Study on tool wear mechanism under cryogenic CO2-assisted minimum quantity lubrication technology

Cryogenic CO2-assisted minimum quantity lubrication milling technology is a green processing technology with broad application prospects. Aiming at the problem of tool wear in the application of cryogenic CO2-assisted minimum quantity lubrication in difficult-to-machine materials and the influence of relevant parameters on tool wear, this study used coated cemented carbide tools to perform milling experiments under cryogenic CO2-assisted minimum quantity lubrication technology conditions. The micro-morphology of the tool and chip was observed, and the energy spectrum of the tool chip contact area was analyzed. The results show that reducing CO2 temperature and increasing the oil flow of minimum quantity lubrication can improve the tool wear. The tool wear mechanisms under cryogenic CO2-assisted minimum quantity lubrication are mainly abrasive wear, diffusion wear, and oxidation wear. The chip sawtooth degree of the optimal parameter group is more conducive to chip breaking than that of dry-cutting and wet-cutting groups. The temperature of the tool-chip contact area is an important factor affecting tool wear; the higher the temperature, the faster the tool wear. At the same time, it is verified that cryogenic CO2-assisted minimum quantity lubrication technology can replace cutting fluid in hard-to-machine materials under certain conditions.


Introduction
The use of cutting fluids is widely in machining. However, conventional cutting fluid cooling has high consumption and production costs [1][2][3], causes serious environmental pollution, and harms the health of operators [4][5][6]. Green cutting technology is developed to solve these problems. At present, green cutting technology mainly includes dry cutting, minimum quantity lubrication (MQL), cryogenic cutting, cryogenic minimum quantity lubrication, and so on [7,8]. In the machining of difficult-to-machine materials, dry cutting has the lowest cost, but it has high-cutting temperature and cutting force [9,10]. MQL can be applied to the machining of aluminum alloy, stainless steel, and carbon structural steel [11,12]. However, it is prone to insufficient cooling, and the cooling effect is poor when the cutting temperature is high [13][14][15]. Cryogenic cutting technology includes cryogenic CO 2 cooling, cryogenic nitrogen cooling, and cryogenic air cooling. Quite a few examine have researched that cryogenic cutting technology is mainly used in the processing of titanium alloy, magnesium alloy, superalloy, and highstrength steel [16][17][18], which can improve the tool life and processing quality [19]. However, the lubrication effect of cryogenic cutting is unfavorable [20,21]. Previous studies of the machining of difficult-to-machine materials have presented that minimum quantity lubrication technology at low temperature has significant advantages in improving the processing quality and tool life [22][23][24]. Cryogenic minimum quantity lubrication technology is a method combining cryogenic cutting and minimum quantity lubrication, which includes cryogenic air-assisted MQL, cryogenic nitrogenassisted MQL, and cryogenic CO 2 -assisted MQL. In the machining of difficult-to-machine materials, although the cryogenic air-assisted MQL can reduce the tool wear and improve the tool life, the cooling effect is limited [20]. Cryogenic nitrogen-assisted MQL can reduce the cutting force and improve processing quality, but the effect of reducing tool wear is less impressive [25,26]. However, previous studies of the cryogenic CO 2 -assisted MQL have emphasized favorable advantages in reducing tool wear and improving tool life [27][28][29]. In addition, cryogenic CO 2 -assisted MQL has the advantages of low cost, non-toxic, and non-pollution, and has widespread use in practical production. The use of cryogenic CO 2 is beneficial to the realization of carbon neutralization and peak target to a certain extent [29].
However, cryogenic CO 2 -assisted MQL is a green cutting technology that is more likely to replace cutting fluid, and there are a few pieces of research in the field. In this study, the effect of cryogenic CO 2 temperature and minimum quantity lubrication oil flow in milling experiments of difficult-to-machine materials was examined. In regards to this study, the tool wear form, wear mechanism, and chip morphology in the cryogenic CO 2 -assisted MQL were studied, compared with those in the wet and dry cutting. This work provided a theoretical basis for the application of cryogenic CO 2 -assisted MQL in the processing of difficult-to-machine materials.

