At the submicroscopic level, the surface of the substrate is rough, full of pores and depressions. The liquid adhesive with good fluidity flows into and fills these pores and depressions. After drying and curing, mechanical connection forces such as hook anchors, tenon joints and riveting are formed. The higher the roughness of the substrate and the larger the surface area, the greater the adhesion. Only when the adhesive completely penetrates into the irregular interface of the rough surface is it beneficial to the adhesion.
As long as the coating film has slightly fluidity, it is less likely to generate unreleasable stress. However, with the increase of the viscosity and rigidity of the coating film and the formation of adhesion to the substrate, a large amount of stress will be generated. The thickness of the adhesive at the concave and convex parts of the substrate is obviously different, which leads to different physical properties. The non-uniform coating will produce great internal stress, which may even cause the rupture of the film layer.