Silica deposition treatment of 2024 aluminum alloy for improved coating adhesion

Silica deposition treatment (trade name: ITRO treatment) is a flame treatment process to form silica deposition on treated surfaces, and could be an innovative, rapid, automated, and clean alternative to reduce costs and hazardous chemicals for aircraft coatings. In this study, the bonding state and adhesion strength of aircraft coatings on an aluminum alloy following silica deposition treatment were evaluated. The chemical structure of the treated surface was examined using scanning electron microscopy and Fourier-transform infrared spectroscopy. The silica deposit consisted of aggregated nanoparticles and contained silica, silanol groups, and water. The amount of silica and silanol groups increased linearly with the number of treatments. The adhesion performance of the coating following the silica deposition treatment using pull-off testing revealed that one-time silica deposition treatment resulted in a high strength comparable to that achieved by anodizing treatments. Flame treatment without organosilicon compounds resulted in low adhesion strength, indicating that the presence of the silica deposit contributes to improving the adhesion strength. Cross-sectional observation of the coated substrate interface revealed that the aggregated nanoparticles formed a thin porous layer of several micrometers. The coating resin completely penetrated this layer, and mechanical interlocking between the silica deposit layer and coating resin was presumed to arise. The fracture surfaces from the pull-off tests revealed that fracture propagated within the silica deposit layer and that part of the silica deposit and coating resin adhered to the silica deposit remained on the substrates. It could thus be concluded that the silica deposit layer was the weakest link in the coated test pieces.