Surface & Coatings Technology 242 (2014) 214–225

Contents lists available at ScienceDirect

Surface & Coatings Technology

j ourna l homepage: www.e lsev ie r .com/ locate /sur fcoat

History of diamond-like carbon films — From first experiments toworldwide applications

Klaus Bewilogua a,⁎, Dieter Hofmann b

a Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germanyb AMG Coating Technologies, Hanau, Germany

⁎ Corresponding author. Tel.: +49 531 2155 642; fax:E-mail address: klaus.bewilogua@ist.fraunhofer.de (K.

0257-8972/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.surfcoat.2014.01.031

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 December 2013Accepted in revised form 20 January 2014Available online 25 January 2014

Keywords:Diamond-like carbonHistoryIndustrial application

Diamond-like carbon (DLC) films combine several excellent properties like high hardness, low friction coeffi-cients and chemical inertness. The DLC coating material can be further classified in two main groups, the hydro-genated amorphous carbon (a-C:H, ta-C:H) and the hydrogen free amorphous carbon (a-C, ta-C). By adding otherelements likemetals (a-C:H:Me) or non-metal elements like silicon, oxygen, fluorine or others (a-C:H:X), severalmodifications of the properties can be adjusted according to application requirements. First reports on hardamorphous carbon films were published in the 1950s and about 20 years later there began worldwide intensiveresearch activities on DLC. In the following years the number of publications increased continuously and the im-portance for industrial applications becamemore andmore evident. Several deposition techniques were appliedto prepare a-C:H, ta-C, metal containing a-C:H:Me and non-metal containing a-C:H:X coatings. In parallel thestructure and deposition mechanisms of DLC coatings were extensively studied. An essential obstacle for abroad industrial application was the high compressive stress level in a-C:H films causing delamination and lim-iting the film thicknesses.Withmetal based intermediate layer systemsmost adhesion problems could be solvedsatisfactorily and thus from themid-1990s the pre-conditions for a broad application especially in the automotiveindustry 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2142. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

2.1. Initial developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2152.2. Superhard hydrogen free ta-C and hydrogen containing ta-C:H coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2172.3. Structure and composition of a-C:H and ta-C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2182.4. Growth mechanisms of a-C:H and ta-C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2202.5. Metal containing diamond-like carbon coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2202.6. Modified of a-C:H and a-C films — incorporation of additional elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

3. Transfer to industrial applications and mass production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2234. Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

1. Introduction

Amorphous diamond-like carbon (DLC) coatings are known for theiroutstanding properties such as low friction coefficients, high hardnessand wear resistance, chemical inertness, optical transparency in the IRspectral range and low electrical conductivities. These properties and

+49 531 2155 900.Bewilogua).

ghts reserved.

their combinations are very promising for a variety of technicalapplications.

TodayDLCfilms are usedworldwide on a large industrial scale to im-prove the performance of tools and of components, especially for auto-motive applications [1–3] with more than 100 million coated parts peryear and a market volume of several €100 million. Other applicationfields with large market shares are protective DLC films (a few nmthick) for high storage density hard disks and recording heads [4,5] aswell as razor blades where thin DLC films improve the performance ofsharp cutting edges. As an example the well-known Mach 3 razor

215K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

from Gillette should be mentioned. DLC films are also applied for wearprotection of cutting and forming tools [6,7], for biomedical compo-nents [8,9], as IR antireflection films (Section 2.1), components of thinfilm sensors [10], barrier films in PET bottles [9], or as a dielectric layer[11]. For a summary of themost important applications of DLC coatingsa recent paper by Robertson [12] is recommended.

Currently the term DLC describes a large variety of carbon basedcoating modifications. In order to devise a classification regarding theuse of different names for DLC coatings, especially in Germany, Japanand Korea attempts were made to describe the different modificationsof DLC systematically. Here the German guideline VDI 2840 ‘Carbonfilms — Basic knowledge, film types and properties’ should be mentioned[13].

In this guide line it will be distinguished between the followingtypes of DLC coatings:

• Hydrogen free amorphous carbon films — a-C• Hydrogen free tetrahedral amorphous carbon films— ta-Cwith a highfraction of tetrahedral coordinated sp3 bonded carbon atoms

• Metal containing hydrogen free amorphous carbon films — a-C:Me,where the metal often is a carbide forming metal like titanium ortungsten

• Hydrogenated amorphous carbon films— a-C:H• Hydrogenated tetrahedral amorphous carbon films— ta-C:H• Metal containing hydrogenated amorphous carbon films — a-C:H:Me• Modified hydrogenated amorphous carbon films— a-C:H:X, where Xis related to such non-metal elements such as silicon, oxygen, nitro-gen, fluorine, and boron.

In order to obtain a comparison of DLC films with diamond andgraphite in Table 1 some typical properties of both these crystallinephases with ta-C and a-C:H are summarized.

All types of coatings mentioned, are usually prepared by physicalvapor deposition (PVD) methods like sputtering or arc evaporation orby plasma-enhanced chemical vapor deposition (PECVD) methods.PECVD techniques base on glow discharge processes applying hydrocar-bon gases like acetylene (C2H2) and negatively biased substrates work-ing at radio frequencies (r.f. — 13.56 MHz) [14] or mid-frequencies(m.f.— some 10 to some 100 kHz) [15]. Besides the outstanding coatingproperties of DLC it is an important aspect that these coatings can bedeposited at low substrate temperatures (b200 °C), e.g. on temperaturesensitive steel components.

The aim of the present paper is to discuss historical developments ofpreparation techniques, structuremodels, growthmechanisms, proper-ties and applications of different types of DLC coatings. Furthermore ex-amples for developments of industrial deposition processes and newapplication fields of DLC coatings will be discussed.

Table 1Comparison of typical properties of diamond, ta-C, a-C:H and graphite (references:[12,38,39,55,56,68,87,102,145]).

Diamond ta-C a-C:H Graphite

Crystal system Diamond cubic Amorphous Amorphous Hexagonal

Mass density/g/cm3 3.51 2.5–3.3 1.5–2.4 2.26sp3 Content/% 100 50–90 20–60 0Hydrogen content/ at.% 0 ~1 10–50 0Hardness/GPa 100 50–80 10–45 b5Friction coefficientsIn humid air 0.1 0.05–0.25 0.02–0.3 0.1–0.2In dry air 0.1 0.6 0.02–0.2 N0.6Band gap/eV 5.5 1–2.5 1–4 −0.04Electricalresistivity/Ω cm

1018 106–1010 104–1012 10−6–10−2

Thermal stabilityin air/°C

800 400–600 300–350 N500

2. History

2.1. Initial developments

Hard amorphous carbon films were first mentioned in 1953 bySchmellenmeier in a paper on the influence of an ionized acetylene(C2H2) atmosphere on surfaces of tungsten–cobalt alloys [16]. Themain objective of his investigation was to find out whether at relativelylow temperatures via a glow discharge in hydrocarbon atmospherestungsten carbide hard metal (so called “Widia”) surface layers can begenerated. As an additional observation the author found that blackand very hard amorphous films were deposited on the cathode of thed.c. glow discharge if the discharge currentwas not too high. In a secondpaper published in 1956 Schmellenmeier [17] reported that these someμm thick hard films consist of “structure-less” regions and, under specif-ic process conditions, of crystallites which were identified by X-ray dif-fraction as diamond.

Already in 1951 König and Helwig [18] investigated direct current(d.c.) glow discharge processes in a benzene (C6H6) atmosphere. How-ever, they analyzed only the films grown on the discharge-anode. Thesefilms were yellow and had a relatively low density (1.4 g/cm3). LaterHeisen [19,20], using nearly the same experimental set-up as Königand Helwig and also benzene as hydrocarbon precursor, observed onthe cathode of the discharge considerably higher deposition rates andhighermass densities of the deposited films than on the anode and con-cluded that the interaction cross sections of positive ionswith adsorbedgas molecules are much higher than those of electrons. FurthermoreHeisen [19] observed that the growth rates of the films depend consid-erably on the substrate geometry as well as that insulating substrateswere charged under ion bombardment and consequently the filmgrowth stops.

The next remarkable publication concerning hard amorphous carbonfilms was that of Aisenberg and Chabot [21] in 1971 where for the firsttime the term “diamond-like carbon”was used. These coatings were pre-pared by an ion beam deposition technique on room-temperature sub-strates. The ion beam consisted of carbon and argon ions generated in adischarge system using graphite as material for the active electrodes.Fig. 1 shows the scheme of this equipment. Positive carbon and gas ionswere extracted from the discharge region and deposited on a negativelybiased substrate and the ion energy could be adjusted by the substrate po-tential. The deposited films were intensively investigated and specifiede.g. as scratch resistant, electrically insulating, optically transparent andchemically resistant. The structure was described as partially crystallinewith lattice constants similar to those of diamond. Furthermore thepreparation of thin-film transistors using the insulating carbon filmswas described. In 1973 the same authors reported that the cutting perfor-mance of paper cutting blades could be markedly increased by applyingdiamond-like carbon films. Wear tests with the coated blades revealedthat these films “apparently reduce the coefficient of friction” [22].

