Surface & Coatings Technology 207 (2012) 135–142Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Fretting wear and friction reduction of CP titanium and Ti–6Al–4V alloy by ultrasonic nanocrystalline surface modification Auezhan Amanov a, b, In-Sik Cho c, Dae-Eun Kim d,⁎, Young-Sik Pyun a,⁎⁎ a Department of Mechanical Engineering, Sun Moon University, Asan 336‐708, South Korea Center for Nano-Wear, Yonsei University, Seoul 120‐749, South Korea Department of Hybrid Engineering, Sun Moon University, Asan 336‐708, South Korea d Department of Mechanical Engineering, Yonsei University, Seoul 120‐749, South Korea b c a r t i c l e i n f o a b s t r a c t Application of surface modification techniques is expected to be a viable solution to mitigate fretting damage and to reduce friction. In this paper, the aim was to improve the fretting wear and friction characteristics of commercially pure titanium (CP Ti) and Ti–6Al–4V alloy by using an ultrasonic nanocrystalline surface modification (UNSM) technique. Lubricated fretting wear and friction tests were conducted with a ball-on-flat configuration on untreated and UNSM-treated specimens using silicon nitride (Si3N4) balls. The results showed that the fretting wear and friction coefficient characteristics of the UNSM-treated specimens were improved compared to those of the untreated specimens. Moreover, it was found that the fretting wear scar diameter and depth of the UNSM-treated specimens were smaller and shallower compared to those of the untreated specimens. Surface analysis was performed using a scanning electron microscope (SEM). © 2012 Elsevier B.V. All rights reserved. Article history: Received 17 January 2012 Accepted in revised form 14 June 2012 Available online 26 June 2012 Keywords: CP Ti Ti–6Al–4V alloy Friction Fretting wear Ultrasonic surface modification 1. Introduction Fretting is a wear phenomenon that occurs when two contacting solids are subjected to a relative oscillatory tangential motion of small displacement amplitude typically less than 100 μm [1]. The damage due to fretting wear can accelerate fatigue failure of components by creating crack initiation sites on the surface. Fretting wear is commonly encountered in various types of materials that are used as machine components, engineering structures and aerospace parts that experience vibration. Particularly, CP Ti and Ti–6Al–4V alloy that are mostly used in aerospace, biomedical and other applications are quite susceptible to fretting related failures. Despite their attractive mechanical and physical properties CP Ti and Ti–6Al–4V alloy display relatively poor fretting and wear resistance [2–6]. This is due to their high surface energy which promotes metal transfer, seizure and adhesive wear in tribological applications. It is therefore important to improve the friction and wear properties of these materials, particularly under fretting wear conditions [7,8]. In order to prevent fretting wear, modification of the surface to improve the tribological properties is necessary. To this end, it has been a great challenge to develop an effective surface modification technique for fretting applications. ⁎ Correspondence to: D.-E. Kim, Department of Mechanical Engineering, Yonsei University, Seoul 120‐749, South Korea. Tel.: +82 2 2123 2822; fax: +82 2 312 2159. ⁎⁎ Correspondence to: Y.-S. Pyun, Department of Mechanical Engineering, Sun Moon University, Asan 336‐708, South Korea. Tel.: +82 41 530 2333; fax: +82 41 530 2307. E-mail addresses:
[email protected] (D.-E. Kim),
[email protected] (Y.-S. Pyun). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.06.046 Currently, the dovetail surfaces of compressor blades are coated with plasma sprayed coatings and dry film lubricants to impede fretting wear and prolong the life of the blades and disks [9]. However, there are many on-going investigations with the purpose of developing longer lasting coatings for compressor parts [10]. Surface modification techniques such as ion-implantation (II), laser beam quenching (LBQ), shot peening (SP), laser shot peening (LSP), plasma nitriding (PN), plasma immersion ion implantation (PIII) and surface mechanical attrition treatment (SMAT) have already been identified as effective methods to enhance the ability of materials to resist fretting wear [11–17]. The SP process is probably the most popular surface modification technique among the above cited techniques. It is also adopted generally by industry due to its versatility in treating components of non-planar geometries. In the SP technique, spherical shots (balls) with sizes in the range of 0.25–1 mm are blasted onto the workpiece surface at impact velocities in the range