Study on Surface Interaction between Atomic Oxygen and MoS_2 and Ultra Low Friction (ULF) Using Their Recombination PRocesses

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The atomic oxygen generator/ultra-high vacuum tribometer/Auger emission spectrometer has newly been designed and manufactured, and tribolocal properties of single crystal, sputtered, and organic binded MoS_2 have been investigated. In order to simulate the environment in low earth orbit, the energy and flux of atomic oxygen beam were set at 5 eV and 5.5*10^<15>atoms/m^2・_S. For MoS_2 singlecrystal, friction coefficient increased from the initial value of 0.03 to larger values, and after 5 hours'irradiation, the friction coefficient over 2.0 resulted. None of S and Mo Auger peaks were identified from that surface, and transfer of Ti, which is the mating material to MoS_2, was detected. Similar tendency was recognized for sputtered MoS_2, and it was found by AES and XPS that the increased friction is caused by the formation of MoO_3, MoO_2, and MoO_<x2>. Though organic binded MoS_2 showed a relatively high friction coefficient of 0.05 before exposure to atomic oxygen, the friction coefficient after 5 hours' exposure to atomic oxygen was low at 0.1. For organic binded films, C and O incorporated in polyamide-imide may have a possible suppressing action from the gasification of S.The oxydized layr for organic binded MoS_2 was thin, so that more stable tribological properties of organic binded MoS_2 than those of single crystal and sputtered MoS_2 can be caused by less gasification of S.By designing a sleeve near the MoS_2 specimen, a recombination of atomic oxygen into molecular state was attempted. Friction experiments at this environment showed a good value of 0.01. In addition, an introduction of CO gas enhanced the ultra low friction, but the irradiation of UV increased the friction. Laser desorption spectroscopy inside the AES chamber detected SO_3 during exposure to atomic oxygen. It was concluded that the ultra low friction requires the stabilization of S on MoS_2 surface and the mesoscopic analysis of surface integrity of MoS_2 crystal.