30 Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 1, 2001,

Pul sed laser deposition (PLD) of diamond-like carbon (DLC) thin film onPolymethylmethacrylate (PMMA) and tool steels

Y. Perera a=, G. Schlaghecken a, J. Gottmann a, T. Klotzbücher v", E. W. Kreutz " and R. Poprawe=

a Lehrstuhlfür Lasertechnik, RWTH Aachen, Steinbachstrafie 15, D-52074 Aachen, Germanyb Present adress: lnstitut für Mikrotechnik Mainz, Carl-Zeiss-Strafie 18-20, D-55129 Mainz, Germany

e Fraunhofer-Institut für Lasertechnik, Steinbachstrafie 15, D-52074 Aachen, GermanyPerera@llt.rwth-aachen.de

ABSTRACT

Pulsed laser deposition with Krf-excimer laser radiation (11,= 248 nm; t = 25 ns) is used to grow diamond like carbon(DLC) thin films on PMMA and tool steel substrates. The investigations are focused on the adherence of the DLC thin films.The pre-treatment of the metallic samples was a corros ion process to passivate the surfaces and of the PMMA samples thedeposition of a titanium buffer layer as well as a mechanical surface modification. The analytical techniques used to determinethe structural and chemical characteristics of the films are Raman spectroscopy, X-ray photoe1ectron spectroscopy, opticaland electron microscopy. The hardness and the elastic modulus of the films are measured by nano-indentation, The adherencecharacteristics of the DLC thin films are compared with the film s of Zr02 and AlP3 deposited by PLD on PMMA.

Keywords: Pulsed Laser Deposition, DLC, tool steel, PMMA.

1. INTRODUCTION

Thin films of DLC can be produced by pul sed laserdeposition (PLD) with a varietjg.of thickness on differentsubstrates. DLC thin films have wide applications intechnology and science owing to its unique properties likeextreme hardness, chemical inertness, optical transparency,high dielectrié constant and electrical resistivity. DLC thinfilms deposited on hard metals is of industrial interest forprotection in applications like cutting tools (drills and mills),medical prosthesis and surgical instruments with excellentbio-compatibility [1,2]. Polymethylmethacrylate (PMMA)has been used in the biomedical industry for manufacturingvarious implants. The long-term failure of the PMMAprostheses is now believed to the directly or indirectly dueto its lower hardness, its wear resistance, and itsvulnerability to biomedical degradation. Therefore, a DLCcoating on PMMA should reduce these problems and extendprostheses life [3].

PLD is an altemative growth technigue ofDLC thin filmsand the purpose of this investigation is to determine whetherPLD can be utilised to grow DLC thin films and to improvethe adhesion of hard DLC thin films on tool steels andPMMA with different pre-treatments. The nucleationphenomena play a key role in the determination of thegrowing morphology and the film guality during depositionof DLC. The nucleation mode strongly depends upon thepre-treatment technique used for the substrate as well asupon the deposition procedure[ 4]. In the case of tool steel a

corrosion process (passivation) is employed and in the caseof the PMMA a titanium buffer layer and also a mechanicaldressing process are used. The application of buffer layersor the mechanical modification of the polymers surfacesare prarnising technigues for the deposition of more adherentDLC films on PMMA.

The morphology of the DLC thin films is characterisedby optical microscopy"and scanning electron rnicroscopy(SEM), the presence of Sp3 and Sp2bonds is analysed bymicro-Raman spectroscopy. Analytical technigue used todetermine the structural and chemical characteristics of thefilms is X-ray photoelectron spectroscopy (XPS). Thehardness and the elastic modulus of the films are measuredby nanoindentation.

