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Characterization of fretting fatigue damage of PVD TiN coated biomedical titanium alloys
Surface & Coatings Technology 200 (2006) 4538 – 4542 www.elsevier.com/locate/surfcoat
Characterization of fretting fatigue damage of PVD TiN coated biomedical titanium alloys Aravind Vadiraj, M. Kamaraj* Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai-600036, India Received 6 December 2004; accepted in revised form 28 March 2005 Available online 26 May 2005
Abstract Fretting fatigue is a form of adhesive wear damage due to small oscillatory movement between two contacting bodies under the action of uniform or non-uniform cyclic loads. Cyclic loads may be experienced due to vibration of one or both the bodies eventually leading to failure at the contact area. Fretting damage is also experienced by load bearing implants within the body environment such as hip joints, knee joints, bone plates, etc. Damage characterization is important from the view of minimizing in-vivo failures. Titanium alloys are frequently used as bioimplants due to its excellent biocompatibility and low modulus of elasticity compared to stainless steel or Co – Cr – Mo alloys. Fretting wear damage of load bearing implants can be minimized through suitable surface modification process. Ti – 6Al – 4V and Ti – 6Al – 7Nb are commonly used for biomedical applications and PVD TiN coated alloys are used for our fretting fatigue studies. Fretting fatigue life of PVD TiN coated alloys improved compared to uncoated alloys. D 2005 Elsevier B.V. All rights reserved. Keywords: Fretting fatigue; Scanning electron microscopy (SEM); Physical vapor deposition; Titanium alloys
1. Introduction Systematic investigation of fretting began in 1927 by Tomlinson who described the possible mechanisms of its occurrences. Fretting is a wear phenomenon caused due to small oscillatory motion between two interacting surfaces under normal load [1]. If fretting action is encountered under heavy pressure, it will produce wear debris and loosening of components depending upon the environment around the joint. Therefore, fretting of components under cyclic load leads to in-service failures without prior warning. Human joints such as hip, knee or shoulder joints are highly prone to degeneration leading to acute pain and joint stiffness commonly termed as osteoarthritis and rheumatoid arthritis. These load bearing joints necessitate the arthroplastic surgery involving complete replacement of diseased joint surfaces with biocompatible devices.
* Corresponding author. Tel.: +91 44 22574769; fax: +91 44 22570545. E-mail address: [email protected] (M. Kamaraj). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.03.036
Fig. 1A shows the schematic sketch of hip prostheses, which will experience compressive cyclic loads within the body. Fig. 1B shows the bone plate in contact with a screw head experiencing fretting motion due to cyclic load. Modular joints and stem-bone interface are the two possible areas of fretting damage in hip prostheses. Kamachi Mudali et al. [2] have made an extensive analysis on failure of stainless steel orthopedic devices. According to their assessment, 74% of the implants failed at the femoral neck region due to fretting fatigue. Elements such as Al and V are known to cause toxicity [3 –5]. Although they are present as stable oxides on the surface, fretting action releases them into the body fluid [6]. Surface engineering is one the emerging areas in the field of material science or tribology aiming at modification of interacting surfaces of the components for improving their life and efficiency while retaining their original bulk properties. In biomedical devices, quality overrules the cost of the product. Therefore, it is necessary to maintain the superior quality of the device for better performance in the human body. Physical vapor deposition (PVD) coatings,
A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
4539
Fig. 2. Fretting fatigue apparatus.
Fig. 1. Fretting fatigue of (A) hip implant and (B) bone plate.
plasma nitriding and ion implantation are some of the popular surface modification processes used for biomedical devices. It favors the formation of hard titanium nitride (TiN) layer on the surface. Titanium nitride (TiN) layer is characterized by high abrasion resistance, low coefficient of friction, high temperature stability and high hardness. The quality of the film depends upon the control of the process. Huang nan et al. [7] have studied the fatigue behavior of titanium based biomaterial coated with 1.4 Am titanium nitride film by ion beam enhanced deposition. Shenhar et al. [8] have characterized residual stresses and fretting wear behavior of 2 Am TiN coating developed on surgical titanium alloys by powder immersion reaction assisted coating method. A major reduction in fretting damage was reported from the coated alloys. R&D group of Multi Arc India Ltd. reported delamination of PVD TiN coatings of higher thickness. So we have made an attempt to study the fretting fatigue behavior of 2 Am thick PVD TiN coated titanium alloys.
