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Applied Surface Science 301 (2014) 410–417 Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: www.elsevier.com/locate/apsusc Microstructure and mechanical properties of multiphase layer formed during depositing Ti film followed by plasma nitriding on 2024 aluminum alloy F.Y. Zhang, M.F. Yan National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China a r t i c l e i n f o Article history: Received 28 November 2013 Received in revised form 12 January 2014 Accepted 16 February 2014 Available online 24 February 2014 Keywords: 2024 Al alloy Plasma nitriding Intermetallic coating Microstructure Wear resistance a b s t r a c t In this study, a novel method was develop to fabricate an in situ multiphase layer on 2024 Al alloy to improve its surface mechanical properties. The method was divided into two steps, namely depositing pure Ti film on 2024 Al substrate by using magnetron sputtering, and plasma nitriding of Ti coated 2024 Al in a gas mixture comprising of 40% N 2 –60% H 2 . The microstructure and mechanical properties of the multiphase layer prepared at different nitriding time were investigated by using X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), microhardness tester and pin-on-disc tribometer. Results showed that multiphase layer with three sub-layers (i.e. the outmost TiN 0.3 layer, the intermediate Al 3 Ti layer and the inside Al 18 Ti 2 Mg 3 layer) can be obtained. The thickness of the Al 18 Ti 2 Mg 3 layer increased faster than TiN 0.3 and Al 3 Ti layer with increasing nitriding time. The hardness of the layer has reached about 593 HV, which is much higher than that of 2024 Al substrate. The wear rate of the coated samples decreased 53% for 4 h nitriding and 86% for 12 h nitriding, respectively, compared with that of the uncoated one. The analysis of worn surface indicated that the coated 2024 Al exhibited predominant abrasive wear, whereas the uncoated one showed severe adhesive wear. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Aluminum alloys have numerous industrial applications due to their attractive properties such as light weight, high specific strength and favorable mechanical formability [1]. However, the widely spread use of these materials has been limited by their low hardness, poor wear resistance and high pitting corrosion tendency. Thus, various hard coatings with nitrides (TiN, AlN) [2,3], oxides (Al 2 O 3 ) [4] and Al-TM (transition element such as Ti, Cr, Fe, Nb and Ni) based intermetallics [5–9] were produced on Al alloy to improve its surface properties. Multiphase coatings containing nitrides and intermetallics have recently attracted increasing attention due to their increased microhardness and improved wear resistance. These coatings were Corresponding author at: Harbin Institute of Technology, School of Materials Science and Engineering, 92 West Dazhi Street, Nan Gang District, Harbin, PR China. Tel.: +86 451 86418617; fax: +86 451 86413922. E-mail addresses: [email protected] (F.Y. Zhang), [email protected], [email protected] (M.F. Yan). usually obtained by duplex treatment involving nitriding. Zhang et al. [10] have reported a TiN/Ti 3 Al composite coating produced by laser nitriding Al coated Ti6Al4V alloy, with increased hardness and modulus. Fe 3 Al based multiphase layer with AlN and Fe 4 N pre- pared by plasma nitriding aluminum layer has improved the surface hardness of plain carbon steel up to eight times [11]. Plasma nitrid- ing was also applied to electro deposited Ni–Cr coating to obtain increased surface hardness for the formation of nano-sized CrN par- ticles [12]. According to reference [13], the electrodeposited Ni–B coating can reach enhanced mechanical properties by gas nitrid- ing treatment. The corrosion resistance and surface hardness of Ni–Al coating were improved due to the formation of hard AlN par- ticles during plasma nitriding [14]. Thus, nitriding treatment could be an efficient method to enhance the properties of multiphase coatings. Several researches [15,6] have reported that Al alloy surface alloyed with Ti could produce Al–Ti based intermetallics, leading to considerable improvement in surface hardness, corrosion resis- tance and wear resistance. In addition, titanium is a strong nitride former alloying element. However, surface alloying the surface of Al with both Ti and N has seldom been investigated. http://dx.doi.org/10.1016/j.apsusc.2014.02.091 0169-4332/© 2014 Elsevier B.V. All rights reserved.

