FRICTION AND WEAR OF ENGINEERING POLYMERS :
STUDIES ON POLYIMIDES/COMPOSITES
By
JAYASHREE BIJWE
Thesis submitted
for the award of the degree of
DOCTOR OF PHILOSOPHY
coaLwit
INDUSTRIAL TRIBOLOGY MACHINE DYNAMICS
AND MAINTENANCE ENGINEERING CENTRE (ITMMEC)
INDIAN INSTITUTE OF TECHNOLOGY, DELHI
NEW DELHI-110016
MARCH 1989
DEDICATED TO
CHINMAY
CHAITANYA
PRADEEP
CERTIFICATE
This is to certify that the thesis "FRICTION AND WEAR
OF ENGINEERING POLYMERS : STUDIES ON POLYIMIDES/COMPOSITES"
being submitted by
Mrs. Jayshree Bijwe
to the Indian
Institute of Technology,Delhi for the award of Degree of
Doctor of Philosophy, is a record of bonafide research work
carried out by her under our guidance and supervision.
To
our knowledge, it has reached the standard of fulfilling
requirements of the degree. The matter embodied in this
thesis has not been submitted for the award of any degree or
diploma of any other University or Institute.
(Dr. (Mrs) P. Vasudevan
Professor & Head
Centre for Rural Development
and Appropriate Technology,
Indian Institute of Technology,
New Delhi-110016 (India).
(Dr. U.S. Tewari)
Sr. Design Engineer
Industrial Tribology Machine
Dynamics and Maintenance
Engineering Centre (ITMMEC)
Indian Institute of Technology, New Delhi-110016.
ACKNOWLEDGEMENTS
It is a pleasant duty to express my profound gratitude
to Dr. U.S. Tewari, Sr. Design Engineer in Industrial
Tribology, Machine Dynamics and Maintenance Engineering
Centre and Prof. (Mrs.) P. Vasudevan, Head Centre for Rural
Development and Appropriate Technology for their constant
guidance and encouragement in pursuing this research work.
It is my privilege to have received their help at various
stages of the investigation.
I take this opportunity to thank Prof. O.P. Chawla,
Head, ITMMEC and Prof. B.C. Nakra, Former Head, ITMMEC, for
providing necessary facilities to carry out this work.
Thanks are also due to Prof. (Mrs.) I.K. Varma (CMST) for
providing the facility of thermal analysis.
I am grateful to Prof. Sethuramiah (ITMMEC), Prof. K.
Tanaka (Kanzawa University) and Dr. J.K. Lancaster for their
precious suggestions during the progress of work. I wish to
place on record my sincere thanks to Dr. Amitabh Jain, Dr.
C.R. Jagga (SDE), Dr. B.K. Gupta (SDE) and Mr. Brahm Prakash
(SDE) for their constant interest and fruitful discussions
during the progress of the work. Thanks are due to Dr.
Chattar Singh for his help while preparing scanning electron
micrographs.
I wish to thank to Dr. L.J. Male, Former
Manager, Product Development, GEC (USA) for providing
material for this study. I wish to acknowledge the help
rendered by supporting staff (ITMMEC) especially Mr. P.S.
Thakkar, Mr. Avtar Singh, Mr. J.P. Singh, Mr. Raghubir and
Mr. M.P. Verma.
I am thankful to Mr. J.C. Tuteja for
tracing of figures.
Finally, I thank my husband Dr. P.R. Bijwe, Asstt.
Professor, Electrical Engg. Deptt., IIT,Delhi, for his
patience, understanding, co-operation and constant interest
in this work.
(Jayashree Bijwe)
aBsmacr
Engineering polymers form one of the most important
class of tribo-materials because of their excellent property
profile such as self lubricity, low density corrosion and
impact resistance, vibration and noise absorption, ease in
processing, better wear and antiwelding characteristics.
Thermoplastics are a better choice
than thermosets
because of special features such as higher impact resistance
and interlaminar fracture toughness, ease and speed in
processing, repairs and recyclability.
