Scratch testing is frequently used to assess the behaviour of thin films under the influence of an applied load. In particular, scratch testing can be used to assess coating adhesion when this is the failure mode occurring. Experimentally, an increasing load may be applied to a spherical diamond indenter whilst the specimen is moved. The onset of adhesion failure is normally identified through subsequent examination of the surface by optical microscopy, or by continuously monitoring the frictional force or acoustic emission.

The critical load for adhesion failure is easiest to identify in the case of a hard, relatively brittle film on a softer substrate. For soft films on hard substrates, film ploughing through to the substrate often occurs before failure of the adhesive bond.

Scratch testing as usually performed (i.e., where the applied load is increased until coating failure is clearly detected) may not always be a realistic guide to what might be expected in practical applications. Here, the loads are generally smaller than the scratch test critical load, and, furthermore, may be applied repetitively over the same area. The NanoTest repetitive scratch test is a complementary technique which may elucidate such effects.

The sample investigated consisted of a hydroxylapitite coating deposited on a titanium alloy. Scratches were performed in the NanoTest instrument with a 200 µm radius Rockwell C diamond. The scratch length was 1 mm, the scan speed was 5 µm.s^-1, and the applied load was increased linearly to about 200 mN. On-load probe displacement and tangential frictional force were monitored continuously.

To eliminate background effects due to the expected gradual probe penetration, differential changes in displacement and friction are used.

Two sets of probe displacement and frictional force data are shown below. In each case, significant changes in both parameters are evident at intermediate loads. The tendency towards broad topography peaks with increasing load is often due to material pile-up and adhesion failure. On the other hand, minimal topography changes together with large friction changes are indicative of ploughing through the film to the substrate surface.

Such results are clearly statistical in nature. However, as with indentation mapping, repetition of nano scratch tests and averaging of the data is an extremely useful indicator of film consistency.