Superlubricity of nanometer-thick Ag, Au films and the two dimensional structure of the slid surface - Study for designing a slide surface which works in space circumstance

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First objective of this work was designing a slide surface which works in a vacuum circumstance. We carried out sliding experiments and analyses of nanometer-thick Ag layers which was deposited on Si, and observed surface structures and characteristics of the slid surfaces. As a result, super lubricity (coefficient of friction : 0.007 at the minimum) was found on it.Crystal orientation of Ag and crystallographic analyses of the slid surface in ultrahigh vacuum were studied using Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), tunneling electron microscopy (STM), in-situ, and by synclotron orbital radiated X-ray diffraction (SOR-XD), ex-situ. These results revealed that the single atomic layers stack on the slid surfaces, and provided useful information to understand the mechanism of super lubricity. The results were summarized as follows,(1)If the polycrystalline film of Ag were deposited, Ag(111) planes have faced to random direction. During the … More reciprocal and repeated sliding cycles on it, Ag (111) sliding plane orientated parallel to the sliding plane. Ag crystallites also have grown 10 times in size and recrystallized at room temperature, during sliding, and C planes were stacked layer by layer parallel to the Si sliding direction, as was proved by STM and SOR-XD observations. Ag crystallites conducted, as if Ag films formed liquid layer, but the stacked Ag layers looked complete crystals without any amorphous fractions. Nearly two dimensional film of Ag metal were proved to have a melting point, as low as the room temperature. The observed superlubricity was understood due to the atomic mobility of Ag atoms along the Si(111) surface and on the Ag nm-thick layers.(2)The Ag atoms were mobile and showed super lubricant function. On the slid surface, therefore, no worn particles were found. The worn particles were supposedly reformed layer structure on the surfaces due to the mobility of the atoms. Ag films were not excluded out or exfoliated by sliding from the sliding tracks, over 1000 cycles of reciprocal sliding.(3)The results may applicable for sliding surface in space, because the experiments were carried out under ultrahigh vacuum. The test specimens were clean that the test results were reproducible without any contaminants on the surfaces. The results were applicable to clean room machines for semiconductor production, and textile products which use non-oil lubricants.(4)The results provided very important and fundamental understandings of super lubrication, and chemical interaction between contact surfaces as the most important factor to reduce friction. Less