Transition metal dichalcogenide-based solid lubricant coatings prepared by high target utilisation sputtering

Abstract

Attempts to reduce friction and wear have been recently heavily investigated worldwide. According to recent studies tribological issues are responsible for approximately 23 % of the world´s total energy consumption. Liquid lubricants and greases have been used for centuries for the friction reduction in many different areas. However, these conventionally used lubricants cannot be used in extreme environments and more demanding applications, such as vacuum, high/low temperatures, high contact pressures, clean environments etc. In such conditions, solid lubrication is the only way for the friction reduction. The objective of this study was to use a new emerging technology called High Target Utilisation Sputtering (HiTUS) for the preparation of the solid lubricant thin films, extended investigation of their properties and testing.
The three types of the coatings were developed, two of them were reported for the first time here. HiTUS technology was employed in preparation of the low friction coatings also for the first time. In this work, transition metal dichalcogenides (TMDs) were doped with other atoms to improve the overall tribological performance of the coatings. Molybdenum disulphide (MoS₂) and molybdenum diselenide (MoSe₂) were doped with nitrogen (N) and the early trials were done on doping of the MoSe₂ with titanium (Ti). It was found that addition of the third element (N or Ti) to the TMD films improved all the properties namely; density, hardness, load-bearing capacity, coefficient of friction (COF) and the lifetime of the films. Enhancements of these properties were assigned to the formation of the tribofilm during sliding combined with strong mechanical support provided by the dense film morphology
The influence of the testing environment on the sliding was studied. All coatings were evaluated in humid air and some of them even under the vacuum, N₂ gas atmosphere and at elevated temperatures. Eventually correlations of the coatings chemistry and structure with the results of the tribological tests were made. Importance of the choice of the doping element and its optimal ii concentration in the TMD film was discussed. It was shown that there is a relatively well-defined upper limit of ~ 30 atom % of the doping element in which the properties are improved. Above this limit, films losing their self-lubricating properties due to the low S/Mo or Se/Mo ratios. We found that ratios of chalcogen to transition metal above 0.9 are required to preserve lubricity.
Herein developed solid lubricant coatings were compared to the other coatings reported in the literature. Some of the here synthetized coatings outperformed even the best coatings published in the literature or currently used in the real applications. Therefore, this thesis provided new interesting knowledge in the field of self-lubricating thin films that can lead to the more durable and long-lasting solutions for the future industrial applications.

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ORCID for Tomas Polcar: orcid.org/0000-0002-0863-6287

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