High temperature plasma nitriding to modify Ti coated C17200 Cu surface: Microstructure and tribological properties

High temperature plasma nitriding to modify Ti coated C17200 Cu surface: Microstructure and tribological properties

Accepted Manuscript High temperature plasma nitriding to modify Ti coated C17200 Cu surface: Microstructure and tribological properties Y.D. Zhu, J.W...

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Accepted Manuscript High temperature plasma nitriding to modify Ti coated C17200 Cu surface: Microstructure and tribological properties Y.D. Zhu, J.W. Yao, M.F. Yan, Y.X. Zhang, Y.X. Wang, Y. Yang, L. Yang PII:

S0042-207X(17)30908-9

DOI:

10.1016/j.vacuum.2017.10.011

Reference:

VAC 7640

To appear in:

Vacuum

Received Date: 12 July 2017 Revised Date:

9 October 2017

Accepted Date: 10 October 2017

Please cite this article as: Zhu YD, Yao JW, Yan MF, Zhang YX, Wang YX, Yang Y, Yang L, High temperature plasma nitriding to modify Ti coated C17200 Cu surface: Microstructure and tribological properties, Vacuum (2017), doi: 10.1016/j.vacuum.2017.10.011. 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 proof before it is published in its final 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.

ACCEPTED MANUSCRIPT High temperature plasma nitriding to modify Ti coated C17200 Cu surface: Microstructure and tribological properties 1

Y.D. Zhu , J.W. Yao1, M.F. Yan *1, Y.X. Zhang1, Y.X. Wang 1, Y. Yang 1 and L. Yang1, National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and

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1

Engineering, Harbin Institute of Technology, Harbin, 150001, China *Correspondent author: Prof. M.F. Yan

O. Box 433, Harbin 150001, P. R. China Tel: 0086-451-86418617; Fax: 0086-451-86413922

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E-mail: [email protected], [email protected]

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Corresponding address: School of Materials Science and Engineering, Harbin Institute of Technology, P.

Abstract

In this work, we studied the microstructure formation and mechanical properties of the Ti alloyed C17200 Cu surface produced by plasma nitriding at 750

. The results

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firstly show that the multiphase coating consists of amorphous and nano-crystalline phases, and various types of Cu-Ti intermetallics, including CuTi2, CuTi, Cu3Ti2, . The formation of

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Be3Ti2Cu, Cu2Ti, Cu3Ti, formed after the plasma nitriding at 750

the specific type of Cu-Ti intermetallic depends on the processing duration, where a

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longer duration contributes to the formation of intermetallics with a higher Cu/Ti ratio. The evolution of the phase compositions is responsible for the change in the mechanical properties. The surface hardness of the modified C17200 Cu alloy improves to be 400 HV0.01 to a depth of 15 µm after plasma nitriding for 6 h, which contributes to a higher surface tribological resistance with a friction coefficient of 0.3 and a wear rate of 4.15×10-9 g/(r•N) for the modified suface.

ACCEPTED MANUSCRIPT Keywords: Plasma nitriding; Copper alloy; Cu-Ti intermetallics; Hardness; Tribological properties 1. Introduction

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C17200 Cu alloy has been widely applied in electronics, aeronautics and optical molding fields for its excellent conductivity and good machinability [1, 2]. However,

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the typical problem met by the devices is the insufficient wear resistance, where the dominated wear mechanism of the original Cu surface is severe adhesion wear [3, 4].

obtain better performance.