Further, use of
short fibre reinforced thermoplastics (FRPs) for triboapplications is a fast emerging area, because of their high
specific stress,
stiffness and possibility of tailoring
their properties through control of fibre and matrix
combination.
Most of the tribo-applications demand materials which
can dependably perform at elevated temperatures and have
better load carrying capacity besides improved friction and
wear properties. Polyimides form excellent class of such
materials and their potential for tribo-applications is
worth exploring.
In the light of the above, a thermoplastic polyimide,
commercially known as ULTEM polyetherimide (PEI) and its
various composites having very good property profile were
selected for the present studies. Since the details on the
constituents of the composites were not available, these
were analysed in the laboratory by various techniques.
was seen that the composites contained
glass
It
fibre
reinforcement, and various solid lubricant additives, viz.
polytetrafluoroethylene, MoS and graphite. It was of
2
interest to study the effect of these fillers and
reinforcement on the friction and wear properties of PEI,
since filler effect in tribology is not always predictable.
It was worthwhile to compare tribological properties of PEI
and its composites with those of commercially established
thermosetting polyimide. Hence, thermosetting polyimide,
commercially known as Vespel PI and its various composites
were selected for the studies.
The friction and wear
studies were performed by sliding polymer pins against
smooth metallic discs under different conditions such as
varying load, sliding distance, speed, oounterface roughness
and fibre orientation.
It was of interest to investigate the effect of fillers
and/or fibres on the wear in the abrasive condition also,
since fillers affect the abrasive wear phenomenon in a
complex way which cannot be predicted apriori.
Hence,
abrasive wear studies of various composites of PEI and PI
(including bulk polymers) were performed under various
abrading conditions of load, abrading particle size,
ii
distance and fibre orientation. The studies in singlepass
condition were done to investigate the relationship between
wear performance and appropriate mechanical properties.
Studies in multipass conditions were done to see the effect
of clogging of the counterface by the fillers and insitu
solid lubrication under extreme abrasive conditions.
In the case of materials sliding against metal discs
(adhesive transfer conditions) it was observed that PEI, a
glassy yet ductile polymer does not wear by film transfeir
mechanism.
mode.
Fatigue was seen to be the predominant wear
The existence of an incubation period (cycles of
sliding required to generate wear particles) indicated that
it has good fatigue wear resistance in the selected
conditions of experiment. However, once the wear started
-13 3
wear rate was high (2:10 m /Nm). Its PTFE filled composite
showed film transfer due to PTFE. The friction coefficient
exhibited by the composite was lowest amongst all the
materials tested. However, wear rate was still high
-14 3
(=10 m /Nm). With the incorporation of glass fibres (GF)
15 3
m /Nm).
However,
wear rate reduced dramally (4.5x10
friction coefficient was highest. Inclusion of graphite
along with GF resulted in a decrease in friction as well as
-15 3
lower wear rate (1.2x10 m /Nm). Addition of three solid
lubricants (PTFE, graphite, and MoS ) along with fibre
2
reinforcement (GF), ultimately resulted in a good
iii
combination of friction and wear (p : 0.12 and wear rate
-16 3
8x10 m /Nm). The comparison of this composite of PEI with
that of PI revealed that friction and wear properties of
both were comparable. Although PI composite was slightly
-16 3
better, Ou :0.1 and wear rate 5x10 m /Nm), ease in
processing and lower cost were the two benefits associated
with the thermoplastic (PEI).
The ,abrasive wear studies of selected materials
revealed that PEI was better in this respect than PI.
However, almost all the composites exhibited higher wear
rates than their parent polymers. This was attributed to
the reduction in Se factor where S and e are rupture stress
and elongation to break. Size effect was observed in most
of the materials.
It is possibly due to a transition
between delamination and cutting wear at a critical
grit
abrasive wear 1.3rformance of these materials
strongly depended on the experimental conditions.
