| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014) | |
| Thin Films | Monday Sessions |
| Session TF-MoM |
| Session: | Self-Organized and Nanostructured Thin Films |
| Presenter: | Albano Cavaleiro, University of Coimbra, Portugal |
| Authors: | A. Cavaleiro, University of Coimbra, Portugal M. Evaristo, University of Coimbra, Portugal T. Polcar, University of Southampton, UK |
| Correspondent: | Click to Email |
Transition metal dichalcogenides (TMD) have a layered structure and weak inter-layer bonding allowing to display very low friction coefficient when a tangential force is applied. Being sliding contact a surface phenomenon, these materials have been largely studied in the form of thin coatings. Whenever conditions exist for establishing stronger bonds between the layers, the friction coefficient can significantly increase. This is the reason why the industrial applicability of these coatings is still very limited due to the deficient tribological behavior in humid atmospheres, for which strong bonds can be formed through oxygen. In order to overcome this problem different approaches were followed based on alloying TMD with different elements. Among these elements, our group has developed a deep study on the addition of carbon and nitrogen. We have proved, as it was already known, that the friction coefficient could increase from the range [0.005 – 0.05] up to [0.05 – 0.3] when the coatings were tested in dry or in humid conditions, respectively.
The aim of this talk is to present a systematic study concerning the influence of humidity in the tribological behavior of TMD+C coatings. We deposited W-S-C coatings with increasing C content up to 60 at.% with two S/W ratios, close to 1.4 and 1.0. These coatings were tested by pin-on-disk in different humidity range from RH=20% up to RH=95%. Contrarily to what we have observed in previous studies, we could not find any case where the friction coefficient went down lower than 0.1. Furthermore, there was no clear trend on the effect of either the humidity or the S/W ratio on the friction coefficient. The detailed analysis of the sliding surfaces allowed to conclude that, in all tests, orientation of WS2 crystals in the top sliding contact could not be achieved. The comparison with previous deposited W-S-C coatings allowed to conclude that the different tribological behavior could be attributed to a different nanostructure arrangement in the as-deposited conditions.