In the 1970s several other papers on DLC were published. Spenceret al. [23] analyzed DLC films prepared by techniques similar to thoseof Aisenberg and Chabot [21]. Using X-ray diffraction and transmissionelectron microscopy (TEM) they found indications for small (size 5–10 nm) and large (up to 5 μm) crystallites. The observed diffractionreflections were assigned to the diamond phase.

Whitmell and Williamson [24] prepared hard and insulating filmswith thicknesses up to 4 μmon differentmetallic substrates using a sim-ilar d.c. based deposition technique like that of Schmellenmeier andHeisen but with a gas mixture of ethylene (C2H4) with 5% argon. Overthe substrates a domed aperture was placed (Fig. 2). Although thedeposited filmswere insulating and a positive charge should be expect-ed, surprisingly high thicknesses could be achieved. Holland [25]explained this assuming that secondary electrons generated at theedges of the domed aperture compensate for the positive charges.Later such a compensation effectwas verified for a d.c. based ion platinga-C:H deposition process on insulating glass substrates [26].

Fig. 1. Scheme of ion-beam deposition system.Adapted according to [21].

216 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

Since the mid-1970s several research groups have published resultsof a-C:H deposition experiments. Especially the work of Holland andOjha [27,25] can be regarded as a break through because they used aradio frequency (r.f. — 13.56 MHz) glow discharge in butane (C4H10)gas to overcome problems with deposition of insulating a-C:H films(for a process-scheme see Fig. 3). Hard and insulating films could beprepared even on dielectric substrates such as glass. Furthermore itseems to be important that similar equipment was available in manylaboratories and could be used simply to prepare DLC coatings. BesidesHolland and Ojha [25,27], Andersson, Berg and co-workers [28,29] usedthe r.f. glow discharge method to prepare a-C:H films and they consid-ered aspects concerning the growth mechanism of a-C:H films like theinitial phases of film growth on different substrate materials [28]. Fur-thermore, different hydrocarbon precursors (methane CH4, ethaneC2H6, propane C3H8, n-butane C4H10 and iso-butane C4H10) were com-pared [29]. The highest deposition rates were observed for both bu-tanes, the lowest one for methane. The authors stated that there wasno simple relationship between growth rate and precursor structure in-dicating complex ionization and dissociation processes of the hydrocar-bon molecules.

In the early and mid-1980s profound work on a-C:H films preparedby r.f. glowdischargewas done. Bubenzer et al. [30] from the FraunhoferInstitute for Applied Solid State Physics in Freiburg, Germany, describedthe negative r.f. self-bias VB of the substrate and the hydrocarbon gaspressure P as themost important parameters controlling the depositionprocess. E.g. the film density ranged between 1.5 and 1.8 g/cm3 and

Fig. 2. Scheme of d.c. glow discharge deposition technique.Adapted according to [24].

depended linearly on the term VB · P−0.5 which on the other handwas found to be proportional to the average energy of the film formingenergy. The objective of the research group in Freiburg was to developa-C:H as transparent wear resistant anti-reflection coatings on infrared(IR) optical components. Ongermaniumand silicon substrates (refractingindex n ≈ 4) a-C:H films with n ≈ 2 would be an ideal transparentantireflection layer [30–32]. Catherine and Couderc [33] compared a-C:H glow discharge deposition processes and growth kinetics both forr.f. (13.56 MHz) and m.f. (50 kHz) plasma excitation. The film propertieswere found to be related to the product VB · P−0.5 for the r.f. process (like-wise Bubenzer et al. [30]) and to the product J · P−0.5 for them.f. processwhere J is the mean current density.

In the period since the mid-1970s also other techniques for a-C:Hdeposition were developed and the films were characterized in detail.Weissmantel and co-workers [34,35] reported on two differentmethods for the preparation of DLC coatings. With the so called dualbeam technique, a carbon target was sputtered with argon ions andthe carbon film growing on the substrate was simultaneouslybombarded by a second ion source operated at about 1 kV with argonand methane (Fig. 4). The deposited carbon films were hard and thestructure was characterized as amorphous with crystallites embeddedin the substrate regions exposed to the highest ion current densities[35].

On the other hand in the Weissmantel group a-C:H coatings wereprepared applying a completely d.c. based “ion plating” process wherehydrocarbon ions were generated in a benzene atmosphere by hot

Fig. 3. Scheme of r.f. glow discharge deposition technique.Adapted according to [25,27].

Fig. 4. Scheme of dual beam deposition technique.Adapted according to [34,35].

217K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

cathode ionization. The a-C:H filmswere grown by acceleration of theseions to a negatively d.c. biased (up to 800 V) substrate [35,36]. Fig. 5shows the scheme of this deposition method. These films were hard,partially optically transparent and electrically insulating. The structurewas described as a mixture of nanocrystalline components consistingof graphite-like and diamond-like elements [36].

In the early 1990s Martinu et al. [37] increased the effectiveness ofthe r.f. glowdischarge processes by applying a simultaneously operatingmicrowave (MW — 2.45 GHz) radiation. With this combined r.f.–MWprocess increased ion fluxes on the substrate, higher deposition rates,lower hydrogen contents and increased hardness of the deposited a-C:H films were achieved.

Enke, Dimigen and co-workers reported in the early 1980s on verylow friction coefficients of hard diamond-like carbon films againststeel counterparts [38–40]. The investigated a-C:H films were preparedby a r.f. glow discharge technique similar to that of Holland and Ojha[27]. In contrast to graphite, thesefilms had extremely low friction coef-ficients (μ) at low humidity (≈0.01) and under “normal” atmospherethe μ values remained rather low (b0.2). Memming et al. [41] measuredfriction coefficients of the same type of a-C:Hfilms against steel in ultra-high vacuum (μ ≈ 0.02), humid air or nitrogen (0.25) as well as in drynitrogen (0.02) and dry oxygen (≥0.6) and explained the differenceswith different developments of the transfer layers which is generatedbetween the sliding partners.

Fig. 5. Scheme of d.c. ion plating deposition technique.Adapted according to [35,36].

Due to the combination of low friction coefficients with high hard-ness and wear resistance, a-C:H films became very interesting for solv-ingdiverse engineeringproblems. Consequently the number of researchgroups working on DLC began to increase considerably in the followingyears. Fig. 6 shows, referring to the database Scopus, how the number ofpublications from a small number up to the 1980s increased continu-ously and reached maximum output after the year 2000. In the periodfrom 1990 to 2000 DLC coatings becamemore andmore interesting re-garding industrial applications, especially for the automotive industrywhere they are well established today.

Nowadays tribological and mechanical properties of DLC coatings areof outstanding interest for industrial applications especially in the auto-motive industry. First references to the potential of DLC films were, asalready mentioned, given by Aisenberg and Chabot [22] and by Enkeet al. [39,40]. As far back as the 1980s, more detailed studies followed,discussing the dependence of friction coefficients on deposition condi-tions [42] and humidity or surrounding gas atmosphere [41,43,44]. Fur-thermore, in the following years, a lot of investigations on hardness,wear and friction were presented whereas especially in the 1990s andlater, the number of research papers on these topics continuously in-creased (see e.g. the historical overview of Donnet and Erdemir and fol-lowing contributions on Tribology of Diamond-like Carbon Films in [45]).

In the late 1970s and in the 1980s, several papers concerning data onother than mechanical or tribological properties of a-C:H films werepublished. In this period, electrical conductivities as well as opticalconstants (refractive indices and extinction coefficients) of a-C:H filmsprepared by r.f. glow discharge processes were investigated in depthby Anderson [46] and Meyerson and Smith [47,48]. As well, already in1980, Meyerson and Smith [49] investigated the electrical conductivityof doped (B and P) a-C:H films and found conductivity gains of 5 ordersof magnitude, for example for a coating deposited at 250 °C from10−12 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 theearly 1980s by research groups from Sydney, Australia [50–52]. Thecharacteristic 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 “Colorchart 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) consideringinterference colors.

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

Superhard hydrogen free ta-C films are characterized by a high frac-tion of tetrahedrally bonded (sp3) carbon atoms. To prepare such films,

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).

218 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

carbon specieswith energies in the range of 100 eV are needed (for a re-view see e.g. [54]). The most appropriate method to realize such condi-tions is the cathodic arc evaporation. The pioneer work in this field wasdone by Strel'nitskij and Aksenov and co-workers in Kharkov in the for-mer Soviet Union (today Ukraine). In 1978 Strel'nitskij et al. [55,56] re-ported on extremely hard and electrically insulating (ρ ≈ 1010 Ω cm)diamond-like carbon films deposited by arc evaporation using a puregraphite cathode. Refractive indices n N 2.2 were another indicationfor diamond-like properties (n of diamond: 2.4). The substrates werepowered by r.f. and d.c. voltages. Themaximummeasuredfilmhardnesswas stated as near to or even higher than that of natural diamond. Thefilms were nanocrystalline but a clear assignment of the X-ray diffrac-tion peaks to known carbon phases was not possible.

