Sliding wear of nanocrystalline Nb-Ag at elevated temperatures: Evolution of subsurface microstructure and its correlation with wear performance

Author’s Accepted Manuscript Sliding wear of nanocrystalline Nb-Ag at elevated temperatures: evolution of subsurface microstructure and its correlation with wear performance Kangjie Chu, Jian Zhou, Fuzeng Ren www.elsevier.com/locate/wear

PII: DOI: Reference:

S0043-1648(18)30665-3 https://doi.org/10.1016/j.wear.2018.08.018 WEA102493

To appear in: Wear Received date: 5 June 2018 Revised date: 22 August 2018 Accepted date: 27 August 2018 Cite this article as: Kangjie Chu, Jian Zhou and Fuzeng Ren, Sliding wear of nanocrystalline Nb-Ag at elevated temperatures: evolution of subsurface microstructure and its correlation with wear performance, Wear, https://doi.org/10.1016/j.wear.2018.08.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Sliding wear of nanocrystalline Nb-Ag at elevated temperatures: evolution of subsurface microstructure and its correlation with wear performance Kangjie Chu1, 2, Jian Zhou3, Fuzeng Ren1, * 1

Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China

2

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

3

Shagang School of Iron and Steel, Soochow University, 178 Gan Jiang Dong Road, Suzhou, China

Abstract An in-depth understanding of the formation of subsurface microstructure and its correlation with wear performance is a prerequisite towards controlling friction and wear at high temperatures, but such information for nanostructured binary immiscible alloys is rather limited. Here, we systematically investigated the sliding wear of nanocrystalline Nb90Ag10 two-phase alloy against Inconel alloy 600 disks at temperatures from 25 to 500 °C. Detailed characterizations on the subsurface layers show that their microstructures have a strong correlation with the corresponding wear performance. At 25 and 200 °C, no oxidation layer formed. The metallic sliding contacts result in higher coefficients of friction (CoF) and wear rates, and the

1

Authors to whom correspondence should be addressed. e-mail: [email protected] (F. Ren) 1

formation of nanolayered structure near the sliding surface. At 350 °C, the tribo-oxidation reaction occurred only within the counterface material and a mechanical mixing layer was formed below the sliding surface of the pin. The third-body wear yielded pronounced reduction in CoF but the highest wear rate. At 500 °C, the tribo-oxidation occurred within both the pin and the counterface material. A protective glaze layer formed on the top of the pin and thus leads to the lowest wear rate.

Graphical Abstract

Keywords: Wear; Nanocrystalline Nb-Ag alloy; Subsurface microstructure

1. Introduction Sliding friction and wear between two metallic components often lead to severe wear 2

damage and modification of the subsurface microstructure, due to the interactions between the contacting surfaces, and with the environment [1, 2]. Such subsurface microstructure evolution can strongly influence the tribological performance of metallic alloys [3], as previously demonstrated in dry sliding of pure Cu [4, 5] and/or Cu alloys [6-9], nanocrystalline Ni-W alloys [10], Al alloys [11-13], Ti alloys [14] and high entropy alloys [15, 16] at ambient and even in liquid nitrogen [17] temperature. Debris generated during the wear process have been shown to originate mostly from the subsurface layers [18]. Thus, nanoscale characterization of the chemical composition and microstructure of the subsurface layers is of great significance to understand the physical process during sliding wear and further to reveal the wear mechanism.

The tribological processes of metallic alloys at elevated temperatures are more complex, since these involve changes in bulk mechanical properties due to microstructural changes, thermal softening, surface chemical and morphological changes owing to accelerated oxidation and diffusion [19]. Depending on the characteristics of the formed surface oxide layers and subsurface deformation layers, wear rate at elevated temperatures can either be increased or reduced [20]. Generally, if a stable oxide layer has good adherence to the substrate and prevents metal-to-metal contact, one would expect a reduction in friction and wear [21-23]. Conversely, if the oxide layer is not very well compacted, the relatively loose hard oxide particles are removed more easily and wear rate increases [24]. An in-depth understanding of the 3

formation of subsurface microstructure and its correlation with wear performance is a prerequisite towards controlling friction and wear at high temperatures, but such information for nanostructured alloys is still rather limited.

