Surface and Coatings Technology 177–178(2004) 552–557

0257-8972/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0257-8972Ž03.00926-5

The coefficient of static friction of silicon containing diamond-like carbonfilms

U. Muller*, R. Hauert¨

Swiss Federal Laboratories for Materials Testing and Research (EMPA), EMPA Dubendorf – Abt. 124, Uberlandstrasse 129,¨¨CH-8600 Dubendorf, Switzerland

Abstract

Diamond-like carbon(DLC) films are well known for their outstanding tribological properties, which has been shown in manystudies. Because of their extreme hardness, low coefficient of sliding friction(often after some running-in time) and especiallytheir ability to form a transfer layer on the counter-body of the frictional pair, these coatings have found different areas ofapplication. There are many different types of these coatings on the market depending on the deposition process and on thedeposition parameters used. The dependence of the film properties on the deposition conditions allows the tailoring of certainfilm properties to specific applications. Additionally, alloying with different elements allows further adaptations. A specialtribological problem is found in applications where almost no or only little motion takes place and therefore no transfer layer canbuild up. In this case a special coating is needed which has a low coefficient of friction already at the very beginning of anymotion. A similar situation is found in low-load applications where neither a transfer layer is created nor the transformation ofthe topmost surface layer into a more graphitic-like state takes place. In this study, silicon containing DLC films are produced ina RF plasma-activated chemical vapor deposition system. A mixture of acetylene and an organosilicon gas is used to depositsilicon containing DLC films onto hardened steel sample plates. On top of a supporting coating, a 30-nm thick DLC layer withvarying silicon content is deposited using different selected self-bias voltages. The coefficient of static friction against a hardenedstainless steel surface is then measured in dependence of the silicon content in the film and of the self-bias used during deposition.The films display a constant coefficient of static friction even after many single measurements. The results are compared toresults obtained for pure DLC films.� 2003 Elsevier B.V. All rights reserved.

Keywords: PACVD; DLC coatings; Tribological properties; Static friction; Humidity

1. Introduction

For many years, diamond-like carbon(DLC) coatingshave been used as protective coatings in a large varietyof applications. Such films consist of carbon and hydro-gen and are a hard form of amorphous hydrogenatedcarbon films(a-C:H). These films can be deposited bydifferent deposition methods, all having their advantagesand disadvantages. The main reason for their success indifferent applications is their low coefficient of friction(CoF) in combination with a high wear resistance.Excellent overviews on the deposition methods, tribo-logical behavior and other properties of amorphoushydrogenated carbon films have been written by Rob-ertsonw1,2x and Grill w3–5x. Especially the tribological

*Corresponding author. Tel.:q41-1-823-43-67; fax:q41-1-823-40-34.

E-mail address: ulrich.mueller@empa.ch(U. Muller).¨

properties of a-C:H films have been thoroughly investi-gated. In dry conditions, it has been shown that DLCcoatings have an extremely low coefficient of dynamicfriction although only after some running-in time. Inmost instances this is accompanied by a low wear rateof the coated part as well as of the often uncoatedcounterpart. The protection of the counterpart isexplained by the fact that a transfer layer is formed onthe counterpart during operation. Under certain condi-tions, however, this transfer layer cannot build up. Oneexample is in the case of no or only little movemente.g. under static conditions. On the other hand, one maystill expect to obtain a relatively low coefficient offriction considering the fact that DLC is extremely hardand chemically inert resulting in reduced interactionsbetween the contacting surfaces.In recent years, new theoretical work on the coeffi-

cient of static friction has been published by several

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Table 1The measured silicon contents(in at.%) of DLC films deposited froman acetyleneyTMS mixture in dependence of the acetylene and theTMS partial pressure(p ) and the self-bias(V ) used are listedTMS SB

