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Tribology of Diamond-Like Carbon Films
Christophe Donnet • Ali ErdemirEditors
Tribology of Diamond-Like Carbon Films
Fundamentals and Applications
Christophe DonnetUniversity Institute of France and University Jean MonnetLaboratoire Hubert Curien UMR 551318 avenue Professeur Benoît Lauras42000 Saint-Etienne, [email protected]
Ali ErdemirArgonne National LaboratoryEnergy Systems Division9700 South Cass AvenueArgonne, IL 60439, [email protected]
ISBN 978-0-387-30264-5 e-ISBN 978-0-387-49891-1
Library of Congress Control Number: 2007930611
© 2008 Springer Science + Business Media, LLCAll rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science + Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now know or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identifi ed as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.
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Foreword
Diamonds have a very powerful image with both the public and the scientific community, because of their beauty and the historic difficulty in first synthesising it. Diamond-like carbon (DLC) is much less famous, but it has a similar importance in economic terms. DLC coatings are used on razor blades, in magnetic hard-disk drives, on bar-code scanners, on PET bottles, and on some car parts. But how did we develop the underlying technology? This book gives the detailed scientific and technological answer to this question.
The historical overview describes how DLC has a much shorter history than diamond itself, from the 1970s with Aisenberg and Chabot and then Holland using plasma deposition to first grow hard, amorphous carbon films. A great deal of early work was carried out by the group of Koidl in the 1980s. Meanwhile, Russian groups invented the cathodic arc and used it on carbon, and this allowed growth of a second type of hydrogen-free DLC. This material came to be known as tetrahe-dral amorphous carbon, which is abbreviated as ta-C or TAC, or various similar ways.
This range of growth techniques allowed us to understand the range of DLCs, as I summarise in Chapter 1. The thin film growth techniques are of critical impor-tance, because DLC is only possible as a thin film material, not as a bulk glassy solid. However, historically, proving that someone had and had not grown diamond synthetically was at least as important as the growth itself. The same applies to DLC. A specific range of characterisation techniques are employed to measure the properties and bonding of DLC, as is described in Chapter 2.
Diamond is known to possess the most extreme properties of any real three-dimensional solid, such as highest atomic density, highest hardness, highest Young’s modulus, highest room temperature thermal conductivity, etc. But diamond is inconvenient as a coating material, because its growth temperature is high. DLC has the huge advantage over diamond of having room temperature, rather low cost, large-area vacuum deposition methods. DLC is also amorphous which allows it to be the smoothest material known. These advantages are what gives DLC its wide range of applications noted above.
DLC does however have two drawbacks; one is that its thin films tend to have a large compressive stress, and the second that it is not mechanically tough.
v
Nevertheless the amorphous character of DLC means that it can be alloyed with metals and other elements like silicon which goes some way to solving these problems. These issues are described in Chapters 4 and 12.
The main applications of DLC are as a mechanical and protective coating. This is the focus of most chapters of the book, mainly in Chapter 3 and Sections B and C. Some years ago, there were many studies of DLC trying to develop the electronic properties of DLC, as field emission or doping, but these turned out to be unsuc-cessful or uncompetitive with other materials. Thus the focus of DLC research now continues in the area of mechanical properties.
The applications in magnetic hard-disk drives and on razors have existed for 10–25 years. The applications as coatings on tool tips and on car components are more recent, and have followed more recent advances in our understanding of tribology, thin film adhesion, and alloying. These topics are less easily found in the journal literature. This book for the first time brings together all these areas in a convenient form. This book will thereby provide an extremely useful reference for the economic developments in these areas.
