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Determination of Ground-Laboratory to In-Space Effective Atomic Oxygen Fluence for DC 93?500 Silicone

ABSTRACT

The objective of this research was to calibrate the ground-to-space effective atomic oxygen fluence for DC 93-500 silicone
in a thermal energy electron cyclotron resonance (ECR) oxygen plasma facility. Silicones, commonly used spacecraft
materials, do not chemically erode with atomic oxygen attack like other organic materials but form an oxidized hardened
silicate surface layer. Therefore, the effective atomic oxygen fluence in a ground test facility should not be determined based
on mass loss measurements, as they are with organic polymers. A technique has been developed at the Glenn Research Center
to determine the equivalent amount of atomic oxygen exposure in an ECR ground test facility to produce the same degree of
atomic oxygen damage as in space. The approach used was to compare changes in the surface hardness of ground test (ECR)
exposed DC 93-500 silicone with DC 93-500 exposed to low Earth orbit (LEO) atomic oxygen as part of a shuttle flight
experiment. The ground to in-space effective atomic oxygen fluence correlation was determined based on the fluence in the
ECR source that produced the same hardness for the fluence in-space. Nanomechanical hardness versus contact depth
measurements were obtained for five ECR exposed DC 93-500 samples (ECR exposed for 18 to 40 hrs, corresponding to
Kapton effective fluences of 4.2 x 10(exp 20) to 9.4 x 10(exp 20) atoms/sq cm, respectively) and for space exposed DC 93-500
from the Evaluation of Oxygen Interactions with Materials III (EOIM III) shuttle flight experiment, exposed to LEO atomic
oxygen for 2.3 x 10(exp 20) atoms/sq cm. Pristine controls were also evaluated. A ground-to-space correlation value was
determined based on correlation values for four contact depths (150, 200, 250, and 300 nm), which represent the near surface
depth data. The results indicate that the Kapton effective atomic oxygen fluence in the ECR facility needs to be 2.64 times
higher than in LEO to replicate equivalent exposure damage in the ground test silicone as occurred in the space exposed
silicone.

Surfaces at the front edge of the spacecraft in low Earth orbit (eg surfaces facing the direction of velocity), as in the International Space Station, are subject to atomic oxygen attack and certain materials are susceptible to erosion. Therefore, ground laboratory tests of the atomic oxygen durability of spacecraft materials are necessary for the assessment of durability when flight data is not available. For accurate space simulation, the facility is commonly calibrated on the basis of Kapton mass loss (DuPont, Wilmington, DE) as a control sample for the effective determination of creep. This is because Kapton has a well-characterized erosion performance of atomic oxygen (Ey, in cubic centimeters per atom) in the low Earth's orbit (LEO) environment. Silicones, a family of commonly used spacecraft materials, are not chemically eroded by the attack of atomic oxygen like other organic materials that have volatile oxidation products. Instead, the silicones react with atomic oxygen and form a surface layer of oxidized hardened silicate. Often, loss of methyl groups causes contraction of the skin surface and degradation by cracking of "mud bricks". But silicones often do not lose mass, and some silicones actually gain mass during atomic exposure to oxygen. Therefore, the effective atomic oxygen yield for the silicones in a ground test facility should not be determined on the basis of traditional mass loss measurements, as is the case with eroding polymers. Another method for determining effective yielding needs to be employed for silicones.

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