Materials and methods
In this study, N87 alloy steel was used as the workpiece, which is a typical difficult-to-machine material. The main chemical composition of N87 alloy steel is presented in Table 1, and the mechanical and physical properties of N87 are shown in Table 2. The APKT1604PDTR KC725M cemented carbide tool and MIRCOLUBE CRYO 75 minimum quantity lubrication oil were selected for the milling experiments. The cutting tool geometric parameters and coating structure are shown in Fig. 1; the two coatings were TiN and TiAlN, whose thickness was 1 µm and 5 µm, respectively. All the milling experiments were carried out in the VMC-1600 CNC of Shenyang, and the cutting parameter is shown in Table 3.
As shown in Table 4, different CO 2 temperatures and minimum quantity lubrication oil flow were designed in the milling experiments. In addition, the experiments of dry cutting and wet cutting were carried out, which have been seen as comparative milling experiments. The cutting process is shown in Fig. 2. The angle and position of nozzle and

The tool wear form of rake face
In Fig. 3, the wear form of the rake face under dry cutting was shown, and the overall wear was serious. Due to the high-cutting temperature and poor lubrication conditions, the coating of the main cutting edge was peeled off seriously, and micro-chipping occurred in the fillet area of the tool tip. As seen in Fig. 4, the wear form of rake face under wet cutting was normal wear of cutting edge and peeling of the coating. This benefited from the low temperature and favorable lubrication of the cutting area. Figures 5,6,and 7 show the wear form of rake face under cryogenic CO 2 -assisted MQL experiments when oil flow was kept unchanged at 60 ml/h and cryogenic CO 2 temperature was set to − 10 °C, − 30 °C, and − 50 °C. As shown in Fig. 5, the main cutting edge of the tool under − 10 ℃ had a large area of chipping, and the coating of the minor cutting edge peeled off seriously. Figure 6 clearly shows that the micro-chipping and crater wear appeared on the main cutting edge of the tool under − 30 °C, and the serious coating material damage appeared in the fillet area of the tool tip. In addition, when the CO 2 temperature was − 50 °C, the main cutting edge of the tool was in good condition and the coating was worn evenly, as shown in Fig. 7. It can be found that the tool wear decreases gradually with the decrease of cryogenic CO 2 temperature when comparing the tool wear forms of three experimental groups. When the cryogenic CO 2 temperature was − 50 °C, the tool wear was lighter than that of − 10 °C and − 30 °C.
The reason is that the heat transfer between cryogenic CO 2 and tool-workpiece follows Newton's convection heat transfer formula [30], as shown in Eq. (1), and the heat exchange is proportional to the temperature difference. So, the temperature  difference in the cutting area is larger if the cryogenic CO 2 temperature is − 50 °C, which is conducive to heat transfer and the cooling effect is obviously better. In addition, the viscosity of the lubricating oil will decrease under high temperature and pressure, which will lead to the thinning of the lubricating oil film in the cutting area, and then the lubrication may fail. However, the oil film thickness can be kept in an effective lubrication state under the appropriate cryogenic CO 2 temperature, and the cryogenic CO 2 carrier can also make the oil mist reach the cutting area stably, which can make the lubrication conditions well [20].
where q is the heat flux (W/m), h is the surface heat transfer coefficient (W/(m 2 ·K)), A is the heat transfer area (m 2 ), and ΔT is temperature difference (°C).
