Influence of Porosity on Ultra-High Vacuum Gas-Tightness in Cold-Sprayed Aluminum Coatings

Vacuum chambers used in high-energy particle accelerator experiments are conventionally made of bulk beryllium, which shows significant drawbacks due to cost and toxicity. An alternative solution could be to develop chambers made of polymer-based composites. Since these materials exhibit high outgassing not compatible with an ultra-high vacuum environment, a suitable gas-tight coating is required. Cold spray deposition of aluminum can be a solution, provided that the coating behaves as a perfect vacuum barrier. Porosity, especially percolating porous networks, is key to coating gas tightness issues. This work addresses the relationship between porosity and gas-tightness in cold spray coatings. To do so, coatings with different porosity were achieved playing with powder morphology, composition, and process parameters. Their gas tightness was evaluated by helium leak tests. Classical microscopy, being essentially a 2D analysis, is strongly limited when dealing with 3D properties as porosity percolation. For this reason, 3D X-ray microtomography images of coatings were obtained and treated by image analysis methods: pores were compared in terms of size and shape. Overall porosity properties, including percolation and a homogeneity criterion, were also investigated. Percolating porosity was highlighted for several samples which showed poor gas-tightness properties. The permeability of percolating pore structures was then numerically computed by a fast Fourier transform-based method, to quantify the mass flow through the coating. Results of those computations were finally compared to experimental coating leak rate measurements, in an effort to elucidate the link between gas tightness and morphology of the pore space.