*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

material (alumina). This phase will be followed by a transitional stage: wear of transferred PVD coatings. The second phase (high wear regime) happens as soon as the trapped debris are oxidized. Gradually, wear is accentuated on both sides of contact (on PVD coatings and alumina ball). For larger displacements (δ > 50 μm) and a high normal load, fatigue wear induced by multicracks has been detected **Figure 11**. One hypothesis is that the generated plastic strain leads to a brittle layer, which can be associated to the tribologically transformed structure (TTS) [24, 25]. Enduring cyclic loading, multicracks are activated which induce debris formation and wear. De Wit [16] showed that the transition period corresponds, in the case of the PVD layer, to the formation of debris made up of amorphous rutiles and nanocrystallines. Beyond this transition, the amorphous phase is transformed into a crystalline phase and contributes to further wear. SEM observations showed that the debris appeared on this TiAlCN/TiAlN/TiAl multilayer coating during the first cycles, in the form of particles less than 1 μm in size [3].

The main wear mechanism of the TiAlZrN-coated AISI4140 steel was the brittle cracks of TiAlZrN, before destroying the coating. After destroying the TiAlZrN coating, the main wear mechanism was cracks due to a low normal force and high slip amplitude and cumulated plastic flow on the edge of the fretting scar and adhesive and abrasive wear at high slip amplitude. Oxidation was observed on most the worn surface.

Moreover, the effect of surface roughness before deposition on the wear behavior of multilayer coatings deposited on an AISI 4041 steel substrate. After deposition, the surface roughness of the coating was approximately half of the original substrate surface roughness. While the frictional behavior was not apparently affected, the wear rate of the coatings increased significantly with increase in the substrate surface roughness. Wear rate increased rapidly when the substrate surface roughness exceeded Ra 1 μm. Above this substrate roughness, the dominant wear mechanism also changed from adhesion to chip/flake formation and fragmentation of the coatings. Chipping/flaking of the coatings initially occurred mainly at the tops of asperities of the surface texture. The Archard's specific wear

#### **Figure 11.**

*Fretting wear scar morphology of multilayered TiAlZrN/TiAlN/TiAl coating (gross slip regime), FN = 200 N.*  δ *= ±100 μm.*

**329**

**Figure 12.**

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

contact mechanics analysis results.

*3.3.2 Prediction energy wear coefficient*

rate increased with the increase in total load for coatings on the rough substrate surfaces; however, this was almost invariant with the increase in load for coatings on the smoother substrate surfaces, nominally following the Archard's wear law. Contact pressure distributions over the real area of contact between the ball and the rough coating surfaces have been analyzed by applying the elastic foundation model of contact mechanics. It has been shown that the contact pressures increase significantly with the increase in surface roughness of the coatings. Plastic yielding is highly possible in the coatings deposited on rough substrate surfaces above Ra 1 μm. The observed apparent effect of surface roughness on wear and wear mechanism transitions of the multilayer coatings can be explained according to the

Investigations at various loads and slip amplitudes confirm that there exists a correlation between the wear volume extension of the TiAlZrN/TiAlN/TiAl multilayer coating and dissipated energy. For all the different loading conditions previously defined in Section 2.4, **Figure 12** shows the rates of the wear volume as a function of the dissipated energy. The used volume is measured using a 3D optical profilometer, and the dissipated energy is estimated directly by the area of the fretting loop for each cycle. As a separate form of this behavior, when the energy approach is applied, all of the test parameters are represented by the one and only linear equation from which a single overall energy wear coefficient (α = slope of the curve ) can be detreminated for each antagonist. In fact, the energy coefficient represents the slope of the straight line which connects the lost volume and the energy dissipated by fric-

/J, and TiAlCN

/J [3].

tion: VLost = αEd. In the case under consideration, αTiAlZrN = 104 μm3

coating provides an energetic wear coefficient of αTiAlCN = 23.103 μm3

*Fretting wear scar morphology of multilayer TiAlZrN/TiAlN/TiAl coating (partial slip regime).*

### *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*

cycles, in the form of particles less than 1 μm in size [3].

the worn surface.

material (alumina). This phase will be followed by a transitional stage: wear of transferred PVD coatings. The second phase (high wear regime) happens as soon as the trapped debris are oxidized. Gradually, wear is accentuated on both sides of contact (on PVD coatings and alumina ball). For larger displacements (δ > 50 μm) and a high normal load, fatigue wear induced by multicracks has been detected **Figure 11**. One hypothesis is that the generated plastic strain leads to a brittle layer, which can be associated to the tribologically transformed structure (TTS) [24, 25]. Enduring cyclic loading, multicracks are activated which induce debris formation and wear. De Wit [16] showed that the transition period corresponds, in the case of the PVD layer, to the formation of debris made up of amorphous rutiles and nanocrystallines. Beyond this transition, the amorphous phase is transformed into a crystalline phase and contributes to further wear. SEM observations showed that the debris appeared on this TiAlCN/TiAlN/TiAl multilayer coating during the first

The main wear mechanism of the TiAlZrN-coated AISI4140 steel was the brittle cracks of TiAlZrN, before destroying the coating. After destroying the TiAlZrN coating, the main wear mechanism was cracks due to a low normal force and high slip amplitude and cumulated plastic flow on the edge of the fretting scar and adhesive and abrasive wear at high slip amplitude. Oxidation was observed on most

Moreover, the effect of surface roughness before deposition on the wear behavior of multilayer coatings deposited on an AISI 4041 steel substrate. After deposition, the surface roughness of the coating was approximately half of the original substrate surface roughness. While the frictional behavior was not apparently affected, the wear rate of the coatings increased significantly with increase in the substrate surface roughness. Wear rate increased rapidly when the substrate surface roughness exceeded Ra 1 μm. Above this substrate roughness, the dominant wear mechanism also changed from adhesion to chip/flake formation and fragmentation of the coatings. Chipping/flaking of the coatings initially occurred mainly at the tops of asperities of the surface texture. The Archard's specific wear

*Fretting wear scar morphology of multilayered TiAlZrN/TiAlN/TiAl coating (gross slip regime), FN = 200 N.* 

**328**

**Figure 11.**

δ *= ±100 μm.*

rate increased with the increase in total load for coatings on the rough substrate surfaces; however, this was almost invariant with the increase in load for coatings on the smoother substrate surfaces, nominally following the Archard's wear law. Contact pressure distributions over the real area of contact between the ball and the rough coating surfaces have been analyzed by applying the elastic foundation model of contact mechanics. It has been shown that the contact pressures increase significantly with the increase in surface roughness of the coatings. Plastic yielding is highly possible in the coatings deposited on rough substrate surfaces above Ra 1 μm. The observed apparent effect of surface roughness on wear and wear mechanism transitions of the multilayer coatings can be explained according to the contact mechanics analysis results.
