Tribological performance of TiAl matrix composites containing silver and V₂O₅ nanowires at elevated temperatures

Abstract

TiAl alloys (TAs) are widely used in aircraft and automotive industries, but their poor wear resistance restrains further applications. In this paper, the tribological properties of TiAl matrix self-lubricating composites containing lubricants of varying amounts (0.5 wt%, 1.5 wt% and 2.5 wt%) of V₂O₅ nanowires (NWs) and 5 wt% silver against Si₃N₄ balls were investigated from room temperature to 600 °C under the same conditions of 20 N load per bearing section and 0.35 m s⁻¹ gliding speed. TiAl–5Ag–1.5V₂O₅ NWs (TB) exhibited excellent tribological properties over a wide temperature range. Moreover, at 450 °C, TB showed a lower friction coefficient of 0.19 and a lower wear rate of 1.87 × 10⁻⁴ mm³ N⁻¹ m⁻¹, which were attributed to a continuous lubricating film containing V₂O₅ NWs and silver on the friction surface. Furthermore, in the formed lubricant films, silver was used as a solid lubricant to provide good lubrication, while V₂O₅ NWs played the role of a support with high shear strength. The investigation indicated that V₂O₅ NWs and silver exhibited an excellent synergistic effect for improving the tribological properties of TB.

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1. Introduction

TiAl alloys (TAs) are used in aerospace and automotive engine heat resistant structures owing to their advantages of low density, high specific strength and specific modulus.^1–3 However, the poor wear resistance of TAs restrains their applications in frictional working conditions. For example, the turbine blades have fatigue cracks on their surfaces due to abrasive wear during the process of using, resulting in disastrous consequences;⁴ as exhaust valves of cars, the wear surface is one of the main causes of component failure.⁵ Therefore, it is necessary to improve the anti-wear property and prolong the service life of TiAl matrix frictional components.

Silver with a large diffusion coefficient is easy to form into a lubricant film on the friction surface, so has been widely used as a solid lubricant to provide good lubrication among various kinds of engineering components. Shi et al.⁶ investigated the tribological behavior of TiAl matrix self-lubricating composites (TMSC) containing Ag from 25 to 800 °C. The results indicated that the friction coefficients of TMSC were lower than those of the TiAl based alloy at 25–400 °C, which was attributed to a smooth and dense lubricating film containing Ag that formed on the worn surface. Meanwhile, V₂O₅ NWs had attracted tremendous attention because they had stable crystal structures and high temperature oxidation resistance. More recently, oxides of transition metals like vanadium⁷ have attracted much research interest. Due to their oxidation stability and low tribo oxidation sensitivity, the lamellar oxide of V₂O₅ has been considered a good candidate for low friction coatings.⁸ Meanwhile, Magnéli oxides like V₂O₅ have also been explored for their friction and wear properties.⁹ Because V₂O₅ NWs have a relatively high mechanical strength, they can provide a support and effectively improve the friction and wear performance for the solid lubricant coating.¹⁰ Hence, as a kind of important functional material, it is feasible to improve the anti-friction and the wear resistance of the composites with the addition of V₂O₅ NWs. Nevertheless, researchers such as Xiong et al.¹¹ and Xu et al.¹² reported that when only a single lubricant was added to the composites, the lubrication effect was very limited over a wide temperature range. Hence, the composites containing two or more solid lubricants were researched to find out the mechanisms of synergetic effects.^13,14 The synergetic effects of Ti₃SiC₂ and Ag solid lubricants have been illustrated by Xu et al.¹⁵ The results showed that a rich-silver smooth tribo-film was formed on the worn surface from 25 to 400 °C, while the Ti₃SiC₂ oxidation reaction formed a rich-oxide tribo-film on the worn surface at higher temperatures of 600 and 800 °C. Yao et al.¹⁶ investigated the effects of TiB₂ on tribological properties of TMSC containing Ag at elevated temperatures. They observed that the reinforced phase TiB₂ and the lubricating phase Ag exhibited a good synergistic effect for improving the tribological properties at elevated temperatures. Furthermore, Shi et al.¹⁷ investigated the tribological performance of TMSC containing Ag, Ti₃SiC₂ and BaF₂/CaF₂ tested from room temperature to 600 °C. The results reported that the tribological performance was improved due to the synergetic effect of Ag, Ti₃SiC₂ and BaF₂/CaF₂ lubricants. However, few studies reported the synergetic effect of Ag and V₂O₅ NWs on the tribological performance of TMSC. Hence, this work investigated the sliding friction and wear of TMSC containing V₂O₅ NWs and Ag from room temperature (25 °C) to 600 °C.

