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3.3. Wear mechanisms of PCBN tools
Presence of a deposit of wear products on the rake face, when machining with PCBN tools, is typically attributed either to formation of low-melting-point eutectic between tool and workpiece materials which is subsequently ejected from the cutting zone, or to chemical reactions of tool material with workpiece, coolant, etc. [16], or both. As the tool material is a composite with cBN and binder grains, chemical reactions and eutectics with both can be decisive wear mechanisms. Indeed, Klimenko et al. [16] have shown formation of Fe-Fe2B eutectics with cBN, while Gimenez et al. [17] found formation of Fe-C perlite-like structures of as a result of interaction of steel with TiC binder of PCBN insert. Other reactions of cBN with Ni (Ni3N), Mo (Mo2N), Cr (Cr2B), etc. in superalloys are possible at temperatures developing in the cutting zone [3].
Thermal cracking on PCBN tools; (b) coating delamination on the minor cutting edge
Focused Ion Beam milling of the crater has revealed that cracks are not superficial and extend deep into the tool bulk. Cracks are perpendicular to the edge line, have significant length and stretch outside crater and flank wear land. Cracking was not found to lead either to edge chipping or tool failure, probably due to its minor intensity, averaging 2-4 cracks per edge. Figure 5.a also depicts the edge line where it can be seen that edge radius (rβ) has smaller size than for a new tool. 3-D optical measurement of edge radius for worn out tools has confirmed that radius decreases, as compared to the original size, down to rβ=5-15 μm for both tool types. Closer to the minor cutting edge, where wear land becomes uniform and absent of grooving, edge radius increases up to rβ=25 μm. Delamination of coating in the same minor cutting edge region was observed, Figure 5.b. Removal of coating in the surface formation region of the edge can play both positive and negative role. It leads to decrease in the size of edge radius and through that to reduction of h1min and ploughing effect. This plays a positive role in the formation of surface roughness and subsurface deformation [14]. Additionally, removal of the coating, serving as the thermal barrier, is expected to reduce local temperature in the region, which normally is attributed to formation of undesirable tensile residual stresses [15]. On the other hand, application of TiN coating reduces friction coefficient and through that
. SEM and EDX analysis of deposit of wear products on CCBN rake (vc=250 m/min, f=0.15 mm/rev)
Figure 6 presents SEM image of a fracture surface of the rake face of coated PCBN insert showing tool bulk, coating and the deposit of two layers of wear products. Two layers are a result of two passes during which the tool life criterion of VBmax=0.3 mm was reached. It can be seen that the deposit has a porous structure consisting of inpidual particles bound together. According to conclusions of Klimenko et al. [16] such structure can be a result of ejection of eutectic melt from the cutting zone in the shape of droplets and subsequent their reaction with environment and coolant. EDX analysis of the wear products has detected high concentrations of iron and oxygen implying formation of iron oxides. Evident peaks corresponding to aluminium and titanium come from the binder of tool material consisting of TiC, TiB2 and small concentrations of Al2O3 and AlN, see Figure 1.b. Presence of Mg, Ca and F is a result of dilution of coolant concentrate with hard water, which has relatively high
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concentrations of Ca2+ and Mg2+ ions (>100 ppm) in the region.
Several studies [5, 13] addressing hard machining of highly alloyed steels have identified formation of holes on surfaces of worn tools, which was attributed to diffusion-or chemical-related decomposition of the binder and successive adhesive pull out of cBN particles. Atomic Force Microscopy (AFM) was applied to analysis of topography of worn surfaces in order to identify if similar wear mechanisms take place when machining Ni-based Inconel 718. Figure 7 presents the results of AFM taken on the tool crater and on the wear land. Prior AFM the tool was subjected to etching with Kallings #2 etchant affecting only matrix -phase of superalloy. On the contrary to the hard machining, topography of the crater proved to be very smooth and without appreciable damages. Severe grooving on the tool flank (see Figure 4) posed a limitation to the size of AFM image, see Figure 7.b. Behavior opposite to the one found on the crater surface and in typical cases of hard turning was observed. Figure 7.b clearly shows significant amount of protruding particles of tool material, thus making the assumption of dominance of adhesive wear mechanism not sustained.