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Therefore, investigation on the surface modification of Cu alloy is of great necessity to

Among the different surface processing technology, plasma nitriding or ion nitriding has been widely used to modify the metal surface, including the Ti alloys, steel,

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Al alloys, since it was developed in 1930 [5-12]. However, for the natural incorporation of Cu with N and the thermo instability of Cu3N, direct nitriding of the Cu alloy seems

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to be ineffective. Based on the effective role of plasma nitriding on the surface modification of Ti alloy [13-15], Ti alloying is proposed and introduced to modify the

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Cu alloy surface by the plasma nitriding process [16, 17]. To make full use of this scheme, out Ti sources should be firstly introduced for the Cu alloys that do not contain Ti element. The basic concept is to fabricate a layer of titanium coating firstly, and plasma nitriding of the Ti coated substrate is then performed to fabricate a multi-phase coating, where the super hardness of the titanium nitride layer and the Cu-Ti intermetallics is attractive for the improvement of the tribological properties of the Cu

ACCEPTED MANUSCRIPT alloy [4, 18, 19]. In addition, it has been reported that surface hardening could be achieved for the Cu-Ti alloy at 800-850

plasma nitriding not only by the formation of

a nitride layer but also the solid solution [17, 20]. During the plasma nitriding and Ti

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alloying process at a specific temperature, the precipitation of the Cu-Ti intermetallics appeared [14], and the strengthening effects can further improve the mechanical

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properties of the fabricated coating by providing an optimal support for the hard layer [21]. Fig. 1 shows the crystal structure of the typical Cu-Ti intermetallics. Among the

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different Cu-Ti intermetallics, α-Cu4Ti, Cu3Ti2, Cu4Ti3, CuTi, CuTi2 and CuTi3 are tetragonal crystals, while β-Cu4Ti, Cu3Ti and Cu2Ti are orthorhombic crystals. In the previous work, the dependence of the microstructure and mechanical properties on the nitriding temperature has been explored, where the formation of Ti2N contributed to the

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obvious improved mechanical properties, but the microstructure and surface hardness of the multi-phase coating treated at 750

shows some peculiar results, even though there

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was no obvious titanium nitride layer in the coating [4]. However, the effects of plasma nitriding duration, as another significant factor that bears great influence on the

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micro-structure and mechanical properties, has not been investigated in Ref. [4]. Even though the surface hardness and wear resistance of the modified Ti alloy by plasma nitriding grew with increasing duration [13, 22], the conclusion might not apply to the surface treatment of Cu alloy. Actually, for the surface modification of Cu alloys by the duplex surface processing at a specific plasma nitriding temperature, the inter-diffusion of Cu-Ti atoms prompts the evolution of the microstructure, and the thickness of the

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existence of Cu also makes it different from the nitriding process of steel [23].

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Fig. 1 Crystal structures of the different Cu-Ti intermetallics, with the green balls and red balls representing Ti and Cu, respectively Based on the above discussion, the present study is conducted to investigate the microstructure formation and evolution mechanism of C17200 Cu surface during the Ti

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alloying process by plasma nitriding. Then, the influence of plasma nitriding duration on the tribological properties is explored. In addition, the microhardness which is

investigated.

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closely related to the tribological properties and its dependence on the treatment time is

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2. Materials and methods

The C17200 Cu substrate was prepared in a size of Φ20×5 mm, and the detail

chemical composition is listed in Table 1. C17200 Cu is an age-hardenable alloy and can be heat treated to produce a wide variety of microstructures [2]. It has been shown that the precipitation hardening can be achieved due to the phase transformation at 750-800

and the following quench into water [1, 21]. Therefore, before the substrates

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for 50 min and quenched into water to improve the mechanical

properties of the C17200 Cu substrate. The hardness of the Cu substrate after heat

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treatment can reach 85 HV0.01. Surface polishing of the sunbstrates was then undertaken by the SiC abrasive paper from 200# to 2000# to obtain a smooth surface, and the

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obtained surface roughness (Ra) was around 15 nm.

