Electrical Discharge Machining of Al 2 O 3 Using Copper Tape and TiO 2 Powder-Mixed Water Medium

: Aluminum-based ceramics are used in industry to produce cutting tools that resist extreme mechanical and thermal load conditions during the machining of Ni-based and high-entropy alloys. There is wide ﬁeld of application also in the aerospace industry. Microtexturing of cutting ceramics reduces contact loads and wear of cutting tools. However, most of the published works are related to the electrical discharge machining of alumina in hydrocarbons, which creates risks for the personnel and equipment due to the formation of chemically unstable dielectric carbides (methanide Al 3 C 4 and acetylenide Al 2 (C 2 ) 3 ). An alternative approach for wire electrical discharge machining Al 2 O 3 in the water-based dielectric medium using copper tape of 40 µ m thickness and TiO 2 powder suspension was proposed for the ﬁrst time. The performance was evaluated by calculating the material removal rate for various combinations of pulse frequency and TiO 2 powder concentration. The obtained kerf of 54.16 ± 0.05 µ m in depth demonstrated an increasing efﬁciency of more than 1.5 times with the closest analogs for the workpiece thickness up to 5 mm in height. The comparison of the performance (0.0083–0.0084 mm 3 /s) with the closest analogs shows that the results may correlate with the electrical properties of the assisting materials.


