Abstract

Suspension plasma spray (SPS) is one of the very important types of thermal spray processes for generating fine microstructured coatings. This type of coating usually functions in harsh environment; therefore, the coating requires strain tolerant behaviour. To achieve this strain tolerant behaviour and durable functionalities of the coatings, substrate surface preparation plays a significant role. Proper surface preparation helps to improve the adhesion of the SPS coatings. Conventionally, grit blasting is used for surface preparation; but, there are some major limitations like grit residue inclusion, generation of compressive stress and formation of microcracks which cannot be avoided especially when surface preparation is performed for the ductile metals like aluminum and copper. To overcome these limitations, in this research, an aerospace grade aluminum (Al-2024) was selected to perform the surface preparation by two advanced techniques, they are atmospheric pressure plasma jet (APPJ) and femtosecond laser (FSL) where grit blasting was the benchmark technique. Surface profiles of the prepared substrates by those techniques were characterized by confocal microscope, morphology was evaluated by SEM, and surface chemistry of the prepared substrates was evaluated by EDSX, XRD, XPS, and ATR-FTIR analytical tools. Under certain processing conditions, in the case of the surface prepared by APPJ a self-organized patterned multilayer and multiscale microstructures was obtained. In the case of FSL at different process parameters, two periodic structures and one random structure were obtained.
For the coating by SPS process, alumina was used to coat the surface prepared by APPJ, FSL and also grit blasting, since alumina is a widely accepted ceramic material for insulating and wear resistant applications. Two spray conditions were selected for depositing alumina coatings in the view of producing dense type and also porous type coating. Coated substrate surface profiles were evaluated by confocal microscopy and the coating microstructures were studied by SEM from the top surface and also from the cross section. Adhesion strength of the deposited coatings were evaluated by ASTM C-633 pull test protocol. Significant adhesion strength was noticed in the case of femtosecond laser, at least 4.5 factors more than the conventional grit blasting and in the case of APPJ-treated substrates the adhesion strength was 2.4 factors higher than that of the grit blasting. In addition, failed samples after the adhesion tests were studied by digital image analysis, SEM, and also by FESEM to understand the adhesion mechanism on the three types of prepared substrates. The results found in this research for the surface preparation show a milestone for the advance surface preparation of aluminum metal by APPJ and FSL. In addition, both surface preparation techniques can eventually be integrated in coating production as an alternative to grit blasting since both techniques are clean and no toxic chemicals are used or produced.