A review of low-temperature plasma-assisted machining: from mechanism to application

Abstract

Materials with high hardness, strength or plasticity have been widely used in the fields of aviation, aerospace, and military, among others. However, the poor machinability of these materials leads to large cutting forces, high cutting temperatures, serious tool wear, and chip adhesion, which affect machining quality. Low-temperature plasma contains a variety of active particles and can effectively adjust material properties, including hardness, strength, ductility, and wettability, significantly improving material machinability. In this paper, we first discuss the mechanisms and applications of low-temperature plasma-assisted machining. After introducing the characteristics, classifications, and action mechanisms of the low-temperature plasma, we describe the effects of the low-temperature plasma on different machining processes of various difficult-to-cut materials. The low-temperature plasma can be classified as hot plasma and cold plasma according to the different equilibrium states. Hot plasma improves material machinability via the thermal softening effect induced by the high temperature, whereas the main mechanisms of the cold plasma can be summarized as chemical reactions to reduce material hardness, the hydrophilization effect to improve surface wettability, and the Rehbinder effect to promote fracture. In addition, hybrid machining methods combining the merits of the low-temperature plasma and other energy fields like ultrasonic vibration, liquid nitrogen, and minimum quantity lubrication are also described and analyzed. Finally, the promising development trends of low-temperature plasma-assisted machining are presented, which include more precise control of the heat-affected zone in hot plasma-assisted machining, cold plasma-assisted polishing of metal materials, and further investigations on the reaction mechanisms between the cold plasma and other materials.

Graphical abstract

Keywords

low-temperature plasma / difficult-to-cut material / machinability / hydrophilization effect / Rehbinder effect

1 Introduction

Titanium alloy, high temperature alloy, engineering ceramics, and other materials with superb properties have been widely used in special equipment, such as aero-engines, gas turbines, aircraft landing gear, nuclear main pumps, and seawater valves [1–10]. Although these materials generally have good mechanical strength and high ductility or hardness, they may also contain a variety of metal compounds and unevenly distributed hard particles, resulting in poor machinability [11–18]. During the machining process, problems such as high temperature, large cutting force, and serious tool wear are likely to occur, making it difficult to meet the increasingly high requirements of special service conditions for machining accuracy, surface integrity, and fatigue life [19].

To date, researchers have employed various cooling media to achieve the high-surface integrity machining of difficult-to-cut materials, including high-pressure coolant-assisted machining [20–22], liquid nitrogen-assisted machining [23,24], supercritical carbon dioxide-assisted machining [25–31], and minimum quantity lubrication (MQL)-assisted machining [32–36]. An et al. [25–31] creatively proposed supercritical CO₂-based MQL technology and applied the technique into materials, such as carbon fiber-reinforced polymer composite material, TC4 titanium alloy, and graphite iron. Their results indicated that the hybrid cooling technology could effectively reduce cutting temperature, alleviate sub-surface damage, and improve surface quality. Li et al. [33–35] innovatively combined compressed gas, degradable vegetable oil, and nanoparticles to improve the cooling and lubrication environments in the grinding area and found that the nanofluid MQL could significantly reduce specific grinding energy, avoid workpiece burn, and improve surface integrity. Apart from the cooling media, external energy fields, such as ultrasonic vibration, have also been applied to improve machinability [37–39]. For example, Bhaduri et al. [39] conducted ultrasonic vibration-assisted grinding experiments of γ-TiAl alloy and found that ultrasonic vibration could reduce grinding force and surface roughness Ra by ~35% and ~10%, respectively.

Although the abovementioned methods can reduce cutting temperature and cutting force, thus improving surface integrity, the material properties have not been changed. As a result, problems caused by the special mechanical strength, ductility, and hardness can still influence the machining process. At the same time, when surface temperature is higher than a certain value, a vapor layer will form when the cooling media are in contact with the surface due to the Leidenfrost effect, and the certain value obtained is called the “Leidenfrost temperature” [40,41]. The Leidenfrost effect leads to a significant decrease in solid–liquid contact area, making it difficult for the coolant to enter the cutting area for effective lubrication and cooling, resulting in relatively lower critical heat flux and heat transfer coefficient [42]. Therefore, it is of great significance to develop machining methods that can adjust material properties, so that material machinability can be effectively improved.