Engineering Department, John RobertsonUniversity of Cambridge,Cambridge CB2 1PZ, UK.E-mail: [email protected]
vi Foreword
Acknowledgments
The strength of this book primarily lies in its very comprehensive and up-to-date nature. It provides an excellent overview of the state of the art in DLC films, in general, and their tribology, in particular. It also offers an exhaustive survey of the recent developments from a fundamental and industrial point of view. We thank our colleagues in the field for their invaluable contributions to this book. Without their time and effort, it would not have been possible to publish this book, which we hope will help us in furthering our quest to develop even better coatings in the near future. We also acknowledge the support of our institutions (Argonne National Laboratory, USA, the University Jean Monnet, France, and the University Institute of France) and funding agencies (the United States Department of Energy, Office of Energy Efficiency and Renewable Energy, Freedom Car and Vehicle Technologies Program; and Centre National de la Recherche Scientifique de France). Last but not least, we thank our families for their unwavering support and understanding during the preparation of this book.
vii
Contents
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Diamond-like Carbon Films: A Historical Overview . . . . . . . . . . . . . . . . . . 1C. Donnet and A. Erdemir
Section A: General Overview on DLC Coatings
1 Classifi cation of Diamond-like Carbons . . . . . . . . . . . . . . . . . . . . . . . . . 13J. Robertson
2 Non-destructive Characterisation of Carbon Films . . . . . . . . . . . . . . . . 25A. C. Ferrari
3 Mechanical Characterisation and Properties of DLC Films . . . . . . . . . 83P. Lemoine, J. P. Quinn, P. D. Maguire and J. A. McLaughlin
4 Residual Stresses in DLC Films and Adhesion to Various Substrates . . . 102Y. Pauleau
Section B: Tribology of DLC Coatings Fundamentals and Experimental Studies
5 Fundamentals of the Tribology of DLC Coatings . . . . . . . . . . . . . . . . . 139J. Fontaine, C. Donnet and A. Erdemir
6 Environmental and Thermal Effects on the Tribological Performance of DLC Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155H. Ronkainen and K. Holmberg
ix
7 Third Bodies and Tribochemistry of DLC Coatings . . . . . . . . . . . . . . 201T. W. Scharf and I. L. Singer
8 An Overview of Superlubricity in Diamond-like Carbon Films . . . . . 237A. Erdemir, J. Fontaine and C. Donnet
9 Hard DLC Growth and Inclusion in Nanostructured Wear-protective Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263A. A. Voevodin
10 Environmental and Surface Chemical Effects on Tribological Properties of Carbon-based Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . 282F. M. Borodich, Y. -W. Chung and L. M. Keer
11 Triboemission and Triboplasma Generation with DLC Films . . . . . . 291K. Nakayama
12 Doping and Alloying Effects on DLC Coatings. . . . . . . . . . . . . . . . . . . 311J. C. Sánchez-López and A. Fernández
13 Tribology of Carbon Nitride Coatings . . . . . . . . . . . . . . . . . . . . . . . . . 339K. Adachi and K. Kato
14 Tribology of DLC Films Under Fretting Conditions . . . . . . . . . . . . . . 362R. Wäsche and D. Klaffke
15 Tribology of DLC Films Under Slip-Rolling Conditions . . . . . . . . . . . 383C. Manier, D. Spaltmann and M. Woydt
16 Tribological Behavior of DLC Films in Various Lubrication Regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410B. Podgornik
Section C: Applications and Future Trends in DLC’s Tribology
17 Industrial Production of DLC Coatings . . . . . . . . . . . . . . . . . . . . . . . . 457H. G. Fuß and M. Frank
18 DLC Films in Mechanical and Manufacturing Industry . . . . . . . . . . 469C. Héau
19 Wear Resistance of Amorphous DLC and Metal Containing DLC in Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484G. J. van der Kolk
x Contents
20 DLC Films in Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . 494R. Hauert
21 Nanotribology of Ultrathin and Hard Amorphous Carbon Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510B. Bhushan
