Micro/Nanosilver Contribution in Modifying the Lubrication Film to Improve Friction and Wear Behaviors of TiAl-10wt.% Ag Composite

Tribological performances of solid lms are studied to improve their application range while retaining the longevity and high accuracy of TiAl alloy. For a ball-on-at tribopair system, addition of micro/nanosilver improved the tribological behaviors of TiAl-10 wt.% Ag self-lubricating composite (TASC), compared to those of TiAl alloy. Notably, during microstructure evolution of the wear scar cross-section, a large amount of silver migrated from the TASC to the wear scar, and the silver distributions increased. ­is inux continuously enriched silver on the wear scar cross-section and formed a low-hardness lubrication lm. Concurrently, a large amount of wear debris from the dry sliding wear contributed enough renements to form a high hardness grain renement layer. ­e results conrmed the formation of a 0.75GPa hardness lm with solid lubrication on a grain rened layer of 6.86GPa hardness, resulting in excellent antifriction and wear resistance from 72min to 90min of the TASC.­ese results can help to evaluate the tribological characteristics for their commercial application and can conceptualize possible solutions for TiAl alloy-base components.


Introduction
Due to excellent lubrication performances and con rmed tribological applications, the friction and wear behaviors of the solid lms have attracted extensive interest in aviation, aerospace, and automobile industries [1][2][3]. eir frictionreducing and wear-resistant enhancements increased the service lifetime and usage precision of TiAl alloy-based components [4,5]. Hence, controlling the friction and the wear behaviors of solid lms has been increasingly recognized as an essential strategy for improving the lubrication of TiAl-base component [6,7], for sustainable reductions in the energy dissipation and power consumption [8,9].
To address this issue, constructing a solid lm of the macroscaled thickness was identi ed as a good starting point to improve the tribological behaviors [10,11]. e results showed that the experimental lms exhibited and accepted the tribological functions in the macroscaled assemblies such as pumps and turbines [12], and micro/nanoscaled technologies such as electromechanical systems [13,14]. However, high alternating loads and variable high temperatures caused noticeable damages to these lms, that led to the unexpected separations of these lms from their substrates, thus causing the induced lubrication failures.
In recent years, laser additive manufacturing (LAM) was employed to prepare the custom-designed microstructures into the friction interfaces [15,16]. Soft lubricants were lled into the interface of the microstructures that led to the observed migration of these lubricants on the sliding wear surface. It resulted in an e ective formation of solid lm with excellent lubrication. But, the modi ed surface microstructure signi cantly reduced the load-bearing properties, and this subsequently decreased the wear resistance and service life.
