# Superior Mechanical Behavior and Fretting Wear Resistance of 3D-Printed Inconel 625 Superalloy
## 3 Results and Discussion
### Tribological Tests
Therefore, it can be inferred that the AM-Inconel 625 would show a high fracture toughness, leading to the superior wear-resistance at varying load condition. Figure 6: Relationship between the (**a**) friction torques and (**b**) friction coefficient of AM-Inconel 625 and the number of cycles at room temperature; (**c**) typical friction torque versus angular displacement amplitude curves (\(T\)-\(\theta\) curves) at the fretting cycles of \(10^{0}\), \(10^{1}\), \(10^{2}\), \(10^{3}\), \(10^{4}\) and \(2\times 10^{4}\). Figure 8: (**a**) Wear volume and (**b**) depth of the fretting wear track of AM-Inconel 625 after fretting wear tests; SEM images of the corresponding wear tracks of AM-processed Inconel 625 after tests at (**c**) 40 and (**d**) 106 N; the insets in b show 3D surface topographies of the wear track at 40 and 106 N load, respectively. Figure 7: Images of the fretting wear tracks of AM-Inconel 625 at (**a**) 40; (**b**) 62; (**c**) 84 and (**d**) 106 N.  
During fretting, the wear volume has been proved to be related to the energy dissipated in the contact. Energy dissipation per cycle of fretting can be obtained by:  
\[E_{f}=2\int_{0}^{\delta}F_{s}dr=2F_{s}\delta=2\mu F_{N}\delta \tag{6}\]  
where \(F_{s}=\mu\cdot F_{N}\) is the frictional force, \(\mu\) and \(F_{N}\) are the friction coefficient and the normal load, respectively, \(\delta\) is the displacement amplitude. With the increased normal load, kinetic energy dissipation increases and the thermal effect plays a significant role in the tribo-oxidation. Meanwhile, the abrasion action of wear debris could be aggravated, leading to the increased wear volume. The calculated values of the dissipation energy with increasing normal load are shown in Table 3.  
It can be observed that at the same displacement amplitude, an increase of the normal load leads to the increase of dissipated energy because the dissipated energy is related with both the friction coefficient and the normal load (Equation (6)). This dissipated energy can promote the thermo-mechanical induced transformation of surface layer (such as the transformed layer, plastic flow, delamination) and tribo-oxidation. It was found that the wear volume increased linearly with increasing dissipated energy. Figure 8b presents the wear depth of wear tracks at different loads, where the quantified wear can be more clearly observed. Each test of the wear depth was carried out at the same condition for three times. An increasing trend can be seen and the maximum depth (\(\sim\)2.4 \(\upmu\)m) is obtained at the load of 106 N after 2 \(\times\) 10\({}^{4}\) cycles. The wear depth of the commonly used nickel-based protective thin coatings, such as NiTi film [56] usually varies as 40-140 \(\upmu\)m per 200 cycles depending on the operating conditions. Thus, it can be demonstrated that the AM-processed Inconel 625 possess extraordinary anti-wear property, which suppresses wear of the fretting contact surface for as long as 2 \(\times\) 10\({}^{4}\) cycles. The 3D surface topographies of the wear track at 40 and 106 N load are further taken as illustrated in the insets of Figure 8b. No obvious wear can be observed with the AM-processed Inconel 625 alloy. Detailed SEM images of the corresponding wear tracks of AM-processed Inconel 625 after tests at 40 and 106 N are shown in Figure 8c,d, respectively and the underlying wear mechanisms for the formation of various topographical features are proposed to be as follows. The presence of shallow discontinuous grooves and a small number of abrasive fragments attached on the wear track at 40 N (see Figure 8c) reveals the mild abrasive wear and materials transfer during fretting wear. At a high applied load of 106 N, the wear track of Inconel 625 becomes rather smooth and is found to be covered with an adhesion tribo-layer. In the previous works, microstructural changes and tribochemical reaction are often observed near wear tracks in microcrystalline materials, most commonly in the form of the nanocrystalline tribo-layer [57; 58; 59; 60; 61; 62; 63; 64; 65; 66].