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.