An obvious drawback of the used arc technique was the appearanceof macro particles in the growing films. To overcome this, Aksenov et al.[57,58] proposed an arc system equipped with a curvilinear separatingfilter allowing a drastic reduction of themacro particles. The carbon spe-cies arriving at the substrate were completely ionized, predominantlysingle charged [58]. The diamond-like-carbon coatings prepared bythis technique had highly pure surfaces and were still extremely hard[58].

A few years later several papers concerning cathodic arc depositionof superhard hydrogen freeDLCfilmswere published.Martin,McKenzieand co-workers [59,60] used a filtered arc system similar to that intro-duced by Aksenov and co-workers (see [58]) and discussed the atomicstructure of the deposited tetrahedral amorphous carbon (ta-C) filmsin detail, taking into account the data of electron energy loss spectrosco-py (EELS) and electron diffraction analyses. In the very comprehensivepaper of McKenzie and co-workers [60], in addition to the atomic struc-ture, the data on the compressive stress (up to 8 GPa) as a function ofthe ion energy as well as a model for the stress development was pre-sented. Furthermore optical properties and possible electronic devicesbuilt with ta-C films were discussed.

Another concept for the arc deposition of ta-C films, the so calledlaser–arc technique, was proposed in the early 1990s by a researchgroup in Dresden, Germany, led by Pompe and Scheibe [61,62]. Theyused short pulses (b1 ms) of a Nd:YAG laser to ignite a vacuum arcrealizing a time and position controlled arc of about 50 ms duration.The deposited carbon films were amorphous, very hard and hadrefractive indices between 2.05 and 2.5. An additional advantage ofthe laser–arc technique is that by moving the target against the laserbeam the material to be deposited can be selected. Thus multilayer

films can be prepared [61]. Later industrially utilizable laser–arctechniques to deposit ta-C were developed [63].

Other PVD techniqueswere revealed to be principally appropriate toprepare carbon films with high sp3 contents. Here the pulsed laser de-position (PLD — for an overview see [64] and references therein), theunbalancedmagnetron sputtering (UBM) [65,66], should bementioned.

Weiler et al. [67] reported on tetrahedral hydrogenatedDLCfilms (ta-C:H) prepared from a plasma beam source based on a r.f. (13.56 MHz)discharge at relatively low working gas pressures (0.05 Pa) and amagnetic confinement allowing generation of a highly ionized plasmabeam. The prepared films had a sp3 fraction up to 0.75, a maximummass density of 2.9 g/cm3 and hydrogen contents between 22 and28 at.%. High hardness (N60 GPa) but also high compressive stress(8.5 GPa) corresponded to the high sp3 contents.

Although the properties of tetrahedrally coordinated DLC films, likehigh hardness and wear resistance and low friction coefficients underambient conditions and under oil lubrication [68], are very attractive,a broad breakthrough of ta-C deposition techniques in the industrialmass production is still outstanding.

2.3. Structure and composition of a-C:H and ta-C

In the abovementioned first publications on diamond-like carbonfilms their structure was specified as amorphous with crystallineregions [17] or partially crystalline where the crystalline phasecorresponded to diamond [21,23]. Anderson [46] found that highly in-sulating a-C:H films prepared by r.f. glow discharge are amorphousand proposed a structuremodel consisting of three- and four-fold coor-dinated carbon atomswhere the latter prevent the formation of extend-ed graphite-like components.

In the 1980s, detailed investigations on the structure of a-C:H werepresented. Craig and Harding [52] analyzed composition and structureof magnetron sputtered a-C:H films (see Section 2.1.). The hydrogencontent, determined by pyrolysis, was in all as-deposited films higherthan 32 at.% and the oxygen content (from residual gas) higher than6 at.%. The proposed structure model consisted of a random networkof tetrahedral coordinated carbon atoms modified by carbon–carbondouble bonds and carbon–hydrogen bonds. This model was consistentwith the low densities (1.0 to 1.6 g/cm3) and high resistivities (N107

Ω cm). McKenzie et al. [50,51,69] developed an a-C:H model wherethreefold coordinated carbon clusters are embedded in an insulatingpolymer-like hydrocarbon material containing many methyl groups.

Fig. 7. Relation between sp3/sp2 ratio and hydrogen concentration in a-C:H films, calculat-ed according to [76], experimental data, derived from Kaplan et al. [86], Robertson [12],and Hofmann et al. [145].

219K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

Assuming such a structure, the results of annealing experiments whichled 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 anddeveloped similar structural concepts for as-grownas 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, Weissmantelet al. [71–73] revealed by electron diffraction and electron energy lossspectroscopy studies a high similarity of ion plated a-C:H films [35]and of evaporated a-C films. The best fit for the experimentally derivedelectron diffraction intensity functions was obtained assuming smallclusters (b1 nm) consisting of puckered hexagonal and of pentagonalrings. To explain the remarkable differences of the a-C:H and a-Cproperties (hardness, electrical conductivity) for a-C:H, strong sp3 con-taining cross-linkages between the clusters were assumed which in a-Cdo not exist. A corresponding hand built model was presented byWeissmantel [73].

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 thegaps of hydrocarbon ring structures indicated that the gap varies rough-ly inversely to the cluster size. Considering this relation, evaporated a-Cfilms dominantly consist of disordered graphite clusters with dimen-sions up to 1.5 nm. On the other hand a-C:Hfilms contain a high amountof sp2 sites in ring structures and the observed gap sizes are in accor-dance with the existence of clusters consisting of four or more aromaticrings.

Angus [75] proposed an empirical categorization of DLC films bytheir gram atom number density ρN and their hydrogen fraction xHwhere ρN is the quotient of the mass density ρm and molar mass:ρN = ρm / (xH + 12(1− xH)) with the atomic masses 1 for hydrogenand 12 for carbon. In Table 2 some examples for ρN and cH data aresummarized. Films with ρN N 0.2 g atom/cm3 were called dense carbo-naceous (hard) films or with significant hydrogen amounts densehydrocarbon (soft) films.

In 1988 Angus and Jansen [76] applied random covalent networkconcepts to dense hydrocarbon films. The theories suitable to describerandom covalent networks were developed some years ago by Phillips[77] and Döhler et al. [78]. Accordingly a random covalent network iscompletely constrained if the number of constraints per atom is equalto the number of degrees of freedom per atom. Applied to hydrocarbonsystems containing sp2 and sp3 bonded carbon and hydrogen atoms, forcompletely constrained structures, hydrogen contents between 17 and62 at.% were derived [76]. In Fig. 7 the described relation (sp3/sp2 ratiovs. hydrogen fraction xH) and an extended set of experimental dataare shown. This mode of representation allows a very clear distinctionbetween soft (high hydrogen content) and hard (low hydrogen con-tent) coatings.

Fink et al. [79] analyzed as-deposited and annealed a-C:H films byhigh-resolution EELS. The films were prepared by r.f. glow dischargein benzene vapor. The content of bonded hydrogen was determined

Table 2Examples for mass densities, hydrogen contents, number densities (calculated accordingto reference [75]) and hard–soft classification of diamond, graphite diamond-like carbonfilms and a polymer.

Mass density(g/cm3)

Hydrogencontent (at.%)

Number density(1/cm3)

Hard/soft

Diamond 3.51 0 0.29 SuperhardGraphite 2.25 0 0.19 Softta-C 3.2 1 0.27 Superharda-C:H (1) 2.3 11 0.21 Harda-C:H (2) 2.0 17 0.20 Harda-C:H (3) 1.4 60 0.26 SoftPolystyrene(C8H8)

1.05 50 0.16 Soft polymer

from infrared absorption spectra and the total hydrogen contentwas es-timated by an effusion technique. With increasing self-bias voltages thestructure changed from weakly bonded benzene-like rings to graphiticring structures. After annealing the number of sp2 bonded carbonatoms increased and above 600 °C hydrogen was released.

An interestingmodel for a-C:Hwas presented in 1990 by Tamor andWu [80] who proposed defected graphitic networks where hydrogenatoms saturate open bonds in vacancies of graphite clusters. Applyingthis model nearly the same lower (20 at.%) and upper (60 at.%) limitsof the hydrogen content like those from Angus and Jansen [76] wereestimated.

In 1993Möller and co-workers [81,82] described structure and com-position of different hydrocarbon films by a ternary phase diagramwhich assumed as the three phases in these films sp3 and sp2 hybridizedcarbon and hydrogen. This ternary phase diagram was afterward usedinmany papers concerning a-C:H films (e.g. [67,83]). Fig. 8 shows an ex-ample for such a phase diagram.

Beginning in the late 1980s, Australian research groups investigatedthe structure of arc deposited ta-C in detail using electron opticalmethods especially EELS [59,84,85] and electron diffraction [60,85]. Itwas proved that in these film sp3 bonded carbon dominates. In thelow energy part of the EELS spectra, the plasmon peak was found inthe range of the diamond peak (33 eV) and from electron diffraction,nearest neighbor distances (r1)and coordination numbers (n1) as wellas the bond angles (α) were determined as very near to the diamondvalues (diamond: r1 = 0.154 nm, n1 = 4 and α = 109.47°).