Introduction
Cutting ceramics are used in industry to produce cutting tools that resist extreme mechanical and thermal load conditions during the machining of Ni-based and high-entropy alloys [1][2][3][4] due to achieving temperatures up to 800 • C in the contact area and other specific properties of these materials [5][6][7]. The wide field of applications includes the aerospace, automotive, chemistry and nuclear industries [8][9][10][11]. In cutting tools, microtexturing of cutting ceramics reduces contact loads and significantly reduces the wear of cutting tools and coatings [12][13][14]. Microtexturing is often provided by a different group of the machining technologies, such as:
Electrical discharge machining has multiple known advantages related to the physics of the process and geometrical features: • Ability to work with the preproduced electrode of any shape that allows microtexturing in one single approach of the electrode tool to the workpiece (sinking) [24][25][26]; • Ability to produce high-precision linear surfaces by a constantly rewinding electrode tool that excludes any influence of the tool wear on the tolerance of the shaping [27,28]; • Low tool wear rate that is especially relevant in the case of sinking in comparison with other technologies using a preproduced tool where the operational life of the tool can count by a few minutes that often requires multilayer and nanocoatings [29][30][31][32]; • Ability to process the material despite their hardness and viscosity (in the case of aluminum and titanium alloys, composites) [33][34][35][36]; • Accuracy up to 80-100 nm due to the use of the new class of linear engines [37,38]; • Ability to perform electrical and acoustic monitoring of process efficiency [39,40].
The problem of applying electrical discharge machining methods to the dielectric materials such as cutting ceramics such as Al 2 O 3 , AlN, SiAlON, Si 3 N 4 , ZrO 2 , etc., is in the absence of required electrical properties of these materials [41][42][43][44][45][46]. That can be solved by adding a conductive phase into the ceramic structure of the sintered composite [47][48][49] that influence as well on the physical and mechanical properties of a new material depending on the conductive and main material fraction or by adding assisting conductive coating or powder in the working area [50,51]. The method was first proposed and patented in 1986 by Russian scientists [52]. In this context, electrical discharge machining of ZrO 2 ceramic in hydrocarbons showed the most outstanding results [53][54][55][56][57] that can be related to the thermochemical properties of this material when thermodissociated Zr 4+ and metal-like carbide ZrC (delocalized metal bond d-element carbide) exhibit similar electrical conductivity (≤44.1 µΩ·cm at a normal temperature depending on purity) and assists processing [58,59]. However, the works published on other ceramics do not show any significant advances in efficiency-maximal depth of 1693 µm from the coating surface after 150 min of processing by the copper-tungsten electrode of Ø308 µm for AlN, for example [46,[60][61][62].
It should be noted that the published works are related to the electrical discharge machining of alumina in hydrocarbons [51,[63][64][65][66], where obviously during thermochemical dissociation of Al 2 O 3 [67,68], released aluminum ion Al 3+ binds with dissociation products of hydrocarbon to form methanide Al 3 C 4 and acetylenide Al 2 (C 2 ) 3 [44,[68][69][70]. In other words, it leads to the formation of chemically unstable dielectric carbides exhibiting dielectric properties that not only hampers processing but also creates risks for personnel and equipment.
The alternative combined approach for wire electrical discharge machining of Al 2 O 3 in the water-based dielectric medium was proposed for the first time ( Figure 1).  Deionized water was chosen as a basis for assisting powder fluid to avoid formation of the mentioned carbides. Powder-mixed electric discharge machining (PMEDM) was first developed as an advanced technique to improve machining productivity [71][72][73]. The use of powder material reduces dielectric medium strength, creating a more uniform density of the electrical discharge at low pulse energy (Figure 2a-c), reducing surface roughness, improving productivity, and reducing tool electrode wear compared with conventional electrical discharge machining. A powder material (micro-or nano-sized) of titanium dioxide, cesium dioxide, and other ceramic components is introduced into the working area (interelectrode gap), which, upon reaching the temperatures of the discharge channel, would acquire better conductive properties ( Figure 3) and provoke denser discharges of pulses during processing conductive materials. It should be noted that many conductive materials exhibit a reversal trend [74][75][76]. The powder is mixed with the dielectric liquid in the working tank. The literature describes two powder-mixed dielectric fluid circulating systems: closed and open. The working reservoir with this suspension can be isolated from the dielectric fluid circulation system in a closed system. In an open system, a powdermixed dielectric fluid circulates continuously [73].    . Specific electrical conductivity of some substances: the field of limit values for conductive materials is marked orange; for insulating materials, it is marked green; the boundary zone is marked yellow; the materials chosen for the current research are marked red.
The present study used a closed system where current discharges were addressed to the introduced micro-sized particles, uniformly suspended in the volume of the dielectric medium (suspension) in the interelectrode gap. TiO₂ material was chosen for assisting powder due to the following reasons: • It is commercially available and relatively safe for the personnel when working with micro-sized particles in suspension (health hazard code 1: exposure may cause only irritation with minimal residual damage, according to NFPA 704), toxic in the form of nanoparticles; • Insoluble in water; • Exhibits increased electrical conductivity to a level close to the one for conductive materials in the presence of heat (more than 1000 °C) ( Figure 3); • Exhibits semiconductive properties at normal temperatures (n-type with the band gap Eg = 3.0 eV for rutile, Eg = 3.20 eV for anatase [77]; the less the band gap is, the more conductive properties exhibit material [78,79]); • Transforms into rutile form when heated.
The purpose of the study is to increase the productivity of electrical discharge machining of alumina using assisting means such as conducting coating powder-mixed dielectric medium based on water. The performance is evaluated by calculating the Figure 3. Specific electrical conductivity of some substances: the field of limit values for conductive materials is marked orange; for insulating materials, it is marked green; the boundary zone is marked yellow; the materials chosen for the current research are marked red.
The present study used a closed system where current discharges were addressed to the introduced micro-sized particles, uniformly suspended in the volume of the dielectric medium (suspension) in the interelectrode gap. TiO 2 material was chosen for assisting powder due to the following reasons:

•
It is commercially available and relatively safe for the personnel when working with micro-sized particles in suspension (health hazard code 1: exposure may cause only irritation with minimal residual damage, according to NFPA 704), toxic in the form of nanoparticles; • Insoluble in water; • Exhibits increased electrical conductivity to a level close to the one for conductive materials in the presence of heat (more than 1000 • C) ( Figure 3); • Exhibits semiconductive properties at normal temperatures (n-type with the band gap E g = 3.0 eV for rutile, E g = 3.20 eV for anatase [77]; the less the band gap is, the more conductive properties exhibit material [78,79]); • Transforms into rutile form when heated.
The purpose of the study is to increase the productivity of electrical discharge machining of alumina using assisting means such as conducting coating powder-mixed dielectric medium based on water. The performance is evaluated by calculating the volumetric The scientific novelty consists of establishing stable relationships between machining performance using the assisting electrode technique and the experimental factors, the electrical properties of the assisting materials, and developing an alternative approach in micro texturing cutting ceramics.

