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Torsional fretting wear of a biomedical Ti6Al7Nb alloy for nitrogen ion implantation in bovine serum
Tribology International 59 (2013) 312–320
Contents lists available at SciVerse ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Torsional fretting wear of a biomedical Ti6Al7Nb alloy for nitrogen ion implantation in bovine serum Zhen-bing Cai a, Guang-an Zhang b, Yong-kui Zhu a, Ming-xue Shen a, Li-ping Wang b, Min-hao Zhu a,n a b
Tribology Research Institute, Key Lab of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu, 610031, China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2011 Received in revised form 4 May 2012 Accepted 11 June 2012 Available online 21 June 2012
Ti6Al7Nb is a high-strength titanium alloy used in replacement hip joints that possesses the excellent biocompatibility necessary for surgical implants. Ti6Al7Nb treated with nitrogen gas (N2) plasma immersion ion implantation–deposition (PIII–D) was investigated. Torsional fretting wear tests of untreated and nitrogen-ion-implanted Ti6Al7Nb alloys against a Zr2O ball (diameter 25.2 mm) were carried out under simulated physiological conditions (serum solution) in a torsional fretting wear test rig. Based on the analyses of the frictional kinetics behavior, the observation of 3D profiles, SEM morphologies and surface composition analyses, the damage characteristics of the surface modification layer and its substrate are discussed in detail. The influence of nitrogen ion density on the implantation and torsional angular displacement amplitudes were investigated. The results indicated that ion implantation layering can improve resistance to torsional fretting wear and thus has wide potential application for the prevention of torsional fretting damage in artificial implants. The damage mechanism prevented by the ion implantation layer on the Ti6Al7Nb alloy is a combination of oxidative wear, delamination and abrasive wear. An increase in ion implantation concentration inhibited detachment by delamination. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Fretting wear Torsional fretting Plasma immersion ion implantation Titanium alloy
1. Introduction Many clinical conditions, including but not limited to arthritis, trauma and congenital or acquired joint diseases, lead to a loss of cartilage tissue in human joints. The intrinsic avascular and aneural nature of cartilage provides little scope for natural selfrepair. As a result, millions of people require treatment to repair damaged cartilage, usually at the major load-bearing articular joints such as the hips and knees [1]. Early clinical results provide evidence that loosening of one or both components of the Total Joint Replacement Surgery (TJR) occurs in 10% to 70% of patients 10 years after surgery [2]. Recent large-scale studies report that 10–20% of new joints must be replaced within 15–20 years, with aseptic loosening accounting for approximately 80% of these revisions [3]. Joint contacts experience very complex kinematics during walking, which is a combination of rolling and sliding. Three different sliding modes (transversal, radial, and circumferential) have been defined in the literature [4,5]. Torsional wear is one of the main motion modes of joints, and torsional fretting can be defined as the relative
n
Corresponding author. Tel.: þ86 28 87600715; fax: þ 86 28 87601342. E-mail addresses: [email protected], [email protected] (M.-h. Zhu). 0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2012.06.009
motion induced by reciprocating torsion under an oscillatory vibratory environment. Several solutions have been proposed to understand torsional contact and the mechanics of its behavior [6–8]. Titanium alloys are attractive materials for the manufacture of implants for medical and dental applications due to their superior biocompatibility and outstanding corrosion resistance compared with other conventional metallic materials [9,10]. Commercially pure titanium and Ti6Al4V alloy are the most commonly used materials in the manufacture of implants. Due to controversy around the potential toxic effects of vanadium compounds, V-free alloys such as Ti6Al7Nb have been recently developed for biomedical applications [11,12]. Ti6Al7Nb alloy is widely used because of its advantages in biocompatibility, mechanical properties and corrosion resistance [13,14]. Plasma immersion ion implantation–deposition (PIIID) is a rapidly developing surface modification technique that has shown effectiveness in modifying the physicochemical characteristics of thin films and mixing layers. In PIII–D, the target is typically enshrouded in self-excited plasma generated by applying a large negative voltage to the target [15,16]. However, the torsional wear behavior of Ti6Al7Nb and its implantation-treated nitrogen ion layer have not been clearly discussed. For a given torsional contact configuration, the resulting contact zone kinematics were found to profoundly influence accumulation, compaction and displacement of the debris particles generated during contact.
Z. Cai et al. / Tribology International 59 (2013) 312–320
When the torsional angular displacement amplitude is very small, accommodation by elastic deformation and a small plastic deformation are observed, and torsional fretting occurs in the partial-slip regime (PSR). When the torsional angular displacement amplitude is very large, which means that gross slip occurs throughout the process, torsional fretting occurs in the gross slip regime (GSR). The intermediate state is a mixed fretting regime (MFR). One dramatic phenomenon that occurs in MFR is a change in wear morphology from annular to an entire worn zone, accompanied by the disappearance of the sticking zone as a function of the number of cycles [7,8,20]. In this research, a new testing method was applied to reveal and compare the torsional fretting behavior of untreated and nitrogen-ion-implantation-treated biomedical Ti6Al7Nb alloy.
