Ceramic Cutting Materials and Tools Suitable for Machining High-Temperature Nickel-Based Alloys: A Review

Nickel-based superalloys are attractive to many industrial sectors (automotive, military, energy, aerospace, etc.). However, their physical properties make them difficult to machining using traditional tools. Therefore, new materials for the machining of Ni-based alloys are required. Ceramic-based composites could act as a tool to replace the current materials. The incentives for this paper are to provide an overview of existing ceramic composites and draw some conclusions that will help in solving the problem of choosing materials for the processing of Ni-based superalloys. Despite the diversity of ceramic composites in this work, the possibility of using the SiAlON ceramic for Ni-based alloy machining is highlighted.

Different alloying elements have their own contributions in affecting the properties of various grades of the superalloys. Al and Cr primarily impart resistance to oxidation, whereas elements such as Cr, Mo, W, Nb, Ta, Ti, and Hf are used to reinforce grain boundaries by forming carbides and precipitating along grain boundaries. is a series of problems in the usual processing of nickel-based superalloys including the following: • Due to the low ability to conduct heat, the thermal generated while processing of workpieces is not passed to the chips or the rough part but is concentrated on the cutting edges, which leads to crater wear and extensive plastic deformation of the cutting tool edge; • Increased crater wear results in severe damage of cutting instrument and plastic strain causes high resistance of the material against during the intrusion of the cutting tool; • Metal pickup and coating delamination due to the chemical reaction of Ni-based superalloys during machining reduces tool life; • High wear on the cutting edge of the tool generated by friction because of the occurrence of solid and abrasive intermetallic compounds in the microstructure of these alloys; • Changes in the microstructure of the alloys due to high heat generation during machining and generated residual stresses cause a further reduction in the cyclic life of the component; • In addition, the machining of these alloys also leads to high cutting emulsion consumption, the cost of which corresponds to the cost of cutting instruments [75].
Based on the above-listed drawbacks, the manufacturers are interested in minimising the losses at all stages in the processing chain. Therefore, new tool materials or other technical solutions for Ni-based superalloys machining are needed. The use of ceramics as a cutting tool could be proposed as a possible solution to these challenges. Ceramic tool materials exhibit very high hardness and wear resistance, high resistance to plastic deformation, and chemical stability. The diversity of ceramic tool grades appearing each year on the market indicates that these materials are promising. Ceramic tools are used for a broad range of materials and processing operations, including also high-speed finishing machining of superalloys. The most promising group of ceramic materials for machining of Ni-based superalloys is mixed nitride and oxide ceramic-namely, SiAlON [76][77][78][79][80][81][82][83][84][85][86]. SiAlON provides excellent resistance to high temperatures, mechanical strength even when subjected to high temperatures, good thermal shock resistance and good wear resistance, high toughness, and outstanding thermal shock resistance. In addition, this class of ceramic possesses improved sinterability, compared to silicon nitride. In our previous article, we provided basic information about this type of material [87]. Briefly, SiAlONs are formed when silicon nitride (Si 3 N 4 ), aluminium oxide (Al 2 O 3 ), and aluminium nitride (AlN) react together. α-SiAlON and β-SiAlON represent the two main phases of this type of ceramic. Each phase features its own properties, which can be contributed to the composites. This review paper provides an overview of ceramic tools for machining nickel-based superalloys including the effect of cutting parameters, use of coolant supply, and the integrity of the machined surface. Special emphasis is placed on SiAlON ceramic as the currently most promising candidate for the processing of these alloys.

