Cold welding in hold down points of space mechanisms due to impact and fretting

Abstract

This paper presents an overview of a special vacuum or space related effect, which is referred to as ‘cold welding’, ‘adhesion’, ‘sticking’ or ‘stiction’. These terms refer to ‘welding or sticking’ of contacting bodies, which only open vertically (no sliding), i.e. according to European Cooperation for Space Standardization ‘separable contact surfaces’. Initiated by a reported failure on a spacecraft, test facilities were developed and a verification study was performed to show the relevance between the test results and the reported failure. In the following, a standardized test method was set-up, in order to compare technological levels of different space-relevant materials and coatings for their tendency to cold welding. Since then several studies were performed. Within them, common experience and numerical results were gained on space relevant combinations of materials and coatings with surface states typical for space engineering. Adhesion forces under fretting were found to exceed the loading forces. As an example, nickel containing stainless steels should be avoided. Based on this knowledge, recommendations for use of certain combinations will be drawn. An assessment of coatings for steels and Aluminium (e.g. hard anodizing, Keronite) is given.

Cold Welding in Hold Down Points of Space Mechanisms Due to Fretting When Omitting Grease

“…Together with ESA, a test philosophy was also agreed to compare different materials (as their mechanical properties differ strongly, between "soft" aluminium and "hard" steel [2]). Experimental data gained with this setup confirmed the previous assumption that fretting causes much higher adhesion forces up to a range of 10 N [3]. The adhesion force of several material combinations in high vacuum was measured and is available via [3,5].…”

“…These first results have been used to set up a standard test method together with ESA to fix parameters for establishing a kind of a data-base to compare typical material combinations used for spacecrafts [2]. In the following years, tests were conducted and results were published [3]. For ESA-member states, data can also be obtained [5].…”

“…Further, also for gears stainless steels are required from point of view on corrosion and stress corrosion cracking. Unfortunately, the austenitic steels show higher tendencies to cold welding than, e.g., martensitic steels [2,3].…”

Friction and solid-solid adhesion on complex metallic alloys

“…More advanced pieces of equipment, housed in vacuum chambers (figure 2), were used to assess the friction and fretting behaviour, respectively, when specimens are placed in aerospace conditions. A full account of the technical specifications of this equipment can be found in [11]. Preparation of the surface for tribology measurements was performed using carborundum abrasive paper down to 4000 grit under water, followed by cleaning in an ultrasound bath of pure ethanol.…”

“…These pieces of equipment were used in the present study in collaboration with Austrian Research Centres, Seibersdorf, Austria. See [11] for a detailed description of the technical specifications of these instruments.…”