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Surface & Coatings Technology

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Effect of different surface asperities and surface hardness induced by shot-peening on the fretting wear behavior of Ti-6Al-4V

Qi Yanga, Wenlong Zhoua, Zhiqiang Niua, Xiaobing Zhenga, Qi Wanga, Xuesong Fua,⁎,Guoqing Chena, Zhiqiang Lib

a Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University ofTechnology, Dalian 116085, PR Chinab Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, PR China

A R T I C L E I N F O

Keywords:Shot-peeningTi-6Al-4VFretting wearSurface delamination

A B S T R A C T

An experimental investigation on the fretting wear of Ti–6Al–4V was conducted using a self-designed test ma-chine. The wear morphologies and the cracking phenomena of as-received and shot-peened specimens werecomparatively analyzed by the combining the applications of a laser scanning confocal microscope and ascanning electron microscope. The results showed that shot-peening exhibited an increase in the wear volumeduring the early fretting wear period, while it reduced the material loss in the long-term fretting wear process.The dominant wear mechanism was transformed from adhesion/peeling to delamination in both the as-received/shot-peened specimens. The wear rates under the peeling and delamination mechanism was explained by theequivalent friction coefficient of the contacting point and the thickness of cracked surface layer. The optimalstrengthening effect was obtained under a moderate shot-peening intensity of 0.3mmA, which produced aproper curvature radius of asperity and a good combination between hardness and toughness of the surfacematerial.

1. Introduction

Fretting is surface damage that is generated from low amplitudeoscillatory sliding between two contacting components. The removal ofthe surface material caused by fretting action is defined as the frettingwear, which could accelerate the crack nucleation rate, resulting in asignificant reduction in fatigue resistance of many metals and alloys [1,2]. The reduced life of machine components under fretting-fatiguewhen compared to plain fatigue (conventional fatigue without fretting)is shown in a number of studies [3–5]. Fretting damage often occurs atbolted and riveted joints in various structures and machine parts, par-ticularly in disk/blade attachments in gas turbine engines where highcontact stresses and small displacements are induced, due to the pro-longed centrifugal loading and high-frequency vibration [6–8]. Thecontacting forms are currently divided into three types: sphere-planecontact, cylinder-plane contact, and plane-plane contact. Consideringthe great hazards triggered by the premature failure of machine com-ponents, particularly for engine components, researchers have in-vestigated the fretting behavior of various metals and alloys [9–13]. Amajority of the equipment that is used to simulate the fretting conditionof practical engineering situations are self-designed. Venkatesh et al.

[9] designed a test fixture that obtained sphere-plane contact with afretting action. Fouvry et al. [10, 11] and Everitt et al. [12] conductedfretting tests using cylinder-plane contact arrangements. Livingstoneand Mcpherson [13] developed a plane-plane test device to evaluate thefretting damage in complete contacts. Previous test apparatuses couldonly accomplish a certain contact form. A test device that couldtransform the contact form through changing various types of frettingpads have not been published.

Shot-peening is a well-known surface treatment that prevents crackinitiation and propagation in components subjected to fatigue andfretting fatigue loading [14]. Shot-peening alters the physical and themechanical properties of a material. These alterations include: (1) in-troducing a residual compressive stress in the material surface and acompensating residual tensile stress in the inner substrate, (2) alteringthe surface roughness of the material, and (3) stimulating work hard-ening of the material near the surface [15]. The effects that differentsurface modifications caused via shot-peening in plain fatigue havebeen commonly investigated [16–18]. A high surface roughness likelyresults in a significant stress concentration and accelerates the crackinitiation. Compressive residual stresses could retard crack propagationas the dominant effect in a property improving regime. There is debate

https://doi.org/10.1016/j.surfcoat.2018.06.092Received 16 May 2018; Received in revised form 27 June 2018; Accepted 30 June 2018

⁎ Corresponding author.E-mail address: xsfu@dlut.edu.cn (X. Fu).

Surface & Coatings Technology 349 (2018) 1098–1106

Available online 02 July 20180257-8972/ © 2018 Published by Elsevier B.V.

