Surface & Coatings Technology 242 (2014) 214–225

Contents lists available at ScienceDirect

Surface & Coatings Technology

journal homepage: www.elsevier.com/locate/surfcoat

History of diamond-like carbon films — From first experiments to

worldwide applications
Klaus Bewilogua a,⁎, Dieter Hofmann b
a
Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germany
b
AMG Coating Technologies, Hanau, Germany

a r t i c l e i n f o a b s t r a c t

Article history: Diamond-like carbon (DLC) films combine several excellent properties like high hardness, low friction coeffi-
Received 3 December 2013 cients and chemical inertness. The DLC coating material can be further classified in two main groups, the hydro-
Accepted in revised form 20 January 2014 genated amorphous carbon (a-C:H, ta-C:H) and the hydrogen free amorphous carbon (a-C, ta-C). By adding other
Available online 25 January 2014
elements like metals (a-C:H:Me) or non-metal elements like silicon, oxygen, fluorine or others (a-C:H:X), several
modifications of the properties can be adjusted according to application requirements. First reports on hard
Keywords:
Diamond-like carbon
amorphous carbon films were published in the 1950s and about 20 years later there began worldwide intensive
History research activities on DLC. In the following years the number of publications increased continuously and the im-
Industrial application portance for industrial applications became more and more evident. Several deposition techniques were applied
to prepare a-C:H, ta-C, metal containing a-C:H:Me and non-metal containing a-C:H:X coatings. In parallel the
structure and deposition mechanisms of DLC coatings were extensively studied. An essential obstacle for a
broad industrial application was the high compressive stress level in a-C:H films causing delamination and lim-
iting the film thicknesses. With metal based intermediate layer systems most adhesion problems could be solved
satisfactorily and thus from the mid-1990s the pre-conditions for a broad application especially in the automotive
industry were given. With modified a-C:H:X and a-C:X coatings a considerable friction reduction or surface en-
ergy adjustments could be achieved.
© 2014 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
2. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
2.1. Initial developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
2.2. Superhard hydrogen free ta-C and hydrogen containing ta-C:H coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
2.3. Structure and composition of a-C:H and ta-C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
2.4. Growth mechanisms of a-C:H and ta-C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
2.5. Metal containing diamond-like carbon coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
2.6. Modified of a-C:H and a-C films — incorporation of additional elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
3. Transfer to industrial applications and mass production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
4. Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

1. Introduction their combinations are very promising for a variety of technical

applications.
Amorphous diamond-like carbon (DLC) coatings are known for their Today DLC films are used worldwide on a large industrial scale to im-
outstanding properties such as low friction coefficients, high hardness prove the performance of tools and of components, especially for auto-
and wear resistance, chemical inertness, optical transparency in the IR motive applications [1–3] with more than 100 million coated parts per
spectral range and low electrical conductivities. These properties and year and a market volume of several €100 million. Other application
fields with large market shares are protective DLC films (a few nm
thick) for high storage density hard disks and recording heads [4,5] as
⁎ Corresponding author. Tel.: +49 531 2155 642; fax: +49 531 2155 900. well as razor blades where thin DLC films improve the performance of
E-mail address: klaus.bewilogua@ist.fraunhofer.de (K. Bewilogua). sharp cutting edges. As an example the well-known Mach 3 razor

0257-8972/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.surfcoat.2014.01.031
 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225 215

