dlc review

7/29/2019 DLC Review

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Wear, 168 (1993) 143-153 143

Review of the tribology of diamond-like carbonA. GrillIBM Research llivirion, T..T. Watson Research Center, ~o~t~wn ~e~~ht~ AT 10598 (USA)(Received July 20, 1992; accepted January 4, 1993)

AbstractDiamond-like carbon (DE) films are characterized by very low friction coefficients, high wear resistance andhigh corrosion resistance. Depending upon the testing environment, the coefficient of friction can be as low as0.01. As-deposited films are wear resistant in vacuum as well as in atmospheric ambient. However, the tribologicalproperties of DLC are strongly affected by the deposition method. This paper reviews the friction and wearproperties of DLC, and of similar materials derived from DLC, as a function of the preparation method andtesting environment. Mechanisms proposed to explain the tribological properties are presented and discussed.

Diamond-like carbon (DLC) is a term used to describehard carbon films which are mostly metastable amor-phous materials but can include a microcrystalline phase.DLC films have been prepared by a variety of methodsand precursors, ~n~~udin~ r.f. or d.c. plasma-assistedchemical vapor deposition (CVD), sputtering, vacuumarc, and ion beam deposition, from a variety of carbon-bearing solid or gaseous source materials [l]. DLCfilms are characterized by an extreme hardness, whichis measured to be in the range 2000-5000 kg mm-[2], a generally low friction coefficient and usually veryhigh internal stresses 13-51. As a function of the de-position conditions, the films may contain varyingamounts of hydrogen. The films deposited by plasma-assisted CVD (PACVD) usually incorporate up to 60%hydrogen f6], while those deposited by sputtering ora vacuum are may contain only small amounts ofhydrogen or no hydrogen at all.The high hardness and chemical resistance of theDLC films makes them good candidates as wear-resistantprotective coatings for metals, optical, or electroniccomponents. The use of DLC is especially attractivein applications where it is required that the thicknessof the protective film be less than 50 nm, as for examplein the case of magnetic recording media. One suchapplication is in hard magnetic disks, where the trendtowards higher density data storage has led to therequirement of very low flying heights between a diskand a recording head 171. The protective coating mustbe resistant to wear and corrosion but also thin enoughnot to impede the achievement of high recording density.

Therefore, special attention has been given to thetribological study of the head-disk interfaces in magneticrecording drives using thin film media, in order todevelop reliable, high capacity disk files. During normaloperation, the read/write head flies above the disksurface; however, when the disk starts or stops, theslider rubs on the surface of the disk. The friction andwear which can develop between the siider and diskduring contact can result in the failure of the recordingmedia. In addition, very little debris can be allowedto form during the start and stop cycles, because thedebris can disrupt the flight of the head and lead tothe failure of the disk surface.

At the operating conditions of magnetic-recordingmedia and micromechanics in general, microtribologybecomes a key technology for interfaces [$I. Microtri-bology addresses the changes taking place at the atomiclevel in the very superficial layers in contact. Therefore,the characterization of microtribological properties hasto be performed at ultralow loads. Microtribologicaltesting tools, such as a contact profilometer [9] or pointcontact microscope flO] have been developed for thispurpose by modi~ing a scanning tunnelling microscope.Measurements have been performed with this tool atloads as small as 1 pg [9].

In microtribology, the required surface material isnot a self-sacrifice-type solid lubricant but has to beboth wear resistant and lubricating. According to Miyakeand Kaneko [8], to reduce atomic-scale wear, the struc-ture of the tribological material has to satisfy thefollowing requirements.

~43-1#8/93/$6.~ 0 1993 - E Isevier Sequoia. All rights reserved


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(1) top layer - shear should always occur at thesliding interface but not within which requires a materialof low surface energy as the topmost layer;

(2) bulk of tribological film - fracture formationand defect growth by sliding should be negligible, whichrequires a high strength material;

(3) interface to substrate - this should have a largeadhesive strength, which should prevent delaminationof the tribological structure from the substrate.The third requirement can be fulfilled by a suitablebonding layer, such as a-Si:H, for deposition of DLCon silicide-forming metals [ll], or by grading the in-terfacial layer with a mixture composition of film andsubstrate, such as Si-C for deposition of DLC on silicon[S]. The other two requirements will be discussed later,showing that DLC, especially the type prepared byPACVD, and materials derived from it can satisfy theserequirements.

