Spray‐pass effects on microstructure and mechanical properties of APS ZrO ₂ ‒Al ₂ O ₃ coating on SUS304 steel

Abstract The performance of ceramic coatings depends strongly on thermal spray parameters, particularly the number of spray passes. This study investigates the influence of one, three, and five passes on the microstructure, mechanical properties, and chemical stability of ZrO 2 –Al 2 O 3 coatings on SUS304 stainless steel using atmospheric plasma spraying. Characterization was performed by laser scanning microscopy, scanning electron microscopy, Vickers microhardness, nanoindentation, and energy‐dispersive X‐ray spectroscopy. Results show that coating quality improves with additional passes. A single pass produced porous coatings with weak splat bonding, poor adhesion, and elemental diffusion from the substrate. Three passes improved densification and mechanical uniformity, although localized defects remained. Five passes yielded the densest lamellar structure with porosity below 2%, hardness above 1050 HV, higher elastic modulus, and stronger chemical barrier properties. Although direct cohesion or adhesion strength measurements were not conducted in this study, these microstructural and mechanical improvements suggest enhanced structural integrity. Overall, the number of spray passes plays a key role in tuning process–structure–property relationships. The five‐pass configuration offered the most refined microstructure and performance, providing insights for optimizing plasma spray processes and developing durable coatings for structural, thermal, and corrosion‐resistant applications.