Ag nanocomposite—the structure, mechanical and tribological properties

Surface & Coatings Technology 201 (2006) 1469 – 1476 www.elsevier.com/locate/surfcoat

Thin films of Mo2N/Ag nanocomposite—the structure, mechanical and tribological properties Witold Gulbiński ⁎, Tomasz Suszko Technical University of Koszalin, 15–17 Racƚawicka Street, 75-620 Koszalin, Poland Received 6 January 2006; accepted in revised form 8 February 2006 Available online 6 March 2006

Abstract Thin films of Mo2N/Ag composites were deposited on steel substrates by the reactive, double source magnetron sputtering. Their structure, chemical and phase composition were examined by X-ray diffraction, EDS/WDS analysis and scanning microscopy. Raman microscopy was used for the local phase identification at the coating surface after high temperature tribological tests carried out up to 400 °C in laboratory air of normal humidity. An addition of silver induced pronounced Mo2N crystallite size refinement accompanied by the slight hardness increase with the maximum at 2.5 at.% of Ag content. An addition of silver led also to the nitride lattice deformation and its partial transformation into β-phase. Except the room temperature tribological tests, where the soft metal addition resulted in an increase of friction, silver plays a positive role manifesting in friction coefficient reduction. After 400 °C friction tests, the silver molybdate phase, Ag2Mo4O13, has been found in the oxide scale covering silver rich (> 15 at.% of Ag) composite coatings. Hardness and tribological properties of Mo2N/Ag composite coatings are discussed in terms of their structure and the phase composition. © 2006 Elsevier B.V. All rights reserved. Keywords: High temperature friction; Molybdenum nitride; MoN; Molybdenum oxide; MoO3; Silver molybdate; Hard coatings; Tribological properties

1. Introduction In the last decade, the concept of superhard nanocomposite materials consisting of two or more mutually immiscible phases, induced numerous research teams to undertake studies focused on the synthesis and properties of these materials, particularly in the thin film form. Beside already classical nanocomposite structures where a hard phase like metal nitrides (TiN, VN, W2N) coexists with the amorphous matrix (Si3N4) [1–3], there is a second group of nanocomposites, where the crystallite size reduction of a nitride (TiN, CrN, ZrN) or carbide (TiC) phase has been achieved by an addition of soft metals (Cu, Ag). They are immiscible with the nitride phase and do not create stable nitrides [4]. Although nitrides of copper (Cu3N and CuN3) and silver (AgN3) are known, their thermal stability is very limited and they easily decompose into free metals.

⁎ Corresponding author. E-mail address: [email protected] (W. Gulbiński). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.017

High hardness and long term stability of the nanostructure have been reported for nitride containing composites such as ZrN/Cu [5–7], ZrTiN/Cu [8], CrN/Cu [9], CrN/Ag [10], TiN/Cu [11,12], TiN/Ag [13], Mo2N/Cu [14] and Mo2N/Ag [15] as well as carbide/silver (TiC/Ag, WC/Ag) combinations [16,17]. These authors however, focused their attention on mechanical properties of these coatings, trying to optimize the nanocomposite structure for the maximum hardness. Tribological properties have been studied in rather limited extend. Few results of room temperature tests under vacuum [16,17] and in laboratory air [12,10] have been published. The data regarding high temperature friction are limited to the single report [14] on the Mo2N/Cu system. The Mo2N/Ag system is particularly interesting for several reasons. The Mo2N thin film coatings oxidize to the lamellar oxide MoO3 playing a key lubricating role in high temperature (HT) friction process on that nitride [18]. An addition of silver to molybdenum oxide leads to the synthesis of low-melting silver molybdates, exhibiting low friction in the wide temperature range [19]. Thus, the silver doped molybdenum

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nitride coatings, beside their composite nanostructure [15], are expected to show interesting HT tribological properties. The aim of this work was to synthesize nanocomposite Mo2N/Ag thin film coatings and to study their structure as well as tribological behavior during dry friction in atmospheric air at elevated temperatures. In the present paper we describe the deposition of Mo2N/Ag composite coatings by double-source, reactive magnetron sputtering. Results of structure studies, microhardness measurements as well as analysis of tribological behavior of these films in the wide temperature range (20– 400 °C) are also reported. Furthermore the Raman microscopy has been used for identification of phases in the wear track.