Sintering of the Samples
The corundum α-Al 2 O 3 A16SG (Alcoa, New York, NY, USA) was used for producing samples. The average particle diameter was 0.53 µm. Granulometric analysis of powder was carried out on an optical granulomorphometer 500NANO (Occhio, Liege, Belgium). Powder mixtures were prepared and ground in a Turbula multidirectional shaker (Eskens B.V., Alpen aan den Rijn, The Netherlands) in ethanol in a polyethylene container at 150 rpm for 24 h. Suspensions were dried in a FreeZone2.5 lyophilizer (LabConco, Kansas, MO, USA) at −50 ± 2 • C, while the sheath temperature was +23 ± 2 • C. During the entire processing, the pressure in the chamber was 0.02 ± 0.01 mbar [80]. The powder mixtures were freely poured into mechanically produced graphite molds [81] fabricated of MPG-6 grade cold-pressed blanks of 1 µm-sized powder particles. Graphite molds machining was carried out on a CTX beta 1250 TC turning and milling machine (DMG MORI, Bielefeld, Germany) with a roughing spindle speed of 100-300 m/min, feed of 0.1 mm/tooth, finishing speed of 150-600 m/min, feed of 0.013-0.05 m/min, by a carbide tool with a multi-layer combined PVD-coating [82,83]. Machining was accompanied by parameter control using a diagnostic system (MSUT Stankin, Moscow, Russia) [84][85][86]. The powder mixture was sintered in a graphite mold on a spark plasma sintering machine KCE FCT-H HP D-25 SD (FCT Systeme GmbH, Rauenstein, Germany) at 1400 • C with a heating rate of 100 • C min −1 at uniaxial pressure 80 MPa in vacuum for 10 min [48,[87][88][89]. The sintered discs were 65.5 mm in diameter and 10 mm in thickness.
Optical control was carried out on an Olympus BX51M instrument (Ryf AG, Grenchen, Switzerland). Scanning electron microscopy of mechanically produced fracture and chemical analyses were conducted on a VEGA3 instrument (Tescan, Brno, Czech Republic). Sample density measurements were carried out in distilled water (ceramic samples were boiled for 5 h and then soaked for 24 h) using the Archimedes principle and compared with theoretical values (3.89 g/cm 3 for Al 2 O 3 ). X-ray diffraction analysis of the samples was conducted on an Empyrean diffractometer (PANalytical, the Netherlands) using a tube with a copper anode. The cross sections of the samples were prepared on ATM Machine Tools (ATM Machine Tools Ltd., Wokingham, UK).