Moreover, for binary immiscible alloy systems subjected to sliding wear at elevated temperatures, stabilization of the subsurface layers involves a dynamic competition process between shear-induced mixing and thermally activated diffusion. Severe plastic deformation (SPD) imposed by wear tends to homogenize the composition within the layers, while thermally activated diffusion promotes phase separation [25, 26]. The outcome of such a competition depends on temperature, strain rate and even length scale of the microstructures [7, 27-30]. Detailed characterization of the subsurface chemical composition and microstructure evolution will also provide deep insights into better understanding on the processes active in these layers.

Here, we have selected nanocrystalline Nb-Ag binary alloy for three reasons: 1) nanocrystalline Nb-Ag binary alloys have demonstrated high wear resistance upon sliding against martensite stainless steel at room temperature [31]; 2) niobium alloys also have promising applications in high temperature environment due to their high melting point and excellent oxidation resistance [32, 33]; 3) Nb and Ag are moderately immiscible and SPD can force the chemical mixing of Nb and Ag [34], but the temperature effects on the subsurface microstructural evolution during wear is yet to explore. The Inconel alloy 600, a typical high temperature alloy, has been 4

chosen as the counterface material not only because it is an oxidation-resistant material and can retain strength over a wide temperature range [35], but also because the transfer of elements from the counter surface (nickel-chromium-iron) to the Nb-Ag pins can be traced unambiguously.

In the present work, we have systematically investigated the sliding wear behavior of Nb-Ag two-phase alloy against Inconel alloy 600 disks at temperatures from 25 to 500 °C. We have combined scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) with X-ray energy-dispersive spectroscopy (EDX) to characterize the cross-sections of the worn pins. On the basis of detailed microscopic examination of the subsurface microstructure, their correlation with the tribological performance was revealed. The obtained results are expected to contribute to a better understanding on the formation of the tribolayers and the wear mechanisms of nanostructured two-phase alloys at elevated temperatures.

2. Experimental procedures 2.1 Fabrication of bulk Nb90Ag10 two phase alloy Commercially pure niobium and silver powders (Alfa Aesar, 1-5 µm, 99.99%) were mixed with a nominal atomic composition of 90% Nb and 10% Ag and then subjected to high energy ball milling (SPEX 8000D) for 12 h at ambient temperature in an argon glove box to force the formation of a supersaturated Nb-Ag solid solution. More 5

details can be found in [34]. The ball milled powders were then compacted into bulk by a spark plasma sintering system (SPS-211 Lx, Fujidempa Kogyo. Co., ltd) at the temperature of 900 °C and pressure of 60 MPa in a vacuum of ~ 6 Pa for 5 min, producing cylinders with density exceeding 97% of the theoretical density. During sintering, Ag precipitated into nearly equiaxed particles with an averaged diameter of 20 nm distributed in the Nb matrix with grain size of ~ 120 nm (Fig. 1a&b).

Fig. 1. Microstructure of the bulk Nb90Ag10 alloy. (a) HAADF-STEM image; (b) BF-TEM image with the corresponding SAD pattern.

2.2 Pin-on-disk wear tests at elevated temperatures The pins were cut into 3  mm in diameter and 5  mm in length from the as-sintered cylinder using a wire-cut electrical discharge machine. Prior to wear testing, both the contact surfaces of pins and disks were mechanically polished with SiC paper down to 1200 grit and ultrasonically cleaned with acetone for 5 min. Pin-on-disk wear tests were performed (Anton Paar High Temperature Tribometer) in air with a relative 6

humidity 50%, under a load of 5 N (with the nominal contact pressure of ~ 0.71 MPa) and at a sliding velocity of 0.1 m·s-1. Inconel alloy 600 (with hardness of 202 HV) was selected as the counterface disk material. The disk has a diameter of 55 mm and thickness of 10 mm. Such pin-on-disk geometry has been widely used to determine the wear of materials during sliding, referring to the ASTM G99-17 (Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus). The test temperatures were chosen as 25, 200, 350 and 500 °C. Steady-state wear rates have been calculated by direct measurement of weight loss of the pin after sliding distance of 300 m to the accuracy of ± 0.1 mg. Three separate tests were run for each specimen, and the average wear rates and coefficients of friction were provided with standard deviations.

