Surface and Coatings Technology 174–175(2003) 421–426

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(03)00421-3

Investigations of the coefficient of static friction diamond-like carbonfilms

U. Muller*, R. Hauert¨

Swiss Federal Laboratories for Materials Testing and Research (EMPA)Uberlandstrasse 129, Duebendorf CH-8600, Switzerland¨

Abstract

Hydrogenated amorphous carbon films or diamond-like carbon films have shown their outstanding tribological properties inmany studies and in many applications. This is due to their extreme hardness, low coefficient of sliding friction(after somerunning-in time) and especially, their property to form a transfer layer on the counter-body of the frictional pair. Hydrogenatedamorphous carbon coatings describe a large class of different coatings with properties ranging from relatively soft to extremelyhard, from highly amorphous to more graphitic-like and from high hydrogen contents to hydrogen free, as well as any combinationof these. A special problem is found in applications, where almost no or only little motion takes place, because either the staticfriction cannot be overcome or the running-in cannot be completed, because the transfer layer cannot form. In all these cases, aspecial coating is needed, which has a low coefficient of friction, already at the very beginning of any motion. A similar situationis found in low load applications, where neither a transfer layer is created nor a transformation of the topmost surface layer intoa more graphitic-like state takes place. Diamond-like carbon films in this study are produced in a r.f. plasma-activated chemicalvapour deposition system from acetylene and deposited onto hardened steel plates. The coefficient of static friction against ahardened stainless steel surface is then measured in dependence of the self-bias, used for the topmost coating layer. Differentmeasurement methods have been used to prove the reproducibility. It was furthermore shown that these films displayed a constantcoefficient of static friction even after 200 single measurements. The overall average coefficient of static friction in air was0.158"0.002, and in a dry nitrogen atmosphere 0.126"0.003 as compared to 0.210"0.011 for uncoated surfaces in air.� 2003 Elsevier Science B.V. All rights reserved.

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

1. Introduction

Diamond-like carbon(DLC) coatings have been usedas protective coatings in tribological applications formany years. These films consist mainly of carbon andhydrogen and are a subclass of the amorphous hydro-genated carbon films. There are many deposition meth-ods common, each one having its advantages anddisadvantages. The technical reason for today’s wide-spread use of DLC coatings in all kinds of applications,is its low coefficient of friction(CoF) combined with ahigh wear resistance. More in depth information on theproperties and deposition methods of a-C:H and DLCcoatings can be found in referencesw1–5x.

Since the development of amorphous hydrogenatedcarbon films, their tribological properties have thorough-

*Corresponding author. Tel.:q 41-1-823-43-67; fax:q 41-1-823-40-34.

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

ly been investigated. It is well established that DLC has,especially in dry conditions, an extremely low dynamicfriction coefficient after some running-in time and alsoshows low wear. In tribological applications, it further-more protects the counterpart, because in most instancesa transfer layer forms on the counterface. It seemstherefore, only natural that DLC coatings also evokeinterest in applications with no or almost no movement,which consequently, resulted in this investigation on thecoefficient of static friction of DLC.

Several theoretical papers have been published inrecent years, regarding the coefficient of static frictiondemonstrating a growing interest in this topicw6–11x.Experimental work regarding the coefficient of staticfriction, however, is still scarce, especially on diamond-like carbon coatings. A very extensive study of coeffi-cients of static friction of 20 metals was realised byRabinowicz w12x. Many of his considerations haveinfluenced this investigation. To our knowledge, the first

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authors to mention static friction of DLC were Grill etal. in 1990 w13x. However, they did not present anyresults, and did not elaborate on that issue until theirnext paperw14x. There they found that the static frictioncoefficient of their as-deposited DLC-films was0.20"0.05. These DLC films were deposited by RFplasma decomposition of acetylene onto Si(100) sub-strates at a self-bias betweeny80 andy200 V, and apressure of 6.6 Pa. They did not, however, elaborate onthe measurement conditions. Deng and Ko were inter-ested in the static friction in conjunction with micro-electromechanical systems(MEMS) w15,16x. Theystudied 200 nm thick DLC films deposited by RF-PACVD, at a self-bias ofy500 V and 3.3 Pa hydrocar-bon gas pressure. The coefficient of static friction ofDLCyDLC contacts(after baking at 2008C for 20 min)were 0.040"0.005 in argon atmosphere or in ultra-highvacuum(UHV). The as-deposited films showed a CoFof 0.060"0.005 in UHV, therefore 50% higher than theone after baking. Bhushan and Koinkarw17x measuredcoefficients of friction on thin film rigid disks coatedwith DLC (DC magnetron sputtered in an ArqH2

