**2. Theoretical background**

The operation conditions for ceramic cutting tools significantly differ from those under which tools of high-speed steel and carbides are used. A substantial increase in cutting speed changes the mechanism of chip formation and contact processes during the cutting, as well as the nature and level of power and temperature loads, thermomechanical stress, and mechanisms of tool wear [13–15]. The results of the studies focused on the stress state in the cutting part of a ceramic tool [16] indicate the presence of tensile and compressive stresses (**Figure 1**). At the

**Figure 1.** *Distribution of principal stresses* σ*1 and* σ*2 along the rake face of a cutting tool [1, 2].*

same time, in most cases, the area of tensile stresses begins at the end of the contact area of the chips with the rake face. When section thickness is small, compressive stresses prevail, while at larger thicknesses, tensile stresses begin to play a significant role [17, 18].

The most stressed section in the tensile area is located on the rake face of the tool at a distance equal to (2–2.5)*lc*, where *lc* is the length of contact between the chips and the rake face of the tool.

With a decrease in the rake angle γ, the compression area extends, and the tensile area decreases or disappears at all. As a result, the negative angle γ is typical for a ceramic cutting tool and makes it possible to achieve a change in the stress state in the direction of the predominantly compressive stresses [17, 18]. Deposition of a coating on a cutting tool significantly changes the nature of the interaction between the material being machined and the tool. In [19, 20], the studies revealed that the coating parameters had a significant influence on the characteristics of the contact processes and the chip formation. To prevent a sudden failure of a ceramic cutting tool as a result of brittle fracture, it is necessary to control the processes of the contact interaction between the tool material and the material being machined by depositing coatings on the working surfaces of the tool. The composition and structure of such coatings will increase the length of the full contact between the chips and the rake face of the tool through enhancement of the adhesion to the material being machined and the improvement of heat removal from the cutting area due to increased thermal conductivity of the tool material. Thus, the specific thermomechanical loads on the cutting edge of the ceramic tool can be reduced (see **Figure 2**).

There are a number of studies considering the use of coatings to improve the performance properties of ceramic cutting tools, with both oxide and nitride ceramics as the ground. In [21], the investigation is focused on PVD of the (Ti,Zr) N-(TiN/ZrN) and TiN-(TiAl)N-(TiN/(TiAl)N) nanostructured multilayer coatings, deposited on ceramic cutting inserts of Al2O3 + ZrO2 + Ti(C,N) and Al2O3 + TiC, with the external layer formed by the alternating nanolayers of TiN and ZrN or TiN and (Ti,Al)N, respectively. With the total coating thickness of 3–5 μm and the microhardness of about HV 29 GPa, the above coatings prolonged the tool life by 20–80% during the dry cutting of NC6 steel (HRC 48–52), at vc = 150 m/min, f = 0.10 mm/rev, ap = 0.5 mm.

### **Figure 2.**

*Differences in (a) stresses and (b) isotherms for a coated and uncoated tool: Cγ and Cγn - the total lengths of the contact between the chips and the rake face of the uncoated (the dashed line) and coated (solid line) tools, respectively; changes in stresses and isotherms occur in the direction of the arrows [1, 2].*

In [22], the studies considered the properties of a ceramic cutting tool with the PVD coating of TiN-(Ti,Al,Si)N-TiN with the thickness of 2–4 μm with a nanostructured layer of (Ti,Al,Si)N and the CVD coating of TiN-Al2O3 with the thickness of 2.6–10 μm, when inserts of nitride ceramics of Si3N4 and oxide ceramics of Al2O3 + ZrO2 were used as substrates. The cutting properties were used during the turning of EN-GJL-250 gray cast iron and C45E steel. The cutting process was carried out under the following conditions: f = 0.10, 0.15, and 0.20 mm/rev; ap = 1 and 2 mm; vc = 200 and 400 m/min. The cutting tools with coatings of all types demonstrated the longer tool life compared to that of the uncoated tools, while the longest tool life was detected for the tools with the PVD coating of TiN-(Ti,Al,Si)N-TiN with the nanostructured middle layer. In [23], the studies considered the cutting properties of tools made of mixed ceramics of Al2O3 + TiCN with the TiN commercial coating during the turning of hardened American Iron and Steel Institute (AISI) 52,100 (HRC 63) steel under the following cutting conditions: f = 0.07, 0.11, and 0.14 mm/rev, ap = 0.5 mm, vc = 100, 150, 200, 250, and 300 m/min. The studies found that for the uncoated cutting inserts, wear in the form of cracking and chipping was more typical, while for the coated tools, the formation of a wear crater on the rake face was typical. The cutting path for the coated tool was about 8 times longer, and the temperature in the cutting area was substantially lower than for the uncoated tool.

According to the results of [24], which studied the cutting properties of tools made of silicon nitride (Si3N4) with the CVD coating of TiN-Al2O3 during the continuous turning of gray cast iron with various depths of cut, the prevailing failure mechanism for the above cutting tools was abrasive wear during the continuous turning and a combination of abrasive wear and brittle fracture during the machining with variable depths of cut. The coated cutting tools demonstrated much longer tool life compared to the uncoated cutting tools: the length and time of cutting were about 3.5 times longer at the cutting speed of 300 m/min and 2 longer at the cutting speed of 380 m/min.