Table 1 Chemical compositions of the C17200 Cu-Be alloy (wt. %)

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Be Co Ni Fe Si Cu 1.96 0.10 0.12 0.15 0.12 Balance

Magnetron sputtering was firstly utilized to fabricate a 6.3 µm thick pure Ti film on the polished substrates and the detail processing parameters could be found in the previous study [4]. The magnetron sputtering process was conducted in the Ar gas at a

was undertaken at 750

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constant pressure of 0.49 Pa. Then, plasma nitriding of the Ti film coated Cu substrates for different duration (4 h, 6 h, 8 h) in a hollow cathodic

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plasma nitriding system (HCPN), and the specific parameters can be found in Ref. [4]. The mixture gas used during the plasma nitriding process was 50%N2+50%H2 at a

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pressure of 260 Pa.

X ray diffraction (XRD, D/max-2000) was firstly applied to examine the modified

surface. Both the Bragg-Brentano X-ray diffraction (BBXRD) and the grazing incidence X-ray diffraction (GIXRD) at 0.5° and 1° were conducted with CuKα radiation. The surface morphology of the fabricated multi-phase coating and its cross-section was then examined by scanning electron microscopes (SEM, FEI QUANTA 200F and SUPRA 55

ACCEPTED MANUSCRIPT SAPPHIRE), and the corresponding element composition was checked by the equipped energy dispersive spectrometer (EDS). Before the EDS examination, the samples were ultrasonic cleaned in acetone and alcohol for 10 min, respectively. The ZAF correction

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of the used SEM instrument was firstly conducted by default. During EDS analysis, the elemental composition both in Normalized at. % concentration and Normalized wt. %

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concentration was obtained. The standards for the quantitative analysis of the elements is from the database rather than the standard sample. In addition, the error is dependent

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on the content of the elements. For Cu and Ti (>20 wt. %), the relative error is less than 5%, while it even reaches more than 30% for other elements, such as N, Fe, O. Thus, only the distribution of Cu and Ti along the cross-sectional profile was analyzed. In addition, a transmission electron microscope (TEM, JEM-2100) was used to

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characterize the phase structure of the Cu-Ti-N multi-phase coating after plasma nitriding. To prepare the TEM sample, the sectional surface of the specimen was firstly

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ground by silicon paper of different grain size to obtain a thin sample, with the modified surface of the specimen left. Then, the ion thinning of the sectional side was undertaken

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to meet the requirement for the TEM observation. A punching equipment was then utilized to get a Φ3 mm TEM sample. Therefore, the examined area is the outmost surface for the TEM.

To measure the micro-hardness of the modified surface, a Vickers hardness indenter was applied, and three repeated times of indentation were conducted for each desired position. In addition, the wear resistance was investigated by the ball-on-disk

ACCEPTED MANUSCRIPT method, where tungsten carbide (WC) balls were applied as friction pairs, and the wear test was undertaken by the ball-on-disk method, with a load of 2 N and a sliding time of 1800 s at a rotational speed of 200 r/min. The surface morphology and element

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composition of the worn track was also examined by the scanning electron microscope, equipped with EDS. The wear rate of the prepared multi-phase coating is evaluated by

W = ∆M /( F ⋅ n )

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the following equation,

(1)

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where W is the wear rate (g·(N·r)-1), ∆M is the weight loss after the wear test (g), F is the load (N), n is the total rotation number of the sample. 3. Results and discussion

Fig. 2 shows the BBXRD results of the fabricated Ti coating by magnetron for different time. As

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sputtering and Cu-Ti-N coating after plasma nitriding at 750

shown in Fig. 2(a), crystalline titanium grains (α- and β-Ti) formed. The crystal structure of titanium at ambient temperature and pressure is close-packed hexagonal α

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phase with a c/a ratio of 1.587, but it undergoes the alpha-to-beta (Body-centered cubic allotropic form of titanium) transition at a higher temperature or with alloying elements.

