Effect of Roughness and Surface Topography on Adhesion of PVB to AA2024-T3 using the Blister Test

Corrosion protection through protective coatings depends on the adhesion of the coating system to the base metal, which is controlled by surface treatments. Measuring adhesion strength is a challenging task. However, the Blister Test (BT) is a quantitative and reproducible technique not exploited in the corrosion field. In this investigation, the effect of roughness and surface topography is studied using this technique. Samples were abraded using SiC paper up to 120, 180, 600 and 1200 in a randomly or aligned manner to obtain different topographies and roughness levels in the range of 0.026 to 1.324 μm. The surfaces were characterized using Optical Profilometry before polyvinyl butyral (PVB) coating was applied. Then the adhesion strength was determined using the BT. The substrates abraded randomly presented lower average roughness than the ones abraded in an aligned manner due to continual cross abrasion of grooves. The adhesion strength results from the BT were reproducible and could rank different mechanical treatments. Roughness degree and surface topography were found to be very important factors for adhesion strength. Adhesion strength was found to increase with roughness for both abrasion methods; however the random samples exhibited the highest adhesion strength at similar roughness values. The groove peaks were found to be stronger barriers than the groove valleys as a result of a higher peeling angle needed for delamination to take place, increasing the energy used for plastic deformation and therefore decreasing the energy available for blister growth. An adhesion strength indicator (AS) was defined based on peeling rate and found to be effective in sensing adhesion strength.