Effect of laser parameters on the tribological behavior of Ti6Al4V titanium microtextures under lubricated conditions

Wear 426–427 (2019) 1272–1279

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Effect of laser parameters on the tribological behavior of Ti6Al4V titanium microtextures under lubricated conditions

T

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J. Salgueroa, , I. Del Sola,b, J.M. Vazquez-Martineza, M.J. Schertzerb, P. Iglesiasb a

Department of Mechanical Engineering and Industrial Design, Faculty of Engineering, University of Cadiz, Av. Universidad de Cádiz 10, Puerto Real, Cádiz E-11519, Spain b Department of Mechanical Engineering, Rochester Institute of Technology, 72 Lomb Memorial Drive, Rochester, NY 14623, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Ti6Al4V Surface modification Laser texturing Tribological wear Friction Lubricated conditions

Surface texturing of metals and alloys has recently been identified as an environmentally friendly alternative to the use of high-performance lubricants with complex formulations. Adding micro-scale textures to one or both sliding surfaces of mechanical components can reduce friction and wear compared to conventional/untextured surfaces. This study investigates the effect of laser textured surfaces on the tribological behavior of titanium Ti6Al4V. Multiple texture types were created by varying the energy density of pulse and scanning speed of the laser. These variations modify the outer layers of the alloy, rising the generation of specific topographies and changing the initial properties by means of microstructural modifications and oxidation processes. The performance of these surfaces was evaluated using a ceramic ball in a ball-on-flat reciprocating tribometer under lubricated conditions. Wettability of the tribological system was examined by measuring the contact angle of the oil used on textured and conventional surfaces. Tribological performance of textured surfaces was found to strongly depend on the laser patterning parameters. Replacing conventional surfaces with textured surfaces reduced friction up to 62% and wear up to two orders of magnitude. Wear mechanisms are discussed from optical microscopy and SEM/EDS observation of wear tracks on titanium disks and ceramic balls.

1. Introduction Titanium alloys are attractive in aerospace and biomedical applications due to their excellent physical properties including high strength, low density, corrosion resistance, and biocompatibility [1–5]. Unfortunately, the low plastic deformation ratios of these alloys result in unstable friction coefficients and other undesirable wear mechanisms under tribological wear situations [1,6–11]. These undesirable effects can be reduced using specific lubricants that facilitate the sliding behavior between contact surfaces [9,12–19]. However, the lubricant retention capability of the contact surface is presented as one of the main relevant parameters to obtain better results in tribological applications [14]. Surface modification by unconventional techniques may vary the initial features and properties of materials in order to adapt the surface to specific applications and overcome limitations of use [20–22]. Surface modification by laser texturing can be used to modify the micro-geometrical characteristics of a material. This can provide

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specialized topographies on the surface while influencing materials properties of the metal near the surface. This technique can be used to alter wetting behavior to improve lubricant retention of the modified surface. The geometry and direction of the grooved texture has an important effect on the lubrication conditions and wear behavior [23,24]. This has been shown to dramatically reduce wear effects, especially abrasion and adhesion [10,25–30]. The initial contact between elements of the tribological pair is defined by singular points and small areas of asperities from the sliding surfaces. The reduced area may develop an important increase on the contact pressures, involving a significant growth of temperature. High contact pressures and friction result in removing processes of the higher asperities and the loss of material by different wear mechanisms. When a lubricant layer is applied between friction couple elements, local contact pressure is reduced as the force is distributed across a larger area. This reduces wear effects and the volume of material debris [31,32].

Corresponding author E-mail address: [email protected] (J. Salguero).

https://doi.org/10.1016/j.wear.2018.12.029 Received 1 September 2018; Received in revised form 31 October 2018; Accepted 12 December 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.

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scanning electron microscopy (SEM) performed using a FEI Quanta 200 system. Cross-sections of the modified layer were evaluated to complement the study of geometrical characteristics of the textures traces. Adding texturing to the samples may affect their lubricant retention capacity. For this purpose, wetting behavior in terms of contact angle measurement were evaluated using a Ramé-Hart system and DropImage Advanced as image processing software.

