NiTi Thermal Spray Splats Morphology On Different Substrates

The present work deals with the study of the splashing behavior of NiTi particles depending on the substrate material with the aim to understand and improve the adhesion of thermally sprayed NiTi. By changing the substrate, the solidification rate of the first layer is mainly affected; therefore coating adherence and phase transitions at the interface can be modified, as adhesive strength to the substrate is especially dependent on splat behavior of individual particles.

 Vacuum Plasma Spraying has been used to produce coatings onto steel substrates. This work deals with the study of splat morphology of the NiTi alloy sprayed by VPS onto different substrates (aluminum, copper, stainless steel, glass and alumina). Previous characteristics are discussed in terms of wettability and thermal conductivities regarding the rapid cooling involved in the process. Although identical conditions were used during thermal spraying, a wide variety of splat formations were observed; commonly, slushy or splash/disc splats are formed. Plasma spray reaches temperatures higher than 3800ºC leading to molten droplets that result in “splash” or “disc” splats depending on whether the splat solidifies before (T < T_m) or after it has come to rest (T > T_m), defining a “transition” substrate temperature T_m [12]. The splat morphology and therefore the coating microstructure will finally determine the coating properties. The thermal effusivity of the substrate material, which is a measure of its ability to exchange thermal energy with its surroundings, plays an important role promoting more or less spreading. The higher the thermal effusivity is, the more rapidly the splats are cooled, thus starting the solidification before they come to rest and, changing their morphology. Then, the thermal effusivity of the substrate determines the transition temperature if the projection conditions are kept constant, enabling an easy interpretation of the splat behavior.