Electrical Discharge Machining
A two-axis wire electrical discharge machine ARTA 123 Pro (NPK "Delta-Test", Fryazino, Moscow Oblast, Russia) was used for experiments ( Table 1). The open tank system of the machine allows using any possible dielectric medium (deionized water, mineral oil, water-or oil-based suspensions) out of the filtration system. A brass wire electrode of 0.25 mm in diameter fabricated of CuZn35 brass provided texture formation on alumina without taking into account the spark gap (path offset). The  Table 2. The previously conducted work with alumina showed that the optimal value of the operational current is 15 A. During the study, it was decided to limit the research area to EDM factors such as pulse frequency, and concentration of the powder in the fluid to make it more straightforward since the main correlation between current, operational voltage, and character of the obtained wells on the surface or material removal rate is known and understandable [90]: where ΣF imp is the summarized force of working impulses in the system's action, N; I is current, A; and V O is operational voltage, V. Machining is carried out according to the control program of the translational movement of the wire electrode along the X-axis from the zero position to a depth of 0.98 mm (taking into account the spark gap) [91][92][93].
The development of factors was carried out following the results of pre-implemented experiments. The factors depended on the machine's capabilities and were limited by the results of each experiment. Each failed experiment minimized the range of the factors and assisted in determining the optimal values in terms of the material removal rate (productivity) [94].
The coated blank was fixed on the machine table during experiments, ensuring immobility. The basing was carried out by wire tool approach along the Xand Y-axes; the surface of the coated workpiece was taken as zero. Before the start of processing, a spark adjusted the wire electrode vertically along the Z-axis. The position from +2 to +3 mm from the surface of the assisting coating was taken as the initial position of the wire electrode. The nozzles of the working fluid are brought as close as possible to the coated blank after the wire electrode location to ensure adequate flushing of the working area. The nozzles from which the dielectric is supplied are located on two sides of the coated blank, directing the jet into the interelectrode gap between the coated blank and wire electrode to ensure its turbulent character. Processing is carried out with complete immersion in the dielectric. When the workpiece is immersed, the dielectric level is provided above the workpiece (blank) level by 1-2 mm. Before machining, the workpiece was held for 8-10 min in a dielectric to avoid the influence of thermal fluctuations due to the difference in ambient temperatures, electrodes, workpiece, and the dielectric fluid. At the end of the electrical discharge machining, the blank was taken out from the tank. The resulting sample was wiped with a rag in place. Formed textures (kerfs) were controlled optically. The scheme of coated blank fixing on the equipment is shown in [95].
The parameters of the kerf were controlled by optical microscopy. Here and below, the optical measurement error was calculated by the formula [96][97][98][99]: where δ l is the longitudinal measurement error, µm; δ t is the transversal measurement error, µm; L is the measured length, mm; and g is the product height above microscope table glass (taken equal to zero), mm. The volumetric material removal rate (MRR) was calculated through the feed rate of the wire electrode into the working area and the volume of removed material (Figure 4) [100]: where S is the kerf area in the plan, mm 2 ; r w is the radius of the electrode tool taking into account the spark gap (r w + ∆), mm; and α is the angle of the formed segment in the plan, rad: where h is the depth of the kerf, µm. The radius of the electrode tool taking into account the spark gap r w is calculated as follows: where r w is the wire electrode radius (125 µm), µm; the spark gap ∆ was taken as 48 ± 9.6 µm for the materials with the threshold conductivity from the previously conducted studies [39,90,91,101].
where δl is the longitudinal measurement error, µm; δt is the transversal measurement error, µm; L is the measured length, mm; and g is the product height above microscope table glass (taken equal to zero), mm. The volumetric material removal rate (MRR) was calculated through the feed rate of the wire electrode into the working area and the volume of removed material (Figure 4) [100]: where S is the kerf area in the plan, mm²; r'w is the radius of the electrode tool taking into account the spark gap (rw + Δ), mm; and α is the angle of the formed segment in the plan, rad: where h is the depth of the kerf, µm. The radius of the electrode tool taking into account the spark gap r'w is calculated as follows: where rw is the wire electrode radius (125 µm), µm; the spark gap Δ was taken as 48 ± 9.6 µm for the materials with the threshold conductivity from the previously conducted studies [39,90,91,101]. The volume of removed material is calculated using the formula [90]: where l is the length of the kerf, µm, then the material removal rate is: where t is the processing time calculated from the feed rate vF of the electrode tool, s: The volume of removed material is calculated using the formula [90]: where l is the length of the kerf, µm, then the material removal rate is: where t is the processing time calculated from the feed rate v F of the electrode tool, s:

Assisting Suspension
Deionized water (LLC "Atlant", pos. Marusino, Lyubertsy district, Moscow region, The Russian Federation) following ASTM D-5127-90 with specific electrical resistivity up to 18.0 MΩ·cm was chosen as a suspension basis to avoid formation insulating and explosive debris that may hamper electrical discharge machining and may have dramatic consequence in terms of safety (formation of Al 4 C 3 or Al 2 (C 2 ) 3 ) [68][69][70].
The powder was subjected to granulometric analysis and optical microscopy. An EL104 (Mettler Toledo, Columbus, OH, USA) laboratory balance with a measurement range of 0.0001-120 g weighed the powder with an error of 0.0001 g in preparing the suspension. Initial TiO 2 powder was sifted using an analytical sieving machine AS200 basic (Retsch, Dusseldorf, Germany) with a test sieve (10 µm by ISO 3310-1). It should be noted that the smallest size of suspended particles led to the highest rate of volumetric material removal and the lowest wear of the primary tool electrode [102][103][104][105][106]. Another reason for using 10 µm granules is the narrow range of the discharge gap: for the conductive materials, it is about 170-200 µm [90,95]; for materials with the threshold conductivity, it is 48-50 µm [49,80,91]. In this case, the electrical conductivity and the discharge gap are in direct proportion [101]. The prepared suspension was constantly stirred during experiments. After processing, the samples were cleaned with alkali.

