The cross-cut test is a fast and simple method of assessing how well a coating adheres to a metal. Good adhesion is important. Because if a coating is easily removed from a surface, this can no longer protect the surface below from environmental influences.
This test can be briefly described as follows: a lattice is cut into the surface coating that is to be tested. Following this, adhesive tape is attached to the cut surface before being abruptly torn off. This method is a further development of the tape test. ISO standard 2409 of the International Organization for Standardization (ISO) exists to ensure the comparability of the results of such cross-cut tests. The German version is DIN EN ISO 2409 coating material – cross-cut test.
Some more details of the cross-cut test: six parallel cuts are firstly made into a coating with a cutter knife. Their distance to one another depends on the thickness of the coating. If the thickness is less than 60 µm, the distance between cuts is 1 mm. If the coats are between 60 and 120 µm thick, then the distance is 2 mm. If protective coatings are thicker, it is 3 mm. In the second stage six further cuts are applied at right angles. The result is an even lattice pattern.
A clear or crepe adhesive tape is then applied to the resultant rectangle. This is removed abruptly at an angle of 60°. The adhesive force of the tape is to be noted. Standard ISO 10683 entitled Fasteners – Non-electrolytically applied zinc flake coatings clearly states: if the test is intended to see how such coatings adhere to sheet metal, adhesive tapes with an adhesive force of 6 to 8 N/25 mm are to be used.
For evaluation, the remaining lattice is observed and the adhesiveness assessed using comparative pictures from standard ISO 2409. The less coating is removed, the better the adhesiveness. To assess such experiments, experts also use cross-cut parameters (Ct). There are six of these: Ct 0 to Ct 5. Parameter 0 corresponds to excellent adhesiveness, 5 very poor adhesiveness:
Ct. 0: No peeling off or flaking. The edges of the cuts are completely smooth. None of the squares of the lattice has flaked off. Ct. 1: Traces of peeling or flaking along the cut. Small splinters from the coating have flaked off at the interfaces of the lattice lines. Flaked-off areas not larger than 5% of the lattice surface area.
Ct. 2: Jagged flaking along the cuts. The coating has flaked off along the edges of the cuts or at the interfaces of the lattice lines. Flaked-off areas larger than 5% but not larger than 15% of the lattice surface area.
Ct. 3: Jagged flaking along the cuts. The coating has flaked off partially or wholly along the edges of the cuts in broad strips and/or individual squares have flaked off partially or completely. Flaked-off areas larger than 15% but not larger than 35% of the lattice surface area.
Ct. 4: Flaking-off of nearly all of the entire surface area of the square cut. The coating has flaked off along the edges of the cuts in broad strips and/or individual squares have flaked off partially or completely. Flaked-off areas larger than 35% but not larger than 65% of the lattice surface area.
Ct. 5: Flaking-off beyond the surface area of the cut square. Any flaking-off that can be assessed as not more than cross-cut parameter 4.
The cross-cut test is usually conducted in the form of a yes/no test. Coatings with a cross-cut parameter of 0 are desired, parameters smaller or equal to 1 – i.e. from zero to 1 – are usually accepted.
Two notes in conclusion:
The cross-cut test is not suitable for coat thicknesses of more than 250 µm or for structured surfaces.
As applying cuts by hand with a cutter knife may result in minor fluctuations, automated processes are also available.