Engineering the microstructure of solution precursor plasma sprayed coatings

International audienceThis study examines the fundamental reactions that occur in-flight during the solution precursor plasma spraying (SPPS) of solutions containing Zr- and Y-based salts in water or ethanol solvent. The effect of plasma jet composition (pure Ar, Ar-H2 and Ar-He-H2 mixtures) on the mechanical break-up and thermal treatment of the solution, mechanically injected in the form of a liquid stream, was investigated. Observation of the size evolution of the solution droplets in the plasma flow by means of a laser shadowgraphy technique, showed that droplet break-up was more effective and solvent evaporation was faster when the ethanol-based solution was injected into binary or ternary plasma gas mixtures. In contrast with water-based solutions, residual liquid droplets were always detected at the substrate location. The morphology and structure of the material deposited onto stainless steel substrates during single-scan experiments were characterised by SEM, XRD and micro-Raman spectroscopy and were shown to be closely related to in-flight droplet behaviour. In-flight pyrolysis and melting of the precursor led to well-flattened splats, whereas residual liquid droplets at the substrate location turned into non pyrolysed inclusions. The latter, although subsequently pyrolysed by the plasma heat during the deposition of entire coatings, resulted in porous ''sponge-like'' structures in the deposit

Leidenfrost on a ratchet

International audienceAs discovered by Leidenfrost, liquids placed on very hot solids levitate on a cushion of their own vapour. These model hovercrafts are remarkably mobile: placed on a hot ratchet, a droplet not only levitates, but also self-propels, in a well-defined direction, at a well-defined velocity (typically, 10 cm s -1). The challenge is to understand the origin of the phenomenon, which contrasts with other situations where an asymmetry in the solid/liquid contact was used to generate liquid self-propulsion. We consider Leidenfrost solids that directly sublimate on hot substrates, and show that they also self-propel on ratchets. This leads to a scenario for the motion: the vapour flow escaping below the Leidenfrost body gets rectified by the presence of asymmetric textures, so that a directional thrust drives the levitating material. Using fishing lines to catch drops, we measure the force acting on them, and discuss both the driving force and the special friction generated by the textures