22 Laser Processing of Tribological DLC Films: An Overview . . . . . . . . 571G. Dumitru
23 New Trends in Boundary Lubrication of DLC Coatings . . . . . . . . . . . 591M. I. De Barros Bouchet and J. M. Martin
24 Fullerene-like Carbon Nitride: A New Carbon-based Tribological Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620E. Broitman, J. Neidhardt and L. Hultman
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655
Contents xi
Contributors
Koshi AdachiTribology Laboratory, Graduate School of Engineering, Tohoku University, Sendai 980–8579, Japan, [email protected]
Maria-Isabel De Barros BouchetEcole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes UMR 5513, 36 avenue Guy de Collongue 69134 Ecully, France, [email protected]
Bharat BhushanNanotribology Laboratory for Information Storage and MEMS/NEMS, 201 W. 19th Avenue, Ohio State University, Columbus, OH 43210–1142, U.S.A,[email protected]
Feodor M. BorodichSchool of Engineering, Cardiff University, Queen’s Buildings, Cardiff CF24 3AA, UK, [email protected]
Esteban BroitmanDepartment of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA, [email protected]
Yip-Wah ChungCenter for Surface Engineering and Tribology, Robert R. McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL 60208, USA, [email protected]
Christophe DonnetUniversity Institute of France and University Jean Monnet, Laboratoire Hubert Curien UMR 5513, 18 avenue Professeur Benoît Lauras, 42000 Saint-Etienne, France, [email protected]
Gabriel DumitruUniversity of Applied Sciences Northwestern Switzerland, School of EngineeringInstitute for Product and Production Engineering (IPPE), Steinackerstrasse 55210 Windisch, Switzerland, [email protected]
xiii
Ali ErdemirArgonne National Laboratory, Energy Systems Division, 9700 South Cass Avenue Argonne, IL 60439, USA, [email protected]
Asuncion FernándezInstituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla)Avda. Américo Vespucio 49, 41092 Sevilla, Spain, [email protected]
Andrea FerrariEngineering Department, University of Cambridge, Cambridge CB2 1PZ, UK, [email protected]
Julien FontaineEcole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes UMR 5513, 36 avenue Guy de Collongue, 69134 Ecully, France. [email protected]
Martin FrankCemeCon AG, Adenauerstrasse 20 A 4, 52146 Würselen, Germany, Martin, [email protected]
Hans-Gerd FußCemeCon AG, Adenauerstrasse 20 A 4, 52146 Würselen, Germany,HansGerd, [email protected]
Roland HauertSwiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, 8600 Dübendorf, Switzerland, [email protected]
Christophe HeauHEF Group, Rue Benoît Fourneyron, 42166 Andrézieux-Bouthéon, France, [email protected]
Kenneth HolmbergVTT Technical Research Centre of Finland, Metallimiehenkuja 6, Espoo, PL 1000, FIN-02044 VTT, Finland, [email protected]
Lars HultmanThin Film Physics Division, Department of Physics, Chemistry and Biology (IFM) Linköping University, 58183 Linköping, Sweden, [email protected]
Koji KatoTribology Laboratory, Graduate School of Engineering, Tohoku University, Sendai 980–8579, Japan, [email protected]
Leon M. KeerCenter for Surface Engineering and Tribology, Robert R. McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL 60208, USA, [email protected]
xiv Contributors
Dieter KlaffkeFederal Institute for Materials Research and Testing (BAM), 12200 Berlin, Germany, [email protected]
Patrick LemoineNanotechnology Research Institute, University of Ulster at JordanstownShore road, Newtownabbey, BT37OQN, County Antrim, Northern Ireland, United Kingdom, [email protected]
James A. McLaughlinNanotechnology Research Institute, University of Ulster at JordanstownShore road, Newtownabbey, BT37OQN, County Antrim, Northern Ireland, United Kingdom, [email protected]
Paul D. MaguireNanotechnology Research Institute, University of Ulster at JordanstownShore road, Newtownabbey, BT37OQN, County Antrim, Northern Ireland, United Kingdom, [email protected]
