Table Of ContentTRIBOLOGY SERIES
Editor
D. Dowson (Gt. Britain)
Advisory Board
W.J. Bartz (Germany) W.A. Glaeser (U.S.A.)
R. Bassani (Italy) H.E. Hintermann (Switzerland)
B. Briscoe (Gt. Britain) K.C. Ludema (U.S.A.)
H. Czichos (Germany) T. Sakurai (Japan)
K. Friedrich (Germany) W.O. Winer (U.S.A.)
N. Gane (Australia)
VOl. 1 Tribology - A Systems Approach to the Science and Technology
of Friction, Lubrication and Wear (Czichos)
VOl. 2 Impact Wear of Materials (Engel)
VOl. 3 Tribology of Natural and Artificial Joints (Dumbleton)
Vol. 4 Tribology of Thin Layers (Iliuc)
VOl. 5 Surface Effects in Adhesion, Friction, Wear, and Lubrication (Buckley)
Vol. 6 Friction and Wear of Polymers (Bartenev and Lavrentev)
Vol. 7 Microscopic Aspects of Adhesion and Lubrication (Georges, Editor)
Vol. 8 Industrial Tribology - The Practical Aspects of Friction, Lubrication
and Wear (Jones and Scott, Editors)
VOl. 9 Mechanics and Chemistry in Lubrication (Dorinson and Ludema)
VOl. 10 Microstructure and Wear of Materials (Zum Gahr)
VOl. 11 Fluid Film Lubrication - Osborne Reynolds Centenary
(Dowson et al., Editors)
VOl. 12 Interface Dynamics (Dowson et al., Editors)
Vol. 13 Tribology of Miniature Systems (Rymuza)
Vol. 14 Tribological Design of Machine Elements (Dowson et al., Editors)
Vol. 15 Encyclopedia of Tribology (Kajdas et al.)
Vol. 16 Tribology of Plastic Materials (Yamaguchi)
Vol. 17 Mechanics of Coatings (Dowson et al., Editors)
Vol. 18 Vehicle Tribology (Dowson et al., Editors)
VOl. 19 Rheology and Elastohydrodynamic Lubrication (Jacobson)
VOl. 20 Materials for Tribology (Glaeser)
Vol. 21 Wear Particles: From the Cradle to the Grave (Dowson et al., Editors)
VOl. 22 Hydrostatic Lubrication (Bassani and Piccigallo)
Vol. 23 Lubricants and Special Fluids (Stepina and Vesely)
Vol. 24 Engineering Tribology (Stachowiak and Batchelor)
Vol. 25 Thin Films in Tribology (Dowson et al., Editors)
Vol. 26 Engine Tribology (Taylor, Editor)
Vol. 27 Dissipative Processes in Tribology (Dowson et al., Editors)
Vol. 28 Coatings Tribology - Properties, Techniques and Applications in Surface
Engineering (Holmberg and Matthews)
TRIBOLOGY SERIES, 28
EDITOR: D. DOWSON
COATINGS TRIBOLOGY
Properties, Techniques and Applications
in Surface Engineering
Kenneth Holmberg
VTT - Technical Research Centre of Finland
Laboratory of Production Engineering
Espoo, Finland
Allan Matthews
Research Centre in Surface Engineering
The University of Hull
Hull, U.K.
ELSEVIER
Amsterdam - Lausanne - New York - Oxford - Shannon - Singapore - Tokyo
ELSEVIER SCIENCE B.V.
Sara Burgerhartstraat 25
P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands
First printing: 1994
Second impression: 1998
Library of Congress Cataloging-in-Publication Data
Holmberg. K. (Kenneth)
Coatings tribology properties. techniques. and applications in
surface engineering / Kenneth Holmberg. Allan Matthews.
p. cm. -- (Tribology series ; 28)
Includes bibliographical references and index.
ISBN 0-444-88870-5 (acid-free paper)
1. Triboiogy. 2. Coatings. I. Matthews. A. (Allan1 11. Title.
111. Series.
TJ1075.H57 1994
621,8’9--dc20 94- 17604
CIP
Illustrations: Erkki Makkonen, VTT, Technical Research Centre of Finland
ISBN 0 444 88870 5
’
1994 ELSEVIER SCIENCE B.V. All rights reserved.
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of any methods, products, instructions or ideas contained in the material herein.
This book is printed on acid-free paper
Printed in The Netherlands
To our wives,
Andrea and Helen.
vii
PREFACE
This book gives a comprehensive description of thin surface coatings, their behaviour
and potential uses in tribological applications. The deposition techniques, tribological
mechanisms, properties of coatings, characterization, evaluation and selection
methodology in addition to applications examples are described and discussed.
