**4.2 Turning of AISI 52100 (HRC 56–58) hardened steel with tools with (Cr,Al,Si)N-(DLC-Si)-DLC-(DLC-Si), and (Cr,Al,Si)N-DLC coatings**

The investigation of the DLC-1 coating structure using TEM reveals the presence of a wear-resistant layer in the amorphous DLC and a (Cr,Al,Si)N transition layer with the columnar structure (**Figure 10a** and **b**) [68]. In its turn, the DLC layer structure includes sublayers of DLC-Si at the border of the (Cr,Al,Si)N layer and the coating surface. The (Cr,Al,Si)N layer is about 0.4 μm thick, and the thickness of the DLC layer is about 1.4 μm.

The DLC-2 coating also has a two-layer structure with a (Cr,Al,Si)N transition layer and a DLC wear-resistant layer (**Figure 10c** and **d**). The thickness of the transition layer is about 0.3 μm, while the thickness of the DLC layer is about 1.1 μm.

*Nanostructured Multilayer Composite Coatings for Cutting Tools DOI: http://dx.doi.org/10.5772/intechopen.94363*

### **Figure 10.**

The TEM analysis of the cross-section image for the DLC-1 coating (**Figure 10a**) reveals the absence of any clear structure in the DLC layer. The electron diffraction patterns depict a broadened halo, typical for an amorphous structure, while a structure close to columnar can be noticed in the DLC-Si sublayer (**Figure 10b**). The chemical composition of the DLC-Si sublayer includes about 56 at% Si + 43 at% C + 1 at% O. The DLC layer includes about 1 at% Si + 97 at% C + 2 at% O (**Figure 10b**). The (Cr,Al,Si)N layer includes 70 at% Cr + 23at% Al + 7 at% Si. The surface layer of the ceramic substrate Al2O3 exhibits signs of diffusion of Cr in the volume of 0.3–0.4 at%. The area around the (Cr,Al,Si)N layer, adjacent to the border of the DLC-Si layer is characterized by an increase content of Si (about 10 at%) to ensure better adhesion with the DLC-Si layer. The high (above 50 at%) content of Si is detected in the transition layer and the surface layer of DLC.

The earlier studies have found that the structure of (Cr,Al,Si)N is characterized by a face-centered NaCl-type lattice with various crystal orientations: (111), (200), and (220) [26, 27]. In the presence of the CrN phase, the phases of Cr, Cr2N, CrSi2, and Si3N4 were also detected [27]. The investigation has also revealed that Si is

present either in the form of a substitutional solid solution in the Cr-Al-N lattice, or in the form of an amorphous Si-N compound that accumulates at the grain boundaries of Cr-Al-N [27, 31, 32]. **Figure 11** exhibits the nanostructure of the (Cr,Al,Si) N layer, characterized by the presence of various phases differing with interplanar spacings and orientations of crystal planes [68].

The formation of a fan-shaped network of microdroplets in the DLC layer, caused by microdroplets of (Cr,Al,Si), embedded in the structure of the DLC layer, is depicted in **Figure 12** [68]. During the process of cutting with a coated tool, active fracture can occur in those areas of the DLC layer. So, during the deposition of the coatings with the structures under study, the minimized number of microdroplets is a key condition to ensure the required working efficiency of the coating.

The (Cr,Al,Si)N layer with its smoothing function plays an essential role during deposition of a DLC-Si layer with regard to the elimination of possible stress concentrators on the surface. In particular, a microdefect which appeared on the surface of the ceramic substrate as a result of conjunction of two grains is depicted in **Figure 13** [68]. When the DLC-Si layer is deposited directly onto a ceramic substrate, such a microdefect can provoke cracking in the DLC-Si structure under the abovedescribed mechanism. The above microdefect is being smoothed by the (Cr,Al,Si)N layer, which forms a smooth surface without serious defects that would be able to act as stress concentration in the DLC layer. With the prime function of the (Cr,Al,Si)

*High-resolution TEM image showing the crystalline structure of the (Cr,Al,Si)N transition layer for DLC-1 [68].*

**Figure 12.**

*Influence of embedded microdroplets on forming cracks in the DLC layer for (a) DLC-3 and (b) DLC-2 coatings [68] (TEM).*

### *Nanostructured Multilayer Composite Coatings for Cutting Tools DOI: http://dx.doi.org/10.5772/intechopen.94363*

N layer consisting in the provision of good adhesion between the ceramic substrate and the DLC-Si coating and the formation of a composite structure with a combination of high hardness and brittle fracture resistance, the smoothing functions of the (Cr,Al,Si)N layer are also crucial to secure good coating performance.

Cutting tests are crucial in the assessment of the working efficiency of a coated tool and the performance properties of the very coating. The tests were also conducted to study the dynamics of wear on the rake faces of the uncoated cutting tools made of ceramics and the tools with the DLC-1 and DLC-2 coatings. The tool with the DLC-2 coating proved to have the best resistance to the wear crater formation on the rake face (see **Figure 14**) [68].

The most active formation of a wear crater was detected on the uncoated tool. The active formation of adherents during the cutting is a typical feature of the tool with the DLC-1 coating. This proves high adhesion between the DLC-1 coating and the material being machined. As a result, strong adhesive bond bridges are formed and broken, which leads to a high adhesive-fatigue wear of the tool. At the same time, an abrasive wear with significantly lower adherent formation is typical for the tool with the DLC-2 coating.