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Copper of BCC structure (Body-centered cubic) is one of the beta stabilizers [24], and it promotes the formation of β-Ti. After 4 h of plasma nitriding, as shown in Fig. 2(b), varied types of Cu-Ti intermetallics appeared, including CuTi2, Cu3Ti2, CuTi and Cu3Ti, and the diffraction intensity for Cu3Ti2 is the strongest. In addition, due to the existence of Be element in the C17200 Cu alloy, the Be3Ti2Cu compounds also formed during the plasma nitriding. With the further increase of the plasma nitriding time, the peak intensity of Cu2Ti changed to be the strongest, and the diffraction peak for CuTi2, CuTi,

ACCEPTED MANUSCRIPT Cu3Ti2 became weak, as shown in Fig. 2(c). Finally, the phase composition of the modified surface shows that CuTi2 and Cu3Ti turn to be the main Cu-Ti intermetallics after the plasma nitriding for 8 h, as shown in Fig. 2(d). At the side of higher Ti

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concentration, CuTi2 and CuTi formed preferentially, while Cu3Ti appeared at the side of Cu substrate. This indicates that the composition of the multi-phase coating develops during the plasma nitriding process, which is attributed to the inter-diffusion of the Cu

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and Ti atoms.

Fig. 2 XRD results of the fabricated Ti coating and Cu-Ti-N multi-phases coating

after plasma nitriding at 750

for different time: a) Ti coating; b) after plasma nitriding

for 4 h; c) after plasma nitriding for 6 h; d) after plasma nitriding for 8 h Fig. 3 shows the grazing incidence X-ray diffraction (GIXRD) pattern for the multi-phase coating after the plasma nitriding for 4 h. It can be found that titanium nitride formed in the coating. However, the diffraction peaks for TixN around 2θ=37° is

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nitriding time for the processing of the Ti alloy or the substrates that did not consume Ti [9]. However, for the Cu substrate, the chemical reaction between Cu and Ti and the reaction between N and Ti competed with each other. The competitive reaction between

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Cu and Ti can limit the formation of Ti-nitride layer. Besides, the competition also led to

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the formation of the nanocomposites or the ternary solid solution [25, 26].

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Fig. 3 GIXRD (with the incidence angle of 0.5° and 1°) pattern of the modified surface after plasma nitriding for 4 h

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To get a further insight into the microstructure, the typical TEM image of the

multi-phase coating after plasma nitriding is shown in Fig. 4. The overall morphology of the multi-phase coating indicates that many particles formed in the coating, as shown in Fig. 4(a). Moreover, both nano-crystalline grains and amorphous region are identified by the selected area electron diffraction (SAED) result and the HRTEM images, as shown in Fig. 4(b)-(d). Specifically, the particles are actually in nanoscale, and the layer contains nano-crystalline TiN0.3, TiN and Fe3O4. The matrix is amorphous. As shown in

ACCEPTED MANUSCRIPT the EDS result of the surface, it can be found that an amount of Cu atoms appeared in the surface layer, but the TEM results did not show the information of crystal Cu. Presumably, the amorphous phase should be amorphous Cu with a certain amount of

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soluted Ti. Considering the particle reinforce effects, the dispersed nano-grains

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contributes to the obviously improved strength and hardness [27].

Fig. 4 (a) TEM image of the multi-phase coating after plasma nitriding for 4 h; (b)

SAED result of the multi-phase coating after plasma nitriding for 4 h; (c) and (d) HRTEM image of the nanocrystalline grain and amorphous region after plasma nitriding for 4 h Fig. 5 shows the surface morphology and the EDS result of the multiphase coating

ACCEPTED MANUSCRIPT after plasma nitriding for 4 h. As shown in Fig. 5(a), many finer grains appeared in the outmost layer after plasma nitriding. Much O element is detected in the fabricated coating, as shown in Fig. 5(b). According to Ref. [28] and [29], oxidation of the TiNx

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might appear to form rutile TiO2 at high temperature, which resulted in the decreasing TiNx amount. In the present work, the GIXRD and TEM results indicated that the no

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rutile TiO2 formed obviously in the multi-phase coating.