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Applied Surface Science 301 (2014) 410–417

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

icrostructure and mechanical properties of multiphase layer formeduring depositing Ti film followed by plasma nitriding on 2024luminum alloy

.Y. Zhang, M.F. Yan ∗

ational Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin50001, PR China

r t i c l e i n f o

rticle history:eceived 28 November 2013eceived in revised form 12 January 2014ccepted 16 February 2014vailable online 24 February 2014

eywords:024 Al alloylasma nitridingntermetallic coating

a b s t r a c t

In this study, a novel method was develop to fabricate an in situ multiphase layer on 2024 Al alloy toimprove its surface mechanical properties. The method was divided into two steps, namely depositingpure Ti film on 2024 Al substrate by using magnetron sputtering, and plasma nitriding of Ti coated 2024Al in a gas mixture comprising of 40% N2–60% H2. The microstructure and mechanical properties of themultiphase layer prepared at different nitriding time were investigated by using X-ray diffractometer(XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), microhardnesstester and pin-on-disc tribometer. Results showed that multiphase layer with three sub-layers (i.e. theoutmost TiN0.3 layer, the intermediate Al3Ti layer and the inside Al18Ti2Mg3 layer) can be obtained. Thethickness of the Al18Ti2Mg3 layer increased faster than TiN0.3 and Al3Ti layer with increasing nitriding

icrostructureear resistance

time. The hardness of the layer has reached about 593 HV, which is much higher than that of 2024 Alsubstrate. The wear rate of the coated samples decreased 53% for 4 h nitriding and 86% for 12 h nitriding,respectively, compared with that of the uncoated one. The analysis of worn surface indicated that thecoated 2024 Al exhibited predominant abrasive wear, whereas the uncoated one showed severe adhesivewear.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Aluminum alloys have numerous industrial applications dueo their attractive properties such as light weight, high specifictrength and favorable mechanical formability [1]. However, theidely spread use of these materials has been limited by their lowardness, poor wear resistance and high pitting corrosion tendency.hus, various hard coatings with nitrides (TiN, AlN) [2,3], oxidesAl2O3) [4] and Al-TM (transition element such as Ti, Cr, Fe, Nb andi) based intermetallics [5–9] were produced on Al alloy to improve

ts surface properties.

Multiphase coatings containing nitrides and intermetallics have

ecently attracted increasing attention due to their increasedicrohardness and improved wear resistance. These coatings were

∗ Corresponding author at: Harbin Institute of Technology, School of Materialscience and Engineering, 92 West Dazhi Street, Nan Gang District, Harbin, PR China.el.: +86 451 86418617; fax: +86 451 86413922.

E-mail addresses: [email protected] (F.Y. Zhang), [email protected],[email protected] (M.F. Yan).

ttp://dx.doi.org/10.1016/j.apsusc.2014.02.091169-4332/© 2014 Elsevier B.V. All rights reserved.

usually obtained by duplex treatment involving nitriding. Zhanget al. [10] have reported a TiN/Ti3Al composite coating producedby laser nitriding Al coated Ti6Al4V alloy, with increased hardnessand modulus. Fe3Al based multiphase layer with AlN and Fe4N pre-pared by plasma nitriding aluminum layer has improved the surfacehardness of plain carbon steel up to eight times [11]. Plasma nitrid-ing was also applied to electro deposited Ni–Cr coating to obtainincreased surface hardness for the formation of nano-sized CrN par-ticles [12]. According to reference [13], the electrodeposited Ni–Bcoating can reach enhanced mechanical properties by gas nitrid-ing treatment. The corrosion resistance and surface hardness ofNi–Al coating were improved due to the formation of hard AlN par-ticles during plasma nitriding [14]. Thus, nitriding treatment couldbe an efficient method to enhance the properties of multiphasecoatings.

Several researches [15,6] have reported that Al alloy surfacealloyed with Ti could produce Al–Ti based intermetallics, leading

to considerable improvement in surface hardness, corrosion resis-tance and wear resistance. In addition, titanium is a strong nitrideformer alloying element. However, surface alloying the surface ofAl with both Ti and N has seldom been investigated.

F.Y. Zhang, M.F. Yan / Applied Surface Science 301 (2014) 410–417 411

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Table 1Chemical composition of 2024 Al alloy (wt%).

ig. 1. X-ray diffraction patterns for Ti coated 2024 Al alloy before and after plasmaitriding.