T

over the role of strain hardening in crack propagation [16, 17]. Thebeneficial effects are believed to be obtained under proper shot-peeningintensities. If the shot-peening is too extreme, the surface integritytends to degrade the fatigue performance, due to the low toughness ofthe surface material with a higher notch sensitivity [18, 19]. The effectthat the surface roughness and the strain hardening induced by shot-peening has on the fretting wear behavior of Ti-6Al-4V is not ofteninvestigated.

In this paper, a fretting test device was developed for the frettingwear experiments. This device primarily aimed to imitate the actualfretting condition of the dovetail joints in gas turbine engines. Thedevice allowed the convenient transformation between various contactgeometries and the regulating operation on various fretting wearparameters such as normal load, vibration frequency, and relative dis-placement. The micromorphology of the fretting region, the subsurfacecracks, and the wear volume of the as-received (AS) and the shot-peened (SP) specimens were observed and analyzed with a combinationof a scanning electron microscope (SEM) and a laser scanning confocalmicroscope (LSCM). The effect that the various surface asperities andsurface hardness induced by shot-peening had on the fretting wear

behavior of Ti–6Al-4V were evaluated and discussed based on the ex-perimental results.

2. Experimental

2.1. Materials and specimens

The material used in the experiments was an alpha/beta titaniumalloy (Ti–6Al–4V) commonly used in aeronautics, particularly in fanblades and disks. Both specimens and fretting pads were machinedusing a Ti-6Al-4V alloy. Four types of specimens were studied: AS, SPwith the intensity of 0.2 mmA (SP-0.2), SP with the intensity of0.3 mmA (SP-0.3), and SP with the intensity of 0.4mmA (SP-0.4). TheAS specimens were machined milled to achieve a similar surface state asthe actual aerospace components. The SP specimens were mechanicalpolished prior to SP treatments. The cylinder fretting pads (10mm by5mm) were manufactured by turning and polishing. The SP specimenswere annealed at 400 °C for 2 h to eliminate residual stress induced byshot-peening.

Fig. 1. Fretting wear test apparatus.

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2.2. Fretting tests

The fretting wear tests were conducted on a self-designed test ma-chine, as seen in Fig. 1. A cylinder-on-flat configuration was used tointroduce the fretting effect. The cylinders could be replaced with a ballor with blocks to change the contact form. The applied normal load wasmaintained at 100 N. The displacement amplitude was set as 150 μm,accompanied with the frequency of 10 Hz. The test device was fittedwith an automatic control system to sustain the accuracy and the sta-bility during long period testing. Each type of specimen was fretted forvarious times between 5min to 120min. This type of experimentalmode is more conducive to observing and analyzing the evolution of thefretting wear behavior. All of the experiments were performed at room

temperature in a dry condition under ambient atmosphere.

2.3. Fretting scar examinations

After each test, the specimens were ultrasonically cleaned withethanol for 10min to remove all dust and wear debris. The wear scars ofeach specimen were examined on a LSCM, as shown in Fig. 2. The threedimensional (3D) surface profiles were performed to determine thewear volume (V=? μm3) generated on the specimen. The two di-mensional (2D) wear profiles were extracted to obtain the average weardepth of each scar (H=? μm). These profiles were in the transversaldirection (i.e. perpendicular to the sliding direction) in the middle of ascar. The 3D and 2D wear profile analysis provided representative

Fig. 2. Schematic illustration of the fretting scar examination.

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evidence of the local wear damage degree in the interface and alloweddirect comparisons between specimens with various surface conditions.

3. Results and discussion

3D surface scans of the specimens prior to the fretting were madeusing a ZYGO 3D surface profiler. The 1×1mm close-up for the var-ious types of specimens is shown in Fig. 3. Each type of specimen had aunique topography, making them easily distinguishable. The surface ofthe AS specimen retained the shallow and narrow grooves followingmachine milling (Fig. 3(a)). Specimens treated with shot-peening hadsimilar characteristics, the generation of surface asperities, as shown inFigs. 3(b)–(d). The differences in shot-peening intensities were clearlyreflected in the number and shape of the asperities. The quantity ofasperities formed under 0.2mmA was larger than when generated in0.3 mmA and 0.4mmA. While the maximum height of the asperitiesincreased from 6.97 μm to 13.97 μm as the shot-peening intensity rosefrom 0.2mmA to 0.4mmA. The surface scans were used to measureroughness values for each type of specimen. The results are shown inFig. 3. The shot-peening significantly promoted the surface roughnessof the specimen, where the roughness value increased with the in-creased shot-peening intensity. The roughness values generally re-flected the differences in surface topography between the various spe-cimens.

