Surface & Coatings Technology 215 (2013) 386–392

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Mechanical and tribological properties of sputtered Mo–O–N coatings

J. Musil a,⁎, P. Novák a, M. Hromádka a, R. Čerstvý a, Z. Soukup a, J. Savková b

a Department of Physics, Faculty of Applied Sciences, University of West Bohemia, Univerzitní 22, CZ-30614 Plzeň, Czech Republicb New Technologies - Research Centre, University of West Bohemia, Univerzitní 8, CZ-30614 Plzeň, Czech Republic

⁎ Corresponding author at: University of West BohemPlzen, Czech Republic. Tel.: +420 37763 2225; fax: +

E-mail address: musil@kfy.zcu.cz (J. Musil).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.06.090

a b s t r a c t

a r t i c l e i n f o

Available online 5 November 2012

Keywords:Mo–O–N coatingsStructureMechanical propertiesFrictionWearMagnetron sputtering

The article reports on mechanical and tribological properties of Mo–O–N coatings prepared by reactive magne-tron sputtering using a continuous and pulsed inlet of oxygen and nitrogen. Main attention was devoted to a de-tailed investigation of correlations between the coating structure and its mechanical and tribological properties.The structure of coatings was characterized using X-ray diffraction (XRD), the hardness H, the effective Young'smodulus E⁎ and the elastic recoveryWe weremeasured by a microhardness tester, and the coefficient of frictionμ and the coefficient of wear k were determined using a pin-on-disk tribometer. It was found that (1) μ of theMo–O–N coating correlates quite well with their hardness H and effective Young's modulus E⁎ and dependson its structure, (2) k of the Mo–O–N coatings is almost independent on the value of E⁎ and its value is verylow of about ~0.15×10−6 mm3/Nm, (3) the incorporation of oxygen in the Mo–N coating increases its coefficientof friction μ at room temperature (RT), and (4) the coefficient of friction μ of both Mo–N and Mo–O–N coatings atfirst increaseswith increasing temperature and above ~350 °Cdecreases to the value of μof the as-deposited coating(μ≈0.4 and 0.5 for the Mo–N and Mo–O–N coatings, respectively).

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The tribological properties of nitrides and oxides of transition metalcoatings deposited by reactive magnetron sputtering have been inten-sively studied already for three decades. These coatings exhibit relativelyhigh hardness and excellent wear resistance. However, a surface oxida-tion of some of these coatings results in a reduction of the friction athigh temperatures above ~250 °C in the air [1,2]. Many papers reporton the formation of lubricious surface oxides on (i) nitride coatings, e.g.TiN [3,4], CrN [3], VN [3], WN [5], MoN [6–9], and MoN/Me [4,10–12],and (ii) oxide coatings, e.g. W–O [2,5] and V–O [2], which reduce thefriction at elevated temperatures.

Molybdenum nitride (MoN) has interesting properties which pre-determine it to be used as a good material for protective coatings[6,7,10,11]. Therefore, a lot of investigations on Mo–N and Mo–Nbased coatings have been carried out. The molybdenum nitride coatingcan exhibit two phases with different crystal structures: hexagonalδ-MoN and cubic γ-Mo2N [8,12]. Two-phase nanocomposite coatingof the type nc-Mo2C/(a-C+a-Mo2N) exhibits a high hardness (up to~50 GPa) [13]. The oxidation resistance of the Mo–N coating is lowmainly due to a loss of its protective ability and the volatility of oxidesalready at slightly increased temperatures; oxidation starts between350 and 400 °C [9]. Considerable attention is also devoted to alloyingof the MoN with selective metals such as Ag and Cu [4,10–12]. These

ia, Univerzitní 22, CZ-30614420 37763 2202.

rights reserved.

coatings were investigated at elevated temperatures up to 400 °C. Itwas found that the coefficient of friction μ oscillates around 0.4 atroom temperature (RT) in ambient air. This low value of μ is typical fornitride coatings and is explained by a water lubrication mechanism. Attemperatures above 100 °C, the water desorbs from the coating surfaceand the water lubrication mechanism fails. At temperatures rangingfrom 100 °C to 200 °C the coefficient of friction μ increases to ~0.8. Athigher temperatures ranging from 200 °C to 400 °C the coefficient offriction μ decreases from 0.8 to 0.5 due to surface oxidation [10–12].Therefore, it could be interesting to incorporate the oxygen directly inthe Mo–N coating and to investigate tribological properties of theMo–O–N coatings. However, such study was not carried out so far.Papers devoted to the MoNxOy coatings report on their structure,hardness H, Young' modulus E, and the effects of (i) the oxygen onamorphization of nitride phases [14] and (ii) thermal annealing onthe coating structure [15]. At present, there is no paper describingtribological properties of MoNxOy coatings.

