Adhesion Tester

Cohesion and adhesion forces are decisive for the adhesion and thus the durability of coatings and adhesive bonds. Cohesive forces hold the molecules of the coating or adhesive layer together. The adhesive forces determine the adhesion between the applied coating and the substrate. How well the coating or bond holds can be tested using various methods. For some of the methods, a special adhesion tester is needed, for the others, suitable equipment makes it easier to carry out the test. The adhesion of the coatings is important for the corrosion protection and the mechanical properties of the composite components. The adhesion between two layers always depends on the condition and material properties of both layers.

Adhesion testers are used both in the development of new coatings, adhesives and bonding techniques, composites and composite components, as well as in quality control in ongoing production, to study the resistance of the bonds when subjected to stress. As a rule, the tests with the adhesion tester involve more or less damage to the test surfaces. Non-destructive tests are only possible with some very complex methods, for example with the photothermal method. The test sites or test samples must therefore be carefully selected and the test results – carefully evaluated.

Here is a list of common methods for testing adhesion strength:

- Bend test (90°, 180°)
- Mandrel bending test (4mm mandrel, 90°)
- Tape test (according to EN ISO 10683)
- Cross cut test
- Cupping test (according to Erichsen)
- Electrolytic test (only Cu, Ni, Cr)
- Torsion test (strips, wires)
- Winding test (wires)
- Friction test
- File test
- Hammer blow test (1,500 ... 6,000/min)
- Thermal shock test (galvanic plastics)
- Cobalt cone test (for Ni on steel)
- Ollard test
- Peel test
- Tear-off test

Only a few of the methods listed provide concrete numerical values for the bond strength of the joint. In many of the tests, adhesion is assessed visually based on the damage to the coating caused by the test. In order to be able to objectively record and compare the measured values for certain compounds, some special test methods and adhesion testers have therefore been developed with which the adhesion strength can be quantitatively determined. In these methods, the material layers are separated by specifically applied forces and the the force required for this is measured.

Adhesion tester for cross-cut tests
In cross-cut tests, the coating is cut according to a predetermined pattern and thus divided into small square areas. A special adhesion tester with several parallel cut edges simplifies the application of the cuts with even spacing. These small areas are then carefully covered with adhesive tape. Then an attempt is made to detach the coating from the substrate by peeling off the adhesive tape. Adhesion is then assessed visually based on the amount and distribution of the detached areas in the grid squares.

For meaningful results, the spacing and arrangement of the cut lines must be adapted to the coating thickness and the type of substrate. According to ASTM standards, for example, the X-Cut method according to ASTM D 3359 must be used for coating thicknesses greater than 125 micrometres and the cross-cut method according to ASTM D 3359 for thicknesses up to 125 micrometres. EN ISO 2409 describes cross-cut tests for coating thicknesses up to 250 micrometres for hard and soft substrates. The specified spacing of the cutting surfaces varies from 1 mm for coatings up to 60 µm on hard substrates to 3 mm for coatings from 120 to 250 µm on hard or soft substrates.

In many operations it is necessary to perform cross-cut tests with different cutting distances. When selecting the appropriate equipment, consider whether it is better to purchase several fixed-blade units or one adhesion tester with a replaceable blade. If the cutting device is to be used at changing locations for grid cuts, it is recommended to choose an adhesion tester that is already supplied with suitable accessories such as adhesive tape, brush and magnifying glass in a practical carrying case.

Adhesion testers for indentation tests
Indentation tests or cupping tests according to EN ISO 1520 (or BS 3900-E4 or ASTM E643) are carried out on coated sheets and plates. The test is used to assess the functionality of the coating when the substrate is deformed. For this purpose, the plate to be tested is clamped in the adhesion tester and slowly pressed in from below with a hemisphere of defined size at a uniform speed of between 0.1 and 0.3 mm/s. The coated surface to be evaluated lies on the surface of the plate and is observed during the indentation.

The ball plunger is pressed in during the cupping test until a specified force is reached or until the coating cracks or peels off because of the increasing bulging of the plate. Adhesion testers for cupping tests are available in very different designs. The important criteria for such an adhesion tester include:

- the diameter of the ball punch
- the maximum indentation depth
- the maximum compression force
- the type of force application (manual or motorised)

Some high-quality models of the adhesion testers for cupping tests are combined with a digital microscope. This allows the bulging surface to be immediately magnified and additionally illuminated. In addition, the images of the microscope can be stored for documentation of the test.

Adhesion tester for peel tests
Peel tests are used to check the adhesion between rigid base material and flexible cover material, for example, for coatings on chipboard or metals. Among other things, it allows checking the adhesion of adhesives and adhesive foils, of nickel coatings on steel and of copper laminations on printed circuit boards with plastic carriers. If the tensile strength of the applied layer is too low, the peel test can be terminated early by tearing the peeled layer.

The adhesion tester can be designed for specific materials and component dimensions or allow the test fixture to be adapted to different dimensions and test specifications. Important criteria in the selection of the adhesion testers for the peel test are:

- the test angle
- the possible width of the peeled strip
- the permissible thickness of the test sample
- the measuring range of the force
- the speed during pulling
- the maximum travel of the test fixture
- manual or automatic test sequences
- data memory
- data interfaces

Some of the specifications result from the components to be tested, the others from industry-specific standards. If many peel tests have to be carried out and documented, the adhesion tester should be chosen that can store the measurement data or transfer them directly to a computer.

Adhesion tester for tear-off tests
The tear-off method is generally used for thick-surface coatings. In this process, a stamp is bonded over the entire surface of the coating to be tested. Depending on the test description, the coating is also slightly cut along the circular shape of the stamp surface or all the way to the substrate. The stamp, which is fixed to the adhesion tester, is then subjected to tensile load at right angles to the surface of the substrate after the adhesive has hardened until it detaches from the substrate or the maximum tensile force is reached.

In DIN EN ISO 4624 - Coating materials - Peel test to determine adhesion strength three versions are described. Depending on the test requirements, either one or two test stamps are bonded to the prepared coated substrates or one of the test stamps serves as a substrate for the coating. Peel tests can be performed on both single-layer and multi-layer coatings. The adhesion testers for peel tests can be used not only for tests in product development or quality control. A mobile adhesion tester can also be used to test coatings on buildings, bridges, dams and other structures.

When deciding on a specific model of the adhesion tester, attention must be paid not only to functionality and operability but also to the specifications of the standards applicable to the particular application, for example: