Effect of target material on deposition and properties of metal-containing DLC (Me-DLC) coatings

Surface and Coatings Technology 127 Ž2000. 224]232

Effect of target material on deposition and properties of metal-containing DLC ž Me-DLC/ coatings K. Bewiloguaa,U , C.V. Cooper b, C. Specht a , J. Schroder ¨ a , R. Wittorf a , c M. Grischke a

Fraunhofer-Institut fur Bienroder Weg 54 E, D-38108 Braunschweig, Germany ¨ Schicht- und Oberflachentechnik, ¨ b United Technologies Research Center, East Hartford, CT 06118-1127, USA c Balzers Verschleißschutz, Balzers, Liechtenstein Received 3 November 1999; accepted in revised form 10 March 2000

Abstract Metal containing diamond-like carbon ŽMe-DLC. coatings were prepared by magnetron sputter deposition using tungsten, tungsten carbide, niobium and titanium as target materials. An essential parameter for the process characterization is the target voltage. The substrate heating during the film growth depends on the target material. For the tungsten target, the contribution of energetic neutrals to the heat flux is quite high. From the viewpoint of resistance to abrasive wear, the best properties were found for W-DLC. The Nb-DLC coatings have an intermediate wear rate but offer the lowest friction coefficients and a high performance under adhesive wear loading. All Me-DLC coatings investigated in this study exhibit clearly higher wear rates than metal-free DLC films prepared by a radio frequency glow discharge technique. Metallic intermediate layers allow the preparation of coatings with a very good adhesion. Q 2000 Elsevier Science S.A. All rights reserved. Keywords: Diamond-like carbon films ŽDLC.; Metal containing DLC; Reactive magnetron sputtering; Substrate heating; Wear; Adhesion

1. Introduction For many years, it has been known that diamond-like carbon ŽDLC. films are hard and have low friction coefficients Žm ., especially against steel. These low m values Ž; 0.2. were revealed for amorphous hydrogenated Žabbreviated as a-C:H or DLC. as well as for metal containing hydrocarbon ŽMe-C:H or Me-DLC. films w1]3x. Today, for both types of coatings, several applications, above all in the fields of machine elements and of tools, are known w4,5x. Besides the coating properties it is an important aspect that DLC as well as

U

Corresponding author. Tel.:q49-531-2155-642; fax: q49-5312155-900. E-mail address: [email protected] ŽK. Bewilogua.

Me-DLC coatings can be deposited at low substrate temperatures Ž- 2008C.. A comparison of DLC and Me-DLC shows that there are advantages and disadvantages to both coating materials as well as to the corresponding deposition techniques. Hard DLC coatings, consisting of a highly cross-linked network of carbon atoms, have high compressive stress Ža few GPa w2,6x.. In many cases such DLC coatings are characterized by a high sp 3rsp 2 ratio of C]C bonding in their structure. The mentioned high stress values often lead to poor adhesion with the substrate, especially on steel and, therefore, limit its use in practical applications. Many methods for the preparation of DLC films have been developed. The most commonly applied method is the radio frequency Žr.f.. glow discharge of

0257-8972r00r$ - see front matter Q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 6 6 - 6