The properties of DLC films make them useful fora variety of protective applications [12]. Nevertheless,improvement of their tribological properties and ex-panding the use of DLC films for tribological protectionin new applications requires a better understanding oftheir friction and wear properties, and the dependenceof those on the deposition parameters and on theoperating environment. This is important taking intoconsideration that the properties of DLC films can besignificantly affected by the deposition system (i.e.method, precursor, parameters) [6]. In an effort toimprove the tribological properties of DLC films, avariety of related materials have been developed andinvestigated.

This paper presents a review of tribological studiesof hard carbon films and related materials, and discussesthe mechanisms proposed to explain these tribologicalproperties.

2. Review of DIE trhologyThe structure of hydrogenated DLC can be described

as a randbm network of covalently bonded carbon inhybridized tetragonal, (sp) and trigonal (sp) localcoordination, with some of the bonds terminated byhydrogen [13]. Robertson [14,15] claimed that, in ad-dition to the short-range order defined by carbon hy-bridization and hydrogen content, a substantial degreeof medium-range order on about the 10 8, scale alsoexists in the films. According to Robertson, the DLCcan be described as a network of graphitic clusters,linked into islands by sp3 bonds. Sputtered hard carbonfilms are often mostly graphite-like, while, for certaindeposition methods, the hard carbon films are consti-tuted of a mixture of graphite and diamond micro-crystallites. Because of this spectrum of microstructures,

the tribological properties of diftcrent carbon films caiibe related to the tribological behavior of graphite 01diamond. These properties will be first discussed toserve as a reference to the properties of DLC

The friction coefficient of diamond sliding on diamondis relatively low and adhesion plays an important rolein determining the friction of diamond on diamond inair. The diamond surface is normally terminated witha layer of chemisorbed hydrogen and oxygen. This layerrenders the surface relatively unreactive and preventsstrong C-C bonding across the sliding interface. Whendiamond-diamond sliding experiments are performedin a high vacuum, the sliding action wears off thechemisorbed hydrogen and oxygen from the surface.Strong bonding then occurs across the interface, causinga large increase in the friction coefficient. The frictioncan be decreased again by bleeding in hydrogen in thechamber containing the sample 1161. During sliding inair, the adsorbates are also removed by sliding but arereplenished through adsorption from the environment,and the friction coefficient remains low. It was foundthat the friction coefficient of diamond on diamond inair can be changed by heating the samples to 100 Cor by exposing it to water [17]. The heating causes thedesorption of materials adsorbed on the surface ofdiamond, leading to better interfacial contact and anincreased friction coefficient. In contrast, water has theopposite effect, reducing the friction coefficient. Lu-bricating oils do not change the friction coefficient ofdiamond on diamond [17].Wear of diamond occurs through chipping of smallfragments from the surface, while wear through a surfacegraphitization mechanism, as a cause for diamond wear,is generally rejected [16]. Even a soft material, suchas gold, can cause wear of diamond. This is explainedthrough a fatigue mechanism which causes the formationand growth of small cracks, leading finally to fractureof the diamond particles. There is also evidence thatthe debris formed during sliding contributes to the lowfriction in diamond [18].It has been shown that water acts as a lubricant forgraphite and improves its wear and lubricating behavior[19-211. The lubricating action of the water has beenexplained as a result of a three-body interaction withthe adsorbed water, which causes reduced adhesionbetween the surfaces in contact. Hydrogen has beenfound to have a similar lubricating effect on graphite.Zaidi et al. [22] investigated the sliding wear of graphiteagainst graphite in an atomic hydrogen atmosphereproduced by passing hydrogen over a tungsten ribbonat 2000 K. The authors found that atomic hydrogen isan effective lubricant for graphite and that there is aclose analogy between the lubrication of the graphiteby water vapor and by atomic hydrogen. It appearsthat hydrogen passivates dangling bonds at the edges


