Mercury/Apollo 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.

Forward reference to Friday's lecture. Graphite is a lubricant on Earth (water vapor intercalates between planes) but an abrasive in vacuum; early Mercury shots seized when this was not understood.

When we get together again — I'll be here on Friday, Dr. Bellmore will be here the next two days, I'll be back on Friday from the red-eye, and we can talk about some things again. I will tell you about how graphite is a lubricant on Earth and an abrasive in space. That was a big surprise at the beginning of the Mercury program. Some of the first space shots, things quit turning. They had put them together with graphite as the lubricant. When they got into space, they found that graphite was no longer a lubricant; it was an abrasive. It all has to do with these fundamentals of bonding and contamination that we're talking about.