plasma) of 5, 10 and 25 nm thickness. Static and kineticfriction measurements were performed using a commer-cial single-disk test stand. The static friction was cap-tured in the first 0.02 s, after the startup of the disk. Atypical value of 0.5–0.6 for the coefficient of staticfriction was measured. Snitka et al.w18x measured thestatic friction of synthesised diamond films, and inves-tigated the change of the coefficient of friction vs. thepolishing time. From initial values of 0.53, they obtainedfinal coefficients of static friction of 0.12. Unfortunately,the authors did not elaborate on the measurement pro-cedure. Beerschwinger et al.w19x conducted a frictionalstudy of micrometer bearings. They found valuesbetween 0.24"0.04 and 0.45"0.12 for the coefficientof static friction of DLC against polysilicon dependingon the contact pressure and the bearing design. Theydid not specify the conditions nor the method used forthe DLC deposition.

After this short literature review, it can be concludedthat so far only little work has been done to determinethe coefficient of static friction of diamond-like carbonfilms. Additionally, the large scatter of the publishedvalues of the coefficient of static friction indicates thehigh sensibility of these values on the experimentalsetup. Furthermore, no systematic investigation on thissubject has been conducted, and it is therefore, one ofthe aims of this study to present a first basic set ofresults. Different 10 nm thick DLC films produced byRF plasma, activated chemical vapour deposition(PACVD) by only changing the self-bias under, other-wise identical conditions are investigated. Resultsobtained in air with approximately 75% relative humid-ity and in dry nitrogen atmosphere are presented here.

2. Experimental

The DLC films were produced by RF(13.56 MHz)plasma activated chemical vapour deposition from acet-ylene gas in an all stainless steel high-vacuum system,with a base pressure better than 5=10 Pa. They6

substrates were mounted on the RF powered electrode,which was capacitively coupled to the RF generator.The power was regulated to yield a constant self-bias.In the deposition process, the substrates were firstcleaned with argon plasma for 30 min at a self-bias ofy600 V and 2.5 Pa pressure. Then an intermediatelayer to insure adhesion was deposited from tetrame-thylsilane at a pressure of 0.5 Pa, followed by thedeposition of a 2mm thick DLC film at y600 V self-bias and 1 Pa acetylene pressure. On top of this coating,the final DLC films with a thickness of 10 nm weredeposited at the specified self-bias. The whole depositionprocess is computer controlled and the plasma is contin-uously on without any interruption. The gas exchangetakes place within approximately 30 s.

All test substrates were made of hardened steel100Cr6 (DIN 1.3505, AISI 52100) with a diameter of23.5 mm and weight of approximately 15.7 g, corre-sponding to a contact pressure of 355 Pa in the tribolog-ical measurements. The surfaces were mirror polishedusing 3 mm diamond suspension. For each self-biasvoltage chosen four substrates were polished just priorto introduction into the vacuum system and were thensimultaneously coated. Five values within the availableself-bias voltage range were chosen for this investiga-tion, namelyy80, y150,y300,y600 andy1000 V,giving a total of 20 samples.

For the measurement of the coefficient of staticfriction, the samples were placed on a 10=34=2.5 cmstainless steel X90CrMoV18 block(DIN 1.4112, AISI440B) hardened to 54 HRC. The surface of this blockwas electrolytic-polished to a surface roughness of betterthan R -0.2 mm. All the measurements in air werea

done in a clean bench to avoid contamination of thesurfaces with dust particles. The environmental condi-tions of the experiments performed in air were 22–248C and 72–76% relative humidity. The measurementsof the CoF in dry nitrogen atmosphere were done in aglove bag, with a constant flow of pure nitrogen(99.9%purity) at a temperature of 238C. The glove bag wasfirst purged several times, until a relative humidity ofless than 1% was measured.