Carbide for Machining Nickel-Based Alloys
The use of carbide as a cutting tool in the processing of nickel-based heat resistant super alloys (HRSA) was due to their high ratio of hardness, tensile strength, and fracture toughness [123]. It should be noted that the basic research of physical and mechanical, chemical, and operational processing conditions of carbide instruments during turning and milling of various structural materials, including also machining of hard materials, began from the 1970s to the 1980s of the previous century [108,124]. Koseki et al. [125] performed a comprehensive analysis of damage analysis of carbide instruments with commercial TiN PVD-coated at turning Ni-based superalloy (Inconel 718) [118]. Electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses showed that the treated material mechanically adheres to the tool, but there are no chemical bonds. These facts confirm the absence of diffusive and oxidative deterioration. The authors highlighted the development of extended wear in places of micro-defects in the form of pores and voids in the coating. Thus, it is necessary to minimise such defects. Hu et al. [126] suggested using the rake face chemical-mechanical polishing (CMP) to improve the cutting performance and the tool life. The study showed that in turning GH4169 nickel-based superalloy, this method made it possible to reduce the components of the cutting forces F x , F y, and F z by 18.26%, 11.37% and 13.81%, respectively, and the service life compared to the insert without processing by 35.92%. Currently, the main vector of the development of carbide instruments is aimed at using complexstructured, multifunctional, wear-resistant coatings [127][128][129]. Vereschaka et al. [127] noted that the architecture of multilayer wear-resistant coatings should be designed based on the properties of the material being processed. First, an intermediate high-adhesion layer must be applied, followed by functional layers, which make it possible to resist abrasive and diffusion wear during the turning process. It is also worth noting that these studies on the use of carbide tools for machining nickel-based HRSA have not been interrupted and are being carried out at the present time, making it possible to use this tool as the main material in the industry. Despite the low cost of carbide tools, they are generally employed at relatively low cutting speeds (~50-80 m/min), because they cannot withstand the high temperature and stresses in the cutting zone encountered during machining. Therefore, other classes of ceramics are needed for machining nickel-based HRSA alloys, which will be discussed in the following sections.

Cubic Boron Nitride for Machining Nickel-Based Alloys
One such material is cubic boron nitride (CBN), which is the second hardest of all known materials and has received significant attention as a material for cutting tools. Costes et al. [113] proposed using cubic boron nitride instead of carbide as a cutting tool for the machining of Inconel 718. It was noted that the main causes of tool wear in processing CBN nickel alloys are adhesion and diffusion, due to their chemical affinity and conventional abrasion. The formation of wear along the front surface was observed, as well as uneven wear along the front surface in the form of a dimple, caused by thermal effects from the edge to the centre. Arunachalam et al. [130] indicated, that in the process of turning nickelbased alloy, diffusion of Nb, Cr, Fe, and Ni from the workpiece into the tool occurs. Mutual diffusion was observed not in the entire volume of the material but in specific areas, which allowed them to conclude that certain conditions were created during the cutting process, in which the chemical reaction between the tool and the processed materials was minimal. In their study, Harris et al. [131] aimed to find a relationship between the CBN content, the TiC/TiN bonding phases, the grain size (1-15 µm), and the physical and mechanical properties of the tool (hardness/temperature) using Vickers indentation method as an evaluation tool by heating under various conditions. Kaya et al. [132] conducted a study of the development of wear of polycrystalline diamond (PCD) and cubic boron nitride (CBN) tools during high-speed (cutting speed Vc = 70-250 m/min) turning of NiTi alloys. It was found that both tool materials are sensitive to increased cutting speed. At cutting speeds up to Vc = 130 m/min, the dominant wear is adhesion wear and abrasion along the insert main flank surface caused by mechanical stress. With an increase in the cutting speed, thermal effects, mainly spalling of grains from pressure on the front surface were observed, thus resulting in the observed diffusion, chemical, and oxidative types of wear, leading to premature failure of the CBN. This study allows us to conclude that there is a limited increase in cutting speed due to the physical and mechanical characteristics of CBN.
Based on literature data, it should be noted that these tools are used to machine nickel-base alloys of hardness equal to or greater than 340 HV. The recommended speed range of these tools to machine Inconel 718 is from 120 to 240 m·min −1 . However, the manufacturing process of the CBN cutting tool is an extremely difficult, expensive, and time-consuming task.