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Fig. 5 SEM image and EDS result of the the modified surface after plasma nitriding for 4 h

Fig. 6 shows the cross-sectional morphology and EDS results along the

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cross-sectional profile of the fabricated Cu-Ti-N coating by plasma nitriding. The surface morphology of the fabricated coating changed with increasing processing

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duration. As shown in Fig. 6(a), many black Cu-Ti intermetallic particles appeared in the multi-phase coating after plasma nitriding for 4 h. With the further increase of plasma nitriding time, the formed Cu-Ti intermetallics grows to be columnar or flake, and the coating is obviously stratified, as shown in Fig. 6(c) and (e). After the plasma nitriding of the pre-fabricated Ti coating on the C17200 Cu alloy, the Cu-Ti-N multiphase coating of varied thickness were obtained by the plasma nitriding at 750 for different time, as illustrated by the EDS results in Fig. 6. It can be found that the

ACCEPTED MANUSCRIPT Cu-Ti intermetallic layer were formed hierarchically. Combining the EDS with the XRD results, the specific and main composition of the fabricated coating can be obtained. It can be found that the overall Cu/Ti ratio for the fabricated coating after plasma nitriding

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for 4 h kept to be 2.3, as shown in Fig. 6(b), and the composition fluctuation should be caused by the distributed particles. The formation of Cu3Ti in the multi-phase coating should account for the higher Cu/Ti ratio, where the main Cu-Ti intermetallic is

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identified to be Cu3Ti2 by BBXRD, as shown in Fig. 2(b). For the surface after plasma nitriding for 6 h, the Cu/Ti ratio changed gradually, as shown in Fig. 6(d). CuTi

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intermetallic appeared at the outmost surface, and Cu2Ti phase occurred in the sub-layer, which was only detected in the surface after plasma nitriding for 6 h. With the further increase of the plasma nitriding time, the color of some areas in the multi-phase coating turned to be even the same with the Cu substrate, as shown in Fig. 6(e), and the atom

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ratio for Cu reaches 80%. On this basis, it is believed that the diffusion of Cu after the plasma nitriding for 8 h results in the formation of Cu grains again in the outmost surface. Compared with the previous studies [4, 30, 31], it can be readily seen that two

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types of surface coating could be obtained on Cu alloy by the plasma nitriding process, which is dependent on the processing parameters. One is TixN/Cu-Ti intemetallics/Cu

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substrate, and the other is the Cu-Ti intermetallics/Cu substrate with only small amount of titanium nitride. During the plasma nitriding at a lower temperature, an obvious TixN layer formed [4], where the 6.3 µm Ti film was not depleted and the Cu atoms did not diffuse to the surface to react with Ti atoms.

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Fig. 6 SEM images of the cross-section and EDS results along the cross-section profile after plasma nitriding at 750

for different time: a) and b) after plasma nitriding for 4 h;

c) and d) after plasma nitriding for 6 h; e) and f) after plasma nitriding for 8 h The formation energy of the Ti(Cu)N compounds and the solution energy of the N

ACCEPTED MANUSCRIPT atoms in Cu-Ti intermetallics could be calculated based on Eq. (2) and (3),

[

(

[

(

E f = Et − x ⋅ ETi + y ⋅ E N + z ⋅ ECu Es = Et − EtCuTi + z ⋅ E N

)] (x + y + z )

)] (x + y + z)

(2) (3)

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where Ef is the formation energy for Ti(Cu)N compound, Es is the solution energy of N in Cu-Ti compound, Et is the total energy of the calculated cell, EtCuTi is the total energy of the Cu-Ti cell, ETi, EN, and ECu is the energy per atom of the pure constituent in solid

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state for Ti, N and Cu, x, y, z is the number of Ti, N and Cu atoms, respectively. Based

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on the calculated Gibbs free energies of the titanium nitrides, as shown in Fig. 7(a), the formation of Ti0.3N, TiN and Ti2N are energy favorable, which has been identified after [4, 30, 31]. Moreover, the change of the formation energy for

plasma nitriding at 650

Ti(Cu)N compounds with increasing Cu atoms is shown in Fig. 7(b). It has been reported that the segregation of Cu around the TiNx grain boundaries prevents the

