Scratch testing is often used to investigate the adhesion of thin films to substrates. However, it is sometimes difficult to produce delamination of polymer films using this method because of the tendency for ploughing to occur, i.e., for the diamond probe to simply cut through the film and to slide along the substrate surface.

A repetitive stress cycling technique has been developed for the NanoTest instrument to enable the investigation of interfacial fatigue resistance. The technique utilises specimen oscillation such that a test probe is caused to periodically compress and relax the contact point. In this case, the interface is gradually weakened until failure occurs. Even multilayer polymer coatings have been separated using the contact fatigue/impact method. It is characteristic of the technique that an abrupt and easily-recognised probe depth change occurs upon adhesion failure.

The frequency, amplitude, normal force, oscillation angle, and diamond geometry are variable. At higher oscillation amplitudes, the test probe actually bounces on the surface to permit the investigation of impact phenomena.

In the example shown, the specimen was mounted at an angle of 30° relative to the probe axis. A sharp, tapered probe was used with an oscillation frequency of 30 Hz. The instrument was programmed to apply a constant normal load of 2.0 mN. Probe displacement is plotted against time for two tests.

Before beginning the sample oscillation, the probe was left in contact for a short period with the 2 mN load applied in order to provide an initial reference position. The oscillation was then started and allowed to continue. Finally, the oscillation was removed to provide a final, static depth measurement.

Rapid probe penetration occurred after specimen oscillation was started and this was followed by a plateau region during which the fatigue process occurred. After a certain period, adhesion failure occurred and the probe displacement rapidly increased.