Ag composites

Tribology International 95 (2016) 324–332

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Tribology International journal homepage: www.elsevier.com/locate/triboint

Study on the formation mechanism of the glaze film formed on Ni/Ag composites Wenbo Duan a, Yanhua Sun a, Chunhui Liu a, Shihui Liu a, Yayun Li a, Chunhua Ding a,n, Guang Ran b, Lie Yu a a b

State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an, Shaanxi, China College of Energy, Xiamen University, Xiamen, Fujian 361102, China

art ic l e i nf o

a b s t r a c t

Article history: Received 3 June 2015 Received in revised form 16 November 2015 Accepted 21 November 2015 Available online 28 November 2015

The results of the study on the sliding wear behavior of a Ni/Ag-15 at% material showed that the wellcompacted glaze film mainly consisting of NiO, Ni and Ag was formed on the contact surface at 600 °C, which played an important part in decreasing friction coefficients. Microscopic observations showed the film possessed a multi-layered structure with silver-rich, outer layer and moderately-oxidized, nonsilver-containing inner layer. The formation of the multi-layered structure is believed to have developed as a result of the joint factors involving repeated oxidization, fracturing, mixing and consolidation, followed by heat-induced silver diffusion from the dense wear debris layer to the outmost surface under the flash temperature. The effect of the second diffusion and coalescence of silver on the tribological performance of the Ni/Ag material was briefly discussed. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Film High temperature Friction Ag

1. Introduction Self-lubricating composite materials have received growing attention for their remarkable properties, such as high strength, toughness, wear resistance and low friction [1,2]. which are highly desirable for high temperature (HT) tribological applications to increase the lifetime and performance of mechanical systems. The self-lubricating concept was developed with a series of tribological composite materials consisting of a metal/ceramic matrix and lubricants, which can provide lubrication in a wide range of temperatures [3,4]. Over the past decades, Ni-based alloys have been widely used as matrix materials in sliding contact conditions due to their good mechanical properties and thermal stability at high temperatures [5], and the addition of one or several lubricants into Ni-based alloys, such as noble metals, oxides, fluorides, molybdates, sulfates, etc. have been attempted in order to improve their tribological performance during HT sliding contact [6–8]. The early investigations of Ni-based alloys, such as the NASA PS200 and PS300 series HT self-lubricating coatings, showed that the coatings provided lubrication by silver film formation at 300– 500 °C and by forming fluorides rich glaze films above 500 °C due to the diffusion of Ag and fluorides from the matrix to the contact surface [9,10]. The primary mechanism for the friction reduction of n

Corresponging author. E-mail address: [email protected] (C. Ding).

http://dx.doi.org/10.1016/j.triboint.2015.11.031 0301-679X/& 2015 Elsevier Ltd. All rights reserved.

the self-lubricating materials is based on their low shear strengths, which allow easy deformation of the surface lubricants under friction-induced stress. Recently, Ouyang et al. investigated the tribological performance of the NiCr composite with BaCr2O4 addition and reported that the tribo-oxidation reaction of BaCr2O4 resulting in the presence of BaCrO4 was responsible for the formation of the protective glaze film during HT wear, which effectively reduced friction and wear [11]. Chen et al. reported that the SiC–Ni coating exhibits self-lubricating properties at 800 °C due to the formation of a laminated tribofilm consisting of silica and nickel monoxide (NiO) in the wear track [12]. NiO is considered as a lubricant at temperatures above 400 °C, which is usually formed on the contacting surface due to the tribo-oxidization/oxidization of Ni-based alloys as reported by other researchers [13]. From the above observations, it can be noted that the formation of tribofilm on the contact surface, which obviously improves the HT tribological performance of self-lubricating materials, strongly depends on the thermodynamic diffusion of lubricants which have larger thermal expansion coefficients, tribochemical/chemical reactions at the contact surface [14,15]. So far, the compositional, microstructural, chemical and crystallographic characteristics of the tribofilm have been extensively studied such as the distribution, sizes and contents of lubricants in both the tribofilm and the matrix [16,17]. However, few studies were reported on the compositon distribution development and microstructure evolution of the tribofilm, which can provide some insight on the