Fig. 4 shows profilometer surveys of the AS and SP surfaces prior tofretting. The typical asperity dimensions under various shot-peeningintensities were acquired. Both the width and the height of asperityincreased with the increasing shot-peening intensities. The 0.3mmAasperities were more spherical, with a greater radius in the asperity roofthan the 0.2mmA and 0.4 mmA asperities. This feature could reducethe stress concentration in the fretting contact.

Fig. 5 shows the surface hardness gradients of the shot peenedspecimens. The hardness near the surface was stimulated by the shot-peening treatment. The maximum hardness value was at least 55 HVhigher than the matrix hardness around 300 HV. A strain hardeninglayer was formed in the surface of specimen subjected to shot-peening.The depth of strain hardening layer increased from 90 μm to 135 μm asthe shot-peening intensity rose from 0.2mmA to 0.4 mmA.

Fig. 6 shows the wear volume of AS and SP specimens in 5, 10, 20,40, 60, and 120min tests. The wear volume of the AS specimen (WAS)

was less than the SP specimens (W0.4 for 0.4 mmA, W0.3 for 0.3mmA,and W0.2 for 0.2 mmA) during the early fretting stage prior to 60min.The wear rate of the AS specimen seemed to increase gradually as thefretting duration increased and became obviously larger than that theSP specimens. The wear volume of the AS specimen transformed to themaximum of all the specimens after 240min fretting. Similar experi-mental results were obtained by K. Kubiak and S. Fouvry et al. [20, 21],who also conducted the fretting wear test with a cylinder/plane con-figuration device. The quantitative results showed that the fretting wearvolume of the AS specimen was initially lower and subsequently higherthan the SP specimen along with the increased cumulated dissipatedenergy (proportion to the test duration). They indicated that shot-pe-ening has little impact on the wear resistance, since the final wearvolumes were close for the AS and the SP specimens. The value of theAS specimen was slightly higher. They attributed the results to the se-vere contact strain loadings and suggested additional investigations toexplain this behavior [21]. We found that shot-peening distinctly im-proved material's fretting wear resistance. The strengthening effectappeared to dominate during the long-term fretting process. During theearly stage (within a certain amount of fretting cycles), shot-peeningcould lead to a reverse effect that accelerates the material loss in thecontact region. Fu et al. [22] performed a series of long-term frettingtests of Ti-6Al-4V with a cylinder/plane contact geometry. The ex-perimental fretting cycles ranged between 106 and 107. The resultsrevealed that shot-peening significantly reduces the wear volume whenthe fretting cycles reaches 107. The leading fretting wear mechanism ofthe AS and the SP specimens in the early stage and long-term processeswere likely different. The corresponding analysis for wear mechanismsis detailed in the following section. The early wear volume of the0.2 mmA and the 0.3mmA specimens were approximately the same andlower than the 0.4 mmA specimen. After 60min, the wear volume andthe wear rate of the 0.3 mmA seemed to be less than the other two SPspecimens, until the entire fretting process was finished. The results forSP specimens will be explained in this study, based on the theoreticalcontact model of the surface asperity and the cross-sectional observa-tions of subsurface cracks.

The results of the 3D morphology measurement for the wear regionunder various fretting stages were collected and analyzed. The evolu-tion of wear behavior was represented using the typical wear mor-phology collected after 20min, 40min, and 60min, as shown in Fig. 8.

Fig. 3. 3D surface profiles and roughness values of specimens.(a) AS, (b) 0.2 mmA, (c) 0.3 mmA, and (d) 0.4mmA.