2. Experimental procedures 2.1. Materials Biomedical Ti6Al7Nb alloy ((wt %): 5.88Al, 6.65Nb, 0.03Fe, 0.10C, 0.20O, 0.07N, 0.02H and 87.05Ti) was used in the present investigation as a rod supplied by the Northwest Institute for Nonferrous Metal Research, Xi’an, China. The rod was machined flat to dimensions of 10 10 25 mm3. One side (10 mm 25 mm) of each sample was ground with 1500-grit diamond paper and then polished to a roughness of about Ra ¼0.5 mm before plasma immersion ion implantation [11]. 2.2. PIII–D treatment Ti6Al7Nb alloy was laid on a stainless steel substrate attached to an insulated stainless steel electrode in the center of the vacuum chamber, and a negative voltage was applied to the electrode. The facility was equipped with an RF plasma source, hot filament glow discharge source, vacuum arc source, etc. The chamber was 1200 mm in height and 1000 mm in diameter. Before PIII, the samples were sputter-cleaned with argon plasma ion bombardment. The pre-treatment instrumental parameters were: RF forward energy¼1000 W with reflected power of
313
approximately 20 W, bias voltage¼ 2.5 k V, gas flow¼ 10 sccm, and clean time¼40 min. Nitrogen (N2) was bled into the vacuum chamber and nitrogen plasma was sustained using an RF power supply with a power of 1000 W, a work pressure of 5.5 10 4 Torr, and gas flow of 20 sccm. The implantation time was 40 min and the nitrogen-ion densities of implantation were 3 1017,5 1017, 7 1017 and 9 1017. After the immersion tests, optical microscopy was used to observe the surface morphology. 2.3. Fretting wear tests and analysis method Torsional fretting wear tests of untreated and plasma implantation nitride Ti6Al7Nb alloy against a Zr2O ball (with diameter of 25.2 mm and a roughness of Ra¼0.2 mm) were performed under simulated physiological conditions in a torsional fretting wear test rig [17]. The test medium was a 20% bovine serum solution (Shanghai Bao Man Biological Technology Co., Ltd.). All tests were performed at 25 1C in naturally aerated solution. Following immersion tests, an optical microscope was used to observe the surface morphology. Angular displacement of the contact pair was measured and controlled by a sensor in the motor system and then acted on as the control signal fed back to the control unit of the tester. In this study, torsional fretting tests were performed under a normal load of 100 N at a constant rotary speed of 0.21/s. The torsional angular displacement amplitudes were set at 0.51, 51, 151, and 451, and the number of cycles varied from 1 to 1000. After the torsional fretting wear tests, the 3D profiles were observed using a profilometer (Nano Map-Dual Mode), and surface morphologies and chemical analyses were investigated using a scanning electron microscope (SEM, KYKY2800) with an energy-dispersive X-ray spectrum (EDX).
3. Results and discussion 3.1. Surface analysis After PIII treatment without auxiliary heating, all surface samples were gold in color, typical of titanium nitride, although the formation of this phase was not detected by XRD. Fig. 1 includes SEM images of the original polished and nitrogen-ion PIIID-processed surfaces.
Fig. 1. Surface SEM morphologies of pre-test specimens. (a) substrate, (b) 3 1017N þ , (c) 5 1017N þ , (d) 7 1017N þ and (e) 9 1017N þ .
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The PIIID processes resulted in obvious changes in the surface topography. Compared to the mirror-like surface of the original substrate, surface roughness increased after the PIIID process, with grain-boundary embossing on the surface. Table 1 also shows the surface hardness of the untreated and treated specimens. The treatment resulted in a 30% increase in surface hardness over the original specimen (Hv ¼347.2), increasing to 465.8 (3 1017 N þ )-613.1(9 1017 N þ ), as shown in Table 1. This result indicates that nitrogen ion implantation by PIII can increase the surface hardness of the titanium alloy.
Table 1 Surface hardness and roughness of specimens. Surface
Hardness (Hv) Roughness Ra (mm)
3 1017 Nþ
5 1017 Nþ
7 1017 Nþ
347.2
465.8
492.4
513.6
613.1
0.5
4.2
4.7
5.6
5.4
Substrate
9 1017 Nþ
3.2. Frictional kinetics behavior For torsional fretting tests, kinetic behavior can be described from the friction torque versus angular displacement amplitude curves (T y curves) [6,7], which can estimate the running condition of fretting wear. The T y curves as a function of the number of cycles are shown in Figs. 2 and 3. At a lower angular displacement amplitude such as 0.51, the loops of torsional fretting wear were elliptical for both counterpairs (i.e., the treated and untreated titanium alloy against the ball specimen). This indicates that fretting was mostly accommodated by elastic-plastic deformation of the contact zones. The height of the curves decreased with the cycle number due to damage accumulation and work hardening. However, with an increase in ion implantation concentration, the loop shape narrowed and retained the same shape even with additional cycles. These variations are clearly related to the ion implantation concentration, and increased surface hardness induced a decrease in looping.
