Analysis of fracture behavior of thin polycrystalline diamond films

The effect of the substrate temperature and CH₄ concentrations on the fracture behavior of thin polycrystalline diamond films was systematically investigated by X-ray diffraction and scanning electron microscopy. The results show that the fracture behavior of thin polycrystalline diamond films synthesized by direct current plasma jet chemical vapor deposition is closely related to the substrate temperature and CH₄ concentrations. A high substrate temperature, due to difference in the thermal expansion coefficients of the substrate and the diamond film, causes thin polycrystalline diamond films to generate high residual stresses, which usually exceed fracture strength of thin diamond film and even that of diamond. The fracture toughness is found to drop with the increasing ratio of CH₄ concentration. In case of high CH₄ concentrations, various defects and impurities, such as cracks, microscopic holes, graphite, and amorphous carbon were observed in the films. Thus, the substrate temperature and CH ₄ concentrations should be strictly controlled within an appropriate range