Charles-Alix ManierFederal Institute for Materials Research and Testing (BAM) 12200 Berlin, Germany, [email protected]
Jean-Michel MartinUniversity Institute of France and Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes UMR 5513, 36 avenue Guy de Collongue, 69134 Ecully, France, [email protected]
Keiji NakayamaNational Institute of Advanced Industrial Science and Technology (AIST), Namiki 1–2, Tsukuba, Ibaraki 305–8564, Japan, [email protected]
Jörg NeidhardtChristian Doppler Laboratory for Advanced Hard Coatings, Department of Physical Metallurgy and Materials Testing, University of Leoben, 8700 Leoben, Austria and Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, 58183 Linköping, Sweden, [email protected]
Yves PauleauNational Polytechnic Institute of Grenoble, Centre National de la Recherche Scientifique CNRS-LEMD, B.P. 166, 38042 Grenoble 9, France, [email protected]
Bojan PodgornikUniversity of Ljubljana, Centre for Tribology and Technical DiagnosticsLjubljana, Bogisiceva 8, SI-1000 Ljubljana, Slovenia, [email protected]
Contributors xv
John P. QuinnNanotechnology Research Institute, University of Ulster at Jordanstown, Shore road, Newtownabbey, BT37OQN, County Antrim, Northern Ireland, United Kingdom, [email protected]
John RobertsonEngineering Department, University of Cambridge, Cambridge CB2 1PZ, UK,[email protected]
Helena RonkainenVTT Technical Research Centre of Finland, Metallimiehenkuja 6, EspooPL 1000, FIN-02044 VTT, Finland, [email protected]
Juan Carlos Sánchez-LópezInstituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla)Avda. Américo Vespucio 49, 41092 Sevilla, Spain. [email protected]
Thomas W. ScharfThe University of North Texas, Department of Materials Science and Engineering, Denton, TX 76203–5310, USA, [email protected]
Irwin L. SingerUS Naval Research Laboratory, Code 6176, Washington, DC 20375–5342, USA,[email protected]
Dirk SpaltmannFederal Institute for Materials Research and Testing (BAM), 12200 Berlin, Germany, [email protected]
Gerry Van Der KolkIonbond Netherlands b.v., Groethofstraat 22b, NL 5916 PB Venlo, The Netherlands, [email protected]
Andrey A. VoevodinMaterials and Manufacturing Directorate, Air Force Research Laboratory,Wright-Patterson Air Force Base, Ohio, USA. [email protected]
Rolf WäescheFederal Institute for Materials Research and Testing (BAM) 12200 Berlin, Germany, [email protected]
Mathias WoydtFederal Institute for Materials Research and Testing (BAM) 12200 BERLIN, Germany, [email protected]
xvi Contributors
Diamond-like Carbon Films: A Historical Overview
C. Donnet1 and A. Erdemir2
1 Introduction
Carbon is one of the most remarkable elements among all others in the periodic table. It exists in more than 90% of all known chemical substances and has the larg-est number of allotropes. Carbon-based solid materials exhibit exceptional proper-ties such as super-high hardness and thermal conductivity, like in diamond, or unusual softness and lubricity, like in graphite. Besides these, carbon is the building block of carbon-based allotropes including white carbon (or ceraphite), nanotubes, buckyballs, other fullerenes, carbon–carbon composites, glassy c arbons, and car-bon nanofibers.
During the last three decades or so, carbon has also been the key element in the synthesis of thin coatings of diamond, diamond-like carbon (DLC), carbon nitride, boron carbide, and a myriad of transition metal carbide, and carbo-nitride coatings. Because of their exceptional mechanical and tribological properties, these coatings are now used in a wide range of engineering applications to control friction and wear. Figure 1 is a ternary diagram (proposed by Robertson and Ferrari) that illustrates the specific domains of various carbon-based coatings with respect to their sp2- and sp3-type bonding characteristics and hydrogen contents. A more comprehensive overview of these and other DLC films is provided by Robertson in Chapter 1. The fullerene-type allotropes of carbon represent a new class of materials and their usefulness is being explored in novel nano- to microscale devices.
