Adhesion tests show how well a coating is bonded to the substrate and between its own layers. They are used to qualify systems, check work on site and investigate failures. Because most are destructive, the specification should say where and how often they are carried out.
Pull-off testing: ISO 4624
A metal dolly is glued to the coating. Once the adhesive has cured, a test instrument pulls the dolly off at right angles, and the force needed is recorded in MPa.
The number on its own is only half the result. The inspector also records where the break happened:
- between the substrate and the first coat (adhesive failure)
- within a coat (cohesive failure)
- between coats
- in the glue itself, which means the coating was at least as strong as the result
ISO 16276-1 sets out how pull-off results are assessed for acceptance on site.
Cross-cut testing: ISO 2409
A lattice pattern is cut through the coating to the substrate, tape is applied and pulled off, and the result is compared with reference images. It is classified from 0 (edges completely smooth, no detachment) to 5 (severe flaking).
The spacing of the cuts depends on the film thickness, and the method suits thinner films. For thicker coatings an X-cut (ISO 16276-2) is often used instead.
Reading the results sensibly
- Test on representative areas, not just the easiest place to reach.
- Record the failure mode, not just the value.
- Repair every test site in line with the specification.
- Compare with the system’s data sheet and qualification results, not a generic number.
How we help
We carry out adhesion testing as part of inspection and failure investigations, and interpret the results against the specification. See failure investigation.
This guide summarises the standard for general understanding. Always work to the current edition and the project specification.