In this study, a novel method was developed to fabricate a mul-iphase layer containing both nitride and intermetallics on 2024 Allloy, with the aim to harden its surface properties. The method cane divided into two steps, namely depositing pure Ti film on 2024 Alubstrate by using magnetron sputtering, and plasma nitriding of Tioated 2024 Al in a gas mixture comprising of N2 and H2. The effectf nitriding time on the microstructure and mechanical propertiesas also investigated.

. Experimental details

.1. Materials

Table 1 shows the chemical composition of 2024 aluminumlloy used in this study. The alloy was machined into substrate

Fig. 2. Surface morphologies for Ti coated sample: (a) before

Cu Mg Mn Fe Zn Si Ti Cr Al

3.81 1.40 0.40 0.40 0.14 0.22 0.05 0.05 Balance

samples (disk-shaped with diameter of 20 mm and height of 4 mm).The surface of each substrate was grinded using silicon carbidepapers from 400 to 2000 grade, and then polished with diamondpastes to a mirror-like finish. Before coating process, samples areultrasonically cleaned in acetone.

2.2. Coating process

The multiphase layer was fabricated through a duplex treatmentwith two-steps. In the first step, the pure Ti film was deposited ontothe 2024 Al alloy disk by using a closed field unbalanced magnetronsputtering ion plating system (UDP-450, Teer Coatings Ltd, UK) [16]with a high quality Ti target (99.999%, 330 mm × 134 mm × 8 mm).The substrates were placed on a rotary holder in the vacuum cham-ber (base pressure, 4 × 10−3 Pa), with a target-substrate distance of80 mm. Prior to deposition, the substrates were sputter cleaned for40 min in an Ar plasma discharge (sputtering pressure, 0.2–0.3 Pa)at a bias of −400 V. Then, the Ti film deposition was conductedunder the following conditions: target power E = 1.8 kW, substratebias voltage U = −70 V, deposition pressure P = 0.4–0.5 Pa, holdingtime t = 4 h and the substrate temperature T ranging from roomtemperature to 81 ◦C.

In the second step, plasma nitriding of Ti coated samples was

carried out in a plasma nitriding unit (LDMC-30, 30 kW) [17]. Beforethe application of a glow discharge, the chamber was evacuated tobelow 8 Pa by a rotary pump. Then, the nitriding treatments wereconducted in a gas mixture (N2 0.2 L/min + H2 0.3 L/min) at 430 ◦C.

nitriding, (b) after 4 h nitriding, (c) after 12 h nitriding.

412 F.Y. Zhang, M.F. Yan / Applied Surface Science 301 (2014) 410–417

ple: (a

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Fig. 3. Cross-section micrographs (BSE image) for Ti coated sam

wo levels of nitriding time (4 h and 12 h) were investigated. Theoltage and the working pressure during nitriding were 650 V and30 Pa, respectively. After the treatment, the samples were cooledown slowly inside the chamber with a nitrogen gas flow.

.3. Characterization

After plasma nitriding treatment, the crystal structure of theurface layer was examined by X-ray diffraction (type, Philips’Pert diffractometer) with Cu-K� radiation (� = 0.15406 nm). Theorking voltage is 40 kV and the scanning speed is 3◦/min. The

ross-sectional morphology of the surface layer is observed by acanning electron microscope (SEM, FEI QUANTI 200F). Addition-lly, the energy dispersive X-ray spectrometer (EDS) was usedor chemical composition analysis. Focused ion Beam (FIB, FEIelios 600 NanoLab) instrument was employed to prepare theross-sectional TEM specimen of the surface layer and the fineicrostructure was characterized by transmission electron micro-

cope (TEM, TECNAI G2 FEG, 200 kV).The surface hardness of the as coated sample was measured

sing a Vickers microhardness tester (HV-1000) at the indentationoad 10 g for 15 s. Five measurements were taken for each test andhe mean value was used.

