Adhesion evaluation of CVD diamond films and metal reinforced composite diamond films

Poor adhesion of diamond films limits the use of CVD diamond films as coatings for cutting tools. The adhesion of these films is limited by stresses in the film caused by thermal expansion mismatch between the substrate and the film and by voids present at the interface due to the morphology of the crystal growth. A three step process of making diamond composite films has been developed, involving nucleation of individual diamonds on the substrate, electroplating a metal binder in the voids between the crystals, and lastly growing a complete film over the composite layer. The metal binder acts both to fill the voids at the interface and to absorb energy during fracture processes at the interface. Diamond growth was performed in a DC Triple Torch reactor using a mixture of methane and hydrogen with a molybdenum substrate. Measurements to determine the amount of improvement of the film adhesion have been performed. These tests include indentations using conventional hardness testing equipment and four point bend tests with the film in tension and compression. A correlation is shown between the plastic zone of the substrate and the area of the film which delaminated during indentation. Bend tests with the film in tension did not delaminate the film, instead the film underwent intergranular fracture. Bend tests in compression act similarly to pile up around an indentation, and cause film delamination, Residual stress measurements in the single step film show a compressive stress of 650 MPa.