Abstract
Solid lubricants, antiwear coatings, and self-lubricating composites are used in applications on spacecraft where oils and greases cannot be used because of the need to avoid lubricant volatility/migration, and where the application requires significant temperature variation, accelerated testing, higher electrical conductivity, or operation in boundary conditions. The purpose of this review is to provide spacecraft designers with tools that can aid in the effective use of solid-based tribological materials, both to increase their usage, and to reduce anomalies. The various tribological material formulations are described, including how their materials, physical, and chemical properties affect their performance. Included are typical solid lubricants like PTFE and bonded or sputter-deposited MoS2, as well as low shear metal coatings, hard coatings, and composite materials (including bulk composites and nanocomposite coatings). Guidance is given on how to develop mechanisms that meet performance requirements, but also how to optimize robustness, so that success is achieved even under unforeseen circumstances. Examples of successful applications are given, as well as how to avoid potential pitfalls, and what the future of solid tribological materials may hold.
Highlights
Solid-based materials are especially useful when used in applications with large temperature extremes or variations, such as cryogenic sensors [3] and turbopumps for LH2/LOX fueled engines [4]
There are applications for which either solid or liquid lubricants can be used, with the choice between them driven by more subtle requirements; such applications include slip ring assemblies [6,7], actuators [8], gimbal bearings [9], and even reaction wheels (RWs) [10]
First is the vacuum environment, necessitating using materials with very low vapor pressures. This has limited liquid-based lubricants to those based on pefluoro-polyalkyl ethers (PFPEs), MAC, and PAO oils [11]. This concern is mostly moot for solid lubricants: in typical spacecraft operating environments, they exhibit negligible vapor pressure
Summary
TMDs like MoS2 and WS2 are lamellar solids consisting of layers of material with strong chemical bonds within the layers but weak physical-type (van der Waals) bonding between layers The COF for MoS2 is inversely proportional to the Hertzian contact stress, unlike solids that follow Amontons’ Law, where the COF is independent of load [27] This behavior is caused by the low shear strength of TMD’s, as opposed to harder materials where the COF is related to surface interaction. MoS2 has proven consistently superior at typical operating temperatures in vacuum than WS2 [31,32] and MoSe2 This superiority may be due to the less diffuse electron orbitals on the smaller Mo and S atoms, resulting in a surface with poorer bonding properties. Like the layers in MoS2, these helixes do not form chemical bonds to other atoms or molecules, resulting in a low-energy, low-friction surface. The reaction layer can lessen deformation of the underlying soft metal surface; such deformation could cause a solid lubricant coating deposited on the part’s surface to weaken or debond
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