(Color online) The scheme for fabrication of micro patterns. (a) 10 μm thickness of spin coated photoresist on steel plate for 30 s at 3000 rpm. (b) After UV exposure (200 mJ) and developed photoresist in developer for 10 min. (c) Etching the surface of steel plate in immersion using Nital etchant (nitric acid and ethyl alcohol mixed solution, v/v = 1:9). (d) After removal of phtoresist using SU-8 remover PG at 80 °C for 120 min. (e) Steel plates were adhered to a polymer composite with very thin epoxy adhesive layer without precrack area (12.5 × 5 mm2).
(Color online) SEM images of surface treated steel: (a) surface mirror polished successively by 9-, 6-, 3-, 1-, and 0.25-μm-diameter diamond particle papers; (b) randomly ground surface by sand paper (#60); (c) microline patterned surfaces with a feature width of 60 μm; (d) square-shaped dot patterned surfaces with feature width of 80 μm.
(Color online) (a) Scheme of end notched flexure (ENF) test to measure fracture loads. When bispecimens are bent by upper pressure, shear stress occurs from precrack area to patterned area between steel and polymer composite. The shear stress causes fracture of the epoxy adhesive. (b) Load vs displacement of various surface treated steel-polymer composites: mirror polishing (MP), sand paper grinding (SG), microscale line patterns (MLP) and microscale square-shaped dot patterns (MDP) with various feature sizes.
SEM images of the surface of the steel plate after ENF test. (a) cross-section view of SEM image of the specimens in which crack occurs after ENF test. (b)–(d) images are top view images after removing polymer composites from steel; (b) image of the randomly ground surface with sand paper, (c) image of 60 μm line patterns, and (d) image of 40 μm dot patterns. As shown in (c) and (d), fracture area on the channel parts of dot patterns was wider as 1/4 of total pattered area (5 × 5 mm) compared to line patterns.
(Color online) (a) Data of fracture loads for all etched surface without any micro regular pattern, 60 μm square dot patterns and mirror polished surface. (b) Image of all etched surface. In (b), somewhat wide peeling area of epoxy adhesive in relatively bright region could be found due to weak interlocking effect.
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