Microstructure, sliding wear and corrosion behavior of bulk nanostructured Co-Ag immiscible alloys

Microstructure, sliding wear and corrosion behavior of bulk nanostructured Co-Ag immiscible alloys

Journal of Alloys and Compounds 748 (2018) 961e969 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 748 (2018) 961e969

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Microstructure, sliding wear and corrosion behavior of bulk nanostructured Co-Ag immiscible alloys Weiwei Zhu a, b, Cancan Zhao b, Jian Zhou c, Chi Tat Kwok a, d, Fuzeng Ren b, * a

Institute of Applied Physics and Materials Engineering, Faculty of Science & Technology, University of Macau, Macau Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China c Shagang School of Iron and Steel, Soochow University, 178 Gan Jiang Dong Road, Suzhou, Jiangsu, China d Department of Electromechanical Engineering, Faculty of Science and Technology, The University of Macau, Macau b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 December 2017 Received in revised form 14 March 2018 Accepted 16 March 2018 Available online 19 March 2018

Bulk nanostructured Co90Ag10 immiscible alloys with Ag particle size of 10 nm have been fabricated by a combination of high energy ball milling and warm pressing. The Ag particle size increased to be 58 nm after annealing at 900  C for 1 h. Pin-on-disk dry sliding wear tests show that even with over 20% hardness reduction, the annealed sample has similar wear resistance with the as pressed one. Wear induced strain in Co-Ag immiscible alloys could not force the formation of self-organized nanolayered structure or a supersaturated solid solution below the sliding surface. Electrochemical polarization tests show that such nanostructured Co-Ag alloys have no passive behaviour in the artificial saliva solution (ASS) but with high corrosion current densities. Energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis show that the corrosion products are composed of insoluble cobalt (II) phosphate, cobalt hydroxide and silver chloride. Such corrosion behaviour in ASS suggests CoAg alloys are not suitable candidate for dental applications. © 2018 Elsevier B.V. All rights reserved.

Keywords: Microstructure Nanostructured Co-Ag Wear Corrosion

1. Introduction In the large pool of candidates for metallic biomaterials, Co-base alloys are reported to be suited for dental and orthopedic implants since they have combined high strength, hardness, wear resistance and adequate corrosion resistance [1e4]. Addition of alloying elements like carbon, chromium and molybdenum conventionally employed to improve the strength, hardness and corrosion resistance of cobalt matrix [5,6]. However, the microstructure of such Co-base alloys strengthened by solid-solution and carbide precipitation usually consists of irregular hard interdendritic Cr- and Morich carbides distributed in the cobalt matrix, which can cause brittleness and are detrimental to the wear and corrosion resistance if the distribution, volume fraction, size and/or morphology are not well controlled [7e11]. In addition, the release of chromium ions from corrosion of Co-Cr alloys could alter the immune system function in patients [12e14] and the continuous release of Ni ions in the cast Co-Cr-Mo alloys during in vivo service may possibly cause an allergy and carcinogenicity [15,16]. Therefore, extensive

* Corresponding author. E-mail address: [email protected] (F. Ren). https://doi.org/10.1016/j.jallcom.2018.03.213 0925-8388/© 2018 Elsevier B.V. All rights reserved.

efforts have also been devoted to the removal or diminishing of Ni from biomedical Co-Cr-Mo alloys [17e22]. An alternating strategy to enhance the strength and wear performance is by grain refinement. Previous reports have demonstrated that nanostructured metals have unusual physical and mechanical properties compared to conventional coarse-grained counterparts, and can thus be potentially explored for biomedical applications [23e25]. More specifically, nanostructured cobalt and ultra-fine grained Co-28Cr-6Mo alloy have demonstrated increased hardness and sliding wear resistance [26,27]. Silver, on the other hand, is often used as solid lubricant to reduce friction [28] and also used as antibacterial element to prevent implant infection [29,30]. In the equilibrium phase diagram, Co and Ag are completely immiscible [31]. Therefore, nanostructured Co-Ag binary alloys have promise for dental or load-bearing orthopedic implants. On the other hand, our previous studies on sliding wear of twophase alloy systems with soft nano-precipitates dispersed into a hard matrix, such as Cu-Ag and Nb-Ag which have face centered cubic (fcc) and body centered cubic (bcc) matrix, respectively, have shown that with controlled Ag precipitate size, severe plastic deformation imposed during wear can induce the spontaneous formation of self-organized nanolayered microstructures, and such