Fig. 2. T y curves as function of the number of cycles, y ¼0.51.
Z. Cai et al. / Tribology International 59 (2013) 312–320
315
Fig. 3. T y curves with increasing cycles, y ¼51.
When the angular displacement amplitude increased to y ¼ 51, the loops became wider and the T y curves in all cycles presented quasi-parallelogram shapes after the first few cycles (Fig. 3). According to the early research on torsional fretting wear [6,7,17], determination of the fretting running regime must be based on a combination of T y curves and the evolution of damage morphologies. From the evolution of wear morphologies, torsional fretting exists in the mixed fretting regime (MFR) under y ¼51. The tangential stress largely increased with an increase in angular displacement amplitude, which induced higher plastic deformation between the contact interfaces. Fig. 3 shows that frictional torque constantly increased during each sliding process due to the plastic deformation of the contact interfaces. The same principle occurred for processes under higher angles after an increase in ion implantation concentration; simultaneously, the shape of the fretting loops narrowed and the loop size decreased as a result of the higher surface hardness, retaining their shape even with additional cycles. The variation of frictional torque as a function of the number of cycles under an imposed normal load is displayed in Fig. 4.
Frictional torque has a close relationship with the imposed angular displacement amplitudes; that is, frictional torque increased with increased angular displacement amplitude. At smaller angles of 0.51 and 51 (Fig. 4(a) and (b)), the torque presented a smooth change that is carefully documented by the steady torque value (Fig. 5). The steady value of the torque decreased with surface modification treatment under all test conditions, possibly due to less deformation and damage to the ion implantation layers. Therefore, ion implantation also reduces the contact interface friction under torsional fretting wear. Under a higher angular displacement of 151 (Fig. 4(c)), fretting was conducted in the gross slip regime (GSR), and all of the torque curves presented three stages [7]. In the initial stage (stage I), the frictional torques were relatively low because of the protection and lubrication of adsorbed and polluted surface films. After approximately 100 cycles, the curves entered the ascent stage (stage II), where frictional torque gradually increased as a function of the number of cycles due to adhesion, abrasion, or plastic deformation of the contact interfaces. Finally, in the steady stage (stage III), the curves achieved a steady state in which frictional
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Fig. 4. Frictional torque of Ti6Al7Nb alloy and its nitrogen-ion implantation surface under various torsional angular displacements and implant concentrations.
torque fluctuated only within a narrow range and remained stable at a relatively high level. It can be assumed that the ion implantation improved the surface strength and hardness of the Ti6Al7Nb alloy, and the contact zone size and scar depth both decreased. Thus, the frictional torque for the ion implanted samples was reduced, and surface damage was delayed. As a result, stage III was not observed in specimens with N þ values above 5 1017(Fig. 4(c)).
3.3. Wear scar observation At y ¼0.51, there was only a surface indentation on the specimen as a result of very slight wear damage. The indentation could not be imaged under the optical microscope because of the reflection of light. The optical morphologies under different angular displacement amplitudes are shown in Figs. 6 and 7. For the Ti6Al7Nb alloy under the lower angular displacement amplitude of y ¼51, the torsional fretting occurred in the MFR according to the T–y curves in Fig. 3 (a). The morphology of a typical fretting annulus is presented in Fig. 6 (a); the contact center stuck, and the micro-slip occurred at the edge of circular contact zone. Similarly, the torsional fretting regime was identified as MFR for the four ion implantation specimens, according to the
T–y curves in Fig. 3(b) to (e). Only slight damage and shallow annular indentation were observed. The torsion fretting exhibited different behavior from the tangential fretting mode. The frictional force (i.e., tangential force) varied along the direction of the radius of the torsional fretting contact zone, which may explain the partial slip in the inner contact zone and the gross slip in the outer contact zone. In general, the frictional coefficients of materials varied with the number of cycles. Therefore, the MFR of torsional fretting often appeared in the central sticking zone and gradually lessened with additional cycles. In short, the wear scar morphology usually displayed an annular shape similar to the typical morphology under the tangential fretting mode. With the increase in angular displacement amplitude (y ¼151), the radius of the central stick zone (Fig. 7(a) (b)) fell to zero, i.e. sticking-zone damage disappeared. For untreated specimens, the wear scar size increased (Fig. 8) with increased damage, while the ion implantation surfaces presented smaller scars and less damage (Figs. 7(b)–(e)). In Fig. 7, the increased ion implantation concentration alleviated the detachment of particles, i.e., delamination decreased. Therefore, ion implantation alleviates delamination of this titanium alloy. Wear also decreased significantly with increased implantation concentration, an effect which can be attributed to the effective range of N þ ion implantation, the
Z. Cai et al. / Tribology International 59 (2013) 312–320
existence of a implanted layer.