The wear pattern on the rake face of the tool was considered in detail (**Figure 10**). The first typical feature was a much more active formation of an adherent of the material being machined for the tool with the DLC-1 coating. At the same time, the uncoated tool and the tool with the DLC-2 coating demonstrated an insignificant formation of such adherents. Based on the obtained results, it can be concluded that there is an

**Figure 14.** *Dynamics of changes in the crater on the rake face of the tool [68].*

increased adhesion between the material of the DLC-1 coating (with a surface sublayer with the high Si content) and the material being machined. The uncoated tool and the tool with the DLC-2 coating are characterized by primarily abrasive wear, accompanied by the formation of typical grooves in the direction of chip flow. The tool with the DLC-1 coating also has similar grooves, but there are also signs of the cutting edge chipping.

Since the flank wear is usually assumed as a limiting factor, its dynamics was considered (**Figure 15**) [68]. The most active flank wear was detected on the tool with the DLC-1 coating. As already stated, this fact can be explained by the high adhesion between the coating and the material being machined. As a result, the tool demonstrated the increased adhesive-fatigue wear. Meanwhile, less active adhesion to the material being machined and lower tendency to chipping were detected on the uncoated tool. At the same time, there is minor chipping in the area of the cutting edge. Finally, the tool with the DLC-2 coating demonstrated a low tendency to the formation of adherents and no visible signs of brittle fracture. Thus, the purely abrasive wear mechanism was typical for the tool with the DLC-2. The tool with the coating DLC-2 exhibited a 17% higher resistance to the flank wear. In combination with its noticeably higher resistance to the formation of a wear crater on the rake face and the more favorable wear pattern (implying the balanced abrasive wear instead of chipping and brittle fracture), the above proves the good prospects of the DLC-2 coating.

**Figure 15.** *Tool wear on the flank face depending on the cutting time [68].*

### **Figure 16.**

*Investigation of the wear pattern on a cross-section of the tool with the DLC-1 coating [68] (SEM). Through cracks and tears (a, c), minor cracks and intact coating (b, d).*

*Nanostructured Multilayer Composite Coatings for Cutting Tools DOI: http://dx.doi.org/10.5772/intechopen.94363*

#### **Figure 17.**

*Investigation of the fracture pattern on a cross-section of the tool with the DLC-2 coating [68] (SEM). General view and a crack passing into a ceramic substrate (a), tearing out of a coating fragment (b), cracks in a coating turning into a substrate (c, d).*

For a better understanding of the wear patterns on the samples, cross-sections were made passing through the centre of the wear craters.

**Figure 16** demonstrates that the sample with the DLC-1 coating bears clear signs of cutting edge chipping. It is also clear that the coating is retained on the rake face of the tool in the area adjacent to the boundary of the wear crater. On the flank face, in the area adjacent to the flank wear land, the DLC layer failed while the (Cr,Al,Si) N layer was preserved. In general, the coating retains good adhesion to the substrate, and it does not separate from the substrate on the sample with the DLC-1 coating.

The wear process on the DLC-1 coating, especially its upper layer, is characterized by the formation of inclined cracks (**Figure 16a–c**), which in some cases also penetrate into the (Cr,Al,Si)N transition layer (**Figure 16a** and **c**). It is also clear that the surface of the ceramic substrate has a rather complex relief, and the (Cr,Al,Si)N transition layer fills in the microroughness of the surface and thus forms a basis for the DLC layer (**Figure 16d**) [68].

During the consideration of the wear process on the tool with the DLC-2 coating, it can be noticed that the transition layer of this coating demonstrates a higher tendency to brittle fracture compared to the transition layer of the DLC-1 coating (**Figure 17**) [68]. The DLC layer of the DLC-2 coating also bears signs of active brittle fracture. In the area, adjacent to the wear crater (Area B, **Figure 17**), almost complete failure of the DLC layer and partial failure of the transition layer are detected (**Figure 17b**). The brittle fracture of the ceramic substrate, accompanied by microchipping, also takes place. The signs of such brittle fracture are detected throughout the whole contact area on the rake face of the tool.

### **5. Conclusions**

The chapter considered the specific features of the use of wear-resistant coatings deposited to improve the performance properties of ceramic cutting tools. The application of the Ti-(Ti,Al)N-(Zr,Nb,Ti,Al)N multilayer composite coating with a nanostructured wear-resistant layer and the (Cr,Al,Si)N-(DLC-Si)-DLC-(DLC-Si) and (Cr,Al,Si)N-DLC composite coatings increase the tool life of a ceramic cutting tool by 50–80%.

Following the studies, it has been found that the intensity of the thermal effect on structures of the ceramic substrate may be reduced through the deposition of the developed coatings under study on contact areas of ceramic cutting tools, since such coatings provide better heat removal from the cutting area due to an increased length of the plastic contact.

The use of the DLC-based coatings can both prolong (due to an enhanced resistance to abrasive wear) and shorten (due to higher adhesion) the tool life. It is not advisable to deposit a coating with an increased Si content in the surface layers of the coating, at least for the considered cutting conditions, despite a bit higher hardness of such coating. At the same time, a coating with the high Si content demonstrated a lower tendency to brittle fracture compared to the DLC-2 coating. The use of the (Cr,Al,Si)N transition layer is advisable, since it improves the adhesion between the ceramic substrate and the DLC coating.

The results of the conducted studies prove the prospects for the application of multilayer composite coatings, including those with DLC layers introduced to improve the cutting properties of ceramic cutting tools (both assembled and onepiece tools).