Therefore, this paper is devoted to a detailed investigation of thestructure, mechanical and tribological properties of theMoNxOy coatings.The aim of our study is to find correlations between the mechanical andtribological properties of theMoNxOy coating and to investigate the effectof annealing temperature Ta on its friction and wear.

2. Experimental

The MoN coatings were reactively sputtered using a round unbal-anced magnetron equipped with a molybdenum target (Ø=100 mm,99.95% purity and fixed to the cathode by a stainless steel ring with

Fig. 1. XRD patterns from Mo–N coating sputtered at Ts=300 °C as a function of pN2. The last XRD pattern is from Mo–N film sputtered at Ts=500 °C. The peak labeled S is thereflection from substrate.

Table 1Mechanical (H, E⁎ and We) and tribological (μ and k) properties of Mo–N coatings,sputtered at Ts=300 °C and 500 °C, Us=Ufl, pT=pN2+pAr=1 Pa as a function of pN2,H, E⁎ and We were measured at diamond indenter load L=30 mN. Friction μ and weark were measured at L=2 N and sliding distance l=1000 m.

pN2 h aD H E⁎ We H/E⁎ μ k

[Pa] [nm] [nm/min] [GPa] [GPa] [%] [10−6 mm3/Nm]

0 2800 233 12.2 214 39 0.057 0.63 4.810.1 3400 283 29.2 254 71 0.115 0.40 0.110.2 5900 295 27.9 258 68 0.108 0.37 0.150.4 3900 195 28.2 246 72 0.115 0.38 0.150.6 2800 145 26.8 244 72 0.110 0.33 0.160.8 3000 140 31.1 261 76 0.119 0.35 0.190.8 2800 140 25.8 254 70 0.101 0.37 0.19

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Øinner=98 mm) in a mixture of Ar and N2 gases continuously intro-duced in the deposition chamber. For deposition of Mo–O–N coatingseither a continuous inlet of O2 or a pulse inlet of both O2 and N2 gaseswas used. Two kinds of gas pulsing were used: (1) the length of oxygenpulse τO2 and the length of nitrogen pulse τN2was the same (τO2=τN2)and the repetition frequency of pulses fr=1/2τO2 and (2) the length ofboth pulses was different (τO2≠τN2) and fr=1/(τO2+τN2). In bothcases during the oxygen pulse-on the nitrogen was switch-off andfrb1 Hz. The argon was introduced continuously. The coatings weresputtered onto 17240 stainless steel substrates (25×25×0.5 mm3)under the following conditions: discharge current Id=1.5 A, substratetemperature Ts=300 and 500 °C, substrate bias Us=Ufl, total pres-sure of sputtering gas pT=pAr+pN2=1 Pa for MoN coatings andpT=pO2+pN2+pAr=1 Pa for Mo–O–N coatings prepared at τO2=τN2and τO2≠τN2, respectively, and substrate-to-target distanceds-t=70 mm with typical deposition rate aD ranging from ~140 to~295 nm/min for the MoN coatings and aD ranging from ~130 to~175 nm/min for the Mo–O–N coatings.

