Film adhesion test

Scratch test ASTM C1624, DIN V ENV 1071-3

Film adhesion test scratch test ASTM C1624, DIN V ENV 1071-3
© Fraunhofer IST
Scratch test of DLC on steel
© Fraunhofer IST

The scratch test is used to determine the bonding strength between a coating and the base material. A diamond stylus is drawn with increasing load across the coating system being tested. Then the resulting scratch mark is evaluated. Various coating defects such as cracks or spalling can be observed in the course of the scratch test. For example, layer separation can be analysed by means of microscopic examination. The critical load under which spalling is first observed is determined as a measurement for layer adhesion. In addition, the frictional force and acoustic emissions are recorded during the measurement and used to support the interpretation of the results.

Scratch test of DLC on steel. Critical load at 35 N. The critical load is determined through optical analysis of crack formation and spalling of the layer, or by analysing the acoustic emissions during scratching. Changes in the friction coefficient during scratching can also be used for evaluation.

Rockwell, VDI Directive 3198

For the Rockwell test, a diamond cone is pressed into the coating surface with a defined force. The coating is damaged in the area surrounding the hardness impression. This can be discerned under the microscope as a network of cracks and/or fractures of the coating at the edge of the impression.

Spalling around the impression  through digital image analysis of the spalled surface areas
© Fraunhofer IST

Spalling around the impression can be analysed according to VDI Directive 3198 through assignment to adhesion classes 1–6, by using the later ISO standard 26443 (4 classes) or, as demonstrated in the image shown here, through digital image analysis of the spalled surface areas which results in a more objective evaluation and detailed classification.

Rockwell spalling

Rockwell spalling
© Fraunhofer IST

Top graphic: spalling 39.2 % | adhesion class 5

Bottom graphic: spalling 0.3 % | adhesion class 1