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growth of titanium nitrides as far as the content of Cu atoms reached an appropriate level [32, 33], and the mechanism of the formation of nano TiNx grains is proposed to

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be that Cu resulted in more nuclei of TiNx and thus smaller grains. Actually, the solution of N atoms is energy feasible, as shown in Fig. 7(c). It has been reported that the

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formation of the titanium nitride layer prevented the diffusion of N atoms [13], so the further formation of TixN is affected. The negative formation energy of the Cu-Ti intermetallics and the Ti-nitrides also shows good possibility between Cu and Ti, as well as N and Ti. Therefore, as the Cu atoms diffused into the pre-fabricated Ti coating, the competitive chemical reaction of Cu with Ti limited the formation of TiNx layer. Thus, the modified surface is mainly composed of Cu-Ti intermetallics. During the diffusion process of the Cu-Ti couple, the formed phase in the multi-phase coating changed with

ACCEPTED MANUSCRIPT the varied Cu and Ti content. Specifically, with the decrease of the Ti concentration, the Cu-Ti intermetallics appeared in the following order: CuTi2/CuTi/Cu3Ti2/Cu2Ti/Cu3Ti. Besides, the formation of the solutions of N in Cu-Ti intermetallics improves the

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strength and the hardness of the multi-phase coating.

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Fig. 7 (a) Formation Gibbs free energies of the titanium nitrides at different temperature; (b) the change of formation energy with Cu fraction for titanium nitrides; (c) the change

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of the solution energy for N in Cu-Ti intermetallics

Fig. 8 shows the measured hardness of the fabricated Cu-Ti-N multiphase coating

along the cross-sectional profile. It can be easily seen that the surface hardness of the modified C17200 Cu alloy changed depending on the formed compounds. After plasma nitriding for 4 h, the formation of Ti-nitride layer contributed to the improved hardness of HV0.01=825.00. The further increase of plasma nitriding time resulted in the formation of the higher Cu/Ti ratio intermetallics, and the Cu atoms even diffused into

ACCEPTED MANUSCRIPT the surface layer to form pure Cu grains, which both contributed to the decrease of the surface hardness. From the cross-sectional hardness shown in Fig. 8, it can be readily found that the hardness of the Cu-Ti intermetallic layer kept to be stable around

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HV0.01=400. In addition, the hardness of the C17200 Cu substrate kept to be higher after plasma nitriding for 6 h and 8 h than that of 4 h. This is attributed to the solid solution and the precipitation hardening during the cooling stage [21, 34].

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effects at 750

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Fig. 8 Micro-hardness of the fabricated Cu-Ti-N multi-phase coating along the cross-sectional profile: a) after plasma nitriding for 4 h; b) after plasma nitriding for 6 h; c) after plasma nitriding for 8 h

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The friction coefficient and wear rate of the fabricated multi-phase coating is

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shown in Fig. 9. The results showed that the modified surface after plasma nitriding for 6 h exhibits the lowest friction coefficient which is only about 0.3. Correspondingly, the wear rate of 4.1×10-9 g/(r·N) for the multi-phase coating after plasma nitriding for 6 h is also the most excellent. On the one hand, the coarse grain after plasma nitriding for 4 h contributed to the higher coefficient of friction (~0.5). However, for a longer plasma nitriding time (8 h), the evolution of the microstructure resulted in the increasing surface roughness which led to the higher friction coefficient. In addition, the diffusion of Cu atoms resulted in the formation of Cu clusters at the outmost surface, and the

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wear rate grew.