Due to few studies being conducted to research the formation mechanism and structural features of lubricant films, which play an important role on the wear behavior, the present investigation is aimed at preparing TMSC containing 5 wt% silver and varying amounts of V₂O₅ NWs (TX) by an in situ technique using spark plasma sintering (SPS). The possibility of an excellent synergetic effect of silver and V₂O₅ NWs lubricants has been explored by carrying out dry sliding wear from 25 to 600 °C. In this study, TiAl as the metal matrix, Ag as the solid lubricant, and V₂O₅ NWs playing the supporting role were used to fabricate TiAl matrix self-lubricating composites. The dry sliding wear experiments of TX under the same conditions of 20 N load per bearing section and 0.35 m s⁻¹ gliding speed from 25 to 600 °C were carried out on a ball-on-disk high-temperature tribometer to obtain lubricating films. The formation and acting mechanisms of lubricating films were investigated at the different testing temperatures. These results provide a clue for the optimized design of TiAl composite materials related to their self-lubricating function.

2. Experimental details

2.1 Preparation of TX

According to the previous experiment data of our laboratory, silver exhibited an excellent lubrication performance with the better choice for the content ranges from 1 to 10 wt%. In this work, silver powder was added with a weight fraction of 5%, while the weight fractions of V₂O₅ nanowires (NWs) were fixed at 0.5, 1.5 and 2.5 wt%, respectively. TiAl matrix self-lubricating composites containing lubricants of varying amounts of V₂O₅ nanowires (NWs) and 5 wt% silver (TX) were fabricated by SPS at a temperature of 1100 °C and at a pressure of 35 MPa for 10 min in a protective Ar atmosphere. TA, TB and TC represent TiAl–5Ag–0.5V₂O₅ NWs, TiAl–5Ag–1.5V₂O₅ NWs and TiAl–5Ag–2.5V₂O₅ NWs composites, respectively. The heating rate was 100 °C min. Commercially available powders of Ti, Al, Nb, Cr and B (10–25 μm average size, 99.9 wt% purity; Peng, et al.¹⁸), Ag (1.0 μm average size, 99.95% purity), as well as V₂O₅ (0.16–0.40 μm average size, 99.5% purity) were used as the starting powders in this study. The starting powders were mixed by vibration milling with a frequency of 45 Hz in teflon vials. The composite powders of the TX matrix consist of Ti, Al, Nb, Cr and B powders with the molar ratio of 48 : 47 : 2 : 2 : 1. After being mixed and dried, the mixtures were then sintered by SPS using a D.R. Sinter® SPS3.20 (Sumitomo Coal & Mining, now SPS Syntex Inc.) apparatus. The as-prepared specimen surfaces were ground to remove the layer on the surfaces and polished mechanically with successive grades of emery papers down to 1, 200 grit, 5 μm up to a mirror finish.

2.2 Vicker’s microhardness and density

The Vicker’s microhardness of TX samples were measured according to the ASTM standard E92-82 (American Society for Testing and Materials¹⁹) by using a HVS-1000 Vicker’s hardness instrument with a load of 1 kg and a dwell time of 8 s. The tests were carried out at three locations to reduce random errors. The densities of specimens were measured based on Archimedes’ principles and according to the ASTM Standard B962-08 (American Society for Testing and Materials²⁰). Eight tests were conducted. The mean values of Vicker’s hardness and measured density of TX were given (see Table 1).