2.3 Materials characterization Surface morphology and chemical composition of both the worn pins and wear tracks on the disks were examined by SEM (TESCAN MIRA3) with attached EDX (AZtec EDX system with an X-MaxN 50 mm2 silicon drift detector, Oxford instruments). Topography maps of the worn pin surfaces were obtained with a 3D optical microscope (ContourGT-K, Bruker). The morphology and size of the wear debris were also analyzed by SEM. Phase compositions of the wear debris were identified by X-ray diffraction (XRD, Smartlab, Rigaku). The subsurface microstructures of the worn pins were characterized by secondary electron (SE) images of focused ion beam (FIB, Helios NanoLab™ 600i)-milled cross sections and a series of TEM and STEM (FEI Tecnai G2 F30 S-TWIN) based analytical techniques, including bright field TEM 7

(BF-TEM), dark field TEM (DF-TEM), high resolution TEM (HRTEM), HAADF-STEM and selected area diffraction (SAD) patterns. Cross-sectional TEM samples parallel to sliding direction (SD) were prepared by FIB standard lift-out technique. Chemical analysis on the cross-sections was also performed by EDX in STEM mode operated at 300 kV.

3. Results 3.1 Coefficients of friction and wear rates Continuous monitoring of the CoFs (Fig. 2a) established that the wear systems at the four temperatures all reached steady state when sliding distance exceeded 45 m. The sliding wear systems at 25 and 200 °C display similar friction behavior with the steady state CoFs of ~ 0.7. In contrast, the wear systems at 350 and 500 °C exhibit distinct friction behavior, where the steady state CoFs decreased to be ~ 0.5 (Fig. 2b) and fluctuated occasionally at a large amplitude (Fig. 2a). From 25 to 350 °C, the calculated wear rate increases with temperatures from 1.82 ± 0.41 × 10-4 mm3/(N·m) to 7.24 ± 0.52 × 10-4 mm3/(N·m). However, the wear rate has a sharp decrease at 500 °C to a value of 0.83 ± 0.11 × 10-4 mm3/(N·m) (Fig. 2c).

8

Fig. 2. (a) Coefficient of friction as a function of sliding distance at various temperatures for Nb90Ag10 alloy sliding against Inconel alloy 600 disk; (b) and (c) are steady state coefficient of friction and wear rate varying with temperatures, respectively.

3.2 Surface morphology, composition and profile of the worn pins Fig. 3 presents comparison of the typical surface morphology, chemical composition and surface profile of the worn pins at varying temperatures. At 25 and 200 °C, the worn pin samples have similar surface morphology, both showing parallel grooves along SD, but the size of the grooves (in term of depth and width) at 200 °C is significantly increased which indicates that the pins underwent more severe wear. No oxides and materials transferred from the counterface disk were detected (insets in Figs. 3a2 and 3b2). However, upon wear exposure at 350 °C, discontinuous patches 9

were located on the worn surface (Fig. 3c2). EDX spectrum (inset in Fig. 3c2) shows that a small amount of oxygen was detected. When the temperature reaches 500 °C, the worn pin surface shows distinct appearance, mostly covered by the oxidation/glaze layer composed of niobium, nickel, chromium, iron and their oxides. Such a glaze layer has a sharp increase in the oxygen concentration, as shown in the EDX spectrum in Fig. 3d2.

Fig. 3. SEM images, EDX spectra and 3D profiles of the worn Nb90Ag10 pin surfaces after sliding against Inconel alloy 600 disks under a load of 5 N and velocity 10

of 0.1 m·s-1 for 300 m at varying temperatures.

