

govern the process. Erosion and abrasive wear situations can also be subdivided into

more specific categories. Examples of these are cavitation erosion, solid particle

erosion, gouging abrasion, and slurry erosion (30).

2.6 STANDARDIZATION OF THE TESTING METHOD

A particular type of wear problem usually motivates wear testing. It can be basic-

research-oriented or application related. Frequently, the developer of a new material

or surface treatment wants to know how the new material compares with other

existing materials. If there is a specific application in mind, the selection of a

particular wear test method is easier because the type of motion, contact conditions,

and environment are dictated by the application. If no application is in mind,

conducting a series of different standard wear tests can be appropriate. Motivations

to do a wear testing are (43):

1. To conduct basic scientific research on the characteristics and mechanism of a

particular type of wear,

2. To evaluate the relative wear resistance of set of materials or the anti-wear

properties of a lubricant,

3. To evaluate the relative wear resistance of a set of materials, including lubricants,

for a specific application,

13

4. To evaluate the characteristics of a particular type of test procedure,

5. To aid in the development of a new wear-resistant materials or treatment,

6. To ensure uniform quality of a particular product.

The standardization of wear testing conditions is not consistent with all the above

motivations to do wear testing. In basic research, for example, it is often desirable to

vary testing conditions over a large range, and enforcing a standard set of testing

conditions would be inappropriate. In screening materials for a specific application,

there may or may not be standard tests available for that application, and

extrapolation of data for use in a new design or set of operating conditions that

differs significantly from the one on which the existing standard based is generally

ill-advised. The best correlation of testing with performance in a particular

application may be obtained with a custom-design simulator, which may bear little

resemblance to configurations in standard wear testing methods.

The diversity of wear test methods being applied to materials has created problems

in comparing results and in establishing a coherent wear technology based for these

materials. Standardization of wear testing is a means to alleviate many of these

problems. American Society for Testing and Materials (ASTM) is attempting to

develop standard wear tests specifically suited for ceramic materials, either by

modifying existing methods developed for other materials, or by developing new

methods. In May 1987, ASTM Committee G-2 on wear and erosion conducted a

symposium on “Selection and Use of Wear Tests for Ceramics”, and a publication

with the same name resulted (43).

Standards in the field of tribology can extend beyond the specifications for

conducting tests. They can involve standards for specimen preparation, specimen

material characterization, and even standards for the completion and presentation of

friction and wear data. In addition to standard test methods, there are standard

practices (31,44,45). Each has a role in tribology. For example; for abrasive wear,

ASTM test for measuring abrasion using the dry sand/rubber apparatus G65, for

14

erosive wear; ASTM practice for conducting erosion tests by solid particle

impingement using gas jets G76, for rolling contact fatigue and sliding wear; ASTM

practice for ranking resistance of materials to sliding wear using block on ring wear

test G77 or ASTM test for wear testing with a pin on disc apparatus G99 could be

used. Even if a material displays poor wear behaviour in one wear mode, it could

still be shown to have superior wear in another, and the value of a new material

would not be overlooked by restricting wear testing to only one kind.

2.7 PIN ON DISC TRIBOLOGICAL TESTING METHOD

The sliding wear caused by a loaded spherical pin contacting a rotating disc is

typical of that which occurs in pin-on-disc tests used to study friction and wear

phenomena. In a pin on disc test, the pin is held stationary under a specified load,

while the disc rotates beneath it at a constant velocity. If a sliding wear mechanism

is being examined, the pin generally has a spherical head and the disc is fabricated

from the material whose wear behavior is investigated which is usually much softer

than the pin material.

A typical pin on disc testing machine, tribometer, is shown in Figure 2.4. The

machine can be used for testing the friction and wear characteristics of dry or

lubricated sliding contact of a wide variety of materials including metals, polymers,

composites, ceramics, lubricants, cutting fluids, abrasive slurries, coatings, and heat-

treated samples (33). Rotating a counter-face test disc against a stationary test

specimen pin performs the test. Wear, friction force, and interface temperature can

be monitored using winlube, the supplied windows-based data acquisition software.

The normal load, rotational speed, and wear track diameter can be adjusted in

accordance with test standard.

15

Figure 2.4 Photograph of a tribometer.

Usually the 'pin' consists of a bearing steel ball, which is clamped in place with a

chuck. Tests are also carried out using pins made of harder materials e.g. silicone

nitride, aluminum oxide, and zirconium oxide. The standard pin-on-disk tribometer

uses a simple load arm with a tangential force sensor mounted close to the contact

point so as to reduce errors due to arm compliance. The load is applied on the end of

the cantilever arm (connected to the pin). Sliding speed can be varied. Friction

coefficient and wear rate are determined. Geometry of wear track, radius, and forces

on disc is schematically represented in Figure 2.5.

16

Figure 2.5 Geometry of wear track, radius, and forces on disc (33).

The results obtained from a pin on disc test are usually expressed in the form of a

wear rate, defined as the volume of material removed per sliding distance for a

given load (33).

17

CHAPTER 3

EXPERIMENTAL PROCEDURE

3.1 SPECIMEN PREPERATION

The C/C composite used in this study was a commercial material (K-Karb) obtained

in a panel form from Kaiser Aerotech Company, San Leandro, CA, USA.

The nominal dimensions of the panel were 15 x 3 x 0.6 cm as length x width x

depth, respectively. The composite consists of graphite fiber reinforced graphite

matrix developed for aerospace applications. The fiber reinforcement was in a plain

weave woven fabric form. The wave pattern of the fiber fabric is shown

schematically in Figure 3.1. A warp yarn is interlaced with every other fill yarn, and

a fill yarn is interlaced with every other warp yarn.

Figure 3.1 Schematic illustration of fiber fabric wave pattern of the C/C composite

investigated.

18

Properties of the C/C composite used in the present study are given in Table 3.1.

Test specimens were prepared by cutting the composite panel into three small

rectangular shaped forms using a diamond saw. The nominal dimensions of the

forms were approximately 5 x 3 x 0.6 cm. The forms were surface polished to

assure the surface smoothness and parallelism. The polishing was performed with

the application of first a series of silicon carbide grinding papers beginning with

240 grit and gradually advancing to 800 grit, and then 0.3 µm alumina powder

solution on a cloth. After the surface polishing the specimens were ready for the

tribological tests.

Table 3.1 Properties of the C/C composite used in the present study (29,46).

Property Unit Range

Fracture Toughness MPa.m1/2 5.7 - 6.3

Density mg/m3 1.68 - 1.72

Elastic Modulus GPa 8.98 - 9.03

Tensile Strength GPa 88.1 - 97.5

Hardness HV 5.7 - 6.3

3.2 TESTING

3.2.1 Tribological Testing

A pin on disc type of tribometer supplied from CSEM Instruments, Switzerland,

was employed to conduct tribological tests under the conditions listed in Table

3.2. A photograph of the tribometer used in this study is shown in Figure 2.4. The

tests were performed with the application of a lubricant as well as without using

any lubricant in accord with ASTM G99-95A, entitled as “Standard Test Methods

for Wear Testing with a Pin on Disc Apparatus”.

19

Table 3.2 Tribological test conditions

Ball Material High purity zirconia

Disc Material Tightly woven C/C composite

Load 2.5, 5, 10 N

Sliding Speed 0.5 cm/s, 1 cm/s

Sliding Distance 1, 10, 100, 1000 m

Application Diameter 2 cm

Temperature Room temperature

Environment Ambient laboratory atmosphere

Lubrication Motor oil

Zirconia was chosen as the counterface because of its relatively high hardness and

low specific wear rate against C/C composite. Commercial zirconia balls were

used as the counterface. The elastic modulus and Vickers hardness of the zirconia

balls were 158-241 GPa and 7-8 GPa, respectively (47).

Loads of 2.5 N, 5 N, and 10 N were applied onto the samples in order to determine

the effects of the load on the tribological (friction, wear, and lubrication)

behaviour of the composite.

Sliding speeds of 0.5 cm/s and 1 cm/s were chosen to compare the effect of the

increasing sliding speed on the tribological behaviour of the composite.