The samples were then pulled and the forceF (equalP

to the friction forceF ) needed to start the slipping ofR

a sample was used to calculate the coefficient of staticfriction according tom sF yF . Three different meth-s P N

ods were used to apply the pulling force. In the firstone, the samples were pulled horizontally using a springbalance with a resolution of 0.1 g and 10 g maximumload. The vertically mounted spring was connected to

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the sample with a fine thread redirected by 908 using astiff stainless steel wire of 1.5 mm diameter. The secondmethod was using the same spring balance connectedwith the same thread to the sample, but this timeredirected by 908 with the use of a roller with a ballbearing. In the third experimental setup, the sample wasplaced on an inclinable plane, and the angle when thesample started slipping was used to calculate the CoFaccording tom stan a. In the first two methods, thes

sample stopped slipping by itself after a certain distance,because of the reducing pulling force due to the con-traction of the loading spring. In these cases, the distancetravelled varies between 1–2 cm. For the inclined plane,a metal wire placed less than 1 cm away from thesample stopped the slipping sample, the inclination wasthen reduced and the stopper moved to the next position.In each run 12 of these slipping cycles were madeconsecutively, keeping the idle time between the cyclesas short as possible, approximately between 5 to 15 s,to avoid the known effect of increasing stiction withelapsing time. The results of the first two cycles wereconsequently discarded allowing the measurements tobe taken in a quasi steady state. Also, before startingeach complete measurement series, the stainless steelblock was whipped using pure alcohol and before eachsample eventual dust particles were blown off usingCO gas. Also each sample was blown with CO gas,2 2

just prior to placing it on the surface for themeasurement.

Each complete series consisted of measuring a set of10 measurements of all 20 samples consecutively withinapproximately 3 h. In total, 5 series have been measuredin air within a week, yielding a total of 1000 singleCoF values(in air). The first two series were measuredusing the stainless steel wire type redirection, the nexttwo with the roller as redirection and the last one wasmeasured on the inclined plane. In dry nitrogen atmos-phere, only one measurement series was made at somelater time using the spring balance with the roller asredirection. The measuring sequence of the samples waseach time randomly chosen, and all the calculations andanalysis were done only after completing the experi-ments of one series. For the first two methods using aredirection of the pulling force, a calibration had to bemade to correct for the additional force required toovercome the resistance of the pulling thread on thesteel wire or the roller. This calibration was done using,in each case, two identical 908 redirections and meas-uring the apparent weight of known masses, e.g. theforce needed to start lifting these masses. The correctionfactor is then the square root of the ratio of real massto apparent mass. In the case of the metal wire used asredirector, the correction factor is 0.678, whereas in thecase of the roller it is 0.869. For all statistical calcula-tions of the error bars, the 99% confidence interval is

equal to 2.58 times and the standard deviation of themean value is used.

For reference purposes, uncoated samples have alsobeen measured in air. These measurements were quiteunstable and not really reproducible. Each sample, threein total, gave consistent results only during the firstmeasurement series. Already after this first series, thesurfaces of the samples showed some streaks, although,not yet really scratches. For the sake of consistency inthe reported results and even more important reproduc-ibility, the area of the stainless steel block on whichthese measurements were done was not used thereafterfor any other measurement again.

3. Results

Fig. 1 and Fig. 2 present the measured coefficients ofstatic friction of DLC against hardened stainless steel inair vs. the self-bias of the topmost diamond-like carbonfilm. In Fig. 1, the data points represent the average ofthe four samples with the same DLC film separate foreach measurement series to illustrate the scatteringbetween the five measurement series(3 different meth-ods and 2 repeated ones), whereas in Fig. 2, the datapoints represent the average of all measurement seriesfor each single sample to illustrate the scatter betweenindividual samples. In both figures, the data points arespread horizontally at each self-bias voltage for clarityonly. In Fig. 1 additionally, curly brackets encompassthe points, which belong to the same bias voltagemarked by the corresponding thick vertical lines. Ateach self-bias voltage value, the symbols are arrangedfrom left to right in chronological order, the samesymbols at different self-biases, but at the same respec-tive position belonging together to the same series. Thesquares, connected by a line as a guide for the eye,represent the average of the measured CoF values forthe corresponding self-bias. The dashed horizontal linedenotes the average of all measurements done in air, thetwo thin lines above and below indicating the corre-sponding error bar. The exact values of these measure-ments are listed in Table 1.

