In this study, suspension plasma spraying is used to produce cast iron coatings that benefit from a graphite structure. In order to increase the graphite content, different hydrocarbons in the form of liquid suspension (hexane and toluene) and gas precursor (methane) were injected into the plasma stream along with iron powder. Besides promoting the formation of a soot carbon structure, liquid hydrocarbon injection also prevents in-flight particle oxidation, which is a major concern when spraying metals. In addition, it has been observed that using a shroud during spraying significantly increases the amount of soot carbon in cast iron coatings, which can be transformed into graphite by post annealing.

Cast iron coatings containing solid lubricant of graphite are an attractive candidate for wear resistant applications of an aluminum alloy substrate. It is difficult to generate a graphite structure in coatings sprayed with as-atomized cast iron powder which contains a few graphite owing to their high solidification rate. Although a graphite structure is remained in coatings sprayed with fully annealed cast iron powder, graphite carbon becomes lower than that in the annealed powder because of the in-flight oxidation and dissolution of graphite into molten iron. The present study is focused on an increase in graphite carbon through flying droplet diagnostic at a certain spray distance, that is, in-situ measurements of droplet temperature and velocity. Water-atomized cast iron powder which was annealed at 900°C for 3.6ks, was supplied as a spray material. The fully annealed powder was plasma-sprayed onto an aluminum alloy substrate, as well as carrying out flying droplet diagnostic. The amount of graphite carbon can be estimated by flying droplet temperature and velocity, which are controlled by spray parameters such as plasma gas flow rate and plasma current. It is confirmed that droplet velocity exhibits stronger influence on graphite carbon compared with droplet temperature. High velocity causes an increase in graphite carbon, so that it is possible to fabricate graphite-graded cast iron coatings with high amount at the surface and low at the interface.