Fig. 9 (a) Friction coefficient and (b) wear rate of the fabricated multi-phase coating

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after plasma nitriding for 4 h, 6 h and 8 h

Generally, the wear behavior is closely related to the surface hardness which is dependent on the microstructure. The distribution of the layered structure of the multi-phase coating plays a significant role on the mechanical properties. For the

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modified surface after plasma nitriding for 4 h at 750

,the appearance of Ti-nitrides

contributed to the high hardness of the surface, but it dropped rapidly with increasing

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depth, where the thin Cu-Ti intermetallics sub-layer could not provide enough support during the wear test. So the material removal rate increased rapidly as soon as the

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surface layer was removed. In comparison, the surface hardness of the modified surface for plasma nitriding for 6 h and 8 h kept to be the same level, which is mainly attributed to the particle, pillar or flake Cu-Ti intermetallics formed during the plasma nitriding process. By comparing the surface morphology of the cross-section, it can be found that the Cu atoms gradually diffused into the surface with increasing plasma nitriding time to be 8 h. This leads to the decrease of the surface hardness and prevents the further

ACCEPTED MANUSCRIPT formation of Ti-nitrides. Therefore, the wear performance for the surface treated for 8 h became worse. From the surface morphology of the worn track, as shown in Fig. 10, it could be

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found that the wear mechanism changes for the coatings after plasma nitriding for different duration. Abrasive wear of the modified surface appeared for the specimen after plasma nitriding. In addition, the adhesion of WC materials during the wear test

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played a positive role to prevent the surface wear, as indicated by the EDS results listed in Table 2, and the weight also had effects on the calculated wear rate. However, the

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modified surface after plasma nitriding for 8 h experienced obvious surface peeling and shedding under the scratching effects of WC ball. Moreover, the longer processing time also resulted in the higher content of Fe element from the sputtering of the stainless steel mesh, and the oxidative activity caused the more serious surface oxidation during

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the wear test.

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Fig. 10 SEM image of the worn track of the fabricated multi-phase coating (a) after plasma nitriding for 4 h; (b) after plasma nitriding for 6 h; (c) after plasma nitriding for 8h

ACCEPTED MANUSCRIPT Table 2 Semi-quantitative EDS results for the area A-F (at.%) in Fig. 12 Area A

Area B

Area C

Area D

Area E

Area F

N O W Ti Fe Co Ni Cu

00.00 43.14 00.38 17.34 00.49 00.31 00.46 37.89

00.00 49.87 00.08 26.36 00.61 00.23 00.30 22.56

03.61 61.92 03.18 14.73 09.00 00.28 00.59 06.70

08.98 50.57 00.67 21.26 02.31 00.07 00.00 16.15

04.91 68.09 00.54 16.59 02.06 0.065 0.075 07.68

02.14 70.89 04.99 07.58 10.49 00.55 00.72 02.64

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Elements

4. Conclusions

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In the present work, surface modification of C17200 Cu alloy was conducted by duplex processing at a solid solution temperature of 750

and the dependence of

mechanical properties on the microstructure was investigated. Based on the results and

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discussion, the following conclusions could be achieved:

(1) The composition of the multi-phase coating prepared at the solid solution temperature of 750

is closely related to the processing duration. With the

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increasing processing duration, the main type of Cu-Ti intermetallic changed,

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and CuTi2/CuTi/Cu3Ti2/Cu2Ti/Cu3Ti/Be3Ti2Cu formed in the modified surface; (2) The Cu-Ti intermetallics coating on Cu substrate was successfully fabricated to modify the C17200 Cu alloy. The fabricated Cu-Ti multiphase coating is composed of nanocrstalline grains and amorphous regions. The competitive chemical reaction between Cu and Ti limited the formation of an obvious TixN layer in the fabricated coating; (3) The surface hardness of the C17200 Cu alloy was improved after plasma

ACCEPTED MANUSCRIPT nitriding by 6 h, which therefore contributed to the improved surface tribological resistance with the friction coefficient of 0.3 and the wear rate of 4.15×10-9 g/(r·N) for the modified suface. In addition, the wear mode was

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mainly abrasive wear for the modified Cu substrate which also caused the adhesion of WC.