Cathode current (A)

20 15 10 5 0 -5 0.00

0.05

0.10

0.15

0.20

Time (ms) Fig. 2. The waveform of the target current pulse.

2. Experimental About 2 μm thick, pure and doped Mo2N coatings containing up to 35 at.% of silver, were deposited by double source, reactive magnetron co-sputtering. Steel substrates (440C) in the form of disks, 28mm in diameter and 3 mm thick were mirror polished (Ra < 35 nm) and hardened to 6 GPa (Vickers microhardness at 1 N load). Two planar magnetron guns, equipped with circular 4 in. targets, were mounted at the extremities of horizontal tubular chamber, in the distance of 180mm. The volume of deposition chamber was divided into two equal parts by a steel curtain perpendicular to the gun axis. The curtain had a window in the center (Fig. 1). The stationary, rectangular substrate holder was positioned in the window, with the substrate facing molybdenum target. The window size was slightly larger than the substrate holder to enable the diffusion of sputtered silver atoms to the “molybdenum side” of the chamber. The width of the gap between the substrate holder and the curtain was about 5mm. Molybdenum and silver targets (purity 99.9%) were sputtered in unbalanced mode controlled by two external electromagnet coils. Two pulsed power supplies (MSS10DORA System) working with the repetition frequency of 1 kHz were used. An additional modulation (120 kHz) was used to

prevent arcing effects at the magnetron cathode. A wave-form of the discharge current, registered using 150 MHz digital storage osciloscope GDS-820S has been shown in Fig. 2. The power supply units worked in the constant power mode. Using such a deposition configuration we were able to tune the silver content in the films with changing the silver target power from zero to 600 W. Simultaneously, the discharge conditions at the molybdenum target, working with the constant power of 750 W, were only slightly affected by the second magnetron. The surface distribution homogeneity of silver in the films was found as satisfactory by EDS analysis. The nitrogen content in the coating was controlled by the adjustment of nitrogen partial pressure in the chamber by means of a optical emission spectroscopy (OES) feedback loop. The loop was driven by the plasma optical signal composed of three atomic lines of molybdenum, situated at 550.8, 553.5 and 557.2 nm. The desired Mo2N stoichiometry was achieved for the molybdenum emission lines intensity equal to 60 ± 2% of the maximum signal observed during sputtering in pure argon. An addition of nitrogen to the sputtering atmosphere did not affect significantly the discharge conditions at the silver source. The pressure of Ar + N2 atmosphere in the technological chamber

Valve unit Spectrometer

Ar, N2

Optical signal Magnetron sources (Mo, Ag)

Pulsed power supply

Power supply of external coils

Pulsed power supply Substrate heating Curtain Pump unit

Fig. 1. Scheme of the deposition rig.

Substrate bias

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3. Results and discussion

suggests the presence of non-reacted molybdenum in the coatings. Taking into account that the standard free energy of formation for molybdenum nitride (50 kJ/mol) is about 6 times lower than for TiN [22], the coexistence of molybdenum with its nitride is not surprising. An addition of silver induced gradual broadening of (200) nitride peak [ICDD card 25-1366] and the drop of its intensity. The (200) preferential orientation, characteristic for silver free nitride films, was also disturbed. The (111) peak appeared already at the lowest silver content (Fig. 3). A detailed analysis of nitride peak positions and shape led us to the conclusion that besides the orientation change, an addition of silver induces some deformations of cubic lattice of γ phase. For the lowest silver concentration range, from zero to about 2.5 at.%, a gradual shift of (200) and (111) peaks to higher angles indicates an increase of the lattice constant from 0.416 nm for pure nitride to 0.422 nm for silver containing films. We assume that at this low concentration, silver atoms, which were co-deposited with the nitride, are dispersed in the Mo2N lattice and responsible for the observed lattice expansion. In this concentration range, the segregation of silver in the form of clusters is not excluded. Their size is however far below the detection limit of diffraction. Simultaneously, silver atoms and subnanometer clusters act as nucleation centers and lead to pronounce refinement of the crystallite size of the host nitride phase as will be shown later. Further increase of silver content from 2.5 at.% up to about 6 at.%, results in a partial transformation of cubic γ phase into tetragonal β-Mo2N phase [ICDD card 24-768]. This transformation manifests in the γ-Mo2N (111) peak asymmetry 4

2

x 10

γ+β Mo N (200)