TixNy
phase
and
the
Fig. 5. End-stage torque values
thickness
of
the
317
As observed in the SEM micrographs of the untreated wear scars in Fig. 9, delamination and abrasive wear were manifested by grooves, pits and scratches on the worn, flat specimen surfaces. The distinct plastic grooves and deformation, detached metallic particles and debris accumulation were clear under high magnification of the worn surfaces, as shown in Fig. 9(b) and (c). EDX analysis shown a higher oxygen peak on the spectrum for the detached area of the titanium alloy; therefore, titanium alloy samples exhibited very low torsional fretting resistance [18,19]. The external third body particles (debris) were definitively eliminated from the contact zone and distributed in a crown flake around the fretting scar. As a result, the wear mechanisms of the Ti6Al7Nb alloy were abrasive wear, oxidative wear and delamination. Ion implantation surface damage was slight (Fig. 10) compared with the above untreated wear scar. Only microcracks and minimal oxidative debris appear in the superficial zone. The 3D morphologies (Fig. 11) of the wear scar clearly illustrate this difference. The damage mechanisms for the implanted Ti6Al7Nb layer were slight abrasive wear and oxidative wear.
Fig. 6. Morphologies of the fretting wear scar, y ¼ 51.
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Fig. 7. Morphologies of the fretting wear scar, y ¼ 151.
4. Conclusion Nitrogen ion implantation into a biomedical Ti6Al7Nb alloy using the PBIII system was carried out, and the compositional and performance changes of the surface of Ti6Al7Nb were examined as a function of the nitrogen-ion density in the range of 3 1017 N þ to 9 1017 N þ . The main conclusions are as follows:
Fig. 8. Diameter of the wear contact zone.
The results indicate that the ion implantation layer can improve resistance to torsional fretting wear, suggesting potentially broad applications in artificial implants to inhibit fretting damage.
(1) Nitrogen ion implantation significantly increased the surface hardness and surface roughness of the Ti6Al7Nb alloy. (2) Compared with the substrate material, damage to ionimplanted surfaces was slighter, with smaller scars. Resistance against torsional fretting wear improved with increasing ion implantation concentration. (3) The damage mechanism of the Ti6Al7Nb alloy and its nitrogen-ion implanted layers was a combination of abrasive wear, oxidative wear, and delamination. However, detachment by delamination was inhibited with increased ion implantation concentration.
Z. Cai et al. / Tribology International 59 (2013) 312–320
Fig. 9. Morphologies of the wear scar in untreated alloy. (a) y ¼ 51, (b) y ¼151 and (c) y ¼151.
Fig. 10. Morphologies of the wear scar in nitrogen-ion implantation specimens. (a) 3 1017N þ , y ¼ 151, (b) 3 1017N þ , y ¼151 and (c) 7 1017N þ , y ¼151.
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for the Doctoral Program of Higher Education of China (20100184120002).
References
Fig. 11. 3D-morphologies of the wear scar, y ¼151, (a) un-treated and 7 1017N þ .
Acknowledgements The authors are grateful to Li-chun Bai (State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences), for the preparation of the ion implantation layer. This work was supported by the Open Project of the State Key Laboratory of Solid Lubrication (0903), Specialized Research Fund
[1] Jayanth Katta Zhongmin. Jin, Eileen Ingham,et al., Biotribology of articular cartilage—A review of the recent advances. Medical Engineering & Physics 2008;30:1349–63. [2] Margaret. AM, Donald WH, Kerry. C, et al. Implant retrieval studies of the wear and loosening of prosthetic joints: a review Wear 2000;241:158–65. [3] Geeth. M, Singh. AK, Asokamani. R, et al. Ti based biomaterials, the ultimate choice for orthopaedic implants–A review. Progress Mater Sci 2009;54: 397–425. [4] Gallego L, Ne´lias D, Deyber S. A fast and efficient contact algorithm for fretting problems applied to fretting modes I, II and III. Wear 2010;268: 208–22. [5] Mohrbacher H, Celis JP, Roos JR, et al. Laboratory testing of displacement and load induced fretting. Tribol Int 1995;28:269–78. [6] Cai ZB, Zhu MH, Zhou ZR. An experimental study of torsional fretting behavior of LZ50 steel. Tribol Int 2010;43:361–9. [7] Cai Zhen-bing, Gao Shan-shan, Zhu Min-hao, et al. Tribological behavior of polymethyl methacrylate against different counter-bodies induced by torsional fretting wear. Wear 2011;270:230–40. [8] Cai Zhen-bing, Zhu Min-hao, Yang Sha, et al. In situ observations of the realtime wear of PMMA flat against steel ball under torsional fretting. Wear 2011:2242–51. [9] Long M, Rack HJ. Titanium alloys in total joint repLacement-a materials science perspective. Biomaterials 1998;19:1621–39. [10] Rautray Tapash R, Narayanan R, Kim Kyo-Han. Ion implantation of titanium based biomaterials. Progress in Materials Science 2011;56:1137–77. [11] Henriques VAR, Sandim HRZ, Coelho GC, et al. Microstructural evolution during hot pressing of the blended elemental Ti6-%Al-7%Nb alloy. Materials Science and Engineering 2003;A347:315–24. [12] Sittig C, Haehner G, Marti A, et al. The implant material, Ti6Al7Nb: surface microstructure, composition and properties. Journal of Materials Science:Matertials in Medicine 1999;10:191–8. [13] Masumoto K, Horiuchi Y, Inamura T, et al. Effects of Si addition on superelastic properties of Ti–Nb–Al biomedical shape memory alloys. Materials Science and Engineering A 2006;438–440:835–8. [14] Viritpon Srimaneepong Takayuki, Yoneyama Equo Kobayashi, et al. Comparative study on torsional strength, ductility and fracture characteristics of laser-welded a þ b Ti–6Al–7Nb alloy, CP Titanium and Co–Cr alloy dental castings. Dental materials 24 2008:839–45. [15] Wang J, Huang N, Yang P, et al. The effects of amorphous carbon films deposited on