Simulation of Mechanical and Thermal Loads and Microtexturing of Ceramic Cutting Inserts in Turning a Nickel-Based Alloy

Abstract

This paper is devoted to the problem of wear resistance in square Si3N4 ceramic cutting inserts, which exhibit high hardness and strength, in combination with brittleness, and are subject to increased mechanical and thermal loads in machining super alloys for aviation purposes (e.g., a nickel-based alloy of Inconel 718 type). Microtextures were proposed to reduce the intensity of the contact loads on the pad between the cutting edge and the workpiece. The simulation of the mechanical and thermal loads demonstrated the superior ability of the faces with the preformed microgrooves (125 µm in width) compared to microwells (ø100 µm). The tense state was 4.97 times less, and deformations were 2.96 times fewer. The microtextures hamper the development of thermal fields at 900 °C. Two types of microtextures (210 µm-wide microgrooves and microwells 80 µm in diameter) were produced on the rake faces of the cutting inserts via an innovative and integrated approach (the electrical discharge machining of dielectrics using a multifunctional electro-conductive assisted and wear-resistant TiN coating and TiO₂ powder mixed suspension). The TiN coating was deposited via magnetron vacuum plasma sputtering (95%N₂/5%Ar). The failure criterion in turning was 400 µm. An increase of 30% in tool wear resistance was demonstrated.

1. Introduction

The problem of the intense wear of the cutting edge under increased mechanical and thermal loads during cutting is well known [1,2,3]. Many scientists have devoted their research to the effect of microtexturing on the performance and operational life of cutting tools [4,5]. Most studies have addressed the problem of cutting when turning mild steel [6,7], iron-nickel-based superalloys [8,9], titanium [10], and aluminum [11] alloys by using a carbide tool with lubricant (

Table 1. Literature devoted to research into the effect of microtexturing on the performance of the cutting tool.

Cutting Tool	Coating	Workpiece	Microtextures	Testing	Performance Effect	Reference
Carbide cutting insert (tungsten carbide, K313)	Uncoated	1045 steel	Microwells of 70 µm in a diameter by a femtosecond laser	Turning, V = 120 m/min, t = 0.1 mm, s = 0.3 mm/rpm, with lubricant	Reduced the mean cutting forces by 10–30%, and the chip–tool contact length by 30%	[6]
Carbide cutting insert (WC/Co-based)	Uncoated	1045 steel	Grooves/wells of by a femtosecond laser	Turning, with lubricant	Reduced the temperature by 21.7%	[7]
Carbide cutting insert (CNMA120408-KR-3225)	TiCN + Al2O3 + TiN composition, CVD	GH4169, Fe/Ni-based superalloy	Grooves/wells of 50 µm in a width/diameter by a femtosecond laser	Turning, V = 50 m/min, t = 0.2 mm, s = 0.2 mm/rpm, with lubricant	Reduced the temperature by 23%	[8]
Carbide cutting insert (TK20)	Multi-nanocomposite coating ((CrTi)N/(AlTi)N layers alternation + (AlTiCr)N/SiN nanostructure), PVD	C0.3Cr15Ni35Mo7Mn7Fe, Fe/Ni-based superalloy	4 lines of microwells of 30 µm in a width by laser and nanoindentation	Turning, V = 17–27 m/min, s = 0.1–0.15 mm/rpm, t = 0.3–0.5 mm, with MoS2-based lubricant	Increased durability by 1.3–1.5 times	[9]
Carbide cutting insert (WC/Co-based)	Uncoated	Ti-6Al-4V alloy	Microwells by a side-pumped solid-state laser and EDM	Turning, V = 90 m/min	Reducing in the cutting forces	[10]
Carbide cutting insert	Uncoated	Al7076-T6 alloy	Microgrooves of 10 µm in a width by a femtosecond laser	Turning, with lubricant	Reduced the temperature by 16%	[11]
Carbide cutting insert (WC/TiC/Co-based)	Uncoated	Not relevant	4 microwells of 150 µm in a diameter by micro-EDM	Sliding, with MoS2 solid lubricant	Reduced the coefficient of friction by 5 times	[12]

). These studies give practical recommendations on various texture microgeometries, the technology of their production (considering thermal and mechanical loads in friction pads in the context of natural experiments), and tests of friction-sliding [12]. The microtexturing of the cutting tool has proved effective in increasing the durability of the tool by reducing cutting forces, residual stresses, cutting temperature, and the chip–tool contact pad for reducing the forces of friction. It also reduces the chemical activity of the workpiece material on the tool material, reducing its adhesion to the cutting edge, which can be seen in examples of carbide tools [13,14] and Al₂O₃/TiC cutting ceramics [15,16], including those that use soft-coating [17]. Meanwhile, one of the most promising materials for machining modern aviation nickel-based heat-resistant alloys, superalloys, anti-corrosive and hardened steels, namely cutting ceramics (Si₃N₄, SiAlON, Al₂O₃, etc.), remains beyond the due attention of the scientific community, but is of practical interest to the industrial sector of the economy. As can be seen from the published works, mainly SiAlON [18,19], Al₂O₃ + SiC [20], and Al₂O₃ + TiC [21] ceramic tools are used for cutting nickel-based superalloys.

