Wear resistant CVD diamond tools for turning of sintered hardmetals

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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.

Introduction

More and more, ecological demands are imposing dry conditions in machining processes due to the pollution effects of the cooling lubricants and electrolyte liquids. In particular, for hardmetal parts finishing, efforts must be put to replace electro-discharge machining (EDM) and diamond wheel grinding. In fact, both are time-consuming techniques, the former being very expensive due to the electrodes cost, while the latter suffer from tools short lifetime, the wheels requiring frequent dressing and sharpening. Superhard cutting tools appear as an alternative for machining of sintered cemented carbide, a very hard and abrasive material. This kind of application is not referred in the literature, although some tool manufacturers include in their brochures polycrystalline diamond (PCD) and cubic boron nitride (c-BN) grades for cutting hardmetal alloys. Chemical vapour deposition (CVD) diamond is a more recent superhard tool material. Compared to PCD, CVD diamond is harder and exhibits a lower friction coefficient, higher abrasion resistance, higher thermal conductivity, and better chemical and thermal stability. It lags behind PCD only in terms of fracture toughness (KIc=5.5 MPa m1/2 for CVD instead of KIc=9 MPa m1/2 for PCD [1]). Other drawbacks associated to PCD tools are the presence of a cobalt binder, which limits the cutting speed, and its high cost [2]. The excellent thermal conductivity of CVD diamond [3] allows the tools to run in dry conditions without generating harmful levels of heat.

CVD diamond tools can be produced by direct coating or by vacuum brazing free-standing slabs onto tip inserts. These materials can be used mainly in the machining of non-ferrous metal alloys and fibreglass reinforced polymers [4], [5], [6], [7], [8], [9], [10], [11], [12]. In the machining of Al–Si alloys with directly coated CVD diamond on Si3N4 or WC–Co, the main wear mechanism is micro attrition whereby individual grains or agglomerates are plucked out on the tool flank [4], [6], [11]. The exposed substrate areas grow rapidly until a critical size is attained, suddenly leading to tool failure. A second failure mechanism reports the appearance of cracks near the cutting edge originating the coating breakthrough at the intersection of the micro-fracture pattern of two cracks [4]. In this application, diamond coated tools showed a dramatic betterment on the cutting action when compared to the uncoated material [5]. Vandevelde et al. [7] also reported that CVD diamond coated WC–Co tools are a good alternative to PCD tipped tools for dry machining of Al–Si metal matrix SiC reinforced composites. The limit of tool life is induced by breakage of the cutting edge while small built-up edges are observed. CVD diamond protects other crucial parts of the tool, as chip breakers, leading to a better workpiece surface finishing. The main wear mechanism of diamond coated ceramics during the machining of the fibre reinforced plastics is flank wear by the abrasive effect of the reinforced fibres [6]. The performance of CVD diamond brazed tools on cutting Al–Si alloy seems to be better than that of PCD tool which is attributed to the softening effect of Co in the latter [10], [12]. PCD displays crater wear as a result of grain pullout after intergranular Co wear, whereas CVD suffers chipping of individual grains on the tool flank. The roughness of the Al–Si alloy turned surface was also better in case of the CVD diamond brazed tools than the PCD tool [10], [12].

In the present work, CVD diamond thick films were grown, cut and brazed onto WC–Co insert holders in order to evaluate its performance in turning of sintered WC–27 wt.% Co workpieces. This hardmetal grade is often used in the fabrication of dies for metal forming processes. The roughness of both the cutting tool edge and the machined surface were monitored along the experiments. Wear mechanisms are discussed and compared with those found on PCD and c-BN superhard tools with the same geometry.

Section snippets

Experimental

The CVD diamond cutting inserts were fabricated by brazing free-standing diamond plates to hardmetal insert holders. The diamond films were grown on silicon wafers by microwave plasma CVD, in a ASTeX PDS 18 equipment, with the following growth parameters: microwave power=4.25 kW; total pressure=14.67 kPa; H2/CH4 flow rate=400/33 sccm. A total deposition time of 48 h allowed the growth of a ∼300-μm thick diamond film that was laser cut (pulsed Nd-YAG 1064 nm, P=50 W, v=10 mm/s, ν=2500 Hz) in the

Results and discussion

Due to its hardness and high abrasive nature, the machining of sintered cemented carbide is a severe task, opening a new applicability field for superhard tools. Under the referred experimental conditions, CVD diamond withstands this operation without debonding from the insert support or suffering any other catastrophic failure, as depicted in Fig. 1a. Moreover, edge retention without signs of flank wear or chipping is patent in Fig. 1b. The main wear mode of CVD inserts is cratering on the

Conclusions

The dry machining of sintered WC–Co forging dies was successfully accomplished with CVD diamond brazed tools. A tenfold decrease on the machining time compared with the conventional diamond wheel grinding method reveals the high potential of the proposed solution.

The CVD diamond tools survive the hard cutting operation without any signs of flank wear or chipping. The main wear mode is cratering on the rake face (KM ∼55 μm) with an almost negligible depth (KT ∼2 μm). The highly abrasive

Acknowledgements

The financial funding from FCT Project POCTI/1999/CTM/35940 and POE/SIME no 50/126 is gratefully acknowledged.

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