Keywords: direct bonding, mechanical strength, bonding energy, double shear test, annealing temperature, roughness, humidity, annealing time, silica glasses, Zerodur® glasses, interface model..

Direct bonding consists of joining two surfaces without the use of any adhesives or additional material. Usually, by bringing two flats, well-polished surfaces into contact at room temperature, they are locally attracted to each other by Van der Waals or hydrogen bonds and adhere or bond. This technology is already used in optical system manufacturing as a result of the very high precision of the process moreover complex geometries are able to bond. More recently, this process was used in the manufacturing of high performance optical systems for terrestrial application such as Fabry-Perot interferometers, prism assemblies, etc. For instance, this bonding process has been used in the manufacturing of the largest slicer ever used in the Multi Unit Spectroscopic Explorer for the Very Large Telescope. Nowadays direct bonding is of particular interest for optical system manufacturing for spatial application. However, even if a first spatial prototype already passed with success space environment test, quantification and improvement of the mechanical strength of assemblies are essential to validate the assembly's life expectancy and to validate the European Space Agency standards. Thus, paper describes the influence of some process parameters, such as roughness, relative air humidity during room temperature bonding, annealing time and temperature, on the mechanical strength of an elementary mechanical structure using a double shear test procedure and cleavage tests. A confrontation is also proposed between the performances of silica and Zerodur® glasses. For the Winlight Optics process considered in this paper, a parallel is presented between chemical phenomena, surface roughness and mechanical strength. Then the choice of the optimal process parameters is confirmed with cleavage tests and they highlight a damaging phenomenon of bonded interfaces with successive re-adhesion. In the same time, an interface mechanical model of the direct bonding is developed. The implemented law relates the bonding energy, the mechanical critical strain energy, the process parameters and the kinetic of chemical reactions with a multi-physics and multi-scale formalism. An usual wedge test is also developed to measure the bonding energy versus process parameters in order to identify the law of the direct bonding model. Then the direct bonding model is implemented in a finite elements code. Finally a comparison between numerical and experimental results is described.

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