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self-lubricating mechanisms that lead to the significantly improved tribological properties. Hence, the current study is focused on the microstructure evolution, silver diffusion development and contact surface oxidation processes for the Ni/Ag-15 at% material in sliding contacts at high temperatures. The microstructure of the tribofilms and the distribution of lubricious phases after sliding wear were observed by using SEM equipped with EDS, GI-XRD and XPS. The friction coefficients were measured at different temperatures, and were examined in a close relationship with the contact surface changes in microstructure and chemistry. The results of our investigations reveal the formation mechanism of the well-compacted multilayered tribofilm for the Ni/Ag-15 at% composite material at high temperatures.

2. Experimental The starting materials included Ag (15 at%) powder with a size range of 50–100 nm and Ni powder (85 at%) with a size range of 10–20 μm. Both Ag and Ni powders were first put into pure ethanol and stirred sufficiently in order to disperse the Ag agglomerations effectively and to eliminate the organic components on the surface of Ag powder. After 15 min stirring, the mixed composite powders were heated to 80 °C in a vacuum oven and maintained at that temperature for 15 min. Upon completion of the heating, the mixed powders were unidirectional cold pressed at a pressure of 300 MPa for 5 min to obtain compacted Ni/Ag samples (Φ15
5 mm2). Then each sample was individually placed in a quartz tube and subjected to induction sintering in a chamber which was evacuated to a base pressure of 10 Pa. The sintered samples were, therefore, machined to be cylinder (Φ5
5 mm2). The detailed technical parameters of induction sintering are present in Table 1. After the end face of the cylinder samples was polished with 600-grit sandpaper, the friction coefficients in air were measured with a high temperature pin-on-disk tribometer under a load of 4.9 N (stress of 2.5
105 Pa) at 25, 200, 400 and 600 °C. The rotation velocity was 45 RPM and the rotation radius was approximately 20 mm. The counterpart disks were made of a stainless steel with composition presented in Table 2. The tribotests were started after the samples were heated to the desired temperature in about 15–30 min and the wear time was 20 min. Wear test was repeated two times at each selected temperature and better tribological performance was chosen. The surface roughness values (0.7770.13 μm) of the Ni/Ag cylinders were measured with Surftest SJ-310. The microhardness values of polished samples were determined by Vickers microhardness tester using 100 g indenting load and a dwell time of 15 s. Before and after tribotests, sample surfaces and wear tracks were examined using a JSM 6390a scanning electron microscope (SEM), which was equipped with an energy dispersive X-ray analysis system (EDX). The constituents of the sample were characterized by X-ray diffraction (XRD) with a Philips X’Pert Pro diffractometer (PANalytical, Holland) using filtered CuKα radiation (λ ¼0.1541 nm) at 20 kV, 40 mA. The surface layer of the specimen was critically investigated by glancing incidence X-ray diffraction (GI-XRD) at incident beam angle of 2°. X-ray photoelectron

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spectroscopy (XPS) analysis was performed with an ULVAC-PHI 1800 spectrometer using Al kα radiation.

3. Results Fig. 1 shows the uniform distribution of bright Ag in Ni/Ag15 at% material with a porosity level of approximately 5% according to image analysis. The inset shows the corresponding XRD patterns indicating that no other new phases are generated during sample preparation process. Fig. 2 and Table3 exhibit the friction coefficients and wear rates of the samples tested at different temperatures, respectively. At room temperature, the friction coefficient is the highest and the fluctuation of friction coefficients is great. The fluctuation of friction coefficients during the running-in period was probably caused by the adjustment of the pin on the disc at the start of the wear test. With increasing temperature, both the friction coefficient and wear rate decreased and reached the lowest value with almost no fluctuations of friction coefficient at 600 °C. Hence, it can be noted Table 2 stainless steel compositions. 9Cr18

C

Si

Mn

S

P

Cr

Ni

Mo

%

0.9-1.0

r 0.8

r 0.8

r 0.030

r 0.035

17–19

r0.6

r 0.75

Fig. 1. SEM micrograph of Ni/Ag-15 at% material. The inset is its XRD spectra.