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nanolaminates can significantly improve wear resistance [32]. However, the binary system with hexagonal close packed (hcp) matrix with much more limited slip is yet to be explored. It is known that cobalt has hcp crystal structure below 690 K above which it will transform into fcc [33]. Therefore, Co-Ag binary alloys may serve as an ideal model system for the deformation of a hcp matrix with dispersed, soft fcc particles. In this context, the present study was undertaken to fabricate the nanostructured Co-Ag two-phase alloys and further to investigate their dry sliding wear behaviour and the corrosion resistance in artificial saliva solution (ASS). The findings are expected to contribute to fundamental understanding on the plastic deformation of nanostructured hcp-fcc two-phase alloys during wear and provide preliminary evaluation on the feasibility of bulk nanostructured Co-Ag as a potential candidate for dental implants. 2. Experimental procedure 2.1. Fabrication of the bulk Co90Ag10 alloys Commercially pure cobalt (Alfa Aesar, 1.6 mm, 99.8%) and silver (Alfa Aesar, 1e3 mm, 99.99%) powders with nominal atomic composition of Co90Ag10 and hardened steel balls (with ball to powder weight ratio of 5:1) were loaded into a 65 ml hardened steel vial and subjected to high energy ball milling for 12 h at ambient temperature, using a SPEX 8000D mill in an argon glove box. Then the milled powder was consolidated into bulk at temperature of 300  C under a constant load of ~1 GPa and high vacuum of ~2  106 Pa. To investigate the thermal stability of the aspressed Co90Ag10 alloy and to adjust the Ag particle size, the as pressed cylinders were annealed at 900  C for 1 h in an argon tube furnace. 2.2. Phase and microstructure of bulk Co90Ag10 alloys The density of the as-pressed and annealed cylinders was measured by Archimedes method (ASTM B962-15). The phase composition of all the samples were identified by X-ray diffraction (XRD) pattern recorded on a PANalytical Philips X0 pert Pro-MPD diffractometer in the 2q range from 30 to 100 using Cu-Ka radiation (l ¼ 1.54056 Å, 40 kV, 40 mA) with a step size of 0.02 and a count time of 1s. The microstructures of the samples were characterized by scanning electron microscopy (SEM) of dual beam focused ion beam (FIB; Helios NanolabTM 600i) system and transmission electron microscopy (TEM) using JEOL 2010 microscope operated at 200 kV. The TEM samples were prepared by FIB milling using the standard lift-out technique.

