Fretting wear and friction reduction of CP titanium and Ti–6Al–4V alloy by ultrasonic nanocrystalline surface modification

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    catier, tanded feateefcecimswere smaller and shallower compared to those of the untreated specimens. Surface

    at occutory tathantigue f

    components, engineering structures and aerospace parts that experi- treatment (SMAT) have already been identied as effective methods

    Surface & Coatings Technology 207 (2012) 135142

    Contents lists available at SciVerse ScienceDirect

    Surface & Coatin

    j ourna l homepage: www.e lseence vibration. Particularly, CP Ti and Ti6Al4V alloy that are mostlyused in aerospace, biomedical and other applications are quite suscepti-ble to fretting related failures. Despite their attractive mechanical andphysical properties CP Ti and Ti6Al4V alloy display relatively poorfretting andwear resistance [26]. This is due to their high surface ener-gy which promotes metal transfer, seizure and adhesive wear in tribo-logical applications. It is therefore important to improve the frictionand wear properties of these materials, particularly under frettingwear conditions [7,8]. In order to prevent fretting wear, modicationof the surface to improve the tribological properties is necessary. Tothis end, it has been a great challenge to develop an effective surface

    to enhance the ability of materials to resist fretting wear [1117]. TheSP process is probably the most popular surface modication tech-nique among the above cited techniques. It is also adopted generallyby industry due to its versatility in treating components of non-planargeometries. In the SP technique, spherical shots (balls) with sizes inthe range of 0.251mm are blasted onto the workpiece surface atimpact velocities in the range of 20150m/s under a controlled atmo-sphere [18]. The impact of the shots induces compressive residualstresses and work hardening to the surface region of the workpiece[19], but does not always generate a nanocrystalline surface layer[20,21].modication technique for fretting application

    Correspondence to: D.-E. Kim, Department of MUniversity, Seoul 120749, South Korea. Tel.: +82 2 212 Correspondence to: Y.-S. Pyun, Department of MechUniversity, Asan 336708, South Korea. Tel.: +82 41 53

    E-mail addresses: (D.-E. Kim), p(Y.-S. Pyun).

    0257-8972/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2012.06.046etting wear is commonlyat are used as machine

    shot peening (SP), laser shot peening (LSP), plasma nitriding (PN), plas-ma immersion ion implantation (PIII) and surface mechanical attritioncreating crack initiation sites on the surface. Frencountered in various types of materials thUltrasonic surface modication

    1. Introduction

    Fretting is a wear phenomenon thsolids are subjected to a relative oscilladisplacement amplitude typically lessdue to fretting wear can accelerate fa 2012 Elsevier B.V. All rights reserved.

    rs when two contactingngential motion of small100m [1]. The damageailure of components by

    Currently, the dovetail surfaces of compressor blades are coatedwith plasma sprayed coatings and dry lm lubricants to impede frettingwear and prolong the life of the blades and disks [9]. However, there aremany on-going investigations with the purpose of developing longerlasting coatings for compressor parts [10]. Surface modication tech-niques such as ion-implantation (II), laser beam quenching (LBQ),Fretting wear analysis was performed using a scanning electron microscope (SEM).Ti6Al4V alloyFriction theUNSM-treated specimenFretting wear and friction reduction of CPnanocrystalline surface modication

    Auezhan Amanov a,b, In-Sik Cho c, Dae-Eun Kim d,, Ya Department of Mechanical Engineering, Sun Moon University, Asan 336708, South Koreab Center for Nano-Wear, Yonsei University, Seoul 120749, South Koreac Department of Hybrid Engineering, Sun Moon University, Asan 336708, South Koread Department of Mechanical Engineering, Yonsei University, Seoul 120749, South Korea

    a b s t r a c ta r t i c l e i n f o

    Article history:Received 17 January 2012Accepted in revised form 14 June 2012Available online 26 June 2012

    Keywords:CP Ti

    Application of surfacemodito reduce friction. In this papcially pure titanium (CP Ti)(UNSM) technique. Lubricaton untreated and UNSM-trfretting wear and friction coto those of the untreated sps.