The tests were performed for the sliding distances (time periods) of 1 m, 10 m, and

100 m (223 s, 2210 s, and 22100 s) in order to see the effect of increasing sliding

distance (time). A single test was also performed for the sliding distance of 1000

m (221000 s) at the load of 5 N and sliding speed of 0.5 cm/s so that the long term

as well as short term tribological behaviour of the samples could be predicted

accurately. Wear track diameter of 2 cm was fixed during tribological testing to

20

provide the same conditions for all experiments. The sliding distances of 1 m, 10

m, 100 m, and 1000 m corresponded to 16th, 159th, 1590th and 15900th rotational

laps, respectively.

The samples were tested also with the application of a lubricant at the load of 5 N

and sliding speed of 0.5 cm/s in order to see the effects of lubricant on the

tribological properties of C/C composite studied. Motor oil was chosen as the

lubricant. The motor oil used in this study was Shell Helix Plus 10W-40. The

kinematic viscosity of the motor oil was 15.1 mm2/s at 20 ºC, 90.8 mm2/s at 40 ºC

and 14.1 mm2/s at 100 ºC (48). The density of the motor oil was 871 kg/m3. The

motor oil was dripped from a dripper onto the contact surface in every 30 min

during the test. The weight of one drip was approximately 0.194 g.

All of the tests were conducted at ambient atmospheric conditions at room

temperature.

Tribological tests were performed by placing the sample into the rotating holder

against a stationary test pin performs the test. The normal applied load, rotational

speed and sliding distance were adjusted before the test conducted. Friction

coefficient values were detected by means of the deflection of the elastic arm.

Friction coefficients were monitored using winlube, the supplied window-based

data acquisition software program in µ.

Surface profiles of the specimens were measured before starting and after

completing each one of the tribological tests in order to determine the worn area

developed during the tests.

3.2.2 Surface Profile Measurement

A portable surface roughness tester, Precision Surtronic 3+, supplied from Taylor

Hobson, England, was employed to measure the surface profile and hence worn

area. Surface profile was detected by tracing the wear track from randomly

21

selected cross-sectional areas. For the consistency and trustability of the data, at

least five cross-sections in the wear track were traced for measurements. Ra

(center line average) surface roughness and worn area values were determined

from the profilemeter software program directly.

The mean average of the data obtained from five measurements was taken into

account to calculate the worn volumes.

3.3 WORN VOLUME AND SPECIFIC WEAR RATE CALCULATIONS

The worn area measured from wear track was multiplied by the circumference of

the wear track to determine the worn volumes according to the following formula:

Vw = A x Л x d .................................................................................................Eq.1

Where Vw is the worn volume in mm3, A is the worn area in mm2, Л is the

constant (3.14) and d is the mean wear track diameter. Wear in the pin material

was not significant. Therefore pin wear has not been taken into consideration

when wear volume is calculated.

The specific wear rates of specimens were calculated by Tribox 2.0 Software

program in mm3/N.m according to the following formula (41).

Ws = Vw / F x S ................................................................................................Eq.2

Where Ws is the specific wear rate of the specimen in mm3/N.m, Vw is the worn

volume, F is the friction force applied in Newton (N), S is the decrement of

specimen length in meter (m).

22

3.4 MICROSCOPIC OBSERVATIONS

3.4.1 Optical Microscopy

At the end of each one of the tribological tests, the wear track surface of the

samples were examined using an Optical Microscopy (OM), Nicon Optiphot-100,

for micro-structural analyses of the wear track.

3.4.2 Scanning Electron Microscopy

A Scanning Electron Microscopy (SEM), Jeol JSM-6400, was employed to

provide information on the mechanisms of the material removal. Specimens were

coated with gold for SEM observations.

3.5 EXPERIMENTAL FLOWCHART

Schematic representation of the experimental procedure for determining the

tribological behaviour of the C/C composite studied is shown in Figure 3.2.

23

STARTING MATERIAL

SAMPLE PREPARATION

SURFACE POLISHING TRIBOLOGICAL TESTING

Friction Wear Lubrication

SURFACE PROFILE MEASUREMENT

Surface roughness Wear area

MICROSCOPIC OBSERVATIONS

Optical Microscopy

Figure 3.2 Schematic representation of the experimental procedure for

determining the tribological properties of the C/C composite studied.

24

CHAPTER 4

RESULTS AND DISCUSSION

4.1 GENERAL

Data obtained during experimental studies of the thesis work were presented and

discussed in this chapter.

The test specimens were prepared from a tightly woven carbon/carbon (C/C)

composite panel according to the procedure as described in Section 3.1. Side view

micrograph taken from optical microscope observation of a representative test

specimen in Figure 4.1 illustrates the texture of the composite in general.

Tribological tests were conducted as described in Section 3.2 in order to

understand the tribological behavior and to determine the tribological properties

(friction, wear, and lubrication) of the composite at adverse conditions. Data on

worn volume and specific wear rate were gathered through quantitative

measurements and calculations according to the formulae given in Section 3.3.

Figure 4.1 Optical micrograph of a specimen showing

composite (X 20).

25

10 µm

the texture of the

4.2 FRICTION COEFFICIENT

Friction coefficients of the composite samples were determined as functions of

sliding distance, sliding time, applied load, sliding speed, and lubricant. Data was

obtained by performing the tests up to the sliding distances of 1 m, 10 m and 100

m with the applied loads of 2.5 N, 5 N and 10 N at sliding speeds of 0.5 cm/s or 1

cm/s. Sliding distances of 1 m, 10 m, and 100 m represent the short, intermediate,

and long term, respectively, behavior of the composite. A single test was also

performed up to the sliding distance of 1000 m at the load of 10 N and at the

sliding speed of 0.5 cm/s in order to determine the far-long term tribological

behaviour of the composite.

In order to evaluate the data, mean friction coefficient (µ) between the first and

certain sliding distance were taken into consideration rather than the coefficient

obtained at the end of certain sliding distances throughout this study.

Mean friction coefficients measured without lubricant for different sliding

distances under different experimental conditions were tabulated in Table 4.1. In

general, mean friction coefficients increased with increasing applied load and

sliding distance but decreased with sliding speed. Mean friction coefficient of the

composite varied between 0.083 µ and 0.135 µ depending upon the test

parameters. Data indicate that the composite could be utilized for the applications

requiring low friction. DeLong et. al. (49) suggested that the friction coefficients

less than 0.1 µ correspond to low level and those between 0.4 µ and 0.9 µ

correspond to the high level. According to this classification the friction

coefficients obtained in this study could be interpreted as low or medium level of

friction.

As seen from Table 4.1, in general for a given sliding speed mean friction

coefficients increased with increasing applied load and sliding distance. This is

attributed to occurrence of different wear mechanisms as will be discussed in

26

Table 4.1 Mean friction coefficient of the C/C composite measured without

lubricant at loads of 2.5 N, 5 N and 10 N, at sliding speeds of 0.5 cm/s and 1 cm/s

for different sliding distances.

Mean Friction Coefficient (µ)

2.5 N 5 N 10 N

Sliding

Distance

(m) 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s

1 0.104 0.119 0.105 0.122 0.130 0.093

10 0.083 0.096 0.100 0.124 0.122 0.106

100 0.132 0.098 0.135 0.113 0.121 0.111

1000 - - - - 0.103 -

Chapter 4.4 as well as surface roughness of the specimen. The roughness of the

contact area increased due to the surface worn out by zirconia pin as seen from the

comparison of the center line average (Ra) values in Tables 4.2 and 4.3. Mean

wear track areas and Ra values were obtained according to Section 3.2.2. It was

not possible to have the same surface roughness values prior to the tribological

tests at different test conditions. Ra values of the C/C composite for different

sliding distances under different experimental conditions prior to and after the

tribological testing were tabulated in Tables 4.2 and 4.3, respectively.

