Study of the Splat-Substrate Interface for a PEEK Coating Plasma-Sprayed onto Aluminum Substrates

The quality of coatings made using thermal spray processes depends greatly on the degree of adhesion between the substrate and its coating. Yet the bonding mechanisms between a substrate and coating are not well understood. In this study, polyetheretherketone (PEEK) powder was plasma-sprayed to form single splats on aluminum substrates, which had undergone various surface treatments, including boiled (BT), etched (E, ET), and polished (PT), all of which had also been thermally treated to remove water from the substrate surface, with the exception of one etched aluminum substrate. Scanning electron microscopy was used to give an overview of the surface and splat morphology. The splat-substrate interfaces were studied in detail using focused ion beam imaging and transmission electron microscopy, to characterize microstructural features within the splat-substrate interface, including inter/intrasplat pores, pores along the splat-substrate interface, level of contact between the splat and the substrate, etc. The results showed that the splat-substrate interface for the BT and the E substrate surface had poor level of contact, with a high number of small pores (<1 μm) along the splat-substrate interface for the BT splat-substrate interface, and the formation of a near-continuous crevice between the PEEK splat and the aluminum substrate for the E substrate surface. The presence of the fine needle-like network of oxide layer on the BT substrate surface may have restricted the flow of the molten PEEK on the aluminum substrate, and the possible presence of physisorbed and chemisorbed water on the E substrate surface may have reduced the level of contact between the PEEK and the aluminum substrate. In contrast, specimens which had undergone thermal treatment to minimize the presence of water on the substrate surface, such as the ET and PT substrate surface, exhibited high level of contact at the splat-substrate interface. The number of pores for the ET and the PT splat-substrate interfaces were substantially lower than of the BT and E splat-substrate interface.