a constant slow sliding velocity of 0.1 m/s. Both stainless steel 440C (SS440C) and alumina were selected as our counterface materials, considering that the alloys may show different wear mechanisms for metal-on-metal (MoM) and metal-on-ceramics (MoC) tribosystems. The other reason we selected two counterface disks is for direct comparison of its wear behaviour with those of Cu90Ag10 [32,34] and pure nanostructured Co [26]. Note that previous studies on wear of Cu90Ag10 [32,34] two-phase alloys use SS440C as counterface disks while wear of nanostructured Co uses alumina disk. Steady-state wear rates were calculated by weight loss measurements after sliding distance of 720 m. Three separate tests were run and the average wear rate and coefficient of friction (CoF) were provided. The morphology of the worn pin surface was analyzed by SEM. The size, morphology and chemical composition of the wear debris were characterized by SEM and energy dispersive X-ray spectroscopy (EDX). For relating the microstructures to the wear test geometry, a laboratory frame of reference was defined by the sliding direction (SD), the transverse direction (TD) in the sliding plane, and the direction normal to the worn surface (ND). The subsurface microstructures parallel to sliding direction (ND-SD plane of view) of worn samples were characterized by SEM and bright field TEM. 2.4. Electrochemical corrosion tests in ASS The corrosion tests of the polished as-pressed and annealed samples were performed by electrochemical polarization using a Princeton Applied Research Versa Studio (PARSTAT 4000). The tests were performed in a conventional three-electrode cell configuration using a saturated calomel reference electrode (SCE) as reference electrode and a platinum plate as counter electrode. The samples (diameter of 10 mm) were insulated with epoxy resin with 78.5 mm2 exposed area as the working electrode. All the potential values are reported vs. SCE hereafter. Measurements were performed at 37  C in ASS: 1.5 g/L NaCl, 1.5 g/L NaHCO3, 0.5 g/L NaH2PO4$2H2O, 0.5 g/L KSCN, and 0.9 g/L lactic acid, with pH of 6.2 [35]. Each test was repeated three times to confirm reproducibility of the data. Prior to potentiodynamic measurements, the specimens were immersed in ASS for 1 h to stabilize the surface at the open circuit potential (OCP). Potentiodynamic polarization curves were recorded by scanning from (OCP value, 0.75 V) to (OCP value, þ0.75 V) at a scanning rate of 10 mV/min. The corrosion potential Ecorr as well as the corrosion current densities icorr was automatically extracted from the polarization curves by the Versa Studio 2.44 software through Tafel slope extrapolation. The corroded surfaces after polarization tests were examined using an SEM. The chemical and phase compositions of corrosion products were analyzed by EDX and X-ray Photoelectron Spectroscopy (XPS, ESCALB MK-II, VG Instruments, UK) using monochromatic Mg Ka radiation.

2.3. Hardness measurements and pin-on-disk sliding wear tests 3. Results and discussion Hardness (ASTM E384-16, 2016) of both as pressed and annealed Co90Ag10 alloys was measured using a Vickers diamond pyramidal indenter (HXD-1000 TM C/LCD, Shanghai Taiming Optical Instrument Co., Ltd., Shanghai, China) under a load of 300 gf for 30 s. Prior to wear tests, the pin samples were cut into 3 mm in diameter and length of 10 mm from the bulk cylinders using a wirecut electrical discharge machine. The contacting surfaces of both the pins and the disk were first polished with sequential silicon carbide papers down to 1200 grit and then vibrationally polished with alumina suspension of 1 and 0.05 mm, and finally silica suspension of 0.02 mm. Pin-on-disk wear tests (Anton Paar Pin-on-Disk High Temperature Tribometer) were performed in air under load of 9.8 N (corresponding to nominal contact pressure of 1.38 MPa) and

3.1. Phase and microstructure The as pressed Co90Ag10 alloy has the relative density of 91%, while after annealing at 900  C for 1 h, the relative density increases to be 93%. Fig. 1 shows the XRD patterns of the as-pressed and annealed Co90Ag10 alloys. All the diffraction peaks are indexed to be Co and Ag. However, both hcp and metastable fcc Co are present. The characteristic reflections of hcp Co are located at 41.7 and 47.6 , corresponding to (1 0e1 0) and (1 0e1 1), respectively. The volume fraction of the two Co crystallographic phases was calculated using the method developed by Sage and Gillaud [36,37]. The volume fraction of hcp-Co (fhcp) of the as pressed Co90Ag10 alloy is 0.10 (±0.03), while after annealing at 900  C for 1 h, fhcp

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are illustrated in Fig. 2. Bright field TEM image (Fig. 2a) shows that the as pressed sample has Ag particles of ~10 nm, which is hardly distinguished by SEM. Selected area electron diffraction (SAED) (Fig. 2b) further confirms the co-existence of fcc and hcp Co. A secondary electron image of a FIB-milled cross section of annealed Co90Ag10 is shown in Fig. 2c. The Co and Ag are distinguishable since they have different secondary electron yields, with Co showing as dark and Ag as bright. The Ag particles are homogeneously distributed in the Co matrix. After annealing at 900  C for 1 h, statistical analysis (Fig. 2d) shows that the Ag particle size follows normal distribution with an average of 58 nm. 3.2. Hardness and wear performance

