*3.5.1 Tribological properties for TiN and TiCrN nitride coatings*

**Figure 13** shows the tribological behavior of individual TiN and TiCrN coatings as well as the [TiN/TiCrN] multilayer system as a function of the bilayers number.

#### **Figure 11.**

*Nanoindentation results: (a) Load-displacement curves for the TiCN, CrAlN and BCN single layers, (b) Hardness and (c) Elastic modulus values.*

**Figure 12.**

*Load-depth curves for the Si3N4 coatings and mechanical properties as a function of the material: hardness and reduced modulus of elasticity Si3N4 coatings.*

**Figure 13.**

*Tribological study of single layer coatings [TiN and TiCrN] and multilayer system [TiN/TiCrN]n as a function of the bilayers number (a) Friction coefficient versus distance and (b) Friction coefficient values for single layer [TiN and TiCrN] and multilayer system [TiN/TiCrN]n.*

Through this behavior, two characteristics stages were identified. Stage I, known as the starting period, where there is a rapid increase in the friction coefficient due to the direct contact between the asperities and the counterpart (Steel 440), in this way, these asperities are eliminated and deformed. Stage II, known as running-in, in which the deformation of the asperities is maintained together with the appearance of defects of the coating, leading to the formation of wear particles or debris [24]. **Figure 13b** show the value of the friction coefficient in the stabilization stage. From this result, it is evident that the TiCrN coating showed a decrease compared to the TiN coating, this decrease in the coefficient is attributed to the deformation of the crystalline structure by the incorporation of chromium atoms, which modifies its surface and mechanical properties, as show above (**Figures 6** and **10**). Furthermore, this tribological behavior can be related to the mechanical friction model proposed by Archad, where the friction coefficient of each coating depends on surface factors such as roughness R(s,a), and elastic–plastic properties (hardness H, or elastic modulus Er). By means of this model, it is established that when the surface of the coating has a low roughness (**Figure 6**) and a high hardness (**Figure 10**) the friction coefficient will be lower since there will be less wear on the surface [31].

The study of the adhesion of coatings was carried out by means of the scratch teste. For this, **Figure 14a** and **b** show the behavior of TiN and TiCrN coatings

*Analysis of the Tribological Evolution of Nitride-Based Coatings DOI: http://dx.doi.org/10.5772/intechopen.100629*

#### **Figure 14.**

*Friction coefficient and normal strength versus distance and critical load Lc2 for single layers (TiN and TiCrN) and optical micrographs of the wear track of the dynamic scratch test at a resolution of x10.*

respectively, where two characteristics stages known as (Lc1 and Lc2) could be characterized. Lc1, is known as the cohesive failure where the first cracks or first failure in the coating start to occur and Lc2 known as the adhesive type failure where delamination occurs at the edge of the scratch track presented in **Figure 14c**. In addition, **Figure 14a** and **b** show the adhesion strength for the single layer coatings as a function of Lc1 and Lc2 failures, where the change in slope corresponding to the adhesive and cohesive failure are observed. Thus, there results were corroborated with the micrographs of the wear tracks of each test where the morphological changes suffered by the surface due to the cohesive and adhesive failure can be appreciated [32].

Finally, **Figure 14d** show the value of the critical load (Lc2) for the TiN and TiCrN single layer coatings as well as the multilayer system [TiN/TiCrN] as a function of the bilayers number. From these results it was possible to show that the TiCrN coating presented a higher resistance to be delaminated, this increase of the Lc2 load in comparison to the TiN coating is attributed to physical factors, such as the change produced within its crystalline structure by the incorporation of chromium atoms in its structure as corroborated in **Figure 1**. This change in the crystalline structure due to the increase of compressive stresses generated that a higher amount of external energy is required to cause a delamination of the coating. In addition, for the multilayer system, it was determined that the increase in the number of interfaces directly affects the delamination resistance of the coatings, because the interfaces restrict movement of the cracks through the coating.

#### *3.5.2 Tribological properties for TiCN, BCN and CrAlN coatings nitride coatings*

**Figure 15** show the tribological behavior of TiCN, BCN and CrAlN coatings deposited on AISI 1045 steel substrate when in direct contact with a 100Cr6 steel counterpart in lubricated and non-lubricated environments. These results showed two characteristic stages, stage I, known as the starting period, which is related to the

**Figure 15.**

*Tribological results of the AISI 1045 steel substrates with TiCN, BCN and CrAlN single layer coatings with and without lubrication: friction coefficient as a function of the sliding distance and friction coefficient for different coatings (TiCN, CrAlN and BCN): without lubrication and with lubrication.*

interference of the friction mechanism due to the initial surface contact associated with the surface and counterpart; therefore; this contact generated a rapid increase in the friction coefficient and stage II is characterized by the friction coefficient presents a settlement period, where a deformation and elimination of the asperities takes place, causing a stabilization of the friction coefficient. Thus, at the settling distance there is an equilibrium of the friction coefficient in relation to the adhesive and interferential friction mechanisms. Therefore, the value of the friction coefficient will depend on the predominant effect related to the adhesive and interferential mechanisms. Finally, **Figure 13b** shows the value of the friction coefficient for all coatings and substrate (AISI 1045) in the non-lubricated environment, where they were obtained for Substrate = 0.82; TiCN = 0.74; CrAlN = 0.66 and BCN = 0.6. On the other hand, the values of the friction coefficient obtained in a lubricated environment were Substrate = 0.26; TiCN = 0.24; CrAlN = 0.23 and BCN = 0.21. Taking into account the last result, it was established that the BCN coating presented the best tribological behavior for both environments (dry and lubricated), this good behavior is attributed to its surface and mechanical properties presented above.

Through the results obtained by the scratch test presented in **Figure 16**, it was possible to evidence an increase of the critical load (Lc2) as a function of the nature of the coating (TiCN, CrAlN and BCN). Moreover, the change of the critical load is related to the increase of the mechanical properties of the coatings (**Figure 11**), the reduction of the surface roughness (**Figures 7** and **8**) and the reduction of the friction coefficient (**Figure 15**). In addition, factor such as resistance to plastic

#### **Figure 16.**

*Friction coefficient as a function of the applied load for all TiCN, BCN and CrAlN coatings showing the cohesive failure (LC1) and adhesive failure (LC2) and critical load as a function of the coating's nature (TiCN, BCN and CrAlN).*

deformation and elastic recovery prevent the propagation and displacement of cracks through the coating, thus requiring a higher applied external load to cause failure between the coating and the substrate (adhesive failure). Therefore, an increase in the critical adhesive load (Lc2) of 46.03% was found for the boron carbide nitride (BCN) coating relative to the coating with lower mechanical properties (TiCN).
