.Diamond and Related Materials 10 2001 18461849

Diamond-like carbon films in multilayered interferencecoatings for IR optical elements

O.M. Kutsay a,, A.G. Gontar a, N.V. Novikova, S.N. Duba, V.N. Tkacha,B.A. Gorshteinb, O.V. Mozkovab

aInstitute for Superhard Materials of the National Academy of Sciences of Ukraine, 2, Atozaodska Str., Kyi, 04074, Ukraineb Arsenal Central Design Bureau, 8, Moskoska Str., Kyi, 03601, Ukraine

Abstract

. .In the majority of modern IR interference multilayer coatings MLC , conventional film-forming materials FFM of fluorideand chalcogenide types are used. Such coatings are characterized by relatively low mechanical strength and stability againstenhanced humidity and, therefore, require surface protection. Our present results support the view that mechanical strength of

.these MLCs can be improved by applying a diamond-like carbon DLC film as an external layer. Nanoindentation measurementsshow that the addition of a DLC film to ZnSeBaF Y O IR antireflection MLC increases the combined hardness of the2 2 3coatings from 0.5 to 3.6 GPa. The formation of an indent on the upper and subsequent layers of MLC has been studied by SEMand X-ray spectrum microanalysis. The resistance of DLC films applied onto MLC against light irradiation, organic solvents as

.well as against environmental factors was also studied. Atomic force microscopy AFM was used to study variations of thesurface morphology of the initial MLC components before and after DLC film deposition. 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Diamond-like carbon; Applications; Mechanical properties; Wear

1. Introduction

The majority of modern interference coatings for .optical elements applied in infrared IR spectral re-

.gion use fluorides PbF , ThF , BaF , chalcogenides2 4 2 . .Sb S , ZnSe, PbS and oxides TiO , ZrO , Y O .2 3 2 2 2 3Unfortunately, chalcogenides and, especially fluorides,are characterized by insufficient mechanical strengthand stability under climatic conditions. Multilayer coat-

. .ings MLC made of such film forming materials FFMrequire upper protective layers with improved operat-

ing characteristics high mechanical stability and adhe-.sion . Oxide films used for these purposes have very

Corresponding author. .E-mail address: kutsay@ism.kiev.ua O.M. Kutsay .

large absorption coefficient in the low-frequency IR .region 820 m . Therefore, oxide layers no more

than 3050 nm in thickness are used. Besides, theoxides have a rather high refractive index, while theupper layers of anti-reflection MLC should have thelowest refractive index. Thus, the search for new possi-bilities to eliminate the above disadvantages of conven-tional interference MLC is urgent to improve theiroperating characteristics. Applying films of new FFMand optimization of the deposition parameters could

.solve this problem. Diamond-like carbon DLC films offer promise as protective layers 13 . The use of a

DLC film as a component of interference MLC isattractive due to the possibility of varying the filmrefraction index over a wide range by changing the

deposition parameters 2 .

0925-963501$ - see front matter 2001 Elsevier Science B.V. All rights reserved. .PII: S 0 9 2 5 - 9 6 3 5 0 1 0 0 4 5 7 - 5

( )O.M. Kutsay et al.Diamond and Related Materials 10 2001 18461849 1847

2. Experimental

Electron-beam deposited polycrystalline ZnSeBaF Y O films were used as the MLC standard for2 2 3

comparison. Silicon and germanium wafers which are.optically transparent in the IR region were used as

substrates. The thickness of the quarter-wave layers ofZnSe and BaF was approximately 2 m; the thickness2of the protective Y O layer was approximately 50 nm.2 3DLC films from 50 to 100 nm in thickness have been

.deposited at low substrate temperatures 300330 Kby plasma-enhanced chemical vapor deposition .PECVD in the methanehydrogen atmosphere. Biasvoltage was 300 V and methane concentration varied

in the range of 3070 vol.% 3 .To measure the mechanical properties of multilayer

coating, depth-sensing nanoindentation tests were used.Tests were performed on a Nano Indenter II device . MTS, USA using a Berkovich indenter 46 . TheOliver and Pharr technique is the most widespread for

the analysis of the load-displacement curve 7 . It al- .lows one to find the average contact pressure ACP in