Microtextures can be applied to the rake face using any type of suitable processing method: laser ablation, plastic deformation (nanoindenter), or electrical discharge machining.

Laser ablation provides an unsuitable surface condition of the ceramics in using for microtexturing of the ceramic inserts:

• the presence of microcracks due to the shrinkage of the molten material as it solidifies [22,23];
• the presence of surface morphology subjected to repeated focal overheating with the creation of near-track/weld stress zones (heat-affected zones) [24,25];
• active evaporation of the lightest materials from the processing zone [26,27];
• formation of a transitional nanoporous interlayer consisting of more refractory material components [28,29];
• increased formation of brittle structures in the near-surface layer (the formation of second-order secondary compounds on the heating surface, due to the material’s active oxidation as a result of the interaction of the molten material of the cutting insert/substrate and coating with the environment) [30];
• splashing of material from the processing zone due to thermal shock [31,32].

Nanoindenters have demonstrated outstanding results in laboratory testing [9], allowing meshes with improved surface properties to be obtained via plastic deformation with a force of 20 N. Square-shaped microtextures were formed on the rake face of the carbide cutting insert, with a width of 30 µm and a maximum depth of 7 µm. We tested the operational life of a cutting tool coated additionally with multilayer and nanocomposite coatings of the following compositions.

• (TiCrAlSi)N coating was used as a basis;
• The basic layer consisted of (CrTi)N and (AlTi)N layers that alternated;
• An (AlTiCr)N/SiN nanocomposite layer was on top of this;
• The nanocomposite layer was formed as an amorphous Si₃N₄ with evenly distributed (AlTiCr)N crystals 5 nm in size.

Resistance tests were conducted with a cutting depth of 0.3–0.5 mm and a feed of 0.1–0.15 mm/rev; the maximum temperature in the cutting area was 240–330 °C. The operational life of the cutting insert was increased by ~1.4 times. However, the nanoindenter allowed them to obtain a microtexture geometry of a restricted form of meshes, when simulation results identify a better option, that is, linear grooves up to 25 µm deep with a step of 0.3 mm.

The difference in the surface and subsurface layers’ microstructures produced using various machining methods is presented in [33]. For conductive materials, precision finishing can be replaced by electrical discharge machining [34]. Electrical discharge machining makes it possible to obtain a more ordered geometry of microtextures:

• linear microtextures with a continuously rewinding tool electrode (wire electrical discharge machining); each groove may be identical to the previous one due to the constant updating of the tool electrode in the processing area [35];
• regular microtextures made using electrical discharge die sinking with a prefabricated electrode [36];
• regular microtextures made using electrical discharge drilling with a universal hollow wire electrode [37].

The only obstacle to using high-performance electrical discharge machining methods in microtexturing the cutting inserts may be the absence of the electrical conductive properties of oxide and nitride ceramics (such as SiAlON, Si₃N₄, and Al₂O₃) that are used in producing the most wear-resistant cutting inserts for cutting heat-resistant materials mainly used in the aviation industry (as gas turbine engines and other key parts of airplanes are subjected to the extreme operating conditions): nickel-based superalloys. However, the problem of machinability can be solved using innovative methods of electrical discharge machining of the insulating materials, which allow us to produce kerfs in the Al₂O₃ insulating ceramics with a depth of up to 54.16 ± 0.05 µm and a blank thickness of up to 5.00 mm, using a combination of the assisting electrode technique and powder-mixed electrical discharge machining [38,39]. This approach has also shown promising results in the electrical discharge machining of Si₃N₄ ceramics. However, the published outstanding results have not been reproduced [40]. The combined technique is discussed in detail in [41,42]. The assisting electrode technique uses an additional conductive coating (self-adhesive tape, PVD coating) to provide the outer layer of the insulating workpiece with conductive properties, to address, therein, the initial electrical impulses and discharges that are present when an underlayer of the workpiece is subjected to the thermochemical dissociation [43,44,45]. Powder-mixed electrical discharge machining enhances the productivity of the method by addressing a part of the electrical pulses to the evenly dispersed powder granules in the interelectrode gap, thus improving the electrical conditions between electrodes [46,47,48].