The structure of coatings was determined by X-ray diffraction(XRD) using a PANalytical X'Pert PRO diffractometer in the Bragg–Brentano configuration with CuKα (λ=0.1541874 nm) radiation.The phase composition of all coatings was determined by standardXRD phase analysis. All obtained XRD patterns were treated to get dvalues and integrated intensities of all significant peaks and thesedata were compared with known standards in the PDF-2 file [16].The mechanical properties of coatings, their microhardness H, effectiveYoung's modulus E⁎ and elastic recovery We were evaluated from theload vs. displacement curves determined by a computer controlledmicrohardness tester Fischerscope H 100 equipped with a Vicker's

diamond indenter. The mechanical properties of the coatings weremeasured at a low value of the diamond indenter load L=30 mNto ensure that the ratio d/h≤0.1 which is a necessary condition toeliminate the effect of the substrate on the indentation measure-ments; here d is the depth of the diamond tip impression and h isthe thickness of the coating. The values of H and E⁎ were measuredwith the accuracy of ±10%. The coating thickness h, surface rough-ness Ra and the macrostress σ generated in the coating during itsgrowth were determined from the electro-mechanical Dektak 8 StylusProfiler analysis. The coefficient of friction μ of all Mo–N and Mo–O–Ncoatings was measured using a ball-on-disk tribometer (CSM) in ambi-ent air with a WC ball (Ø=6 mm) rotating at radius r=3 mm underthe load L=2 N at the velocity v=0.05 m/s and the sliding distance

Fig. 2. Evolution of XRD patterns from Mo–O–N coating, sputtered at Us=Ufl, Ts=300 °C, pAr=0.2 Pa, pT=pAr+pO2+pN2=1 Pa and continuous inlet of O2 and N2 reactive gases,with increasing pO2 in Ar+O2+N2 sputtering gas mixture.

388 J. Musil et al. / Surface & Coatings Technology 215 (2013) 386–392

l=1000 m (53,052 revolutions). The wear rate k of coatings wasmeasured under the same conditions as those used in the frictiontests. The wear loss, defined as the coating volume loss per the slidingdistance l, wasmeasured by the Dektak 8 Stylus Profiler. CSM high tem-perature tribometer has been used to evaluate tribological properties ofcoatings up to 500 °C against WC ball under the following conditions:load L=2 N, linear velocity 0.05 m/s, and sliding distance 660 m(35,000 cycles).

3. Results and discussion

3.1. Mo–N coatings

The structure and phase composition of the Mo–N coating stronglydepends on the amount of nitrogen incorporated in the coating, seeFig. 1. The amount of atomic nitrogen N in the coating is controlled bythe partial pressure of nitrogen pN2 in a sputtering gas mixture Ar+N2.

The coating deposited in a pure argon atmosphere is formed by abody-centered cubic (BCC) Mo (ICDD card 42-1120 [16]) with aslightly preferred (110) orientation (texture). The additionof thenitrogenN2 in the sputtering gas results in gradual nitriding of Mo and in gradualformation of (i) the two-phase coating composed of the mixtureγ-Mo2N and Mo, (ii) the one-phase β-Mo2N coating, (iii) the one-phaseB1-MoN coating, (iv) the two-phase coating composed of the mixture of

Table 2Evolution of H, E⁎, We, μ and k ofMo–O–N coatings, sputteredwith (i) pulsed inlet of O2 and N2

and (ii) continuous inlet of O2, N2 and Ar and pT=1 Pa at Us=Ufl, Ts=300 and 500 °C, with inpO2, respectively; f=1/(T=τO2+τN2) is the repetition frequency of pulses. Friction μ and we

τO2 τN2 pO2 pN2 h aD H E⁎

[s] [s] [Pa] [Pa] [nm] [nm/min] [GPa] [GPa]

Pulsed inlet of O2 and N2 gas5 20 3000 150 25.4 22710 30 3200 160 26.7 23115 45 3100 155 23.4 211

continuous inlet of O2 and N2 gas Ts=300 °C0 0.8 2600 130 26.9 2440.1 0.7 2600 130 22.2 1850.2 0.6 2800 140 18.9 1670.3 0.5 2900 145 18.8 162

Ts=500 °C0 0.8 2800 140 25.8 2540.2 0.6 3400 170 20.9 1760.4 0.4 3500 175 16.9 1750.6 0.2 3400 170 4.8 94

B1-MoN and δ-MoN and (v) the single-phase δ-MoN coating with in-creasing pN2; here γ-Mo2N is a face-centered cubic (FCC) nitride (ICDDcard 25-1366 [16]), β-Mo2N is a tetragonal nitride and (ICDD card25-1368 [16]) with the lattice parameter ranging from 0.416 to0.419 nm, and the B1-MoNphase is ametastable nitrogen supersaturatedγ-Mo2Nphasewith the lattice parameter ranging from0.420 to 0.427 nm[17–19] and δ-MoN is a hexagonal nitride (ICDD card 25-1367 [16]). Theintensity of the Mo reflection decreases with increasing pN2 and disap-pears at pN2≈0.3 Pa. The Mo–N coatings sputtered at pN2 ranging from0.6 to 0.8 Pa are hexagonal δ-MoN nitrides.