As the surface worn out by zirconia pin, the roughness of the contact area

increased and this situation led to an increase in friction coefficient. On the other

hand, with the same exceptions on the data, the friction coefficient decreased with

increasing sliding speed. Lower friction coefficient at higher sliding speed was

assumed to cause by the easy formation and well-developed friction films at

higher sliding speed (high energy mode). As the kinetic energy loading was

decreased, the particulate-type debris became more dominant on the frictional

surface (50). However, the mean friction coefficient values given in Table 4.1 for

different sliding distances do not accommodate with this explanation. The

difference might be due to the experimental conditions and surface properties of

27

Table 4.2 Center line average surface roughness value of the C/C composite

measured without lubricant at loads of 2.5 N, 5 N and 10 N, at the sliding speeds

of 0.5 cm/s and 1 cm/s for different sliding distances prior to the wear.

Ra Value ( µm )

2.5 N 5 N 10 N

Sliding

Distance


1 0.300 0.338 0.282 0.320 0.319 0.260

10 0.317 0.350 0.228 0.310 0.252 0.341

100 0.299 0.290 0.179 0.233 0.258 0.300

1000 - - - - 0.281 -

Table 4.3 Center line average surface roughness value of the C/C composite

measured without lubricant at loads of 2.5 N, 5 N and 10 N, at the sliding speeds

of 0.5 cm/s and 1 cm/s for different sliding distances after the wear.

Ra Value ( µm)

2.5 N 5 N 10 N

Sliding

Distance


1 0.402 0.374 0.314 0.410 0.499 0.360

10 0.555 0.381 0.364 0.412 0.500 0.462

100 0.304 0.399 0.356 0.283 0.501 0.370

1000 - - - - 0.305 -

the samples. At the beginning of the test, friction surface of the sample was

smooth and even but surface irregularities were formed due to friction and wear

during experiment. The effect of lubricant on the mean friction coefficient of the

composite was determined only at a constant load of 5 N and a sliding speed of 0.5

cm/s for different sliding distances. The mean friction coefficients measured with

lubricant for different sliding distances were given in Table 4.4.

28

As seen from Table 4.4, the mean friction coefficient measured with lubricant

varied between 0.106 µ and 0.113 µ corresponding to a medium level of friction

coefficient. Results were similar to those gathered without lubricant. Initial

decrease followed by an increase in friction coefficient with increasing sliding

distance is noticed because of the reasons related with surface conditions as

explained earlier in this section.

Table 4.4 Mean friction coefficient of the C/C composite

measured with lubricant at a load of 5 N and a sliding

speed of 0.5 cm/s for different sliding distances.

Sliding Distance

(m)

Mean Friction Coefficient

(µ)

1 0.109

10 0.106

100 0.113

Ra values of the C/C composite prior to tribological tests and after tribological

tests at the load of 5 N and sliding speed of 0.5 cm/s for different sliding distances

with lubricant were tabulated in Tables 4.5 and 4.6, respectively. The increase in

the surface roughness with lubricant was less than the increase in that without

lubricant. When Ra values measured from different test conditions were evaluated,

in general, surface roughness increased approximately 72 % in ambient condition

and increased 28 % in lubricated condition after tribological testing. When a

comparison was made between the Ra values in ambient condition and lubricated

condition; the increase of the surface roughness in lubricated condition was

smaller than the increase of the surface roughness in ambient condition.

29

Table 4.5 Center line average surface roughness values of the C/C

composite measured with lubricant at a load of 5 N and a sliding

speed of 0.5 cm/s for different sliding distances prior to the wear.

Sliding Distance (m)

Ra value (µm)

1 0.262

10 0.247

100 0.198

Table 4.6 Center line average surface roughness values of the C/C

composite measured with lubricant at a load of 5 N and a sliding

speed of 0.5 cm/s for different sliding distances after the wear .


Ra value (µm)

1 0.330

10 0.304

100 0.270

A representative figure showing the variation of the friction coefficient of the C/C

composite studied as functions of sliding distance, number of rotational laps, and

sliding time were illustrated in Figures 4.2 (a)-(c). The tests were performed under

ambient atmospheric conditions at a sliding speed of 0.5 cm/s and a load of 10 N.

Although 100 % filtering was applied to get rid of the fluctuations in small

intervals such as ¼ seconds, the fluctuations in small intervals were not

completely eliminated. These fluctuations mainly resulted from the local friction

coefficient, which is a function of the local shear strength at the contact interface

and the local contact geometry (51).

30

Fric

tion

Coe

ffici

ent (

µ)

(a)

(b)

Fric

tion

Coe

ffici

ent (

µ)

Fric

tion

Coe

ffici

ent (

µ)

Sliding Distance/ Number of Laps/ Sliding Time

(c) Figure 4.2 Variation of the friction coefficient of the C/C composite studied as a

function of sliding distance, number of rotational laps, and sliding time. Data were

obtained at the sliding speed of 0.5 cm/s and at the applied load of 10 N in

ambient condition up to;

a) 1 m

b) 10 m

c) 100 m

31

Various statistical variables, such as microstructures, surface roughness, test

temperature, local contamination, adhesive transfers, free wear particles and

tribochemical reactions on the contact surfaces on the microscale are all related to

the constants through the local values of the friction coefficient and the wear

resistance of tested material (51). If the fluctuations were ignored, there would be

a smooth line having the mean friction coefficient of 0.130 µ, 0.122 µ, and 0.121

µ for the sliding distances of 1 m, 10, m and 100 m, respectively.

An increase followed by a decrease in the friction coefficient was observed within

the initial rotational laps for all of the tests. Thereafter the mean friction

coefficients remained more or less the same. Kopalinsky and Black (52) observed

similar behavior and explained this increase followed by decrease in the friction

coefficient at the early stage of testing with the surface properties. The surface

irregularities (roughness) of the starting material had a profound effect on the

friction coefficient. Lee et. al. (53) reported similar results and concluded that the

friction and wear behavior of the composite was sensitive to the sliding surface

condition, and the initial surface condition had a significant effect on friction

behavior of the composite.

The variation in friction coefficient from one experiment to the others might be

due to either the difference in the starting surface roughness of the samples prior

to testing or the microstructural arrangements that occurs during the test in the

composite. The surface roughnesses of the specimens prior to the tests were 0.319

µ, 0.252 µ, and 0.258 µ for the tests conducted for 1 m, 10 m, and 100 m,

respectively. In addition, experimental errors due to the calibration of the

tribometer and the arrangement between the pin and the disc might be among the

reasons. Calibration of the tribometer gives a maximum 2 % uncertainty and

deviations from the parallelism between the pin and the disc gives much more

uncertainty. Moreover, it could also be the result of the decrease in the true area of

the contact. The true area of contact was explained by most of the friction theories

(54), and assumed that force per unit area, which resists sliding (the shear

32

strength) is constant from which it follows that frictional force is proportional to

the true area of contact. It is quite appropriate with the Amonton’s second law

(54), for the sum of all contact points, which is established by microscopic surface

irregularities, determines the true area of contact and, hence, the observed

frictional force. Another reason for the increase or decrease at the initial rotational

laps may be attributed to the stabilization of the machine to the friction between

pin and surface of the material. Therefore the values obtained at the initial number

of rotational laps may not be reliable and representative.

In order to see the effect of the applied load on the friction coefficient of the C/C

composite, the tests were conducted at the loads of 2.5 N, 5 N, and 10 N at the

sliding speeds of 0.5 cm/s and 1 cm/s. The variation of the friction coefficient with

sliding distance, number of rotational laps, and sliding time at different loads were

illustrated in Figures 4.3 (a)-(c). The tests were conducted under ambient

atmospheric conditions at the sliding speeds of 0.5 cm/s. The mean friction

coefficient increased as the applied load increased from 2.5 N to 5 N, but

decreased as the applied load increased from 5 N to 10 N. This change might be

due to the surface roughness properties as discussed earlier. An increase in the

mean friction coefficient with increasing applied load was an expected result

according to the Amonton’s Law of Friction, which states that the relationship

between the friction force (F) and normal load (N) is linear; that is, F/N=µ (55).

