**3.2 Tribological properties**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

Preliminary work was carried out to determine the fretting running map of PVD

multilayered coatings in contact with alumina sphere. The tangential force (Q ) and displacement (δ) amplitudes are determined for each cycle, and each sliding rate is reported on a 2D map of the fretting displacement and friction force. After a certain number of cycles, the partial slip regime (PSR) is manifested as a change in the hysteresis loop form, whereas the gross slip regime (GSR) maintains the buckle form with a variation of tangential force [3]. Running condition fretting maps (RCFMs) can then be determined from this map [15]. **Figure 7** shows the boundary lines of both sliding rates for different multilayered TiAlCN/TiAlN/TiAl and TiAlZrN/TiAlN/TiAl coatings. It can be seen that the gross slip regime region of the PVD-coated AISI4140 steel is enlarged due to the presence of the TiAlZrN, then the TiAlCN layers. From a phenomenological consideration, the gross slip regime corresponds to wear, and in the partial slip regime, the wear is associated with cracking, and the contribution of TiAlZrN/TiAlN/TiAl-reducing coatings should be interpreted positively. Indeed, in such a situation, wear is favored with the cracking of the covered part, which makes it possible to sacrifice the surface in order to

The TiAlZrN coating thus reduces the partial slip regime field, which is the most detrimental for fretting. However, sliding amplitudes are rather large and seen to be related more to the reciprocating condition. However, it is fundamental to relate the displacement value to the contact dimension. The boundary between the fretting and reciprocating conditions can be related to the ratio between the displacement amplitude and the contact radius, e = d/a [19]. It transpires that when e remains little than 1, a nonexposed surface exists and grosses slip fretting conditions prevail, whereas if e is above 1, the whole surface is exposed to the

**3. Test results and discussion**

protect the volume of the part [3].

**3.1 Fretting wear**

**324**

**Figure 7.**

*Effect of multilayered coatings (TiAlZrN/TiAlN/TiAl) on the running condition.*

**Figure 8** shows the evolution of the coefficient of friction as a function of the total number of cycles for the two multilayered PVD coatings under same loading parameters. The first cycle systematically presents a low friction coefficient around 0.11, and the incipient low friction coefficients can be explained by the presence of surface oxides. During the test, the friction coefficient increases progressively toward a level known as the stabilized friction coefficient. Such a difference of friction behavior between the two antagonists (TiAlZrN and TiAlCN) is clearly illustrated in the graph of evolution of the friction versus the fretting cycles. It confirms the previous fretting cycle analysis and outlines the difference of friction kinetics between the two antagonists. The transition period is systematically longer in the presence of the TiAlZrN coating. In the case of the PVD layer, De wit [20] showed that the transition period corresponds to the formation of debris made of amorphous retiles and nanocrystallines. Beyond this transition, the amorphous phase is transformed into a crystalline phase and contributes to further wear. Depending on the loading condition, the film of TiAlCN or TiAlZrN can be eliminated, thus favoring a significant increase of the friction coefficient **Figure 9**. It explains the influence of the pressure and displacement amplitude on the evolution of the friction coefficient on a coated specimen, taking into account a slight surface degradation at low friction, which must be introduced into the friction cycle [21, 22]. It can be seen that by increasing the displacement amplitude or the pressure, the wear depth will grow faster and the elimination of surface porosity will be accelerated. As soon as the surface porosity and the aluminum oxide, which plays the role of a solid lubricant, are removed, low friction conditions can no longer be maintained and high metal/metal interactions with high friction coefficients in the range of 0.5–0.6 are observed, indicating that the PVD film has been breached. When a microarc oxidation coating was used, the fretting friction coefficient of modified PVD coatings alloy under higher loading condition remains as high as about 0.6; however,

#### **Figure 8.**

*Evolution of the friction coefficient for the TiAlZrN/TiAlN/TiAl and the TiAlZrN/TiAlN/TiAl (FN = 50 N,*  δ *= 20 μm).*

**Figure 9.**

*Stabilized friction coefficient as a function of normal load and slip amplitude of PVD multilayered coating. (a) TiAlCN/TiAlN/TiAl [3] and (b) TiAlZrN/TiAlN/TiAl.*

the cracking domain and severe adhesive nature was limited. Under low loading condition, the friction coefficient was significantly reduced and remained favorable stable at 0.4–0.3 in the long term, which indicates that the multilayered coatings lowered the shear and adhesive stresses between contact surfaces, consequently alleviating the possibility of initiation and propagation of cracks in the inner layer of multilayered coatings.