of 20–150 m/s under a controlled atmosphere [18]. The impact of the shots induces compressive residual stresses and work hardening to the surface region of the workpiece [19], but does not always generate a nanocrystalline surface layer [20,21]. Recently, a new surface modification technique called ultrasonic nanocrystalline surface modification (UNSM) which utilizes an ultrasonic vibration at 20 kHz was developed. The UNSM technique involves higher kinetic energies than the other techniques cited above since the ball (tip) strikes the workpiece surface under a high frequency of 20 kHz. Also, the surface roughness of the specimen after the UNSM treatment tends to be much smoother than what can be achieved with the SP process. In the UNSM technique, the ball (tip) trace can be Amanov et al. In this work.01 N 0. Material Frequency. In this work. In previous studies.4 14 CP Ti 20 Ti–6Al–4V alloy . was calculated from the wear scar diameter using the approximate equation given by Halling [31]: V ¼ πRh ð1−h=4RÞ 2 4 ð1Þ where R is the radius of the ball and h is the maximum depth of the wear scar. UNSM technique was applied to CP Ti and Ti–6Al–4V alloy materials in order to improve their tribological properties. Ball diameter.2 Al – 6. V. and surgical implants.47 V – 3. the CP Ti and Ti–6Al–4V alloy specimens were treated under the UNSM conditions as shown in Table 1. Also.305 89. However. while V stabilizes the beta phase [30]. The high-frequency fretting wear behavior of UNSM-treated and untreated specimens made of AISI304 stainless steel was investigated in a previous study [23]. The technique utilizes ultrasonic vibration energy.17 Ti 99. kHz μm load. respectively. The surface roughness was measured using a surface profilometer (Mitutoyo SJ-400). The friction coefficients of both untreated and UNSM-treated disk specimens were obtained using a microtribometer (UMT-2 CETR. The wear volume loss. all fretting wear tests in this study were conducted at a high-frequency of 20 kHz by using a newly developed fretting wear test rig. It should be noted that Al helps to stabilize the alpha phase.%) of CP Ti and Ti–6Al–4V alloy specimens. the thickness of the nanocrystalline surface layer can be controlled with better accuracy than process such as SP. In friction and fretting wear tests.136 A. / Surface & Coatings Technology 207 (2012) 135–142 Table 2 Chemical composition (wt. 2. All experiments were conducted at a constant test frequency of 20 kHz for up to 1×10 6 cycles. all the specimens were prepared to have the same surface roughness value of Ra = 0. Under the very high-frequency of 20 kHz. fretting wear properties of CP Ti and Ti–6Al–4V alloy were investigated at a frequency range from 2 to 300 Hz in ambient environment at room temperature [26–29].6mm were used as the counter surface in all the friction tests and silicon oil was used as a lubricant. The α phase has an HCP structure whereas the β phase has a BCC structure. especially for the compressor blades and disks. Detailed description of the UNSM process and its effects on metal and alloy properties as well as microstructure is available in the literature [22–25].07 2. As shown in the schematic the test specimen was mounted on a fixture and placed on top of the ball attached to the ultrasonic horn.000 shots per square millimeter. Fig. Silicon nitride (Si3N4) balls with a diameter of 1.89 H 0. The energy generated from the oscillations is used to impact the workpiece surface from 20.2. and deeper surface regions with high compressive residual stress.1 0. Values UTS (MPa) Yield Stress.03 0.38 Table 3 Mechanical properties of CP Ti and Ti–6Al–4V alloy specimens. At least three tests were conducted under each experimental condition. 2.015 – O 0. 1 shows the microstructure of CP Ti and Ti–6Al–4V alloy identifying the alpha/beta phases.52 20 Ti–6Al–4V alloy 1020 970 4.006 Fe 0. The UNSM is a novel surface modification technique which improves the tribological properties of interacting surfaces in relative motion. Amplitude. The fretting contact consisted of a nominally flat specimen with dimensions of 40×20× 10 mm in contact with a Si3N4 ball with a diameter of 12. In Fig.10 μm in order to eliminate the surface roughness effect on the tribological behavior. Static Spindle Feed rate. The oscillatory motion of the ball was controlled by the ultrasonic booster system. The bright spots in α phase are due to the etching effect. Ultrasonic nanocrystalline surface modification process UNSM is a technique that can be used to improve the mechanical surface properties of metals and alloys. UNSM technique can result in homogenous microstructure. Subsequently. Therefore.25 0. the temperature of the contact zone may increase significantly and this may affect the fretting wear mechanism due to oxidation as well as alteration in the mechanical properties of the specimen.1. in order to prevent high temperature at the contact zone the contact region was cooled by an air spraying system. Fretting experiments were conducted using a newly developed high-frequency fretting wear test rig under the conditions as shown in Table 5. The peak-to-peak displacement (stroke) was automatically maintained to be constant throughout the test by a photonic sensor. 