1.8

2

1.6 1.4 1.2

Counts

was measured by the Leybold CTR90 capacitance gauge. The total pressure during deposition was about 1 Pa. The films were deposited at the substrates heated resistively to the temperature of 350 ± 10 °C and polarized negatively (− 30 V) to the ground. The average substrate current density changed from 0.5 to 1 mA/cm2, dependent on the power of silver source. Low bias potential was chosen to avoid resputtering of silver known as possessing very high sputtering yield. The deposition process was controlled by the PC based data acquisition system built on DASYLab v.5.0 software. Structural characterization of deposited coatings was based on X-ray diffraction (Co Kα radiation) in the ϑ–2ϑ geometry. The mean size of molybdenum nitride crystallites was calculated from the diffraction peak broadening, using Scherrer equation and Warren-Biscoe correction factor for instrumental peak broadening [20]. The peak parameters (position and width) have been carefully calculated using fityk 0.5.0 fitting software. For every single composition three samples has been analyzed. The elemental composition was checked by the X-ray microprobe working in wave (WDS) and energy (EDS) dispersive spectroscopy mode. The composition analysis uncetrainity was less than 0.2 at.% for heavy elements (Mo, Ag) (EDS) and 0.5 at.% for nitrogen and oxygen (WDS). Vickers micro-hardness of coatings was calculated from multiple measurements (six per sample) at different loads (0.1– 1 N) and fitted according to the model proposed by Korsunski [21]. Jeol 5500LV SEM was used for the size determination of the indentation prints and for the film morphology analysis as well. Friction tests were carried out in ball-on-disc geometry at the normal load of 1 N and sliding speed about 60mm/s (1rps). The duration of single test was 80 min. The friction was tested in ambient air of normal humidity (about 50%) at the room temperature (RT), 100, 250 and 400 °C. The 10 mm alumina balls, polished to the roughness less than 0.07μm, have been used as counterparts. The friction force, sample temperature and sliding speed were measured by PC controlled data acquisition system built on DASYLab v.5.0 package. The friction coefficient values used for further analysis have been derived as an average of 3 tests (3 samples) for single composition of the coating. The phase identification inside and outside of the wear track was supported by the analysis of Raman spectra registered using T640000 Yvon-Jobin triple grating Raman microscope equipped with an Argon laser (514.5 nm) providing the beam of about 6 μm in diameter and the maximum power of 3.5 mW.

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γ –Mo N (111) 2 β–Mo N (112)

1

Fe

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0 at. % Ag

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0.5 at. % Ag 1.6 at. % Ag

0.6

2.0 at. % Ag

3.1. The structure

0.4

The γ-Mo2N/Ag composite films were deposited with an increasing silver content, up to 35 at.%. The oxygen impurity level was below 3 at.% for whole set of studied samples. Silver free, γ-molybdenum nitride films have been highly textured with strong (200) preferential orientation. At the low angle side of (200) peak we have found a diffuse shoulder. It

0.2

2.4 at. % Ag 6.5 at. % Ag 15.3 at. % Ag 35.5 at. % Ag

0 35

40

45

50

55

2ϑ (°) Fig. 3. X-ray diffraction patterns of Mo2N/Ag films.

60

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a) γ+β Mo N (200) → 2 ← γ Mo2N (111)

Counts

800 600

← Fe

β Mo2N (112)

400

Mo (112)

200 0

←Ag (111)

35

40

←Ag (200)

45

50

55

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2ϑ (°)

b)

Counts

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← γ Mo2N (111)

600

← Fe

γ Mo N (200) →

400

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Mo (112)

200 0

←Ag (111)

35

40

← Ag (200)

45

50

55

60

2ϑ (°) Fig. 4. XRD peak fitting results for the coatings containing (a) 6 at.% and (b) 15 at.% of silver.