Fig. 1. X-ray diffraction patterns of the as pressed and annealed Co90Ag10 alloys.

increases to be 0.61 (±0.04). Such results are consistent with previous study on Co-Cr-Mo-W biomedical alloy produced by laser sintering that thermal treatment induces the formation of the hexagonal phase [38]. In addition, the annealed Co90Ag10 alloy has much sharper diffraction peaks, indicating grain size increase after annealing. The microstructure of as pressed and annealed Co90Ag10 alloys

The as pressed Co90Ag10 alloy has the hardness of 530 (±20 H V), while after annealing at 900  C for 1 h, the hardness decreases to be 423 (±5 H V). With the measured weight loss, density, applied load and sliding distance, the calculated wear rate of the annealed Co90Ag10 alloy upon against SS440C is 3.41  105 mm3/(N.m), which is over twice that of Cu90Ag10 with similar Ag particle size [32,34]. The wear rates of the as pressed and annealed Co90Ag10 alloys upon against alumina were calculated to be 4.0  104 mm3/(N$m) and 4.11  104 mm3/(N$m), respectively. Of particular interest is that even with over 20% hardness reduction, the annealed Co90Ag10 sample has pretty close wear resistance to that of the as pressed one. It is noted that the wear rate of Co90Ag10 is almost four times that of pure nanostructured Co [26], which

Fig. 2. Microstructure of the as pressed and annealed Co90Ag10 alloys. (a) and (b) are bright field TEM image and corresponding selected area electron diffraction pattern of the as pressed sample, respectively; (c) SEM image of the annealed sample (dark matrix is Co and bright are Ag particles); (d) Ag particle size distribution in annealed sample (average precipitate size: 58 nm).

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Table 1 A comparison of wear rate (105 mm3/N.m) and CoF in Co, Co90Ag10 and Cu90Ag10 alloys. Counterface disk Wear rate CoF

Nanostructured Co [26]

As pressed Co90Ag10

Annealed Co90Ag10

Cu90Ag10 with dAg ¼ 58 nm [32,34]

10 e 0.33 e

40 e 0.242 (±0.037) e

41 3.41 0.389 (±0.015) 0.464 (±0.083)

e 1.49 e 0.63

Al2O3 SS440C Al2O3 SS440C

Note: dAg represents diameter of Ag precipitate.

suggests that addition of 10 at.% Ag in the Co matrix significantly lowers the wear resistance. A comparison of CoF and wear rate in Co, Co90Ag10 and Cu90Ag10 alloys is shown in Table 1. Fig. 3 shows the CoFs of the two alloys sliding against SS440C and alumina disks as a function of sliding distance. The CoF of the annealed Co90Ag10 upon against SS440C is 0.464 (±0.083), much lower than that of Cu90Ag10 (0.64) [32,34]. The annealing treatment at 900  C for 1 h increases the CoF of Co90Ag10 from 0.242 (±0.037) to 0.389 (±0.015) upon wear against alumina disk. In contrast to CoF of as pressed pure nanostructured Co (0.33) upon against alumina disk, the addition of Ag could reduce the CoF. Such results indicate that there is no direct correlation between the CoF and the wear resistance of the materials. To explore the wear mechanism and explain the wear performance of Co90Ag10 alloys with varying Ag particle size, we also analyzed the worn surface, wear debris and subsurface

Fig. 3. Coefficients of friction vs. sliding distance of the as pressed and annealed Co90Ag10 alloys sliding against alumina and SS440C disks under a load of 9.8 N and sliding velocity of 0.1 m/s.

