Influence of Bio-Lubricants on the Tribological Properties of Ti6Al4V Alloy

Influence of Bio-Lubricants on the Tribological Properties of Ti6Al4V Alloy

Journal of Bionic Engineering 10 (2013) 84–89 Influence of Bio-Lubricants on the Tribological Properties of Ti6Al4V Alloy Yong Luo, Li Yang, Maocai T...

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Journal of Bionic Engineering 10 (2013) 84–89

Influence of Bio-Lubricants on the Tribological Properties of Ti6Al4V Alloy Yong Luo, Li Yang, Maocai Tian School of Material Science and Engineering, China University of Mining and Technology, Xuzhou 221116, P. R. China

Abstract Titanium alloy is one of the best materials for biomedical applications due to its superior biocompatibility, outstanding corrosion resistance, and low elastic modulus. However, the friction and wear behaviors of titanium alloys were sensitive to the environment including lubrication. In order to clarify the wear mechanism of titanium alloy under different lubrications including deionized water, physiological saline and bovine serum, the friction and wear tests were performed between Ti6Al4V plates and Si3N4 ball on a universal multi-functional tester. The friction and the wear rate of titanium alloy were measured under dry friction and three different lubrication conditions. The worn surfaces were examined by scanning electron microscopy. The results revealed that under the dry friction, the wear resistance of titanium alloy was the worst since the wear mechanism was mainly the combination of abrasive wear and oxidation wear. It was also found that Ti6Al4V alloy had low friction coefficient and wear rate under three lubrication conditions, and its wear mechanism was adhesive wear. Keywords: titanium alloy, lubrication, friction coeffitient, wear mechanism Copyright © 2013, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(13)60202-4

1 Introduction Titanium and its alloys are known as the most appropriate materials for biomedical applications due to their superior biocompatibility, outstanding corrosion resistance, and low elastic modulus comparable with that of human bone[1–6]. However, the poor tribological performance of titanium alloy limits its use in wear-related engineering applications, especially in artificial joints[7]. There were a lot of reports on the friction and wear of titanium alloys during the past two decades and many efforts were made attempting to provide a better understanding of wear mechanisms of titanium alloys. According to the research of Molinari et al.[8], the poor tribological properties of titanium and its alloys were attributed to the low resistance to plastic shearing, the low work-hardening and the low protection exerted by the surface oxide. As we known, the surface oxide was easily removed by spalling and could not protect the subsurface layers against wear. Güleryüz and Cimenoğlu highlighted that the formation of wear debris fallen of the surface oxide and release of metal ions caused adverse tissue reactions, implant loosening and eventual Corresponding author: Yong Luo E-mail: [email protected]

revision surgery[9]. Therefore, there were many early works focusing on different surface modification ways to improve the tribological performance of titanium alloys, including Physical Vapor Deposition (PVD), plasma immersion ion implantation, thermal oxidation, plasma and laser nitriding and so on. Liu et al. reviewed some methods about surface modification to enhance the wear resistance of titanium and its alloys for biomedical applications[10]. Recent works indicated that different components of the lubricants such as phospholipid, proteoglycan molecules, hyaluronic acid, and gelatin had significant effects on the friction and wear of ultra-high molecular weight polyethylene[11–14]. Scholes and Unsworth investigated the effects of proteins on the friction and lubrications of artificial joints, including the CoCrMo alloy and UHMWPE[15]. However, there are few reports on the effects of different lubricants on the tribological behaviors of titanium alloys. Therefore, it is essential to fully understand the wear mechanisms of titanium alloy under biolubrication systems. Ti6Al4V alloy was chosen in this experiment because it is one of the most widely used titanium alloys. The wear behavior of a sliding system depends on many factors, including the properties of the specimen

Luo et al.: Influence of Bio-Lubricants on the Tribological Properties of Ti6Al4V Alloy

and counter-face materials, their interaction with the environment and the experimental conditions[16,17]. In this work, the friction and wear tests were performed between Ti6Al4V plates and Si3N4 ball on a universal multi-functional tester to research the effects of different lubrication systems including deionized water, physiological saline and bovine serum on the wear mechanisms of titanium alloys.

2 Experimental In the present study, the Ti6Al4V specimens were machined in a 20 mm × 20 mm square shape with 5 mm thickness. Before performing experiment, the samples of Ti6Al4V were polished to reach the roughness of 0.04 μm, and then they were ultrasonically cleaned by ethanol for 30 min. Si3N4 ball with 4 mm diameter and 0.02 μm roughness was chosen for wear test due to its super wear resistance and excellent chemical stability. During the experiment, the friction and wear behaviors of titanium alloy were investigated under four conditions including the dry friction, deionized water, physiological saline and bovine serum, respectively. The sliding wear test of Ti6Al4V was performed on a Universal Multifunctional Tester (UMT) with the ball-on-flat style under dry friction and three different lubrication conditions. The schematic of contact between Si3N4 ball and Ti6Al4V plate was shown in Fig.1. During the process, the titanium alloy specimen was fixed to the specimen holder and the Si3N4 ball was implemented reciprocating motion continuously. Si3N4 ball Reciprocating motion Z-carriage

Strain gauge sensor

Normal force

Lubricants

Si3N4 ball

Ti6Al4V

Fig. 1 The schematic of ball-on-flat style wear test.

