The mechanical properties of freestanding near-frictionless carbon films relevant to MEMS

Abstract

Amorphous, diamond-like carbon films with a mixture of sp2 and sp3 hybridizations have exhibited excellent material properties such as chemical stability, wear resistance and optical transparency resulting in their wide use as protective coatings in numerous applications. The hydrogenated forms of these films, a-C:H, and specifically the near-frictionless carbon (NFC) films developed at Argonne National Laboratory have exhibited the lowest ever recorded friction coefficient, 0.001, and ultra-low wear rates of 10−11−10−10 mm3 N−1 m−1, even under dry sliding conditions and at very high contact pressures (Robertson 2002 Diamond-like amorphous carbon Mater. Sci. Eng. R 37 129). Application of these films to sliding or rotating microelectromechanical systems (MEMS) would open up an entirely new class of commercialized MEMS devices. With this in mind, this paper reports on thin-film mechanical property measurements of the NFC films relevant to MEMS. The membrane deflection experiment was employed to subject microfabricated freestanding films to pure tension and measure mechanical properties such as Young's modulus, residual stress and fracture strength. Young's modulus was consistently measured at 35.13 ± 2.29 GPa. The fracture strength varied from 0.12 GPa to 0.90 GPa and the residual stress state was compressive and ranged from 79 MPa to 310 MPa. Width and thickness effects of the membranes were also observed in this work, where fracture strength increased with decreasing membrane width and thickness. Weibull analysis of the fracture strength is also presented in the paper.