Table 1 Vicker’s hardness and measured density of TX

Samples	Microhardness (HV1)	Measured density (g cm^−3)
TiAl–5Ag–0.5V2O5 NWs (TA)	587.2	4.29
TiAl–5Ag–1.5V2O5 NWs (TB)	612.5	4.97
TiAl–5Ag–2.5V2O5 NWs (TC)	564.6	3.86

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2.3 Tribological tests

The tribological tests were conducted in ambient air using a HT-1000 ball-on-disk high-temperature tribometer (made in Zhong Ke Kai Hua Corporation, China) according to the ASTM Standard G99-95.²¹ The disks were TX and the counterparts were commercial Si₃N₄ ceramic balls with a diameter of 6 mm (about HV 15 ± 0.5 GPa). The frictional tests were carried out at different temperatures of 25, 150, 300, 450 and 600 °C under the same conditions of 20 N load per bearing section and 0.35 m s⁻¹ gliding speed. A schematic diagram of the ball-on-disk wear test is shown in Fig. 1. The as-prepared TX were cleaned by acetone and dried by hot air before the test. Furthermore, the tests were performed at a relative humidity of 55–65% and for an 80 min dry sliding wear test.

Fig. 1 The schematic diagram of ball-on-disk wear test.

The friction coefficients were recorded by the computer system of the tribometer as sliding continued. The friction coefficients were measured when steady state was obtained in the friction test. The cross-sectional profiles of worn surfaces were measured by the surface profiler of ST400 (Nanovea, USA). The wear results of TX had been presented as a specific wear rate and could be calculated according to the formula presented as follows:

W = V/(S × F) (1)

where V is the wear volume (mm³), F is the applied load (N) and S is the sliding distance (m). V was determined by the profiles of the cross section of the wear scars, which were measured using a surface profile meter:

V = A × L (2)

where A is the cross-sectional area of the wear scar (mm²) and L is the perimeter of the wear track (mm). Fig. 2 shows the representative morphology of the wear scar of TX sliding against the Si₃N₄ ball obtained after tests at 150 °C. As shown in Fig. 2, when the measuring stylus of the surface profiler of ST400 slowly moved across the wear scar along the straight line AA (see 3D profile of the wear scar in Fig. 2a), the coordinate positions of the measuring stylus was continuously recorded to form a 2D cross-section profile (see Fig. 2b). Similarly, the cross-sectional area A was measured eight times at different locations, and then the mean cross-sectional area A of the wear scar was acquired. Three tribological tests were repeatedly carried out under different testing conditions.

Fig. 2 Representative morphology of the wear scar of TX sliding against the Si₃N₄ ball after tests at 150 °C: 3D profile of the wear scar (a) and 2D profile of the wear scar (b).

2.4 Microstructure analysis

X-ray diffraction (XRD, D/MAX-RB, RIGAKU Corporation, Japan) studies with Cu-Kα radiation at 30 kV and 40 mA at a scanning speed of 0.01° s⁻¹ were carried out on TX to analyze the phase compositions. The morphologies and element contents of worn surfaces of TB were characterized using an electron probe microanalyzer (EPMA, JXA-8230, JEOL Corporation, Japan) and energy dispersive spectroscopy (EDS, Inca X-Act, Oxford Instruments, Britain). The magnification morphologies of worn surfaces of TB, V₂O₅ NWs and silver powders were characterized using field emission scanning electron microscopy (FESEM, ULTRA-PLUS-43-13, Zeiss Corporation, Germany). The cross-sections of TB were characterized using FESEM.

3. Results and discussion

3.1 XRD of TX and TB

In order to ensure the reliability of technological conditions of SPS, the XRD patterns of TX are shown in Fig. 3. It shows that the as-prepared TX were mainly composed of TiAl, V₂O₅ and Ag phases. TiAl peaks appeared in the XRD pattern of TX, indicating that the elements of Ti and Al had gone through synthesis reactions to obtain the intermetallic phase of TiAl. V₂O₅ and Ag did not react with Ti and Al, thereby eliminating possible reaction products at the interfaces between the additives and the matrix.^22,23 Moreover, it could be clearly seen that the peaks of Ag were weak, which were similar to the results observed by Ding et al.²⁴

Fig. 3 The typical XRD patterns of TX.