polyethylene terephthalate on bacterial adhesion. Biomaterials 2004;25:3163–70. [16] Shalnov KV, Kukhta VK. Uemura, et al. Applications of combined ion implantation for improved tribological performance. Surface & Coatings Technology (2011) (in press). [17] Zhenbing Cai Minhao, Zhu Huoming, Shen, et al. Torsional fretting wear behaviors of 7075 aluminum alloy in various relative humidity environment. Wear 2009;267:330–9. [18] Lei MK, Ou YX, Wang KS, et al. Wear and corrosion properties of plasmabased low-energy nitrogen ion implanted titanium. Surface & Coatings Technology 2011;205:4602–7. [19] Fernandes AC, Vaz F, Ariza E, et al. Tribocorrosion behaviour of plasma nitrided and plasma nitridedþ oxidised Ti6Al4V alloy. Surf Coat Technol 2006;200:6218. [20] Zhen-bing Cai Shan-shan, Gao Xue-qi Gan, et al. Torsional fretting wear behavior of nature articular cartilage in vitro. International Journal of Surface Science and Engineering 2011;5(5/6):348–68.
Contents lists available at SciVerse ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Torsional fretting wear of a biomedical Ti6Al7Nb alloy for nitrogen ion implantation in bovine serum Zhen-bing Cai a, Guang-an Zhang b, Yong-kui Zhu a, Ming-xue Shen a, Li-ping Wang b, Min-hao Zhu a,n a b
Tribology Research Institute, Key Lab of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu, 610031, China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2011 Received in revised form 4 May 2012 Accepted 11 June 2012 Available online 21 June 2012
Ti6Al7Nb is a high-strength titanium alloy used in replacement hip joints that possesses the excellent biocompatibility necessary for surgical implants. Ti6Al7Nb treated with nitrogen gas (N2) plasma immersion ion implantation–deposition (PIII–D) was investigated. Torsional fretting wear tests of untreated and nitrogen-ion-implanted Ti6Al7Nb alloys against a Zr2O ball (diameter 25.2 mm) were carried out under simulated physiological conditions (serum solution) in a torsional fretting wear test rig. Based on the analyses of the frictional kinetics behavior, the observation of 3D profiles, SEM morphologies and surface composition analyses, the damage characteristics of the surface modification layer and its substrate are discussed in detail. The influence of nitrogen ion density on the implantation and torsional angular displacement amplitudes were investigated. The results indicated that ion implantation layering can improve resistance to torsional fretting wear and thus has wide potential application for the prevention of torsional fretting damage in artificial implants. The damage mechanism prevented by the ion implantation layer on the Ti6Al7Nb alloy is a combination of oxidative wear, delamination and abrasive wear. An increase in ion implantation concentration inhibited detachment by delamination. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Fretting wear Torsional fretting Plasma immersion ion implantation Titanium alloy
1. Introduction Many clinical conditions, including but not limited to arthritis, trauma and congenital or acquired joint diseases, lead to a loss of cartilage tissue in human joints. The intrinsic avascular and aneural nature of cartilage provides little scope for natural selfrepair. As a result, millions of people require treatment to repair damaged cartilage, usually at the major load-bearing articular joints such as the hips and knees [1]. Early clinical results provide evidence that loosening of one or both components of the Total Joint Replacement Surgery (TJR) occurs in 10% to 70% of patients 10 years after surgery [2]. Recent large-scale studies report that 10–20% of new joints must be replaced within 15–20 years, with aseptic loosening accounting for approximately 80% of these revisions [3]. Joint contacts experience very complex kinematics during walking, which is a combination of rolling and sliding. Three different sliding modes (transversal, radial, and circumferential) have been defined in the literature [4,5]. Torsional wear is one of the main motion modes of joints, and torsional fretting can be defined as the relative
n
Corresponding author. Tel.: þ86 28 87600715; fax: þ 86 28 87601342. E-mail addresses: [email protected], [email protected] (M.-h. Zhu). 0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2012.06.009
motion induced by reciprocating torsion under an oscillatory vibratory environment. Several solutions have been proposed to understand torsional contact and the mechanics of its behavior [6–8]. Titanium alloys are attractive materials for the manufacture of implants for medical and dental applications due to their superior biocompatibility and outstanding corrosion resistance compared with other conventional metallic materials [9,10]. Commercially pure titanium and Ti6Al4V alloy are the most commonly used materials in the manufacture of implants. Due to controversy around the potential toxic effects of vanadium compounds, V-free alloys such as Ti6Al7Nb have been recently developed for biomedical applications [11,12]. Ti6Al7Nb alloy is widely used because of its advantages in biocompatibility, mechanical properties and corrosion resistance [13,14]. Plasma immersion ion implantation–deposition (PIIID) is a rapidly developing surface modification technique that has shown effectiveness in modifying the physicochemical characteristics of thin films and mixing layers. In PIII–D, the target is typically enshrouded in self-excited plasma generated by applying a large negative voltage to the target [15,16]. However, the torsional wear behavior of Ti6Al7Nb and its implantation-treated nitrogen ion layer have not been clearly discussed. For a given torsional contact configuration, the resulting contact zone kinematics were found to profoundly influence accumulation, compaction and displacement of the debris particles generated during contact.
