Elucidation of Elemental Steps of Thermal Spraying Accompanied by In-Process Reaction and Structure Control in Composite Coatings

众所周知，在热喷涂过程中，复合喷涂材料中的WC等热力学不稳定的硬质颗粒容易分解为W2C和钨。基于过程反应的反应热喷涂技术是近年来研究的热点，目的是制备以氮化物或碳化物为增强相的复合涂层。本研究分为两大类：(1)研究了铝合金表面等离子喷涂铸铁涂层的基本特征，包括(1)预热温度对涂层形貌的影响；(2)反应层和气孔的形成；(3)低压等离子喷涂涂层的微观组织。随着衬底温度的升高，溅射形貌由溅射状转变为圆盘状和星形。在较高的衬底温度下，变形的衬底隆起，主要是由于表面的轻微熔化，在衬底外围附近被识别出来。平坦化的…随着衬底温度的升高，更多的圆盘基板的g比降低，这是因为隆起是基板扩展的障碍。由铁、铝和氧组成的反应层准备在较高的基片温度下形成，这与变形的脊线一起提高了拼板的粘合强度。然而，在较低的衬底温度下，孔洞出现在衬底界面，阻碍了反应层的形成。反应超音速火焰喷涂具有火焰速度快、火焰温度低的特点，可用于原位复合涂层的形成。将SiO_2/Al-Mg颗粒喷涂到铝基上，制备了复合涂层。通过SiO_2的还原和放热反应，复合涂层有望由镁铝和Al-Si合金组成。然而，喷涂层实际上是由镁铝、铝硅和附加的镁硅组成。这是因为虽然镁铝氧化物的热力学稳定性比镁铝硅高，但镁铝硅的形成速度比镁铝酸镁快得多。随着喷涂距离的增加，虽然熔滴的飞行时间较长，但生成的镁铝氧化物的量减少。因此，形成MgAl_2O_4等过程中的反应主要发生在衬底上。较少

It is known that thermodynamically unstable hard particles such as WC in composite spray materials are ready to decompose to W_2C and tungsten during the thermal spray process. Reactive thermal spraying based on in-process reactions has been recently focused on the fabrication of composite coatings containing nitrides or carbides as a reinforcement. The present study is classified into two main categories:(1) Fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate, including (i) the effects of preheat substrate temperature on the splat morphology, (ii) the formation of a reaction layer and pores and (iii) the splat microstructure, were investigated in low pressure plasma spraying. With an increasing substrate temperature, the splat morphology changes from a splash-type to a disk and star-shape. Deformed substrate ridges, mainly due to the slight surface melting, are recognized adjacent to the splat periphery at high substrate temperatures. The flattenin … More g ratio of disk splats decreases with substrate temperature because the ridges act as an obstacle for splat expansion. A reaction layer composed of iron, aluminum and oxygen is ready to form at high substrate temperatures, which, along with the deformed ridges, improves the adhesive strength of splats. However, the pores appear at the splat interface at low substrate temperatures, which hinder the formation of a reaction layer. The amount of graphitized carbon increases in cast iron splats with an increase in substrate temperature.(2) reactive HVOF spraying characterized by high flame velocity and low flame temperature was applied to the formation of in situ composite coatings. Granulated particles of SiO_2/Al-Mg were sprayed onto an aluminum substrate to fabricate the composite coatings. Through the reduction of SiO_2 accompanied by an exothermic reaction, the composite coatings are expected to consist of MgAl_2O_4 and Al-Si alloy matrix. However, the sprayed coatings actually consist of MgAl_2O_4, Al-Si and additional Mg_2Si. This is because the formation rate of Mg_2Si is much faster compared with that of MgAl_2O_4, although MgAl_2O_4 is more thermodynamically stable than Mg_2Si. The amount of formed MgAl_2O_4 decreases with an increase in the spraying distance, although it gives a longer in-flight period of droplets. Hence, the in-process reaction such as MgAl_2O_4 formation mainly occurs on a substrate. Less