Table 1 Technical parameters of induction sintering. Output voltage (kV)

Sintering time (min)

Vacuum pressure (Pa)

Cooling time (min)

2.0

10

10–12

20 Fig. 2. Variation of friction coefficients with sliding time at different temperatures.

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that the Ni/Ag-15 at% material exhibited improved the tribological performance at 400 and 600 °C compared to that at room temperature. In addition, it can be seen that the running-in period is approximately 4 min at room temperature, which gradually decreased with increasing temperature. This is attributed to faster diffusion of Ag particles from matrix to surface with temperature due to the higher thermodynamic drive [18], which is beneficial to decrease the friction coefficient rapidly and prolong the wear lifetime of high precision components such as foil air bearings. Fig. 3 presents SEM micrographs of the worn surfaces tested at 25, 200, 400 and 600 °C, showing that the morphology of worn surfaces is strongly dependent on testing temperature. At 25 and 200 °C, the worn surfaces exhibited a similar rough morphology with a lot of wear debris, as well as a few smeared patches and scratches, which indicated microfracture to be the main wear mechanism. With increasing temperature to 400 °C and above, the worn surfaces (Figs. 3c and d) were covered with glaze films and the amount of wear debris was significantly reduced, which may explain the significantly improved tribological performance. Table 4 presents the EDS elemental analyses of the whole worn surfaces at different temperatures. Fig. 4a and b presents the backscattered SEM micrographs of the cross-sectional worn surface tested at 600 °C, showing the existence of a distinct film on the worn surface. This verified the existence of the lubricating film which was presented as the glaze film in Fig. 3. GI-XRD analysis was used to identify the phases of

the lubricant film, as shown in Fig. 4c, which determined that the detected layer contained Ni, Ag and NiO. Based on the Eq. (1) related to GI-XRD, the effective penetration depth of GI-XRD for NiO, Ni and Ag is 0.78, 0.50, 0.09 μm, respectively. t¼

sin α U sin β
μ U sin α þ sin β

ð1Þ

where t is the penetration depth of GI-XRD; μ is the linear absorption coefficient; α is the incident angle onto the sample surface and β is the angle between the sample surface and the diffracted beam (2θ–α). This data indicated that the effective penetration depth of GI-XRD did not exceed the lubricant film thickness of approximately 1–2 μm. That is to say, the components detected by GI-XRD resulted from the bulk of the lubricating film formed on the worn surface. The high NiO intensity in the GI-XRD patterns, which suggested the high NiO content in the lubricating film [14], indicated that the massive Ni at the contact surface was oxidized into NiO at 600 °C, confirming that the Ni and O elements measured by EDS in Fig. 3 originated from Ni and NiO. For further studying the microstructure and the distribution of lubricious phases in the glaze film, XPS is used to characterize the surface components and chemical states of the glaze film after wear at 600 °C, as shown in Fig. 5. XPS is a surface-sensitive quantitative spectroscopic technique that measures the elemental composition from the top 0–10 nm of the material being analyzed. It can be seen from Fig. 5a, the full scan spectra of the film, that the

Table 3 Wear rates of the samples at different testing temperatures. Wear rates (g N  1 m  1)

25 °C

200 °C

400 °C

600 °C

5.7 7 1.5
10  4

6.9 7 2.3
10  4

2.3 7 1.9
10  4

7.6 71.6
10  5

Fig. 3. Worn surface micrographs of samples tested at (a) 25 °C, (b) 200 °C, (c) 400 °C and (d) 600 °C.