The family of DLC coatings is perhaps the largest and represents one of most-studied among all other coatings. These coatings were first discovered in the early 1950s by Schmellenmeier [1] but did not attract much attention until the work of Eisenberg and Chabot almost two decades later [2]. During the 1980s, a few more researchers developed interest in these films, while during the 1990s, the research on DLC films gained momentum [3,4]. As can be deduced from Figs 2–4, almost
1 University Jean Monnet, Saint-Etienne, France, Member of the University Institute of France, Paris, France
2Argonne National Laboratory, Argonne, IL, USA
1
C. Donnet and A. Erdemir (eds.), Tribology of Diamond-Like Carbon Films: Fundamentals and Applications.© Springer 2008
2 C. Donnet and A. Erdemir
every year since the early 1990s, numerous papers and patents have been devoted to DLC films. Since 2000, these films have attracted even more interest, and they are still the subject of numerous scientific studies. In the following sections, we will provide a brief overview of recent developments in the field of DLC films, in gen-eral, and some of the major milestones, in particular. We will also summarize the current status and future trends in these films.
More comprehensive overviews of DLC films are provided in Section A of this book. In this section, Chapter 1 reviews various forms and compositions of DLC films, while Chapter 2 tackles the challenging subject of characterization. These films are metastable forms of carbon combining both sp2 and sp3 hybridizations, including hydrogen when a hydrocarbon precursor is used during deposition. These two chap-ters do not deal with tribology, but understanding the tribological behavior of DLC films requires a solid background on the chemical and structural nature of these films, which, in turn, depends on the deposition process and/or parameters. The chemical
sp3
sp2H
No f i lm
Hydrocarbon polymers
Glassy C Evap. C
ta-C
Sputtered C
ta-CH
a-CH
Fig. 1 Ternary phase diagram for various DLC films with respect to their sp2, sp3, and hydrogen contents. (After Robertson, [35].)
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Fig. 2 Number of publications per year on DLC coatings (black bar) and on tribology of DLC coatings (grey bar), deduced from (a) the Web of Science and (b) the INSPEC data bases
Diamond-like Carbon Films: A Historical Overview 3
composition, such as the hydrogen and/or nitrogen content or the presence of other alloying elements, controls the mechanical and tribological properties of a sliding pair consisting of DLC on one or both sliding surfaces. Among the many properties of an engineering material, hardness and stiffness play the most important roles in its ability to carry the load and, hence, in its wear resistance. Chapter 3 reviews the mechanical
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Fig. 3 Variation of the percentage of publications related to the tribology of DLC films within all other tribology-related publications, as deduced from the Web of Science and INSPEC data bases
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Fig. 4 Number of US patents issued per year on DLC coatings (black bar) and on their tribology (grey bar)
4 C. Donnet and A. Erdemir
properties of DLC films extending from organic polymers for highly hydrogenated DLC films to diamonds for tetrahedral unhydrogenated DLC films. Whatever the nature of the DLC, one should keep in mind that the industrial success of DLC films in tribological contacts is strongly dependent on their adhesion properties. Basically, DLC films adhere well on substrates containing elements forming carbides, such as silicon or titanium. In most cases, intermediate layers need to be deposited in situ on the substrate before the deposition of the DLC films. This practice is detailed in the chapters of this book dedicated to industrial applications. Nevertheless, one of the most critical aspects of DLC films is the presence of stress, mainly resulting from the quenching of the impinging species during film growth. Chapter 4 reviews the various mechanisms responsible for stress in DLC films.
2 Inception and Early Studies
Historically, the earliest attempts to produce DLC films can be traced back to 1953 when Heinz Schmellenmeier reported a black carbon film derived from C
2H
2 gas
in glow-discharge plasma. His film exhibited great hardness and, hence, was very resistant to scratching by other hard objects. Later work by Eisenberg and Chabot in the early 1970s produced such films on negatively biased metallic substrates by using an ion beam deposition system [2]. Their films were very hard and hence resistant to scratching and also possessed a high dielectric constant, high index of refraction, excellent optical transparency, and high resistance to corrosion in strongly acidic solutions. Because DLC films are typically amorphous and dense, they were less prone to pin-hole defects. During the mid-1970s, Holland et al. and a few other researchers were also able to synthesize DLC films from other hydro-carbon sources by simply applying a radio frequency (RF) bias to the substrate materials and thus creating a glow-discharge plasma [3–5]. Because of the impres-sive mechanical properties of these films, some researchers had speculated that they were perhaps composed of crystalline diamond, but systematic microscopic studies by Weissmantel et al. during the late 1970s dispelled these speculations by confirming that the DLC films had an amorphous structure [5].