emerging at its high-angle side. The peak deconvolution, shown in Fig. 4(a), gives a component at the position corresponding to (112) line of β-Mo2N phase. An intensity increase of this β component suggests that the volume fraction of β-Mo2N phase rises when Ag concentration changes between 2.5 and 6 at.%. The (200) peak position remains not affected since the difference of d spacing for both phases is very small. Finally, the metallic silver emerges in diffracting patterns, when its content becomes higher than 6 at.%. The Ag (111) peak lies close to β-Mo2N (112) one, whereas Ag (200) peak becomes visible at the low angle slope of the α-Fe (110) peak, originating from the steel substrate (Fig. 4(b)). 3.2. Crystallite size At low Ag concentrations (below 2.5 at.%), silver which is immiscible with Mo2N, plays a role of the nucleation centers

and leads to the pronounce drop of crystallite size from 40nm for pure nitride to about 10 nm when the silver content approaches 3 at.%, as shown in Fig. 5. At higher concentrations, Ag segregating at grain boundaries hinders the nitride crystallite growth and leads to further, gradual structure refinement. The Mo2N crystallites become as small as 7nm for silver rich samples. 3.3. Microhardness The hardness of coatings slightly increases from 24GPa for pure nitride films to about 29 GPa for films containing 2.5 at.% of silver, as shown in Fig. 6. It corresponds to the nitride crystallite size of about 10nm. Similar hardness enhancement was reported by Turutoglu et al. [15] for Mo2N/Ag composites. An increase of silver concentration above this limit results in the slight decrease of hardness to about 25GPa. Appearance

(200) (111)

35 30

20

Hardness (GPa)

Crystallite size (nm)

25

15

10

25 20 15 10

5

0

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30

40

Ag content (at. %)

5

0

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30

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Ag content (at. %) Fig. 5. Mo2N crystallite size versus silver content in the coating. Crystallite size error bars (±2 nm) are not shown to keep clarity of the graph.

Fig. 6. Dependence of composite coating microhardness on the silver content.

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0.9 RT 100°C 250°C 400°C

Friction coefficient

0.8 0.7 0.6 0.5 0.4 0.3

0

5

10

15

20

25

30

35

Ag content (at. %) Fig. 7. Friction coefficient versus silver content for different test temperatures. Friction coefficient error bars (±0.05) are not shown to keep clarity of the graph.

of free metallic silver in the composites rich in Ag (more than 6 at.% of Ag) leads, as expected, to the further reduction of hardness, down to 7 GPa. In the case of studied Mo2N/Ag coatings consisting of hard phase doped with the soft metal, immiscible with the host phase and inert to nitrogen, the mechanism of hardness enhancement is different from the one observed for nanocomposites like TiN/ a-Si3N4 [23]. Starting from the lowest concentrations, silver atoms dispersed in the nitride lattice induce, as shown above, its distortion resulting in the hardness enhancement. Simultaneously, silver sub-nanometer clusters play a role of nucleation centers leading to the nitride crystallite size refinement. The drop of the grain size also contributes to the hardness increase according to the Hall-Petch relation [24,25]. For coatings showing the maximum hardness, which is observed when the silver content approaches about 2.5 at.%, a rough calculation shows that the silver amount is not enough to build a continuous 1500

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monolayer covering Mo2N grains of average size 10nm. Similar observations have been reported for other soft metal containing composites like TiN/Cu [12,11] or ZrN/Cu [4]. These observations suggest that the hardening mechanism in studied composites is different from the one, occurring in “classical” superhard materials like TiN/a-Si3N4 composites. In that case the maximum of hardness is reached when the silicon nitride content passes the percolation threshold and the continuous monolayer covering TiN grains is formed [26]. In the case of Mo2N/Ag composites, the Ag content necessary to form the monolayer covering nano-sized (8– 10 nm) nitride crystallites, has to exceed about 10 at.%. At this concentration, the hardness of studied composite is already far below the maximum (Fig. 6). For silver rich films, the presence of soft component facilitates the grain boundary sliding, leading to the further drop of hardness. 3.4. Tribological properties The tribological behavior of Mo2N/Ag films at room temperature differs significantly from the one observed above 100 °C. As shown in Fig. 7, the friction coefficient for pure and slightly Ag doped films, oscillates around 0.35. This low value is typical for pure nitride films studied at room temperature in humid laboratory air [18,27,28]. The sliding is facilitated by adsorbed water, which together with surface oxide traces, builds the lubricating layer. The presence of this lubricating medium to a large extent masks intrinsic tribological properties of the coating. Actually, the RT friction for Mo2N/Ag-alumina tribocontact, measured in humidity-free conditions, is high (about 0.8) and decreases with increasing silver content. The same trend is also observed at higher temperatures (100 °C and above) where the “water lubrication” mechanism fails due to water desorption.

a) 15 at. % Ag Ag Mo O

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1000

4 13

MoO

3

Mo N 2

Ag

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0 10

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30

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50

60

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90

2ϑ (°) 1500

b) 35 at. % Ag

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Ag2Mo4O13 MoO3

1000

Mo2N

500

0 10

Ag

← Fe

20

30

40

50

60

70

80

90

2ϑ (°) Fig. 8. X-ray diffraction pattern after the 400°C friction test for the Mo2N/Ag composite coating containing (a) 15 at.% Ag and (b) 35 at.% Ag.

