Effect of gas flow rates and nozzle throat width on deposition of α-alumina films of granule spray in vacuum

Deposition of α-alumina film by spraying alumina granules in a vacuum at room temperature was performed using a modified aerosol deposition system that had a supplemental gas flow line in addition to the carrier gas flow line. The pressure difference between the nozzle inlet and deposition chamber was increased by increasing the supplemental gas flow rate or by decreasing the nozzle throat width. For the same nozzle, the alumina film thickness per granule consumption increased as the pressure difference was increased. However, the film obtained by using the wide throat nozzle was thicker than that by using the narrow throat nozzle in spite of the smaller pressure difference except for the case without using the supplemental gas. The deposition behaviors according to the supplemental gas flow rate and the nozzle throat width were explained in part by the friction-affected layer near the nozzle throat wall and a critical granule velocity for film deposition. Alumina granules and films were characterized by scanning electron microscopy, X-ray diffractometry, surface profilometry, and transmission electron microscopy to assess and correlate the film quality to the deposition conditions.