Insights into the wear track evolution with sliding cycles of carbon-alloyed transition metal dichalcogenide coatings

Highlights

• Compact and polycrystalline Mo-Se-C coatings are deposited by magnetron sputtering.

• The coatings are tribologically tested in ambient air conditions at various sliding cycles.

• The evolution of tribolayers is studied using Raman SEM and HRTEM.

• The tribological response in different sliding environments is governed by the TMD phase.

Abstract

This study is aimed towards a better understanding of the tribological performance of the optimized Mo-Se-C dry lubricant coatings. The coatings are deposited with a lower carbon content (< 30 at. %), as compared to the literature. The detailed analysis of the wear track and the tribolayer evolution as a function of the C content and the number of sliding cycles is discussed, using Raman spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) analyses. The coatings are highly compact, homogeneous, with (002) basal planes parallel to the surface as observed by X-ray diffraction (XRD) studies. In unidirectional tests, under ambient conditions, the friction coefficient and the specific wear rate decrease with the number of sliding cycles. After 25,000 cycles, the wear tracks are not fully covered with MoSe₂ tribolayer, instead displayed zones of the tribolayer as well as the as-deposited coating. After 100,000 cycles, the wear track is covered by a thick MoSe₂ tribolayer. Scanning-TEM (STEM) chemical composition mapping also confirms that MoSe₂ tribolayer is dominating during sliding in ambient air. Reciprocating sliding tests are also performed in ambient air, dry N₂, and at 200 °C. The transition metal dichalcogenide (TMD) phase is always governing the low friction, with coatings not showing the chameleon behavior claimed in the literature for C-containing TMD coatings.

Introduction

Solid lubricant coatings are under research for the past few decades. They are potential candidates for applications involving aerospace, tool manufacturing and automotive industries [[1], [2], [3], [4]]. Transition metal dichalcogenides (TMD) lie among the most developed classes of solid lubricant coatings [5]. TMDs in their pure sputtered form are porous, with columnar cross-section morphology that makes them very soft. This causes very low load-bearing capacities and low adhesion to the substrate, leading to poor tribological properties. One other major drawback of pure sputtered TMDs is their low resistance to moisture and oxidation. The dangling bonds in TMDs can react with moisture and increase the interlamellar interactions, thus affecting the low frictional properties [6]. These issues of pure TMD coatings can be solved by doping/combining with other elements that can either react with TMDs to form metal compounds or support them by forming nanocomposite coatings. The most common doping elements include metals e.g. Ti, Au, Pb, Zr and Cr [[7], [8], [9], [10], [11]] or non-metals e.g. C and N [[12], [13], [14], [15], [16]]. Among these elements, carbon is considered one of the most promising elements for achieving stable nanocomposite TMD coatings intended for low friction applications in diverse environments.

Voevodin et al. [17] were among the first researchers to introduce TMD-C nanocomposite coating consisting of WS₂/WC/DLC. According to the authors, carbon was governing the frictional properties when sliding in ambient air and WS₂ was playing a dominant role in the vacuum and the dry conditions, which was called at that moment as a “chameleon behavior”. Following their works, many other researchers published works on TMD-C coatings [6,[18], [19], [20]]. In some cases, the results contradicted the findings of Voevodin et al. [17] and it was claimed that whether it is dry, vacuum or ambient atmosphere, TMD phases were playing the main role in easy shear tribological properties [6,21,22]. This will be discussed in this work for Mo-Se-C coatings. The usage of MoSe₂ as a TMD phase in nanocomposite coatings is rarer, as compared to MoS₂ or WS_2. The interest for the Mo-Se-C system relies on the scarce literature available, which shows that the MoSe₂ phase is less sensitive to humidity [23]. An additional advantage of the MoSe₂ is the smaller difference in the atomic mass of the chemical elements of the compound when compared to, e.g. WS₂ [24]. This is important for the synthesis of the coatings by magnetron sputtering as the scattering behavior of the species will be more similar. Moreover, the preferential re-sputtering of the chalcogen atom due to the bombardment of the growing film with energetic species will be attenuated. In this context, the Se/Mo ratio, which is very important for TMD containing films, can be more easily controlled. Also, in our recent work [25], we reported the optimization of the compositional, morphological, structural and mechanical properties of Mo-Se-C coatings as compared to either other TMD-C systems or previous works on these coatings. This study is mainly focused on the ambient air tribological performance of the optimized coatings.

Most of the literature studies only deal with short duration tests which are not the best indication of the long-term performance [[26], [27], [28], [29], [30], [31]]. Also, in the literature, the changes in the coefficient of friction (COF) with the number of sliding cycles have been reported without a very precise explanation [21,32]. Likewise, the literature on TMD-C coatings lacks the TEM investigation of the (evolution of) wear tracks achieved after different number of sliding cycles in ambient air. Based on the shortcomings of literature, in this work: i) the tribological performance has been tested at different sets of sliding cycles (5000, 25,000 and 100,000 cycles) to analyze the difference between the running-in and the steady-state stages during the tribological test. This will also unfold the ability of the coatings for long-duration sliding / long-term applicability. ii) Another important domain of focus is the exploration of the role of sliding cycles on the evolution of the wear track composition and the mechanism behind the wear track coverage by tribolayer. The study of the wear tracks composition and tribolayer evolution using SEM, Raman spectroscopy and HRTEM will reveal the science behind the decrease of the COF with an increasing number of sliding cycles as well as the mechanism of tribolayer formation. iii) Finally, the sliding performance of the coatings in diverse environments has been studied to reveal whether the Mo-Se-C coatings display or not, a chameleon character.