As a first result, three general conclusions can bedrawn from this figure:(1) no difference was observedfor the different measurement methods used,(2) noaging of the films as such, or due to environmentalreactions of the surfaces, was observed(at least notwithin a week) and (3) no changes(surface roughness,wear or removal of the top 10 nm layer) of the filmsdue to repeatedly measuring the coefficient of staticfriction was observed. It further seems thatm shows as

slight dependence of the self-bias with the lowest valueat approximatelyy150 toy300 V.

Fig. 2 shows the CoF data of each sample averagedover all five measurement series. As in Fig. 1, the dataare again spread horizontally for clarity. And again the

424 U. Muller, R. Hauert / Surface and Coatings Technology 174 –175 (2003) 421–426¨

Fig. 1. The coefficient of static friction in air vs. the self-bias voltage is shown for each measurement series. For each self-bias the data pointsare spread horizontally for clarity, but belong to the same self-bias voltage as denoted by the thick black line and the curly bracket. In each group,the symbols are arranged from left to right in chronological order, the same symbols at the same respective position denoting the same series.The error bars are the corresponding 99% confidential intervals. The big squares show the average for each self-bias voltage(as given in Table1) and are connected by a line as a guide for the eye. The dashed thick line is the average of all measurements, and the two thin lines aboveand below denote the error bar of the total average.

Fig. 2. The coefficient of static friction vs. the self-bias voltage is shown for each sample as a single point. The dashed horizontal line togetherwith the two thin lines shows the same as in Fig. 1. The squares are also the same ones as in Fig. 1. The error bars show again the corresponding99% confidential intervals.

squares, connected by a line as guide for the eye,represent the average of the measured CoF values forthe corresponding self-bias. Also, the horizontal dashedline together with the two thin lines above and below isthe same as in Fig. 1. Despite the different representa-tion, the overall view is very similar to Fig. 1, except

for the two samples far away from the others. Because,the preparation was done as identical as possible, themost probable cause for this difference lies presumablyin the grinding step, since this was the only one donemanually one after the other. The final polishing with 9mm followed by 3mm diamond suspension was made

425U. Muller, R. Hauert / Surface and Coatings Technology 174 –175 (2003) 421–426¨

Table 1The averages of all measurements of the coefficient of static friction in air for each single self-bias voltage are given together with the statisticalerror based on the 99% confidence limit of the standard deviation of the mean value of all corresponding measurements together. The overallaverage of all the different DLC coatingsm in air is 0.158"0.002s

Self-biaswyVx Coefficient of static frictionm (SB)s Error (99% confidence limit)

80 0.162 "0.004150 0.152 "0.004300 0.154 "0.004600 0.156 "0.0041000 0.167 "0.006

Fig. 3. The filled circles represents the coefficients of static friction of the different DLC coatings measured in a dry nitrogen atmosphere(relativehumidity -1%) with the dotted line showing the total average of the CoF in dry nitrogen. The diamonds with the shortly dashed line are theaverage results in air from Fig. 1. The line with long dashes represents the initial coefficient of static friction of uncoated steel samples in air.

on a polishing machine, everytime with all four samplesdestined for the same DLC coating together. However,no differences regarding scratches, dust or texturebetween the sample surfaces could be seen and optically,the surface finish was the same for all samples. Unfor-tunately, the two outliers are located at the end pointsof the self-bias measurement series, and influence theaveragem at these points, shifting it in both cases tos

higher values increases the impression of increasingcoefficients of static friction at the end points of theself-bias range investigated. A closer look at Fig. 2,however, shows that the increase of the CoF aty80 Vandy1000 V self-bias would still be present, althoughless pronounced. Because we have no direct and obviousevidence for the special result of these two samples, wedid not discard them and kept them in the analysis.

