NASA missions employing mobility systems and other moving mechanical assemblies for application on Mars, the Moon, and in deep space depend on the reliable operation of these assemblies and their tribological components. Wet lubricants are sometimes used in space applications, but in order to avoid solidification, they often require active heating due to the extreme cold temperatures that are encountered. Dry lubricants, such as molybdenum disulfide (MoS2), are more commonly chosen for space mechanisms because they are not subject to the low-temperature limitations of wet lubricants while also providing superior lubricating properties. A major drawback of dry lubricants is low wear resistance that eventually leads to failure of the assembly as the lubricant is removed.
A practical, near-term solution to extending the life of dry lubricants is to replenish the material in situ. A limited number of wet lubricant replenishment systems have been developed, most of which are not applicable to NASA missions, and there are no known practical systems for delivering dry lubricants. A pulsed plasma lubricator (PPL) has been developed that can deposit films of lubricating materials such as MoS2, copper, and Teflon on a remote substrate (such as a moving mechanism), with lubricating and wear resistance properties equal to or greater than pre-applied films of dry lubricants.
Controlled testing with the PPL depositing Teflon on remote substrates has been performed. The substrates were later subjected to a wear characterization through an in-house diagnostics developed for this task. The testing revealed that PPL-deposited films had better wear properties than pre-applied films such as Teflon, and PPL films could be re-applied to a substrate to replenish the lubricating film in order to minimize the total wear over a given period of time or extend the lifetime far in excess of the life capable with preapplied lubricants.
The innovation of the PPL is to modify a space-qualified device, the pulsed plasma thruster (PPT), to efficiently sputter and transport lubricants. The PPT is a simple device, consisting of little more than a pair of electrodes separated by a solid propellant. A capacitively charged arc is struck across the propellant, causing ablation. Electro dynamic and gas dynamic forces accelerate the propellant and produce thrust. In the PPL, the challenge is to optimize the arc to ablate lubricant without causing dissociation, and then to transport it with sufficient energy that good adhesion is obtained at the substrate. It was demonstrated that the PPL is capable of transporting lubricants that adhere to the surface of substrates, and those films provide lubrication and wear resistance equal to or greater than pre-applied films.
This work was done by Richard R. Hofer, Donald B. Bickler, and Saverio A. D’Agostino of Caltech for NASA’s Jet Propulsion Laboratory.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, NASA Management Office–JPL. Refer to NPO-47881.