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Prior to 20min, the dominant wear mechanism of the AS specimenseemed to be adhesion, since the morphology of the material trans-ferred (noted by the blue dotted ellipse) is observed in Fig. 7(a1). Theadhesion was generated by the formation of “cold welding points” at

some small contact regions of the fretting pads. When the “cold weldingpoints” were broken, due to the continuous fretting action, they mayfall off and transform into wear debris. Others will cause materialtransfer between the contact surfaces. SP specimens prior to 20min hadsome surface asperities induced by shot-peening that appeared to becrushed due to the fretting action. It was probably that the shot-peeningtreatment increased the surface hardness but reduced the toughness ofthe surface material. The surface asperities ere easily broken whensubjected to severe contact stress under the fretting condition. The leftasperities are noted by the red dotted ellipses in Fig. 7(b1), Fig. 7(c1),and Fig. 7(d1). The leading wear mechanism of the SP specimens in theinitial fretting stage was the peeling of asperities. When the specimenswere fretted for 40min, the surface delamination appeared to becomethe primary wear mechanism. Small wear pits formed at the delami-nated positions. The pits were observed in the corresponding 3D tmorphology image, which are noted by black dotted ellipses inFig. 7(a2). For SP specimens, asperities that formed under 0.2 mmAwere peeled off and the surface delamination began, as shown inFig. 7(b2). However, some of the asperities of the 0.3mmA and the0.4 mmA specimen remained after 40min (Fig. 7(c2) and Fig. 7(d2)).When the fretting continued for 60min, the specimens were dominatedby the wear mechanism of the surface delamination, as shown inFig. 7(a3)–(d3). The specimens should enter into the stable stage for along-term fretting wear process, until the macrocracks appeared on thecontact regions.

Fig. 8 and Fig. 9 show the SEM images of the AS and the SP spe-cimens after 20min, 40min, 60 min, and 240min of fretting. Fig. 9(a)shows the typical morphology of the smooth pit that was produced bythe “cold-welding point” in adhesion prior to 20min. The pit was sur-rounded by some wear debris that came from the other broken “cold-welding points” adjacent to the pits. Fig. 9(b) exhibits the flake-mentaldelamination on the surface after 60min of fretting. The wear debriswas continuously ground under the fretting condition when the frettingduration reached 120min, as shown in Fig. 9(c). After 240min, afretting crack originated at the contact edge where severe stress con-centration existed. Fig. 10(a) and Fig. 10(b) show the peeling of surfaceasperities in the SP specimens prior to 40min. Fig. 10(c) shows thecharacteristics of the surface delamination after 60min, which was si-milar to the peeling behavior. We believe that the differences betweenpeeling and delamination primarily existed in three aspects: the peelingcould occur in a short time due to the severe local stress concentration.The formation of delamination requires a certain number of frettingcycles. The delamination was often observed after peeling during thefretting process. The delamination was formed by the initiation and thepropagation of subsurface cracks [23, 24]. Cracks that extended parallelto the fretting surface were found in the subsurface via cross-sectionobservations [23–26]. Peeling was caused by the local material crush ofcontact asperities were not companied by subsurface cracks. Thepeeling tended to generate as small shallow pits with irregular shapes,as shown in Fig. 9(a) and Fig. 9(b). The pits produced by delaminationwere larger than the pits caused by peeling (Fig. 9(c)). Fig. 10(d) showsthat no cracks were found in the SP specimens after 240min of fretting.The amount of wear debris was generated and accumulated in thefretting area. The SEM images were in good accordance with the 3Dmorphology analysis shown in Fig. 7, providing additional evidence forthe conclusions of wear mechanism evolution of the AS and the SPspecimens. The fretting wear performance of the material was improvedby the shot-peening treatment in the long-term fretting process. Thisimprovement was reflected not only in the wear volume reduction, butalso in the resistance to the fretting crack initiation in accordance withthe experimental results shown in Figs. 6–9. The AS and the SP speci-mens were dominated by the adhesion and the asperity peeling duringthe early fretting wear stage. The fretting wear behavior during thelong-term fretting process was led by the surface delamination in boththe AS and the SP specimens. During the early fretting stage, the re-lative displacement in AS specimen was possibly constrained by the

Fig. 4. 2D surface profiles of specimens and typical asperities of SP surfaces.(a) AS, (b) 0.2 mmA, (c) 0.3 mmA, and (d) 0.4mmA.