Soft micro/nanosilver has been the preferred lubricant due to its excellent ductility, low shear resistance, and good plastic deformation [17][18][19], leading to the optimized friction and wear behaviors of the TiAl-base components. Xu et al. [19] successfully prepared the TiAl/Ag self-lubrication composites using the spark plasma sintering (SPS), and explored their tribological behaviors sliding against the fixed Si 4 N 3 balls. ey found large amounts of nanosilver appeared on the wear scars, that formed the solid films which resulted in low friction and high wear resistance. Furthermore, Shi et al. [20] reported the friction and wear behavior of TiAl/Ag with a wide temperature range of 25°C to 800°C. e results showed that a large quantity of silver existed on the wear scar that formed a solid film at 450°C; thus, TiAl/Ag acquired excellent tribological properties. Because of the film formation during a dry sliding process, this film was resistant to wear-induced damages, had improved forcebearing properties, and could self-adapt to high loads and variable temperatures, thus obtaining the TiAl alloy-base components with enhanced self-lubricating properties.
Previous studies have reported an important influence of the micro/nanoscaled silver on the formation mechanisms of solid films. But, silver migration and the film-forming process during dry sliding were rarely mentioned. Hence, this study on the formation mechanisms and lubrication functions was central to an in-depth understanding of the silver contributions in forming solid films and in reducing the friction and wear of TiAl-base components. It will also provide an important reference for the tribological applications of solid films.
In this work, TiAl-10 wt.% Ag self-lubricating composite (TASC) was prepared using the SPS instrument (D.R. Sinter ® SPS3.20). Friction and wear properties of the TASC sliding against the fixed Si 4 N 3 balls were examined under high-temperature and on a ball-on-disk tribometer (HT-1000). Primary wear mechanism of the TASC was analyzed by using an electron probe microanalyzer (EPMA, JAX-8230). e main element contents of the wear scars were measured by energy dispersive spectroscopy (EDS). e surface texture of the wear scar was studied using the surface profilometer (ST400) and a cross-sectional morphology of the wear scar was investigated using the field emission scanning electron microscope (FESEM, SIRION 200). e cross-sectional hardness of the wear scar was characterized using the cooperative methods by theoretical calculations and experimental tests.
Before the SPS process, the raw powders were mixed for 30 min in Teflon vials, using vibration milling at 45-55 Hz. After mixing, the powders were loaded into cylinder graphite molds of 25 mm diameter and then sintered on the SPS furnace of D.R. Sinter ® SPS3.20 to synthesize TiAl and TASC. In the sintering environment of pure argon, the heating rate, sintering temperature, the holding time, and the applied pressure were selected as 100-120°C/min, 1100-1200 C, 20 min, and 30-35 MPa, respectively. e main reactions of Ti and Al elements for preparing TiAl alloys could be written as follows: ΔG 0 > 0 No synthesis reaction, ΔG 0 � 0 Equilibrium synthesis reaction, ΔG 0 < 0 Spontaneous synthesis reaction, (1) Herein, ΔG 0 1 , ΔG 0 2 , and ΔG 0 3 were Gibbs free energies. In synthesis process of TiAl alloy, Ti, Al, TiAl 2 , Ti 3 Al, and TiAl 3 have been ensured as the intermediate reactants to form the TiAl alloy-based materials. Figure 1 shows a typical backscattering morphology and the main elemental distributions of the TASC. As can be seen from Figures 1(a) and 1(b), the back-scattering results of EPMA tests indicated that the silver mainly disperses in the white regions; subsequently, the EDS results of an EPMA indicate approximately 10 wt.%-silver in a TASC sample.

