| Abstract: |
In the Plasma Spray-Physical Vapor Deposition (PS-PVD) process, major fractions of the powder feedstock can be evaporated so that the coating builds up mainly from the vapor phase. In this work, a substrate plate setup was designed to investigate the deposition mechanisms in dependence on deposition conditions in PS-PVD. From the leading edge to the end of the plate, the columns in the coatings possess successively pyramidal, cauliflower, and laminar shaped tops. In addition, the different microstructures show characteristic crystallographic textures. In particular, the randomly orientated cauliflower structure indicates that the deposits in PS-PVD are mainly nano sized clusters rather than atoms or molecules in contrast to electron beam-physical vapor deposition (EB-PVD).
The evolution of the morphology and texture of the coatings can be understood based on the assumption of a boundary layer (B-L) formed on the substrate surface. Within the B-L, the steep gradients of the flow velocities and temperatures promote supersaturaion, leading to nucleation and growth of nano-sized clusters. Furthermore, as the distance from the leading edge gets larger, the B-L provides considerably different deposition conditions, i.e. increasing boundary layer thickness while the substrate temperatures decrease. Under such assumption, the different growth patterns of the columnar coatings in PS-PVD can be explained by the different B L conditions.
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