K. Bewilogua et al. r Surface and Coatings Technology 127 (2000) 224]232

hydrocarbon gases with negatively self-biased substrates Žsee e.g. w7x.. However, there are several problems in the up scaling of this r.f. technique to industrially relevant dimensions and geometries. Commonly Me-DLC films having low metal contents Žatomic ratios of MerC up to approx. 0.3. have markedly lower compressive stress than a-C:H Ž- 1 GPa w8,9x.. Such films consist of a network of amorphous carbon ŽDLC. with incorporated metal carbides. The friction coefficients of such coatings are rather similar to those of DLC coatings. However, the wear resistance of Me-DLC coatings is lower than that of DLC. At present, the lowest abrasive wear rates reported for Me-DLC coatings are at least a factor of 2 higher than those reported for Me-free DLC coatings w10x. Until today there was no data on the sp 3rsp 2 ratios of bonding in Me-DLC films available. Although this ratio is a useful quantity, to correlate structure and properties of hydrogen-free amorphous carbon films, such data cannot be taken into account for our Me-DLC coatings. Therefore, in the following sections the state of the amorphous carbon]hydrogen network will be expressed only in terms of weakly and highly crosslinked structures. Today, Me-C:H ŽMe-DLC. coatings are prepared in industrial batch coaters by reactive magnetron sputtering in argon]hydrocarbon gas mixtures using metal or metal carbide targets w10,11x. This technique has a great potential for further scale-up and also for the realization of in-line deposition processes. Comparing the two types of coatings, it should be noted that the electrical resistivity of DLC coatings Ž) 10 6 V cm. is much higher than that of Me-DLC Ž10y3 ; 1 V cm. w12x. It is noteworthy that recently some papers on metalcontaining hydrogen free amorphous carbon composites were published. Voevodin et al. w13,14x used a magnetron-sputtering assisted pulsed laser deposition technique to prepare titanium carbide and tungsten carbide containing amorphous diamond-like carbon films. Wei et al. w15x used a pulsed laser technique with a special target configuration, which allowed to ablate graphite and a dopant ŽCu, Ti, Si.. A few percent of these elements incorporated into the carbon matrix caused a markedly improved adhesion. However, it seems to be difficult to compare the properties of the mentioned composite films to our Me-DLC films. On the one hand the DLC composites are hydrogen-free and our films contain considerable amounts of hydrogen Žsee below., on the other hand it is problematic to compare both types of preparation techniques Žpure sputteringrlaser ablation vs. reactive magnetron sputtering.. In nearly all papers concerning Me-DLC coatings, only one target material has been considered, e.g. titanium w11,16x, tungsten w5,17x or tungsten carbide

225

ŽWC. and WCrNi w18x. Bergmann and Vogel w9x, on the other hand, compared hardness and internal stress of coatings deposited using targets composed of WC, Ti, and Cr. The aim of the present paper is to report on Me-DLC coatings prepared by use of different target materials: Ti, Nb, W, and WC Žhard compound.. It is known that the transition metals of group IV to VI are forming hard carbide phases. With respect to high wear resistant coatings one metal of each group was selected as a target for our investigations. Of particular interest in the present paper are studies of tribological properties, such as abrasive wear and sliding friction, as well as the adhesion behavior of coating with substrate. Furthermore, the effect of substrate heating during the deposition process will be discussed.

2. Experimental details Metal hydrocarbon ŽMe-DLC. coatings were prepared by reactive dc magnetron sputtering in unbalanced mode using HTC 1000r4 ŽABS. coater ŽHauzer Techno Coating, the Venlo, The Netherlands.. Fig. 1 schematically shows a cross-section of the process chamber, which has a volume of approximately 1 m3. Before initiating the deposition runs, the residual pressure in the vacuum chamber was - 10y3 Pa. The Me-DLC deposition process consisted of four main steps: Ži. Ar ion etching for substrate cleaning; Žii. sputter deposition of a metal or metal]carbide film; Žiii. increasing the reactive gas flow up to a maximum value; and Živ. deposition of the final layer under constant process conditions. In all Me-DLC deposition steps Žiii. and Živ. two diametrically opposed targets were used. The most important deposition parameters are summarized below: v v

targets: Ti, Nb, W, WCŽqCo.; reactive gas: acetylene ŽC 2 H 2 .;

Fig. 1. Schematic illustration of the magnetron sputter machine, HTC 1000.

K. Bewilogua et al. r Surface and Coatings Technology 127 (2000) 224]232

226 v

v v

v v v

v

v

v

gas flows: Ar, ; 300 sccm Žconstant.; C 2 H 2 variable, flow in%: wC 2 H 2 xrwArq C 2 H 2 xrup to 50%; total gas pressure: 0.3]0.6 Pa; substrate potential: floating potential to y750 V Žd.c.. Žbias voltage, Ub .; electromagnetic coil: 1000 turns; current: F 8 A; substrate sputter cleaning: Ar Ž0.3 Pa.; substrate holder temperature: commonly up to 2008C; substrate materials: steel substrates, e.g. ball bearing steel Ž100Cr6., Si wafers. substrate rotation: up to 12 rev.rmin of planetary, twofold rotation of substratesrsingle rotation for temperature measurements; and deposition rate: 2]3 mmrh.