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A. Grill I Tribology of diamond-like carbon 145

of the graphite crystallites, leaving only the possibilityof weak interactions with the r bonds, thus resultingin reduced friction.2.1. Friction coefficientsDespite the high chemical inertness of DLC films,their tribological behavior is controlled to a large extent,as we shall see in the following, by the surface chemistryof the films. The surface chemistry can be affected bythe environment. However, it is also dependent on themethod used for the preparation of the films, whichdetermines their structure.In 1980, Enke et al. [3] reported a very low frictioncoefficient of DLC films deposited by PACVD fromacetylene. In nitrogen at a relative humidity (RH) ofless than l%, the friction coefficient between a steelball and a DLC-coated silicon wafer was p = 0.01-0.02.It increased with increasing RH, attaining values of0.05 at RH= 10% and up to 0.19 at RH= 100%. Asimilar behavior was reported for films deposited byPACVD from ethylene [23]. The films showed a con-tinuous increase in the friction coefficient with increasingRH and the friction exhibited a hysteresis effect betweenRH =50% and 100%. Values as low as p= 0.005 weremeasured in a vacuum [23]. Enke et al. explained thelow friction coefficients by assuming that graphitizationof the surface layer of the DLC films takes place duringtesting.Low friction coefficients were also reported by Weiss-mantel et al. [4] for carbon films deposited by ion beammethods. The films had a friction coefficient of 0.19against steel, decreasing to 0.04 after 10 000 passes ina pin-on-disk apparatus.Memming et al. [18] studied the tribological behaviorof amorphous hydrogenated carbon (a-C:H) depositedby r.f. PACVD from acetylene, toluene or benzene.Stationary (as referred to by the authors) friction coef-ficients of DLC against steel were found to decreaseto very low values, i.e. p cO.02, at RH < 1% and thislow value was maintained in ultrahigh vacuum (UHV).The very low friction coefficient was attained after 25rotations from an initial value of ~~0.2. The frictioncoefficient remained low in dry nitrogen (p= 0.02);however, it increased drastically to about 0.6 whennitrogen was substituted by dry oxygen. In humid ni-trogen or humid oxygen, the friction coefficient was0.25. The authors found by Auger electron spectroscopythat, in the conditions of very low friction, carbonaceousmaterial was transferred from the DLC film to thesteel rider, while, in the conditions of high friction (indry oxygen), iron transferred to the DLC surface. Thematerial transfer changed the chemistry of the interfacelayer and, according to the authors, this tribochemicalbehavior explained the different values obtained forthe friction coefficients under various conditions [18].

Memming et al. also found that the loss of hydrogenafter annealing the DLC films above 550 C causedlarge changes in the friction coefficient, which reachedvalues of ~=0.68 in UHV or dry nitrogen, indicatingthat the presence of hydrogen in these films was essentialfor obtaining a low friction in dry nitrogen or IX-IV.However, in a humid atmosphere, the friction coefficientdid not change significantly after annealing 1181.Weiss-mantel et al. also found previously [24] an increase ofthe friction coefficient of DLC upon heating above 400C and attributed this increase to a loss of hydrogenrather than to graphitization of the DLC.

Miyoshi and co-workers studied the tribological be-havior of hydrogenated DLC films deposited on siliconnitride by r.f. PACVD of methane or butane in slidingwear against spherical silicon nitride riders [25,26]. Thefriction coefficient was found to be 0.1 in dry nitrogenand 0.18 in laboratory air. For films deposited at apower density of 0.08 W cme2, the coefficient of frictionwas found to increase with increasing number of slidingpasses in both air and dry nitrogen. In air, however,the film broke through and was removed from thesliding zone after only 1000 passes, probably as a resultof poor adhesion. The coefficient of friction of a filmdeposited at a higher power density of 0.40 W cm-decreased, between 10 and 10 000 passes, reaching avalue of 0.01 in dry nitrogen. The authors explainedthe decrease of the friction coefficient above 1000 passesby assuming the formation of a hydrocarbon rich layerat the sliding interface. The differences in the behaviorof the films deposited at different power levels wereattributed to differences in film densities; however,density measurements were not reported.According to Miyoshi and co-workers, annealing invucuo above 500 C causes hydrogen loss from thesurface of the DLC and the formation of a graphite-like layer through a two-step process [25,26], i.e. acarbonization stage in which the film loses hydrogen,followed by a polymerization stage, forming graphiticcrystallites or sheets. During heating in z)acuo, thefriction coefficient remained low up to 500 C (~


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surfaces, resulting in increased friction in UHV or drynitrogen, similar to the cases reported for diamond orgraphite [16,22]. Humidity reduces the friction coef-ficient to an intermediate value, as it would do forgraphite, but also for diamond, although the values arecloser to that of graphite.