The FESEM images of V₂O₅ NWs and silver powders are shown in Fig. 4. As shown in Fig. 4a, silver had droplet structures, which was beneficial to the formation of a lubricant film on the frictional surface. Meanwhile stable crystal structures of V₂O₅ NWs were found in Fig. 4b, which might provide a support with high shear strength and effectively improve the friction and wear performance of lubricant films. As shown in Table 1, it could be obviously seen that Vicker’s hardness and the measured density of TX obtained the optimal values with the addition of 1.5% V₂O₅ NWs. It could be that a certain added amount of V₂O₅ improved the microstructure of TB, and homogeneous precipitation of the fine grain in the TB substrate promoted the increase of microhardness and measured density of TB.

Fig. 4 FESEM micrographs of silver (a) and V₂O₅ NW (b) powders.

Microstructure and elemental distribution of TB are shown in Fig. 5. In Fig. 5a, according to the EDS analysis, the gray areas like A were the continuous bulk TiAl phase, the dark gray areas like B were the V₂O₅ phase and the light gray areas like C were the Ag phase. As shown in Fig. 5b–d, the TiAl and Ag phases had a dense and uniform distribution in the sample, while the V₂O₅ phase had a relatively scattered distribution (see Fig. 5e and f). It could see that the isolated Ag and V₂O₅ phases were surrounded by the continuous bulk TiAl phase. It was clear that TiAl, V₂O₅ and Ag phases had an approximately uniform distribution in TB.

Fig. 5 Microstructure and elemental distributions of TB.

3.2 Tribology

3.2.1 Tribological behavior. Fig. 6 shows the variations of friction coefficients of TX and dynamic friction coefficient curves of TB with sliding time, as well as the variations of wear rates with testing temperatures for TB sliding against Si₃N₄ balls from 25 to 600 °C. Fig. 6a shows the variations of friction coefficients of TX at different temperatures of 25, 150, 300, 450 and 600 °C under the same conditions of 20 N load per bearing section and 0.35 m s⁻¹ gliding speed. It could be obviously seen that TB obtained the smaller friction coefficients at elevated temperatures. As shown in Fig. 6b, small fluctuations of friction coefficients and the minimum value obtained at 450 °C could be found, if compared to those at 25, 150, 300 and 600 °C. Fig. 6c shows the variations of wear rates of TB with the sliding time under different temperature conditions. As shown in Fig. 6c, the overall variation trend of wear rates was basically similar under the five different experimental temperature conditions. It increased firstly at 25–150 °C, slowly decreased at 150–450 °C and then finally increased at 450–600 °C. Meanwhile, it was also found that the small wear rate of 1.87 × 10⁻⁴ mm³ N⁻¹ m⁻¹ was acquired at 450 °C, if compared to those at 25, 150, 300 and 600 °C. Obviously, the excellent tribological behavior of TB was obtained at 450 °C, which might be attributed to the good lubrication of silver and a support of V₂O₅ NWs with high shear strength.

Fig. 6 Tribological testing results of TB sliding against Si₃N₄ balls under different conditions: (a) the variations of friction coefficients of TX at different temperatures, (b) the varying dynamic friction coefficient curves of TB with time and (c) the wear rate variations of TX at different temperatures.

According to the above friction and wear results, it could be concluded that TB (TiAl–5Ag–1.5V₂O₅ NWs) exhibited the lower friction coefficients and wear rates over a wide temperature range. Zhu et al.²⁵ pointed out that the ideal composition of a wide temperature self-lubricating composite should be composed of a high temperature oxidation resistant and high strength matrix, favorable solid lubricant, as well as a wear resistant phase. In order to better understand the corresponding mechanisms, it was imperative to analyze the worn surfaces and cross-sections of wear scars of TB with the optimum content of V₂O₅ NWs under the aforementioned testing conditions.

