Influence of the Surface Roughness on the Adhesion of Thermal Plasma Spray Al₂O₃ Coatings

Atmospheric thermal plasma spraying is an innovative coating process, widely used in aerospace, medicine, and other industries to enhance the characteristic work piece surface properties, e.g. wear resistance, corrosion resistance, thermal insulation, etc. For surface customization, masks are used to protect areas of the workpiece from unwanted coatings. However, due to the nature of plasma spraying, a gradual accumulation of spray material on masks can lead to a change in the flow dynamics near the edges causing a loss of contour accuracy and thickness homogeneity of the actual coating. Additionally, accumulated particles can fall off the masking and get embedded in the coating. This poses a problem, e.g. for conductive coatings, as these particles represent impurities that can lead to a change in the conductivity or resistance.<br/>To tackle this problem, research is done on generating masks’ surface conditions that affect film adhesion and promote swift delamination, while optimum plasma spraying conditions are maintained. Samples were spray coated with Al₂O₃ (2kg/h) using atmospheric pressure plasma spraying. As device served an Oerlicon Metco F4MB-XL Spray Gun (DC, I=600A, U=32V, gases: Ar, H2). In thermal spray processes, adhesion between the coating material and the substrate surface is of complex character. However, besides of physical (van der Waals) and chemical interactions, and metallurgical processing, adhesion of coatings to a substrate surface is mainly established by mechanical anchoring [1, 2].<br/>In order to improve the understanding of relevant coating mechanisms, the influence of the surface roughness on coating adhesion is studied on a methodical basis. For this reason, prior to applying the spray coating, the surface roughness R_a of stainless steel (Type: 1.4301) was enhanced by using corundum blasting to provide a defined reference value. Subsequently, R_a was systematically reduced by means of Plasma Electrolytic Polishing (PEP), in order to provide a sample series with a defined range of surface roughness. PEP is an environmentally friendly hybrid process, assumed to combine classical electrolysis and atmospheric plasma processes. It is used to efficiently smooth out surfaces, clean off surface residues, and deburr edges, by using an electrolyte that is harmless during disposal and therefore relatively environmentally friendly, compared to classical electrolysis.<br/>Tactile profilometry was carried out to measure the surface roughness obtained in PEP treatment (measured with KLA+ Alpha-Step® D-600). Moreover, the effects of additional interface layers (SiOx, TiN) and substrate materials (quartz glass) on film adhesion were investigated. During PEP, with increasing polishing time the surface roughness could be reduced to a minimum of R_a ≈ 0.05 µm. Further reduction was not achieved, presumably, because the (selective) material removal during PEP of steel stimulates both processes; surface smoothing and roughening.<br/>Whereas the sandblasted specimens provide sufficient coating adhesion, the formation of adhering coatings is efficiently suppressed at smaller R_a. Below a critical roughness, swift and complete delamination is observed for Al₂O₃ coatings with a typical thickness above 100 µm. Additionally, it was observed that Al₂O₃ coatings exhibit low adhesion on smooth quartz glass, whereas solid coatings on corundum blasted glass surfaces could be attained.<br/><br/>(Funded by the European Union and the State of Mecklenburg-Western Pomerania, TBIV-1-321).<br/>[1] Ruzbarský, J. & Panda, A., Plasma and Thermal Spraying, Springer International Publishing, 2017, DOI: 10.1007/978-3-319-46273-8<br/>[2] Kewitz, T. et al., Atmosphärendruck-Plasmaspritzen in der Hochspannungstechnik - thermische und elektrische Isolierschichten aus Al₂O₃, Jena, MEOX Projektmanagement GbR, 2019, 59-68, ISBN: 978-3-00-063646-2