MECHANISTIC STUDY OF THE SYNERGISM BETWEEN SB2O3 AND MOS2 LUBRICANT SYSTEMS USING RAMAN-SPECTROSCOPY

A number of materials have been added to MoS2 to improve its lubricati ng properties. However, the mechanism underlying this improvement is n ot fully understood. Perhaps the most widely used and studied additive is Sb2O3. While not a lubricant itself, it acts synergistically with MoS2 to improve friction and wear properties. This paper is directed t owards developing a better fundamental understanding of the synergism between Sb2O3 and MoS2 in adhesively bonded films. A commercial prepar ation was used to form the films and laser Raman spectroscopy was used to analyze them before and after rubbing. It is shown that films laye r as a result of tribostress -MoS2 preferentially coats film surfaces and Sb2O3 becomes enriched in the next deeper layer. The mechanism pro posed to explain the synergistic behavior is that only the thin layer of MoS2 residing on top is exposed to degradation from the environment . The Sb2O3 layer acts as a thermal and oxygen diffusion barrier to re tard oxidation deeper into the film. Sb2O3 also acts as a beneficial s upport for MoS2 as was demonstrated earlier by Centers (Tribol. Trans. , 31/32 (1987) 149). The proposed mechanism suggests that protection i s provided against tribo-oxidation but not necessarily thermal oxidati on. To validate this concept, wear oxidation is evaluated in situ usin g a Raman tribotester and the results are compared with thermal oxidat ion data. Films containing Sb2O3 are indeed more resistant to tribo-ox idation than MoS2 films. It is also shown that the transfer film surfa ce is enriched with MoS2. This indicates that friction is primarily go verned by MoS2 rubbing against MoS2 and that Sb2O3 plays a secondary r ole.