Toughening of chemically-vapor deposited diamond by means of active incorporation of carbon allotropes

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This research project aimed at toughening of chemically-vapor deposited (CVD) diamond by means of actively incorporating carbon allotropes besides diamond. We studied the following two points and have succeeded to develop CVD diamond with adhesive toughness more than twice as much as that of usual one.Elucidation and control for the toughening mechanism by means of the incorporation of non-diamond carbon allotropes The relations between deposition condition and crystalline structure, and between crystalline structure and strength were examined for the case of CVD diamond deposited under various conditions. As a result, it is for the first time clarified that non-diamond carbon concentrates on the grain boundaries which results in weaker strength of grain boundaries than that of grains, and that selective fracture of grain boundary dissipates the energy which leads to a higher adhesive toughness for the case of the adequate grain size. On the contrary, it was confirmed that toughness of CVD diamond films monotonically decreases with respect to the increase of non-diamond carbon with finer grain size.Realization of thin films with higher toughness by means of structure control Application of bias voltage to the substrates, which is an effective method to control the state of crystalline nuclei on the interface of CVD diamond and substrates, was studied. Together with the knowledge on the effect of deposition condition mentioned above, we examined the possibility to optimize the condition to obtain higher toughness. As a consequence, we succeeded to obtain a CVD diamond coating on tungsten carbide cutting tools with the adhesive toughness of 30 J/m^2 which is more than two times of the conventional one being currently used in industry.