One of the more commonly used chemicals in plasma treating process is Hexamethyldisiloxane (HMDSO).
Primarily used for barrier coatings and to help protect the coating post treatment. HMDSO dispensed with argon into the vacuum chamber, is a big part of the hydrophilic coating process. Though, this chemical can be used for hydrophobic processes as well.
By adjusting the ratio of HMDSO and Oxygen that will be in the chamber, a thin film can be produced with the hardness of the film is customizable by the settings of the oxygen and HMDSO. Even after time, the film will hold its strength to protect the surface. The coating is known for increasing the contact angle of a surface, and with more time exposed to the HMDSO the contact angle will increase.