.the indent Meier hardness only at peak loads.Nanohardness at the peak loads for MLC will averagethe values for layers having different properties. Tomeasure nanohardness as a function of depth using theloading segment data, we applied the technique pro-

posed in 8 . Measurements at a peak load of 120 mNallow us to study the mechanical behavior of MLC to adepth of approximately 2000 nm. Nanohardness tests

performed at a minimal load of 0.3 mN at an indent.depth of approx. 50 nm give information on mechani-

cal properties of the upper layer.The wear resistance tests were carried out according

to the industrial requirements for optical coatings; 3000revolutions of a rubber tip radius of a spherical sur-

.face of 3.0 mm covered with a cambric cloth. Loading .on the tip was 200 g. A Digital Instrument USA

.atomic force microscope AFM was used to studyvariations of surface morphology in the MLC. The IRtransmission spectra in the wavelength range of2.520.0 m were recorded by a Carl Zeiss Specord

.M80 Germany spectrophotometer.The stability of MLC covered with a DLC film MLC

. DLC under the action of climatic factors enhancedhumidity, i.e. a relative humidity of 98% at 35 and

.100C; action time was 40 and 1 h, respectively , or-ganic solvents acetone, toluene, carbon tetrachloride,

.isopropyl spirit boiling for 30 min and ultraviolet irra- 2diation a UV light flux of 60 Wm power; irradiation

.time240 h was assessed by comparing the opticaltransmission of the MLCDLC sample before andafter the tests.

3. Results and discussion

Optical transmission spectra were measured for ini-tial MLC samples and after deposition of DLC films .MLCDLC onto them. After DLC film deposition,all optical elements retained their transparency values.

The loaddisplacement curve of the MLC down to adepth of 50 nm is given in Fig. 1. In this case, mechani-cal properties of Y O and underlying BaF films have2 3 2been analyzed. The hard and brittle oxide film de-posited onto soft BaF , has been destroyed by the2indenter at a depth of 10 nm. Consequently, the inden-ter sharply descends to a depth of 30 nm and thenpenetrates into soft fluoride. Once failed, the oxide filmdoes not affect indenter penetration. The low

.nanohardness 0.55 GPa at a depth of 50 nm and theunloading curve shape point to this fact. The unloadingcurve demonstrates slight elastic recovery of the indentdepth that is typical for such a soft material as BaF .2The elastic recovery of oxide films is much greater.Thus, the protective properties of the Y O films ap-2 3pear to be poor. The films fail even at a load of 0.005mN.

A DLC film deposited onto a MLC has significantlyimproved mechanical properties of the sample surface .Fig. 2 . In this case, penetration to a depth of 50 nmrequires 10 times higher loading. The nanohardness atthis depth has increased to 3.6 GPa. Also, the essentialincrease of the indent depth elastic recovery has beenobserved during unloading. Almost all of the indenterdisplacement during the test is due to elastic deforma-tion of the sample surface. The high elastic recoveryduring nanohardness tests is typical for carbon andhydrocarbon diamond-like films and is one of the main

reasons for their high wear resistance 9 .The dependence of nanohardness on indenter dis-

placement is shown in Fig. 3. For the initial MLCsample, the nanohardness was approximately 0.5 GPain the displacement range from 500 to 1000 nm. The

Fig. 1. Loaddisplacement curve for MLC.

( )O.M. Kutsay et al.Diamond and Related Materials 10 2001 184618491848

Fig. 2. Loaddisplacement curves for MLC and MLCDLC.

observed surface nanohardness increase is due to theinfluence of the Y O film at a depth500 nm. At2 3displacement greater than 1000 nm, the indenter be-gins to penetrate into the ZnSe layer, which is harderthan the BaF layer. As a result, the nanohardness at a2depth1000 nm begins to increase.