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lubricating film contained Ni, O, Ag and Fe elements. Fig. 5b–e presents the narrow scan analyses of the surface elements, indicating that the worn surface contained small amounts Ni2O3 and Fe2O3, in addition to Ni, Ag and NiO which had been already observed in the GI-XRD result. Fig. 6 presents SEM micrograph of the stainless steel counterpart tested at 600 °C, showing that the worn surface was also covered with a continuous tribofilm mainly consisting of Ni, Ag and O elements, along with a small amount of Fe and Cr elements, as identified by the inserted EDS patterns. Fig. 7a and 7b present the surface and the cross-section of a Ni/ Ag sample after annealing at 500 °C for 15 min, showing that many Ag particles diffused from the Ni/Ag matrix to the surface due to their larger thermal expansion coefficient (19
10  6 K  1) compared to that of Ni (13
10  6 K  1). This suggests that Ag can spontaneously diffuse from the Ni/Ag matrix to the open surface under the thermodynamic activation. Simulations by MD (molecular dynamics simulation) have been performed in order to identify the thermodynamic behavior of silver inside the nickel matrix. Fig. 8 presents Ag diffusion from the Ni matrix to the open surface under the actions of temperature and friction force. In the figure, blue and purple balls are Ni and Ag Table 4 EDS analyses of the worn surfaces at different testing temperatures Temperature (°C)

25 200 400 600

Elements (at %) O

Fe

Ni

Ag

Cr

6.0 7 3.2 16.0 7 5.4 26.2 7 4.3 25.8 7 5. 7

2.2 7 1.1 5. 97 3.6 3.9 7 2.0 4.17 0. 8

77.3 7 6.7 59.6 7 7. 4 33.07 4.4 44.17 4.8

14. 67 3. 6 18.5 7 5.5 35.77 6.2 25.0 7 4.1

/ / 1.2 7 0.2 1.17 0.4

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atoms, respectively. It can be seen that due to the larger thermal expansion coefficient, Ag atoms diffuse out of Ni matrix under thermodynamic activation.

4. Discussion 4.1. Worn surface analyses at 25 and 200 °C EDS analysis revealed that the predominant constituent of the smeared patches formed at 20 and 200 °C was Ni element, suggesting that Ni on the contact surface suffered plastic deformation during wear process. In studies of Ni coatings tested at room temperature, similar results were reported, namely, the formation of smeared patches of Ni is the typical feature of plastic deformation for metal-based materials during wear process [19]. Compared to smeared Ag patch which has a low shear strength and is favorable for the significantly decreased friction coefficient [20], the shear strength of the smeared Ni patch is much higher, which makes negligible contribution to a decrease in the friction coefficients of the samples tested at 25 and 200 °C. It was confirmed by EDS that the major constituent of the wear debris was Ni element, along with small amounts of Ag, O and Fe elements. The existence of O may be due to the oxidation of the contacting surfaces during wear process, while Fe may result from the transfer of the counterpart material (stainless steel). The presence of wear debris indicated that fragmentation and fracture occurred on the wear surface as a result of abrasive, adhesive and fatigue wear [21,22]. It is well known that at the initial stage of sliding wear, the actual contacts between the sliding surfaces are at local asperities, which produces a high compressive pressure and large shear strains in the asperities. As the shear stress is sufficiently higher than the strength of the materials, wear debris

Fig. 4. Backscattered micrograph of (a) the cross-sectional worn surface at 600 °C, (b) the lubricating film and (c) GI-XRD patterns of the lubricating film.

