It is known that when surface roughness is changed, that the adhesiveness of CVD diamond thin film also changes. However, it has not been clarified whether the cause of this is based on an increase of mechanical bonding strength or based on an increase in chemical bonding strength accompanying the change in nucleation density due to the change in surface roughness. To clarify whether the underlying factor is mechanical bonding strength or chemical bonding strength, nucleation density and adhesiveness were evaluated.
In order to evaluate the influence of surface roughness only, polishing was performed with single-phase Mo abrasive paper to prepare substrates with different surface roughness, and film formation of diamond thin films was done using the microwave plasma CVD method. Measurement of nucleation density of the diamond thin film was done using an SEM (Scanning Electron Microscope) and adhesiveness was evaluated using the ultrasonic cavitation method.
No significant difference in nucleation density was observed as a result of changing the surface roughness. However, ultrasonic cavitation tests revealed that the damaged area tended to increase as the surface roughness became smaller. In particular, it deteriorated sharply when the elevation difference of polishing marks was approximately equal to or less than the film thickness (Ra=200nm) of the diamond thin film. These results indicate that, rather than the influence of nucleation density, changing the surface roughness itself has a direct effect on the mechanical bond : when the elevation difference of polishing marks is relatively small compared to the film thickness, the mechanical bond (anchor effect) becomes weaker, allowing the surface to become damaged more easily.
