*2.3. Characterization*

The structures and the phase compositions of all samples were analyzed with an X-ray diffractometer (XRD, D500, Bruker, Beijing, China) with a 40 kV operating voltage and Cu Kα radiation in the 2θ range of 25◦–80◦. The chemical states before the friction test were examined with an X-ray photoelectron spectroscope (XPS, PHI-5702, Thermo Fisher Scientific, Waltham, MA, USA). The Raman spectrometer (Labram-010, an excitation wavelength of 632 nm, HORIBA, Beijing, China) further surveyed the phase constitutions of all samples before and after the wear test. The elemental compositions of selected regions and the morphologies of the wear tracks for all samples were analyzed using a scanning electron microscope (SEM, JSM-6700F, JEOL, Beijing, China), equipped with backscattered electrons (BSEs) and an energy dispersive spectrometer (EDS).

#### **3. Result and Discussion**

#### *3.1. Composition and Microstructure*

The XRD patterns of all samples before the wear test are presented in Figure 2, which includes the un-implanted GH4169 alloy. It was found that the γ matrix phase could be observed in all samples, which mainly came from the GH4169 alloy. In Figure 2ab, the characteristic peaks of Ag (2θ = 37.92◦) and Mo (2θ = 40.56◦) correspond to FCC Ag (111) and BCC Mo (110), respectively, excluding the GH4169 base. Both Ag (111) and Mo (110) peaks have a smaller angle shift. It may be related to the lattice distortion induced by the bombardment of the high-energy ion beam. Moreover, the XRD results of the co-implanted GH4169 alloy allowed us to identify the Ag and Mo phases in the co-implanted sample (Figure 2d).

**Figure 2.** XRD spectra of (**a**) the un-implanted alloy (N); (**b**) silver (A); (**c**) molybdenum (M); and (**d**) silver and molybdenum (AM) before the wear test.

Figure 3a, in which peaks derived from O, Ag, and Mo are all evident, exhibits the XPS full spectrum for the co-implanted GH4169 alloy before the wear test. As can be seen in Figure 3b, the XPS peaks of Ols at 531.8 eV and 530.5 eV yielded two Ag2MoO4 Gaussian peaks. The Ag3d5/2 and 3d3/2 peaks deviate from those of the Ag-implanted sample (Figure 3c), indicating that the binding energy of Ag3d5/2 and 3d3/2 slightly shifted to a lower value. Similarly, the Mo3d5/2 and 3d3/2 peaks deviate from those of the Mo-implanted sample (Figure 3d). Based on the Ag3d peak position, the peaks at 368.18 eV and 374.23 eV can be assigned to Ag3d5/2 and 3d3/2 in the Ag2MoO4 phase, respectively. In addition, the peaks located at 232.73 eV and 235.93 eV can be assigned to Mo3d5/2 and 3d3/2 in Ag2MoO4, respectively. This result is consistent with reports of Suthanthiraraj and Liu [11,22].

Compared with the XPS result above, no obvious characteristic peaks of the Ag2MoO4 phase were found in the XRD pattern of AM before sliding. This may be due to the low content and good dispersion of Ag2MoO4. To confirm this, the phase compositions of AM were determined via a laser Raman spectrometer (Figure 4). Monoclinic silver molybdate with a broad peak can be detected with Raman spectra [2,11–13]. We inferred that silver molybdate indeed formed during the preparation process. Because of the bombardment of the high-energy ion beam, the co-deposited particles on the base surface have substantially high activity. The chemical reaction between Ag, Mo, and the residual oxygen from the vacuum chamber or the oxygen adsorbed on the base surface might happen under an unbalanced fabrication process at RT. XPS and Raman analysis reveal that silver molybdate can be generated directly via ion-beam-assisted bombardment at RT.

**Figure 3.** (**a**) XPS full spectra; (**b**) O1s XPS spectra for AM; (**c**) Ag3d XPS spectra of A and AM; and (**d**) Mo3d XPS spectra of M and AM before the wear test.

**Figure 4.** Raman spectra of AM before the wear test.

#### *3.2. Friction and Wear Properties*

Figure 5a presents the friction coefficient evolution of all samples with sliding time under a normal load of 2 N. Compared with the bare GH4169 alloy (N), the AM shows a lower friction coefficient at the beginning of the test, which is attributed to the formation of silver molybdate with an atomically layered structure during the wear test. As the test proceeded, the friction coefficient steadily increased to reach the base nature. The average friction coefficient and wear rate under the dry friction and RT conditions are presented in Figure 5b for all samples. Compared with the bare GH4169 alloy, there is no obvious change in the average friction coefficient at the steady wear state. It was speculated that the

Ag-implanted sample has a smaller mean friction coefficient due to the synergistic lubrication effect of Ag and Ag2O. The wear rate of the co-implanted sample (AM) was the lowest of all samples, as shown in Figure 5b, from 4.88 × 10−<sup>4</sup> mm3·N−1·m<sup>−</sup><sup>1</sup> to 1.22 × 10−<sup>4</sup> mm3·N−1·m<sup>−</sup><sup>1</sup> compared with the base, due to the presence of silver molybdate. The wear rate for the Mo-implanted sample is much higher, the highest of all the samples, than that of the base. It is also estimated that MoO3 formed on the worn surface leads into the worst anti-wear performance for M. Research shows that MoO3 is brittle and has no lubrication at low temperature [23].