During the 1980s, there were a few other reported papers on DLC films. In a 1985 review paper, Arnoldussen and Rossi from IBM predicted and discussed in detail the potential usefulness of thin DLC films for magnetic recording media [6]. Such a possibility had already been explored in 1981 by King, who was able to demonstrate far superior performance and efficiency for the DLC-coated disk media than the other types of overcoat materials used in hard disks at that time [7]. Based on his experimental findings, King concluded that the use of these films in magnetic recording media may lead to some major advances in the hard disk industry, and obviously his predictions were right. During the 1980s, only a few other tribo-logical studies were performed on DLC [8–10]. In 1986, Robertson published a comprehensive review paper on amorphous carbons, including DLC films, and summarized the details of the main characteristics of such materials [11].
Since the early 1990s, the popularity of DLC films has gained momentum. We have witnessed an explosion in DLC-related research activities, including the number of publications and granted patents (see Figs 2–4). During these years, many novel types of DLC films were formulated; DLC films were produced on an industrial scale and systematic tribological studies were carried out. As a result of such increased industrial and scientific activities, our knowledge base on DLC films increased tremendously. Numerous key publications that appeared in books and journals focused on the synthesis, characterization, and industrial application of DLC films. In particular, papers published by Grill et al. [12], Miyoshi and his coworkers [13], Ronkainen et al. [14], Erdemir et al. [15], and Donnet et al. [16] provided the foundation for more in-depth studies and reinforced the notion that DLC films deserve more scientific and industrial attention. During the 1990s, systematic studies were also carried out to elucidate the effect of incorporation of hydrogen and other heteroatoms (F, N, Si, etc.) on the friction and wear properties of these films [17–28]. Since the beginning of 2000, several review articles have been published that provided more detailed insight into the unique structural, mechanical, and tribological properties of these films [18, 29–34]. In particular, a recent comprehensive review article by Professor Robertson is an excellent source for further information on DLC films and their properties [35].
3 State of the Art
Owing to their very unique structures and attractive properties and performance characteristics, DLC films continue to draw significant attention from both the scientific and industrial communities. As can be seen from Figs 2 and 3, the number of scientific papers published in recent years increased steadily, while the number of granted patents (see Fig. 4) reached record numbers during the same period, thus reinforcing the notion that these films are industrially useful and relevant. They are currently used in numerous industrial applications where high resistance to wear, scuffing, corrosion, and erosion is paramount. Some of the chapters in Section C of this book treat recent industrial applications, as does the next section in the current chapter.
Dedicated scientific studies on DLC films in recent years have led to the devel-opment of more exotic versions consisting of unique nanophases and/or structures. Specifically, with recent advances in deposition processes (hybrid arc-magnetron systems, femtosecond pulse laser deposition, etc.) it has now become rather easy to control the properties of DLC films and tailor them to meet the ever-increasing performance and durability requirements of industrial applications. Some of the latest films, based on nanoscale multilayers, provide excellent toughness and much improved tribological properties [36–37], while others are extremely hard and resilient, and hence well-suited for protection against wear under severe sliding conditions [38–40]. Another class of recent DLC films is not as hard but can provide what may be the lowest friction coefficients on any known material [41].
Diamond-like Carbon Films: A Historical Overview 5
6 C. Donnet and A. Erdemir
DLC is the only material or coating that can provide both high hardness and low friction under dry sliding conditions. Most of the DLC films identified in Fig. 1 are inherently hard and lubricious under typical sliding conditions. Figure 5 illustrates this point better by categorizing various classes of tribological coatings with respect to their typical hardness and friction values. There is neither direct nor universal correlation between hardness and friction coefficients of engineering materials. This observation is especially true for DLC and other hard coatings, as shown in Fig. 5. Nonetheless, most DLC films available today are capable of providing not only high hardness but also low friction. Materials with high hardness and stiffness have, in general, high wear resistance. Diamond represents a prime example for this point. It has the highest known hardness and thus provides superior wear resistance when used as a bulk material or a thin coating.