Ceramic Instruments for Machining Nickel-Based Superalloys
In the processing of difficult-to-machine steels and alloys, ceramic tool materials are slowly replacing hard alloys due to their physical and chemical properties [3]. Ceramic tools are used for a wide range of operations and materials, including high-speed finishing of high-temperature alloys. The specific properties of nickel-based heat-resistant alloys when properly used can provide high productivity. The variety of grades of ceramic instruments and compositions appearing on the market every year makes it possible to assert the promise and interest in the material. Currently, ceramic cutting tools (ceramic inserts) are mainly represented by NTK Cutting Tools (USA), NikkoTools (Italy), Sandvik Coromant (Sweden), ISCAR (Israel), Tungaloy Corporation (Japan), CeramTec (Germany), etc. Silicon nitride-and alumina-based ceramics are particularly highlighted in the processing of Nibased superalloys. Zhao [133] identified four groups of instrumental materials in his work: Al 2 O 3 -based ceramics, Si 3 N 4 -based ceramics, SiAlON-based ceramics, and ceramic matrix composites. He noted the growing need for ceramic instruments and highlighted the stages of development from 1938 to the present day. One of the most promising ceramic materials for efficient turning Nickel-Based HRSA is Al 2 O 3 with the addition of TiC and other hard particles such as TiN, (W, Ti) C, Ti (C, N), TiB 2 , and SiCw [134,135]. Table 2 presents various categories, compositions, and properties of Al 2 O 3 -based ceramic instruments for turning hardened steels and heat-resistant alloys. Usually, Al 2 O 3 /carbide or Al 2 O 3 /nitride composite grades are selected for machining ferrous metals (such as steels and cast irons) at high cutting speeds. This is because of their hardness, wear and heat resistance, chemical stability, as well as their ability to achieve a high level of accuracy and finish. Interrupted cuts are only recommended with very small chip sections and strong cutting-edge designs, as found in round or square inserts with T-lands. Al 2 O 3 /SiC (w) composite grades are used for machining heat-resistant alloys, such as those which are nickel based, in virtue of their increased resistance to fracturing. by adding micro-sized WC and nano-sized TiC particles into the micro-sized Al 2 O 3 matrix, Al 2 O 3 /WC/TiCn micro-nanocomposite ceramic tool material LWT-1 ( Table 2) was developed at Shandong University. This was achieved through the use of the hot-pressing technique [135]. To prevent the formation of agglomerative TiC nanoparticles (average grain size of 140 nm), the surfactant polyethylene glycol (PEG) and deionised water were used as a dispersant and dispersing medium, respectively. This was carried out with the aim of obtaining a well reagglomerated and uniform suspension of TiC nanoparticles. These particles were then mixed with micro Al 2 O 3 (average grain size of 0.5 µm), WC powder (average grain size of 0.4 µm), and the sintering additives MgO and NiO. The mixed slurry was ball milled for 48 h and then dried at 330 • C in a vacuum. The powder mixture was sieved through a 120-mesh sieve and loaded into a cylindrical graphite die with an inner diameter of 42 mm. The specimens were then sintered via the hot-pressing technique with flowing N 2 at a temperature of 1700 • C for 10 min under a fixed uniaxial pressure of 30 MPa. Grade AWT10 (Table 2) is another micro-nanocomposite ceramic tool material that was hot-pressed by adding micro-sized (W,Ti)C and nano-sized Al 2 O 3 particles into a micro-sized Al 2 O 3 matrix [135]. In their review, Xikun et al. highlighted a line of commercial and experimental Si 3 N 4 -based ceramic instruments for processing difficult-to-machine alloys due to their high thermal resistance, high thermal conductivity, low coefficient of thermal expansion, and fracture toughness. There has been a great success in toughening research and application of cutting tool materials with Si 3 N 4 as raw materials, Y 2 O 3 , Al 2 O 3 , MgO, ZrO 2 , HfO 2 , as sintering additives, additives of carbide strengthening phase, or SiC w , SiC p (Table 3) [115,133].

