Residual Stress and Microhardness of DLC Multilayer Coatings

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ELSEVIER Diamond and Related Materials 5 (1996) 478-482

Residual stress and microhardness of DLC multilayer coatingsJianguo Deng, Manuel Braun

Royal I nsti tu te of Technology, Phy sics Department, Frescati vdgen 24, S-104 05 Stockhol m, Sweden

bstract

The residual stress and microhardness of diamond-like carbon (DLC) multilayer coatings (about 2.5 pm thick), prepared byunbalanced magnetron sputtering with various methane gas flow rates, substrate bias voltages and substrate bias ion currentdensities, have been measured. The experimental results show that the, residual stress value is strongly affected by the substratebias voltage and ion current density. The residual stress value increases with substrate bias voltage, reaches a maximum of 8.5GPa at a bias of - 150 V, and then decreases slightly. However, at a constant bias voltage of - 80 V, the residual stress increaseswith bias ion current density in the range studied here, and reaches a maximum of 8.0 GPa at a bias ion current density of3.0 mA cm-. Furthermore, a close correlation between microhardness and residual stress is observed. Increasing the methane gasflow rate to a certain value leads to higher residual stresses and microhardnesses. However, a further increase in the gas flow ratehas no obvious effect.Keyw ords: DLC; Sputtering; Residual stress; Microhardness; Multilayer coatings

1 IntroductionHard ceramic and diamond-like carbon (DLC) coat-

ings find numerous applications in industry; DLC anddiamond coatings are particularly attractive because oftheir excellent thermal conductivity and electrical resis-tivity, extremely high hardness and chemical inertness.These unique properties of DLC coatings make themexcellent candidates, not only for tribological applica-tions as a mechanically hard layer, but also for electronicpackaging, passivation, thermal heat sinks, etc. Ongoingresearch work, including different deposition techniques,has been extensive in recent years. After fabrication,many coated systems are left in a state of high residualstress. The presence of a residual stress may be beneficialor detrimental, depending on its value and the potentialapplication. In particular, brittle ceramic and DLCcoatings [l-3] are usually under high compressive stresswhich greatly increases their apparent strain to failure.If the stresses are too large, they can produce bucklingof the coating or plastic deformation of the substrateleading to failure of the interface. Also, residual stressesin coatings deposited onto thin substrates can causeunacceptable bending effects. Therefore in order to avoidthese and other potential problems, a knowledge of theresidual stresses in coated systems is important.Large compressive stresses are present in DLC coat-ings prepared at low temperature. This problem may0925-9635/96/ 15.00 0 1996 Elsevier Science S.A. All rights reservedSS Z 0925-9635(95)00463-7

lead to poor adhesion between the coating and thesubstrate, with large areas tending to spa11 off [4]. Thegrading of hard alloy nitride films through variousintermediate compounds of varying composition hasbeen shown to increase the scratch adhesion levels byat least 100% [5,6]. Earlier studies in the field haveshown that the optimum coating sequence in this caseis Ti, TiN, TiCN, Tic, DLC. The various compositionsare graded to give smooth boundaries between the layersby varying gradually the reactive gases [ 1,7]. The pur-pose of this study is to investigate the influences of themethane gas flow rate, substrate bias voltage and biasion current density on the residual stress and microhard-ness of DLC multilayer coatings, i.e. DLC/TiC/TiCN/TiN/Ti/substrate.

2. Experimental detailsThe coatings were deposited onto AISI 420 steelsubstrates using reactive unbalanced magnetron sputter-ing deposition techniques. High purity argon (99.99%)was used as the sputtering gas, and high purity methane(99.99%) and/or nitrogen (99.99%) were used as reactive

gases for coating deposition. The target material con-sisted of pure titanium (99.5%). The size of the targetwas 20 cm in diameter.The AISI 420 steel material had a hardness of 6.256


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J. Deng, M . BraunjD iam ond and Related Mat eri als 5 (1996) 478-482 419

HK (GPa). The compod.tion of the steel material was0.38% C, 0.8% Si, 13.6% Cr, 0.5% Mn, 0.3% V withFe balance. The samples were mechanically ground andpolished progressively to give a surface finish of about1 urn centre line average. The finished specimens wereultrasonically cleaned in acetone, methanol and distilledwater, and then dried and loaded into the magnetronsputtering chamber for deposition.

