Influences of repetition rate of arc discharges on hardness and modulus of ultrananocrystalline diamond films prepared by coaxial arc plasma deposition

Abstract

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. The UNCD 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 nm and from 71 to 62%, respectively. Near-edge x-ray absorption fine-structure and Fourier transform infrared spectroscopic measurements revealed enlarged σ*C–H and sp2-CH peaks, respectively. The formation of sp2-CH bonds at grain boundaries owing to the reduced grain size results in degraded hardness.