Features of SiAlONs and Their Application in the Processing of Nickel-Based HRSA
It is known that nickel alloys are very difficult to cut materials due to the high chemical affinity and poor thermal conductivity that they have, in conjunction with the presence of hard-abrasive carbides in their microstructure and the work-hardening that occurs during machining. For this reason, in order to carry out efficient machining of this type of alloy, it is necessary to select the appropriate cutting tool and define the most optimal machining parameters for the tool-part pairs. In mass production, the use of high-speed machining is essential. Unfortunately, this type of machining for nickel alloys using cemented carbide is not feasible due to the loss of the characteristics of the cutting tool mainly related to the ultra-high local temperature that occurs in the tool-part contact zone, which leads to a reduction in the useful life of the tool. One of the ways to avoid the negative effects (high temperature and stresses in the cutting zone) that occur by machining nickel alloys at high cutting speeds (V = 200-750 m·min −1 ) is the use of ceramic cutting tools, which can keep its properties (chemical stability, hardness, thermal shock resistance, and high wear resistance) for a long time even at high temperatures. The silicon nitride (Si 3 N 4 )-based ceramic known as SiAlON has better thermal properties and toughness than Al 2 O 3 and is used widely to machine superalloys. SiAlON cutting tools have some special features such as good fracture toughness, hardness (even at high temperatures), and resistance to sudden temperature changes, which make it a suitable material for machining various difficult-to-cut materials including nickel alloys. SixAlxOxNx is characterised by high chemical inertness and low coefficient of thermal expansion; therefore, during cutting, there is more uniform and predictable wear, without grooves and chipping. There are several kinds of SiAlON ceramics, such as α-SiAlON, β-SiAlON, and their combination (α + β). The most commonly used compositions at present are β-SiAlON and (α + β) SiAlONs, which contain a substantial excess of sintering aids. However, the field is still changing with compositions developing to suit specific applications.

α-SiAlON
α-SiAlONs are isostructural with α-Si 3 N 4 in which chemical binding of m and n (Si-N) are substituted by Al-N (m) and Al-O (n) bonds, respectively, the schematic diagram of which is shown in Figure 2 [136][137][138]. In addition, supplementary cation for the stability of crystal lattice of α-SiAlONs is needed. α-Sialons can be presented by the chemical formula M v+ x Si 12-m-n Al m+n O n N 16-n , where Li, Mg, Ca, Y, and/or a rare earth element refer to M, and by considering its ionic radius [138] or with using quenching. For entry of cations with considerable size in structure, the presence of other cations is required. At high temperatures, the α-phase, specifically with high x (x is typically between 0.3 and 1.0) is stable, but during the thermal processing, it can transform to β-phase sialon or other crystalline or glassy phases. The α-SiAlONs is used due to its high hardness and temperature resistance due to a decrease in the amount of the residual glassy phase, but for the same reasons, these materials have increased fragility, which requires the introduction of materials with improved fracture toughness but without a significant decrease in hardness [139][140][141][142][143]. In his study, Ekström [144] carried out a series of experiments with α-SiAlON, from which a number of conclusions can be drawn as follows: 1.
α-SiAlON can be used as a base in a composite material with the addition of silicon carbide or molybdenum-disilicide-reinforcing fibres, which have sufficient chemical compatibility. However, with the introduction of SiC w , it was not possible to increase the strength, and with the addition of MoSi 2 , a sharp drop in hardness is observed, which is unacceptable.

2.
It was found that it is impossible to use rare earth elements with large ion sizes (for example, Sm, and Nd) as a stabiliser of α-phases in ceramics used at temperatures above 1000 • C since they decompose. However, these types of ceramics can be used at low temperatures.

3.
The most favourable alloying elements, from the point of view of stability at high temperatures, are rare earth elements with small ion sizes, such as yttrium or ytterbium, which form α-phases.
In order to prolong the useful life of the α-SiAlON-based cutting tool, different works have been developed based on the addition of different phases to the ceramic structure. For example, Salamon et al. and Rosenflanz et al. [145][146][147] used α-SiAlON with other second phases additives to design new ceramic instruments, which provide a significant enhancement in the mechanical behaviour and chemical stability. All starting powders were homogenised in water (using the buffer) and freeze granulated and biaxially pressed. Heating regimes were carried out in the HIP furnace at 1760 • C for 1 h in ≈100 MPa Ar (N 2 ). Different heating and cooling rates were applied to obtain different phase compositions from the same starting powders. Heating regimes are labelled as HIP 1 (fastest cooling rate) and HIP 5 (slowest cooling rate). Table 4 shows the chemical compositions studied in this work. It was found that hardness (HV1) and fracture toughness (K 1C ) for 1-5 compositions are around 20.2 and 6.8; 14.2 and 6.2; 17.0 and 6.7; 15.1 and 6.7; 17.7 and 6.2; 14.4 and 7.0; 19.7 and 6.9, respectively. With regards to the tool life of studied materials, the best behaviours (longer life) are shown in samples 3, 5 and 6 (31.0, 27.5 and 23.3 min, respectively), compared with standard cutting instruments made from β-Si 3 N 4 . The cooling influence was evaluated by comparison of phase composition of sample number 7. It was found that the presence of defects and polytype 21R (SiAl 6 O 2 N 6 ) has an adverse impact on cutting behaviour [145]. However, the presence of homogeneity of the microstructure and pure αand β-SiAlONs with a good ratio between elongated and submicron grains provide a positive influence on cutting performance. Moreover, it was shown that pure αor β-SiAlONs have better cutting performance, compared with β-Si 3 N 4 , and inhomogeneities and polytype 21R have higher effects on cutting performance than on macro-mechanical properties (HV1, K 1c ); it was also assumed that changing cooling parameters also leads to change in the phase composition (1 vs. 7).

