In long-lived ball bearings operated in vacuum, there apparently exists a synergism between a pre-coated MoS2 film and a transfer film of the PTFE composite. The objectives of this study are to confirm this synergism in friction tests, and to examine how the transfer film improves tribological performance. Roll/slide friction tests were carried out in vacuum using a two-disk configuration. A PTFE composite (PTFE + glass fiber + Mo) pin was rubbed against one disk on the roll/slide friction track. The test results showed that the transfer film on 440C steel disk gave relatively high coefficient of friction, over 0.25, but when the disk specimens had a sputtered MoS2 film, the duration of the low friction period, under 0.02, was dramatically extended by rubbing the PTFE composite pin. The synergism between a sputtered MoS2 film and the PTFE composite was clearly demonstrated. To examine what happened at the interface, another type of roll/slide friction tests with a disk/roller configuration was performed in a preparation chamber of XPS, and the friction track of the roller was analyzed without exposure to the atmosphere. A PTFE composite pin was rubbed against the disk. The XPS analysis showed that metal fluoride, possibly iron fluoride, was easily formed on 440C steel disk and transferred to the roller surface. When the disk and the roller had a sputtered MoS2 Nm, the Life of the MoS2 film remarkably extended by rubbing the PTFE composite pin, the same as in the case of the roll/slide friction tests with the two-disk configuration. Nevertheless, no fluorine peak was detected on the roller surface in the XPS analysis, indicating that the PTFE composite did not react with, nor transfer on the MoS2 film. This suggests that both the transfer of PTFE and the formation of metal fluoride were not the essential factor for the synergism. Other mechanical and/or chemical factors seem to enhance the durability of the MoS2 film, although the mechanisms have yet to be clarified. (C) 1999 Elsevier Science S.A. All rights reserved.