As a material state rich in active particles, plasma can effectively improve surface wettability as well as reduce material strength, hardness, and deformation resistance. When plasma is induced into a cutting area, its effects on material properties can effectively promote material fracture and the permeation of the cooling media into the cutting area, thus realizing the highly efficient, precise, and low-damage machining of difficult-to-cut materials. According to electron temperature T_e, plasma can be classified as high-temperature (T_e: ~10⁸ K) and low-temperature plasma (T_e < 10⁵ K). Compared with the high-temperature plasma, the low-temperature plasma is easier to obtain and maintain. In recent years, researchers have applied the latter into machining process, i.e., low-temperature plasma-assisted machining, which can significantly improve material machinability and surface integrity. However, the mechanisms, application ranges, and development trends of this promising technique have been rarely discussed. It should be of great significance to systematically discuss these issues, so as to provide enlightenments for further application and development of this technology. In this paper, after introducing basic characteristics of the low-temperature plasma, we expound the action mechanism of the low-temperature plasma-assisted machining, and then describe the application of the low-temperature plasma in the auxiliary processing of difficult-to-cut materials; finally, we prospect the development trend of the low-temperature plasma-assisted machining. Schematic diagram showing the overall structure of the paper is shown in Fig.1.

2 Basic characteristics of low-temperature plasma and its mechanism in auxiliary machining

Plasma is an unbound macroscopic system consisting of charged particles and is generally regarded as the fourth state of matter after solid, liquid, and gas [43]. Plasma exists widely in nature, and more than 99% of matter in the universe, such as aurora, sun, and lightning, are in a plasma state. The classification of plasma according to the electron temperature T_e is shown in Tab.1. On the one hand, T_e in the high-temperature plasma can reach 10⁸ K, the plasma is completely ionized, and particle density is very high, which can be generated by the solar core in nature and tokamak devices in laboratories. On the other hand, T_e in the low-temperature plasma is lower than 10⁵ K, the plasma is partially ionized, and it is generally generated by gas discharge in laboratories. Low-temperature plasma can be further divided into hot and cold plasma according to different thermodynamic equilibrium states [44,45]. Hot plasma is in a local thermodynamic equilibrium state, its T_e and ion temperature (T_i) are relatively high and approximately equal (T_e ≈ T_i in the range of 10⁴‒10⁵ K), and the macroscopic temperature can reach up to 10⁵ K. By contrast, cold plasma is in a non-local thermodynamic equilibrium state, the value of T_e (10⁴‒10⁵ K) is much higher than T_i (10³ K), and the macroscopic temperature is relatively low, even close to room temperature.

The schematic diagram and experimental setup of the low-temperature plasma-assisted machining process is shown in Fig.2 [46,47]. The low-temperature plasma influences the machining process by physical and chemical effects. For hot plasma, the main effect is the thermal softening effect contributed by the high temperature. For cold plasma, the main mechanisms are reducing hardness, improving surface wettability, and promoting material fracture through chemical reactions. The mechanisms of the hot and cold plasma during the assisted machining process are summarized below.

Hot plasma: The high macroscopic temperature (> 10000 K) of hot plasma can effectively reduce mechanical strength and hardness of materials via the thermal softening effect. Thus, hot plasma can be applied in machining processes of high-strength and high-hardness materials, such as bearing steel, titanium alloy, and nickel-base superalloy (Fig.2(a) and Fig.2(b) [46]), thereby reducing cutting forces via the thermal softening effect, which ultimately improves material machinability. The mechanism of hot plasma-assisted machining (HPAM) has similarities with laser-assisted machining, which uses high-temperature plasma torch or laser beam to soften materials. Compared with laser-assisted machining, the use of hot plasma is relatively more economical, providing comparable heating rates at lower costs [48]. At the same time, the spot size of a plasma torch is larger, resulting in relatively lower heating accuracy, while also ensuring the heating of all radial cutting depths in the milling process [49,50].

