Combined Processing of Micro Cutters Using a Beam of Fast Argon Atoms in Plasma

: We present a new method for coating deposition on micro cutters without an increase in their cutting edges radii caused by the deposition. For this purpose, the cutting edges are sharpened before the coating deposition with a concentrated beam of fast argon atoms. The sharpening decreases the initial radius and, hence, limits its value after the coating deposition. The concentrated beam of fast argon atoms is generated using an immersed in the gas discharge plasma concave grid under a negative high voltage. Ions accelerated from the plasma by the grid pass through the grid holes and are concentrated in the focal point of the grid. As a result of the charge exchange in the space charge sheaths of the grid, they are transformed into fast atoms. A uniform sputtering by the fast atoms of the micro-cutter surface reduces the radius of its cutting edge.


Introduction
The useful life of cutting tools substantially increases after the deposition of thin wear-resistant films [1,2]. This is due to the film hardness of about 25-30 GPa [3] exceeding by many times the hardness of the bulk material. Magnetrons [4] and vacuum arc [5] are mainly used to produce the metal vapor needed for coating synthesis. The density of magnetron discharge plasma near the product surface and the sputtering rate are substantially enhanced when pulsed DC magnetrons [6][7][8][9][10][11][12][13] are used. Metal vapor can be also produced due to sputtering a target at the bottom of a hollow cathode [14].
To prevent the formation of metal droplets in the coatings, the vacuum arc plasma can be filtered using magnetic field. An overview of filtered vacuum arc deposition systems based on magnetic ducts is presented in [15]. The droplets can be also removed from the plasma by transformation of radial plasma streams emitted by the arc cathode spots on the side surface of a cylindrical cathode into an axial stream by means of "single bottle neck" magnetic field [16]. Filtered vacuum arc with ion-species-selective bias has been applied to the synthesis of metal-doped diamond-like carbon films [17]. A magnetic island filter comprising three external coils, which generate a uniform magnetic field, and an internal coaxial coil with a cylindrical permanent magnet in its core, placed within the magnetic island, that generate a field in the opposite direction to the external field is described in [18]. Instead of coils, an internal curvilinear spiral was used in [19], which was transparent and allowed observation of the plasma movement from the arc cathode to the substrate. Analysis of the workpiece surface in [20] revealed that its roughness and the number of cathode spots show no direct relation because the current density per cathode spot does not change according to the number of cathode spots. In a pulsed vacuum arc discharge, the microparticles can be charged, and a roughly quadratic dependence of particle charge on the particle diameter was observed [21] with a 1-µm-diameter particle having a positive charge of ≈1000 electronic charges and a 5-µm-diameter particle having We processed end mills by Cerin (IT) under catalog number 105.030031240 with the nominal diameter of the working part amounting to 3 mm (h10) and length of 12 mm as well as the nominal angle of inclination of the chip groove equal to 30 degrees for machining light alloys, aluminum, and titanium ( Figure 1). The material of the end mills is solid carbide type HM CK10-20-MG Micrograin: monocarbide (WC-94%; Co-6%) with the carbide grain size of 0.5-0.8 μm, hardness Rockwell 91.8 HRA, and transverse rupture strength 3000 N/mm 2 . The work surfaces of the end mills are polished and uncoated. According to the catalog, their application areas are light alloys, plastics, reinforced plastics, and titanium alloys. While studying the bombardment by a beam of fast argon atoms, it is necessary to compare the initial geometric parameters of the cutting tools with the parameters obtained after processing. Analysis of the changes in the controlled parameters will make it possible to conclude the effectiveness of the fast atom beam. The controlled parameters were the profiles of the front surface located at different distances from the end of the cutter, as well as the angular parameters of the cutting edge, the front, and rear corners, and the cutting edge radius.