**Figure 5.** (**a**) The friction coefficient and (**b**) the average friction coefficient and wear rate for all samples tested under a normal load of 2 N and a fixed sliding speed of 47 mm/s.

Figure 6 displays the cross-section profiles and surface 3D topographies of the wear scars for all samples after the 10 min friction test. Among all samples, the sample AM exhibits the smoothest wear tracks with the smallest wear volumes as seen in Figure 6d, indicating the best wear resistance performance. The visible adhesive marks are observed on the selected zone marked with a red box for the worn surface of N, A, and M as seen from Figure 6a–c. The Mo-implanted sample shows the deepest wear tracks (Figure 6c), which is consistent with Figure 5.

The SEM morphologies of the wear tracks after sliding are given for all samples in Figure 7. The selected dark zone (named I) and gray zone (named II) on the wear scars for all samples were analyzed with EDS. The oxygen element was enriched in the dark regions. In Figure 7a, the worn surface is characterized with small bumps and delamination pits, indicating abrasive wear and brittle fracture in the bare GH4169 alloy. The discontinuous dark regions covering the surface in Figure 7b are believed to be the discontinuous oxide layer. It can be inferred that the predominant wear mechanism for A is oxidation and adhesive wear. The Mo-implanted sample, accompanied by a large amount of the wear debris and some abrasive grooves on the worn surface, exhibits the most serious wear condition (Figure 7c). The continuous tribo-layer covered on the worn surface, together with some shallow grooves for the co-implanted sample, can be seen in Figure 7d. As discussed before, the continuous oxide layers (the corresponding continuous dark regions) formed in AM with the smoothest wear scar, which accounts for the lower wear rate. The formation of a tribo-layer, as a protective and lubricating layer [24], can effectively reduce the direct contact of the sample and the Si3N4 ball. It has been illustrated that the oxidation wear dominates the wear processing of the co-implanted GH4169 alloy [25].

**Figure 6.** The cross-section profiles, surface 3D topographies of wear tracks for (**a**) N, (**b**) A, (**c**) M, and (**d**) AM after the wear test.

**Figure 7.** The SEM micrographs of the worn surfaces after the fiction test for (**a**) N, (**b**) A, (**c**) M, and (**d**) AM.

Adhesion wear can be seen in the SEM images in Figure 8 on the worn surface of the counterface ball against the AM. The SEM image of the worn surface of the counterface ball against A presents clear abrasion wear. This is because, as the wear test continued, the contact between the counterface and the sample (A and AM) led to material transfer via adhesive wear. Table 3 shows the chemical compositions of the counterface balls determined via EDS, providing further evidence for oxidation wear and material transfer via adhesion wear.

The tribo-chemical reactions and phase changes on the worn surfaces were further studied with the micro-Raman. In the following Raman spectra (Figure 9a), Fe2O3, resulting from the oxidation of Fe and Cr in the matrix, can be detected on the worn surfaces of all samples. The generation of Fe2O3 is expected to contribute to the lubricating effect [23,26,27]. Ag2O and MoO3 can be found on the worn surfaces, respectively, of A and M. In addition, the Ag2MoO4 phase is detected on the worn surface after the sliding process of AM. It has been inferred that silver molybdate can be combined via

tribo-reaction between MoO3 and Ag on high-energy-contacted surfaces during RT wear tests [2,11]. Based on the Raman spectra of the sample surfaces, as shown in Figure 9b, before and after the wear test, the peak intensity of Ag2MoO4 increased substantially after the friction test of AM. It can be inferred that Ag2MoO4 content rose after the wear test due to the further generation of Ag2MoO4 during sliding. That the Ag2MoO4 phase can be synthesized during the preparation and rubbing processes is also demonstrated.

**Figure 8.** The SEM images of the counterface balls (Si3N4 ball) after sliding against (**a**) A, (**b**) B, (**c**) C, and (**d**) D and corresponding higher magnification SEM images against (**e**) A, (**f**) B, (**g**) C, and (**h**) D.


**Table 3.** The element compositions of the worn surfaces of all counterface balls.

**Figure 9.** (**a**) Raman spectra of the worn surfaces for all samples after the wear test; and (**b**) Raman spectra before and after the wear test for AM.

As demonstrated above, the unbalanced fabrication method results in the generation of Ag2MoO4 during the preparation process at RT. In addition, more silver molybdate formed on the worn surface of AM during the RT friction-wear test through a series of tribo-chemical reactions on the high-energy-contacted surface. The co-implanted sample has the best wear resistance performance, attributed to the formation of Ag2MoO4. In the friction test, the wear mechanism was dominant by oxidation wear in AM covered by a continuous oxide layer. A large amount of silver molybdate with a layered structure [11,28], formed on the worn surface during sliding, promotes the formation of a continuous oxide film. Furthermore, the Ag2MoO4 phase with weaker Ag–O bridging bonds can easily break to form rich silver lubrication layers at a lower wear rate. It is generally known that silver molybdate is an excellent high-temperature solid lubricant, and little research on the lubrication behavior of Ag2MoO4 at RT has been conducted. However, our experimental results prove that silver molybdate can greatly improve the wear resistance performance at RT.