The idea to write the book arose from a conversation with Professor Duncan Dowson.
The book is a natural continuation of more than ten years fruitful research cooperation
between the tribology research group at the VTT Technical Research Centre of Finland
in Helsinki, and the Research Centre in Surface Engineering at the University of Hull
in the UK. It brings together accumulated knowledge both on coating processes and on
the tribology of coatings. It was possible to cover the brief of the book only by
combining two areas of expertise, the fields of tribological understanding and of coating
technology .
During the process of writing the book the authors have tried to cover and take into
account the most important published works in the field. This was a most stimulating
but at the same time a very huge task, which can be seen from the large number of
references cited, over 800 altogether, in the reference list at the end of the book. By
considering about one thousand five hundred articles, and even then still probably
missing some relevant contributions, one aim of the book has been to bring together,
systematically in a single volume, the state-of-the-art knowledge on tribological
coatings. This has previously been difficult to achieve and overview because the
information has only been available in the form of numerous separate articles not linked
logically together.
If, by writing this book, we have managed to take a first step towards a systematic
structure for this complex field of technology then one of our main aims has been
achieved. At the same time we are fully aware that this is, by no means, the last word
but rather a first attempt to cover a topic that is in the process of rapid development -
with new results contributing to our understanding being published all the time.
Helsinki and Hull
March 1994
Kenneth Holmberg Allan Matthews
...
Vlll
ACKNOWLEDGEMENTS
A number of people have through the years supported and encouraged the authors with
their work in the coatings and tribology field. First of all we would like to thank
Professor John Halling for originally enspiring us to enter this field, not least with his
most enjoyable lectures, Dr Heikki Sundquist for stimulating our collaboration and
Professor Duncan Dowson for continuous support and encouragement during the writing
of the book.
We are indepted for helpful and critical comments on the manuscript to Dr John
Lancaster, Dr Steven Franklin, Mr Peter Anderson and Dr Juha-Pekka Hirvonen.
Professor Holmberg wishes to express his sincere gratitude for the continuous support,
encouragement and understanding given when writing the manuscript by his Laboratory
Director at the VTT, Professor Karri Vartiainen, and his thanks to the tribology research
group for helpful discussions and enjoyable cooperation. Professor Matthews records his
sincere thanks to the research team in the Research Centre in Surface Engineering at the
University of Hull for their good natured help and advice, especially Dr Adrian Leyland
and Dr Adrian James for their critical appraisal of the manuscript.
The authors express their thanks to Mrs Terhi Topp for skilful text processing and
layout work and to Mr Erkki Makkonen for meticulous preparation of the illustrations.
Although many people have helped and advised us in writing this book, any errors
which may remain are entirely our own, and we would welcome constructive comments
and critisism on the text contained herein.
The financial support of the Swedish Academy of Engineering Sciences in Finland and
the VTT Technical Research Centre of Finland is gratefully acknowledged. Professor
Holmberg wishes to thank the Library of the Helsinki University of Technology for
provision of a silent room, which gave an ideal environment in which to concentrate on
writing the book.
Finally we want to warmly thank our wives and children for their understanding and
support while we have spent much of our free time, both evenings and weekends,
researching and writing this book.
...
XI11
NOTATION
Symbols
A area, e.g. contact area or prqjected area of indentation
A, contact area contributed by deformed asperities
A‘ load support area of the coating
4, debris contact area
A, load support area of the substrate
b half contact length
C constant
d diameter or indentation diagonal length
E Young’s modulus of elasticity
E’ reduced modulus of elasticity, I/E’ = OS((1 - v,~)/E+, (1 - v2*)/E2)
F frictional force
G materials parameter, G = a . E’, or indentation load-crack length function
H hardness
H’ film thickness parameter, H’ = NR’
h film thickness
k constant
K wear rate, K = V/ws, or fracture toughness
1 length of contact perpendicular to flow
L‘ critical load
m indentation size effect (ISE) index
n number of cycles
P pressure
r radius
R radius
+
R’ radius of conformity, 1/R’ = (UR,) (l/R2)
R, centre line average value (C.L.A or R,) for the roughness of the surface
Rc Rockwell hardness (c-scale)
RH relative humidity
Rm,, maximum height of irregularities
R, peak-to-valley height value for the roughness of the surface
s distance
S shear strength
t’ dummy variable
T temperature or transition zone
U velocity
U’ displacement in x-direction
U speed parameter, U = (17, . u’)/(E’. R’)
V velocity
V’ displacement in y-direction
V volume of worn or deformed material
W normal load
xiv
W’ load per unit length
W load parameter, W = w/(E’. R’. 1)
W;, work of adhesion
X distance in x-direction or length of distance
Y distance in y-direction or length of distance
Z distance in z-direction
Greek symbols
pressure exponent of viscosity or
coefficient of thermal expansion
presentage of asperity contact area under compression
Boussinesq function
deviator stress
strain
absolute dynamic viscosity
angle
empirical interface parameter
specific film thickness, h = h,,/R’,
coefficient of friction, p = F/w
Poissons ratio
tensile stress or strength
stress or strength in shear
shear strength within asperity adhesion area
Subscripts
a adhesion or asperity
C coating or critical
cornp composite
d deformation or debris
f film
Hz Hertzian
i counter, i = 1, 2, ... N
I interface
.i junction
m melting or hydrostatic
max maximum
min minimum
0 ambient
P ploughing
r radial or rough
S substrate
V Vickers
vM von Mises
Y yield
1
CHAPTER 1
INTRODUCTION
Tribology is the field of science and technology dealing with contacting surfaces in
relative motion - which means that it deals with phenomena related to friction and wear.