In order to see the effect of sliding speed on the friction coefficient of the

composite, the tests were conducted at the same experimental conditions but at a

faster sliding speed of 1 cm/s. The variation of the friction coefficient as a function

of sliding distance, number of rotational laps, and sliding time at the sliding speed

of 1 cm/s at the applied load of 10 N were depicted in Figures 4.4 (a)-(c). The

curves of the variation of friction coefficient with increasing sliding distance

reveal that the changes in the sliding distance had an influence on the friction

coefficient of the composite.

33

Sliding Distance/ Number of Laps/ Sliding Time (a)


(b)


Fric

tion

Coe

ffici

ent (

µ)

Fric

tion

Coe

ffici

ent (

µ)

Fric

tion

Coe

ffici

ent (

µ)


(c)

Figure 4.3 Variation of the friction coefficient of the C/C composite studied as a


obtained at the sliding speed of 0.5 cm/s and for the sliding distance of 100 m in

ambient condition at;

a) 2.5 N

b) 5 N

c) 10 N

34


(a)


Fric

tion

Coe

ffici

ent (

µ)

Fric

tion

Coe

ffici

ent (

µ)

(b)


Fric

tion

Coe

ffici

ent (

µ)


(c)



obtained at the sliding speed of 1 cm/s and at the applied load of 10 N in ambient

condition up to;

a) 1 m

b) 10 m

c) 100 m

35


Fric

tion

Coe

ffici

ent (

µ)

(a)

(

(b)

Fric

tion

Coe

ffici

ent (

µ)


Fric

tion

Coe

ffici

ent (

µ)


(c)



obtained at the sliding speed of 1 cm/s and at the sliding distance of 100 m in

ambient condition at;

a) 2.5 N

b) 5 N

c) 10 N

36

The variation of the friction coefficient as functions of number of rotational laps,

sliding time and sliding distance between 0 and 100 m at different applied loads

were illustrated in Figures 4.5 (a)-(c). The tests were conducted under ambient

atmospheric conditions at a constant sliding speed of 1 cm/s. A comparison made

between Figures 4.5 (a) and (b) revealed that there was an increase in the mean

friction coefficient with increasing applied load from 2.5 N to 5 N. This increment

was due to the change in the surface roughness as the surface worn out by the

zirconia pin. However, the mean friction coefficient did not increase when the

applied load was increased further as seen in Figure 4.5 (c). This situation might

be the result of the formation of surfaces with reattached debris on them.

In order to determine the effect of lubricant on the friction coefficient of the

composite, the tests were conducted for different sliding distances but a constant

sliding speed of 0.5 cm/s and applied load of 5 N. Figures 4.6 (a)-(c) represent the

variation of the friction coefficient studied as a function of sliding distance,

number of rotational laps, and sliding time with lubricant. As expected and seen

from the figures that in the long term, 100 m, the mean friction coefficient of the

composite decreased from 0.135 µ to 0.113 µ in lubricated condition. The results

indicate that, the lubricant had a positive effect on the mean friction coefficient of

the composite.

In order to determine the effect of far-long term tribological behavior of the

composite, the tests were conducted for 1000 m sliding distance at the sliding

speed of 0.5 cm/s and applied load of 10 N. Figure 4.7 illustrates the variation of

the friction coefficient as a function of sliding distance, number of rotational laps,

and sliding time, at ambient atmosphere. The mean friction coefficient of the C/C

composite was 0.103 µ. The well-developed friction films at higher sliding

distances led to the low value of friction coefficient. Moreover, the formation of

surfaces with reattached debris on them gave rise to the decrease in coefficient of

friction with increasing number of cycles. Schön (26) has observed similar results

and indicated that the reattached debris layer is probably mainly made up of

37

Fric

tion

Coe

ffici

ent (

µ)

(a)

Fric

tion

Coe

ffici

ent (

µ)


(b)

Fric

tion

Coe

ffici

ent (

µ)



(c)



obtained at the sliding speed of 0.5 cm/s and at the applied load of 5 N with

lubricant up to;

a) 1 m

b) 10 m

c) 100 m

38


Fric

tion

Coe

ffici

ent (

µ)



function of sliding distance, number of rotational laps, and sliding time. Data was

obtained at the sliding speed of 0.5 cm/s and at the applied load of 10 N in

ambient condition up to the sliding distance of 1000 m.

matrix particles. Carbon fibers are hard and they do not reattach to the surface

easily but, there could be some fiber pieces in the reattached debris which could

act as reinforcement. Gomes et. al. (5) conducted a tribological study on a

commercial two dimensional C/C composite by using unidirectional C/C

composite as pin material. A pin-on-disk type friction and wear test machine was

used in that study. Although the experimental conditions were a little bit different

in their study, the material tested was alike. When compared with the study of

Gomes et. al. (5), this thesis work includes more information on the tribological

behavior of the C/C composite. First, the friction coefficients obtain in this study

include short, intermediate, long and far-long term tribological behavior of the

composite. Second, the tests were conducted by using high purity zirconia ball as

pin material, which had a higher hardness than the C/C composite pin material.

39

Finally, oil lubricant was used in the tests and effects on wear properties of the

composite were determined. This thesis study clarifies the tribological behavior of

the C/C composite in many aspects. A comparison of the two studies, in general,

has led to a conclusion that the increase in the sliding distance and applied load

decreases the friction and wear resistance of the composite. The lubricant plays a

protecting role on the friction surface and decreases the mean friction coefficient

of the C/C composite.

The variation of the mean friction coefficient of the composite as a function of

sliding distance at the sliding speeds of 0.5 cm/s and 1 cm/s were depicted in

Figures 4.8 (a) and (b), respectively. Mean friction coefficient tended to decrease

for all loads applied up to 10 m at the sliding speed of 0.5 cm/s. Beyond this point

it either increased or remained unchanged until the end of the experiment

depending on the applied load. At the load of 10 N, it remained constant while

increased at the loads of 2.5 and 5 N between the sliding distances of 10 m and

100 m. At the sliding speed of 1 cm/s, mean friction coefficient decreased at the

load of 2.5 N and increased at the loads of 5 N and 10 N up to 10 m. After this

point, it either decreased or increased until the end of the experiment depending on

the applied load. At the load of 5 N, it decreased while increased at the loads of

2.5 N and 10 N between the sliding distances of 10 m and 100 m.

Variation of the mean friction coefficient of the C/C composite measured at a load

of 5 N and a sliding speed of 0.5 cm/s for different sliding distances under

lubricated and unlubricated conditions was shown in Figure 4.9. When the sliding

distance was increased, similar frictional behavior was observed in the same

specimen for both lubricated and unlubricated condition. Apart from the initial

rotational laps, mean friction coefficient increased as the sliding distance

increased. However, the increase in mean friction coefficient in the unlubricated

condition was greater than that in the lubricated condition.

40

0

0,04

0,08

0,12

0,16

0 20 40 60 80Sliding Distance (m)

Mea

n Fr

ictio

n Co

effic

ient

2.5N5N10N

Mea

n Fr

ictio

n C

oeffi

cien

t (µ)

100

(a)

0

0,04

0,08

0,12

0,16

0 20 40 60 80 100

Sliding Distance

2.5 N5 N10 N

Mea

n Fr

ictio

n C

oeffi

cien

t (µ)

(b)

Figure 4.8 Variation of the mean friction coefficient of the C/C composite studied

as a function of sliding distance for the applied loads of 2.5 N, 5 N and 10 N at

ambient condition. Data was obtained at the sliding speeds of;

a) 0.5 cm/s,

b) 1 cm/s

41

0

0,05

0,1

0,15

0 20 40 60 80 100

Sliding Distance (m )

Mea

n Fr

ictio

n C

oeffi

cien

t

lubricated

unlubricated

Mea

n Fr

ictio

n C

oeffi

cien

t (µ)

Figure 4.9 Variation of the mean friction coefficient as a function of sliding

distance for lubricated and unlubricated conditions. The sliding speed was 0.5

cm/s and the applied load was 5 N.

In the lubricated condition, the friction coefficient was more or less the same for

the sliding distances of 1 m, 10 m, and 100 m being equal to approximately 0.1 µ.