(1) q = h ⋅ A ⋅ ΔT Figures 7,8,9,and 10 show the wear form of rake faces under cryogenic CO 2 -assisted MQL experiments when cryogenic CO 2 temperature was kept unchanged at − 50 °C and oil flow was set to 0 ml/h, 40 ml/h, 60 ml/h and 80 ml/h. As seen in Fig. 8, the crater wear and coating peeling appeared on the main cutting edge of the tool when the oil flow was 0 ml/h. In addition, the local coating peeling occurred on the minor cutting edge. As illustrated in Fig. 9, although the tool crater wear was reduced, the coating peeling was still uneven and also occurred on the minor cutting edge. However, when the oil flow is 60 ml/h, the main cutting edge was worn evenly without abnormal wear, as shown in Fig. 7. In Fig. 8, the wear form of the rake face when oil flow was 80 ml/h is shown. Although the wear of the main cutting edge was uniform, the coating peeling was more serious than that when oil flow was 60 ml/h and the small range of coating peeling occurred on the minor cutting edge. Compared with the tool wear forms of the four experiment groups, the tool wear was generally reduced with the increase of minimum quantity lubrication oil flow. The main reason for the improvement of tool wear is that the lubrication conditions in the cutting area become better with the increase of oil flow. Thus, the friction between the tool and workpiece or chip is reduced, and the tool wear is improved. However, when the lubricating oil flow is too large, the particles of oil mist will increase, making it more difficult for oil mist to enter the cutting area. Consequently, the lubrication effect was decreased and the tool wear was increased.

The tool wear form of flank face
In this section, the wear form of the tool flank face under different cooling and lubrication was analyzed. Figures 11  and 12 show the wear form of the tool flank face under dry cutting and wet cutting. The main wear form of tool flank face was abrasion, chipping, crack, and notching in dry cutting. This was because there is no cooling and lubrication in the cutting area, which leads to high temperature and high friction in the contact area between the tool and workpiece, and the tool wear of flank face was more serious in the cutting process. Due to the good cooling and lubrication condition of the cutting area, the temperature of the contact area between the tool and the workpiece was relatively low, and the friction was small, so the wear of flank face of wet cutting was light and uniform. Figures 13 and 14 show the several typical wear form of flank face under cryogenic CO 2 -assisted MQL. And the tool wear land width (VB) of different experimental groups is shown in Fig. 15. The VB of experiments 3 and 4 was relatively large, especially experiment 3, and the wear form of flank face was mainly notching, crack, and abrasion, similar to that shows in Fig. 14. This is because the temperature of cryogenic CO 2 was insufficient, and the cooling effect on the tool flank face is poor. In addition, most of the lubricating oil  volatilizes before reaching the contact area between the tool and workpiece due to the high temperature, which cannot achieve the lubrication effect. The wear form of other experimental groups was similar to that shows in Fig. 13. It can be seen from Fig. 15 that the temperature of cryogenic CO 2 had a great influence on the VB value of the tool flank face, and the VB value decreased with the decrease of cryogenic CO 2 temperature. The reason is that when the temperature of CO 2 decreased, the cooling effect of cryogenic CO 2 on the tool flank face became better, and the temperature of the cutting area was decreased. At the same time, with the help of cryogenic CO 2 , the lubricating oil could reach the cutting area for lubrication. And the influence of lubricating oil on VB value was relatively small.
