Demonstration and challenges for joining technology based on direct bonding usable for construction of (large) structures in space

Abstract

A direct bonding process relying on van der Waals forces offers an ideal combination of easiness to assemble and material compatibility. Such a bonding procedure, also denoted as optical bonding, is already known and used frequently in different applications. However, there are strict requirements to achieve optical bonding: (1) a high level of cleanliness of the surfaces and (2) low roughness (RMS roughness <2 nm and preferably <0.5 nm). The first condition will be possible to realize in space, since the vacuum prevents the absorption of a thin water layer present on all surfaces under atmospheric conditions and forces a desorption of all residual water. Also, particle contamination will be minimized, due to the absence of strong capillary forces which attract particles if present and the absence of particles in space, providing a suitable and without residue removable protection prior to bonding. The second condition is now state of the art of silicon carbide (SiC) polishing and can hence be realized. While meeting the requirements, theoretically very large adhesion forces can be realized in vacuum. Surfaces have been polished according to the requirements. The forces measured on these surfaces are nearly as high as theoretically predicted and demonstrate the proof of principle of direct bonding of SiC under ambient conditions and in vacuum. However, the realization of the required flatness over large contact areas is still a challenge. Furthermore, since the surfaces display a really low roughness, extremely clean handling and bonding conditions need to be realized to avoid the spontaneous adhesion of small particles which would as such prevent direct bonding of larger areas.