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Low-speed spacecraft mechanisms like solar array drives and pay load steering mechanisms use high precision gears of SUS 17-4PH material. In order to have the lowest overall weight of the mechanisms, simplest design with solid lubricated bearings and gears are preferred against complicated design with fluid lubrication involving hermetic sealing for retaining the fluid lubricant against evaporation. The operating speed of these mechanisms is in the range of 1–15 revolutions per day. Hence to optimally meet the above requirements like the need for the tribo-component to function for many years unattended and exposed to the vacuum of space, solid lubrication of these gears with thin films of Pb ion-plated (Pb-IP) is widely adopted. The long life requirement of these mechanisms with low jitter depends on the friction, wear and endurance life of the lubricant coating. The latest upgraded design of such mechanisms, with requirements of high torque and more than 15 years of orbital life, needs to be supported with appropriate enhancement/improvement of the tribological aspects of the solid lubricant coatings. In regard to the present study of ion-plated Pb coating on SUS 17-4PH steel, a newly developed Pb-IP coating processed with higher substrate bias has shown an increase in the coating life cycles while testing in a ball-on-disk tester in vacuum of 10 −6 Pa. The cross-sectional examination of the graded interfaces by SEM–EDS showed an increase in thickness of the graded interface for the new Pb-IP coating compared to that of the conventional Pb-IP coating. |
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