During the study, three samples of this type of end mill were measured. To control the initial geometric parameters, the algorithms of the Walter Helicheck Plus (Walter, Germany) measuring system was used, which are based on the use of machine vision methods. The measuring was carried out with cameras of transmitted and reflected light. Measurement inaccuracy by positioning stability for measuring diametrical and linear parameters does not exceed 0.32 μm. The measurement was carried out in a non-contact manner using a CCD (Charge-Coupled Device) matrix camera of transmitted light and a CCD camera of reflected light with ×400 magnification.
A hydraulic chuck with an SK 50 tool cone was used for fixing the measured samples of cutters. The obtained results of primary measurements for the working geometry of three samples are presented in Table 1.  While studying the bombardment by a beam of fast argon atoms, it is necessary to compare the initial geometric parameters of the cutting tools with the parameters obtained after processing. Analysis of the changes in the controlled parameters will make it possible to conclude the effectiveness of the fast atom beam. The controlled parameters were the profiles of the front surface located at different distances from the end of the cutter, as well as the angular parameters of the cutting edge, the front, and rear corners, and the cutting edge radius.
During the study, three samples of this type of end mill were measured. To control the initial geometric parameters, the algorithms of the Walter Helicheck Plus (Walter, Germany) measuring system was used, which are based on the use of machine vision methods. The measuring was carried out with cameras of transmitted and reflected light. Measurement inaccuracy by positioning stability for measuring diametrical and linear parameters does not exceed 0.32 µm. The measurement was carried out in a non-contact manner using a CCD (Charge-Coupled Device) matrix camera of transmitted light and a CCD camera of reflected light with ×400 magnification.
A hydraulic chuck with an SK 50 tool cone was used for fixing the measured samples of cutters. The obtained results of primary measurements for the working geometry of three samples are presented in Table 1. The main geometrical parameters of a solid carbide end micro mill and radius of cutting on the radial section are shown in Figure 2. The main geometric parameters of the cutting edge were measured in three areas: A, B, and C. In the range of each area, the cutting edge radius was controlled in sets of sections: r eA1 , r eA2 , r eA3 , and r eAn for A area, r eB1 , r eB2 , r eB3 , and r eBn for B, and r eC1 , r eC2 , r eC3 , and r eCn for C.
The main geometrical parameters of a solid carbide end micro mill and radius of cutting on the radial section are shown in Figure 2. The main geometric parameters of the cutting edge were measured in three areas: A, B, and C. In the range of each area, the cutting edge radius was controlled in sets of sections: reA1, reA2, reA3, and reAn for A area, reB1, reB2, reB3, and reBn for B, and reC1, reC2, reC3, and reCn for C. Cutting edge radius was measured with an optical 3D measuring system MicroCAD Premium plus. Its measuring algorithm is to measure a strip of light using a micromirror projector. Figure 3a shows a photograph of the measuring system and Figure 3b elucidates the measurement of the cutting edge radius.   Cutting edge radius was measured with an optical 3D measuring system MicroCAD Premium plus. Its measuring algorithm is to measure a strip of light using a micromirror projector. Figure 3a shows a photograph of the measuring system and Figure 3b elucidates the measurement of the cutting edge radius. The main geometrical parameters of a solid carbide end micro mill and radius of cutting on the radial section are shown in Figure 2. The main geometric parameters of the cutting edge were measured in three areas: A, B, and C. In the range of each area, the cutting edge radius was controlled in sets of sections: reA1, reA2, reA3, and reAn for A area, reB1, reB2, reB3, and reBn for B, and reC1, reC2, reC3, and reCn for C. Cutting edge radius was measured with an optical 3D measuring system MicroCAD Premium plus. Its measuring algorithm is to measure a strip of light using a micromirror projector. Figure 3a shows a photograph of the measuring system and Figure 3b elucidates the measurement of the cutting edge radius.     The radius of the cutting edge was measured in three areas: A, B, and C, which were located from each other with an equal step of 5 mm at a distance of 2, 7, and 12 mm. In each of the areas, the value of the radius , , and is found as the arithmetic mean of the measured values for four sections located at 0.05 mm distance from each other (Equations (1)-(3)).