Solid
lubricants effectively reduced the abrasivity of the
abrasive papers reducing further wear. Thus, it was seen
that the composites which performed very well in adhesive
wear contidions did not perform so well in abrasive wear
conditions.
Scanning electron microscopy and energy dispersive Xray analysis were found to be very effective techniques for
investigating wear mechanisms.
iv
Abbreviations
FRPs
Fibre reinforced plastics
PTFE
Polytetrafluoroethylene
PEEK
Polyether-ether ketone
PEK
Polyether ketone
PE
Polyethylene
LDPE
Low density polyethylene
HDPE
High density polyethylene
UHMWPE Ultrahigh molecular weight polyethylene
PA
Polyimide (nylon)
PC
Polycarbonate
PI
Polyimide
PAI
Polyamide-imide copolymer
PEI
Polyetherimide
BM
Bismalemide
PPS
Polyphenylene sulphide
POM
Polyoxymethylene (Acetal)
PU
Polyurethane
PS
Polystyrene
PMMA
Polymethyl methacrylate
PET
Poly ethylene terephthalate
PBTP
Poly butylene terephthalate
Pc
Phthalocyanine
PVC
Poly vinyl chloride
PP
Neropylene
EP
Epoxy resin
ABS
Acrylonitrile butadiene styrene copolymer
LCP
Liquid crystalline polymer
M S
Mild steel
GF
Glass fibre
CF
Carbon fibres
AF
Aramide fibres
Fp
Fibres parallel to the sliding surface
F
N
Fibres normal to the sliding surface
Friction coefficient
Initial friction coefficient
/Li
Stable friction coefficient
Peak friction coefficient
p
K
Specific wear rate
0
S
Stress before rupture (ultimate tensile strength)
e
elongation before rupture (ultimate tensile
elongation)
H
Hardness.
CONTENTS
Page
No.
ACKNOWLEDGEMENTS
i -iv
ABSTRACT
CHAPTER I
INTRODUCTION
1
1.1
Friction of Polymers
1.2
Wear of Polymers
3
Adhesive or transfer wear
of polymers
7
1.2.1.1 Wear mechanism in adhesive
or transfer mode
8
1.2.1
1.2.1.2 Adhesive (transfer) wear
of composites
10
1.2.1.3 Effect of various parameters
on friction and wear of polymers
13
and their composites
1.2.2
Abrasive (grooving) wear of
polymers and composites
20
1.2.2.1 Abrasive wear of rigid polymers 22
1.3
1.4
1.2.2.2 Abrasive wear of polymer
composites
24
Tribology of High Performance
Engineering Polymers
26
Tribology of Polyimides and Composites
28
1.4.1. Tribology'of Polyimides
29
1.4.2. Tribology of PI composites
31
1.5
Scope of the Work
34
Page
No.
CHAPTER II
EXPERIMENTAL
Part A
2.1
Materials Studied
41
2.2
Techniques for Characterisation
for the Materials
42
2.2.1
Elemental analysis
42
2.2.2
Infrared spectroscopy
42
2.2.3
Thermal methods of analysis
42
2.2.4
Micros,copic techniques
43
Tribological Studies
44
Friction and wear studies
in the present work
45
2.3
2.3.1
2.3.1.1
2.3.1.2
2.3.2
Friction and wear studies
against metallic surfaces
46
Materials and their properties
48
Abrasive wear studies
2.3.2.1
Singlepass and multipass
conditions
50
51
Part B (Analysis of the Composites)
52
2.4.1
Material A
52
2.4.2
Material B
54
2.4.3
Material C
55
2.4.4
Material D
56
2.4.5
Material E
57
2.4.6
Polyimide (PI)
58
2.4
Page
No.