2. THEORETICAL BACKGROUND

2.1 Pulsed Laser Deposition

In PLD, the laser radiation is directed on a target locatedin a vacuum chamber. The impact of the laser radiation onthe target surface results in various complicated processesincluding removal, melting, evaporation of material and/or plasma generation due to excitation and ionisation ofthe species ejected from the target by the laser photons. AlIthese processes are triggered by a conversion into thermal,chernical, and mechanical energy. The material ejected fromthe target is deposited on a substrate generally positionedopposite the target [5].

y. Perera y col. /Revista Latinoamericana de Metalurgia y Materiales

2.2 Passivation

A number of reactive metals that come in contact withcorrosive environments can abruptly turn into an extreme1ycorrosion resistant state due to a phenomenon called"passivation" [6]. In passivating, a metal reacts to form alayer of corrosion products so thin that it is invisible and socomplete a barrier that it slows corrosion by several ordersof magnitude. This thin passivation layer is largely composedof amorphous oxides and hydroxides of the metal [6].

oApparently during the HzSO/HzOz treatment of Co

containing tool steels a thin CoO/CoS04 film forms [7],which prior to the diamond deposition by chemical vapordeposition (CVD) would be reduced to Co/CoS, controllingthe diamond growth [7]. Previous work reported that theCo in the hard metals binder phase could howeverdetrimentally influence the chemical vapor deposition ofdiamond. Also the Co vapour pressure, surface migrationand carbon solubility are main parameters influencingdiamond nucleation and growth and coating adhesion [7].

3. EXPERIMENTAL DETAILS

3.1 Set-up

A KrF excimer laser (AL=248 nm,L 'tL= 25 ns) is usedfor pulsed laser deposition in a UHV chamber which wasevacuated by a turbo molecular pump to a base pressure oftypically 106 mbar before deposition. The pulsed laserradiation was focused on a high purity (99.999%) graphitetarget with an angle of incidence of 45°. The laser typicallyruns at a repetition rate of 1 Hz for the production of aninitial coating for 1000 pulses that helps the nucleation ofDLC on the pre-treated surfaces and for the deposition ofthe main coating a repetition rate of 10Hz is used for 5000pulses. The target substrate distance is 4 cm. The depositionis carried out using argon as processing gas with pressuresof 10-5 - 10-4 mbar, and fluence EeL = 3,3 Jrcm-.The substrates are cleaned before the deposition of DLCusing a r.f system by Ar" bombardment for 5 min, employinga processing gas pressure of l O? mbar, to reduce thecontamination by oxygen and OH- arising from residualwater vapour.

3.2 Pretreatment of tool steels

Distinct improvement of the adhesion of diamond-likecarbon coatings on the tool steel substrates is achieved bysubstrate surface pre-treatment: The samples are polishedwith a diamond solution and washed in ultra-sonic withethanol for 5 min before the corros ion treatment. For thetool steel substrates two different passivation processes are

31

used: the first is to grow CrZ03 coating from the Cr into thetool steels using a 50% HN03 solution [6], the second is toform an amorphous CoO/CoS04 film from the cobalt contentin the tool steels using a HzSOiHP2 solution (8% H2S04in 54% HP) [7]. For the both cases the solutiantemperature was 80°C, and the time was about 5 mino Thereaction of the samples during the corrosion process ishigher and the gas bubbles sweep across the surface, afterthat the samples were washed in ultra-sonic with ethanolfor 5 minoThe chemical composition of the tool steels beforethe pre-treatment is shown in the table l.

3.3 Pre-treatment of PMMA

Two different processes are used to increase the adhesionof DLC coating on virgin PMMA. The first pre-treatrnentis to deposit a Ti buffer layer on PMMA by DC sputtering,the second is a mechanical grinding of the PMMA with agrinding paper 1200. The surface shows a homogeneousdistribution of scratches and this new morphology is usedto improve the adherence and to prevent the delaminationof the DLC coatings.

3.4 Surface analysisThe morphology of the DLC films is analysed by opticaland scaning electron microscopy (SEM). The amount ofSp2 and Sp3 bonds are determined by micro-Ramanspectroscopy (the spectra were excited with Ar ion laserradiation, A= 488nm). Analytical technique used todetermine the structural and chemical characteristics of thefilms was X-ray photoelectron spectroscopy (XPS) usingnon-rnonochromatised MgKa radiation (hv = 1253.6 eV).The hardness and the e1astic modulus of the films aremeasured by nanoidentation using an nanoindenter XP witha target depth of 50 nm.