The chamber was pumped down to a backgroundpressure of at least 1.5 x lop4 Pa. The total pressure inall the deposition runs was in the range (2.0-3.0) x 10-lPa depending on the different coating processes. Thetarget power was set at 4 kW. The temperature of thesubstrate during deposition was maintained at about300 C. After deposition, the substrates were cooled toroom temperature in the vacuum chamber.2.1. Coat i ng procedure

DLC coatings with multi-interlayers, i:e. DLC/TiC/TiCN/TiN/Ti, were deposited onto AISI 420 steel sub-strates under varying reactive gas compositions. Thecompositions of the gas mixtures were controlledthrough the relative gas flow rates which were monitoredusing a combination of solenoid valves and gas flowmeters. Alternatively, the multi-interlayer compoundswere deposited by operating the reactive gases sequen-tially to form graded coatings which combine excellentsubstrate adhesion with high surface hardness [7].

During deposition, the volume flow rates of methaneand nitrogen were varied in the range O%-80% and23%-O% respectively. The volume flow rate of argonwas kept constant at 70%. In order to obtain coatingswith different stress and microhardness values, threeseries of DLC layers were prepared in which the varyingparameter in each series was: (i) the volume flow rate ofmethane which was increased gradually to final valuesof 30%, 40%, 50%, 60%, 70% and 80% CH,; (ii) thebias voltage which was varied in the range 0 V to - 300V; (iii) the ion current density measured on the substratewhich was varied in the range 1.02-3.0 mA cm-.

2.2. St ress and mi croha rdness measurementsVarious techniques, such as X-ray diffraction [8],

Newton ring observation [9,10], laser beam deflection[ 111, thickness gauge [ 1:2] and thin microfilm buckling[13], were used to obtain the strain or curvaturenecessary to determine the stress level. The most widelyused technique adopted in this study was to coat a thinsubstrate and measure the bending resulting from resid-ual stress after deposition.The intrinsic stress of the deposited coating is given

by Stoneys equation [ 141E,T, 1 1

c=6(1-v,)T, z -R,( >where E, is the Youngs modulus of the substrate mate-rial, v, is the Poissons ratio of the substrate, T, and Tfare the substrate and film thicknesses respectively andR0 and R are the radii of curvature of the substratebefore and after coating respectively.Specimen preparation was quite straightforward. Allthe substrates for the stress measurements were first cutto the desired shape, i.e. 40 mm x 10 mm. The substrateswere then thinned to the desired thickness of 0.18 mm.With the sample rigidly magnetically clamped flat, thesubstrate was thinned using a surface grinder with anSic abrasive wheel. This process must be performedvery slowly and carefully and the wheel should be welllubricated to prevent spurious residual stress being intro-duced into the substrate which would affect the specimencurvature.The radius of curvature measurements were made usingoptical interferometry [ 15,161 which gave surface heightreadings to about 0.1 pm resolution. The specimen wasplaced on a large, flat base plate with the coating facingupwards. The plate was moved under the optical interfer-ometer and height measurements were taken at regularintervals along the centre line of the specimen. Thisprocedure was repeated to ensure that the readings werereproducible. The height measurements were then pro-duced to evaluate the radius of curvature and stress levels.It should be noted that the measured stresses in allthe DLC coatings studied here consist of intrinsic stressesdue to microstructural imperfections of the interlayersand top DLC layer and thermal stress due to mismatchof the temperature expansion coefficients. All stressmeasurements were performed at room temperature,since low temperature stress levels are of interest in thisinvestigation. Although the measurements do not giveabsolute values of the DLC coating stresses, they indicatethe relative stress values of the DLC multilayer coatingsdeposited on the same substrate material.For the microhardness measurements, substratesamples consisted of discs 5 mm thick and 40 mm indiameter. Microhardness measurements were carried outwith a Fischerscope HlOO apparatus. This instrumentperforms microhardness tests dynamically under loadingwith a diamond Vickers indenter. During testing, theload is applied in quadratic increments for a fixed timeduring successive steps until a maximum load is reached.

