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Tribological behavior of PIII treated AISI 316 L austenitic stainless steel against UHMWPE counterface
Wear 261 (2006) 264–268
Tribological behavior of PIII treated AISI 316 L austenitic stainless steel against UHMWPE counterface N. Saklako˘glu a,∗ , ˙I.E. Saklako˘glu b , K.T. Short c , G.A. Collins c a
Celal Bayar University, Faculty of Engineering, Mechanical Engineering Department, Muradiye Kampusu, Manisa 45140, Turkey b Ege University, Faculty of Engineering, Mechanical Engineering Department, Izmir, Turkey c Australian Nuclear Science and Technology Organisation, NSW, Australia Received 15 February 2005; received in revised form 31 August 2005; accepted 24 October 2005 Available online 18 January 2006
Abstract The aim of this work was to study the tribological benefits of PIII treated austenitic stainless steel by nitrogen ions and/or C ions in a ringer solution, which simulates the environment of the human body. This was achieved by hardness, wear and friction testing, and atomic force microscopy and XRD studies. The results showed that the samples, both treated and untreated, exhibited virtually no wear from contact with the ultra high molecular weight polyethylene (UHMWPE) pins, however, the pins themselves exhibited wear. The amount of wear of the pins was found to decrease with increasing PIII treatment temperature, but addition of C to the chamber caused some increase the amount of wear on the pins. Although C ions reduced to improve the hardness, friction characteristic was improved by formation of carbon-expanded austenite. © 2005 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Austenitic stainless steel; Tribology; Surface modification; UHMWPE
1. Introduction Modern implants such as hip and knee prostheses are made of a metallic femoral component (316 L stainless steel, Ti6AI4V, or cobalt–chromium), which is in sliding contact with an ultra high molecular weight polyethylene (UHMWPE) acetabular cup. However, numerous clinical studies have raised concerns related to the detrimental effect of wear debris from artificial hip prostheses to the tissue surrounding the implants. Several solutions to this problem have been proposed. Improvements in the tribological properties of stainless steel achieved by ion implantation have been widely described elsewhere [1–3]. Over the last 10 years, Collins and co-workers from ANSTO have studied the surface treatment of austenitic stainless steel using the plasma immersion ion implantation (PIII) technique [4–6], developed by ANSTO. PIII is a non-line-of-sight technique for the surface modification of materials, but it is a hybrid treatment involving implantation and thermodynamically driven diffusion of nitrogen [7–10].
∗
Corresponding author. Tel.: +90 5337275727; fax: +90 2362412143. E-mail address: [email protected] (N. Saklako˘glu).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.10.008
In this paper, the tribological characteristics of AISI 316 L stainless steel in a medium called ‘ringer solution’ have been studied, because this solution chemically simulates the environment surrounding metallic implants in the human body. 2. Experimental details The samples chosen for this investigation were AISI 316 L stainless steel 30 mm in diameter and 4-mm thick. Each sample was ultrasonically cleaned in ethanol before loading into a treatment chamber. PIII treatment was performed at ANSTO, using the system which has been previously described elsewhere [4,8]. Samples were treated at 320, 380 and 450 ◦ C respectively with and without C ions. The sample chamber was pumped to a base pressure of <2 × 10−7 mbar before each PIII treatment was begun. Nitrogen implantations were performed, applying negative high voltage pulses of 30 kV. Prior to the implantation, an Ar/H2 preclean at 15 kV/100 Hz HV pulsing during heating up was performed. For nitrogen + carbon implantations 90% N2 + 10% CH4 gases were used. The details of the treatment parameters are given in Table 1. Surface damage occurring after the PIII treatment was examined using an atomic force microscope (AFM).
