Towards a Complete Model of DC Plasma Spray Coating Process

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Improving the performance of industrial systems depends on the ability of its components to withstand demanding process conditions. For example, to improve the efficiency of a gas turbine, higher engine temperature is required. The maximum operating engine temperature is currently limited by the material properties and not by the combustion process. To enhance the performance of some engine components they are coated with a layer of high melting point, insulating materials that protects them from higher engine temperature. One of the most commonly used techniques to deposit protective coatings is thermal spray coating. This is an enabling technology and its applications are wide ranging and include aerospace, automotive, chemical processing, medical implants, power generation, pulp and paper, etc. One of the most versatile thermal spray techniques is the dc (direct current) plasma spray. The technology is widely used for the deposition of thermal barrier coatings, as well as corrosion and wear resistant coatings. In this process, powder materials are injected into a high temperature plasma jet issued from a plasma torch. The powders are then accelerated, heated and melted before deposited on a component. The quality of the coatings depends on the state of the impacting particles which is a function of the thermal history and trajectory of the powders within the plasma jet. Importantly, the state of the plasma jet is determined by the dynamics of arc fluctuations inside the torch. Thus, the process which leads to the deposition of the coatings has three complementary regions: • Region 1: Plasma torch where gases are heated to high temperatures by a fluctuating arc struck between a cathode and an anode. The amplitude of these fluctuations depends on the type of plasma gases as well as power supply design. • Region 2: Plasma jet region where powders are injected and are heated, melted and accelerated towards the substrate. • Region 3: Substrate region where powders are deposited. The state of the impacting particles is most crucial in determining the microstructure of the deposit. Based on our earlier modeling work, a 3-dimensional, time-dependent integrated model of plasma spray process which includes all three regions of this process will be developed. The model will be able to predict the microstructure of the coating as a function of plasma torch operating conditions. These include the type of plasma gases, flow rate, arc current, powder material and feed rate, powder size distribution, carrier gas flow rate and substrate conditions, e.g., roughness, material, and temperature. The model will try to answer some important questions such as: a) What is the effect of arc fluctuations on the state of impacting particles? b) What is the effect of plasma gas on the amplitude of arc fluctuations and how does it affect particle heating? c) How does porosity forms and what is the size distribution of these pores?