The tribological properties of DLC films depositedon silicon by r.f. PACVD from a methane and hydrogenmixture have been reported by Kim et al. 1271. Thefriction was measured against a silicon nitride ball inargon and air at different humidities. The frictioncoefficient varied from p= 0.2 in 50% humid argon and100% humid air to ~=0.06 in dry argon. This variationof the friction coefficient was attributed to materialtransferred between DLC and the ball in the contactarea. In dry argon, material was transferred from theDLC to the silicon nitride ball and this reduced thefriction coefficient. In dry or humid air, an interfaciallayer - identified by microprobe Fourier transform IRas a carbonyl compound (oxidized hydrocarbon) ---formed by a tribochemical reaction covered both theDLC and silicon nitride contact areas and increasedthe friction coefficient [27].

The effects of the deposition temperature and sub-strate bias on the tribological properties of DLC filmsdeposited by r.f. PACVD from acetylene have beenreported by Grill et al. [28]. The DLC films weredeposited on silicon wafers at temperatures between100 and 250 C and substrate biases of - 80 and - 150V (d.c.). The films were annealed in z)acuo at tem-peratures up to 590 C for 3-4 h. The original hydrogencontent in the films decreased from 40% in the as-deposited film to 22% after annealing at 590 C. Thefriction coefficients were measured in air atRH=40%-70% against a spherical steel rider. Thefriction coefficients of the as-deposited DLC films werefound to decrease from 0.35 f 0.04 for films depositedat 100 C to 0.20f 0.04 for films deposited at 250 C,with the surface bias having a negligible effect. Withinthe experimental error, the friction coefficients of theannealed films were essentially the same as those ofthe as-deposited films. The insensitivity of the frictioncoefficients to the annealing temperature was similarto what was observed by Memming et al. [18] andMiyoshi [26] in humid air.The tribological properties of sputtered DLC filmshave been reported on by several authors [29-321.Agarwal et al. [29] measured the sliding friction ofamorphous carbon films 30 nm thick sputtered onmagnetic recording fihns (CoNi and COP) against single-crystal, hemispherical sapphire. The tests were per-formed at a linear speed of l-5 m s- up to 20 000disk revolutions. After an initial decrease, as a resultof surface burnishing, the friction coefficient increased

from an initiar value of 0.3I .O with increasing number

Marchon nr~d co-worker 0.02. The films were identified as having agranular structure consisting of nanocrystalline metalor carbide particles in a DLC matrix. The frictioncoefficients and adhesive wear were measured with apin-on-disk apparatus, using uncoated steel or cementedcarbide spheres.

The friction coefficient of the Me-DLC films hasbeen found to be strongly dependent on the metalincorporated into the films. In an earlier paper 1421,Dimigen et al. reported that, in humid air or humidnitrogen, the coefficient of friction was found to be~~0.2. At a very low relative humidity, i.e. RH l%, the friction coefficient of the metal-containingfilms was generally below 0.2 [42]. More recent resultsobtained in ambient air at RH= 18%-41% showed aminimal friction coefficient of 0.04 at 12 at.% metalin Ta-DLC films 1431. For W-DLC films containing lessthan 13 at.% W, the friction coefficient against steeldecreased from 1.5 at RH=40%-80% to about 0.02in dry nitrogen (RH


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However, the wear behavior of DLC has been onlyrecently addressed in a systematic way. The followingstudies will be discussed according to the type of DLC:(corresponding to the deposition method) and not inchronological order.

Namura et al. f7] investigated the failure mechanismsof hard magnetic disks coated with sputtered carbonby contact start-stop (CSS) tests against Al,O,-TiCsliders. The carbon films, 20-30 nm thick were sputteredon top of a sputtered magnetic layer of Co-Cr andpermalloy. The tests were performed at rise times inthe range of 2-24 s and relatively low humidities of15%-20% to prevent slider stiction. The slider waspressed on the disk with a load of 5 mN before startingthe tests. The acceleration time of the disk was identicalto the deceleration time. The wear of the carbon layerwas found to be minimal during CSS testing under theconditions investigated. Disk failure was found to occurabruptly. Fatigue cracks were observed to develop inthe disk with repeated CSS cycIes. The cracks extendedin-plane and propagated to the surface of the disks.When the crack reached the surface, flaking occurredabruptly. The authors concluded that the cyclic loadis dominant over the forces acting at the start, becauseof higher static friction coefficient and dynamic loadfactor.