Phase Analysis of As-Prepared Samples.
FESEM crosssection microstructures and the X-ray di raction (XRD) patterns of as-prepared samples are shown in Figure 2. As indicated in Figures 2(a) and 2(c), the FESEM morphologies con rmed the successful synthesis of TiAl and TASC for their compaction structures. After the tests using the XRD instrument at a scanning speed of 0.01 s − 1 , the main phases of the TASC, determined from the intensities of the main di raction peaks as shown in Figures 2(b) and 2(d), were TiAl alloy and silver lubricant.

Hardness and Density of the TASC.
e mean hardness of the TASC was approximately 5.96 GPa on Vicker's apparatus of model No. HVS-1000 (ASTM standard of E92-82 [21]). e mean density of 4.24 g/cm 3 was obtained following the signi cant reference to the ASTM standard No. B962-08 [22] and after employing Archimedes' principle. Subsequently, the nanomechanical instruments of TI-950 and 1341 were applied for acquiring 125-135 GPa in elastic modulus and 900-1000 MPa in tensile strength.

Friction and Wear Behavior of the TASC
e typical schematic diagram of the TASC/Si 3 N 4 tribo-pair is presented in Figure 3(a). As shown in Figure 3(a), in accordance with the ASTM Standard No. G99-95 [23], at 450°C, friction and wear behaviors of the TASC sliding against the Si 3 N 4 balls were tested on the ball-on-disk tribometer (HT-1000). e friction coe cients were recorded in succession for 90 min using a controlled system of HT-1000. A wear rate W of the TASC sample was calculated from the following equation; W V/(F·L) [24]; here L and F are the sliding distance and applied load, respectively. After the tests, the mean wear volume V was calculated from the 3D and 2D pro les of the wear scars. e microcosmic morphologies of the wear scars have been measured using the surface pro lometer (ST400), and the results on 3D and 2D pro les have been respectively exhibited in Figures 3(b) and 3(c). Figure 4 shows sliding friction coe cients and wear rates of the TASC samples with increasing sliding times from 0 min to 90 min. As can be seen in Figure 4, the friction coe cients and wear rates of the TASC are smaller than those of TiAl alloy. Further, frictional coe cients and wear rates of the TASC gradually reduces during the 0-72 min, and this subsequent reduction from 72 min to 90 min, ensures excellent tribological performances.    Figure 5 shows EPMA morphology, silver distribution, and indentation morphology of the wear scar cross-sections. As shown in Figure 5(a), as the sliding time increases to 90 min, the lubrication lm, the grain re nement layer, and the microdeformation layer are observed in the wear scar crosssections. As shown in Figure 5(b), the silver lubricant is abundantly enriched in the lubrication lm. As indicated in Figure 5(c), the nanoindentation morphology of the wear scar cross-section can be clearly observed. Figure 6 shows the typical FESEM morphologies of the wear scar cross-sections of a TASC at 18, 36, and 90 min. As indicated in Figure 6, the layered microstructures were mainly composed of work hardening and microdeformation layers at 18 min, as can be seen in Figure 6(a). Figure 6(b) shows that with the increase in the sliding time from 18 to 36 min, layer structures of the cross-sections were increasingly transformed into particle re nement and microdeformation layers, respectively. When the sliding time increased from 36 min to 90 min, the layer microstructures evolved into lubrication lm, grain re nement, and microdeformation Advances in Materials Science and Engineering layers, as can be seen in Figure 6(c). During the wear process at 0-90 min of dry sliding, the layer evolution of the wear scar cross-section at 0-72 min helped to reduce friction coe cients and wear rates. Subsequently, at 72-90 min, the lubrication lm formed on the grain re ned layer, which resulted in low friction and low wear of the TASC.     of the plowing grooves of the wear scars. As can be seen from Figure 7(b), big peeling pits appeared on the wear scar, showing that the primary wear mechanism of the TASC was severe peeling at 36 min. Figure 7(c) shows that the plastic deformation appeared on the wear scar, causing the wear mechanism of plastic deformation at 72 min. Continuous transitions of the wear mechanisms from severe plowing to plastic deformation demonstrated that the friction force and material loss were reduced in succession, thus resulting in the low friction coe cient and a small wear rate of the TASC at 72 min. Herein, the back-scattering EPMA instrument was introduced to characterize the silver distribution of the wear scar, presented in Figure 8. A large amount of silver mainly appeared in the white regions of the wear scar. Silver distributions in the wear scars at 18, 36, and 72 min are shown in gures 8(a)-8(c), respectively. e distribution areas of the silver on the wear scar gradually increased, with the increase in sliding time from 18 min to 72 min. Figure 9(a) shows the typical EPMA morphology of the wear scar at 90 min. Plastic deformation bodies existing on wear scar resulted in the wear mechanism of plastic deformation. Figures 9(b) and 9(c) display the microcosmic morphologies determined by the FESEM of the wear scars, showing the microcosmic deformation. As shown in Figure 9(d), XPS results of the wear scars indicated that a large amount of silver distributed in the wear scar. In addition, an EDS result con rmed approximately 52.21 wt.% silver.

