<p>This study explores the structural and mechanical characteristics of alumina (Al₂O₃) nanoceramic coatings deposited on A37 steel substrates via atmospheric plasma spray (APS) at an operating temperature of 12,000°C. By varying the number of plasma torch passes, coatings of differing thickness were fabricated and analyzed using Rietveld refinement of X-ray diffraction (XRD) patterns and Vickers microhardness testing. The results reveal a composite microstructure consisting of nanocrystalline γ-Al₂O₃ domains embedded in an amorphous alumina matrix. Notably, the amorphous phase fraction increases with coating thickness, while a reduction in γ-Al₂O₃ crystallite size enhances microhardness. This crystallite size-dependent hardening effect underscores the tunability of mechanical properties via process parameters. These findings demonstrate the potential of plasma-sprayed alumina coatings for surface engineering applications requiring high wear resistance and tailored stiffness. The integration of phase analysis, microstructure control, and mechanical optimization highlights APS as a versatile method for advanced ceramic–metal interface design. </p>