Based on the above analysis of rake face and flank face, it can be found that the experimental group in which oil flow is 60 ml/h and the cryogenic CO 2 temperature is − 50 °C has the lightest tool wear on the rake face and the light wear of flank face, and the experimental parameters are optimal. In the next section, the tool wear mechanism of dry cutting, wet cutting, and the optimal parameter array of cryogenic CO2-assisted MQL will be studied. Figures 16,17,and 18 show the comparison diagram of the rake face before and after tool etching in the experiments of dry cutting, wet cutting, and the optimal parameter array of cryogenic CO 2 -assisted MQL. Through the comparison, the chip adhesion and adhesive wear area can be clearly seen. In the cutting process, the high-cutting temperature and high pressure will be produced in the tool-chip contact area zone, resulting in cold welding, which makes the workpiece material easy to bond to the cutting edge of the tool [31,32]. The adhesion workpiece material fells off under intermittent mechanical impact load during the high-speed milling, and the coating material will be taken away, which results in adhesive wear on the tool surface. With the progress of cutting, the bonding and peeling process of workpiece materials are repeated, resulting in the aggravation of adhesive wear. As shown in Fig. 16, the adhesive wear of the tool during dry cutting was serious on the main cutting edge and the fillet area of the tool tip, and also occurred on the minor cutting edge. In addition, the damage caused by bonding and tearing on the fillet area of the tool tip is also shown in Fig. 16. This was mainly due to the poor cooling and lubrication condition, which leads to the high temperature under alternating mechanical stress and thermal stress. In this case, the bonding of workpiece material was more serious under high temperature and pressure, and the adhesion layer was easier to fall off during processing due to the high friction in the tool-chip contact area, resulting in serious adhesive wear. As seen in Fig. 17, the adhesive wear of the tool was relatively light under wet cutting cooling and the chip adhesive area was small and mainly occurred on the main cutting edge. This benefitted from the favorable cooling and lubrication condition and the low temperature and friction in the tool-chip contact area zone. Figure 18 shows the tool adhesive wear of the optimal parameter array (oil flow 60 ml/h, CO 2 temperature − 50 °C); it can be found that the chip adhesive area was small and mainly occurred on the main cutting edge. The area and degree of adhesive wear were better than that of dry-cutting tool, and more uniform than that of wet-cutting tool. The reason is that the extremely low temperature and the phase transformation of liquid CO 2 when it reached the tool-chip contact area zone can effectively reduce the cutting temperature, maintain the hardness of tool surface, and reduce the impact of the adhesive layer falling off on the tool coating. In addition, under the protection of cryogenic CO 2 at temperature − 50 °C, the lubricating oil mist can stably and effectively enter the tool-chip contact area zone, which can reduce the cutting force and the adhesive wear of the tool.

Diffusion wear
The tool and the workpiece are always in contact during the cutting process, and some alloy elements in the workpiece material diffuse into the tool under the action of high temperature. At the same time, some elements in the tool material will also diffuse into the workpiece material to form mutual diffusion [33,34]. This mutual diffusion will reduce the strength of the tool to a certain extent and accelerate the wear of the tool [32].
The diffusion behavior of elements can be described by Fick's law, as shown in Eqs. (2) and (3). However, the tool temperature is relatively not large because of the intermittent contact of milling [35], and the cutting time is relatively (a) Before etching (b) After etching   short in this experiment, so the element diffusion mainly occurs on the tool surface and the shallow layer of the tool, and we focus on analyzing the constituent diffusion of the workpiece material to the cutting tool.
where J is the flux, D, C, and t are the diffusion coefficient (m 2 /s), concentration of diffusing species (kg/m 3 ), and time (s), and x is the distance between the inner section of the tool and the rake face (m). For high-speed milling, the diffusion between cutting tool and chip belongs to unsteady diffusion, and its element concentration gradient changes with the progress of cutting. So, it conforms to Fick's second law when analyzing the element diffusion of the rake face, as shown in Eq. (3). During the cutting process, the chip is constantly generated with the movement of the tool and the workpiece. Therefore, the concentration of Fe, Cr, and other elements in the workpiece material remains unchanged for the element diffusion process of the rake face, which belongs to the diffusion problem of constant concentration at one end. For Fick's second law, two boundary or initial conditions are required to obtain a unique solution, and the boundary or initial conditions of this study are as follows: The above boundary or initial conditions are applied to the partial differential diffusion equation of Fick's second law to obtain the solution, as shown in Eq. (4).