DATEC servo-hydraulic UTM. Normal pressure was applied through calibrated proving ring-contact pad arrangement as shown in Fig. 2. Contact pressure of 40 MPa was applied before beginning the test. Friction coefficient is continuously recorded from strain gages bonded to ring and pad holders, which is connected to five channel strain amplifier. Fretting fatigue studies of titanium alloys is more meaningful in a physiological medium representing the body fluids. Ringer solution, phosphate buffered solution (PBS) and Hank’s balanced salt solution (HBSS) are some of the widely accepted mediums for preliminary biomaterial fretting fatigue studies [9]. Starosvetsky and Gotman [10] have studied the corrosion behavior of 1 Am titanium nitrided coating on Ni –Ti alloy in ringer solution although the protein may influence the surface. Laure Duisabeau et al. [11] have conducted fretting wear test for Ti– 6Al –4V and AISI 316 L stainless steel in ringer solution. They comment that the presence of a solution containing chloride ions activates a localized corrosion phenomenon, which leads to modification of the displacement regime. Wear tests of biomedical alloys are normally conducted with dilute
2. Experimental details Ti– 6Al– 4V bars (hot rolled and annealed) of dimension 200 mm 60 mm 10 mm were obtained from Vikram Sarabhai Space Center (VSSC), India, and Ti –6Al – 7Nb in the form of precision ground rod with diameter 16 mm and length 127 mm were procured form Carpenter Tech, USA. Fatigue test specimens were machined from Ti–6Al– 4V and fretting pads from Ti–6Al – 7Nb alloys, respectively. Both the pads and specimens were polished to less than 0.1R a and coated with 2 Am thick PVD TiN at Multi-Arc India Limited. The hardness of TiN coating was reported as 2800 HVN with adhesion strength of 90 N. Flat-on-flat contact fretting fatigue tests were conducted in 100 kN
Fig. 3. S – N curve for fretting fatigue of titanium alloys.
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A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
bovine serum in varying concentration. It is also observed that high temperatures generated during fretting will denature the protein and increases the viscosity of the medium. Although the fretting motion is affected by the proteins in the body, the effect of chloride ions is considered in our experiments. When all the necessary parameters are optimized from our pilot studies, the fretting fatigue tests will be conducted in an enclosed chamber with protein and saline liquid, which will perfectly simulate the body environment. Ringer solution maintained at 37 -C is continuously streamed on pads and collected in a container below. This flow is adjusted to obtain complete immersion of contact area similar to fretting action of hip joints
surrounded by continuous flow of physiological medium. Cyclic loads of 3 to 7.5 kN at 5 Hz frequency were applied during the test.
3. Results and discussion Fig. 3 shows the S – N curve for fretting fatigue of titanium alloys. Fretting fatigue of uncoated Ti– 6Al –4V failed within 3% of the plain fatigue life. Plain fatigue life of PVD TiN coated Ti– 6Al – 4V at 500 MPa cyclic load did not fail even after 106 cycles. This indicates TiN coatings prevent the formation of surface intrusions and extrusions, which are generally responsible for fatigue failures.
Fig. 4. (A) Fretting scar of uncoated Ti – 6Al – 4V and (B) EDS spectra of fretting scar.
A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
Fig. 5. Comparison of friction coefficient of uncoated and PVD TiN coated alloys at 500 MPa cyclic load.
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Fretting fatigue of uncoated titanium alloys is characterized by surface damage in the form of deep scratches, particle detachment and transfer from counter surface as shown in Fig. 4A. EDS peaks of the scar as shown in Fig. 4B indicate Nb from the particle of the pad surface. The specimen and pad are almost the same material. So they exhibit high metallurgical compatibility, which is the primary reason for the observed damage. Comparison of friction coefficients of the uncoated and TiN coated alloys is as shown in Fig. 5. Higher friction is being experienced without TiN coatings. For uncoated alloys, friction coefficient gradually increases and later decreases with the on going fretting damage. Squeaking
Fig. 6. Edge of contact failure area of PVD TiN coated alloy.
