Definition - What does Coating Adhesion mean?
Coating adhesion is the minimum adhesion required for a coating to stick to the substrate. It governs a coating's durability in many applications and determines the coating performance. Good coating adhesion depends on many attributes of the interface region:
- Atomic bonding structure
- Elastic moduli and state of stress
- Thickness, purity and fracture toughness
Therefore, coating adhesion is strongly affected by the interactions between the coating and the substrate.
Three tests are used to measure coating adhesion: cross-cut test, scrape adhesion and pull-off test.
Corrosionpedia explains Coating Adhesion
A coating's durability depends on two basic properties:
- Cohesion - The inner strength of a material, which is determined by the strength of the molecular forces in the bulk.
- Adhesion - The strength of the bonds formed between one material and another.
Lacking these two properties can create failures:
- Cohesive failures usually occur in the paint film itself (e.g., abrasion, cracking due to aging and dissolving in solvent).
- Adhesive failures, which can be a blister forming at the interface, lifting of the paint film or any other situation that results from low adhesion at the interface.
Adhesion failures determine the lifespan of the paint system. Poor adhesion results in coating failure whereas good adhesion leads to durable coatings. Good adhesion will be evident when:
- Adsorption forms interfacial bonds.
- Chemical bonds are formed at the interface between the paint and the substrate.
- Mechanical interlocking occurs when the paint dries.
All of these mechanisms do not have to be present all the time to form good adhesion. Different mechanisms may be at work depending on the paint system, but a good wetting or adsorption is always required for good adhesion.
If coating adhesion is poor, a rubbing action can cause a localized rupture at the coating/substrate interface, leading to blistering or even complete spalling off of the coating. Coating adhesion is enhanced by reducing internal stress, surface tension and incorporating surface reactive groups.