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Fig. 5. XPS scan spectra of the worn surface: (a) Full spectrum; (b) Fitting chart of Ag3d; (c) Fitting chart of Fe2p; (d) Fitting chart of O1s; (e) Fitting chart of Ni2p.

are produced due to the occurrence of local fragmentation and fracture at the asperities. As pointed out by Rigney [23], these initial fragments, which are highly strained and work hardened, can cause severe third body wear, which in turn produce more wear debris. However, the plastic deformability of these wear debris particles mainly consisting of Ni is still low and they are incapable of being deformed to form smeared patches under external loads. Under the effect of centrifugal force, part of the wear debris could be swept away from the contact zone resulting in a small amount of wear debris existing on the rough worn surface. In addition, few Ag lubricating patches were observed on the worn surface, which also led to high friction coefficients and wear rates at 20 and 200 °C. Similar findings were reported by Dellacorte that few Ag lubricating films were generated on the worn surface of PS304 coatings containing 10 at% Ag in sliding against Ni-based alloy in the temperature range from room temperature to 300 °C [24]. This may be due to the low content of Ag in the wear track, which is incapable of forming Ag patches under the external load. At 200 °C, although more Ag particles can overflow from the matrix to the worn surface due to its high diffusion coefficient and may form several Ag lubricating patches, the main wear mechanism of the sample is still microfracture, as shown in Fig. 3b, and accordingly part of the wear debris produced during wear process may inevitably embed into the Ag lubricating patches formed in the wear track. This not only damages the integrity of the Ag lubricating patches reducing their plastic deformability, but also causes the severe three-body abrasion of Ag patches, thus leading to rapid destruction of the Ag patches. It can be seen from Table 4 that although the Ag content on the worn surface at 200 °C is higher than that at 25 °C, the increase in Ag content is lower compared to that noticed at high temperatures. This confirms the above notion that the Ag content in the wear track tested at 25 and 200 °C is too low to form Ag lubricating patches. Due to the lack of Ag lubricating patches together with the presence of wear debris in the wear track, the friction coefficients at 25 and 200 °C are relatively high and the corresponding fluctuations of the friction coefficients are great, as shown in Fig. 2.

Fig. 6. SEM micrograph of the stainless steel disk counterpart tested at 600 °C.

4.2. Worn surface analyses at 400 and 600 °C Increasing the temperature not only promoted oxidation but also caused an increase in the ductility of Ni phase. During the friction and wear process, these NiO and Ni phases obviously suffered plastic deformation due to their excellent ductility at 400 °C. Consistent with this notion is that only a small amount of wear debris was present on the worn surface at 400 °C compared to at 25 and 200 °C. This also indicates that the ductility of wear debris mainly consisting of Ni and NiO, as identified by EDS, significantly increased with increasing temperature from 25 to 400 °C. Metal oxides are known to soften in the temperature range round 0.4-0.7 of their melting temperature [25], namely 792– 1386 °C for NiO, which is obviously different from our results that the transition of NiO phase occurred from brittle to ductile in the temperature range of 200–400 °C. This can be explained by the flash temperature on the local asperity contacts being high enough to induce the brittle-to-ductile transition of NiO phase though the ambient temperatures are only 400 and 600 °C, which can also account for the presence of 6 at% O in EDS analysis of the worn

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Fig. 7. SEM micrographs of (a) the surface and (b) the cross-section for a Ni/Ag sample after annealing at 500 °C for 15 min.

Fig. 8. MD simulations of the purple Ag atoms inside the blue Ni atoms: (a) and (b) at 25 °C; (c) and (d) at 500 °C.

surface tested at 25 °C. As a result, under a joint action of the flash and ambient temperatures, these NiO, Ni and heat-overflowed Ag phases on the contact surface were plastically deformed under the external load and friction force, and accordingly forming glaze films, thus leading to the decreased friction coefficient. However, compared to the film formed at 400 °C, EDS analysis of the whole worn surface tested at 600 °C exhibited lower Ag contents, as shown in Table 4, which is not obviously consistent with the fact that more Ag diffuse from the matrix to the surface with increasing temperature. The reason for this may be that during the friction process, the soft contacting surface suffers micro-cutting/plowing effect of the protuberance of the harder counterpart. The wear debris is generated and accumulated on wear track and some wear debris may be transferred and adhered to the counterpart surface. In the present case, the Ag component in the film formed on the worn surface got softer with increasing temperature from 400 to 600 °C leading to more Ag being detached and transferred to the counterpart and eventually resulting in a difference in the component contents between the glaze films formed at 400 and 600 °C. Similar findings were

