Elastic constant of nanocrystalline diamond film

Abstract

Three elastic stiffness coefficients of nanometer grain-size polycrystalline CVD diamond (NCD) films and micrometer grain-size diamond (MCD) film were measured by resonant-ultrasound spectroscopy coupled with laser-Doppler interferometry (RUS/Laser method). The diagonal stiffness coefficients C 11 and C 44 of the NCD films are found to be smaller than those of bulk polycrystalline diamond and MCD film, and decrease as the film thickness decreases. The off-diagonal coefficient C 12 of the NCD films was much larger than those of the bulk diamond and MCD films. Large C 12 was consistently explained by the micromechanics model with random-oriented pancake-shape graphite plates at the grain boundaries. This indicates that the NCD films consist of sp 3-bonded diamond grains and sp 2-bonded grain boundaries. Raman spectra showed two peaks due to the D-band and G-band of sp 2-bonded graphite phase. These peaks increased with an increase of C 12. Thus, the coefficient of C 12 can predict the grain boundary chemistry or structure of the CVD polycrystalline diamond film.