Towards a Complete Model of DC Plasma Spray Coating Process

提高工业系统的性能取决于其部件承受苛刻工艺条件的能力。例如，为了提高燃气轮机的效率，需要更高的发动机温度。目前，发动机的最高工作温度受材料特性的限制，而不受燃烧过程的限制。为了提高一些发动机部件的性能，它们被涂上了一层高熔点的绝缘材料，以保护它们免受更高的发动机温度的影响。沉积防护涂层最常用的技术之一是热喷涂。这是一项使能技术，其应用范围很广，包括航空航天、汽车、化工加工、医疗植入物、发电、纸浆和造纸等。 最通用的热喷涂技术之一是直流等离子喷涂。该技术广泛用于热障涂层的沉积，以及耐腐蚀和耐磨涂层的沉积。在这个过程中，粉末材料被注入到从等离子体炬发出的高温等离子体射流中。然后，粉末被加速、加热和熔化，然后沉积在一个组件上。涂层的质量取决于撞击颗粒的状态，这是等离子射流中粉末的热历史和轨迹的函数。重要的是，等离子射流的状态是由火炬内部的电弧波动动态决定的。因此，导致涂层沉积的过程具有三个互补区域： ·区域1：等离子体炬，通过阴极和阳极之间的波动电弧将气体加热到高温。这些波动的幅度取决于等离子体气体的类型以及电源设计。 ·区域2：等离子喷射区，粉末在这里被注入，并被加热、熔化并向基材加速。 ·区域3：沉积粉末的衬底区域。碰撞颗粒的状态是决定沉积物微观结构的最关键因素。 在前面建模工作的基础上，我们将建立一个包括等离子喷涂过程所有三个区域的三维、时变的综合模型。该模型将能够预测涂层的微观结构作为等离子炬操作条件的函数。这些因素包括等离子体气体的类型、流量、电弧电流、粉末材料和进给速度、粉末粒度分布、载气流量和衬底条件，例如粗糙度、材料和温度。 该模型将尝试回答一些重要的问题，例如： A)弧光波动对碰撞粒子的状态有何影响？ B)等离子体气体对电弧波动幅度有什么影响，对粒子加热有何影响？ C)孔洞是如何形成的，这些孔洞的大小分布是什么？

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?