reported by Ouyang [26] that softer constituents in the glaze film can be readily transferred to the counterpart surface for ZrO2(Y2O3)–CaF2–Ag composites. It can be seen from Fig. 4b that the film exhibited a multilayered microstructure with a total thickness of 1–2 μm. EDS determined that the bright upper layer of the film, as marked by a black arrow, contained a large amount of Ag, while the dark lower layer of the film, as marked by a white arrow, contained amounts of Ni and O. Thus, it can be inferred that the main component of the bright upper layer of the film was Ag, while those of the dark lower layer were a mixture of NiO and Ni. Based on these studies, it can be identified that during sliding of Ni/Ag-15at.% against stainless steel at 600 °C, a composite lubricating film was generated on the worn surface with a Ag-rich top layer and moreoxidized NiO containing lower layer. The formation of silver cap layer on the film may have two competing effects on the tribological performance of the film. On the one hand, metallic Ag has a low hardness (Mohs hardness 2.5) compared to metallic Ni (Mohs hardness 4.0) and NiO (Mohs hardness 5.0). According to Archard's proposal [27], the wear rate

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varies inversely with the hardness, that is to say, the softer Ag cap layer can accelerate the wear of the film and cause faster abrasive wear. On the other hand, the Ag cap layer with better ductility can respond kinematically to the velocity mismatch of the contact surfaces and accordingly causes less wear. The softer Ag cap layer formed at 600 λ°C is effective in distributing contact stress [17], and the shear and slipping in the soft layer could act to accommodate frictional force and impact load caused by cyclic contact between the tribocouple, which can prevent the film from wear and help to decrease the friction coefficient. Furthermore, Ag solid lubrication effect reduces interface shear stresses at the contact point, which reduces the occurrence of adhesive and fatigue wear. Thirdly, the combination between the film and the substrate is just a kind of mechanical occlusion, thus the film had a tendency to be detached from the substrate under the repeated action of the frictional force. The better ductility of the film is beneficial in increasing its adhesive strength to the underlying substrate by increasing the contact area at the interface. Fig. 4b also shows that the worn surface is rough and uneven, indicating that before the formation of lubricating film, the worn surface suffered significant material microfracture and accordingly, a large amount of wear debris was generated. It is consistent with the behavior of the friction coefficient for the sample tested at 600 °C, as shown in Fig. 2, that during the running-in period at 600 °C, the friction coefficient of the sample was high and the fluctuation was great since the sliding process was dominated by microfracture. With further sliding, the wear debris suffered repeated oxidation, mixing, fracture, agglomeration, compacting and smearing between the contacts, resulting in the formation of a compacted film consisting of a mixture of Ni, Ag and oxidation produces. In addition, it can be also noted from Fig. 4b that the lubricating film was relatively dense and free of pores. This suggested that the debris generated by wear has a good plastic deformability, resulting in the rapid formation of dense glaze film under the repeated load and friction force. The dense film free of pores is incapable of providing space requirements for silver particle growth and coalescence, thus restricting the silver particle growth and coalescence inside the film matrix, eventually leading to the silver diffusion from the film bulk towards the outer surface. Tribochemical reaction is considered to be another important factor influencing the tribological performance of metal materials [28]. In this study, it can be noted that the dark lower layer in the film formed at 600 °C contained massive NiO, which resulted from both oxidation and tribo-oxidation. This lower NiO-containing layer can also provide limited lubrication at the moment when the Ag cap layer is breached during the wear process as it does for the non-Ag containing Ni-based materials. However, the present results are different from findings conducted by Joardar for Ti (CN)–WC–Ni cermets [29]. It was reported that instead of the formation of lubricating films, the presence of a large amount wear debris of NiO in the surface layer played a major role in the poor wear resistance of the cermet because the formation of brittle and porous NiO is likely to cause a premature loss of Ni and the dislodgement of hard phase grains at the surface layer. The reason why the film of NiO is not formed on the wear surface is not yet understood. The XPS results of the glaze films formed at 600 °C indicated that in addition to Ni, Ag and NiO, the worn surface contained small amounts Ni2O3 and Fe2O3. Compared with GI-XRD result in Fig. 4c, the Ni2O3 and Fe2O3 phases were invisible in GI-XRD. This may be explained from the following two aspects: (1) the contents of Ni2O3 and Fe2O3 are too low to be detected by GI-XRD; (2) XPS has higher detection accuracy than GI-XRD. It is known that the probe depth of XPS is approximately  10 nm, which is much smaller than that of GI-XRD with a probe depth of  several microns. This suggests that not only the contents of Ni2O3 and

