Characterization of Thermal Spray Coatings

The ability to characterize fully the microstructure of a coating is paramount for understanding the in-service properties and eventual optimization of the coating. This article discusses sample preparation and subsequent analytical techniques (LOM, SEM, XRD, WDS, and QIA) for several cermet thermal spray coatings and provides a detailed analysis of as-sprayed microstructures in addition to processing trends for several FeCrAIY-carbide coatings. It was found that the splats produced in these high velocity oxy-fuel (HVOF) coatings tended to exhibit a predominantly dendritic structure most likely retained from the gas atomization process that produced the original powder. Chemical analysis showed that the carbides tend to breakdown during spraying producing a complex mixture of oxides and various carbides. Finally, image analysis revealed that as the carbides in the pre-sprayed powder were increased, more carbides and oxides with less FeCrAIY and thinner coatings were found. These techniques allow the thorough characterization of thermal spray cermet coatings, which in turn should further the understanding of the thermal spray processes and help provide superior coatings in the future.