Low-pressure and atmospheric pressure plasma polymerized silica-like films as primers for adhesive bonding of aluminum

Plasma processes, including plasma etching and plasma polymerization, were investigated for the pretreatment of aluminum prior to structural adhesive bonding. Since native oxides of aluminum are unstable in the presence of moisture at elevated temperature, surface engineering processes must usually be applied to aluminum prior to adhesive bonding to produce oxides that are stable. Plasma processes are attractive for surface engineering since they take place in the gas phase and do not produce effluents that are difficult to dispose off. Reactive species that are generated in plasmas have relatively short lifetimes and form inert products. The objective of this work was to develop plasma etching and plasma polymerization as environmentally compatible processes for surface engineering of aluminum. Plasma polymerized silica-like films of thickness less than 200 nm were deposited on pretreated aluminum substrates using hexamethyldisiloxane (HMDSO) as the "monomer" and oxygen as a "co-reactant" in low-pressure RF-powered (13.6 MHz) reactor. Recently, plasma deposition at atmospheric pressure has become a promising technology because they do not require vacuum systems, can be applied to large objects with complex shapes, and adapted easily for continuous processing. Therefore, atmospheric pressure plasma processes were investigated and compared with their more traditional counterparts, low-pressure plasmas. Molecular structure and morphology of the plasma polymerized films were determined using surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM).; The effectiveness of plasma etching and plasma polymerization as surface engineering processes for aluminum were probed by determining the initial strength and durability of aluminum/epoxy lap joints prepared from substrates that were plasma pretreated, coated with silica-like film, and then bonded together using a 1-part epoxy adhesive. The durability of joints was evaluated using a "stressed durability test" which involved applying a static load to joints, exposing them to a cyclically varying, corrosive environment, and determining the number of cycles required to produce failure.; Atmospheric pressure plasma polymerized HMDSO films exhibit RAIR spectra with prominent features similar to those observed for the low-pressure plasma films. These films had less than 7% carbon, revealing the films to be silica-like in nature. Durability results show that reducing plasma pretreatment of aluminum substrates was better compared to oxygen plasma pretreatment. Joints prepared from aluminum substrates that were acid etched, and then primed with silica-like film had exceptional durability. Durability of these joints was related to the acid etching, which formed a uniform and dense aluminum oxide structure with low magnesium content and high surface topography, and to the primer film which prevented hydration of the oxide. Also, joints prepared from substrates that were atmospheric pressure plasma pretreated exhibited better durability compared to similar joints prepared from substrates that were pretreated in low-pressure reactor. These results show that the atmospheric pressure plasma pretreatment have potential as pretreatment processes that can be applied to metals such as aluminum prior to finishing operations.