The values for the friction coefficient in the unlubricated condition for the same

sliding distances were 0.105 µ, 0.100 µ, and 0.135 µ, respectively. It is commonly

known that lubrication decreases the frictional effect. However, the higher

frictional coefficients obtained for the sliding distances of 1 m and 10 m in the

lubricated condition compared to the unlubricated condition (though the values

were essentially the same; that is, the differences were within the experimental

error limits) was attributed to initial uneven surface roughness values.

Data gathered in lubricated condition were comparable to or slightly higher than

those gathered in unlubricated condition at different sliding distances. In an oil

environment, the behavior was similar to that observed for the samples tested in

air, with slightly lower wear rates of the samples in all cases, indicating some

positive influence of oil environment. In general, the oil environment does not

cause the samples rapidly wear (56).

42

The current lubrication is primarily based on two principles (57): fluid pressure to

separate the surfaces to avoid contact and surface chemical films to protect the

surfaces from shear stresses, rubbing and abrasion. When the surfaces come into

contact, many of the asperities undergo elastic deformation. This condition is

generally referred to the elastohydrodynamic lubrication (EHL). Further increase

in the contact pressure beyond the EHL regime causes the asperities to deform

plastically and the thickness of the fluid film to decrease. Under such conditions,

the temperatures at the asperity tips promote to form a chemical film and this film

protects the surface from wear. As seen from the figures that in the long term, 100

m, mean friction coefficient of the C/C composite has decreased in lubricated

condition and the lubricant had a positive effect on the mean friction coefficient of

the C/C composite.

Figure 4.10 illustrates the variation of the mean friction coefficient of the C/C

composite studied as a function of sliding distance at the sliding speeds of 0.5

cm/s and 1 cm/s at ambient condition. Data was obtained at 1 m, 10 m and 100 m

at the applied load of 5 N.

Mean friction coefficient obtained for the sliding speed of 0.5 cm/s initially

decreased from 0.105 µ to 0.100 µ as sliding distance was increased from 1 m to

10 m, but then increased from 0.100 µ to 0.135 µ as sliding distance was increased

from 10 m to 100 m. For the sliding speed of 1 cm/s, mean friction coefficient

tended to increase from 0.122 µ to 0.124 µ as sliding distance was increased from

1m to 10 m, but then decreased from 0.124 µ to 0.113 µ as sliding distance was

increased from 10 m to 100 m.

43

0

0,05

0,1

0,15

0 20 40 60 80


Mea

n Fr

ictio

n C

oeffi

cien

t M

ean

Fric

tion

Coe

ffici

ent (

µ)

0.5 cm/s1 cm/s

100

Figure 4.10 Variation of the mean friction coefficient as a function of sliding

distance for the sliding speeds of 0.5 cm/s and 1 cm/s at ambient environment. The

applied load was 5 N.

At relatively higher speed, fatigue effects and frictional heating are intensified

causing a surface damage (56), resulting in high friction coefficient at the sliding

distances of 1 m and 10 m. However, as the sliding distance increased, the mean

friction coefficient of the composite decreased at relatively higher speed. The

decrease in coefficient of friction with increasing number of rotational laps might

be the result of the change in the contact surface during the mechanical

arrangement after surface profile analysis. The arrangement between the pin and

the surface of the composite might cause the decrease. In addition, clearing worn

particles away might have some decreasing effect. The results indicated that

friction behavior is sensitive to sliding surface conditions as well as the initial

surface conditions of the composite.

Adhesion is the major cause of friction of smooth surfaces (58). The deformation

component of friction significantly increases in the case of debris formation at the

interface that gives rise to greater friction coefficient and abrasive wear due to

44

ploughing by wear particles (56). Friction force is governed not only by

heterogeneity of the contacting materials but to a larger degree by the surface local

tilt at micro-and nanolevel (58).

Particle mobility is the key factor that affects the wear. The low friction

coefficient and low wear rate may be achieved by the elimination of wear particles

from the contact region just after their formation. One way to do this is to design

surfaces of microgrooves or undulations at the sliding interface for trapping wear

particles in the grooves. If there is no way for particles to escape from the contact

region they will agglomerate and form larger particles. This results in increase of

both abrasive wear and deformation component of friction (58).

The review of previous studies (56-58) shows that wear particles at the sliding

interface dramatically change friction characteristics. These changes are strongly

affected by the way in which the particles are held against the surface and time of

interacting with the surface.

4.3 WORN VOLUME

The geometry of contact surface changed as a result of wear. Wear particles

agglomerated with time and covered the contact surfaces. After the end of each

test, the agglomerated surfaces were cleaned and mean worn areas were

determined by using surface profile measurements. Worn volumes were calculated

by inserting the data obtained from the surface profile measurements according to

the Equation 1 as described in Section 3.3. The worn volumes calculated under

different loads, at different sliding speeds for the C/C composite were presented in

Table 4.7. The tests were also conducted for the distance of 1000 m under the

applied load of 10 N in order to observe the far-long term tribological behavior of

the composite and presented in Table 4.7.

45

The effect of lubricant on the worn volume of the C/C composite was calculated

and represented in Table 4.8. The data presented in Tables 4.7 and 4.8 were

obtained after surface profile analysis upon completion of the tribological tests for

1, 10 and 100 m. In general, worn volumes increased as the sliding distance,

applied load and sliding speed were increased. Worn volumes of the composite

varied between 8.50 mm3 and 57.44 mm3 depending upon the test parameters. The

worn volume was lower in lubricated condition than unlubricated condition for a

given test condition i.e., at the same load and at the same sliding speed.

Table 4.7 Worn volume of the C/C composite measured without lubricant at loads

of 2.5 N, 5 N and 10 N, at sliding speeds of 0.5 cm/s and 1 cm/s for different

sliding distances.

Worn Volume (mm3)

2.5 N 5 N 10 N

Sliding

Distance


1 8.50 15.21 20.20 24.00 35.03 37.78

10 24.73 28.34 26.25 33.58 39.80 43.30

100 25.60 30.00 37.70 38.50 44.90 50.04

1000 - - - - 57.44 -

Table 4.8 Worn volume of the C/C composite measured with

lubricant at a load of 10 N and a sliding speed of 0.5 cm/s

for different sliding distances.


Worn Volume (mm3)

1 12.40

10 20.60

100 29.24

46

In order to determine the effect of sliding speed on the worn volume of the

composite, the tests were conducted at the same experimental conditions but at a

faster sliding speed of 1 cm/s. The variation of the worn volume of the composite

studied as a function of sliding distance for sliding speeds of 0.5 cm/s and 1 cm/s

were illustrated in Figures 4.11 (a) and (b), respectively. As seen from the figures,

in general, worn volume increased with increasing applied load and sliding speed.

The increase in the worn volume with increasing applied load was an expected

behavior according to the Amounton’s Law. The data fit almost the line. Therefore

the wear mechanism occurred in the C/C composite suits Type II behavior as

described in Section 2.4.

Worn volumes tend to increase for all loads applied up to a sliding distance of 10

m. Beyond this point it either continued to increase or remained unchanged until

the end of the experiment depending on the applied load. At the load of 2.5 N, it

remained constant while an increase was observed at the loads of 5 N and 10 N.

When the sliding speed increased, similar behaviour was observed in the same

specimen. However the increase in worn volume at the sliding speed of 1 cm/s

was greater than that at the sliding speed of 0.5 cm/s. At the sliding speed of 0.5

cm/s, as the sliding distance increased from 1 m to 10 m, the increase in the

amount of worn volume was 13 %. As the sliding distance increased from 10 m to

100 m the increase in the amount of worn volume was only 12.8 %. The increase

in the amount of worn volume with increasing sliding distance was more or less

the same. At a higher sliding speed of 1 cm/s, between the sliding distances from 1

m to 10 m and 10 m to 100 m, the increase in the amount of worn volume were

15.7 % and 15.6 %, respectively. The test results suggest that this composite is

suitable for short, intermediate, long, and far-long term wear applications under

different applied loads and sliding speeds.