Fe2O3 components were low but also they mainly existed within the ultrathin outmost layer of the lubricating film, which can be evidenced by microscopic observation of Fig. 4b where a few dark ultrathin discontinuous layers can be observed on the top of Ag cap layer, as shown by black arrow. It can be inferred that during the wear process, Fe in the stainless steel counterpart was oxidized into Fe2O3 at high temperatures and then, transferred to the contacting surface of the Ni/Ag sample. In addition, it can be also observed that Ni2O3 phase appeared in the lubrication film, which suggests that though NiO is generally the only stable bulk oxide and Ni2O3 can decompose into NiO and O2 at 600 °C, some undecomposed Ni2O3 still exist in the lubricating film. Using XPS, Kim and Winograd also found the existence of some Ni2O3 in the surface oxide at temperatures between 80 and 250 °C in air, with an increased ratio of NiO to Ni2O3 with increased temperature [30]. The ratio increases because Ni2O3 is reduced thermally to NiO, since NiO is thermodynamically favored at higher temperatures. The presence of Ni2O3 in the wear track at 600 °C was considered to act as gross surface defect in the structure of the formed NiO layer, which may inhibit the friction reduction capability of the more stable NiO film [31]. These dark, ultrathin, discontinuous layers may originate from the counterpart surface since the material transfer between contact surfaces during wear is mutual even during a steady sliding contact wear period. Material transfer is a common phenomenon during wear process, and is strongly related to the wear resistance of contacting surface materials, as well as their plastic deformability and chemical affinities. Since the above components possess sufficient ductility at 600 °C and Ag, Ni elements are siderophile, Ni, Ag and NiO were readily transferred and adhered to stainless steel counterpart, hence forming the transferred film. The presence of the transferred film shown in Fig. 6 might confirm the above notion that some silver transfer led to a decrease in Ag content in the glaze film formed in the wear track of Ni/Ag. Fig. 9 provides a schematic diagram of the formation process of multi-layered film during wear at 600 °C to facilitate a discussion on the thermodynamic and oxidation behaviors of silver and nickel inside the matrix and the film. Fig. 9a exhibits the cross section of the Ni/Ag-15 at% at room temperature, showing that Ag particles (white) are distributed in the Ni matrix. After increasing temperature up to 600 °C and prior to wear, Ag particles with higher thermal expansion coefficient of 19*10  6 K  1overflow from the Ni (13*10  6 K  1) matrix to the surface, as shown in Fig. 9b, accordingly, leading to the formation of black pores in near-surface of the matrix. Meanwhile, a number of NiO (green) particles were generated on the surface of the matrix due to oxidation, along with a few Ni2O3 (blue) particles. Also, the stainless steel counterpart experiences oxidation leading to the formation of Fe2O3 (purple particles) which can be removed and transferred to the Ni/Ag surface during wear process. At the beginning of wear process (Fig. 9c), the wear process is dominated by fracture and fragmentation resulting in the generation of wear debris particles, as well as pits and scratching on the worn surface. Although more Ag patches may be formed in the wear track compared to the behavior at 200 °C under the external load and friction force, numerous wear debris particles formed on the worn surface may inevitably force against and cut into the Ag patches, thus destroying their integrity, which makes them incapable of forming a continuous Ag film. Further wear may cause more oxidation of Ni phase both in the wear debris and in the near-surface of the Ni/Ag material and more Ag diffusion from the matrix to the surface, accompanied by the formation of numerous wear debris particles mainly comprised of Ni, NiO and Ag, along with a few Ni2O3 and Fe2O3 on the rough worn surface, as shown in Fig. 9d. Since the plastic deformability of Ni and NiO are substantially improved due to thermal softening at 600 λ°C, the wear debris particles of Ni, Ag