The radius of the cutting edge was measured for the basic type of end mill with grinding clearance and rake surfaces. The average value of the cutting edge radius per 2-mmdistanced area (A) was equal to 10.8 μm ( Figure 5); the average value of the cutting edge radius per 7-mm-distanced area (B) was equal to 10.5 μm; and the average value of the cutting edge radius per 12-mm-distanced area (C) was equal to 10.25 μm. The radius of the cutting edge was measured in three areas: A, B, and C, which were located from each other with an equal step of 5 mm at a distance of 2, 7, and 12 mm. In each of the areas, the value of the radius r eA , r eB , and r eC is found as the arithmetic mean of the measured values for four sections located at 0.05 mm distance from each other (Equations (1)- (3)).
The radius of the cutting edge was measured for the basic type of end mill with grinding clearance and rake surfaces. The average value of the cutting edge radius per 2-mm-distanced area (A) was equal to r eA 10.8 µm ( Figure 5); the average value of the cutting edge radius per 7-mm-distanced area (B) was equal to r eB 10.5 µm; and the average value of the cutting edge radius per 12-mm-distanced area (C) was equal to r eC 10.25 µm. The complete experimental process of the present work comprises: 1. Measurement of cutting edge radii, geometry, and roughness of micro cutters. 2. Etching the micro cutters to reduce their cutting edge radii. 3. Measurement of the cutting edge radii after the etching. The complete experimental process of the present work comprises: 1. Measurement of cutting edge radii, geometry, and roughness of micro cutters.

2.
Etching the micro cutters to reduce their cutting edge radii.

3.
Measurement of the cutting edge radii after the etching. 4.
Measurement of the geometry and roughness of micro cutters after etching.
Final measurement of the cutting edge radii of coated micro cutters.

Etching the End Mills with Fast Atoms
The experimental system for processing the end mills is presented in Figure 6. A 20-cm-diameter concave grid with a surface curvature radius of 20 cm is fixed to the high-voltage feedthrough in the center of the vacuum chamber. Figure 5. Cutting edge radii reA1, reA2, reA3, and reA4 in four sections distanced from the mill end at 2 mm and distanced from each other at 0.05 mm.
The complete experimental process of the present work comprises: 1. Measurement of cutting edge radii, geometry, and roughness of micro cutters. 2. Etching the micro cutters to reduce their cutting edge radii. 3. Measurement of the cutting edge radii after the etching. 4. Measurement of the geometry and roughness of micro cutters after etching. 5. Deposition of a wear-resistant coating. 6. Final measurement of the cutting edge radii of coated micro cutters.

Etching the End Mills with Fast Atoms
The experimental system for processing the end mills is presented in Figure 6. A 20cm-diameter concave grid with a surface curvature radius of 20 cm is fixed to the highvoltage feedthrough in the center of the vacuum chamber. On the grid surface, holes with a diameter of 7 mm are evenly distributed at a distance of 8 mm between their centers. At a distance of 6 cm from the chamber wall is On the grid surface, holes with a diameter of 7 mm are evenly distributed at a distance of 8 mm between their centers. At a distance of 6 cm from the chamber wall is installed a rotating holder for the tools being processed by the beam. The axis of the holder rotation passes through the focal point of the concave grid surface.
At an argon pressure in the chamber p ≈ 0.5 Pa, an increase in the voltage between the anode and the chamber to several hundred volts leads to establishing a gas discharge with a current in the anode circuit I a up to 4 A and a discharge voltage of U d = 400-500 V [42]. The chamber is filled with a brightly glowing homogeneous plasma, which is separated from its walls by a cathode sheath and from the grid by a grid sheath of the ion space charge. The sheath width d can be calculated using the derived from the Child-Langmuir law [43] following expression where ε o is the electric constant equal to 0.885 × 10 −11 F/m, e is the electron charge, M is the ion mass, U is the grid bias voltage, and j is the ion current density. When the ion mass M = AM A is measured in atomic mass units A = 1.66 × 10 −27 kg, the sheath width is equal to When U = 0, the chamber and the grid are equipotential, and both sheaths are of the same width. At p ranging from 0.2 to 1 Pa, the voltage U d between the chamber and the anode is virtually independent of the argon pressure p at a constant current I a in the anode circuit. However, at p < 0.2 Pa, a decrease in pressure causes an increase in the discharge voltage U d , and at p = 0.02 Pa, it reaches a value of U d ≈ 1 kV. With an increase in the voltage U between the chamber and the grid from zero to 5 kV, the current I in the grid circuit at a constant current I a in the anode circuit approximately doubles, the width of the sheath d between the plasma and the grid grows to 5-10 cm, and the discharge voltage U d decreases approximately by two times.