During sputter-cleaning and deposition experiments, the substrate temperatures were measured by a thermocouple ŽChromel-Alumel. mounted at a typical substrate position. These experiments were carried out at single rotating substrates only. Composition and structure of the films were mainly characterized by electron probe microanalysis ŽEPMA., secondary ion mass spectroscopy ŽSIMS., and scanning electron microscopy ŽSEM., respectively. Furthermore, the resistance to abrasive wear was determined with a Calo tester operating with an alumina suspension. To quantify the results, the volume of coating removed by the Calo device, generated by a rotating ball, was divided by the normal force and the track length of the rotating ball. For these abrasive wear experiments, the unit of wear is 10y1 5 m3rNrm. Data on adhesive wear were obtained using a pin-on-disc device without lubrication. The counterpart was a sapphire ball. Friction coefficients were estimated using a pin-on-disc arrangement with a normal load of 1 N, a velocity of approximately 40 mmrs, and a relative humidity of approximately 50%. Post-deposition characterization of the coatings was accomplished using static- and scratchindentation adherence, respectively, using a Rockwell ŽHRC. indenter and a scratch device, the latter built by Fraunhofer-IPA, Stuttgart. The tribological properties of the Me-DLC films were compared with those of metal-free DLC films. These films were deposited by a radio frequency Ž13.56 MHz. glow discharge technique using C 2 H 2 as a carbon source.

3. Results and discussion 3.1. Target ¨ oltage The target voltage, UT , is a rather sensitive parameter indicating the coverage of metallic targets. In Fig. 2, UT for Ti, Nb, and W targets are plotted vs. C 2 H 2 flow

Fig. 2. Target voltage, UT , as a function of acetylene flow for W-, Nb-, and Ti-targets.

rate at constant power levels. For all targets, UT is observed to increase with increasing flow. Only for films deposited using the Ti target is a pronounced intermediate maximum in UT Žat approx. 15% acetylene flow. observed. Such a maximum obviously is typical for titanium and does not depend on the sputter machine used. The reason for this exceptional development in the behavior of Ti is not well understood at this time. The observed behavior of UT for all three metals may be interpreted as follows w17x. The power of the magnetron discharge is given by Ps UT Ii Ž1 q g i ., where Ii is the ion current and g i is the secondary electron emission coefficient Žfor ion excitation.. With increasing C 2 H 2 flow, the target surface will be covered with carbides and with amorphous carbon; that is, the targets will become ‘poisoned’. If both carbides and carbon have lower g i than the pure metals and Ii is assumed to be constant, UT will increase. The slight drop in UT at flows ) 40% can be correlated with an increasing contribution of acetylene to the plasma. This suggestion is supported by the observation that, after decreasing the C 2 H 2 flow rate, the target voltage suddenly and markedly increases. 3.2. Substrate heating Fig. 3 shows the time dependence of the substrate temperatures for Ti-DLC as affected by different substrate bias voltages. The first strong increase corresponds to the sputter cleaning wstep Ži., Section 2x. As expected, during the Me-DLC deposition wstep Živ.x, the temperature clearly depends on the applied bias voltage. In Fig. 4, the temperature]time curves for the target materials Ti, Nb, and W are graphed. The measurements were carried out in a non-reactive mode working only with argon at identical target power values and substrate bias voltages for all target materials. As shown in Fig. 4, the highest substrate temperatures

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Fig. 3. Time dependence of substrate temperatures for Ti-DLC at substrate bias voltage between y120 and y600 V. The corresponding main steps Ži. ] Živ. of the deposition process Žsee Section 22. . are labeled.

were measured for the tungsten target. This observation was confirmed by further experiments. To explain these data, the following contributions to substrate heating in magnetron sputter processes Žsee e.g. w19]21x. were identified and estimated: v v v v v

heat of recombination Žneutralization of Ar ions.; heat of condensation; ion kinetic energy; sputtered atoms; and energetic neutrals.

The contribution of photons from the target glow discharge w21x and of thermal electrons were neglected.

The contribution from high-energy electrons Žsecondary electrons generated at the target surface . could not be determined because no reliable data were available. For deposition conditions similar to those emw20x estimated a ployed in the present study, Kalber ¨ heat flux on the order of 10 mWrcm2 . The estimated heat fluxes to the substrate for the above mentioned contributions are summarized in Table 1. In addition to the metals W, Nb, and Ti, carbon was considered. The following details of the calculations shall be noted: 3.2.1. Heat of recombination Taking into account an average ion current density,

Fig. 4. Time dependence of the substrate temperature for sputter deposition of metal films ŽW, Nb, Ti. with argon as sputter gas. Each curve was measured under the same conditions Žtwo targets, each 6 kW, substrate bias voltage Ub s y100 V.