Agarwal et al. [29] described the wear of the sputteredcarbon films by a wear coefficient K, defined as theratio of the vertical cross-section of the wear trackproduced in the pin-on-disk tester to the horizontalcontact area of the slider. The values of K obtainedwere 5 X 10M6 or CoNi and 9 x 10e6 for COP with hardcarbon over-coating, U.Y.50x lo- for uncoated COP,indicating an increase in wear resistance by a factorof 6 as a result of coating the magnetic metallic filmwith the carbon layer, despite the high friction coef-ficient. The wear was found to increase with normalload and the number of revolutions but was independentof the linear speed. According to the authors, theprincipal contribution to the friction coefficient of DLCis inelastic in nature; the mechanical energy is dissipatedprimarily in the form of heat through inelastic processesrather than the generation of lattice defects or othermechanisms of plastic defo~ation~ Nevertheless thesmall fraction of the total energy consumed to formlattice defects causes plastic deformation and this,coupled with fragmentation of surface material, resultsin macroscopic wear.Wear tests performed by Strom el al. [32] on thinfilm magnetic disks with sputtered carbon overcoats,using commercial read/write ceramic heads as sliders,indicated that the carbon overcoats wear through atribochemical mechanism in the presence of oxygenand through a mechanical mechanism in the absenceof oxygen. In dry gas without oxygen, debris is produced

almost mnnedrately through mecnatncal wear irr,o trr;debris precludes the development of high friction itn


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A. Grill 1 Tribology of diamond-lip carbon 151

L AsDee. 390 440 490 59002 3.000; 1.000

E3001003010

In Ai r1(a)

As Dep. 390 440 490 590Temperature ( C )

DeposTempBeIs

Fig. 1. Wear of DLC films vs. annealing temperature: (a) innitrogen; (h) in air.

nitrogen and humid air 1491. The film deposited at-80 V (dc.) at 250 C and annealed to 440 C did

wear at a higher rate than the as-deposited film andwas worn through in nitrogen after annealing at 490C, while in air it lost its wear resistance after annealingat 400 C. DLC films deposited at 180 or 250 C at abias of - 150 V showed negligible wear in nitrogenafter annealing at 390 C, wore more rapidly afterannealing at higher temperatures but remained wearresistant in nitrogen, even after annealing at 590 C(see Fig. 1). In humid air, the wear resistance decreasedand the film deposited at 180 C at - 150 V lost itswear resistance after annealing at 490 C. However,the film deposited at 250 C at - 150 V remained wearresistant in humid air, even after annealing at 590 C[491.These results also showed that the wear of the DLCfilms was not directly related to the values of theirfriction coefficients. In addition, while other physicalproperties showed only small differences in behaviorbetween the films deposited at various temperaturesand substrate biases [48], the wear resistance of thefilms and their thermal stability were strongly affectedby the deposition conditions. A higher wear resistanceafter annealing was obtained for DLC deposited athigher temperatures and films deposited at the higherbiases remained wear resistant, even after annealingat 590 C.

Em et al. 1271 measured the wear rates of the r.f.PACVD DLC films against a silicon nitride ball in apin-on-disk tribometer. The wear tests were performedin argon and air at different humidities. The wear ratesof the silicon nitride ball and the DLC at RH =50%were lop8 and 10e7 mm3 N- m- respectively. In dryargon, dry air and 100% humid air, the wear rates ofDLC were two orders of magnitude lower and that ofthe ball was undetectable. An interfacial oxidized layerformed from mechanically scissored particles, whichcombined to produce macrowear soft debris, was ob-served between the sliding parts. Kim el al. claimedthat, in 100% humid air, water molecules producestrong adhesion of the wear debris and that a denselayer covered the track on the DLC and reduced theoxidation rate in the wear track. This oxidized debrislayer was thick enough to separate the DLC from thesilicon nitride and so reduce the wear. In 50% humidair, the debris was found to be a mixture of oxidizedhydrocarbon and hydrated SiO, from the ball. However,the debris appeared to be loose and not covering thewear track. According to the authors, the wear of DLCagainst silicon nitride is determined by the transferlayer of wear debris. The wear is determined by anadhesive (mechanical) wear mechanism in dry argonbut by tribochemistry in air and humid environments[27]. The authors found no correlation between wearand the friction coefficients in the various testing con-ditions.

