Hard corrosion-resistant bilayer coatings, were grown in a high-density microwave electron cyclotron resonance discharge. The bilayer coatings consist of a relatively soft (0.6-1.5 GPa) polymer-like coating as the adherent bottom layer and a much harder (8-12 GPa) top layer. The polymer underlayers were grown from 100% octamethylcyclotetrasiloxane, (OMCTS) while the hard top layer was either silicon dioxide grown from OMCTS in an oxygen plasma or silicon nitride grown from hexamethyldisiloxane in an ammonia plasma. The bilayer structures combined high surface hardness values with good corrosion resistance, surviving 1800-2600 h in an ASTM B 117 salt-fog corrosion test. (C) 2004 American Vacuum Society.