Standard motors are not an appropriate choice for a vacuum application. In general, standard motors will not survive in a vacuum of 10 -4 Torr or lower. Some materials like petroleum-based grease vaporize so quickly that they literally create clouds of vapor in the vacuum chamber. Other materials vaporize more slowly, but can become an application nightmare. Silicone is one of these nightmares, since once a vacuum chamber has been contaminated with silicone, it is nearly impossible to clean it all out, and it will continue to spread to anything and everything that is placed into the chamber.
Motor cooling in a vacuum is also a problem.
Conventional motors are cooled primarily by convection into the air that normally surrounds them, and to a lesser degree by conduction through the mounting surface. When a motor is energized in a vacuum, convection cooling is not available, and heat is dissipated primarily by conduction from the motor to the mounting structure. As a result, when operating a motor in a vacuum, provisions for heat exchange and higher temperature operation must be made. Even if a standard motor does not outgas contaminants in a vacuum, it is likely that the vacuum process will be hampered with the motor present. Leakage is the slow release of captured or clinging gas molecules from minute cracks in the motor laminations, windings, bearings and metal surfaces.