Effect of combination of microstructure and surface treatment on shear strength of precision bonded joints

To improve the bonding strength of precision adhesive joints, mechanical and electrochemical surface treatments were combined to modify the original bonding surfaces. Microgrooves with different aspect ratios were manufactured on aluminum-alloy surfaces and phosphoric acid anodizing (PAA) was then applied to the surfaces. Wettability, shear strength, and fracture morphology were tested and analyzed. PAA treatment could change the original flat surface to be hydrophilic, and microgrooves could further enhance these characteristics. Compared with the original flat surface, the shear strength of the micro-structured surfaces without PAA treatment increased by 8.54% at an aspect ratio of 0.02, but decreased by 0.22% and 1.90% as the aspect ratio increased to 0.04 and 0.08, respectively. The shear strength of the flat surface only treated by PAA increased by 22.88%, while combining microgrooves could further improve the shear strength by 38.17%, 39.09%, and 42.24% at aspect ratios of 0.02, 0.04, and 0.08, respectively. Fracture morphology analysis and observation of interfacial wetting indicated that manufacturing microgrooves and applying PAA treatment on the aluminum-alloy surfaces could eliminate many tiny interface failures and greatly increase the effective bonding area, which significantly improved the shear strength.