Tribological properties of gradient Mo–Se–Ni–C thin films obtained by pulsed laser deposition in standard and shadow mask configurations

doi:10.1016/j.tsf.2013.12.058

Thin Solid Films

Volume 556, 1 April 2014, Pages 35-43

https://doi.org/10.1016/j.tsf.2013.12.058 Get rights and content

Highlights

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Pulsed laser co-deposition from MoSe₂(Ni) and graphite targets was carried out.
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Use of shadow mask avoids the deposition of micron- and nanometre-sized particles.
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Influence of the mask on tribological properties of the films was investigated.
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Use of the mask did not provide an increase of wear resistance of the films.
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Accelerated wear of steel ball counterpart was observed for standard deposition.

Abstract

Solid lubricant films were obtained by pulsed laser deposition (PLD) on steel substrates in such a way as to form an underlayer of diamond-like carbon film, and then the concentration of carbon was gradually reduced in Mo–Se–Ni–C films to obtain a pure MoSe_x(Ni) top layer. The use of a shadow mask configuration (SMPLD) avoids the deposition of micron- and nanometre-sized metallic particles (Ni, Mo), but the SMPLD films were characterised by relatively high Se content, reduced density and low hardness. The tribological properties of the films were evaluated using a ball-on-disk sliding test in humid air after long-term storage in laboratory conditions. For the PLD films, a relatively high friction coefficient was measured during running-in (~ 0.08). The friction coefficient decreased to 0.06 and did not change during the deepening of the wear track. The micro- and nano-particles were embedded into the film matrix during initial running-in and did not cause any apparent acceleration of the wear that was inhibited by the formation of a thick MoSe₂ tribolayer on the surface of the wear track. However, an increased wear rate of the counterpart was detected. The smooth surface of SMPLD films provides a low friction at the beginning of the test; then the friction coefficient increased gradually from 0.05 to 0.2 during the film wear, and sufficiently thick low-friction tribolayer was not found. The improved ability of the PLD films to form tribolayers could be due to their structure peculiarity and the relatively low concentration gradient of carbon in these films.

Introduction

Transition metal dichalcogenides (TMDs) are well known for their excellent lubricating properties in sliding contact. For most tribological applications, the protection of the bulk material by thin films is a suitable method for improving the antifriction properties and wear resistance. The most convenient and increasingly popular method to prepare these films is deposition by magnetron sputtering. In this method, the deposition parameters can be empirically optimised by varying the discharge mode (radio frequency, conventional or pulsed direct current) and changing the discharge power, the chemical composition and pressure of the gas, the location of the substrate relative to the sputtered target, the bias voltage, and the substrate temperature [1], [2], [3], [4], [5], [6], [7]. Parameter optimisation allows the formation of TMD coatings with the required structure, texture, and chemical composition.

Nevertheless, there are some common drawbacks to pure TMD coatings: a very low load-bearing capacity, low adhesion to the substrate, and a detrimental effect of the air moisture on the tribological contact. One of the most successful approaches, which has been actively developed in the present work, is to deposit a composite material, associating high strength materials with self-lubricants. Voevodin et al. [8], [9] and then Nossa and Cavaleiro [10] developed the concept of nanocomposite structured coatings combining hard (WC) and self-lubricant (WS₂) phases embedded into an amorphous carbon matrix. The hardness and wear resistance of such coatings were significantly improved compared to pure WS₂; however, the coatings exhibited relatively high friction coefficients in humid air. Recently, a more complex study of TMD films doped with carbon was carried out by Polcar and Cavaleiro. In [11], [12] they reviewed the results of the tribological behaviour of nanocomposite coatings composed of nanoplatelets of TMD immersed in a C-rich amorphous matrix. Three TMD materials were produced with carbon content varying between 25 and 70 at.%, namely W–S–C, Mo–Se–C, and W–Se–C systems. The parameters of the magnetron deposition process were adapted in such a way that the film did not contain hard carbide nanograins. The films showed exceptionally low friction in humid air, negligible wear and, particularly high load-bearing capacity.

In [13], [14], TMD + C composite films, including Mo–Se–C and W–Se–C, were grown by pulsed laser deposition (PLD). When using PLD in standard configuration, the structure of the composite films showed several notable differences from the structure of the films obtained by magnetron sputtering. The PLD of TMD + C films resulted in the formation of a dense amorphous matrix containing nanometre-sized metal-based particles. Particle sizes varied in a range from ~ 5 nm to ~ 50 nm. These particles consisted of metal core and TMD-based shell. Fominski et al. [15] established that the metal core is formed at the stage of laser ablation of a synthesised TMD target (W for WSe₂ target, Mo for MoSe₂ target), while the shell grows as a result of condensation, migration, and redistribution of atoms during the deposition of a laser-initiated atomic flow on the surface of a growing film. For PLD of Mo–Se–Ni–C films, local ordering of atoms was detected within the amorphous matrix, causing the formation of a mixture of amorphous carbon, Mo–C, and Mo–Se phases. Increasing the carbon content caused an increase in the content of sp³ bonds in the carbon phase, and an increase in the hardness of the films. In the case of laser ablation of TMD targets containing nickel, which is the currently used intercalating element, micron-sized Ni particles were also found on the surface of the films [13].