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A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
Fig. 7. Roughness profile of various surfaces.
sound from the contacts was also witnessed intermittently during fretting process indicating that the contact is dwelling in stick-slip regime. Initially, the fretting is between the natural oxide screen, which later gets disrupted during the sliding process exposing bare substrate for frequent adhesion and fracture. Reduction of friction coefficient at the later stages of fretting of uncoated alloys can be attributed to continuous flow of ringer solution providing lubricating effect at the contact. Fretting cracks were mostly observed to have nucleated within the contact area for all the specimens. Nucleated cracks initially propagate inclined to the loading axis and later align perpendicular. Friction is minimized with TiN coatings due to its excellent tribological properties compared to uncoated alloys. But it could not withstand the fretting process for a long time as observed in Fig. 6A. The film ruptured at the later stages of fretting leaving networks of cracked layers Fig. 6B shows EDS spectra of ruptured film indicating Al and V peak from the damaged site. Delaminated TiN particles acted as potential abrasives for inducing damage on both the surfaces. Their sizes vary from less than a micron to more than 10 Am. Particles with sharper edges will be more harmful as they easily dig into the surface during fretting. Fig. 7 shows the surface roughness developed due to fretting fatigue damage. Fretting damage seems to be more severe for uncoated surface compared to TiN coated alloys.
4. Conclusions Fretting fatigue failures of biomedical implants is one of the serious concerns in the area of orthopedic implant surgery. Surface damaged is characterized by deep scratches with debris particles within the contact area. Wear debris are known to cause toxicity and loosening of implants and must be prevented by surface modification of the implants. Fretting fatigue crack propagation is inclined from the surface during initial stages and later aligns perpendicular to the stress axis.
PVD TiN coated alloys have shown improvements in fretting fatigue life compared to uncoated alloys. Steady state friction coefficients are less for PVD TiN compared to uncoated specimens. The contact suffered damage when TiN film ruptured. Roughness profile indicates that the surface damage severity is less compared to uncoated alloys. EDS spectra of failure edge of PVD coated alloys indicates Al and V present in the substrate.
Acknowledgement We wish to thank Mr. J.A. Suresh for all his technical assistance during the course of our experiments. We express our genuine gratitude to Carpenter Technology, USA for providing us the medical grade Ti6Al– 7Nb alloys. We are also grateful to Mr. Prabhakar, Multi Arc India Ltd. for his assistance in developing PVD TiN coatings for titanium alloys.
References [1] R.B. Waterhouse, Fretting Corrosion, ch. 7, Pergamon press, 1972. [2] U. Kamachi Mudali, T.M. Sridhar, N. Eliaz, Baladev Raj, Corros. Rev. 21 (2 – 3) (2003) 231. [3] N. Hallab, et al., J. Bone Joint Surg., 2001-Am. 83-A (2001) 428. [4] S.H. Teoh, Int. J. Fatigue 22 (2000) 825. [5] Marc Long, H.J. Rack, Biomaterials 19 (1998) 1621. [6] R.A. Antoniou, T.C. Radtke, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 237 (1997) 229. [7] Huang nan, Chen Yuanru, Cai Guangjun, Lin Chenggang, Wang Zhongguang, Yiao Guo, Su Huehe, Liu Xianghuai, Zhen Zhihong, Surf. Coat. Technol. 88 (1996) 127. [8] A. Shenhar, I. Gotman, S. Radin, P. Ducheyne, E.Y. Gutmanas, Surf. Coat. Technol. 126 (2000) 210, (ISBN: 0-08-044150-5). [9] M. Sumita, T. Hanawa, I. Ohnishi, T. Yoneyama, Comprehensive Structural Integrity, vol. 9. ISBN: 0-08-044150-5, 2003 p. 131. [10] D. Starosvetsky, I. Gotman, Biomaterials 22 (2001) 1853. [11] Laure Duisabeau, Pierre Combrade, Bernard Forest, Wear 256 (2004) 805.