In industrialized societies there is a growing need to reduce or control friction and wear
for several reasons, such as to extend the lifetime of machinery and bio-systems, to
make engines and devices more efficient, to conserve scarce material resources, to save
energy, and to improve safety.
Historically these aims have been achieved by design changes, selecting improved bulk
materials, or by utilizing lubrication techniques. Bulk material changes might involve
applications with ceramics or polymers, whilst the lubrication techniques would include
the use of liquid lubricants such as mineral or synthetic oils or solid lubricants such as
molybdenum disulphide.
Recently tribologists have made increasing use of another approach to friction and wear
control - that is to utilize surface treatments and coatings. This has led to, and to some
extent been fuelled by, the growth of a new discipline called surface engineering. The
growth has been encouraged by two main factors. The first has been the development
of new coating and treatment methods, which provide coating characteristics and
tribochemical properties which were previously unachievable. The second reason for the
growth of this subject area has been the recognition by engineers and materials
scientists that the surface is the most important part in many engineering components.
It is at the surface that most failures originate, either by wear, fatigue or corrosion.
The surface also has many functionally important attributes, not confined to mechanical
or chemical properties, such as the electronic, magnetic and optical characteristics.
Mechanical components and tools are today facing higher performance requirements.
The use of surface coatings opens up the possibility for a material design in which the
specific properties are located where they are most needed. The substrate material can
be designed for strength and toughness while the coating is responsible for the
resistance to wear, thermal loads and corrosion.
2 I Introduction
Tribologists developed an understanding of the behaviour of surfaces in contact,
providing a theoretical basis for the prediction of the desirable attributes of surfaces,
even before fully optimized coatings were available.
It is against this background that the newly developed coating and treatment methods
are being developed, and already making a significant impact. Devices and bearing
systems which operate under near-vacuum conditions, as in space mechanisms or
satellites, or engine components operating under hot corrosive and erosive conditions,
as in acro gas-turbines, could not function without advanced tribological coatings.
Whilst coatings for thc applications cited are in commercial use, there are many others
which are still at a developmental stage. In this book we set out the background to
present developments in tribological coatings, emphasising the newer processes, and
place this in the context of current thinking on the theories and experimental findings
relating to the use of surface coatings in tribology.
We shall take as our definition of tribological coatings, those which are sufficiently
thin that the substrate material plays a role in determining the friction and wear
performance. Thus, we exclude coatings which are so thick that there is little or no
substrate intluence on the tribological behaviour - the coating in effect acts as a bulk
material. Weld deposits are a typical example of such thick coatings that have been
excluded from in-depth study.
The cvating processes can conveniently be divided into four generic groups - gaseous,
solution, molten and solid - depending on the state of the depositing phase (Rickerby
and Matthews, 1991a). Our definition of tribological coatings means that we shall
concentrate mainly on the gaseous state processes, which are attracting considerable
scientific and commercial interest.
A general design appraisal of the tribological requirements on a contact with coated
surfaces can be formulated as follows:
1. The initial coefficient of friction, the steady state coefficient of
friction and the friction instability must not exceed certain design
values.
2. The wear of the coated surface and that of the counterface must
not exceed certain design values.
3. The lifetime of the system must, with a specified probability, be
longer than the required lifetime. The lifetime limit of the system
may be defined as occurring when even one of the earlier
requirements is not maintained.