N. Saklako˘glu et al. / Wear 261 (2006) 264–268 Table 1 PIII treatment details Sample name
Temperature (◦ C)
Time (h)
Nitrogen dose (at. cm−2 )
Untreated 320 ◦ C–5 h 380 ◦ C–5 h 380 ◦ C–5 h–10% CH4 450 ◦ C–5 h–10% CH4
– 320 380 380 450
– 5 5 5 5
– ∼2 × 1018 ∼2 × 1018 ∼2 × 1018 ∼2 × 1018
Microhardness was measured using a UMIS-2000 ultra microhardness tester with a Berkovich indenter. This was achieved by performing hardness tests directly on the surface at loads of 50, 250 and 1000 mN. Five indents were made at each load on each sample from which average values were calculated. Wear measurement of the treated and untreated 316 discs were performed on a pin-on-disc tribometer (CSEM) using a
265
UHMWPE counterface (6-mm diameter) in the ringer solution. It was carried out under a load of 5 N and a constant speed of 0.3 m/s. Each disc was tested for 500 m (corresponding to 9969 turns). The coefficient of friction was recorded during wear testing by a transducer on the load arm of the tribometer. The quantitative value of wear was obtained by measuring the cross sectional area of the wear track and then converting this to the wear volume.
3. Results and discussion 3.1. Surface examination After treatments, all samples exhibited slight discolouration on the surface. The surface appearance of the untreated and treated samples was observed by AFM scanning. Fig. 1 shows 2D representations of the surface appearance of samples.
Fig. 1. The surfaces of untreated and treated samples by AFM.
266
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Fig. 2. Friction coefficients of the samples as a function of the distance.
3.2. Friction coefficient As is seen in Fig. 2, all friction curves belong to PIII treated samples lied under untreated sample’s, but no clear improvement. PIII treatments with C ions were located in the bottom reducing the friction coefficient. Notably, the 450 ◦ C–5 h–10% CH4 process reduced the friction coefficient by a factor of ∼1.6. 3.3. Microhardness The hardness results for the PIII treated and untreated samples can be seen in Fig. 3. It was observed that the hardness increased on the near surface with increasing process temperature. The addition of C ions was decreased to improve the hardness.
with increasing PIII process temperature. The addition of C ions to the chamber during PIII process was increased the wear of UHMWPE pins compared to PIII treatment without C ions. 3.5. XRD-examination Glancing angle X-ray diffraction patterns for untreated and PIII treated samples are shown in Fig. 6. The peaks in the PIII treated samples, below the positions of the standard austenite peaks, indicate the formation of expanded austenite. The peak shifts are smaller for the carbon-expanded austenite. According to Blawert et al. [10] that the nitrogen and carbon is randomly distributed in the middle of the cube edges, expanding
3.4. Wear The samples, both treated and untreated, exhibited virtually no wear. The amount of wear of the samples was so slight that it was not possible to measure the amount of wear volume. There were merely slight tracks showing a colour change on the sample surfaces. It was seen that there was a large amount of wear debris deposited on the samples from the UHMWPE pins after the wear tests (Fig. 4). We measured the diameter of the wear scar of UHMWPE pins which is spherical headed by a metal microscope. The graph which shows the wear loss of UHMWPE pins is given in Fig. 5. It was observed that the wear amount of UHMWPE pins decreased
Fig. 3. The hardnesses plotted as a function of the applied load in mN.
Fig. 4. The particles on the 450 ◦ C–5 h–% 10 ◦ C sample surface after wear test.
Fig. 5. The wear loss of UHMWPE pins.
N. Saklako˘glu et al. / Wear 261 (2006) 264–268
267
Fig. 6. Glancing angle X-ray diffraction traces for untreated and PIII treated samples at various temperatures with and without C ions.