Cold plasma: Although the macroscopic temperature of cold plasma is quite low, it contains a variety of high-energy active particles, including electrons (1–10 eV), excited atoms or molecules (0–20 eV), and photons (3–40 eV) [51]. The energies of these particles are generally higher than those of typical chemical bonds, such as C–C (3.45 eV), C–F (4.69 eV), and C–H (4.3 eV). Therefore, when cold plasma is employed to treat materials, its active particles can break the chemical bonds of molecular chains; as dangling bonds appear at the broken positions, they generate free radicals. The active particles in cold plasma, such as O and OH, may combine with these free radicals and form hydrophilic oxygen-containing groups (e.g., C–O and C=O), thereby improving surface hydrophilicity. During the machining process of difficult-to-cut materials, the difficulties involved as coolants enter the cutting area as a result of the Leidenfrost effect contribute to the high cutting temperature and poor machinability [42]. Surfaces with better hydrophilicity have higher Leidenfrost temperature and heat transfer coefficient. Therefore, when cold plasma and other coolants are simultaneously induced into the cutting area (Fig.2(c) and Fig.2(d) [47]), the hydrophilization effect of the cold plasma may promote the coolants to permeate the cutting area, significantly improving the cooling and lubricating condition.

Aside from the effect in improving the permeability of coolants through hydrophilization, cold plasma may also reduce material strength and ductility due to the Rehbinder effect, which claims that active particles absorbing on cracks may promote crack propagation [52–54]. Cold plasma contains various active particles and may promote the expansion of cracks and lattice gaps. When applied into the machining process, cold plasma may reduce the ductility and deformation resistance of materials, effectively reducing cutting force, restraining tool wear, alleviating chip adhesion, and improving surface quality. When investigating the effects of cold plasma on these properties, surface wettability is generally characterized by measuring water contact angles. Meanwhile mechanical properties, such as ductility and deformation resistance, are characterized by nanoindentation measurement, single particle scratching test, and tensile experiment.

Cold plasma can effectively change surface properties without inducing obvious structural damage; thus, it has significant application values in other fields. For instance, radio frequency (RF) plasma-enhanced physical vapor deposition has been widely studied and applied in improving coating adhesion, i.e., the physical-chemical bonding between coatings and substrates [55–57]. Cold plasma can also be used for improving the painting ability [58] and corrosion resistance [59,60] of various materials.

According to the above discussion, with varying thermodynamic properties and macroscopic temperatures, the action mechanisms of hot plasma and cold plasma in assisted machining are radically different. The mechanism of hot plasma is the thermal softening effect, while that of cold plasma can be mainly attributed to chemical reactions. The different mechanisms contribute to the dissimilar applications of HPAM and cold plasma-assisted machining (CPAM), which are respectively described below.

3 Hot plasma-assisted machining

The demand for materials with high strength and good heat resistance has been increasing, particularly in aerospace field. However, material properties, such as high strength and low thermal conductivity, bring about problems, including large cutting forces, high cutting temperatures, and serious tool wear. Given that the strain hardening rate and yield stress of materials significantly decrease with the increase of temperatures contributed by the thermal softening effect, thermally enhanced machining can effectively improve material machinability. Compared with flame heating and laser-assisted machining, HPAM has relatively higher heating efficiency and lower cost [48] and has been widely applied in the machining of difficult-to-cut materials.

During HPAM, most equipment adopts the plasma transferred arc, whose generating system is illustrated by the schematic diagram shown in Fig.3. A hot plasma jet is first generated around the electrode by closing switch S₁; then, switch S₂ is closed to alter the electric circuit and form a plasma arc between the electrode and workpiece. The high temperature of the hot plasma requires large current (tens to hundreds of amperes) and a high power of thousands of watts [46,61]. The high temperature generated by the high power can effectively reduce material hardness through the thermal softening effect. As a result, less spindle power is required during machining, and higher productivity can be achieved [61].

Kitagawa et al. [62] first reported the HPAM technique for high-hardness metals, such as 18% Mn steel and 2.25% Cr cast iron. Their turning experiments indicated that the hot plasma could reduce cutting forces, alleviate flank wear, and improve surface roughness Ra. Madhavulu and Ahmed [61] applied hot plasma in the turning of stainless and alloy steels and revealed that the HPAM could improve material removal rate and tool life by 1.8 times and 1.67 times, respectively. López de Lacalle et al. [49] employed the HPAM to machine Haynes 25, Inconel 718, and Ti‒6Al‒4V, and found that cutting forces could be reduced by up to 25% while productivity was improved by 350% for the Haynes 25. For Inconel 718, the HPAM could significantly alleviate notch wear, thus contributing to the increase of ~200% in tool life. Lee et al. [50,63] developed an HPAM device (Fig.4 [63]) and conducted HPAM experiments of Ti‒6Al‒4V, AISI 1045 steel, and Inconel 718. Their experimental results indicated that the surface roughness Ra and cutting force could be reduced by 70.5% and 60.2%, respectively, for Ti‒6Al‒4V [50]. Furthermore, when machining AISI 1045 steel, the HPAM decreased the cutting force by up to 61% and 15% compared with conventional machining and laser-assisted machining, respectively [63]. Farahnakian and Razfar [68] combined hot plasma and ultrasonic vibration to assist in the turning of hardened steel AISI 4140; compared with conventional turning (CT), the cutting force decreased by up to ~60%, and surface roughness Ra was improved by 5%–20%. Rao [48] investigated the hot plasma-assisted turning process of hardened AISI 4340 alloy steel and found that surface roughness Ra and tool wear were influenced by the heating temperatures of hot plasma and machining time. Under higher heating temperatures, the surface roughness Ra became relatively lower and increased more slowly with the machining time (Fig.5 [48]). They also found that tool wear form changed from notch wear to diffusion wear (873.15 K).