3.3 Surface morphology and composition of wear tracks To better understand the tribo-oxidation process and the formation of the tribolayer, we have examined the surface morphology and composition of the wear tracks on the counterface disks (Such information are included in Supplementary Material). At 25 °C, parallel grooves along with a minor amount of patches and pits were observed in the wear track (Fig. S1a-b). Elemental maps show that the attached patches cover only a small part of the surface area and their compositions are identified to be Nb and Ag transferred from the pin material (Fig. S1c). At 200 °C, the wear track shows much shallower grooves (Fig. S2a-b) and over half of the surface area is covered by Nb-Ag particles with sizes scattered from 5 to 50 μm (Fig. S2b-c), which indicates a significant amount of material transfer from the pin to counterface disk. No oxygen was detected at both 25 and 200 °C, indicating that the tribo-pairs have metal-to-metal sliding contacts. At 350 °C, large Nb-Ag flakes with size up to over 100 μm were compacted on the surface of the wear track (Fig. S3a-b). Consistent with EDX spectrum obtained from the pin surface, a minor amount of oxides forms at this temperature (Fig. S3e). Surprisingly, at 500 °C, the wear track has a clean and fresh surface (Fig. S4a-b). Almost no material transferred from the pin to the disk and oxides were detected (Fig. S4c-e).

3.4 Morphology, size and composition of the wear debris 11

Typical debris generated during wear at varying temperatures and their corresponding XRD patterns are shown in Fig. 4. After wear exposure at 25 °C, the debris are thin flakes with average size of ~ 30 μm (Fig. 4a), while at 200 °C, the debris shows large flakes with multilayered structure and average size of up to ~ 100 μm (Fig. 4c). XRD patterns confirm that the wear debris at 25 and 200 °C have the same phase compositions (Figs. 4b and 4d). Both are Nb phase from the pins and Ni-Cr-Fe alloy phase from the Inconel alloy 600 counterface disk. At temperature of 350 °C, the wear debris show remarkable difference from those generated at 25 and 200 °C. Besides much larger size of ~ 150 μm (Fig. 4e), the debris are predominantly niobium and silver from the pin material with a minor amount of NiCr2O4 oxides (Fig. 4f). A very limited amount of counterface disk material was detected. These can be evidenced by the sharp increase in the diffraction intensity of niobium at 2θ = 38.47° and the drastic drop in the diffraction intensity of Ni-Cr-Fe at 2θ = 44.65° (Fig. 4f). These results are consistent with the highest wear rate (Fig. 2c) and the presence of minor amount of oxides on the surfaces of both the pin (Fig. 3c1) and corresponding wear track (Fig. S3) at this temperature. Upon wear at 500 °C, the debris also shows multilayered structure but with much smaller size. Of particular interest is that the predominant composition switches to be Ni-Cr-Fe alloy removed from the counterface disk with a small amount of niobium oxides (Fig. 4h). This also agrees well with above observations that under this wear condition, the sample has the lowest wear rate, the composition of the glaze layer on the worn pin surface is mainly from the counterface material (Fig. 3d1) and the wear track shows clean and fresh surface. 12

Fig. 4. SEM micrographs and corresponding XRD patterns of the debris generated upon wear at varying temperatures.

3.5 Subsurface microstructure In order to better understand the above wear behavior, evolution of the subsurface microstructures was in-depth characterized by cross-sectional FIB/TEM analysis. Fig. 13

5 presents FIB machined ND-SD cross-sectional secondary electron (SE) images of Nb90Ag10 after wear at different temperatures, which yield good contrast over large fields of view. At temperatures of 25 (Fig. 5a) and 200°C (Fig. 5b), the samples have similar microstructures below the sliding surface. The wear imposed plastic deformation extends to a depth of ~ 2.5 μm at 25 °C and ~ 3 μm at 200 °C. The silver precipitates (shown in bright contrast) are gradually elongated along the SD and evolve to be nanolayers upon approaching the sliding surface. No oxidation or mechanical mixing layers were observed. After wear exposure at 350 °C, the subsurface can be two divided into two distinct regions: 1) the top is an ~ 2 μm thick mechanical mixing layer (MML) with a minor amount of nano/submicroscale pores inside; and 2) below the MML is the plastic deformation region with the depth extending from 2 to 4.5 μm (Fig. 5c). However, it should be noted that only a small part of the worn pin surface was covered by such MML, as shown in the patchy area in Fig. 3c2. After wear exposure at 500 °C, an ~ 4 μm thick glaze layer was present below the sliding surface. A considerable amount of nano to microscale irregular pores were located in this region. Below the glaze layer, from 4 to 5.5 μm, is the plastic deformation region with silver precipitates much less plastically deformed as compared with those found after wear exposure below 200 °C. A sharp interface between the glaze layer and the pin substrate was observed (Fig. 5d).

