CVD diamond coated cutting tools are used for machining of abrasive and hard materials such as Al-Si alloys and tungsten carbide. The knowledge of the mechanisms governing the diamond/substrate interfacial strength is crucial in cutting tools design. The most adequate substrate material for maximizing the adhesion of diamond films is the silicon nitride (Si3N4) ceramic that possesses a thermal expansion coefficient similar to that of diamond. Buchkremer-Hermanns and co-workers consider the formation of a SiC interlayer between diamond and Si3N4, which may favour chemical bonding to diamond, although they could not detect it by glazing incidence X-Ray diffraction. They believed that insufficient detection sensitivity for very thin films, texture effects or presence of amorphous layers are possible reasons. In the case of TiN substrates, a graded interlayer of amorphous TiCN of only 8 Å was suggested by Contreras, as observed by HRTEM images and EDS measurements. Due to the difficulty in the detection of such layers, which can be in the order of a few angstrons, a definite evidence of their nature is yet to be demonstrated.