Surface preparation of aluminum by atmospheric-pressure plasma jet for suspension plasma sprayed ceramic coatings

Abstract

Surface preparation is a key step preceding the deposition of suspension plasma spray (SPS) coatings, a type of thermal spray coatings. This work evaluates the effect of surface preparation by atmospheric-pressure plasma jet (APPJ) treatment on the adhesion of SPS-deposited Al2O3 ceramic coatings on aluminum (Al-2024) substrates. For comparison, the substrate surfaces were also prepared by grit-blasting. Surface roughness (R) parameters and surface morphology of both grit-blasted and APPJ-treated substrates were characterized by confocal microscopy and scanning electron microscopy (SEM). In addition, surface chemistry was evaluated by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Based on morphological analyses, the grit-blasted surfaces exhibit typical random peaks and valleys, whereas those subjected to APPJ treatment show self-organized patterns with multilayered and multiscale features. ATR-FTIR spectroscopy analyses also demonstrated a significant difference in surface chemistry for both surface preparation types. After the characterization of both types of prepared substrates, SPS coating was deposited on the substrates under two spray conditions. One spray condition demonstrated poor adherence of the coatings for both types of the prepared substrates; the other spray condition produced coatings with good adherence. The microstructures of the coating were evaluated for both spray conditions. The Adhesion strength of the coating was evaluated for the condition, which produced a coating with good adhesion. ASTM-C-633 pull test protocol was followed for the adhesion evaluation, and the pull test revealed significant adhesion improvements when the surface was prepared by APPJ. Improved adhesion strength was linked to the surface morphology and also to the surface chemistry of the surface prepared by APPJ. In addition, splat morphology was investigated on both grit-blasted and APPJ-treated substrates to correlate the improved adhesion.