At a sliding speed of 0.5 cm/s low friction coefficient values were obtained, even

though the normal applied load on the pin was relatively high. Similar results were

found by Gomes et.al. (5) who have reported that fine scale polishing is the

47

prevailing wear mechanism, flattening both mating surfaces, and which improves

their load-carrying capability. In this normal low friction/low wear regime, it is

well accepted that (5) water vapor molecules adsorb to the surface and passivate

the dangling covalent carbon bonds. At relatively high speed of 1 cm/s, fatigue

effects and frictional heating are intensified causing a surface damage in the form

of fiber pullouts and matrix-matrix fracture, resulting in high worn volumes.

The direct exposure of the matrix surfaces indicates that the matrix-matrix bond

strength is exceeded and the carbon matrix, which is more brittle than the

reinforcement phase and weakly attached to it, is preferentially removed. High

rotation speeds also intensify the debris removal by centrifugal forces avoiding the

formation of protective layers in open tribological systems (2).

In tribological tests, the main problem changing the worn volume is the

accumulation problem, which in turn results in causing a change in the dominant

wear mechanisms from abrasive to adhesive. Furthermore ‘Archard Wear Law’,

which states that a linear relation between incremental wear volumes and local

loads and sliding distances, emphasizes that even if every element at the loaded

and wearing interface in the field application behaves locally, it is not immediately

obvious how the overall or global volumetric wear loss from component will be

related to the total applied load or global sliding history (59). This is because the

interfacial pressure adjusts itself as wear proceeds so that the demands of

equilibrium are satisfied and the overall geometry is consistent with the

maintenance of geometric compatibility between both elements of the tribological

pair. Therefore the increase in the worn volume at some distances may also be

related to the ‘Archard Wear Law’ (60,61). The area of apparent contact changes,

so that, although the Archard relation may still be applicable on the microscale,

the relation between either the macroscopic wear dimension, or the total wear

volume, may be other than a linear function of sliding distance or load.

48

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100


2.5N5N10N

Wor

n V

olum

e (m

m3 )

(a)

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100


2.5N5N10N

Wor

n V

olum

e (m

m3 )

(b)

Figure 4.11 Variation of worn volume of the C/C composite studied as a function

of sliding distance for the applied loads of 2.5 N, 5 N and 10 N. Environment was

ambient. Data was obtained at the sliding speeds of,

a) 0.5 cm/s

b) 1 cm/s

49

In order to get rid of the accumulation problem by conducting experiments, it is

necessary to use lubricant, which cleans the surface while experiments are run.

Hence, to determine the effect of lubricant on the worn volume of the composite,

tests were carried out with oil lubricant. The results were illustrated graphically in

Figure 4.12. It is clearly seen that, the wear process was more or less the same in

both lubricated and unlubricated conditions. Though, the worn volume was higher

in the unlubricated condition than the lubricated one. The friction and wear profile

at various sliding speeds in a tribological system is affected by the mobility of the

lubricant, which varies according to its viscosity and affinity to the disk surface.

Consequently, the type of lubricant and the sliding speed can determine the

optimum amount of lubricant (62).

In this study the oil lubricant, which had the viscosity of 15.1 mm2/s at room

temperature, was used. The maximum worn volume obtained for the lubricated

composite was 1.5 times lower than that of the unlubricated composite.

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90 100Sliding Distance (m)

with lubricant

without lubricant

Wor

n V

olum

e (m

m3 )

Figure 4.12 Variation of worn volume of C/C composite studied as a function of

sliding distance with and without lubricant. The applied load was 10 N and the

sliding speed was 0.5 cm/s. Data was obtained for 1 m, 10 m and 100 m.

50

Mean wear track areas for different sliding distances under different experimental

conditions were tabulated in Table 4.9. The mean wear track areas were

determined by averaging the values of the track areas obtained from five of the

reference points. The mean wear track area of the composite varied between

134.75 µm2 and 914.25 µm2. As expected, the wear track area increased with

increasing sliding distance. The dominating effect of the surface profile before

testing on the wear track area was explained in Section 4.2. The effect of lubricant

on the mean wear track area is also represented in Table 4.10 at the load of 5 N

and sliding speed of 0.5 cm/s for different sliding distances. The data varied

between 280.9 µm2 and 347.4 µm2, and increased with increasing sliding distance

as the surface worn out by the zirconia pin.

Table 4.9 Mean wear track area of the C/C composite measured without lubricant

at loads of 2.5 N, 5 N and 10 N, at sliding speeds of 0.5 cm/s and 1 cm/s for

different sliding distances.

Mean Wear Track Area (mm2)

2.5 N 5 N 10 N

Sliding

Distance


1 134.75 242.09 321.85 317.85 557.53 326.00

10 393.71 312.80 417.89 534.50 633.13 582.80

100 407.37 332.25 599.81 550.75 714.60 650.00

1000 - - - - 914.25 -

Table 4.10 Mean wear track area of the C/C composite

measured with lubricant at a load of 5 N and a sliding

speed of 0.5 cm/s for different sliding distances.


Mean Wear Track Area (µm2)

1 280.90

10 311.50

100 347.40

51

Schematic representations of the typical wear track profiles obtained after

tribological testing at a load of 10 N and a sliding speed of 0.5 cm/s for different

sliding distances were illustrated in Figures 4.13 (a)-(c). The mean wear track

areas were 557.53 µm2, 633.13 µm2 and 714.60 µm2 for sliding distances of 1m,

10 m and 100 m, respectively. The Ra values were 0.499 µm, 0.500 µm and 0.501

µm for sliding distances of 1m, 10 m and 100 m, respectively. Maximum depth

distances were 4.0 µm, 3.5 µm and 12.9 µm, respectively. As seen from the data,

the wear track area increased with increasing sliding distance.

The results obtained after tribological testing at loads of 2.5 N, 5 N and 10 N,

sliding speed of 0.5 cm/s for sliding distance of 100 m were presented in Figures

4.14 (a)-(c). The mean wear track areas were 407.37 µm2, 599.81 µm2 and 714.60

µm2; the Ra values were 0.304 µm, 0.356 µm and 0.501µm; maximum depth

distances were 3.0 µm, 13.9 µm and 12.9µm, for the loads of 2.5 N, 5 N, and 10 N

respectively. As seen from the data, the wear track area increased with increasing

sliding distance and load.

52

µm

-12-10

-8-6-4-20

246

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 mm

Dep

th D

ista

nce

(µm

)

(a)

µm

-6-5-4-3

-2-1012

3

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 mm

Dep

th D

ista

nce

(µm

)

(b)

µm

-25

-20

-15

-10

-5

0

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 mm

Dep

th D

ista

nce

(µm

)

(c)

Wear Track Distance (mm)

Figure 4.13 Schematic representation of the wear track of the C/C composite.

Data were obtained after tribological testing at a load of 10 N and at a sliding

speed of 0.5 cm/s for the sliding distances of;

a) 1 m

b) 10 m

c) 100 m

53

µm

-8

-7

-6

-5

-4

-3

-2

-1

0

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 mm

Dep

th D

ista

nce

(µm

)

(a)

(a)

µm

-25

-20

-15

-10

-5

0

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 mm

Dep

th D

ista

nce

(µm

)

(b)

µm

-25

-20

-15

-10

-5

0

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 mm

Dep

th D

ista

nce

(µm

)

Wear Track Distance (mm)

(c)

Figure 4.14 Schematic representation of the wear track of the C/C composite.

Data were obtained after tribological testing for a sliding distance of 100 m at

sliding speed of 0.5 cm/s at the loads of;

a) 2.5 N

b) 5 N

c) 10 N

54

4.4 SPECIFIC WEAR RATE

The specific wear rates for all of the experimental conditions were calculated

employing Equation 2 as discussed in Section 3.3. Data were tabulated in Tables

4.11 and 4.12. Specific wear rates of the composite varied between 60.520x10-4

mm3/N.m and 0.058x10-4 mm3/N.m in the ambient condition, and 35.11x10-4

mm3/N.m and 0.437x10-4 mm3/N.m in the lubricated condition, depending upon

the test parameters. As seen from Tables 4.11 and 4.12, the specific wear rate was

the maximum for the sliding distance of 1 m. As sliding distance increased,

specific wear rate decreased for all applied loads.