Z. Cai et al. / Tribology International 59 (2013) 312–320
When the torsional angular displacement amplitude is very small, accommodation by elastic deformation and a small plastic deformation are observed, and torsional fretting occurs in the partial-slip regime (PSR). When the torsional angular displacement amplitude is very large, which means that gross slip occurs throughout the process, torsional fretting occurs in the gross slip regime (GSR). The intermediate state is a mixed fretting regime (MFR). One dramatic phenomenon that occurs in MFR is a change in wear morphology from annular to an entire worn zone, accompanied by the disappearance of the sticking zone as a function of the number of cycles [7,8,20]. In this research, a new testing method was applied to reveal and compare the torsional fretting behavior of untreated and nitrogen-ion-implantation-treated biomedical Ti6Al7Nb alloy.
2. Experimental procedures 2.1. Materials Biomedical Ti6Al7Nb alloy ((wt %): 5.88Al, 6.65Nb, 0.03Fe, 0.10C, 0.20O, 0.07N, 0.02H and 87.05Ti) was used in the present investigation as a rod supplied by the Northwest Institute for Nonferrous Metal Research, Xi’an, China. The rod was machined flat to dimensions of 10 10 25 mm3. One side (10 mm 25 mm) of each sample was ground with 1500-grit diamond paper and then polished to a roughness of about Ra ¼0.5 mm before plasma immersion ion implantation [11]. 2.2. PIII–D treatment Ti6Al7Nb alloy was laid on a stainless steel substrate attached to an insulated stainless steel electrode in the center of the vacuum chamber, and a negative voltage was applied to the electrode. The facility was equipped with an RF plasma source, hot filament glow discharge source, vacuum arc source, etc. The chamber was 1200 mm in height and 1000 mm in diameter. Before PIII, the samples were sputter-cleaned with argon plasma ion bombardment. The pre-treatment instrumental parameters were: RF forward energy¼1000 W with reflected power of
313
approximately 20 W, bias voltage¼ 2.5 k V, gas flow¼ 10 sccm, and clean time¼40 min. Nitrogen (N2) was bled into the vacuum chamber and nitrogen plasma was sustained using an RF power supply with a power of 1000 W, a work pressure of 5.5 10 4 Torr, and gas flow of 20 sccm. The implantation time was 40 min and the nitrogen-ion densities of implantation were 3 1017,5 1017, 7 1017 and 9 1017. After the immersion tests, optical microscopy was used to observe the surface morphology. 2.3. Fretting wear tests and analysis method Torsional fretting wear tests of untreated and plasma implantation nitride Ti6Al7Nb alloy against a Zr2O ball (with diameter of 25.2 mm and a roughness of Ra¼0.2 mm) were performed under simulated physiological conditions in a torsional fretting wear test rig [17]. The test medium was a 20% bovine serum solution (Shanghai Bao Man Biological Technology Co., Ltd.). All tests were performed at 25 1C in naturally aerated solution. Following immersion tests, an optical microscope was used to observe the surface morphology. Angular displacement of the contact pair was measured and controlled by a sensor in the motor system and then acted on as the control signal fed back to the control unit of the tester. In this study, torsional fretting tests were performed under a normal load of 100 N at a constant rotary speed of 0.21/s. The torsional angular displacement amplitudes were set at 0.51, 51, 151, and 451, and the number of cycles varied from 1 to 1000. After the torsional fretting wear tests, the 3D profiles were observed using a profilometer (Nano Map-Dual Mode), and surface morphologies and chemical analyses were investigated using a scanning electron microscope (SEM, KYKY2800) with an energy-dispersive X-ray spectrum (EDX).