    echanical Engineering, Yonsei3 2822; fax: +82 2 312 2159.anical Engineering, Sun Moon0 2333; fax: +82 41 530

    rights reserved.tanium and Ti6Al4V alloy by ultrasonic

    ng-Sik Pyun a,

    on techniques is expected to be a viable solution tomitigate fretting damage andhe aimwas to improve the fretting wear and friction characteristics of commer-Ti6Al4V alloy by using an ultrasonic nanocrystalline surface modication

    retting wear and friction tests were conducted with a ball-on-at congurationd specimens using silicon nitride (Si3N4) balls. The results showed that theient characteristics of the UNSM-treated specimens were improved comparedens. Moreover, it was found that the fretting wear scar diameter and depth of

    gs Technology

    v ie r .com/ locate /sur fcoatRecently, a new surface modication technique called ultrasonicnanocrystalline surface modication (UNSM) which utilizes an ultra-sonic vibration at 20kHz was developed. The UNSM technique involveshigher kinetic energies than the other techniques cited above since theball (tip) strikes the workpiece surface under a high frequency of20kHz. Also, the surface roughness of the specimen after the UNSMtreatment tends to be much smoother than what can be achieved withthe SP process. In the UNSM technique, the ball (tip) trace can be

  • controlled by a computer numerical control (CNC) machine. Subse-quently, UNSM technique can result in homogenous microstructure,thicker nanocrystalline and work-hardened surface layers, and deepersurface regions with high compressive residual stress. Also, the thick-ness of the nanocrystalline surface layer can be controlled with betteraccuracy than process such as SP.

    In this work, UNSM technique was applied to CP Ti and Ti6Al4Valloy materials in order to improve their tribological properties. TheUNSM is a novel surface modication technique which improves the

    the phase has a BCC structure. In Fig. 1 the white phase is and

    Table 2Chemical composition (wt.%) of CP Ti and Ti6Al4V alloy specimens.

    Material C N Fe Al V H O Ti

    CP Ti 0.1 0.03 0.3 0.015 0.25 99.305Ti6Al4V alloy 0.01 0.006 0.2 6.47 3.89 0.17 89.254

    Table 3Mechanical properties of CP Ti and Ti6Al4V alloy specimens.

    Values CP Ti Ti6Al4V alloy

    UTS (MPa) 434 1020Yield Stress, Y02 (MPa) 275 970Density (g/cm3) 4.52 4.4Elongation (%) 20 14

    136 A. Amanov et al. / Surface & Coatings Technology 207 (2012) 135142tribological properties of interacting surfaces in relative motion. De-tailed description of the UNSM process and its effects on metal andalloy properties as well as microstructure is available in the literature[2225].

    In previous studies, fretting wear properties of CP Ti and Ti6Al4Valloy were investigated at a frequency range from 2 to 300Hz in ambi-ent environment at room temperature [2629]. However, all frettingwear tests in this study were conducted at a high-frequency of 20kHzby using a newly developed fretting wear test rig. The high-frequencyfretting wear behavior of UNSM-treated and untreated specimensmade of AISI304 stainless steel was investigated in a previous study[23]. However, the effect of UNSM on the tribological properties ofCP Ti and Ti6Al4V alloy was expected to be different from that ofAISI304 steel because of the difference in the material structureand properties. The fretting wear mechanism and tribological charac-teristics of the specimens were investigated systematically throughcontrolled fretting tests and rigorous surface characterization. Theobjective of this research was to improve the friction and frettingwear characteristics of CP Ti and Ti6Al4V alloy by applying theUNSM technique.

    2. Experimental details

    2.1. Ultrasonic nanocrystalline surface modication process

    UNSM is a technique that can be used to improve the mechanicalsurface properties ofmetals and alloys. The technique utilizes ultrasonicvibration energy. The principle of UNSM is based on the instrumentalconversion of harmonic oscillations of an acoustically tuned body intoresonant impulses of ultrasonic high-frequency. The energy generatedfrom the oscillations is used to impact the workpiece surface from20,000 to 40,000 shots per square millimeter. The roughness of theworkpiece can be readily controlled by varying the impact load duringthe UNSM process. In this work, the CP Ti and Ti6Al4V alloy speci-mens were treated under the UNSM conditions as shown in Table 1.

    2.2. Materials and test conditions

    The materials under investigation in this study were CP Ti andalpha/beta Ti6Al4V alloy that are widely used in aeronautics, espe-cially for the compressor blades and disks, and surgical implants,automotive and marine parts, reactor vessels and heat exchangers.Their chemical composition and mechanical properties are given inTables 2 and 3, respectively. It should be noted that Al helps to stabi-lize the alpha phase, while V stabilizes the beta phase [30]. Fig. 1shows the microstructure of CP Ti and Ti6Al4V alloy identifyingthe alpha/beta phases. The phase has an HCP structure whereas

    Table 1UNSM treatment process conditions for CP Ti and Ti6Al4V alloy specimens.

    Material Frequency,kHz




    Feed rate,mm/rev


    CP Ti 20 30 30 30 0.07 2.38Ti6Al4V alloy 60the gray phase is . The bright spots in phase are due to the etchingeffect.

    The friction coefcients of both untreated and UNSM-treated diskspecimens were obtained using a microtribometer (UMT-2 CETR,USA) wi