2. The principle of UNSM is based on the instrumental conversion of harmonic oscillations of an acoustically tuned body into resonant impulses of ultrasonic high-frequency.7 mm and a hardness of HV 1700. Experimental details 2. However. The objective of this research was to improve the friction and fretting wear characteristics of CP Ti and Ti–6Al–4V alloy by applying the UNSM technique. All the fretting tests were performed under lubricated conditions at room temperature with a relative humidity of 42%. 1 the white phase is β and the gray phase is α. mm/rev mm rpm N 30 30 60 30 0. h is related to the measured mean wear scar diameter by the following equation: 1=2 2 2 h ¼ R− R −d =4 ð2Þ Table 1 UNSM treatment process conditions for CP Ti and Ti–6Al–4V alloy specimens. The average fretting wear scar dimensions measured using a surface profilometer were used for calculating the fretting wear volume loss. The fretting wear mechanism and tribological characteristics of the specimens were investigated systematically through controlled fretting tests and rigorous surface characterization. automotive and marine parts. reactor vessels and heat exchangers. Materials and test conditions The materials under investigation in this study were CP Ti and alpha/beta Ti–6Al–4V alloy that are widely used in aeronautics. USA) with a ball-on-disk configuration under the conditions as shown in Table 4. The normal load was applied by dead weight. Their chemical composition and mechanical properties are given in Tables 2 and 3. A schematic of the highfrequency fretting wear test rig is shown in Fig. thicker nanocrystalline and work-hardened surface layers.000 to 40. Material CP Ti Ti–6Al–4V alloy C 0. the effect of UNSM on the tribological properties of CP Ti and Ti–6Al–4V alloy was expected to be different from that of AISI304 steel because of the difference in the material structure and properties. The roughness of the workpiece can be readily controlled by varying the impact load during the UNSM process. speed.254 controlled by a computer numerical control (CNC) machine.3 0. σY02 (MPa) Density (g/cm3) Elongation (%) CP Ti 434 275 4. 6 mm. For SEM observations.51 1 15 5.42 2 2.62 10 00. Microstructure of CP Ti (a) and α + β Ti–6Al–4V alloy (b). . the specimens were again ultrasonically cleaned with acetone and ethanol for 5min each to eliminate the wear debris trapped in the fretting scar. / Surface & Coatings Technology 207 (2012) 135–142 137 (a) (b) α β α+β Elem ment Ti K Total l We eigh ht% 100 0. 1.13 87 7. An average of at least four measured fretting wear scar diameter values were used for calculating the wear scar depth. mm 6 Maximum contact pressure.A. all the specimens were cleaned with acetone and ethanol for 5min each to remove all the contaminants from the surface. where d is the mean wear scar diameter.68 Testing track radius.41 1 1.51 Temperature.51 2.0 00 α-ph hase e tom mic% % At 10 0. .33]. °C 21 .49 Ti–6Al–4V alloy 0. This method of quantifying fretting wear has been employed in other works as well [32. mN 50 Rotational speed.3 36 E Elem ment t A Al K T Ti K VK F Fe k T Tota al Weig ght% % W 4.00 0 E Elem ment t A Al K T Ti K VK T Tota al W Weig ght% % 5. rpm 100 Sliding distance. Prior to testing.17 78 8.2 25 76 6. Amanov et al.00 0 100 0. . The microstructure of the fretted surfaces was analyzed using an SEM (JEOL JSM-5610).00 0 Ato omic c% 100 0. the specimens were polished with 1 μm diamond paste and etched with a solution of 2ml HF+5ml H2O2 +100ml H2O. The Vickers microhardness Table 4 Ball-on-disk friction test conditions performed using a silicon nitride ball with a diameter of 1.6 61 Fig. .94 14 4.0 00 β-ph hase e At tom ic% % 7. GPa CP Ti 0. After the tests.20 1.91 10 00. Normal load.96 91. m 37. In order to obtain the residual stress values with respect to depth the surface layers of the specimens were removed by electropolishing which can remove the top surface layer without inducing additional residual stress. Thus. 3. As shown in Fig. 3 and 4. 4 that some compressive stresses exist at the surface of the untreated specimens. 2. N 100. 