In the present research, the microtextures were developed and adapted for production by two electrical discharge machining technologies that use a universal electrode tool:

• Wire electrical discharge machining, taking into account the spark gap and wire diameter of 0.25 mm;
• Electrical discharge drilling, taking into account the minimum possible diameter of the tool electrode.

The developed microtextures were subjected to a simulation of the typical mechanical (cutting depth of 0.5 mm and feed of 0.5 mm/rev) and thermal loads (900 °C) involved in cutting nickel-based alloys. Based on the simulation data, two types of microtextures were formed on the cutting inserts by electrical discharge machining using multifunctional TiN-coating, a TiO₂ powder suspension, a wire tool electrode of ø0.25 mm, and a tungsten rod of ø0.1 mm. The produced microtextures (microgrooves of 0.21 mm in width and 0.04 mm in depth and microwells of ø0.08 mm in diameter and 0.03 mm in depth) were subjected to optical microscopy and profilometry. The wear resistance tests were conducted to confirm the results of the simulations (the failure criterion was 400 µm). The developed models and the method of simulation may be used for pre-evaluation of the developed microtextures.

2. Materials and Methods

2.1. Ceramic Cutting Inserts and Microtextures

Cutting inserts made of silicon nitride (Si₃N₄) tool ceramics were chosen as the object of study, with the following ANSI coding (Figure 1a):

• SNGN150716T02520 6190, Sandvik Coromant (Sandviken, Sweden) (a square-shaped indexable insert for turning cast iron and hard alloy).

This type of insert was modeled in the Solid Works computer modeling system (Dassault Systèmes SE, Vélizy-Villacoublay, IDF, France). For the developed inserts, computer drawings were generated following the requirements of GOST 2.051 (a unified system for design documentation in the Russian Federation). The requirements for manufacturing microtextures (in terms of their shape, dimensions, and roughness parameters) are formed based on technological experience. Therefore, it is recommended to control the design and technological elements of a given microtexture’s shape with a tool (electrode), thereby observing the roughness requirements and exercising visual control.

2.2. Modeling of Mechanical and Thermal Loads

Loads corresponding to the experimental data obtained during the machining of the titanium and nickel alloys [49,50] were imposed on each insert using the finite element method, using mechanical load modeling in the ANSYS engineering simulation software environment (version R19.2, ANSYS, Inc., Canonsburg, PA, USA) and Solid Works (systems for finite element analysis (FEM) for computer-aided engineering calculations (CAE)), and according to the following dependencies [9,51]:

$$P_z=262·t^{0.8}·s^{0.75},$$

(1)

$$P_y=200·t^{0.7}·s^{0.6},$$

(2)

$$P_y=121·t^{1.1}·s^{0.5},$$

(3)

where P_z, P_y, P_x are the components of cutting forces along the corresponding axes, N; t is cutting depth, mm; s is the feed, mm/rev. In the developed system, they were −89.475, −81.225, and −39.915 N when t = 0.5 mm and s = 0.5 mm/rev, respectively. Mesh was program-generated as follows: the mesh type was standard, and the global size was 0.1256 mm, with an accuracy of 0.0063 mm. The contact wedge dimensions were 0.987 × 0.695 mm, with an angle of 90°; the contact area was 0.4468 mm². The obtained data were approximated using PlanetCalc software (LLC “PLANETCALC”, Moscow, Russia).

Additionally, thermal loads were imposed in the contact zone of the cutting insert and workpiece, also in the ANSYS and Solid Works software. The thermal load was applied to the cutting edge of the insert, rake and flank faces at an incline of α = 10°, γ = 10°, according to the scheme of the cutting insert mounted in the holder (Figure 1b).

2.3. Microtexturing using Electrical Discharge Machining

An ARTA 123 Pro two-axis wire electrical discharge machine (NPK “Delta-Test”, Fryazino, Russia), with a wire electrode ø0.25 mm in diameter and made of CuZn35 brass, was used for microtexturing in the form of microgrooves (

Table 2. Main characteristics of wire electrical discharge machine ARTA 123 Pro.

Parameters	Description
Max axis motions X × Y × Z, mm	125 × 200 × 80
Tool positioning accuracy, µm	±1
Average surface roughness parameter Ra, µm	0.6
Dielectric medium	Any
Max power consumption, kW/kVA	<6/<7.5

). The machine enables the use of any type of working medium (water- or oil-based) [52].

The chosen range of factors based on [52,53] is presented in

Table 3. Wire electrical discharge machining factors.

