Topographic Changes on the Surfaces of PVD Coatings in Humid Air: an AFM/LFM Study

Tribological investigations of self-mated physical vapour deposition (PVD) coatings have shown in part very low friction in humid air and a strong dependence of the tribological behaviour on the relative humidity during the test. This behaviour can be interpreted in terms of the growth of surface layers, which act as solid lubricants and reduce friction and wear considerably. The use of surface sensitive analytical methods like SEM, XPS and AES for getting more information about this kind of surface layer, however, can have an adverse effect on these layers because of the necessary vacuum conditions. Therefore, atomic force microscopy (AFM) and lateral force microscopy (LFM) were used to study the topographical changes of two different PVD coatings under the influence of humidity. The aim of this work was to verify and to explain the well-examined macroscopic behaviour of two different engineering coatings on a microscopic scale. Because of the spatial resolution of the LFM technique it is possible to discern species with a different frictional behaviour on the same surface. This allows the beginning of (tribo)chemical reactions to be detected locally in contrast to conventional integrating tribological testing. Depending on the chemical nature of the PVD coatings and the stock holding period in certain environments, topographical changes can be detected by means of AFM/LFM. Under humid conditions small hillocks (droplets) begin to grow on TiN surfaces, the number and size of these hillocks depending on the duration of stock holding. On CrN surfaces this hillock formation does not occur even after extended storage for several weeks in humid air.