Assisiting Electrode
The most typical coating for the assisting electrode technique used in the works of many authors is copper foil or sheet placed adhesively on the surface of the ceramic blank [51,65,107,108]. Copper and copper group metals and alloys based on them form insulating oxides that are unstable in the presence of hydrogen of medium (in the case of Cu, reduction to metallic copper) [109][110][111][112][113]: CuO + H 2 ↑ → Cu + H 2 O + Q↑ (exothermic reaction) (10) or dissociates at temperatures above 280 • C for Ag 2 O, above 100 • C for Ag 2 O 2 , and above 225 • C for Au 2 O, when Au 2 O 3 is stable and exhibits conductive properties [114,115]. This property of copper (II) oxide, combined with its inertness to the media and other components of the working area, makes copper unrepeatable when designing technology for a wide group of materials. A HomaFix 404 (20 m × 10 mm) copper tape (JSC Electroma, Lipetsk, Russia) was used as an assisting coating. The main properties of the tape are provided in Table 3. The specific electrical resistance of the assisting coating (Table 4) was controlled by a Fischer Sigmascope SMP10 device (Helmut Fischer GmbH, Sindelfingen, Germany) by evaluating the electric conductance at normal temperature in Siemens and the percentage of the electrical conductance of the control sample fabricated of annealed bronze in the range of 1-112%.  Before coating, the ceramic samples were placed in an ultrasonic tank and cleaned using a soap solution at a temperature of 60 • C for 20 min and alcohol for 5 min [116]. Approbation of the coated samples was conducted in a deionized water medium. Coating removal was conducted by a complex method: washing in an ultrasonic tank and mechanical cleaning.

Characterization of Al 2 O 3 Samples
A microphotograph of Al 2 O 3 powder is presented in Figure 5a. It shows a predominantly favorable spherical shape of particles that contributes to reducing the porosity of the samples. The variety of particle sizes has a positive effect on the mechanical properties of the samples. Tape width, mm 10 Before coating, the ceramic samples were placed in an ultrasonic tank and cleaned using a soap solution at a temperature of 60 °C for 20 min and alcohol for 5 min [116]. Approbation of the coated samples was conducted in a deionized water medium. Coating removal was conducted by a complex method: washing in an ultrasonic tank and mechanical cleaning.

Characterization of Al₂O₃ Samples
A microphotograph of Al₂O₃ powder is presented in Figure 5a. It shows a predominantly favorable spherical shape of particles that contributes to reducing the porosity of the samples. The variety of particle sizes has a positive effect on the mechanical properties of the samples. The polished surface of the sintered sample is presented in Figure 5b. The density of the samples after sintering was 99.2 ± 0.2%. X-ray diffraction analyses did not reveal any contamination by other phases nor traces of any side reactions during sintering. The The polished surface of the sintered sample is presented in Figure 5b. The density of the samples after sintering was 99.2 ± 0.2%. X-ray diffraction analyses did not reveal any contamination by other phases nor traces of any side reactions during sintering. The chemical analysis of the samples at the fracture site corresponds to the theoretical chemical composition of the ceramics.

Characterization of TiO 2 Powder
Granulometric analysis (Figure 6a, Table 5) of TiO 2 powder showed that the powder sample had an average inner diameter of 10.84 µm and 9.29 µm for 50% of the particles, while the average area diameter was 16.12 µm and 13.94 for 50% particles. The average circularity of the titanium dioxide powder is about 0.656 µm and about 0.700 µm for 50% of the particles. Titanium dioxide granules have a specific character. Optical microscopy of the granules is shown in Figure 6b. The powder was sieved before further processing.
Technologies 2022, 10, x FOR PEER REVIEW 11 of 22 chemical analysis of the samples at the fracture site corresponds to the theoretical chemical composition of the ceramics.

Characterization of TiO₂ Powder
Granulometric analysis (Figure 6a, Table 5) of TiO₂ powder showed that the powder sample had an average inner diameter of 10.84 µm and 9.29 µm for 50% of the particles, while the average area diameter was 16.12 µm and 13.94 for 50% particles. The average circularity of the titanium dioxide powder is about 0.656 µm and about 0.700 µm for 50% of the particles. Titanium dioxide granules have a specific character. Optical microscopy of the granules is shown in Figure 6b. The powder was sieved before further processing.