Factor	Measuring Unit	Value
Experiment number	-	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
Operational voltage, Uo	V	108
Pulse frequency, f	kHz	5	5	5	5	10	10	10	10	15	15	15	15	20	20	20	20
Pulse duration, D	µs	0.5	1	1.5	2	0.5	1	1.5	2	0.5	1	1.5	2	0.5	1	1.5	2
Rewinding speed, vW	m/min	5
Feed rate, vF	mm/min	0.1
Wire tension, FT	N	0.25

. The operational current was 0.3–0.4 A [53]. Variations of two technological factors, pulse frequency f and duration D, were chosen for the experiments. This is because the machine has an open control system, and if the influence of the operational current and voltage on the kerf parameters is quite well studied [54,55], the impact of the pulse parameters will be of interest to the scientific community.

The scheme of microtexturing is presented in Figure 2. A cutting insert was immersed in the TiO₂ powder suspension [38]. The dielectric fluid level was 1–2 mm above the cutting zone, and the cutting insert was held for 5–7 min in a medium before processing to avoid thermal fluctuations. The obtained sample was wiped with a rag [56]. A total of 5 kerfs were produced for each factor’s set.

An electrical discharge super drill AgieCharmilles DRILL 20 (GF Machining Solutions, Biel, Switzerland) was used for microtexturing in the form of microwells (

Table 4. Main characteristics of electrical discharge super drill AgieCharmilles DRILL 20.

Parameter	Description
Max axis motions X × Y × Z, mm	300 × 200 × 300
Tool positioning accuracy, µm	Not provided
Average surface roughness parameter Ra, µm	Not provided
Dielectric medium	Deionized water
Max power consumption, kW/kVA	3.2/4

). The machine uses deionized water as a dielectric medium. During the experiments, the water supply was turned off, and electrical discharge machining was conducted with the full immersion of the workpiece in a TiO₂ powder and deionized water medium [38].

The microtexturing of the microwells was conducted using a tungsten wire ø0.1 mm in diameter. A hollow brass electrode with an inner hole diameter of ø0.1 mm was used as the main tungsten rod electrode holder. The choice of tungsten rod as a tool electrode was due to the high stiffness and thermal stability of tungsten. A scheme of microtexturing using an electrical discharge super drill is presented in Figure 3. The range of the chosen factors based on [57] is presented in

Table 5. Electrical discharge drilling factors.

Factor	Measuring Unit	Value
Experiment number	-	1	2	3
Operational voltage, Uo	V	10	8	6
Servo voltage, Us	%	50	40	60
Voltage pulse duration, Ton	µs	99
Pause duration, Toff	µs	5

. Variations of two technological factors, operational voltage U_o and servo voltage U_s, were chosen for the experiments. This choice is because the practical experience and conducted investigations [58] showed that among all the T_on/T_off combinations at this machine, only 99 µs/5 µs enables us to work with the insulating ceramics; meanwhile, the influence of operational and servo voltages on the microwell parameters remains poorly studied. A HomaFix 404 (20 m × 10 mm) copper tape 0.035 ± 0.0002 mm in thickness (JSC Electroma, Lipetsk, Russia) was used for the formation of a local reservoir for the TiO₂ powder suspension.

Formed microtextures (microgrooves and microwells) were controlled optically. The optical measurement error was calculated as follows [59,60,61,62]:

$${\mathsf{\delta }}_l=\pm 3+\frac{L}{30}+\frac{g·L}{4000},$$

(4)

$${\mathsf{\delta }}_t=\pm 3+\frac{L}{50}+\frac{g·L}{2500},$$

(5)

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. Optical control was carried out on an Olympus BX51M microscope (Ryf AG, Grenchen, Switzerland). The microrelief of the microtextures was controlled with a Dektak XT probe profilometer (Bruker Nano, Inc., Billerica, MA, USA) with a vertical accuracy of 5 Å (0.5 nm) and a radius of 12.5 µm.

2.4. Assisting Powder-Mixed Medium

A water-based suspension of titanium (IV) oxide, TiOx-271, grade (LLC “Titanium Investments”, Armyansk, Republic of Crimea, Russia), followed by GOST 9808-84 with a concentration of 150 g/L, was used as a dielectric medium. The average powder granule diameter was d₅₀ = 9.29–13.94 µm. Deionized water of ASTM D-5127-90 standard (LLC “Atlant”, pos. Marusino, Lyubertsy district, Moscow region, Russia) was the basis of the suspension. The choice of powder is grounded by conductive debris (TiN_x) forming in the discharge gap during the thermochemical dissociation of the insulating workpiece and powder material [63].

Titanium dioxide is an n-type semiconductor with a band gap E_g = 3.0 eV for rutile, and E_g = 3.20 eV for anatase [64] (anatase transforms to rutile when heated). The smaller the band gap is, the higher electrical conductivity the semiconductive material demonstrates [65,66]. The choice of suspension powder was grounded in [38]. The main advantages are that it is relatively safe in micro size (exposure may only cause irritation with minimal residual damage, according to NFPA 704), it exhibits chemical inertness to water, and it increases electrical conductivity in the presence of high heat (>1000 °C).