Microt~bological studies performed by Miyake et al.[9] for Si-DLC films and fluorinated Si-DLC filmsshowed an increase in the lifetime of the films withincreasing silicon content, with a maximal lifetime beingobtained for films containing lo%-20% Si. Under theconditions of microtribological testing, wear tracks 3nm in size were obtained at a load of 7 FN onun~uorinated film, while wear of the ~uorinated filmwas undetectable. The wear increased with load andwas lower for the fluorinated films at all loads. Thewear of Si-DLC films was also reported on by Oguriand Arai [41]. The wear of a steel rider against Si-DLC was found to be the same as that against DLCcoatings; however, the wear of the Si-DLC coating wasfound to be higher than that of DLC.

In the study of the wear of Me-DLC films, for W-DLC films with 12 at.% W, the authors observed atwo-order reduction in the sliding wear of the coateddisk against steel compared with that for the baresubstrate and a drastic reduction in the wear of thesliding counterparts 1431. When the test was carriedout against TiN or T&N films, significant wear of thesteel balls was observed, with the transfer of steel tothe nitride films. For the Me-DLC films, such a transferof steel onto the W-DLC was not observed. Dimigenand Klages [43] also measured the abrasive wear rates


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152 A. Grill I Triboloa of diamond-l& ccrrix I!of the Me-DLC using a glycerine suspension of Al,O,particles 5 pm in size. The abrasive wear rates showeda minimum at about 13 at.% W in W-DLC films (acomposition at which the lowest friction coefficient wasobtained). The minimal abrasive wear rates were twoand eight times lower than the abrasive wear rates ofTiAlN and TiN respectively. However, the abrasivewear rate of Me-DLC was two times larger than thatof pure DLC films. The combination of a low energysurface (low S/ H ratio) with a relatively strong, cross-linked microstructure, resulting in high hardness andhigh H/ E values, could explain the low adhesion to theMe-DLC films and their low friction coefficient andlow wear [43].

The DLC films deposited by vacuum arc discharge,as reported by Aksenov et al. [37], were found to havea high wear resistance both in air and vacuum. Thebest wear resistance was obtained for the combinationof DLC coatings with highest hardness. DLC coatingsimproved the lifetime of drills made of tool steel bya factor of 1.5-3 during drilling of abrasive glass-reinforced polymers and increased by a factor of fourthe lifetime of carbide tools for turning titanium. Theapplication of DLC on burnishers made of hardenedtool steel made it possible to reduce the surface rough-ness of non-ferrous metals by two or three classes [37].Tochitsky et al. [38] found that the service life of thesteel parts coated with arc-discharge-deposited DLCincreased by 3-40 times.

3. Summarizing remarks

The excellent tribological performance of DLC filmsand their derivatives might be attributed to a combi-nation of favorable properties which are ceramic-like,on the one hand, i.e. high hardness, and polymer-like,on the other hand, i.e. high elasticity and H/ E and lowsurface energy. The results reviewed show that, althoughthe tribological properties of DLC or derived materialsdeposited by different methods span a wide range, thematerials appear to have several common characteristics:

the value of the friction coefficient of hydrogenatedDLC deposited by PACVD is low in humid nitrogenor oxygen, extremely low in dry nitrogen or UHV, andvery high in dry oxygen;the loss of hydrogen through annealing at high tem-peratures causes a marked increase in the frictioncoefficient in UHV but affects only slightly its valuein a humid atmosphere.In UHV or an inert atmosphere, these films display africtional behavior similar to that of diamond. However,in a humid atmosphere, their behavior seems to becloser to that of graphite. The mixed type of frictional

behavior reflects the mixture ot sp and sp- carbotihybridization in the hydrogenated carbon films.

The behavior of DLC deposited by other method:,is less systematic, though very h triction coefftcicntsin UHV were found for vacuum-arc-deposited filmsand a behavior similar to that


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A. Grill I Tnbology of diamond-like carbon 153

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