Reducing Seal Adhesion in Low Impact Docking Systems
- Monday, 01 November 2010
Atomic oxygen is used to treat mating silica surfaces to reduce unwanted adhesion during docking/undocking operations.
Silicone elastomers, used in seals for airlocks or other sealing surfaces in space, are sticky in their as-received condition. Because of the sticking, a greater force may be needed to separate the mating surfaces. If the adhesion is sufficiently high, a sudden unpredicted movement of the spacecraft during undocking, vibration, or uneven release could pull off the seal, resulting in a damage that would have to be repaired before another docking. The damaged seal can result in significant gas leakage and possibly in a catastrophic mishap impacting the safety of the crew. It is also possible that a compromised seal could result in a delayed but sudden gas leak that could put the crew at unexpected risk. This is especially of concern for androgynous seals, which have identical mating surfaces on both sides for interchangeability and redundancy. Such seals typically have elastomer-on-elastomer sealing surfaces. To reduce sticking, one could use release agents such as powders and lubricants, but these can be easily removed and transferred to other surfaces, causing uneven sealing and contamination. Modification of the elastomer surface to make a more slippery and less sticky surface that is integral with the bulk elastomer would be more desirable.The analysis of materials retrieved from early space shuttle missions such as the Long Duration Exposure Facility indicated that silicone surfaces were converted to SiO2 silica glass as a result of the low Earth orbital atomic oxygen exposure. With a controlled atomic oxygen exposure, the converted silica surface is integral with the underlying silicone and is sufficiently thin enough that the elastomers retain their flexibility and sealing properties, yet the silicone does not stick to surfaces because the surface is essentially a thin film of SiO2 glass.
Silicone seals are mounted on the surface of electrically grounded aluminum exposure plates or suspended such that the interfacing silicone seal surfaces are exposed to a low-pressure atmospheric plasma. The seal surfaces are then exposed to atomic oxygen, until the desired reduction in seal adhesion is achieved. Polyimide Kapton H atomic oxygen fluence witness samples are also placed into the vacuum chamber during air plasma exposure to measure the Kapton effective fluence by weight loss of vacuum dehydrated Kapton H samples.
Functional operation of the seals is such that the atomic oxygen treated surface will then interface during docking with another atomic oxygen treated surface or a metal surface. Thus, the mating is between two opposing silica surfaces on silicone or silica surfaces on smooth metal surfaces.
The atomic oxygen treated silicone seal geometry can be varied as well as the composition of the silicone. Parts of the seal can be treated rather than all of the seal if it is desirable to have adhesion in a specific direction or at a specific interface.
This work was done by Bruce A. Banks and Sharon K. Miller of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Manufacturing & Prototyping category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18572-1.
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