the fee unit cells. It can be explained by assuming a high density of stacking faults. 320 ◦ C–5 h sample included “nitrogen-expanded austenite” (γ N ) peaks beside austenite peaks but austenite peaks had a low intensity compared to the γ N peaks. At the diffraction pattern for 380 ◦ C–5 h sample austenite peaks disappeared and the diffraction pattern was consisted of “expanded austenite” peaks completely. This indicates that expanded austenite has formed to a considerable depth. At the diffraction pattern for 380 ◦ C–5 h–10% CH4 sample austenite peaks and a small γ N peak appeared again but the diffraction pattern was dominated by “carbon-expanded austenite” (γ c ) peaks. This shows that addition of C to the chamber during PIII treatment suppresses the formation of “nitrogenexpanded austenite” and decreases the nitriding effect causing
carbon-rich surface. This is the reason for the relatively lower surface hardness and higher wear loss of the samples which were nitrided with C ions. 450 ◦ C–5 h–10% CH4 sample included a higher γ c and a lower γ (1 1 1) peaks according to 380 ◦ C–5 h–10% CH4 sample. That means it consists of a higher amount of γ c phase. It is known that carbon implantations yield lower friction coefficient [11–13]. γ c phase is responsible for the relatively lower friction coefficient. The shift of the peaks from their fee positions due to excessive carbon or nitrogen atoms causes to change the lattice constant. The interplanar spacing for cubic structures is given by a d= 2 (h + k2 + l2 )
(1)
Table 2 The lattice constants for untreated and treated samples
Phase 2θ d-spacing a
Untreated
320 ◦ C–5 h
380 ◦ C–5 h
380 ◦ C–5 h–CH4
450 ◦ C–5 h–CH4
γ (1 1 1) 43.54 2.078 3.599
γN 40.35 2.235 3.871
γN 40.25 2.240 3.879
γc 41.16 2.193 3.798
γc 40.78 2.213 3.833
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where a is the lattice constant and h, k and l are the Miller indices [14]. Using the Eq. (1), the lattice constants can be calculated for untreated and treated samples. For (1 1 1) peak, the results are given in Table 2. As it seen, nitrogen-expanded austenite causes more expansions in the lattice constant.
• It is clear that the UHWMPE material must be hardened for use as an implant material as a counterface for hard AISI 316 L SS. Otherwise, it produces wear debris, which can be detrimental to the human body. References
4. Conclusion Based on our work, several conclusions may be drawn, as follows: • PIII treatment of AISI 316 L stainless steel at temperatures from 320 to 450 ◦ C has caused slight discolouration on the surface. • The microhardness for near surface increased noticeably as a consequence of the PIII treatment as the process temperature increased but no significant increase for deeper. The C ions were decreased to improve the hardness. • The wear tests of the untreated and treated samples resulted minor colour changes on the surfaces, which could not be measured by a profilometer, but the worn material came from UHWMPE pins. The wear amount of UHMWPE pins decreased with increasing PIII process temperature. The C ions was increased the wear of UHMWPE pins compared to PIII treatment without C ions. • The friction coefficient decreased notably as a result of the 450 ◦ C–5 h–10% CH4 PIII process, but a slight decrease for other PIII process condition. • As it is seen that nitriding using the PIII system produced an expanded austenite phase which is a nitrogen or carbon-rich layer can be associated with a hardening effect resulting from the PIII treatment. PIII can promote hardening by three mechanisms: supersaturation, stresses in the layer and defect density on the properties of the layers. The supersaturation causes stresses and the stresses causes defects and vice versa, so the separation of the different contributions is almost impossible. • Nitrogen causes a considerable expansion of the austenite lattice, but carbon yields a lower increase of the fee lattice constant. So PIII treatment with nitrogen + carbon ions results in less hardness than obtained using just nitrogen. However, it yields lower friction coefficient due to formation of carbonexpanded austenite. The nature of nitrogen-expanded and carbon-expanded austenite has been widely described elsewhere [9,10].