Characterization of fretting fatigue damage of PVD TiN coated biomedical titanium alloys Aravind Vadiraj, M. Kamaraj* Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai-600036, India Received 6 December 2004; accepted in revised form 28 March 2005 Available online 26 May 2005
Abstract Fretting fatigue is a form of adhesive wear damage due to small oscillatory movement between two contacting bodies under the action of uniform or non-uniform cyclic loads. Cyclic loads may be experienced due to vibration of one or both the bodies eventually leading to failure at the contact area. Fretting damage is also experienced by load bearing implants within the body environment such as hip joints, knee joints, bone plates, etc. Damage characterization is important from the view of minimizing in-vivo failures. Titanium alloys are frequently used as bioimplants due to its excellent biocompatibility and low modulus of elasticity compared to stainless steel or Co – Cr – Mo alloys. Fretting wear damage of load bearing implants can be minimized through suitable surface modification process. Ti – 6Al – 4V and Ti – 6Al – 7Nb are commonly used for biomedical applications and PVD TiN coated alloys are used for our fretting fatigue studies. Fretting fatigue life of PVD TiN coated alloys improved compared to uncoated alloys. D 2005 Elsevier B.V. All rights reserved. Keywords: Fretting fatigue; Scanning electron microscopy (SEM); Physical vapor deposition; Titanium alloys
1. Introduction Systematic investigation of fretting began in 1927 by Tomlinson who described the possible mechanisms of its occurrences. Fretting is a wear phenomenon caused due to small oscillatory motion between two interacting surfaces under normal load [1]. If fretting action is encountered under heavy pressure, it will produce wear debris and loosening of components depending upon the environment around the joint. Therefore, fretting of components under cyclic load leads to in-service failures without prior warning. Human joints such as hip, knee or shoulder joints are highly prone to degeneration leading to acute pain and joint stiffness commonly termed as osteoarthritis and rheumatoid arthritis. These load bearing joints necessitate the arthroplastic surgery involving complete replacement of diseased joint surfaces with biocompatible devices.
* Corresponding author. Tel.: +91 44 22574769; fax: +91 44 22570545. E-mail address: [email protected] (M. Kamaraj). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.03.036
Fig. 1A shows the schematic sketch of hip prostheses, which will experience compressive cyclic loads within the body. Fig. 1B shows the bone plate in contact with a screw head experiencing fretting motion due to cyclic load. Modular joints and stem-bone interface are the two possible areas of fretting damage in hip prostheses. Kamachi Mudali et al. [2] have made an extensive analysis on failure of stainless steel orthopedic devices. According to their assessment, 74% of the implants failed at the femoral neck region due to fretting fatigue. Elements such as Al and V are known to cause toxicity [3 –5]. Although they are present as stable oxides on the surface, fretting action releases them into the body fluid [6]. Surface engineering is one the emerging areas in the field of material science or tribology aiming at modification of interacting surfaces of the components for improving their life and efficiency while retaining their original bulk properties. In biomedical devices, quality overrules the cost of the product. Therefore, it is necessary to maintain the superior quality of the device for better performance in the human body. Physical vapor deposition (PVD) coatings,
A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
4539
Fig. 2. Fretting fatigue apparatus.
Fig. 1. Fretting fatigue of (A) hip implant and (B) bone plate.
plasma nitriding and ion implantation are some of the popular surface modification processes used for biomedical devices. It favors the formation of hard titanium nitride (TiN) layer on the surface. Titanium nitride (TiN) layer is characterized by high abrasion resistance, low coefficient of friction, high temperature stability and high hardness. The quality of the film depends upon the control of the process. Huang nan et al. [7] have studied the fatigue behavior of titanium based biomaterial coated with 1.4 Am titanium nitride film by ion beam enhanced deposition. Shenhar et al. [8] have characterized residual stresses and fretting wear behavior of 2 Am TiN coating developed on surgical titanium alloys by powder immersion reaction assisted coating method. A major reduction in fretting damage was reported from the coated alloys. R&D group of Multi Arc India Ltd. reported delamination of PVD TiN coatings of higher thickness. So we have made an attempt to study the fretting fatigue behavior of 2 Am thick PVD TiN coated titanium alloys.
