New Process Improves Adhesion of Diamond to Cemented Carbide
Cemented carbide is a commonly used material for industrial cutting tools. While it is the tungsten carbide that lends this metal its hardness, the toughness that makes it equal to many tasks is ensured by the cobalt, which binds the tungsten carbide grains in the material structure. A diamond coating’s special hardness is designed to significantly increase the wear resistance of the tool once more. "But this is where the challenge lies, because the cobalt present in the carbide prevents stabilisation of the diamond structure during the coating process. Instead, a graphite-like structure of carbon is formed", explains Dr. Manuel Mee, a scientist of the Tribological Coatings group at Fraunhofer IWM. To prevent the cobalt from interacting with the diamond, the cobalt phase on the surface of the carbide tool is removed using a wet-chemical process. This type of pre-treatment, however, causes the peripheral zone of the cemented carbide to become porous and lose its toughness. The irregular stresses caused particularly while cutting inhomogeneous materials facilitate the breakdown of the peripheral zone, which is now more vulnerable. The result is that the diamond coating flakes off.
Dr. Mee’s new pre-treatment process combines various approaches into a single process in order to take into account all the factors which affect the adhesion of the coating. "With the new method we can now maintain the stability of the peripheral zone and even slightly increase it", Mee explains. The carbon is first removed from the surface of the cemented carbide at high temperature. Renewed enrichment with carbon then leads to a large proportion of the unwanted cobalt in the peripheral zone being vaporised, while improving the surface structure, hardness and fracture toughness. Wetting of the surface by a very thin cobalt film cannot be prevented, however. Mee has therefore added a further step to the process where the application of a silicon-based interlayer should finally keep the cobalt away from the diamond layer.
According to Fraunhofer IWM, every step of the process can be implemented with a microwave plasma, without having to interrupt the process chain, saving extra work steps and time. Once a tool becomes worn, its coating can be removed and the process applied again. This therefore reduces the material required. This is particularly beneficial in the case of the tungsten used in this process since this raw material is mainly mined in China and cannot be reliably obtained on the world market.