*3.5.3 Tribological properties for Si3N4 coatings nitride coatings*

**Figure 17a** shows the friction coefficient of Si3N4 based coating deposited on AISI 316 steel substrates in a lubricated and dry environment. The results of the tribological study in dry environment evidenced two characteristic stages during the test. Stage I, known as the starting period, is associated with the interferential friction mechanism due to the direct contact between the surface roughness of the coating and the counterpart (100Cr6 steel), whereby, which, the roughness decreases and generates wear particles on the surface [5]. These particles cause a rapid increase in the friction coefficient followed by a slight decrease until it stabilizes. Subsequently, in stage II, the reduction of these roughness is maintained along with the appearance of new defects in the coating, leading to a stabilization of the friction coefficient [33].

In the lubricated environment, the curves show a different behavior in relation to the tribological study in dry environment (not lubricated), since the incorporate of the lubricant inside the tribological contact generated a large decrease in the friction coefficient. Therefore, the decrease in the friction coefficient is attributed to the fact that the lubricant supports the applied external load, decreasing the roughness reduction and caused a lower amount of wear particles (debris) on the tribo-system surface. **Figure 17b** shows the value of the friction coefficient as a function of the material, in dry and lubricated environment. This behavior is related to the friction model proposed by Archad [31]. This model correlates the mechanical (H, Er) and morphological (roughness) properties of the coating, where surface with better mechanical properties and lower roughness will present a lower friction coefficient, as was the case for the Si3N4 coating. This is due to the fact that the Si3N4 coating is able to withstand the continuous passage of the counterpart in relation to the uncoated steel substrate, thus producing a lower wear rate on its surface.

#### **Figure 17.**

*Friction coefficient for Si3N4 coatings in lubricated and non-lubricated environment: (a) friction coefficient versus sliding distance (b) friction coefficient as a function of the material evaluated.*

**Figure 18.**

*SEM micrographs of the wear tracks generated in the scratch test for Si3N4 coatings deposited on 316 stainless steel substrates.*

**Figure 18** shows the friction coefficient as a function of critical load for Si3N4 coating, in addition to the SEM micrograph of the scratch track where the types of failures, cohesive failure (Lc1) and adhesive failure (Lc2), were determined. In the cohesive failure (Lc1) the first cracks are produced by the applied external load, and in the adhesive failure (Lc2) a delamination is generated at the edge of the scratch track. These tribological characteristics are due to the mechanical and surface properties of the coating.

#### **4. Conclusions**

From the study of the mechanical and tribological behavior of TiN and TiCrN coating. It was determined that the TiCrN coating presented the best set of properties, these better properties were attributed to the incorporation of chromium (Cr) atoms within the crystalline structure of TiN. Therefore, structural, morphological and mechanical changes were produced, which influenced its behavior under applied load states.

The above results determined that the boron nitride (BCN) coating had the lowest friction coefficient (0.208) in a lubricated environment and had a friction coefficient of 0.6 in a non-lubricated environment. This tribological behavior is associated with its low roughness and high mechanical properties with respect to the coatings (TiCN and CrAlN). In this research, a decrease in the friction coefficient was obtained comparing non-lubricated and lubricated environments by 68.2%, 65% and 65.3% for TiCN, CrAlN and BCN coatings, respectively. On the other hand, cohesive failure (Lc1) and adhesive failure (Lc2) were obtained for BCN coating with 38.41 N and 49.32 N, respectively.

The tribological properties analyzed by Pin On Disk in dry and lubricated environment for the Si3N4 coating presented the lowest friction coefficient in relation to uncoated steel. This behavior is attributed to its structural, mechanical and morphological properties, so the Si3N4 coating proved to be a suitable candidate to be implemented in the food and pharmaceutical industry.

#### **Acknowledgements**

This research was supported by Universidad Militar Nueva Granada, Bogotá, Colombia; CIC biomaGUNE, San Sebastian, Spain; Centro de Desarrollo Tecnológico y Asistencia Técnica a la Industria del Servicio Nacional de Aprendizaje (CDT-ASTIN-SENA), Cali, Colombia; Universidad Autónoma de Occidente, Cali, Colombia.

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