from Gillette should be mentioned. DLC films are also applied for wear 2. History
protection of cutting and forming tools [6,7], for biomedical compo-
nents [8,9], as IR antireflection films (Section 2.1), components of thin 2.1. Initial developments
film sensors [10], barrier films in PET bottles [9], or as a dielectric layer
[11]. For a summary of the most important applications of DLC coatings Hard amorphous carbon films were first mentioned in 1953 by
a recent paper by Robertson [12] is recommended. Schmellenmeier in a paper on the influence of an ionized acetylene
Currently the term DLC describes a large variety of carbon based (C2H2) atmosphere on surfaces of tungsten–cobalt alloys [16]. The
coating modifications. In order to devise a classification regarding the main objective of his investigation was to find out whether at relatively
use of different names for DLC coatings, especially in Germany, Japan low temperatures via a glow discharge in hydrocarbon atmospheres
and Korea attempts were made to describe the different modifications tungsten carbide hard metal (so called “Widia”) surface layers can be
of DLC systematically. Here the German guideline VDI 2840 ‘Carbon generated. As an additional observation the author found that black
films — Basic knowledge, film types and properties’ should be mentioned and very hard amorphous films were deposited on the cathode of the
[13]. d.c. glow discharge if the discharge current was not too high. In a second
In this guide line it will be distinguished between the following paper published in 1956 Schmellenmeier [17] reported that these some
types of DLC coatings: μm thick hard films consist of “structure-less” regions and, under specif-
ic process conditions, of crystallites which were identified by X-ray dif-
• Hydrogen free amorphous carbon films — a-C fraction as diamond.
• Hydrogen free tetrahedral amorphous carbon films — ta-C with a high Already in 1951 König and Helwig [18] investigated direct current
fraction of tetrahedral coordinated sp3 bonded carbon atoms (d.c.) glow discharge processes in a benzene (C6H6) atmosphere. How-
• Metal containing hydrogen free amorphous carbon films — a-C:Me, ever, they analyzed only the films grown on the discharge-anode. These
where the metal often is a carbide forming metal like titanium or films were yellow and had a relatively low density (1.4 g/cm3). Later
tungsten Heisen [19,20], using nearly the same experimental set-up as König
• Hydrogenated amorphous carbon films — a-C:H and Helwig and also benzene as hydrocarbon precursor, observed on
• Hydrogenated tetrahedral amorphous carbon films — ta-C:H the cathode of the discharge considerably higher deposition rates and
• Metal containing hydrogenated amorphous carbon films — a-C:H:Me higher mass densities of the deposited films than on the anode and con-
• Modified hydrogenated amorphous carbon films — a-C:H:X, where X cluded that the interaction cross sections of positive ions with adsorbed
is related to such non-metal elements such as silicon, oxygen, nitro- gas molecules are much higher than those of electrons. Furthermore
gen, fluorine, and boron. Heisen [19] observed that the growth rates of the films depend consid-
erably on the substrate geometry as well as that insulating substrates
In order to obtain a comparison of DLC films with diamond and were charged under ion bombardment and consequently the film
graphite in Table 1 some typical properties of both these crystalline growth stops.
phases with ta-C and a-C:H are summarized. The next remarkable publication concerning hard amorphous carbon
All types of coatings mentioned, are usually prepared by physical films was that of Aisenberg and Chabot [21] in 1971 where for the first
vapor deposition (PVD) methods like sputtering or arc evaporation or time the term “diamond-like carbon” was used. These coatings were pre-
by plasma-enhanced chemical vapor deposition (PECVD) methods. pared by an ion beam deposition technique on room-temperature sub-
PECVD techniques base on glow discharge processes applying hydrocar- strates. The ion beam consisted of carbon and argon ions generated in a
bon gases like acetylene (C2H2) and negatively biased substrates work- discharge system using graphite as material for the active electrodes.
ing at radio frequencies (r.f. — 13.56 MHz) [14] or mid-frequencies Fig. 1 shows the scheme of this equipment. Positive carbon and gas ions
(m.f. — some 10 to some 100 kHz) [15]. Besides the outstanding coating were extracted from the discharge region and deposited on a negatively
properties of DLC it is an important aspect that these coatings can be biased substrate and the ion energy could be adjusted by the substrate po-
deposited at low substrate temperatures (b200 °C), e.g. on temperature tential. The deposited films were intensively investigated and specified
sensitive steel components. e.g. as scratch resistant, electrically insulating, optically transparent and
The aim of the present paper is to discuss historical developments of chemically resistant. The structure was described as partially crystalline
preparation techniques, structure models, growth mechanisms, proper- with lattice constants similar to those of diamond. Furthermore the
ties and applications of different types of DLC coatings. Furthermore ex- preparation of thin-film transistors using the insulating carbon films
amples for developments of industrial deposition processes and new was described. In 1973 the same authors reported that the cutting perfor-
application fields of DLC coatings will be discussed. mance of paper cutting blades could be markedly increased by applying
diamond-like carbon films. Wear tests with the coated blades revealed
that these films “apparently reduce the coefficient of friction” [22].
In the 1970s several other papers on DLC were published. Spencer
Table 1 et al. [23] analyzed DLC films prepared by techniques similar to those
Comparison of typical properties of diamond, ta-C, a-C:H and graphite (references: of Aisenberg and Chabot [21]. Using X-ray diffraction and transmission
[12,38,39,55,56,68,87,102,145]).
electron microscopy (TEM) they found indications for small (size 5–
Diamond ta-C a-C:H Graphite 10 nm) and large (up to 5 μm) crystallites. The observed diffraction
Crystal system Diamond cubic Amorphous Amorphous Hexagonal
reflections were assigned to the diamond phase.
3
Whitmell and Williamson [24] prepared hard and insulating films
Mass density/g/cm 3.51 2.5–3.3 1.5–2.4 2.26
with thicknesses up to 4 μm on different metallic substrates using a sim-
sp3 Content/% 100 50–90 20–60 0
Hydrogen content/ at.% 0 ~1 10–50 0 ilar d.c. based deposition technique like that of Schmellenmeier and
Hardness/GPa 100 50–80 10–45 b5 Heisen but with a gas mixture of ethylene (C2H4) with 5% argon. Over
Friction coefficients the substrates a domed aperture was placed (Fig. 2). Although the
In humid air 0.1 0.05–0.25 0.02–0.3 0.1–0.2
deposited films were insulating and a positive charge should be expect-
In dry air 0.1 0.6 0.02–0.2 N0.6
Band gap/eV 5.5 1–2.5 1–4 −0.04 ed, surprisingly high thicknesses could be achieved. Holland [25]
Electrical 1018 106–1010 104–1012 10−6–10−2 explained this assuming that secondary electrons generated at the
resistivity/Ω cm edges of the domed aperture compensate for the positive charges.
Thermal stability 800 400–600 300–350 N500 Later such a compensation effect was verified for a d.c. based ion plating
in air/°C
a-C:H deposition process on insulating glass substrates [26].
216 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

Fig. 1. Scheme of ion-beam deposition system.

Adapted according to [21].

Since the mid-1970s several research groups have published results depended linearly on the term VB · P−0.5 which on the other hand
of a-C:H deposition experiments. Especially the work of Holland and was found to be proportional to the average energy of the film forming
Ojha [27,25] can be regarded as a break through because they used a energy. The objective of the research group in Freiburg was to develop
radio frequency (r.f. — 13.56 MHz) glow discharge in butane (C4H10) a-C:H as transparent wear resistant anti-reflection coatings on infrared
gas to overcome problems with deposition of insulating a-C:H films (IR) optical components. On germanium and silicon substrates (refracting
(for a process-scheme see Fig. 3). Hard and insulating films could be index n ≈ 4) a-C:H films with n ≈ 2 would be an ideal transparent
prepared even on dielectric substrates such as glass. Furthermore it antireflection layer [30–32]. Catherine and Couderc [33] compared a-C:
seems to be important that similar equipment was available in many H glow discharge deposition processes and growth kinetics both for
laboratories and could be used simply to prepare DLC coatings. Besides r.f. (13.56 MHz) and m.f. (50 kHz) plasma excitation. The film properties
Holland and Ojha [25,27], Andersson, Berg and co-workers [28,29] used were found to be related to the product VB · P−0.5 for the r.f. process (like-
the r.f. glow discharge method to prepare a-C:H films and they consid- wise Bubenzer et al. [30]) and to the product J · P−0.5 for the m.f. process
ered aspects concerning the growth mechanism of a-C:H films like the where J is the mean current density.
initial phases of film growth on different substrate materials [28]. Fur- In the period since the mid-1970s also other techniques for a-C:H
thermore, different hydrocarbon precursors (methane CH4, ethane deposition were developed and the films were characterized in detail.
C2H6, propane C3H8, n-butane C4H10 and iso-butane C4H10) were com- Weissmantel and co-workers [34,35] reported on two different
pared [29]. The highest deposition rates were observed for both bu- methods for the preparation of DLC coatings. With the so called dual
tanes, the lowest one for methane. The authors stated that there was beam technique, a carbon target was sputtered with argon ions and
no simple relationship between growth rate and precursor structure in- the carbon film growing on the substrate was simultaneously
dicating complex ionization and dissociation processes of the hydrocar- bombarded by a second ion source operated at about 1 kV with argon
bon molecules. and methane (Fig. 4). The deposited carbon films were hard and the
In the early and mid-1980s profound work on a-C:H films prepared structure was characterized as amorphous with crystallites embedded
by r.f. glow discharge was done. Bubenzer et al. [30] from the Fraunhofer in the substrate regions exposed to the highest ion current densities
Institute for Applied Solid State Physics in Freiburg, Germany, described [35].
the negative r.f. self-bias VB of the substrate and the hydrocarbon gas On the other hand in the Weissmantel group a-C:H coatings were
pressure P as the most important parameters controlling the deposition prepared applying a completely d.c. based “ion plating” process where
process. E.g. the film density ranged between 1.5 and 1.8 g/cm3 and hydrocarbon ions were generated in a benzene atmosphere by hot