C H pressure2 2 TMS pressure Self-bias voltage(V)(Pa) (Pa)

y80 y150 y300 y600

1.00 0.000 0.7 0.2 0.1 0.30.95 0.025 3.4 3.2 3.0 2.50.90 0.050 6.2 5.8 5.3 4.20.80 0.100 11.4 10.6 9.8 8.90.50 0.250 23.2 21.6 20.7 18.00.00 0.500 41.3 38.9 39.9 38.3

authorsw6–11x. However, experimental work regardingthe coefficient of static friction is still scarce, especiallyin the case of DLC films. In 1971, Rabinowicz publisheda very extensive study on the coefficients of staticfriction of 20 metalsw12x; whereas, the first authors tomention static friction of DLC were Grill et al. in 1990w13x. This was the motivation for our recent study ofthe static friction coefficient of DLC filmsw14x whereinalso a short literature review can be found.In the present work, we are investigating the influence

of the addition of silicon into the DLC films on thecoefficient of static friction. In earlier studies by Oguriand Arai w15,16x, it was shown that the coefficient ofdynamic friction stayed below 0.1 in ambient conditionsbetween 50 and 70% relative humidity(RH) for siliconcontents between 20 and 30 at.%. In later studies on theinfluence of silicon on the tribological properties ofDLC films in dynamic friction, it was shown by Tamorw17,18x, Gilmore et al. w19x and Gangopadhyayw20xthat silicon reduced the influence of the humidity in theenvironment on the tribological behavior. A similarresult was obtained by Smeets et al.w21x in pure nitrogenatmosphere, dry and at 50% RH. In ambient air at 50–60% RH, it was further shown by Kim et al.w22,23xthat silicon incorporation into DLC had a beneficialeffect on the tribological behavior in the sense that therunning-in time was reduced by up to factor of 10 andthe increase in the friction coefficient during the run-ning-in time was almost completely eliminated. Michleret al. w24x measured the dynamic CoF in dry artificialair and ambient air with 47% RH showing that the CoFnot only depends on the silicon content and the RH butalso on the oxygen content. Racine et al.w25x investi-gated the dependence of the friction coefficient on theenvironmental humidity for silicon containing tetrahe-dral hydrogenated amorphous carbon films.All the results mentioned above were obtained for

dynamic friction; whereas, the aim of this study was toinvestigate the combined influence of the self-bias andthe silicon concentration on the coefficient of staticfriction. Therefore, following the Taguchi method, alimited set of deposition parameters was chosen allowingan evaluation of the combined influence of siliconincorporation and self-bias on the static friction behavior.Results obtained in ambient air at 40 and 75% RH arepresented and compared to earlier results obtained forpure DLC films w14x.

2. Experimental

DLC films were produced from an acetyleneytetra-methylsilane(TMS) gas mixture by means of a RF(13.56 MHz) plasma-activated chemical vapor deposi-tion process. The chamber was an all stainless steelhigh-vacuum system with a base pressure better than5=10 Pa. The RF-powered electrode was capacitivelyy6

coupled to the RF generator and the power was regulatedto yield a constant self-bias. The substrates were loadedthrough a load-lock onto the RF-powered electrode. Thedeposition process consisted of first argon plasma clean-ing for 30 min at a self-bias ofy600 V and 2.5 Papressure. Then, an intermediate layer to insure adhesionwas deposited from TMS at a pressure of 0.5 Pafollowed by the deposition of a 2-mm-thick DLC filmat y600 V self-bias and 1 Pa acetylene pressure. Astop coating silicon containing DLC films with approxi-mately 30 nm thickness were deposited at the specifiedself-bias. The whole deposition process is computercontrolled and the plasma is on continuously withoutany interruption. The gas exchange takes place withinapproximately 30 s.Table 1 lists the measured silicon content in depend-

ence of the acetylene partial pressure, the TMS partialpressure and the self-bias used for the deposition. Thesilicon concentrations were measured using X-ray photo-electron spectroscopy(XPS) and values are alwaysgiven in units of atomic percent(at.%). Because hydro-gen cannot be measured using XPS, concentrations arenormalized to a total of 100 at.% neglecting the hydro-gen contribution. As can be seen from Table 1, thesilicon content varies slightly with self-bias. However,it depends linearly on the TMS partial pressure. Thedata in Table 1 can therefore be fitted to the TMS partialpressure and the self-bias using the following equation:

