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Surface treatment and tribology

Why is molybdenum disulfide (MoS₂) better suited than graphite as a solid lubricant for mechanisms used in vacuum, such as on satellites?

  1. AMoS₂ is much harder than graphite, so it survives long as a film in vacuum without being worn away even when rubbing directly against a metal counterface
  2. BGraphite's interlayer sliding relies on adsorbed water and the like, so its friction rises in vacuum, whereas MoS₂'s does not
  3. CGraphite has a high vapor pressure, so in vacuum it gradually sublimes even near room temperature, thinning until it quickly disappears
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Answer: B. Graphite's interlayer sliding relies on adsorbed water and the like, so its friction rises in vacuum, whereas MoS₂'s does not

Graphite and MoS₂ are both layered crystals that give low friction because layers stacked by weak van der Waals forces slide over one another. In graphite, however, water molecules and gases adsorbed at layer edges and elsewhere help the layers slide, so in vacuum or dry gas the friction coefficient becomes large and wear increases. MoS₂ layers slide easily by nature, with a low friction coefficient of about 0.05 or less in vacuum, so it is used in space hardware. Conversely, in humid air MoS₂ oxidizes easily and its performance drops. Both are soft materials and are not chosen for hardness. Graphite is lost by sublimation only at high temperatures above about 2000 °C, which is not a concern at the temperatures where mechanisms operate.

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