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The core of the diffusion theory of coating adhesion is that two compatible macromolecular chain segments diffuse and interweave with each other to form a transition layer, thereby enhancing adhesion; this theory is mainly applicable to polymer substrates and polymer coating systems. The specifics of diffusion theory: When a coating (polymer coating) and its substrate, which is also a polymer, become fully wetted to the point where molecular contact is achieved, short chains of macromolecules will move across the interface through Brownian motion and diffuse toward the other side. Eventually, an interwoven network structure is formed, which eliminates the original distinct interface and creates a transition layer that possesses the properties of both materials, thereby enabling strong adhesion. Complete macromolecules find it difficult to diffuse completely across the interface; only short local segments can diffuse more easily, resulting in the formation of a diffusion interface layer at the interface with a thickness of 10–1000 angstroms. Applicability and Limitations: Application scenarios: This theory is primarily applicable to polymer systems in which both parties are mobile long-chain molecules, such as solvent bonding and thermal welding of thermoplastics, or interlayer adhesion of coatings within the same system, with bonding being achieved through molecular diffusion. Limitation: It is impossible to explain the adhesion between polymer materials and hard materials such as metals and glass, which lack the ability to diffuse, as it is difficult for polymers to diffuse into such materials ； At the same time, dissimilar polymers are usually incompatible and it is difficult for effective diffusion to occur. Practical significance: In actual coating applications, the diffusion theory is rarely used as the sole mechanism for adhesion; it usually works in combination with mechanisms such as mechanical anchoring, chemical bonding, and intermolecular forces to enhance the overall adhesion of the coating.