Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films were prepared at different repetition rates of arc discharge by coaxial arc plasma deposition. With increasing repetition rate from 5 to 20 Hz, the hardness decreased from 23 to 11 GPa. TheUNCD grain size and sp3/(sp2 + sp3) estimated from powder X-ray diffraction (XRD) and X-ray photoemission spectra decreased from 2.6 to 1.8 nmand from 71 to 62%, respectively. Near-edge X-ray absorption fine-structure and Fourier transform infrared spectroscopic measurements revealed enlarged σ∗C-Hand sp2-CH peaks, respectively. The formation of sp2-CH bonds at grain boundaries owing to the reduced grain size results in degraded hardness.
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