microstructure after wear. Top-down SEM images of as pressed (Fig. 4a) and annealed (Fig. 4b) pins after sliding against alumina show that the dominant wear is abrasion, as revealed by scratches and grooves parallel to SD. The morphology of the annealed Co90Ag10 pin surface after wear testing was similar for both counterface materials, as illustrated in Fig. 4b and c, respectively. The wear mechanism is distinct from Cu90Ag10 two-phase alloys upon wear against SS440C, which is dominantly adhesive wear [34]. The generated wear debris show flake-like morphology with size ranging from several to a hundred microns, as illustrated in Fig. 5. EDX spectra (Fig. 5b and d) reveal that the compositions of the debris upon wear against alumina are mainly Co and Ag from the pins and their oxides with a minor amount of Al and O from the counterface alumina disk. The composition of debris generated upon wear against SS440C is also mainly from the pin material but with a significant amount of oxides and a minor amount of iron from the SS440C disk (Fig. 5f). It is reasonable that much more oxides will be generated for MoM than MoC tribosystems since the tribochemical reactions much more easily occur for MoM couples in atmospheric environment. For the as pressed Co90Ag10 alloy with Ag particle of 10 nm, the ND-SD cross-sectional subsurface microstructure (Fig. 6a) shows no much difference from the starting microstructure (Fig. 2a) after wear against alumina. SAED pattern taken below the sliding surface (Fig. 6b) confirms that the Co and Ag remain phase separated. For the annealed Co90Ag10 alloy with Ag particle of 58 nm, wear induced severe plastic deformation below the sliding surface is observed upon wear against both alumina (Fig. 6c) and SS440C (Fig. 6d) , but they have different deformation depths with the former extending to ~1 mm and the latter only ~0.5 mm. No transfer layer/mechanical mixing layer was observed in all Co90Ag10 alloys upon wear against the two counterface disks. To explain the wear induced microstructure in Co-Ag immiscible alloys, it is necessary to compare them with Cu-Ag and Nb-Ag immiscible alloys with similar Ag precipitate/particle size. In fccCu matrix, upon Ag precipitate size of 58 nm, the worn microstructure comprised three regions: 1) extending from the wear

Fig. 4. SEM images of worn surfaces. (a) and (b) are the worn surface of as pressed and annealed Co90Ag10 alloys after sliding against alumina disk, respectively; (c) worn surface of annealed sample after sliding against SS440C disk for 720 m under a load of 9.8 N and velocity of 0.1 m/s.

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Fig. 5. SEM images and typical EDX spectra of wear debris of the as pressed and annealed Co90Ag10 alloys. (a) and (b) are from the as pressed sample against alumina; (c) and (d) are from the annealed sample against alumina; (e) and (f) are from the annealed sample against SS440C.

surface to a depth of ~0.6 mm is a single phase Cu-Ag solid solution; 2) below this solid solution layer, and extending over ~0.5 mm, was a chemically self-organized nanolayered structure; and 3) at greater depths to ~4 mm, a microstructure with individual Ag-rich precipitates was recovered. In bcc-Nb matrix, upon Ag precipitate size of 55 nm, the worn microstructure comprised of two regions: 1) the bcc Nb-rich and fcc Ag-rich nanolayered structure to a depth of ~0.7 mm; and 2) followed by a region containing individual plastically deformed, elongated Ag precipitates to a depth of ~2.5 mm. Here, with mainly hcp-Co matrix and Ag particle size of 58 nm, strain imposed by wear could not force the formation of nonequilibrium supersaturated Co-Ag solid solution or self-organized nanolayered structure. We have proposed that the chemical layering results from the plastic deformation and stretching of Ag precipitates guided by geometrically necessary boundaries (GNBs) that

progressively form in the matrix as the strain increases [32,39], as found for pure elements [40,41]. While GNBs form between regions of different strain patterns to accommodate the accompanying difference in lattice rotation [42] and such different strain patterns further depend on the difference in the operating slip systems [43]. Since slip in hcp metals is much more limited than in fcc and bcc crystal structures, which typically requires a much higher resolved shear stress, thus, wear induced strain in Co-Ag immiscible alloys could not cause the formation of nanolayered structure or even to a non-equilibrium supersaturated solid solution. 3.3. Corrosion behaviour of bulk nanostructured Co90Ag10 alloys Fig. 7 shows the typical potentiodynamic polarization curves of as pressed and annealed Co90Ag10 alloys. The polarization curves

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Fig. 6. ND-SD cross sectional subsurface microstructure of as pressed and annealed Co90A10 alloys after sliding against the alumina and SS440C disks. (a) and (b) are bright field TEM image and corresponding selected area electron diffraction pattern of as pressed Co90Ag10 after sliding against alumina, respectively; (c) secondary electron image of annealed Co90Ag10 after sliding against alumina; (d) secondary electron image of annealed Co90Ag10 after sliding against SS440C.