c s2.43q77.3p q0.00396VSi TMS SB

wherec is the silicon content in atomic percent,pSi TMS

is the TMS partial pressure in pascal andV is theSB

self-bias voltage in volt.Hardened steel 100Cr6(DIN 1.3505, AISI 52100) is

used for the sample substrates. The samples were tem-pered at 2508C to a hardness of 58 HRC, and had adiameter of 23.5 mm and a weight of approximately15.8 g giving an apparent contact pressure of 358 Pa inthe tribological measurements. The surfaces were mirrorpolished using 3-mm diamond powder. Five points werechosen within the two-dimensional parameter area ofself-bias, ranging fromy80 toy1000 V, and silicon

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Table 2Overview of the deposition parameters of the sample sets prepared

Sample Self-bias Si-content Surfaceset (V) (at.%)

Si (at.%) O (at.%)

Set 1 y80 4.3 2.6 12.9Set 2 y150 18.3 14.0 12.3Set 3 y300 0.0 0 8.0Set 4 y300 8.6 6.4 11.5Set 5 y600 1.7 1.3 10.5

Each set consists of four samples, coated in the same run. The self-bias is the RF self-bias used during deposition, the silicon content wasmeasured in the bulk part of the thin film on a piece of Si-wafercoated at the same time as the four samples. The surface concentra-tions of silicon and oxygen are also measured on the same film, afew weeks after the deposition. Hydrogen could not be measured andthe carbon content is the balance to 100 at.%.

content, ranging 0–20 at.%. For each of the five param-eter sets, four samples were polished just prior tointroduction into the vacuum system and were thensimultaneously coated together with a piece of Si(1 1 1)wafer for the XPS measurements. Therefore, a total of20 samples were used in this investigation. A summaryof the samples and the DLC film parameters is given inTable 2. The silicon content referenced to is the onemeasured in the bulk of the thin film during XPS depthprofiling. The silicon and oxygen concentrations meas-ured at the top surface are also included in Table 2.For the measurement of the coefficient of static

friction, the samples were carefully placed on a10=34=2.5 cm stainless steel X90CrMoV18 block3

(DIN 1.4112, AISI 440B) hardened to 54 HRC. Thesurface of this block was electrolytic-polished to asurface roughness of better thanR -0.2 mm. All thea

measurements in air were carried out in a clean benchto avoid contamination of the surfaces with dust parti-cles. The environmental conditions of the experimentsperformed in air were 238C and 40% RH. An additionalexperiment was carried out at 76% RH for comparisonpurposes with earlier experiments.The samples were then pulled horizontally by a slowly

increasing force and the forceF (equal to the frictionP

force F ) needed to start the sliding of a sample wasR

used to calculate the coefficient of static friction accord-ing to m sF yF (F being the normal force due tos P N N

the weight). To pull the samples, a vertically mountedspring balance was connected to the sample with a finethread redirected by 908 with the use of a roller with aball bearing. The sample stopped sliding after a certaindistance because of the reducing pulling force due tothe contraction of the loaded spring. The distancestraveled varied generally between 1 and 2 cm. In eachmeasurement run of one sample, 12 of these slidingcycles were made consecutively, keeping the idle timebetween the cycles as short as possible, approximatelybetween 5 and 15 s, to avoid the known effect of

increasing adhesion with elapsing time. The results ofthe first two cycles were consequently discarded allow-ing the measurements to be taken in a quasi-steadystate. Before starting each complete measurement series,the stainless steel block was whipped using pure alcohol.Also, each sample was cleaned with CO gas just prior2