Analysis in Wear Scar Morphology
is indicated that a large amount of silver had migrated on the wear scars, causing plastic deformation that self-repaired the wear scars, and resulted in the excellent surface texture, as can be seen from Figure 9(e).
From the above discussions, it can be concluded that a large amount of silver migrated on the wear scar to increase the silver distribution. Because of the sliding friction force, the silver distribution regions on the wear scar improved with the continuous increase in sliding times. It resulted in bene cial formation of the silver-enriched lm. Sliding wear scars were continuously well-repaired because of a plastic deformation of silver, which smoothened the wear scar to result in an excellent texture. Arresting reductions in friction resistance and material loss were mainly attributed to silverenriched lm and the excellent texture of wear scar, which resulted in exceptional friction and wear behavior of the TASC. Figure 10 shows typical FESEM morphology, surface texture, and primary elemental composition of the wear scar of the xed Si 3 N 4 ball at 90 min. As shown in Figure 10(a), plastic deformation bodies appeared on the wear scar to cause main wear mechanism of the plastic deformation. As can be seen from Figure 10(b), an excellent texture of the wear scar was con rmed by the surface height parameters, such as Sa: 0.11 μm, Sq: 0.13 μm, and Sku:3.25. As shown in Figure 10(c), an EDS result demonstrated that approximately 12.91 wt.% silver was found on the wear scar of the Si 3 N 4 ball. In the sliding wear process from 0 min to 90 min, a large amount of silver continuously migrated from the TASC to the tribopair surface, and was subsequently transferred to the frictional surface of the Si 3 N 4 ball. e result was that almost 12.91 wt.% silver was utilized to repair the wear scar of a Si 3 N 4 ball, which pro tably obtained an excellent texture of the wear surface. us, a large quantity of silver and the very smooth texture appeared into the contact    Advances in Materials Science and Engineering interface of the TASC/Si 3 N 4 tribo-pair; and the frictional resistance and material loss were significantly reduced, which helped to achieve the low friction and low wear rate of the TASC. Figure 11 indicates the schematic diagrams of the structural evolution of wear scar cross-sections with increased sliding time from 15 min to 80 min. As indicated in Figure 11(a), because of the high contact stress applied on the friction surface at 15 min, the silver was repeatedly extruded, produced plastic deformation and the microscopic flow, causing its migration onto the wear surface, and generating an inhomogeneous distribution. Micro/nanoscaled silver spread out into an incomplete lubrication film. Concurrently, insufficient sliding wear caused the big crystalline grains to form into a work hardening layer. e applied stress was transferred below the work hardening layer to produce a microscopic plastic deformation, thus forming a microdeformation layer. is improved the tribological behaviors of TASC due to the formation of a layered structure mainly composed of an incomplete lubrication film, the work hardening, and microdeformation layers.

Formation Schematic of Lubrication Film
As shown in Figure 11(b), when sliding time increased from 15 min to 25 min, sliding friction numbers increased significantly, and then caused microscopic cracks to occur and extend in succession. Subsequently, big fatigue cracks appeared into the work hardening layer. After reiterative wear process of high contact stress, cross-sectional cracks continuously propagated onto the wear scar surface and destroyed the work hardening layer. As indicated in Figure 11(c), a destroyed work hardening layer produced a large amount of wear debris under the rolling compaction of sliding force. Due to the surface stress and frictional heating, the wear debris accumulated in succession, thus leading to the formation of particle accumulation layer at 30 min.
When the dry sliding wear continued for 40 min, the big wear debris was extruded into small particles and they gradually accumulated to form particle refinement layers, as can be seen in Figure 11(d). It caused the friction coefficients and wear rates to further reduce from 30 min to 40 min. As sliding wear continued to 60 min, small wear particles produced plastic deformation and local fracture. is resulted in possible occurrences of grain dislocation during crystal-plane slipping. Crystal orientation transformation caused the formation of the grain refinement layer on a microdeformation layer, as shown in figure 11(e).

Advances in Materials Science and Engineering
Simultaneously, an incomplete solid film appeared on the grain refinement layer, probably because of insufficient grain refinement and the heterogeneous silver distributions. Sliding friction coefficients and wear rates continuously decreased as the time increased from 40 min to 60 min. When the sliding wear time increased to 80 min, a large amount of silver constantly spread out onto the wear scar and formed a homogeneous distribution, as shown in Figure 11(f). Additionally, a further prolonged sliding wear caused the interfacial grains to experience more refinements.
is was beneficial for the silver distribution on the wear scar. Hence, sufficient grain refinement and enhanced silver distribution caused the solid films to form in the grain refined layer. is resulted in the formation of an exceptional lubrication microstructure, consisting of a lubrication film, grain refined layer, and microdeformation layer, and this allowed the TASC to achieve an excellent sliding wear behavior at 80 min.
Consequently, during a dry sliding wear, a large quantity of micro/nanosilvers were continuously extruded out of the TASC. Silver gradually spread out on the wear scar to form the solid lubrication film, for the reiterative spread of contact stress. A large amount of silver migrated onto the wear scar to form the lubrication film at 80 min, which caused the low shear resistance of solid film. Subsequently, friction resistance and material loss of the TASC were observably reduced, thus resulting in an excellent tribological behavior.