Dynamic evolution of oxide scale on the surfaces of feed stock particles from cracking and segmenting to peel-off while cold spraying copper powder having a high oxygen content

Abstract

The oxide scale present on the feedstock particles is critical for inter-particle bond formation in the cold spray (CS) coating process, therefore, oxide scale break-up is a prerequisite for clean metallic contact which greatly improves the quality of inter-particle bonding within the deposited coating. In general, a spray powder which contains a thicker oxide scale on its surface (i.e., powders having high oxygen content) requires a higher critical particle velocity for coating formation, which also lowers the deposition efficiency (DE) making the whole process a challenging task. In this work, it is reported for the first time that an artificially oxidized copper (Cu) powder containing a high oxygen content of 0.81 wt.% with a thick surface oxide scale of 0.71 μm., can help achieve an astonishing increment in DE. A transition of surficial oxide scale evolution starting with crack initiations followed by segmenting to peeling-off was observed during the high velocity particle impact of the particles, which helps in achieving an astounding increment in DE. Single-particle deposit observations revealed that the thick oxide scale peels off from most of the sprayed powder surfaces during the high-velocity impact, which leaves a clean metallic surface on the deposited particle. This makes the successive particles to bond easily and thus leads to a higher DE. Further, owning to the peeling-off of the oxide scale from the feedstock particles, very few discontinuous oxide scale segments are retained at inter-particle boundaries ensuring a high electrical conductivity within the resulting deposit. Dependency of the oxide scale threshold thickness for peeling-off during the high velocity particle impact was also investigated.