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5. Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of

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China (Grant No. 51371070 and U1537201) for the financial support of this research work. References

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austenitic stainless steel produced by high temperature plasma nitriding in short time, Appl. Surf. Sci. 298 (2014) 243-250.

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[7] B.S. Yilbaş, A.Z. Şahin, A.Z. Al-Garni, S.A.M. Said, Z. Ahmed, B.J. Abdulaleem, M. Sami, Plasma nitriding of Ti  6Al  4V alloy to improve some tribological

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properties, Surf. Coat. Technol. 80 (1996) 287-292.

[8] P. Vissutipitukul, T. Aizawa, Wear of plasma-nitrided aluminum alloys, Wear 259 (2005) 482-489.

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conventional plasma nitriding of gamma-TiAl alloy, Vacuum 131 (2016) 89-96. [10] K. Kyzioł, K. Koper, M. Środa, M. Klich, A. Kaczmarek, Influence of gas mixture

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during N+ ion modification under plasma conditions on surface structure and mechanical properties of Al–Zn alloys, Surf. Coat. Technol. 278 (2015) 30-37.

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[11] K. Kyzioł, K. Koper, A. Kaczmarek, Z. Grzesik, Plasmochemical modification of aluminum-zinc alloys using NH3-Ar atmosphere with anti-wear coatings deposition,

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of Cu-3 at.% Ti alloy aged in a hydrogen atmosphere, Mater. Sci. Eng. A 517 (2009) 105-113.

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[15] K. Farokhzadeh, J. Qian, A. Edrisy, Effect of SPD surface layer on plasma nitriding of Ti-6Al-4V alloy, Mater. Sci. Eng. A 589 (2014) 199-208.

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[18] L. Yang, F.Y. Zhang, M.F. Yan, M.L. Zhang, Microstructure and mechanical properties of multiphase layer formed during thermo-diffusing of titanium into the

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surface of C17200 copper-beryllium alloy, Appl. Surf. Sci. 292 (2014) 225-230. [19] L. Liu, H.H. Shen, X.Z. Liu, Q. Guo, T.X. Meng, Z.X. Wang, H.J. Yang, X.P. Liu,

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Wear resistance of TiN(Ti2N)/Ti composite layer formed on C17200 alloy by

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[23] H. Aghajani, M. Torshizi, M. Soltanieh, A new model for growth mechanism of nitride layers in plasma nitriding of AISI H11 hot work tool steel, Vacuum 141

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(2017) 97-102.

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Press, Boca Raton, 2006.

[25] P. Patsalas, G. Abadias, G.M. Matenoglou, L.E. Koutsokeras, C.E. Lekka, Electronic and crystal structure and bonding in Ti-based ternary solid solution

1324-1330.

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nitrides and Ti-Cu-N nanocomposite films, Surf. Coat. Technol. 205 (2010)

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characterization of TiN-Cu films using EXAFS spectroscopy, Surf. Coat. Technol. 204 (2010) 1933-1936.

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ACCEPTED MANUSCRIPT titanium and plasma nitriding to modify C61900 Cu-Al alloy, Vacuum 126 (2016) 41-44. [31] Y.D. Zhu, M.F. Yan, Y.X. Zhang, H.T. Chen, Y. Yang, Microstructure formation

thermo plasma nitriding, Vacuum 134 (2016) 25-28.

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and evolution mechanism of Cu-Ti coating during dual-magnetron sputtering and

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Alloy. Compd. 728 (2017) 863-871.

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Jpn. Inst. Met. 10 (1969) 166-173.

ACCEPTED MANUSCRIPT CuTi2/CuTi/Cu3Ti2/Cu2Ti/Cu3Ti/Be3Ti2Cu mainly formed in the modified surface depending on the processing duration; The competitive reaction between Cu and Ti limits the formation of Ti-nitride

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layer; A friction coefficient of 0.3 and a wear rate of 4.15×10-9 g/(r·N) is obtained for

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C17200 Cu alloy;