3. Results and discussion 3.1. Surface analysis After PIII treatment without auxiliary heating, all surface samples were gold in color, typical of titanium nitride, although the formation of this phase was not detected by XRD. Fig. 1 includes SEM images of the original polished and nitrogen-ion PIIID-processed surfaces.
Fig. 1. Surface SEM morphologies of pre-test specimens. (a) substrate, (b) 3 1017N þ , (c) 5 1017N þ , (d) 7 1017N þ and (e) 9 1017N þ .
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The PIIID processes resulted in obvious changes in the surface topography. Compared to the mirror-like surface of the original substrate, surface roughness increased after the PIIID process, with grain-boundary embossing on the surface. Table 1 also shows the surface hardness of the untreated and treated specimens. The treatment resulted in a 30% increase in surface hardness over the original specimen (Hv ¼347.2), increasing to 465.8 (3 1017 N þ )-613.1(9 1017 N þ ), as shown in Table 1. This result indicates that nitrogen ion implantation by PIII can increase the surface hardness of the titanium alloy.
Table 1 Surface hardness and roughness of specimens. Surface
Hardness (Hv) Roughness Ra (mm)
3 1017 Nþ
5 1017 Nþ
7 1017 Nþ
347.2
465.8
492.4
513.6
613.1
0.5
4.2
4.7
5.6
5.4
Substrate
9 1017 Nþ
3.2. Frictional kinetics behavior For torsional fretting tests, kinetic behavior can be described from the friction torque versus angular displacement amplitude curves (T y curves) [6,7], which can estimate the running condition of fretting wear. The T y curves as a function of the number of cycles are shown in Figs. 2 and 3. At a lower angular displacement amplitude such as 0.51, the loops of torsional fretting wear were elliptical for both counterpairs (i.e., the treated and untreated titanium alloy against the ball specimen). This indicates that fretting was mostly accommodated by elastic-plastic deformation of the contact zones. The height of the curves decreased with the cycle number due to damage accumulation and work hardening. However, with an increase in ion implantation concentration, the loop shape narrowed and retained the same shape even with additional cycles. These variations are clearly related to the ion implantation concentration, and increased surface hardness induced a decrease in looping.
Fig. 2. T y curves as function of the number of cycles, y ¼0.51.
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Fig. 3. T y curves with increasing cycles, y ¼51.
When the angular displacement amplitude increased to y ¼ 51, the loops became wider and the T y curves in all cycles presented quasi-parallelogram shapes after the first few cycles (Fig. 3). According to the early research on torsional fretting wear [6,7,17], determination of the fretting running regime must be based on a combination of T y curves and the evolution of damage morphologies. From the evolution of wear morphologies, torsional fretting exists in the mixed fretting regime (MFR) under y ¼51. The tangential stress largely increased with an increase in angular displacement amplitude, which induced higher plastic deformation between the contact interfaces. Fig. 3 shows that frictional torque constantly increased during each sliding process due to the plastic deformation of the contact interfaces. The same principle occurred for processes under higher angles after an increase in ion implantation concentration; simultaneously, the shape of the fretting loops narrowed and the loop size decreased as a result of the higher surface hardness, retaining their shape even with additional cycles. The variation of frictional torque as a function of the number of cycles under an imposed normal load is displayed in Fig. 4.
Frictional torque has a close relationship with the imposed angular displacement amplitudes; that is, frictional torque increased with increased angular displacement amplitude. At smaller angles of 0.51 and 51 (Fig. 4(a) and (b)), the torque presented a smooth change that is carefully documented by the steady torque value (Fig. 5). The steady value of the torque decreased with surface modification treatment under all test conditions, possibly due to less deformation and damage to the ion implantation layers. Therefore, ion implantation also reduces the contact interface friction under torsional fretting wear. Under a higher angular displacement of 151 (Fig. 4(c)), fretting was conducted in the gross slip regime (GSR), and all of the torque curves presented three stages [7]. In the initial stage (stage I), the frictional torques were relatively low because of the protection and lubrication of adsorbed and polluted surface films. After approximately 100 cycles, the curves entered the ascent stage (stage II), where frictional torque gradually increased as a function of the number of cycles due to adhesion, abrasion, or plastic deformation of the contact interfaces. Finally, in the steady stage (stage III), the curves achieved a steady state in which frictional
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Fig. 4. Frictional torque of Ti6Al7Nb alloy and its nitrogen-ion implantation surface under various torsional angular displacements and implant concentrations.
torque fluctuated only within a narrow range and remained stable at a relatively high level. It can be assumed that the ion implantation improved the surface strength and hardness of the Ti6Al7Nb alloy, and the contact zone size and scar depth both decreased. Thus, the frictional torque for the ion implanted samples was reduced, and surface damage was delayed. As a result, stage III was not observed in specimens with N þ values above 5 1017(Fig. 4(c)).
