Cold welding’was first diagnosed as the cause of some spacecraft mechanism
failures in the late 1980s and early 1990s. It was clear that laboratory testing
was needed in order to assess the effects of different surfaces making ‘static
contact’ under vacuum.
This was done by constructing two dedicated sets of equipment – an ‘impact
facility’ and a ‘fretting facility’, both developed at AIT – which have been
used to investigate several combinations of bulk materials and coatings for
their tendency to ‘cold welding’. The test philosophy is based on repeated
closing and opening of a pin-to-disc contact. In an impact test, in each cycle,
the contact is closed by an impact with a defined energy (no fretting applied).
During a fretting test, the contact is closed softly (without impact), and while
closed, fretting is applied to the contact. For both tests, the adhesion force, i.e.
the force required to re-open the contact, is measured at each opening. Basic
studies [4] were carried out to show the influence of the main parameters,
the impact energy and the static load (contact pressure). These first results
have been used to set up a standard test method with fixed parameters [5]. An
overview of the test combinations is given in the following sections.