4. RESULTS AND DISCUSSION

4.1 Pre-treatments

For de tool steel substrates passi vated with the corrosionprocess, the nucleation of DLC on the chromia after thepassivation is not possible and graphite is produced, maybebecause the Crp3 is not a tip of surface protrusion for apreferential nucleation of DLC as occurring with carbidephases, such W, Si, Mo and so on. This implication of themodel is supported by a large number of observations ondiamond nucleation on other surface protrusions[4]. Theadherence on the tool steels was improved by the productionofCoO/CoS04 passivating layers. In the case ofthe PMMAsubstrates, the samples with a surface modification showstronger adherence that the one with a Ti buffer layer

32 Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 1, 2001.

Table 1 Chemical composition of the tool steels used as substrates

:Mteria C% ~% Ml% P::::%ss% Co% 0% Ml% V% W%13.202 1.30-1.45 ::::0.45 ::::0.40 0.030 O.()3() 45),5.00 3.ID4.50 0.70-1.00 3.:n4.00 11.5-125

13.207 1.20-1.35 ::::0.45 ::::0.40 O.()3() O.cr.JÜ 9.50-10.5 3.ID4.50 3.20-3.~ 3.00-3.50 9.00-10.0

(a) (b)

Fig. 1 SEM of DLC deposited by PLD on PMMA and tool steels; 3,3 Jzcm-, 6000 pulses, p = 10-'mbar. (a) PMMA with thesurfaces grinding modification, (b) DLC on the tool steel 1.3207 passivated, (e) DLC on the tool steel 1.3202 passivated.

4.2 Optical and Scanning Electron Microscopy

Fig. 1 shows the SEM results of the systems with theimproved adherence and hardness obtained for thisinvestigation. The growth of DLC on PMMA with themechanical surface modification (fig. la) is observed. Theinhomogeneous surface morphology and microcracks in thefilms are also observed. For the virgin PMMA samples thedeposition of DLC as observed by optical microscopy showdelamination of the coating. According to [8] a partialdelamination of A1203 films with more than 500 nm inthickness takes place at the A1203-PMMA interface. Thedelamination is caused by large compressive stresses in thefilms and low adherence of the film. A titanium buffer layera few nanometer in thickness between the PMMA substrateand the DLC film does not change the adherencesignificantly. Similar results are obtained using a buffer Zr02

layer for the deposition of A1P3 on PMMA and also PCsubstrates [8],-

The adherence of the films deposited on polished toolsteels without a passivation process was not possible andthe thin films are easy removable from the surface. For thedeposition on passivated tool steels the formation of a CoO/CoSO 4 film is the more interesting method because thislayer improves the adherence between the DLC/steelsinterface [7]. Figs. lb and lc show the DLC films depositedon tool steels after the passivation process and these coatingshow an excellent adherence, but the pinholes on the

. coatings are produced from the previous pinholes in theoriginal steels surface. This problem was detected from theanalysis by optical microscopy before the corrosión-passivation process.

4.3 Raman Spectroscopy

The Raman spectra of the films deposited on toolsteels are shown in fig 2.

~ •...ni

ji,j,&.-?·c,;~.< (e) 1.3202 Passivated p == lO-s mbar '

~""::~'O'fr,m".'·,1QW.,~.. .1ilW·.· .: o .~ '1-roy' :l!..'1ÓO'::~

?ek~~l&o~'Fig. 2 Raman Spectra from DLC deposited by PLD on tool steels,3,3 J/cm2,6000 pulses. (a) Both steels show a identical spectra,p = 1O·4mbar; (b) Both passivated steels show the same spectra,p = lO-4mbar; (e) 1.3202 passivated tool steel, p = IOvrnbar: (d)1.3207 passivated tool steel, p = 10-5mbar.

With increasing processing gas pressure during the PLDthe intensity of the G-band decreases and the D-bandappears, in agreement with the previous observations [9].For all the samples deposited at 10-5 mbar and differentpre-treatment the spectra are identical with a peak at 1594.5cm", corresponding to a spt-fraction of 60% [10] The D-band is

y. Perera y col. /Revista Latinoamericana de Metalurgia y Materiales

not observed. By increasing the processing gas pressureto 10-4mbar the films show the two usual features seen insp--bonded carbon, the G peak around 1560 cm' and theD-band with a mode around 1360 cm 1, attributed to adisorder-activated mode of sp--bonds [10]. This isreasonable, since during PLD the kinetic energy of the film-forming partic1es is drastically reduced with increasingprocessing gas pressure [8], so that less kinetic energy andmomentum is available for the formation of spt-bonds inthermal spikes or by subplantation effects.