[1] S. Mandl, R. Sader, G. Thorwarth, D. Krause, H.-F. Zeilhofer, H.H. Horch, B. Rauschenbach, Investigation on plasma immersion ion implantation treated medical implants, Biomol. Eng. 19 (2–6) (2002) 129– 132. [2] S. Mandl, D. Krause, G. Thorwarth, R. Sader, F. Zeilhofer, H.H. Horch, B. Rauschenbach, Plasma immersion ion implantation treatment of medical implants, Surf. Coat. Technol. 142–144 (2001) 1046–1050. [3] S. Mandl, B. Rauschenbach, Improving the biocompatibility of medical implants with plasma immersion ion implantation, Surf. Coat. Technol. 156 (1–3) (2002) 276–283. [4] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, C.H. Van Der Valk, Development of a plasma immersion ion implanter for the surface treatment of metal components, Surf. Coat. Technol. 84 (1–3) (1996) 537–543. [5] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, X. Li, M. Samandi, Nitriding of austenitic stainless steel by plasma immersion ion implantation, Surf. Coat. Technol. 74–75 (Part 1) (1995) 417–424. [6] C. Blawert, B.L. Mordike, G.A. Collins, K.T. Short, J. Tendys, Influence of process parameters on the nitriding of steels by plasma immersion ion implantation, Surf. Coat. Technol. 103–104 (1998) 240–247. [7] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Ion-assisted surface modification by plasma immersion ion implantation, Surf. Coat. Technol. 103–104 (1998) 212–217. [8] G.A. Collins, R. Hutchings, J. Tendys, Plasma immersion ion implantation—the role of diffusion, Surf. Coat. Technol. 59 (1–3) (1993) 267–273. [9] M.P. Fewell, D.R.G. Mitchell, J.M. Priest, K.T. Short, G.A. Collins, The nature of expanded austenite, Surf. Coat. Technol. 131 (1–3) (2000) 300–306. [10] C. Blawert, H. Kalvelage, B.L. Mordike, G.A. Collins, K.T. Short, J. Jiraskova, O. Schneeweiss, Nitrogen and carbon expanded austenite produced by PI3, Surf. Coat. Technol. 136 (1–3) (2001) 181–187. [11] R. Sanchez, J.A. Garcia, A. Medrano, M. Rico, R. Martinez, R. Rodriguez, C. Fernandez-Ramos, A. Fernandez, Successive ion implantation of high doses of carbon and nitrogen on steels, Surf. Coat. Technol. 158–159 (2002) 630–635. [12] G. Thorwarth, S. M¨andl, B. Rauschenbach, Plasma immersion ion implantation of cold-work steel, Surf. Coat. Technol. 125 (2000) 94– 99. [13] F.E. Kennedy, D. Lidhagen, A. Erdemir, J.B. Woodford, T. Kato, Communication, tribological behavior of hard carbon coatings on steel substrates, Wear 255 (2003) 854–858. [14] P.L. Mangonon, The Principles of Materials Selection for Engineering Design, Prentice-Hall Inc., 1999, ISBN 0-13-242595-5.
Tribological behavior of PIII treated AISI 316 L austenitic stainless steel against UHMWPE counterface N. Saklako˘glu a,∗ , ˙I.E. Saklako˘glu b , K.T. Short c , G.A. Collins c a
Celal Bayar University, Faculty of Engineering, Mechanical Engineering Department, Muradiye Kampusu, Manisa 45140, Turkey b Ege University, Faculty of Engineering, Mechanical Engineering Department, Izmir, Turkey c Australian Nuclear Science and Technology Organisation, NSW, Australia Received 15 February 2005; received in revised form 31 August 2005; accepted 24 October 2005 Available online 18 January 2006
Abstract The aim of this work was to study the tribological benefits of PIII treated austenitic stainless steel by nitrogen ions and/or C ions in a ringer solution, which simulates the environment of the human body. This was achieved by hardness, wear and friction testing, and atomic force microscopy and XRD studies. The results showed that the samples, both treated and untreated, exhibited virtually no wear from contact with the ultra high molecular weight polyethylene (UHMWPE) pins, however, the pins themselves exhibited wear. The amount of wear of the pins was found to decrease with increasing PIII treatment temperature, but addition of C to the chamber caused some increase the amount of wear on the pins. Although C ions reduced to improve the hardness, friction characteristic was improved by formation of carbon-expanded austenite. © 2005 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Austenitic stainless steel; Tribology; Surface modification; UHMWPE
1. Introduction Modern implants such as hip and knee prostheses are made of a metallic femoral component (316 L stainless steel, Ti6AI4V, or cobalt–chromium), which is in sliding contact with an ultra high molecular weight polyethylene (UHMWPE) acetabular cup. However, numerous clinical studies have raised concerns related to the detrimental effect of wear debris from artificial hip prostheses to the tissue surrounding the implants. Several solutions to this problem have been proposed. Improvements in the tribological properties of stainless steel achieved by ion implantation have been widely described elsewhere [1–3]. Over the last 10 years, Collins and co-workers from ANSTO have studied the surface treatment of austenitic stainless steel using the plasma immersion ion implantation (PIII) technique [4–6], developed by ANSTO. PIII is a non-line-of-sight technique for the surface modification of materials, but it is a hybrid treatment involving implantation and thermodynamically driven diffusion of nitrogen [7–10].