For structure characterization of DLC films beside electron diffrac-tion, EELS, IR spectroscopy [70] or nuclear magnetic resonance (NMR)[86] the Raman spectroscopy was an important characterization tool[87–89]. An essential advantage of the Raman spectroscopy is that noextensive sample preparation is necessary. Although the interpretationof 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].Modeling of a-C structures started in 1984 by Beeman et al. [91]

using hand models with several 100 carbon atoms and in 1989 byGalli et al. [92,93] using first-principle molecular-dynamic methods. Inthe 1990s several groups carried out extensive computer modeling ofpure a-C [94], ta-C [95,96] or both pure and hydrogenated amorphouscarbon [97–99] and compared the results with experimentally deter-mined data likemass densities, hydrogen contents or interference func-tions and radial distribution functions. The best points of concurrenceof experiment and model could be used to draw conclusions e.g. onthe contributions of sp3, sp2 and sp1 bonds (sp1: one-dimensional,acetylene-like), and made it possible to calculate electronic densitiesof states [95,97,98]. Fig. 9 shows an example for an a-C:H film modelwith a dominant sp2 share and 30 at.% hydrogen.

Fig. 8. Ternary phase diagram adapted according to and using data from [81–83,12,145].

220 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

For a survey of the different structure characterization methods ofamorphous and nanocrystalline carbon films a paper of Chu and Li[100] is recommended.

2.4. Growth mechanisms of a-C:H and ta-C

Already Heisen [19] (see Section 2.1.) presented first considerationson the growth of hydrocarbon films on discharge cathodes. Later, begin-ning in the 1980s, the roles of ions and radicals on the growth behaviorof a-C:H filmswere consideredmore in detail by Catherine and Couderc[33] and by Bubenzer et al. [30] and Koidl et al. [101] for r.f. based a-C:Hdeposition processes. Koidl et al. [101] investigated different processgases as precursors (benzene, cyclo-hexane, n-hexane, methane) andfound that the film properties are nearly independent of the usedprecursor gas if the bias voltage is high enough for an effective fragmen-tation of the hydrocarbon molecules. On the other hand, the findingthat the deposition rates depended clearly on the precursor gascorresponded to the results reported by Andersson et al. [29]. The depo-sition rates increased with increasing molecule masses. Furthermore,the ionization energy was seen as another rate-determining factor(see also [102]).

Fig. 9. Structure model of an a-C:H filmwith 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.Image: with kind permission of Thomas Frauenheim, Bremen Center of ComputationalMaterials Science.

In the 1990s, several theoretical studies on deposition mechanismsboth of hydrogen free ta-C and of hydrogen containing a-C:H filmswere published. Möller [103,104] from the Max-Planck Institute forPlasma Physics in Garching, Germany analyzed the elementary mecha-nisms of the growth of a-C:H films from low-pressuremethane plasmasand developed models which predicted the sp3/sp2 ratios for varyingion energies [103] and explained, amongst others, the dependence ofthe 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 thefollowing years several papers ondeposition experiments andmodelingof a-C:H film growth were published by the research group from theMax-Planck Institute for Plasma Physics in Garching, Germany, for ex-ample by von Keudell et al. [105] who discussed the processes in thenear surface growth zones and the mechanisms of hydrogen releasefrom growing a-C:H films under ion bombardment. Hydrogen ionshave a considerably higher penetration depth than carbon ions andform via recombination H2 molecules, which can desorb and thuslower the hydrogen content in the films. Additionally, such a mecha-nismexplains the experimentalfindings that the composition and prop-erties of a-C:H films prepared from very different precursors likemethane CH4 or benzene C6H6 are often rather similar.

For more details, a review of von Keudell [106] on “Formation ofpolymer-like hydrocarbon films from radical beams of methyl andatomic hydrogen” is recommended.

In 1989 and 1990 Lifshitz et al. [107,108] suggested a subplantationmodel to describe the ion-enhanced growth of carbon films wheresubplantation means the penetration of impinging hyperthermal spe-cies (1–1000 eV) into the top layers of the bombarded sample causingphase transformations. In the case of carbon this model explained thehigh sp3 contents assuming a considerable difference of the displace-ment energies Ed between graphite (25 eV) and diamond (80 eV) lead-ing to preferred displacement of the low-Ed and remaining high-Edcarbon.

Robertson [109] developed a similar model where ions enter thesubsurface region which will be densified. The experimentally deter-mined density maxima of ta-C film at ion energy around 100 eV wereinterpreted as a consequence of a densification due to the ion strikingthe surface and, at higher energies, to an energy dissipation accompa-nied by an annealing and density decrease. The impact of the ions canbe 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 filmgrowth 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 retardingindustrial applications of DLC as tribological coatings was their disap-pointing adhesion caused by relatively high compressive stresses inthe 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 ina hydrocarbon–argon atmosphere. Alternatively, instead of pure metaltargets, metal–carbon (metal carbide) targets could be utilized. SuchMe-DLC films with metal to carbon ratios in the range Me/C = 0.1–0.2were 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 slightlylower 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 processstep to prepare an adhesion improving metallic interlayer. Moreover,it was remarkable that the electrical conductivity ofMe-DLCwas severalorders of magnitude higher than that of a-C:H [113]. Later Benndorf

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].

221K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

et al. [114] developed a structure model which explained the high elec-trical conductivities even at relatively low metal contents. The authorsassumed that metal, not bonded in carbides, modifies the carbide sur-rounding a-C:H matrix.

The reactive sputter deposition technique was developed and ap-plied in the same period and even earlier by Australian scientists forthe preparation of solar absorbing surfaces. Ritchie and Harding [115]prepared Fe–C films by d.c. diode sputtering in an Ar–CH4 mixture.Wilson et al. [116] used a dual-post cathode sputter coater with two in-dependently controlled cathodes consisting of different materials oper-ating with acetylene so that graded metal–carbon films could beproduced.

Themost important factors for industrial applications of Me-DLC (ora-C:H:Me) coatings were the tribological properties, the good adhesionand especially thehigh potential for transferring the deposition process-es to industrial dimensions (see Section 3). Industrially relevant deposi-tion processes could not be realized with r.f. processes. Thereforetransferring the process to magnetron sputter techniques wasdemanded. Bergmann and Vogel [117] used a magnetron sputter ma-chine equipped with four planar targets to prepare W-DLC coatingsfrom a tungsten carbide target as well as Ti-DLC and Cr-DLC coatingsfrom the corresponding metal targets. Deposition rate, hardness andstress of the coatings depended on the reactive gas (acetylene) flowduring sputter deposition and these properties were clearly differentfor the three investigated metals.

A summary of the situation at the end of the 1980s concerning depo-sition processes, structure and composition as well as optical, electrical,mechanical and tribological properties of Me-DLC films was presentedby Klages and Memming [44].

Already in 1980, Richie [118] investigated the structure of Ti-DLCfilms prepared by reactive d.c. magnetron sputtering applying anargon–methane mixture and a pure titanium target and reported on ti-tanium carbide particles of 5–10 nm diameter embedded in an amor-phous carbon matrix. In the 1990s some very detailed studies on Me-DLC growth processes and the structure were published. Wang et al.[119,120] analyzed the structure and composition of Ti-, Ta- and W-DLC films prepared by reactive r.f. sputtering and measured therebythe hydrogen contents by elastic recoil detection (ERD). Sjöström et al.[121] investigated the structure of Mo-DLC and W-DLC films by high-resolution transmission electron microscopy, X-ray diffraction andAuger electron spectroscopy. In the amorphous a-C:Hmatrix small crys-tallites with dimensions of few nmwere embedded. In the case of mo-lybdenum, metallic bcc Mo clusters (1–4 nm) and of tungsten 1 nmclusters, probably consisting of tungsten carbide, were identified. Acomparative study on the structure and cluster distribution in Me-DLCfilms for carbide forming metals (W, Fe) and non-carbide formingmetals (Au, Pt) was published by Schiffmann et al. [122] in 1999.

To control reactive magnetron sputter deposition processes and toprevent excessive target poisoning a plasma emission monitor couldbe used [123,124]. Using this control tool it was revealed that themain contribution to film growth originates from target near discharge

processes and that the contribution of direct plasma polymerization atthe substrates is small [124]. For reactive r.f. sputter processes thesame two contributions to the a-C:H:Me film growth were revealedfor Ag-DLC films [125].

Me-DLC coating preparation using magnetron sputter machineswith rotating substrates caused a columnarmorphologywith character-isticmultilayer structures consisting of oscillatingmetal rich and carbonrich single layers coatings [126–130]. These oscillations could be ex-plained by periodical changes of the substrate position compared tothe targets with a maximum metal content deposited directly in frontof the metal target. Fig. 11 shows a scheme of such a multilayer Me-DLC coating with a columnar structure.

Me-DLC coatings consist of an a-C:Hmatrixwith embedded carbideswhich commonly are even harder than a-C:H [130]. Therefore it issomewhat surprising that Me-DLC coatings are generally softer and ex-hibit higher wear rates than metal free a-C:H films [126,131]. An expla-nation for this observation is that the a-C:H matrix of Me-DLC hasweaker cross-linkages, higher hydrogen contents and therefore alower mechanical stability [131].

