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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. |