3.2.2 Wear mechanisms. Fig. 7 shows the typical electron probe morphologies of friction surfaces of TB over a wide temperature range from 25 to 600 °C obtained after tests. At 25 °C, the deep parallel grooves and a small amount of wear debris appeared on the worn surface, as shown in Fig. 7a. It revealed that the wear mechanisms were mainly micro-cutting wearing and mild abrasive wear. At 150 °C, the shallow parallel grooves appeared as shown in Fig. 7b. In addition, many delamination pits were also found. It indicated that the wear mechanisms were mainly micro-cutting wearing and furrow. At 300 °C, it was observed that there were more wear debris and more rough parallel grooves as could be seen from Fig. 7c, if compared to Fig. 7a. It was obvious that the main wear mechanisms were abrasive wear and micro-cutting wearing. At 450 °C, as shown in Fig. 7d, the abundant and tiny wear particles adhered to the worn surface could be found and the main mechanism was mild abrasive wear during the whole sliding process, which was beneficial to the lower mean friction coefficient of 0.19 and lower wear rate of 1.87 × 10⁻⁴ mm³ N⁻¹ m⁻¹, as shown in Fig. 6a and c. As temperature increased up to 600 °C, it could been clearly found that the friction condition deteriorated sharply, a large number of wear debris gathered, and coarse and deep parallel grooves appeared (see Fig. 7e). The morphology of the worn surface undoubtedly suggested that the main wear mechanism was the serious micro-cutting wearing.

Fig. 7 EPMA images of worn surfaces of TB obtained after tests at elevated temperatures: (a) 25 °C, (b) 150 °C, (c) 300 °C, (d) 450 °C and (e) 600 °C.

In order to know the synergistic effect of Ag and V₂O₅ NWs on the friction and wear behavior, the chemical compositions of the worn surfaces of TB at different testing temperatures were analyzed by EDS (the mean value of three measurements) and the results of the areas marked by points on the worn surfaces of TB shown in Fig. 7 are given in Table 2. It shows that the contents of V (V₂O₅ NWs) and Ag elements were higher at 450 °C, if compared to those at 25, 150, 300 and 600 °C. At 600 °C, the content of the V (V₂O₅ NWs) element was much lower due to the extremely softened V₂O₅ NWs moving away easily from the worn surface of TB during the wear test. It suggested that Ag and V₂O₅ NWs might have played a beneficial role in improving the tribological performance of TB at 450 °C.

Table 2 EDS analysis of worn surfaces of TB at different testing temperatures

Element (wt%)
Temperatures	V	Ag	O	Al	Ti	Fe	Nb	Cr	B
25 °C(A)	5.32	1.43	5.14	26.79	48.74	0.53	8.97	1.67	0.64
150 °C(B)	7.53	2.31	6.07	37.77	34.39	0.37	7.13	3.07	0.63
300 °C(C)	6.03	7.25	8.39	31.11	40.05	0.27	4.12	2.29	0.48
450 °C(D)	10.25	15.13	6.35	25.48	39.16	0.33	1.66	0.53	0.84
600 °C(E)	0.81	10.01	10.63	29.89	42.48	0.31	2.06	1.98	0.99

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According to the above analysis, Ag in combination with V₂O₅ NWs exhibited a good synergistic effect for improving the tribological performance of TB. In order to study the anti-wear mechanisms, detailed microstructural investigations of worn surfaces and cross-sections of worn surfaces of TB against Si₃N₄ counterface balls were further performed by FESEM, as shown in Fig. 8 and 9.

Fig. 8 FESEM micrographs of worn surfaces of TB obtained after tests at different temperatures: (a) 150 °C, (b) 300 °C, (c) 450 °C and (d) 600 °C.

