Splat Morphology and Graphite Formation in Plasma-Sprayed Cast Iron

Fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate are investigated in the present study: focusing on the effects of preheat substrate temperature (T_S) and chamber pressure (P_C) on the splat morphology, the adhesive strength of splats, the formation of a reaction layer and graphite. Splash-type splats appear at low T_S but disk and star-shaped splats arise at high T_S. Deformed substrate ridges, mainly due to the slight surface melting, are formed adjacent to the splat periphery at high T_S. At low T_S, pores are observed at the splat/substrate interface, which cause a decrease in the adhesion of splats. In contrast, a reaction layer composed of iron, aluminum and oxygen is ready to form at high T_S. The amount of graphitized carbon increases in cast iron splats with T_S. At a low P_C of 26.3 kPa, disk-type splats are in the majority at a constant T_S of 473 K. As P_C increases, star-shaped splats appear along with disk splats. The flattening ratio of disk splats decreases with the increase of P_C, because of a decrease in the kinetic energy and temperature of molten droplets. An interfacial oxide layer composed of iron, aluminum and oxygen is ready to form at high P_C. The number of pores intensively increases with P_C, which leads to a decrease in the adhesive strength of splats. The amount of formed graphite in cast iron splats slightly increases with P_C, however, that of a rapidly solidified phase of Fe-Si-C decreases because of lowering of the solidification rate.