to placing it on the surface for the measurement. Acomplete experimental measurement series consisted ofperforming a set of 10 measurements on each of the 20samples consecutively within approximately 1.5 h. Themeasuring sequence of the samples was, each time,randomly chosen and all the calculations and analysiswere carried out only after completing the experimentsof one series. Because the measurement of the pullingforce was done using a redirection, a calibration had tobe made to correct for the additional force required toovercome the resistance of pulling the thread over theroller. This calibration was carried out with two identical908 redirections, measuring the apparent weight ofknown masses, i.e. the force needed to start lifting thesemasses. The correction factor was then the square rootof the ratio of real mass to apparent mass. This correc-tion factor also depended on the environmental condi-tions; it therefore was measured each time immediatelyafter the completion of the measurements of the CoFsof one series. This calibration factor amounted, for aRH of approximately 40%, to 0.757–0.766 and to 0.833at 72% RH. For all statistical calculations of the errorbars, the 99% confidence interval equal to 2.58 timesthe standard deviation of the mean value is used.In total, three measurement series were measured in

air at 22–238C and 39–41% RH within a week. A fewdays later an additional series was measured in air at 248C and 75–77% RH to allow a comparison with theearlier measurements.

3. Results

Fig. 1 shows the measured coefficients of staticfriction of silicon containing DLC films in air vs. siliconcontent in the topmost DLC film. The upper curve wasmeasured in ambient air at approximately 238C and39–41% RH; whereas, the lower curve was measuredat 24 8C and 75–77% RH. The respective data pointsare slightly offset to the left and to the right for clarityonly and belong pairwise to the same silicon content.Each pair is further labeled with the self-bias used fordeposition. In the upper curve, each data point representsthe average of three measurements series of the foursamples of each set of the five Si-contentyself-biascombinations given in Table 2; whereas, in the lowercurve, each data point represents the average of onlyone measurement series. In both cases, the error barsdenote the corresponding 99% confidence interval. Theexact values of all points are all listed in Table 3. Thedotted lines between the data points serve only as guides

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Table 3Summary of the measured coefficients of static friction of Si-DLCfilms at two levels of RH

Sample Self-bias Si-content mSset (V) (at.%)

40% RH 76% RH

Set 3 y300 0.0 0.166"0.007 0.149"0.008Set 5 y600 1.7 0.176"0.005 0.156"0.008Set 1 y80 4.3 0.167"0.006 0.145"0.006Set 4 y300 8.6 0.170"0.007 0.155"0.009Set 2 y150 18.3 0.159"0.008 0.148"0.011

The statistical errors given are the 99% confidence limits of thestandard deviations of the mean value of all corresponding measure-ments together.

Fig. 1. The coefficient of static friction in air vs. the silicon content is shown for two different RH levels. The lines act only as guides to the eyeconnecting points measured at the same level of humidity. The data points(all listed in Table 3) are slightly spread horizontally for clarity andbelong pairwise to the same silicon content. Each pair is further labeled with the self-bias used for deposition. The error bars are the corresponding99% confidential intervals.

to the eye connecting points measured at the same levelof humidity.The first conclusion one can draw from this figure is

that the influence of the silicon concentration onto thecoefficient of static friction is hardly larger than theerror bars of the data and no obvious trend is visiblewithin a curve. The second conclusion to draw is that arelatively clear lowering of the CoF is observed whenthe humidity increases from 40 to 75% RH. The reduc-tion of the friction coefficient is between 10 and 14%.As a third conclusion, a slightly better reduction can befound at moderate silicon contents, namely at 1.7 and4.3 at.%. Towards lower, as well as towards higher,silicon contents the difference diminishes.In Fig. 2, the static CoF of silicon containing DLC is

compared to the CoF of pure DLC films, in both casesmeasured at approximately 75% RH. The results frompure DLC films, taken from Ref.w14x, are shown assmall dots connected by a thin line as a guide to theeye. The new data points represented by large squaresare labeled with their respective silicon content. In bothcases, the error bars correspond again to the 99%confidential interval. There is only one data point,located at the lowest self-bias, which is significantlydifferent between pure and silicon containing DLC;whereas, at higher self-biases practically no differencecan be seen independent of the silicon content.