CHAPTER III FRICTION AND WEAR STUDIES OF PEI
AND COMPOSITES AGAINST METALLIC
SURFACES
3.1
3.1.1
3.2
3.2.1
Friction and Wear Characteristics
of Bulk Polyetherimide
63
Friction and wear studies of PEI
on a pin and disc machine
66
3.1.1.1 Friction of•PEI
71
3.1.1.2 SEM and EDAX studies of
worn pin and disc surfaces
73
3.1.1.3 Wear mechanism of PEI
74
Friction and Wear Characteristics
of Composite B (PEI+PTFE)
76
Tribo-studies at various roughnesses
of the composites
77
3.2.2 Friction and wear at various loads
3.2.3
Wear as a function of sliding
distance
3.2.4 Wear mechanism and SEM studies
of worn surfaces
3.3
3.3.1
3.3.2
61
80
82
83
Friction and Wear Characteristic
of Composite (PEI+GF 20%)
87
Friction studies of composite C
under various experimental conditions
89
3.3.1.1 Effect of counterface roughness
on u
91
3.3.1.2 Effect of load on u
93
3.3.1.3 Effect of sliding speed on
friction and wear
93
Wear studies
95
3.3.2.1 Steady state wear of composite C 95
3.3.2.2 Effect of load on wear
96
Page
No.
3.3.2.3 Effect of counterface Ra on
wear
3.3.3
3.4
3.4.1
3.3.2.4 Load supporting action of
fibres
100
Worn surface analysis
101
3.3.3.1 Surface prefilometry
101
3.3.3.2 SEM studies
102
Friction and Wear Characteristics
of Composite D
104
Friction studies of composite D
107
3.4.1.1 Friction studies with very
smooth and moderate counterfact roughness
107
3.4.1.2 Friction studies at two extreme
loads
3.4.2 Friction studies at two exteeme loads
3.5.1
109
109
3.4.1.1 Steady state wear
110
3.4.1.2 Wear studies under various
loads
110
3.4.1.3 as a function of sliding
distance
111
3.4.3 Scanning electron microscopy and
wear mechanism
3.5
98
Friction and Wear Characteristics of
Composite E (PEI+GF+Graphite+PTFE+MoS )
2
-Steady state wear of composite E
112
117
117
3.5.2 Friction and wear studies at
various loads
118
3.5.3
119
Wear as a function of sliding distance
3.5.4 Friction and wear of various sliding
speeds
120
Page
No,
3.5.5
Friction and wear as a function of
fibre orientation
121
3.5.6
Scanning electron microscopy
123
3.6
Comparative Friction and Wear Studies
of PEI and PI Composites
127
WEAR AGAINST ABRASIVE PAPERS
(ABRASIVE WEAR)
131
Abrasive Wear Studies in Singlepass
Condition
133
Abrasive wear studies of PEI and
composites in singlepass condition
134
4.1.1.1 Abrasive wear as a function
of load
135
4.1.1.2 Abrasive wear as a function of
abrading particle size
143
CHAPTER IV
4.1
4.1.1
4.1.2 Abrasive wear studies of PI and
composites in singlepass condition
4.2
4.2.1
4.2.2
147
4.1.2.1 Abrasive wear as a function
of load
147
4.1.2.2 Abrasive wear as a function of
abrading particle size
149 .
Abrasive Wear Studies in Multipass
Condition
150
Abrasive wear studies of PEI and
composites in multipass condition
151
Abrasive wear studies of PI and
composites in multipass condition
151
4.2.2.1 Abrasive wear studies as a
function of abrading distance
151
4.2.2.2 Abrasive wear as a function
of load
153
Page
No.
4.3
4.3.1
4.2.2.3 Abrasive wear as a function
of particle size
155
Scanning Electron Microscopic Studies
156
Paper surface analysis with SEM
156
4.3.2 Pin surface examination with SEM
CHAPTER V
5.1
158
CONCLUSIONS AND FUTURE SCOPE OF
THE WORK
Conclusions
162
5.1.1
Adhesive wear
163
5.1.2
Abrasive wear
169
Scope of the Future Work
172
5.2
,REFERENCES
175
Bio-data
191
List of Publications
192