The properties of a-C:H:Me coatings depend considerably on themetal content which can be simply adjusted by the reactive gas contentin the sputter gas. Fig. 12 illustrates typical dependencies for hardnessand abrasivewear (a) and for the friction coefficients of tungsten and ti-tanium containing a-C:H:Me coatings (b). The pronounced hardnessmaxima (Fig. 12 a) can be interpreted as an effect of a MeC/a-C:Hnanocomposite structure consisting of metal carbide nanocrystalssurrounded by an amorphous carbon binder phase [132,130].

2.6. Modified of a-C:H and a-C films— incorporation of additional elements

In this section the so-called modified or doped DLC films, accordingto the VDI 2840 guideline [13] denoted as a-C:H:X where X is related tonon-metal elements, will be considered.

In themid-1970s and 1980s several authors reported on silicon con-taining a-C:H:Si films which were prepared by glow discharge decom-position of silane (SiH4) and hydrocarbon gases (C2H4 [133], CH4

[134,135]) as well as by sputter techniques [135,136]. McKenzie et al.[136] applied the d.c. magnetron technique to prepare a-SixC(1−x)Hy

films. The same magnetron sputter technique had already been previ-ously successfully used for the deposition of pure a-C:H films [50–52](see also Section 2.1.). Theworking gas for the a-SixC(1−x)Hy depositionwas a silane–acetylene–argon mixture. Because the motivation of thisresearch was to develop low emittance solar selective coatings on cop-per substrates, the optical propertieswere studied in detail. It is remark-able that, beside single layer coatings, also a-C:H/a-C:Si multilayercoatings were prepared by varying the parameter x [136].

In the early 1990s Japanese research groups investigated friction co-efficients (μ) of silicon containing a-C:H:Si coatings prepared byplasma-enhanced CVD methods. Oguri and Arai [137,138] measured μvalues using a ball-on disk tester under un-lubricated conditions atroom temperature in humid and dry atmospheres. In both atmospheres

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

222 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

for films with Si/C ratios between 0.15 and 0.4 the friction coefficientswere extremely low (b0.05) and thereby clearly lower than those ofpure a-C:H coatings in humid air (0.12–0.2). To explain the low frictioncoefficients in humid air it was assumed that fine silica particles formedduring the sliding process will be surrounded by a water layer and thesilica-sol acts as a liquid lubricant [139]. Miyake [140] found a similarfriction behavior under vacuum conditions. As-deposited films andfilms annealed at 400 °C exhibited the extremely low μ values (marked-ly lower than 0.1) for a Si concentration range around 10%. After anneal-ing at 600 °C the friction coefficients increased drastically (N0.2). Acomparison of a-C:H:Si, a-C:H:Ge and a-C:H:Ti revealed that obviouslyonly a silicon incorporation leads to a drastic reduction of the friction co-efficients [141]. In the following years many details on structure andproperties of silicon containing a-C:H:Si and other doped coatings, espe-cially with respect to their tribological properties, were reported (for anoverview see [142] and references therein).

Another remarkable property of a-C:H:Si is the clearly higher ther-mal stability compared to that of pure a-C:H. For a-C:H:Si films preparedby r.f. PECVD the temperature at which structural changes started couldbe considerably increased by adding silicon to a-C:H [143,144]. It wasassumed that higher silicon contents lead to more sp3 bonded carbonand thus the carbon network will be stabilized against graphitization[144]. Recently also for sputter deposited a-C:H:Si a clearly higher ther-mal stability compared to that of a-C:H was reported by Hofmann et al.[145]. Both hardness and friction coefficients remained nearly constanteven after annealing at 500 °C in air whereas a-C:H coatings alreadyfailed at temperatures below 400 °C.

Fig. 12. Typical dependencies of hardness, wear (a) and friction coefficAdapted according to [130], hardness of a-C:H:Ti from [132].

In the mid-1990s it was discovered that incorporation of non-metalelements into the a-C:H can alter the surface energies of a-C:H coatingsto lower (X: F, Si, Si+O) and also to higher (B, N, O) values [146]. Thus itbecame possible to combine the high mechanical stability of diamond-like carbon with low surface energies comparable with those of thewell-known hydrophobic material polytetrafluorethylene (PTFE —

surface energy about 19 mN/m, hardness b1 GPa). Grischke et al.[147,148] reported on low surface energies of modified a-C:H:Si(31 mN/m, 10–15 GPa), a-C:H:Si:O (24 mN/m, 7–10 GPa) and a-C:H:F(20 mN/m, 2 GPa) coatings. To realize these modifications in r.f.PECVD deposition processes the precursors TMS (Si(CH3)4), HMDSO(O–Si2(CH3)6) and C2F4 were used. For the corresponding pure a-C:Hcoatings the surface energy was 41 mN/m and the hardness of20–30 GPa. Potential applications of this type of a-C:H:X films weredemonstrated for example for heat exchanger surfaces [149,150].

Also hydrogen freemodified a-C:X coatings were found to be hydro-phobic. Schulz et al. [151] investigated a-C:F and a-C:Al films preparedby laser–arc technology (see Section 2.2.). In both cases not only thesurface energy, but also the Young's modulus decreasedwith increasingX-content.

Also in the mid-1990s Meneve et al. [152,153] reported on a-C:H/a-Si1 − x Cx:Hmultilayers prepared in a r.f. PECVD process usingmethaneand alternatively added silane as process gas. Under optimum processconditions remarkable properties like low friction coefficients of a-C:H:Si combined with low wear rates of pure a-C:H could be achieved.The high potential of DLC basedmultilayerswas confirmed by later pub-lications in the 2000s [154–156].

ients (b) on metal/carbon ratios, shown for a-C:H:W and a-C:H:Ti.

223K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

3. Transfer to industrial applications and mass production

In the late 1980s and early 1990s Me-DLC processes were trans-ferred to large scale d.c. magnetron sputter machines, mostly equippedwith 4 targets, and became applicable for industrial applications[157,158]. To prepare hard and wear resistant Me-DLC coatings highion current densities at the rotating substrates were necessary. Thiswas realized by applying special magnetic field arrangements like theso called Plasma Booster [157,159] or closed field unbalanced magne-tron (CFUBM) arrangements [158,160]. Furthermore, multi-chambersystems allowing in-line processes to prepare tungsten containing DLC(W-DLC or a-C:H:W) coatings were developed [161,162].

Hydrogen free carbon and metal containing carbon coatings for tri-bological applicationswere developed by Teer and co-workers using in-dustrial CFUBM machines [163,164]. The tribological properties ofcarbon/chromium (C/Cr) multilayers depend considerably on the Crcontents, and it was found that pure carbon coatings have good wearproperties at low loads while coatings with optimum Cr contents haveexcellent tribological performances at high loads [164].

Increasing tribological stresses of engineering components requireda replacement of Me-DLC by the harder and more wear resistant metalfree pure a-C:H coatings [1,162]. At the end of the 20th and beginning ofthe 21st century the adhesion problems of pure a-C:H coatings could besolved reliably for industrial scale deposition processes by preparationof interlayer systems based on metal and metal nitride layers (seee.g. [165–167]). In industrial practice, hybrid deposition processes be-came established. These hybrid processes consisted of a sputter deposi-tion of the interlayer system followed by a PECVD process (m.f. glowdischarge) for deposition of the a-C:H top layer (see e.g. [167,162]).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].

Alternatively the a-C:H top layer can be deposited by a reactivemag-netron sputter process using graphite targets and acetylene as reactivegas [145,168].

Thus the a-C:H coatings could complete the spectrum of availableDLC coatings especially in the application area of highly loaded compo-nents where a-C:H:Me coatings are no longer sufficient. On the otherhand, the mentioned complex interlayer systems are often too thick(clearly more than 1 μm) and their preparation is not compatible witheconomic deposition processes. In fact in many cases for industrialapplications of DLC coatings interlayers thinner than 1 μm are required.

Another approach to realize a high adhesion level is based on theHIPIMS (high power impulse magnetron sputtering) [168,169]. Highenergy metal ions were used to etch the substrates before starting thedeposition process and the first part of the interlayer (tungsten carbideWC) was deposited using HIPIMS.

Arc technologies for ta-C preparation on an industrial scale are stillnot widely established. However, now laser–arc modules which canbe added to industrial coating machines are available [170].

In the late 1990s [171] and intensively in the 2000s the tribologicalbehavior of DLC coatings under lubricated conditions was investigatedbecause such conditions are found in industrial applications of coatedcomponents. It became obvious that friction and wear sensitively de-pend on the system coating — lubricant- (oil + additive) counterpart[172] and that modifications of pure carbon films can lead to superiortribological properties as shown for chromium containing a-C:H films[173,174]. Kano [175] tested a-C:H and ta-C coatings in two different lu-bricants and detected at boundary lubrication a maximum friction re-duction for ta-C films operating in an ester containing oil. For anoverview considering the type of DLC coating, the influence of additivesand contact conditions can be seen in [172].

