Cold-gas dynamic-spray process (or cold spray)

Since the early 2000s, dynamic cold gas spraying - more commonly known as "cold spray", much to the (justified) chagrin of French purists - has established itself as the leading new process for producing powder coatings. With it, the material is given a facelift to improve its surface properties. Its principle is based on keeping the particles entirely in a solid state while they are being sprayed; their densification, on impact with the substrate, resulting from their very high speed (potentially largely supersonic). The quality of densification depends on the ability of the particles to plastically deform to ensure that they are properly stacked and bonded together. As a result, the cold spray process is mainly used for metals and, to a lesser extent, polymers. Ceramics are out of the question, except in the rare case of very specific process conditions and powder selection. These are still little-known, but they take into account the fact that it is fundamentally different mechanisms from plastic deformation that lead to the formation of a deposit. When ceramics are used in cold spray processes, they are usually mixed with a binder (metal or polymer) to create a composite deposit.

The resultant coatings are distinguished by their "cleanliness" (absence of oxidation in particular) and high density, without altering the main metallurgical characteristics of the initial powder. What's more, compared with other processes such as plasma spraying, investment costs are often low, while material yields are high (usually in excess of 80%). This easily justifies the popularity of cold spray. This is all the more the case as its latest developments also place it in the current concert of additive manufacturing processes for the production of parts. This means it can venture far beyond the field of coatings. The industrial sectors interested in cold spray are numerous and overlap with those of conventional thermal spraying: aerospace, automotive, energy, luxury goods... via its major groups as well as its network of SMEs. Transversally, among the most promising applications for cold spray are those linked to repair.

The aim of this article is to provide an overview of the cold spray process, covering issues relating both to the various process technologies and to the materials used for deposition. The main basic deposition mechanisms are discussed, but always in the light of the targeted properties and applications.

At the end of the article, readers will find a glossary and a table of acronyms and notations.