Electrical Discharge Machining in TiO 2 Powder-Mixed Water Medium
The general view of the working area and results of the optical microscopy for the machined kerf for the various powder concentration and pulse frequencies are shown in Figure 7. Due to the visual optical whiteness of the ceramic material, the formed texture (kerf) is not distinguishable; however, traces of adsorbed copper and deposed drops are clearly visible and cannot be removed mechanically with effort. The deepest kerf in Al 2 O 3 sintered blank using TiO 2 powder-mixed water medium was obtained for a concentration of 150 g/L, and pulse frequency f = 2 kHz and f = 5 kHz.

Scanning Electron Microscopy (Chemical Analyses)
The results of quantitative and qualitative analysis of the obtained kerfs after electrical discharge machining of alumina using a monolayer copper tape coating are shown in

Scanning Electron Microscopy (Chemical Analyses)
The results of quantitative and qualitative analysis of the obtained kerfs after electrical discharge machining of alumina using a monolayer copper tape coating are shown in Figure 8 and Table 6. The images show a uniform distribution of chemical elements such as aluminum and oxygen (Figure 8a,b) and the assisting electrode coating of copper deposited on the surface (Figure 8c). When using a suspension, the amount of deposed copper increases many times (by 81.1%) (Figure 8f, Table 6), while zinc (2.6%) of the wire tool and titanium (1.5%) of the powder are also presented ( Table 6). The presence of carbon corresponds to normal atmospheric contamination of the samples.
Technologies 2022, 10, x FOR PEER REVIEW 14 of 22 Figure 8 and Table 6. The images show a uniform distribution of chemical elements such as aluminum and oxygen (Figure 8a,b) and the assisting electrode coating of copper deposited on the surface (Figure 8c). When using a suspension, the amount of deposed copper increases many times (by 81.1%) (Figure 8f, Table 6), while zinc (2.6%) of the wire tool and titanium (1.5%) of the powder are also presented ( Table 6). The presence of carbon corresponds to normal atmospheric contamination of the samples.

Material Removal Rate
An analysis of the processing performance of alumina using the combined approach of assisting electrode techniques was carried out for TiO₂ concentration of 50, 100, and 150

Material Removal Rate
An analysis of the processing performance of alumina using the combined approach of assisting electrode techniques was carried out for TiO 2 concentration of 50, 100, and 150 g/L in a water medium for the pulse frequency (f ) range of 2-11 kHz. The measured values of the kerf parameters and the calculated values of the machined segment angle are shown in Table 7. The material removal rate is shown in Table 8. A graphical presentation of the relationship between material removal rate, pulse frequency, and TiO 2 concentration is shown in Figure 9. The higher performance was achieved for pulse frequencies 2 and 5 kHz and a concentration of 150 g/L.  Figure 9. The performance of electrical discharge machining of alumina using a copper tape c and TiO₂ powder-mixed water medium.

Discussion
A comparison of the performance of electrical discharge machining alumina wa ried out for the current study's data and previously published ( Table 9). The prese sults were compared with those obtained with a brass electrode tool in a water-base dium and those obtained using hydrocarbons. The data indicate that the deve method is superior to:

•
The previously developed technique of electrical discharge machining using a electrode tool in water-based working fluid [80] and; • The published data on electrical discharge machining alumina in hydrocarbons monolayer copper foil of 6 µm in thickness [65].
The results published for the technique using carbon tape [66] exceed the valu the present study but demonstrate significant shortcomings typical for sinker type o chining and working with carbons and hydrocarbons: • The manufacture of a unique electrode is required; • There is no possibility of processing ruled surfaces; • Graphite particles exhibit electrical anisotropy (electrical properties are differ different directions of the crystal lattice) [117]; • It creates risks of the formation of chemically unstable dielectric carbides with a ensuing consequences for equipment and personnel (Al₃C₄ or Al₂(C₂)₃) [44,