While hot plasma can reduce material strength and cutting forces by heating the cutting area, the heating effect may also cause an excessively high temperature of the cutting tools, leading to a serious reduction in tool life [46]. To reduce tool temperature and restrain groove wear, researchers have proposed a hybrid machining method combining hot plasma heating and liquid nitrogen cooling. Wang et al. [64] first employed the hybrid machining method to turn Inconel 718, in which the workpiece was heated by hot plasma, while the cutting tool was cooled by liquid nitrogen, as shown in Fig.6(a) [64]. Experimental results demonstrated that the surface roughness Ra could be reduced by 250%, cutting forces were decreased by about 30%–50% (Fig.6(b) [64]), and tool life was extended by 170% over CT. Feyzi and Safavi [65] employed hot plasma to soften the Inconel 718 workpiece; and simultaneously utilized liquid nitrogen to ensure that the tool insert was not overheated. Compared with conventional machining, tool flank wear was effectively alleviated, tool life improved by at least 4–8 times, and surface roughness Ra improved from 2.5 to 0.2 μm. Khani et al. [46] induced both hot plasma and liquid nitrogen into the turning process of 17-4PH stainless steel and found that the cutting force could be reduced by up to 48%. In addition, the tool flank wear decreased by 48%, and tool life improved by 117% thanks to the cooling of the cutting insert contributed by the liquid nitrogen (Fig.7 [46]).

The experimental results of the HPAM indicate that the hot plasma can effectively improve material machinability. Apart from the machining experiments, researchers have also developed theoretical models depicting the HPAM process to further investigate the influence of hot plasma. Leshock et al. [66] systematically characterized the temperature distribution of the HPAM process of Inconel 718 through numerical modeling (Fig.8 [66]) and experimental measurement, and the results of the numerical analysis and experimental investigation showed good agreement. HPAM experiments were then performed based on these analyses, and the results indicated that the plasma heating of the workpiece reduced resultant cutting forces by up to 30% and that specific shear energy of the HPAM was much lower. Meanwhile, the relatively higher chip temperature of the HPAM led to higher flank wear rates, whereas notch wear could be effectively alleviated and tool life was improved by 40%. Shao-Hsien and Tsai [67] conducted predictive analysis for the thermal diffusion of the HPAM process of Inconel 718 through exponential smoothing, and the exponential smoothing model was able to estimate cutting temperatures depending on plasma heating current and feed rate. Rao [48] established regression models for surface roughness Ra, tool wear, and material removal rate during the hot plasma-assisted turning process of AISI 4340 alloy steel, and the models could predict these measured values with accuracies all above 98%. The error between the predicated and experimental values were estimated to be 2.45%–9.22%, and the models could predict the surface roughness and tool life with a confidence interval of 99.8%. Moon and Lee [63] employed the ANSYS workbench thermal analysis software to predict preheating temperature distribution of the HPAM (Fig.9 [63]) and found that the effective depth of cut was about 0.3 mm for AISI 1045 steel and 0.4 mm for Inconel 718.