The dry sliding wear tests were conducted using a pin-on-discear testing machine (POD-1). During wear tests, the cylindricalins of the 2024Al alloy (before and after treatment) were rotatedgainst a stationary hardened chromium steel (Rc64) ball of 5 mmiameter at the speed of 200 r/min (0.1 m/s) for 1200 s. The normalontact load was 2 N. All tests were performed in air and without

ubrication (temperature 20 ◦C, humidity 60% RH). The worn sur-aces and wear debris of selected samples were characterized usingEM equipped with energy dispersive X-ray. The weight loss waseasured by an electronic balance with minimum measuring range

) before nitriding, (b) after 4 h nitriding, (c) after 12 h nitriding.

of 0.1 mg. Weight wear rate WL, was calculated according to the fol-lowing equation: WL = �M/L × n, where �M (g) is the weight loss,L (N) is the contact load and n (r) is the total number of turns.

3. Results and discussions

3.1. Microstructure

Fig. 1 shows the XRD patterns of Ti coated 2024 Al alloy beforeand after plasma nitriding. It can be seen that Ti film with hexag-onal structure was obtained by magnetron sputtering method,which was in accordance with previous studies [18,19]. Afternitriding treatments, nitride (TiN0.3) and intermetallics (Al3Ti andAl18Ti2Mg3) formed on the surface of 2024 Al alloy. Obviously,the relative intensity of the newly formed phase increased withincreasing nitriding time. TiN0.3 has the lowest nitrogen contentamong all titanium nitrides and it was also detected on the sur-face of titanium alloys nitrided in the literatures [20,21]. Lengauerreported that TiN0.3 has a similar hexagonal structure to �-Ti, butthe former has a larger crystal lattice [22].Thus, in present study, itcan be noticed that TiN0.3 has a similar diffraction pattern to �-Ti,but the peaks of TiN0.3 show a left shift compared with that of �-Ti,due to the expansion of the crystal lattice [23]. In addition, Lengaueralso showed that the lattice parameters of TiN0.3 depended on thecomposition. With increasing the nitriding time, more nitrogenatoms could be solute in TiN0.3, thus, the peaks of TiN0.3 also shiftedto a much smaller 2� value after 12 h nitriding. The formation ofAl3Ti and Al18Ti2Mg3 indicated that diffusion reaction between

Ti film and Al alloy containing Mg occurred under present nitrid-ing temperature. In a word, a multiphase (TiN0.3/Al3Ti/Al18Ti2Mg3)layer has been successfully fabricated on the surface of 2024 Alalloy.

F.Y. Zhang, M.F. Yan / Applied Surface Science 301 (2014) 410–417 413

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Al3Ti and the large crystal was Al18Ti2Mg3 (the crystal structureof this ternary phase was reported in Ref. [29]). The TEM analyseswere in accord with the previous XRD and EDS results. Especially,

Fig. 4. Elements depth profiles of the multiphase la

Fig. 2 shows the surface morphologies of Ti coated Al alloy beforend after plasma nitriding. A dense Ti film with facet microstructurerystallized in the range of about 250 nm can be seen from Fig. 2a.fter plasma nitriding, fine particles with nano-scale size (about0 nm) formed on the larger facet particle. According to the XRDnalysis in Fig. 1, the fine particles could be TiN0.3 particles. Withncreasing the nitriding time, TiN0.3 particles grow larger and theitriding surface become denser and smoother.

The backscattered electron (BSE) images of the cross section ofhe Ti coated Al alloy before and after plasma nitriding are shownn Fig. 3. As can be seen from Fig. 3a, the deposited Ti film withbout 3.0 �m thickness is uniform and without diffusion layer at thelm–substrate interface. It can be seen from Fig. 3b and c that inten-ive interdiffusion between Ti film and Al alloy substrate occurredfter plasma nitriding. The single Ti film was converted into multi-hase layer with three distinct sub-layers on the surface of Al alloy.he multiphase layer was metallurgical bonded to the substrate. Inddition, no cracks and pores can be found in the layers.

