Application of MOS2-based solid film coating

Manoj Rajankunte Mahadeshwara

October, 28 2024
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Introduction

Solid lubricants are used in tribological systems to reduce friction and improve wear resistance. Amongst most of the solid lubricants molybdenum disulfide (MoS2) is frequently used in extreme environments, like gas turbine engines, due to its excellent tribological properties such as low friction and wear. MoS2 has a layered structure with strong covalent bonds between molybdenum and sulphur atoms, and weak Van der Waals forces between layers. However, its performance is highly sensitive to environmental conditions. MoS2 can degrade in humid or ambient environments which leads to higher friction and potential failure that is often due to MoO3 formation. To improve performance, MoS2 is often doped with metals or composites like Au, Ti, Ni, Ag, and others.

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Figure-1 Structures of the polytypes of MOS2

Need for solid lubricants

Liquid lubricants have limitations in extreme conditions which necessitates the use of solid lubricants to overcome these challenges. In high vacuum, liquid lubricants evaporate and may contaminate devices, while at high temperatures, they decompose or oxidize. Additionally, at cryogenic temperatures, they solidify or become too viscous. Under conditions such as radiation or exposure to corrosive gases, they tend to decompose, and in high-pressure environments, they may exceed their load capacity. Liquid lubricants also attract dust and contaminants, increasing contamination risks. Therefore, solid lubricants are needed to address these limitations effectively.

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MOS2 in space application

The development of solid lubricants, such as MoS₂ has driven an increasing demand for space applications. The space missions which involve extreme temperature variations and long durations without maintenance need solid lubricants to operate at those extreme conditions. A notable example of lubricant failure was the Galileo spacecraft’s high-gain antenna malfunction which was caused by MoS₂-based solid lubricant failure in space despite of successful Earth-based tests. Research is ongoing to improve the performance of solid lubricants in particularly MoS₂ for future missions. The friction-reducing effect of MoS₂ coatings involves two key processes: (1) the formation of a transfer film on the counter-surface as it slides against the MoS₂-coated component, and (2) the shear-induced alignment of the MoS₂ basal planes in the sliding direction, both in the original coating and in the transfer film or wear particles.

Future Challenges

Doped MoS₂ coatings show improved tribological performance under challenging conditions, making them valuable for critical technologies like space exploration. However, challenges remain in scalability, doping material options, and testing setups that can simulate long-term missions in vacuum and cryogenic temperatures. The deposition methods and parameters significantly impact performance, but systematic studies are lacking due to limited disclosure of commercial coating details. Simulations offer a promising approach to study these coatings, particularly the hidden sliding interface, and to predict new dopants. The success of simulations depends on developing accurate interaction potentials, a key research focus moving forward.

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Reference

[1] Savan, A., Pflüger, E., Voumard, P., Schröer, A. and Simmonds, M., 2000. Modern solid lubrication: recent developments and applications of MoS2. Lubrication Science, 12(2), pp.185-203.

[2] Vazirisereshk, M.R., Martini, A., Strubbe, D.A. and Baykara, M.Z., 2019. Solid lubrication with MoS2: a review. Lubricants, 7(7), p.57.

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I am a postgraduate researcher at the University of Leeds. I have completed my master's degree in the Erasmus Tribos program at the University of Leeds, University of Ljubljana, and University of Coimbra and my bachelor's degree in Mechanical Engineering from VTU in NMIT, India. I am an editor and social networking manager at TriboNet. I have a YouTube channel called Tribo Geek where I upload videos on travel, research life, and topics for master's and PhD students.