DATEC servo-hydraulic UTM. Normal pressure was applied through calibrated proving ring-contact pad arrangement as shown in Fig. 2. Contact pressure of 40 MPa was applied before beginning the test. Friction coefficient is continuously recorded from strain gages bonded to ring and pad holders, which is connected to five channel strain amplifier. Fretting fatigue studies of titanium alloys is more meaningful in a physiological medium representing the body fluids. Ringer solution, phosphate buffered solution (PBS) and Hank’s balanced salt solution (HBSS) are some of the widely accepted mediums for preliminary biomaterial fretting fatigue studies [9]. Starosvetsky and Gotman [10] have studied the corrosion behavior of 1 Am titanium nitrided coating on Ni –Ti alloy in ringer solution although the protein may influence the surface. Laure Duisabeau et al. [11] have conducted fretting wear test for Ti– 6Al –4V and AISI 316 L stainless steel in ringer solution. They comment that the presence of a solution containing chloride ions activates a localized corrosion phenomenon, which leads to modification of the displacement regime. Wear tests of biomedical alloys are normally conducted with dilute
2. Experimental details Ti– 6Al– 4V bars (hot rolled and annealed) of dimension 200 mm 60 mm 10 mm were obtained from Vikram Sarabhai Space Center (VSSC), India, and Ti –6Al – 7Nb in the form of precision ground rod with diameter 16 mm and length 127 mm were procured form Carpenter Tech, USA. Fatigue test specimens were machined from Ti–6Al– 4V and fretting pads from Ti–6Al – 7Nb alloys, respectively. Both the pads and specimens were polished to less than 0.1R a and coated with 2 Am thick PVD TiN at Multi-Arc India Limited. The hardness of TiN coating was reported as 2800 HVN with adhesion strength of 90 N. Flat-on-flat contact fretting fatigue tests were conducted in 100 kN
Fig. 3. S – N curve for fretting fatigue of titanium alloys.
4540
A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
bovine serum in varying concentration. It is also observed that high temperatures generated during fretting will denature the protein and increases the viscosity of the medium. Although the fretting motion is affected by the proteins in the body, the effect of chloride ions is considered in our experiments. When all the necessary parameters are optimized from our pilot studies, the fretting fatigue tests will be conducted in an enclosed chamber with protein and saline liquid, which will perfectly simulate the body environment. Ringer solution maintained at 37 -C is continuously streamed on pads and collected in a container below. This flow is adjusted to obtain complete immersion of contact area similar to fretting action of hip joints
surrounded by continuous flow of physiological medium. Cyclic loads of 3 to 7.5 kN at 5 Hz frequency were applied during the test.
3. Results and discussion Fig. 3 shows the S – N curve for fretting fatigue of titanium alloys. Fretting fatigue of uncoated Ti– 6Al –4V failed within 3% of the plain fatigue life. Plain fatigue life of PVD TiN coated Ti– 6Al – 4V at 500 MPa cyclic load did not fail even after 106 cycles. This indicates TiN coatings prevent the formation of surface intrusions and extrusions, which are generally responsible for fatigue failures.
Fig. 4. (A) Fretting scar of uncoated Ti – 6Al – 4V and (B) EDS spectra of fretting scar.
A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
Fig. 5. Comparison of friction coefficient of uncoated and PVD TiN coated alloys at 500 MPa cyclic load.
4541
Fretting fatigue of uncoated titanium alloys is characterized by surface damage in the form of deep scratches, particle detachment and transfer from counter surface as shown in Fig. 4A. EDS peaks of the scar as shown in Fig. 4B indicate Nb from the particle of the pad surface. The specimen and pad are almost the same material. So they exhibit high metallurgical compatibility, which is the primary reason for the observed damage. Comparison of friction coefficients of the uncoated and TiN coated alloys is as shown in Fig. 5. Higher friction is being experienced without TiN coatings. For uncoated alloys, friction coefficient gradually increases and later decreases with the on going fretting damage. Squeaking
Fig. 6. Edge of contact failure area of PVD TiN coated alloy.