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Table 1 Friction coefficients for various Me-DLC coatings against sapphire and their sliding-distances-to-failure a Contribution to substrate heating ŽmWrcm2 .

Target material W

Nb

Ti

C

Heat of recombination Heat of condensation Ion kinetic energy ŽUb s y100 V. Sputtered atoms Energetic neutrals

10 5 100 6 105

10 4 100 3 19

10 3 100 2 4

10 7 100 1 1

a Measured under conditions of dry sliding in a laboratory-air environment.

ji , of 1 mArcm2 and an ionization potential of 15.6 eV for Ar, a heat flux of approximately 10 mWrcm2 was calculated for this contribution. 3.2.2. Heat of condensation This quantity was calculated using data on bond energies, mass densities of the materials, and growth rates of the films. 3.2.3. Ion kinetic energy The heat flux is approximately the product of ion current density and bias voltage at the substrate. For Ub s y100 V and ji s 1 mArcm2 , the heat flux is approximately 100 mWrcm2 . 3.2.4. Sputtered atoms The used formula is Jsp s b Me = n = Esp , where Jsp is the heat flux to the substrate, n the number of atoms arriving at the substrate per second per centimeter squared, and Esp is the average energy per sputtered atom at the target surface. The thermalization coefficient, b Me , is the ratio of particle energy at the substrate to that at the target. This ratio depends on the product of gas pressure, p, and substrate ]target distance, D S ] T . From curves published by Somekh w22x for tungsten and a p? D S ] T product of 100 Pa ? mm, a b W value of approximately 0.1 was derived. The same b Me values are assumed for the other materials. According to Windischmann w23x, Esp s 10 eV for titanium; a linear relationship between Esp and the atomic number, Z, has been assumed for the present study. 3.2.5. Energetic neutrals A fraction, R p , of ions striking the target will be neutralized and backscattered with high energies. This contribution was estimated using the equation, J N s h = bAr = R p = g = PD w20x, where PD is the power density at the target Ž7 Wrcm2 in the present study., R p is the particle reflection coefficient, and g is the energy reflection coefficient w23x. The fraction, h, of energy of the neutrals deposited into the substrate is assumed to be approximately 0.8 w23x. For the thermalization co-

efficient, bAr , a value of approximately 0.5 is presumed to be realistic w22x. From the curves published by Windischmann w23x for tungsten, R p is taken to be approximately 0.17, and g is taken to be 0.22. Using the relations R ; Ž M2rM1 . 2 and g ; Ž M2rM1 .1r3 w23x, the values for the other elements were calculated, where M2 is the atomic mass of the sputtered metal and M1 is the mass of the sputtering gas ŽAr in the present study.. The estimated heat fluxes for the different target materials are summarized in Table 1. Obviously, the effect of energetic neutrals is very pronounced for the tungsten target. The estimated heat fluxes provide a good explanation for the observed differences in substrate heating for different target materials, as shown in Fig. 4. Fig. 5 shows the results of an experiment carried out to confirm the effect of energetic neutrals on substrate heating. Tungsten targets were sputtered with Ar ions. After reaching an equilibrium temperature, C 2 H 2 was added to the sputter gas, and voltage and current at the substrate were kept constant. After a few minutes, the substrate temperature was observed to decrease. This can be correlated to the coverage of the target Žsee Section 3.13.1. . and a smaller quantity of argon ions reflected by the tungsten target. The reduced amount of energetic neutrals leads to a temperature decrease. 3.3. Film composition and structure The composition of the Me-DLC films depends strongly on the acetylene flow rate. Fig. 6 shows that clear differences in the functional relationship between C 2 H 2 flow rate and metal concentration arise depending on the composition of the sputtering target utilized. To achieve the same composition ŽMerC ratio. using a sputtering target composed of W, much higher C 2 H 2 flow rates are required than for titanium and niobium. A possible explanation for these differences is that the adsorption energies, Ea , of C 2 H 2 molecules on the metal surfaces are different. Unfortunately, no data regarding the adsorption rates could be found in the literature. A portion of the adsorbed molecules will react with the metal to form the carbide phase or will

Fig. 5. Substrate temperatures for sputter deposition processes of a W film Žsputter gas Ar. and a W-DLC film Žsputter gas Arq C 2 H 2 . at constant substrate voltage and current in both modes.