Fig. 5. Surface hardness gradients for the shot peened specimens.

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adhesive effect. The materials were likely to transfer from the cylinderpad to the specimen surface. These effects induced by adhesion pro-vided a reasonable explanation for the low material loss of the ASspecimen during the early fretting stage. The surface asperities inducedby shot-peening in the SP specimens were rapidly peeled, due to thegreat local stress concentration, which led to the relatively higher wearvolume compared to the AS specimen.

Fig. 10 shows the wear volume analysis of the SP specimens duringthe asperity peeling stage using the model of contact point proposed byKragel'Skiĭ [27]. This model is based on the elastic-plastic contactprinciple. The contact point of asperity was simplified as a sphere-planeconfiguration, the curvature radius of asperity was r, the pressed depthwas h, and the radius of the contact region was a. The equivalentfriction coefficient of the top of the asperity under the plastic condi-tions, fM, was as follows:

⎜ ⎟≈ ∙ = ∙ ∙⎛⎝

⎞⎠

f h rr

Ncσ

0.55 / 0.31 1M

T

12

(1)

N is the normal stress on a single asperity, c is the material coefficient,and σT is the yield strength after strain hardening. The value of c ∗ σTwas proven to be roughly equal to the value of the local microhardness

Fig. 6. Evolution of the wear volume of various specimens versus the frettingduration.

Fig. 7. Fretting behavior evolution of the specimens elucidated by a typical 3D wear morphology: (a1)–(a3) AS specimen, (b1)–(b3) 0.2 mmA specimen, (c1)–(c3)0.3 mmA specimen, and (d1)–(d3) 0.4 mmA specimen.

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of the SP material [27]. Kragel'Skiĭ indicated that the normal stress Ncould be regarded as a constant in the theoretical analysis. Theequivalent friction coefficient fM was primarily influenced by the as-perity curvature radius r and the local micro hardness HV. Fig. 10shows that the curvature radius of asperity in the 0.2mmA and the0.4 mmA specimens were smaller than in the 0.3 mmA specimen.

Considering the rank of surface hardness: HV0.4 > HV0.3 > HV0.2, itwas determined that the equivalent friction coefficient of the 0.4 mmAspecimen was the highest of all the SP specimens. For the 0.2 mmA andthe 0.3 mmA specimens, their values of equivalent friction coefficientwere approximately identical, due to the opposite rank orders in thecurvature radius and the microhardness. This theoretical analysis for fM

Fig. 8. SEM images of the typical wear morphologies of AS specimen in various fretting stages.(a) AS, (b) 0.2 mmA, (c) 0.3 mmA, and (d) 0.4mmA.

Fig. 9. SEM images of the typical wear morphologies of SP specimen in various fretting stages.(a) AS, (b) 0.2 mmA, (c) 0.3 mmA, and (d) 0.4mmA.

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provided a qualitative explanation for the wear behavior of the SPspecimens in the early fretting stage (Fig. 6) when the dominant me-chanism was asperity peeling.

Fig. 11 shows the cross-sectional SEM images of the fretting regionin the AS and the SP specimens after 240min of fretting. Subsurfacecracks were observed in all the specimens. The cracks were about2–8 μm beneath the surface and propagated along the parallel direction

to the surface. These cracks were initially observed and investigated bySuh et al. [23, 24], where they are widely recognized as the basic ex-perimental evidence for surface delamination. The cracked surfacelayer would become flake-like wear sheets when the subsurface crackpropagated to the fretting surface. The observational results wereconsistent with the above analysis in Figs. 7–9 that stated delaminationwas the dominant wear mechanism for long-term fretting process in

Fig. 10. Wear volume analysis of the SP specimens during the asperity peeling stage using the model of contact point proposed by Kragel'Skiĭ [27].

Fig. 11. Cross-sectional SEM images for the subsurface cracks of the AS and the SP specimens after 240min of fretting.(a) AS, (b) 0.2 mmA, (c) 0.3 mmA, and (d) 0.4mmA.

