Hexamethyldisiloxane (HMDSO)-plasma-polymerised coatings as primer for iron corrosion protection: influence of RF bias

Silicone-like coatings were prepared by plasma enhanced chemical vapor deposition on steel substrates using a hexamethyldisiloxane-oxygen (80 : 20) precursor mixture, in a microwave reactor. The plasma composition, the structure and barrier properties of the coatings were studied as a function of the RF bias. OES was carried out in order to identify the excited species in the plasma. The coatings were characterized by several ex situ diagnostics including FTIR, SEM, gravimetry and EIS. OES, using argon as an actinometer, showed no modification in the HMDSO dissociation as the RF bias varies; but it indicated changes in the chemical recombination reactions. Coating analyses revealed that radio frequency (RF) bias induced a densification of the coatings associated with modifications in their morphology and chemical composition. Results were explained using the general mechanism in the literature relative to plasma-deposited thin films. EIS results indicated that the best barrier properties during immersion in NaCl solutions were obtained with coatings deposited under high RF bias.