Elastic constant of nanocrystalline diamond film

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 C11 and C44 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 C12 of the NCD films was much larger than those of the bulk diamond and MCD films. Large C12 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 sp3-bonded diamond grains and sp2-bonded grain boundaries. Raman spectra showed two peaks due to the D-band and G-band of sp2-bonded graphite phase. These peaks increased with an increase of C12. Thus, the coefficient of C12 can predict the grain boundary chemistry or structure of the CVD polycrystalline diamond film.