The powder subjected to granulometric analysis and optical microscopy is shown in [38]. An EL104 (Mettler Toledo, Columbus, OH, USA) laboratory balance with a measurement range of 0.0001–120 g ± 0.0001 g was used to weigh the powder portion. An AS200 basic analytical sieving machine (Retsch, Dusseldorf, Germany) with a test sieve (10 µm by ISO 3310-1) was used to pre-sift the powder. The choice of the test sieve is based on research results that proved that the smallest size led to the highest material removal rate and the lowest wear of the electrode [67]. At the same time, a higher conductivity of the material to be processed requires a larger discharge gap, which is about 170 and 200 µm for mild steel and copper [68], and 48–50 µm for materials with threshold electrical conductivity [69]. Thus, the electrical conductivity and the discharge gap are proportional [70]. The suspension was instantly stirred using an IL100-6/1 ultrasonic unit (LLC “Ultrasonic Technology—INLAB”, Saint Petersburg, Russia) at a frequency of 22 kHz to avoid powder conglomerations [71]. A frequency of 30 kHz–1 MHz is harmful to the human body (cavitation effect). After processing, the samples were cleaned with alkali.

2.5. Multifunctional Coating

It should be noted that any conductive coating can be used to provide conductivity to the outer layer on the surface of the insulating sample. Our research group has devoted a few studies to this; we proposed using a Ni-Cr coating (plasma vapour deposition), which has an effect on the electrical discharge machining ability of aluminum oxide [53], and a Cu-Ag multilayer sandwich coating also showed remarkable results in the electrical discharge machining of Al₂O₃ [38,39]. All possible options for the wide range of insulating ceramics are provided in [41]. However, difficulties in cleaning samples may be encountered after microtexturing; with that said, TiN-coating can be multifunctional, providing electrical conductivity to the outer layer and positively affecting the operational life of the cutting tool as a wear-resistant coating.

The cutting inserts were pre-coated with a multifunctional electroconductive TiN coating of 3.8–4.0 µm thickness. The thickness of the coating was chosen based on practical experience, since coatings with a thickness of less than 1.8–2.0 µm exhibit uneven electroconductive properties when deposited on insulating ceramic samples [72]. At the same time, this classical composition of the TiN coating provides excellent wear-resistant properties [73]. The technological process of the classical TiN coating was carried out with a multifunctional STANKIN unit (MSUT Stankin, Moscow, Russia), equipped with systems and devices that carry out the purification of the samples to be processed, as well as coating deposition via vacuum-arc evaporation of cathodes and chemical vapor deposition [74,75]. The nitriding process is described in detail in [76]; the complex nitride-based nanostructured and nanolayered coating deposition process is described in [77,78]. The coating was deposited via vacuum-arc evaporation of the cathode materials. The technological cycle included four stages: sample heating, gas discharge purification, ion purification, and a TiN coating deposition.

Table 6. Range of TiN coating deposition factors.

Factor	Measuring Unit	Value
Working gas	-	95%N2/5%Ar
Current strength	A	100
Operational voltage	V	−40
Pressure	Pa	0.9
Holding time	min	90

presents the TiN-coating deposition stage factors’ values. The factors were approbated previously and selected to provide maximum adhesive strength of the coating to the sample [79,80]. As it is known, oxide and nitride ceramics are not electrically conductive [81], and exhibit lower adhesion strength [82] than ceramic composites with electrical conductivity above the percolation threshold [83]. The coating deposition process was accompanied by diagnostics based on a Prony–Fourier multichannel inductive spectral analysis sensor [84]. Before coating, the ceramic samples were cleaned in an ultrasonic tank with a soap solution at 60 °C for 20 min, and with alcohol for 5 min [85,86].

The specific electrical resistance ρ of the deposited TiN coating was controlled with a Fischer Sigmascope SMP10 instrument (Helmut Fischer GmbH, Sindelfingen, Germany). The method is described in [39]. The coating thickness was measured with a Calowear instrument (CSM Instruments, Needham, MA, USA), using the spherical notch method (diameter of a ball of 20 mm) [87,88]. The coatings’ adhesion strength was determined with a M1 macroscratch tester (Nanovea, Irvine, CA, USA) according to the method from the ASTM C1624-05 standard [89]. A diamond cone with an apex angle of 120° and an apex radius of 100 μm was used as the indenter. The load was increased linearly in the range of 0.2–40 N. The loading rate was 1–3 N/min. Each sample was subjected to five tests.

