The Dynamics of Deposit Formation in Thermal-Spray Processes
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Deposit Formation in Thermal-Spray Processes
Armelle Vardelle, Christian Moreau, and Pierre Fauchais Introduction In thermal spraying, coatings are formed by particles flattening and piling up on the substrate. At impact, the sudden deceleration of the particle causes a pressure buildup at the particle-surface interface; the high pressure inside the particle forces melted material to flow laterally and ductile material to deform. The particle spreads outward from the point of impact and forms a “splat.” The arresting of spreading results from the conversion of particle kinetic energy into work of viscous deformation and surface energy. Solidification constraint (when the solidification front is advancing from the substrate surface fast enough to interact with the liquid during spreading) and mechanical constraint (due to the roughness of the substrate surface) can interfere with the flattening process.1 The spreading kinetics of the impacting droplets govern the splat shape and thickness, the ability of the sprayed material to fill existing voids, and the quality of contact between the splat and the underlying layer, thereby controlling the local cooling rate and overall heat transfer. The efficient heat extraction by the substrate results in a high cooling rate (106 –108 K/s) through both liquid and solid phases, leading in turn to undercooling and rapid solidification.2,3 The process of splat formation depends on the velocity, size, molten state, chemistry, and angle of impact of the droplets onto the surface. It is also subject to the surface topography of the substrate, its temperature, and reactivity. In addition, as the thermal contraction of splats is constrained by the underlying solid, tensile stress is developed during deposition. This
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quenching stress can be relieved by various mechanisms such as microcracking, plastic yielding, creep, and so on.4,5 As a result, thermal-sprayed coatings have a characteristic microstructure of lamellae, pores, and cracks. Metal coatings may also include oxides formed during particle flight to the substrate or after impact. In thermal sprays, coatings are generally deposited in a layer-by-layer manner. Therefore, the deposition process presents two characteristic stages. The first stage is the formation of a single splat, and the second stage is the building-up of a layer resulting from the movement of the plasma gun over the coated part. Both stages exhibit time constants: (a) the time required for a particle to flatten and solidify on the substrate (or for a layer to be formed), and (b) the time interval between two successive impacts or two passes of the gun over the same location. The former time interval depends mainly on the relative gunto-substrate motion and feed rate of the feedstock material, while the latter is essentially controlled by the gun velocity. The time required for a particle to flatten and solidify is sufficiently short so that each impact can be considered to be inde-
pendent of the others. The values of these various time constants for dc pl
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