∗
Corresponding author. Tel.: +90 5337275727; fax: +90 2362412143. E-mail address: [email protected] (N. Saklako˘glu).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.10.008
In this paper, the tribological characteristics of AISI 316 L stainless steel in a medium called ‘ringer solution’ have been studied, because this solution chemically simulates the environment surrounding metallic implants in the human body. 2. Experimental details The samples chosen for this investigation were AISI 316 L stainless steel 30 mm in diameter and 4-mm thick. Each sample was ultrasonically cleaned in ethanol before loading into a treatment chamber. PIII treatment was performed at ANSTO, using the system which has been previously described elsewhere [4,8]. Samples were treated at 320, 380 and 450 ◦ C respectively with and without C ions. The sample chamber was pumped to a base pressure of <2 × 10−7 mbar before each PIII treatment was begun. Nitrogen implantations were performed, applying negative high voltage pulses of 30 kV. Prior to the implantation, an Ar/H2 preclean at 15 kV/100 Hz HV pulsing during heating up was performed. For nitrogen + carbon implantations 90% N2 + 10% CH4 gases were used. The details of the treatment parameters are given in Table 1. Surface damage occurring after the PIII treatment was examined using an atomic force microscope (AFM).
N. Saklako˘glu et al. / Wear 261 (2006) 264–268 Table 1 PIII treatment details Sample name
Temperature (◦ C)
Time (h)
Nitrogen dose (at. cm−2 )
Untreated 320 ◦ C–5 h 380 ◦ C–5 h 380 ◦ C–5 h–10% CH4 450 ◦ C–5 h–10% CH4
– 320 380 380 450
– 5 5 5 5
– ∼2 × 1018 ∼2 × 1018 ∼2 × 1018 ∼2 × 1018
Microhardness was measured using a UMIS-2000 ultra microhardness tester with a Berkovich indenter. This was achieved by performing hardness tests directly on the surface at loads of 50, 250 and 1000 mN. Five indents were made at each load on each sample from which average values were calculated. Wear measurement of the treated and untreated 316 discs were performed on a pin-on-disc tribometer (CSEM) using a
265
UHMWPE counterface (6-mm diameter) in the ringer solution. It was carried out under a load of 5 N and a constant speed of 0.3 m/s. Each disc was tested for 500 m (corresponding to 9969 turns). The coefficient of friction was recorded during wear testing by a transducer on the load arm of the tribometer. The quantitative value of wear was obtained by measuring the cross sectional area of the wear track and then converting this to the wear volume.
3. Results and discussion 3.1. Surface examination After treatments, all samples exhibited slight discolouration on the surface. The surface appearance of the untreated and treated samples was observed by AFM scanning. Fig. 1 shows 2D representations of the surface appearance of samples.
Fig. 1. The surfaces of untreated and treated samples by AFM.
266
N. Saklako˘glu et al. / Wear 261 (2006) 264–268
Fig. 2. Friction coefficients of the samples as a function of the distance.
3.2. Friction coefficient As is seen in Fig. 2, all friction curves belong to PIII treated samples lied under untreated sample’s, but no clear improvement. PIII treatments with C ions were located in the bottom reducing the friction coefficient. Notably, the 450 ◦ C–5 h–10% CH4 process reduced the friction coefficient by a factor of ∼1.6. 3.3. Microhardness The hardness results for the PIII treated and untreated samples can be seen in Fig. 3. It was observed that the hardness increased on the near surface with increasing process temperature. The addition of C ions was decreased to improve the hardness.