where C(x, t) is the concentration at distance x and time t, C 0 is the initial concentration of Fe, Cr, and other elements in in the workpiece material, and erf(z) is the Gauss error function, and the value of it can be obtained by querying the error function table, and it is defined as Eq. (5). Figure 19 shows the energy spectrum analysis of the main cutting edge in the experimental group of dry cutting, wet cutting, and optimal parameters experimental group (oil flow 60 ml/h, CO 2 , temperature − 50 °C). It can be seen from the figure that in addition to the elements C, N, Al, Ti, and W contained in the tool itself, there were also alloy elements such as Fe, Cr, Ni, and Si contained in the workpiece material, which showed that the alloy elements in the workpiece material diffuse into the tool. The Fe, Ni, and Co were the congeners, and the diffusion of Ni, Fe, and Cr from the workpiece to the tool was because of the affinity for Co [36]. Figure 20 is the element distribution diagram of two typical elements Fe and Cr diffused from the workpiece material to the tool. It can be found that the alloy elements such as Fe and Cr diffused from the workpiece material to the tool were relatively enriched in the wear area, indicating that the diffusion wear mainly occurred in the contact and wear area between the tool and the workpiece. There was little difference in the diffusion distribution of elements in the three groups of experiments, and the main difference was still in proportion. Figure 21 shows mass percentage of Fe and Cr elements on the tool-cutting edge on the transversal crosssection in 8 groups of experiments. It can be found from Fig. 21 that during dry cutting, the wet cutting experimental group had a better effect in controlling diffusion wear. In addition, the mass percentage of Fe and Cr elements at the cross-section decreased with the decrease of cryogenic CO 2 temperature. This is because the diffusion coefficient D in Eq. (4) is a function of temperature, as shown in Eq. (6). When the cryogenic CO 2 temperature is high, the temperature of the cutting area is relatively high, and the element diffusion coefficient is large. Therefore, the mass percentage of Fe and Cr elements is larger at the same time (x) and distance (t).
where D 0 is the diffusion constant, Q is the activation energy of diffusion (J/mol), R is the gas constant (8.315 J/mol·K), and T is the absolute temperature (K).
In the process of high-temperature diffusion, the two-way diffusion process of Fe and Cr elements in the workpiece material and W and Co elements in the tool material leads to the reduction of the hardness and wear resistance of the tool material. And in the cutting process, the tool surface with reduced hardness was gradually worn, resulting in repeated element diffusion and wear.

Oxidation wear
Oxidation wear refers to the chemical reaction between some elements in the tool material and oxygen in the air under the high temperature in the cutting area, resulting in oxides with low strength and hardness. With the progress of cutting, these oxides were taken away by the workpiece and chips, forming wear on the tool surface [37,38]. The mass percentage of the O element on the rake face of the tool in 8 groups of experiments are shown in Fig. 22. It can be found that the mass percentage of the O element was large, indicating that the tool surface was oxidized during cutting. Figure 23 shows the distribution of the O element; it can be seen that the O element was relatively enriched on the tool wear area. By analyzing the mass percentage and distribution of the O element, it can be seen obviously that the optimal parameter array of cryogenic CO 2 -assisted MQL had the best effect on the control of tool oxidation wear. The Co element and WC in the tool substrate material and Ti and Al element in the tool coating reacted with oxygen in the high temperature condition [39]. This oxide was the mixture of Al 2 O 3 , titanium dioxide, and others, which made the tool wear more serious in the process of repeated oxidation and wear.
The experiments 3, 4, and 5 in Fig. 22 show the mass percentage of the O element on the main cutting edge area of the tool when the oil f low was kept unchanged at 60 ml/h and the CO 2 temperature was − °10 °C, − 30 °C, and − 50 °C, respectively. It can be found that the mass percentage of the O element decreased from 12.32 to 5.34% with the decrease of CO 2 temperature from − 10 to − 50 °C, indicating that the oxidation wear of tools decreased with the decrease of cooling temperature. This was because the cryogenic CO 2 reduced the temperature of the contact area between the tool and workpiece, slowed down the oxidation of the tool material, and inhibited the oxidation wear. However, the effect of oil flow on tool oxidation wear was not obvious. Compared with the conditions of dry cutting and wet cutting, the mass percentage of the O element in the optimal parameter array was the lowest, and the O element was not very enriched. It indicated that when the experimental conditions were appropriate, the inhibition effect of cryogenic CO 2 -assisted MQL on tool oxidation wear was better, even better than wet cutting. Furthermore, it also provided a basis for the possibility of cryogenic CO 2 -assisted MQL to replace the traditional cutting fluid.

