The tribological behavior of carbon/silicon bi-layer coatings deposited on a silicon substrate by DC magnetron sputtering was assessed and compared to that of amorphous carbon and silicon coatings. The motivation was to develop a wear resistant coating for silicon using thin layers of amorphous carbon and silicon. Wear tests were conducted by sliding a stainless steel ball against the coating specimens under applied normal loads in the range of 20 50 mN. Results showed that the wear rate of the bi-layer coating was strongly dependent on the ratio of thickness between the carbon and silicon layers. The wear rate of the bi-layer coating with 25 nm thick carbon and 102 nm thick silicon layers was about 48 and 20 times lower than that of the single-layer amorphous carbon and amorphous silicon coating, respectively. In addition, the steady-state friction coefficient of the bi-layer coating could be decreased to 0.09 by optimizing the thickness of the layer. Finally, a model for the wear reduction mechanism of the carbon/silicon bi-layer coating was proposed.
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Finally, Fig. 8c depicts the model of wear for the bi-layer C/Si specimen with the optimum thickness ratio. The relatively stiff and hard amorphous silicon coating is supported by a compliant amorphous carbon coating just below. Upon contact with the pin under the applied normal load the coating is elastically deformed sufficiently to distribute the load over a larger contact area. This allows the contact stress to be maintained below the critical stress level needed to cause significant wear. Also, the inherently low friction coefficient of the amorphous silicon coating can be achieved over a prolonged period since wear debris is not sufficiently generated to cause plowing. As can be seen in the SEM image of the wear track shown in Fig. 6d and illustrated in Fig. 8c, a distinct wear track is not formed and only evidence of slight burnishing wear can be found. Thus, with the optimum thickness ratio between the two coating layers, friction and wear can be minimized by lowering the interfacial shear stress and distributing the load over a larger area.
Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2012-0001232).
All Science Journal Classification (ASJC) codes
- Mechanics of Materials
- Mechanical Engineering
- Surfaces and Interfaces
- Surfaces, Coatings and Films