Fig. 2. Scheme of d.c. glow discharge deposition technique. Fig. 3. Scheme of r.f. glow discharge deposition technique.
Adapted according to [24]. Adapted according to [25,27].
 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225 217

Fig. 4. Scheme of dual beam deposition technique.

Adapted according to [34,35].

cathode ionization. The a-C:H films were grown by acceleration of these Due to the combination of low friction coefficients with high hard-
ions to a negatively d.c. biased (up to 800 V) substrate [35,36]. Fig. 5 ness and wear resistance, a-C:H films became very interesting for solv-
shows the scheme of this deposition method. These films were hard, ing diverse engineering problems. Consequently the number of research
partially optically transparent and electrically insulating. The structure groups working on DLC began to increase considerably in the following
was described as a mixture of nanocrystalline components consisting years. Fig. 6 shows, referring to the database Scopus, how the number of
of graphite-like and diamond-like elements [36]. publications from a small number up to the 1980s increased continu-
In the early 1990s Martinu et al. [37] increased the effectiveness of ously and reached maximum output after the year 2000. In the period
the r.f. glow discharge processes by applying a simultaneously operating from 1990 to 2000 DLC coatings became more and more interesting re-
microwave (MW — 2.45 GHz) radiation. With this combined r.f.–MW garding industrial applications, especially for the automotive industry
process increased ion fluxes on the substrate, higher deposition rates, where they are well established today.
lower hydrogen contents and increased hardness of the deposited a-C: Nowadays tribological and mechanical properties of DLC coatings are
H films were achieved. of outstanding interest for industrial applications especially in the auto-
Enke, Dimigen and co-workers reported in the early 1980s on very motive industry. First references to the potential of DLC films were, as
low friction coefficients of hard diamond-like carbon films against already mentioned, given by Aisenberg and Chabot [22] and by Enke
steel counterparts [38–40]. The investigated a-C:H films were prepared et al. [39,40]. As far back as the 1980s, more detailed studies followed,
by a r.f. glow discharge technique similar to that of Holland and Ojha discussing the dependence of friction coefficients on deposition condi-
[27]. In contrast to graphite, these films had extremely low friction coef- tions [42] and humidity or surrounding gas atmosphere [41,43,44]. Fur-
ficients (μ) at low humidity (≈0.01) and under “normal” atmosphere thermore, in the following years, a lot of investigations on hardness,
the μ values remained rather low (b0.2). Memming et al. [41] measured wear and friction were presented whereas especially in the 1990s and
friction coefficients of the same type of a-C:H films against steel in ultra- later, the number of research papers on these topics continuously in-
high vacuum (μ ≈ 0.02), humid air or nitrogen (0.25) as well as in dry creased (see e.g. the historical overview of Donnet and Erdemir and fol-
nitrogen (0.02) and dry oxygen (≥0.6) and explained the differences lowing contributions on Tribology of Diamond-like Carbon Films in [45]).
with different developments of the transfer layers which is generated In the late 1970s and in the 1980s, several papers concerning data on
between the sliding partners. other than mechanical or tribological properties of a-C:H films were
published. In this period, electrical conductivities as well as optical
constants (refractive indices and extinction coefficients) of a-C:H films
prepared by r.f. glow discharge processes were investigated in depth
by Anderson [46] and Meyerson and Smith [47,48]. As well, already in
1980, Meyerson and Smith [49] investigated the electrical conductivity
of doped (B and P) a-C:H films and found conductivity gains of 5 orders
of magnitude, for example for a coating deposited at 250 °C from
10− 12 Ω− 1 cm− 1 to 10− 7 Ω− 1 cm− 1.
Similar data for the optical properties of a-C:H films deposited by d.c.
magnetron sputtering in Ar–C2H2 gas mixtures were published in the
early 1980s by research groups from Sydney, Australia [50–52]. The
characteristic of this unusual deposition technique was that metal tar-
gets (e.g. stainless steel) were over coated with carbon from the disso-
ciation of hydrocarbon species in the plasma of the glow discharge.
Thus a-C:H films could be prepared on different substrates.
It is noteworthy that also in 1980 Moravec [53] proposed “Color
chart for DLC films on silicon”, a simple but rather helpful tool to mea-
sure the thicknesses of thin DLC films (up to about 300 nm) considering
interference colors.

