Enhanced adhesion of diamond coatings

The residual stresses in the diamond coatings are analysed using Raman microprobe spectroscopy, and compared with the predictions of the analytical model. The adhesion of the diamond coatings on various substrates with and without an intermediate layer of TiC is quantitatively evaluated by measuring the length of the delamination crack surrounding through-thickness holes in the coating and comparing with the relationship derived between crack length and strain energy release rate. The measured adherence on WC-Co substrates, as characterised by the critical strain energy release rate for growth of the delamination crack, was found to be significantly higher in the presence of the TiC intermediate layer developed during the course of this work. Diamond coatings are of interest for a wide range of applications due to the unique properties of crystalline diamond. Many applications require that the coating adhere strongly to metallic substrates which may have a large difference in thermal expansion coefficient with diamond. These substrates may also have undesirable chemical interactions with carbon during the deposition of the coatings. Intermediate layers are a possible solution to both of these problems. Such layers can act as diffusion barriers preventing the deleterious chemical interactions, and may help to accommodate the thermal expansion mismatch strains. Several aspects of these issues are addressed in this work. The requirements for an effective intermediate layer between diamond films and Fe-group containing substrate materials are described, and two potential layers identified: TiN and TiC. Crystalline diamond coatings are subsequently deposited on these layers by hot filament CVD. A large grained TiC coating with a relatively rough surface was found to provide the best adhesion to the diamond layer. As judged qualitatively by the extent of spallation adjacent to hardness indentation, this intermediate layer performs better than similar TiC layers reported in the literature. The mechanics of the interface for a coating-substrate system loaded by thermal expansion mismatch is modeled. Both continuous coatings and coatings containing a through-thickness hole surrounded by an annular delamination crack are examined. Analytic expressions for the stress distribution in the film and in the substrate are derived by representing the thermal expansion mismatch loads as tractions and moments acting along the outer free edge of the specimen and along the tip of the annular crack. The loads near the center hole are found to vary with the size of the delamination crack, and hence constitute a driving force for growth of such a delamination. The strain energy release rate for the growth of the annular crack surrounding the central hole is derived, and expressed in terms of the thermal expansion misfit between film and substrate; their thickness, elastic moduli and Poisson's ratios; and the characteristic dimensions of the film-substrate system. The crack driving force is found to decrease as the delamination crack surrounding the hole propagates, and hence a relationship between crack length and crack driving force is established.