Testing the Adhesion of Coatings
Having prepared a surface, applied a coating and measured the wet and dry film thickness, a lot of time and money will have been wasted if the coating does not adhere to the substrate.
The greatest single cause of paint adhesion failure is poor surface preparation. The adhesion of a coating to a substrate is typically affected by the cleanliness of the surface and the profile, such as the mechanical key provided for the coating to stick to the surface.
There are several methods for testing the adhesion of a coating to a surface. They range from a simple cross cut test; through cross hatch testing, where a visual assessment is made; to pull off testing where the force required to induce failure is measured. In most cases, the tests are destructive. There are two types of possible failure:
Adhesive Failure: where the failure is between layers.
Cohesive Failure: a failure within the coating film, the glue used to fix the test piece to the coated surface or the substrate.
Adhesion tests carried out in the field can be differentiated between three different test methods: Cross Hatch Method, Pull Off Adhesion, Push Off Adhesion
Cross Hatch Method
This basic test method involves cutting an “X” through the coating using a sharp utility knife
Pull Off and Push Off Methods
Pull and push off tests involve gluing a dolly to the coating and measuring the force required to remove it
Preparing a Dolly or Pull Stub for Test
Setting up an adhesion test for all styles and gauges is very similar, these points should be taken into account
Pulling the Dolly Off the Coating
The load fixed to the dolly should be applied in a steadily increasing manner over a specified time period