Critical particle velocity under cold spray conditions

Cold spray process is an emerging technique that produces high density coatings. Particles (1 to 50 μm in diameter) are carried by a supersonic gas stream through a de Laval nozzle and, finally, impact on a substrate with high kinetic energy. Low gas temperatures (< 600 °C) make it possible to maintain sprayed material in solid state during the whole process. If the particles reach a given velocity, called “critical velocity”, they can bind to the surface and create a coating. This velocity is clearly dependent on both sprayed material and substrate properties. This work presents an imaging technique that allows a fast measurement of critical velocity. The measuring method is first evaluated by comparing the critical velocity of copper (sprayed on copper substrate) found in the literature, with the measured one. Its accuracy is then tested with other materials and, finally, some improvements of the method are proposed.