Table 4.11 Specific wear rate of the C/C composite measured without lubricant at

loads of 2.5 N, 5 N and 10 N at sliding speeds of 0.5 cm/s and 1 cm/s for different

sliding distances.

Specific Wear Rate x 10-4 (mm3/N.m)

2.5 N 5 N 10 N

Sliding

Distance


1 33.690 60.520 40.230 39.730 34.850 20.370

10 9.843 3.935 5.224 6.723 3.982 3.665

100 1.024 0.836 0.754 0.529 0.445 0.409

1000 - - - - 0.058 -

Table 4.12 Specific wear rate of the C/C composite measured

with lubricant at a load of 5 N and a sliding speed of

0.5 cm/s for different sliding distances.


Specific Wear Rate x 10-4

(mm3/ N.m)

1 35.11

10 3.918

100 0.437

55

For all the tests performed, mild wear can be recognized easily on the specimens

by the naked eye. Mild wear is a polished appearance, corresponding to the

spreading of a coherent debris layer giving a relatively flat surface. It always

shows a shiny and smooth wear track as a result of running in. Severe wear is a

rough morphology, resulting from uneven wear rates of the matrix and fibers or

the presence of low adhered powdery debris. It shows a rough surface

accompanied by lot wear debris beside the wear track. In the literature, if the

specific wear rate is approximately 2x10-6 mm3/N.m, it is defined as mild wear and

if it is in between 1x10-3 mm3 / N.m and 3x10-2 mm3 / N.m, it is defined as severe

wear (5). Therefore, the wear shown in Figures 4.13 (a) - (c) and Figure 4.14 (b) -

(c) are accounted as mild to severe wear.

The worn surface morphology was categorized into three types (I, II, and III) as

discussed in Section 2.4. The pre-transitional friction coefficients of the

composites are generally low. When transition occurs, the initial thin, smooth film

(Type I morphology) suddenly disrupts into a rough powdery debris layer (Type II

morphology), causing the friction coefficient rise (64).

Generally at the beginning of the experiments, the specific wear rate is higher.

Under certain conditions, the powdery debris subsequently compacts into

smoother and denser lubricative film (Type III morphology) that causes both

friction and wear to decrease. The wear rate all increases sharply when the

transition from Type I to Type II occurs. When the powdery Type II debris

compacts to form the lubricative Type III debris, the wear rate decline, although

never approach their initial Type I levels (64).

It is generally believed that the high frictional behaviour of carbon materials

during dusting is due to the interaction between unsaturated covalent bonds of

carbon atom. While not nearly as effective as water vapor in preventing dusting,

molecular and atomic oxygen can also have lubricative effect on carbon materials

at room temperature. It is, thus, conceivable that the adsorption of oxygen at

56

elevated temperatures can reduce the interaction between the dangling covalent

bonds of carbon atoms by forming various types of oxygenated complex on

carbon-carbon surfaces (64).

Figures 4.15 (a) and (b) illustrate the variation of the specific wear rate studied as

a function of sliding distance at the sliding speeds of 0.5 cm/s and 1 cm/s at

ambient condition. Data was obtained at 1 m, 10 m, and 100 m at different applied

loads. Similar behaviour was observed graphically in both cases. Specific wear

rates were the maximum for sliding distance of 1 m, than it decreased sharply as

the sliding distance increased to 10 m, and it continued to decrease smoothly until

the end of the experiment. The decrease in the specific wear rate is mainly due to

the abrasive particles. When abrasive particles sandwiched between two surfaces

are loose, the wear rate is less by than when one material slides against hard

protuberances of the counterface. This is because the loose abrasive particle

spends most of its time to roll at the sliding interface (53). Also, the formation of

powdery type lubricative debris film that can improve the wear resistance of the

composites through the filling of open pores located at the friction surfaces with

the wear debris and create a stable friction films across the surface of the

composite (64).

It can be stated that, as sliding speed increased from 0.5 cm/s to 1 cm/s the

specific wear rate decreased or increased with a few exceptions generally within

the first laps in the initial unsteady state as seen from Figures 4.15 (a) and (b).

This is because the specific wear rate changes through the repeated contact

process under constant load and sliding speed and is generally high in an initial

unsteady state and relatively low in the later steady state according to Kato (51). In

addition, it is concluded that specific wear rate depends on the sliding distance as

it was the highest at a sliding distance of 1 m and the lowest at a sliding distance

of 100 m.

57

Sliding Distance (m) (a)

Spe

cific

Wea

r Rat

e x

10-4

(mm

3 /N.m

) S

peci

fic W

ear R

ate

x 10

-4 (m

m3 /N

.m)

Sliding Distance (m) (b)

Figure 4.15 Variation of specific wear rate studied at loads of 2.5 N, 5 N and 10 N

at ambient condition. The data were obtained at the sliding speeds of;

a) 0.5 cm/s

b) 1 cm/s

In specific wear rate prediction it is important to consider the critical condition for

the transition of the wear mode from one to another (63). In the present study, to

predict the specific wear rate and to compare whether lubrication plays a critical

role on the C/C composite, specific wear rates were calculated and depicted in

Figure 4.16. When the sliding distance is increased similar wear behaviour is

observed in the same specimen for both lubricated and unlubricated condition.

However, specific wear rate is greater in the unlubricated condition than the

58

lubricated condition. This was an expected result due to the protective layer

occurred at the surface of the composite. The basic principal of the lubrication is

either to avoid contacts using fluid pressures or when inevitable, use chemistry to

generate a sacrificial film to protect the surfaces. The film also functions to

redistribute the stresses at the interface; provide a sacrificial easily sharable layer;

increase the real area of contact by physically smoothing out the relative

roughness thereby lowering the contact pressure (52).

The future studies would be conducted on the effect of different lubricants with

different velocities and determine whether or not tribochemical reactions play role

in the wear process of the C/C composite and evaluate the optimum amount of

lubricant to preserve the composite from severe wear.

0

10

20

30

40

50

0 20 40 60 80 1Sliding Distance (m)

00

with lubricantwithout lubricant

Spe

cific

Wea

r Rat

e x

10-4

(mm

3 /N.m

)

Figure 4.16 Variation of specific wear rate of C/C composite studied as a function

of sliding distance with and without lubricant. The applied load was 5 N and the

sliding speed was 0.5 cm/s. Data was obtained for 1 m, 10 m and 100 m.

59

Since the absolute value of specific wear rates depend also on the abrasive pin

used, it can only express a value for that particular pair of interacting surfaces and

cannot be compared to the absolute values obtained by other researchers. It is

reasonable to compare the relative behavior of similar materials tested under the

same standard conditions. However, the tribometer used in this study had a

maximum loading capacity of 10 N. Therefore the highest load that could be used

in this study was 10 N so the comparison with the other researches was restricted.

As the specific wear rates strongly depend on the mechanism of the wear process,

and which also depends on both the material properties and the conditions under

which the material used, it can easily be concluded that at the beginning of the test

the wear mechanism is different from at the end.

There was a polished surface at the beginning and as the experiment proceeded,

surface began to be damaged and caused to complicated wear mechanism. Since

abrasive wear process was dominant in some interval, which depends on the

loading condition and surface properties, as load increased the time of this

dominant mechanism disrupts and another mechanism became dominant.

Material subjected on the wear process has got two important parameters in

predicting the specific wear rate; the hardness and fracture toughness; since the

hardness defines the load concentration at the asperities and whether material

removal can occur by fracture depends on toughness (65).

The observed load-independent wear behavior at high loads may be related to a

combination of tribochemical and mechanical processes, since material removal

by mechanical action alone should be load-dependent. The processes of

tribochemical reactions, film formation, film fracture, and dissolution of reaction

product can occur simultaneously. At high loads, the rate of film growth may be

sufficiently large to compensate for dissolution or increase mechanical wear; thus,

resulted in a load-independent behavior, as explained by Nagarajan et. al. (66).