The cutting properties of ceramic tools of Si3N4 with the PVD coating of (Ti,Al) N-(Al,Cr)O during the turning of HT250 gray cast iron and AISI 4340 steel were studied in [25]. The thicknesses of the (Ti,Al)N and (Al,Cr)O layers were about 2.0 and 0.6 μm, respectively. The tool life of a tool made of silicon nitride with the (Ti,Al)N-(Al,Cr)O coating was longer compared to uncoated inserts during the turning of gray cast iron and steel. In [26], authors investigated the cutting properties of tools based on silicon nitride with the PVD coatings of (Ti0.5,Al0.5) N and (Cr0.3,Al0.7)N during the dry turning of gray cast iron. For the tools with the (Ti0.5,Al0.5)N and (Cr0.3,Al0.7)N coatings, the tool life was at least 2 times longer compared to the uncoated tools. The tools with the (Ti0.5,Al0.5)N coating demonstrated longer tool life compared to the tools with the (Cr0.3,Al0.7)N coating. In [27], the studies are focused on the cutting properties of tools made of the Al2O3 + TiC mixed ceramics with the TiN-(Al,Cr)N multilayer coating during the dry turning of AISI 4340 (HRC 46) hardened steel at vc = 125–175 m/min, ap = 0.25–0.63 mm, f = 0.10–0.25 mm/rev. After 9 minutes of cutting, the wear VB was on average 45% higher for the uncoated tool.

The application of diamond-like carbon (DLC) coatings for ceramic cutting tools should be considered separately. While several studies consider the properties of DLC, deposited on a ceramic substrate, there are hardly any investigations focused on ceramic cutting tools with DLC coatings. For example, in [28, 29], the studies consider the challenges of improving the performance properties of ceramic tribological pairs (sliding bearings). A significant decrease in the coefficient of friction (COF) was noted with the use of samples with DLC coatings. Ceramic products made of SiC with DLC coatings demonstrate excellent chemical stability, low COF, and very good wear resistance [30]. The properties of DLC coatings deposited on the Si3N4 substrate were also considered. Gomes et al. [31] studied the tribological properties of uncoated samples and samples with the DLC and DLC-Si coatings under friction, paired with counterbodies made of stainless steel. Both coatings demonstrated good tribological properties, but samples with the DLC-Si coating separated from the substrate, and the wear coefficient for samples with the DLC coating was much lower compared to samples with the DLC-Si coating. As a result of the studies considering the properties of mechanical face seals of nitride ceramics with the DLC and DLC-Si coatings, it was found that the use of these coatings significantly reduced the COF and improved wear resistance of products. At the same time, the DLC coatings look more preferable compared to DLC-Si coatings [32]. Following the results of the investigation focused on the properties of the DLC coating, deposited on the substrate of Si3N4 and M50 steel, it was found that the normal stresses on the boundary of the "coating–substrate" interface were higher (by about 10%) for the ceramic substrate, which could be explained by the higher value of the elastic modulus of Si3N4 [33]. It has also been found that as the coating thickness grows from 200 up to 400 nm, the stresses decrease at the boundary of the "coating–substrate" interface in accordance with the quadratic expression, and such a decrease slows down with the growth of the coating thickness [34]. A sample with the DLC coating demonstrates a lower COF compared to a sample with the MoS2 coating [35]. Following the investigation focused on the tribological properties of a sample with the Cr-DLC coating under friction, paired with uncoated counterbodies of Al2O3, ZrO2, Si3N4, and WC in air and in the helium atmosphere, it was found that the tribological properties of the samples with the DLC coating were significantly higher in air than in the helium atmosphere [36]. Two-dimensional finite element modeling of the properties of the DLC coating, deposited on the substrate of Al2O3 exhibited that a growth of the DLC coating thickness led to an increase in its hardness and crack resistance [37]. The studies revealed the ability of the DLC coating to minimize surface defects on the substrate and significantly reduce the intensity of oxidation processes [38]. The DLC coatings deposited on the substrate of β-SiAlON increase the surface hardness and improve the surface quality [39]. The comparison of the tribological properties of the Cr2O3-based samples with the DLC, TiN, and TiAlN coatings and of the uncoated samples in contact with cast iron counterbodies found that the samples with the DLC coatings demonstrated the highest scuffing resistance and the lowest coefficient of friction (COF) [40]. The studies carried out in air and in water environment, in nitrogen atmosphere, and in vacuum revealed a significant decrease in the COF, an increase in wear and oxidation resistance after deposition of the DLC coatings on the substrates of SiC, Si3N4, and ZrO2 [41, 42]. Close values of the elastic modulus of the coating and the substrate is an important factor able to reduce internal stresses and thus improve the service life and reliability of products with the DLC coatings. At the same time, the deposition of coatings, in particular, the DLC coatings, on ceramic products increases their wear resistance, significantly reduces the COF, and enhances the oxidation resistance. Another important factor is also a leveling effect of a coating, which minimizes the influence of microconcentrators of stresses (pores, microcracks, etc.) on the reliability and service life of a product, while reducing the surface roughness value is important. Meanwhile, many studies note such a problem as the low strength of adhesive bonds between a DLC coating and a ceramic substrate, which leads to failure of the coating due to its separation from the substrate. Another important challenge is to study the influence of Si on the properties of the DLC coatings and the cutting properties of tools with such coatings.