β-SiAlON
β-Sialon are formed by substituting silicon in the β-Si 3 N 4 structure by aluminiumprovided valence compensation by replacement equivalent concentration of N by O. A general formula of β-SiAlONs is Si 6-z Al z O z N 8-z , with 0 < z < 4.2, where z (Si-N) bonds are replaced by z (Al-O) bonds [147]. β-SiAlONs are isostructural with β-Si 3 N 4 . β-SiAlON ceramics are promising materials for applications involving high temperatures, highly corrosive environments, and high mechanical stress because they exhibit high strength retention and high thermal shock resistance at elevated temperatures and good erosion resistance and low creep [148][149][150][151][152]. Compared with Si 3 N 4 , β-SiAlON ceramics are easily sintered and fabricated and can be easily combined with other materials to strengthen or improve the properties of ceramics [153,154].
Due to the fact that for consolidation of β-SiAlON-based ceramic samples with complex geometry and improved mechanical properties (σ f up to 1000 MPa and K 1c~8 MPa·m 1/2 ), conventional sintering techniques were possible, this type of ceramic dominated at the initial stage [155]. Based on high mechanical characteristics, recently, the β-SiAlON has been used even in additive technologies to obtain parts with complex spatial shapes; for instance, Tian et al. [78] used Si 3 N 4 , Al 2 O 3 , and AlN particles as raw materials for the design of β-SiAlON composition with particular base crystalline phases. In this work, materials for performing ceramic specimens with a β-SiAlON matrix were obtained by stereolithography technology. The manufacturing technology consisted of the following procedures: the green body β-SiAlON with other SiAlON phases were degreased and then sintered under the following technological modes: working medium-inert gas, sintering temperature T = 1800 • C, holding time 1 h, pressure P = 3 GPa. Moreover, the authors studied the effects of the different Al 2 O 3 contents on shrinkage loss, phase composition, relative density, microstructure, mechanical property, and hardness. XRD methods have shown that the major phase of the ceramics was β-SiAlON, and the other phase impurities of it were Si 5 AlON 7 , Si 3 . 1 Al 2 . 9 O 2 . 9 N 5 . 1, and Si 2 Al 4 O 4 N 4 , with mainly intergranular phases of Y 3 Al 5 O 12 , YAlO 3 , and Al 2 O 3 . The sintered β-Si 5 AlON 7 ceramics had a relative density within the order of 90 percent, with Vickers hardness (HV 5 = 16.7 GPa) bending strength G f = 465 MPa and fracture toughness K 1C = 4.84 MPa·m 1/2 . In addition, Zhuo et al. [156] found that the shrinkage of the material was 28.7 percent. The highest values of fracture toughness at ambient temperature were observed for dense β-SiAlON ceramic with low z values (z~1) and microstructures consisting of elongated crystals and grain boundary glass. However, the hardness values (14-15 GPa) were found relatively low due to the presence of the residual grain boundary glass (~10 GPa) [157]. Therefore, in cutting tools, β-SiAlON is used as a reinforced phase that provides strength and toughness.

