Fig. 5 illustrates the evolution of flank wear as a function of cutting time at different cutting speeds and feed rates. This figure implies that with progression of cutting time, flank wear land increased. There are three distinct regions which can be seen in such wear curves. In the first region, the wear rate is relatively high due to wear of the tool layers damaged during manufacturing processes. The second region is the region of steady-state wear in which wear is uniform. In the third region of the wear curve, owing to high cutting forces, temperatures and severe tool vibrations, the tool wear rate is accelerated
Variation of cutting speed and feed rate affects the tool wear rate. As is depicted in Fig. 5, the increase of cutting speeds and feed rates led to a higher flank wear rate and consequently wear curves inclined to the left. This can be attributed to shorter contact area between chip-tool interface resulting in high concentration on temperature very close to the cutting edge
The SEM micrographs of worn tools at a relatively low (50 m/min), medium (70 m/min) and high (90 m/min) cutting speeds, 1 mm depth of cut and 0.15 mm/rev feed rate are shown in Figs. 6–8, respectively. According to these figures, the most dominant tool wear mechanism was adhesion. Adhesion is a result of welding of the work or chip material on the cutting tool. Although TiAlN coating of cutting tool suppresses the adherence of workpiece material on the rake and flank faces, further cutting led to flaking of the coating, due to the crack initiation at the interface between coating and substrate (Figs. 6–8). Flaking results in exposure of substrate of the tool to the freshly generated underside of the chip and consequently the workpiece material is readily to adhere on to the rake and flank faces under the conditions of high stresses and high temperature acting on the interfaces [32].
The adhesion of the strained-hardened workpiece material, causes formation of built-up edge (BUE) on the cutting edge and rake face of the cutting tool (Figs. 6 and 8). The thin layer of adhered material on the rake and flank faces changes the physical properties of the tool material and consequently reduces the tool capability [33]. BUE is not completely stable and when falls off, it takes away tool material and then results in chipping and crack formation in tool substrate [34]. In Figs. 7 and 8, the chipping of tool edge can be observed.
At 70 and 90 m/min cutting speeds, another tool failure mode was seen; namely, crater wear. Crater is widely observed at high cutting speeds and may be attributed to diffusion wear mechanism [15]. Excessive heat generation and close contact between the tool-chip and tool workpiece resulted from machining at high speeds accelerate diffusion process and consequently formation of cratering on the rake face of the cutting insert [7].