The increase of the substrate temperature Ts from 300 °C to 500 °Cin the deposition of the coating at pN2=0.8 Pa results in the conversionof the hexagonal δ-MoN phase to the B1-MoN phase. This conversion isvery probably due to a desorption of N from the surface of the growingcoating. Moreover, a pronounced (200) texture of this coating indicatesits columnar structure andweak diffraction peaks (100) and (300) referto an ordered structure of B1-MoN.

All Mo–N coatings exhibit a high hardness H ranging from ~26 to~31 GPa, high elastic recovery We ranging from ~68 to ~76%, high ratioH/E⁎>0.1, and very low wear k≈0.16×10−6 mm3/Nm. Very lowvalues of k can be explained by high values of H>25 GPa. The coefficientof friction μ ranging from ~0.40 to ~0.33 decreases with decreasing E⁎

and is minimal at the lowest value E⁎, see Table 1. This experimentshows that μ and k of the Mo–N coating is more influenced by its

at pN2=0.6 Pa, continuous inlet of Ar at pAr=0.4 Pa and pT=1 Pa at Us=Ufl, Ts=300 °C,creasing length of oxygen pulse τO2, and nitrogen pulse τN2 and partial pressure of oxygenar k were measured at L=2 N and sliding distance l=1000 m.

We H/E⁎ μ k Structure film

[%] [10−6 mm3/Nm]

74.2 0.112 0.39 0.17 m-MoO2+γ-Mo2N (δ-MoN)74.7 0.116 0.38 0.19 m-MoO2+γ-Mo2N (δ-MoN)72.2 0.111 0.41 0.22 m-MoO2+γ-Mo2N (δ-MoN)

74.5 0.110 0.5 0.68 γ-Mo2N73.6 0.120 0.50 0.68 γ-Mo2N69.5 0.110 0.80 – γ-Mo2N69.5 0.116 0.48 0.12 m-MoO2

70.0 0.101 0.37 0.19 γ-Mo2N71.5 0.119 0.41 0.10 γ-Mo2N62.7 0.097 0.49 0.16 m-MoO2

34.1 0.012 – – m-MoO3

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mechanical properties (H and E⁎) than by its structure and phasecomposition.

3.2. Mo–O–N coatings

The Mo–O–N coatings were prepared either at different values ofpN2 and pO2 or at different lengths of oxygen pulse τO2. The effect ofan addition of oxygen in the Ar+N2 mixture on the XRD pattern ofthe coating is illustrated in Fig. 2. This figure shows that the increase

Fig. 3. (a) Coefficient of friction μ and elastic recovery We, (b) coefficient of wear k and(c) hardness H of Mo–N coating (open symbols) and Mo–O–N coating (full symbols)sputtered under different combinations of Ts, pO2, pN2, and τO2 as a function of effectiveYoung's modulus E⁎.

Fig. 4. Comparison of structure of MoO2 oxide coating A and γ-Mo2N nitride coating B.

of pO2 at first reduces intensities of the Mo2N peaks (pO2=0.2 Pa)and that these peaks are fully converted to the MoO2 peaks with astrong (111) preferred crystallographic orientation at pO2>0.2 Pa.

Changes in the coating structure result in a decrease of its hardnessH and elastic recovery We but almost no change of the friction, seeTable 2. To obtain more information on interrelationships betweenthe mechanical and tribological properties further Mo–O–N coatingswere sputteredwith continuous and pulsed inlet of O2 andN2; obtainedresults are also given in Table 2. The Mo–O–N coatings prepared withpulsed inlet of O2 and N2 gases are slightly harder and more elasticthan those prepared with continuous inlet of O2 and N2 gasesand exhibit slightly lower friction μ but almost comparable wear k.The increase of the substrate temperature Ts from 300 to 500 °C resultsin decrease of the friction andwear of theMo–O–N films prepared withcontinuous inlet of O2 and N2 gases.

