Plasma polymer films as adhesion promoting primers for aluminum substrates. Part I: Characterization of films and film/substrate interfaces

Plasma-polymerized hexamethyldisiloxane (HMDSO) films (similar to 800 Angst rom in thickness) were deposited onto aluminum substrates (6111-T4 alloy) i n radio frequency (RF) and microwave (MW) powered reactors to be used as pr imers for structural adhesive bonding. Processing variables such as substra te pre-treatment, carrier gas, and him posttreatment were adjusted to produ ce films that had different structures and properties. The plasma polymeriz ed films were characterized by reflection-absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscop y (SEM), and ellipsometry. Films deposited using argon as the carrier gas w ere siloxane-like. When siloxane-like films were post-treated with an oxyge n plasma, a silica-like surface layer was produced. Silica-like films were deposited using oxygen as the carrier gas. Plasma polymerized silica-like f ilms were formed of spheroidal particles deposited next to each other and i mpinging upon one another. These spheres were thought to be islands formed by the growth of stable nuclei. Etching aluminum substrates in argon and ar gon/hydrogen plasmas before deposition of the primer removed adsorbed water from the surface, possibly creating a more stable oxide surface to which t he silica-like primer could bond. The effect of etching silica-like films i n an Ar plasma was to promote the condensation of adjacent silanol groups t o form Si-O -Si bonds but with little change in the film thickness or the c oncentration of isolated silanol groups.