The redistribution of the elements in the multiphase layer washown in Fig. 4. Combined with the XRD analysis, the outmost whiteayer with a high N content (up to 20 at%) refers to the TiN0.3 layer.

hile in the intermediate layer, the ratio of Al/Ti is lose to 3/1,ndicating the Al3Ti layer. As can be seen from Fig. 4, Mg atomsssembled as a horizontal step in the inside layer, in which the atomatio of Al: Mg: Ti is close to 18:2:3, indicating the Al18Ti2Mg3 layer.he above EDS analyses of the three layers are in good agreementith the XRD results in Fig. 1. Thus, it is to say that the multiphase

ayer is divided into three compound layers, i.e. the outmost TiN0.3ayer, the intermediate Al3Ti layer, the inside Al18Ti2Mg3 layer.he thickness of each sub-layer after plasma nitriding is shown inig. 5. With increasing nitriding time, the thickness of the outmostiN0.3 layer is nearly constant (about 2.1 �m). The intermediatel3Ti layer also grow slowly, the thickness of which increased from.8 �m for 4 h to 1.2 �m for 12 h. While the thickness of Al18Ti2Mg3

ayer increased apparently, from 2.2 �m for 4 h to 5.3 �m for 12 h.Obviously, the formation of the three compound layers was

elated to two reaction diffusion systems (i.e. N–Ti and Ti–Al). Asor N–Ti system, N atoms reacted with top surface of Ti film toorm TiN0.3 and diffused into Ti film to form continuous TiN0.3 layer,hich can be reflected by the N distribution profiles in Fig. 4. Simul-

aneously, interdiffusion between Ti and Al alloy substrate alsoccurred to form Al3Ti and Al18Ti2Mg3 compounds. The formationf Al based compounds inhibited the further diffusion of N into Al

ide. Thus, the N distribution profiles declined dramatically at thentersection of Ti and Al distribution profiles in Fig. 4. According torevious researches [24–26], Al3Ti was usually observed as the firsteactant in Ti/Al system due to its most negative formation energy

tained by different nitriding time: (a) 4 h, (b) 12 h.

than the other titanium aluminides. In addition, Al3Ti also occurredfirst in the Al–Ti–Mg ternary system based on Gao’s study [27].In present study, the formation of Mg rich compound Al18Ti2Mg3could result from the segregation of Mg atoms at the interface ofAl3Ti and Al. The similar phenomenon of Mg segregation in Fig. 4was also observed at the matrix/reinforcement interface in manyAl (Mg) matrix composites, which was related to the decrease ofinterface energy [28]. Due to the fast diffusion rate of Mg in Al, thegrowth of Al18Ti2Mg3 layer was more apparent than that of Al3Tiwith increasing nitriding time.

In order to further study the microstructure of the multiphaselayer, TEM observations were carried out (Fig. 6). Focused ion Beam(FIB) instrument was employed to prepare the cross-sectional TEMspecimen of the multiphase layer obtained by 12 h nitriding treat-ment. The overview microstructure on the cross section of themultiphase (TiN0.3/Al3Ti/Al18Ti2Mg3) layer is shown in Fig. 6a. Theoutside layer was found to be composed of fine particles with thesize about 200 nm (Fig. 6b), while the intermediate layer containedultrafine precipitates (<50 nm, Fig. 6c). In addition, large rectanglecrystals with the size range from 100 nm to 600 nm can be seen fromFig. 6d. The corresponding SAED patterns of the formation phasesare showed in Fig. 6e–g, which indicated that the fine particles inthe outmost layer was referred to TiN0.3, the ultrafine phase was

Fig. 5. TEM images of the multiphase layer obtained by 12 h nitriding: (a) BF micro-graph of the cross-sectional multiphase layer, (b)–(d) BF micrographs of the outsideTiN0.3 layer, the Al3Ti layer and the Al18Ti2Mg3 layer, (e)–(g) SAED patterns of TiN0.3,Al3Ti, and Al18Ti2Mg3.

414 F.Y. Zhang, M.F. Yan / Applied Surface Science 301 (2014) 410–417

he sub

tpstcs

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Fig. 6. The thickness of the Ti film and t

he fine microstructure of Al18Ti2Mg3 was firstly observed in theresent study. Moreover, this ultrafine-grained Al3Ti phase waseldom reported in previous studies. According to Musil’s investiga-ion [30], the ultra-fine grained Al3Ti and fine TiN0.3 crystals couldome from the fine structure of the deposited Ti film by magnetronputtering method.