2.2. Superhard hydrogen free ta-C and hydrogen containing ta-C:H coatings

Fig. 5. Scheme of d.c. ion plating deposition technique. Superhard hydrogen free ta-C films are characterized by a high frac-
Adapted according to [35,36]. tion of tetrahedrally bonded (sp3) carbon atoms. To prepare such films,
218 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

Fig. 6. Number of publications on diamond-like carbon coatings in four decades (data from Scopus database for all DLC modifications described in the Introduction).

carbon species with energies in the range of 100 eV are needed (for a re- films can be prepared [61]. Later industrially utilizable laser–arc
view see e.g. [54]). The most appropriate method to realize such condi- techniques to deposit ta-C were developed [63].
tions is the cathodic arc evaporation. The pioneer work in this field was Other PVD techniques were revealed to be principally appropriate to
done by Strel'nitskij and Aksenov and co-workers in Kharkov in the for- prepare carbon films with high sp3 contents. Here the pulsed laser de-
mer Soviet Union (today Ukraine). In 1978 Strel'nitskij et al. [55,56] re- position (PLD — for an overview see [64] and references therein), the
ported on extremely hard and electrically insulating (ρ ≈ 1010 Ω cm) unbalanced magnetron sputtering (UBM) [65,66], should be mentioned.
diamond-like carbon films deposited by arc evaporation using a pure Weiler et al. [67] reported on tetrahedral hydrogenated DLC films (ta-
graphite cathode. Refractive indices n N 2.2 were another indication C:H) prepared from a plasma beam source based on a r.f. (13.56 MHz)
for diamond-like properties (n of diamond: 2.4). The substrates were discharge at relatively low working gas pressures (0.05 Pa) and a
powered by r.f. and d.c. voltages. The maximum measured film hardness magnetic confinement allowing generation of a highly ionized plasma
was stated as near to or even higher than that of natural diamond. The beam. The prepared films had a sp3 fraction up to 0.75, a maximum
films were nanocrystalline but a clear assignment of the X-ray diffrac- mass density of 2.9 g/cm3 and hydrogen contents between 22 and
tion peaks to known carbon phases was not possible. 28 at.%. High hardness (N 60 GPa) but also high compressive stress
An obvious drawback of the used arc technique was the appearance (8.5 GPa) corresponded to the high sp3 contents.
of macro particles in the growing films. To overcome this, Aksenov et al. Although the properties of tetrahedrally coordinated DLC films, like
[57,58] proposed an arc system equipped with a curvilinear separating high hardness and wear resistance and low friction coefficients under
filter allowing a drastic reduction of the macro particles. The carbon spe- ambient conditions and under oil lubrication [68], are very attractive,
cies arriving at the substrate were completely ionized, predominantly a broad breakthrough of ta-C deposition techniques in the industrial
single charged [58]. The diamond-like-carbon coatings prepared by mass production is still outstanding.
this technique had highly pure surfaces and were still extremely hard
[58]. 2.3. Structure and composition of a-C:H and ta-C
A few years later several papers concerning cathodic arc deposition
of superhard hydrogen free DLC films were published. Martin, McKenzie In the abovementioned first publications on diamond-like carbon
and co-workers [59,60] used a filtered arc system similar to that intro- films their structure was specified as amorphous with crystalline
duced by Aksenov and co-workers (see [58]) and discussed the atomic regions [17] or partially crystalline where the crystalline phase
structure of the deposited tetrahedral amorphous carbon (ta-C) films corresponded to diamond [21,23]. Anderson [46] found that highly in-
in detail, taking into account the data of electron energy loss spectrosco- sulating a-C:H films prepared by r.f. glow discharge are amorphous
py (EELS) and electron diffraction analyses. In the very comprehensive and proposed a structure model consisting of three- and four-fold coor-
paper of McKenzie and co-workers [60], in addition to the atomic struc- dinated carbon atoms where the latter prevent the formation of extend-
ture, the data on the compressive stress (up to 8 GPa) as a function of ed graphite-like components.
the ion energy as well as a model for the stress development was pre- In the 1980s, detailed investigations on the structure of a-C:H were
sented. Furthermore optical properties and possible electronic devices presented. Craig and Harding [52] analyzed composition and structure
built with ta-C films were discussed. of magnetron sputtered a-C:H films (see Section 2.1.). The hydrogen
Another concept for the arc deposition of ta-C films, the so called content, determined by pyrolysis, was in all as-deposited films higher
laser–arc technique, was proposed in the early 1990s by a research than 32 at.% and the oxygen content (from residual gas) higher than
group in Dresden, Germany, led by Pompe and Scheibe [61,62]. They 6 at.%. The proposed structure model consisted of a random network
used short pulses (b1 ms) of a Nd:YAG laser to ignite a vacuum arc of tetrahedral coordinated carbon atoms modified by carbon–carbon
realizing a time and position controlled arc of about 50 ms duration. double bonds and carbon–hydrogen bonds. This model was consistent
The deposited carbon films were amorphous, very hard and had with the low densities (1.0 to 1.6 g/cm3) and high resistivities (N107
refractive indices between 2.05 and 2.5. An additional advantage of Ω cm). McKenzie et al. [50,51,69] developed an a-C:H model where
the laser–arc technique is that by moving the target against the laser threefold coordinated carbon clusters are embedded in an insulating
beam the material to be deposited can be selected. Thus multilayer polymer-like hydrocarbon material containing many methyl groups.
 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225 219