2.6. Resistance Tests

A workpiece with a diameter of 100 mm was made of XH45MBTJuBP nickel-based heat-resistant alloy, the national standard of the Russian Federation GOST 5632-2014. This alloy is the analog of Inconel 718 used in many parts of gas turbine engines operating under increased mechanical and thermal loads. The composition and the main properties of the alloy are shown in [90]. The workpiece was machined on a ZMM CU500MRD lathe machine (ZMM, Nova Zagora, Bulgaria) as follows: cutting speed V = 300 m/min, feed s = 0.5 mm/rev, and cutting depth t = 0.5 mm. The testing was replicated ten times for each type of cutting insert. The flank wear pad was controlled every 2 min on a Stereo Discovery V12 Zeiss optical microscope (Carl Zeiss AG, Oberkochen, Germany). The failure criterion was 400 µm.

3. Results

3.1. Modeling and Development of the Microtextures

Various textures were formed, with a width of up to 50–200 µm, a depth of up to 20–30 µm, and a step of 150–300 µm, on the faces of the inserts at a distance of 0.150–0.175 mm from the cutting edge (

Table 7. Geometry of the developed microtextures.

Width/Diameter, µm	Depth, µm	Step, µm	Distance from the Cutting Edge, mm
50; 100; 125, 150, 200	20; 25; 30	150; 300	0.150; 0.175

). The microtextures were as follows:

• 2–3 microgrooves at an inclination of 45° to the axis of symmetry of the cutting edge on the insert’s rake face (designed for production via wire electrical discharge machining with a universal brass/tungsten wire electrode with a diameter of 0.125–0.1 mm);
• 6–9 evenly distributed microwells/holes symmetrically located to the axis of symmetry of the cutting edge on the insert’s rake face (designed for production using an electrical discharge super drill with a universal brass/tungsten wire ø0.1 mm in diameter);
• 2–3 wavy grooves with a radius of R300 µm at an inclination of 45° to the axis of symmetry of the cutting edge on the insert’s rake face (designed for production via electrical discharge die sinking with a complex profile electrode made of M2 copper, a second-order tool);
• 3 × 3 lattice microtexture with a step of 200 µm at an inclination of 45° to the axis of symmetry of the cutting edge on the insert’s rake face (designed for production via electrical discharge die sinking with a complex profile electrode made of M2 copper, a second-order tool).

The developed microtextures were applied to the insert model (Figure 4). Thus, four variants of insert models with microtextures were obtained and optimized using computer simulation tools.

3.2. Simulation of Mechanical and Thermal Loads

The following types of microtextures were subjected to load modeling:

-

an insert without microtextures;

-

an insert with three straight microgrooves at an inclination of 45°;

-

an insert with a set of nine microwells/holes of ø0.125 mm in diameter;

-

an insert with three wavy microgrooves at an inclination of 45°;

-

an insert with a 3 × 3 lattice microtexture.

First, the loads were superimposed superficially. The results of modeling the tense state and deformations of four types of microtextures are shown Figure 5. It can be seen that the microwells demonstrated the lowest values for the tense state and deformations, and the microwells of four diameters were additionally subjected to simulation of mechanical loads. The data regarding the maximum stress and deformation at the cutting edge, when simulating mechanical loads, as the wedge of surface contact (pad of 0.987 × 0.695 mm) between the workpiece with the rake face of the cutting insert, are presented in

Table 8. Results of mechanical loads’ simulation at the cutting edge for a square-shaped Si₃N₄ cutting insert.

Developed Microtextures	Tense State, N/m2	Deformations, mm
Without microtextures	2.25 × 108	0.00074
Straight microgrooves	7.1 × 108	0.00246
Wavy microgrooves	1.21 × 109	0.00260
Lattice microtexture, 3 × 3	8.66 × 109	0.00977
Microwells of ø0.100 mm	1.43 × 108	0.00083
Microwells of ø0.125 mm	1.38 × 108	0.00056
Microwells of ø0.150 mm	1.54 × 108	0.00068
Microwells of ø0.200 mm	1.86 × 108	0.00616

. A graphical presentation of the results for microwells with various diameters is shown in Figure 6. The inserts with nine microwells with a diameter of ø0.125 and ø0.100 mm showed the best result in reducing the surface stress deformations at the cutting edge when applying mechanical loads in the zone of contact between the workpiece and the cutting insert.

The thermal loads were imposed for the following insert types:

-

an insert without microtextures;

-

an insert with three straight microgrooves at an inclination of 45°;

-

an insert with a set of nine microwells/holes of ø0.125 mm in diameter;

-

an insert with three wavy microgrooves at an inclination of 45°;

-

an insert with a 3 × 3 lattice microtexture.

The thermal modeling results showed that the insert without microtextures is subject to the greatest thermal load (Figure 7a). At the same time, the inserts with microgrooves and a lattice microtexture hamper the development of the contact thermal loads, and demonstrate differences in the pattern of the thermal fields’ development (Figure 7b,d,e). The straight microgroove demonstrated the best results in hampering the thermal fields’ development.