The density of the gas atoms at room temperature and pressure p = 0.02 Pa amounts to n = 0.5 × 10 19 m −3 [44], and the average path of argon ions between charge exchange collisions, called the charge exchange length, is equal to λ = 1/nσ = 1 m. We took into account that for argon ions with an energy of 5 keV, the charge exchange cross-section is equal to σ = 2 × 10 −19 m 2 [45,46].
At a pressure of p = 0.02 Pa, the width of the grid sheath d ≈ 0.05 m is much lower than the charge exchange length λ, and therefore, no formation of fast atoms in the sheath occurs. All ions extracted from the plasma bombard the grid and cause the emission from its surface of the secondary electrons [47]. Hence, the grid emits only two beams of electrons with energy e(U + U d ), propagating in opposite directions.
With a pressure increase to 0.2 Pa, accelerated ions turn in the sheath into fast atoms escaping from the sheath. The energy of each fast atom, eϕ, corresponds to the potential ϕ of the point in the sheath where it appears. With a pressure increase from 0.2 to 2 Pa, the number of fast atoms increases tenfold, their energy decreases from 5 to 500 eV, and the neutral beam current grows up.
For an estimate of the beam diameter, the distribution of the etching rate by fast atoms of a polished titanium target covered with a mask on its surface was measured. The straight boundary between them is located horizontally (Figure 7). The target was etched with fast atoms, and then, the height of the step between the covered by the mask and open substrate surfaces was measured along the border between them using a mechanical profiler Dektak XT. The etching rate distribution at various distances Z between the target and the grid revealed the beam diameter dependence on Z. As Z increases, the diameter D decreases from 45 mm at Z = 17 cm to 6 mm at Z = 20 cm, and as the distance increases from Z = 21 cm to Z = 24 cm, the diameter D increases from 7 mm up to 52 mm.     The results of the mills etching are presented in Table 2. They show that at the beginning, for the sections distanced from the mill end at 7 mm, the etching rate is maximum and amounts to 3 μm/h. Further on, the etching rate falls down, and after a 3-h-long etching, it is close to zero for the section distant from the mill end at 7 mm and to 0.2 μm/h for the sections distant from the mill end at 2 and 12 mm.  The results of the mills etching are presented in Table 2. They show that at the beginning, for the sections distanced from the mill end at 7 mm, the etching rate is maximum and amounts to 3 µm/h. Further on, the etching rate falls down, and after a 3-h-long etching, it is close to zero for the section distant from the mill end at 7 mm and to 0.2 µm/h for the sections distant from the mill end at 2 and 12 mm. The average value of the cutting edge radius after etching per 2-mm-distanced area (A) was equal to r eA 4.25 µm; the average value of the cutting edge radius per 7-mm-distanced area (B) was equal to r eB 3.65 µm (Figure 9); the average value of the cutting edge radius per 12-mm-distanced area (C) was equal to r eC 4.1 µm.
Coatings 2021, 11, x FOR PEER REVIEW 9 of 17 The average value of the cutting edge radius after etching per 2-mm-distanced area (A) was equal to 4.25 μm; the average value of the cutting edge radius per 7-mmdistanced area (B) was equal to 3.65 μm ( Figure 9); the average value of the cutting edge radius per 12-mm-distanced area (C) was equal to 4.1 μm. Figure 9. Profiles of the cutting edge radii after etching in four sections distanced from the mill end at 7 mm and distanced from each other at 0.05 mm.