Examples for the industrial exploitation especially of a-C:H and a-C:H:Me coatings in the automotive industry (diesel injection systems, tappets,piston pins and others) have been reported many times [1,162,176,177].Today nearly all high pressure diesel injection systems are coatedwith a-

C:H and other components like tappets and piston pins will be routinelycoated with a continuously increasing proportion [2].

4. Summary and outlook

The first experimental evidence of DLC coatings was reported about60 years ago. From the mid-1970s the research activities on DLC coat-ings continuously increased until the end of the 1990s and thereafterremained on a high level. During the course of this development, newdeposition methods like reactive sputtering and cathodic arc processesas well as modifications of diamond-like amorphous carbon-basedfilms were created. After reliably solving the adhesion problems thenumber of industrial applications increased continuously. Today alarge number of applications, especially in the automotive industry,are established and, for example for diesel injection systems, DLC coat-ings are indispensable.

Future developments of coating deposition processes should takeinto consideration the following points:

– Reliable homogeneous deposition of DLC-based coatings on three-dimensional parts in industrial scale deposition machines,

– Reduction of the costs of ownership for the coatings by optimizationof the deposition processes, which is connected with higher deposi-tion rates and reliable processes at retaining coating quality,

– Optimization of DLC based coatings with respect to operation underlubricated conditions, aiming to a further reduction of frictioneffects,

– And increase in the thermal stability of DLC coatings by optimizationof modified coating types including multilayer coatings designs.

References

[1] C.P.O. Treutler, Surf. Coat. Technol. 200 (2005) 1969–1975.[2] G.J. van der Kolk, in: C. Donnet, A. Erdemir (Eds.), Tribology of Diamond-like Car-

bon Films, Springer Science + Business Media, LLC, New York, 2008, pp. 484–493.[3] A. Matthews, S.S. Eskildsen, Diamond Relat. Mater. 3 (1994) 902–911.[4] P. Goglia, J. Berkowitz, J. Hoehn, A. Xidis, L. Stover, Diamond Relat. Mater. 10 (2001)

271–277.[5] A. Ferrari, Surf. Coat. Technol. 180–181 (2004) 190–206.[6] H. Fukui, J. Okida, N. Omori, H. Moriguchi, K. Tsuda, Surf. Coat. Technol. 187 (2004)

70–76.[7] M. Weber, K. Bewilogua, I. Bialuch, M. Keunecke, H. Thomsen, R. Wittorf, 49th An-

nual Technical Conference Proceedings of the Society of Vacuum Coaters, 2006,pp. 529–537.

[8] R. Hauert, Tribol. Int. 37 (2004) 991–1003.[9] A. Shirakura, M. Nakaya, Y. Koga, H. Kodama, T. Hasebe, T. Suzuki, Thin Solid Films

494 (2006) 84–91.[10] S. Biehl, S. Staufenbiel, F. Hauschild, A. Albert, Microsyst. Technol. 16 (2010)

879–883.[11] A. Grill, Diamond Relat. Mater. 10 (2001) 234–239.[12] J. Robertson, Phys. Status Solidi A 205 (2008) 2233–2244.[13] German Guideline VDI 2840 ‘Carbon films— Basic knowledge, film types and prop-

erties’, Beuth-Verlag, 2005. (available as a German/English version).[14] S. Peter, K. Graupner, D. Grambole, F. Richter, J. Appl. Phys. 102 (2007)(053304-1-18).[15] A. Hieke, K. Bewilogua, K. Taube, I. Bialuch, K. Weigel, 43rd Annual Technical Con-

ference Proceedings of the Society of Vacuum Coaters, 2000, pp. 301–304.[16] H. Schmellenmeier, Exp. Tech. Phys. 1 (1953) 49–68.[17] H. Schmellenmeier, Z. Phys. Chem. 205 (1956) 349–360.[18] H. König, G. Helwig, Z. Phys. 129 (1951) 491–503.[19] A. Heisen, Ann. Phys. (Berlin) 457 (1–2) (1958) 23–35.[20] A. Heisen, Optik 18 (2) (1961) 59–68.[21] S. Aisenberg, R. Chabot, J. Appl. Phys. 42 (1971) 2953–2958.[22] S. Aisenberg, R. Chabot, J. Vac. Sci. Technol. 10 (1973) 104–107.[23] E.G. Spencer, P.H. Schmidt, D.C. Joy, F.J. Sansalone, Appl. Phys. Lett. 29 (1976)

118–120.[24] D.S. Whitmell, R. Williamson, Thin Solid Films 35 (1976) 255–261.[25] L. Holland, J. Vac. Sci. Technol. 14 (1977) 5–15.[26] K. Bewilogua, D. Wagner, Vacuum 42 (1991) 473–476.[27] L. Holland, S.M. Ojha, Thin Solid Films 38 (1976) L17–L19.[28] L.P. Andersson, S. Berg, Vacuum 28 (1978) 449–451.[29] L.P. Andersson, S. Berg, H. Norström, R. Olaison, S. Towta, Thin Solid Films 63

(1979) 155–160.[30] A. Bubenzer, B. Dischler, G. Brandt, P. Koidl, J. Appl. Phys. 54 (1983) 4590–4595.[31] A. Bubenzer, B. Dischler, A. Nyaiesh, Thin Solid Films 91 (1982) 81–87.[32] B. Dischler, A. Bubenzer, P. Koidl, Appl. Phys. Lett. 42 (1983) 636–638.[33] Y. Catherine, P. Couderc, Thin Solid Films 144 (1986) 265–280.

224 K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

[34] C. Weissmantel, Proc. 7th Int. Vacuum Congress and 3rd Int. Conference on SolidSurfaces 1977, Berger, Vienna, 1977, pp. 1533–1544.

[35] C. Weissmantel, G. Reisse, H.-J. Erler, F. Henny, K. Bewilogua, U. Ebersbach, C.Schürer, Thin Solid Films 63 (1979) 315–325.

[36] K. Bewilogua, D. Dietrich, L. Pagel, C. Schürer, C. Weissmantel, Surf. Sci. 86 (1979)308–313.

[37] L. Martinu, A. Raveh, A. Domingue, L. Bertrand, J. Klemberg-Sapieha, S.C. Gujrathi,M.R. Wertheimer, Thin Solid Films 208 (1992) 42–47.

[38] H. Dimigen, H. Hübsch, Trockengeschmiertes Gleitlager, Verfahren zu seinerHerstellung und seine Verwendung, EP 0 022 285 B1, 1984. (Priority 1979).

[39] K. Enke, H. Dimigen, H. Hübsch, Appl. Phys. Lett. 36 (1980) 291–292.[40] K. Enke, Thin Solid Films 80 (1981) 227–234.[41] R. Memming, H.J. Tolle, P.E. Wierenga, Thin Solid Films 143 (1986) 31–41.[42] A. Imamura, T. Tsukamoto, K. Shibuki, S. Takatsu, Surf. Coat. Technol. 36 (1988)

161–169.[43] L.P. Andersson, Thin Solid Films 86 (1981) 193–200.[44] C.-P. Klages, R. Memming, Mater. Sci. Forum 52&53 (1989) 609–644(and refer-

ences therein).[45] C. Donnet, A. Erdemir, in: C. Donnet, A. Erdemir (Eds.), Tribology of Diamond-like

Carbon Films, Springer Science + Business Media, LLC, New York, 2008, pp. 1–10,(and following Sections).

[46] D.A. Anderson, Phil. Mag. 35 (1977) 17–26.[47] B. Meyerson, F.W. Smith, J. Non-Cryst. Solids 35&36 (1980) 435–440.[48] F.W. Smith, J. Appl. Phys. 55 (1984) 764–771.[49] B. Meyerson, F.W. Smith, Solid State Commun. 34 (1980) 531–534.[50] D.R. McKenzie, L.M. Briggs, Solar Energy Mater. 6 (1981) 97–106.[51] D.R. McKenzie, R.C. McPhedran, N. Savvides, L.C. Botten, Phil. Mag. B 48 (1983)

341–364.[52] S. Craig, G.L. Harding, Thin Solid Films 97 (1982) 345–361.[53] T.J. Moravec, Thin Solid Films 70 (1980) L9–L10.[54] Y. Lifshitz, Diamond Relat. Mater. 5 (1996) 388–400.[55] V.E. Strel'nitskij, V.G. Padalka, S.I. Vakula, Sov. Phys. Tech. Phys. 23 (1978) 222–-

224(Translation from Russian original publication in: Zh. Tekh. Fiz. 48 (1978) 377).[56] V.E. Strel'nitskij, I.I. Aksenov, S.I. Vakula, V.G. Padalka, V.A. Belous, Sov. Tech. Phys.

Lett. 4 (1978) 546–547(Translation from Russian original publication in: Pisma Zh.Tekh. Fiz. 4 (1978) 1355).

[57] I.I. Aksenov, V.A. Belous, V.G. Padalka, V.M. Khoroshikh, Sov. J. Plasma Phys. 4(1978) 425–428(Translation from Russian original publication in: Fiz. Plazmy 4(1978) 758).

