Impacting and Wetting Dynamics of Single Polysilazane Droplet on Inconel Surface

Abstract

Silicon-based polymers are promising materials for making polymer-derived ceramic (PDC) coatings for a variety of applications. One way to obtain thermally stable and chemically resistant PDCs is to, firstly, spray liquid polymers onto the target surface and then, cure these precursors through thermal, radiative, or chemical approach. Yet, the interactions during the spray coating process between those polymer droplets and target surfaces are not well characterized throughout current literature. This study presents a detailed characterization of the single droplet impacting and spreading dynamics for a specific type of ceramic precursor, Polysilazane, on the Inconel surface, which has wide applications in seawater and aerospace fields. In the experiment, the Polysilazane droplet diameter was fixed around 1.6mm while its impact velocity was varied between 0.2m/s and 4.2m/s. The same parameter space was repeated with water droplets for comparison. High-speed (4,400 fps), short (<2s) videos and low-speed (30fps), long (>300s) videos were taken simultaneously to cover both the instantaneous impacting process and the whole slow spreading process. It is revealed that Polysilazane droplets have significantly different spreading characteristics from water droplets on Inconel surface. Water droplets show strong retraction after impacting on Inconel surface. However, for Polysilazane droplets, no retraction occurs. Instead, a slower wetting process continues after the "plateau" relaxation stage. Analysis of the droplet maximum spread factor shows that the variation of both liquids can be well correlated by an existing universal rescaling method.