Chip analysis
The chips of dry cutting, wet cutting, and the optimal parameter array of cryogenic CO 2 -assisted MQL were collected, prepared, grounded, and polished. And the micro-morphology was observed by scanning electron  Fig. 26. It can be found that the chips obtained from the milling experiment were sawtooth chips. The formation of the sawtooth chip was mainly due to the increase of cutting temperature in the cutting process, which made the thermal softening effect of workpiece material much greater than the work hardening effect. This will reduce the shear deformation resistance and lead to adiabatic shear fracture. The repeated action of this process will lead to the formation of serrated chips. The degree of serrated chip and frequency of sawtooth can be expressed by Eqs. (7) and (8) [40]. And the analysis data of serrated G and f at 5 mm and 10 mm were manifested in Tables 5 and 6.
where G is the serrated degree, H, h, d, and 1 are the average addendum height, the average root height, pitch, and the basic angle, as shown in Fig. 27, v is the speed of cutting, and is the shear angle which can be obtained from the theory mentioned by Merchant [41]. As shown in Fig. 28, the serrated degree and frequency of dry cutting, wet cutting, and the optimal parameter array were compared. It can be clearly seen that the serrated degree at 5 mm and 10 mm of the optimal parameter array was the largest; the sawtooth generation frequency of the optimal parameter array was similar to that of the wet cutting and far lower than that of dry cutting at 5 mm, and it was the lowest at 10 mm. The reason was that the excellent cooling and lubrication conditions make the lower temperature of the cutting area, which led to less plastic deformation region and more fracture region, and resulted in high serrated degree and low frequency [42]. And the cutting process was relatively stable [43].
In the secondary deformation zone, a lot of heat was generated due to the adhesion and friction between the Micro-morphology of dry cutting, wet cutting, and the optimal parameter array at 5 mm chip and the rake face under high pressure, so the temperature of the second deformation zone was high. And the f low stress of the chip materials was decreased because of the thermal softening effect, which resulted in plastic deformation of the chip material near the rake face. Therefore, the state of the second deformation zone can largely reflect the temperature during cutting and the quality of cooling and lubrication, and the width of the second deformation zone was described by Π , as shown in Fig. 27. The Π of the three experiments was measured and shown in Fig. 29. It can be found that the Π of dry cutting at 5 mm and 10 mm was the largest, it was because of the high temperature during the cutting without the cooling and lubrication, and the flow stress was decreased. It meant that the friction in the second deformation zone was very large, resulting in serious tool wear as shown in Fig. 16. In addition, it can be clearly seen that the Π of the optimal parameter array at 5 mm and 10 mm was similar to that of the wet cutting and far lower than that of dry cutting. This indicated that the cutting fluid and cryogenic CO 2 can exchange heat well, and reduce the temperature rise in the second deformation area. In addition, the good lubrication conditions made the friction coefficient and friction force of the tool-chip contact area decrease, and the temperature of tool-chip contact area decreased relatively. Therefore, cutting fluid and cryogenic CO 2 -assisted MQL can improve the adhesion and friction between chip and the tool rake face, which was beneficial to improve a) Dry cutting b) Wet cutting c ) Experiment 5

Fig. 25
Micro-morphology of dry cutting, wet cutting, and the optimal parameter array at 10 mm tool wear. And it can also found that the cooling and lubrication condition of cryogenic CO 2 -assisted MQL is similar to that of cutting fluid, and the feasibility of the machining application of cryogenic CO 2 -assisted MQL is proved. The chip thickness of the three experiments was similar at 5 mm, and the optimal parameter array of cryogenic CO 2 -assisted MQL at 10 mm was the smallest. With the increase of the serrated degree, the chip thickness decreased. At the same time, it was conducive to the separation of chips and played a certain role in improving tool wear.