As will be further realized from the other chapters of this book, DLC films have come a long way. They now represent some of the most interesting and important tribological materials that have ever been developed and have found numerous prac-tical applications. Section B of the book provides an excellent overview of the fun-damental tribological aspects of the DLC films that have been studied during the last 15 years. The chapters of this section cover almost all of the relevant topics, ranging from the role of hydrogen and carbon hybridization in friction and wear to the influ-ence of incorporated heteroatoms as dopants or alloying elements in the carbona-ceous network. The effects of third bodies on the tribology of DLC films are also covered, as are other important topics like environmental and thermal effects, super lubricity, and triboemission, as well as specific physical and chemical phenomena observed on sliding DLC surfaces under dry and lubricated sliding conditions.
4 Practical Applications
Since their initial discovery in the early 1950s, DLC films have emerged as one of the most valuable engineering materials for various industrial applications, includ-ing microelectronics, optics, manufacturing, transportation, and biomedical fields.
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Fig. 5 A schematic representation of hardness and coefficients of friction (COF) of carbon-based and other coatings
Diamond-like Carbon Films: A Historical Overview 7
In fact, during the last two decades or so, DLC films have found uses in everyday devices ranging from razor blades to magnetic storage media [29]. For example, the thickness of the carbon overcoats used on magnetic hard disks has decreased almost linearly with time since the mid-1980s, and the DLC films used today are only a few nanometers thick, yet their durability and efficiency are much better than their predecessors. Moreover, these days most of us also appreciate the comfort and lon-gevity of DLC-coated razor blades offered by several manufacturers.
Until the mid- to late 1990s, few industrial applications took advantage of the unique properties of DLC films. In fact, except for magnetic storage media, DLC films were not used in large volumes by industry. A few companies tried DLC films on eyeglasses and laser barcode scanners to improve their resistance to abrasion and/or scratching. Over the last decade, however, several new versions of DLC films (such as W-, Ti-, and Cr-doped films) have been synthesized, and with the introduction of industrial-scale, more robust coating systems, the production of high-quality DLC films has become rather easy and inexpensive. Over the past few years, researchers have made great strides in controlling the chemistry and hence the properties and performance of DLC films in actual applications. Current DLC films thus have the capacity to meet the increasingly stringent application require-ments of advanced mechanical systems [29,42–44].
As a consequence, DLC films have made a significant positive impact on the manufacturing and transportation industries. These films are used in large vol-umes in various manufacturing industries to prevent wear and material transfer during drawing, stamping, molding, and rolling operations. In the transportation industry, many fuel injectors in high-performance diesel engines are now coated with DLC, as are a variety of other engine parts employed in race cars. In coming years, there is no doubt that DLC films will be found on several other critical engine components since future engine systems are expected to be much more compact, yet far more robust and efficient, than their predecessors. Downsizing with no compromise in performance and efficiency is likely to continue at an accelerated pace in the coming years and necessitate greater use of hard and low friction coatings like DLC.
We certainly owe much of the progress in the production and uses of DLC films to a very strong and progressive technology base acquired over the last two decades in the field of surface engineering. The current knowledge base and expertise in these fields are at a point where a coating specialist can produce a DLC coating tailored to meet or exceed the multifunctional property and performance demands of a given application. In particular, recent developments in magnetron sputtering, plasma-enhanced chemical vapor deposition, pulse-laser deposition, and cathodic arc tech-nologies provide the kinds of flexibility that a specialist needs in designing and developing multifunctional DLC coatings having a multilayered or nanostructured/nanocomposite architecture that can provide excellent hardness, friction, toughness, and corrosion resistance. In fact, some nanocomposite films are quite hard, moisture insensitive, and self-lubricating, thus raising the prospect for truly “chameleon” coat-ings that can adapt their superior tribological properties to the surrounding environ-mental and thermal conditions [45]. When combined with duplex/multiplex surface
8 C. Donnet and A. Erdemir
treatments, DLC coatings can enjoy additional functionalities that can meet the ever-increasing performance demands of more severe applications. Near-frictionless and near-wearless DLC films have also been synthesized in recent years and are currently being evaluated for a wide range of applications [46–48]. Several other forms of DLC films have greatly improved the performance, durability, and effi-ciency of numerous advanced mechanical systems.