The above experimental results indicated that two diffusioneaction systems were stimulated simultaneously to produce thehree compound layers, i.e. Ti–N system and Ti–Al (Mg) system. The

ost important role in this duplex treatment is the existence of Tiiffusion source on the surface of Al alloy substrate. Both nitridend intermetallic phase exhibit high hardness and strength. Thus,

he multiphase layer containing the two kinds of compounds wasxpected to exhibit good surface properties. In the following sec-ion, the mechanical properties of 2024 Al alloy coated with the

ultiphase layer were investigated.

-layers after plasma nitriding at 430 ◦C.

3.2. Mechanical properties

3.2.1. HardnessFig. 7 shows the surface hardness of 2024 Al alloy substrate and

the multiphase layer prepared at 430 ◦C for different time. It shouldbe noticed that a hardness of 593 HV was obtained on the multi-phase layer of the nitrided Al alloy, which is significantly higherthan that of the untreated substrate (98 HV) and Ti coated substrate(177 HV). However, the hardness of the multiphase layer increasesslightly with increasing nitriding time. In such a case, it could besay that the hardness of TiN0.3 increased a little with increasing theN content after 12 h nitriding.

3.2.2. Wear resistanceThe dry sliding wear behaviors of the uncoated and coated

samples were evaluated using a ball-on-disc tribometer. The

F.Y. Zhang, M.F. Yan / Applied Surface Science 301 (2014) 410–417 415

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Afcos

The SEM morphologies of the wear surface are shown in Fig. 10.

Fig. 7. Surface hardness of the uncoated and coated 2024 Al alloy.

riction coefficients vs sliding time curves of the test samples arehown in Fig. 8. It is noticed that the stable friction coefficients0.49) of the coated samples was lower than that (0.54) of thencoated one. In addition, compared with the uncoated Al alloyubstrate, the friction curves of samples coated with multiphaseTiN0.3/Al3Ti/Al18Ti2Mg3) layer exhibited smoother.

The larger fluctuations in the friction curve for the uncoatedl alloy, usually indicating sever adhesive wear behavior, resulted

rom the soft substrate. However, the hard fine TiN0.3/Al3Ti parti-

les could play a role of abrasive medium, leading to the decreasef the fluctuations of friction curves for the coated samples. Con-equently, the wear rate of the coated samples has a remarkably

Fig. 8. Frictional coefficient curves of the uncoated and coate

Fig. 9. Wear rate of the uncoated and coated 2024 Al alloy with a GCr15 ball under2 N for 1200 s.

decrease compared to the uncoated one (see in Fig. 9). Withincreasing the nitriding time, the friction coefficient and wear ratedecrease. The wear rate of the coated samples decreased 53% for 4 hnitriding and 86% for 12 h nitriding, respectively, compared withthat of the uncoated one. After nitriding for 12 h, the coated sam-ple exhibited the lowest friction coefficient (0.49) and wear rate(0.2 × 10−7 g r−1 N−1). Thus, it can be concluded that the multiphaselayer can significantly improve the wear resistance of 2024 Al alloy.

Obviously, the wear track width for coated samples was much nar-rower than that of the uncoated one, leading to the decrease inwear rate. Under the load of 2 N for 20 min, grooves in the sliding

d 2024 Al alloy with a GCr15 ball under 2 N for 1200 s.

416 F.Y. Zhang, M.F. Yan / Applied Surface Science 301 (2014) 410–417

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ig. 10. Worn surface morphologies of the uncoated and coated 2024 Al alloy with a

irection, plastic deformation regions and material smearing wereisible on the worn surface of all the tested samples. As can be seenrom Fig. 10a, the uncoated Al alloy exhibited typical characteristicf adhesive wear [31]. However, a notable reduction of craters, finerrooves, less smearing were observed on the worn surface of theoated samples (Fig. 10b and c), resulting in less material removedrom the tested surface. The scars in the worn surface for the coatedamples are evidence of abrasive wear caused by the hard particlesloughing into the pin.

To understand the changes that occurred on the sliding surface,he EDS analysis was carried out. As can be seen from the resultshown in Table 2, oxidation reaction occurred on the surface area (And B in Fig. 10a) of the uncoated alloy. As for the coated samples,he EDS results shown that the worn surface contained a high con-ent of Al (area C), indicating that the multiphase layer was wornhrough under present test condition. The hard particles detachedrom the TiN0.3/Al3Ti/Al18Ti2Mg3 layer can be observed in area D,hich is evidenced by the high content of Ti and N in Table 2. Thus,

he wear mechanism for the coated 2024 Al alloy under present test

ondition could be a mixture of abrasive wear and adhesive wear,hich can be also reflected by the lower fluctuation in the sliding

urves (Fig. 8).

able 2hemical composition of the marked area in Fig. 10 (at%).