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150 ← MoO3

Intensity (a.u.)

6.5 at. % Ag silver molybdate ←– – – →

100

50

← MoO3

within the wear track

outside the wear track

0 100

200

300

400

500

600

700

Raman shift (cm-1)

800

900

1000

1100

Fig. 9. Micro-Raman spectra registered outside and inside the wear track at the Mo2N/6 at.% Ag composite coating after the 400°C friction test.

Another question is why the RT friction for silver rich coatings (> 15 at.% Ag) is not as low as 0.35 and oscillates around 0.6. We tentatively assume that surface properties of Agdoped coatings change to hydrophobic and thus the conditions for water adsorption become less favourable. As long as the test temperature remains below the oxidation threshold, which for studied nitride is about 350 °C [29], the friction reduction can be exclusively explained by the appearance of low shear strength component (σAg ≈ 15 MPa) in the tribo-contact. The situation changes when the test temperature rises to 400 °C, i.e. exceeds the nitride oxidation threshold. The molybdenum oxide α-MoO3 grows at the entire surface of the sample. The sliding process becomes oxide controlled or more precisely saying, oxidation product controlled, since this

temperature is also high enough to activate the chemical reaction between molybdenum oxide and silver. As we have shown in our earlier papers on the high temperature tribological properties of silver doped oxides [19,30], the molybdenum oxide and metallic silver react readily already at 450°C. Dependent on their molar ratio, the reaction products are various silver molybdate phases, i.e. Ag2Mo4O13, Ag6Mo10O33, Ag2Mo2O7 and Ag2MoO4, ordered according to the increasing silver content. The oxidation product of Mo2N/15 at.% Ag composite contains up to 11 mol% of Ag2O. According to the MoO3– Ag2O phase diagram [31], it is enough to reduce the melting temperature of oxide scale from 800 to 660°C. The reduction of melting temperature is directly connected with the onset of oxide softening above its brittle-to-ductile transition, which

Fig. 10. SEM pictures together with corresponding silver distribution EDS maps for Mo2N/Ag coatings, surface-oxidized during 400°C friction tests, and containing (a) 6 at.%, (b) 15 at.% and (c) 35 at.% of silver.

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occurs usually at 0.4–0.7 Tm [32,33]. This temperature induced drop of oxide hardness and shear strength contributes to the reduction of friction. The X-ray diffraction analysis of coatings containing 15 at.% Ag, after the 400 °C friction test shows also the first sign of Ag2Mo4O13 phase [ICPDS card 21-1342], emerging in the oxide scale, as shown in Fig. 8(a). The content of that phase significantly increases when silver content in the film reaches 35 at.%. It is noteworthy that the molybdate phase in a large extent replaces molybdenum oxide, growing at its expense (Fig. 8(b)). The Raman microscopy confirms entirely these observations, giving simultaneously better insight into the local phase composition inside and outside the wear track. For samples containing 6 at.% Ag, the first traces of phases other than MoO3 have been found in Raman spectra taken from the wear track area after 400°C tests (Fig. 9). Outside the track only molybdenum oxide was observed. Thus, the molybdate synthesis seems to be tribo-activated. As expected, further enrichment of the composite with silver results in an increase of molybdate content in the surface oxide scale. The intensity of Raman modes (865, 903 and 953cm− 1) assigned to the molybdate phase increases also outside the track. The Ag2Mo4O13 phase, in the form of elongated Ag2Mo4O13 crystals, can be found at the entire surface of the coating (Fig. 10 (c)). The molybdate, with the melting temperature of 528°C, plays an important lubricating role during high temperature (400 °C) friction tests. Besides the temperature induced oxidation of studied coatings, accompanied by the silver molybdate synthesis, the metallic silver outwards diffusion and segregation at the surface was observed during heating of silver-rich films. Similar observations have been already reported by Endrino et al. for TiC/Ag and WC/Ag [16,17] and by Mulligan et al. for CrN/Ag coatings [10]. Despite the relatively high deposition temperature (350 °C) we used in this study, the segregation has not been observed at the surface of as deposited Mo2N/Ag coatings, containing less than 15 at.% Ag. Above this limit, silver emerges at the composite surface in the form of nanosize aggregates. After tribological tests lasting 80 minutes at the temperature of 400 °C in air, silver can be observed at the surface of oxide layer in the form of spherical aggregates of about 1μm in diameter, as shown in Fig. 10(a–c). With an increasing fraction of silver, the size of Ag aggregates does not change while their quantity per area unit increases. It leads to the further facilitation of sliding. Despite the elevated silver concentration at the surface, the synthesis of silver-rich molybdates like Ag2Mo2O7 and Ag2MoO4 was not observed. The limitation factors are in this case time and temperature of the test. 4. Conclusions Mo2N/Ag nanocomposite thin film coatings have been deposited by doublesource, reactive magnetron sputtering. An addition of silver up to 2.5 at.% led to the slight increase of

