## 3 Results and Discussion
### Assessment of Tool Wear  
Hard turning faces severe tool wear issues in the absence of suitable cooling application. Therefore, the current research utilized a sustainable cooling technology (ZnO nano-cutting fluid through dual-nozzle MQL) to control the growth of wear in turning hardened bearing steel (57 \(\pm\) HRC). The tool wear numeric value was obtained using an optical microscope and is displayed in Table 1. It is evident that the tool flank wear was greatly subsidized due to the effective cooling and lubricating capability of ZnO nano-cutting fluid. The dual-nozzle coolant injecting system provided two directions of cooling (toward perpendicular to rake face and tool flank face); however, rapid heat transfer from the tool-work contact region to the environment was noticed resulting lower growth in tool wear. The obtained wear values (ranging from 0.027 mm to 0.095 mm) were much lower for hard turning concerns, which ensured the suitability of the TiAlN-PVD-coated carbide tool for machining hardened steel under dual-nozzle dispersed ZnO nano-cutting fluid. Moreover, wear mechanism was addressed using optical images, SEM images and EDS results. The optical images of tool wear (at 0.1 mm of depth cut) are displayed in Figure 4. It was noted that abrasion was the main wear mechanism responsible for growth in flank wear. Deep grooves on the tool edge were noticed due to the continuous flow of the heated chip over the tool edge, as shown in Figure 4. At the highest-speed (240 m/min) conditions, the highest tool wear was found. The least wear (0.027 mm) was produced when turning was executed at the lowest speed (70 m/min) with a combination of the highest depth of cut (0.2 mm), moderate feed (0.1 mm/rev), and the highest flow rate (50 mL/hr). Further, the fresh tool SEM and EDS tests were carried out and their results are disclosed in Figure 5. Moreover, SEM and EDS tests were also performed for tools having the least (Figure 6) and largest tool wear (Figure 7). From the fresh tool analysis, it can be determined that the tool has titanium (Ti), aluminum (Al), and nitrogen (N) elements, which are the constituents of the coating layer, TIAIN. Also, the elements carbon (C) and tungsten (W) are found in EDS test. The mapping result of the fresh tool (Figure 5) ensured the presence of 5% of C, 17% of N, 53% of Al, 25% of Ti, and 1% of W, while the lowest wear (Figure 6) tool contains 4% of C, 10% of N, 61% of Al, 21% of Ti, and 1% of O, Fe, Mn, and W each. Therefore, it can be stated that the worn tool lost some amount of C, N, and Ti elements due to friction from rubbing the tool tip with the workpiece. Similarly, the percentage amount of other elements like Al, O, Fe, and Mn were improved due to the adhesion of work materials or chip particles on the tool tip region. Similarly, if the results of spot 1 (Figure 6) were compared with a fresh tool, then it can be said that the % of C, Al, and W are increased on worn tools. The increment in C is possible due to the adhesion of the chip's portion or workpiece material (C is the element of the workpiece). The increment in Al and W are possible due to the increasing exposure of tool coating/tool substrate after the chipping of the tool edge. The other element like Ti has been greatly reduced due to the chipping of the tool. Moreover, the elements like Cr, Mn, and Fe are found in that region, which ensures the presence of workpiece elements in that region through adhesive action. Moreover, for a more in-depth analysis, the elemental analysis was accomplished at the edge of the tool as shown in spot 2 (Figure 6). The percentage of C and N was improved in comparison to the fresh tool. The percentage increment of C ensured the adhesion of workpiece elements, while N might be gained due to the exposure of the element N of the coating layer due to the rubbing action. Similarly, decrements in other elements like, Al, Ti, and W are noticed due to gradual wear occurring at the tool tip due to friction between the tool edge and the workpiece during cutting action.