Functionally-Gradient Sol-Gel Coatings for Aircraft Aluminum Alloys

Foster-Miller, Inc. approach is to generate functionally gradient sol-gel coatings in a single step process to replace the standard and water intensive process of chemical or electrolytic pretreatment (generating an alumina layer) followed by primer (containing corrosion inhibitors) application. These coatings would chemically bond to the native oxide on aluminum to form a corrosion protection barrier that gradually transistions into a region with suitable functionality for chemically interacting with the adhesive. Fundamental research on functionally gradient sol-gel coatings derived from tetraethoxysilane/aminoalkyltriethoxysilane mixtures has produced lap-shear joints exhibiting significantly improved environmental resistance. However, these coatings are subject to abrasive damage, resulting in substrate corrosion. Altering the composition of this functionalized region and electrophoretic processing (if required) to increase the coating hardness and density would allow more damage tolerant coatings to be formulated. Incorporating modified-transition metal alkoxides (whose oxides are harder than silica) and modified-alkoxysilanes (for adhesive bonding) will improve the hardness and adhesion characteristics of the coating. In Phase I, Foster-Miller will team with Lord Corporation and Lockheed to evaluate and select materials and apporaches to generate functionally gradient sol-gel coatings on aluminum alloys that provide corrosion protection and good adhesive surfaces. Phase II involves the optimization of the coating characteristics and process parameters.