Section C of this book is devoted to the industrial applications and future trends in DLC films. Although some of the applications are well-established and routine, others are still emerging. In the automotive field, as already mentioned, the use of DLC films in fuel injectors has become a routine operation. The other high-volume applications that are currently being explored for automobiles include tappets, valve lifters, and wrist pins. Even the door hinges and locking systems of some passenger cars are being coated with DLC rather than a thick layer of grease. Another rapidly growing practical application for DLC is in the textile industry, where various needles and components are coated with DLC. Computer hard disks have been using DLC for more than two decades, and their applications in other nano- to microscale devices (like MEMS) are being explored. Another emerging application for DLC films is in invasive and implantable medi-cal devices. These films are currently being evaluated for their durability and performance characteristics in certain biomedical implants including hip and knee joints and coronary stents.
At present, high-quality DLC films are readily available from various commer-cial sources (such as CemeCon, Hauzer, Balzers, Ion Bond, HEF). Some of these DLC coatings are extremely hard and resilient, while others are relatively soft but capable of very low friction and wear coefficients. Films that contain unique crys-talline nanostructures and/or nanophases are also available and have the ability to meet the increasingly stringent application conditions of advanced mechanical devices. These multifunctional nanocomposite DLC films are now routinely pro-duced by both chemical and physical vapor deposition. These nanocomposite DLC films can also provide very impressive mechanical and tribological properties, while most of the recently developed nanocomposite coatings are able to provide superhardness but lack lubricity or low friction [49].
5 Future Directions
The field of carbon-based materials and coatings has enjoyed strong and growing interest from all kinds of scientific disciplines and industries during the last two decades. In particular, DLC coatings have attracted the most attention in recent years, mainly because they are cheap and easy to produce and offer exceptional properties for demanding engineering applications. Accordingly, in this book, we attempt to highlight some of the most important developments in the field of DLC films, in general, and their tribological properties, in particular. From the survey of the state of the art in scientific research and industrial practices that involve DLC
Diamond-like Carbon Films: A Historical Overview 9
films, some chapters in the last section of this book discuss a range of future developments related to friction and wear of DLC films. Large-scale and low-cost manufacturing of DLC films will certainly lead to much broader applications in the near future. The present levels of industrial production of DLC films are probably still not reflective of their future potential, especially in the high-volume, large-scale production sectors (such as microelectronics, transportation, and manufacturing). There is a critical need to further reduce the coating costs and also improve process reliability to fulfill the quality and consistency requirements of these industrial sec-tors. More robust, low-temperature (<200°C) processes, with reduced sensitivity to unexpected changes in process parameters, are still needed for the deposition of DLC coatings in large-volume applications. There is also a need for processes that can produce high-quality coatings on a large number of different substrates, includ-ing not only low and high alloy steels, but also metallic (Al, Cu, Mg, etc.), ceramic, and polymeric (or rubber-like) materials.
For the last two decades, solid lubrication by thin coatings has mainly been used to stretch the limits of classical oils and/or fluid-based lubricants. An increasing demand to develop environment-friendly lubricants is leading to the development of new additives that are compatible with thin coatings, especially with the DLC films. In a related attempt to further improve the performance and durability of DLC films under lubricated sliding conditions, researchers have lately been creating special textures on their sliding surfaces. In particular, high-precision patterns created on DLC surfaces by excimer or femtosecond lasers have been shown to improve the tribological properties of these films under boundary-lubricated sliding conditions. Among the classical forms of DLC films, frontier research will also be devoted to the investigation of new forms of carbon-based solid lubricants, including multicomponent, nanotube, fullerene-like, and other composite carbon-based films.
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