(α + β) SiAlON
The α-β SiAlON combinations with a minimum of glassy phase are currently the most promising materials for further use, both at low and high temperatures [2]. Figure 3 shows microstructure of as-sintered α/β-SiAlON composite. At the same time, α-SiAlON ceramic with equiaxed structure exhibits outstanding values of hardness (~22 GPa), compared to any other sialon ceramic. By varying the ratio of the αand β-phases in the raw material mixture, it is also possible to optimise the characteristics of the α/β-SiAlON mixed ceramics, where αand β-phases give high hardness and high-strength toughness, respectively [158]. Moreover, the strength, wear behaviour, and creep resistance of this ceramic can be significantly improved due to the low content of intergranular glass in produced details [157]. Ekström [144] noted that the most favourable ratio of α-β phases is approximately 1: 1. This α-β SiAlON ratio allows achieving high hardness HV 10 = 22 GPa and sufficient indentation fracture toughness K 1C = 5.5 MPa·m 1/2 . Due to the excellent properties of (α + β)-SiAlONs, they are used in the production of cutting tools. For example, Mandal et al. [159] developed (α + β)-SiAlON ceramics, which were fabricated in an aquatic environment using various oxides as additives to obtain the dense samples. Then, cast iron and superalloy were machined using sintered samples and commercially available ceramic inserts by turning and milling operations. It was found that the sintered (α + β)-SiAlONs show a hardness value of about 19 GPa, while the values of commercial Si 3 N 4 inserts are 15-16 GPa. Therefore, the authors argue that careful selection of starting composition with sintering additives and mixing technique allows the production of (α + β)-SiAlON ceramic that can be used in machining. Bitterlich et al. [160] introduced, for the first time, particles of SiC, WC, MoSi 2, and Ti (C, N) into the composition of (α + β) -SiAlON composites to use the material as a cutting tool. The authors noted an unsatisfactory result; however, points of growth of further optimisation of the composition are given in the work. The (α + β)-SiAlON composites with different reinforcing particles (SiC, Ti(C30N70), MoSi 2 , WC) were prepared by gas pressure sintering. After sintering the new composite material, diffusion of MoSi 2 and WC with the matrix appeared, leading to a change in the composition of the reinforcing phase, which affected their properties. The best tribological parameters were obtained for the material with 10 vol.% SiCw.

SiAlON Application in Machining Nickel-Based Alloys
Due to the physical and mechanical properties of SiAlONs used as cutting tools, their use in the processing of nickel alloys is very widespread. Much research has been conducted to determine the effectiveness of sialon application, compared to other ceramics [159][160][161]. For instance, Krebs et al. [162] investigated the use of ceramic tools whisker-reinforced SiAlON when turning on the clad alloy steel AISI 625 4130 via an automatic system for cladding a tungsten inert gas (TIG). It has been experimentally established that the use of cutting inserts from SiAlON for rough turning of clad alloy 625 is very effective. The use of corundum ceramics (Al 2 O 3 ) reinforced with whiskers (SiC w ) under the same conditions is ineffective [162]. The application of SiAlON inserts contributes to an increase in the removal rate of the work material when compared with the possibilities of using a carbide tool, including those with wear-resistant coatings. It should be noted that the kinetics of wear of SiAlON inserts was identical in a fairly wide range of technological parameters [163][164][165]. It was noted that most of the wear was formed uniformly over the main surface with the formation of a V-shaped groove with a notch. When turning with low cutting speed limits (Vc = 140 m/min), the most intense diffusion wear was observed along the main flank surface, which contributed to the formation of burrs on the workpiece surface. The optimal turning mode was achieved at Vc = 160 m/min and a feed rate of 0.25 mm/rev. In addition, it was noted that prolonged turning with worn inserts led to microstructural changes in the processed material and the formation of defects in the form of cavities. Lima et al. [166] conducted a study on turning a nickel-based superalloy Inconel 751 using argon and oxygen atmospheres. Three groups of materials were used as cutting tools: SiAlON (Si 3 N 4 + Al 2 O 3 ), whisker (Al 2 O 3 + SiCw), and mixed (Al 2 O 3 + TiC). Experiments have shown that cutting speed, tool geometry, and cutting fluid affect the type of wear and tool life. However, the best results have been achieved with SiAlON tools. Zheng et al. [81] investigated the wear mechanisms of a Sialon ceramic tool in ultra-high-speed turning Inconel 718. The main wear mechanisms identified in the experimental part were adhesive and mechanical wear. SEM and EDS, during critical wear on the inserts' edge, deposits in the form of separate elements Ni, Fe, and Cr were found, contributing to the more active development of wear. The author also noted a serrated morphology shaving, indicating that thermal deformation during the cutting process. Li et al. [167] determined in their work the optimal cutting speed when processing Inconel 718, at which there are no diffusion phenomena that contribute to the formation of cuts on the back surface and craters on the front surface. Nalbant et al. [168] studied the wear mechanisms of SiAlON tools when machining a nickel alloy with plates having different geometric shapes (RNGN 120700, and SNGN 120712). Practice has shown that the predominant failures of square plates are crater wear along the front surface and mechanical wear along the rear surface, while for round plates, it is the formation of notches and mechanical wear on the rear surface. The authors explain this by the fact that round inserts allow the use of different plunge angles, depending on the processing modes. This allows piercing and notching to be controlled. Zheng et al. [169] proposed a new composition of SiAlON tools, strengthening it by introducing graded nano-α-Si 3 N 4 into its composition. The experimental material made it possible to reduce the coefficient of friction and reduce the adhesive reactions. Mechanical tests showed that the introduction of α-Si 3 N 4 made it possible to reduce microcracks on the working parts of the inserts, which made it possible to increase the cutting speed up to 300 m/min. Shalaby et al. [63] used SiAlON ceramic tools in the processing of aerospace superalloy Inconel 100 (IN100). The effect of cutting speed on chip formation in dry finish turning was investigated. It is noted in the work that a change in cutting speeds, in a certain range, leads to the formation of various lubricating and protective tribofilms that contribute to the elimination of the development of micro-cracks and defects.