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331

Fig. 9. Schematic diagram of the thermodynamic formation of multi-layered glaze film.

and NiO certainly undergo multiple mechanical mixing, compacting, agglomeration and sintering leading to the formation of the fine-structured, well-compacted glaze film on the Ni/Ag substrate. When the well-compacted glaze film become dense enough and free of voids, the Ag particles due to their higher thermal expansion coefficient compared to that of Ni and NiO (14*10  6 K  1 [32]) in the compacted film diffuse from the compacted film to film surface under the action of flash temperature and eventually form an upper Ag-rich layer in the glaze film with a moderately-oxidized layer left underneath, as shown in Fig. 9e and f. The microstructure characterizations provided the evidence of second silver diffusion and coalescence mechanism from the wellcompacted glaze film formed on Ni/Ag to the surface under flash temperature, which in turn protected the glaze film, even the Ni/ Ag substrate. The mechanism not only improves the tribological performance of the Ni/Ag material, but also prevents the glaze film and the underneath substrate from oxidation to some extent, which is expected to enhance the oxidation resistance and extend the wear lifetime of the Ni/Ag composites.

5. Conclusions Ni/Ag-15 at% composite material produced by induction sintering exhibited the highest friction coefficient of approximately 0.9 with great fluctuation at room temperature. The SEM/EDS analyses of the worn surface demonstrated a rough morphology without any lubricating films formed in the wear track, and the dominant wear mechanism was micro-fracture which resulted in the formation of wear debris mainly resulting from Ni/Ag. At 200 °C, although silver partly diffused from the matrix to the surface forming silver patches at the beginning of testing, the number of silver patches on the worn surface was too small to form a continuous silver film. Thus, the numerous wear debris particles generated and accumulated on the worn surface inevitably cut into the plastic-deformed silver patches leading to the break of the patches, even severe three-body abrasive wear. As a result, the worn surface tested at 200 C was still rough and few silver patches were observed in the wear track. At 400 and 600 °C, the friction coefficients of the materials were maintained at lower values with almost no fluctuations during wear process. The heating-induced silver diffusion and oxidation of Ni resulted in material microstructure and chemistry changes with continuous lubricating films formed on the worn surface and a silver-deficient

subsurface of the matrix left underneath. It was confirmed that the silver diffusion and nickel oxidation at the surface played an important part in improving the HT tribological performance of Ni/Ag composite material. Characterization of the composite films revealed that the glaze film exhibited a multi-layered structure with outermost, ultrathin, transferred layer, Ag-rich, thin outer layer and moderately-oxidized, Ni-containing inner layer. These layers are believed to develop as a result of the joint action of oxidation, mixing, fracturing, compacting, etc. The effect of the second activation of silver, namely diffusion and coalescence, on the formation of the tribofilms was discussed. This phenomenon of second activation can be observed in many application fields such as nanotechnology and microelectronics, where Ag lubricant is added to a matrix in order to lower friction and wear.

Acknowledgment The work was supported by National Basic Research Program of China 973 Grant nos. 2013CB035704 and 2013CB035706; the National Natural Science Foundation of China (Grant nos. 51171143, 51275386 and 51375366); and the Fundamental Research Funds for the Central Universities, NCET-11-0418 and 2015GY127.

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