How is unwanted cold welding prevented in space?

Question

Metallic surfaces touching in the vacuum can stick to each other and fuse. This is known as the "cold welding" and it seems to be a possible issue in space if it happens where it should not. It was discussed for example in the 2009 paper Assessment of Cold Welding between Separable ContactSurfaces due to Impact and Fretting under Vacuum STM-279 from ESA which summarizes the tests of the cold welding phenomena under various conditions and concludes that it can be a problem in space.

I would like to know whether there are some specific examples how this is prevented in spacecraft design. For example the ISS Solar Array Rotary Joints (SARJ) have fairly large rotating metallic areas exposed to the vacuum. Are there any countermeasures implemented in the SARJ against the cold welding?

There are also other moving devices on the ISS: Canadarm2, Dextre, Mobile Base System, Heat Rejection System Radiators, etc. Was it necessary to take the cold welding into account when some (or all) of these were manufactured?

Other spacecraft also have moving parts: the Space Shuttle had the payload bay doors, space probes carry solar arrays and deployable high-gain antennas, etc.

Answer 1

I'll mostly focus on prevention of accidental cold-welding in vacuum of space, but for reference, one of the possible causes for the Galileo high-gain antenna deployment anomaly that was considered ⁽¹⁾ was also "Retention of the ribs at the mid-point restraint due to friction, cold welding, or adhesion".

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Preventing accidental cold-welding:

• Materials selection: Contact surfaces could either be made of different materials that do not cold-weld, and/or of materials with low contact adhesion (dissimilar metals). Aerospace & Advanced Composites (AAC) publishes Cold Weld Database listing materials combinations and their adhesion in four categories also for fretting and impact contacts.
• Coated surfaces: Selected materials can be coated (painted, anodised, oxidized, chemical film coated,...) to reduce contact adhesion. Coating effectiveness can be reduced through fretting and impact damage.
• Reduce actuator dependency: By reducing number of moving parts, surfaces prone to fretting damage, triboelectric wear and exposure of unprotected materials to cold-welding also reduce in size.
• Secure passive structures: Secure latches, locks, fasteners, and so on to prevent wear and abrasive particle contaminants finding way between other moving parts.
• Cleanliness: Reduce particle and molecular contamination of the assembly to reduce abrasion, wear and corrosion of surface coated materials. Environmental sealing, flushing or purging with inert gases or liquids can be used before deployment, and in some cases also during use.
• Lubrication: Use of specific material surface properties or an applied material between two contacting or moving surfaces in order to reduce friction, wear or adhesion.
• Reduce contact surface: Either by increasing margin between them, reducing surface size altogether, and/or using non-polished finish low contact area surfaces where possible.
• Reduce environmental exposure: Thermally insulate parts exposed to thermal cycling, shield against micrometeorite, radiation damage, stray light induced emissions, electrostatic charge and chemical interaction (e.g. atomic oxygen) that can degrade or damage coating and/or introduce contaminants and expose bare metals prone to cold-welding.

And so on and these are just some of the mitigation techniques that I thought were specific to preventing accidental cold-welding, the list is by no means complete. More specifically, ESA adheres to ECSS Space engineering Mechanisms (ECSS-E-ST-33-01C) that covers this in the section 4.7.5.4.5 Separable contact surfaces ⁽²⁾.

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For more general overview of cold-welding in space applications, I suggest reading ESA's Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum ⁽³⁾. From its abstract:

A common failure mode seen during the testing and operation of spacecraft is termed ‘cold welding’. European laboratories refer to this as ‘adhesion’, ‘sticking’ or ‘stiction’. This publication is intended to provide the space community with the most recent understanding of the phenomenon of ‘cold welding’ in relation to spacecraft mechanisms with separable contact surfaces. It presents some basic theory and describes a test method and the required equipment. Cold welding between two contacting surfaces can occur under conditions of impact or fretting. These surfaces may be bare metals, or inorganically or organically coated metals and their alloys. Standard procedures for quantifying the propensity of material surface pairs to cold weld to each other are proposed. Of particular interest will be the contact data of different materials, which are presented in numerical form and as tables summarising contacts between materials that can be either recommended or considered unsuitable for use under vacuum. The data have been compiled in a database that can be accessed online.

Sources:

1. Galileo High Gain Antenna Deployment Anomaly, Michael R. Johnson, JPL (PDF)
2. Space engineering Mechanisms, ECSS Secretariat, ECSS-E-ST-33-01C, 6 March 2009 (PDF)
3. Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum, B. D. Dunn, ESA STM-279, November 2009 (PDF)