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My book says that it is a better lubricant on the Moon because of absence of gravitational pull on the Moon. However, I also read that it's lubricant nature is because of a film of moisture or gas molecules absorbed on the surface of its layers and in vacuum, it gets dried up and would be a bad lubricant. Which is correct?

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  • $\begingroup$ Googled enough to convince myself that "regular" graphite would be a bad lubricant in the vacuum of space, but couldn't find a good authoritative reference. $\endgroup$
    – MaxW
    Oct 29 '18 at 17:56
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    $\begingroup$ Gravitation pull has no impact on lubricity (contact forces perhaps). $\endgroup$
    – Jon Custer
    Oct 29 '18 at 18:33
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    $\begingroup$ By curiosity: please state your source (here: your book) you cite. Perhaps SE Space Exploration has contributors who happens to know about the lubricants used for lunar rover and lunochod by NASA and pre-Roscosmos and their experiences. $\endgroup$
    – Buttonwood
    Oct 29 '18 at 22:36
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Graphite, like most solid lubricants, has a layered structure which makes it a good lubricant. The atoms of a layer are strongly bonded to each other, while the interlayer (van der Waals) forces are much weaker. The layers align themselves between the sliding surfaces and the layers slide one over the other providing lubrication.

Since it are the shear forces that come into play, gravity has no influence on the system. From that point of view, there is no difference between its use here or on the Moon. But, there are also extrinsic factors which influence its performance.

Graphite works best in a humid environment. This is probably due to the weakening effect of the water molecules on the interlayer forces. So, graphite should not work well in vacuum.

Another solid lubricant, $\ce{MoS2}$, which has also a layered structure like graphite, works much better in vacuum and should be the lubricant of choice on the Moon.

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