4.1. Validation of Workpiece Modification
Cutting force observation is an effective method to verify the influence of SAM on a work material. Figure 3
a shows the drastic reduction in cutting force by approximately 30% when the tool removes the surface-affected layer, and correspondingly produces thinner deformed chips (Figure 3
b) during orthogonal microcutting. Figure 3
c shows a top view of the chip formation from the orthogonal microcutting tests on iron with variations in chip morphology according to the order of surface-active media-affected sections. It is evident that the thicker chips exhibit multiple folds that are attributed to the plastic deformation of the chip, while an immediate change occurs from the mushroom-like features for the unaltered surface to the sharp edges for the surfactant-affected segments. This can be attributed to the embrittlement of the chips as a result of the reduction in plastic deformation (i.e., brittle failure of the chips) under the influence of the surface-active medium [33
Although not commonly reported, the mechanochemical effect also improves the machined surface quality [23
]. A sharp distinction in machined surface topography can be observed between that produced with and without SAM, as shown in Figure 4
. Unique surface defects in the form of dimples are prominent in the regions without ink, which is similar to the observations in the micromachining of AISI 1045 steel [34
]. In microcutting, considerations for material deformation characteristics of individual grains must be adopted, such that plastic deformation occurs by shearing along the slip planes of the grain crystallographic orientation. The randomly oriented grains then result in changes to the material removal process such as the deformation features in the chips [35
] and the machined surface [36
]. The misalignment of shearing forces for favourable slip deformation would result in machined surface defects [36
], taking the form of the dimples observed in Figure 4
. On the contrary, smooth defect-free machined surfaces can be produced with the application of ink, which suggests that the shearing action occurs with ease along the slip system under the mechanochemical effect. It is good to note that the transition to a smoother machined surface is not dependent on the coating thickness, which rules out the mechanical implications of the ink as a solidified coating that was observed by Lee et al. [37
] during the micromachining of brittle materials. The observations of the characteristic features of the Rehbinder effect in machining [26
] verify the effectiveness of the mechanochemical effect on the iron workpiece and serve as the fundamental mechanism to explain the differences in diamond tool wear.
4.2. Diamond Tool Wear
Significant tool wear can be observed on the flank face of all cutting tools after the diamond turning of iron over a cutting distance of 58.8 m. The wear patterns show vertical streaks perpendicular to the cutting edge, which is a characteristic feature for chemical tool wear on diamond tools. Although chemical wear is inevitable due to the high chemical affinity between the workpiece and diamond, the wear land is reduced under the mechanochemical effect as shown by the wear progression in Figure 5
The wear progression of the tool under conventional cutting conditions presents the typical flank wear development plot where rapid wear initially occurs within a short time, followed by a stable progression and finally ending off with an acceleration before failure. The increased rate of wear under conventional cutting occurs as the cutting distance goes beyond 42 m. In contrast, the wear development under the mechanochemical effect remains as a gentle inclined gradient, which indicates that the tool has not reached the critical stage before failure. The wear land, defined by the maximum height of the wear marks (VBmax
), is lower by 56% under the Rehbinder effect. Naturally, the lower tool wear led to a 27% improvement in the final surface roughness of 41.8 nm Ra as compared to 57.2 nm Ra under normal conditions (Figure 5
b). The produced surface roughness is dependent on the cutting tool geometry (i.e., wear morphology) and tool-tip vibrations induced by the material microstructure characteristics [38
]. Hence, the disparities in surface roughness are determined to be caused by the differences in tool profile as a result of tool wear and the enhanced material removal mechanism that reduces tool-tip vibrations.
The second set of tool wear tests, using 0.4 mm nose-radius cutting tools, demonstrates the magnitude of the mechanochemical effect on tool wear at different feed rates as shown in Figure 6
. Although the overall cutting distance for each cutting parameter was the same, the wear lands are drastically different with increasingly devastating wear at higher feeds regardless of any mechanochemical influence. This is due to the effectively larger volume of the engaged material that increases the cutting forces during machining as shown in Figure 7
, and translates to the overall heat generation that leads to tribo-chemical wear. Moreover, the cutting forces increase over time due to the degradation of the cutting edge geometry that results in the instabilities of the machining process, such as vibrations and the irregular build-up of work materials.