Assuming such a structure, the results of annealing experiments which

led to properties similar to those of glassy carbon, could be simply ex-
plained. More or less simultaneously Dischler et al. [70] analyzed IR ab-
sorption spectra and developed similar structural concepts for as-grown
as well as for annealed a-C:H films prepared by r.f. glow discharge.
It had been proved reasonable to compare structural features of a-C:
H films with those of evaporated a-C films. Bewilogua, Weissmantel
et al. [71–73] revealed by electron diffraction and electron energy loss
spectroscopy studies a high similarity of ion plated a-C:H films [35]
and of evaporated a-C films. The best fit for the experimentally derived
electron diffraction intensity functions was obtained assuming small
clusters (b 1 nm) consisting of puckered hexagonal and of pentagonal
rings. To explain the remarkable differences of the a-C:H and a-C
properties (hardness, electrical conductivity) for a-C:H, strong sp3 con-
taining cross-linkages between the clusters were assumed which in a-C Fig. 7. Relation between sp3/sp2 ratio and hydrogen concentration in a-C:H films, calculat-
do not exist. A corresponding hand built model was presented by ed according to [76], experimental data, derived from Kaplan et al. [86], Robertson [12],
Weissmantel [73]. and Hofmann et al. [145].
Robertson and O'Reilly [74] discovered that the optical gaps of a-C
(~0.5 eV) and a-C:H (1.5–2.5 eV) are clearly different. Analyses of the
gaps of hydrocarbon ring structures indicated that the gap varies rough- from infrared absorption spectra and the total hydrogen content was es-
ly inversely to the cluster size. Considering this relation, evaporated a-C timated by an effusion technique. With increasing self-bias voltages the
films dominantly consist of disordered graphite clusters with dimen- structure changed from weakly bonded benzene-like rings to graphitic
sions up to 1.5 nm. On the other hand a-C:H films contain a high amount ring structures. After annealing the number of sp2 bonded carbon
of sp2 sites in ring structures and the observed gap sizes are in accor- atoms increased and above 600 °C hydrogen was released.
dance with the existence of clusters consisting of four or more aromatic An interesting model for a-C:H was presented in 1990 by Tamor and
rings. Wu [80] who proposed defected graphitic networks where hydrogen
Angus [75] proposed an empirical categorization of DLC films by atoms saturate open bonds in vacancies of graphite clusters. Applying
their gram atom number density ρN and their hydrogen fraction xH this model nearly the same lower (20 at.%) and upper (60 at.%) limits
where ρN is the quotient of the mass density ρm and molar mass: of the hydrogen content like those from Angus and Jansen [76] were
ρN = ρm / (xH + 12(1 − xH)) with the atomic masses 1 for hydrogen estimated.
and 12 for carbon. In Table 2 some examples for ρN and cH data are In 1993 Möller and co-workers [81,82] described structure and com-
summarized. Films with ρN N 0.2 g atom/cm3 were called dense carbo- position of different hydrocarbon films by a ternary phase diagram
naceous (hard) films or with significant hydrogen amounts dense which assumed as the three phases in these films sp3 and sp2 hybridized
hydrocarbon (soft) films. carbon and hydrogen. This ternary phase diagram was afterward used
In 1988 Angus and Jansen [76] applied random covalent network in many papers concerning a-C:H films (e.g. [67,83]). Fig. 8 shows an ex-
concepts to dense hydrocarbon films. The theories suitable to describe ample for such a phase diagram.
random covalent networks were developed some years ago by Phillips Beginning in the late 1980s, Australian research groups investigated
[77] and Döhler et al. [78]. Accordingly a random covalent network is the structure of arc deposited ta-C in detail using electron optical
completely constrained if the number of constraints per atom is equal methods especially EELS [59,84,85] and electron diffraction [60,85]. It
to the number of degrees of freedom per atom. Applied to hydrocarbon was proved that in these film sp3 bonded carbon dominates. In the
systems containing sp2 and sp3 bonded carbon and hydrogen atoms, for low energy part of the EELS spectra, the plasmon peak was found in
completely constrained structures, hydrogen contents between 17 and the range of the diamond peak (33 eV) and from electron diffraction,
62 at.% were derived [76]. In Fig. 7 the described relation (sp3/sp2 ratio nearest neighbor distances (r1)and coordination numbers (n1) as well
vs. hydrogen fraction xH) and an extended set of experimental data as the bond angles (α) were determined as very near to the diamond
are shown. This mode of representation allows a very clear distinction values (diamond: r1 = 0.154 nm, n1 = 4 and α = 109.47°).
between soft (high hydrogen content) and hard (low hydrogen con- For structure characterization of DLC films beside electron diffrac-
tent) coatings. tion, EELS, IR spectroscopy [70] or nuclear magnetic resonance (NMR)
Fink et al. [79] analyzed as-deposited and annealed a-C:H films by [86] the Raman spectroscopy was an important characterization tool
high-resolution EELS. The films were prepared by r.f. glow discharge [87–89]. An essential advantage of the Raman spectroscopy is that no
in benzene vapor. The content of bonded hydrogen was determined extensive sample preparation is necessary. Although the interpretation
of Raman spectra is rather complex [83], this technique allows compar-
ison of different phases of carbon materials and estimation of sp3/sp2
ratios [83,90].
Table 2 Modeling of a-C structures started in 1984 by Beeman et al. [91]
Examples for mass densities, hydrogen contents, number densities (calculated according using hand models with several 100 carbon atoms and in 1989 by
to reference [75]) and hard–soft classification of diamond, graphite diamond-like carbon
Galli et al. [92,93] using first-principle molecular-dynamic methods. In
films and a polymer.
the 1990s several groups carried out extensive computer modeling of
Mass density Hydrogen Number density Hard/soft pure a-C [94], ta-C [95,96] or both pure and hydrogenated amorphous
(g/cm3) content (at.%) (1/cm3)
carbon [97–99] and compared the results with experimentally deter-
Diamond 3.51 0 0.29 Superhard mined data like mass densities, hydrogen contents or interference func-
Graphite 2.25 0 0.19 Soft tions and radial distribution functions. The best points of concurrence
ta-C 3.2 1 0.27 Superhard
a-C:H (1) 2.3 11 0.21 Hard
of experiment and model could be used to draw conclusions e.g. on
a-C:H (2) 2.0 17 0.20 Hard the contributions of sp3, sp2 and sp1 bonds (sp1: one-dimensional,
a-C:H (3) 1.4 60 0.26 Soft acetylene-like), and made it possible to calculate electronic densities
Polystyrene 1.05 50 0.16 Soft polymer of states [95,97,98]. Fig. 9 shows an example for an a-C:H film model
(C8H8)
with a dominant sp2 share and 30 at.% hydrogen.
220 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