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Fig. 6. Metal-to-carbon ratio ŽMerC. in Me-DLC coatings as a function of acetylene flow for sputtering targets composed of W, WC, Nb, and Ti.

cross-link to build up the carbon film on the target surface. W-DLC coatings prepared both from W and from WC targets reach nearly the same composition ŽWrC ratio. at high C 2 H 2 flow rates. Typical oxygen contents of high quality Me-DLC were in the range between 1 and 2 at.%. Quite importantly, a clear correlation between wear resistance ŽSection 3.43.4. . and oxygen content has been observed. Due to instabilities in the deposition process, the wear rates of Me-DLC coatings, in some cases, were rather high compared to baseline behavior. Nearly all of the coatings were determined to contain considerably higher oxygen contents Ž) 5 at.%.. The most probable explanation for this finding is that the amorphous carbon network surrounding the metal carbide particles is not so strongly cross-linked and can react much more readily with oxygen after venting the deposition chamber. Weakly cross-linked carbon films commonly are softer and exhibit higher wear rates. SIMS measurements revealed the depth profiles of metal and carbon as well as of the elements of oxygen and hydrogen. These data allowed a quantitative determination of the hydrogen contents Žsee w24x.. In films with low metal contents ŽMerC up to 0.3., hydrogen concentrations between 15 and 25 at.% were measured. The HrC ratios were in the range of 0.2]0.4. Fig. 7 shows a cross-sectional SEM image of a WDLC coating and an accompanying schematic illustration of the expected variation in metal concentration at a given location on a substrate within the processing chamber as a function of time. Initially, the flow rate for C 2 H 2 is nil, during which time ionized Ar sputters the metallic target and deposits onto the substrates. Following the deposition of the metals ŽW. innermost layer, the flow of C 2 H 2 is initiated and increased, and

229

a multilayer-like structure grows with a periodicity of approximately 30 nm. According to an approximate rough estimation, any point on a rotating substrate needs approximately 75 s to reach an identical position in front of a target. In that time interval, the increase of the film thickness is calculated to be between 20 and 40 nm. This indicates a relationship between microstructural periodicity and substrate rotation. Clearly, the bright fringes correspond to W-rich layers and the dark fringes to C-rich layers. SIMS measurements have confirmed the periodic oscillations of the tungsten content in the film. The dependence of the metal content on the substrate position was demonstrated by deposition experiments with fixed substrate positions and by operating two targets of the same composition. Fig. 8 shows that immediately in front of the target Ž08 position., the WrC ratios are clearly higher than in the position perpendicular to the targets Ž908 position.. Similarly, it is expected that the WrC ratio at a given position within a film being deposited onto a rotating substrate lies between these extremes. A plausible explanation for the positional dependence of the WrC ratios can

Fig. 7. Cross-sectional SEM image of a W-DLC coating with schematic depiction of oscillations in metal content.

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be given by considering the different scattering behavior of sputtered atoms with different atomic numbers, Z. The sputtered atoms will be scattered by Ar Ž Z s 18. on their way from the target to the substrate. In the present study, the tungsten sputtering target Ž Z s 74. is expected to have adsorbed species consisting of carbon Ž Z s 6. or hydrocarbon species, as discussed in Section 3.13.1. . These adsorbed species will be removed by Ar sputtering. Motohiro and Taga w25x used Monte Carlo simulations to predict that particles of lower atomic mass than that of the sputtering gas can reach a substrate shaded from the target more easily than atoms with a higher atomic number. Furthermore, in our experiments, the substrate-target distance, D S ] T , in the 908 position was clearly larger than that in front of the target Ž08 position.. The arrival rate of sputtered atoms at the substrate shows a much stronger decrease for heavy elements than for light elements with increasing D S ] T w25x. The dimensions of the carbide crystallites for the magnetron-sputtered coatings were not determined. However, Schiffmann et al. w26x investigated radio frequency Žr.f.. sputter-deposited Me-DLC films and reported a decrease in the particle radii with increasing melting point of the metal. According to these data, it is expected that the radii of the tungsten carbide particles in the current study are approximately 1 nm. In addition, the magnetron-sputter-deposited films reported here were investigated by X-ray diffraction. All samples with metal-to-carbon ratios, MerC, less than 0.3 showed only broad peaks, characteristic of amorphous material. To gain more insight on the structure of these films, better resolving methods, especially Transmission Electron Microscopy ŽTEM., are needed w26x. 3.4. Wear and friction beha¨ ior The abrasive wear behavior of the coatings was evaluated by using the above mentioned calotte tester. Fig.

