Adhesive bonding is a process where two similar or dissimilar materials are fastened together. As the name implies, in adhesive bonding, an adhesive, typically some kind of glue is used to bond the two surfaces. In the adhesive bonding process, both adhesive and cohesive forces are involved. Depending on the strength of these forces, the adhesion failure can be either adhesive, cohesive, or substrate failure.
When the glue is applied on the surface, its ability to wet the surface is of utmost importance for good adhesion. Good wettability ensures that the glue can penetrate surface structures which leads to better mechanical interlocking at the interface. The adhesive forces between the glue and the substrate as well as the cohesive forces in the glue determine how well the glue can wet the surface. If the cohesive forces inside the glue (or any other coating formulation for that matter) are high, the glue prefers to stay together rather than interact with the surface. However, if the adhesive interaction between the substrate and the glue is high enough, the glue might spread after all. Once the glue has hardened and the interfaces are formed, these same forces determine whether the bonding is permanent or adhesion failure is likely to happen.
To understand why bonding fails, we need to first define the ways the failure can occur. 
These can be divided into three categories:
Adhesive failure, or delamination, is one of the most common types of failure mechanisms. There the two dissimilar materials detach from each other. The failure can happen between the adhesive and either of the two substrates, it is bonding together. Cohesive failure happens in the adhesive itself or inside the layer of a coating. Substrate failure is not related to the bonding process itself as it is a problem in a substrate.
As you might guess already, the cohesive failure will happen if the cohesive forces inside the adhesive are not strong enough. Then, on the other hand, the adhesive failure happens if the glue has not spread properly on the substrate. To make sure that the spreading of the glue (or other coating formulation) is sufficient, contact angle measurements can be used.
To read more about adhesion and how contact angle measurements can help to predict its success, please download the white paper through the link below.
Susanna is an Application Scientist at Biolin Scientific. In her PhD thesis, she developed fabrication methods for a new type of inorganic-organic polymers. Microfabricated polymer chips were utilized as tool for biomolecule separation in analytical chemistry.