4542
A. Vadiraj, M. Kamaraj / Surface & Coatings Technology 200 (2006) 4538 – 4542
Fig. 7. Roughness profile of various surfaces.
sound from the contacts was also witnessed intermittently during fretting process indicating that the contact is dwelling in stick-slip regime. Initially, the fretting is between the natural oxide screen, which later gets disrupted during the sliding process exposing bare substrate for frequent adhesion and fracture. Reduction of friction coefficient at the later stages of fretting of uncoated alloys can be attributed to continuous flow of ringer solution providing lubricating effect at the contact. Fretting cracks were mostly observed to have nucleated within the contact area for all the specimens. Nucleated cracks initially propagate inclined to the loading axis and later align perpendicular. Friction is minimized with TiN coatings due to its excellent tribological properties compared to uncoated alloys. But it could not withstand the fretting process for a long time as observed in Fig. 6A. The film ruptured at the later stages of fretting leaving networks of cracked layers Fig. 6B shows EDS spectra of ruptured film indicating Al and V peak from the damaged site. Delaminated TiN particles acted as potential abrasives for inducing damage on both the surfaces. Their sizes vary from less than a micron to more than 10 Am. Particles with sharper edges will be more harmful as they easily dig into the surface during fretting. Fig. 7 shows the surface roughness developed due to fretting fatigue damage. Fretting damage seems to be more severe for uncoated surface compared to TiN coated alloys.
4. Conclusions Fretting fatigue failures of biomedical implants is one of the serious concerns in the area of orthopedic implant surgery. Surface damaged is characterized by deep scratches with debris particles within the contact area. Wear debris are known to cause toxicity and loosening of implants and must be prevented by surface modification of the implants. Fretting fatigue crack propagation is inclined from the surface during initial stages and later aligns perpendicular to the stress axis.
PVD TiN coated alloys have shown improvements in fretting fatigue life compared to uncoated alloys. Steady state friction coefficients are less for PVD TiN compared to uncoated specimens. The contact suffered damage when TiN film ruptured. Roughness profile indicates that the surface damage severity is less compared to uncoated alloys. EDS spectra of failure edge of PVD coated alloys indicates Al and V present in the substrate.
Acknowledgement We wish to thank Mr. J.A. Suresh for all his technical assistance during the course of our experiments. We express our genuine gratitude to Carpenter Technology, USA for providing us the medical grade Ti6Al– 7Nb alloys. We are also grateful to Mr. Prabhakar, Multi Arc India Ltd. for his assistance in developing PVD TiN coatings for titanium alloys.
References [1] R.B. Waterhouse, Fretting Corrosion, ch. 7, Pergamon press, 1972. [2] U. Kamachi Mudali, T.M. Sridhar, N. Eliaz, Baladev Raj, Corros. Rev. 21 (2 – 3) (2003) 231. [3] N. Hallab, et al., J. Bone Joint Surg., 2001-Am. 83-A (2001) 428. [4] S.H. Teoh, Int. J. Fatigue 22 (2000) 825. [5] Marc Long, H.J. Rack, Biomaterials 19 (1998) 1621. [6] R.A. Antoniou, T.C. Radtke, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 237 (1997) 229. [7] Huang nan, Chen Yuanru, Cai Guangjun, Lin Chenggang, Wang Zhongguang, Yiao Guo, Su Huehe, Liu Xianghuai, Zhen Zhihong, Surf. Coat. Technol. 88 (1996) 127. [8] A. Shenhar, I. Gotman, S. Radin, P. Ducheyne, E.Y. Gutmanas, Surf. Coat. Technol. 126 (2000) 210, (ISBN: 0-08-044150-5). [9] M. Sumita, T. Hanawa, I. Ohnishi, T. Yoneyama, Comprehensive Structural Integrity, vol. 9. ISBN: 0-08-044150-5, 2003 p. 131. [10] D. Starosvetsky, I. Gotman, Biomaterials 22 (2001) 1853. [11] Laure Duisabeau, Pierre Combrade, Bernard Forest, Wear 256 (2004) 805.