During the deposition of metallic cold sprayed coatings, it could be observed that only a thin layer is formed on the substrate and further building-up of a thick coating is not enabled. As for other thermal spray techniques, the formation of cold sprayed coatings can be divided to two stages: the creation of the first layer onto the substrate and the building-up of the coating itself onto as-sprayed layers. This two-stage build-up process was evidenced according the study of two Ti-6Al-4V powders exhibiting different characteristics (particle size, morphology, oxygen content, hardness, etc) which were sprayed by cold gas dynamic spraying onto substrates of different nature with various hardnesses (Ti-6Al-4V, AISI 304L, Al-alloy 2017). The phenomenology of the two-stage process is investigated in the present study. Cold spray conditions with pure nitrogen or pure helium as processes gas were applied to achieve a significant difference for particle velocities. The first stage of the process was completed by both powders with the formation of a first coating layer onto the various substrates. However, very different features for particle-substrate interactions (penetration depth and comparative deformation) were observed. For the particle-particle interaction (the second stage of the process), despite similar spraying conditions for both powders, the results were completely different since the formation of thick coating was achieved only with one of the powders. It was found that the intrinsic ductility of the material powder is the main parameter to promote the successful completion of both stages in order to achieve thick coatings.

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