Wüest M. 51 Wykes M. 82 Yamaguchi M. 17 Ybarra G. 129 Yubero F ...

JUNE 28 WEDNESDAY MORNING
RIVA-SS-WEM-OR.8 THE TRIBOLOGICAL BEHAVIOUR OF W-S-C FILMS IN PIN-
ON-DISK TESTING AT ELEVATED TEMPERATURE. M. Evaristo, T. Polcar, A. Cavaleiro,
ICEMS -Faculdade de Ciências e Tecnologia da Universidade de Coimbra, Dep. Eng. Mecânica, Rua
Luís Reis Santos, 3030-788 Coimbra, Portugal
Transition metal dichalcogenides (TMD) are well known for their self lubricant properties, due to
unique crystal structure. However, the structure of TMD presents some drawbacks, particularly, they
have low hardness making them inappropriate to applications requiring high load bearing capacity.
Moreover, TMD films exhibit morphologies with high porosity promoting oxidation, particularly in
humid atmosphere or elevated temperature.
Several approaches have been used to increase the mechanical properties of TMD thin films. One of
the most successful is either doping TMD with a metal, which increases hardness, or the deposition
of an interlayer improving adhesion. MoS 2 is one of the most studied TMD, showing good results
when deposited with an interlayer and doped with Ti, Cr, and other metals.
There is a reference in literature showing that WS 2 has higher oxidation resistance than MoS 2 (about
100ºC), therefore, it is a good candidate for films with good tribological behaviour at high temperatures.
Previous works showed that the hardness and adhesion of W-S films doped with C or N deposited
with a Ti interlayer significantly increased compared to pure WS 2 . The analysis of friction and
wear behaviour at elevated temperatures was the main aim of this study.
W-S-C films were deposited by magnetron sputtering in an Ar atmosphere with a Ti interlayer. A
carbon target with several pellets of WS 2 incrusted in the zone of the preferential erosion was used.
The number of pellets was changed to modify the carbon content in the films, which varied from 26
up to 70at.%. Doping W-S films with carbon lead to a substantial increase of the hardness in the
range 4 to 10GPa; the maximum of hardness was obtained for coatings with the carbon content of
40at.%.
The XRD diffraction patterns showed that there was a loss of crystallinity with the increase of the
carbon content in the film. SEM images of the cross section of the films showed decrease of the dimensions
of the columnar structure and decrease of the porosity as the carbon content in the films increased.
The coatings were tested by pin-on-disk from room temperature (RT) up to 400ºC. At RT, the friction
coefficient was in range 0.3-0.1, and the wear rate decreased with the carbon content. At temperatures
higher than 100ºC, the friction is below 0.1 for all compositions, which can be explained
by the atmosphere drying.
The tribological behaviour of the coatings with increasing temperatures depends on the films carbon
content. For low carbon content up to 40at.%, the wear rate was almost independent of the temperature
up to 300º, while it increased dramatically in case of coatings with high carbon content. The
highest carbon content (∼70at.%) coatings were peeled off from the substrate at temperatures exceeding
100 ºC. In general, the limiting temperature for W-S-C coatings is 400ºC.
96