[58] I.I. Aksenov, I. Vakula, V.G. Padalka, V.E. Strel'nitskij, V.M. Khoroshikh, Sov. Phys.Tech. Phys. 25 (1980) 1164–1166(Translation from Russian original publicationin: Zh. Tekh. Fiz. 50 (1980) (1980) 2000).

[59] P.J. Martin, S.W. Filipczuk, R.P. Netterfield, J.S. Field, D.F. Whitnall, D.R. McKenzie, J.Mater. Sci. Lett. 7 (1988) 410–412.

[60] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Z.H. Wang, E. Kravtchinskaia, D. Segal,P.B. Lukins, P.D. Swift, P.J. Martin, G. Amaratunga, P.H. Gaskell, A. Saeed, DiamondRelat. Mater. 1 (1991) 51–59.

[61] H.-J. Scheibe, P. Siemroth, IEEE Trans. Plasma Sci. 18 (1990) 917–922.[62] W. Pompe, H.-J. Scheibe, Thin Solid Films 208 (1991) 11–14.[63] H.-J. Scheibe, D. Drescher, B. Schultrich, M. Falz, G. Leonhardt, R.Wilberg, Surf. Coat.

Technol. 85 (1996) 209–214.[64] A.A. Voevodin, M.S. Donley, Surf. Coat. Technol. 82 (1996) 199–213.[65] N. Savvides, B. Window, J. Vac. Sci. Technol. A 4 (1986) 504–508.[66] J. Schwan, S. Ulrich, H. Roth, H. Ehrhardt, S.R.P. Silva, J. Robertson, R. Samlenski, R.

Brenn, J. Appl. Phys. 79 (1996) 1416–1422.[67] M. Weiler, S. Sattel, K. Jung, H. Ehrhardt, V.S. Veerasamy, J. Robertson, Appl. Phys.

Lett. 64 (1994) 2797.[68] H. Ronkainen, K. Holmberg, in: C. Donnet, A. Erdemir (Eds.), Tribology of

Diamond-like Carbon Films, Springer Science + Business Media, LLC, New York,2008, pp. 155–200.

[69] D.R. McKenzie, L.C. Botten, R.C. McPhedran, Phys. Rev. Lett. 51 (1983) 280–283.[70] B. Dischler, A. Bubenzer, P. Koidl, Solid State Commun. 48 (1983) 105–108.[71] K. Bewilogua, D. Dietrich, G. Holzhüter, C. Weissmantel, Phys. Status Solidi A 71

(1982) K57–K59.[72] C. Weissmantel, K. Bewilogua, K. Breuer, D. Dietrich, U. Ebersbach, H.-J. Erler, B.

Rau, G. Reisse, Thin Solid Films 96 (1982) 31–44.[73] C. Weissmantel, in: K.J. Klabunde (Ed.), Thin Films From Free Atoms and Particles,

Academic Press, New York, 1985, pp. 153–201.[74] J. Robertson, E.P. O'Reilly, Phys. Rev. B 35 (1987) 2946–2957.[75] J. Angus, Thin Solid Films 142 (1986) 145–151.[76] J. Angus, F. Jansen, J. Vac. Sci. Technol. A 6 (1988) 1778–1782.[77] J.C. Phillips, J. Non-Cryst. Solids 34 (1979) 153–181.[78] G.H. Döhler, R. Dandoloff, H. Bilz, J. Non-Cryst. Solids 42 (1980) 87–96.[79] J. Fink, T. Müller-Heinzerling, J. Pflüger, B. Scheerer, B. Dischler, P. Koidl, A.

Bubenzer, R.E. Sah, Phys. Rev. B30 (1984) 4713–4718.[80] M.A. Tamor, C.H. Wu, J. Appl. Phys. 67 (1990) 1007–1012.[81] P. Reinke, W. Jacob, W. Möller, J. Appl. Phys. 74 (1993) 1354–1361.[82] W. Jacob, W. Möller, Appl. Phys. Lett. 63 (1993) 1771–1773.[83] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095–14107.[84] S.D. Berger, D.R. McKenzie, P.J. Martin, Phil. Mag. Lett. 57 (1988) 285–290.[85] D.R. McKenzie, D.C. Green, P.D. Swift, D.J.H. Cockayne, P.J. Martin, R.P. Netterfield,

W.G. Sainty, Thin Solid Films 193–194 (1990) 418–430.[86] S. Kaplan, F. Jansen, M. Machonkin, Appl. Phys. Lett. 47 (1985) 750–753.[87] J. Robertson, Adv. Phys. 35 (1986) 317–374.[88] A. Richter, H.-J. Scheibe, W. Pompe, K.-W. Brzezinka, I. Mühling, J. Non-Cryst. Solids

88 (1986) 131–144.

[89] K.W.R. Gilkes, H.S. Sands, D.N. Batchelder, J. Robertson, W.I. Milne, Appl. Phys. Lett.70 (1997) 1980–1982.

[90] A.C. Ferrari, J. Robertson, Phys. Rev. B 64 (2001)(075414-1-13).[91] D. Beeman, J. Silverman, R. Lynch, M.R. Anderson, Phys. Rev. B 30 (1984) 870–875.[92] G. Galli, R.M. Martin, R. Car, M. Parrinello, Phys. Rev. Lett. 62 (1989) 555–558.[93] G. Galli, R.M. Martin, R. Car, M. Parrinello, Phys. Rev. B 42 (1990) 7470–7482.[94] C.Z. Wang, K.M. Ho, Phys. Rev. B 50 (1994) 12429–12436.[95] N.D. Marks, D.R. McKenzie, B.A. Pailthorpe, M. Bernasconi, M. Parrinello, Phys. Rev.

B 54 (1996) 9703–9714.[96] H.-P. Kaukonen, R.M. Nieminen, Phys. Rev. Lett. 68 (1992) 620–623.[97] P. Blaudeck, T. Frauenheim, G. Jungnickel, U. Stephan, Solid State Commun. 85

(1993) 997–1000.[98] T. Frauenheim, P. Blaudeck, U. Stephan, G. Jungnickel, Phys. Rev. B 48 (1993)

4823–4834.[99] M. Weiler, R. Kleber, S. Sattel, K. Jung, H. Ehrhardt, G. Jungnickel, S. Deutschmann,

U. Stephan, P. Blaudeck, T. Frauenheim, Diamond Relat. Mater. 3 (1994) 245–253.[100] P.K. Chu, L. Li, Mater. Chem. Phys. 96 (2006) 253–277.[101] P. Koidl, C. Wild, B. Dischler, J. Wagner, M. Ramsteiner, Mater. Sci. Forum 52–53

(1990) 41–70.[102] J. Robertson, Progr. Solid State Chem. 21 (1991) 199–333.[103] W. Möller, Appl. Phys. Lett. 59 (1991) 2391–2393.[104] W. Möller, Appl. Phys. A 56 (1993) 527–546.[105] A.v. Keudell, M. Meier, C. Hopf, Diamond Relat. Mater. 11 (2002) 969–975.[106] A.v. Keudell, Thin Solid Films 402 (2002) 1–37.[107] Y. Lifshitz, S.R. Kasi, J.W. Rabalais, Phys. Rev. Lett. 62 (1989) 1290–1293.[108] Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Phys. Rev. B 41 (1990)

10468–10480.[109] J. Robertson, Diamond Relat. Mater. 2 (1993) 984–989.[110] J. Robertson, Diamond Relat. Mater. 3 (1994) 361–368.[111] H. Hübsch, H. Dimigen, Kohlenstoff enthaltende Gleitschicht, EP 0 087 836 B1,

1987. (Priority 1982).[112] H. Dimigen, H. Hübsch, Philips Tech. Rev. 41 (1983/84) 186–197.[113] H. Dimigen, H. Hübsch, R. Memming, Appl. Phys. Lett. 50 (1987) 1056–1058.[114] R. Benndorf, M. Grischke, H. Koeberle, R. Memming, A. Bauer, F. Thieme, Surf. Coat.

Technol. 36 (1988) 171–181.[115] I.T. Richie, G.L. Harding, Thin Solid Films 57 (1979) 315–320.[116] H.R. Wilson, D.R. McKenzie, L.M. Briggs, Thin Solid Films 91 (1982) 123–130.[117] E. Bergmann, J. Vogel, J. Vac. Sci. Technol. A 5 (1987) 70–74.[118] I.T. Richie, Thin Solid Films 72 (1980) 65–71.[119] M. Wang, K. Schmidt, K. Reichelt, H. Dimigen, H. Hübsch, J. Mater. Res. 7 (1992)

667–676.[120] M. Wang, K. Schmidt, K. Reichelt, X. Jiang, H. Dimigen, H. Hübsch, J. Mater. Res. 7

(1992) 1465–1472.[121] H. Sjöström, L. Hultman, J.-E. Sundgren, L.R. Wallenberg, Thin Solid Films 232

(1993) 169–179.[122] K.I. Schiffmann, M. Fryda, G. Goerigk, R. Lauer, P. Hinze, A. Bulack, Thin Solid Films

347 (1999) 60–71.[123] J. Güttler, J. Reschke, Surf. Coat. Technol. 60 (1993) 531–535.[124] K. Bewilogua, H. Dimigen, Surf. Coat. Technol. 61 (1993) 144–150.[125] J.T. Harnack, C. Benndorf, Diamond Relat. Mater. 1 (1992) 301–306.[126] K. Bewilogua, C.V. Cooper, C. Specht, J. Schröder, R. Wittorf, M. Grischke, Surf. Coat.