Table 1 Ti6Al4V alloy weight composition (wt%). Element

Al

V

Fe

C

O

N

H

Ti

wt%

6.26

3.91

0.18

0.011

< 0.10

< 0.10

< 0.10

Rest

This work examines the effects of laser processing parameters on the micro-geometrical surface modification of the titanium alloy Ti6Al4V. Laser surface texturing is used to develop specific topographies that improve the friction and wear behavior of the initial surfaces. It is hypothesized that increased performance is the result of variations of the lubricant retention capability on the textured surfaces.

2.3. Tribological tests Pin-on-flat reciprocating tests were carried out over the 24 different textured surfaces. All tests were performed with a load of 5 N, a sliding distance of 50 m and a sliding speed of 0.012 m/s. Tests were repeated at least twice on each sample, using a tungsten carbide pin with 1.5 mm diameter. This configuration provides an average Hertz contact pressure of 1.81 GPa with a maximum value of 2.71 GPa. All the sliding tests were carried out under lubricated conditions, using 15–20 µl volume rate of Synton PAO 40 poly alpha olefin fluid, with the characteristics shown in Table 3. Tribological tests were carried out under boundary lubrication regime. The observed deviation for friction and wear data in repeated experiments was lower than 5%. This is consistent with ASTM G 133 [35] that recommends maximum deviations lower than 20%.

2. Experimental 2.1. Materials All titanium alloy (Ti6Al4V) samples (Table 1) in this investigation had a thickness of 5.0 mm and an initial roughness of Ra < 0.05 µm. Samples were textured using a 20 W Ytterbium fiber infrared laser (Rofin EasyMark F20). Laser texturing was performed at three different pulse energy densities (Ed) and 8 scanning speeds (Vs) (Table 2). Energy density was altered by selecting different pulse rates. Energy density decreases as pulse rate increases. This reduces the aggressiveness of the texturing process. Lowering the scanning speed increases the thickness of the subjected layer by focusing the beam on the same area for a longer time. Each texture was created on a 10 mm × 10 mm square, setting a 60 µm spot diameter and 100 ns pulse width. Surface treatment was carried out through linear bidirectional layout with a 0.1 mm separation between laser tracks in an open-air atmosphere. Under this conditions, linear textures allow to contain higher volume of lubricant than dimples, favoring the improvement of the wear performance for reciprocating wear test.

2.4. Worn surfaces characterization Characterization of worn surfaces were performed by optical microscopy using an Olympus SZX12 stereoscopic system. Image processing methods were used to evaluate the area of adhered material to the sphere surface. Three-dimensional (3D) profilometry techniques (Nanovea ST400) have been used to characterize the amount of material subjected to friction and sliding effects, however, mainly due to the irregularities of the texture surface, volume loss using a profilometer as indicated in ASTM G133 [35] is not recommended. Under these conditions, an evaluation based on the Eq. (1) [36], has been selected to calculate the volume loss (Vf ), approximating the sliding track to an ellipsoid. Volume loss was calculated as

2.2. Textured surface characterization Laser textured effects on Ti6Al4V were characterized by measurement of surface finish, inspection of surface by microscopy and examination of wetting behavior through contact angle measurement. Surface finish was characterized by average roughness (Ra), maximum height on single measurement length (Rz) described in ISO 4287 [33], and reduced peak height (Rpk) described in ISO 13565–2 [34]. Reduced peak height gives the height of the protruding peaks above the roughness core profile (Fig. 1). It provides insight into tribological behavior of the textured surface by representing the fraction of material that will be eliminated in the first instants of sliding contact. Measurements were performed using a MAHR Concept PGK120. In all cases, at least ten measurements distributed over the modified area were performed and error bars on respective figures represent standard deviations from the mean result. Shape features of the textured tracks were examined by optical and

Vf =

Energy density of pulse (J/ cm2)

Scanning speed (mm/ s)

F1

20.0

Ed1

17.68

F2

50.0

Ed2

7.07

F3

80.0

Ed3

4.42

V1 V2 V3 V4 V5 V6 V7

(1)

where R o is the initial radio of the carbide sphere used as pin, R w is the pin radio after the tribological test, a is the major axis of the ellipse and b is the minor axis of the ellipse. The terms h o and h w are determined by the following equations.

h w = Rw −

ho = R o −

Rw 2 − b2

R o 2 − b2

(2) (3)

3. Results and discussion Modification of the initial surface features of a Titanium alloy (Ti6Al4V) was carried out through a laser texturing processes. Processing was performed in a non-protective atmosphere, resulting in variations in the surface properties and features of the alloy resulting from chances in the micro-geometry. Changes in the alloy texture and the wetting of the samples may affect their tribological behavior under lubricated conditions of the friction pair.