3.3. Wear scar observation At y ¼0.51, there was only a surface indentation on the specimen as a result of very slight wear damage. The indentation could not be imaged under the optical microscope because of the reflection of light. The optical morphologies under different angular displacement amplitudes are shown in Figs. 6 and 7. For the Ti6Al7Nb alloy under the lower angular displacement amplitude of y ¼51, the torsional fretting occurred in the MFR according to the T–y curves in Fig. 3 (a). The morphology of a typical fretting annulus is presented in Fig. 6 (a); the contact center stuck, and the micro-slip occurred at the edge of circular contact zone. Similarly, the torsional fretting regime was identified as MFR for the four ion implantation specimens, according to the
T–y curves in Fig. 3(b) to (e). Only slight damage and shallow annular indentation were observed. The torsion fretting exhibited different behavior from the tangential fretting mode. The frictional force (i.e., tangential force) varied along the direction of the radius of the torsional fretting contact zone, which may explain the partial slip in the inner contact zone and the gross slip in the outer contact zone. In general, the frictional coefficients of materials varied with the number of cycles. Therefore, the MFR of torsional fretting often appeared in the central sticking zone and gradually lessened with additional cycles. In short, the wear scar morphology usually displayed an annular shape similar to the typical morphology under the tangential fretting mode. With the increase in angular displacement amplitude (y ¼151), the radius of the central stick zone (Fig. 7(a) (b)) fell to zero, i.e. sticking-zone damage disappeared. For untreated specimens, the wear scar size increased (Fig. 8) with increased damage, while the ion implantation surfaces presented smaller scars and less damage (Figs. 7(b)–(e)). In Fig. 7, the increased ion implantation concentration alleviated the detachment of particles, i.e., delamination decreased. Therefore, ion implantation alleviates delamination of this titanium alloy. Wear also decreased significantly with increased implantation concentration, an effect which can be attributed to the effective range of N þ ion implantation, the
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existence of a implanted layer.
TixNy
phase
and
the
Fig. 5. End-stage torque values
thickness
of
the
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As observed in the SEM micrographs of the untreated wear scars in Fig. 9, delamination and abrasive wear were manifested by grooves, pits and scratches on the worn, flat specimen surfaces. The distinct plastic grooves and deformation, detached metallic particles and debris accumulation were clear under high magnification of the worn surfaces, as shown in Fig. 9(b) and (c). EDX analysis shown a higher oxygen peak on the spectrum for the detached area of the titanium alloy; therefore, titanium alloy samples exhibited very low torsional fretting resistance [18,19]. The external third body particles (debris) were definitively eliminated from the contact zone and distributed in a crown flake around the fretting scar. As a result, the wear mechanisms of the Ti6Al7Nb alloy were abrasive wear, oxidative wear and delamination. Ion implantation surface damage was slight (Fig. 10) compared with the above untreated wear scar. Only microcracks and minimal oxidative debris appear in the superficial zone. The 3D morphologies (Fig. 11) of the wear scar clearly illustrate this difference. The damage mechanisms for the implanted Ti6Al7Nb layer were slight abrasive wear and oxidative wear.
Fig. 6. Morphologies of the fretting wear scar, y ¼ 51.
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Fig. 7. Morphologies of the fretting wear scar, y ¼ 151.
4. Conclusion Nitrogen ion implantation into a biomedical Ti6Al7Nb alloy using the PBIII system was carried out, and the compositional and performance changes of the surface of Ti6Al7Nb were examined as a function of the nitrogen-ion density in the range of 3 1017 N þ to 9 1017 N þ . The main conclusions are as follows:
Fig. 8. Diameter of the wear contact zone.
The results indicate that the ion implantation layer can improve resistance to torsional fretting wear, suggesting potentially broad applications in artificial implants to inhibit fretting damage.
(1) Nitrogen ion implantation significantly increased the surface hardness and surface roughness of the Ti6Al7Nb alloy. (2) Compared with the substrate material, damage to ionimplanted surfaces was slighter, with smaller scars. Resistance against torsional fretting wear improved with increasing ion implantation concentration. (3) The damage mechanism of the Ti6Al7Nb alloy and its nitrogen-ion implanted layers was a combination of abrasive wear, oxidative wear, and delamination. However, detachment by delamination was inhibited with increased ion implantation concentration.
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Fig. 9. Morphologies of the wear scar in untreated alloy. (a) y ¼ 51, (b) y ¼151 and (c) y ¼151.
Fig. 10. Morphologies of the wear scar in nitrogen-ion implantation specimens. (a) 3 1017N þ , y ¼ 151, (b) 3 1017N þ , y ¼151 and (c) 7 1017N þ , y ¼151.
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for the Doctoral Program of Higher Education of China (20100184120002).
References
Fig. 11. 3D-morphologies of the wear scar, y ¼151, (a) un-treated and 7 1017N þ .
Acknowledgements The authors are grateful to Li-chun Bai (State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences), for the preparation of the ion implantation layer. This work was supported by the Open Project of the State Key Laboratory of Solid Lubrication (0903), Specialized Research Fund
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