Grain boundaries and mechanical properties of nanocrystalline diamond films

Phase-pure nanocrystalline diamond thin films grown from plasmas of a hydrogen-poor carbon argon gas mixture have been analyzed regarding their hardness and elastic moduli by means of a micro indenter and a scanning acoustic microscope. The films are fully elastic, superhard and the moduli rival single crystalline diamond In addition, Raman spectroscopy with an excitation wavelength of 1064 nm shows a peak at 1438 1/cm and no peak above 1500 1/cm, and X-ray photoelectron spectroscopy a shake-up loss at 4,2 eV. This gives strong evidence for the existence of solitary double bonds in the films. The hardness and elasticity of the films are then explained by the assumption that the solitary double bonds interconnect the nanocrystals in the films, leading to an intergrain boundary adhesion of similar strength as the intragrain diamond cohesion. The results are in good agreement with recent simulations of high-energy grain boundaries.