Synthesis of RF Plasma-sprayed Coatings Based on the Controls of Droplet Flattening, Solidification and Reaction Step

点击翻译按钮获取中文摘要

This research has been performed to understand the fundamental phenomenon in the synthesis of metal and intermetallic matrix composite coatings by reactive RF plasma spraying with premixed elemental powder, which is based on the droplet flattening, solidification and reaction with gases.Typical splat morphology of impinged titanium droplets on a substrate is a disk-type with an outer peripheral fringe. If the supplied titanium powder size becomes finer or the nitrogen partial pressure, splats containing prominent asperities (TiN) with a smaller flattening ratio appear along with the plain disk-type. Aluminum splats are also classified into two categories : a disk-type with an irregular outer periphery and a semi-nodular-type. Oxygen exists on the splat surfaces, on which there are nitrogen concentrated areas corresponding to AIN. Consequently, the nitride formation proceeds on titanium and aluminum droplets during the flight as well as on the substrate. If the substrate temperature in … More higher than 873 K just before spraying with premixed titanium and aluminum powder, the formation of TiAl and TiィイD22ィエD2SIN proceeds on the substrate.In situ composite coatings containing titanium carbide (TiC) precipitates were synthesized by reactive RF plasma spraying with elemental titanium powder, in which the effect of methane concentration premixed into the plasma gas on the TiC formation was examined through the process. The absorption of decomposed carbon into molten titanium droplets occurs during their flight in the plasma flame, so that fine TiC precipitates which increase in volume with mixed methane are recognized in the sprayed titanium particles. The flattering ratio of splats impinged on a substrate decreases with increasing premixed methane. The amount of TiC precipitates in titanium matrix coatings is approximately five times higher than that in sprayed particles during their flight regardless of premixed methane. Hence, the TiC formation is dominated at the splat depositing step, because the time exposed by the methane mixed plasma flame is much longer than the flight time of droplets. The maximum volume fraction of TiC precipitates is approximately 30 vol% in the coatings. Less