Hardness and fracture roughness of CVD diamond film

The hardness and fracture toughness of a chemically vapor deposited (CVD) diamond film were measured by the Vickers indentation method. A free-standing 90 mu m thick diamond film with a grain size of approximately 25 mu m was tested. For comparison, the (011) plane of a natural type Ia diamond single crystal was also tested. The indentation testing of the diamond crystal and CVD diamond film was performed using a microhardness tester under a load of 5 N. The indent size was used to measure the hardness. From the length of radial cracks emanating from the corners of the indent, the fracture toughness was evaluated. To reveal the indents and radial cracks, the samples were etched in KNO3 melt at about 600 degrees C. The hardness of natural diamond varied from 64 to 67 GPa. In natural diamond, long radial cracks emanate from the indent corners. The length and direction of propagation of these cracks depend on the orientation of the indent. After prolonged etching we observed the emergence of internal cracks under the indent. There was evidence of stress-corrosion cracking owing to a high level of residual stresses around the indents. The hardness value of 75 GPa was obtained for CVD diamond film. In contrast to natural diamond, the CVD diamond film, which we tested does not show as a rule any radial cracks emanating from the corners of the Vickers indent (at 5 N load). This is likely to be due to a high level of internal compression stresses in the diamond film. After etching, stress-corrosion cracking around indents in CVD diamond was observed. Stress-corrosion cracking at fine grain groups, which was not associated with the hardness indents, was also observed. This fact supports the presence of high internal stresses in CVD diamond film.