Elements (at%) N O Mg Al Ti Fe Cu

A – 19.32 1.92 76.89 – 0.20 1.67B – 3.78 2.47 91.87 – 0.23 1.65C 1.30 18.94 3.33 69.51 4.65 0.26 2.01D 11.00 20.18 4.29 35.28 27.32 0.39 1.53

5 ball under 2 N: (a) the uncoated one, (b) after 4 h nitriding, (c) after 12 h nitriding.

The wear debris of the untreated Al alloy displayed metal-lic shining with white color while the treated ones exhibitedblack color. Fig. 11 shows the wear debris morphologies for thetested specimens. It can be seen from Fig. 11a, the worn debrisof the untreated Al alloy were large flake-like particles (about100 �m) with stratified structure, indicating a significant plas-tic deformation during wear test. However, the wear debris ofthe coated samples was much smaller, a mixture of fine pow-ders and small amount of large flake-like particles (Fig. 11b andc). The morphologies of the wear debris changed a little withincreasing nitriding time. The fine powders exhibited two typesof size as displayed in Fig. 11b. The rectangular particle (markedby area A) with the size of 10 �m contained high content of Nand Ti (Table 3), indicating the TiN0.3 particles. While the finerpowder in 5 �m size (area B in Fig. 11b) contains high Mg andCu content, indicating the intermetallic particles. The large flake-like particle (area C in Fig. 11b) with high content of Al and Oreferred to the material delaminated from the Al alloy substrate.According to the above analysis, the multiphase layer can sig-nificantly improve the hardness and wear resistance of 2024 Al

alloy. The coated 2024 Al alloy exhibited mainly abrasive wearbehavior.

Table 3Chemical composition of the marked area in Fig. 11 (at%).

Elements (at%) N O Mg Al Ti Cu Fe Cr

A 10.08 17.72 0.81 7.06 59.66 0.58 3.27 0.82B 1.52 14.12 11.37 62.14 4.32 6.31 0.16 0.06C 2.13 33.20 2.88 55.33 5.15 1.06 0.25 –

F.Y. Zhang, M.F. Yan / Applied Surface Science 301 (2014) 410–417 417

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ig. 11. Wear debris morphologies of the uncoated and coated 2024 Al alloy with a

. Conclusion

In this study, a novel method was developed to synthesize aultiphase layer on 2024 Al alloy. The microstructure evolution,

ardness and wear resistance of the multiphase layer with differentitriding time were investigated. The following conclusions can beighlighted:

(1) A multiphase layer was successfully obtained by plasmaitriding Ti coated 2024 Al alloy. The multiphase layer was madef three sub-layers, i.e. the outmost TiN0.3 layer, the intermediatel3Ti layer and the inside Al18Ti2Mg3 layer. The TiN0.3 and the Al3Tirystals were in nano-scale. The thickness of the Al18Ti2Mg3 layerncreased faster than TiN0.3 and Al3Ti layer with increasing nitridingime.

(2) The formation of the multiphase layer led to remarkablyncrease of surface hardness of 2024 Al alloy. The hardness of the

ultiphase layer increased slightly with increasing nitriding time.he friction coefficient of 2024 Al alloy coated with multiphaseayer decrease with increasing nitriding time. The wear rate of theoated Al alloy decreased nearly 86% compared with that of 2024l alloy. It indicated that the wear resistance of the 2024 Al alloyas improved significantly by the multiphase layer. The predomi-ant wear mechanism in coated 2024 Al alloy was abrasive wear,hereas severe adhesive wear in the uncoated one.

cknowledgments

The authors gratefully acknowledge the Specialized Research

und for the Doctoral Program of Higher Education of China (Granto. 20112302130006) and National Natural Science Foundationf China (Grant no. 51371070) for the financial support of thisesearch work.

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ball under 2 N: (a) the uncoated one, (b) after 4 h nitriding, (c) after 12 h nitriding.

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