3. Results and discussion3.1. I nfl uence of methanejow rat e on residual stress andmicrohardness

Figs. 1 and 2 show the effect of CH4 gas flow rate (orpartial pressure) on the residual stress and microhardness


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2;; 22 0 1 I I I I I I25 35 45 55 65 75 85

Gas flow rate (vol.%)Fig. 1. The influence of the CH, gas flow rate on the residual stressof the DLC coatings at a constant bias voltage of -80 V.

10 25 35 45 55 65 75 65

Gas flow rate (vol.%)Fig. 2. The influence of the CH, gas flow rate on the microhardnessof the DLC coatings at a constant bias voltage of - 80 V.

of the DLC multilayer coatings respectively. It is obviousthat increasing the CH4 partial pressure, after completionof the Tic interlayer deposition, generally gives rise toa slight increase in both the stress and microhardnessvalues up to a CH4 flow rate of about 70%. This effectis probably related to the increasing carbon concen-tration in the DLC coatings. For all the DLC multilayercoatings with different CH4 gas flow rates reported inthis work, the residual stress values were in the range3.3-6.5 GPa and the microhardness values were in therange 27.7-41.3 GPa.In order to investigate the carbon content in the DLCcoatings prepared at different CH4 gas flow rates, theC/Ti ratio of the DLC coatings was analysed byRutherford backscattering spectrometry (RBS) using2 MeV H+ ions with a scattering angle of 150. Thesemeasurements are summarized in Fig. 3, which presentsthe total carbon and titanium concentrations measuredin the deposited DLC coatings as a function of the CH,gas flow rate. The drastic decrease in the titaniumcontent in the DLC coatings reflects the reduction ofsputtered titanium for high CH, gas flow rates due toan increasing degree of target poisoning. In this case,carbon deposition onto the substrate coating occurs viatwo processes, namely direct sputtering of carbon (andsome titanium) from the poisoned target and directdeposition from the gas phase [7]. Both processes

201 i---- 10 t

25 35 45 55 65 75 85Gas flow rate (vol.%)

Fig. 3. The carbon and titanium concentrations in the DLC coatingsas a function of increasing CH, gas flow rate at a constant biasvoltage of -80 V.

contribute to the carbon content in the DLC coatingsand increase the C/Ti concentration ratio by loweringthe titanium concentration. This result confirms that acarbon- rich DLC coating enhances the hardness value.3.2. Influence of bias volt age on residual str ess andmicrohardness

The variation of the residual stress values of the DLCcoatings as a function of the bias voltage is presented inFig. 4. For bias values between 0 and -75 V, the stressvalue of the DLC coatings is lower than 4.5 GPa. Themaximum stress value is 8.5 GPa at a bias of - 150 V.A further increase in the bias voltage leads to a slightdecrease in the stress value. The change in stress is dueto ion bombardment of the DLC coating during itsformation. When the bias voltage increases, both theion current density and ion energy increase. Higher ionbombardment energy during deposition leads to anincreased lattice distortion and disorder effect, and finelyproduces a higher strain distribution in the coating.However, the situation changes when the bias voltageincreases above a certain voltage level. In this case, thehigher energy ions impinging on the surface of the DLCcoating during the coating procedure give rise to a

0. 10 50 100 150 200 250 300 360Substrate bias voltage (-V)Fig. 4. The influence of the substrate bias voltage on the residualstress of the DLC coatings at a constant CH, gas flow rate of 65%.


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J. Deng, M . BraunJDi amond and Related Ma terial s 5 (1996) 478-482 481

10 j 10 50 100 150 200 250 300 360Substrate bias voltage (-V)

Fig. 5. The influence of the substrate bias voltage on the microhardnessof the DLC coatings at a constant CH, gas flow rate of 65%.

resputtering effect, and the result is a more damagedcoating structure.Fig. 5 shows the results of the hardness measurementsas a function of the bias voltage. There is a strongcorrelation between the bias voltage and surface hard-ness. The hardness value increases with the substratebias voltage, reaches a maximum value of 41.2 GPa ata bias of - 150 V, and then decreases slightly. This effectis correlated with ion bombardment as the coating isformed. The energy imparted to the growing surface byion bombardment helps to anneal out imperfections inthe coating, but above a certain energy level there ismore damage induced by ion bombardment than isannealed out. It appears that this effect is observed whencomparing the hardness values of the coatings producedat bias values of zero to - 150 V and - 150 V to - 300 V.From the results above, both the residual stress andmicrohardness of the DLC coatings increase to a maxi-mum value at a bias voltage of about - 150 V.Additional bias voltage inputs during deposition haveno effect except to induce more damage in the DLCcoatings.3.3. nfuence of ion current density on residual stress andmicrohardness