3 by noting the depth at which the microhardness equaled to that of the untreated specimen.34].96.7 mm. X-ray diffraction (XRD) is the most appropriate method for quantifying the residual stress produced by surface treatments [35]. 1. while the residual stress of the untreated specimens stabilized at a depth of 60 μm. Results and discussion 3. The depth of the hardened layer could be estimated from Fig. However. This is attributed to the surface finishing process that was used to prepare the specimen. 2. respectively.55 measurement was carried out using Mitutoyo HM-103 micro-Vickers hardness testing machine on the specimens at a load of 50gf. The residual stress measurement was performed using the X 3000 (X stress 3000) equipment with a tube current of 40mA at a tube voltage of 40kV. the highest compressive residual stress at the top surface of CP Ti and Ti–6Al–4V alloy reached up to − 1279. Schematic of the high-frequency fretting wear test rig.24 and 2.138 A. it is Fig. 200. The type and magnitude of the compressive residual stress state is directly related to the machining and polishing conditions which vary significantly with the technique used [40]. 300 and 400 Displacement amplitude. The high residual compressive stresses are beneficial for increasing the fretting wear resistance and the high hardness can be helpful to deter mechanical surface damage [36]. respectively.55. compared with those of the untreated specimens suggests that the surface and subsurface deformation in these specimens may be attributed to the greater extent of dislocation generation. GPa CP Ti 1.6. The magnitude of compressive residual stress decreased with increasing depth from the top surface.47 Ti–6Al–4Valloy 1. also demonstrated by using a thin plate bending method that surface finishing procedures such as polishing and grinding can generate compressive residual stress on the workpiece surface [38]. Relatively low load and speed conditions were applied during the specimen cutting process in order to minimize the possibility of specimen surface modification. Chou et al. Vickers-hardness and residual stress tests for the untreated and UNSM-treated CP Ti and Ti–6Al–4V alloy specimens were obtained as shown in Figs. / Surface & Coatings Technology 207 (2012) 135–142 Table 5 Fretting wear test conditions performed using a silicon nitride ball with a diameter of 12. Amanov et al. Normal load.7 MPa. while the hardness of Ti–6Al–4V alloy increased from 328 HV to 379 HV. The increase in hardness due to UNSM treatment can be attributed to both grain refinement and work-hardening effects on the surface layer following the Hall–Petch relationship [17]. as a result of the UNSM treatment due to local plastic deformation and increased strain hardening. respectively. the depth of the hardened layer was about 176 μm and 200 μm for the CP Ti and Ti–6Al–4V alloy specimens. the observation of increased compressive residual stress in the UNSM-treated specimens. 4 that the depth of the hardened layer become equal at a depth of about 250 μm since the residual stress of the UNSM-treated specimens kept decreasing as a function of depth from the top surface. the psi-splitting X-ray diffraction method was applied to determine the residual stress along the axial direction of the specimen by using the diffraction pattern of the Fe (2 1 1) crystal plane obtained by Cu Kα radiation. .1. Microhardness and compressive residual stress results The microhardness and compressive residual stress values with respect to depth from the surface were obtained to assess the effective depth of UNSM treatment. 3 that at the top surface the hardness of CP Ti increased from 146HV to 193 HV. 2. 2. The presence of compressive residual stress at the surface is normally thought to be a result of local plastic deformation [39]. cycles 1×10 6 Maximum contact pressure. showed that surface finishing can introduce large compressive residual stress at the surface [37]. It could be seen from Fig.4 and − 1142. The specimens were cut perpendicular to the surface using a diamond cutter and microhardness tests were performed on the cross section of the specimens. In this study.31and 2. It could also be found from Fig.02. However. Zhao et al. It has been previously reported that improvement in surface hardness by SMAT was beneficial in increasing the friction and fretting wear resistance [17. it was also shown in Fig. However. μm ±30 Frequency. 4. kHz 20 Fretting time. This plastically deformed layer leads to strain hardening and induces compressive stress at the surface of the metallic materials [43]. the friction coefficient fluctuated after it . 