Transonic Flow Phenomena of the Cold Spray Deposition Process

Kurzfassung

The cold spray deposition process is an emerging technology for coating surfaces. The process is based on the impact on and the subsequent adherence to a substrate of micron-sized particles of high kinetic energy. In contrast to well-known thermal spray processes particles (metals, polymers) never become melted throughout the whole process, hence the prefix cold. Acceleration and transport of particles is achieved by means of supersonic nozzle flows and free jets, respectively. This results in a number of different transonic flow regimes encountered in the cold spray process. Phenomena characteristic of the various transonic regimes will be introduced and discussed making reference to results obtained at the cold spray facility of DLR. Cold spray deposition depends crucially on high impact velocities of particles. Therefore, acceleration of particles is a major issue. Particles are accelerated by means of a carrier gas in a converging-diverging nozzle. The diverging section is normally chosen to be very long resulting in extremely slender nozzles. In this manner the time available for acceleration is increased. On the other hand, boundary layers formed at the nozzle walls can no longer be neglected. This may result in a fully viscous flow field inside the nozzle. When the gas issues from the nozzle exit, turbulent mixing at the shear layer formed between the supersonic free jet and the ambient air causes a reduction in flow velocity. In order to avoid the negative effect on particle velocity the distance between nozzle exit and substrate is taken to be small in the cold spray process. Normally, it amounts to several exit diameters of the nozzle. Therefore, the interaction of the supersonic core flow of the jet with the substrate results in an inherently transonic flow field. Ahead of the substrate a bow shock is formed. Shocks that are normally present in the free jet impinge on the bow shock. Furthermore, under certain conditions the flow may separate periodically in the stagnation point region on the substrate. All these effects cause the particles to traverse an unsteady flow field before impact on the substrate. The relative velocity between particles and the gas flow can be supersonic, i.e., the particle Mach number, Mp, formed with the relative velocity between particles and gas flow, and with the local sound speed, is Mp = O(1). This typically happens both, near the location of particle injection into the mean flow and near the site of impact. In the former case the particle velocity is still small. In the latter case the gas velocity is abruptly decreased across the bow shock formed ahead of the substrate while the particle velocity is only little changed due to the great inertia of particles. Particles impinging on the substrate can be reflected from the surface, move upstream and interact with the bow shock. Similarly, particle-particle interactions can be present in the form of intersecting particle bow shocks. Finally, high-velocity impacts of particles on the substrate result in very high pressures within the two materials. This results in a plastic deformation of both, particles and substrate. The pressure increase and basic features of subsequent materials flow can be explained in terms of shock polars.