Fig. 8. Atomic ratio of WrC as a function of acetylene flow for two fixed substrate positions on a rotating substrate.

Fig. 9. Abrasive wear rate as a function of atomic ratio, WrC, for W-DLC coatings.

9 shows the wear development of W-DLC coatings depending on the tungsten-to-carbon ŽWrC. ratio. The ordinate axis presents arbitrary units. To get the complete wear rate the number must be multiplied by 10y1 5 m3rNrm Žsee Section 22. .. The minimum wear rates were found at different C 2 H 2 flow values for the different target materials. Data from a large number of coatings were taken into account. To investigate the reliability and repeatability of the deposition process, the C 2 H 2 flow values were varied by "10 sccm around the value found for the wear minimum. Fig. 10 shows the results of these parametric variations. The columns in the diagram represent the wear range that can be achieved within the corresponding range of metal-tocarbon ratio. A comparison of different target materials demonstrates that W-DLC coatings offer the lowest abrasive wear rates by a significant margin. In contrast, WC-DLC coatings show remarkably higher values. Compared to W-DLC deposited using a W target, Ti- and Nb-DLC coatings possess intermediate wear rates. Nonetheless, all Me-DLC coatings investigated in this study clearly have lower wear resistance than metal-free DLC coatings. It is expected that the amorphous carbon matrix of DLC films is cross-linked much more strongly than that of Me-DLC coatings. The effect of metal constituent on the wear behavior remains unclear. It is likely that the dimensions of the crystallites influence the stability of the amorphous carbon network, with smaller crystallites favoring greater stability. However, to support such hypotheses, more information on the film structure, including the crystallite size and distribution, is required. Furthermore, an essential effect of the fast neutrals bombarding the growing film can be assumed. The bombardment of neutrals could explain the clearly lower wear rates of W-DLC coatings. The adhesive wear behavior, as indicated by the friction coefficient, is summarized in Table 2 for W-, Nb-, and Ti-DLC coatings. All measurements have been performed with a pin-on-disc tribometer using a

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Fig. 10. Range of abrasive wear rates for Me-DLC coatings prepared using various target materials. For comparison, the wear rate of metal-free DLC is indicated.

Table 2 Estimated contributions to substrate heating for sputtering targets composed of W, Nb, Ti, and C Žgraphite. a Coating

Friction coefficient m

Distance to fail Žm.

W-DLC Nb-DLC Ti-DLC

0.3 0.2 0.25

2600 5000 300

a

The numbers in the table correspond to heat fluxes in Wrcm2 .

sapphire ball as the counterpart at a sliding velocity of 0.8 mrs in a laboratory environment having 50]75% relative humidity. These conditions produce a Hertzian contact stress of approximately 2 GPa. In contrast to the abrasive wear behavior, the Nb-DLC coatings display the greatest adhesive wear resistance. In addition, the Nb-DLC films exhibit the lowest friction coefficients of the three Me-DLC coating compositions. Ex-

Fig. 11. Adhesion classifications for Me-DLC coatings prepared using various sputtering target compositions. The Cr interlayers with a thickness of 0.5 mm were deposited at Ub s y50 V.

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periments using steel counterparts instead of sapphire produce the same trend for the friction coefficients. 3.5. Adhesion The adhesion classification of the coatings was carried out by means of static Rockwell indentation at a 150-kg load. According to the well known VDI classification, a value 1 means excellent adhesion, and a value of 6 indicates complete delamination around the indentation. Fig. 11 shows the mean values Ž10 processes. for Me-DLC coatings prepared using target materials of different compositions. Especially for the W- and WCDLC films, a chromium interlayer leads to considerable improvement in the adhesion behavior. It is noteworthy that, for Nb-DLC coatings, no interlayers are required to achieve very good adhesion.