3.3. Correlations between mechanical and tribological properties

Mechanical and tribological values of the Mo–N and Mo–O–Ncoatings prepared under different deposition conditions werebrought together to see general trends. Dependencies (i) μ, We, (ii) k,We and (iii) H of all Mo–N and Mo–O–N sputtered coatings (Tables 1and 2) as a function of E⁎ are displayed in Fig. 3. These dependenciesindicate general trends in correlations between the μ, k, H, E⁎, We,and H/E⁎ of the coating and its phase composition. Fig. 3a showsthat (i) μ decreases with increasing E⁎, and (ii) μ of the coatingwith the same E⁎ can be reduced if its elasticity (We) is increased,compare the XRD structure of the coatings A and B in Fig. 4. The coatingA is the MoO2 oxide and thus it exhibits a lower H compared to theMo2N nitride coating B, see Table 2. This comparison clearly showsthat the structure of coating and its phase also strongly influence the

Fig. 5. Coefficient of friction μ as a function of sliding distance ofMo–O–N coatings, preparedat Us=Ufl, Ts=500 °C, and pT=pAr+pN2+pO2=1 Pa.

Fig. 6. The coefficient of friction μ of (a) the Mo–N coating sputtered with continuousinlet of N2 at Us=Ufl, Ts=300 °C, pAr=0.4 Pa, pN2=0.6 Pa, and pT=pAr+pN2=1 Paand (b) the Mo–O–N coating sputtered with pulsed inlet of O2 and N2 at τO2=10 sand τN2=30, pT=1 Pa, Us=Ufl, and Ts=300 °C.

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friction μ and wear k. Fig. 3b shows that almost all coatings exhibit verylow wear k independently of the value of E⁎. Fig. 3c shows that theMo–N coatings are harder than the Mo–O–N coatings. It is a reasonwhy the Mo–N coatings exhibit a lower friction μ despite the fact thatboth Mo–N and Mo–O–N coatings are highly elastic (We≥60%). Also,it is worthwhile to note that both Mo–N and Mo–O–N coatings exhibithigh ratio H/E⁎which correlateswellwith high values of their elastic re-coveryWe≥60%. On the contrary, a pure Mo is too soft; this fact results

Fig. 7. Structure, hardness H, effective Young's modulus E⁎, H/E⁎ ratio and friction μ of as-depT=pAr+pN2=1 Pa and the same coating after tribological test at temperatures T=100, 2

in a low elastic recovery We≈40%, a high friction μ (Fig. 3a) and a highwear k (Fig. 3b).

3.4. Tribological properties of selected Mo–N and Mo–O–N coatings athigher temperatures

3.4.1. Effect of incorporation of O in Mo–N on friction of Mo–O–N coatingThe effect of an incorporation of O in theMo–N coating on its friction

was investigated by measurement of the coefficient of friction μ of theMo–O–N coatings prepared at Us=Ufl, Ts=500 °C, pT=1 Pa anddifferent values of pO2. Obtained results are displayed in Fig. 5.This experiment shows that the incorporation of O in the MoNcoating increases its friction.

3.4.2. Friction of Mo–N and Mo–O–N coatings at elevated temperaturesTribological properties of the Mo–N and Mo–O–N coatings were

measured not only at RT but also at higher temperatures T=100,250, 350 and 500 °C. The evolution of the friction coefficient μ of theMo–N and Mo–O–N coating as a function of sliding distance l isdisplayed in Fig. 6a and b, respectively. From Fig. 6 it is seen thatthe friction μ of both Mo–N and Mo–O–N coatings is relatively high atRT (~0.4 forMo–N and ~0.5 forMo–O–N). The friction further increaseswith increasing temperature T at up to ~350 °C. However, the friction μdecreases at higher values of T; μmeasured after the tribological test atT=500 °C approaches to that measured at RT. To explain this decreaseof μ at T=500 °C, the XRD patterns, the hardness H and the effectiveYoung's modulus E⁎ of the Mo–O–N coatings after tribological tests attemperatures T=100, 250, 500 °C were measured, see Fig. 7. Fromthis figure it is seen that the Mo–O–N coating is the composite whosestructure changes from the mixture of two nitrides MoN+Mo2N at RTto the mixture of one oxide and two nitrides MoO3+MoN+Mo2Nafter the tribological test at T=500 °C. This change of the structureresults also in the dramatic decrease of the coating hardness from~28 GPa to ~3 GPa only. This experiment clearly shows that structurechanges of any tribological coating at all range of operation tempera-tures T must be avoided.

