RF power and SiOxCyHz deposition efficiency in TEOS/O2 discharges for the corrosion protection of magnesium alloys

The effect of the applied voltage of 27.12 MHz TEOS/O2/He discharges on the deposition process of SiOxCyHz thin films over rough as-cast Mg alloys was investigated by applying plasma diagnostics, surface characterization techniques and electrochemical impedance spectroscopy. The process efficiency in terms of power usage and deposition rate was calculated from plasma electrical and deposition rate measurements and was found to follow an inverse relation to the applied voltage, while saturating at higher voltages. Films with good substrate step coverage and free of cracks and structure failures were deposited at intermediate applied voltages and total power dissipation. As a consequence, the corrosion resistance of the films was optimized at the same conditions, indicating that in the case of no special pre-treatment of the Mg substrates, the corrosion performance is mainly determined by the SiOxCyHz adhesion to the substrate rather than the chemical composition of the films.