In the 1990s, several theoretical studies on deposition mechanisms

both of hydrogen free ta-C and of hydrogen containing a-C:H films
were published. Möller [103,104] from the Max-Planck Institute for
Plasma Physics in Garching, Germany analyzed the elementary mecha-
nisms of the growth of a-C:H films from low-pressure methane plasmas
and developed models which predicted the sp3/sp2 ratios for varying
ion energies [103] and explained, amongst others, the dependence of
the hydrogen content on the energy of the ions striking the substrates,
depth profiles of carbon and hydrogen in an a-C:H film and the influ-
ence of the substrate temperature on the deposition rate [104]. In the
following years several papers on deposition experiments and modeling
of a-C:H film growth were published by the research group from the
Max-Planck Institute for Plasma Physics in Garching, Germany, for ex-
ample by von Keudell et al. [105] who discussed the processes in the
Fig. 8. Ternary phase diagram adapted according to and using data from [81–83,12,145].
near surface growth zones and the mechanisms of hydrogen release
from growing a-C:H films under ion bombardment. Hydrogen ions
For a survey of the different structure characterization methods of have a considerably higher penetration depth than carbon ions and
amorphous and nanocrystalline carbon films a paper of Chu and Li form via recombination H2 molecules, which can desorb and thus
[100] is recommended. lower the hydrogen content in the films. Additionally, such a mecha-
nism explains the experimental findings that the composition and prop-
erties of a-C:H films prepared from very different precursors like
2.4. Growth mechanisms of a-C:H and ta-C methane CH4 or benzene C6H6 are often rather similar.
For more details, a review of von Keudell [106] on “Formation of
Already Heisen [19] (see Section 2.1.) presented first considerations polymer-like hydrocarbon films from radical beams of methyl and
on the growth of hydrocarbon films on discharge cathodes. Later, begin- atomic hydrogen” is recommended.
ning in the 1980s, the roles of ions and radicals on the growth behavior In 1989 and 1990 Lifshitz et al. [107,108] suggested a subplantation
of a-C:H films were considered more in detail by Catherine and Couderc model to describe the ion-enhanced growth of carbon films where
[33] and by Bubenzer et al. [30] and Koidl et al. [101] for r.f. based a-C:H subplantation means the penetration of impinging hyperthermal spe-
deposition processes. Koidl et al. [101] investigated different process cies (1–1000 eV) into the top layers of the bombarded sample causing
gases as precursors (benzene, cyclo-hexane, n-hexane, methane) and phase transformations. In the case of carbon this model explained the
found that the film properties are nearly independent of the used high sp3 contents assuming a considerable difference of the displace-
precursor gas if the bias voltage is high enough for an effective fragmen- ment energies Ed between graphite (25 eV) and diamond (80 eV) lead-
tation of the hydrocarbon molecules. On the other hand, the finding ing to preferred displacement of the low-Ed and remaining high-Ed
that the deposition rates depended clearly on the precursor gas carbon.
corresponded to the results reported by Andersson et al. [29]. The depo- Robertson [109] developed a similar model where ions enter the
sition rates increased with increasing molecule masses. Furthermore, subsurface region which will be densified. The experimentally deter-
the ionization energy was seen as another rate-determining factor mined density maxima of ta-C film at ion energy around 100 eV were
(see also [102]). interpreted as a consequence of a densification due to the ion striking
the surface and, at higher energies, to an energy dissipation accompa-
nied by an annealing and density decrease. The impact of the ions can
be direct (carbon ion) into the surface near region or indirect by dis-
placement of a surface atom into an interstitial site (Fig. 10).
In a continuative publication Robertson [110] extended the film
growth model to plasma deposited a-C:H films.

2.5. Metal containing diamond-like carbon coatings

In the 1980s and still in the 1990s the main reason of the retarding
industrial applications of DLC as tribological coatings was their disap-
pointing adhesion caused by relatively high compressive stresses in
the range of some GPa. To overcome this obstacle, Dimigen and co-
workers [111,112] prepared metal containing DLC films (Me-DLC,
today abbreviated as a-C:H:Me) by reactive r.f. sputtering of a metal in
a hydrocarbon–argon atmosphere. Alternatively, instead of pure metal
targets, metal–carbon (metal carbide) targets could be utilized. Such
Me-DLC films with metal to carbon ratios in the range Me/C = 0.1–0.2
were found to have, under non-lubricated conditions, similar low fric-
tion coefficients like a-C:H. In comparison to a-C:H, not only their hard-
ness and wear resistance, but also the compressive stress was slightly
lower but, on the other hand, due to a strongly cross-linked carbon–
hydrogen matrix, considerably higher than the hardness of metal con-
taining polymers [44]. Another essential advantage of the Me-DLC pro-
cess was that the metal target could simply be used in a first process
Fig. 9. Structure model of an a-C:H film with a density of 1.7 g/cm3, 30 at.% hydrogen, 65%
sp2, 20% sp3 and 15% sp1 bonds. A cluster consisting of graphite-like rings is accented.
step to prepare an adhesion improving metallic interlayer. Moreover,
Image: with kind permission of Thomas Frauenheim, Bremen Center of Computational it was remarkable that the electrical conductivity of Me-DLC was several
Materials Science. orders of magnitude higher than that of a-C:H [113]. Later Benndorf
 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225 221

Fig. 10. Ion-film interactions during growth of ta-C and a-C:H: direct (a) and indirect (b) subplantations, scheme of a-C:H growth (c).
Adapted according to [109,110,105].

