Splat Formation Mechanism in Thermal Spraying

Abstract

In order to understand the splat formation process of individual splat deposited by thermal spraying, commercially available Nickel powders with diameter of several tens micrometers were thermally sprayed onto mirror polished AISI304 substrate surface. The deposited splat shows transition phenomenon from splash type to disk one in flattening on collision onto substrate surface according to both substrate temperature and ambient pressure change. That is, transition temperature, Tt and transition pressure, Pt can be defined as critical values for the transitions. The three-dimensional map was set up to control the thermal spraying process. The observation on the bottom surface morphology on single splat indicates that the initial solidification may induce the splashing occurrence. Moreover, as the simulation of the real thermal spraying process, free falling experiment was conducted in this study. The thermal history of the free falling Cu droplet onto AISI304 substrate was investigated, indicating the flattening pattern is decided just after collision onto solid surface, which is enough earlier to finalization of the flattening.

Recent Developments in the Research of Splat Formation Process in Thermal Spraying

“…However, only few attentions have been paid to the splat formation process under reduced pressure conditions. Despite this, existence of a similar transition was indicated recently as indicated in Figure 4 [48,49], that is, the transition from splash splat to disk-shaped one occurred by reducing the ambient pressure [18,20,21,[48][49][50][51]. Fukumoto et al systematically investigated the effect of desorption of adsorbates on particle �attening behavior in low pressure plasma spraying, while the substrate temperature being kept at room temperature [18,19].…”

“…However, it is not fully understood yet. Similarly, the coating adhesion strength has a close relation with the shapes of the depositions through changing ambient pressure [21]. To summarize, splat shapes changed transitionally from splash type to disk-shaped one as increasing substrate temperature or reducing ambient pressure.…”

“…Ambient pressure (kPa) Fraction of disk-shaped splat (%)Shear adhesion strength (MPa) Dependence both of fraction of disk-shaped splat and coating adhesion strength on ambient pressure[20,21].…”

Failure Mechanism of 7YSZ Splat as Thermal Barrier Coating

“…During the spreading of splat, the heat transfer from molten droplet to bond coating becomes faster. Thus, the viscosity inside the droplet increased significantly in such a short period, this layer could restrain the movement of the boundary because a driving energy for the flattening transfers to viscous energy effectively, and promote the formation of disk-shaped splat [15,16]. Besides, a common feature evidence from the image is the uniform cracks on the splat.…”