3.5. Effect of structure of Mo–O–N coating on its wear

The Mo–N and almost all Mo–O–N coatings exhibit very low wearrate ranging from ~0.1 to ~0.2×10−6 mm3/Nm, see Table 2. This can

posited Mo–N coatings sputtered at Us=Ufl, Ts=300 °C, pAr=0.4 Pa, pN2=0.6 Pa, and50, and 500 °C.

Fig. 8. Surface profile and SEM image of the wear track for Mo–O–N coating with (a) γ-Mo2N structure and (b) m-MoO2 structure, sputtered at Us=Ufl, Ts=300 °C, pO2=0.1 Pa,pN2=0.7 Pa, and h=2600 nm and pO2=0.3 Pa, pN2=0.5 Pa, and h=2900 nm, respectively, after tribology test with WC ball at L=2 N.

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be explained by relatively high hardness H ranging from ~17 to~30 GPa, low value of the effective Young's modulus E⁎ satisfying ahigh ratio H/E*≥0.1 and high value of the elastic recovery We≥65%of the coating. The correlation between the wear k and the coatingstructure shows that the value of k, however, strongly depends onthe coating structure, see Table 2. This fact demonstrates the depthprofile of the racetrack and its surface morphology after the tribologicaltest, see Fig. 8. From this figure it is clearly seen that theMo–O–N coatingwith m-MoO2 structure exhibits ~5 times lower wear k compared to theMo–O–N coating with γ-Mo2N structure. It means that the high value ofWe of the hard Mo–O–N coating with H/E⁎≥0.1 is insufficient conditionnecessary to ensure lowvalues ofwear k and the coating structure plays adominant role in the optimization of its tribological properties.

4. Conclusions

The main results of the investigation of mechanical and tribologicalproperties of the Mo–N and Mo–O–N films can be briefly summarizedas follows.

1. The Mo–N coatings sputtered on a floating substrate (Us=Ufl) atTs=300 °C and 500 °C are crystalline. The addition of oxygen Oin the Mo–N coating decreases crystallinity.

2. The Mo–N andMo–O–N coatings are highly elastic and exhibit quitehigh hardness H up to ~25 GPa, low effective Young's modulus E⁎

resulting in a high ratio H/E⁎>0.1 and high elastic recoveryWe≥60%.

3. The friction μ and wear k of the Mo–N and Mo–O–N coatings corre-late quite well with the effective Young's modulus E⁎. The friction μdecreases with increasing E⁎. On the contrary, the coatings exhibitvery low wear k (~0.15×10−6 mm3/Nm) independently of thevalue of E⁎.

4. The high value of We of the Mo–O–N coating in combination with alow absolute value of E⁎ satisfying the condition H/E⁎>0.1 is aninsufficient condition necessary to ensure low values of the

coefficient of friction μ and the wear k as was recently demonstrat-ed for the nc-TiC/a-C nanocomposite thin films [20]. The structureof the coating is also very important.

5. The incorporation of oxygen O in the Mo–N coating results in theincrease of its coefficient of friction at RT.

6. The friction of both Mo–N and Mo–O–N coatings increases with in-creasing temperature T up to ~350 °C, decreases at higher valuesof T and approaches to that of as-deposited films (μ≈0.4 and 0.5for the Mo–N and Mo–O–N coatings, respectively, sputtered atTs=500 °C).

7. A good function of the tribological coating can be guaranteed onlyin the case when no change of its structure takes place in thewhole range of operating temperatures T.

Acknowledgements

This work was supported in part by the Ministry of Education of theCzech Republic under Project MSM# 4977751302 and the Grant Agencyof the Czech Republic under Project No. P108/12/0393.

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