Mo–Se–Ni–C films obtained by shadow mask PLD (SMPLD) had no micro- or nano-particles, but these films were characterised by high selenium content and reduced density. Doping with carbon caused the formation of composite films containing Mo–Se and amorphous carbon phases as in the PLD films, but the hardness of the composite SMPLD films was significantly lower than even the hardness of the pure PLD MoSe_x(Ni) films [13]. A mask in the form of a thin disk was installed in the path of the laser plume from the MoSe₂(Ni) target, which protected the substrate from the deposition of Ni and Mo particles moving away from the target by rectilinear trajectories. The laser plume ejected from the graphite target propagated freely from the target to the substrate, bypassing the mask. In the shadowed region behind the mask, the atomic flux from TMD target was deposited only after scattering by molecules of the buffer gas introduced into the chamber for deposition. Thus, the use of the mask may affect the tribological properties of the SMPLD films both by removing the micro- and nano-particles from their volume and by changing the film growth conditions (composition, energy and angular characteristics of the deposited atomic flux).

The influence of carbon on the mechanical properties of the composite TMD + C films prepared by PLD and SMPLD was established, but the tribological properties of the composite films were not studied in [13], [14]. In this paper, the results of comparative studies of tribological properties of Mo–Se–Ni–C thin film coatings obtained on steel substrates by PLD and SMPLD are presented. The films had a gradient depth distribution of solid lubricant, i.e. the MoSe_x compound concentration was 100% on the film surface and decreased to zero at the interface with the substrate. This provided a relatively smooth change in the mechanical properties along the film thickness and could help to improve the adhesion of the films to the substrate. After fabrication, the samples were stored for 2 years and then the tribological tests were conducted by the pin-on-disk method in humid air.

It should be noted that the PLD regimes selected for gradient film deposition were similar to the regimes used by Fominski et al. [13] to obtain homogeneous composite Mo–Se–Ni–C films with different concentrations of carbon. It is possible to use these results to characterise the specific features of gradient Mo–Se–Ni–C films and reduce to some extent the structural and chemical analyses of the films.

Section snippets

Experimental details

A schematic diagram of the experimental set-up for PLD and SMPLD of Mo–Se–Ni–C films can be found in [14]. Laser radiation penetrated into the vacuum chamber and was scanned over the target holder by an automatic device controlled by a computer program. Two targets – MoSe₂(Ni) and graphite – were mounted on the holder. The angle between the laser beam and the target surface was ~ 45°. The substrate was placed parallel to the target surface, thus perpendicular to the particle flow. A laser beam

Chemical composition, thickness, and morphology of deposited films

RBS study showed that the thickness of gradient Mo–Se–Ni–C films obtained by PLD slightly decreased and the thickness of the films obtained by SMPLD increased a little when measured from the centre to the edge of the substrate. The thickness change did not exceed 15%. RBS spectra for the PLD and SMPLD gradient films measured in the centre of the substrates are shown in Fig. 1. The results of mathematical fitting of RBS spectra illustrating the depth distribution of Mo, Se, Ni, and C atoms in

Discussion

Differences in roughness of the as-deposited gradient Mo–Se–Ni–C films could be one of the reasons for the difference in the friction coefficients for the PLD (~ 0.8) and SMPLD films (~ 0.6) during running-in. Film surfaces with lower protrusions allow a faster friction decrease [24]. When the number of sliding cycles increased, the wear of the PLD and SMPLD films developed under the influence of several competitive processes. Fominski et al. [13] showed that the hardness of pure MoSe_x(Ni) films

Conclusion

PLD was used to obtain gradient Mo–Se–Ni–C thin film coatings in which the concentration of the MoSe_x phase increased smoothly with distance from the interface with the DLC underlayer and reached 100% at the film surface. The gradient Mo–Se–Ni–C PLD films contained metallic micro- (Ni) and nano-particles (Mo) generated by laser ablation of an MoSe₂(Ni) target. These particles were absent in the films produced by using a mask, which was placed between the substrate and the MoSe₂(Ni) target. For

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Tribological properties of gradient Mo–Se–Ni–C thin films obtained by pulsed laser deposition in standard and shadow mask configurations

Highlights

Abstract

Introduction

Section snippets

Experimental details

Chemical composition, thickness, and morphology of deposited films

Discussion

Conclusion

Thin. Solid Films

Thin. Solid Films

Thin. Solid Films

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Thin Solid Films

Surf. Coat. Technol.

Surf. Coat. Technol.

Thin Solid Films

Surf. Coat. Technol.

Appl. Surf. Sci.

Surf. Coat. Technol.

Thin Solid Films

Catal. Today

Wear

Wear

Wear