3.3. Properties of Deposited Multifunctonal Coatings

The specific electrical resistance ρ of the coated samples was ~30.79 ± 5.54 µm·Ω·cm. The results of the scratch tests are shown in Figure 8. The destruction of the coating on the samples takes the form of chips and cracks, and traces of plastic deformation are visible. The average value of Lc₁ was 37.07 N. No areas of complete destruction of the coating were found on any of the samples; therefore, Lc₂ was not determined. This confirms the strong adhesion between the deposited coating and sample.

3.4. Characterization of Microtextures

Figure 9 presents the results of the experiments with variations in the factors involved in the wire electrical machining of microgrooves (Figure 9a,b) and the electrical discharge drilling of microwells (Figure 9c,d). The experiments showed that the maximum depth of 63 ± 3 µm was achieved at f = 15 kHz, D = 1.5 µs in wire electrical discharge machining, while the kerf width was 210 ± 3 µm, which is suitable for forming microgrooves. At the same time, the maximum depth of the produced microwells was about 27 ± 3 µm, with a diameter of 80 ± 3 µm in electrical discharge drilling. This is smaller than the simulated microwell diameters of 100, 125, 150, and 200 µm, but can also be of practical interest to the industry.

The microphotographs and microrelief of the produced microtextures are presented in Figure 10. Microgrooves (Figure 10a,c) are well-formed recesses with even edges and drop-like erosion products (secondary structures of the second-order oxides [52]) deposited to the surface subjected to electrical erosion. Microwells (Figure 10b,d) are sphere-shaped recesses with traces of coating damage formed due to continuous rotation of the tungsten rod electrode. Microrelief (Figure 10d) demonstrates the drop-like structure deposited on the rake face with a height of up to 21.8 µm, which can be subjected to intensive wear during the first stage of the cutting process during the contact of friction surfaces, i.e., the workpiece and the cutting insert. Those irregularities are formed by secondary structures (Figure 10b). Based on the obtained data, microtextures were produced on the faces of the cutting inserts for resistance tests.

3.5. Wear Resistance of Microtextured Cutting Inserts

Figure 11 shows the dependences of wear chamfer size on the flank face of the cutting inserts, in turning the XH45MBTJuBP nickel-based heat-resistant alloy, for three types of TiN-coated Si₃N₄ cutting inserts: without microtextures, with microgrooves, and with microwells. The measured flank wear data are provided in

Table 9. The average flank wear of TiN-coated Si₃N₄ cutting inserts in turning the XH45MBTJuBP nickel-based heat-resistant alloy.

Cutting Time, min	Wear Chamfer on the Flank Face of TiN-Coated Si3N4 Cutting Inserts, µm
	Without Microtextures	Straight Microgrooves, z = 0.210 mm	Microwells, d = 0.080 mm
2	183.20 ± 3.00	122.60 ± 3.00	156.20 ± 3.00
4	282.80 ± 3.00	219.50 ± 3.00	189.80 ± 3.00
6	348.30 ± 3.00	305.20 ± 3.00	257.10 ± 3.00
8	418.50 ± 3.00	342.70 ± 3.00	286.30 ± 3.00
10	612.90 ± 3.00	421.50 ± 3.00	327.60 ± 3.00

. Wear chamfer images are shown in Figure 12.

The experimental results confirm the modeling of the mechanical and thermal loads and demonstrate that the operational life of the TiN-coated Si₃N₄ cutting insert was improved by more than 1.3. The average durability of the cutting insert with microwells of ø80 µm exceeded 10 min in turning the nickel-based heat-resistant alloy. The comparison of the experimental measurements of the flank wear chamfer after 10 min of turning the XH45MBTJuBP nickel-based heat-resistant alloy and the results of the computer model simulations are provided in

Table 10. Comparison of the experimental measurements of the flank wear chamfer after 10 min of XH45MBTJuBP nickel-based heat-resistant alloy turning, and the results of the computer model simulations.