Micro-hardness analysis was car ried out with a force of 300 gf on the non-free surface of chips obtained from the three groups of experiments, and the sampling points were shown in Fig. 30. As shown in Fig. 31, the non-free surface Vickers hardness of dry cutting and wet cutting were similar. However, the hardness of the optimal parameter array of cryogenic CO 2 -assisted MQL was the highest. The reason was that during the process of cryogenic CO 2 -assisted MQL, cryogenic CO 2 increased the surface hardness of workpiece materials to a certain extent. When the   Fig. 29 The width of the second deformation zone Π of dry cutting, wet cutting, and the optimal parameter array cryogenic CO 2 temperature was high, the protection effect of cryogenic CO 2 on oil mist was unfavorable. It led to an increase in workpiece surface hardness and poor lubrication, which speeded up the tool wear. However, when the cryogenic CO 2 temperature met the requirements and the oil flow was appropriate, the protection effect of cryogenic CO 2 on oil mist was superior, which made the lubrication effect of the processing area better. Furthermore, the adverse effect on tool wear by the increase of workpiece surface hardness was reduced.

Conclusion
This study based on the cryogenic CO 2 -assisted MQL researched the influence mechanism of CO 2 temperature and oil flow on tool wear in the processing of difficult-tomachine materials, analyzed the tool wear mechanism and chip morphology, and drew the following conclusions.
(1) Under the condition of cryogenic CO 2 -assisted MQL, the lower the CO 2 temperature is, the better the improvement effect of tool wear, and there is an optimal value of oil flow. In addition, the lower the temperature of the CO 2 carrier, the better the protective effect of the oil mist, so that the oil mist can reach the cutting area stably and effectively. Too small oil flow of MQL will lead to oil mist unable to meet the lubrication requirements. Excessive oil flow will lead to the enlargement of oil mist particles, resulting in unfavorable capillary permeation and accessibility of oil mist. (2) Tool wear is mainly adhesive wear, diffusion wear, oxidation wear, and so on. Among the dry cutting, wet cutting, and the optimal parameter array of cryogenic CO 2 -assisted MQL, wet cutting has a better control effect on tool diffusion wear; cryogenic CO 2 -assisted MQL has a better control effect on tool adhesive wear and oxidation wear. (3) The chip of cryogenic CO 2 -assisted MQL is the serrated chip. Compared with dry cutting and wet cutting, the optimal parameter array of cryogenic CO 2 -assisted MQL has a greater degree of the serrated chip, lower frequency, smaller width of the second deformation zone Π , and smaller thickness, which are conducive to chip breaking and beneficial to improve tool wear. Cryogenic CO 2 increases the surface hardness of the workpiece, but its adverse effect on tool wear can be effectively restrained under appropriate cryogenic CO 2 -assisted MQL. (4) The temperature of the tool-chip contact area is an important factor affecting tool wear. The higher cutting temperature accelerates the progress of adhesive wear, diffusion wear, and oxidation wear, which makes the coating material on the tool surface fall off quickly, forming crater wear, edge collapse, etc., and finally leads to the tool failure. In addition, the higher temperature leads to the high frequency and small serrated degree of chips, larger width of the second deformation zone Π , and increases the hardness of the non-free surface of the chip which speeds up the wear of the tool.
Author contribution Feng Jiang provided ideas and guidance for the paper. Lin Cheng collected and analyzed the data and wrote the paper. Tian Qiu and Shizhan Huang helped collected and analyze the data. Hong Xie and Yan Shui provided experimental support. Chao Liu, Yousheng Li, Liangliang Lin, and Zhiyang Xiang provided guidance and advice. Fuzeng Wang, Xian Wu, and Lan Yan provided constructive suggestions on the structure and some contents of the paper.