J. Sacramento

Wear resistant CVD diamond tools for turning of sintered hardmetals

Abstract Sintered hardmetals are very hard materials that are usually machined using diamond grinding wheels and electro-discharge machining. Dry cutting with super-hard cutting tools like cubic boron nitride (c-BN), polycrystalline diamond (PCD) and chemical vapour deposition (CVD) diamond is an ecological alternative to reduce operation times and, therefore, to improve the productivity. In the present work, cylindrical forging dies of WC–27 wt.% Co hardmetal grade were turned at fixed operating parameters (cutting speed=15 m/min; depth-of-cut=0.2 mm; feed rate=0.03 mm/rev.) using CVD diamond tipped hardmetal inserts. Commercial PCD and c-BN inserts were tested for comparison. The cutting tool behaviour was studied in terms of both the tool wear and the finishing quality of the workpiece. The tool damage was evaluated using a special probe for edge roughness evaluation, together with scanning electron microscopy observations. The CVD diamond tools survived the task showing slight cratering, whereas flank wear was the main wear mode for the other superhard tools. Amongst all the tested tools, PCD presented the worst performance in terms of tool wear and workpiece surface quality. Furthermore, the operation time was reduced to one tenth with respect to conventional diamond wheel grinding.

Abrasive Resistance of CVD Diamond Brazed Thick Films in Machining of WC Pre-Sintered Forms

Machining of cemented carbide pre-sintered forms is a strong abrasive operation, leading to significant expenses on tool consumption. To overcome this problem, the industry often uses very hard polycrystalline diamond (PCD) inserts. However, the presence of a soft binder in the PCD diamond layer weakens its abrasion resistance. An alternative to PCD is the use of cobalt-free CVD diamond films as cutting tools. In this work, CVD diamond films brazed to metal holders were used as cutting tips in machining of green WC forms and the performance compared to PCD. Diamond films of thickness ~0.4mm were grown by microwave plasma assisted CVD (MPCVD) on silicon substrates. The wafers so obtained were processed into small triangles (tips), through laser cutting and subsequent removal of the silicon substrate using chemical etching. The tips were then brazed to steel holders with reactive Ag/Cu/Ti filler metal. Abrasion resistance was tested in rough turning conditions on pre-sintered and isostatic pressed cemented carbide bars at 10ms -1 of cutting speed. The worn surfaces were observed by SEM. The onset of flank wear on PCD inserts was detected at about 30 hours of working time, while the cutting edge of the CVD diamond tool kept its sharpness even after almost 40 hours. Micro-cracking and crystal pullouts at the cutting edge were identified as the main wear mechanisms of the PCD tool.