14

Fig. 5. Secondary electron images of FIB-milled ND-SD cross sections of the worn Nb90Ag10 pins at varying temperatures.

To obtain more detailed microstructure information in the plastic deformation region after wear exposure below 200 °C and to extract the chemistry, crystallography and microstructure information in the MML/glaze layer at higher temperatures of 350 and 500 °C, TEM and HAADF-STEM based analyses have also been performed. We first investigated the subsurface microstructure of the pins with wear exposure at 25 °C (Fig. 6a-b) and 200 °C (Fig. 6c-d) with the absence of MML/glaze layer. At both temperatures, sliding wear induced the formation of alternating Nb and Ag nanolayered structures upon approaching the sliding surface. Differing from the Nb90Ag10 alloy upon wear against stainless steel 440C at room temperature which forced the formation of a supersaturated solid solution right below the sliding surface [31], Nb and Ag are still phase separated in this study, as evidenced by the 15

HAADF-STEM images and SAD patterns. This suggests that the counterface disk material also affects the plastic strain gradient generated below the sliding surface of the pins. The notable subsurface microstructure difference between the two temperatures (25 and 200 °C) lies in the thickness of generated Nb and Ag layers. As shown in Fig. 6, just below the sliding surface, the Ag layer is ~ 5 nm thick at 25 °C while increases to be ~ 10 nm at 200 °C.

Fig. 6. ND-SD cross sectional BF-TEM (a, c) and HAADF-STEM (b, d) images of Nb90Ag10 alloy after sliding wear at 25°C (a-b) and 200 °C (c-d), respectively.

16

Fig. 7 shows the ND-SD cross-sectional HAADF-STEM images and EDX spectra of Nb90Ag10 after wear against Inconel alloy 600 at 350 °C. A close inspection reveals that the MML roughly exhibits two distinct contrasts. EDX analyses (Fig. 7b) confirm that the bright area is niobium and silver transferred from the pin (Location  in Fig. 7b) and the dark area is mainly composed of oxides (Location  in Fig. 7b). BF-TEM image (Fig. 8a) further reveals that such niobium and silver are in the form of mechanically mixed fragmented nanolayers (Location  in Fig. 8a). Corresponding SAD pattern confirms the niobium and silver in the MML are phase separated, as indicated by the non-uniform intensity (Fig. 8b). The oxides are present as equiaxed nanograins (Location  in Fig. 8a). Corresponding SAD pattern shows such oxides are mainly nickel and chromium oxides, i.e. Cr2O3, NiCr2O4 (Fig. 8c). A few nano/submicron scale irregular pores were also observed in the HAADF-STEM image. Microcracks parallel to the surface near the interface between the MML and the severely plastically deformed pin substrate (marked in Fig. 7a) provide a valuable clue to the detachment of the niobium and silver nanolayers during wear. Below the MML is a SPD region with alternating niobium and silver nanolayered structure with the niobium layer of ~ 50 nm thick and Ag layer of 30 nm thick (Fig. 7a and Fig. 8d). Corresponding SAD pattern taken from this region suggests that Nb and Ag nanolayers have relatively strong crystallographic texture (Fig. 8e). A few undeformed black nanoparticles in the SPD region (a typical one is marked in Location  in Fig. 7c) were identified to be niobium oxides, which shows much higher concentration of oxygen in comparison with that obtained from the pin matrix (Location  in Fig. 7a). 17

It should be noted that the Cu peaks in the EDX spectra (Fig. 7b) are from the TEM copper grids. As the depth increases, such nanolayers are progressively recovered to the base material with equiaxed grains.

Fig. 7. ND-SD cross sectional HAADF-STEM images (a, c) and EDX spectra (b) of Nb90Ag10 alloy after sliding wear at 350 °C.

18

Fig. 8. ND-SD cross sectional BF-TEM images and corresponding SAD patterns of Nb90Ag10 alloy after wear at 350°C.

