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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 :
iii
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.
vii
To my dear parents;
Halise and Timur KARAVELİ
viii
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.
ix
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
xii
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
xiii
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
xiv
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
4.3 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 sliding distance of
100 m in ambient condition at;
a) 2.5 N.................................................................................................34
b) 5 N....................................................................................................34
c) 10 N....................................................................................................34
4.4 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 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
4.5 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 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
4.6 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
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
4.7 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
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
(m) 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s
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
(m) 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s
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 .
Sliding Distance (m)
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)
Sliding Distance/ Number of Laps/ Sliding Time
(b)
Sliding Distance/ Number of Laps/ Sliding Time
Fric
tion
Coe
ffici
ent (
µ)
Fric
tion
Coe
ffici
ent (
µ)
Fric
tion
Coe
ffici
ent (
µ)
Sliding Distance/ Number of Laps/ Sliding Time
(c)
Figure 4.3 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 for the sliding distance of 100 m in
ambient condition at;
a) 2.5 N
b) 5 N
c) 10 N
34
Sliding Distance/ Number of Laps/ Sliding Time
(a)
Sliding Distance/ Number of Laps/ Sliding Time
Fric
tion
Coe
ffici
ent (
µ)
Fric
tion
Coe
ffici
ent (
µ)
(b)
Sliding Distance/ Number of Laps/ Sliding Time
Fric
tion
Coe
ffici
ent (
µ)
Sliding Distance/ Number of Laps/ Sliding Time
(c)
Figure 4.4 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 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
Sliding Distance/ Number of Laps/ Sliding Time
Fric
tion
Coe
ffici
ent (
µ)
(a)
(
(b)
Fric
tion
Coe
ffici
ent (
µ)
Sliding Distance/ Number of Laps/ Sliding Time
Fric
tion
Coe
ffici
ent (
µ)
Sliding Distance/ Number of Laps/ Sliding Time
(c)
Figure 4.5 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 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)
Sliding Distance/ Number of Laps/ Sliding Time
(b)
Fric
tion
Coe
ffici
ent (
µ)
Sliding Distance/ Number of Laps/ Sliding Time
Sliding Distance/ Number of Laps/ Sliding Time
(c)
Figure 4.6 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 5 N with
lubricant up to;
a) 1 m
b) 10 m
c) 100 m
38
Sliding Distance/ Number of Laps/ Sliding Time
Fric
tion
Coe
ffici
ent (
µ)
Sliding Distance/ Number of Laps/ Sliding Time
Figure 4.7 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 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
Sliding Distance (m)
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
(m) 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s
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.
Sliding Distance (m)
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
Sliding Distance (m)
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
Sliding Distance (m)
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
(m) 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s
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.
Sliding Distance (m)
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
(m) 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s 0.5 cm/s 1 cm/s
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.
Sliding Distance (m)
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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