The Splat Formation Issue in Thermal Spray Processes

Abstract

Transition phenomenon in the flattening behavior of the thermally sprayed metallic particles has been recognized in our series of experimental works. Based on the results, a hypothesis has been proposed, namely, ultra-rapid cooled chill structure, preferentially formed at the bottom part of the splat, plays an essential role for the generation of the disk splat. Universality of this hypothesis beyond material difference was verified experimentally in our recent study, by using several kinds of ceramic materials with different thermal properties. To perform this, Al2O3, Y2O3 and YSZ powder materials were plasma sprayed onto AISI304 stainless steel substrate, and a fractional change of the disk splat with a substrate temperature increase was investigated, followed by a precise observation of the cross-section microstructure of the splats. The results obtained showed that unique amorphous and chill structures were observed in Al2O3 and Y2O3 splat at the bottom, respectively, indicating that similar disk formation mechanism in metallic material may act in these materials. On the other hand, only normal columnar structure was recognized in YSZ splat. It was indicated that a rapid increase in viscosity may act on this material. Consequently, our hypothesis was verified partially, beyond the materials difference. Keywords: thermal spray, splat, transition temperature, chill structure, viscosity. INTRODUCTIONObjective of our series of observations [1] on the flattening behavior of a thermally sprayed single splat is to establish the controllability or reliability of the thermal spray process. Since an individual particle is a fundamental element for the coating formation, the flattening behavior of thermally sprayed particles onto the flat substrate surface has been precisely investigated. The results showed that a transitional phenomenon was recognized in the flattening behavior both with the substrate temperature and ambient pressure changes. That is, the flattening of the splat changes transitionally from a distorted shape with splash to a disk-shaped splat without splash at certain critical temperature and pressure values, respectively. A transition temperature, Tt and transition pressure, P t were defined and introduced by the author [2], respectively, for those critical conditions. Correspondingly, it was confirmed that the coating adhesive property on the blasted substrate surface changes transitionally on those critical temperature and pressure ranges, respectively. Based on the results, three-dimensional transition curvatures by combining both transition curves was proposed for each sprayed material, as a controlling principle for the practical usage in the thermal spray industries [3]. Namely, information given in the single splat has a meaningful contribution onto the practical usage of the thermal spraying.On the other hand, based on the fluid dynamics, namely the splashing parameter or so-called Sommerfeld Number, K [4], restraint mechanism for the splashing have to be clarified from an academic vie...

Mathematical Modeling and Analysis Using Nondimensionalization Technique of the Solidification of a Splat of Variable Section

“…On the other hand, the properties mentioned above, which influence the cooling rate, change with temperature, so to make a more accurate model, it is necessary to include these variations. In addition, it is necessary to include the irregularity that the splat presents in the model [25][26][27][28][29][30][31][32][33].…”

Modelling of Alumina Splat Solidification on Preheated Steel Substrate Using the Network Simulation Method

“…Yang et al [34] have also evaluated the effect of substrate preheating on the splat grain structure. Fukumoto et al [35] investigated the fractional change of the disk splat with an increase in the substrate temperature as well as the microstructure of the cross section of the splats using Y 2 O 3, YSZ and Al 2 O 3 powder materials that were plasma sprayed on an AISI304 stainless steel substrate.…”