4. Summary and conclusions Me-DLC coatings prepared using different target materials and the corresponding deposition processes were studied and compared. The substrate heating during the film growth depends on the target material as well as on other deposition parameters. Obviously, for targets with higher atomic masses such as tungsten, the contribution of fast Ar atoms to the heat flux can not be neglected. From the viewpoint of resistance to abrasive wear, the best properties were found for W-DLC prepared using a W target. However, it should be taken into account that the cost of W sputtering targets is much greater than that of titanium by a factor of approximately 5. The Nb-DLC coating has an intermediate wear rate but offers the lowest friction coefficients and a high performance under adhesive wear loading. All investigated Me-DLC coatings exhibit clearly higher wear rates than metal-free DLC films. A possible means to reduce this difference is to increase the amount of cross-linking in the amorphous carbon matrix, e.g. by higher ion current densities at the substrate.

Acknowledgements The authors would like to thank their colleagues, Drs

P. Willich and K. Schiffmann, and M. Lutansieto from the Fraunhofer-IST, for the analysis ŽEPMA, SIMS. and tribological characterization of the Me-DLC and DLC coatings. References w1x C.-P. Klages, R. Memming, Mater. Sci. Forum 52r53 Ž1989. 609. w2x K. Enke, Thin Solid Films 80 Ž1981. 227. w3x H. Dimigen, H. Hubsch, R. Memming, Appl. Phys. Lett. 50 ¨ Ž1987. 1056. w4x A. Matthews, S.S. Eskildsen, Diamond Relat. Mater. 3 Ž1994. 902. w5x J. Guttler, J. Reschke, Surf. Coat. Technol. 60 Ž1993. 531. ¨ w6x G. Gille, B. Rau, Thin Solid Films 120 Ž1984. 109. w7x J. Robertson, Surf. Coat. Technol. 50 Ž1992. 185. w8x H. Dimigen, C.-P. Klages, Surf. Coat. Technol. 49 Ž1991. 543. w9x E. Bergmann, J. Vogel, J. Vac. Sci. Technol. A 5 Ž1987. 70. w10x K. Bewilogua, M. Grischke, J. Schroder ¨ et al., SVC 41st Annual Technical Conference Proceedings, Boston 1998, Published by Society of Vacuum Coaters, 1998, p. 75. w11x D.P. Monaghan, D.G. Teer, P.A. Logan, I. Efeoglu, R.D. Arnell, Surf. Coat. Technol. 60 Ž1993. 525. w12x M. Grischke, K. Bewilogua, K. Trojan, H. Dimigen, Surf. Coat. Technol. 74r75 Ž1995. 739. w13x A.A. Voevodin, S.V. Prasad, J.S. Zabinski, J. Appl. Phys. 82 Ž1997. 855. w14x A.A. Voevodin, J.P. O9Neill, S.V. Prasad, J.S. Zabinski, J. Vac. Sci. Technol. A 17 Ž1999. 986. w15x Q. Wei, R.J. Narayan, A.K. Sharma, J. Sankar, J. Narayan, J. Vac. Sci. Technol. A 17 Ž1999. 3406. w16x V. Yu Kulikowsky, F. Fendrych, L. Jastrabik, D. Chvostova, Surf. Coat. Technol. 91 Ž1997. 122. w17x K. Bewilogua, H. Dimigen, Surf. Coat. Technol. 61 Ž1993. 144. w18x O. Wanstrand, M. Larsson, P. Hedenquist, Surf. Coat. Technol. ¨ 111 Ž1999. 247. w19x M. Andritschky, F. Guimaraes, V. Teixeira, Vacuum 44 Ž1993. 809. w20x T. Kalber, Thesis, Technical University Braunschweig, Berichte ¨ aus Forschung und Entwicklung, 5, IRB-Verlag, Fraunhofer, 1998. w21x B. Window, Surf. Coat. Technol. 71 Ž1995. 93. w22x R.E. Somekh, J. Vac. Sci. Technol. A 2 Ž1984. 1285. w23x H. Windischmann, Crit. Rev. Solid State Mater. Sci. 17 Ž1992. 547. w24x P. Willich, R. Bethke, in: A. Benninghoven ŽEd.., Secondary Ion Mass Spectroscopy SIMS X, John Wiley, Chichester, 1997, p. 609. w25x T. Motohiro, Y. Taga, Thin Solid Films 112 Ž1984. 161. w26x K.I. Schiffmann, M. Fryda, G. Goerigk, R. Lauer, P. Hinze, A. Bulack, Thin Solid Films 347 Ž1999. 60.