Table 2 Laser Texturing parameters. Pulse rate (kHz)

a π [ho2 (3Ro − ho) − hw 2 (3Rw − h w )] 3b

10 20 40 80 100 150 250

3.1. Roughness of the textured surfaces The evaluation of the textured surface through micro-geometrical characteristics shows three stages, related to scanning speed of laser beam ranges, in which different behavior was detected for Ra, Rz and Rpk parameter (Figs. 2, 3 and 4). Similar soft reductions in Ra were 1273

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Fig. 1. Maximum height and reduced peak height (Rpk) definition [34].

Table 3 Viscosity and thermal stability of Synton PAO 40. Density 20 °C (g/cm3)

Kinematic Viscosity 100 °C (cSt)

Dynamic Viscosity (cP)

0.842

4

25 °C 810.34

40 °C 339.14

Thermal Stability (onset °C) 100 °C 38.63

295

Fig. 4. Rpk roughness behavior as a function of Vs. Fig. 2. Ra roughness behavior as a function of Vs.

of uniformity in the textured grooves (Fig. 5). Second stage (100–150 mm/s) is characterized by an important decrease in the values of Ra and Rz (Figs. 2 and 3) for the higher Vs analyzed and the appearance of a singular behavior point at 150 mm/s where Ra and Rz values reaches a minimum. In the case of Rpk parameter (Fig. 4), a relevant dependence has been confirmed with Ed in the first stage, finding a decreasing trend for the 100–150 mm/s range and giving rise to a stability in the reduced peak height for higher Vs. The study of the Rpk parameter allows a better understanding between the micro-geometry (developed from laser texturing) and the sliding and friction behavior, mainly due to the production of higher wear debris volume. Cross-sectional images of samples subjected to 150 mm/s scanning speeds are provided to provide deeper insight into laser textured grooves (Fig. 5). Textured grooves show a singular point through cross section images that may detect a phenomenon whereby the vaporized material from the laser pulse impacts solidifies on the top of the texturized grooves, causing a total or partial obstruction of the textured track and decreasing the roughness values (Fig. 5). Obstruction of the texturing grooves by solidified material involves a decrease in the deep of the laser beam tracks, resulting in lower roughness values for the modified layer.

Fig. 3. Rz roughness behavior as a function of Vs.

observed for low scanning speeds (10–100 mm/s) at energy densities of 4.42 J/cm2 and 17.68 J/cm2 (Fig. 2). Interestingly, average roughness Ra increased over the same scanning speeds at an energy density of 7.07 J/cm2. This increased roughness appears to be mainly due to a loss 1274

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Fig. 5. Cross section of Vs = 150 mm/s textured surfaces for (a) Ed = 17.68 J/cm2, (b) Ed = 7.07 J/cm2 and (c) Ed = 4.42 J/cm2.

Fig. 7. Friction coefficient behavior for textured and untreated surfaces.

Fig. 6. Contact angle (lubricant) behavior for textured and untreated surfaces.

caused by vaporization and solidification processes on the irradiated layer, giving place to variations in the geometry and size of the induced grooves, as can be observed in Fig. 5. Wear rate for the 100–150 mm/s Vs range is shown again as the variation stage for the studied Ed. These results were not unexpected as the trends described here were similar to those observed for friction coefficient as a function of Vs. Morphologically, wear track geometry and size are strongly influenced by the texturized conditions (Fig. 8). However, mainly due to the irregularities defined by the textured surface features, a reference plane for the study of the wear volume (recommended by ASTM G133) can not be determined. For this reason, the study of wear has been carried out by the measurement of the width and depth of the sliding track following the equations presented by Qu et al. [36]. When sliding tracks are analyzed, different wear effects are developed with a direct dependency of the laser processing parameters. Taking as starting point the tribological properties of the Ti6Al4V untreated surface, wear mechanisms developed on the different textured samples have been analyzed. As can be seen in Fig. 9, the non-textured wear track shows plastic deformation phenomena. Two-body abrasion marks are detected on the contact surface due to wear debris from cold working titanium. EDX results show no significant differences between wear track and plastically deformed material of the non-textured contact. For the laser treated samples, abrasion effects on the sliding track have also been observed. In this case, an important increase in the hardness of the Ti alloy based on the oxidation layer may cause the appearance of harder wear debris that scratch the successive layers of the sample [21,29,40,41]. The effect of scanning speed on wear behavior of irradiated sample was examined at an intermediate energy density of 7.07 J/cm2. At this energy density, track dimensions decreased as Vs increased (Fig. 11). Low scanning speeds result in more aggressive laser processing conditions due to the laser beam remains longer time on the same section. This results in thicker textured layers at lower scanning speeds that

