Process control of reactive thermal spraying based on flying droplets diagnosis

作为反应热喷涂的基本沉积步骤，除了将热能和动能转化为飞行液滴，液滴撞击基底之外，还增加了原位反应。在无原位反应的常规热喷涂中，液滴与基体的碰撞、液滴的压扁和凝固等几个μs量级的短过程控制着整个沉积过程。然而，预期的原位反应在喷涂过程中通过液滴飞行和沉积阶段对涂层特性起着重要作用，测量了喷涂过程中原位反应进行的液滴飞行温度和速度。将获得的数据直接反馈到喷涂参数，如等离子体电流和等离子体气体流量，以控制或监测原位反应，即所需的涂层特性。研究结果表明：（1）液滴的发射率决定了液滴温度和速度的测量精度。(2)可以根据一定的喷雾参数对液滴温度和速度进行反馈控制。(3)在通过改变喷涂工艺参数来控制复合涂层组织的研究中，铸铁涂层中石墨碳含量主要受液滴速度即在等离子火焰中的停留时间的控制。

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.