with increasing PIII process temperature. The addition of C ions to the chamber during PIII process was increased the wear of UHMWPE pins compared to PIII treatment without C ions. 3.5. XRD-examination Glancing angle X-ray diffraction patterns for untreated and PIII treated samples are shown in Fig. 6. The peaks in the PIII treated samples, below the positions of the standard austenite peaks, indicate the formation of expanded austenite. The peak shifts are smaller for the carbon-expanded austenite. According to Blawert et al. [10] that the nitrogen and carbon is randomly distributed in the middle of the cube edges, expanding
3.4. Wear The samples, both treated and untreated, exhibited virtually no wear. The amount of wear of the samples was so slight that it was not possible to measure the amount of wear volume. There were merely slight tracks showing a colour change on the sample surfaces. It was seen that there was a large amount of wear debris deposited on the samples from the UHMWPE pins after the wear tests (Fig. 4). We measured the diameter of the wear scar of UHMWPE pins which is spherical headed by a metal microscope. The graph which shows the wear loss of UHMWPE pins is given in Fig. 5. It was observed that the wear amount of UHMWPE pins decreased
Fig. 3. The hardnesses plotted as a function of the applied load in mN.
Fig. 4. The particles on the 450 ◦ C–5 h–% 10 ◦ C sample surface after wear test.
Fig. 5. The wear loss of UHMWPE pins.
N. Saklako˘glu et al. / Wear 261 (2006) 264–268
267
Fig. 6. Glancing angle X-ray diffraction traces for untreated and PIII treated samples at various temperatures with and without C ions.
the fee unit cells. It can be explained by assuming a high density of stacking faults. 320 ◦ C–5 h sample included “nitrogen-expanded austenite” (γ N ) peaks beside austenite peaks but austenite peaks had a low intensity compared to the γ N peaks. At the diffraction pattern for 380 ◦ C–5 h sample austenite peaks disappeared and the diffraction pattern was consisted of “expanded austenite” peaks completely. This indicates that expanded austenite has formed to a considerable depth. At the diffraction pattern for 380 ◦ C–5 h–10% CH4 sample austenite peaks and a small γ N peak appeared again but the diffraction pattern was dominated by “carbon-expanded austenite” (γ c ) peaks. This shows that addition of C to the chamber during PIII treatment suppresses the formation of “nitrogenexpanded austenite” and decreases the nitriding effect causing
carbon-rich surface. This is the reason for the relatively lower surface hardness and higher wear loss of the samples which were nitrided with C ions. 450 ◦ C–5 h–10% CH4 sample included a higher γ c and a lower γ (1 1 1) peaks according to 380 ◦ C–5 h–10% CH4 sample. That means it consists of a higher amount of γ c phase. It is known that carbon implantations yield lower friction coefficient [11–13]. γ c phase is responsible for the relatively lower friction coefficient. The shift of the peaks from their fee positions due to excessive carbon or nitrogen atoms causes to change the lattice constant. The interplanar spacing for cubic structures is given by a d= 2 (h + k2 + l2 )
(1)
Table 2 The lattice constants for untreated and treated samples
Phase 2θ d-spacing a
Untreated
320 ◦ C–5 h
380 ◦ C–5 h
380 ◦ C–5 h–CH4
450 ◦ C–5 h–CH4
γ (1 1 1) 43.54 2.078 3.599
γN 40.35 2.235 3.871
γN 40.25 2.240 3.879
γc 41.16 2.193 3.798
γc 40.78 2.213 3.833
268
N. Saklako˘glu et al. / Wear 261 (2006) 264–268
where a is the lattice constant and h, k and l are the Miller indices [14]. Using the Eq. (1), the lattice constants can be calculated for untreated and treated samples. For (1 1 1) peak, the results are given in Table 2. As it seen, nitrogen-expanded austenite causes more expansions in the lattice constant.
