Microstructural Features and Defect Formation in Plasma-Sprayed Al2O3 Coatings Deposited by a Water-Stabilized Plasma Torch

Al2O3 coatings were deposited on S235J2 steel substrates using a water-stabilized plasma (WSP) spraying system equipped with a WSP PAL-160 plasma torch. The microstructure, phase composition and adhesion behaviour of the deposited coatings were investigated. Cross-sectional and surface observations were performed using scanning electron microscopy (SEM), while phase composition was evaluated by X-ray diffraction analysis. The coatings exhibited a characteristic lamellar architecture formed by successive deposition and rapid solidification of molten particles. The most frequently observed defects were interlamellar pores, isolated rounded pores and fine microcracks located within individual lamellae. Despite the presence of these discontinuities, the coating formed a continuous layer without extensive delamination. X-ray diffraction analysis revealed that the deposited coating consisted predominantly of γ-Al2O3, whereas only a minor fraction of α-Al2O3 was detected. The predominance of γ-Al2O3 was attributed to rapid cooling of molten particles after impact on the substrate surface. Adhesion testing performed according to ASTM C633 yielded an average adhesion strength of approximately 10.58 ± 0.36 MPa and revealed a mixed adhesive–cohesive failure mode. The obtained results confirm that water-stabilized plasma spraying is capable of producing Al2O3 coatings with a typical lamellar microstructure, adequate adhesion and phase composition characteristic of rapidly solidified alumina deposits.