The analysis of the entire set of measured cutting edge radii showed that the best results were obtained for the 3-h-long etching.
A further increase in the etching time had no benefit for the tool sharpening. It should be mentioned that after quite long etching of the end mills, no significant increase in the  The analysis of the entire set of measured cutting edge radii showed that the best results were obtained for the 3-h-long etching.
A further increase in the etching time had no benefit for the tool sharpening. It should be mentioned that after quite long etching of the end mills, no significant increase in the surface roughness has been observed ( Figure 10). The analysis of the entire set of measured cutting edge radii showed that the best results were obtained for the 3-h-long etching.
A further increase in the etching time had no benefit for the tool sharpening. It should be mentioned that after quite long etching of the end mills, no significant increase in the surface roughness has been observed ( Figure 10). After a 3-h-long etching of the end mill, a wear-resistant coating was synthesized on its surface. The synthesis was carried out according to the standard technology on a Platit π 311 system for the deposition of wear-resistant coatings manufactured by Platit (Switzerland). The deposited diamond-like coating (DLC) was a two-layer composition: an adhesive sublayer based on a complex nitride (CrAlSi)N and an outer wear-resistant DLC layer (Figure 10b). The choice of this particular coating (CrAlSi)N/DLC as an object for research is not accidental. This compound can be used to improve the performance of hard-alloy tools, including small-sized ones. To estimate the thickness of the synthesized After a 3-h-long etching of the end mill, a wear-resistant coating was synthesized on its surface. The synthesis was carried out according to the standard technology on a Platit π 311 system for the deposition of wear-resistant coatings manufactured by Platit (Switzerland). The deposited diamond-like coating (DLC) was a two-layer composition: an adhesive sublayer based on a complex nitride (CrAlSi)N and an outer wear-resistant DLC layer (Figure 10b). The choice of this particular coating (CrAlSi)N/DLC as an object for research is not accidental. This compound can be used to improve the performance of hard-alloy tools, including small-sized ones. To estimate the thickness of the synthesized coating, a cylindrical mask with an inner diameter of 3 mm was preliminary put on the end mill.
Within an hour, a DLC coating was synthesized on the end mill and its mask. After removing them from the chamber, the mask was detached from the end mill. Using a Dektak XT mechanical profilometer, the height of the step between the coating surface and the end mill surface screened with the mask was measured (Figure 11a). The thickness of the wear-resistant coating synthesized on the end mill is equal to the step height of 2.4 µm. coating, a cylindrical mask with an inner diameter of 3 mm was preliminary put on the end mill.
Within an hour, a DLC coating was synthesized on the end mill and its mask. After removing them from the chamber, the mask was detached from the end mill. Using a Dektak XT mechanical profilometer, the height of the step between the coating surface and the end mill surface screened with the mask was measured (Figure 11a). The thickness of the wear-resistant coating synthesized on the end mill is equal to the step height of 2.4 μm.
(a) (b) Figure 11. Profilogram of the end mill surface without the detached mask (a) and construction of a two-layer DLC coating deposited on a 3-mm-diameter end mill (b). Figure 12 shows the experimentally obtained measurement results, which were performed in four sections of the cutting edge of a 3-mm diameter end mill after a 3-h-long etching and deposition of a two-layer DLC coating. The average value of the cutting edge  Figure 12 shows the experimentally obtained measurement results, which were performed in four sections of the cutting edge of a 3-mm diameter end mill after a 3-h-long etching and deposition of a two-layer DLC coating. The average value of the cutting edge radii after etching and deposition of a two-layer DLC coating per 2-mm-distanced area (A) was equal to r eA 6.25 µm; the average value of the cutting edge radius per 7-mm-distanced area (B) was equal to r eB 5.5 µm; and the average value of the cutting edge radius per 12-mm-distanced area (C) was equal to r eC 6.2 µm (Figure 12) (Table 3).