• It is clear that the UHWMPE material must be hardened for use as an implant material as a counterface for hard AISI 316 L SS. Otherwise, it produces wear debris, which can be detrimental to the human body. References
4. Conclusion Based on our work, several conclusions may be drawn, as follows: • PIII treatment of AISI 316 L stainless steel at temperatures from 320 to 450 ◦ C has caused slight discolouration on the surface. • The microhardness for near surface increased noticeably as a consequence of the PIII treatment as the process temperature increased but no significant increase for deeper. The C ions were decreased to improve the hardness. • The wear tests of the untreated and treated samples resulted minor colour changes on the surfaces, which could not be measured by a profilometer, but the worn material came from UHWMPE pins. The wear amount of UHMWPE pins decreased with increasing PIII process temperature. The C ions was increased the wear of UHMWPE pins compared to PIII treatment without C ions. • The friction coefficient decreased notably as a result of the 450 ◦ C–5 h–10% CH4 PIII process, but a slight decrease for other PIII process condition. • As it is seen that nitriding using the PIII system produced an expanded austenite phase which is a nitrogen or carbon-rich layer can be associated with a hardening effect resulting from the PIII treatment. PIII can promote hardening by three mechanisms: supersaturation, stresses in the layer and defect density on the properties of the layers. The supersaturation causes stresses and the stresses causes defects and vice versa, so the separation of the different contributions is almost impossible. • Nitrogen causes a considerable expansion of the austenite lattice, but carbon yields a lower increase of the fee lattice constant. So PIII treatment with nitrogen + carbon ions results in less hardness than obtained using just nitrogen. However, it yields lower friction coefficient due to formation of carbonexpanded austenite. The nature of nitrogen-expanded and carbon-expanded austenite has been widely described elsewhere [9,10].
[1] S. Mandl, R. Sader, G. Thorwarth, D. Krause, H.-F. Zeilhofer, H.H. Horch, B. Rauschenbach, Investigation on plasma immersion ion implantation treated medical implants, Biomol. Eng. 19 (2–6) (2002) 129– 132. [2] S. Mandl, D. Krause, G. Thorwarth, R. Sader, F. Zeilhofer, H.H. Horch, B. Rauschenbach, Plasma immersion ion implantation treatment of medical implants, Surf. Coat. Technol. 142–144 (2001) 1046–1050. [3] S. Mandl, B. Rauschenbach, Improving the biocompatibility of medical implants with plasma immersion ion implantation, Surf. Coat. Technol. 156 (1–3) (2002) 276–283. [4] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, C.H. Van Der Valk, Development of a plasma immersion ion implanter for the surface treatment of metal components, Surf. Coat. Technol. 84 (1–3) (1996) 537–543. [5] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, X. Li, M. Samandi, Nitriding of austenitic stainless steel by plasma immersion ion implantation, Surf. Coat. Technol. 74–75 (Part 1) (1995) 417–424. [6] C. Blawert, B.L. Mordike, G.A. Collins, K.T. Short, J. Tendys, Influence of process parameters on the nitriding of steels by plasma immersion ion implantation, Surf. Coat. Technol. 103–104 (1998) 240–247. [7] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Ion-assisted surface modification by plasma immersion ion implantation, Surf. Coat. Technol. 103–104 (1998) 212–217. [8] G.A. Collins, R. Hutchings, J. Tendys, Plasma immersion ion implantation—the role of diffusion, Surf. Coat. Technol. 59 (1–3) (1993) 267–273. [9] M.P. Fewell, D.R.G. Mitchell, J.M. Priest, K.T. Short, G.A. Collins, The nature of expanded austenite, Surf. Coat. Technol. 131 (1–3) (2000) 300–306. [10] C. Blawert, H. Kalvelage, B.L. Mordike, G.A. Collins, K.T. Short, J. Jiraskova, O. Schneeweiss, Nitrogen and carbon expanded austenite produced by PI3, Surf. Coat. Technol. 136 (1–3) (2001) 181–187. [11] R. Sanchez, J.A. Garcia, A. Medrano, M. Rico, R. Martinez, R. Rodriguez, C. Fernandez-Ramos, A. Fernandez, Successive ion implantation of high doses of carbon and nitrogen on steels, Surf. Coat. Technol. 158–159 (2002) 630–635. [12] G. Thorwarth, S. M¨andl, B. Rauschenbach, Plasma immersion ion implantation of cold-work steel, Surf. Coat. Technol. 125 (2000) 94– 99. [13] F.E. Kennedy, D. Lidhagen, A. Erdemir, J.B. Woodford, T. Kato, Communication, tribological behavior of hard carbon coatings on steel substrates, Wear 255 (2003) 854–858. [14] P.L. Mangonon, The Principles of Materials Selection for Engineering Design, Prentice-Hall Inc., 1999, ISBN 0-13-242595-5.