Atomic-modeling and Simulation of Copper Sulfide as Micro Solid Lubricant
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Atomic-modeling and Simulation of Copper Sulfide as Micro Solid Lubricant Tomohiro Sato1, Yoshimasa Hirai1, Takehisa Fukui1, Takahiro Ejima2, Masanori Takuma3 and Ken-ichi Saitoh3 1 Technology Development Division, Kurimoto, LTD, 2-8-45 Shibatani Suminoe-ku, Osaka-shi, Osaka 5590021, Japan 2 Graduate School of Science and Engineering, Kansai University, Suita-shi, Osaka 5648680, Japan 3 Department of Mechanical Engineering, Kansai University, Suita-shi, Osaka 5648680, Japan ABSTRACT Many industries have developed new materials as substitutes for lead in solid lubricants. For example, lead bronze, a Cu-Pb alloy that has been used for slide bearings, has been replaced by a Cu alloy containing sulfide. The development of Pb-free Cu alloys has received considerable attention recently. Pb and sulfide are types of solid lubricants; in particular, MoS2 (molybdenum disulfide) is a popular sulfide lubricant. This study focuses on a material that contains Cu2S. The properties of Cu2S as a solid lubricant are unknown. First principles (FP) and molecular dynamics (MD) are used to clarify the lubrication mechanism of Cu2S. The atomiclevel stable structure of Cu2S is evaluated by FP under specific sliding conditions. The Cu2S lubrication mechanism is clarified by MD using the FP results for the interatomic potential functions. It is clarified that there is a specific slip system and the Cu-S bonds that exists above and below the layers of the slip system are very strong. INTRODUCTION Recently, practical applications of lead-free Cu alloys have been promoted. For example, sulfide dispersed Cu alloys have been developed for slide bearings as tribology assemblies [1]. These alloys primarily containing Cu2S have good sliding properties under specific conditions [2]. Figure 1 shows the results of energy dispersive spectroscopy (EDS) analysis. Each bright color indicates the peak of elements. EDS and X-ray diffraction results indicate that the developed Cu alloy contains Cu2S. However, the lubrication mechanism of Cu2S is unknown, and it is difficult to clarify the mechanism experimentally. The properties of the sulfide dispersed in the alloy could not be evaluated because it could not be extracted. In contrast, the sliding properties of MoS2, a popular sulfide lubricant, can be evaluated macroscopically and microscopically [3] because MoS2 has a hexagonal crystal structure and can be considered a single substance. The lubrication mechanism of MoS2 is based on exfoliation between S atoms layers. In this study, an atomistic model of Cu2S is constructed and first principles (FP) and molecular dynamics (MD) are used to clarify the lubrication mechanism of Cu2S. In a simulation, Cu2S can be treated as a single substance. The simulation procedure is as follows. 1. Determination of atomistic Cu2S structure, 2. Evaluation of stable structure of Cu2S by FP, 3. Evaluation of the mechanism of Cu2S lubrication by MD.
(b) Cu (c) S (a) SEI image Figure 1 EDS analyses of developed Cu alloy. Each bright color indicates the peak of elements.
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