(a) (b) Figure 11. Profilogram of the end mill surface without the detached mask (a) and construction of a two-layer DLC coating deposited on a 3-mm-diameter end mill (b). Figure 12 shows the experimentally obtained measurement results, which were performed in four sections of the cutting edge of a 3-mm diameter end mill after a 3-h-long etching and deposition of a two-layer DLC coating. The average value of the cutting edge radii after etching and deposition of a two-layer DLC coating per 2-mm-distanced area (A) was equal to 6.25 μm; the average value of the cutting edge radius per 7-mmdistanced area (B) was equal to 5.5 μm; and the average value of the cutting edge radius per 12-mm-distanced area (C) was equal to 6.2 μm (Figure 12) (Table 3).     Figure 13 presents SEM images (magnification 5000 times) of the end mill cutting edges after the diamond-like coatings deposition on the end mills sharpened through grinding (a) and by fast argon atoms (b).

Discussion
The above results showed that the etching of end mills with a fast atom beam allows an appreciable sharpening of the tools. Due to the etching, the cutting edge radius diminishes to ≈3−4 μm from the minimum value of ≈10−11 μm available with the tool sharpen-

Discussion
The above results showed that the etching of end mills with a fast atom beam allows an appreciable sharpening of the tools. Due to the etching, the cutting edge radius diminishes to ≈3−4 µm from the minimum value of ≈10−11 µm available with the tool sharpening through grinding. The sharpening occurs due to a quite homogeneous sputtering of the tool surface near the edge at low gas pressure p < 1 Pa. Homogeneous plasma inside the chamber is generated by the glow discharge with a large hollow cathode [48], which is the vacuum chamber itself.
It was discovered in [49] that the lower limit of the glow discharge operating pressure is proportional to the aperture of electron losses from the hollow cathode, and due to a decrease in the aperture, it can be diminished to ≈0.01 Pa. This finding made it possible to use the discharge for the plasma immersion processing of products [50,51], in broad beam sources of gaseous ions [52][53][54][55] and electron beam sources [56][57][58]. The present research is the first attempt to use the discharge for the micro tools processing.
In our case, the aperture of electron losses is equal to the surface area of the anode immersed in the discharge plasma. The volume of the vacuum chamber with a diameter of 50 cm and length of 55 cm amounts to V = 0.12 m 3 , and its internal surface area is equal to S = 1.5 m 2 . When the anode area S a is less than a critical value where S is the chamber surface area, m and M are the electron mass and the ion mass, and e is the Naperian base, a positive anode fall of potential U a occurs [42]. With a pressure decrease from 0.1 to 0.01 Pa, the anode fall can grow from U a ≈ 10 to U a ≈ 500 V. It results in the anode overheating and melting by electrons accelerated in the negative space charge sheath near the anode surface. For the discharge in argon and S = 1.5 m 2 , the critical value amounts to S * = 0.008 m 2 . Not to have problems with the discharge anode, its surface area was chosen to be equal to S a = 0.02 m 2 . This area exceeds the critical anode area of S * = 0.008 m 2 and, hence, it prevents the positive anode fall of potential. The discharge plasma uniformity at the gas pressure of 0.01-1 Pa makes it possible to accelerate ions from the main plasma volume in the center of the chamber using a concave grid ( Figure 6) and transform the concentrated ion beam into a fast atom beam. The decrease in the cutting edge radius of the end mills after etching with such a beam can be explained in Figure 14. the positive anode fall of potential. The discharge plasma uniformity at the gas pressure of 0.01-1 Pa makes it possible to accelerate ions from the main plasma volume in the center of the chamber using a concave grid ( Figure 6) and transform the concentrated ion beam into a fast atom beam. The decrease in the cutting edge radius of the end mills after etching with such a beam can be explained in Figure 14. When a negative voltage is applied to the cutting edge in the plasma, ions are extracted from the plasma and enter the sheath. The width of the sheath at a voltage from 100 to 1000 V exceeds 1 mm. Therefore, the radius of the plasma boundary emitting ions onto the cutting edge surface exceeds about one hundred times the cutting edge radius of 10 μm. In a homogeneous plasma, the ion current density is constant at the entire plasma boundary. However, the area of the plasma emitting ions on the cutting edge is one hundred times larger than the edge area.
