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Tool life comparison for uncoated and coated tools under variation of cutting speed: (a) vc=250 m/min; (b) vc=300 m/min; (c) vc=350 m/min
Relatively low performance of coated PCBN tools in terms of roughness is partly countered by higher tool life. Figure 3 presents the tool life comparison between coated and uncoated tools for all test conditions at wear criterion of VBmax=0.3 mm. Tool life for PCBN tools proved to have a limited dependency on feed rate, showing a small (about 15%) reduction in with increase in feed. This can be explained by the simultaneous action of mechanical and thermal loads. According to Proskuriakov [4], when finish turning Ni-based superalloys with PCBN tools,
doubling feed rate leads to increase in cutting temperature by approximately 40–60 C. This is expected to intensify tool material softening and chemical wear for PCBN tools.
It can also be seen (Figure 3.a) that at speed 250 m/min coated PCBN tools have approximately 20% longer tool life than uncoated. The gap is rapidly closing with increase in speed and becomes negligible at speed 350 m/min. This behavior can be explained by the fact that the protective mechanism of TiN coating has a temperature-limited range. It is known [10] that titanium nitride begins to oxidize with formation of rutile (TiO2) even at 650 C, yet the reaction achieves significant intensity at temperature above 1000 C. According to Proskuriakov [4] cutting temperature reaches this level at speed around 240–270 m/min for PCBN tools with high cBN content and correspondingly high thermal conductivity.
The same effect is attributed to the significant decrease in tool life with the increase in cutting speed (see Figure 3). Increase in speed from 250 m/min to 350 m/min leads to a drop in tool life by more than 250%. At these temperatures chemical wear due to reactions of cBN with alloying elements (Cr, Ni, Fe, Nb, etc.) is believed to be one of the main wear mechanisms when machining Ni-based superalloys [3].
3.2. Morphology of tool wear
Appearance of intensive rake cratering and grooving on the tool clearance (see Figure 4) is normally attributed to chemical wear when machining superalloys [11]. It can be seen that grooving has varying intensity along the edge line. Closer to minor cutting edge grooving ceases and flank wear becomes uniform. Such behavior closely follows temperature field found in hard machining with PCBN round tools and tools with large nose radius [12]. Increase in the cutting speed and application of coating leads to extension of grooving to the minor cutting edge and increase in its depth.
(a) SEM of worn out UCBN tool (vc=250 m/min, f=0.1 mm/rev);
(b) SEM of worn out CCBN tool (vc=350 m/min, f=0.1 mm/rev)
Additionally to grooving, deposits of wear products were found on the rake face of PCBN tools, their intensity increasing with cutting speed. Apart from the above defects PCBN tools have experienced fracture beneath the wear land, Figure 4.b. For both uncoated and coated tools fracture was found to be dependent mostly on the cutting speed. Feed has a less significant influence on fracture.
V. Bushlya et al. / Procedia CIRP 3 (2012) 370 – 375 373
When the tool life criterion (VBmax=0.3 mm) was reached, cutting conditions of above vc≥300 m/min and f≥0.15 mm/rev resulted in an appearance of fracture on the tool flank, yet severe fracture was observed under vc=350 m/min and f≥0.15 mm/rev for both coated and uncoated PCBN tools.
No transfer layer was observed on the crater or on the wear land, which is regarded as a typical feature of tool wear in hard machining of alloyed steels [13]. Several other tool deterioration mechanisms were observed: thermal cracking and delamination of coating (see Figure 5), but their influence on the tool life was limited.
shearing force acting on the tool clearance face and machined surface. This in turn is expected to reduce subsurface deformation of the machined component.