Discussion
A comparison of the performance of electrical discharge machining alumina was carried out for the current study's data and previously published ( Table 9). The present results were compared with those obtained with a brass electrode tool in a water-based medium and those obtained using hydrocarbons. The data indicate that the developed method is superior to:

•
The previously developed technique of electrical discharge machining using a brass electrode tool in water-based working fluid [80] and; • The published data on electrical discharge machining alumina in hydrocarbons using monolayer copper foil of 6 µm in thickness [65]. The results published for the technique using carbon tape [66] exceed the values of the present study but demonstrate significant shortcomings typical for sinker type of machining and working with carbons and hydrocarbons:

•
The manufacture of a unique electrode is required; • There is no possibility of processing ruled surfaces; • Graphite particles exhibit electrical anisotropy (electrical properties are different in different directions of the crystal lattice) [117]; • It creates risks of the formation of chemically unstable dielectric carbides with all the ensuing consequences for equipment and personnel (Al 3 C 4 or Al 2 (C 2 ) 3 ) [44,[68][69][70].
The relationship between productivity and electrical properties of coatings and assisting powders is shown in Figure 10. The study [65] was not considered due to the measured data absence for the electrical resistance/conductivity of the developed coating and the absence of the assisting powder used in the study. For [66], the available data for the graphite were taken into consideration. The specific electrical conductivity is provided in Table 4. The band gap (E g ) was taken as an average value of 0.4-1.0 eV [118][119][120]. The relationship between productivity and electrical properties of coatings and assisting powders is shown in Figure 10. The study [65] was not considered due to the measured data absence for the electrical resistance/conductivity of the developed coating and the absence of the assisting powder used in the study. For [66], the available data for the graphite were taken into consideration. The specific electrical conductivity is provided in Table 4. The band gap (Eg) was taken as an average value of 0.4-1.0 eV [118][119][120].

Conclusions
The paper solves a scientific and technical problem relevant to modern mechanical engineering and metalworking, which consists of increasing the productivity of electrical discharge machining of dielectric material, alumina, using the combined approach using assisting copper tape coating of 40 µm and TiO₂ powder-mixed dielectric medium. Using TiO₂ powder-mixed water-based dielectric medium was proposed for the first time. The choice of water as a dielectric basis is substantiated by the safety of the personnel and equipment since the dissociation of hydrocarbons creates risks of the formation of chemically unstable dielectric carbides (methanide Al₃C₄ and acetylenide Al₂(C₂)₃). The choice of the powder is substantiated by the electrical properties of titanium dioxide.
The conducted work allowed us to establish relationships between the performance of the machining Al₂O₃ ceramics and experimental factors such as pulse frequency and powder concentration. The highest values for the productivity of 0.0083-0.0084 mm 3 /s were achieved for pulse frequency 2 kHz and 5 kHz and TiO₂ powder concentration of 150 g/L. This exceeds the analog for copper tape coating by 1.64 times and has the advantage of not using hydrocarbons.

Conclusions
The paper solves a scientific and technical problem relevant to modern mechanical engineering and metalworking, which consists of increasing the productivity of electrical discharge machining of dielectric material, alumina, using the combined approach using assisting copper tape coating of 40 µm and TiO 2 powder-mixed dielectric medium. Using TiO 2 powder-mixed water-based dielectric medium was proposed for the first time. The choice of water as a dielectric basis is substantiated by the safety of the personnel and equipment since the dissociation of hydrocarbons creates risks of the formation of chemically unstable dielectric carbides (methanide Al 3 C 4 and acetylenide Al 2 (C 2 ) 3 ). The choice of the powder is substantiated by the electrical properties of titanium dioxide.
The conducted work allowed us to establish relationships between the performance of the machining Al 2 O 3 ceramics and experimental factors such as pulse frequency and powder concentration. The highest values for the productivity of 0.0083-0.0084 mm 3 /s were achieved for pulse frequency 2 kHz and 5 kHz and TiO 2 powder concentration of 150 g/L. This exceeds the analog for copper tape coating by 1.64 times and has the advantage of not using hydrocarbons.
The comparison of the performance (0.0083-0.0084 mm 3 /s) with the performance of the analogs shows that the results may correlate with the electrical properties of the assisting materials: the electrical resistance of the deposed coating and the band gap of the used powder. However, it requires further research.