60

In experiments, the primary problem was the accumulation of the worn particles

on the wear track section. Therefore it would have been better to use a continuous

process during the experiment that will clear away the worn particles from the

surface and simultaneously detect the worn volume and the friction coefficients.

A comparison of mean friction coefficient and specific wear rate values obtained

for C/C composite in this study with those reported in the literature for similar

material and test conditions were depicted in Table 4.13. Mean friction coefficient

of the C/C composite used in this study varied between 0.083 µm and 0.135 µm.

Mean friction coefficient of the other materials such as C/C composite, ceramic

(SiC, TiB2, Mullite, Al2O3), and metal (cast iron) varied between 0.1 µm and 0.98

µm. The mean friction coefficient of the C/C composite used in this study was

smaller when compared to other C/C composite, ceramic and metal materials

reported in the literature. Specific wear rate of the C/C composite used in this

study varied between 0.058x10-4 mm3/N.m and 60.520x10-4 mm3/N.m. Specific

wear rate of other materials such as C/C composite, ceramic (SiC, TiB2, Mullite,

Al2O3), and metal (cast iron) varied between 3x10-2 mm3/N.m and 65.5x10-5

mm3/N.m. Specific wear rate of the C/C composite used in this study was lower or

higher than other C/C composite, ceramic and metal materials. The tribological

test conditions examined in this study were not identical with those given in the

literature.

61

Table 4.13 A comparison of friction coefficient and specific wear rate values

obtained for C/C composite in this study with those reported in the literature for

selected materials.

Material Mean Friction

Coefficient

(µm)

Specific Wear Rate

(mm3/ N.m)

Reference

Number

C/C composite 0.083 - 0.135 0.058 - 60.520x10-4 This study

C/C composite 0.42 - 0.55 45.0 - 22.0x10-3

(mm3/m)

14

C/C composite 0.4 - 0.9 1x10-3 - 3x10-2 70

C/C composite 0.1 - 0.25 10-3 -10-5 5

C/C composite 0.71 - 0.98 - 22

SiC 0.53 - 0.72 12.5 - 20.2 x 10-5 9

TiB2 0.63 - 0.77 11.2 - 31.1 x 10-5 9

Al2O3 0.62 28.2 x 10-5 9

Mullite - 12.0 - 68.5 x 10-5 9

Cast iron 0.4 - 0.55 - 67

Cast iron - 0.89 - 1.4 x 10-6 (g/MPa.m)

68

Steel 0.2 - 0.75 - 69

4.5 SURFACE CHARACTERIZATION

4.5.1 Optical Microscopy (OM)

At the end of the tests, the wear tracks of the disks were examined by OM.

Representative images of the surface characteristics taken after long term, 100 m,

sliding distance at a load of 10 N and at sliding speeds of 0.5 cm/s and 1 cm/s in

unlubricated condition are shown in Figures 4.17 (a) and (b). A comparison

between these figures reveal that the wear track depth and surface damage formed

62

at 0.5 cm/s sliding speed was less than those formed at 1 cm/s. Although wear

damage was formed just on the surface and was very light in both images, it is

more obvious in Figure 4.17 (b) than in Figure 4.17 (a).

(a)

Figure 4.17 OM image take

sliding distance at a load of 10

Data was obtained at sliding sp

a) 0.5 cm/s

b) 1 cm/s

The surface characteristics aft

at sliding speed of 0.5 cm/s fo

depicted in Figures 4.18 (a) an

Figure 4.18 (a) was lesser w

lubricant film covered the sur

that the wear damage was less

lubricant on the surface of the

out the relative roughness.

5 µm

(b)

n after the tribological test performed fo

N in unlubricated condition (x 10).

eeds of;

er 100 m sliding distance taken at a load o

r both lubricated and unlubricated conditi

d (b). The wear track depth and surface d

hen compared with those in Figure 4.18

face of the sample and formed a protective

in lubricated condition. The film that deve

sample supported the load and functioned t

63

5 µm

r 100 m

f 5 N and

ons were

amage in

(b). The

layer so

loped by

o smooth

5 µm 5 µm

(a) (b) Figure 4.18 OM image taken after the tribological test performed for 100 m

sliding distance at a load of 5 N at a sliding speed of 0.5 cm/s (x 10).

Data was obtained;

a) with lubricant

b) without lubricant

Figure 4.19 represents the surface characteristics after 1000 m sliding distance at a

load of 10 N and at sliding speed of 0.5 cm/s in unlubricated condition. As seen in

the photographs in Figures 4.17 (a) and 4.18 (b), the wear track depth and surface

damage were much severe when compared with that obtained after 100 m. The

wear damage of the surface increased as the sliding distance increased and the

wear track on the surface was apparent. It could have been seen even with the

naked eye. The wear surface became rougher as the applied load and sliding

distance were increased.

5 µm

Figure 4.19 OM images taken after the tribological test performed for 1000 m

sliding distance at a load of 10 N, at a sliding speed of 0.5 cm/s without lubricant

(x 10).

64

4.5.2 Scanning Electron Microscopy (SEM)

After the far-long term testing of the sample, the wear track of the disk was

examined by SEM. Representative images showing the surface characteristics

after 1000 m sliding distance taken at a load of 10 N at a sliding speed of 0.5 cm/s

in unlubricated condition were illustrated in Figures 4.20 (a) and (b). The images

were taken from two different sections at different magnifications. The debris

agglomeration seen on the surface which formed during the preparation of

specimen for SEM analysis or during the tribological test enabled us to observe

the surface damage mechanisms clearly. The composite showed Type II wear

mechanism corresponding to the medium level of friction. The lubricative film

evolved from the Type II powdery debris, rolled, compacted and piled up resulting

the wear mechanism change from abrasive to adhesive.

SEM micrographs show that cracks are present throughout the composites and the

powdery debris agglomerates are packed into the cracks on composite surfaces.

The difference between the worn and unworn surfaces are seen in Figure 4.20 (a).

The left hand side of the figure shows the unworn surface while right hand side of

the figure shows the worn surface. The texture of the unworn part was smooth and

even. However, the texture of the worn part was rougher. The irregularities formed

due to the friction and wear is seen on this part. There was a debris agglomeration

all over on the specimen. The debris agglomeration was spread on the surface

during the tribological test and sample preparation for the SEM examination. The

fiber debonding, fracture and pull-out energy absorbing mechanisms are seen in

Figure 4.20 (b).

65

(a)

(b)

Figure 4.20 SEM images taken after the tribological test performed for 100 m

sliding distance at a load of 10 N and at a sliding speed of 0.5 cm/s without

lubricant. (a) X 300

(b) X 1000.

66

CHAPTER 5

CONCLUSIONS

1. C/C composite exhibited a low friction coefficient when tested against a

zirconia pin. Friction coefficient increased with increasing applied load and

sliding distance and decreased with increasing sliding speed.

2. The surface quality of the sample, applied load, sliding speed, sliding distance

or sliding time as well as lubrication had an influence on the friction

coefficient and specific wear rate of the composite. Friction coefficient varied

within the range of 0.083 µ to 0.135 µ depending on the test parameters.

3. C/C composite had a specific wear rate in the range of 60.520x10-4 mm3/N.m

and 0.058x10-4 mm3/N.m. The specific wear rate changed through the repeated

contact process under constant load and sliding speed and was generally high

in an initial unsteady state and relatively low in the later steady state.

4. The lubricant played a protecting role on the friction surface and decreased the

mean friction coefficient of the C/C composite.

5. Structural characterizations revealed that the composite showed Type II wear

mechanism corresponding to the low and medium level of friction.

6. The dominant wear mechanism changed from abrasive to adhesive at higher

loads and / or higher sliding distances.

67

FUTURE WORKS

1. In order to understand the tribological behavior of the C/C composite better,

the tests should be performed for shorter sliding distance intervals.

2. Far-long term, 1000 m, tribological behavior of the C/C composite might be

studied for all applied loads and sliding speeds.

3. Effect of temperature on the tribological behavior of this composite might be

studied.

4. A study, which includes the hardness, and fracture toughness parameters might

be done to predict the specific wear rate of this composite.

68

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