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

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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