4.4 XPS Analysis

The XPS spectra of the films deposited on tool steelsand PMMA are compared with previously reported valuesfor DLC (fig. 3). Po is the Cls line and the loss peaks PI' P2

and P3 have been referenced to Po' For the graphite sample,a P 1 at about 7 eVis characteristic for rt-type plasmon losses,while a P3 at 28 eV arises due to bulk plasmon losses.Diamond does not have n-type electrons and hence doesnot exhibit the peak P, at 7 eVoData from pure diamondreport an intense loss peak (P3) about 34 ± 4 eV and themaximum of this bulk plasmon loss (P3) is centred at 33eV for the DLC coatings. There is another loss peak (P)around 12 eV which was unidentified for a ion beamdeposited diamond-like carbon film and is not present onthe DLC deposited by PLD or in pure diamond [11].

4.5 Hardness Measurements

The hardness of the films deposited on PMMA and toolsteels is measured by nanoindentation. As the load iscontinuously increased, the maximum indenter depth is10% of the film thickness. The state-of-the-art of highquality DLC thin films is characterised by the high valuesof the Young modulus and hardness of E ~ 800 GPa andH, ~40 GPa, respectively [9]. For DLC thin films depositedin this investigation, the elastic modulus and hardness showa middle values of E = 300 GPa and H, = 20 GPa for the1.3207 tool stee1, and E = 310 GPa and 30 GPa for the1.3202 too1 stee1, respectively, because the presence ofpinholes in the substrate decrease these values. For the caseof PMMA substrate an influence of the PMMA is indicated(EPMMA = 5.2 GPa, H¿ = 300 MPa). The amount of Sp3-bonds is determined from the Raman spectra by the positionof the G-band. The content of sp=bonds in the films isabout 60%. This amount of sp=bonds may be has alsoinfluences in the value of hardness of this coatings, becausethe increase of the hardness in the DLC thin film is directlyproportional to the amount of sp'<bonds present in the thinfilm [9]

33

320 210

Biná~ Enetgy (eV)

Fig. 3 XPS spectra from DLC deposited by PLD on tool steels andPMMA with surface grinding modification; 3,3 Jzcm";6000 pulses.Both polished steels show identical graphite spectra, p = l Or'mbar,For PMMA, 1.3207 passivated steel and 1.3202passivated steelthe DLC spectra are shown, p = lO? mbar.

5. SUMMARY

The adherence of the DLC thin films on too1 stee1s isimpraved significantly by the passivation pre-treatment withH2SO/H202 solution. Pinholes appear on the substratesurface because the DLC thin film made a exact copy of thesurfaces on which it is deposited and the substrate shown apinholes in its structure, with this morphology influencingthe measured values of hardness and elastic modulus. Onpolished steel the nuc1eation ofDLC was not possible. Therewas no adhesion of the thin films. With increasing processinggas pressure the amount of sp--bonds in the thin films ontool stee1 decreases (G-band decreasing and the D-bandappear in Raman spectra). The deposition of adherent DLCon PMMA was not possible. The titanium buffer layerlocated at the DLCIPMMA interface had no influence onincreasing the adherence of DLC thin films on PMMA,improved coatings were produced on PMMA with amechanical pre-treatment, but more work is needed to checkother pre-treatments. The values of hardness and elasticmodulus for the DLC thin films produced by PLD on PMMAwith a mechanical surface modification indicate an influenceof the PMMA (E = 3 GPa) on the nanoidentationmeasurements. Further investigations have to concentrateon the substrate pre-treatment and the development ofbetterbuffer layers to allow the increase the film thickness to 2-10Ilm and also high adherence, which are desirable fortribological applications.

34 Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 1, 2001.

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