Mechanical properties of diamond films: A comparative study of polycrystalline and smooth fine-grained diamonds by Brillouin light scattering

Brillouin light scattering, Raman light scattering and x-ray diffraction we re used to investigate the elastic and microstructural properties of polycr ystalline and smooth fine-grained diamond films of varying diamond quality. They were deposited on a titanium alloy by a two-step microwave plasma-ass isted chemical vapor deposition process at 600 degreesC. Their morphology a nd roughness were studied by scanning electron microscopy and atomic force microscopy. Their refractive indices were determined by the Wine spectrosco py technique. The diamond purity of all these coatings in terms of the sp(3 ) bonding fraction was deduced from visible and UV Raman spectroscopy as a function of the deposition conditions. All the samples were found to be tex tured with a (011) crystallographic direction normal to the film plane, lea ding to essentially hexagonal symmetry of the elastic tensor. By taking adv antage of the detection of a number of different acoustic modes, complete e lastic characterization of the films was achieved. The elastic constants C- 11 and C-66, respectively, were selectively determined from the frequency o f the longitudinal and shear horizontal bulk modes traveling parallel to th e film surface. The three remaining elastic constants, namely, C-44, C-33 a nd C-13, were obtained from detection of the Rayleigh surface wave a bulk s hear wave and the bulk longitudinal wave propagating at different angles fr om the normal to the surface. The values of the elastic constants depend on the deposition conditions and on the microstructural properties of the fil ms, especially the diamond quality and the polycrystalline or smooth fine-g rained nature of the diamond. For the polycrystalline diamond film with the best quality, the elastic constants are rather close to the Voigt or Reuss average estimate values using known bulk elastic constants of diamond, whe reas those of the smooth fine-grained diamond films are reduced because of the poorer diamond quality leading to lower residual stress in the films. ( C) 2001 American Institute of Physics.