| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Wednesday Sessions |
| Session TF-WeP |
| Session: | Thin Films Posters Session II |
| Presenter: | Junko Masuda, Tohoku University, Japan |
| Authors: | J. Masuda, Tohoku University, Japan K. Adachi, Tohoku University, Japan T. Takeno, Tohoku University, Japan |
| Correspondent: | Click to Email |
Molybdenum disulfide (MoS2) is widely used as a solid lubricant in vacuum. It produces very low friction due to its lamellar structure. However, one of the drawback of MoS2 is weak for oxidation. Recently, the coating that MoS2 nanoparticles are dispersed into diamond-like carbon (DLC) host matrix is proposed to overcome this problem utilizing gas barrier effect of DLC in addition, its hardness increases . Such coating is called MoS2-containing DLC (MD). The coating shows lower friction, m ~ 0.01, than MoS2 in vacuum when the coating is prepared under specific condition. When it shows low friction, unique transfer layer onto counter surface is formed. The structure of the transfer layer is consisted of 5 nm carbon layer on counter surface and mixed layer composed of MoS2 and C on it. These facts clearly indicate that such specific structure is organized thanks to the coating. We have also demonstrated that low friction can be achieved depending not only on MoS2 concentration but also bias voltage during deposition. Carbon matrix is important for achieving low friction. It is indicated that in order to form unique transfer layer, carbon has something potential to support MoS2 to arrange lamellar structure. The purpose of this study is to clarify mechanisms of low friction.
Friction tests were conducted with ball-on-disk type apparatus in well-controlled environmental conditions. Dry nitrogen, dry oxygen and humid nitrogen gases are used to change atmospheric condition during friction tests. Various observation techniques including scanning electron microscope (SEM) and transmission electron microscope (TEM) equipped with energy dispersive X-ray spectrometer (EDS) are employed in this study.
Under nitrogen gases, both MoS2 and MD coatings show low friction with vacuum. Actually, it is natural that MD coating shows low friction in nitrogen because MoS2 and DLC give low friction in this condition. In case of oxygen condition, MoS2 shows higher friction coefficient, ~ 0.2, in oxygen condition than in vacuum. On the other hand, friction coefficient of 0.02 is achieved utilizing MD coating, and very thin transfer layer was formed on the counter surface, which clearly shows the potential for low friction utilizing MD coating. A cross-sectional TEM image of the layer shows that structure of the transfer is also same as the one obtained in vacuum. In addition, EDS mapping on the TEM image revealed that oxygen from atmosphere is included in the whole transfer. It suggests that composite structure of MD coating can contribute the formation of a thin transfer layer.