The substrate ion current density during deposition,under the conditions studied here, has a large effect onthe residual stress and microhardness of the DLC coat-ings as can be seen in Figs. 6 and 7. The results showthat the residual stress and microhardness of the DLCcoatings are strongly dependent on the substrate ioncurrent density, which was set after the deposition of theTic interlayer. The residual stress values increase slightlywith the ion current density. The highest value is 8.0GPa at an ion current density of about 3.0 mA cme2.The microhardness values increase more rapidly as theion current density is increased, reaching a maximumvalue of 43.6 GPa.In the light of the experimental results, it was foundthat the DLC coatings pre:pared at an ion current density

0 1 . 10.6 1.0 1.5 2.0 2.5 3.0 3.6Ion current density (mA cm-)

Fig. 6. The influence of the ion current density on the residual stressof the DLC coatings at constant bias voltage (-80 V) and CH, gasflow rate (65%).

z.+ 254 20 t /b2 15

110 t 1 10.5 1.0 1.5 2.0 2.5 3.0 3.5

Ion current density (mA cm-a)Fig. 7. The influence of the ion current density on the microhardnessof the DLC coatings at constant bias voltage (-80 V) and CH, gasflow rate (65%).of less than 2.0 mA cmW2were matt black, had a porousmicrostructure, low microhardness and residual stressand contained a relatively large amount of impurities.In contrast, DLC coatings prepared under the sameconditions, but at an ion current density higher than2.0 mA cmm2, were compact, dense, bright black withless detectable defects on the surface area of the DLCcoating. For higher ion current densities, however, con-siderable residual stress can be generated within thehardest DLC coating, leading to an increase in thestored elastic energy and a reduction in the level ofcoating adhesion. Recent unpublished sputtering experi-ments carried out at floating potential and high ioncurrent densities (greater than 2.0 mA cm-) indicatethat it is possible to deposit low stress, compact DLCcoatings with different structures.

4. ConclusionsFor all the DLC multilayer coatings prepared atdifferent CH4 gas flow rates, the residual stress valuesare in the range 3.3-6.5 GPa and the microhardnessvalues are in the range 27.7-41.3 GPa. Increasing theCH, gas flow rate during deposition leads to an increasedC/Ti ratio, under the conditions studied here, and finally


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482 .I. Deng, M. Braunj Di amond and Related Mat erials 5 (1996) 478-482

influences the residual stress and microhardness of theDLC coatings.

The substrate bias voltage has a strong influence onthe residual stress and microhardness of the sputteredDLC coatings. As the bias is increased negatively from0 to - 300 V, the residual stress of the coating goes from0.7 GPa, to a peak value of 8.5 GPa at a bias of - 150V, and then decreases slightly to 7.9 GPa at a bias of-300 V. At the low bias levels of 0 to -50 V, theresidual stress of the coating is less than 2.7 GPa, whichis lower than that found for all other bias levels. Thehardness value increases with the substrate bias voltage,reaches a maximum value of 41.2 GPa at a bias of - 150V, and then decreases slightly.

High bias ion bombardment of growing DLC coatingshas a crucial effect on the residual stress and microhard-ness. Even at a bias voltage as low as -80 V, higherion current densities during the growth process help toobtain good coatings at low temperature. Ion currentdensities on the substrate of less than 2.0 mA cmm2 leadto low microhardness and residual stress values. Incontrast, DLC coatings prepared under the same condi-tions, but at an ion current density higher than2.0 mA cmm2, are compact, dense, bright black with lessdetectable defects on the surface of the DLC coating.However, higher substantial residual stresses can begenerated within the hardest DLC coating at an ioncurrent density of 3.0 mA cme2 and this leads to areduction in the level of coating adhesion.

AcknowledgementsThe authors wish to express their gratitude to Narings-och Teknikutvecklingsverket (NUTEK) and The

Swedish Research Council for Engineering Sciences(TFR) for financial support.

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Residual Stress and Microhardness of DLC Multilayer Coatings