Detailed chemistry, crystallography and microstructure of the glaze layer formed after wear exposure at 500 °C are shown in Fig. 9. The HAADF-STEM image of the glaze layer (Fig. 9a) shows almost uniform contrast. EDX analysis reveals that the chemical elements are mainly from the disk material with a large amount of oxygen additionally. Obviously, such oxygen mainly stems from the reaction of the disk material with the 19

atmospheric environment. The BF-TEM images (Fig. 9b-c) show the glaze layer mainly consists of equiaxed nanograins. HRTEM image of several individual nanograins (Fig. 9d) further reveals that the grain size of the oxides is ~ 10 nm. The SAD pattern (Fig. 9e) shows such equiaxed nanograins are randomly oriented. An indexation of the dominant diffraction rings shows that the glaze layer mainly consists of NiCr2O4. Dark field TEM image (Fig. 9f) shows the mean grain size is about 10 nm, consistent with the HRTEM image.

Fig. 9. STEM and TEM based characterization on the glaze layer formed on the Nb90Ag10 pin after sliding wear at 500°C. (a) a low-magnification HAADF-STEM image with an inset of a typical EDX spectrum; (b) and (c) are BF-TEM images of the selected area in (a) at different magnifications, respectively; (d) a HRTEM image of the equiaxed oxides nanograins. (e) SAD pattern taken in the area marked in (b); and 20

(e) DF-TEM image of NiCr2O4 oxides.

4. Discussion The present work focuses on the evolution subsurface microstructure in the nanostructured Nb90Ag10 pins and its correlation with wear performance. Microstructural evolution in the Inconel alloy 600 counter body is not considered. The results clearly indicate that the wear mode and wear resistance of the nanostructured Nb90Ag10 binary alloy strongly depend on the evolution of sub-surface microstructure at elevated temperatures.

Upon wear exposure at 25 and 200 °C, no oxides were found either on the worn surfaces (including both the worn pin surfaces and wear tracks) or in the generated wear debris. Such metal-to-metal sliding contacts should be responsible for the relatively high CoFs observed in Fig. 2a and would also result in SPD in the subsurface region of the pins. Due to the wear-imposed SPD, the original equiaxed Nb/Ag grains were transformed into Nb/Ag alternating elongated nanolayered structure (Fig. 6). Such self-organized chemical nanolayering reaction has also been observed previously upon sliding wear of Cu-Ag two-phase alloys at room temperature [7]. However, the thickness of the formed Nb/Ag nanolayers increased from ~ 5 nm at 25 °C to ~ 10 nm at 200 °C just below sliding surface (Fig. 6). This is due to that at 200 °C, the thermally activated diffusion would contribute more during the dynamic competition between homogenization by forced chemical mixing and phase separation by thermally activated diffusion [36, 37]. This observation was 21

consistent with high pressure torsion of Cu-Ag at elevated temperatures [38], which shows much larger self-organized microstructural length scale at higher temperatures. On the one hand, for the layer thickness (h) in the range of few nanometers to a few tens of nanometers, the strength (σ) of the metallic multilayers dependence on h can be described by refined confined layer slip model developed by Misra et al. [39], which shows σ ∝

ℎ b

ln⁡( ) ℎ

, where b is the length of Burgers vector. Recently, Subedi et

al.[40] reported that Hall-Petch with modified coefficients provides a good fit to the experimental data down to h of about 5 nm, σ = σ0 + kh-1/2 with coefficients (σ0, k) slightly different from those in the classical Hall-Petch equation [41]. In the subsurface region, h25°C < h200°C, so that σ25°C < σ200°C is expected. On the other hand, ultrafine/nanostructured metals are known for a rapid decrease in hardness at elevated temperatures [42]. For example, hardness of nanostructured Nb-Cu decreased from 5.4 GPa at room temperature to 3.2 GPa at 200 °C [43]. To verify this hypothesis, we have measured the hardness of Nb90Ag10 at 25 and 200 °C by nanoindentation. As expected, the hardness decreased from 5.4 GPa at 25 °C to 4.3 GPa at 200 °C. In contrast, the Inconel alloy 600 disk could retain strength over a wide temperature range by solid solution strengthening and precipitation strengthening [44]. These explain why the nanostructured Nb90Ag10 alloy at 200 °C underwent more severe wear than the one at 25 °C (Fig. 2c) and much more Nb-Ag were detected on the surface of counterface disk at 200 °C (Fig. S2).

