`Mercury/Apollo program graphite lubricant failure`

Used to teach why graphite is a lubricant at atmosphere (CO₂/O₂ absorbed on basal planes) but an abrasive in vacuum. Astronauts' knobs sticking.

Graphite is a lubricant; diamond is an abrasive. Diamond is an abrasive because it's hard — three-dimensional structure. Why is graphite a lubricant? It turns out NASA found out, to their chagrin, that graphite is a lubricant because these basal planes tend to be sites to absorb carbon dioxide and oxygen as a surface film layer. There's a surface energy associated with the basal plane, and a surface energy associated with diamond, but diamond's is isotropic because the crystal structure is symmetric. Graphite is not symmetric — you have very anisotropic properties.