3.2. Effects of textured layer on contact angle measurement The change in lubricant absorption properties induced by laser texturing were evaluated by comparing contact angles on textured surfaces to an un-textured sample (Fig. 6). The contact angle on all textured surfaces was lower than the untreated Ti6Al4V alloy. This suggests that texturing samples increases lubricant retention, which should in turn improve tribological performance. This behavior indicates a direct dependence between the existence of roughness and variations in the contact angle, according to Cassie-Baxter and Wenzel models [37,38]. Under these conditions, can be ensured the retention of used lubricant into laser texturing tracks, favoring the modification of the wear properties, in sliding and friction conditions, compared to untreated surfaces. This consideration may be an initial condition to obtain a better monitoring of the lubrication layer thickness, being a relevant parameter related to friction effects on the modified surfaces, according to Fan et al. [39]. 3.3. Tribological behavior of textured surfaces under lubricated conditions Laser texturing plays an important role on the friction and wear between sliding surfaces. Modification of micro-geometrical features and surface properties by laser texturing may induce the appearance of different wear phenomena and subsequently the variation of the involved parameters as friction coefficient (μ) or wear volume (Vf). These effects were examined by varying the laser texturing of Ti6Al4V titanium alloy by controlling energy density of pulse (Ed) and scanning speed of the beam (Vs). Friction coefficient generally increased with Vs (> 100 mm/s) for all samples. These results present similar stages as Ra and Rz roughness evaluation. When analyzing the Vs range of 100–150 mm/s present an inflexion point for all Ed studied, tending to stability the μ values for higher (17.68 J/mm2) and lower (4.62 J/mm2) Ed and showing an important growth for Ed = 7.07 J/mm2. Described behavior may be 1275

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Fig. 8. Wear volume of sliding track under different surface conditions.

Fig. 9. Wear track and EDX of untreated Ti6Al4V surface.

Fig. 10. Wear track SEM and EDX of Vs = 10 mm/s laser textured Ti6Al4V surface.

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Fig. 11. Wear track and EDX of Vs = 40 mm/s, Vs = 100 mm/s and Vs = 250 mm/s laser textured Ti6Al4V surface.

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Fig. 12. Adhered effects on carbide spheres surface.

an increment of oxygen level regarding to non-textured surface. Thermal oxidation of Ti alloy takes place in the outer layers, being removed by the friction process. For this reason, a decrease in oxygen levels were detected inside the wear track, keeping constant carbon concentration. Textured surface developed using 250 mm/s presents medium levels of carbon, nevertheless wear track composition is similar to 40 and 100 mm/s where a relevant decrease in oxygen composition have been obtained. This behavior, as previous considerations, is mainly caused by the remove of the modified layer by means of sliding contact.

3.4. Effect on the pin surface The evaluation of carbide pins used in tribological test allow for identification of the wear effects on the harder element of the contact couple. Adhered material area on the sphere surface is strongly influenced by varying the energy density of pulse and/or scanning speed of the beam (Figs. 12 and 13). Evidence of several wear mechanisms was found on the carbide pin. These mechanisms included adhesive effects. In most cases, the adhered material layer of the titanium alloy on the sphere surface suggests that adhesive wear mechanism is an important component in these cases. Based on the study of the adhered area, a relevant dependence has been recognized between Ed and the growth of built up layers over the carbide surface. Under this consideration, an increase in the Ed values implies higher roughness values. More aggressive treatments affect the thickness of the modified layer favoring the appearance of larger asperities, as can be observed by Rz measurements (Fig. 3). This reduces the contact surface between pin and irradiated surface. This leased to increased contact pressure, which facilitates breakage of the highest peaks and gives rise to wear by friction. On the other hand, the decrease of energy of laser pulses seems to reduce the width of the adhesion