5(b) shows the typical microstructure of plastically deformed CP Ti after the UNSM treatment. The grain size was measured by analyzing the electron backscatter diffraction (EBSD) observations using the TexSEM Laboratories (TSL) orientation imagining microscopy (OIM) Analysis 5 Program. respectively. which is a software for EBSD data acquisition and processing.8 μm. The grain size measurement results revealed that the untreated CP Ti specimen had an initial grain size of about 35.5 μm and it was refined to 200nm after the UNSM treatment. the density of β phase decreased from its initial state. / Surface & Coatings Technology 207 (2012) 135–142 139 Fig. It is well known that plastic deformed layers have a strong correlation with microstructures and mechanical properties in many metallic materials [42]. Fig. Variation in residual stress with respect to depth from the surface of untreated and UNSM-treated CP Ti (a) and Ti–6Al–4V alloy (b) specimens.8 μm. Fig. the average friction coefficient increased within 5 m of sliding up to a value of 0. As for the Ti–6Al–4V alloy specimen. 3. and after the UNSM treatment they were refined to 1. 3. Amanov et al.46.3. The grain size refinement effect of the UNSM treatment was much more significant for the CP Ti specimen than the Ti–6Al–4V alloy specimen. Fig.49 and reached a value of about 0. Also. Microstructure characteristics The cross-sectional microstructures of the UNSM-treated specimens were compared to those of the untreated specimens by SEM analysis. For the untreated CP Ti. The precipitates were identified and located at the grain boundaries in the plastic deformation layer. Also. On the other hand. 4. as for the Ti–6Al–4V alloy specimen. the initial grain size of α and β phases was 9. 5(a) and (c) shows the cross-sectional microstructures of the untreated CP Ti and Ti–6Al–4V alloy specimens where the undeformed grains with a second phase can be found on the grain boundaries. 6 shows the average friction coefficient as a function of sliding distance for CP Ti and Ti–6Al–4V alloy specimens before and after the UNSM treatment.9 μm and 3.2. 3. It could be seen from Fig. 5(d) revealed clear evidence of changes in the microstructure and also showed that the initially continuous β phase was fragmented after the UNSM treatment. grain boundaries became less apparent by the deformation of grains and mechanical twins were formed after the UNSM treatment. 5(b) that the plastic deformation layers produced by the UNSM treatment on CP Ti have significantly different features compared to those of the untreated specimen. Fig. Frictional characteristics of untreated and UNSM-treated specimens Fig.A. reasonable to assume that the depth of compressive residual stress region extends to a depth commensurate with the depth of the dislocation generation zone. The refined grains are expected to lead to increase in hardness as a consequence of the predictions based on the Hall–Petch relationship [41]. Variation in microhardness with respect to depth from the surface of untreated and UNSM-treated CP Ti (a) and Ti–6Al–4V alloy (b) specimens. respectively. In addition.2 μm and 0. 4. The wear debris appeared to be adhered to the specimen surface. Variation of the average friction coefficient of CP Ti and Ti–6Al–4V alloy specimens before and after the UNSM treatment. 7 shows the SEM images of the typical fretting wear scars generated in the untreated and UNSM-treated specimens after 1×10 6 fretting cycles at 20kHz and slip amplitude of ±30 μm. . For the UNSM-treated CP Ti. as with fretting wear scars reported by Mohdtobi et al.28 up to a value of 0. [48] the stick and slip regions could be identified within the scars.37 during the first 4 m and gradually decreased slightly. Amanov et al. the average friction coefficient increased up to a value of 0.25 up to a value of 0. Though the SEM images gave qualitative information about the fretting wear behavior. increased abruptly. Also.32 within 25m of sliding and stabilized to a value of about 0. 5. quantification of wear was not possible from these images. The fluctuation of the friction coefficient may be attributed to the localized fracture of the transfer layer and interaction of the particles at the sliding interface. 3. the average friction coefficient increased from the initial value of 0. the average friction coefficient increased from an initial value of 0. probably due to combined effects of shear and compressive stresses imparted by the counter surface. The wear of UNSM-treated specimens Fig. The variation in fretting wear volume of the untreated and UNSM-treated specimens as a function of normal load was obtained as shown in Fig. Fretting wear characteristics of untreated and UNSM-treated specimens The effect of UNSM treatment on the reduction in fretting wear was evaluated by observing the wear characteristics of the untreated and UNSM-treated specimens. Comparable frictional behavior of Ti–6Al–4 V alloy has been reported in previous studies [46. 