The DLC film deposition increases abruptly thehardness at a depth of500 nm. The influence of theDLC film is observed to the 1500-nm depth. At agreater depth, the nanohardness of the DLC-coatedsample differs only slightly from that of the uncoatedsample.

The regularities of MLCDLC failure at nanoin-dentation are confirmed by SEM and X-ray spectrummicroanalysis of an indent profile. To characterize thefailed layer of the indent, the characteristic X-ray radi-ation of Zn, Y and Ba was used. The YL -radiationintensity profile for the 50-nm deep indent is evidencethat the protective DLC coating prevents the MLCfrom being damaged. It is confirmed by the fact that at

.maximal loading 150 mN , DLC decreases the indentdepth. The ZnK -radiation from the bottom layer ofZnSe has not been detected. This fact confirms that theindenter destroys only the upper layers.

Fig. 3. Hardness of MLC and MLCDLC vs. displacement of theindenter.

Fig. 4. Optical images of MLC and MLCDLC surfaces after wear . .test: MLCDLC top ; and MLC bottom .

The results of the wear resistance tests are shown inFig. 4. The surface of a sample with MLCDLC the

.top of the image exhibits no mechanical damages evenafter increasing the total number of wearing tip revolu-tions up to 15 000. The traces of failure of the surfaceof MLC without a DLC film could be seen in Fig. 4 .bottom of the image . The failure of the MLCDLChas been observed only after adding diamond micron

.powder grain size23 m to the contact zone.Even after processing by the micron powder, the MLCDLC showed less mechanical damage than the MLC .Fig. 5 .

The use of DLC film as a protective layer increasesthe stability of the optical and mechanical properties ofthe MLC system under the influence of external fac-tors. The results of preliminary testing of these proper-ties of the DLC-coated surface of the monocrystalline

Fig. 5. Optical images of MLC and MLCDLC surfaces after wear .test diamond micron powder grain size23 m in the contact . .zone : MLCDLC top ; and MLC bottom .

( )O.M. Kutsay et al.Diamond and Related Materials 10 2001 18461849 1849

.Fig. 6. AFM images of MLC surfaces: initial a and with a DLC film .b Scale: x, y0.2 mdiv; and z100.0 nmdiv.

Ge and Si are reported in 10 . The MLC are character-ized by a polycrystalline structure and have some disad-vantages, such as porosity and low stability to theenvironmental influence. Thus, it was necessary to provenot only the mechanical stability of the MLCDLCsystem but also the fact that it retains its opticalproperties. For this purpose, we have examined theMLCDLC samples exposed to enhanced humidity,organic solvents and ultraviolet irradiation. No changeshave been observed in optical transmission after expo-

sure of the MLCDLC system to these environmentalfactors.

Surface morphologies of the MLC before and afterDLC film deposition are shown in Fig. 6. The quantita-

tive parameters of the roughness according to the.ASME B46.1 standard were obtained from statistical

analysis of the frontal image for a scanning area of5.05.0 m. After DLC deposition, the MLC root-

.mean-square RMS roughness was reduced from 30 to20 nm. This is indicative of the ability of DLC films toreduce the roughness of a polycrystalline surface.

4. Conclusions

The possibility of improving the mechanical proper-ties of MLC by DLC deposition has been studied.Multilayer coatings are necessary for parts made of

.materials with high refraction indices Si, Ge and GaAsto increase optical transparency. Considerable im-

provement of MLC mechanical properties increasedcombined nanohardness of the coatings from 0.5 to 3.6

.Gpa whilst retaining their optical characteristics, couldbe achieved by DLC film deposition.

Acknowledgements

This work was supported by the Science and Tech- .nology Center in Ukraine STCU project no. 1356 .

The authors thank P.V. Lytvyn for AFM images ofsample surfaces.

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