Fig. 7. Potentiodynamic polarization curves of bulk nanostructured Co90Ag10 alloys recorded in ASS.

can be divided into four potential domains (AB, BC, CD, and DE). For the as pressed Co90Ag10 alloy, the cathodic domain (AB) includes potentials below - 0.522 V where the current is determined by the reduction of water and partially of dissolved oxygen. The active zone (BC) extends from 0.522 V to 0.303 V. The corrosion potential (Ecorr) and corrosion current density (icorr), as derived from Tafel slopes, are determined to be - 0.522 V and 14.116 mA/cm2. At potentials more positive than Ecorr, corrosion rate increases, and

reaches a maximum at the potential of - 0.303 V, at which insoluble corrosion products start to form and cause a sudden drop in current density in the region of 0.303 V to 0.279 V (the CD zone). The rate of anodic reaction was controlled by the dynamic formation and dissolution of such corrosion products. Above potential of 0.279 V, the current density increases with potential. Compare with pure nanostructured Co [26], addition of Ag causes the corrosion potential to be more negative and also increases the corrosion current density, which suggests that addition of Ag will reduce the corrosion resistance of Co. This is probably due to galvanic corrosion between Co and Ag. The annealed sample follows similar trend with the as pressed one but with more positive corrosion potential (Ecorr ¼ 0.454 V) and larger current density (icorr ¼ 32.393 mA/cm2). The nanostructured Co-Ag alloys do not show any stable passivation. The surface morphology and chemical compositions of the bulk nanostructured Co90Ag10 alloys after potentiodynamic polarization tests in ASS at 37  C are shown in Fig. 8. The surfaces of the alloys are covered by a cracked precipitated layer. The formation of cracks in the corrosion product layer may be due to volumetric contraction during dehydration process after electrolyte removal and the vacuum atmosphere during SEM analysis [44]. EDX spectra (Fig. 8b and d) show that the corrosion products contain Co, Ag, P, O and Cl. The corrosion products of the as pressed and annealed samples have almost identical surface morphology and chemical composition. Due to the layer thickness limit, we can hardly identify the phase compositions of the corrosion products by XRD. Thus, to provide deep insights into the chemical state of Co, Ag, P, O and Cl in the

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Fig. 8. Surface morphology and chemical compositions of nanostructured Co90Ag10 two phase alloys after potentiodynamic polarization tests inASS at 37  C. (aeb) are SEM image and EDX spectrum from the corrosion products of as pressed sample, respectively; (ced) are SEM image and EDX spectrum from the corrosion products of annealed sample, respectively.

Fig. 9. XPS survey scan spectrum of the corrosion products of annealed bulk nanostructured Co90Ag10 alloy.

corrosion products, we also conducted XPS analysis. Considering the as pressed and annealed Co90Ag10 alloys have the same corrosion products, we will only focus on the annealed sample hereafter. The XPS survey spectrum of the corrosion products is shown in Fig. 9. The main peaks can be indexed to Co 2p, 2s; Ag 3p, 3 d; P 2p and 2s; O 1s; Cl 2s, 2p. Si 2s is probably from contamination during XPS sample preparation. Additional Auger lines of Co LMM and O KLL were also observed in the wide scan spectrum. Fig. 10 shows the high-resolution XPS spectra of Co 2p, P 2p and O 1s of the corrosion products. High resolution Co 2p spectrum shows spin-orbit splitting into 2p1/2 and 2p3/2 components and both

