Coating Characteristics - Adhesion

Adhesion is the ability of a coating to remain attached to the substrate under the required operating conditions and this is clearly vital. Many use the words ‘adhesion’ or ‘bond strength’ to describe this ability, but the subject is much more complex than the description of a single strength parameter might suggest. The adhesion between the coating and the substrate should not be mixed up with the adhesion between the coating top surface and the sliding counterface, which is related to adhesive friction and adhesive wear.

Failure of coating adhesion to the substrate results in interfacial de-bonding which for brittle materials is often called cracking. The crack nucleation and propagation to larger surface failure is due to local load levels and directions, resulting stress conditions, strain and deformation. Sometimes a failure that is referred to as adhesive may in fact be a cohesive failure when the cracking occurs in the coating or in the substrate.

Indeed, the complexity is such that it is unlikely that any one measure or test can satisfactorily differentiate between the ability of different coatings to stay bonded to different substrates under different application conditions.

From a scientific point of view, it is preferable to think of adhesion as a fundamental property which can be quantified according to known data about atomic binding forces. Breaking the bonds between the coating and the substrate will produce a lateral crack. 

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