In Fig. 3, the results of the measurements of the CoFin a dry nitrogen atmosphere are shown and comparedto the averaged ones in air. The overall average of theCoF of all DLC coated samples in air is 0.158"0.002,whereas the one in dry nitrogen is 0.126"0.003. This

corresponds to a reduction in friction of slightly morethan 20%. For comparison purposes, the initial staticCoF of uncoated steel samples is included in this figure,it has a value of 0.210"0.011 in air.

4. Discussion

Although the quality of the surface finish certainlyplays an important role in friction measurements, adetailed topographical analysis of the real surfaces wasnot within the scope of this study. However, from theway the experiments were carried out and from ourresults, we can conclude that we measured mostly thereal tribological influence of the DLC coatings on thestatic friction against hardened stainless steel. We espe-cially took great care to a consistent and as identical aspossible preparation of our samples allowing a directcomparison of the results. Even so, it is almost impos-sible to quantify absolute measurement errors restrictingthe possibilities for comparison with experimental datafrom other sources, all the data presented can be com-

426 U. Muller, R. Hauert / Surface and Coatings Technology 174 –175 (2003) 421–426¨

pared with each other on an absolute scale. Therefore,the result that the DLC coated samples exhibit a lowerstatic CoF is clearly the consequence of changes of theinteraction of the sample surface with the one of thecounterpart under the given environment, but not due totopographical changes. Our results of the coefficient ofstatic friction m , are in the same range as the coeffi-s

cients of dynamic frictionm reported consistentlyk

between 0.1 and 0.2w20–23x for DLC in air, which isquite surprising. This agreement may, however, be purelycoincidental considering the many possible differencesbetween all these measurements, e.g. surface finish,load, sliding speed and eventual tribochemical reactionsoccurring due to wear, especially considering the build-up of a transfer layer observed in pin on disk experi-ments of DLC coated surfaces. A very important resultof this study is that DLC yields a stable coefficient ofstatic friction at loads, which already make hardenedsteel surfaces exhibit an unstable static CoF due to wear,although the sliding distances are small.

Regarding the self-bias dependence of the static CoFin air, as well as in dry nitrogen with a minimum ofapproximatelyy150 V, resembles strikingly the inverseof the dependence of the hardness on the self-bias asshown by Robertsonw1x with a maximum hardness atapproximatelyy200 V self-bias obtained for PACVDdeposited DLC films from acetylene. One may under-stand this on the basis that harder surfaces show a lowertendency to adapt to a counter surface, and thereforereducing the effective contact area resulting in a lowerfriction. But other causes, as for example differentinteractions of the surface with the surrounding atmos-phere, have to be considered too.

The standard deviation of a single measurement sethas been found to be approximately 0.017 for air and0.008 for dry nitrogen, calculated from 100 measurementsets in air and 20 in dry nitrogen. The difference instandard deviation could already be observed during themeasurements, and is in accordance with the observationthat the distance the samples travelled, when theyslipped, was much shorter in dry nitrogen atmospherethan in air. This again is in accordance with theexplanation of the sticking depending on the rest timeas due to the development of menisci at the contactareas between the two surfacesw24x. This effect is muchmore pronounced in humid air than in a dry nitrogenatmosphere keeping in mind that even in a dry nitrogenatmosphere there is still enough water around to form athin layer on all surfaces.

5. Conclusions

Summarising, we generally can say that the low CoFvalues obtained for all surfaces are proof of good surfacequality of the stainless steel block, as well as thesamples. The differences between the uncoated and thecoated surfaces show the possible improvements, where-as the reproducibility of the coefficient of static frictionof the DLC coated samples, is proof of the gain inendurance attainable through the protective coating andfor this the most relevant result. As expected, thecoefficient of friction is still further lowered in drynitrogen atmosphere. But in both cases the same depend-ence, although weak, on the self-bias of the depositedDLC films can be seen, namely an inverse behaviour ascompared to the hardness dependence on the self-bias.

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