From 350 °C to 500 °C, the oxidation reaction significantly influenced the wear 22

behavior. Considerable efforts have been devoted to investigating the role of oxidation during wear of alloys [45-47]. The formation of metallic oxides could serve as protective layer on worn surface, called “glaze layer”, leading to the transition from severe wear to mild wear [45, 48-50]. However, depending on the nature of the tribolayer, the hard oxides particles may also cause severe abrasive wear rather than the formation of glaze layer [51]. A critical temperature is required to establish the stable ‘glaze’ surface under a given set of conditions to support the protection effect [3, 4]. Upon wear exposure at 350 °C, the above observations have demonstrated that both wear debris (Fig. 4e-f) and MML (Fig. 7-8) are composed of dominantly Nb-Ag together with a small amount of NiCr2O4, which suggest that the oxidation of the disk material occurred prior to the Nb90Ag10 pin, thus generating NiCr2O4 and Cr2O3 oxides. The presence of oxides should be responsible for the reduction in CoFs (Fig. 2b).

To understand the oxidation process, kinetically, Arrhenius equation [52, 53] has been proposed to identify oxidation rate constant K(T) of metals: K(T) = k0 exp(-Ea/RT). where k0 is the pre-exponential factor which is assumed to be independent of temperature, Ea is the activation energy for parabolic oxidation, R is the ideal gas constant, and T is the temperature of oxidation. It is shown that the oxidation rate has a positive correlation with temperature and negative correlation with the activation energy. Previous studies have shown a very limited oxidation rate of niobium [54-56] 23

below 400 °C. For example, Aylmore et al. [55] have investigated the kinetics of the oxidation of niobium in dry oxygen at 1 atm pressure and temperatures in the range 350-750 °C. The results show that the oxidation rate KNb (at 350 °C) = 0.0018 mg·cm-2·h-1 and almost no oxides generated after heating for 265 h in air condition. This matches well with our present results that no niobium oxides were detected in the wear debris (Fig. 4e-f) and MML (Fig. 7-8) at 350 °C. Therefore, during sliding wear at 350 °C, the Inconel alloy 600 disk starts to oxidize but the Nb90Ag10 pin material not yet. However, such a temperature is not enough to support formation of stable and protective glaze layer, as shown in Fig. 7. The generated disk-sourced oxides, in the form of loose particles with limited compaction, would modify and enhance the wear process

by

abrasive

action

during

sliding

(abrasion-assisted-severe-wear),

accompanied by production of large, predominantly pin-sourced metallic wear debris, as evidenced in Fig. 4e-f. Part of the debris particles were smeared on the metallic wear surfaces to form the MML (shown in Fig. 5c and further analyzed in Fig. 7-8) and covered very local area of the worn pin surface, as the discontinuous patches shown in Fig. 3c2. This helps to explain the highest wear rates at 350 ˚C.

A transition from severe wear to the mild wear was observed when temperature increased to 500 °C. According to Aylmore et al.’s study [55], the oxidation rate KNb at 500 °C rapidly increased to be 4.0 mg·cm-2 ·h-1, more than two thousand times larger than the value at 350 °C. This means that at 500 °C, both the Nb90Ag10 pin and the counterface disk were oxidized. The XRD results clearly show that the wear debris 24

