CVD diamond tool performance in metal matrix composite machining

Metal matrix composite (MMC) has found increasing usages in the industry for lightweight high-strength applications. However, because of abrasive nature of the reinforced phase in MMC, machinability is poor, tool wear is rapid, yet only diamond tools are technically suitable to MMC machining. Furthermore, diamond coatings seem to be more economically viable than polycrystalline diamond for MMC machining. In this study, CVD diamond-coated tools, 30 mu m thick on a tungsten carbide substrates, were investigated by outside diameter turning of MMC of aluminum-alloy reinforced with silicon-carbide particles. Cutting conditions ranged from 1 m/s to 6 m/s of cutting speed, 0.05 mm/ rev to 0.3 min/rev feed, and I mm to 2 mm depth of cut. Tool wear was measured and compared at different machining conditions. Worn diamond-coated tools were extensively characterized by scanning electron microscopy. Cutting forces, chip thickness, and the chip-tool contact area were also measured for cutting temperature simulation by finite element analysis. The results show that tool wear is sensitive to cutting speed and feed rate, and the dominant wear mechanism is coating failure due to high stresses. The catastrophic coating failure suggests the bonding between the coating and substrate is critical to tool performance. High cutting temperatures will induce greater interfacial stresses at the bonding surface due to different thermal expansions between the coating and substrate, and plausibly result in the coating failure. A thermal management device, heat pipe, has been demonstrated for cutting temperature reductions. (c) 2005 Elsevier B.V. All rights reserved.