### **3.3 Wear properties**

In every tribological application, the extent of the damage or surface deterioration is of interest. There are several methods of evaluating the wear volume/loss, which can be roughly classified into three methods: weight measurement, topographical analysis, and 2D analysis by means of empirical equations. In tribological research, where many specimens need to be analyzed, a simple and fast procedure is desirable for wear volume/loss determination. Moreover, the effect of different material combinations, slip amplitude, normal force, and the energy dissipated during sliding are also presented.

**327**

**Figure 10.**

*TiAlCN/TiAlN/TiAl.*

*Fretting Wear Performance of PVD Thin Films DOI: http://dx.doi.org/10.5772/intechopen.93460*

*3.3.1 Prediction of the wear volume evolution*

The linearity of the variation of fretting wear with normal load and displacement obtained by the two multilayered TiAlCN/TiAlN/TiAl- and TiAlZrN/TiAlN/ TiAl-coated specimens are used to impose the same definition as Archard's equation, namely, a quasi-linear function. The wear volume evolution of the two coated steels is shown in **Figure 10**, as a function of slip amplitude and normal force. For all specimens, the beneficial effects of the coating on wear volume diminished with the increasing normal force and fretting stroke. The latter observation is consistent with the work of Santner et al. [23], who reported that TiN was much more effective at suppressing wear under sliding wear conditions. For the steel substrate, the coating TiAlCN or TiAlZrN had no good effect on the fretting wear for sliding amplitudes larger than 50 μm, regardless of the applied normal forces out of 500 N. The wear transition is attributed to the higher variation of both normal load and sliding amplitude. However, for all fretting wear tests, the behavior evolution of wear volume versus displacement is the same. This means in all tests, wear volumes remain constant and are not greatly influenced by the low normal load or sliding distance. In fact, it is only the wear amplitude which changes according to the displacement and high normal load. In every case, there is a constant wear volume which precedes the establishment of the high wear regime. Three suppositions can be made to synthesize all the results presented above. Wear volumes are similar for all loading conditions and consist of two phases. The first one corresponds to the elimination of the contamination layer and the native oxides. Thus, alumina to PVD coatings contact will be established. As a result, the adhesion phenomenon is favored as regards the miscibility antagonists by plastic deformation, which increases the micro-junction density by crushing asperities. Hence, adhesive wear appears by creating transfer of the softer material (PVD coatings) on the harder

*Wear volume as a function of parameters loading multilayered coatings: (Zr) TiAlZrN/TiAlN/TiAl, (C)* 

*Fretting Wear Performance of PVD Thin Films DOI: http://dx.doi.org/10.5772/intechopen.93460*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

the cracking domain and severe adhesive nature was limited. Under low loading condition, the friction coefficient was significantly reduced and remained favorable stable at 0.4–0.3 in the long term, which indicates that the multilayered coatings lowered the shear and adhesive stresses between contact surfaces, consequently alleviating the possibility of initiation and propagation of cracks in the inner layer

*Stabilized friction coefficient as a function of normal load and slip amplitude of PVD multilayered coating.* 

In every tribological application, the extent of the damage or surface deterioration is of interest. There are several methods of evaluating the wear volume/loss, which can be roughly classified into three methods: weight measurement, topographical analysis, and 2D analysis by means of empirical equations. In tribological research, where many specimens need to be analyzed, a simple and fast procedure is desirable for wear volume/loss determination. Moreover, the effect of different material combinations, slip amplitude, normal force, and the energy dissipated

**326**

of multilayered coatings.

*(a) TiAlCN/TiAlN/TiAl [3] and (b) TiAlZrN/TiAlN/TiAl.*

during sliding are also presented.

**3.3 Wear properties**

**Figure 9.**
