Process control of reactive thermal spraying based on flying droplets diagnosis

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As a fundamental depositing step of reactive thermal spraying, besides the transition of thermal and kinetic energy to flying droplets, the droplets impact upon a substrate, the in-situ reaction is added. In the conventional thermal spraying accompanying without in-situ reaction, the short processing period of order of μs consisting of droplets collision on a substrate, droplets flattening and their solidification control the whole depositing process. However, the expected in-situ reactions play an important role upon the coating characteristics through both droplets flying and depositing stages.Temperature and velocity of flying droplets, in which the in-situ reactions proceed, were measured during the spray process. The obtained data were directly feedbacked to the spray parameters such as plasma current and plasma gas flow to control or monitor the in-situ reactions, that is, the required coating characteristics. The following are conclusions in the present study :(1) The measurement accuracy of droplets temperature and velocity is governed by the emissivity of a given droplet.(2) It is possible to feedback-control droplets temperature and velocity based on a certain spray parameter.(3) In the study regarding the microstructure control of composite coatings through changing the spray parameters, graphite carbon content in cast iron coatings is mainly controlled by the droplets velocity, that is, staying period in the plasma flame.