Full Length ArticleFluorine-carbon doping of WS-based coatings deposited by reactive magnetron sputtering for low friction purposes
Graphical abstract
Introduction
The automotive industry is known to have a great impact in the World economy due to the commercialization of a huge amount of diversified electronic systems and mechanical components [1]. Before the assemblage step, many mechanical devices require previous lubrication. The tools, for an efficient and durable lubricants’ application should possess a combination of low friction and specific wettability properties. However, the European commission is nowadays more alerting on the use of greases or oils, applying restrictive rules, since these lubricants are harmful to the environment and human health [2], [3], [4]. Thus, materials and surface engineering is a potential solution for removing or, at least, reducing the lubricants’ use by developing functional surfaces through the application of coatings able to provide self-lubricating properties and suitable contact angles in relation to lubricants (oils).
Transition metal dichalcogenides (TMDs) are materials with interesting electronic, optical, mechanical and magnetic characteristics which also triggered intensive research for self-lubricating purposes [5], [6]. Indeed, their anisotropic layered crystal structure allows to reach low friction coefficients mainly in vacuum or in dry air. TMDs’ materials consist of a transition metal layer (e.g. Mo, W or Nb) sandwiched between two chalcogen layers (e.g. S, Se or Te), in which the atoms are covalently chemically bonded together while these layers are held together by weak van der Waals interactions [7].
Magnetron sputtering is considered a clean, reproducible and cost-effective method, widely used to deposit TMD coatings [8]. Pure sputtered TMDs exhibit columnar morphology, low hardness (<1 GPa), with consequent very low load-bearing capacity, and low adhesion to the substrate materials. Their high porosity allows a high reactivity of the coating with oxygen in humid air environments, causing an increased friction and wear which strongly limit their application as self-lubricant to dry or vacuum atmospheres [9], [10], [11]. Alloying TMDs with selected elements such as, Ti [12] or Cr [13] could improve the mechanical and tribological performances. Another alternative to improve the mechanical performance is to alloy TMD coatings with carbon or nitrogen. Magnetron-assisted pulsed laser deposition or laser ablation allowed to produce dense self-lubricant W-S-C based coatings, less sensitive to air environment than WS2, combining excellent frictional properties and good tribological performance in vacuum (coefficient of friction (COF) of ∼0.03) and in humid air (COF of ∼0.15), when tested against steel balls [14], [15]. Similar TMD-based coatings with carbon or nitrogen were deposited by magnetron sputtering [16], [17], [18]. The W-S-C coatings were also tribologically tested against steel balls at 30% of relative humidity (RH), and the COF values decreased down to ∼0.06 when the load increased from 5 to 47 N [19]. These coatings were also tested at temperatures above 100 °C with COF values below 0.01, and the wear rates were almost independent of the temperature up to 400 °C [20]. Apart from the higher thermal stability depicted for the W-S-C system, lower wear volumes were also observed for this system, withstanding higher loads (1000 N) in humid air than other TMDs alloyed with carbon [21], [22]. Therefore, if it would be possible to add the good mechanical and tribological characteristics of the W-S-C coatings with a suitable surface wettability behaviour (hydrophilic-oleophobic), the final surface functionality could be improved allowing a more efficient and controlled application of the lubricants and, then, reducing their consumption.
Wettability is a complex surface property which is commonly controlled by the roughness or/and by the chemical composition of the surface [23], [24], [25], [26], [27]. In regard to chemical modification for amphiphobic behaviour, polymeric-based coatings containing F radical [28] or silane (H-Si-H radicals) [29], [30] species are often used. These coatings are able to decrease the surface free energy and, then, induce repellence to particular liquids, although they have intrinsically low mechanical resistance and load-bearing capacity. Then, the purpose of this research was to provide a reasonable mechanically strong material with a specific wettability property, through the alloying of W-S-C coatings with fluorine using magnetron sputtering deposition. Fluorine alloying is well-known in many fields for changing specific properties of materials, such as: (a) the refractive index for photovoltaic applications [31], [32]; (b) the electrical properties to achieve more dielectric or conductive films [33], (c) the mechanical properties for achieving ultra-low friction [34], [35] or even, (d) the surface properties to avoid coagulation mechanisms in medical devices [36].
The fluorine is reported to deteriorate the hardness and the elastic modulus of the C-F or F-DLC sputtered coatings [37], [38], [39]. In relation to the water contact angles, the fluorine alloying has shown to increase the coatings’ repellence [40], [41], with no report in regard to oil wettability. Other authors sputtered MoS2 and PTFE (C-F chains) and the produced coatings demonstrated reduced sensitivity to relative humidity in the tribological assay [42]. Similarly, the tribological behaviour of WS2:CFx films deposited by pulsed laser deposition showed less sensitivity to moisture than pure WS2, at least until 50–60% of RH; these films depicted ultralow friction in dry air (COF ≤ 0.01) against stainless steel balls. No results were presented concerning water or oil wettability [43]. The same deposition method was used to deposit WS2:CaF2 coatings with very good lubrication performances (COF = 0.15) at high temperature (500 °C), with great potential for airspace industry applications [44].
In summary, fluorine doping of TMD coatings for wettability purposes is still not explored. Furthermore, the literature is scarce on the fluorine doping of TMDs-C for improvement of the mechanical properties. Therefore, a study on the F alloying of the W-S-C films for tribological applications at high loads in humid environments with further assessment of the surface coatings’ wettability is still missing. In this study, these coatings will be developed to improve the mechanical properties/tribological performance and reduce the surface free energy, in order to validate their applicability on metallic materials, for automotive components’ lubrication purpose.
Section snippets
Production of the fluorine-carbon doped W-S coatings
The coatings were deposited by reactive magnetron sputtering on commercial glass, mirror-polished (1 0 0) Si, and M2 steel disc substrates. The latter were mechanically polished (RotorPol 21 model by Struers with 25 cm plate diameter) with SiC sandpapers, with increasing grit size, and diamond paste (3 µm). Prior to depositions, all samples were firstly degreased in acetone (15 min) and in ethanol (15 min) in ultrasonic bath and, then, air dried.
A semi-industrial DC closed field unbalanced
Morphology and chemical composition
The top-view and cross-section morphologies of the WS-C/F coatings are presented in Fig. 2(a)-(f) as section I and II, respectively. The coatings’ surface has a typical cauliflower morphology for low CF4 flow rates. For higher flows, the films become denser although fissures could be detected on the surface, particularly for WS-CF10 and WS-CF15 coatings. WS2 and WS-CF2 coatings have high porosity all over the sample; however, quite good compactness was observed in the cross-section of WS-CF5
Conclusions
W-S coatings doped with fluorine and carbon were deposited in an Ar/CF4 sputtering plasma. The fluorine incorporation in the coatings reached a maximum value of 9.5 at.% for CF4 flow rate of 5 sccm. For higher values, fluorine content almost vanished and a significant increase of both oxygen and carbon incorporation was observed. These results show that, until a certain CF4 threshold value, this gas contributed to the coating growth whereas, for higher flow rates, an etching regime takes place.
Acknowledgements
This research work was supported through the program COMPETE – Programa Operacional Factores de Competitividade – by national funds through FCT – Fundação para a Ciência e a Tecnologia in the framework of the Strategic Funding UID/FIS/04650/2013, UID/EMS/00285/2013 and with a PhD fellowship PD/BD/112079/2015. This work also acknowledges On-Surf Project – Proj. N° 24521 - PT2020 - SI I&DT Business – Mobilizing Programs. This work was also sponsored by IAPMEI funds through QREN 38587 2014/2015 –
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