**1. Introduction**

In order to achieve a low-friction coefficient and wear rate for some harsh-condition applications, numerous works on the lubrication effects about the addition of solid lubricant phases to metal matrices, such as graphite, oxides, soft metals, MoS2, and fluoride, have been carried out [1–7]. Double metal oxides, including titanates, tungstates, molybdate, and vanadates, are effective high-temperature solid lubricants [8–11]. Among these double metal oxides, more attention has been paid to silver molybdate, which possesses a layer-like structure, because of its excellent lubrication performance, achieving low friction at high temperatures [2,9–17]. For example, Liu et al. [11] investigated the tribological properties of Ni-based self-lubricating composites containing Ag and MoS2 within a wide temperature range. The evolution of the friction coefficient from room temperature (RT) to 700 ◦C ranges from 0.18 to 0.77, and the coefficient decreases down to 0.18 at 700 ◦C as temperature increases. The wear rate from RT to 700 ◦C is reduced by an order of magnitude, and the worn surface is covered by a continuous and smooth film at 700 ◦C. As temperature increases to 500 ◦C, MoS2 is oxidized and becomes molybdenum oxide. Ag and Mo oxide generates silver molybdate, first proposed by Gulbinski [18], good high-temperature lubrication with a layered structure, which may be considered a mixture of AgO and MoO3 layers separated by a silver layer. In fact, the lubrication mechanism of silver molybdate can be attributed to weaker Ag–O bridging bonds that are more easily broken to form a rich silver lubrication film when subjected to sliding, which is responsible for the low friction at the sliding interface [9]. Ni-based composites with an addition of lubricious Ag2MoO4 at various temperatures have also been explored. Due to high-temperature fabrication processing, Ag2MoO4 decomposes but is reproduced in the rubbing process at high temperatures, leading to an improvement in tribological performance. Thus, at 700 ◦C, the lowest friction coefficient can be observed (about 0.26). Furthermore, Chen [2] and Aouadi [13] studied adaptive NiCrAlY–Ag–Mo coatings and nanocomposite Mo2N/MoS2/Ag coatings, respectively. Friction coefficients for both coatings can be lowered to 0.1 at 600 ◦C due to the formation of silver molybdate on the worn surface during sliding. Due to the large enthalpy of formation ( Δ *H*f = +57 kJ/mol), silver molybdate usually forms during sliding at high temperatures and works as an effective high-temperature solid lubricant.

In this study, ion-beam-assisted bombardment (IBAB), an unbalanced process, makes it possible to produce intermetallic compounds or intermediate phases at RT [19–21]. In fact, a silver molybdate phase formed via the co-implanted GH4169 alloy during the preparation process in our experiments, but only a small amount was produced, which may be related to the high surface atomic activity of the Ag and Mo particles on the surface. In addition, a series of tribo-reactions promoted the production of more silver phase on the worn surface during the sliding process. Usually, the powder metallurgy process as a high-temperature fabrication method leads to the decomposition of silver molybdate [11]. The generation of silver molybdate at RT during preparation and sliding has considerable significance, but such generation has scarcely been reported. Moreover, the RT lubrication performance of silver molybdate in the form of a film has not been investigated as much.

This work focuses on the tribological properties of a GH4169 alloy co-implanted with Ag and Mo prepared via IBAB at RT. For comparison, a single Ag and a single Mo are implanted into the same alloy, respectively. A correlation analysis and a discussion of the tribo-chemical reaction on the worn surface and effects on the wear behavior are given. The wear mechanism is further revealed as well.

#### **2. Experimental Details**

#### *2.1. Sample Preparation*

The chemical compositions of the commercial GH4169 alloy, provided by Beijing Iron and Steel Research Institute, are shown in the following Table 1.


**Table 1.** The chemical compositions of the GH4169 alloy.

The polished 15 mm × 15 mm × 5 mm GH4169 alloy substrates were implanted with Ag, Mo, and Ag–Mo via IBAB. These preparation experiments were performed on a homemade multi-functional ion beam system (MIS800, Sykeyou Vacuum Technology Institute, Shenyang, China), the structure schematic of which is shown in Figure 1. Two Kaufman ion sources (named sputtering ion sources 1 and 2) were respectively used for sputtering an Ag target (purity > 99.99%) and an Mo target (purity > 99.9%) with a 2.5 keV Ar+ ion beam. A third Kaufman ion source (named the assisted ion source) was used for bombarding the particles into the alloy surface via an Ar+ ion beam with an accelerating voltage of about several tens of thousands of volts, which was incident along the normal direction of the base surface. The purity of argon as a working gas was about 99.999%, and the working pressures during deposition with single and double targets was set to 1.5 × 10−<sup>2</sup> Pa and 2.3 × 10−<sup>2</sup> Pa, respectively. The experimental time was 1 h. The implanted samples were numbered in accordance with the implanted particle, as shown in Table 2. The un-implanted alloy was marked as N.

**Figure 1.** Schematic diagram of the multi-functional ion beam deposition system.

**Table 2.** The particles implanted into GH4169 alloy substrates and the numbering.


#### *2.2. Friction and Wear Test*

The friction coefficients of all samples were measured under a normal load of 2 N in atmospheric conditions at RT with a wear tribometer (WTM-2E, Lanzhou Institute of Chemical Physics, Lanzhou, China) in a ball-on-disk style (a relative humidity of 70% ± 5%). The wear test was conducted in a circular motion 5 mm in diameter under a fixed sliding speed of 47 mm/s. The upper ball was a counterpart Si3N4 ball 5 mm in diameter with a hardness of 19 GPa. Hertzian contact pressure was 1.2 GPa, according to our calculations. The single wear test time was 10 min and was repeated five times. The friction coefficient curves were recorded automatically with a computer attached to the tribometer. After the wear test, the Wyko NT9100 surface profiler (Chung Ming Automation Equipment Co., Ltd., Shanghai, China) was used to obtain the profiles of the wear tracks. The wear rate was calculated as the equation ω = Δ*V*/(*F*·*L*), ω was the wear rate in mm3·N−1·m<sup>−</sup>1, Δ*V* was the wear volume loss in mm3, *L* was the total motion distance in m, and *F* was the load in N.