et al. [114] developed a structure model which explained the high elec- processes and that the contribution of direct plasma polymerization at
trical conductivities even at relatively low metal contents. The authors the substrates is small [124]. For reactive r.f. sputter processes the
assumed that metal, not bonded in carbides, modifies the carbide sur- same two contributions to the a-C:H:Me film growth were revealed
rounding a-C:H matrix. for Ag-DLC films [125].
The reactive sputter deposition technique was developed and ap- Me-DLC coating preparation using magnetron sputter machines
plied in the same period and even earlier by Australian scientists for with rotating substrates caused a columnar morphology with character-
the preparation of solar absorbing surfaces. Ritchie and Harding [115] istic multilayer structures consisting of oscillating metal rich and carbon
prepared Fe–C films by d.c. diode sputtering in an Ar–CH4 mixture. rich single layers coatings [126–130]. These oscillations could be ex-
Wilson et al. [116] used a dual-post cathode sputter coater with two in- plained by periodical changes of the substrate position compared to
dependently controlled cathodes consisting of different materials oper- the targets with a maximum metal content deposited directly in front
ating with acetylene so that graded metal–carbon films could be of the metal target. Fig. 11 shows a scheme of such a multilayer Me-
produced. DLC coating with a columnar structure.
The most important factors for industrial applications of Me-DLC (or Me-DLC coatings consist of an a-C:H matrix with embedded carbides
a-C:H:Me) coatings were the tribological properties, the good adhesion which commonly are even harder than a-C:H [130]. Therefore it is
and especially the high potential for transferring the deposition process- somewhat surprising that Me-DLC coatings are generally softer and ex-
es to industrial dimensions (see Section 3). Industrially relevant deposi- hibit higher wear rates than metal free a-C:H films [126,131]. An expla-
tion processes could not be realized with r.f. processes. Therefore nation for this observation is that the a-C:H matrix of Me-DLC has
transferring the process to magnetron sputter techniques was weaker cross-linkages, higher hydrogen contents and therefore a
demanded. Bergmann and Vogel [117] used a magnetron sputter ma- lower mechanical stability [131].
chine equipped with four planar targets to prepare W-DLC coatings The properties of a-C:H:Me coatings depend considerably on the
from a tungsten carbide target as well as Ti-DLC and Cr-DLC coatings metal content which can be simply adjusted by the reactive gas content
from the corresponding metal targets. Deposition rate, hardness and in the sputter gas. Fig. 12 illustrates typical dependencies for hardness
stress of the coatings depended on the reactive gas (acetylene) flow and abrasive wear (a) and for the friction coefficients of tungsten and ti-
during sputter deposition and these properties were clearly different tanium containing a-C:H:Me coatings (b). The pronounced hardness
for the three investigated metals. maxima (Fig. 12 a) can be interpreted as an effect of a MeC/a-C:H
A summary of the situation at the end of the 1980s concerning depo- nanocomposite structure consisting of metal carbide nanocrystals
sition processes, structure and composition as well as optical, electrical, surrounded by an amorphous carbon binder phase [132,130].
mechanical and tribological properties of Me-DLC films was presented
by Klages and Memming [44]. 2.6. Modified of a-C:H and a-C films — incorporation of additional elements
Already in 1980, Richie [118] investigated the structure of Ti-DLC
films prepared by reactive d.c. magnetron sputtering applying an In this section the so-called modified or doped DLC films, according
argon–methane mixture and a pure titanium target and reported on ti- to the VDI 2840 guideline [13] denoted as a-C:H:X where X is related to
tanium carbide particles of 5–10 nm diameter embedded in an amor- non-metal elements, will be considered.
phous carbon matrix. In the 1990s some very detailed studies on Me- In the mid-1970s and 1980s several authors reported on silicon con-
DLC growth processes and the structure were published. Wang et al. taining a-C:H:Si films which were prepared by glow discharge decom-
[119,120] analyzed the structure and composition of Ti-, Ta- and W- position of silane (SiH4) and hydrocarbon gases (C2H4 [133], CH4
DLC films prepared by reactive r.f. sputtering and measured thereby [134,135]) as well as by sputter techniques [135,136]. McKenzie et al.
the hydrogen contents by elastic recoil detection (ERD). Sjöström et al. [136] applied the d.c. magnetron technique to prepare a-SixC(1 −x)Hy
[121] investigated the structure of Mo-DLC and W-DLC films by high- films. The same magnetron sputter technique had already been previ-
resolution transmission electron microscopy, X-ray diffraction and ously successfully used for the deposition of pure a-C:H films [50–52]
Auger electron spectroscopy. In the amorphous a-C:H matrix small crys- (see also Section 2.1.). The working gas for the a-SixC(1−x)Hy deposition
tallites with dimensions of few nm were embedded. In the case of mo- was a silane–acetylene–argon mixture. Because the motivation of this
lybdenum, metallic bcc Mo clusters (1–4 nm) and of tungsten 1 nm research was to develop low emittance solar selective coatings on cop-
clusters, probably consisting of tungsten carbide, were identified. A per substrates, the optical properties were studied in detail. It is remark-
comparative study on the structure and cluster distribution in Me-DLC able that, beside single layer coatings, also a-C:H/a-C:Si multilayer
films for carbide forming metals (W, Fe) and non-carbide forming coatings were prepared by varying the parameter x [136].
metals (Au, Pt) was published by Schiffmann et al. [122] in 1999. In the early 1990s Japanese research groups investigated friction co-
To control reactive magnetron sputter deposition processes and to efficients (μ) of silicon containing a-C:H:Si coatings prepared by
prevent excessive target poisoning a plasma emission monitor could plasma-enhanced CVD methods. Oguri and Arai [137,138] measured μ
be used [123,124]. Using this control tool it was revealed that the values using a ball-on disk tester under un-lubricated conditions at
main contribution to film growth originates from target near discharge room temperature in humid and dry atmospheres. In both atmospheres
222 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

Fig. 11. Scheme morphology of a Me-DLC coating grown in a sputter machine with rotating substrates.
Adapted according to [130].

