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

本课题旨在研究化学气相沉积（CVD）金刚石的增韧，方法是在金刚石外主动加入碳同素异形体。我们对以下两点进行了研究，成功地开发出了粘结韧性比普通金刚石提高一倍以上的CVD金刚石。以不同沉积条件下的CVD金刚石为例，研究了沉积条件与晶体结构、晶体结构与强度之间的关系。首次阐明了非金刚石碳集中在晶界处，导致晶界强度弱于晶粒强度，在适当晶粒尺寸的情况下，晶界的选择性断裂耗散能量，使晶界具有较高的粘接韧性。相反，证实了CVD金刚石薄膜的韧性随晶粒尺寸越细的非金刚石碳的增加而单调降低。研究了在衬底上施加偏置电压是控制CVD金刚石与衬底界面晶核状态的有效方法。结合上述关于沉积条件影响的知识，我们考察了优化沉积条件以获得更高韧性的可能性。因此，我们成功地获得了碳化钨刀具上的CVD金刚石涂层，其粘接韧性为30 J/m^2，是目前工业上使用的传统涂层的两倍多。

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.