At the Institute of Aerodynamics and Flow Technology of the German Aerospace Center a pilot facility for cold spraying has been set up. The facility enables the study of a wide variety of different process parameters such as nozzle geometries, reservoir pressures and temperatures. Characteristic properties of the particles and fluid flow can be determined experimentally. The facility is particularly well suited for testing different applications of the newly developed cold spray method. Sample coatings can easily be produced.
In cold spray processes the deposit particles are accelerated and transported to the substrate by means of a supersonic free jet (see the sketch of process below). On impact, the deposit particles stick to the substrate and form a coating. Hence, materials to be deposited are not melted throughout the whole process. The low thermal energy of spray particles is compensated for by a high kinetic energy. Thermal loads are avoided, thus permitting a wide variety of different materials for being deposited. The process itself is extremely simple and allows for applying coatings to locally well confined geometries.
In cold sprays the distance between the nozzle exit and the substrate is quite small (of the order of a few diameters of the nozzle exit). This results in a fluid mechanical behavior that is fundamentally different from that observed with classical thermal sprays. Low temperatures and a well defined jet geometry (little entrainment of ambient air) allow for the application of coatings to small well-defined areas without masking.
Cold spraying allows for coating metallic and non-metallic substrates by metals or polymers. Materials to be deposited are neither exposed to high temperatures nor oxidized throughout the whole process. Furthermore, the carrier gas can be chosen to be an inert gas thus suppressing chemical reactions between carrier gas and particles. This permits a wide variety of different materials to be deposited. Thermal loads applied to substrates remain small.
These features render cold spraying quite different from thermal spraying. In cold sprays the deposition of coatings occurs via high-velocity impacts of solid microscopic particles, i.e., due to a high kinetic energy of the particles. The particles are not melted. Solidification processes and thus the build up of thermal stresses are absent.
Diameter of particles:
10 - 100 µm
Temperature of particles:
less than half the melting temperature
pressurized air / nitrogen