contains niobium oxides (Nb2O5). Even with the high oxidation rate of niobium, the Nb90Ag10 alloy has the lowest wear rate (Fig. 2c). Such lowest wear rate could be explained by the formation of glaze layer on the Nb90Ag10 pin surface (Fig. 5 & 9). Based on the detailed characterization and in-depth analysis of the pin and wear track surface, wear debris and microstructure of the glaze layer, one rational explanation could be that during the early stage of wear, oxides formed on nanostructured pin surface caused the severe abrasive wear of Inconel alloy 600 disk. After the generation of the debris particles, some were lost from between the surfaces, but some were retained and involved in developing compact, load-bearing layers. These particles underwent deformation, fragmentation and comminution and breakdown into smaller particles. A fraction of these finer particles were agglomerated on the pin surface and sintered together to form more solid layers, as confirmed by the equiaxed nanograined oxides observed in Fig. 9. Such sintering was enhanced by high compressive pressure of the wear tests. The increase in temperature not only increased the rate of oxidation of the residual metals in the debris particles but also increased the rate of sintering and consolidation of the surface layer. If the surface become solid before breakdown occurs, the glaze layer is established and the wear rate thus becomes very low [45, 46]. As is shown, the removed Ni-Cr-Fe wear debris transferred to the Nb90Ag10 surface and formed compacted dense glaze layer. Such a glaze layer has much higher hardness and further plows the disk surface. This explanation can be further confirmed by the clean and fresh surface observed in the wear track and the generated wear debris contains dominantly Ni-Cr-Fe large flakes 25

with a minor amount of Nb2O5. Despite the lower part of glaze layer looks porous, the upper part, with several hundred nanometer depth below the sliding surface, is actually very dense, as shown in Fig. 5d. Such glaze layer would protect the pin substrate from severe wear and thus yield reduced wear rate.

5. Conclusions The sliding wear of nanocrystalline Nb90Ag10 binary immiscible alloy against Inconel alloy 600 disks at temperatures from 25 to 500 °C was systematically investigated. On the basis of detailed microstructural and chemical characterizations on the subsurface layers, their correlation with the tribological performance was revealed. The obtained results provide deep insights into the formation of the tribolayers and the wear mechanisms of nanostructured two-phase alloys at elevated temperatures. The following conclusions can be drawn. (1) No oxidation reaction was found upon sliding wear at relatively low temperatures of 25 and 200°C. The metal-to-metal sliding contacts result in higher friction and wear rates. Wear-induced SPD leads to the formation of Nb/Ag alternating elongated nanolayers from the original equiaxed grains near the sliding surface. More thermally activated diffusion contribution yields a larger nanolayer thickness at 200 °C. The higher wear rate at 200 °C should be attributed to the reduced yield strength and hardness. (2) At intermediate temperature of 350 °C, the tribo-oxidation reaction occurred between the counterface material and the environment but the pin has almost no 26

oxidation. The third-body wear caused pronounced reduction in CoF but the highest wear rate. A mechanical mixing layer, consisting of equiaxed nanocrystalline counterface material oxides and fragmented Nb/Ag nanolayers, was found below the sliding surface. (3) At relatively higher temperature of 500 °C, the tribo-oxidation occurred for both the Nb10Ag10 pin and the counterface material. An ~ 4 μm thick protective glaze layer formed on the top of the Nb90Ag10 and thus leads to lowest wear rate. These results show that the tribological performance of the nanostructured Nb-Ag two-phase alloy have a strong correlation with the subsurface microstructure formed during wear at elevated temperatures and also suggest that nanostructured Nb-Ag two-phase alloy could be a potential candidate for high temperature applications.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant no. 51501087), the Fundamental Research Program of Shenzhen (Grant Nos.

JCYJ20170307110418960,

JCYJ20170412153039309,

and

JCYJ20160530185550416), and Guangdong Innovative & Entrepreneurial Research Team Program (No. 2016ZT06C279). This work was also supported by the Pico Center at SUSTech that receives support from Presidential fund and Development and Reform Commission of Shenzhen Municipality.

27

Supplementary Material Surface morphology, elemental maps and typical EDX spectra of the wear track on the Inconel 600 disk after sliding against Nb90Ag10 alloy at 25, 200, 350 and 500 ºC are included.

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Highlights • Sliding wear of nanocrystalline Nb90Ag10 alloy at elevated temperatures was investigated. • Below 200 °C, no oxidation layer formed, resulting in higher coefficients of friction and wear rates. • At 350 °C, the tribo-oxidation occurred only within the disk and a mechanical mixing layer was formed. • At 500 °C, the tribo-oxidation occurred within both the pin and the disk and a protective glaze layer leads to the lowest wear rate.. 34