8.47]. respectively. / Surface & Coatings Technology 207 (2012) 135–142 (a) (b) Precipitates (c) (d) α+β β α Fig.43 during the first 15m and stabilized to a value of about 0. it was observed that the fretting wear volume increased as the normal load increased for both CP Ti and Ti–6Al–4V alloy specimens.42. From the results. Therefore.50]. For the UNSM-treated Ti–6Al–4V. CP Ti and Ti–6Al–4V alloy images showed that the fretting wear scars for untreated and UNSM-treated specimens had a diameter of about 680 and 630 μm and 520 and 480 μm. One interesting point to note was that the general features of the fretting wear scar generated at a high frequency of 20kHz were quite similar to those observed at much lower frequencies reported in other works [49. Cross-sectional SEM micrographs untreated CP Ti (a).45]. For the untreated Ti–6Al–4V. and UNSM-treated Ti–6Al–4V alloy (d) specimens.140 A. It has been well reported that titanium alloys tend to experience material transfer to the counter surface when rubbed against other metals or ceramics [1. further surface analysis was conducted to quantify the fretting wear volume. Fig. It was postulated that the observed reduction in friction coefficient of the UNSM-treated specimen compared to that of the untreated specimen was related to the increase in hardness and compressive residual stress as well as alteration in the microstructure after the UNSM treatment.44. untreated Ti–6Al–4V alloy (c). 6. UNSM-treated CP Ti (b).31. The untreated specimens showed significant evidence of wear debris formation on one side of the wear scar. Amanov et al. SEM image of the fretting wear scars of the untreated CP Ti (a) and Ti–6Al–4V alloy (b) and UNSM-treated CP Ti (c) and Ti–6Al–4V alloy (d) specimens after the fretting test (1×106 cycles at 20 kHz. The generation of induced compressive residual stress in the surface layer by UNSM treatment is one of the most important phenomena to mitigate fretting wear. the reduction in coefficient of friction can also improve the fretting wear resistance because of the decrease in the alternating tensile shear stresses. When the specimens were subjected to the UNSM treatment the highest compressive residual stress at the top surface of CP Ti and Ti–6Al–4V alloy reached up to − 1279.2 μm and 0. Surface hardness of CP Ti and Ti–6Al–4V alloy increased from 146 HV to 193 HV and from 328 HV to 379 HV. the surface roughness value for all specimens was kept identical. . The three mechanisms to enhance fretting resistance could be summarized as: (1) increased surface hardness. This improvement may be attributed to the increased hardness and induced compressive residual stress of the UNSM-treated specimens. the following conclusions may be drawn: By UNSM treatment. Conclusions The effect of UNSM-treatment on the high-frequency fretting wear and friction characteristics of CP Ti and Ti–6Al–4V alloy was investigated. it was confirmed that the fretting wear resistance of the UNSM-treated specimens was improved compared to the untreated specimens.4 and − 1142. The UNSM-treated specimens showed an enhanced fretting wear resistance and low friction coefficient compared to those of the untreated specimens at higher loading rates. in this study. 7. respectively. The reason for the improved fretting wear and frictional properties of the UNSM treated specimens was attributed to the increased hardness and compressive residual stress induced by the UNSM process.7 MPa. after the UNSM treatment. However. was significantly lower compared to those of the untreated specimens. respectively.8 μm. Finally. Hence. slip amplitude of ±30 μm. This observation is in accord with other surface treatment processes such as SP and ion-beam-enhanced deposition methods that are also useful in increasing the compressive residual stress at the surface [52. (3) low friction coefficient. Fig. (2) induced compressive residual stress.9 μm and 3. / Surface & Coatings Technology 207 (2012) 135–142 141 (a) (b) Fretting direction Fretting direction 500 µm 500 µm (c) (d) Fretting direction Fretting direction 500 µm 500 µm Fig.53].A. Variation in wear scar volume as a function of normal load for untreated and UNSM-treated specimen of CP Ti and Ti–6Al–4V alloy after 1×106 fretting cycles at 20 kHz and a slip amplitude of ±30 μm. it has been reported that to minimize the fretting wear rough surfaces are preferred [51]. 8. From the experimental results. 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