Vacuum welding of parts that should move is a big problem for spacecraft. On Earth, air provides lubrication for such simple processes as screwing a nut on a bolt. Remove the air and the parts can weld themselves in place.
On the Moon, the process of vacuum welding made the lunar regolith so tough that more than one Apollo astronaut suffered from injury in trying to remove it. Lunar regolith is just dust that has been compressed over time. Essentially, it’s tougher than any Earth rock, including concrete. The lunar rock breccia that makes up all the Lunar highlands is all formed by vacuum welding under relatively low pressure and ambient temperature.
Whenever I try to think of vacuum welding as a lunar technology, and decide how technology can be useful under lunar or even Earth conditions, my brain goes into meltdown after about an hour or so without coming to any definite conclusions.
Here are some thoughts.
- On the Moon, compress surface dust into briquettes in a brick mould and use it to build structures such as lunar habitations. These help keep DNA damage from solar cosmic rays to a minimum. And by not requiring any more than human strength and passive heating are easy to make.
- The mould would consist of an open rectangular box and top plate. Put the top plate on, hit it with a hammer, and voila, instant building material.
- Or would it? The lunar dust may vacuum weld itself to the box and be unable to be removed from the mould. Or the banging from a hammer may not suffice.
- So testing on Earth is required. But how to test?
- A vacuum on Earth could be inside a fixed wall vessel such as a bell jar, or inside a flexible walled vessel such as a plastic bag.
- Would a plastic bag suffice? Or would that leave pockets of air?
- If possible using a plastic bag, that makes the technology useful for manufacturing on Earth. Evacuate the air and then either press or hammer to get the welding. This would be like sand casting, but could produce a much more stable mould for re-use that doesn’t need to be re-created each time.
Then there is the problem of selecting materials for vacuum welding into briquettes. Lunar grains are sharp edged and thus very abrasive. That’s good, except that it means that when grains come together the contact area is small, as in sintering on Earth. Leading to unwanted voids from a strength point of view, wanted voids from a thermal blanket insulation point of view. Compressed lunar regolith would be a superb thermal insulator, easily outperforming fibreglass, foam etc. It could even be used for making tiles for withstanding the heat of re-entry of, for example, a space shuttle. Or as outer protection for a spacecraft to investigate the Sun from close-up.
Anyway, normally the voids are unwelcome. Certainly from a strength point of view. One possibility is of sieving the material into grades of lunar material carefully chosen so that finer grades fill the voids left by larger grades, and still finer grades fill the voilds left by the finer grades, etc. All the way fown to micron size grains.
Another approach would be tom embed the sharp-pointed particles in a ductile matrix made from, for example, aluminium foil. As each grain penetrates the alfoil it produces an extended boundary that has the ability (in vacuum) to bond strongly chemically and not just physically. Giving a composite material potentially as strong as the strongest steels. It would have the hardness of ceramics with the crack-resistance of steel.
But all this requires testing on Earth. Moving away from the plastic bag type vacuum to a bell-jar-like chamber filled with vacuum. It is obvious that we would not use real lunar dust as the testing material, buy a stimulant. Annoyingly, official lunar-dust-simulant will not suffice. And the reason for that is that as soon as dust is exposed to air it reacts with oxygen in Earth’s atmosphere. And because oxygen has only two chemical bonds it seals the surface against further chemical reaction. Such as anodising aluminium and the oxide layer on chromium. But for best bonding we want those chemical bonds available.
And that means that for testing on Earth the dust, and any other materials such as aluminium flakes, need to be manufactured under vacuum. The manufacturing process for out unofficial lunar simulant consists of fracture hammering using either vertical hammering (like pile driving or a gold battery) or rotary hammering (like a car crusher or high speed rotary crusher. I don’t recommend a ball mill or rod mill, because of potential premature vacuum welding. But manufacturing under vacuum risks the problem of vacuum welding of the product to the machine and machine parts to each other. So care is needed.
The manufacturing of dust under vacuum conditions could be as simple as crushing basalt, but could be done using a variety of other source materials, including naturally occurring minerals and metals. (Incidentally making aluminium powder for Earth-based use in explosives). The resulting dust surfaces would be both sharp and chemically active, ready for testing.
The first test would be angle of repose. One way of doing this would be to put a pile of dust in a 90 degree cradle that rocks back and forth, to see if the simple action of low energy movement would cause the dust to coalesce into something with a high angle of repose. A bigger rocking cradle test would see if the slightly greater gravitational potential energy results in a larger angle of repose.
Alternatively, the angle of repose can be measured in a rotating drum, perhaps 10% full, to see if particles coalesce to increase their angle of repose just under the action of sliding due to gravity.
The second test would try out various options for briquette construction. Pressure vs impact loading. Trying out different mould materials and release compounds. And testing the strength of the resulting briquettes. All in vacuum again. Putting and using strength testing apparatus inside a vacuum chamber would again be a challenge.
I wonder how high a vacuum would be needed? There have been records of corrosion of satellites in orbit due to the free oxygen in the far outer fringes of the Earth’s atmosphere. Earth’s atmosphere at 86 km up has a pressure of 0.37 Pa. The lunar atmosphere is about 3*10^-15 bar, about 3*10^-10 Pa. The vacuum in a cyclotron is near 10^-5 Pa. A high vacuum two stage pump tends to have a vacuum near 60 Pa. Hmm. A diffusion pump backed by a rotary pump will get to 1 to 10 Pa. Would need to experiment with several different vacuums to see how rapidly the oxygen present reacts with the dust, OR, simply produce more dust than all the oxygen in the vacuum can react with.