Abstract: This study examines how ball milling parameters, specifically rotational speeds (20, 40, and 60 r/min) in dry and wet conditions, affect the development of mullite/5wt% nano-fly ash coatings on AISI 410 steel, focusing on their impact on feedstock powders and plasma-sprayed coatings. Optimized milling parameters at 60 r/min under wet conditions yielded high-quality feedstock powders with a particle size of 14 µm and limited size distribution. Coatings produced from wet-milled powders demonstrated a higher deposition efficiency (35%) due to their smaller, uniformly distributed particles, which enhanced melting during the spraying process. These coatings also exhibited significantly lower porosity (7.9%), resulting in denser structures with superior mechanical properties, including a hardness of HV₁ 647, fracture toughness of 1.41 MPa·m^0.5, and a smoother surface finish with a roughness (R_a) of 6.1 µm. Residual stress analysis showed that wet-milled coatings had higher residual stresses, reaching up to 165.95 MPa, compared to dry-milled coatings. This increase is attributed to finer particle sizes and rapid thermal cycling during deposition, which intensified tensile stresses within the coating. These results highlight the importance of optimizing milling parameters to enhance coating performance and process efficiency.