The FESEM images of worn surfaces of TB obtained after wear tests at different temperatures of 150, 300, 450 and 600 °C are shown in Fig. 8. At 150 °C, after the Si₃N₄ counterface ball was circularly sliding on the same wear track, it could be clearly seen that massive V₂O₅ NWs were uniformly exposed to the wear scar and a little silver was squeezed out (see Fig. 8a). At 300 °C, the exposed V₂O₅ NWs and a continuous extrusion of silver started to form the lubricating films, with the already formed and incompletely formed lubricating films existing at the same time (see Fig. 8b). Meanwhile, V₂O₅ NWs in combination with Ag on the worn surface of TB had helped to reduce the friction coefficient and wear rate of the worn surface, but they were extremely unstable (see Fig. 6b). As the temperature increased up to 450 °C, the content of silver and the number of V₂O₅ NWs started to achieve an optimum value on the worn surface. Finally, a thick lubricating film containing massive V₂O₅ NWs and Ag was formed as shown in Fig. 8c, which could resist plastic deformation and enhance the anti-friction and anti-wear properties of TB. Furthermore, it was evident that the formation of lubricating films was favorable to the improvement of tribological behavior with a lower mean friction coefficient of 0.19 and lower wear rate of 1.87 × 10⁻⁴ mm³ N⁻¹ m⁻¹ during the whole wear process. At 600 °C, it could be seen that some smooth films were formed in some areas and most of the worn surface was covered by wear particles (see Fig. 8d), but the configurations of films was completely different from those in Fig. 8c. It indicated that excessive silver diffused to the sliding surface and formed a film, while extreme softening of V₂O₅ NWs caused them to be peeled off to become wear debris. Moreover, the films were easily destroyed under the act of the circularly sliding counterpart Si₃N₄ balls at 600 °C due to the high temperature softening, resulting in the higher wear loss. Meanwhile, the rough surface and the deterioration of lubrication conditions led to the increase in the friction coefficient.

From the aforementioned analysis, it can be concluded that the friction and wear behavior of TB can be improved when the worn surface is well covered by the formed lubricating films containing V₂O₅ NWs and silver. Moreover, it was obviously found that the tribological behavior of TB at 450 °C was better than those at 25, 150, 300 and 600 °C, which was attributed to the combination of V₂O₅ NWs and silver and their excellent synergistic effect. In order to clarify the structure and the formation mechanism of the tribo-films, the subsurface analysis was carried out on the worn surfaces of TB by cross-sectioning it perpendicular to the sliding direction and the locations of the cross-sectional position are shown in Fig. 7c–e.

Fig. 9 exhibits the typical FESEM micrographs underneath the worn surfaces of TB after sliding against Si₃N₄ balls at 300, 450 and 600 °C. There were three distinct layers of lubricating films, with a compacted layer and the substrate material existing underneath the worn surfaces as shown in Fig. 9a–c. At 300 °C, it could be found that an obvious separation between the compacted layer and the substrate material could be seen, indicating that the friction layer of TB had a relatively poor combination with the substrate material during the dry sliding friction process. Severe dislocation motion occurred at the corresponding near-surface region of TB due to local deformation, resulting in the formation of the discontinuous lubricating film, as shown in Fig. 9a. As is clear in Fig. 9b, there were a lot of tiny and uniformly distributed submicron grains in the compacted layer, which were beneficial to acquire the homogeneous lubricating films of the wear scar at 450 °C, if compared to those at 300 and 600 °C, leading to the low wear rates and lower friction coefficients. At 450 °C, a thick and homogeneous lubricating film containing the extruded silver and the exposed V₂O₅ NWs was formed on the worn surface of TB. The lubricating films formed low shearing stress junctions at the sliding interface, which has not only a significant anti-friction effect, but also a protective action for the worn surface.²⁶ Furthermore, the formation of the compacted layer was beneficial to form a compact lubricating film on the worn surface of TB, resulting in the improvement of the wear resistance and reduction of the friction coefficient of TB at 450 °C as shown in Fig. 9b. Consequently, TB obtained excellent friction and wear performance with a lower mean friction coefficient of 0.19 and low wear rate of 1.87 × 10⁻⁴ mm³ N⁻¹ m⁻¹ (see Fig. 6). Fig. 9c shows that severe plastic deformation and dislocation motion emerged underneath the lubricating films due to formation of a loosened structure on the cross section. At last, excessive Ag diffused to the sliding surface to form a rich-silver film. The lubricating effect of Ag started to deteriorate at temperatures above 500 °C.²⁶ At 600 °C, due to the failure of lubricating effect of Ag and wear debris of V₂O₅ NWs, as well as the loosened structure beneath the worn surface, the self-lubricating action of TB would fail, resulting in the higher friction coefficient and wear rate as shown in Fig. 6a and c.

