| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Wednesday Sessions |
| Session TF-WeP |
| Session: | Thin Films Posters Session II |
| Presenter: | Takanori Takeno, Tohoku University, Japan |
| Authors: | T. Takeno, Tohoku University, Japan T. Hayase, Tohoku University, Japan K. Adachi, Tohoku University, Japan |
| Correspondent: | Click to Email |
Water is environmentally friendly liquid. When we used it as a lubricant, it gives very low friction. Silicon carbide and silicon nitride can produce very low friction, < 0.01, under water lubricating condition. However, one of the drawback is low load carrying capacity due to low viscosity of water. With increase of load, direct contact between the materials occurs, which leads to seizure. Some researchers have shown increase of load-carrying capacity thanks to DLC that can reduce friction under direct contact condition. Although it works as protective coating to avoid seizure, friction under such severe condition is still high, ~0.1-0.2. So, the target of this study is to produce the coating to show low friction and high load carrying capacity under water lubricating condition. To achieve the goal, we are developing SiC-containing DLC coatings.
Investigated coatings are prepared by RF generated methane plasma and DC magnetron co-sputtering of silicon carbide target (Purity: 99.99%). Silicon carbide disk is used as a substrate material. Various coatings are deposited by changing methane and argon flow rate (CH4/Ar). Composition, structural analyses and hardness are measured by Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS) and nanoindentation technique. Friction tests are performed using ball-on-disk tribometer. Silicon carbide ball is used as a counter material. Ball and disk are immersed in water and friction tests are carried out.
By changing flow rate of methane and argon, various coatings with different composition are achieved. Hardness of the coatings decreases with increase flow rate. Friction properties also vary with composition of the coatings. With CH4/Ar = 0.1, coating is removed at very beginning of the friction test. When the coating is deposited with CH4/Ar = 0.2, very low friction of 0.05 can be achieved. Almost similar friction curve is obtained with further increase of the ratio but fluctuation of the friction coefficient is also observed with CH4/Ar = 0.3. It suggests that these is optimum structure or composition of the coating. We also changed speed and load for friction tests. Surprisingly, friction coefficient of the coating with CH4/Ar = 0.2 decreases with increase of load and decrease of speed condition, meaning lower friction is obtained with more severe contact condition. According to the literature, lowering friction coefficient is due to the termination of OH bonds by reacting SiC surface with water. Thanks to the hydrogen bonding between O from OH and H from H2O, water is captured on the frictional surface and friction force can be derived from shearing the low viscos water.