Research results of the abovementioned experimental and theoretical work were generally consistent, demonstrating that the HPAM could effectively reduce cutting forces, extend tool life, and improve surface roughness Ra through the thermal softening effect, as listed in Tab.2 [46,48‒50,61–66]. The relatively optimal surface roughness Ra obtained by each work varied from ~0.1 to ~0.6 μm due to the differences in machining processes and workpieces materials, while they all showed significant improvement compared with conventional machining. However, the relatively large temperature gradient and limited controllability of the hot plasma heating may result in the excessively high temperature of the cutting area, leading to thermal damage and metamorphic layer of machined surfaces [49,68,69]. As shown in Fig.10 [49], the hot plasma heating generated a metallurgical layer with Widmanstätten microstructures on the TC4 titanium alloy surface. The structure possessed higher hardness but poorer ductility, and its fatigue behavior was difficult to predict, which might restrain the application of machined surfaces. In addition, as it was difficult to precisely control the affected depth, there was still Widmanstätten microstructure after HPAM, as shown in Fig.10(c). Therefore, the HPAM has been mostly applied in rough or semi-finishing machining of high-hardness or high-strength materials, while precision machining with low surface roughness (e.g., Ra < 0.1 μm) has been rarely reported with the use of HPAM.

4 Cold plasma-assisted machining

Compared with hot plasma, cold plasma has much lower macro temperature and is rich in active particles. Therefore, it can effectively adjust material properties, such as surface wettability and mechanical strength, without inducing obvious thermal damages [70–78]. Cold plasma can rapidly hydrophilize surfaces to be superhydrophilic (Fig.11) and water can spread and penetrate more easily. Additionally, compared with hydrophilic or hydrophobic surfaces, the superhydrophilic surfaces can form larger liquid films under bubbles and has relatively higher Leidenfrost temperature, critical heat flux, and heat transfer coefficient [79–83]. Thus, when induced into the cutting area, cold plasma can effectively improve heat transfer efficiency of the coolants. Meanwhile, researchers have recently found that cold plasma can reduce material strength and hardness through chemical reactions [84] and the Rehbinder effect [85]. Currently, cold plasma has been applied into the machining processes of various materials. Given that the influence depth of cold plasma is quite limited, research on CPAM has mainly focused on precision machining.

Cold plasma is generally formed by gas discharge, and a typical system is shown in Fig.12 [86]. As can be seen, working gas flowed through the pressure reducing valve and the flowmeter into the electrode, after which gas pressure and flow rate were respectively adjusted by the pressure reducing valve and the flowmeter. High voltage was applied between the needle and the nozzle to break down the working gas, thereby forming a cold plasma jet. Compared with hot plasma, with high voltage (kilovoltage level) but low current (milliampere level), the discharge power of cold plasma is quite low (can be even lower than 10 W [87,88]), the size of the generator is on the order of several hundred millimeters, and investment costs of the generator can be within 10000 USD. The relatively smaller sizes and lower costs of the cold plasma contribute to its promising application prospects.

4.1 Cold plasma-assisted polishing

The cold plasma-assisted polishing (CPAP) operates based on chemical reactions between the active particles in the cold plasma and material surfaces, so as to reduce material hardness, as well as improve material removal rate and surface integrity. Yamamura et al. [84] first proposed the CPAP technique, which reduced the hardness of hard materials through plasma irradiation, while polishing using soft abrasives (Fig.13 [84]). In the past decade, they have applied the technique in processing a variety of materials, including silicon carbide [84,89–94], aluminum nitride [95,96], gallium nitride [97], and single-crystal diamonds [98–101]. The results demonstrated that cold plasma could oxidize the silicon carbide to turn it into silicon dioxide, leading to the significant decrease in hardness from 37.4 to 4.5 GPa [89]. The silicon dioxide could then be easily removed by hydrofluoric acid dipping or ceria abrasives (Fig.14 [90]), and atomic flattening of the silicon carbide was achieved with a root mean square roughness of ~0.1 nm. When applying the CPAP into the aluminum nitride ceramic, the aluminum nitride was modified to be aluminum fluoride, and obvious improvements in surface integrity and material removal rate were achieved (Fig.15 [95]). For single-crystal diamond, the cold plasma could convert the carbon atoms to carbon dioxide; thus, the material removal rate was 20 times greater than that of conventional polishing, and surface roughness Sq of 0.13 nm could be obtained [98]. Bastawros et al. [102] developed a CPAP platform (Fig.16 [102]) and utilized atmospheric pressure He-H₂O plasma to assist the polishing of single crystal sapphire. The results indicated that the materials removal rate could be improved by two times, which can be attributed to the improvement of hydride formation on the sapphire surface by active chemical species. Lyu et al. [103] reported a novel polishing approach combining plasma treatment and low-pressure polishing for lutetium oxide. Their innovative polishing technique could obtain atomically smooth lutetium oxide surface with a surface roughness Sa of 0.47 nm.