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hardness (from 24 to 29 GPa) induced by the nitride crystallite size refinement to about 10 nm. At higher Ag concentrations, the composite hardness drops down to 7GPa. The silver addition induces a distortion of the γ-Mo2N nitride lattice and consequently leads to the appearance of tetragonal β-Mo2N phase. Free metallic silver emerges in diffraction patterns when its concentration approaches 15 at.%. The positive influence of silver on tribological behavior of studied coatings has been observed during high temperature tests. The optimum content of Ag is slightly above 6 at.%. The hardness of composite at this Ag concentration is still sufficiently high (about 20 GPa), whereas the friction coefficient is clearly lowered. We have shown that during high temperature tribological tests (400 °C) of silver rich coatings, the thermally- and triboactivated synthesis of the silver molybdate (Ag2Mo4O13) occurs. This phase, beside the metallic silver, contributes to the lowering of friction. The effect of silver addition on high temperature tribological properties of molybdenum nitride coatings is not as high as it was expected, taking into account our earlier study [19]. The friction coefficient on pure Ag2MoO4 coatings, measured with the same test rig, was as low as 0.2 at 400 °C. However, the temperature and the time of tests conducted in the framework of the presented study, were not enough for the synthesis of silver-rich molybdates. Acknowledgements Authors would like to express their thanks to Dr. BogdanWarcholiński, Mr. Jan Kwiatkowski and Mr. Zbigniew Kukliński for their help by microhardness and X-ray diffraction measurement. Our thank are also due to Mr. Tomasz Halamus form the Department of Molecular Physics at the Technical University of L̷ódz for his help by Raman studies and Mr. Paweƚ Kochmański from Technical University of Szczecin for EDS/WDS analysis. References [1] S. Vepr̆ek, Surface and Coatings Technology 97 (1997) 15. [2] S. Vepr̆ek, Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films 17 (5) (1999) 2401. [3] S. Vepr̆ek, M. Jilek, Vacuum 67 (2002) 443. [4] J. Musil, Surface and Coatings Technology 125 (2000) 322. [5] J. Musil, P. Zeman, H. Hrubý, P.H. Mayrhofer, Surface and Coatings Technology 120–121 (1999) 179. [6] J. Musil, P. Zeman, Vacuum 52 (1999) 269. [7] P. Zeman, R. Čerstvý, P.H. Mayhofer, C. Mitterer, J. Musil, Material Science and Engineering A 289 (2000) 189. [8] J. Musil, R. Daniel, Surface and Coatings Technology 166 (2003) 243. [9] M.A. Baker, P.J. Kencha, M.C. Josephb, C. Tsotsos, A. Leyland, A. Matthews, Surface and Coatings Technology 162 (2–3) (2003) 222. [10] C. Mulligan, D. Gall, Surface and Coatings Technology 200 (2005) 1495. [11] J.L. He, Y. Setsuhara, I. Shimizu, S. Miyake, Surface and Coatings Technology 137 (2001) 38. [12] H.S. Myung, H.M. Lee, L.R. Shaginyan, J.G. Han, Surface and Coatings Technology 163–164 (2003) 591. [13] J.G. Han, H.S. Myung, H.M. Lee, L.R. Shaginyan, Surface and Coatings Technology 174–175 (10) (2003) 738.

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