bands were curve-fitted, including the shake-up satellites of the cobalt ions although both components qualitatively contain the same chemical information [45]. The energy difference between corresponding Co 2p3/2 and Co 2p1/2 peaks is approximately 15 eV. The intensive peak at 780.7 eV and the broad satellite peak at 786.3 eV can be assigned to cobalt hydroxide (Co(OH)2) [26]. Moreover, another Co 2p3/2 fitted at 782.5 eV for Co(OH)2 further confirms the Co in the corrosion product is in the form of Co(II) state. Correspondingly, two primary signals at 796.8 eV and 798.5 and a satellite peak at 802.3 eV for Co 2p1/2 were observed. Conclusively, the peak positions and satellite structure of the Co 2p lines indicate that oxidation state of Co in the corrosion product is Co (II). The broad P 2p peak at around 133 eV, deconvoluted into 2p3/ 2 at 132.7 eV and 2p1/2 at 133.6 eV, is characteristic of the tetrahedral PO4-group [46,47]. In the high resolution O 1s spectrum, two peaks are required to fit the O 1s broad band at around 531 eV which suggests that oxygen has two distinct chemical environments in the corrosion products. The lower binding energy peak at 530.7 eV is assigned to be the O from the hydroxide [48] and the higher binding energy peak at 531.8 eV is ascribed to phosphate [49]. Since the Co 2p and O 1s core level binding energies of hydroxide are in the same range as those of cobalt phosphate [48,50], it can be deduced that the cobalt in the corrosion product are cobalt (II) phosphate hydrate (Co3(PO4)2$8H2O) and cobalt hydroxide (Co(OH)2). The high resolution XPS spectra of Ag 3 d and Cl 2p obtained from the corrosion products are shown in Fig. 11. The two peaks at approximately 368.1 and 374.1 eV can be ascribed to the binding

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4. Conclusions Bulk nanostructured Co-Ag two phase alloys have been fabricated by a combination of high energy ball milling and warm pressing. Annealing at 900  C for 1 h increases the average Ag particle size from 10 nm to 58 nm and decreases the hardness from 530 (±20 H V) to 423 (±5 H V) but only slightly reduces the wear resistance. Distinct from Cu-Ag and Nb-Ag binary alloy systems, wear induced strain in Co-Ag immiscible alloys could not force the formation of self-organized nanolayered structure or a supersaturated solid solution. This is probably due to that slip in hcp-Co matrix is much more limited than in the matrix with fcc and bcc crystal structures. Electrochemical polarization tests show that such nanostructured Co-Ag alloys have no passive behaviour in ASS but with high corrosion current densities. The corrosion products are composed of insoluble cobalt (II) phosphate, cobalt hydroxide and silver chloride. Such corrosion behaviour in ASS suggests that the corrosion resistance of Co-Ag alloys should be further improved for dental applications. Acknowledgments

Fig. 10. High resolution XPS spectra of Co 2p, P 2p and O 1s obtained from the corrosion products.

This work was financially supported by the National Natural Science Foundation of China (Grant no. 51501087), the Fundamental Research Program of Shenzhen (Grant Nos. JCYJ20170412153039309, JCYJ20170307110418960 and JCYJ20160530185550416), Guangdong Innovative & Entrepreneurial Research Team Program (No. 2016ZT06C279), and Science and Technology Development Fund (FDCT) of Macau S.A.R. (Grant No. 095/2014/A2). 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. References

Fig. 11. High resolution XPS spectra of Ag 3 d and Cl 2p obtained from the corrosion products.

energies of Ag 3d5/3 and Ag 3d3/2 of silver chloride (AgCl), respectively. The Cl 2p peak in the wide scan spectrum can also be fitted by two typical peaks, Cl 2p3/2 at 198.0 eV and Cl 2p1/2 at 199.6 eV of AgCl, respectively [51]. Thus, Ag is present in the form of AgCl in the corrosion products. Based on above XPS analysis, we tentatively describe the phase compositions of the corrosion products are a mixture of Co3(PO4)2$8H2O, Co(OH)2 and AgCl. Preliminary evaluations on the corrosion behaviour of the fabricated nanostructured Co-Ag alloys show that the alloys do not show any passivation behaviour but with high corrosion current densities and produce the insoluble cobalt phosphate hydrate, cobalt hydroxide and silver chloride after electrochemical polarization tests. Therefore, such nanostructured Co-Ag alloys might not be suitable candidates for dental applications.

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