Fig. 13. Adhered area on pin as a function of Vs.

improve wear resistance (Fig. 10). The increase of height and deep of the irradiated asperities may facilitate the retention of lubricant on the surface resulting in the improvement of wear resistance and preventing the friction effects on the untreated layers of Ti6Al4V. Adhesion effects over the friction contact are detected from 40 mm/ s to 250 mm/s. Under these texturing conditions, adhered particles (AP) from wear debris were located on the sliding path, also three-body abrasion mechanisms (3BA) were observed as a result of the friction contact with removed particles from the modified layer subjected to oxidation (harder than initial alloy) (Fig. 11). EDX analysis on the bottom track particles from wear debris at 250 mm/s shows an important growth of carbon (C) content, mainly due to pins (WC-Co) removed material, initial composition of the alloy and lubricant diffusion phenomena. Laser modified layer is related to 1278

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mark (Fig. 12). A slight increase trend has been observed for 17.68 J/cm2 and 4.42 J/cm2. However, as can be confirmed, an unexpected trend was observed for 7.07 J/cm2, showing a singular point for 150 mm/s. In this sense, values obtained of adhered material area from high scanning speeds are similar to non textured contact, this fact is specially due to the decrease in the thickness of the irradiated layer and the softening of the textured tracks (Fig. 12). Similar behaviors were detected between adhered area and the results for friction coefficient (Fig. 7). and wear rate (Fig. 8). Together, these results support the hypothesis that wear debris rising from rough surfaces. Results from this investigation also suggest that laser processing parameters on the retention effects of lubricant and the wear mechanisms involved.

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4. Conclusions Laser texturing processes may induce the development of modified layers with special topographies for specific applications. Through variations of laser processing parameters, the dimensions and geometry of the texturing tracks can be controlled. Scanning speed of the beam (Vs) is shown as one of the main variables that govern the laser modification of micro-geometrical textures. An increase of Vs results in the incidence of less energy on the surface, giving rise to a decrease in the maximum dimensions and shape of the texturized grooves. Lubricant absorption can be promoted by textured surfaces by the generation of parallel micro-grooves. Better retention of lubricant was obtained for the modified layer in all laser processing treatments, improving the wear and friction behavior in sliding conditions compared to untreated surfaces. A significant dependence has been confirmed between laser processing parameters and the wear resistance under the conditions studied. All texturing treatments applied allow reducing the wear effects, in terms of volume of affected material. A reduction of approximately 80% of wear track volume have been obtained for lower scanning speeds (< 100 mm/s). This improvement is mainly due to the modification of initial surface properties of the alloy including oxidation processes and microstructural changes. Adhesion and abrasion phenomena has been identified as most relevant mechanisms involved in the wear process of the lubricated sliding contact of textured Ti6Al4V and WC-Co. The energy density of the pulse (fluence) (J/cm2) and scanning speed of the beam are the main variables that govern the formation of modified material layers with different natures and properties affecting the friction wear behavior. Acknowledgements This work has received financial support from Spanish Government (MINECO/AEI/FEDER, Grant Project DPI2017-84935-R) and the University of Cadiz (Training plan UCA/REC01VI/2016 and Ph.D. research stay UCA/R20REC/2017). We also acknowledge the support from the Mechanical Engineering Department at the Rochester Institute of Technology. References [1] C. Veiga, J.P. Davim, A.J.R. Loureiro, Properties and applications of titanium alloys: a brief review, Rev. Adv. Mater. Sci. 32 (2012) 133–148. [2] H.J. Rack, J.I. Qazi, Titanium alloys for biomedical applications, Mater. Sci. Eng. C26 (2006) 1269–1277. [3] A.R.H. Vinicius, Titanium production for aerospace applications, J. Aerosp. Technol. Manag. 1 (2009) 7–17. [4] A.P. Mouritz, Introduction to Aerospace Materials, Woodhead Publishing Limited, Cambridge, UK, 2012, pp. 202–223. [5] Z.M. Jin, J. Zheng, W. Li, Z.R. Zhou, Tribology of medical devices, Biosurf. Biotribol. 2 (4)

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