4. Discussion

At the beginning of the discussion, we have toemphasise that the purpose of this paper is not toinvestigate the influence of surface morphology androughness on the coefficient of static friction. We there-fore did not use high contact pressures and took greatcare to prepare surfaces with the same morphology androughness. In consequence, each set of coating parame-ters was applied to four different samples, such as firstlyto eliminate statistically contributions from differentmorphologies and different contact areas. Secondly,using a thick DLC layer as a base layer and then onlya very thin layer of 30 nm as top layer lets one expect

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Fig. 2. The coefficient of static friction vs. the self-bias voltage is shown for pure DLC(small dots, from Ref.w14x) and silicon containing DLC(big squares, from Table 3), both measured at approximately 75 % RH. The thin line connecting the data from pure DLC(small dots) servesonly as a guide to the eye. The single data points are further spread horizontally for clarity but belong to the same self-bias voltage as denotedby the thick black line and the curly bracket. The new data are additionally labeled with the corresponding silicon content. The error bars showagain the corresponding 99% confidential intervals.

that further changes of the morphology are negligible.Additionally all samples were prepared according to thepredefined procedure as identically as possible. There-fore any difference in the static CoF is due to thedifferent deposition parameters, e.g. silicon content andself-bias. Furthermore, the fact that differences areobserved is due to changes of the surface chemistry andallows the conclusion that the observed behavior is notprimarily due to adsorbates. Naturally we cannot distin-guish between changed chemical interactions of the twotribologically interacting surfaces on one side andyordifferent adsorbing behavior of the adsorbates on theother side.Analyzing the data presented in Fig. 1 further reveals

a very slight decrease of the coefficient of static frictionwith increasing silicon content in air at 40% RH. At76% RH this slight decrease has vanished and overall,it has to be acknowledged that the static CoF dependsto a much higher degree on the ambient atmospherethan on the film properties determined by the self-biasand silicon content. As in our previous investigationw14x of the static CoF, we however find again a stablestatic CoF over many sliding cycles(a total of 40 inthis study) although the samples, this time did notundergo as many sliding cycles as in the last study.Quite surprisingly the increase in the RH from 40 to

76% resulted in a reduced coefficient of static frictionwhich may not have been expected from the results ofdynamic friction on pure DLC and silicon containing

DLC w17,19,24x. In the model by Oguri and Araiw15,16x, this behavior was explained by the promotionof water molecules bonding to the surface through thehydrogenation of silicon oxide present at their surfaces.In our case, however, we have a stable, oxidized silicon-containing surface which is from the beginning inequilibrium with the ambient atmosphere. On the otherhand, it was shown by Zhao et al.w26x in their studyon the stiction behavior of laser-textured hard disk mediathat the coefficient of stiction, which is comparable tothe coefficient of static friction, increases with RH. Thesame result has been obtained by Riedo et al.w27x inAFM experiments on CrN and DLC surfaces.The comparison of the new data on Si-DLC with the

old ones on pure DLC as shown in Fig. 2 is proof ofthe good reproducibility of our results, especially con-sidering the fact that all surfaces were freshly made forthe present study. The very small differences in thecoefficient of static friction between the two sets, exceptfor case of 5 at.% Si with a self-bias ofy80 V showsthe small, almost negligible influence of Si-content andself-bias on the static CoF. Therefore, it is to assess therelevance of the difference in the CoF aty80 V self-bias between pure DLC and Si-DLC with 5 at.% silicon.In all this considerations, we have furthermore to bearin mind that we could not discuss the influence of thehydrogen content in the films which also changes whenchanging the deposition conditions, e.g. self-bias voltageand reactive gas composition.

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5. Conclusions

We have shown that the coefficient of static frictionis not heavily influenced by the incorporation of siliconinto DLC films, in contrast to findings in the case ofdynamic friction. On one side this is disappointing buton the other hand it emphasizes that the effect of siliconin Si-DLC under dynamic friction is to change theinteractions between freshly exposed surfaces and theambient atmosphere, e.g. water but also oxygen. On theother hand, we have shown that well-prepared surfacescoated with DLC or Si-DLC exhibit a stable lowcoefficient of static friction. This makes DLC films,modified or not, a well-suited coating for applicationswhere little or no movement occurs but nevertheless astable and low coefficient of friction is required.

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