<p>Aerosol deposition (AD) is a room-temperature solid-state spray process for fabricating dense ceramic coatings. Achieving successful, dense coatings in AD requires careful selection and pretreatment of feedstock powders. In this study, alumina (Al₂O₃) powders composed of dense, submicron-sized particles were mechanically milled using 3&#xa0;mol.% yttria-stabilized tetragonal zirconia polycrystal (referred to as ZrO₂ for simplicity) balls and jars for 0 to 9 h to enhance their deposition behavior. Ball milling reduced particle size, introduced defects into the powder particles, and increased their deposition rate in AD. However, it also led to the gradual adherence of ZrO₂ debris to the Al₂O₃ particle surfaces, increasing with milling time. Coatings deposited on silicon substrates using the 7- and 9-h ball-milled powders showed greater thickness, improved uniformity, and fewer defects compared to coatings from untreated or 2- and 4-h milled powders. The coatings showed a composite structure with distinctive zebra-like patterns, comprising approximately 40-50 wt.% ZrO₂. Nanoindentation revealed high hardness, attributed to the dense coating structure. These findings highlight that powder pretreatment can serendipitously lead to composite coating formation and provide valuable insight into the formation mechanism of these composite coatings.</p>