1. The dust and the contact

Arthur C. Clarke’s A Fall of Moondust traps tourists in a sea of lunar dust so fine it behaves like a liquid. The engineering problem is not the dust itself but the uncertainty it creates: you cannot see what is really touching what. A contact interface in orbit has a similar problem. The apparent mating surface is actually a landscape of tiny peaks and valleys.

This entry reads about how contact resistance and thermal resistance change under thermal cycling.

2. Contact resistance

Electrical contact resistance depends on the area of actual metal-to-metal contact, which is much smaller than the apparent area. Under thermal cycling:

  • Fretting: small relative motion between surfaces can wear the contact spots and increase resistance.
  • Oxide growth: in vacuum, oxidation is slower, but surface films can still form.
  • Stress relaxation: the force holding the contacts together may decrease as materials creep.
  • Cold welding: in vacuum, small contact spots can weld, which may either lower or raise resistance depending on whether the weld survives the next cycle.

The result is that contact resistance often drifts upward over many cycles.

3. Thermal resistance

Thermal contact resistance is the inverse of thermal contact conductance. It depends on:

  • Surface flatness and roughness: smoother surfaces have lower resistance.
  • Interface pressure: higher pressure deforms the peaks and increases contact area.
  • Thermal interface material: a compliant TIM fills voids but may pump out or degrade.
  • Differential expansion: if the two sides expand differently, the pressure distribution changes.

Under thermal cycling, thermal resistance can increase due to TIM degradation, surface fretting, or loss of clamping force.

4. What the literature recommends

The literature recommends:

  • Use materials with matched CTE where possible.
  • Maintain adequate and stable clamping force.
  • Choose TIMs with low outgassing and stable properties over temperature.
  • Design for inspectability or testability.

These are all applicable to the rack-to-cell interface.

5. What this changes

  • Contact and thermal resistance are expected to drift under thermal cycling.
  • The ground test should measure this drift.
  • Material selection and clamping design matter more than the room-temperature test suggests.