Plasma Polymer Films as Adhesion Promoting Primers for Aluminum. Part II: Strength and Durability of Lap Joints

Abstract

Plasma-polymerized hexamethyldisiloxane (HMDSO) films (∼800 Å in thickness) were deposited onto 6111-T4 aluminum substrates in radio frequency and microwave powered reactors and used as primers for structural adhesive bonding. Processing variables such as substrate pre-treatment, carrier gas and film post-treatment were adjusted to produce films that had different structures and properties. Films deposited using argon as the carrier gas were siloxane-like and were not wetted by typical structural adhesives. These films performed poorly as primers. When siloxane-like films were post-treated with an oxygen plasma, a silica-like surface layer was produced which was wetted by structural adhesives. However, these films were mechanically weak and were not useful primers. Silica-like films were deposited using oxygen as the carrier gas. These films had good cohesive strength, adhered well to the substrates, were easily wetted by epoxide adhesives, and were outstanding primers for structural adhesive bonding of aluminum. The initial strength of aluminum epoxy lap joints prepared from substrates coated with silica-like primers was determined by loading joints to failure. Durability was determined by applying a static load to lap joints, exposing them to a cyclically-changing, corrosive atmosphere, and measuring the time to failure. Pretreatment of the substrates before deposition of the primers was required in order to obtain high initial strengths and outstanding durability. The best results were obtained when the substrates were etched in chronic/sulfuric acid to remove the thick magnesium-rich oxide typically found on 6111-T4 aluminum and then etched in an argon plasma before deposition of the primer. The argon plasma pre-treatment removed adsorbed water from the surface of the aluminum before primer deposition, creating a more stable oxide surface to which the silica-like primer could bond.