**3.1 Fretting wear**

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 protect the volume of the part [3].

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

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**Figure 8.**

δ *= 20 μm).*

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

gross slip fretting conditions.

**3.2 Tribological properties**

ambient and the contact is under reciprocating conditions [18]. The maximum e value calculated for all performed test situations remains lower 0.5, which implies

**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,

*Evolution of the friction coefficient for the TiAlZrN/TiAlN/TiAl and the TiAlZrN/TiAlN/TiAl (FN = 50 N,* 

**Figure 7.** *Effect of multilayered coatings (TiAlZrN/TiAlN/TiAl) on the running condition.*

ambient and the contact is under reciprocating conditions [18]. The maximum e value calculated for all performed test situations remains lower 0.5, which implies gross slip fretting conditions.