The CPAP technology generates new compositions with relatively lower hardness through chemical reactions, thus reducing material hardness and significantly improving material removal rate and surface integrity. At present, the application of this technology mainly concentrates on non-metallic materials, such as silicon carbide and sapphire. Thus, the application of the CPAP in high-efficient and precision machining of engineering metal materials has great significance.

4.2 Cold plasma-assisted cutting

During the cold plasma-assisted cutting (CPAC) process, the movement speeds between the cutting tools and workpieces are relatively higher, and the tool-workpiece contact areas are smaller. Therefore, atmospheric pressure cold plasma jets (Fig.17), which may more easily penetrate into the cutting area, have been widely used. Cold plasma jets were first reported by Koinuma et al. [104], who proposed a microbeam cold plasma-generating device to achieve the separation of discharge and treatment areas. Atmospheric pressure cold plasma jet could produce active particles in the surrounding air as well as eliminate the limitation of sample size caused by discharge devices [105–111]. In recent years, researchers have developed a variety of atmospheric pressure cold plasma jets [87,112–118], showing promising application prospects in the field of auxiliary cutting.

Liu et al. [119,120] first applied cold plasma jets in the machining of difficult-to-cut metal materials, such as NAK80 die steel and 304 stainless steel, as well as conducted surface wettability modification, friction and wear tests, and turning experiments. The results indicated that cold plasma jets could improve the surface wettability of metal materials and reduce friction coefficient between tool and friction pair, thus effectively reducing cutting forces, prolonging tool life, and improving surface quality. On this basis, Huang et al. [86,121,122] combined the cold plasma jet and ultrasonic elliptical vibration and proposed a hybrid turning (HYT) method for the diamond cutting of ferrous metals, as shown in Fig.18 [121]. The ultrasonic vibration contributed to intermittent contact between the tool and workpiece, which promoted the cold plasma jet to enter the cutting area. Turning experiments demonstrated that the flank wear of the HYT was less than 50% of the CT, and surface roughness Ra was largely reduced. As coolants were not used in these experiments, the main influence mechanisms of the cold plasma were the Rehbinder effect and chemical reactions induced by reactive particles. The Rehbinder effect contributed to the reduction in friction coefficient, and the chemical reactions formed nitrides, such as Fe_xC_yN_x, which inhibited graphitization of the diamond tool in cutting of ferrous metals [121].

The above CPAC methods could improve the machinability of various materials; however, these did not involve coolants, and the hydrophilization effect of the cold plasma jet was not utilized to adjust the cutting processes. Recently, Liu et al. [85,123‒125] proposed a new hybrid machining method that combined the cold plasma jet and MQL, and applied the method in the turning, micro-grinding, and micro-milling processes of metal materials, including pure iron, GCr15 bearing steel, TC4 titanium alloy, and Inconel 718 superalloy, as shown in Fig.19 [125]. The machining experiments indicated that the hybrid machining method could obviously reduce cutting temperatures and cutting forces, alleviate tool wear, and improve surface integrity. For the TC4 titanium alloy, compared with dry micro-milling, the main cutting force was reduced by 25%, surface roughness Ra was improved by 50%, and cracks on the machined surface could be obviously alleviated (Fig.20 [85]). The cold plasma jet rapidly improved surface wettability without inducing obvious structural damages, thereby facilitating permeation of the MQL coolant into the cutting area. In addition, the cold plasma jet promoted material fracture due to the Rehbinder effect, which led to the decrease of cutting forces and alleviation of cracks on the machined surface.

Katahira et al. [47] investigated the cold plasma jet-assisted micro-milling of silicon carbide and found that the cold plasma jet obviously reduced surface roughness Ra and improve surface quality. During the CPAC process, the relatively optimum Ra reached ~0.73 nm and remained around 1–2 nm after micro-milling for 3000 mm. By contrast, for the conventional micro-milling, the Ra gradually increased from ~3 to ~6 nm with the increase of cutting distance. According to their investigation on the influence mechanism of the cold plasma jet, the plasma treatment led to better hydrophilicity by generating silicon dioxide layers on the silicon carbide surface. Thus, coolants could more easily permeate into the cutting area (Fig.21 [47]), which contributed to the improvements of cooling and lubrication environment, surface quality, and tool life.

