Humidity resistant MoS₂ coatings deposited by unbalanced magnetron sputtering

Highlights

• MoS₂ coatings applicable for varying environmental conditions were developed.

• Effects of deposition conditions on film properties were examined using DoE.

• High amounts of residual stress inside the coatings result in high wear resistance.

• The amount of residual stress can be controlled in a targeted manner.

• Best performing coatings show a (002)-oriented nanocrystalline structure.

Abstract

MoS₂ is a suitable solid lubricant for environments free of oxygen or water vapor (i.e. vacuum). Humid air degrades film properties due to oxidation accompanied by high wear and increasing coefficients of friction. The present study aims at the further development of sputtered pure MoS₂ coatings, extending their applicability to varying environmental conditions by increasing the resistance against humidity. The systematic coating development process is supported by using an experimental Box–Behnken design with variations of the deposition parameters cathode voltage, target/substrate distance, temperature and argon gas pressure. In contrast to common one-factor-at-a-time (OFAT) studies, this approach enables a determination of interactions between deposition process parameters and tribological–mechanical MoS₂ film properties. The tribological improvement focuses on a maximization of wear resistance in air and vacuum measured in ball-on-disk experiments. The evaluated mechanical properties are hardness, elastic modulus and residual stresses. These stresses were determined by the substrate curvature method. The study reveals that the residual stress state in the films and the hardness-to-modulus ratio are crucial for their tribological performance in humid air and vacuum environments. After a detailed determination of the relationships between deposition conditions and film properties, some selected microstructural analyses are presented which show that a substantially basal orientation of the lattice has positive effects on wear but also causes anisotropic film properties which result in fissile fracture of the coating if strong shock or point loads occur.

Introduction

Molybdenum disulfide (MoS₂) is known to be a well-established solid lubricant for vacuum applications. MoS₂-films are therefore used in a variety of mechanisms of space technology. The prerequisites for the selection of MoS₂ as a common lubricant in space were successful field tests in early space travel. A notable early example was its use on the extendible legs of the Apollo Lunar Module in 1969 [1]. Its excellent lubrication properties result from a layered lattice structure with easy shear in the basal plane direction [2]. The advantages over other solid lubricants such as PTFE or graphite are its high stability in high temperature, cryogenic temperature and vacuum environments, as well as a low coefficient of friction (COF) under space conditions and at high loads [3]. On the other hand, it is commonly known that the tribological properties of MoS₂ are very sensitive to moisture. The reaction of MoS₂ and H₂O to MoO₃ and H₂S (gaseous) causes a higher coefficient of friction because of increasing shear strength of MoO₃ [1], [4]. This effect particularly leads to increased wear and thus to a reduced lifetime of coated components.

Generally, many moving mechanical assemblies and tribological components must perform well in extreme space environments, as well as on earth, where they will be assembled and tested to guarantee correct and reliable operation [5]. In this case, lubricants additionally have to deal with humid air. In consequence, the creation of application-oriented test conditions cannot be achieved easily. Moreover, the mechanical stresses during these tests result in a more intense oxidation which degrades film properties. Further challenges are docking operations in human space travel, which are safety-critical and thus need increased attention. These interlocking mechanisms have to work not only in vacuum (UHV-conditions in space) but also in very moist air from inside the spaceship when the lock is open. The performance of currently existing or validated solid lubricants is limited concerning their suitability for varying environmental conditions and most of them do not meet these requirements [6].

Since the 1960s, sputtered MoS₂ films have been under continuous improvement [7], [8]. Some important influences of manufacturing and application as well as their limitations are known [9], [10], [11], [12]. However, despite the most recent improvements, sensitivity to moisture is still an issue. For example, the humidity in the assembly hall of the WENDELSTEIN 7-X nuclear fusion reactor has to be controlled at below 50% RH. One reason for this expensive approach is to protect the MoS₂ coatings of the support elements for superconducting coils from oxidation [13].

From the literature, some methods are known to reduce the moisture sensitivity of sputtered MoS₂ coatings. Dopants like Ti, Cr, C, Al, Au and TiN [14], [15], [16], [17], [18], [19], [20], [21] ensure increased wear resistance in air due to an increase in hardness on the one hand and an oxygen gettering effect during the co-deposition and wear process on the other hand [22], [23], [24]. These additions have the effect of interrupting the MoS₂ crystal growth, causing a smaller grain size and enhancing disorder inside the coating [25].

There are also other possible ways to reduce the moisture sensitivity of pure MoS₂ coatings. Dense microstructures with less porosity ensure a smaller surface that is exposed to oxidation because common columnar coatings show a space of about 100 Å between the crystallites [2], [13], [26]. A formation of a (002)-oriented structure with the crystal's basal plane parallel to the substrate surface additionally slows oxidation of the coating because closed sulfur layers on the surface create barriers, whereas oxygen in random-oriented films can react with molybdenum preferentially at the crystallite edges or defects rather than on the flat faces of the lamellae [1], [23]. Therefore, oxidation occurs to a depth of at least 10 nm in random-oriented films, but only to a depth of 1 nm for basal-oriented ones after storage in humid environments [27].

The current study is focused on developing sputtered pure MoS₂ coatings in order to extend their outstanding tribological properties in dry and inert media to humid and oxidizing environments (with attention focused on wear). By making use of design of experiments (DoE) a determination of interactions between deposition process parameters and tribological and mechanical MoS₂ film properties is possible. The statistical evaluation includes a description of the complex relationship between deposition parameters and residual stress of the generated films.