Page 21 - Demo

19CUTTING MATERIALSCVD utilizes chemical reactions in a gas environment at temperatures of about 900-1000%u00b0C (1650-1830%u00b0F) that facilitate high coating adhesion. This process is well suited for multiple coating by different coating material. CVD forms a relatively thick coating that increases wear resistance and hightemperature strength of a coated cemented carbide. At the same time, the process requires strict thermal control because the high environmental temperature may affect the carbide microstructure and thus diminish the cutting performance.Historical Notes: Introduction of CoatingsDespite the CVD coating that was applied to cemented carbide tools in the 1960s, most hard metal tools continue to be uncoated for quite some time. To make cemented carbides more universal and applicable to machining various engineering materials, tool manufacturers invented hard metals that contained various additives. The progress in CVD processes and the adoption of PVD coating method for cutting tools in the 1980s dramatically changed the world of machining; now most cemented carbides are coated. The addition of this new technology permitted the cemented carbides to focus on cutting specific material groups. The substrates contained fewer additives; therefore, their structures became more uniform and stable, which further improved control during production.The temperature of the PVD method, based on vacuum deposition processes such as sputtering, evaporation, cathodic arc deposition and others, is approximately twice as low when compared to CVD. This eliminates changes in the metallurgical properties of a carbide substrate. Compared with CVD, PVD coatings are thinner, and therefore perfect for tools that require sharp cutting edges, such as solid carbide endmills or threading tools. However, a line-of-sight character of common PVD techniques can lead to variations in coating thickness. To overcome this problem, the coated tools are mounted on specially designed movable parts of PVD coating equipment. On the other hand, the areas of cutting tools that do not require coating (the shank of a solid drill, for instance) can be masked when PVD methods are applied, while for CVD technology this is problematic.Combining CoatingMethodsModern technology allows both methods %u2013 CVD and PVD %u2013 to be combined for coatings of replaceable cutting inserts, as a means of controlling coating properties. ISCAR%u2019s carbide grade DT7150 features a tough substrate and a dual MT CVD and TiAlN PVD coating. This was originally developed to improve the productive machining of special-purpose hard cast iron.Reducing the risk of negatively affecting the carbide microstructure changes with CVD method %u2013 MT CVD (%u201cMT%u201d stands for medium or moderate temperature). It utilizes lower temperatures %u2013 around 800%u00b0C (1470%u00b0 F).