The normal force during the friction and wear test was set to 9.8 N, which corresponded to the contact pressure of 1.45 GPa. The sliding speed and recipro-

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cating displacement of Si3N4 ball were 4 mm·s−1 and 6 mm respectively for the wear test. The coefficient of friction was digitally recorded throughout 30 min test by a load transducer. After wear test, the loss of weight was measured by electronic balance with an accuracy of 0.00001 g. Then the wear rate was calculated by Eq. (1). The worn surface of Ti6Al4V alloy was examined by Scanning Electron Microscope(SEM), and the Ti6Al4V specimens under lubrication conditions were cleaned to remove the liquid from the surfaces before placing them into the SEM.

ω = Δm / ( F ⋅ S ),

(1)

where ω is wear rate, Δm is the weight loss, F equals to 9.8 N, and S is the total sliding distance, S = 4 mm·s−1 × 30 min× 60 s·min−1 = 7200 mm = 7.2 m.

3 Results and discussion 3.1 Coefficient of friction and wear rate Fig. 2 shows the curves of the friction coefficient of Ti6Al4V against the Si3N4 ball under dry friction and three different lubrication conditions. The common features of friction curves were heavy fluctuations under dry friction while lubrication conditions result in a significant reduction in both the value and the fluctuation of friction coefficient curves. Table 1 shows the average Coefficient of Friction (CoF) and the variance of Ti6Al4V under four conditions, including the dry friction, deionized water lubrication, physiological saline lubrication and bovine serum lubrication, respectively. The average value and the variance of CoF were calculated when the three CoF curves under lubrication remained constant after certain testing periods (600 s). It was also found that the value of CoF and the variance of Ti6Al4V alloy under three lubrication conditions decreased compared to dry friction. The average values of CoF in the deionized water, physiological saline, and bovine serum dropped by 10.47%, 19.50% and 31.32% compared with the CoF around 0.68 under dry friction condition, and the variance of CoF had a greater reduction. The results of wear rate of Ti6Al4V under four conditions are shown in Table 2. It was found that Ti6Al4V alloy had the most severe wear under dry friction. The wear rate of Ti6Al4V under the lubrication of physiological saline, deionized water, and bovine serum was decreased by 17.0%, 29.8%, and 32.3%, respec-

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tively. It should be noticed that the value of CoF under deionized water lubrication was greater than that under physiological saline lubrication, but the wear rate of Ti6Al4V specimen under deionized water lubrication was lower than that under physiological saline lubrication.

higher than that in the edges because the pressure and the shear force in the center were higher than those in the edges. The Ti6Al4V specimen had mild plastic deformation under the bovine serum lubrication. However, shallow and narrow scratches were observed on the worn area of Ti6Al4V specimen under bovine serum lubrication, which was quite different from other specimens.

(a) Dry friction

Fig. 2 Friction coefficient of Ti6Al4V under dry friction and three different lubrication conditions. Table 1 Friction coefficient of Ti6Al4V alloy under dry friction and three different lubrication conditions (after 600 s) Dry friction

Deionized water

Physiological saline

Bovine serum

Average friction coefficient

0.675

0.604

0.543

0.464

Variance

0.290

0.018

0.014

0.005

(b) Deionized water

Table 2 Wear rate of Ti6Al4V specimen under dry friction and different lubrication conditions

Wear rate (kg·N−1·m−1)

Dry friction

Deionized water

Physiological saline

Bovine serum

1.88×10−9

1.32×10−9

1.56×10−9

1.28×10−9

3.2 Worn surface Fig. 3 shows SEM micrographs of worn surfaces of Ti6Al4V specimen under dry friction and lubrication conditions (low magnification). Worn surface morphology exhibited the similar characteristics, which could be distinguished by the rough surface appearance as a result of the heavy ploughing action and scratch damage of Si3N4 ball. Severe plastic deformation along with grooves parallel to the sliding direction were found on the worn surface of the Ti6Al4V alloy under dry friction and three lubrication conditions, but the depth and width of grooves were different. It was also found that the depth of scratch in the center was obviously

(c) Physiological saline

(d) Bovine serum

Fig. 3 Worn morphology of Ti6Al4V specimen under dry friction and three lubrication conditions (low magnification).