Type of Cutting Insert	Experimental Data	Theoretical Data
	Wear Chamfer after 10 min of Turning, µm	Tense State, N/m2	Deformations, µm
Without microtextures	612.90 ± 3.00	2.25 × 108	0.74
Straight microgrooves	421.50 ± 3.00 (z = 0.210 mm)	7.1 × 108	2.46 (z = 0.125 mm)
Microwells	327.60 ± 3.00 (d = 0.080 mm)	1.43 × 108	0.83 (d = 0.100 mm)

. It should be noted that modern simulation software to imitate the wear process is not available. However, the technological issues of microtexture production limit comparisons even on a bigger scale. The production of microtextures on insulating ceramics is technologically limited, as seen in Figure 9. For all the possible types of microtextures, achieving a wide range of size groups was impossible due to varying electrical discharge machining factors, and the use of relatively universal tools, equipment, and specially designed assisting tools. Obviously, for a more detailed experimental study of the influence of microtexture dimension types on the wear chamfer value and the tool’s service life, the development of electrical discharge machining technology on the microscale and the appropriate tooling and equipment are required. A comparison between the experimental and theoretical data in the context of the dependencies of the obtained data on the summarized area of the contact pad is provided in Figure 13. Obviously, the experimental results depend not only on the contact area of the tool (the cutting insert) with the workpiece; the theoretical data have an exponential dependence on the contact area (y = y₀ e^a−b·x, where a and b are empiric constants; the dependence may be approximated to y = e^{3.85−9.52·x} with an average approximation error of ~10.34%, correlation coefficient was 0.99, determination coefficient was 0.98). The obtained results were compared with recently published work devoted to the microtexturing of a carbide cutting insert using nanoindentation method (

Table 11. Comparison of performance parameters in turning using a microtextured cutting insert.

Cutting Insert	Coating	Microtexture	Method of Production	Workpiece Material	Turning Condition	Performance Effect	Reference
Si3N4, square-shaped	TiN coating of 3.8–4.0 µm	3 lines of 9 microwells of ø80 µm, at a distance of 150–175 µm from the cutting edge	Electrical discharge drilling with a tungsten wire of ø100 µm in TiO2-powder suspension (150 g/L)	XH45MBTJuBP (C0.1Cr15Ni45Mo4.6Mn0.6Fe) nickel-based heat-resistant alloy	V = 300 m/min, s = 0.5 mm/rpm, t = 0.5 mm, no lubricant, Tmax = 900 °C	Increased the durability by 1.3 times	Current study
TK20 carbide alloy, square-shaped	Multilayer nATCRo3 coating ((CrTi)N/(AlTi)N layers alternation + (AlTiCr)N/SiN nanocomposite layer where (AlTiCr)N crystals in amorphous Si3N4)	4 lines of doubled square-shaped prints with a width of 30 µm, a depth of 7 µm, at a distance 181 µm from the cutting edge	Nanoindentation (plastic deformation) with a force of 20 N	C0.3Cr15Ni35Mo7Mn7Fe iron–nickel heat-resistant alloy	V = 17–27 m/min, s = 0.1–0.15 mm/rpm, t = 0.3–0.5 mm, MoS2-based lubricant, Tmax = 240–330 °C	Increased the durability by 1.3–1.5 times	[9]

).

4. Conclusions

The paper showed that applying developed measures, such as microtexturing, can be very effective in improving the durability of highly brittle materials, such as cutting ceramics of the Si₃N₄ type, during intensive mechanical and thermal loads (e.g., turning a nickel-based heat-resistant alloy, V = 300 m/min, s = 0.5 mm/rpm, t = 0.5 mm); the operational life of the cutting insert was improved by 1.3 times, compared with an insert without microtextures (which had a durability of 7.5 min). The developed measures include

• developing the various shaped microtextures;
• the adaptation of the developed microtextures according to the chosen technology of production (innovative assistance electrode electrical discharge machining in a TiO₂ powder mixed suspension);
• simulation of the mechanical (P_x = −89.475 N, P_y = −81.225 N, P_z = −39.915 N) and thermal loads (T_max = 900 °C) in the contact pad.

Innovative electrical discharge machining with an assisting electrode in a TiO₂ powder mixed suspension (an assisting electrode powder mixed electrical discharge machining) demonstrated that it is possible to produce the required microtextures (microgrooves of 210 µm width and microwells of 80 µm diameter) on an industrial scale. The results obtained during wear resistance tests showed that the numerical data are adequate for the results of the experiment.

Further research is possible in the context of applying the developed microtextures to other types of oxide and nitride ceramic cutting tools in cutting hard-to-machine metal alloys (Ni- and Ti-based) and hardened steels, including more detailed research on the electrical discharge machining effect on the microstructure, chemical content, and properties of the surface and subsurface layers. Identifying the dependence of the flank wear chamfer on the contact area of the tool (cutting insert) and the workpiece, and on other machining/turning factors, may be of practical interest to the industry, and requires additional theoretical and experimental research.

The development of microtexturing techniques for the electrical discharge machining of insulating ceramics at the microscale using multifunctional conductive and wear-resistant coatings and a powder mixed suspension has potential in the context of improving the operational life of the cutting tools in machining hard-to-machine metal alloys, and can be easily transformed from laboratory scale to industrial one, in order to accelerate the switch to the sixth technological paradigm.