Therefore, the ion current density at the edge is a hundred times higher than at the rest of the wedge surface. Etching of the edge leads to a significant increase in its radius, and the tool becomes blunt. To avoid bluntness of the tool, it is necessary to etch not by ions accelerated from the plasma by a voltage applied to the tool but by a broad beam of When a negative voltage is applied to the cutting edge in the plasma, ions are extracted from the plasma and enter the sheath. The width of the sheath at a voltage from 100 to 1000 V exceeds 1 mm. Therefore, the radius of the plasma boundary emitting ions onto the cutting edge surface exceeds about one hundred times the cutting edge radius of 10 µm. In a homogeneous plasma, the ion current density is constant at the entire plasma boundary. However, the area of the plasma emitting ions on the cutting edge is one hundred times larger than the edge area.
Therefore, the ion current density at the edge is a hundred times higher than at the rest of the wedge surface. Etching of the edge leads to a significant increase in its radius, and the tool becomes blunt. To avoid bluntness of the tool, it is necessary to etch not by ions accelerated from the plasma by a voltage applied to the tool but by a broad beam of accelerated ions or fast atoms. They sputter the cutting edge of the tool with the same intensity as the rest of its surface. When removing a surface layer of the same thickness, the radius of the cutting edge decreases. Therefore, the tool is sharpened, when it is etched by a broad beam.
In our experiments, the cutting edge radius diminished to ≈3−4 µm from the minimum value of ≈10−11 µm available at the tool sharpening through grinding. In the beginning, the etching rate amounted to ≈2 µm/h, and after a 3-h-long etching, it diminished to zero. The reason for this phenomenon can be related to the structure of the end mill material. In our case, it was a carbide type HM CK10-20-MG Micrograin of carbide group: tungsten-cobalt single-corpus with the carbide grain size of 0.8 µm. One can hardly imagine a tool with a cutting edge radius of ≈1 µm made of a material with a grain size of 0.8 µm. In our experiments, the minimum cutting edge radius exceeds the grain size by five times.
The results of the end mills etching presented in Table 2 exhibit another specific feature to be explained. They reveal a difference in the dependencies on the etching time of the cutting edge radius for the end mill sections distant from the mill end at 2, 7, and 12 mm. The 12-mm-long cutting part of the end mill rotated in the center of the 20-mm-diameter beam and should be etched quite homogeneously. Nevertheless, the etching rate for the section distant from the mill end at 7 mm was a little higher than for the sections distant from the mill end at 2 and 12 mm. It could be caused by a radial distribution of the fast atom flow density with a maximum at the beam axis.
The use of wear-resistant coatings prevents the intensive wear of the cutting tool; however, the coating affects the microgeometry of the cutting edge by increasing the radius of its rounding. With an increase in the radius of the cutting edge about the thickness of the cut layer, the deformation area of the workpiece increases, and as a result, the force load on the tool tooth increases [31]. Cutting forces have a great influence on the chip formation nature, the process of material cutting, vibration characteristics of cutting, the integrity of the cutting tool, and the surface quality [59][60][61][62]. Additional surface treatment by fast atoms will reduce the cutting edge radius and prepare surfaces for the deposition of a wear-resistant coating while providing a relatively smaller radius of the cutting edge than when coating the surface without preparation of this kind.
