Quantification of physical properties of thin films

Experimental work was undertaken to study the physical properties of thin layered coatings for optical applications. The properties of particular interest were wear, friction and adherence. A computer-controlled wear tester containing a mechanical probe, having a sapphire stylus, was used to make scratches on combinations of silver and ZnO films deposited on glass. Scratches were made under controlled loads of 0.05 and 0.1 N. During an experiment, both the load and the lateral force were measured at a high sampling rate from which the coefficient of friction was calculated. The volume of the material worn during repeat scratches was deduced from profilometer traces. The coefficient of friction data showed that, for silver on glass, there was the characteristic saw-tooth wear pattern which flattened out for multiscratches. For silver on ZnO, the Coefficient of friction decreased with the number of scratches indicating removal of silver. This was confirmed by scanning electron microscopy-energy-dispersive X-ray analysis. For ZnO/Ag/ZnO, the initial coefficient of friction was low, but it increased as the silver layer was approached, showing the typical saw-tooth behavior before reaching a constant value comparable with that for ZnO. The wear data showed the differences in the manner in which wear occurs for the different materials. Both ZnO and ZnO/Ag/ZnO tended to exhibit brittle wear whereas silver showed an adhesive wear pattern. It was also found that, in most cases, ZnO tended to remain adherent to glass even when subjected to a severe scratch test.