Conclusions
Different types of cutting tools with diverse characteristics are now being used in the industry for hard machining. Carbides are the most commonly used tool materials in the machining of Ni-based superalloys currently, owing to their excellent balance between cost and performance. However, the recommended cutting speeds range from 10 to 30 m·min −1 when machining nickel-based alloys with these tools. Higher speed is not recommended due to the loss of the characteristics of cutting tools, mainly related to the ultra-high local temperature that occurs in the tool-part contact zone, which leads to a reduction in the tool life [170][171][172]. Cubic boron nitride cutting tools appear to give better overall performance than cemented carbides when machining nickel-based alloys. Amongst all known materials, its hardness is second only to that of diamonds. It has excellent wear durability, high hot hardness, good thermal resistance a high coefficient of thermal conductivity. However, its tool cracking, due to its high brittleness, and chip breaking difficulty, owing to no chip breaker, are the two major issues of a CBN cutting tool. Moreover, the cost of CBN tools does, however, place a strict limitation on their use. Polycrystalline diamond tools demonstrate wear resistance almost 500 times greater than those of tungsten carbide and extreme hardness. On the other hand, PCD tools are extremely fragile and exhibit low toughness. SiAlON cutting tools were considered as a suitable material for machining a variety of difficult-to-cut materials, including nickel alloys, due to their special characteristics such as good fracture toughness, hardness (even at high temperatures), and resistance to sudden changes in temperature. In addition, the study provided a description of the various types of SiAlON ceramics such as α-SiAlON, β-SiAlON, and their combinations (α + β). Examples of the use of sialons in the processing of nickel-based superalloys were given. The review showed that effective nickel-based HRSA treatment is a pressing issue. The most promising direction of development is the use of progressive methods of introducing alloying elements into the SiAlON and Si 3 N 4 ceramic matrix. It is worth noting that desired material for cutting tools for particular applications is the one that demonstrates a balanced combination of various features such as hardness, toughness, wear resistance, thermal shock resistance, and chemical stability, especially at elevated temperatures. In addition, all related factors as cutting tool failure and geometry, coolant and machining conditions, and cutting chip must be taken into account in the selection of a cutting tool. As is evident throughout this review, the design and development of new tool geometry and new machining method, new tool materials, and functional surfaces remain major driving forces for studies on cutting tool technology in the machining of Ni-based superalloys.