for films with Si/C ratios between 0.15 and 0.4 the friction coefficients In the mid-1990s it was discovered that incorporation of non-metal
were extremely low (b0.05) and thereby clearly lower than those of elements into the a-C:H can alter the surface energies of a-C:H coatings
pure a-C:H coatings in humid air (0.12–0.2). To explain the low friction to lower (X: F, Si, Si+O) and also to higher (B, N, O) values [146]. Thus it
coefficients in humid air it was assumed that fine silica particles formed became possible to combine the high mechanical stability of diamond-
during the sliding process will be surrounded by a water layer and the like carbon with low surface energies comparable with those of the
silica-sol acts as a liquid lubricant [139]. Miyake [140] found a similar well-known hydrophobic material polytetrafluorethylene (PTFE —
friction behavior under vacuum conditions. As-deposited films and surface energy about 19 mN/m, hardness b1 GPa). Grischke et al.
films annealed at 400 °C exhibited the extremely low μ values (marked- [147,148] reported on low surface energies of modified a-C:H:Si
ly lower than 0.1) for a Si concentration range around 10%. After anneal- (31 mN/m, 10–15 GPa), a-C:H:Si:O (24 mN/m, 7–10 GPa) and a-C:H:F
ing at 600 °C the friction coefficients increased drastically (N 0.2). A (20 mN/m, 2 GPa) coatings. To realize these modifications in r.f.
comparison of a-C:H:Si, a-C:H:Ge and a-C:H:Ti revealed that obviously PECVD deposition processes the precursors TMS (Si(CH3)4), HMDSO
only a silicon incorporation leads to a drastic reduction of the friction co- (O–Si2(CH3)6) and C2F4 were used. For the corresponding pure a-C:H
efficients [141]. In the following years many details on structure and coatings the surface energy was 41 mN/m and the hardness of
properties of silicon containing a-C:H:Si and other doped coatings, espe- 20–30 GPa. Potential applications of this type of a-C:H:X films were
cially with respect to their tribological properties, were reported (for an demonstrated for example for heat exchanger surfaces [149,150].
overview see [142] and references therein). Also hydrogen free modified a-C:X coatings were found to be hydro-
Another remarkable property of a-C:H:Si is the clearly higher ther- phobic. Schulz et al. [151] investigated a-C:F and a-C:Al films prepared
mal stability compared to that of pure a-C:H. For a-C:H:Si films prepared by laser–arc technology (see Section 2.2.). In both cases not only the
by r.f. PECVD the temperature at which structural changes started could surface energy, but also the Young's modulus decreased with increasing
be considerably increased by adding silicon to a-C:H [143,144]. It was X-content.
assumed that higher silicon contents lead to more sp3 bonded carbon Also in the mid-1990s Meneve et al. [152,153] reported on a-C:H/a-
and thus the carbon network will be stabilized against graphitization Si1 − x Cx:H multilayers prepared in a r.f. PECVD process using methane
[144]. Recently also for sputter deposited a-C:H:Si a clearly higher ther- and alternatively added silane as process gas. Under optimum process
mal stability compared to that of a-C:H was reported by Hofmann et al. conditions remarkable properties like low friction coefficients of a-C:
[145]. Both hardness and friction coefficients remained nearly constant H:Si combined with low wear rates of pure a-C:H could be achieved.
even after annealing at 500 °C in air whereas a-C:H coatings already The high potential of DLC based multilayers was confirmed by later pub-
failed at temperatures below 400 °C. lications in the 2000s [154–156].

Fig. 12. Typical dependencies of hardness, wear (a) and friction coefficients (b) on metal/carbon ratios, shown for a-C:H:W and a-C:H:Ti.
Adapted according to [130], hardness of a-C:H:Ti from [132].
 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225 223

3. Transfer to industrial applications and mass production C:H and other components like tappets and piston pins will be routinely
coated with a continuously increasing proportion [2].
In the late 1980s and early 1990s Me-DLC processes were trans-
ferred to large scale d.c. magnetron sputter machines, mostly equipped 4. Summary and outlook
with 4 targets, and became applicable for industrial applications
[157,158]. To prepare hard and wear resistant Me-DLC coatings high The first experimental evidence of DLC coatings was reported about
ion current densities at the rotating substrates were necessary. This 60 years ago. From the mid-1970s the research activities on DLC coat-
was realized by applying special magnetic field arrangements like the ings continuously increased until the end of the 1990s and thereafter
so called Plasma Booster [157,159] or closed field unbalanced magne- remained on a high level. During the course of this development, new
tron (CFUBM) arrangements [158,160]. Furthermore, multi-chamber deposition methods like reactive sputtering and cathodic arc processes
systems allowing in-line processes to prepare tungsten containing DLC as well as modifications of diamond-like amorphous carbon-based
(W-DLC or a-C:H:W) coatings were developed [161,162]. films were created. After reliably solving the adhesion problems the
Hydrogen free carbon and metal containing carbon coatings for tri- number of industrial applications increased continuously. Today a
bological applications were developed by Teer and co-workers using in- large number of applications, especially in the automotive industry,
dustrial CFUBM machines [163,164]. The tribological properties of are established and, for example for diesel injection systems, DLC coat-
carbon/chromium (C/Cr) multilayers depend considerably on the Cr ings are indispensable.
contents, and it was found that pure carbon coatings have good wear Future developments of coating deposition processes should take
properties at low loads while coatings with optimum Cr contents have into consideration the following points:
excellent tribological performances at high loads [164].
Increasing tribological stresses of engineering components required – Reliable homogeneous deposition of DLC-based coatings on three-
a replacement of Me-DLC by the harder and more wear resistant metal dimensional parts in industrial scale deposition machines,
free pure a-C:H coatings [1,162]. At the end of the 20th and beginning of – Reduction of the costs of ownership for the coatings by optimization
the 21st century the adhesion problems of pure a-C:H coatings could be of the deposition processes, which is connected with higher deposi-
solved reliably for industrial scale deposition processes by preparation tion rates and reliable processes at retaining coating quality,
of interlayer systems based on metal and metal nitride layers (see – Optimization of DLC based coatings with respect to operation under
e.g. [165–167]). In industrial practice, hybrid deposition processes be- lubricated conditions, aiming to a further reduction of friction
came established. These hybrid processes consisted of a sputter deposi- effects,
tion of the interlayer system followed by a PECVD process (m.f. glow – And increase in the thermal stability of DLC coatings by optimization
discharge) for deposition of the a-C:H top layer (see e.g. [167,162]). of modified coating types including multilayer coatings designs.
For industrial applications in large scale deposition machines, m.f. pro-
cesses will be preferred because adequate bias voltages cannot be gen-
erated at large substrate surface areas [165]. References
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