Unlike the constant undeformed chip thickness (UCT) and direction of cutting speed in orthogonal micro-cutting, both vary in a time-dependent manner in micro-milling. The micro-milling process can be considered as the composition of varied orthogonal micro-cutting in the time domain [63][64][65]. In end micro-milling (Figure 15a), the thickness (i) initially increases from zero, (ii) reaches the maximum value (approximately equal to the feed per tooth), and (iii) reduces to zero, while in a side micro-milling (Figure 15b), the thickness decreases from maximum to zero. The above stages may occur sequentially for a single cutting edge, or, in most common scenarios, simultaneously for multiple cutting edges depending on the employed tool geometries (the cutting edge radius and tooth number) and the machining parameter (feed per tooth) [66]. This would result in very complex material behaviors in comparison with orthogonal micro-cutting.
initially increases from zero, (ii) reaches the maximum value (approximately equal to the feed per tooth), and (iii) reduces to zero, while in a side micro-milling (Figure 15b), the thickness decreases from maximum to zero. The above stages may occur sequentially for a single cutting edge, or, in most common scenarios, simultaneously for multiple cutting edges depending on the employed tool geometries (the cutting edge radius and tooth number) and the machining parameter (feed per tooth) [66]. This would result in very complex material behaviors in comparison with orthogonal micro-cutting. Consider the side micro-milling process based on the two-tooth micro milling cutter as an example. If the UCT at the entrance is larger than the minimum undeformed chip thickness (MUCT), the chip begins to form (Figure 15b). However, when the instantaneous UCT becomes smaller than the MUCT, the chip generation would stop. A part of the workpiece material will elastically recover, while other material will undergo plastic deformation after the micro-milling cutter is passed by. The machined surface in a side micro-milling includes both the theoretical residual height (Rt) and the residual height (Rmax) left by the existing MUCT (Figure 16b) [62]. Rmax in micro-milling can be expressed as Equation (7) [62], respectively, and they are primarily related to the feed per tooth (fz) and MUCT (hm), as shown in Equation (8). Consider the side micro-milling process based on the two-tooth micro milling cutter as an example. If the UCT at the entrance is larger than the minimum undeformed chip thickness (MUCT), the chip begins to form (Figure 15b). However, when the instantaneous UCT becomes smaller than the MUCT, the chip generation would stop. A part of the workpiece material will elastically recover, while other material will undergo plastic deformation after the micro-milling cutter is passed by. The machined surface in a side micro-milling includes both the theoretical residual height (R t ) and the residual height (R max ) left by the existing MUCT (Figure 16b) [62]. R max in micro-milling can be expressed as Equation (7) [62], respectively, and they are primarily related to the feed per tooth (f z ) and MUCT (h m ), as shown in Equation (8). As a result of using the etching operation before coating, the radius of the cutting edge can be reduced to 6.25 microns compared to 12.7 after coating a ground micro mill.
Reducing the cutting edge radius allows decreasing the height of residual microroughness formed during cutting in the area of the plunger zone from 1.33 μm (point I in Figure 16a) to 0.58 μm (point II in Figure 16a), minimum undeformed chip thickness hm from 2.97 μm (point I in Figure 16b) to 1.46 μm (point II in Figure 16b), forces in the ploughing-dominated region determined on the base of ref. [65,66] from 7 N/mm (point I in Figure 16c) to 5.7 N/mm (point II in Figure 16c) when machining a steel part with the following parameters β 40°, Poisson's ratio 0.25 (for steel), mill's radius Rf 1.5 mm, feed per tooth 0.005 mm/tooth. After the chip thickness of 3.0 μm, where the effect of ploughing becomes small and that of shearing is more significant, forces for pearlite became higher, but increasing these parameters are actual only for the machining process with small deps of cut [67−69]. As a result of using the etching operation before coating, the radius of the cutting edge can be reduced to 6.25 microns compared to 12.7 after coating a ground micro mill.
Reducing the cutting edge radius allows decreasing the height of residual microroughness formed during cutting in the area of the plunger zone from 1.33 µm (point I in Figure 16a) to 0.58 µm (point II in Figure 16a), minimum undeformed chip thickness h m from 2.97 µm (point I in Figure 16b) to 1.46 µm (point II in Figure 16b), forces in the ploughing-dominated region determined on the base of ref. [65,66] from 7 N/mm (point I in Figure 16c) to 5.7 N/mm (point II in Figure 16c) when machining a steel part with the following parameters β 40 • , Poisson's ratio k 0.25 (for steel), mill's radius R f 1.5 mm, feed