Gear train systems used in the lunar environment should be able to endure limiting conditions such as ultra-high vacuum with minimal energy dissipation. From this point of view, we have obtained the optimum thickness of a silver coating that can influence friction mechanisms of the gear and pinion. To clarify the sliding friction between the gear and pinion, a pin-on-disk tribometer that could be implemented in space tribological applications was constructed according to the specifications of ASTM G99-95a using a solid lubricant at a vacuum pressure of approximately 10-7 Torr. The friction characteristics were compared for atmospheric and vacuum environments. Through these results, an improved understanding of the friction mechanisms between the gears coated with silver is obtained. Moreover, an analysis of energy dissipation due to differences in the gear backlash was conducted.