Technol. 127 (2000) 224–232.[127] N.J.M. Carvalho, J.Th.M. DeHosson, Thin Solid Films 388 (2001) 150–159.[128] C. Strondl, G.J. van der Kolk, T. Hurkmans, W. Fleischer, T. Trinh, N.J.M. Carvalho,

J.Th.M. DeHosson, Surf. Coat. Technol. 142–144 (2001) 707–713.[129] B. Feng, D.M. Cao, W.J. Meng, J. Xu, R.C. Tittsworth, L.E. Rehn, P.M. Baldo, G.L. Doll,

Surf. Coat. Technol. 148 (2001) 153–162.[130] K. Bewilogua, J. Brand, H. Thomsen, M. Weber, R. Wittorf, Z. Metallkd. 96 (2005)

998–1004.[131] K. Bewilogua, R. Wittorf, H. Thomsen, M. Weber, Thin Solid Films 447–448 (2004)

142–147.[132] T. Zehnder, J. Patscheider, Surf. Coat. Technol. 133–134 (2000) 138–144.[133] D.A. Anderson, W.E. Spear, Phil. Mag. 35 (1977) 1–16.[134] Y. Catherine, G. Turban, Thin Solid Films 60 (1979) 193–200.[135] A. Morimoto, T. Miura, M. Kumeda, T. Shimizu, J. Appl. Phys. 53 (1982) 7299–7305.[136] D.R. McKenzie, N. Savvides, D.R. Mills, R.C. McPhedran, L.C. Botten, Sol. Energy

Mater. 9 (1983) 113–126.[137] T. Arai, K. Oguri, Hard and Lubricant Thin Film of Amorphous Carbon–Hydrogen–

Silicon, Iron Base Metallic Material Coated Therewith, and the Process for Produc-ing the Same, EP 0 435 312 B1, 1995. (Priority 1989).

[138] K. Oguri, T. Arai, J. Mater. Res. 5 (1990) 2567–2571.[139] K. Oguri, T. Arai, J. Mater. Res. 7 (1992) 1313–1316.[140] Y. Miyake, Surf. Coat. Technol. 54 (55) (1992) 563–569.[141] K. Oguri, T. Arai, Thin Solid Films 208 (1992) 158–160.[142] J.C. Sánchez-López, A. Fernández, in: C. Donnet, A. Erdemir (Eds.), Tribology of

Diamond-like Carbon Films, Springer Science + Business Media, LLC, New York,2008, pp. 311–338.

[143] S.S. Camargo, J.R.A. Santos, A.L. Baia Neto, R. Carius, F. Finger, Thin Solid Films 332(1998) 130–135.

[144] W.-J. Wu, M.-H. Hon, Surf. Coat. Technol. 111 (1999) 134–140.[145] D. Hofmann, S. Kunkel, K. Bewilogua, R. Wittorf, Surf. Coat. Technol. 215 (2013)

357–363.[146] M. Grischke, K. Bewilogua, K. Trojan, H. Dimigen, Surf. Coat. Technol. 74–75 (1995)

739–745.[147] M. Grischke, A. Hieke, F. Morgenweck, H. Dimigen, Diamond Relat. Mater. 7 (1998)

454–458.

225K. Bewilogua, D. Hofmann / Surface & Coatings Technology 242 (2014) 214–225

[148] K. Trojan, M. Grischke, H. Dimigen, Phys. Status Solidi A 145 (1994) 575–585.[149] G. Koch, D.C. Zhang, A. Leipertz, M. Grischke, K. Trojan, H. Dimigen, Int. J. Heat Mass

Transfer 41 (1998) 1899–1906.[150] W. Augustin, J. Zhang, I. Bialuch, T. Geddert, S. Scholl, in: R.K. Shah, M. Ishizuka,

T.M. Rudy, V.V. Wadekar (Eds.), Proc. Fifth Int. Conf. on Enhanced, Compact andUltra-Compact Heat Exchangers: Science, Engineering and Technology, Engineer-ing Conferences International, Hoboken, NJ, September 2005, pp. 394–402.

[151] H. Schulz, M. Leonhardt, H.-J. Scheibe, B. Schultrich, Surf. Coat. Technol. 200 (2005)1123–1126.

[152] E. Dekempeneer, J. Smeets, J. Meneve, Method for Applying a Friction-ReducingCoating, EP 0 651 069 B1, 1999. (Priority 1993).

[153] J. Meneve, E. Dekempeneer, W. Wegener, J. Smeets, Surf. Coat. Technol. 86–87(1996) 617–621.

[154] E. Dekempeneer, K. Van Acker, K. Vercammen, J. Meneve, D. Neerinck, S. Eufinger,W. Pappaert, M. Sercu, J. Smeets, Surf. Coat. Technol. 142–144 (2001) 669–673.

[155] E. Bertran, E. Martinez, G. Viera, J. Farjas, P. Roura, Diamond Relat. Mater. 10 (2001)1115–1120.

[156] C. Chouquet, C. Ducros, S. Barrat, A. Billard, F. Sanchette, Surf. Coat. Technol. 203(2008) 745–749.

[157] D. Hofmann, S. Beisswenger, A. Feuerstein, Surf. Coat. Technol. 49 (1991) 330–335.[158] D.P. Monaghan, D.G. Teer, P.A. Logan, I. Efeoglu, R.D. Arnell, Surf. Coat. Technol. 60

(1993) 525–530.[159] D. Hofmann, H. Schuessler, K. Bewilogua, H. Hübsch, J. Lemke, Surf. Coat. Technol.

73 (1995) 137–141.[160] T. Hurkmans, F. Hauzer, B. Buil, K. Engel, R. Tietema, Surf. Coat. Technol. 92 (1997)

62–68.[161] M. Hans, R. Büchel, M. Grischke, R. Hobi, M. Zäch, Surf. Coat. Technol. 123 (2000)

288–293.

[162] M. Grischke, R. Herb, O. Massler, J. Karner, H. Eberle, 44th Annual Technical Confer-ence Proceedings of the Society of Vacuum Coaters, 2001, pp. 407–410.

[163] D. Camino, A.H.S. Jones, D. Mercs, D.G. Teer, Vacuum 52 (1999) 125–131.[164] S. Yang, D.G. Teer, Surf. Coat. Technol. 131 (2000) 412–416.[165] O. Massler, M. Pedrazzini, C. Wohlrab, H. Eberle, M. Grischke, T. Michler, Method

and Apparatus for Fabrication of DLC Layer Systems, EP 1 362 931 B1 (2005, Prior-ity 2000); US 6 740 393 B1 (2004).

[166] M. Weber, K. Bewilogua, H. Thomsen, R. Wittorf, Surf. Coat. Technol. 201 (2006)1576–1582.

[167] A. Hieke, T. Hurkmans, G.J. van der Kolk, M. Tobler, R. Bonetti, 48th Annual Techni-cal Conference Proceedings of the Society of Vacuum Coaters, 2005, pp. 556–561.

[168] W.-D. Münz, M. Schenkel, S. Kunkel, J. Paulitsch, K. Bewilogua, Journal of Physics:Conference Series, 100, IOP Publishing, 2008, (082001-1-6).

[169] M. Lattemann, A.P. Ehiasarian, J. Bohlmark, P.A.O. Persson, U. Helmersson, Surf.Coat. Technol. 200 (2006) 6495–6499.

[170] H.-J. Scheibe, M. Leonhardt, A. Leson, C.-F. Meyer, T. Stucky, V. Weihnacht, Vak.Forsch. Prax. 20 (6) (2008) 26–31.

[171] H. Ronkainen, S. Varjus, K. Holmberg, Wear 222 (1998) 120–128.[172] B. Podgornik, in: C. Donnet, A. Erdemir (Eds.), Tribology of Diamond-like Carbon

Films, Springer Science + Business Media, LLC, New York, 2008, pp. 410–453.[173] J. Sun, W. Zhang, Z.-Q. Fu, C.-B. Wang, W. Yue, Z.-J. Peng, X. Yu, S.-S. Lin, M.-J. Dai,

Key Eng. Mater. 492 (2012) 155–159.[174] M. Keunecke, K. Bewilogua, J. Becker, A. Gies, M. Grischke, Surf. Coat. Technol. 207

(2012) 270–278.[175] M. Kano, Tribol. Int. 39 (2006) 1682–1685.[176] T. Lampe, S. Eisenberg, E. Rodriguez Cabeo, Surf. Coat. Technol. 174–175 (2003) 1–7.[177] J. Vetter, G. Barbezat, J. Crummenauer, J. Avissar, Surf. Coat. Technol. 200 (2005)

1962–1968.