Correlation of Particle Impact Conditions and Coating Properties in Cold Spraying

Abstract

Abstract In cold spraying, the high strain rate plastic deformation during particle impact leads to a local temperature rise at the particle/substrate interface. This gives rise to thermal softening and thus further strain and heat generation, finally resulting in adiabatic shear instabilities, which are necessary to supply sufficient heat for successful bonding of the particles. These adiabatic shear instabilities can only occur, if a critical impact velocity is exceeded. A further increase of the impact velocity beyond this critical velocity continuously increases the fraction of well-bonded interfaces up to 95%, thus improving mechanical performance of the coatings. However, at far too high impact velocities, the efficiency again decreases and then changes to erosion due to hydrodynamic penetration. This erosion velocity is approximately two to three times higher than the critical velocity. The optimum velocity range between critical and erosion velocity is defined as “window of deposition”. Both critical and erosion velocity depend on the spray material properties, but also on particle impact temperature and particle size. Furthermore, they are also influenced by the powder purity. This study demonstrates the previously mentioned effects by calculations and experimental investigations. The presented link between fluid dynamics and impact dynamics enables to predict optimum spray parameters as well as the process effectiveness and resulting coating properties for certain cold spray conditions. Following this strategy, it was possible to increase the ultimate cohesive strength of cold-sprayed copper coatings from 80 MPa to more than 400 MPa, using nitrogen as process gas. In the annealed state, the ductility of these coatings corresponds to annealed bulk material. The overall optimization strategy is applicable to a wide variety of other spray materials. These developments should boost several new cold spray applications.