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both the AS and the SP specimens. In addition, it was observed inFig. 11 that the thickness of the cracked surface layers differed invarious specimens. The maximum thickness was about 8 μm, which wasfound in the AS specimen. The minimum value was 2.5 μm, which ex-isted in the 0.3mmA specimen. According to the proposed delamina-tion theory, the wear rate of metals was proportional to the thickness ofthe delaminated wear sheets [23, 24]. There exists a possibility to usethe thickness of the cracked surface layer to characterize the wear be-haviors of various specimens under the delamination mechanism. Thethickness of the delaminated sheets shown in Fig. 11 agreed well withthe wear behavior of the corresponding specimens for the long-termfretting process (after 60min) shown in Fig. 6. The different wear ratesand wear volumes of the specimens under the delamination mechanismwas explained by the various thickness of the cracked layers on thefretting surfaces. It seems that the shot-peening treatment reduced thethickness of the delaminated sheets to improve the fretting wear re-sistance of Ti-6Al-4V during the long-term fretting process. Theminimum thickness of the delaminated sheets was obtained at a mod-erate intensity of 0.3 mmA. This phenomenon indicated that neither asevere or weak shot-peening was conducive to reducing the damage ofdelamination. A weak shot-peening is likely not enough to improve thelocal yield strength of the material surface, which results in a poorresistance to the fretting wear. The severe shot-peening could havesignificantly reduced the toughness of the surface material, whichmotivated the subsurface crack to propagate at a deeper layer via someunknown mechanisms. It was reasonable to assume that the thickness ofthe wear sheets was effectively reduced when the shot-peening treat-ment produced a good combination between the hardness and thetoughness of the surface material. The leading mechanism for thethickness of the cracked surface layer requires further investigations inthe future studies.

4. Conclusion

An experimental investigation on the cylinder/plane fretting wearbehavior of Ti–6Al–4V was conducted using a self-designed test ma-chine. The test device was fitted with an automatic control system thatensured the accuracy and the stability of the long period testing andallowed convenient transformation of the various contact geometries.The micromorphology characteristics of the specimen surface wereanalyzed by the combining application of LSCM and SEM. The com-parison analysis for the wear and cracking phenomena of the AS and theSP specimens were performed and the effect that shot-peening had onthe fretting wear behavior of Ti–6Al–4V was evaluated. The wear rateof the surface peeling and the surface delamination stages were quali-tatively analyzed based on the local friction coefficient analysis of thecontacting asperities and the cross-sectional SEM observation results forthe subsurface cracks. The conclusions taken from the study were asfollows:

• The self-designed equipment was stable during the long-term fret-ting process. The wear scars were clear with regular configuration.

• The surface roughness and the strain hardening layer in Ti-6Al-4Vwere fabricated via shot-peening. The larger shot-peening intensityled to the higher energy and velocity of the shots. The value of thesurface hardness and roughness both increased as the shot-peeningintensity rose.

• Shot-peening led increased the material loss during the early frettingstage. Shot-peening improved and reduced the fretting wear volumeof Ti-6Al-4V during the long-term fretting process.

• During the early fretting stage, the wear behavior of the AS and theSP specimens were dominated by the mechanisms of adhesion andasperity peeling. The AS and the SP specimens were led by thesurface delamination mechanism during the long-term frettingprocess.

• The wear rate of asperities was probably influenced by the

equivalent friction coefficient of the contacting point, which wasrelated to the curvature radius of the surface asperity and the localmicro hardness of the surface layer. The equivalent friction coeffi-cient were theoretically analyzed according to the elastic-plasticcontact model.

• The wear rate under the delamination mechanism were character-ized by the thickness of the cracked surface layer. The minimumthickness of the delaminated sheets was obtained at a moderateintensity that produced the best combination between the hardnessand the toughness of the surface material.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 51405059), Foundation of LiaoningEducational Committee (LZ2015014) and the Fundamental ResearchFunds for the Central Universities (DUT17GF209).

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Xuesong Fu, Doctor, School of Materials Science and Engineering, Dalian University ofTechnology, Dalian 116085, P. R. China.

Q. Yang et al. Surface & Coatings Technology 349 (2018) 1098–1106

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