Fig. 4 shows SEM micrographs of worn surfaces on Ti6Al4V specimen under dry friction and three lubrications conditions (high magnification). The damage

Luo et al.: Influence of Bio-Lubricants on the Tribological Properties of Ti6Al4V Alloy

caused by Si3N4 ball was often considered disastrous, especially under dry friction sliding. The worn surfaces under deionized water and physiological saline were relatively slight although there were still some small fatigue cracks and spalling. In comparison, the worn surface of the Ti6Al4V specimen tested under bovine serum lubrication was very smooth. There were only some small pits on it. No cracking, delamination or spalling were observed in the wear scar under bovine serum lubrication. The pure polishing mode was predominant under bovine serum lubrication, only slight damages such as deformation and scratch were observed.

50 μm (a) Dry friction

(b) Deionized water

(c) Physiological saline

(d) Bovine serum

Fig. 4 Worn morphology of Ti6Al4V specimen under dry friction and three different lubrication conditions (high magnification).

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3.3 Discussion During the wear process of Ti6Al4V specimen under dry friction, the Ti element would react with the oxygen and form TiO2 film in the air due to frictional heat accumulation[18]. Due to the brittleness of TiO2 film, the combination of the matrix is not tough and reliable. In this case, the wear particles were removed easily from the surface of Ti6Vl4V specimen. This process increased the surface roughness of Ti6Al4V and produced wear particles. Therefore, wear particles would participate in the friction and wear process as the new intermediates, which led to a high value and a heavy fluctuation of CoF[19]. Among the four conditions, under dry friction, Ti6Al4V specimen had the highest wear rate, the largest wear scar width and the roughest worn surface, which was resulted from the combination of the oxidation wear and the abrasive wear. The CoF, the wear rate, the width and the roughness of the wear scar Ti6Al4V specimens under three lubrication conditions decreased compared with those under dry friction. That is because the lubricants could prevent the exposure of wear scar to air, which decreased the oxidation rate of titanium and then maintained the steady value of CoF. Therefore, it was suggested that the wear mechanism of Ti6Al4V alloy under lubrications was mainly adhesive wear. The CoF of Ti6Al4V specimen under physiological saline lubrication was lower than that of Ti6Al4V specimen under deionized water lubrication, that is because active anion in physiological saline and active metal on the surface of Ti6Al4V specimen could form chemical reaction film, which had antifriction effect. However, Cl− in the physiological saline was sensitive to crevice corrosion, so Cl− became a main factor to influence the wear rate of Ti6Al4V alloy[20]. The Cl− corrosion not only destroyed the surface structure of Ti6Al4V but also decreased the strength of Ti6Al4V alloy. Therefore, the wear debris was easily fallen off from Ti6Al4V specimen. As well, the Cl− could be preferentially adsorbed on titanium passive film, and combined into a soluble chloride with metal cation upon the passive film. Consequently, it accelerated the corrosive wear of Ti6Al4V specimen. As a result, the wear rate of Ti6Al4V specimen under physiological saline lubrication was higher than that of Ti6Al4V under deionized water lubrication. Under bovine serum lubrication, proteins in the

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bovine serum solution were deposited on the surface of Ti6Al4V specimen and its defects, which formed a sticky film on the surface of Ti6Al4V specimen[21]. The film was supposed to prevent worn surface exposing to air and separate the Si3N4 ball and the Ti6Al4V specimen, which reduced the CoF and prevented propagation of wear track. Therefore, the CoF of Ti6Al4V specimen under bovine serum lubrication was the lowest, and the wear rate of Ti6Al4V specimen and the width of the wear scar under bovine serum lubrication were the smallest. The results show that, bovine serum lubricant was optimal among the three lubrications although the influence of lubrication was not remarkable. The CoF and the wear rate of Ti6Al4V specimen and the width of the wear scar were decreased under three lubrication conditions. The CoF of Ti6Al4V specimen under three lubrication conditions ranged from 0.4 to 0.7, which was in the level of the high CoF. Similarly, the wear rate kept in the same order of magnitude, in high wear state. It fully illustrated that lubrication could not provide a considerably beneficial effect on the wear resistance of the Ti6Al4V alloy.

4 Conclusions In this study, the CoF, the wear rate, the worn surface and the wear mechanisms of Ti6Al4V alloy under dry friction, deionized water, physiological saline and bovine serum lubrications were investigated. The results of this research can be summarized as follows: (a) The CoF of Ti6Al4V alloy under dry friction was high and heavily fluctuant. The wear rate of Ti6Al4V alloy under dry friction was high. The Ti6Al4V alloy had poor tribological properties. The mechanism under dry friction was mainly abrasive wear and oxidation wear. (b) Three different lubrication conditions including deionized water, physiological saline and bovine serum had the effect on reducing the CoF and the wear rate of titanium alloys. The effect of bovine serum lubrication was relatively obvious. (c) The wear mechanism of titanium alloys under deionized water, physiological saline and bovine serum conditions was mainly adhesive wear. But the corrosion wear also played an important role on the titanium alloy under physiological saline lubrication.

Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities under Grant No.2012QNA06.

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