The machined surface is also presented for each cutting condition by pictorally examining the optical quality of the surface. Extremely poor surface conditions at a small feed of 5 μm/rev are observed before achieving near-mirror finishing at higher feed rates. This can be attributed to the relative tool sharpness size effect, which invokes rubbing as the material deformation process when the tool edge radius is significantly larger than the effective uncut chip thickness [39
]. Although it would be interesting to investigate further the correlation between the mechanochemical effect and the material removal mechanism, the topic is out of the scope of this paper, which focuses on the mechanochemical influence on diamond tool wear.
At higher feeds of 10 and 20 μm/rev, where shearing is the material removal mechanism, the reflected images appear to be clearer under the mechanochemical effect when compared with the surfaces produced by conventional cutting. In addition to the enhanced surface quality expected under the mechanochemical influence (Figure 5
), the clearer reflections are also results of the reduced wear lands. As the wear land increase, the machined surface quality is expected to deteriorate due to the degraded tool geometry that could change the material removal mechanism. Nonetheless, diamond tool wear is reduced, regardless of the material removal mechanism, with increasing feeds from 5 to 20 μm/rev, corresponding to magnitudes of 9.4%, 14.78%, and 16.15%. In the meantime, it is good to note that the difference in percentage wear reduction between the two different wear tests is in the size of the nose-radius, where wear rates are often higher for smaller nose-radii [40
As the spindle revolution was kept constant throughout the tests with the 0.4 mm nose radii tools, the change in feed rate changes the feed per revolution, which determines (i) the effective area of the uncut chip thickness during turning and (ii) the surfactant-affected length as shown in Figure 8
. These are two main factors that define the degree of influence that the mechanochemical effect has on the material response. The effective uncut chip thickness corresponds to the degree of plastic deformation during material removal, which would affect the performance of the mechanochemical effect. It was suggested that the physicochemical phenomenon is even more effective to cause embrittlement when dealing with ‘gummy’ metals that exhibit high plasticity during deformation and are challenging to cleanly cut [27
]. Therefore, a thicker uncut chip thickness that should correspond to a higher degree of plasticity in the chips would call for a more significant influence of the mechanochemical effect. The geometrical aspects between the feed and the effective uncut chip thickness have been covered in detail by Chaudhari and Wang [29
] and the principals may be applied to explain the different magnitude of tool wear reduction under the mechanochemical effect. In general, the increase in feed enlarges the effective uncut chip thickness and therefore improves the effectiveness of the mechanochemical effect due to the increase in plastic deformation.
The effective coverage of the surfactant during turning is directly proportional to the feed, such that larger feeds would encounter larger volumes of surfactant-affected material that would influence the manifestation of the Rehbinder effect. However, it is also important to consider the proportion of surfactant-affected material to the main bulk of material that is not covered with ink. Figure 9
illustrates the influence of feed and nominal cutting depth on the proportions of surface lengths that are under the mechanochemical effect. During turning, the length of surfactant-affected material is equivalent to the feed (f
) and the length of unaffected material p
when cutting with a round-nosed tool can be determined by Equation (A3) in Appendix A
. The surfactant coverage can then be calculated by Equation (2):
The percentage increase in surfactant coverage shows an increasing trend with a larger feed, similar to the trend observed in the reduction of cutting forces during the experimental wear test. This verifies the increasing influence of the mechanochemical effect with larger feeds. Curiously, the percentage reduction in wear does not directly coincide with the reduction in cutting forces and the influence of surfactant coverage. While other factors such as the tool-tip vibrations and the tribo-chemical nature of diamond tools may also participate in the wear progression, these factors should not affect the results to this degree. This intriguing question will be answered as the mechanism of the mechanochemical effect on tool wear is progressively discussed in the subsequent sections.
In the meantime, it may appear that larger feeds should be used during machining to a ensure wider coverage of the material under the mechanochemical effect, but the use of larger feeds would increase the theoretical peak-to-valley height of the machined surface and incur larger machining forces as observed in Figure 7
. Hence, it is suggestive to say that there exists an optimal balance between low feeds for better surface finishing and the proportion of surfactant-affected material, which can be achieved through a series of systematic controlled tests involving a large quantity of cutting tools.
While larger feeds may accommodate a larger proportion of surfactant-affected material, the nominal cutting depth also presents its significant influence on the proportion of surfactant coverage where the surfactant coverage increases as the cutting depth decreases. Herein lies a critical need to study the Rehbinder effect in micromachining due to the increasing influence of the mechanochemical effect as the cutting characteristic length decreases further into the microscale. To this end, the working principle of the mechanochemical effect on this work material must be evaluated before explaining the occurrence of wear reduction.