Fig. 9 The typical FESEM micrographs of subsurfaces underneath the worn surfaces of TB after sliding against Si₃N₄ balls at 300 °C (a), 450 °C (b) and 600 °C (c).

TB displayed varying wear mechanisms during sliding progression at different testing temperatures according to the above analysis. Fig. 10 is a schematic illustration to show the wear mechanisms of TB from 300 to 600 °C. At 300 °C, Ag was squeezed out less to wear scar of TB and V₂O₅ NWs were less ground out to the worn surface, resulting in the small formation areas of lubricating films as shown in Fig. 10a. At 450 °C, it could be found that a smooth and continuous lubricating film consisting of Ag and V₂O₅ NWs was well formed on the worn surface as shown in Fig. 10b. Meanwhile, V₂O₅ NWs as a support with high shear strength were dispersed homogeneously in the lubricating films, restricting the plastic flow of Ag and withstanding plastic deformation. As the temperature increased up to 600 °C, extremely softened V₂O₅ NWs with little attachment were easier to form wear debris and removed from the worn surface of TB, hence there was almost no V₂O₅ NWs existing in the worn track and the compacted layer. Moreover, excessive Ag was squeezed out from the wear track to form a film (see Fig. 10c). However, the wear rates were still high as shown in Fig. 6c. According to the EDS analysis, as shown in Table 2, the amount of silver at 600 °C was lower, if compared to that at 450 °C. The reason was that silver was softened and lost its lubricating effect at 600 °C; meanwhile, the films were easily scaled off and formed wear debris, which were easy to be taken away by Si₃N₄ balls. As a result, lubricating films mainly consisting of Ag and massive V₂O₅ NWs had a significant anti-friction effect on worn surfaces and played the role of protecting the worn surface.

Fig. 10 A schematic illustration showing the wear mechanisms of TB from 300 to 600 °C.

Based on the above results, TB showed excellent tribological properties at 450 °C, if compared to those at 25, 150, 300 and 600 °C. At 450 °C, the formation area of lubricating films on the worn surface of TB was continuous and smooth, resulting in the decrease of friction coefficients and wear rates, as well as the improvement of tribological test stability. Meanwhile, it also could be found that the mean friction coefficient was reduced down to 0.19 and the wear rate was lowered to 1.87 × 10⁻⁴ mm³ N⁻¹ m⁻¹. The good tribological performance of TB at 450 °C was attributed to the remarkable synergistic effect of Ag and V₂O₅ NWs. Moreover, the reasonable working conditions were very important for realizing the good tribological performance of TB.

4. Conclusions

In this study, the sliding friction and wear behavior of TB at elevated temperatures of 25, 150, 300, 450 and 600 °C, as well as the microstructure of the lubricating films formed during the sliding wear process on the worn surface of TB were systematically investigated. It was concluded that the excellent tribological performance of TB could be attributed to the formation of a smooth and continuous lubricating film and the synergistic effect of V₂O₅ NWs and Ag. Moreover, the formation of the lubricant film is affected by the applied temperatures. TB showed excellent tribological properties at 450 °C, where the minimum mean friction coefficient was recorded as 0.19, and the minimum value of the wear rate was 1.87 × 10⁻⁴ mm³ N⁻¹ m⁻¹. This behavior was attributed to Ag, as a solid lubricant, diffusing to the sliding surface to provide good lubrication, lowering the friction coefficient, while V₂O₅ NWs played a supporting role with high shear strength, enhancing the wear ability of TB. It was also observed that a good anti-wear and anti-friction film containing V₂O₅ NWs and Ag on the worn surface of TB at 450 °C could be continuously formed, leading to the low friction coefficient and high wear resistance. The investigation showed that TB, due to its excellent tribological behavior at 450 °C, could be chosen as a promising structural material for mechanical components.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51275370) and Self-determined and Innovative Research Funds of WUT (135204008). Authors also wish to gratefully thank the Material Research and Testing Center of Wuhan University of Technology for their assistance. Authors were grateful to M. J. Yang, S. L. Zhao and W. T. Zhu in Material Research and Test Center of WUT for their kind help with EPMA and FESEM.

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