The CPAC technique effectively improved material machinability via the hydrophilization effect, chemical reactions, and the Rehbinder effect induced by the cold plasma jet. It should be of great importance to investigate the reaction mechanisms between the active particles in the cold plasma jet and material surfaces. Meanwhile, for the machining processes under higher temperature and more severe lubrication conditions, such as grinding, techniques to ensure the plasma jet can break through the air barriers remain to be further studied.

In summary, cold plasma could achieve low-damage and high-precision machining of various materials, as listed in Tab.3 [47,84–86,89–102,119,121,122,124]. For the CPAP of non-metallic materials, such as silicon carbide and single-crystal diamonds, the main mechanism involved the chemical reactions between the cold plasma and the materials reduced material hardness. By contrast, the machining mechanism of CPAC was attributed to the hydrophilization and Rehbinder effects.

While the mechanisms of the CPAM have been studied and attributed to chemical reactions and the Rehbinder effect, the influences of these reactions on material properties still require further investigation. When the hot plasma is applied in auxiliary machining, the main mechanisms for different materials are indeed similar, i.e., reducing material hardness and strength via the thermal softening effect. By contrast, for the cold plasma, its influence on properties of various materials can vary widely. Therefore, it is promising to conduct experimental research and molecular dynamic simulation to further reveal the influence mechanisms of cold plasma on material properties, which are expected to largely facilitate the further application of CPAM in the future. In addition, the Rehbinder effect is mainly concerned about the interaction between reactive particles and crystal materials. For non-crystal brittle materials, the influences of cold plasma on mechanical properties and minimum chip thickness also require further investigation.

5 Conclusions

Materials with high hardness, strength, or plasticity have been widely used in aviation, aerospace, military, and other fields, but the machinability of these materials is generally poor. These lead to problems, such as large cutting forces, high cutting temperatures, serious tool wear, and chip adhesions, which seriously affect machining quality. Low-temperature plasma, which contains a variety of active particles, could effectively improve material machinability and has been widely applied in auxiliary machining process of difficult-to-cut materials. However, the mechanisms, applications, and development prospects of this promising technique have been rarely discussed in the literature. In this paper, after introducing the basic characteristics, classifications, and action mechanisms of low-temperature plasma, we respectively discussed the influence mechanisms, machining accuracy, and applications of HPAM and CPAM. The main conclusions and prospects are summarized as follows:

(1) According to the different thermodynamic equilibrium states, low-temperature plasma can be classified as hot plasma and cold plasma corresponding to different macroscopic temperatures. Under a local thermodynamic equilibrium state, the macroscopic temperature of hot plasma is higher than the melting points of most materials, and it can improve material machinability via the thermal softening effect. By contrast, cold plasma is under a non-local thermodynamic equilibrium state with a macroscopic temperature that is close to room temperature. The main mechanisms of CPAM can be attributed to the chemical reactions that reduce material hardness, improve surface wettability, and promote fracture.

(2) Hot plasma can reduce material hardness and strength through the thermal softening effect, thus contributing to the decrease in cutting force, the extension of tool life, and the improvement in surface quality. The relatively optimal surface roughness Ra varied from ~0.1 to ~0.6 μm for different workpiece materials, showing significant improvement compared with conventional machining. However, the relatively large temperature gradient and limited controllability of hot plasma heating may result in thermal damage and thermal stress. Therefore, HPAM has been mostly used in semi-precision and rough machining of difficult-to-cut materials with high hardness or strength. It may be promising to develop methods to better control the heat-affected zones by experimental research and thermal analysis, which can help achieve low-damage and high-efficient machining.

(3) With low macroscopic temperature, cold plasma can realize rapid surface modification without changing surface microstructures or textures. During the machining process, cold plasma can effectively reduce material hardness or improve surface wettability through the chemical reactions between the active particles and material surfaces, as well as promote material fracture of metal materials via the Rehbinder effect. On this basis, cold plasma has been applied into precision machining of various materials, including polishing of non-metallic materials like silicon carbide, as well as the turning, micro-milling, and micro-grinding of metal materials, such as die steel and titanium alloy. In future works, it should be of great significance to focus on high-efficient and high-integrity polishing of metal materials. It may also be a promising research direction to investigate the effects of the cold plasma on mechanical properties and minimum chip thickness of non-crystal brittle materials. In addition, further investigations into the influence mechanisms of cold plasma on material properties may also be conducted through experiments and molecular dynamic simulations, as these can significantly facilitate the other applications of cold plasma in the future.

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