Superhard cutting tool coatings: CVD diamond and PVD cBN

The performance of diamond coated tools highly depends on the properties of tungsten carbide substrates as well as on the pre-treatment. It could be proven that the hardness, the cobalt-content and the surface roughness of the substrate exert the decisive influence on the layer adhesion and on the resistance against surface fatigue. During highly frequent mechanical load, the diamond coating adheres better, if a lower cobalt-content of the substrate and a lower surface roughness are given. A deeper etching during the pretreatment in order to remove cobalt from the sub-surface as well as a thicker layer also have a positive influence on layer adhesion. The influence of the layer morphology could not be proven clearly. The friction behaviour of diamond coated tungsten carbide tools is influenced by the surface condition and the substrate properties only to a small degree. In single stressing three-point bending test, tungsten carbides show the same behaviour, whereas a higher surface roughness generally shows a lower firmness due to its stronger notch damage behaviour. The pre-treatment has an influence on the strength. However, the thermal stress of the tungsten carbide caused by the CVD process is negligible. Tungsten carbides with a higher hardness, bending strength and a medium cobalt-content could be a possibility to overcome the current limitation of diamond adhesion of geometrically highly complex tools. An initial analysis of the cutting process regarding an abrasively effective aluminium silicon alloy shows the potential of these tools. The cBN tool coatings are eligible for the machining of various workpiece materials even at high cutting speeds. Besides the cBN layers show a high resistance against mechanical wear, which was proven in model tests. The cBN layer system show excellent properties in comparison to conventional coatings. The high intrinsic compressive stress of cBN layers, which were deposited by PVD, do not represent a problem for the subsequent treatment anymore. Concepts to use suitable hard material interlayers offer a high capability for the machining of hard workpiece materials. On the basis of the machining tests on alloyed steel and on chrome steel it could be shown that a cBN coating exhibits a large capability for the machining of such materials due to its high hardness and its high temperature stability. In case of milling application with a TiAIN cBN coating the IWF and the IST currently adjusting the layer system. Additionally, in further research the deposition and evaluation of cBN coatings on geometrically complexindexable inserts and on ceramic substrates is intended. The combination of the advantages of a tool coating on a complex formed tool and this high efficient cutting material cBN could become the first choice for many branches of industry in future. The considerable mechanical characteristics of ceramic could be combined by the coating ofceramic tools with those of the cBN layer system. Entnommen aus <a_href="http://www.fiz-technik.de/db/b_tema.htm" target="_blank">TEMA</a>